Whirlin, a cytoskeletal scaffolding protein, stabilizes the paranodal region and axonal cytoskeleton in myelinated axons

BMC Neuroscience

Manzoor A Bhat⁴  Bechara Kachar²  M’hamed Grati²  Jun Yang¹  James A Green³ 

[1] Ophthalmology and Visual Sciences, Moran Eye Center and Neurobiology and Anatomy, University of Utah, Salt Lake City, UT 84132, USA;Laboratory of Cell Structure and Dynamics, National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Bethesda, MD 20892, USA;Department of Cell and Molecular Physiology, University of North Carolina School of Medicine, Chapel Hill, NC 27599, USA;Department of Physiology, University of Texas School of Medicine, Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229, USA

关键词: Whirlin;    Axonal cytoskeleton;    Paranodal domain;    Axonal domains;    Myelinated axons;   

Others  :  1140089
DOI  :  10.1186/1471-2202-14-96

 received in 2013-07-19, accepted in 2013-09-03,  发布年份 2013

【 摘 要 】

Background

Myelinated axons are organized into distinct subcellular and molecular regions. Without proper organization, electrical nerve conduction is delayed, resulting in detrimental physiological outcomes. One such region is the paranode where axo-glial septate junctions act as a molecular fence to separate the sodium (Na⁺) channel-enriched node from the potassium (K⁺) channel-enriched juxtaparanode. A significant lack of knowledge remains as to cytoskeletal proteins which stabilize paranodal domains and underlying cytoskeleton. Whirlin (Whrn) is a PDZ domain-containing cytoskeletal scaffold whose absence in humans results in Usher Syndromes or variable deafness-blindness syndromes. Mutant Whirlin (Whrn) mouse model studies have linked such behavioral deficits to improper localization of critical transmembrane protein complexes in the ear and eye. Until now, no reports exist about the function of Whrn in myelinated axons.

Results

RT-PCR and immunoblot analyses revealed expression of Whrn mRNA and Whrn full-length protein, respectively, in several stages of central and peripheral nervous system development. Comparing wild-type mice to Whrn knockout (Whrn^−/−) mice, we observed no significant differences in the expression of standard axonal domain markers by immunoblot analysis but observed and quantified a novel paranodal compaction phenotype in 4 to 8 week-old Whrn^−/− nerves. The paranodal compaction phenotype and associated cytoskeletal disruption was observed in Whrn^−/− mutant sciatic nerves and spinal cord fibers from early (2 week-old) to late (1 year-old) stages of development. Light and electron microscopic analyses of Whrn knockout mice reveal bead-like swellings in cerebellar Purkinje axons containing mitochondria and vesicles by both. These data suggest that Whrn plays a role in proper cytoskeletal organization in myelinated axons.

Conclusions

Domain organization in myelinated axons remains a complex developmental process. Here we demonstrate that loss of Whrn disrupts proper axonal domain organization. Whrn likely contributes to the stabilization of paranodal myelin loops and axonal cytoskeleton through yet unconfirmed cytoskeletal proteins. Paranodal abnormalities are consistently observed throughout development (2 wk-1 yr) and similar between central and peripheral nervous systems. In conclusion, our observations suggest that Whrn is not required for the organization of axonal domains, but once organized, Whrn acts as a cytoskeletal linker to ensure proper paranodal compaction and stabilization of the axonal cytoskeleton in myelinated axons.

【 授权许可】

   
2013 Green et al.; licensee BioMed Central Ltd.

【 图 表 】

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