Peripheral myelin protein 22 is a constituent of intercellular junctions in epithelia

Alterations in peripheral myelin protein 22 (PMP22) gene expression are associated with a host of heritable demyelinating peripheral neuropathies, yet the function of the protein remains unknown. PMP22 expression is highest in myelinating Schwann cells of peripheral nerves; however, significant levels of PMP22 mRNAs can be detected in a variety of non-neural tissue, including epithelia. To date, PMP22 protein expression and localization in non-neural tissues have not been studied in detail. In adult rat liver and intestine, and cultured epithelial cells, we detected PMP22-like immunoreactivity associated with markers of the tight junctional complex, including zonula occludens 1 (ZO-1) and occludin. Upon disruption of intercellular contacts, PMP22 was internalized into vesicles that were immunoreactive for both anti-occludin and anti-PMP22 antibodies. Nonionic detergent extraction of cultured epithelial cells did not solubilize PMP22, as the majority of the protein remained in the detergent insoluble fraction, as did ZO-1 and occludin. We also observed the targeting of exogenous myc-tagged PMP22 to apical cell junctions in polarized epithelia and to anti-ZO-1 antibody immunoreactive cell contacts of L fibroblasts. These studies support a role for PMP22 at intercellular junctions of epithelia and may indicate a similar function in myelinating Schwann cells. Furthermore, our findings could provide an explanation for certain phenotypes of PMP22 neuropathy mice that cannot be accounted for by dysmyelination

PMP22 transgenic dorsal root ganglia cultures show myelin abnormalities similar to those of human CMT1A.

none9Charcot-Marie-Tooth 1A (CMT1A) neuropathy is caused by duplication of the peripheral myelin protein 22 (PMP22) gene, leading to protein overexpression. Although this protein has a role in regulating Schwann cell growth and peripheral myelin compaction, how altered concentrations of PMP22 impair myelination is unknown. We established dorsal root ganglia (DRG) cultures from a transgenic rat overexpressing PMP22 (PMP22tg) to study the behavior of PMP22tg Schwann cells in early stages of development and myelination. We used reverse transcriptase-polymerase chain reaction and light and electron microscopy to study PMP22 expression and myelin formation. Myelin ultrastructure was evaluated in sural nerves from CMT1A patients to compare experimental and human findings. PMP22tg DRG cultures contained a greater number of internodes devoid of myelin, in the absence of remyelination, and increased periodicity of myelin lamellae compared with normal cultures. Widening of myelin lamellae was also observed in CMT1A biopsy specimens. Our results suggest that both functions of PMP22, in regulating Schwann cell differentiation and contributing to peripheral myelin compaction, are affected by its overexpression. The presence of similar myelin abnormalities in PMP22tg cultures and human nerves emphasizes the importance of developing in vitro models of hereditary neuropathies to study their underlying pathomechanisms.L. Nobbio;G. Mancardi;M. Grandis;G. Levi;U. Suter;K. A. Nave;A. J. Windebank;M. Abbruzzese;A. SchenoneNobbio, Lucilla; Mancardi, GIOVANNI LUIGI; Grandis, Marina; G., Levi; U., Suter; K. A., Nave; A. J., Windebank; Abbruzzese, Michele; Schenone, Angel