Impact of transcription factor Sox8 on oligodendrocyte specification in the mouse embryonic spinal cord

The myelin-forming oligodendrocytes of the mouse embryonic spinal cord express the three group E Sox proteins Sox8, Sox9, and Sox10. They require Sox9 for their specification from neuroepithelial cells of the ventricular zone and Sox10 for their terminal differentiation and myelination. Here, we show that during oligodendrocyte development, Sox8 is expressed after Sox9, but before Sox10. Loss of Sox8 did not impair oligodendrocyte specification by itself, but enhanced the Sox9-dependent defect. Oligodendrocyte progenitors were still generated in the Sox9-deficient spinal cord, albeit at 20-fold lower rates than in the wildtype. Combined loss of Sox8 and Sox9, in contrast, led to a near complete loss of oligodendrocytes. Other cell types such as ventricular zone cells and radial glia remained unaffected in their numbers as well as their rates of proliferation and apoptosis. Oligodendrocyte development thus relies on the differential contribution of all three group E Sox proteins at various phases.     

Hypomyelination Phenotype Caused by Impaired Differentiation of Oligodendrocytes in Emx1-cre Mediated Cdk5 Conditional Knockout Mice

Cyclin-dependent kinase 5 (Cdk5) plays a pivotal role in neuronal migration and differentiation, and in axonal elongation. Although many studies have been conducted to analyze neuronal functions of Cdk5, its kinase activity has also been reported during oligodendrocyte differentiation, which suggests Cdk5 may play an important role in oligodendrocytes. Here, we describe a hypomyelination phenotype observed in Emx1-cre mediated Cdk5 conditional knockout (cKO) mice (Emx1-cKO), in which the Cdk5 gene was deleted in neurons, astrocytes and oligodendrocyte -lineage cells. In contrast, the Cdk5 gene in CaMKII cKO mice was deleted only in neurons. Because the development of mature oligodendrocytes from oligodendrocyte precursor cells is a complex process, we performed in situ hybridization using markers for the oligodendrocyte precursor cell and for the differentiated oligodendrocyte. Our results indicate that hypomyelination in Emx1-cKO is due to the impaired differentiation of oligodendrocytes, rather than to the proliferation or migration of their precursors. The present study confirmed the in vivo role of Cdk5 in oligodendrocyte differentiation.     

Regulation of integrin growth factor interactions in oligodendrocytes by lipid raft microdomains

Individual growth factors can regulate multiple aspects of behavior within a single cell during differentiation, with each signaling pathway controlled independently and also responsive to other receptors such as cell surface integrins. The mechanisms by which this is achieved remain poorly understood. Here we use myelin-forming oligodendrocytes and their precursors to examine the role of lipid rafts, cholesterol and sphingolipid-rich microdomains of the cell membrane implicated in cell signaling. In these cells, the growth factor PDGF has sequential and independent roles in proliferation and survival. We show that the oligodendrocyte PDGFalpha receptor becomes sequestered in a raft compartment at the developmental stage when PDGF ceases to promote proliferation, but is now required for survival. We also show that laminin-2, which is expressed on axons in the CNS and which provides a target-dependent signal for oligodendrocyte survival by amplification of PDGFalphaR signaling, induces clustering of the laminin binding integrin alpha6beta1 with the PDGFalphaR-containing lipid raft domains. This extracellular matrix-induced colocalization of integrin and growth factor receptor generates a signaling environment within the raft for survival-promoting PI3K/Akt activity. These results demonstrate novel signaling roles for lipid rafts that ensure the separation and amplification of growth factor signaling pathways during development.     

Sox9 is essential for outer root sheath differentiation and the formation of the hair stem cell compartment

BACKGROUND: The mammalian hair represents an unparalleled model system to understand both developmental processes and stem cell biology. The hair follicle consists of several concentric epithelial sheaths with the outer root sheath (ORS) forming the outermost layer. Functionally, the ORS has been implicated in the migration of hair stem cells from the stem cell niche toward the hair bulb. However, factors required for the differentiation of this critical cell lineage remain to be identified. Here, we describe an unexpected role of the HMG-box-containing gene Sox9 in hair development. RESULTS: Sox9 expression can be first detected in the epithelial component of the hair placode but then becomes restricted to the outer root sheath (ORS) and the hair stem cell compartment (bulge). Using tissue-specific inactivation of Sox9, we demonstrate that this gene serves a crucial role in hair differentiation and that skin deleted for Sox9 lacks external hair. Strikingly, the ORS acquires epidermal characteristics with ectopic expression of GATA3. Moreover, Sox9 knock hair show severe proliferative defects and the stem cell niche never forms. Finally, we show that Sox9 expression depends on sonic hedgehog (Shh) signaling and demonstrate overexpression in skin tumors in mouse and man. CONCLUSIONS: We conclude that although Sox9 is dispensable for hair induction, it directs differentiation of the ORS and is required for the formation of the hair stem cell compartment. Our genetic analysis places Sox9 in a molecular cascade downstream of sonic hedgehog and suggests that this gene is involved in basal cell carcinoma.     

Functional analysis of Sox8 and Sox9 during sex determination in the mouse

Sex determination in mammals directs an initially bipotential gonad to differentiate into either a testis or an ovary. This decision is triggered by the expression of the sex-determining gene Sry, which leads to the activation of male-specific genes including the HMG-box containing gene Sox9. From transgenic studies in mice it is clear that Sox9 is sufficient to induce testis formation. However, there is no direct confirmation for an essential role for Sox9 in testis determination. The studies presented here are the first experimental proof for an essential role for Sox9 in mediating a switch from the ovarian pathway to the testicular pathway. Using conditional gene targeting, we show that homozygous deletion of Sox9 in XY gonads interferes with sex cord development and the activation of the male-specific markers Mis and P450scc, and leads to the expression of the female-specific markers Bmp2 and follistatin. Moreover, using a tissue specific knock-out approach, we show that Sox9 is involved in Sertoli cell differentiation, the activation of Mis and Sox8, and the inactivation of Sry. Finally, double knock-out analyses suggest that Sox8 reinforces Sox9 function in testis differentiation of mice.