Jibrin A Kama

Institute for Neurodegenerative Disorders, New Haven, Connecticut, United States

Are you Jibrin A Kama?

Claim your profile

Publications (8)36.26 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Huntington disease (HD) is a progressive neurodegenerative disorder caused by expression of polyglutamine-expanded mutant huntingtin protein (mhtt). Most evidence indicates that soluble mhtt species, rather than insoluble aggregates, are the important mediators of HD pathogenesis. However, the differential roles of soluble monomeric and oligomeric mhtt species in HD and the mechanisms of oligomer formation are not yet understood. We have shown previously that copper interacts with and oxidizes the polyglutamine-containing N171 fragment of huntingtin. In this study we report that oxidation-dependent oligomers of huntingtin form spontaneously in cell and mouse HD models. Levels of these species are modulated by copper, hydrogen peroxide, and glutathione. Mutagenesis of all cysteine residues within N171 blocks the formation of these oligomers. In cells, levels of oligomerization-blocked mutant N171 were decreased compared with native N171. We further show that a subset of the oligomerization-blocked form of glutamine-expanded N171 huntingtin is rapidly depleted from the soluble pool compared with “native ” mutant N171. Taken together, our data indicate that huntingtin is subject to specific oxidations that are involved in the formation of stable oligomers and that also delay removal from the soluble pool. These findings show that inhibiting formation of oxidation-dependent huntingtin oligomers, or promoting their dissolution, may have protective effects in HD by decreasing the burden of soluble mutant huntingtin.
    Preview · Article · May 2011 · Journal of Biological Chemistry
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Huntington disease (HD) is a progressive neurodegenerative disorder caused by expression of polyglutamine-expanded mutant huntingtin protein (mhtt). Most evidence indicates that soluble mhtt species, rather than insoluble aggregates, are the important mediators of HD pathogenesis. However, the differential roles of soluble monomeric and oligomeric mhtt species in HD and the mechanisms of oligomer formation are not yet understood. We have shown previously that copper interacts with and oxidizes the polyglutamine-containing N171 fragment of huntingtin. In this study we report that oxidation-dependent oligomers of huntingtin form spontaneously in cell and mouse HD models. Levels of these species are modulated by copper, hydrogen peroxide, and glutathione. Mutagenesis of all cysteine residues within N171 blocks the formation of these oligomers. In cells, levels of oligomerization-blocked mutant N171 were decreased compared with native N171. We further show that a subset of the oligomerization-blocked form of glutamine-expanded N171 huntingtin is rapidly depleted from the soluble pool compared with "native " mutant N171. Taken together, our data indicate that huntingtin is subject to specific oxidations that are involved in the formation of stable oligomers and that also delay removal from the soluble pool. These findings show that inhibiting formation of oxidation-dependent huntingtin oligomers, or promoting their dissolution, may have protective effects in HD by decreasing the burden of soluble mutant huntingtin.
    Preview · Article · Mar 2011 · Journal of Biological Chemistry
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Motor dysfunction, cognitive impairment, and regional cortical atrophy indicate cerebral cortical involvement in Huntington disease (HD). To address the hypothesis that abnormal corticostriatal connectivity arises from polyglutamine-related alterations in cortical gene expression, we isolated layer 5 cortical neurons by laser-capture microdissection and analyzed transcriptome-wide mRNA changes in them. Enrichment of transcription factor mRNAs including foxp2, tbr1, and neuroD6, and neurotransmission- and plasticity-related RNAs including sema5A, pclo, ntrk2, cntn1, and Lin7b were observed. Layer 5 motor cortex neurons of transgenic R6/2 HD mice also demonstrated numerous transcriptomic changes, including decreased expression of mRNAs encoding the Lin7 homolog b ([Lin7b] also known as veli-2 and mals2). Decreases in LIN7B and CNTN1 RNAs were also detected in human HD layer 5 motor cortex neurons. Lin7 homolog b, a scaffold protein implicated in synaptic plasticity, neurite outgrowth, and cellular polarity, was decreased at the protein level in layer 5 cortical neurons in R6/2 mice and human HD brains. Decreases in Lin7b and Lin7a mRNAs were detected in R6/2 cortex as early as 6 weeks of age, suggesting that this is an early pathogenetic event. Thus, decreased cortical LIN7 expression may contribute to abnormal corticostriatal connectivity in HD.
    Full-text · Article · Sep 2010 · Journal of Neuropathology and Experimental Neurology
  • [Show abstract] [Hide abstract]
    ABSTRACT: Background Alterations in cellular calcium homeostasis have been recognised as one of several pathogenic mechanisms in Huntington's disease (HD). The discovery of a direct interaction between the C terminal extremity (C-ter) of inositol-3-phosphate receptor (IP3R) and huntingtin (htt) associated protein 1 (HAP1) has shed light on the function of HAP1 in calcium homeostasis regulation. The C-ter of IP3R consists of a small domain in continuation of the transmembrane region; it has important regulatory functions on the IP3R. Aim We wondered whether this regulatory domain is present in other membrane proteins, which might therefore interact with HAP1 and Htt, and could display altered functions in the presence of mutant htt. We searched for Cter-IP3R homologous sequences and discovered that the C-ter domain of another transmembrane channel—the Ryanodine Receptor (RYR), which is also a key regulator of calcium homeostasis—displays a striking sequence homology with C-ter-IP3R. Methods/techniques GST-pull down, Co-IP, immunocytochemistry, FLIM-FRET. Results In GST-pull down experiments, a specific interaction of HAP1 with C-ter-RYR and C-ter-IP3R, respectively, was observed. Mutant and wt Htt was pulled down in co-immunoprecipitation experiments using four different antibodies against RYR. Transiently expressed GFP tagged C-ter-RYR together with mRFP labelled N-ter-htt colocalised in N2a cells whereas expression of C-ter-RYR with mRFP alone did not. Immunocytochemistry of primary neurons showed a colocalisation of endogenous htt and RYR. To further assess the interaction of htt and RYR in primary neurons we performed FLIM-FRET based assays in a 96 well format using a high resolution fluorescent plate reader. Close proximity of RYR and htt was detected between the two proteins, as indicated by a significant shortening in lifetime of the donor fluorochrome compared with the negative control in which the acceptor fluorophore was not present. Significant lifetime shortening of the donor fluorochrome was observed independently of RYR or htt used as donor molecule. In summary, these results clearly suggest an interaction of RYR with htt and HAP1. Further work is in progress to clarify the effect of mutant htt on the channel activity of the ryanodine receptor.
    No preview · Article · Sep 2010 · Journal of Neurology Neurosurgery & Psychiatry
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion within the huntingtin gene. Mutant huntingtin protein misfolds and accumulates within neurons where it mediates its toxic effects. Promoting mutant huntingtin clearance by activating macroautophagy is one approach for treating Huntington's disease (HD). In this study, we evaluated the mTOR kinase inhibitor and macroautophagy promoting drug everolimus in the R6/2 mouse model of HD. Everolimus decreased phosphorylation of the mTOR target protein S6 kinase indicating brain penetration. However, everolimus did not activate brain macroautophagy as measured by LC3B Western blot analysis. Everolimus protected against early declines in motor performance; however, we found no evidence for neuroprotection as determined by brain pathology. In muscle but not brain, everolimus significantly decreased soluble mutant huntingtin levels. Our data suggests that beneficial behavioral effects of everolimus in R6/2 mice result primarily from effects on muscle. Even though everolimus significantly modulated its target brain S6 kinase, this did not decrease mutant huntingtin levels or provide neuroprotection.
    Full-text · Article · Jun 2010 · Molecular Neurodegeneration
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Huntington's disease (HD) is caused by a dominant polyglutamine expansion within the N-terminus of huntingtin protein and results in oxidative stress, energetic insufficiency and striatal degeneration. Copper and iron are increased in the striata of HD patients, but the role of these metals in HD pathogenesis is unknown. We found, using inductively-coupled-plasma mass spectroscopy, that elevations of copper and iron found in human HD brain are reiterated in the brains of affected HD transgenic mice. Increased brain copper correlated with decreased levels of the copper export protein, amyloid precursor protein. We hypothesized that increased amounts of copper bound to low affinity sites could contribute to pro-oxidant activities and neurodegeneration. We focused on two proteins: huntingtin, because of its centrality to HD, and lactate dehydrogenase (LDH), because of its documented sensitivity to copper, necessity for normoxic brain energy metabolism and evidence for altered lactate metabolism in HD brain. The first 171 amino acids of wild-type huntingtin, and its glutamine expanded mutant form, interacted with copper, but not iron. N171 reduced Cu(2+)in vitro in a 1:1 copper:protein stoichiometry indicating that this fragment is very redox active. Further, copper promoted and metal chelation inhibited aggregation of cell-free huntingtin. We found decreased LDH activity, but not protein, and increased lactate levels in HD transgenic mouse brain. The LDH inhibitor oxamate resulted in neurodegeneration when delivered intra-striatially to healthy mice, indicating that LDH inhibition is relevant to neurodegeneration in HD. Our findings support a role of pro-oxidant copper-protein interactions in HD progression and offer a novel target for pharmacotherapeutics.
    Full-text · Article · Mar 2007 · PLoS ONE

  • No preview · Article · Jan 2006 · Aktuelle Neurologie
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Transcriptional dysregulation has been described as a central mechanism in the pathogenesis of Huntington's disease (HD), in which medium spiny projection neurons (MSN) selectively degenerate whereas neuronal nitric-oxide-synthase-positive interneurons (nNOS-IN) survive. In order to begin to understand this differential vulnerability we compared mRNA levels of selected genes involved in N-methyl-D-aspartate (NMDA) glutamate receptor and calcium (Ca2+) signaling pathways in MSN and nNOS-IN from 12-week-old R6/2 mice, a transgenic mouse model of HD and wild-type littermates. We undertook a laser capture microdissection (LCM) study to examine the contribution of transcriptional dysregulation in candidate genes involved in these two signaling pathways in discrete populations of striatal neurons. The use of LCM in combination with quantitative real-time polymerase chain reaction (Q-PCR) allowed us to quantify the neuronal abundance of candidate mRNAs. We found different transcriptional alterations in R6/2 neurons for both MSN and nNOS-IN, indicating that global transcriptional dysregulation alone does not account for selective vulnerability. Further, we observed a striking enrichment of several mRNAs in the nNOS-IN population, including that for the NMDA receptor subunit NR2D, the postsynaptic density protein 95 (PSD-95) and the huntingtin-associated protein 1 (HAP1) as well as nitric-oxide-synthase (nNOS) mRNA itself. The higher expression levels of these molecules in nNOS-IN when compared with MSN together with an association of nNOS, NR2D and HAP1 in a protein complex with PSD-95 suggest that these proteins may be involved in protective pathways that contribute to the resistance of this interneuron population to neurodegeneration in HD.
    Full-text · Article · Feb 2005 · Human Molecular Genetics

Publication Stats

230 Citations
36.26 Total Impact Points

Institutions

  • 2011
    • Institute for Neurodegenerative Disorders
      New Haven, Connecticut, United States
  • 2010
    • Massachusetts General Hospital
      Boston, Massachusetts, United States
  • 2005-2010
    • Harvard Medical School
      • Department of Neurology
      Boston, Massachusetts, United States
  • 2007
    • Harvard University
      Cambridge, Massachusetts, United States