Glial βII Spectrin Contributes to Paranode Formation and Maintenance

Abstract

Action potential conduction along myelinated axons depends on high densities of voltage-gated Na channels at the nodes of Ranvier. Flanking each node, paranodal junctions (paranodes) are formed between axons and Schwann cells in the peripheral nervous system (PNS) or oligodendrocytes in the CNS. Paranodal junctions contribute to both node assembly and maintenance. Despite their importance, the molecular mechanisms responsible for paranode assembly and maintenance remain poorly understood. βII spectrin is expressed in diverse cells and is an essential part of the submembranous cytoskeleton. Here, we show that Schwann cell βII spectrin is highly enriched at paranodes. To elucidate the roles of glial βII spectrin, we generated mutant mice lacking βII spectrin in myelinating glial cells by crossing mice with a floxed allele of with mice, and analyzed both male and female mice. Juvenile (4 weeks) and middle-aged (60 weeks) mutant mice showed reduced grip strength and sciatic nerve conduction slowing, whereas no phenotype was observed between 8 and 24 weeks of age. Consistent with these findings, immunofluorescence microscopy revealed disorganized paranodes in the PNS and CNS of both postnatal day 13 and middle-aged mutant mice, but not in young adult mutant mice. Electron microscopy confirmed partial loss of transverse bands at the paranodal axoglial junction in the middle-aged mutant mice in both the PNS and CNS. These findings demonstrate that a spectrin-based cytoskeleton in myelinating glia contributes to formation and maintenance of paranodal junctions. Myelinating glia form paranodal axoglial junctions that flank both sides of the nodes of Ranvier. These junctions contribute to node formation and maintenance and are essential for proper nervous system function. We found that a submembranous spectrin cytoskeleton is highly enriched at paranodes in Schwann cells. Ablation of βII spectrin in myelinating glial cells disrupted the paranodal cell adhesion complex in both peripheral and CNSs, resulting in muscle weakness and sciatic nerve conduction slowing in juvenile and middle-aged mice. Our data show that a spectrin-based submembranous cytoskeleton in myelinating glia plays important roles in paranode formation and maintenance.

Epidermal control of axonal attachment via β-spectrin and the GTPase-activating protein TBC-10 prevents axonal degeneration

“…In C. elegans, the spectrin-network has been demonstrated to partially function non-cell-autonomously within the epidermis to limit buckling of touch receptor neuron axons during normal body movement, where it was postulated to affect membrane elasticity 6,7 . Moreover, in vertebrate myelinating glia the spectrinbased cytoskeleton has been implicated in the formation and maintenance of adhesion complexes at the paranodal regions flanking nodes of Ranvier 41,42 , and disruption of axon-glia attachments causes axonal degeneration in Purkinje neurons 43 . In this context, together with the data we present here, this suggests mechanisms governing adhesion play highly conserved roles in axonal protection.…”

“…7b). Indeed, TBC-10 and RAB-35 have known roles in membrane recycling events 14,24 and βspectrin has been demonstrated to both stabilize membrane proteins in epithelia 39 and adhesion complexes mediating axonglia attachment [41][42][43] . Consistent with this notion, our results indicate that UNC-70 and TBC-10 can regulate the levels and balance of the hemidesmosome components on the membrane, with mutations in unc-70 causing a decrease of both VAB-10 and LET-805, whereas mutations in tbc-10 cause an increased level of LET-805.…”

βII spectrin (SPTBN1): biological function and clinical potential in cancer and other diseases

“…In cultured Schwann cells, silencing of βII spectrin could inhibit myelination 48 . Observation of conditional knockout mice lacking glial βII spetrin revealed nodes and paranodes have changed during CNS development 49 .…”

Axonal Spectrins: Nanoscale Organization, Functional Domains and Spectrinopathies

“…In the nervous system, αII-spectrin (non-erythrocytic α-spectrin) is the sole α-subunit as verified at mRNA transcript (Zhang et al, 2014) and protein levels (Huang et al, 2017a). βI-spectrin is concentrated in cortical layer 2 and 4, cerebellar granule cells, and in the soma of Purkinje cells (Stankewich et al, 2010), while βII-spectrin is widely expressed in neurons and glia (Galiano et al, 2012; Zhang et al, 2013; Susuki et al, 2018). βIII-spectrin is found in the soma and dendrites of the cerebellar molecular layer (Stankewich et al, 2010).…”

“…βII-spectrin is also found in Schwann cells at paranodes, and mice lacking βII-spectrin in myelinating glial cells have disrupted paranodal axoglial junctions in young mice. These defects eventually resolve, but then re-appear as mice age, suggesting that glial βII-spectrin is required for the timely formation and long-term maintenance of paranodal junctions (Susuki et al, 2018).…”