From this study, the examined NRs can be divided into two functional categories. The first category includes AR, ERα, ERβ, and PR, which are known to be the classical endocrine receptors that mediate the actions of steroid hormones (Mangelsdorf et al.,1995). Except for ERβ, they are expressed only at P21, implying that this subset of nuclear receptors may play roles in the maintenance of cerebellar function in adulthood after the cerebellum is fully differentiated. Consistent with this hypothesis, it has been reported that estrogen treatment decreases the levels of γ-amino butyric acid α (GABAα) receptors in the cerebellum (Bar-Ami et al.,1993), and that estrogens and anti-estrogens alter the response of Purkinje cells to glutamate (Smith et al.,1988; Smith,1989). Together with our expression data, these studies suggest that estrogen receptors may play a role in the maintenance of cerebellar function.
The second category of nuclear receptors includes COUP-TFI, COUP-TFII, GCNF, ERRγ, LRH-1, and RORα, which are all classified as orphan receptors (Mangelsdorf et al.,1995). Their temporal expression patterns suggest they are more likely to be important in cerebellar development. The observed RORα expression pattern at the different stages we examined was not surprising, since it was previously reported that this receptor plays a crucial role in the cerebellum development such as Purkinje cell differentiation and maturation (Dussault et al.,1998; Steinmayr et al.,1998; Gold et al.,2003). Except for RORα, the detailed expression pattern of COUP-TFI, COUP-TFII, GCNF, ERRγ, and LRH-1 in the cerebellum, however, was not known before this study. Among these five receptors, the COUP-TFs are well-characterized orphan nuclear receptors. COUP-TFI and COUP-TFII show more than 97% amino acid identity in their DNA binding and putative ligand-binding domains (Qiu et al.,1994). However, COUP-TFI and COUP-TFII expression patterns are quite distinct from each other. In general, COUP-TFI is more highly expressed in the neuronal tissue of the central and peripheral nervous systems, whereas COUP-TFII is expressed in the mesenchyme of developing organs (Pereira et al.,1995). Loss of function assays indicate that COUP-TFI is important for neurogenesis and neural crest cell differentiation during embryonic development (Qiu et al.,1997; Zhou et al.,2001; Yamaguchi et al.,2004), while COUP-TFII acts as a major regulator of angiogenesis, vein identity, and organ development (Pereira et al.,1999; Takamoto et al.,2005; You et al.,2005). In the cerebellum, the expressions patterns of these two receptors are not co-localized. COUP-TFI is highly expressed in Bergmann glia cells with lower expression detected in granule and Basket/Stellate cells, while COUP-TFII is only expressed in Purkinje cells. Interestingly, they both show different expression patterns along the anterior-posterior axis in the EGL or in Purkinje cells. Unlike COUP-TFI, COUP-TFII is highly expressed only in the anterior half of the cerebellum in a manner similar to that of Sonic hedgehog (Shh), which is important for cerebellar foliation (Corrales et al.,2004,2006). Since COUP-TFII is more likely to be a downstream target of Shh signaling (Krishnan et al.,1997; Takamoto et al.,2005), it is reasonable to expect that COUP-TFII may play an important role in cerebellar development. Additionally, when one considers the temporal expression pattern of COUP-TFI and ERRγ, it is probable that they may also function during cerebellar development.