The triplet repeat disease X-linked spinal and bulbar muscular atrophy (SBMA) is caused by a polyglutamine expansion in the androgen receptor (AR) gene. Just how this expansion causes degeneration of lower motor neurons is unclear, but in today's Neuron, Albert La Spada and colleagues at the University of Washington Medical Center, Seattle, point an accusing finger at dysregulation of the gene for vascular epithelial growth factor (VEGF).

Previous animal models of SBMA have revealed that the polyglutamine-expanded AR must move to the nucleus in order to cause toxicity (see for example Katsuno et al., 2002), but none of these models used constructs in which the receptor is regulated by its normal promoter elements. To create such a model, first author Bryce Sopher and colleagues created mice in which human androgen receptor with short (20 repeats, AR20) and long (100 repeats, AR100) expansions are expressed from yeast artificial chromosomes.

When Sopher examined these animals, he found that their motor neurons progressively lost function; this caused gait problems, muscle weakness, and eventually paralyzed the mice. This was more pronounced in animals with the longer repeat, and virtually absent in females, in keeping with the X-linked pattern of inheritance and sex-limited expression of the human disease. Overall, the model recapitulates SBMA more faithfully than previous ones did, the authors claim.

Sopher found that the expanded receptor does indeed end up in the nucleus, but curiously, it does not form aggregates detectable by microscope in motor neurons, though small soluble aggregates might have been there. Instead, the authors detected aggregates in spinal cord astrocytes in the AR100 transgenics. (It is worth noting that recent evidence has shown that mutant superoxide dismutase, which causes familial amyotrophic lateral sclerosis (ALS), need not be expressed in neurons to cause disease. See ARF related news story).

To test the role of the expanded receptor in regulating transcription, Sopher and colleagues built on previous observations that the AR might interfere with CREB binding protein (CBP), a well-studied transcriptional coactivator (see McCampbell et al., 2000). When the authors immunoprecipitated CBP, they found that the androgen receptor came along for the ride, and the longer the polyglutamine repeat, the stronger the association between the two proteins.

So what might be the consequences of this liaison? Could expanded AR prevent CBP from activating gene transcription? And which affected gene(s) may be important for motor neuron survival? To answer these questions, Sopher turned to a CBP's downstream target, VEGF. This growth factor is known to be regulated by a CBP binding element. Previous work by Peter Carmeliet’s group in Belgium has shown that abolishing that element induces motor neuron degeneration (Oosthuyse et al., 2001; see also Lambrechts et al., 2003). Contrary to expectations, when Sopher measured VEGF expression in the AR20 and AR100 neurons, it was no different from controls. However, a particular isoform of VEGF—VEGF 164—has been shown to have neurotrophic activity, and when Sopher measured expression of this isoform, he indeed saw a progressive loss of expression. AR100 transgenics had about 30 and 45 percent less VEGF than wild-type mice at 6.5 and 11 months of age, respectively. Moreover, when the authors added VEGF 164 to cultured neurons expressing expanded androgen receptor, the growth factor rescued them: 60 percent of the cells died in the absence of VEGF, versus 20 percent in its presence.

The authors suggest that "decreased expression of VEGF or an inability to upregulate VEGF in the face of injury, ischemia, or stress is a fundamental property of degenerating motor neurons." Note, too, that VEGF has been linked with increased risk for developing amyotrophic lateral sclerosis.—Tom Fagan.

Reference:
Sopher BL, Thomas Jr PS, LaFever-Bernt MA, Hold IE, Wilke SA, Ware CB, Lee-Way J, Libby RT, Ellerby LM, La Spada AR. Androgen receptor YAC transgenic mice recapitulate SBMA motor neuronopathy and implicate VEGF164 in the motor neuron degeneration. Neuron 2004 March 4;41:687-699. Abstract

Comments

Make a Comment

To make a comment you must login or register.

Comments on this content

  1. This is an interesting paper in that it connects CBP and androgen receptor to the expression of VEGF. (Evidence that CBP regulates VEGF expression and for the connection between CBP and androgen receptor already existed.) It is curious that the authors found reductions only in one transcript of VEGF. If the regulation is at the transcriptional level, one would imagine all alternatively spliced forms to be affected. It would have been better to perform the rescue experiment in vivo, but admittedly, that is a tall order. That said, the authors made a huge effort to avoid problems such as exogenous promoter, overexpression, etc., that are frequently associated with traditional transgenic approaches. These mice do recapitulate the phenotypes of the human disease.

  2. In an elegant study, Sopher et al. generated and analyzed transgenic mice that express yeast artificial chromosomes carrying the human androgen receptor (AR) with either 20 (AR20) or 100 (AR100) CAGs. In this model, human AR is expressed between 30 and 100 percent relative to endogenous mouse AR levels. The AR100 male, but not female, mice developed growth retardation, muscle weakness and atrophy, and motor degeneration resembling X-linked spinal and bulbar muscular atrophy (SBMA, or Kennedy’s disease). The polymorphic CAG repeats in the first exon of AR vary in length from 5-34 in healthy controls to 40-66 in SBMA patients.

    The gender-dependent phenotype in AR100 mice is consistent with the requirement of nuclear translocation of mutant AR by testosterone. In transgenic mice expressing AR containing 97 CAG repeats, castration of males rescued the phenotype, while administration of testosterone to the females worsened the manifestations (Katsuno et al., 2002). Based on the requirement for nuclear AR translocation and the fact that AR is a transcription factor, the authors tested whether SBMA transcription is altered by interfering with CREB binding protein (CBP)-mediated transcription of vascular endothelial growth factor (VEGF). CBP is a transcription coactivator that remodels chromatin through its histone acetyltransferase activity. Indeed, coimmunoprecipitation experiments with anti-CBP antibodies revealed more AR in the pulled-down complex with increasing the length of the CAG repeats. In addition, VEGF164, a VEGF isoform expressed in motor neurons and protecting them from apoptotic insults, was reduced in AR100 mice. Finally, adding VEGF164 peptide reduced cell death in a motor neuron cell culture model of AR polyglutamine neurotoxicity.

    Strikingly, the affected motor neurons showed diffuse nuclear AR staining but no nuclear aggregates. Nuclear aggregates were observed in unaffected neurons in the dorsal lateral hypothalamus and tectum, in astrocytes in the spinal cord, and in muscle and liver. These findings support the conclusion that nuclear inclusions might not be required for neurotoxicity, and that soluble forms or microaggregates might be toxic. This is also consistent with a mouse model showing a correlation between neuronal vulnerability and solubility of ataxin-1 with 154 glutamines, while nuclear inclusions only occur late in the course of the disease (Watase et al., 2002).

    The toxicity of intraneuronal protein aggregates might depend on the neuroprotective potential of the affected cells. While in control mice the herbicide paraquat caused the formation of α-synuclein-containing intraneuronal deposits and generation of nigrostriatal neurons, overexpression of α-synuclein (human wild-type or the Ala53Thr mutant) protected against paraquat-induced neurodegeneration in the presence of the intraneuronal deposits. This neuroprotective effect might be due to increased levels of the neuroprotective HSP70 (Manning-Bog et al., 2003). Overexpression of HSP70 showed neuroprotection without affecting nuclear inclusions in SCA1 mice expressing ataxin-1 with expanded CAG repeats (Cummings et al., 2001) and neuroprotection without affecting α-synuclein containing-intraneuronal deposits in a Drosophila model of Parkinson’s disease (Auluck et al., 2002). Interestingly, overexpression of HSP70 also reduced motor dysfunction in mice expressing AR with 97 CAG repeats (Adachi et al., 2003). Other HSPs such as HSP105α are also neuroprotective against toxicity associated with AR with expanded CAG repeats (Ishihara et al., 2003). The ability of HSPs to reduce toxicity and aggregate formation might be linked or independent. HSP70, HSP40, and HSP27 suppressed toxicity independent of suppression of aggregation (Wyttenbach et al., 2003; Adachi et al., 2003). [Editor's note: see also Magrané et al., 2004.] However, both monomeric AR and nuclear-localized AR complexes were also reported reduced under conditions of overexpression of HSP70, suggesting that HSP70 might be neuroprotective by increasing mutant AR turnover.

Comments on Primary Papers for this Article

No Available Comments on Primary Papers for this Article

References

News Citations

  1. ALS—Is It the Neurons or the Glia?

Paper Citations

  1. . Testosterone reduction prevents phenotypic expression in a transgenic mouse model of spinal and bulbar muscular atrophy. Neuron. 2002 Aug 29;35(5):843-54. PubMed.
  2. . CREB-binding protein sequestration by expanded polyglutamine. Hum Mol Genet. 2000 Sep 1;9(14):2197-202. PubMed.
  3. . Deletion of the hypoxia-response element in the vascular endothelial growth factor promoter causes motor neuron degeneration. Nat Genet. 2001 Jun;28(2):131-8. PubMed.
  4. . VEGF is a modifier of amyotrophic lateral sclerosis in mice and humans and protects motoneurons against ischemic death. Nat Genet. 2003 Aug;34(4):383-94. PubMed.
  5. . Androgen receptor YAC transgenic mice carrying CAG 45 alleles show trinucleotide repeat instability. Hum Mol Genet. 1998 Jun;7(6):959-67. PubMed.

Further Reading

Papers

  1. . Androgen receptor YAC transgenic mice carrying CAG 45 alleles show trinucleotide repeat instability. Hum Mol Genet. 1998 Jun;7(6):959-67. PubMed.

Primary Papers

  1. . Androgen receptor YAC transgenic mice carrying CAG 45 alleles show trinucleotide repeat instability. Hum Mol Genet. 1998 Jun;7(6):959-67. PubMed.