Dendritic mitochondria reach stable positions during circuit development
- Washington University School of Medicine, United States
Figures
Figure 1
Schematic of the retinal circuitry and its development.
Top: Illustration of the five classes of neurons in the mammalian retina. Photoreceptors - rods and cones indicated by the shape of their outer segments - translate changes in photon flux into changes in glutamate release. Horizontal cells provide feedback to photoreceptor terminals, whereas bipolar cells (BCs) relay photoreceptor signals from the outer to the inner retina where they provide excitatory input to a diverse class of inhibitory interneurons called amacrine cells and to retinal ganglion cells (RGCs), the output neurons of the eye. Bottom: Timeline of retinal development from birth (postnatal day 0) to circuit maturity (P21).
Figure 2
Mitochondrial distribution in RGC dendrites across development.
(A–C) Representative RGCs expressing mtYFP and PSD95-CFP in P9 (A), P15 (B), and P21 (C) retinas. Top panels show maximum intensity projections (MIPs) through confocal image stacks and bottom panels show MIPs of excerpts of the same stacks at higher magnification. (D) Density of mitochondria in RGC dendrites expressed as a volumetric fraction (see 'Materials and methods') across development. (E) Density of synapses along RGC dendrites given per length of dendrite based on skeletonization of the respective arbors (see 'Materials and methods') across development. (F) Scatter plots comparing the average nearest neighbor distance from synapses to mitochondria (NND-observed) to the mean average NND obtained from Monte Carlo simulations in which the positions of synapses along dendrites were randomized (NND-random). Dots represent individual cells and circles (error bars) indicate the mean (± SEM) at the different ages examined. (G) Normalized mitochondrial density plotted as a function of distance from dendritic branch points at P9 (left), P15 (middle), and P21 (right). Solid lines (shaded areas) indicate the mean (± SEM) across a number of RGCs (P9 n = 6, P15, n = 6, P21, n = 8). mtYFP, mitochondrially targeted yellow fluorescent protein; RGC, retinal ganglion cell.
Figure 3
Motility of dendritic mitochondria and peroxisomes across development.
(A) Kymographs of representative time-lapse imaging series of mitochondria (mtYFP, 0.9 fps, A) at P9 (left), P15 (middle), and P21 (right). Top panels show still frames at t = 0s of the branch segments depicted in the kymographs in the bottom panels. (B) Summary data of the motile fraction of mitochondria across development (P9 n = 10 RGCs, P15 n = 9 RGCs, P21 n = 7 RGCs). (C, D) Analogous to A (C) and B (D) but for time-lapse imaging of peroxisomes labeled with SKL-GFP (P9 n = 11 RGCs, P15 n = 10 RGCs, P21 n = 12 RGCs). (E–G) Bars (error bars) indicating the mean (± SEM) duration of uninterrupted runs (E), duration of pauses (F), and speed during uninterrupted motion (G) for mitochondria at P9 and P15 (P9 n = 65 mitochondria, P15 n = 32 mitochondria). See also Video 1. mtYFP, mitochondrially targeted yellow fluorescent protein; RGCs, retinal ganglion cells.
Figure 4 with 2 supplements
Spontaneous neuronal activity, dendritic Ca2+ transients, and mitochondrial motility during development.
(A) Raster plot of representative spike trains of eight neighboring RGCs recorded in a biolistically labeled retinal explant on an MEA at P9. (B) Representative △F/F traces of GCaMP6s signals recorded from three neighboring RGCs by two-photon imaging. (C) Cross-correlations of firing rates (top panel) and dendritic Ca2+ transients (bottom panel) of neighboring RGCs (<200 μm between recording sites on MEA, <200 μm between cell bodies in Ca2+ imaging) (MEA recordings n = 30 pairs, Ca2+ imaging n = 11 pairs). (D) Average frequencies of waves of correlated activity on MEA (black bar) and global Ca2+ signals in RGC dendrites observed by two-photon imaging (green bar) in biolistically labeled retinal explants at P9 (MEA n = 6 retinas, Ca2+ imaging n = 22 retinas, p>0.6). (E–G) GCaMP6s and mtDsRed signals during simultaneous fast (0.9 fps) time-lapse two-photon imaging. Top: still frames of mtDsRed channel at t = 0s. Left panels: ΔF/F traces aligned with time plotted in kymograph. ΔF/F traces were calculated based on the average GCaMP6s intensity in the dendrite segment shown above the kymograph. Center panels: kymographs of mtDsRed signal. Green overlay indicates the timing Ca2+ transients. Right panels: black lines represent schematized depictions of mitochondrial runs in the kymographs. (F) Break between top and bottom panels represents 10 min between pre-treatment (top) and 30 mM K+ application (bottom). (G) 5 µM DHβE treatment. (H) Top: Plot of the instantaneous mitochondrial motility as a function of time relative to a Ca2+ transient (n = 134 mitochondria, 10 RGCs). Bottom: Normalized Ca2+ imaging trace as a function of time (aligned on t = 0 as the time of the Ca2+ transient). Lines (shaded areas) represent the mean (± SEM). (I, J) Paired plots of RGCs before and after 30 mM K+ (I) or DhβE (J) application. In the left panel of I the percentage of time above threshold measured was measured as percentage of frames in which the average GCaMP6s intensity within the frame was more than 2 SD above mean intensity of across the whole recording. The left panel of J show the frequency of Ca2+ transients. Right panels of I and J show changes in mitochondrial motility between pre-treatment and treatment portions of the recordings (30 mM K+ n = 8 RGCs, DhβE n = 7 RGCs). RGCs, retinal ganglion cells.
Figure 4—figure supplement 1
Miro1 expression and effect on mitochondrial motility in RGCs during development.
(A) Western blot showing Miro1 protein expression levels in whole retinal lysate at P6 – P21. Bottom panel shows β-actin loading control. (B) Bars indicate the intensities of the western blot signal relative to β-actin intensity and normalized to P12 intensity level. (C) Kymographs of representative time-lapse imaging series of mitochondria (mtYFP, 0.9 fps) in P9 RGCs overexpressing Miro1 (left) or expressing mtYFP alone (right). Top panels show still frames at t = 0s of the branch segments depicted in the kymographs in the bottom panels. (D) Summary data of the motile fraction of mitochondria in RGCs overexpressing Miro1, a mutant form of Miro1 unable to bind Ca2+ (MiroKK), or mtYFP alone (control) (Miro1 n = 9 RGCs, MiroKK n = 4, control n = 6). fps, frames per second; mtYFP, mitochondrially targeted yellow fluorescent protein; RGC, retinal ganglion cell.
Figure 4—figure supplement 2
Spontaneous, local dendritic Ca2+ transients and mitochondrial motility during development.
(A) Bars (error bars) indicating the mean (± SEM) frequency of global and local Ca2+ events in P9 RGCs (global n = 32 RGCs, local n = 7). (B) Paired plot of average mitochondrial speed in the 5 s before (Pre) and 5 s after (Post) a local Ca2+ event within a moving mitochondrion’s path (n = 7 mitochondria). (C) Representative images of a P9 RGC dendritic branch during global and local signals. Heat maps of ΔF/F intensities are overlaid onto a maximum intensity projection of the GCaMP6 signal in all three panels. The top panel is a representative image of the baseline GCaMP6 intensity; the middle panel is a global Ca2+ event and the bottom panel shows a local Ca2+ transient. Scale bar: 5 μm. RGC, retinal ganglion cell.
Figure 5 with 1 supplement
Sensory-evoked neuronal activity and mitochondrial motility at maturity.
(A, B) Spike raster plots of eight representative RGCs recorded in darkness (A) and during presentation of a full-field white noise stimulus (B, see 'Materials and methods'). (C) Bars (error bars) indicate the mean (± SEM) firing rates of RGCs (n = 334 RGCs, 3 retinas, p<10–26). (D) Representative RGCs expressing mtCFP at t = 0 min (top panels), t = 30 min (middle panels) after being exposed to white noise stimulus (right panels) or kept in darkness (left and center panels). Bottom panels show merged images of t = 0 and t = 30 min. (E) Bars (error bars) indicating the mean (± SEM) of % mitochondria displaced between t = 0 min and t = 30 min (P9 – no stim. n = 5 RGCs, P21 – no stim. n = 5 RGCs, P21 – stim n = 5 RGCs). RGCs, retinal ganglion cells.
Figure 5—figure supplement 1
Antimycin A and Oligomycin and mitochondrial motility.
(A) Kymographs of representative time-lapse imaging series of mitochondria (mtDsRed, 0.9 fps) in a P9 RGC before (left) and after Oligomycin/Antimycin A application. Top panels show still frames at t = 0s of the branch segments depicted in the kymographs in the bottom panels. (B) Paired plots of RGC mitochondrial motility before and after 30 min of 10 µM Oligomycin/ 4 µM Antimycin A application at P9 (n = 5 RGCs, p>0.2). (C) Representative images of P21 RGCs expressing mtDsRed at t = 0 min (top panels), t = 30 min (middle panels) in the presence (right panels) or absence (left panels) of Oligomycin/Antimycin A. Bottom panels show merged images of t = 0 and t = 30 min. (D) Bars (error bars) indicating the mean (± SEM) of % mitochondria displaced between t = 0 min and t = 30 min (n = 4 RGCs, p>0.4). fps, frames per second; RGC, retinal ganglion cell.
Figure 6
Pathological hyperactivity and dendritic mitochondria in retinal degeneration.
(A, B) Spike raster plots of eight representative RGCs recorded from P21 WT (A) and from P21 Crx-/- retinas (B). (C) Bars (error bars) indicating the mean (± SEM) firing rates of WT and Crx-/- RGCs (WT n = 100 RGCs, 3 retinas, Crx-/- n = 235 RGCs, 3 retinas, p<10–8). (D) Kymographs of representative time-lapse series of a P21 WT RGC (left panels) and Crx-/- RGC (right panels). (E) Bars (error bars) indicating the mean (± SEM) motile fraction of mitochondria in P21 WT and Crx-/- dendrites (P21 WT n = 7 RGCs, Crx-/- n = 6 RGCs). (FH) Bars (error bars) indicating the mean (± SEM) mitochondrial speed during uninterrupted motion (F), duration of uninterrupted runs (G), and duration of pauses (H) for mitochondria in P9 – 15 WT and Crx-/- dendrites (Crx-/- n = 30 mitochondria, pooled WT P9 and P15 n = 97 mitochondria). RGC, retinal ganglion cell.
Author response image 1
(A) 2-photon image of a section of RGC dendrite filled with OGB-1 via a recording electrode.
The green line indicates the position and extent of the line scan. (B) △F/F of the OGB-1 signal (top, green) aligned with the voltage trace (bottom, black) recorded simultaneously from the same RGC in a Crx-/- retina.
Videos
Video 1
Mitochondrial motility across development.
Time-lapse confocal recording of RGC dendrites expressing mtYFP at P9 (top panel), P15 (center panel), and P21 (bottom panel). Scale bar: 5 μm. 0.9 Hz time-lapse, playback speed = 15 frames per second. mtYFP, mitochondrially targeted yellow fluorescent protein; RGC, retinal ganglion cell.
Video 2
Mitochondrial motility during spontaneous global Ca2+ transients.
Simultaneous time-lapse two-photon recording of GCaMP6 and mtDsRed expression in P9 RGC dendrites. Top panel shows Ca2+ events in a section of dendrite. Center panel shows mtDsRed signal in the same dendritic branch. Bottom panel shows dots overlaid at each mitochondrion position tracked for mitochondria in the center panel. In frames in which a mitochondrion is moving, its dot is green; dots are red for stationary mitochondria. Scale bar: 5 μm. 0.9 Hz time-lapse, playback speed = 15 frames per second. RGC, retinal ganglion cell.
Video 3
Spontaneous global Ca2+ transients and pharmacological treatment during retinal waves.
(A) Time-lapse two-photon recording of GCaMP6 signal in a P9 RGC undergoing spontaneous Ca2+ events while participating in retinal waves. (B) A P9 RGC before and during wash-in of 30 mM High K+ mACSF, exhibiting a large sustained increase in GCaMP6 intensity. (C) A P9 RGC before and during wash-in of 5 μM DhβE, exhibiting an abolition of spontaneous wave activity. Scale bars: 5 μm. 0.9 Hz time-lapse, playback speed = 20 frames per second. RGC, retinal ganglion cell.
Video 4
Mitochondrial motility in pathologically hyperactive mature cells.
Time-lapse confocal recording of RGC dendrites expressing mtYFP in a P21 WT dendrite (top panel) and a P21 Crx-/- dendrite (bottom panel). Scale bars: 5 μm. 0.9 Hz time-lapse, playback speed = 15 frames per second.
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Dendritic mitochondria reach stable positions during circuit development
eLife 5:e11583.
https://doi.org/10.7554/eLife.11583
