Localization and functional roles of PMP22 in peripheral nerves of P0-deficient mice

Peripheral nerves of P0-deficient (P0^-) mice show a severe dysmyelination and altered expression of several cell surface molecules. In the present study we investigated the subcellular localization of the peripheral myelin protein (PMP)22 in the abnormal axon-Schwann cell units of the mutants. We show by postembedding immunoelectron microscopy that PMP22 is expressed in both noncompacted and abnormally compacted myelin-like regions of P0^- mice. By the generation of mice deficient for both P0 and PMP22 (P0^-/PMP22^- double mutants) we investigated the functional role(s) of PMP22 in P0^- mice. In 4-week-old double mutants, some abnormally compacted myelin-like sheaths showed slight alterations in compaction with collapsed intraperiod lines, whereas the totally uncompacted axon-Schwann cell units displayed a more irregular cytoarchitecture owing to the presence of more cytoplasm within the loose Schwann cell loops. These findings show an only subtle impact of PMP22 on the structure of P0-deficient myelin-like sheaths. During early stages of myelin formation, peripheral nerves of P0^-/PMP22^- mice resembled those of PMP22^- mutants in that they were characterized by a strongly retarded spiralling of Schwann cell processes. Thus, P0 appears to be the major determinant of myelin structure, whereas PMP22 is the predominant regulator of the timely correct initiation of myelination.