Characterization of the Function of Sox21 During Neurogenesis

Abstract

Neurogenesis, the progression from neural progenitor to committed neuron, is a tightly regulated process that is fundamental for development of the central nervous system. Whereas the SoxB1 transcriptional activators are required for induction and maintenance of neural progenitors, the closely related SoxB2 proteins function as repressors and are proposed to inhibit SoxB1 targets to control the progression of neuronal differentiation. However, conflicting reports compelled us to reassess this relationship. To clarify its role in neurogenesis and determine its mechanism of action, we characterized the function of the SoxB2 protein Sox21 in the African clawed frog Xenopus laevis. Our gain-of-function assays showed that rather than promote differentiation as reported, Sox21 expanded the neural progenitor domain and repressed neuron formation. Strikingly, we found that Sox21prohibited differentiation by binding to Ngn2 to counteract its function in driving differentiation. Our loss-of-function studies demonstrated that the loss of Sox21 also reduced neuron formation, and more importantly caused increased cell death. This suggests that, like other Sox proteins, the level of Sox21 expression determines how it functions; at high levels it expands progenitors and represses differentiation, but a threshold level is required for proper neuron formation and cell viability.

Because all Sox proteins recognize the same DNA-binding sequence, it is understood that they cooperate with other partner proteins to regulate their specific target genes. We propose that the role of Sox21 is dependent on interactions with partner proteins at different levels of expression. We found both novel and candidate Sox21-interacting proteins through literature searches and by identification of Sox21-bound proteins through mass spectrometry. We confirmed that Sox2, a SoxB1 protein, and Gsh2 both interact with Sox21 in vivo. Because these proteins have opposing functions, we predict that the function of Sox21 varies when bound to each of them.

With this report we have redefined the relationship between Sox21 and SoxB1 proteins, suggesting that they have cooperative rather than opposing functions. Furthermore, our work enriches the field of knowledge regarding how Sox proteins work with partners to specify various functions during embryonic development.