Sall3 is required for the terminal maturation of olfactory glomerular interneurons.

The olfactory bulb (OB) is a laminar structure containing two distinct neuronal populations. Excitatory projection neurons populate the mitral cell layer (MCL), and interneuron populations are located in the glomerular layer (GL) and granule cell layer (GCL), superficial and deep to the MCL respectively. OB neuronal populations are sequentially produced and have distinct origins. Mitral cells are the firstborn OB population (Hinds, 1968). These cells arise in the pallium, and the transcription factor Tbr1 is required for their generation (Bulfone et al., 1998; Moreno et al., 2003; Puelles et al., 2000). OB interneurons arise from an ER81-positive population in the subpallial dorsal lateral ganglionic eminence (LGE) and migrate to the OB via the rostral migratory stream (RMS; Doetsch and Alvarez-Buylla, 1996; Lois and Alvarez-Buylla, 1994; Stenman et al., 2003; Wichterle et al., 1999, 2001). Previous studies determined that OB interneurons are continuously generated throughout life (Altman, 1969; Altman and Das, 1966; Doetsch and Alvarez-Buylla, 1996; Hinds, 1968; Lois and Alvarez-Buylla, 1994; Luskin, 1993); however, mechanisms that regulate the generation of this diverse cellular population are not well characterized. OB interneurons begin to express mature interneuron markers as they terminally differentiate and radially migrate in the OB. The OB comprises chemically distinct interneuron populations; at least three subtypes have been identified characterized by γ-aminobutyric acid (GABA), calretinin, and calbindin expression (Kosaka et al., 1995; Toida et al., 2000). Cells in the GCL homogenously express GABA (Parrish-Aungst et al., 2007). Within the GL, GABA, calretinin, and calbindin are expressed by ~20–53%, ~20–27%, and ~10% of neurons, respectively (Kosaka et al., 1995; Parrish-Aungst et al., 2007). The remaining interneuron populations in this layer have not yet been chemically distinguished. The GABAergic population can be further divided into dopaminergic and nondopaminergic populations, characterized by the expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine biosynthesis; ~80% of dopaminergic cells are also GABAergic (Hack et al., 2005; Kosaka et al., 1998; Parrish-Aungst et al., 2007), and 95% of TH-positive cells in the GL are Pax6-positive (Hack et al., 2005). Little overlap is observed between the TH/Pax6 populations and the calretinin (~2% overlap with Pax6)/calbindin (~10% overlap with Pax6)-positive populations (Hack et al., 2005; Kosaka et al., 1995, 1998; Waclaw et al., 2006). In addition, examination of the temporal specification of OB interneurons has identified overlap in the timing of birth of these chemically diverse populations (Tucker et al., 2006). These findings suggest that intrinsic factors play an important role in OB interneuron specification, insofar as diverse interneuron populations born at the same time would be exposed to similar extrinsic cues. Insight into the mechanisms that regulate production of interneuron populations destined for the OB has been obtained from analyses of murine knockout models (Soria et al., 2004; Stenman et al., 2003; Waclaw et al., 2006; Yoshihara et al., 2005; Yun et al., 2003). These studies have demonstrated that interneuron generation can be divided into the following steps; specification of interneuron progenitors within the LGE, entry from the LGE to the RMS, migration in the RMS, exit from the RMS to the OB, radial migration within the OB, and terminal differentiation. The transcription factors Sp8 (Waclaw et al., 2006) and GSH1/2 (Stenman et al., 2003; Yun et al., 2003) are required for the initial specification of OB progenitors in the LGE. Vax1 is necessary for the transition of cells from the LGE to the RMS (Soria et al., 2004). The secreted protein Slit has been implicated in the migration of inter-neurons via the RMS (Chen et al., 2001; Hu, 1999; Wu et al., 1999), and the homeobox protein ARX is required for the entry of interneurons from the RMS to the OB (Yoshihara et al., 2005). Furthermore, Pax6 has been implicated in the generation of dopaminergic OB interneurons (Hack et al., 2005; Kohwi et al., 2005). Thus, we are beginning to understand components of OB cell type generation and migration within the RMS; however, mechanisms regulating terminal differentiation and maturation within the OB are not well characterized. We examined the role of Sall3 in the development of the olfactory system. Sall3 is one of four mammalian members of the Sall gene family, which are widely expressed throughout development, in the peripheral and central nervous system as well as in peripheral organs (Al-Baradie et al., 2002; Buck et al., 2000, 2001; Kohlhase et al., 1996, 1999, 2000, 2002a,b; Ott et al., 1996, 2001). Distinct developmental disorders with some overlapping characteristics are associated with mutation or deletion of members of this gene family in humans (Al-Baradie et al., 2002; Kohlhase et al., 1998, 1999, 2002b). SALL3 is one of several genes deleted in 18q deletion syndrome, characterized by hearing loss, mental retardation, midfacial hypoplasia, delayed growth, and limb abnormalities (Jayarajan et al., 2000; Kohlhase et al., 1999; Mahr et al., 1996; Strathdee et al., 1997; Verhoeven et al., 2006; Wilson et al., 1979). The Sall genes are zinc finger-containing putative transcription factors (Kuhnlein et al., 1994). They have been shown to localize to heterochromatin (Netzer et al., 2001; Sakaki-Yumoto et al., 2006; Sato et al., 2004), and it is hypothesized that they can mediate transcriptional repression via recruitment of histone deacetylase (Kiefer et al., 2002; Lauberth and Rauchman, 2006). Members of the Sall gene family can interact at the protein level (Kiefer et al., 2002; Sakaki-Yumoto et al., 2006; Sweetman et al., 2003) and have been implicated in diverse cellular processes, including cell cycle regulation, cell fate specification, neuronal differentiation, migration, and cell adhesion (Barembaum and Bronner-Fraser, 2004; Bohm et al., 2007; Cantera et al., 2002; de Celis et al., 1999; Franch-Marro and Casanova, 2002; Jurgens, 1988; Kuhnlein and Schuh, 1996; Toker et al., 2003). These data suggest the members of the Sall gene family are important developmental regulators. We have previously shown that Sall3 (previously named msal1) is expressed during olfactory development (Ott et al., 1996, 2001), although a detailed description has not been given. We determined that Sall3 was expressed by cells within the OB and olfactory epithelium. Within the OB, Sall3 was expressed by progenitor cells and subpopulations of differentiated neurons. Sall3-deficient animals die perinatally (Parrish et al., 2004), and we examined development of the olfactory system to P0.5. We identified alterations in interneuron development in Sall3 mutant animals. Our data suggest that Sall3 is required for the terminal maturation of GL interneurons in the developing OB.