Oligodendroglial anillin facilitates septin assembly to prevent myelin outfoldings

Abstract

Myelination of axons facilitates rapid and precise impulse propagation in the nervous system and thus, normal motor, sensory, and cognitive capabilities. In the central nervous system (CNS), myelin is formed by oligodendrocytes, which enwrap axons with several layers of compacted membrane. The axon/myelin-unit is often impaired in myelin-related disorders and upon normal aging. However, only little is known about proteins that maintain the structure of mature myelin, many of them localized to its non-compacted sub-compartments. For example, one of the latest steps of myelin maturation in the CNS is the assembly of septin filaments, which prevent the formation of pathological myelin outfoldings. This work focusses on the analysis of Anlnfl/fl;CnpCre/Wt (Anln cKO) mice, which lack the cytoskeletal adaptor protein anillin (ANLN) in myelinating oligodendrocytes. To assess the role of ANLN for myelination, different biochemical and microscopic analyses have been performed. By quantitative mass spectrometry (MS) analysis of purified myelin and immunoblot validation, a specific reduction of ANLN, SEPT2, SEPT4, SEPT7, and SEPT8 has been detected in Anln cKO mice compared to controls. Electron micrograph analysis revealed the emergence of pathological myelin outfoldings upon conditional ANLN deletion and the observed reduction of myelin septins. Interestingly, these myelin outfoldings are a very specific neuropathology, as other analyzed neuropathological features were not observed in Anln cKO mice. By focused ion beam-scanning electron microscopy (FIB-SEM), it has been observed that these outfoldings are large sheets of multiple compact membrane layers, a plausible explanation for an altered membrane resistance along myelinated fiber tracts and thus, the observed reduced nerve conduction velocity. The results lead to the assumption that anillin-dependent scaffolding of mature myelin by septin filaments is required for its normal function in facilitating rapid nerve conduction in the healthy CNS.