Epitope-tagged P(0) glycoprotein causes Charcot-Marie-Tooth-like neuropathy in transgenic mice.

In peripheral nerve myelin, the intraperiod line results from compaction of the extracellular space due to homophilic adhesion between extracellular domains (ECD) of the protein zero (P(0)) glycoprotein. Point mutations in this region of P(0) cause human hereditary demyelinating neuropathies such as Charcot-Marie-Tooth. We describe transgenic mice expressing a full-length P(0) modified in the ECD with a myc epitope tag. The presence of the myc sequence caused a dysmyelinating peripheral neuropathy similar to two distinct subtypes of Charcot-Marie-Tooth, with hypomyelination, altered intraperiod lines, and tomacula (thickened myelin). The tagged protein was incorporated into myelin and was associated with the morphological abnormalities. In vivo and in vitro experiments showed that P(0)myc retained partial adhesive function, and suggested that the transgene inhibits P(0)-mediated adhesion in a dominant-negative fashion. These mice suggest new mechanisms underlying both the pathogenesis of P(0) ECD mutants and the normal interactions of P(0) in the myelin sheath.

P0-Cre transgenic mice for inactivation of adhesion molecules in Schwann cells.

Normal peripheral nerve myelination depends on Schwann cell-basal lamina interactions. An important component of Schwann cell basal lamina is laminin--predominantly laminins 2 and 4. Mutations in the alpha 2 chain common to these two isoforms are associated with dysmyelination in mouse (dy) and man (congenital muscular dystrophy). Thus, laminin 2 and 4 receptors are also likely to be important for myelin formation. Several laminin 2/4 receptors are detected at the basal lamina surface of myelin-forming Schwann cells, namely, alpha 6 beta 4 and alpha 6 beta 1 integrins and dystroglycan. The evidence linking these receptors to myelination is suggestive, but not conclusive. Genetic studies have not yet confirmed a role for these molecules in myelin formation. Natural or targeted inactivation of alpha 6, beta 4, and beta 1 integrins and of dystroglycan have profound effects on other tissues causing embryonic or perinatal death before myelination. Therefore, to conditionally inactivate these receptors specifically in myelin-forming Schwann cells, we have constructed and initially characterized a P0-Cre transgene that activates Cre-mediated recombination of loxP-containing genes in peripheral nerve.

Peripheral nerve dysmyelination due to P0 glycoprotein overexpression is dose-dependent.

We have previously shown that increased dosage of the mouse protein zero gene (Mpz) causes a dysmyelinating neuropathy in transgenic (Tg80) mice. To ask whether the dysmyelination is dose dependent, we inbred one of the Tg80 lines and compared the resulting phenotype in homozygous and heterozygous mice. Whereas heterozygous mice (30% overexpression) have only transient peripheral nerve hypomyelination at two weeks after birth and normal myelin at four weeks after birth, homozygous mice demonstrated more severely hypomyelinated nerves. In the latter, many Schwann cells had achieved a one-to-one relationship with large axons but formed no myelin at four weeks after birth. Expression analysis confirmed a doubling of Mpz overexpression in the sciatic nerves of the homozygous mice. Thus, a threshold exists for Mpz overexpression, above which dysmyelination results. These data have important implications for replacement therapy in Charcot-Marie-Tooth 1B neuropathies due to loss of P0 function.

Schwann cells synthesize alpha7beta1 integrin which is dispensable for peripheral nerve development and myelination.

Defects in laminins or laminin receptors are responsible for various neuromuscular disorders, including peripheral neuropathies. Interactions between Schwann cells and their basal lamina are fundamental to peripheral nerve development and successful myelination. Selected laminins are expressed in the endoneurium, and their receptors are developmentally regulated during peripheral nerve formation. Loss-of-function mutations have confirmed the importance and the role of some of these molecules. Here we show for the first time that another laminin receptor, alpha7beta1 integrin, previously described only in neurons, is also expressed in Schwann cells. The expression of alpha7 appears postnatally, such that alpha7beta1 is the last laminin receptor expressed by differentiating Schwann cells. Genetic inactivation of the alpha7 subunit in mice does not affect peripheral nerve formation or the expression of other laminin receptors. Of note, alpha7beta1 is not necessary for basal lamina formation and myelination. Nonetheless, these data taken together with the previous demonstration of impaired axonal regrowth in alpha7-null mice suggest a possible Schwann cell-autonomous role for alpha7 in nerve regeneration.