Functional role of the overexpression of the myelin and lymphocyte protein MAL in Schwann cells

For fast propagation of action potentials in the nervous system, higher vertebrates have developed a specialized plasma membrane structure, the myelin, ensheathing nerve fibers. Myelin sheaths are formed by Schwann cells in the peripheral nervous system (PNS), whereas oligodendrocytes are the myelin-forming cells in the central nervous system (CNS). Impairment of the myelin sheath results in severe pathology as seen in multiple sclerosis in the CNS or polyneuropathies such as the Charcot-Marie-Tooth disease in the PNS. During development of peripheral nerves, a coordinated reciprocal signaling between Schwann cells and axons is crucial for accurate Schwann cell development, differentiation as well as maintenance of the myelin sheaths. In addition to orchestrated signal transduction, large amounts of lipids and membrane proteins are synthesized and have to be transported to distinct compartments for proper myelin formation. The transmembrane myelin and lymphocyte protein MAL is associated with lipid rafts, and is important for targeting proteins and lipids to distinct myelin membrane domains. In the PNS, MAL expression starts at around embryonic day 17, implicating a functional role in Schwann cell development. MAL overexpression retards the onset of peripheral myelination, and leads to reduced expression of p75NTR and myelin protein zero (P0). Since accurate expression of p75NTR is essential for proper initiation of myelination, it was suggested that altered p75NTR expression in MAL-overexpressing mice might be the cause of delayed onset of myelination.
To elucidate a functional link between MAL overexpression and retarded myelination, primary mouse Schwann cell cultures were investigated. We could show that the induction of the CREB signaling pathway is functional, indicating that Schwann cells overexpressing MAL are not less responsive to axonal signals. Despite functional activation of the cAMP signaling pathway, significantly reduced mRNA expression levels of P0 and p75NTR were detected in untreated Schwann cells overexpressing MAL. This study revealed that most transcription factors known to modulate P0 expression were not altered in Schwann cells overexpressing MAL. During development, Schwann cells depend on accurate levels of different growth factors. To determine whether the delayed onset of myelination might be caused by a deficient downstream signaling of a particular growth factor, Schwann cells were treated with neuregulin1, nerve growth factor or fibroblast growth factor 1. However, none of the investigated growth factors could ameliorate the reduced expression of P0 and p75NTR in MAL-overexpressing Schwann cells.
MAL-overexpressing Schwann cells were further investigated using a whole genome expression analysis. Most transcripts of genes implicated in Schwann cell development, differentiation and maintenance were not affected by MAL-overexpression. However, we identified a number of genes associated with cytoskeleton organization and components of the basal lamina that are regulated in a MAL-dependent manner. Especially during development and differentiation of Schwann cells, major changes in cellular processes and architecture are crucial for accurate radial sorting and myelination, proposing them as novel candidates for influencing myelination.