Exposure at the cell surface is required for gas3/PMP22 To regulate both cell death and cell spreading: implication for the Charcot-Marie-Tooth type 1A and Dejerine-Sottas diseases.

Gas3/PMP22 is a tetraspan membrane protein highly expressed in myelinating Schwann cells. Point mutations in the gas3/PMP22 gene account for the dominant inherited peripheral neuropathies Charcot-Marie-Tooth type 1A disease (CMT1A) and Dejerine-Sottas syndrome (DSS). Gas3/PMP22 can regulate apoptosis and cell spreading in cultured cells. Gas3/PMP22 point mutations, which are responsible for these diseases, are defective in this respect. In this report, we demonstrate that Gas3/PMP22-WT is exposed at the cell surface, while its point-mutated derivatives are intracellularly retained, colocalizing mainly with the endoplasmic reticulum (ER). The putative retrieval motif present in the carboxyl terminus of Gas3/PMP22 is not sufficient for the intracellular sequestration of its point-mutated forms. On the contrary, the introduction of a retrieval signal at the carboxyl terminus of Gas3/PMP22-WT leads to its intracellular accumulation, which is accompanied by a failure to trigger cell death as well as by changes in cell spreading. In addition, by substituting the Asn at position 41 required for N-glycosylation, we provide evidence that N-glycosylation is required for the full effect on cell spreading, but it is not necessary for triggering cell death. In conclusion, we suggest that the DSS and the CMT1A neuropathies derived from point mutations of Gas3/PMP22 might arise, at the molecular level, from a reduced exposure of Gas3/PMP22 at the cell surface, which is required to exert its biological functions.