Functional relevance of oligodendroglial CDC42 effector proteins CDC42EP1 and CDC42EP2 in myelin morphogenesis

by Sophie Hümmert
Doctoral thesis
Date of Examination:2025-08-28
Date of issue:2025-11-12
Advisor:Dr. Hauke Werner
Referee:Dr. Hauke Werner
Referee:Prof. Dr. Christine Stadelmann-Nessler
Persistent Address: http://dx.doi.org/10.53846/goediss-11621

 

 

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Abstract

English

Myelin sheaths are composed of multiple compacted membrane layers that tightly encase the underlying axon. Structural abnormalities of myelin indicate pathological processes and are frequently associated with neurological disorders or age-related degeneration of the nervous system. The regular structure of healthy central nervous system (CNS) myelin relies on specialized structural proteins, including cytoskeletal filaments assembled from the septin subunits SEPTIN2, SEPTIN4, SEPTIN7, and SEPTIN8. This study addresses the vital question: How is septin filament assembly in CNS myelin regulated? It is hypothesized that a pathway downstream of the master regulator CDC42 influences septin assembly within myelin. This hypothesis was tested by genetically ablating the CDC42 effector proteins CDC42EP1 and CDC42EP2 specifically in oligodendrocytes. A multi-modal approach to characterize the resulting phenotypes was employed, including transmission electron microscopy (TEM) for ultrastructural analysis, immunohistochemical (IHC) imaging to assess myelin septin architecture and neuropathology, qRT-PCR to assess transcriptional changes in septin-associated genes, and quantitative myelin proteome analysis. The results indicate that recombination of the Cdc42ep1 and Cdc42ep2 genes in oligodendrocytes causes a marked reduction in septin protein levels within myelin, as demonstrated by proteomic profiling and IHC analysis. TEM imaging revealed that mutant mice developed myelin outfoldings as specific pathology, a phenotype that closely resembles the defects observed in Septin8-deficient mice, which lack functional myelin septin filaments. Together, the findings suggest that oligodendroglial CDC42 effector proteins regulate myelin septin assembly to facilitate structural integrity of healthy myelin sheaths. Mechanistically, it is demonstrated that CDC42EP1/EP2 directly interact with septin filaments and their loss results in fragmented, disorganized septin structure.
Keywords: Oligodendrocyte; myelination; white matter; septins; CDC42EP1; CDC42EP2
 
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