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 Sptbn1 with Cnp-Cre 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. SIGNIFICANCE STATEMENT 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.

Cognitive and mood impairments are common central nervous system complications of type 2 diabetes, although the neuronal mechanism(s) remains elusive. Previous studies focused mainly on neuronal inputs such as altered synaptic plasticity. Axon initial segment (AIS) is a specialized functional domain within neurons that regulates neuronal outputs. Structural changes of AIS have been implicated as a key pathophysiological event in various psychiatric and neurological disorders. Here we evaluated the structural integrity of the AIS in brains of db/db mice, an established animal model of type 2 diabetes associated with cognitive and mood impairments. We assessed the AIS before (5 weeks of age) and after (10 weeks) the development of type 2 diabetes, and after daily exercise treatment of diabetic condition. We found that the development of type 2 diabetes is associated with significant AIS shortening in both medial prefrontal cortex and hippocampus, as evident by immunostaining of the AIS structural protein βIV spectrin. AIS shortening occurs in the absence of altered neuronal and AIS protein levels. We found no change in nodes of Ranvier, another neuronal functional domain sharing a molecular organization similar to the AIS. This is the first study to identify AIS alteration in type 2 diabetes condition. Since AIS shortening is known to lower neuronal excitability, our results may provide a new avenue for understanding and treating cognitive and mood impairments in type 2 diabetes.

Renal fibrosis, a detrimental feature of irreversible kidney damage, remains a sinister consequence of long-term calcineurin inhibitor (CNI) immunosuppressive therapy. Our study not only incorporates renal fibroblasts into the growing list of cell types negatively impacted by CNIs but also identifies renal fibroblast-to-myofibroblast transition as a process mediated via a TGF-β-dependent mechanism. This insight will direct future studies investigating the feasibility of inhibiting TGF-β signaling to maintain CNI-mediated immunosuppression while ultimately preserving kidney health.