Biomedical subjects

Randell T Libby

Publications and source records attributed to Randell T Libby.

6 recordsLinked to original sources

PubMed2006

Thermoregulatory and metabolic defects in Huntington's disease transgenic mice implicate PGC-1alpha in Huntington's disease neurodegeneration.

Huntington's disease (HD) is a fatal, dominantly inherited disorder caused by polyglutamine repeat expansion in the huntingtin (htt) gene. Here, we observe that HD mice develop hypothermia associated with impaired activation of brown adipose tissue (BAT). Although sympathetic stimulation of PPARgamma coactivator 1alpha (PGC-1alpha) was intact in BAT of HD mice, uncoupling protein 1 (UCP-1) induction was blunted. In cultured cells, expression of mutant htt suppressed UCP-1 promoter activity; this was reversed by PGC-1alpha expression. HD mice showed reduced food intake and increased energy expenditure, with dysfunctional BAT mitochondria. PGC-1alpha is a known regulator of mitochondrial function; here, we document reduced expression of PGC-1alpha target genes in HD patient and mouse striatum. Mitochondria of HD mouse brain show reduced oxygen consumption rates. Finally, HD striatal neurons expressing exogenous PGC-1alpha were resistant to 3-nitropropionic acid treatment. Altered PGC-1alpha function may thus link transcription dysregulation and mitochondrial dysfunction in HD.

Adipose Tissue, Brown↗

PubMed2006

Bergmann glia expression of polyglutamine-expanded ataxin-7 produces neurodegeneration by impairing glutamate transport.

Non-neuronal cells may be pivotal in neurodegenerative disease, but the mechanistic basis of this effect remains ill-defined. In the polyglutamine disease spinocerebellar ataxia type 7 (SCA7), Purkinje cells undergo non-cell-autonomous degeneration in transgenic mice. We considered the possibility that glial dysfunction leads to Purkinje cell degeneration, and generated mice that express ataxin-7 in Bergmann glia of the cerebellum with the Gfa2 promoter. Bergmann glia-specific expression of mutant ataxin-7 was sufficient to produce ataxia and neurodegeneration. Expression of the Bergmann glia-specific glutamate transporter GLAST was reduced in Gfa2-SCA7 mice and was associated with impaired glutamate transport in cultured Bergmann glia, cerebellar slices and cerebellar synaptosomes. Ultrastructural analysis of Purkinje cells revealed findings of dark cell degeneration consistent with excitotoxic injury. Our studies indicate that impairment of glutamate transport secondary to glial dysfunction contributes to SCA7 neurodegeneration, and suggest a similar role for glial dysfunction in other polyglutamine diseases and SCAs.

Age Factors↗

PubMed2005

A SCA7 CAG/CTG repeat expansion is stable in Drosophila melanogaster despite modulation of genomic context and gene dosage.

CAG and CTG repeat expansions are the cause of at least a dozen inherited neurological disorders. In these so-called "dynamic mutation" diseases, the expanded repeats display dramatic genetic instability, changing in size when transmitted through the germline and within somatic tissues. As the molecular basis of the repeat instability process remains poorly understood, modeling of repeat instability in model organisms has provided some insights into potentially involved factors, implicating especially replication and repair pathways. Studies in mice have also shown that the genomic context of the repeat sequence is required for CAG/CTG repeat instability in the case of spinocerebellar ataxia type 7 (SCA7), one of the most unstable of all CAG/CTG repeat disease loci. While most studies of repeat instability have taken a candidate gene approach, unbiased screens for factors involved in trinucleotide repeat instability have been lacking. We therefore attempted to use Drosophila melanogaster to model expanded CAG repeat instability by creating transgenic flies carrying trinucleotide repeat expansions, deriving flies with SCA7 CAG90 repeats in cDNA and genomic context. We found that SCA7 CAG90 repeats are stable in Drosophila, regardless of context. To screen for genes whose reduced function might destabilize expanded CAG repeat tracts in Drosophila, we crossed the SCA7 CAG90 repeat flies with various deficiency stocks, including lines lacking genes encoding the orthologues of flap endonuclease-1, PCNA, and MutS. In all cases, perfect repeat stability was preserved, suggesting that Drosophila may not be a suitable system for determining the molecular basis of SCA7 CAG repeat instability.

Animals↗

PubMed2004

Androgen receptor YAC transgenic mice recapitulate SBMA motor neuronopathy and implicate VEGF164 in the motor neuron degeneration.

X-linked spinal and bulbar muscular atrophy (SBMA) is an inherited neuromuscular disorder characterized by lower motor neuron degeneration. SBMA is caused by polyglutamine repeat expansions in the androgen receptor (AR). To determine the basis of AR polyglutamine neurotoxicity, we introduced human AR yeast artificial chromosomes carrying either 20 or 100 CAGs into mouse embryonic stem cells. The AR100 transgenic mice developed a late-onset, gradually progressive neuromuscular phenotype accompanied by motor neuron degeneration, indicating striking recapitulation of the human disease. We then tested the hypothesis that polyglutamine-expanded AR interferes with CREB binding protein (CBP)-mediated transcription of vascular endothelial growth factor (VEGF) and observed altered CBP-AR binding and VEGF reduction in AR100 mice. We found that mutant AR-induced death of motor neuron-like cells could be rescued by VEGF. Our results suggest that SBMA motor neuronopathy involves altered expression of VEGF, consistent with a role for VEGF as a neurotrophic/survival factor in motor neuron disease.

Animals↗

PubMed2003

Genomic context drives SCA7 CAG repeat instability, while expressed SCA7 cDNAs are intergenerationally and somatically stable in transgenic mice.

Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant cerebellar ataxia caused by a CAG repeat expansion in the ataxin-7 gene. In humans, SCA7 is characterized by marked anticipation due to intergenerational repeat instability with a bias toward expansion, and is thus regarded as the most unstable of the polyglutamine diseases. To study the molecular basis of CAG/CTG repeat instability and its pathological significance, we generated lines of transgenic mice carrying either a SCA7 cDNA construct or a 13.5 kb SCA7 genomic fragment with 92 CAG repeats. While the cDNA transgenic mice showed little intergenerational repeat instability, the genomic fragment transgenic mice displayed marked intergenerational instability with an obvious expansion bias. We then went on to generate additional lines of genomic fragment transgenic mice, and observed that deletion of the 3' genomic region significantly stabilized intergenerational transmission of the SCA7 CAG92 repeat. These results suggest that cis-information present on the genomic fragment is driving the instability process. As the SCA7 genomic fragment contains a large number of replication-associated motifs, the presence of such sequence elements may make the SCA7 CAG repeat region more susceptible to instability. Small-pool and standard PCR analysis of tissues from genomic fragment mice revealed large repeat expansions in their brains and livers, but no such changes were found in any tissues from cDNA transgenic mice that have been shown to undergo neurodegeneration. As large somatic repeat expansions are absent from the brains of SCA7 cDNA mice, our results indicate that neurodegeneration can occur without marked somatic mosaicism, at least in these mice.

3' Untranslated Regions↗

PubMed2002

Polyglutamine-expanded ataxin-7 promotes non-cell-autonomous purkinje cell degeneration and displays proteolytic cleavage in ataxic transgenic mice.

Spinocerebellar ataxia (SCA) type 7 is an inherited neurodegenerative disorder caused by expansion of a polyglutamine tract within the ataxin-7 protein. To determine the molecular basis of polyglutamine neurotoxicity in this and other related disorders, we produced SCA7 transgenic mice that express ataxin-7 with 24 or 92 glutamines in all neurons of the CNS, except for Purkinje cells. Transgenic mice expressing ataxin-7 with 92 glutamines (92Q) developed a dramatic neurological phenotype presenting as a gait ataxia and culminating in premature death. Despite the absence of expression of polyglutamine-expanded ataxin-7 in Purkinje cells, we documented severe Purkinje cell degeneration in 92Q SCA7 transgenic mice. We also detected an N-terminal truncation fragment of ataxin-7 in transgenic mice and in SCA7 patient material with both anti-ataxin-7 and anti-polyglutamine specific antibodies. The appearance of truncated ataxin-7 in nuclear aggregates correlates with the onset of a disease phenotype in the SCA7 mice, suggesting that nuclear localization and proteolytic cleavage may be important features of SCA7 pathogenesis. The non-cell-autonomous nature of the Purkinje cell degeneration in our SCA7 mouse model indicates that polyglutamine-induced dysfunction in adjacent or connecting cell types contributes to the neurodegeneration.

Animals↗