Expression of a novel serine/threonine kinase gene, Ulk4, in neural progenitors during Xenopus laevis forebrain development

We have analyzed the expression pattern of a novel serine/threonine kinase gene Ulk4 during forebrain development in Xenopus laevis. To this aim, we firstly cloned a Ulk4 cDNA fragment from X. laevis and generated a RNA probe that was used for its detection by in situ hybridization. Throughout development xUlk4 expression was detected along the ventricular (vz) and subventricular zones (svz) of all forebrain regions, with the exception of the vz of the striatum. In the adult, xUlk4 was also mainly located in the vz, with some xUlk4 expressing cells reaching the svz/mantle zone (mz). This xUlk4 expression was especially remarkable in forebrain regions involving the homeostatic control of the brain such as the preoptic region, the hypothalamic territory and some neurosecretory circumventricular organs (CVOs). We further combined in situ hybridization for xUlk4 with immunohistochemistry for the neural progenitor cell marker SOX3, the radial glial marker brain lipid-binding protein (BLBP), neuronal markers MAP2 and doublecortin (DCX) and the specific neuronal marker tyrosine hydroxylase (TH). xUlk4 was co-expressed with the neural stem/progenitor cell marker SOX3 in the vz of all the forebrain regions throughout development and in the adult, and this co-expression was also especially evident in the svz of the hypothalamic region. xUlk4 was also expressed in the radial glia along the whole brain. We have also found minor expression of xUlk4 in some DCX- or MAP2-positive cells but not in TH-positive neurons. These findings suggest that Ulk4 may play roles in neural stem/progenitor cells during neurogenesis both in development and in the adulthood, in migrating cells as well as in cells committed to neuronal fate in Xenopus. Moreover, the results obtained in this study argue for an involvement of Ulk4 in the control of the neuroendocrine homeostatic functions in the brain.