The expression patterns of Cdc25A, Cdc25B, Sox2 and Mnb in central nervous system in early chicken embryos

Asian - Australasian Journal of Animal Sciences, June, 2009 by Hui Zhang, Junhui Qin, Jingjing Cao, Nainan Hei, Chunsheng Xu, Ping Yang, Haili Liu, Xiaohong Chu, Huijun Bao, Qiusheng Chen

INTRODUCTION

Cell division cycle (Cdc) genes are required for regulation of cell proliferation. However, others factors are necessary for cell differentiation events. To understand the mechanisms that underlie this regulated process, greater knowledge of the molecular control of the acquisition of cell proliferation and differentiation is required. At present, considerable progress has been made in identifying the signals and elucidating the molecular mechanisms that regulate cell proliferation and/or differentiation. However, it is important to investigate these genes at the transcription level and elucidate regulated factors in relation to each other and their roles in embryo development. Cell cycle progression is regulated by the cyclin-dependent kinase (CDK) family. CDK activity throughout the cell cycle is highly regulated by association with cyclins and with inhibitory proteins (Oogood, 2002). CDK activity is also regulated by phosphorylation. The Cdc25 gene was first identified in the fission yeast Schizosaccharomyces pombe as a positive regulator of the G2/M transition in the cell cycle (Russell and Nurse, 1986). Three members of the Cdc25 family have been identified in mammalian cells while only two isoforms (A and B) have been characterized in the chick. The Cdc25 proteins are 300-600 residues in length and can be divided into two regions. The N-terminal regions are highly divergent in sequence. The more highly homologous C-terminal regions (~60% pair-wise identity over ~200 amino acids) contain the catalytic functionality of the Cdc25s (Rudolph, 2007). Cdc25 phosphatases serve as key activators of the CDK/ cyclins. The Cdc25 phosphatase family activates CDKs and stimulates cell cycle progression by catalysing removal of CDK inhibitory phosphates (Kumagai and Kornbluth, 1991). The Cdc25 phosphatases can dephosphorylate both phospho-tyrosine and phospho-serine/threonine residues, a property shared with several other dual-specificity phosphatases. The dualspecificity phosphatases are related in reaction mechanism to the tyrosine-specific phosphatases (Perry and Kornbluth, 2007).

Cdc25 is itself regulated by phosphorylation. CDKs and polo-like kinases increase Cdc25 phosphatase activity thus contributing to an amplification loop that ensures the faithful activation of CDKs during cell cycle transitions (Powers et al., 2000). Other Cdc25 kinases, including Chk1 (checkpoint kinase 1) and Cds1 (checking DNA synthesis 1) (Blasina et al., 1999), inhibit Cdc25 function by inhibiting phosphatase activity or through generation of a 14-3-3 binding site (Wilker and Yaffe, 2004). Other proteins are also reported to associate or co-distribute with Cdc25, including Ras, p13, Raf-1 and cyclin B (Powers et al., 2000).

Sox2 is a member of the Sox (SRY-related HMG box) gene family that encode transcription factors with a single HMG DNA-binding domain. Sox2 belongs to the Sox B1 subgroup based on homology within and outside the HMG box (Kamachi et al., 2000). Several lines of evidence indicate that Sox2 may act to maintain or preserve developmental potential. Sox2 is a transcription factor that plays multiple critical roles during embryonic development in vertebrates. Sox2 is expressed in cells that retain their ability to proliferate and/or acquire glial fates, whereas it is down-regulated in cells that become postmitotic and differentiate into neurons (Bylund et al., 2003). In all vertebrates studied to date, Sox2 is also a general marker for the very early developing neural plate. The complex expression profile of Sox2 is controlled by multiple regulatory elements, each responsible for directing expression to a specific subset of expression sites (Papanayotoul et al., 2008). To date, no single secreted factor or any combination thereof has been found to induce either Sox2 expression or a neural plate in competent cells not normally fated to form part of the neural plate (Papanayotoul et al., 2008).

"minibrain" (Mnb) kinase is a mutant of Drosophila that exhibits a specific and marked size reduction of the optic lobes and central hemispheres of the adult brain (Fischbach and Technau, 1984). The Mnb gene encodes a Ser/Thr protein kinase that possesses a YXY sequence in the activation loop (Tejedor et al., 1995). Mnb was originally identified as a gene essential to the neuronal proliferation of Drosophila (Miyata and Nishida, 1999). The ortholog of Drosophila Mnb, termed dual specificity tyrosinephosphorylation regulated kinase 1A (Mnb/DYRK1A), was subsequently characterized in many organisms. Similar to Drosophila, Mnb/DYRK1A is involved in the early development of the central nervous system (CNS) of vertebrates (Adayev et al., 2006). The Mnb gene is essential in cell proliferation and neuronal differentiation during postembryonic neurogenesis (Kentrup et al., 1996). DYRKs possess Ser/Thr phosphorylation activity as well as autophosphorylation activity on Tyr residues, suggesting that DYRKs are dual specificity kinases (Kentrup et al., 1996). The kinase activity of DYRK is dependent on the YXY motif in the activation loop (Kentrup et al., 1996), suggesting the existence of a phosphorylation-dependent activation mechanism of DYRK by certain upstream kinases. Thus, from Drosophila to humans, it is suggested that DYRK/Mnb is a key regulator of growth of neuronal cells as is the case for conventional MAP kinases regulating growth in certain cell types. Although the exact cellular function of the DYRK kinases are yet unknown, it may be very interesting to know the physiological role of this family of protein kinases (Miyata et al., 1999; Yang et al., 2001).