No abstract

The nuclear vitamin D receptor (VDR) mediates the actions of its 1,25-dihydroxyvitamin D(3) ligand to control gene expression in terrestrial vertebrates. Prominent functions of VDR-regulated genes are to promote intestinal absorption of calcium and phosphate for bone mineralization and to potentiate the hair cycle in mammals. We report the cloning of VDR from Petromyzon marinus, an unexpected finding because lampreys lack mineralized tissues and hair. Lamprey VDR (lampVDR) clones were obtained via RT-PCR from larval protospleen tissue and skin and mouth of juveniles. LampVDR expressed in transfected mammalian COS-7 cells bound 1,25-dihydroxyvitamin D(3) with high affinity, and transactivated a reporter gene linked to a vitamin D-responsive element from the human CYP3A4 gene, which encodes a P450 enzyme involved in xenobiotic detoxification. In tests with other vitamin D responsive elements, such as that from the rat osteocalcin gene, lampVDR showed little or no activity. Phylogenetic comparisons with nuclear receptors from other vertebrates revealed that lampVDR is a basal member of the VDR grouping, also closely related to the pregnane X receptors and constitutive androstane receptors. We propose that, in this evolutionarily ancient vertebrate, VDR may function in part, like pregnane X receptors and constitutive androstane receptors, to induce P450 enzymes for xenobiotic detoxification.

Background: Mucolipidosis Type IV is currently characterized as a lysosomal storage disorder with defects that include corneal clouding, achlorhydria and psychomotor retardation. MCOLN1, the gene responsible for this disease, encodes the protein mucolipin-1 that belongs to the "Transient Receptor Potential" family of proteins and has been shown to function as a non-selective cation channel whose activity is modulated by pH. Two cell biological defects that have been described in MLIV fibroblasts are a hyperacidification of lysosomes and a delay in the exit of lipids from lysosomes.

The degradation of misfolded proteins is essential for cellular and organism viability. Quality control mechanisms of protein folding involve multi-component systems, which include chaperones, ubiquitylation enzymes, and ultimately degradation by the proteasome. So far, quality control mechanisms have been described in the cytoplasm, the nucleus and endoplasmic reticulum (ER) (Bader et al., 2007;Goldberg, 2003;Hampton, 2002;Jarosch et al., 2003;Meusser et al., 2005;von Mikecz, 2006).The recognition and degradation of misfolded proteins in the ER is called ER-associated degradation (ERAD) (Hampton, 2002;Jarosch et al., 2003;McCracken and Brodsky, 2003;Meusser et al., 2005;Richly et al., 2005;Sitia and Braakman, 2003). Membrane-spanning and secretory proteins are first transported into the ER in an unfolded state through the Sec61p complex (Matlack et al., 1998). Folding of these nascent polypeptides is assisted by a number of ER-resident chaperones. Translocated proteins also undergo modifications to support folding; these include N-terminal glycosylation and disulphide bond formation (Meusser et al., 2005;Schroder and Kaufman, 2005;Sitia and Braakman, 2003). In the ER, proteins that do not fold properly are retro-translocated to the cytoplasm. During retro-translocation, these misfolded proteins are ubiquitylated by several ER-specific E3 ubiquitin ligase complexes. A cytoplasmic ubiquitin-binding and multi-ubiquitylation enzyme complex further modifies these proteins and finally transports them to the proteasome for degradation.The accumulation of misfolded proteins in the ER activates the unfolded protein response (UPR), which is required for cells to survive conditions of stress. The UPR is mediated by three ER transmembrane proteins, IRE1, PERK and ATF6, which get activated at least in part because of the dissociation of the ER chaperone BiP, to which they are normally bound and also because of their sequestration by misfolded proteins (Bertolotti et al., 2000;Cox et al., 1993;Harding et al., 1999;Haze et al., 1999;Iwawaki et al., 2001;Kimata et al., 2004;Lee et al., 2002;Mori et al., 1993;Okamura et al., 2000). IRE1, PERK and ATF6 function to decrease the load on the ER by reducing translation rate and activating the transcription of chaperones, ERAD proteins and other enzymes. UPR activation also results in increased biosynthesis of some lipids, the elaboration of the ER and increased secretion (Sato et al., 2002;Shaffer et al., 2004;Sriburi et al., 2004). A major downstream regulator of UPR is XBP1/HAC1. Upon activation of IRE1, the XBP1 mRNA is directly spliced by an endonuclease activity in the C-terminus of IRE-1; this splice variant of XBP1 functions as a potent transcriptional activator of several genes (Calfon et al., 2002;Cox and Walter, 1996;Sidrauski and Walter, 1997;Yoshida et al., 2001).Derlin proteins are a conserved family that function in ERAD (Schekman, 2004). They have four transmembrane domains and are conserved in all eukaryotes. There are two members in Saccharomyces cerevisiae, Der1p a...

MTM1, MTMR2, and SBF2 belong to a family of proteins called the myotubularins. X-linked myotubular myopathy, a severe congenital disorder characterized by hypotonia and generalized muscle weakness in newborn males, is caused by mutations in MTM1 (Laporte et al., 1996). Charcot-Marie-Tooth types 4B1 and 4B2 are severe demyelinating neuropathies caused by mutations in MTMR2 (Bolino et al., 2000) and SBF2/MTMR13 (Senderek et al., 2003), respectively. Although several myotubularins are known to regulate phosphoinositide-phosphate levels in cells, little is known about the actual cellular process that is defective in patients with these diseases. Mutations in worm MTM-6 and MTM-9, myotubularins belonging to two subgroups, disorganize phosphoinositide 3-phosphate localization and block endocytosis in the coelomocytes of Caenorhabditis elegans. We demonstrate that MTM-6 and MTM-9 function as part of a complex to regulate an endocytic pathway that involves the Arf6 GTPase, and we define protein domains required for MTM-6 activity.