Mechanism and consequence of abnormal calcium homeostasis in Rett syndrome astrocytes

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Version of Record published: April 16, 2018 (This version)
Accepted Manuscript published: March 29, 2018 (Go to version)
Accepted: March 28, 2018
Received: November 7, 2017

1. Of interest
Dynamic compartmentalization of the pro-invasive transcription factor NHR-67 reveals a role for Groucho in regulating a proliferative-invasive cellular switch in C. elegans

Taylor N Medwig-Kinney, Brian A Kinney ... David Q Matus

Research Article Dec 1, 2023
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Abstract

Astrocytes play an important role in Rett syndrome (RTT) disease progression. Although the non-cell-autonomous effect of RTT astrocytes on neurons was documented, cell-autonomous phenotypes and mechanisms within RTT astrocytes are not well understood. We report that spontaneous calcium activity is abnormal in RTT astrocytes in vitro, in situ, and in vivo. Such abnormal calcium activity is mediated by calcium overload in the endoplasmic reticulum caused by abnormal store operated calcium entry, which is in part dependent on elevated expression of TRPC4. Furthermore, the abnormal calcium activity leads to excessive activation of extrasynaptic NMDA receptors (eNMDARs) on neighboring neurons and increased network excitability in Mecp2 knockout mice. Finally, both the abnormal astrocytic calcium activity and the excessive activation of eNMDARs are caused by Mecp2 deletion in astrocytes in vivo. Our findings provide evidence that abnormal calcium homeostasis is a key cell-autonomous phenotype in RTT astrocytes, and reveal its mechanism and consequence.

https://doi.org/10.7554/eLife.33417.001

Introduction

Rett syndrome (RTT) is a debilitating neurodevelopmental disorder (Hagberg, 1985) caused by mutations in the methyl-CpG binding protein 2 (MECP2) gene (Amir et al., 1999). RTT is a complex disease, as all key cell types (neurons, astrocytes, microglia, and oligodendrocytes) in the brain have been shown to contribute to the disease etiology (Ballas et al., 2009; Chao et al., 2010; Derecki et al., 2012; Fyffe et al., 2008; Lioy et al., 2011; Luikenhuis et al., 2004; Maezawa and Jin, 2010; Maezawa et al., 2009; Nguyen et al., 2013; Samaco et al., 2009; Williams et al., 2014). To fully understand the disease etiology and develop effective treatments, it is essential to define the key phenotypes, link the phenotypes with loss of MeCP2 function, and reveal the consequences of the phenotypes in each brain cell type. Moreover, it is also important to define the connection between phenotypes in different cell types.

Earlier research to understand the RTT disease etiology mostly focused on neuronal dysfunction in RTT (Chao et al., 2010; Fyffe et al., 2008; Luikenhuis et al., 2004; Samaco et al., 2009), because MeCP2 is expressed at high levels in neurons in a temporal and spatial pattern closely tracking neuronal maturation (Akbarian et al., 2001; Shahbazian et al., 2002). Not surprisingly, specific subsets of RTT-like phenotypes were observed in mouse models when Mecp2 is deleted in specific neuronal subtypes (Chao et al., 2010; Fyffe et al., 2008; Samaco et al., 2009). Collectively, these studies confirm the essential role of MeCP2 in maintaining the normal function of neurons, suggesting the loss of function or malfunction of MeCP2 in neurons as the main cause of RTT pathogenesis. Recently, a series of studies have reported that astrocytes express MeCP2, loss of MeCP2 in astrocytes causes neuronal defects in a non-cell autonomous manner, and restoring MeCP2 expression to normal level in astrocytes alone rescues some disease symptoms (Ballas et al., 2009; Lioy et al., 2011; Maezawa et al., 2009; Williams et al., 2014). Intriguingly, cell type specific deletion of Mecp2 in astrocytes alone is not sufficient to generate most of the RTT-like phenotypes in mice (Lioy et al., 2011), suggesting the loss of function or malfunction of MeCP2 in astrocytes is not the initial cause of the disease, yet is required to maintain the disease progression. Although a few studies have reported gene expression changes in Mecp2 mutant mouse astrocytes (Forbes-Lorman et al., 2014; Yasui et al., 2013), no major cell autonomous phenotype in astrocytes has been characterized in depth. Thus, identification of cell autonomous phenotypes in astrocytes not only is essential for our understanding of astrocyte dysfunction in RTT, but also will help us to determine the mechanism underlying its non-cell autonomous effects on neurons.

To identify novel cell autonomous phenotypes in RTT astrocytes, we examined intracellular calcium dynamics in astrocytes differentiated from congenic pairs of wild type and mutant RTT iPSC lines. We have used the term ‘isogenic’ to describe these RTT iPSC lines in our previous reports (Ananiev et al., 2011; Williams et al., 2014). However, given that the difference between wild type and mutant iPSC lines generated from the same RTT patients is the entire X chromosome, congenic is a more accurate term to define these lines. We observed abnormal spontaneous and pharmacologically evoked cytosolic calcium activities in cultured mutant RTT astrocytes, when compared to their congenic wild type controls. Similar phenotypes can be observed in Mecp2 knockout mouse astrocytes in vitro, in situ, and in vivo. In addition, re-expression of wild type MeCP2 is sufficient to rescue the abnormal calcium activities in both the mouse and human RTT astrocytes, further confirming that these phenotypes are caused by the loss of MeCP2 function. Moreover, we report that these abnormal calcium activities are mediated by calcium overload in the endoplasmic reticulum (ER), which is caused by abnormal TRPC4-dependent store operated calcium entry (SOCE). Although we provide no direct evidence that TRPC4 is the actual SOCE channel, both loss- and gain-of-function manipulations from our study suggest TRPC4 is upstream of the SOCE and subsequent calcium phenotypes in RTT astrocytes. Finally, we show that these abnormal calcium activities in astrocytes lead to excessive activation of extrasynaptic NMDA receptors (eNMDARs) on neighboring neurons and increased network excitability; and both the abnormal astrocytic calcium activity and the excessive activation of eNMDARs are caused by the loss of Mecp2 in astrocytes (but not neurons) in vivo. Our findings provide the first evidence that suggests abnormal calcium homeostasis as a cell autonomous phenotype in RTT astrocytes, and begin to reveal the mechanism and consequence of such a phenotype.

Results

Abnormal calcium activities in RTT astrocytes

Congenic pairs of wild type and mutant RTT iPSC lines carrying the common R294X mutation were differentiated into glial fibrillary acidic protein positive (GFAP+) astrocytes as described in our previous paper (Williams et al., 2014). On average, more than 90% of the cells were GFAP +in cultures used in our study (Figure 1—figure supplement 1 and Table 1). Using the green-colored calcium-sensitive dye Fluo-4, we monitored spontaneous cytosolic calcium (Ca²⁺) dynamics in live mutant (R294X-MT) and congenic wild type control (R294X-WT) astrocytes (Videos 1–2, Figure 1A–B). Ionophore A23187 was used as a loading control (Figure 1—figure supplement 2). A significantly higher percentage of mutant astrocytes (30 ± 3% in MT vs. 18 ± 2% in WT, n = 26 randomly selected fields in each genotype, p=0.001) showed spontaneous oscillation in cytosolic Ca²⁺ levels (Figure 1C). In each astrocyte showing spontaneous oscillation, we further separated and quantified the calcium transients in the soma from those in the processes (Figure 1A). In the soma, the mutant cells had a significantly higher frequency (0.74 ± 0.02/min in MT vs. 0.41 ± 0.01/min in WT, n = 408 cells in MT and n = 280 cells in WT, p<0.001; Figure 1C) and slightly higher amplitude (Table 2) than their congenic wild type controls. Similar changes were observed in the processes of those astrocytes (Table 2 and Figure 1—figure supplement 3). Moreover, the R294X mutant astrocytes had significantly higher peak amplitude (4.60 ± 0.07 in MT vs. 4.04 ± 0.07 in WT, n = 567 cells in MT and n = 811 cells in WT, p<0.001) of ATP-induced Ca²⁺ rise than the congenic wild type control (Figure 1D–E). The increase in ATP-induced Ca²⁺ rise in human RTT mutant astrocytes was unlikely to be mediated by the P2 receptors, because no difference was detected in the expression level of any P2X or P2Y receptor family members between the wild type and mutant RTT astrocytes by RNA-seq and microarray analysis (data not shown). Similar observations of altered Ca²⁺ activities were made in human astrocytes carrying the rare RTT-causing V247fs mutation (Figure 1—figure supplement 4). Thus, both spontaneous and pharmacologically evoked cytosolic Ca²⁺ activity appeared abnormal in mutant human RTT astrocytes differentiated from patient-specific iPSCs and grown in the absence of neurons.

Video 1

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posterframe for video — Spontaneous calcium activities in wild type human astrocytes
https://doi.org/10.7554/eLife.33417.002

Video 2

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Table 1

Quantification of the percentage of GFAP+ cell in five differentiations from human iPSCs

https://doi.org/10.7554/eLife.33417.004

stereology #		1	2	3	4	5
WT		90%	94%	92%	89%	91%
MT		90%	92%	94%	89%	92%

Table 2

Amplitudes of spontaneous Ca²⁺ elevations in different experiments

https://doi.org/10.7554/eLife.33417.005

Cell type	Wild-type	Mutant
R294X astrocytes	1.660 ± 0.01624 N = 1091	1.808 ± 0.01882 N = 2962
R294X astrocytes (processes)	1.467 ± 0.01601 N = 2228	1.722 ± 0.01994 N = 2743
V247fs astrocytes	1.686 ± 0.02453 N = 902	1.816 ± 0.02128 N = 1760
V247fs astrocytes (processes)	1.560 ± 0.02224 N = 1825	1.640 ± 0.02447 N = 2312
Mouse primary astrocytes	1.787 ± 0.007193 N = 5694	1.822 ± 0.006209 N = 8389
Astrocytes in situ	2.010 ± 0.02998 N = 1367	2.151 ± 0.03079 N = 1997
Astrocytes in vivo	3.463 ± 0.1410 N = 285	3.045 ± 0.05410 N = 1346
Astrocytes in vivo (processes)	3.283 ± 0.03640 N = 2040	3.007 ± 0.01546 N = 8796

Figure 1 with 4 supplements see all

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Abnormal Ca²⁺ activities in mutant human RTT astrocytes.

(A) Pseudocolored Fluo4 fluorescence images from wild type (WT, left) and MeCP2 mutant (MT, right) astrocytes differentiated from human iPSCs. Green ellipses indicate astrocyte cell soma, while yellow rectangles indicate processes. Scale bars = 50 μm. (B) Representative ΔF/F₀ traces showing the spontaneous intracellular Ca²⁺ activity from soma in (A). The black traces are from WT astrocytes and red traces are from MT ones. (C) Quantification of the percentage (left) of astrocytes showing spontaneous Ca²⁺ oscillations and the frequency (right) of such oscillations. (D) Trace of Fluo4 fluorescence changes in wild type (WT) and mutant (MT) human astrocytes stimulated by 10 μM ATP. Average traces are shown with the solid lines. (E) Quantification of the peak amplitude of the ATP-evoked Ca²⁺ elevations in wild type (WT) and mutant (MT) human astrocytes.

https://doi.org/10.7554/eLife.33417.006

Figure 1—source data 1 The numerical data for the graphs shown in Figure 1C and E.: https://doi.org/10.7554/eLife.33417.011
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To determine whether the observed abnormal cytosolic Ca²⁺ activities were caused by the loss of MeCP2 function, we infected mutant human RTT astrocytes with lentiviruses expressing either the green fluorescence protein (lenti-GFP) alone or GFP and wild type MECP2 together (lenti-MECP2/GFP). Three days after the infection, more than 90% of the cells were positive for GFP in both infection types (Figure 2A). In R294X mutant astrocytes infected with lenti-MECP2/GFP, anti-MECP2 immunoreactivity was detected in all the GFP+ cells (right, Figure 2A). Further quantification of MECP2 immunoreactivity in those cells revealed no difference from wild type astrocytes (Figure 2—figure supplement 1), suggesting the exogenous expression of MECP2 was not significant above the endogenous level. When Ca²⁺ activity was recorded (Figure 2B), the percentage of cells with spontaneous Ca²⁺ oscillation was significantly reduced by the expression of wild type MECP2, compared with expression of GFP alone, in the mutant human RTT astrocytes (20 ± 2% in MECP2/GFP vs. 36 ± 4% in GFP, n = 6 randomly selected fields in each group, p=0.006; Figure 2C). In the cells detected with spontaneous cytosolic Ca²⁺ oscillation, both the frequency (0.27 ± 0.06/min in MECP2/GFP vs. 0.59 ± 0.06/min in GFP, n = 21 cells in MECP2/GFP and n = 32 cells in GFP, p<0.001; Figure 2C) and amplitude (1.65 ± 0.08 in MECP2/GFP vs. 1.75 ± 0.03 in GFP, n = 62 events in MECP2/GFP and n = 209 events in GFP, p<0.001) were significantly reduced by the exogenous expression of wild type MECP2 as compared with expression of GFP alone. In addition, expression of wild type MECP2 significantly reduced the elevated level of pharmacologically evoked Ca²⁺ response in mutant human RTT astrocytes (3.90 ± 0.13 in MECP2/GFP vs. 4.20 ± 0.10 in GFP, n = 96 cells in MECP2/GFP and n = 123 cells in GFP, p=0.022; Figure 2D–E). Similar rescue effects were observed when the V247fs mutant astrocytes were used (Figure 2—figure supplement 2). Together, these results further support the cell autonomous nature of the abnormal Ca²⁺ activities in RTT mutant astrocytes.

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Exogenous expression of MeCP2 rescues the abnormal calcium activities in RTT astrocytes.

(A) Representative images of mutant human RTT astrocytes infected with lentivirus expressing GFP alone (left) or with lentivirus co-expressing wild type MECP2 and GFP (right). MECP2 immunoreactivity was detected in GFAP positive cells in the right column, but not the left column. Scale bars, 50 μm. (B) Representative ΔF/F₀ traces showing the spontaneous intracellular Ca²⁺ activity in RTT mutant astrocytes infected with lentivirus expressing either GFP alone (GFP) or co-expressing GFP and wild type MECP2 together (MECP2/GFP). (C) Quantification of the percentage (left) of astrocytes showing spontaneous Ca²⁺ oscillations and the frequency (middle) and amplitude (right) of such oscillations in RTT mutant astrocytes infected with lentivirus expressing either GFP alone (GFP) or co-expressing GFP and wild type MECP2 together (MECP2/GFP). (D) Trace of ATP (10 μM) evoked Fluo4 fluorescence changes in RTT mutant astrocytes infected with lentivirus expressing either GFP alone (GFP) or co-expressing GFP and wild type MECP2 together (MECP2/GFP). Average traces are shown with the solid lines. (E) Quantification of the peak amplitude of the ATP-evoked Ca²⁺. The bar graphs in this figure show the mean ±s.e.m. *p<0.05, **p<0.01, ***p<0.001.

https://doi.org/10.7554/eLife.33417.012

Figure 2—source data 1 The numerical data for the graphs shown in Figure 2C and E.: https://doi.org/10.7554/eLife.33417.015
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Since the phenotype of abnormal Ca²⁺ activities have not been reported in previous studies using RTT mouse models, we next examined whether they can be observed in Mecp2 null mouse astrocytes. Primary hippocampal astrocytes were isolated from newborn male Mecp2 null mice (Mecp2^-/y) and their wild type (WT) littermates, both of which also carried the Rosa-CAG-LSL-GCaMP6s allele (http://jaxmice.jax.org/strain/024106) and the hGFAP-creERT allele (http://www.jax.org/strain/012849). The astrocytes were treated with 4-OH tamoxifen for 24 hr to induce the expression of GCaMP6s, and then imaged for GCaMP6s fluorescence as the indication of cytosolic Ca²⁺ activities. Similar to our findings in human RTT astrocytes, spontaneous Ca²⁺ oscillation (Figure 3A–B) was detected in significantly higher percentage of Mecp2 null astrocytes than in wild type controls (67 ± 5% in Mecp2^-/y vs. 47 ± 6% in WT, n = 19 randomly chosen fields in both Mecp2^-/y and WT, p=0.01; Figure 3C). In cells that showed spontaneous Ca²⁺ oscillation, significantly higher frequency (2.01 ± 0.04/min in Mecp2^-/y vs. 1.64 ± 0.03/min in WT, n = 575 cells in Mecp2^-/y and n = 439 cells in WT, p<0.001; Figure 3C) and slightly higher amplitude (Table 2) were observed in the soma of Mecp2 null astrocytes than in wild type astrocytes. Since the processes were very short in primary mouse astrocyte culture, spontaneous Ca²⁺ activity in processes was not quantified separately because it closely resembled that in the soma. As for pharmacologically evoked Ca²⁺ response, significantly higher amplitude was detected in primary Mecp2 null mouse astrocytes (7.25 ± 0.23 in Mecp2^-/y vs. 6.54 ± 0.12 in WT, n = 153 cells in Mecp2^-/y and n = 278 cells in WT, p=0.04) than in wild type astrocytes (Figure 3D–E).

Figure 3

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Abnormal Ca²⁺ activities in primary astrocytes isolated from *Mecp2* null mice.

(A) The pseudocolored GCaMP6s fluorescence image from astrocytes. (B) Representative ΔF/F₀ traces showing the spontaneous intracellular Ca²⁺ activity in mouse primary astrocytes from wild type and *Mecp2*-/y mice. (C) Quantification of the percentage (left) of astrocytes showing spontaneous Ca²⁺ oscillations and the frequency (right) of such oscillations. (D) Trace of fluorescence changes in wild type and *Mecp2*-/y astrocytes stimulated by 10 μM ATP. Average traces are shown with the solid lines. (E) Quantification of the peak amplitude of the ATP-evoked Ca²⁺ elevations from wild type and *Mecp2*-/y astrocytes.

https://doi.org/10.7554/eLife.33417.016

Figure 3—source data 1 The numerical data for the graphs shown in Figure 3C and E.: https://doi.org/10.7554/eLife.33417.017
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Next, to explore the in situ relevance of this novel phenotype in Ca²⁺ dynamics, we studied spontaneous and agonist evoked Ca²⁺ activities in astrocytes in acute hippocampal slices prepared from 2 to 3 week-old Mecp2 null mice and their wild type littermates using Fluo4. In this series of experiments, SR101 was used to label astrocytes (Figure 4A); TTX (1 μM) was used to suppress spontaneous neuronal activity. Similar to our observation in cultured human patient-specific iPSC derived astrocytes and primary mouse astrocytes, higher frequency (0.59 ± 0.01/min in Mecp2^-/y vs. 0.47 ± 0.01/min in WT, n = 337 cells in Mecp2^-/y and n = 293 cells in WT, p<0.001) and slightly higher amplitude (Table 2) of spontaneous Ca²⁺ oscillation was observed in the soma of SR101+ cells in Mecp2 null slices than in wild type slices (Figure 4B–C). In addition, the astrocytes in Mecp2 null slices had significantly higher peak amplitude (2.75 ± 0.02 in MT vs. 2.22 ± 0.09 in WT, n = 754 cells in Mecp2^-/y and n = 238 cells in WT, p<0.001) of glutamate-induced Ca²⁺ rise than astrocytes in wild type slices (Figure 4D).

Figure 4

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Abnormal Ca²⁺ activities in astrocytes in acute brain slices prepared from *Mecp2* null mice.

(A) Representative images of SR101 labeling (astrocyte marker) and Fluo4 signals in acute hippocampal slices. Note the colocalization of SR101 and Fluo4. Scale bars = 50 μm. (B) Representative ΔF/F₀ traces showing the spontaneous intracellular Ca²⁺ activity in SR101 positive cells in acute hippocampal slices prepared from wild type and *Mecp2^-/y* mice. (C) Quantification of the frequency of the astrocytic Ca²⁺ oscillations in the slice experiments. (D) Traces of Fluo4 fluorescence changes stimulated by 100 μM glutamate in SR101 positive cells in acute hippocampal slices prepared from wild type and *Mecp2*-/y mice. Average traces are shown with solid lines. (E) Quantification of the peak amplitude of the glutamate-evoked Ca²⁺ elevations. The bar graphs in this figure show the mean ±s.e.m. *p<0.05, **p<0.01, ***p<0.001.

https://doi.org/10.7554/eLife.33417.018

Figure 4—source data 1 The numerical data for the graphs shown in Figure 4C and E.: https://doi.org/10.7554/eLife.33417.019
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To further rule out potential artifacts in cultured astrocytes and in acute slices, we examined spontaneous Ca²⁺ activities in astrocytes of the frontal cortex in live mice carrying the GCaMP6s sensor. We mated male Rosa-CAG-LSL-GCaMP6s mice with either female wild type (Mecp2^+/+) or Mecp2^flox/flox mice, injected adeno-associated virus (AAV) expressing mCherry and Cre recombinase under the human GFAP promoter (AAV8-hGFAP-mCherry-Cre) into the lateral ventricles of the postnatal day one male pups from these matings to activate the expression of GCaMP6s and delete Mecp2 predominantly in astrocytes. Based on the mating scheme, male pups would be of the genotype of either Mecp2^+/y;Rosa-CAG-LSL-GCaMP6s (WT) or Mecp2^flox/y; Rosa-CAG-LSL-GCaMP6s (floxed). When immunostaining with MeCP2 antibody on sections prepared from 3 months old floxed mice that receivedAAV8-hGFAP-mCherry-Cre injection at birth, 100% of the GCaMP6s (anti-GFP) positive cells in the cortex were MeCP2 negative. In contrast, GCaMP6s positive cells from WT mice were MeCP2 positive (Figure 5—figure supplement 1). When GCaMP6s fluorescence were recorded in mCherry-positive cells in the frontal cortex of 2–3 months-old live WT and floxed mice injected with AAV8-hGFAP-mCherry-Cre (MT) (Video 3–4, Figure 5A–B), higher frequency of spontaneous oscillation of GCaMP6s fluorescence was detected in both the soma (2.15 ± 0.11/min in MT vs. 1.44 ± 0.20/min in WT, n = 57 cells in MT and n = 18 cells in WT, p=0.003, Figure 5C) and processes (2.14 ± 0.04/min in MT vs. 1.69 ± 0.06/min in WT, n = 373 processes in MT and n = 110 processes in WT, p<0.001, Figure 5—figure supplement 2) of MT astrocytes. However, the amplitude of spontaneous astrocytic Ca²⁺ activity appeared to be slightly smaller in MT mice (Table 2). Taken together, the phenotype of increased frequency of spontaneous cytosolic Ca²⁺ activity was found consistently across species, as well as in vitro, in situ, and in vivo.

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Figure 5 with 2 supplements see all

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Abnormal spontaneous Ca²⁺ activities *in vivo* from *Mecp2* null astrocytes.

(A) Representative GCaMP6s image showing two astrocytes in vivo. The somas and the processes could be clearly identified. Scale bars = 10 μm. (B) Representative ΔF/F₀ traces showing the spontaneous intracellular Ca²⁺ activity in the soma of astrocytes in vivo from wild type and *Mecp2* null astrocytes. (C) Quantification of the frequency (left) and the full width at half maximum (FWHM, right) of the astrocytic Ca²⁺ oscillations *in vivo*. The bar graphs in this figure show the mean ±s.e.m. ***p<0.001.

https://doi.org/10.7554/eLife.33417.022

Figure 5—source data 1 The numerical data for the graphs shown in Figure 5.: https://doi.org/10.7554/eLife.33417.025
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Abnormal calcium load in the ER, calcium leak from the ER, baseline cytosolic calcium level, and TRPC4-dependent SOCE in RTT astrocytes

To reveal the cellular and molecular mechanisms underlying the abnormal Ca²⁺ activities in the RTT astrocytes, we first examined the source of Ca²⁺. Since the abnormal spontaneous cytosolic Ca²⁺ oscillation in mutant human astrocytes persisted after Ca²⁺ was removed completely from extracellular environment (0 mM Ca²⁺ plus 2 mM EGTA, Figure 6—figure supplement 1), we measured the Ca²⁺ load in the endoplasmic reticulum (ER), the major source of intracellular Ca²⁺ storage. Treating astrocytes with thapsigargin (TG, 1 μM), an irreversible inhibitor of sarco-ER Ca²⁺ ATPase, induced a slow elevation of Ca²⁺ in the cytosol, which was mediated by the leakage of Ca²⁺ from the ER. The amplitude of the TG-induced Ca²⁺ elevation was significantly higher in the mutant human RTT astrocytes than in their congenic controls (125 ± 4%. in MT vs. 100 ± 3% in WT, n = 156 cells in MT and n = 178 cells in WT, p<0.001, Figure 6A), suggesting the mutant astrocytes had more releasable Ca²⁺ in the ER. Moreover, when the kinetics of this Ca²⁺ rise was analyzed, the leakage of Ca²⁺ from the ER in the mutant astrocytes was much faster (time constant 54 ± 1 s in MT vs. 59 ± 1 s in WT, n = 156 cells in MT and n = 178 cells in WT, p<0.001, Figure 6B–C).

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RTT astrocytes display abnormal calcium load in the ER, calcium leak from the ER, baseline cytosolic calcium level, and TRPC4-dependent SOCE.

(A) Left, average traces of Fluo4 fluorescence changes in wild type and mutant astrocytes treated with 1 μM TG to release ER calcium. Bath solution contained 0 mM Ca²⁺ plus 2 mM EGTA. Right, Quantification of the peak amplitude of TG induced Ca²⁺ elevations. (B) The rise phase (dots) shown in (A) is fitted with a single exponential curve. (C) Quantification of the time constant of the rise phase in the left panel from congenic mutant and wild type human RTT astrocytes in response to TG. (D) Representative GFP and Rhod-2 images in primary astrocytes from female *Mecp2^+/-* ± with a GFP transgene on the wild type X chromosome. Scale bars = 50 μm. (**E–F**) Quantification of Rhod-2 fluorescence intensity in GFP negative and GFP positive primary astrocytes isolated from either female *Mecp2^+/-* ± with a GFP transgene on the wild type X chromosome (E) or female *Mecp2^+/+* mice with a GFP transgene on the wild type X chromosome (F). (G) The average trace of Fluo4 fluorescence changes in response to extracellular Ca²⁺ (2 mM) after depletion of ER Ca²⁺ store using TG. (H) The Western blot result showing the protein level of TRPC4 in WT and MeCP2 mutant astrocytes. (I) Quantification of the Western blot results. (J) Representative traces of current density of TG-induced inward current in wild type and mutant astrocytes held at −70 mV. This current was partially blocked by TRPC4 selective antagonist, ML204. (K) Quantification of ML204 sensitive current from wild type and mutant astrocytes held at −70 mV. (L) Representative traces of current density of TG-induced inward current in mutant astrocytes infected with lentivirus expressing either GFP alone or co-expressing GFP and shTRPC4. (M) Quantification of ML204 sensitive current at −70 mV. The bar graphs in this figure show the mean ±s.e.m. *p<0.05, **p<0.01, ***p<0.001.

https://doi.org/10.7554/eLife.33417.026

Figure 6—source data 1 The numerical data for the graphs shown in Figure 6A, C, E, F, I, K and M.: https://doi.org/10.7554/eLife.33417.032
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To accurately compare the baseline cytosolic Ca²⁺ level between Mecp2 mutant and wild type astrocytes, we prepared primary mouse astrocytes from Mecp2^+/- mice that carry an X chromosome-linked GFP transgene. The GFP transgene is on the same X chromosome as the wild type Mecp2 gene. Due to random X chromosome inactivation, a mixture of GFP+ and GFP- astrocytes will be present in these cultures, with the GFP+ cells expressing the wild type MeCP2 and the GFP- cells expressing no MeCP2. When the red-colored calcium-sensitive dye Rhod-2 was used to image baseline cytosolic Ca²⁺ level in the Mecp2 null (GFP-) and wild type (GFP+) astrocytes in the same culture (Figure 6D), significantly higher level of Rhod-2 fluorescence was detected in the Mecp2 null cells (154 ± 7% in GFP- vs. 100 ± 5% in GFP+, n = 30 in each group, p<0.0001; Figure 6E). As a control, astrocyte cultures were also prepared from Mecp2^+/+ female mice that carry the same X chromosome-linked GFP transgene. No significant deference was detected in baseline cytosolic Ca²⁺ level between the GFP+ and GFP- cells (107 ± 8% in GFP- vs. 100 ± 6% in GFP+, n = 31 in GFP- and n = 32 in GFP+, p=0.46; Figure 6F). These results reveal Ca²⁺ overload in the ER, the faster Ca²⁺ leak rate, and the elevated baseline cytosolic Ca²⁺ level as significant contributors to the abnormal spontaneous and pharmacologically evoked cytosolic Ca²⁺ activities in the mutant RTT astrocytes.

To further define the cellular events upstream of the abnormal Ca²⁺ homeostasis, we measured Ca²⁺ influx through the store operated calcium entry (SOCE) pathway, which is the primary pathway for reloading Ca²⁺ into the ER in astrocytes. After depleting Ca²⁺ from the ER by treating astrocytes with TG in the absence of extracellular Ca²⁺ (0 mM Ca²⁺ plus 2 mM EGTA), we switched bath solution to that containing 2 mM Ca²⁺ and measured Ca²⁺ influx. Comparing to that in the congenic control, the amplitude of Ca²⁺ influx was significantly higher in the mutant human RTT astrocytes (2.93 ± 0.07 in MT vs. 2.41 ± 0.06 in WT, n = 156 cells in MT and n = 178 cells in WT, p<0.001; Figure 6G), suggesting an abnormal increase of SOCE in the mutant cells.

In search of the molecular mechanism underlying the increased SOCE in mutant astrocyte, we performed RNA-seq and microarray experiments to compare gene transcription profiles in the mutant astrocytes and their congenic controls. Highly relevant to the observed SOCE phenotype, the expression of the transient receptor potential cation channel, subfamily C, member 4 (TRPC4), which has been previously shown to regulate SOCE in lung vascular endothelial cells (Tiruppathi et al., 2002), was found to be significantly higher in the mutant human RTT astrocytes (~27 fold increase by RNA-seq,~14 fold increase by microarray). Western blot analysis using protein lysates from mutant human RTT astrocytes and their congenic controls also confirmed the increased expression of TRPC4 (Figure 6H–I). Immunostaining with TrpC4 antibody also detected significantly higher immunoreactivity in GFAP positive astrocytes in hippocampal sections from the Mecp2 null mouse brains as compared with that in wild type mouse brains (Figure 6—figure supplement 2). Chromatin immunoprecipitation analysis revealed significant MeCP2 occupancy at the promoter of Trpc4 (Figure 6—figure supplement 3), suggesting direct regulation of Trpc4 transcription by MeCP2. To corroborate the expression level changes at the RNA and protein level for TRPC4, we performed whole cell patch clamp recording on the astrocytes to directly assess the current mediated by TRPC4 using a TRPC4-selective antagonist, ML204 (Miller et al., 2011). Significantly higher density of ML204 sensitive inward current (at −70 mV) was detected in the R294X mutant human RTT astrocytes (−0.93 ± 0.01 pA/pF in MT vs. −0.62 ± 0.08 pA/pF in WT, n = 14 cells in MT and n = 15 cells in WT, p=0.01; Figure 6J–K). Moreover, this current was significantly reduced when mutant human RTT astrocytes were infected with a lentivirus expressing an shRNA specific against TRPC4 (−0.61 ± 0.10 pA/pF in MT infected with lenti-GFP-shTRPC4 vs. −1.04 ± 0.05 pA/pF in MT infected with lenti-GFP, n = 9 cells in each condition, p=0.006, Figure 6L–M), suggesting the current is dependent on TRPC4. The I-V curve of the ML204-sensitive current can be found in Figure 6—figure supplement 4. Finally, the phenotypes of increased Ca²⁺ overload in the ER, faster Ca²⁺ leak from the ER, increased SOCE, and increased level of TrpC4 were also observed in primary astrocytes isolated from Mecp2 null mice (Figure 6—figure supplement 5).

To determine the contribution of increased TRPC4 expression to the series of Ca²⁺ related phenotypes in the Mecp2 null astrocytes, we treated these cells with ML204 for 24 hr, and then measured SOCE-mediated Ca²⁺ elevation, Ca²⁺ load in the ER, and recorded spontaneous cytosolic Ca²⁺ activities. Comparing to untreated Mecp2 null astrocytes, ML204-treated cells showed significantly reduced amplitude of SOCE mediated Ca²⁺ elevation (3.15 ± 0.12 in ML204-treated group vs. 6.61 ± 0.86 in untreated, n = 18 in ML204-treated and n = 16 in untreated, p=0.0002; Figure 7A) and lower ER Ca²⁺ load (1.86 ± 0.06 in ML204-treated vs. 2.47 ± 0.24 in untreated, n = 14 in ML204-treated and n = 15 in untreated, p=0.02; Figure 7B). More importantly, ML204 treatment significantly reduced the percentage of Mecp2 null astrocytes showing spontaneous cytosolic Ca²⁺ oscillation (23 ± 3% in ML204-treated vs. 46 ± 7% in untreated, n = 9 randomly chosen fields in ML204-treated and n = 8 randomly chosen fields in untreated, p=0.03; Figure 7C) and the frequency (0.20 ± 0.01/min in ML204-treated vs. 0.31 ± 0.01/min in untreated, n = 91 cells in ML204-treated and n = 203 cells in untreated, p<0.001; Figure 7C) and amplitude (1.64 ± 0.02 in ML204-treated vs. 1.97 ± 0.02 in untreated, n = 168 events in ML204-treated and n = 597 events in untreated, p<0.001; Figure 7C) of such oscillations.

Figure 7

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The role of TRPC4 in regulating calcium homeostasis in astrocytes.

(A) Extracellular Ca²⁺ -induced Fluo4 fluorescence changes after TG-induced ER Ca²⁺ depletion in *Mecp2^-/y* astrocytes in the absence (control) or presence of ML204. (B) Average traces of TG-induced Fluo4 fluorescence changes in *Mecp2^-/y* astrocytes with or without 24 hr of ML204 treatment. (C) Quantification of percentage (left) of astrocytes showing spontaneous Ca²⁺ oscillation and the frequency (middle) and amplitude (right) of such oscillation with or without 24 hr of ML204 treatment. (D) TRPC4 Western blot result showing decreased expression level of TRPC4 in mutant RTT human astrocytes after infected with lentivirus-shTrpc4-GFP, compared with astrocytes infected with lentivirus-GFP. (E) Quantification of the TRPC4 Western blot result. (F) Average traces of extracellular Ca²⁺ (2 mM)-induced Rhod-2 fluorescence changes from mutant RTT astrocytes infected with lentivirus expressing either shTRPC4/GFP or GFP alone, after depletion of ER Ca²⁺ store by TG pre-treatment. (G) Average traces of TG-induced Rhod-2 fluorescence changes from mutant RTT astrocytes infected with lentivirus expressing either shTRPC4/GFP or GFP alone. (H) Quantification of the frequency of spontaneous Ca²⁺ elevations from mutant RTT astrocytes infected with lentivirus expressing either shTRPC4/GFP or GFP alone. (I) Representative images of wild type human RTT astrocytes infected with lentivirus co-expressing TRPC4 and GFP (left) or with lentivirus expressing GFP alone (right). Note anti-TRPC4 immunoreactivity is higher in astrocytes infected with lenti-GFP/TRPC4 than in those infected with lenti-GFP. Scale bar = 50 μm. (J) Average traces of extracellular Ca²⁺ (2 mM)-induced Rhod-2 fluorescence changes from wild type astrocytes infected with lentivirus expressing either GFP/TRPC4 or GFP alone, after depletion of ER Ca²⁺ store by TG pre-treatment. (K) Average traces of TG-induced Rhod-2 fluorescence changes from wild type astrocytes infected with lentivirus expressing either GFP/TRPC4 or GFP alone. (L) Quantification of percentage (left) of astrocytes infected with lentivirus expressing either GFP/TRPC4 or GFP alone that showed spontaneous Ca²⁺ oscillation and the frequency (middle) and amplitude (right) of such oscillations. The bar graphs in this figure show the mean ±s.e.m. *p<0.05, **p<0.01, ***p<0.001.

https://doi.org/10.7554/eLife.33417.033

Figure 7—source data 1 The numerical data for the graphs shown in Figure 7C, H and L.: https://doi.org/10.7554/eLife.33417.034
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In addition to pharmacological manipulation, we infected mutant RTT human astrocytes with lentivirus expressing a shRNA specific against TRPC4 to reduce its level (Figure 7D–E). 72 hr after the infection, we measured SOCE-mediated Ca²⁺ elevation, ER Ca²⁺ load, and recorded spontaneous cytosolic Ca²⁺ activities. Comparing to those infected with lentivirus expressing GFP alone, the mutant astrocytes infected with lentivirus expressing shTRPC4 showed significantly reduced SOCE-mediated Ca²⁺ elevations (1.72 ± 0.05 in MT/Lenti-ShTRPC4-GFP vs. 2.02 ± 0.04 in MT/Lenti-GFP, n = 23 in MT/Lenti-shTRPC4 and n = 26 in MT/Lenti-GFP, p<0.0001; Figure 7F), lower ER Ca²⁺ load (2.37 ± 0.16 in MT/Lenti-shTRPC4-GFP vs. 2.95 ± 0.29 in MT/Lenti-GFP, n = 23 in MT/Lenti-shTRPC4 and n = 26 in MT/Lenti-GFP, p=0.04; Figure 7G), and lower frequency (0.42 ± 0.03/min in MT/Lenti-shTRPC4-GFP vs. 0.63 ± 0.03 in MT/Lenti-GFP, n = 86 cells in MT/Lenti-shTRPC4 and n = 79 cells in MT/Lenti-GFP, p<0.001, Figure 7H) of the spontaneous Ca²⁺ elevations. Together, the pharmacological and molecular manipulations demonstrated that elevated TRPC4 expression is required for the abnormal calcium homeostasis in MeCP2 deficient astrocytes.

Finally, we overexpressed TRPC4 in wild type human astrocytes (Figure 7I), and observed increased SOCE-mediated Ca²⁺ elevations (2.76. ± 0.15 in WT/GFP-TRPC4 vs. 2.36 ± 0.08 in WT/GFP, n = 52 in WT/GFP-TRPC4 and n = 55 in WT/GFP, p=0.02; Figure 7J), higher ER Ca²⁺ load (3.65 ± 0.23 in WT/GFP-TRPC4 vs. 2.62 ± 0.13 in WT/GFP, n = 52 in WT/GFP-TRPC4 and n = 55 in WT/GFP, p=0.0001; Figure 7K), more cells showing spontaneous Ca²⁺ oscillation (29 ± 5% in WT/GFP-TRPC4 vs. 16 ± 3% in WT/GFP, n = 13 in WT/GFP-TRPC4 and n = 12 in WT/GFP, p=0.03; Figure 7L), higher frequency (0.71 ± 0.04/min in WT/GFP-TRPC4 vs. 0.45 ± 0.01/min in WT/GFP, n = 74 cells in WT/GFP-TRPC4 and n = 100 cells in WT/GFP, p<0.001; Figure 7L) and amplitude (1.49 ± 0.04 in WT/GFP-TRPC4 vs. 1.22 ± 0.01 in WT/GFP, n = 523 events in WT/GFP-TRPC4 and n = 445 events in WT/GFP, p<0.001; Figure 7L) of spontaneous Ca²⁺ oscillations in those cells. These results strongly suggest that elevated TRPC4 expression is sufficient to cause abnormal calcium homeostasis in wild type astrocytes.

Abnormal calcium homeostasis in RTT astrocytes leads to excessive activation of extrasynaptic NMDA receptor activation in neighboring neurons and increased network excitability

Previous work has shown that Ca²⁺ oscillation in astrocytes could lead to activation of eNMDARs (Fellin et al., 2004). We measured activation of eNMDAR on mouse hippocampal pyramidal neurons by whole cell patch clamp recording, and detected a significantly higher frequency of a slow inward current (SIC, Figure 8A, left) in acute hippocampal slices prepared from 2 to 3 weeks old Mecp2 null mice than those from the wild type littermate control (3.88 ± 0.46/20 min in Mecp2 null vs. 1.50 ± 0.39/20 min in WT, n = 16 in Mecp2 null and n = 12 in WT, p=0.001; Figure 8A, right). The SICs were sensitive to both D-APV (selective NMDA receptor antagonist, 50 µM, Figure 8B, left) and ifenprodil (selective NR2B-containing NMDA receptor antagonist, 10 µM, Figure 8B, right), suggesting they were mediated by NR2B-containing eNMDARs. Similar increase in SIC frequency was also detected in acute hippocampal slices prepared from 6 weeks old symptomatic Mecp2 null mice (Figure 8—figure supplement 1). Although similar phenotype was recently reported (Lo et al., 2016), the underlying cause of such increased activation of eNMDAR is not well understood.

Figure 8 with 2 supplements see all

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Abnormal Ca²⁺ activities in astrocytes lead to excessive activation of extrasynaptic NMDA receptors in neighboring neurons and increased network excitability.

(A) Left: representative traces of whole-cell patch clamp recording from CA1 pyramidal neurons in acute hippocampal slices prepared from wild type and *Mecp2^-/y* mice showing slow inward currents (SICs). Arrows indicate all SICs. Inset: magnified view of the recording. Right: quantification of the SIC frequency in neurons from wild type and *Mecp2^-/y* mice. (B) Quantification of the amplitude of SICs of neurons under control condition and in the presence of D-APV (left), an antagonist of NMDA receptors, or ifenprodil (right), a selective antagonist of NR2B-containing NMDA receptors. (C) Quantification of the SIC frequency in neurons adjacent to astrocytes with or without intracellular infusion of BAPTA. (D) Orthogonal (x-y view, y-z view, and x-z view) projections of z-scanning images showing immunofluorescence of NeuN and GFAP in regions around the hippocampus. The left image is from a *Mecp2^flox/y* mouse injected with AAV-hSyn-mCherry-Cre, while the right image is from a *Mecp2^flox/y* mouse injected with AAV-GFAP-mCherry-Cre. Note that all of the mCherry positive cells are NeuN positive in the left image, while all of the mCherry positive cells are GFAP positive in the right image. Scale bars = 50 μm. (E) Quantification of the frequency of the spontaneous Ca²⁺ oscillations in astrocytes from *Mecp2^flox/y* mice injected with AAV-mCherry, AAV-hSyn-mCherry-Cre, or AAV-GFAP-mCherry-Cre. ***p<0.001 vs. AAV-mCherry. (F) Quantification of the SIC frequency from *Mecp2^flox/y* mice injected with AAV-mCherry, AAV-hSyn-mCherry-Cre, or AAV-GFAP-mCherry-Cre. *p<0.05 vs. AAV-mCherry. (G) Representative patch clamp recordings in CA1 pyramidal neurons showing characteristic spontaneous epileptiform bursting activity in response to application of the GABA_A-receptor antagonist bicuculline (3 μM) from wild-type (top) and Mecp2^-/y (bottom) mice. (H) Quantification of the latency, frequency, duration and the amplitude of the epileptiform activity. The latency is defined as the time elapsed between Bic application and epileptiform activity onset. The bar graphs in this figure show the mean ±s.e.m. *p<0.05, **p<0.01, ***p<0.001.

https://doi.org/10.7554/eLife.33417.035

Figure 8—source data 1 The numerical data for the graphs shown in Figure 8A, B, C, E, F and H.: https://doi.org/10.7554/eLife.33417.038
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To determine the role of abnormal Ca²⁺ activities in the astrocytes in the increased frequency of SICs, membrane-impermeable BAPTA, along with a small dye that can pass through gap junctions, was injected into a single astrocyte in acute hippocampal slices prepared from the Mecp2 null mice. Thirty minutes after injection, many astrocytes became positive for the dye in the field of injection, indicating diffusion of the dye and BAPTA into astrocytes connected through gap junctions. Whole cell patch clamp recording on hippocampal pyramidal neurons within the field of these dye-labeled astrocytes (i.e. with the intracellular chelator BAPTA) revealed a significant reduction in the frequency of SIC (2.08 ± 0.51/20 min in Mecp2 null with BAPTA infusion into a single astrocyte vs. 5.33 ± 1.04/20 min in Mecp2 null with no treatment, n = 12 in each group, p=0.01; Figure 8C), suggesting that the abnormally high calcium activity in astrocytes is required for the excessive activation of eNMDARs on neighboring neurons.

To further determine the relationship between these two phenotypes, we examined calcium activities and activation of eNMDARs in astrocyte-specific and neuron-specific Mecp2 knockout mice. To generate cell type-specific Mecp2 knockout mice, we mated wild type males with Mecp2^flox/flox females to generate Mecp2^flox/y male pups, and injected AAV viruses (AAV8-hGFAP-mCherry-Cre, AAV8-Syn-mCherry-Cre, or AAV-mCherry) into the lateral ventricles of postnatal-day-1 old Mecp2^flox/y male pups. Consistent with previous report (Kim et al., 2013), mCherry-positive cells showed widespread presence in the forebrain (including cortex and hippocampus), suggesting robust distribution of AAV in this method. Immunohistological analysis of sections from Mecp2^flox/y male mice injected with AAV8-hGFAP-mCherry-Cre showed that 100% of mCherry-positive cells were positive for GFAP (right, Figure 8D, Table 3) and negative for MeCP2 (Figure 8—figure supplement 2), and that ~ 27% of GFAP-positive cells were positive for mCherry (Table 3), suggesting successful deletion of Mecp2 in about one third of the astrocytes in the forebrain. Similar analysis of sections from Mecp2^flox/y male mice injected with AAV8-Syn-mCherry-Cre showed that 100% of mCherry-positive cells were positive for NeuN (left, Figure 8D, Table 3) and negative for MeCP2 (Figure 8—figure supplement 2), and that ~ 74% of NeuN-positive cells were positive for mCherry (Table 3), suggesting successful deletion of Mecp2 in about more than two thirds of the neurons in the forebrain. Further double staining with either anti-GFAP and anti-MeCP2 antibodies or anti-NeuN and anti-MeCP2 antibodies on those brain sections (Figure 8—figure supplement 2) revealed that, while almost all mCherry-positive cells were indeed negative for MeCP2 in mice receiving either AAV virus, there was a negligible number (2%) of NeuN-positive cells that were negative for MeCP2 in the hippocampus of the AAV-GFAP-mCherry-Cre injected mice, and a small number (7%) of GFAP-positive cells that were negative for MeCP2 in the hippocampus of the AAV-hSyn-mCherry-Cre injected mice (Table 4). Thus, mice with predominantly astrocyte-specific deletion of Mecp2 and predominantly neuron-specific deletion of Mecp2 were generated. Mecp2^flox/y male mice injected with AAV-mCherry (no Cre recombinase) were used as wild type control for this series of experiments.

Table 3

Histological results of mice injected with AAVs (I)

https://doi.org/10.7554/eLife.33417.039

	Mice injected with AAV-GFAP-mCherry-Cre	Mice injected with AAV-hSyn-mCherry-Cre
%GFAP + cells in all mCherry + cells	100	0
%NeuN + cells in all mCherry + cells	0	100
%mCherry + cells in all GFAP + cells	28 ± 3	0
%mCherry + cells in all NeuN + cells	0	74 ± 10

Table 4

Histological results of mice injected with AAVs (II)

https://doi.org/10.7554/eLife.33417.040

	Mice injected with AAV-GFAP-mCherry-Cre	Mice injected with AAV-hSyn-mCherry-Cre
%MeCP2- cells in all mCherry + cells	98 ± 0.8	97 ± 1.5
%MeCP2- cells in all GFAP + cells	35 ± 3	7 ± 1.7
%MeCP2- cells in all NeuN + cells	2 ± 0.1	75 ± 11

Abnormal spontaneous calcium activity in astrocytes (Figure 8E, n = 34 in AAV-mCherry group, n = 36 in AAV-Syn-mCherry-Cre group, and n = 47 in AAV-hGFAP-mCherry-Cre group; ANOVA: F = 14.11, p<0.0001) and excessive activation of extrasynaptic NMDA receptors in neighboring neurons (Figure 8F, n = 38 in AAV-mCherry group, n = 21 in AAV-Syn-mCherry-Cre group, and n = 30 in AAV-hGFAP-mCherry-Cre group; ANOVA: F = 4.23, p<0.018) were only observed in predominantly astrocyte-specific, but not in predominantly neuron-specific, Mecp2 knockout hippocampal slices. Results from this series of genetic manipulations strongly suggest that the abnormal calcium activities in astrocytes are caused by the loss of MeCP2 function in astrocytes, and can lead to excessive activation of extrasynaptic NMDA receptors on neighboring neurons (even when these neighboring neurons are wild type).

Beyond eNMDAR activation, several previous studies have suggested that increased astrocytic Ca²⁺ activity may lead to a hyperexcitable network (Gómez-Gonzalo et al., 2010; Kuchibhotla et al., 2009; Tian et al., 2005; Wetherington et al., 2008). Such a hypothesis is highly relevant to RTT, because 80–90% of RTT patients have seizures (Jian et al., 2007), and spontaneous seizures have also been reported in RTT mice (D'Cruz et al., 2010). As the first step to explore a role for abnormal astrocytic Ca²⁺ activity in leading to seizure in RTT, we adopted a widely used slice model of seizure (or epileptiform activity or network hyper-excitability) by adding the GABA_A receptor antagonist biccucullin and increasing bath Ca²⁺ concentration (Borck and Jefferys, 1999; McLeod et al., 2013; Ratté et al., 2011; Tian et al., 2005).

In this model, whole cell patch clamp recording in CA1 pyramidal neurons readily detected epileptiform activity in slices from 5 to 8 week-old wild type and Mecp2^-/y mice (Figure 8G). Comparing with wild type, epileptiform activity in Mecp2^-/y neurons had shorter latency (4.5 ± 0.4 min in Mecp2^-/y vs. 6.5 ± 0.6 min in wild type, Figure 8H), higher frequency (5.6 ± 0.4 Hz in Mecp2^-/y vs. 3.5 ± 0.5 Hz in wild type, Figure 8H), longer duration (755 ± 30 ms in Mecp2^-/y vs. 621 ± 39 ms in wild type, Figure 8H), and higher amplitude (364 ± 15 pA in Mecp2^-/y vs. 279 ± 28 pA in wild type. Figure 8H). More importantly, intracellular infusion of membrane impermeable BAPTA into a single astrocyte significantly reduced epileptiform activity in neighboring neurons (latency: 4.5 ± 0.4 min in Mecp2^-/y vs. 5.7 ± 0.4 min in Mecp2^-/y with BAPTA infusion; frequency: 5.6 ± 0.4 Hz in Mecp2^-/y vs. 3.8 ± 0.5 Hz in Mecp2^-/y with BAPTA infusion; duration: 755 ± 30 ms in Mecp2^-/y vs. 633 ± 31 ms in Mecp2^-/y with BAPTA infusion. Figure 8H). 7–10 neurons from 3 to 4 mice were analyzed from each genotype/treatment in this series of experiments. Similar results were obtained recording field potentials under the same conditions described in Figure 8G–H (data not shown). Together, these data reveal a potential link between increased astrocytic Ca²⁺ activity and increased network excitability. Since such increased network excitability may underlie seizure-a specific RTT symptom, these data further highlight the functional significance of abnormal Ca²⁺ activity in RTT.

Discussion

Since the initial report that astrocytes express MeCP2 and that Mecp2 null astrocytes have non-cell autonomous influence on wild type neurons (Ballas et al., 2009), many gene expression changes (Maezawa et al., 2009; Okabe et al., 2012; Yasui et al., 2013) and a few cellular phenotypes have been identified in Mecp2 null astrocytes (Maezawa et al., 2009; Nectoux et al., 2012). However, it is not clear how these alterations contribute to RTT pathogenesis either by directly changing astrocyte functions or by indirectly changing neuronal functions. In addition, no major cell autonomous phenotype in astrocytes has been clearly defined, with its underlying mechanism and downstream consequences examined in detail.

A cell autonomous phenotype in astrocytes is inherently difficult to define in vivo, because astrocytes normally develop in the presence of neurons and intimately interact with neurons in the intact brain. To circumvent this issue, we first turned to human iPSC-differentiated astrocytes because they are differentiated and maintained in the absence of neurons. Using astrocytes differentiated from congenic pairs of human RTT patient specific iPSCs, we discovered that both the spontaneous and the pharmacologically evoked cytosolic calcium activities are abnormal in mutant RTT astrocytes. We then confirmed that similar phenotypes could be found in Mecp2 null mouse astrocytes in vitro, in situ, and in vivo. While significant difference exists across the astrocyte populations included in our study (species [human and mouse], developmental stage [embryonic and postnatal], and system [in vitro, in situ, and in vivo]), the core phenotype of abnormal astrocytic calcium activity remains consistent. As for the underlying mechanism, we provide evidence that the abnormal calcium activity is mediated by calcium overload in the endoplasmic reticulum (ER) caused by TRPC4-dependent abnormal store operated calcium entry (SOCE). Although our findings don’t directly reveal whether TRPC4 is a component of the SOCE channel, they do suggest elevated TRPC4 expression in mutant RTT astrocytes is upstream of the SOCE, ER, and cytosolic calcium phenotypes. As for the downstream consequences, we demonstrate that these abnormal calcium activities lead to excessive activation of the eNMDARs on neighboring neurons and increased network excitability. Together, our results not only identify a novel cell autonomous phenotype and its underlying mechanism in RTT astrocytes, but also reveal a functional link between the astrocyte phenotype and a novel neuronal phenotype in RTT models and network hyper-excitability that may underlie a significant RTT symptom. For future studies, we hypothesize a linear model in the following order: first, loss of MeCP2 expression in astrocytes leads to increased expression of TRPC4 in astrocytes; second, increased expression of TRPC4 in astrocytes leads to calcium overload in the ER of astrocytes; third, calcium overload in the ER of astrocytes leads to increased cytosolic calcium activity in the astrocytes; fourth, increased cytosolic calcium activity in astrocytes leads to excessive activation of eNMDARs in neighboring neurons and increased network excitability (summarized in Figure 9).

Figure 9

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A schematic summary of the findings in our study.
https://doi.org/10.7554/eLife.33417.041

The abnormal calcium homeostasis in mutant RTT astrocytes manifests in multiple interconnected cellular events, including elevated SOCE, Ca²⁺ overload in the ER, elevated baseline cytosolic Ca²⁺ level, and abnormal spontaneous and evoked Ca²⁺ activities. Both molecular and pharmacological inhibition of the TRPC4 channel in mutant RTT astrocytes rescue the phenotypes of increased SOCE, ER Ca²⁺ overload and abnormal cytosolic Ca²⁺ activities. Conversely, overexpression of TRPC4 in wild type astrocytes can cause the mutant phenotypes. Together, these results strongly suggest that increased TRPC4 expression is upstream of the altered SOCE, and the subsequent ER and cytosol Ca²⁺ phenotypes. As most of our TRPC4-related data came from in vitro experiments, future in vivo studies are needed to ascertain the functional significance of increased TRPC4 expression in RTT disease progression. In addition, it is worth noting that other TRP channels such as TRPA1, TRPC, TRPV4 have been implicated in modulating Ca²⁺ dynamics in astrocytes (Ma et al., 2016; Molnár et al., 2016; Shibasaki et al., 2014; Shigetomi et al., 2013; Shigetomi et al., 2011). Yet, their expression remained unchanged in mutant human RTT astrocytes (data not shown). Of course, other deregulated genes and pathways due to the loss of MeCP2 function may also contribute to these phenotypes, providing interesting topics for future studies to further illustrate the central role of abnormal calcium homeostasis in astrocyte dysfunction in RTT. Furthermore, it is worth noting that abnormal calcium activities in astrocytes have been implicated in other neurological diseases, such as Alzheimer’s disease(Kuchibhotla et al., 2009), suggesting abnormal calcium homeostasis in astrocytes may be a common pathological event in neurological diseases. However, the molecular mechanisms underlying the abnormal calcium activities may be different in different diseases. While the P2Y1 receptor is shown to mediate the astrocytic hyperactivity in Alzheimer’s disease (Delekate et al., 2014), we have identified TRPC4 as a major contributor to the abnormal calcium homeostasis in RTT astrocytes. Since Ca²⁺ is a pleiotropic signal for a diverse range of cellular functions, abnormal Ca²⁺ homeostasis can lead to additional functional consequences in mutant RTT astrocytes. Future studies are therefore needed to define the full spectrum of cellular deficits in RTT astrocytes that are downstream of the altered Ca²⁺ signaling.

Up to now, a major challenge in understanding the contribution of astrocytes to RTT pathogenesis has been the missing link between astrocyte dysfunction and neuronal dysfunction and RTT symptoms. We showed that intracellular infusion of a membrane-impermeable calcium chelator, which abolishes calcium activities in astrocytes only, reduced the excessive activation of eNMDARs on neighboring neurons and rescued network hyper-excitability. Moreover, we provided genetic evidence that the loss of MeCP2 in astrocytes is both necessary and sufficient to cause the abnormal calcium activity in astrocytes and the excessive activation of eNMDARs in neighboring neurons. These results strongly suggest that this novel neuronal phenotype is dependent on a cell autonomous phenotype in the astrocytes. Thus, our findings provide a direct functional link between astrocyte dysfunction and neuronal dysfunction in RTT.

Although it remains debatable whether excessive activation of eNMDARs leads to increased neuronal synchrony and seizure in general (Angulo et al., 2004; Fellin et al., 2004; Tian et al., 2005), increased synchronous neuronal firing has recently been observed in the hippocampus of both male Mecp2^-/y and female Mecp2^+/- mice(Lu et al., 2016). Thus, the relationship between the abnormal astrocytic calcium activity, the excessive activation of eNMDARs, and increased neuronal synchrony is worth further studying in the context of RTT. While 80% of RTT patients have seizures (Jian et al., 2007), the cellular and molecular mechanisms underlying this symptom are not well understood. Even if the excessive activation of eNMDARs and/or the increased neuronal synchrony may not be directly linked to the seizure phenotype in RTT, our data linking the abnormal calcium activities in RTT astrocytes to increased network excitability still provide a new avenue for investigating this often difficult-to-manage symptom in RTT. At this moment, the exact molecular events linking abnormal calcium activities in astrocytes and altered neuronal and network properties remain elusive. One obvious candidate is glutamate, because previous studies have detected elevated glutamate level in primary cultures of astrocytes and microglia prepared from RTT mice (Maezawa and Jin, 2010; Okabe et al., 2012). Beyond glutamate, astrocytic calcium is known to modulate inhibitory synaptic efficacy by controlling GABA level through astrocytic GAT-3 (Shigetomi et al., 2011), and to modulate basal synaptic transmission through the release of purines (Panatier et al., 2011). Both mechanisms may help explain how abnormal calcium activities in RTT astrocytes cause increased network excitability. Therefore, more systematic approaches are needed in future studies to fully characterize the changes in factors present in RTT astrocyte conditioned medium (due to alteration in either release or uptake by astrocytes) and reveal how those changes underlie the non-cell autonomous influence on neurons.

Figure	Statistical methods
Figure 1C	Mann-Whitney U Test
Figure 1E	Mann-Whitney U Test
Figure 2C	Two Tailed Unpaired t-Test
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Figure 6A	Mann-Whitney U Test
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Figure 6F	Two Tailed Unpaired t-Test
Figure 6I	Two Tailed Unpaired t-Test
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Figure 8A	Two Tailed Unpaired t-Test
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Figure 8E	One-way ANOVA followed by post-hoc Holm-Sidak test
Figure 8F	One-way ANOVA followed by post-hoc Holm-Sidak test
Figure 8H	Two Tailed Unpaired t-Test

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Spontaneous calcium activities in wild type human astrocytes

Spontaneous calcium activities in mutant human astrocytes

Quantification of the percentage of GFAP+ cell in five differentiations from human iPSCs

Amplitudes of spontaneous Ca2+ elevations in different experiments

Abnormal Ca2+ activities in mutant human RTT astrocytes.

Figure 1—source data 1

Exogenous expression of MeCP2 rescues the abnormal calcium activities in RTT astrocytes.

Figure 2—source data 1

Abnormal Ca2+ activities in primary astrocytes isolated from Mecp2 null mice.

Figure 3—source data 1

Abnormal Ca2+ activities in astrocytes in acute brain slices prepared from Mecp2 null mice.

Figure 4—source data 1

Spontaneous calcium activities in astrocytes from live wild type mice

Spontaneous calcium activities in astrocytes from live Mecp2 mutant mice

Abnormal spontaneous Ca2+ activities in vivo from Mecp2 null astrocytes.

Figure 5—source data 1

RTT astrocytes display abnormal calcium load in the ER, calcium leak from the ER, baseline cytosolic calcium level, and TRPC4-dependent SOCE.

Figure 6—source data 1

The role of TRPC4 in regulating calcium homeostasis in astrocytes.

Figure 7—source data 1

Abnormal Ca2+ activities in astrocytes lead to excessive activation of extrasynaptic NMDA receptors in neighboring neurons and increased network excitability.

Figure 8—source data 1

Histological results of mice injected with AAVs (I)

Histological results of mice injected with AAVs (II)

A schematic summary of the findings in our study.

List of statistical methods

Author details

Qiping Dong

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Qing Liu

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Ronghui Li

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Anxin Wang

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Qian Bu

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Kuan Hong Wang

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Qiang Chang

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For correspondence

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Further reading

Amplitudes of spontaneous Ca²⁺ elevations in different experiments

Abnormal Ca²⁺ activities in mutant human RTT astrocytes.

Abnormal Ca²⁺ activities in primary astrocytes isolated from Mecp2 null mice.

Abnormal Ca²⁺ activities in astrocytes in acute brain slices prepared from Mecp2 null mice.

Abnormal spontaneous Ca²⁺ activities in vivo from Mecp2 null astrocytes.

Abnormal Ca²⁺ activities in astrocytes lead to excessive activation of extrasynaptic NMDA receptors in neighboring neurons and increased network excitability.