Abstract
Charcot-Marie-Tooth (CMT) disease is a peripheral neuropathy associated with gene duplication and point mutations in the peripheral myelin protein 22 (PMP22) gene. However, the role of PMP22 in Schwann cell physiology and the mechanisms by which PMP22 mutations cause CMT are not well-understood. On the basis of homology between PMP22 and proteins associated with modulation of ion channels, we hypothesized that PMP22 alters ion channel activity. Using whole-cell electrophysiology, we show here that heterologous PMP22 expression increases the amplitude of currents similar to those ascribed to store-operated calcium (SOC) channels, particularly those involving transient receptor canonical channel 1 (TrpC1). These channels help replenish Ca2+ in the endoplasmic reticulum (ER) following stimulus-induced depletion. Currents with similar properties were recorded in WT but not pmp22-/- mouse Schwann cells. Heterologous expression of the CMT-associated PMP22_L16P variant, which fails to reach the plasma membrane and localizes to the ER, led to larger currents than WT PMP22. Similarly, Schwann cells isolated from Trembler J (TrJ; PMP22_L16P) mice had larger currents than WT littermates. Calcium imaging in live nerves and cultured Schwann cells revealed elevated intracellular Ca2+ in TrJ mice compared with WT. Moreover, we found that PMP22 co-immunoprecipitated with stromal interaction molecule 1 (STIM1), the Ca2+ sensor SOC channel subunit in the ER. These results suggest that in the ER, PMP22 interacts with STIM1 and increases Ca2+ influx through SOC channels. Excess or mutant PMP22 in the ER may elevate intracellular Ca2+ levels, which could contribute to CMT pathology.
Highlights
Charcot-Marie-Tooth (CMT) disease is a peripheral neuropathy associated with gene duplication and point mutations in the peripheral myelin protein 22 (PMP22) gene
Because peripheral myelin protein 22 (PMP22) shows homology to several proteins associated with ion transport across the plasma membrane [26], we hypothesized that expression of PMP22 could affect ion channel activity in mammalian cells
Because the intracellular calcium concentration in those experiments was low (Ͻ10 nM), the latter results suggest that the currents observed after the removal of extracellular divalent cations may be associated with store-operated calcium (SOC) channels, which are expressed in HEK293 cells [37, 38]
Summary
Charcot-Marie-Tooth (CMT) disease is a peripheral neuropathy associated with gene duplication and point mutations in the peripheral myelin protein 22 (PMP22) gene. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. The CMT1E-associated L16P mutation results in protein misfolding, accumulation in the ER, and formation of cytoplasmic aggresomes (9 –15) Both duplication and point mutations in PMP22 lead to dys-/demyelination, increased Schwann cell number, and severe secondary axonal loss. Recent analysis of pmp22ϩ/Ϫ mice revealed an essential role for this protein in the formation of tight adherens and adhesion junctions in peripheral myelin [25] This may be related to the fact that PMP22 is homologous to the claudins and likely shares a similar structure [26, 27]. Because PMP22 shows structural homology to proteins associated with modulation of ion channels [26], we hypothesized that PMP22 modifies ion channel activity in a manner that impacts cell function
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