Cc2d1b Contributes to the Regulation of Developmental Myelination in the Central Nervous System

Abstract

Background: Numerous studies have indicated that myelination is the result of the interplay between extracellular signals and an intricate network of transcription factors. Yet, the identification and characterization of the full repertoire of transcription factors that modulate myelination are still incomplete. CC2D1B is a member of the Lgd/CC2D1 family of proteins highly expressed in myelinating cells in the central and peripheral nervous systems. In addition, the absence of CC2D1B limits myelin formation in vitro. Here we propose to delineate the function of CC2D1B in myelinating cells during developmental myelination in vivo in the central and peripheral nervous systems.

Methods: We used a Cc2d1b constitutive knockout mouse model and then performed morphological analyses on semithin sections of sciatic nerves and electron micrographs of optic nerves. We also performed immunohistological studies on coronal brain sections. All analyses were performed at 30 days of age.

Results: In the peripheral nervous system, animals ablated for Cc2d1b did not show any myelin thickness difference compared to control animals. In the central nervous system, immunohistological studies did not show any difference in the number of oligodendrocytes or the level of myelin proteins in the cortex, corpus callosum, and striatum. However, optic nerves showed a hypomyelination (0.844 ± 0.022) compared to control animals (0.832 ± 0.016) of large diameter myelinated fibers.

Conclusions: We found that CC2D1B plays a role in developmental myelination in the central nervous system. These results suggest that CC2D1B could contribute to gene regulation during oligodendrocytes myelination in optic nerves.

Keywords: Cc2d1b; Schwann cell; myelin; myelination; oligodendrocyte.

Figure 1

KO of Cc2d1b does not alter developmental myelination in the PNS. (A,B) Single cells RNA-seq–based gene expression values (RPKM) of Cc2d1b in myelinating Schwann cells (mySC), non-myelinating Schwann cells (nmSC), and all Schwann cells (allSC; Gerber et al., 2021). Schwann cells were isolated from mouse sciatic nerve embryonic day 13.5 (E13.5), E17.5, P1, P5, P14, P24, P60 (Gerber et al., 2021). (C) Western blot analysis shows that CC2D1B protein levels are decreased in sciatic nerves of Cc2d1b KO mice at 30 days of age. CC2D1A protein levels are not affected by the absence of CC2D1B. Calnexin was used as a protein loading control. (D) Myelination in Cc2d1b KO mice. Semithin analysis of control and Cc2d1b KO sciatic nerves at P10, P20, and P60. The thickness of myelin (g ratio) was measured. n = 3–5 mice for each genotype. (E) Measurements of compound muscle action potential amplitude from Cc2d1b KO animals at P60. n = 6 nerves for each genotype. (F) Measurements of nerve conduction velocity (NCV) from Cc2d1b KO animals at P60. n = 6 nerves for each genotype. Data are represented as mean ± s.e.m. Scale bars 10 μm.

Figure 2

KO of Cc2d1b alters myelination of large diameter fibers in the CNS. (A) RNA-seq-based gene expression values (FPKM) of Cc2d1b in mouse brain neurons, astrocytes, microglia, oligodendrocyte precursor cells (OPC), and myelinating oligodendrocyte (My.OL; Zhang et al., 2014). (B) Electron microscopy of the Cc2d1b KO optic nerves. (Up) Electron micrographs of axons in the optic nerves of control and Cc2d1b KO mice, at 30 days of age. Scale bars, 2 μm. (Down) Scatter plot, and bar graph of g-ratio values of myelinated axons per axon diameter. Mean axonal diameter of myelinated axons. One-hundred fibers per animal were analyzed. n = 4 mice for each genotype. Data are presented as mean ± s.e.m. Two-sided Student’s t-test: *P-value ≤ 0.05.

Figure 3

Myelin protein level and OLIG2-positive cell number are not affected in Cc2d1b KOCNS. (A) Representative coronal sections of brain tissue immunostained for MBP and OLIG2 collected from adult Cc2d1b KO animals, at 30 days of age. Scale bar, 200 μm. (B) Integrated fluorescence intensity for MBP, MOG, and PLP1 and the number of OLIG2-positive cells were quantified in the lateral corpus callosum (CC), the cingulate cortex (CX), and striatum (ST). (C) Representative cross sections of optic nerve immunostained for OLIG2 and KI67 collected from adult Cc2d1b KO animals, at 30 days of age. Scale bar, 50 μm. (D) Integrated fluorescence intensity for PLP1, and the number of OLIG2-positive and KI67-positive cells were quantified in the optic nerves. n = 3–4 mice for genotype. Data are presented as mean ± s.e.m.