Cdc42 and Rac1 Signaling Are Both Required for and Act Synergistically in the Correct Formation of Myelin Sheaths in the CNS

“…known for their roles in regulating signaling pathways linking extracellular stimuli to the assembly and organization of the actin cytoskeleton (Hall, 1998), also control microtubule dynamics, cell polarity, membrane trafficking, and gene transcription (Etienne-Manneville and Hall, 2002; Jaffe…”

“To follow the fate of the recombined cells by detection of -galactosidase expression, we bred the conditional LacZ allele from the ROSA26 reporter mouse strain (Soriano, 1999) into control and mutant mice.”

“Cdc42 is not required for OPC proliferation, directed migration, or morphological differentiation Our results do not support a requirement for cdc42 in OPC proliferation, directed migration, or in vitro morphological differentiation; however, there is a significant body of evidence showing a key role for cdc42 in the…”

“Oligodendrocyte progenitor cells (OPCs) were purified from mixed glial cultures (McCarthy and de Vellis, 1980) and differentiated on extracellular matrix substrates (Relvas et al., 2001).”

“…roles in regulating signaling pathways linking extracellular stimuli to the assembly and organization of the actin cytoskeleton (Hall, 1998), also control microtubule dynamics, cell polarity, membrane trafficking, and gene transcription (Etienne-Manneville and Hall, 2002; Jaffe and Hall, 2005).”

“Consequently, in wells to which 25 or 100 ng/ml of netrin-1 was added to the bottom chamber, control OPC migration toward the lower surface of the filter was inhibited significantly ( p 0.01 and 0.009, respectively), as compared with wells to which no netrin-1 was added; however, control and mutant OPCs migrated equal…”

“Cdc42 is not required for OPC proliferation, directed migration, or morphological differentiation Our results do not support a requirement for cdc42 in OPC proliferation, directed migration, or in vitro morphological differentiation; however, there is a significant body of evidence showing a key role for cdc42 in the…”

“This observation raises important questions about the specificity of Rho GTPase dominant-negative approaches and is in line with previous reports showing that dominant-negative mutant expression induced more severe phenotypes than those produced by genetic loss-of-function approaches on identical cell types or organis…”

“Several lines of research have demonstrated crucial roles for Cdc42 and Rac1 in the morphological diferentiation of OLs and in myelin formation activity [18, 19].”

“Consistent with this, the predicted HOX TF activity from our TF network is higher in OPCs than in MOLs, suggesting that priming may support rapid gene activation during regeneration or remyelination and diminishes with differentiation.”

“During central nervous system (CNS) development, oligodendrocyte progenitor cells (OPCs) differentiate into myelin forming oligodendrocytes (OLs).”

“In fact, our bioinformatics analysis identified a variety of cytoskeletal regulators such as Rac1, Pak1, Daam2, Sema6a , all of which are targeted by FMRP and were shown to affect oligodendrocyte morphology (Thurnherr et al. 2006; Cristobal et al. 2022; Brown et al. 2021; Bernard et al. 2012).”

“Myelin sheaths are a significantly stretched and altered plasma membrane that enfold around the nerve axon in a helical style (Raine, 1984; Thurnherr et al., 2006; Benninger et al., 2007; Nodari et al., 2007).”

“, 1987) and, as a result, in vitro OL differentiation assays have been extensively used by many groups and in many different experimental contexts to address fundamental questions of OL biology (Colognato et al., 2007; Huang et al., 2016; Najm et al., 2015; Thurnherr et al., 2006).”

“…its role in synaptic plasticity, Rac1 regulates key cellular processes across different neural cell types, including promoting oligodendrocyte myelination [239], shaping astrocyte morphology [240], and enabling microglia to adapt to their microenvironment within the central nervous system [241].”

“Its activity has been associated with oligodendrocyte differentiation and myelination (Brown et al., 2021), as well as with proper myelin sheath formation in the central nervous system (Thurnherr et al., 2006).”

“As illustrated in Figure 4a, both groups achieved high levels of performance to a criterion of 80% correct trials across two successive days and there was no difference in the number of sessions required to reach criterion between the groups (Figure 4b, GENOTYPE (F(1,14)=0.”

“CDC42 is known to synergise with Rac1 during myelination of CNS [9].”

“Cdc42 flox mice were previously used to delete Cdc42 in myelinating oligodendrocytes, including during development (Thurnherr et al. 2006); Cdc42 flox/flox ; Cnp Cre mice displayed FIGURE 1 | Septins, CDC42, and CDC42-effector proteins CDC42EP1/CDC42EP2 in oligodendrocytes and myelin.”

“Most of these genes are involved in small GTPase signaling and cytoskeleton remodeling (Thurnherr et al., 2006), which suggests extensive oligodendrocyte structural remodeling associated with the CPZ condition.”

“Previous in vitro data demonstrated that Rac1 is a positive regulator of OPC morphological differentiation; however, in vivo data from Rac1 mutants under a Cnp promoter did not reveal any change in the number of myelinated fibers 68 .”

“Pinch2 regulates Rho GTPase activation We and others previously showed that myelin formation depends on tight regulation of Rho GTPase mediated signaling (Nodari et al., 2007, Pereira et al., 2009, Thurnherr et al., 2006, Benninger et al., 2007).”

“Other proteins that may be involved in actin cytoskeleton-dependent myelin formation include neural Wiskott-Aldrich syndrome protein (N-WASP), Wiskott-Aldrich syndrome protein family verprolin homologous protein 1 (WAVE1, also known as Wiskott-Aldrich syndrome protein family member 1), cell division control protein 42…”

“As mentioned earlier, high Rac1 activity has been proposed to mediate radial sorting by allowing Schwann cells to for lateral lamellipodia, while a low Rac1 activity would promote migration and elongation along axons (77, 85, 86, 176, 219).”

“polymerizing proteins that can compensate for each other (Thurnherr et al., 2006).”

“, 2005) and more specifically, the Rho GTPase family members, RhoA and Rac1 have been associated with membrane formation in oligodendrocytes (Thurnherr et al., 2006; Czopka et al., 2009) indicating that activation of these Rho GTPases could be activated and responsible for LPAinduced membranous structure formation in…”

“…is no apparent effect on oligodendrocyte development upon Cdc42 or Rac knockout in vivo, there is an increase in the amount of myelin out-foldings and a reduction in 11 myelin thickness, which points to a role for cytoskeletal modification in appropriate myelin formation (Thurnherr et al. 2006).”