Identification and characterization of a new member of the gas3/PMP22 gene family in C. elegans

Myelin tetraspan family proteins but no non-tetraspan family proteins are present in the ascidian (Ciona intestinalis) genome

Several of the proteins used to form and maintain myelin sheaths in the central nervous system (CNS) and the peripheral nervous system (PNS) are shared among different vertebrate classes. These proteins include one-to-several alternatively spliced myelin basic protein (MBP) isoforms in all sheaths, proteolipid protein (PLP) and DM20 (except in amphibians) in tetrapod CNS sheaths, and one or two protein zero (P0) isoforms in fish CNS and in all vertebrate PNS sheaths. Several other proteins, including 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNP), myelin and lymphocyte protein (MAL), plasmolipin, and peripheral myelin protein 22 (PMP22; prominent in PNS myelin), are localized to myelin and myelin-associated membranes, though class distributions are less well studied. Databases with known and identified sequences of these proteins from cartilaginous and teleost fishes, amphibians, reptiles, birds, and mammals were prepared and used to search for potential homologs in the basal vertebrate, Ciona intestinalis. Homologs of lipophilin proteins, MAL/plasmolipin, and PMP22 were identified in the Ciona genome. In contrast, no MBP, P0, or CNP homologs were found. These studies provide a framework for understanding how myelin proteins were recruited during evolution and how structural adaptations enabled them to play key roles in myelination.

The molecular physiology of tight junction pores

Tight junctions form selective barriers that regulate paracellular transport across epithelia. A large family of tetraspanning cell-cell adhesion proteins called claudins create the barrier and regulate electrical resistance, size, and ionic charge selectivity. Study of inherited human claudin diseases and the outcome of the genetic manupulation of claudins in mice, Drosophila, and Caenorhabditis elegans are furthering our understanding of paracellular physiology.

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.