Heparan sulfate proteoglycans (HSPGs) are major components of the extracellular matrix that play a central role in controlling cell proliferation, differentiation, and morphological development through their interactions with signaling molecules and other extracellular matrix components (Perrimon and Bernfield,2000; Selleck,2000). HSPGs are composed of a protein core surrounded by covalently linked heparan sulfate (HS) chains composed of disaccharide repeats (Bernfield et al.,1999; Prydz and Dalen,2000). Based on the nature of their core protein, they can be classified into three functionally distinct families: the transmembrane syndecans, the glycosylphosphatidylinositol (GPI) -anchored glypicans and the soluble perlecans. During synthesis of HS chains, a specific sulfation pattern of highly (S-domains), partially (transition zones), and nonsulfated regions is generated (Maccarana et al.,1996). This sulfation pattern is established in the Golgi apparatus by specific sulfotransferases at the 2-O position of uronic acid and 6-O, 3-O, and N positions of glucosamine (Ori et al.,2008). It has been proposed that this structural heterogeneity is specific of certain cell types or stages of development and plays an important role in regulating signaling pathways (Gallagher,2006; Kreuger et al.,2006).
Recently, two additional HS-modifying enzymes that generate the sulfation HS pattern have been discovered. These enzymes, called Sulf1 and Sulf2, are extracellular endosulfatases that have the unique ability to eliminate the sulfate group in position 6-O of glucosamine in highly sulfated regions of HS (Morimoto-Tomita et al.,2002; Ai et al.,2003,2007). Genes encoding for Sulf enzymes have been identified in birds, mouse, rat, and human, and more recently in amphibian (Dhoot et al.,2001; Morimoto-Tomita et al.,2002; Ohto et al.,2002; Braquart-Varnier et al.,2004; Nagamine et al.,2005; Ai et al.2007; Freeman et al.,2008). During development, these enzymes are involved in regulating major signaling pathways, including Wnt, FGF, HGF, GDNF, BMP, and Shh (Dhoot et al.,2001; Ai et al.,2003; Wang et al.,2004; Viviano et al.,2004; Danesin et al.,2006, Ai et al.,2007, Freeman et al.,2008).
During development, Sulf proteins are expressed in neural and mesodermal tissues (Dhoot et al.,2001, Wang et al.,2004; Braquart-Varnier et al.,2004; Danesin et al.,2006; Ai et al.,2007). In the embryonic central nervous system (CNS), Sulf1 has been shown to be expressed in floor plate cells and in a subset of neural progenitors of the spinal cord, as well as in the choroid plexus (Dhoot et al.,2001; Ohto et al.,2002; Braquart-Varnier et al.,2004). In the ventral spinal cord, Sulf1 has been detected in oligodendrocyte progenitors, where it functions as a positive regulator of Shh signaling and contributes to trigger neural progenitors from a neuronal to glial fate (Danesin et al.,2006).
In this study, we have performed a detailed analysis of Sulf1 expression in the forebrain of chicken embryo. We have further compared Sulf1 expression with previously described genes such as Nkx2.2, plp/dm20, and Pax6 to precisely position domains of Sulf1 expression within the longitudinal as well as transversal prosomeric limits. Our analysis is based on the prosomeric model developed by L. Puelles, J.R.L. Rubenstein, and coworkers (Bulfone et al.,1993; Puelles and Rubentein,1993; Puelles,1995; Rubenstein et al.,1998; Puelles and Rubenstein,2003). In this model, the anteroposterior (AP) regionalization first causes the forebrain to become subdivided into rostral secondary prosencephalon (hypothalamo-telencephalic complex) and caudal diencephalon (Puelles et al.,1987,2004; Puelles and Rubenstein,1993,2003; Puelles,1995). The diencephalic region next develops three prosomeric transverse units, known as prosomeres 1–3 (p1–p3; Puelles and Rubenstein,2003). Our results show a markedly regional pattern of Sulf1 expression throughout forebrain development.