Huntington’s disease is an autosomal dominant disease associated with a mutation in the gene encoding huntingtin (Htt) leading to expanded polyglutamine repeats of mutant Htt (mHtt) that elicit oxidative stress, neurotoxicity, and motor and behavioural changes1. Huntington’s disease is characterized by highly selective and profound damage to the corpus striatum, which regulates motor function. Striatal selectivity of Huntington’s disease may reflect the striatally selective small G protein Rhes binding to mHtt and enhancing its neurotoxicity2. Specific molecular mechanisms by which mHtt elicits neurodegeneration have been hard to determine. Here we show a major depletion of cystathionine γ-lyase (CSE), the biosynthetic enzyme for cysteine, in Huntington’s disease tissues, which may mediate Huntington’s disease pathophysiology. The defect occurs at the transcriptional level and seems to reflect influences of mHtt on specificity protein 1, a transcriptional activator for CSE. Consistent with the notion of loss of CSE as a pathogenic mechanism, supplementation with cysteine reverses abnormalities in cultures of Huntington’s disease tissues and in intact mouse models of Huntington’s disease, suggesting therapeutic potential.

The inositol pyrophosphates, molecular messengers containing an energetic pyrophosphate bond, impact a wide range of biologic processes. They are generated primarily by a family of three inositol hexakisphosphate kinases (IP6Ks), the principal product of which is diphosphoinositol pentakisphosphate (IP7). We report that IP6K2, via IP7 synthesis, is a major mediator of cancer cell migration and tumor metastasis in cell culture and in intact mice. IP6K2 acts by enhancing cell-matrix adhesion and decreasing cell-cell adhesion. This action is mediated by IP7-elicited nuclear sequestration and inactivation of the tumor suppressor liver kinase B1 (LKB1). Accordingly, inhibitors of IP6K2 offer promise in cancer therapy.I nositol pyrophosphates, conserved eukaryotic messenger molecules with a pyrophosphate bond, mediate numerous physiologic processes including regulation of Akt (1), insulin secretion (2), ATP production (3), DNA repair (4), and damage response (5). Exemplified by diphosphoinositol pentakisphosphate (PPIP5, IP7), inositol pyrophosphates are primarily generated by a family of three inositol hexakisphosphate (IP6) kinases (IP6K) (6, 7) and also may be formed by a more recently described group of IP6/7 kinases (8). IP6K1 and IP6K2 are widely distributed, whereas IP6K3 is expressed primarily in the brain (9). A related enzyme, inositol polyphosphate multikinase (IPMK), converts IP3 to IP4 and IP5, displays physiologic PI3 kinase activity (10), and noncatalytically acts as a transcriptional coactivator (11) and stabilizer of the mTOR complex-1 (12). Whether IP6Ks and IPMK play direct roles in tumor progression remains unexplored.Tumor cell metastasis requires loss of cell-cell adhesion and gain of migratory and invasive properties, a collective program known as epithelial-mesenchymal transition (EMT), which is also critical for embryonic development (13). Cell-cell adhesion and cell migration/invasion are primarily driven by E-cadherinmediated cell clustering (14) and focal-adhesion-based cellmatrix interactions (15), respectively. Whether these two distinct adhesion processes are concurrently or independently regulated remains poorly understood.The tumor suppressor LKB1 has been separately reported to inhibit FAK activation (16) and to enhance E-cadherin expression (17)(18)(19). LKB1 is mutated in Peutz-Jeghers syndrome patients who tend to develop cancer at multiple sites (20). As a master kinase controlling the activity of AMPK and 13 other AMPK-like kinases (21), LKB1 mediates diverse cellular processes such as polarity, adhesion, metabolism, tumor growth/metastasis, and neuronal axon initiation/branching (22). Unlike conventional protein kinases, LKB1 is not activated by activation loop phosphorylation but by protein-protein interaction with two other subunits of the heterotrimeric holoenzyme: STRAD and Mo25. LKB1 can also exist as an inactive nuclear monomer. STRAD binds and stabilizes LKB1 (23) in the cytosol, where it is phosphorylated at serine 428 by PKCζ (24). Whether cytosolic localization enhance...

A common property of G protein-coupled receptors is that they become less responsive with prolonged stimulation. Regulators of G protein signaling (RGS proteins) are well known to accelerate G protein GTPase activity and do so by stabilizing the transition state conformation of the G protein ␣ subunit. In the yeast Saccharomyces cerevisiae there are four RGS-homologous proteins (Sst2, Rgs2, Rax1, and Mdm1) and two G␣ proteins (Gpa1 and Gpa2). We show that Sst2 is the only RGS protein that binds selectively to the transition state conformation of Gpa1. The other RGS proteins also bind Gpa1 and modulate pheromone signaling, but to a lesser extent and in a manner clearly distinct from Sst2. To identify other candidate pathway regulators, we compared pheromone responses in 4,349 gene deletion mutants representing nearly all nonessential genes in yeast. A number of mutants produced an increase (sst2, bar1, asc1, and ygl024w) or decrease (cla4) in pheromone sensitivity or resulted in pheromone-independent signaling (sst2, pbs2, gas1, and ygl024w). These findings suggest that Sst2 is the principal regulator of Gpa1-mediated signaling in vivo but that other proteins also contribute in distinct ways to pathway regulation.G protein-coupled receptors respond to a vast array of chemical and sensory signals, including hormones, neurotransmitters, odors, and light. Approximately one-third of all drugs act by binding directly to receptors of this class (64). Upon agonist stimulation of the receptor, a cognate G protein ␣ subunit will exchange GDP for GTP and undergo dissociation from the G protein ␤␥ subunit dimer. The dissociated subunits bind to effector enzymes, which in turn activate protein kinases, trigger new gene transcription, and ultimately produce programmed changes in cell homeostasis or differentiation (90). Regulators of G protein signaling (RGS proteins) function as GTPase-accelerating proteins (GAPs) and, in this manner, promote rapid inactivation or desensitization of the signal (89).Whereas mammalian genome analysis has revealed at least 16 G␣-and ϳ40 RGS-encoding genes (89, 106), a similar analysis in the yeast Saccharomyces cerevisiae reveals only two G␣ subunits but at least four RGS protein homologues. Gpa1 mediates cellular responses to mating pheromones. These pheromones, called a-factor and ␣-factor, are produced by haploid a and ␣ cells and bind to G protein-coupled receptors on cells of the opposite mating type. Upon activation of pheromone receptors, Gpa1 binds to GTP and dissociates from the G␤␥ dimer Ste4/Ste18, and the dissociated subunits activate a multitude of downstream effectors leading to cell fusion (mating) to form an a/␣ diploid (36, 50). Prominent among the known effectors are components of a MAP (mitogen-activated protein) kinase cascade comprised of Ste20, Ste11, Ste7, and Fus3. A parallel signaling pathway responds to glucose stimulation, leading to activation of a distinct receptor (Gpr1) (66,73,76,99,124), a distinct G protein ␣ subunit (Gpa2), and an atypical G protein ␤␥ complex ...

The inositol hexakisphosphate kinases (IP6Ks) are the principal enzymes that generate inositol pyrophosphates. There are three IP6Ks (IP6K1, 2, and 3). Functions of IP6K1 and IP6K2 have been substantially delineated, but little is known of IP6K3's role in normal physiology, especially in the brain. To elucidate functions of IP6K3, we generated mice with targeted deletion of IP6K3. We demonstrate that IP6K3 is highly concentrated in the brain in cerebellar Purkinje cells. IP6K3 physiologically binds to the cytoskeletal proteins adducin and spectrin, whose mutual interactions are perturbed in IP6K3-null mutants. Consequently, IP6K3 knock-out cerebella manifest abnormalities in Purkinje cell structure and synapse number, and the mutant mice display deficits in motor learning and coordination. Thus, IP6K3 is a major determinant of cytoskeletal disposition and function of cerebellar Purkinje cells.