Inherited brain disorders often cause severe problems for those affected by them. One example is a group of diseases, collectively termed “developmental and epileptic encephalopathies”, or DEE for short. People with these diseases usually have both epilepsy and intellectual disabilities, and in some patients these conditions are associated with two mutations that change a gene called TRPM3.

The TRPM3 gene encodes a protein called an ion channel. Ion channels form pores on the surfaces of cells. When channels are active, the pores open, allowing charged particles – which, in the case of TRPM3, are sodium and calcium ions – to pass through, carrying tiny electrical currents. In the nervous system, ion channels help nerve cells communicate and also allow them to sense changes in the environment.

The TRPM3 channel is known to open in response to heat and certain chemical “activators”. In mice, TRPM3 is found in sensory nerve cells, where it acts as a heat sensor. Although altering TRPM3 in mice affects their ability to sense intense or painful heat stimuli, they are otherwise completely normal and have no symptoms resembling human DEE disorders.

Although TRPM3 is found in the human brain, little is known about its role there or what effects the DEE-associated mutations have on its activity. Zhao et al. therefore set out to determine, whether each of the mutation was a ‘loss of function’, meaning that it stopped the channel from opening, or a ‘gain of function’, meaning it made the channel open more often.

Frog egg cells and mammalian cells grown in the laboratory were engineered to produce the TRPM3 ion channel. Measurements of electrical activity on these cells revealed that the two mutations seen in people with DEE were both ‘gain of function’. Both mutants were more sensitive to heat and chemical activators than the normal protein. They were also more active overall, even without any stimuli. However, one mutation had a greater effect on heat sensitivity, while the other caused a larger increase in chemical-induced activity.

Imaging experiments revealed that both mutant channels also increased the amount of calcium inside the cells. This could explain why the mutations cause disease, since abnormally high calcium levels can damage nerve cells. In addition, the epilepsy drug primidone switched off the mutant channels, pointing to potential treatment of this disease using primidone.

Transient Receptor Potential Melastatin 3 (TRPM3) is a Ca²⁺ permeable, non-selective cation channel activated by heat (Vriens et al., 2011) and chemical activators such as the neurosteroid pregnenolone sulfate (PregS) (Wagner et al., 2008) and the synthetic compound CIM0216 (Held et al., 2015). TRPM3 is a well-established temperature sensor in peripheral sensory neurons of the dorsal root ganglia (DRG); its genetic deletion in mice leads to defects in noxious heat sensation as well as reduced inflammatory heat hyperalgesia (Vriens et al., 2011; Vandewauw et al., 2018). Inhibitors of TRPM3 also reduced both acute heat sensation and inflammatory heat hyperalgesia (Straub et al., 2013; Krügel et al., 2017). While TRPM3^-/- mice show defects in noxious heat sensation, the channel shows increased activity well below the noxious range, when temperature is increased from 15°C to 26°C, with further increases in activity at 37°C (Vriens et al., 2011). TRPM3 is also inhibited by activation of Gi-coupled receptors such as μ-opioid receptors and GABA_B receptors in DRG neurons, and agonists of those receptors reduced nocifensive reactions to local injection of TRPM3 agonists (Badheka et al., 2017; Dembla et al., 2017; Quallo et al., 2017).

TRPM3 is also expressed in tissues other than peripheral sensory neurons, where its functional roles are not well understood. In pancreatic β-cells, it was shown that application of the TRPM3 agonist PregS induced an increase in insulin secretion (Wagner et al., 2008), but TRPM3^-/- mice showed no impairment in glucose homeostasis (Vriens et al., 2011). TRPM3 is also expressed in vascular smooth muscle cells, and PregS was reported to induce contractile responses in freshly isolated aorta, but the concentration required for this response was higher than the plasma levels of this compound (Naylor et al., 2010). TRPM3 is also expressed in various regions of the brain, where its functional role is essentially unexplored (Oberwinkler and Philipp, 2014).

Very little is known about the physiological and pathophysiological roles of TRPM3 in humans. A recent paper showed that two missense mutations in TRPM3 are associated with a neurodevelopmental disorder with intellectual disability, hypotonia and epilepsy, pointing to important roles of this channel in the human brain (Dyment et al., 2019). Seven of the eight patients had a de novo Val to Met substitution in the S4-S5 loop, while one patient had a Pro to Gln substitution in the extracellular segment of S6. The effects of these mutations on channel function however were not reported.

Here, we tested the functional effects of the two disease-associated mutations using electrophysiology and intracellular Ca²⁺ measurements. We find that both disease-associated mutations render the channel overactive. Both mutants showed constitutive activity that was inhibited by the TRPM3 antagonist primidone. As primidone is a clinically used medication (Krügel et al., 2017), our findings offer potential clinical intervention to treat this channelopathy. We also find that the Val to Met substitution in the S4-S5 loop induced a larger left shift in the concentration response relationship to PregS and CIM0216 than the Pro to Gln substitution close to the pore-loop in the extracellular segment of S6. The increase in heat activation on the other hand was more pronounced in the S6 mutant. We conclude that both reported mutants of TRPM3 are gain of function, but the mechanism of increased channel activity is different for the two mutants.

Here we used, patch clamp electrophysiology and intracellular Ca²⁺ measurements in HEK293 cells and two-electrode voltage clamp electrophysiology in Xenopus oocytes to study the effects of disease-associated mutations on TRPM3 function. TRPM3 has a large number of splice variants (Oberwinkler and Philipp, 2014), but there is no information available which splice variants are expressed in the human brain, and relatively little is known about the functional differences between splice variants. To ensure that our results do not only apply to one variant, we used two commonly used and well-characterized splice variants of TRPM3 in our experiments. In HEK293 cells, we expressed the human orthologue of the most studied mouse variant TRPM3α2 that was originally cloned from mouse brain (Oberwinkler et al., 2005; Vriens et al., 2011). In Xenopus oocytes, we expressed the human TRPM3 splice variant we used in several previous studies (Badheka et al., 2015; Badheka et al., 2017), originally described in Grimm et al. (2003) also called the TRPM3₁₃₂₅ variant (Oberwinkler et al., 2005).

Most alternatively spliced exons are in the cytoplasmic N-terminus; thus, the numbering of mutated residues varies between splice variants. The more common S4-S5 segment mutant described as V837M (Dyment et al., 2019) corresponds to V990M in the hTRPM3 splice variant we expressed in Xenopus oocytes, and to V992M in the hTRPM3α2 variant we expressed in HEK293 cells. The S6 mutant P937Q in Dyment et al corresponds to P1090Q in the hTRPM3 variant we used in oocytes, and it is P1092Q in the hTRPM3α2 we used in HEK293 cells. Figure 1—figure supplement 1 shows the location of these residues in the TRPM3 sequence and putative structure.

First, we co-expressed the Ca²⁺ indicator GCaMP6f (Chen et al., 2013) and the hTRPM3α2 isoform and its mutants in HEK293 cells and performed intracellular Ca²⁺ measurements in a 96-well plate reader. We found that the concentration-response relationship to PregS (Figure 1A–D) and CIM0216 (Figure 1E–H) were left shifted in the mutant channels; V992M showed a much larger shift than P1092Q for both agonists. The V992M mutant also showed a larger increase in basal Ca²⁺ levels than P1092Q at room temperature (21°C), and the application of the TRPM3 antagonist primidone decreased basal Ca²⁺ levels for both mutants in a concentration-dependent manner (Figure 1I–L). Primidone had no effect on basal cytoplasmic Ca²⁺ levels in cells transfected with the wild type TRPM3 indicating negligible basal activity of the wild type channel at room temperature (Figure 1I). Primidone robustly inhibited Ca²⁺ signals evoked by EC₅₀ concentrations of PregS both for wild type and mutant channels (Figure 1—figure supplement 2). We also measured PregS responses at 37°C. Consistent with earlier results (Vriens et al., 2011), sensitivity of wild-type TRPM3 to PregS increased at 37°C; the EC₅₀ of activation decreased to 0.99 μM at 37°C compared to 7 μM at room temperature (Figure 1—figure supplement 3A,B). Basal Ca²⁺ levels at 37°C were substantially elevated in cells expressing wild-type channels, which is consistent with the low temperature threshold of TRPM3 (Vriens et al., 2011). Both mutant channels showed very high basal Ca²⁺ levels, which were not further increased by PregS, indicating substantial Ca²⁺ overload when kept at 37°C continuously (Figure 1—figure supplement 3C,D). Primidone (50 μM) reduced basal Ca²⁺ levels at 37°C in wild type channels and to a smaller extent in the mutant channels (Figure 1M–P).

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Next, we transfected HEK293 cells with the mutant and wild-type hTRPM3α2 and used fura-2 Ca²⁺ imaging to study the effects of acutely increased temperatures (Figure 2). We first increased the temperature to 37°C, followed by 10 μM primidone at room temperature to facilitate return of Ca²⁺ to baseline. Then we applied 25 μM PregS (in the absence of primidone), and compared the Ca²⁺ responses induced by 37°C to that induced by PregS. In cells transfected with wild-type TRPM3, the temperature-induced Ca²⁺ response was, on average, 29.4% of that induced by PregS, for the V992M mutant it was 76.5%, whereas for the P1092Q mutant it was ~122.5% (Figure 2D,E). In cells not expressing TRPM3 increasing temperature to 37°C induced only negligible Ca²⁺ signals (not shown).

Figure 2

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Cytoplasmic Ca²⁺ is an indirect measure of TRPM3 activity, thus next, we performed whole cell patch clamp experiments to compare currents induced by increased temperatures and by PregS. These measurements were performed in the absence of extracellular Ca²⁺ to avoid indirect effects of increased cytoplasmic Ca²⁺ such as Ca²⁺ induced desensitization. We stimulated each cell with a temperature ramp from 23°C to 36°C followed by a saturating concentration of PregS (100 μM) at room temperature (Figure 3A–C). Since the current amplitudes induced by PregS were highly variable, presumably due to different expression levels of the channel (Figure 3H), we normalized the currents induced by increased temperatures to those evoked by PregS and plotted these relative currents as a function of temperature (Figure 3D–F). Figure G shows that the slope of the increase of these currents as a function of temperature were significantly steeper for the P1092Q mutant than for V992M, and both mutants were significantly steeper than the wild type.

Figure 3

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Next, we expressed the wild-type hTRPM3₁₃₂₅ and its V990M and P1090Q mutants in Xenopus oocytes and performed full concentration response measurements with the TRPM3 agonist PregS. Consistent with our Ca²⁺ measurements, the concentration response relationships for PregS were left-shifted for both mutants compared to wild-type, but the effect of the V990M mutation was much more pronounced than that of the P1090Q (Figure 4A–D).

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Both disease-associated mutations are de novo, and all known patients are heterozygous. To mimic heterozygous conditions, we co-injected oocytes with wild type cRNA and either mutant in a 1:1 ratio. The PregS dose response was still markedly left shifted for the V990M:TRPM3 combination, but it was only marginally shifted in the P1090Q:TRPM3 combination compared to WT TRPM3 (Figure 4E–G).

To assess basal current levels, we applied 50 μM primidone at room temperature (20–22°C). Consistent with our Ca²⁺ measurements, primidone evoked a significantly larger inhibition of basal activity in the V990M than in the P1090Q mutant both at 100 and −100 mV (Figure 4—figure supplement 1B-D). Primidone did not induce any inhibition in non-injected oocytes (not shown, n = 5), but it evoked a small reduction in oocytes expressing TRPM3 (Figure 4—figure supplement 1A,D). Current amplitudes induced by 100 μM PregS were not different between the two mutants and wild type, at +100 mV, but the V990M mutants showed somewhat larger amplitudes at −100 mV than the wild type or the P1090Q mutant (Figure 4—figure supplement 1E). We also compared the basal current amplitudes before applying any stimuli, and found that oocytes expressing the V990M mutant showed higher currents than those expressing P1090Q, and both mutants had higher basal currents than wild-type TRPM3 (Figure 4—figure supplement 1F).

We also compared the currents induced by increased temperatures to those evoked by PregS in channels expressed in Xenopus oocytes (Figure 4—figure supplement 2). We found that the ratio of currents induced by 30°C over those induced by 50 μM PregS were significantly larger in the P1090Q mutant compared to V990M, and both mutants showed significantly larger current ratios than wild type TRPM3. Increasing temperature to 30°C induced negligible currents in non-injected oocytes, but those currents became larger at higher temperatures (not shown), which prevented us from testing higher temperatures in this experimental setting.

TRPM3 has been shown to be inhibited by activation of Gi-coupled receptors via direct binding of Gβγ (Badheka et al., 2017; Dembla et al., 2017; Quallo et al., 2017). To test if the mutations alter receptor-induced inhibition, we co-expressed TRPM3 or its mutants with Gi-coupled muscarinic M2 receptors in Xenopus oocytes. Figure 4—figure supplement 3A–F show that when we applied acetylcholine (ACh) to stimulate M2 receptors, it evoked a ~ 50% inhibition of TRPM3 currents induced by 50 μM PregS. The inhibition of the P1090Q mutant was similar to wild type, but the V990M mutant was essentially not inhibited. To test whether the lack of inhibition was due to allosteric effects of the increased sensitivity to PregS, we stimulated the oocytes expressing the V990M mutant with 5 μM PregS where ACh induced a ~ 35% inhibition (Figure 4—figure supplement 3C,E). We also tested inhibition at the EC₅₀ of PregS for wild type (17 μM) and mutant channels, 0.6 μM for V990M and 7 μM for P1090Q. Figure 4—figure supplement 3G-K shows that wild type and V990M mutants were inhibited to a similar extent, the P1090Q mutant was inhibited somewhat more than the wild-type channel. These data show that while the V990M mutation affects receptor-induced inhibition at high PregS concentrations, it is not likely to be the primary mechanism of its gain-of-function phenotype.

Overall, our data show that disease-associated mutations in TRPM3 render the channel overactive. Both mutants showed basal activity even at room temperature, which was reduced by the TRPM3 antagonist primidone. Basal activity of the V992M mutant at room temperature was higher than that of the P1092Q mutant, but at 37°C, the difference in basal activity between the two mutants became negligible. Given the increased constitutive activity of the mutants at body temperature, increased neuronal excitability and/or Ca²⁺-induced neuronal damage is a possible disease-causing mechanism. Primidone is a clinically approved antiepileptic drug; it is thought to exert its effects by being converted to barbiturate by the liver, but it crosses the blood brain barrier (Nagaki et al., 1999), and directly inhibits TRPM3 activity even below its therapeutic concentration (Krügel et al., 2017). Our data showing that primidone inhibited the basal activity of the mutant channels, suggests a potential therapy for this newly described channelopathy.

The V990/V992M mutation showed a larger increase in basal activity at room temperature, and also induced a larger left shift in the concentration response curves of the agonists PregS and CIM0216 than the P1090/1092Q mutation. This is expected if both mutations increase the stability of the open state of the channel, with the V990/992M mutation having a larger effect. If increased open state stability is the only explanation for the over-activity of the mutants, we would expect V990/992M to be also more sensitive to activation by increased temperatures than P1090/1092Q. This is however not what we observed. We consistently find that the P1090/1092Q mutant showed more pronounced activation by increased temperatures than the V990/992M mutant. This indicates that the mechanism of over-activity is different for the two mutants.

These two residues are in different locations; V990/992 is in the S4-S5 linker whereas P1090/1092 is in the outer portion of S6 (Figure 1—figure supplement 1). The S4-S5 linker plays essential roles in channel gating, and has been shown to be a hotspot of disease-associated gain-of-function mutations in a number of TRP channels, including TRPV3, TRPV4, TRPM4 and TRPA1 (Hofmann et al., 2017). This channel segment also plays a role in binding of hydrophobic, or amphipathic ligands in TRPM channels (Huang et al., 2020). For example, in TRPM8 channels the S4-S5 linker is in direct contact with both the cooling agent icilin, and the menthol analog WS12 (Yin et al., 2019). While PregS is thought to activate TRPM3 by directly binding to the channel (Drews et al., 2014), its binding site in the channel is not known, and currently there is no structural information available for TRPM3. Because of the clear increase in basal activity of the V990/992M mutant, it is quite likely that the mutation primarily increased the stability of the open state and the decrease in EC₅₀ of agonist activation is a consequence of the change in activation equilibrium constant (Colquhoun, 1998). It cannot be excluded, however, that the mutation also affected PregS binding concurrently, given the general role of this segment in ligand binding in TRP channels.

The membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP₂) is also required for PregS-induced TRPM3 activity (Badheka et al., 2015; Tóth et al., 2015; Uchida et al., 2016). In TRPM8, the equivalent of V990 is two positions upstream from a residue that is in close contact with PIP₂, and it is 4 and 7 positions downstream from two residues in close contact with icilin and the menthol analog WS12 (Yin et al., 2019; Figure 1—figure supplement 1A). PIP₂ is located adjacent to the menthol analogue WS12 and the cooling agent icilin in the TRPM8 structures, and menthol has been shown to allosterically affect PIP₂ activation (Rohács et al., 2005). Therefore, it is also possible that the V990/992M mutation affects PregS activation indirectly via PIP₂. Increased Ca²⁺ levels in cells expressing mutated channels may also modify cellular PI(4,5)P₂ levels, which may alter channel activity. Differentiating between these possibilities will require future studies.

While the mechanism of temperature activation of TRP channels is not fully understood (Clapham and Miller, 2011; Islas, 2017; Arrigoni and Minor, 2018; Castillo et al., 2018), large-scale unbiased mutagenesis studies on TRPV1 (Grandl et al., 2010) and TRPV3 (Grandl et al., 2008) show that mutations in the pore region and the outer portion of S6 in these channels selectively abolished heat-, but not agonist-induced channel activation. The P1090/1092Q mutation in TRPM3 is located in the outer portion of S6, and it had a stronger effect on heat activation than the V990/992M mutation, therefore it is possible that the primary effect of the P1090/1092Q mutation is increasing heat sensitivity. Increased temperatures synergize with PregS in activating wild-type TRPM3 (Vriens et al., 2011), see also Figure 1—figure supplement 3B, therefore it is possible that the increased PregS sensitivity of the P1090/1090Q mutant is secondary to its increased heat activation.

Overall our data show that both disease-associated mutations render TRPM3 overactive, but likely with different mechanisms.

All data generated or analysed during this study are included in the manuscript and supporting files.

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Author details

1. Siyuan Zhao

   Department of Pharmacology, Physiology and Neuroscience, New Jersey Medical School, Rutgers University, Newark, United States

   Contribution

   Conceptualization, Data curation, Investigation, Writing - original draft, Writing - review and editing

   Competing interests

   No competing interests declared

2. Yevgen Yudin

   Department of Pharmacology, Physiology and Neuroscience, New Jersey Medical School, Rutgers University, Newark, United States

   Contribution

   Investigation, Writing - review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-0663-6435

3. Tibor Rohacs

   Department of Pharmacology, Physiology and Neuroscience, New Jersey Medical School, Rutgers University, Newark, United States

   Contribution

   Conceptualization, Supervision, Funding acquisition, Visualization, Writing - original draft, Writing - review and editing

   For correspondence

   rohacsti@njms.rutgers.edu

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-3580-2575

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