Abstract. Nuclear receptors (NRs) are a unique superfamily of transcription factors (TFs) which are involved in and play a crucial role in almost all aspects of mammalian physiology. Small Heterodimer Partner (SHP; NR0B2), an exceptional member of this superfamily of NRs, have been identified as a key regulatory factor of the transcription of a variety of genes involved in diverse metabolic pathways, and are thereby an important factor in a variety of physiological functions. Since its discovery a decade ago, considerable progress has been made in the elucidation of the underlying mechanism by which SHP regulates various metabolic processes, and the results of previous studies support its importance in the maintenance of metabolic homeostasis. In this review, we have evaluated the current state of understanding of the molecular mechanisms and the resultant physiological interpretations governed by SHP. SMALL Heterodimer Partner is a unique member of the mammalian nuclear receptor superfamily, which is clearly distinct from the conventional nuclear receptors, both structurally and functionally. It is classified into the "orphan" subfamily, due to the absence of any known ligand for this receptor, and appears to function as a transcriptional co-regulator of numerous nuclear receptors and transcription factors. The dosage-sensitive sex reversal-adrenal hypoplasia congenital region gene on the X chromosome, gene-1 (DAX-1; NR0B1) is the closest relative to SHP. Both belong to the same subfamily, and both share considerable functional and structural similarities with SHP. SHP owes its place in the nuclear receptor family to the presence of a putative ligand binding domain (LBD), although it lacks the classical DNA binding domain (DBD) which is generally detected among other nuclear receptors. Intensive research conducted following its discovery last decade has resulted in a clearer understanding of the molecular basis for the inhibitory functions of SHP on a variety of metabolic processes, thereby providing us with ample evidence regarding its importance as a key regulator in a number of signaling pathways [1,2]. Hub-based topological analysis of the nuclear receptor network has demonstrated the extensive connections existing among these receptors as the result of their similarity in terms of interaction and tissue expression, as well as a considerable number of feedback loops that work in concert in this network, with SHP playing a principal role as the hub protein within this network [3]. In this review, we have attempted to survey the entirety of the information available thus far concerning the structural and functional aspects of the orphan nuclear receptor SHP, hoping to acquire a detailed understanding of the physiological significance of the activity of SHP.

Feedback regulations are integral components of the cAMP signaling required for most cellular processes, including gene expression and cell differentiation. Here, we provide evidence that one of these feedback regulations involving the cyclic nucleotide phosphodiesterase PDE4D plays a critical role in cAMP signaling during the differentiation of granulosa cells of the ovarian follicle. Gonadotropins induce PDE4D mRNA and increase the cAMP hydrolyzing activity in granulosa cells, demonstrating that a feedback regulation of cAMP is operating in granulosa cells in vivo. Inactivation of the PDE4D by homologous recombination is associated with an altered pattern of cAMP accumulation induced by the gonadotropin LH/human chorionic gonadotropin (hCG), impaired female fertility, and a markedly decreased ovulation rate. In spite of a disruption of the cAMP response, LH/hCG induced P450 side chain cleavage expression and steroidogenesis in a manner similar to wild-type controls. Morphological examination of the ovary of PDE4D-/- mice indicated luteinization of antral follicles with entrapped oocytes. Consistent with the morphological finding of unruptured follicles, LH/hCG induction of genes involved in ovulation, including cyclooxygenase-2, progesterone receptor, and the downstream genes, is markedly decreased in the PDE4D-/- ovaries. These data demonstrate that PDE4D regulation plays a critical role in gonadotropin mechanism of action and suggest that the intensity and duration of the cAMP signal defines the pattern of gene expression during the differentiation of granulosa cells.

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The YrdA protein shows high sequence similarity to γ-class carbonic anhydrase (γ-CA) proteins and is classified as part of the γ-CA protein family. However, its function has not been fully elucidated as it lacks several of the conserved residues that are considered to be necessary for γ-CA catalysis. Interestingly, a homologue of γ-CA from Methanosarcina thermophila and a β-carboxysomal γ-CA from a β-cyanobacterium have shown that these catalytic residues are not always conserved in γ-CAs. The crystal structure of YrdA from Escherichia coli (ecYrdA) is reported here in two crystallographic forms. The overall structure of ecYrdA is also similar to those of the γ-CAs. One loop around the putative catalytic site shows a number of alternative conformations. A His residue (His70) on this loop coordinates with, or is reoriented from, the catalytic Zn(2+) ion; this is similar to the conformations mediated by an Asp residue on the catalytic loops of β-CA proteins. One Trp residue (Trp171) also adopts two alternative conformations that may be related to the spatial positions of the catalytic loop. Even though significant CA activity could not be detected using purified ecYrdA, these structural features have potential functional implications for γ-CA-related proteins.