A mouse model of spinal and bulbar muscular atrophy

Exploring the Role of Posttranslational Modifications in Spinal and Bulbar Muscular Atrophy

Spinal and Bulbar Muscular Atrophy (SBMA) is an X-linked adult-onset progressive neuromuscular disease that affects the spinal and bulbar motor neurons and skeletal muscles. SBMA is caused by expansion of polymorphic CAG trinucleotide repeats in the Androgen Receptor (AR) gene, resulting in expanded glutamine tract in the AR protein. Polyglutamine (polyQ) expansion renders the mutant AR protein toxic, resulting in the formation of mutant protein aggregates and cell death. This classifies SBMA as one of the nine known polyQ diseases. Like other polyQ disorders, the expansion of the polyQ tract in the AR protein is the main genetic cause of the disease; however, multiple other mechanisms besides the polyQ tract expansion also contribute to the SBMA disease pathophysiology. Posttranslational modifications (PTMs), including phosphorylation, acetylation, methylation, ubiquitination, and SUMOylation are a category of mechanisms by which the functionality of AR has been found to be significantly modulated and can alter the neurotoxicity of SBMA. This review summarizes the different PTMs and their effects in regulating the AR function and discusses their pathogenic or protective roles in context of SBMA. This review also includes the therapeutic approaches that target the PTMs of AR in an effort to reduce the mutant AR-mediated toxicity in SBMA.

What is the Pathogenic CAG Expansion Length in Huntington's Disease?

Huntington's disease (HD) (OMIM 143100) is caused by an expanded CAG repeat tract in the HTT gene. The inherited CAG length is known to expand further in somatic and germline cells in HD subjects. Age at onset of the disease is inversely correlated with the inherited CAG length, but is further modulated by a series of genetic modifiers which are most likely to act on the CAG repeat in HTT that permit it to further expand. Longer repeats are more prone to expansions, and this expansion is age dependent and tissue-specific. Given that the inherited tract expands through life and most subjects develop disease in mid-life, this implies that in cells that degenerate, the CAG length is likely to be longer than the inherited length. These findings suggest two thresholds- the inherited CAG length which permits further expansion, and the intracellular pathogenic threshold, above which cells become dysfunctional and die. This two-step mechanism has been previously proposed and modelled mathematically to give an intracellular pathogenic threshold at a tract length of 115 CAG (95% confidence intervals 70- 165 CAG). Empirically, the intracellular pathogenic threshold is difficult to determine. Clues from studies of people and models of HD, and from other diseases caused by expanded repeat tracts, place this threshold between 60- 100 CAG, most likely towards the upper part of that range. We assess this evidence and discuss how the intracellular pathogenic threshold in manifest disease might be better determined. Knowing the cellular pathogenic threshold would be informative for both understanding the mechanism in HD and deploying treatments.

The French national protocol for Kennedy's disease (SBMA): consensus diagnostic and management recommendations

Background

Kennedy’s disease (KD), also known as spinal and bulbar muscular atrophy (SBMA), is a rare, adult-onset, X-linked recessive neuromuscular disease caused by CAG expansions in exon 1 of the androgen receptor gene (AR). The objective of the French national diagnostic and management protocol is to provide evidence-based best practice recommendations and outline an optimised care pathway for patients with KD, based on a systematic literature review and consensus multidisciplinary observations.

Results

The initial evaluation, confirmation of the diagnosis, and management should ideally take place in a tertiary referral centre for motor neuron diseases, and involve an experienced multidisciplinary team of neurologists, endocrinologists, cardiologists and allied healthcare professionals. The diagnosis should be suspected in an adult male presenting with slowly progressive lower motor neuron symptoms, typically affecting the lower limbs at onset. Bulbar involvement (dysarthria and dysphagia) is often a later manifestation of the disease. Gynecomastia is not a constant feature, but is suggestive of a suspected diagnosis, which is further supported by electromyography showing diffuse motor neuron involvement often with asymptomatic sensory changes. A suspected diagnosis is confirmed by genetic testing. The multidisciplinary assessment should ascertain extra-neurological involvement such as cardiac repolarisation abnormalities (Brugada syndrome), signs of androgen resistance, genitourinary abnormalities, endocrine and metabolic changes (glucose intolerance, hyperlipidemia). In the absence of effective disease modifying therapies, the mainstay of management is symptomatic support using rehabilitation strategies (physiotherapy and speech therapy). Nutritional evaluation by an expert dietician is essential, and enteral nutrition (gastrostomy) may be required. Respiratory management centres on the detection and treatment of bronchial obstructions, as well as screening for aspiration pneumonia (chest physiotherapy, drainage, positioning, breath stacking, mechanical insufflation-exsufflation, cough assist machnie, antibiotics). Non-invasive mechanical ventilation is seldom needed. Symptomatic pharmaceutical therapy includes pain management, endocrine and metabolic interventions. There is no evidence for androgen substitution therapy.

Conclusion

The French national Kennedy’s disease protocol provides management recommendations for patients with KD. In a low-incidence condition, sharing and integrating regional expertise, multidisciplinary experience and defining consensus best-practice recommendations is particularly important. Well-coordinated collaborative efforts will ultimately pave the way to the development of evidence-based international guidelines.

Trinucleotide Repeat Expansion Diseases, RNAi, and Cancer

Many neurodegenerative diseases are caused by unstable trinucleotide repeat (TNR) expansions located in disease-associated genes. siRNAs based on CAG repeat expansions effectively kill cancer cell lines in vitro through RNAi. They also cause significant reduction in tumor growth in a human ovarian cancer mouse model with no toxicity to the treated mice. This suggests that cancer cells are particularly sensitive to CAG TNR-derived siRNAs, and explains a reported inverse correlation between the length of CAG TNRs and reduced global cancer incidences in some CAG TNR diseases. This review discusses both mutant proteins and mutant RNAs as a cause of TNR diseases, with a focus on RNAi and its role in contributing to disease pathology and in suppressing cancer.

Selection of reference genes for normalization of quantitative polymerase chain reaction data in mouse models of heart failure

The accurate measurement of mRNA expression levels is crucially dependent on the use of relevant reference genes for the normalization of data. Currently, heart failure is a serious and widespread disease, and multiple mouse models are utilized for the study of this complex disease. Although mouse models are commonly used to study cardiovascular disease, various studies have not employed the appropriate selection strategies. The present study investigated the expression stability of eight candidate reference genes (GAPDH, ACTB, B2M, CycA, TBP, PBGD, HTRP 1 and 18S) in two mouse models of heart failure, including the transverse aortic arch constriction (TAC) model and the myocardial infarction (MI) model, using GeNorm software. The expression of BNP was normalized using different reference gene strategies, and it was demonstrated that its induction following heart failure was most profound with the optimal reference gene combination. The most stable genes were identified as follows: TBP and CycA in the MI model, and PBGD and GAPDH in the TAC model. The present study provides important information for reference gene selection in mouse models of heart failure, and will aid further investigations of the transcriptome in cardiovascular research.

Mechanisms mediating spinal and bulbar muscular atrophy: investigations into polyglutamine-expanded androgen receptor function and dysfunction

Spinal and bulbar muscular atrophy (SBMA, Kennedy’s disease), a late-onset neuromuscular disorder, is caused by expansion of the polymorphic polyglutamine tract in the androgen receptor (AR). The AR is a ligand-activated transcription factor, but plays roles in other cellular pathways. In SBMA, selective motor neuron degeneration occurs in the brainstem and spinal cord, thus the causes of neuronal dysfunction have been studied. However, pathogenic pathways in muscles may also be involved. Cultured cells, fly and mouse models are used to study the molecular mechanisms leading to SBMA. Both the structure of the polyglutamine-expanded AR (polyQ AR) and its interactions with other proteins are altered relative to the normal AR. The ligand-dependent translocation of the polyQ AR to the nucleus appears to be critical, as are interdomain interactions. The polyQ AR, or fragments thereof, can form nuclear inclusions, but their pathogenic or protective nature is unclear. Other data suggests soluble polyQ AR oligomers can be harmful. Post-translational modifications such as phosphorylation, acetylation, and ubiquitination influence AR function and modulate the deleterious effects of the polyQ AR. Transcriptional dysregulation is highly likely to be a factor in SBMA; deregulation of non-genomic AR signaling may also be involved. Studies on polyQ AR-protein degradation suggest inhibition of the ubiquitin proteasome system and changes to autophagic pathways may be relevant. Mitochondrial function and axonal transport may also be affected by the polyQ AR. Androgens, acting through the AR, can be neurotrophic and are important in muscle development; hence both loss of normal AR functions and gain of novel harmful functions by the polyQ AR can contribute to neurodegeneration and muscular atrophy. Thus investigations into polyQ AR function have shown that multiple complex mechanisms lead to the initiation and progression of SBMA.

Extraocular muscle characteristics related to myasthenia gravis susceptibility

Background

The pathogenesis of extraocular muscle (EOM) weakness in myasthenia gravis might involve a mechanism specific to the EOM. The aim of this study was to investigate characteristics of the EOM related to its susceptibility to myasthenia gravis.

Methods

Female F344 rats and female Sprague-Dawley rats were assigned to experimental and control groups. The experimental group received injection with Ringer solution containing monoclonal antibody against the acetylcholine receptor (AChR), mAb35 (0.25 mg/kg), to induce experimental autoimmune myasthenia gravis, and the control group received injection with Ringer solution alone. Three muscles were analyzed: EOM, diaphragm, and tibialis anterior. Tissues were examined by light microscopy, fluorescence histochemistry, and transmission electron microscopy. Western blot analysis was used to assess marker expression and ELISA analysis was used to quantify creatine kinase levels. Microarray assay was conducted to detect differentially expressed genes.

Results

In the experimental group, the EOM showed a simpler neuromuscular junction (NMJ) structure compared to the other muscles; the NMJ had fewer synaptic folds, showed a lesser amount of AChR, and the endplate was wider compared to the other muscles. Results of microarray assay showed differential expression of 54 genes in the EOM between the experimental and control groups.

Conclusion

Various EOM characteristics appear to be related to the increased susceptibility of the EOM and the mechanism of EOM weakness in myasthenia gravis.

Tissue and cell-specific transcriptional activity of the human cytomegalovirus immediate early gene promoter (UL123) in zebrafish

The human cytomegalovirus (CMV) is a member of the herpesvirus superfamily and causes different diseases including encephalitis, gastrointestinal diseases, pneumonitis, hepatitis, and retinitis. The immediate early (IE) gene of the human cytomegalovirus is essential to the viral replication. The proximal promoter region of this gene behaves as a strong enhancer and was commonly used to overexpress genes in vitro and in vivo in numerous cell types and species. However, there was no detailed report on the spatial and temporal transcriptional activity of the human CMV-IE gene promoter in zebrafish. In the present study, we generated stable transgenic zebrafish lines carrying the eGFP reporter gene under the control of the human CMV-IE gene promoter (-602/-14). We demonstrated that the hCMV-IE:eGFP transgene was expressed in numerous tissues but transgene expression was either regionalized or restricted to specific cell types as embryo and larval development progressed. In adult, the global expression pattern was similar but not identical to that described for the simian CMV-IE gene promoter in stable zebrafish with high transgene expression in the spinal cord, olfactory organs, central nervous system, neuromasts, retina, and skeletal muscles. However, we describe additional major expression sites in the hepatocytes, the epithelial cells of the intestine, the epithelial cells of the renal tubules, and the oocytes. Interestingly, our study shows that the tissue and cell specific expression pattern of the human CMV-IE gene promoter is rather well conserved in stable transgenic zebrafish compared to that observed in mouse. The major expression sites described in zebrafish are in agreement with the targeted cells and symptoms resulting from CMV infections in human. Finally, the hCMV:eGFP transgenic lines described in the present study will be valuable tools to trace specific cell lineages in adult zebrafish.

Role of androgen receptor polyQ chain elongation in Kennedy's disease and use of natural osmolytes as potential therapeutic targets

Instability of CAG triplet repeat encoding polyglutamine (polyQ) stretches in the gene for target protein has been implicated as a putative mechanism in several inherited neurodegenerative diseases. Expansion of polyQ chain length in the androgen receptor (AR) causes spinal and bulbar muscular atrophy (SBMA) or Kennedy's disease. Although the mechanisms underlying gain-of-neurotoxic function are not completely understood, suggested pathological mechanisms of SBMA involve the formation of AR nuclear and cytoplasmic aggregates, a characteristic feature of patients with SBMA. The fact that certain AR coactivators are sequestered into the nuclear inclusions in SBMA possibly through protein-protein interactions supports the notion that AR transcriptional dysregulation may be a potential pathological mechanism leading to SBMA. AR conformational states associated with aberrant polyQ tract also modulate the interaction of AR with several coactivators. In many cases, such diseases can be treated through protein replacement therapy; however, because recombinant proteins do not cross the blood-brain barrier, the effectiveness of such therapies is limited in case of neurodegenerative diseases that warrant alternative therapeutic approaches. Among different approaches, inhibiting protein aggregation with small molecules that can stimulate protein folding and reverse aggregation are the most promising ones. Thus, naturally occurring osmolytes or "chemical chaperones" that can easily cross the blood-brain barrier and stabilize the functional form of a mutated protein by shifting the folding equilibrium away from degradation and/or aggregation is a useful therapeutic approach. In this review, we discuss the role of polyQ chain length extension in the pathophysiology of SBMA and the use of osmolytes as potential therapeutic tool.

Mouse models of polyglutamine diseases: review and data table. Part I

Polyglutamine (polyQ) disorders share many similarities, such as a common mutation type in unrelated human causative genes, neurological character, and certain aspects of pathogenesis, including morphological and physiological neuronal alterations. The similarities in pathogenesis have been confirmed by findings that some experimental in vivo therapy approaches are effective in multiple models of polyQ disorders. Additionally, mouse models of polyQ diseases are often highly similar between diseases with respect to behavior and the features of the disease. The common features shared by polyQ mouse models may facilitate the investigation of polyQ disorders and may help researchers explore the mechanisms of these diseases in a broader context. To provide this context and to promote the understanding of polyQ disorders, we have collected and analyzed research data about the characterization and treatment of mouse models of polyQ diseases and organized them into two complementary Excel data tables. The data table that is presented in this review (Part I) covers the behavioral, molecular, cellular, and anatomic characteristics of polyQ mice and contains the most current knowledge about polyQ mouse models. The structure of this data table is designed in such a way that it can be filtered to allow for the immediate retrieval of the data corresponding to a single mouse model or to compare the shared and unique aspects of many polyQ models. The second data table, which is presented in another publication (Part II), covers therapeutic research in mouse models by summarizing all of the therapeutic strategies employed in the treatment of polyQ disorders, phenotypes that are used to examine the effects of the therapy, and therapeutic outcomes.

Generation of a novel rodent model for DYT1 dystonia

A mutation in the coding region of the Tor1A gene, resulting in a deletion of a glutamic acid residue in the torsinA protein (∆ETorA), is the major cause of the inherited autosomal-dominant early onset torsion dystonia (DYT1). The pathophysiological consequences of this amino acid loss are still not understood. Currently available animal models for DYT1 dystonia provided important insights into the disease; however, they differ with respect to key features of torsinA associated pathology. We developed transgenic rat models harboring the full length human mutant and wildtype Tor1A gene. A complex phenotyping approach including classical behavioral tests, electrophysiology and neuropathology revealed a progressive neurological phenotype in ∆ETorA expressing rats. Furthermore, we were able to replicate key pathological features of torsinA associated pathology in a second species, such as nuclear envelope pathology, behavioral abnormalities and plasticity changes. We therefore suggest that this rat model represents an appropriate new model suitable to further investigate the pathophysiology of ∆ETorA and to test for therapeutic approaches.

Necdin protects embryonic motoneurons from programmed cell death

NECDIN belongs to the type II Melanoma Associated Antigen Gene Expression gene family and is located in the Prader-Willi Syndrome (PWS) critical region. Necdin-deficient mice develop symptoms of PWS, including a sensory and motor deficit. However, the mechanisms underlying the motor deficit remain elusive. Here, we show that the genetic ablation of Necdin, whose expression is restricted to post-mitotic neurons in the spinal cord during development, leads to a loss of 31% of specified motoneurons. The increased neuronal loss occurs during the period of naturally-occurring cell death and is not confined to specific pools of motoneurons. To better understand the role of Necdin during the period of programmed cell death of motoneurons we used embryonic spinal cord explants and primary motoneuron cultures from Necdin-deficient mice. Interestingly, while Necdin-deficient motoneurons present the same survival response to neurotrophic factors, we demonstrate that deletion of Necdin leads to an increased susceptibility of motoneurons to neurotrophic factor deprivation. We show that by neutralizing TNFα this increased susceptibility of Necdin-deficient motoneurons to trophic factor deprivation can be reduced to the normal level. We propose that Necdin is implicated through the TNF-receptor 1 pathway in the developmental death of motoneurons.

Androgens and spermatogenesis: lessons from transgenic mouse models

Transgenic mouse models have contributed considerably to our understanding of the cellular and molecular mechanisms by which androgens control spermatogenesis. Cell-selective ablation of the androgen receptor (AR) in Sertoli cells (SC) results in a complete block in meiosis and unambiguously identifies the SC as the main cellular mediator of the effects of androgens on spermatogenesis. This conclusion is corroborated by similar knockouts in other potential testicular target cells. Mutations resulting in diminished expression of the AR or in alleles with increased length of the CAG repeat mimick specific human forms of disturbed fertility that are not accompanied by defects in male sexual development. Transcriptional profiling studies in mice with cell-selective and general knockouts of the AR, searching for androgen-regulated genes relevant to the control of spermatogenesis, have identified many candidate target genes. However, with the exception of Rhox5, the identified subsets of genes show little overlap. Genes related to tubular restructuring, cell junction dynamics, the cytoskeleton, solute transportation and vitamin A metabolism are prominently present. Further research will be needed to decide which of these genes are physiologically relevant and to identify genes that can be used as diagnostic tools or targets to modulate the effects of androgens in spermatogenesis.

Prenatal flutamide enhances survival in a myogenic mouse model of spinal bulbar muscular atrophy

BACKGROUND

Spinal bulbar muscular atrophy (SBMA) is caused by a CAG repeat expansion mutation in the androgen receptor (AR) gene, and mutant AR is presumed to act in motoneurons to cause SBMA. However, we found that mice overexpressing wild-type (wt) AR solely in skeletal muscle fibers display the same androgen-dependent disease phenotype as when mutant AR is broadly expressed, challenging the assumptions that only an expanded AR can induce disease and that SBMA is strictly neurogenic. We have previously reported that AR toxicity was ligand dependent in our model, and that very few transgenic (tg) males survived beyond birth.

METHODS

We tested whether the AR antagonist flutamide could block perinatal toxicity. tg males were treated prenatally with flutamide and assessed for survival and motor behavior in adulthood.

RESULTS

Prenatal treatment with flutamide rescued tg male pups from perinatal death, and, as adults, such perinatally rescued tg males showed an SBMA phenotype that was comparable to that of previously described untreated tg males. Moreover, tg males carrying a mutant endogenous allele for AR--the testicular feminization mutation (tfm)--and thus having functional AR only in muscle fibers nevertheless displayed the same androgen-dependent disease phenotype as adults.

CONCLUSIONS

These mice represent an excellent model to study the myogenic contribution to SBMA as they display many of the core features of disease as other mouse models. These data demonstrate that AR acting exclusively in muscle fibers is sufficient to induce SBMA symptoms and that flutamide is protective perinatally.

Recovery of function in a myogenic mouse model of spinal bulbar muscular atrophy

With this paper, we deliberately challenge the prevailing neurocentric theory of the etiology of spinal bulbar muscular atrophy (SBMA). We offer data supporting an alternative view that androgen receptor (AR) acts in skeletal muscles to cause the symptoms of SBMA. While SBMA has been linked to a CAG repeat expansion in the AR gene and mutant AR is presumed to act in motoneurons to cause SBMA, we find that over-expression of wild type AR solely in skeletal muscle fibers results in the same androgen-dependent disease phenotype as when mutant AR is broadly expressed. Like other recent SBMA mouse models, transgenic (tg) females in our model exhibit a motor phenotype only when exposed to androgens, and this motor dysfunction is independent of motoneuronal or muscle fiber cell death. Muscles from symptomatic females also show denervation-like changes in gene expression comparable to a knock-in model of SBMA. Furthermore, once androgen treatment ends, tg females rapidly recover motor function and muscle gene expression, demonstrating the strict androgen-dependence of the disease phenotype in our model. Our results argue that SBMA may be caused by AR acting in muscle.

Spinal and bulbar muscular atrophy: a motoneuron or muscle disease?

Kennedy disease (KD, or spinal and bulbar muscular atrophy) is caused by a CAG/polyglutamine expansion in the androgen receptor (AR) gene. Both motoneurons and muscles are affected by KD, but where mutant ARs act to initiate this disease is not clear. We discuss recent insights into this disease with two main themes. (1) KD is androgen-dependent, suggesting that blocking androgen action may be an effective treatment. (2) Androgens may trigger KD by acting in muscles, which indirectly affects the motoneurons, suggesting that blocking AR function in muscles may rescue motoneurons from disease and provide an effective treatment. Future research will provide a better understanding of how androgens trigger KD and the relative contributions of motoneurons versus muscles in this disease.

The androgen receptor's CAG/glutamine tract in mouse models of neurological disease and cancer

The androgen receptor (AR) is a ligand-activated transcription factor that is central to androgen-dependent development and diseases. Activity of the receptor is influenced by the length of a CAG/glutamine tract in its N-terminal transactivating domain. Expansions of this tract cause Kennedy disease, a protein aggregation degenerative disorder of motor neurons that occurs only in men, and shorter length tracts have been linked to increased risk of prostate cancer. Here we review recent data from mouse models in which gene targeting was used to humanize the mouse Ar gene and introduce CAG/glutamine tracts of varying lengths. Insertion of an expanded tract encoded by 113 CAG repeats modeled Kennedy disease and revealed an important myopathic contribution to the disease phenotype. Variations in CAG tract length within the range of normal human alleles influenced the onset and progression of prostate cancer when targeted Ar mice were crossed to a transgenic prostate cancer model. This series of mice with different Ar alleles has provided insights into the mechanisms by which variations in the CAG/glutamine tract length influence the occurrence of human disease.

Androgen receptor and Kennedy disease/spinal bulbar muscular atrophy

Kennedy Disease/Spinal Bulbar Muscular Atrophy (KD/SBMA) is a progressive neurodegenerative disease caused by genetic polyglutamine expansion of the androgen receptor. We have recently found that overexpression of wildtype androgen receptor in skeletal muscle of transgenic mice results in a KD/SBMA phenotype. This surprising result challenges the orthodox view that KD/SBMA requires expression of polyglutamine expanded androgen receptor within motoneurons. Theories relating to the etiology of this disease drawn from studies of human patients, cellular and mouse models are considered with a special emphasis on potential myogenic contributions to as well as the molecular etiology of KD/SBMA.

Opposing effects of polyglutamine expansion on native protein complexes contribute to SCA1

Spinocerebellar ataxia type 1 (SCA1) is a dominantly inherited neurodegenerative disease caused by expansion of a glutamine-encoding repeat in ataxin 1 (ATXN1). In all known polyglutamine diseases, the glutamine expansion confers toxic functions onto the protein; however, the mechanism by which this occurs remains enigmatic, in light of the fact that the mutant protein apparently maintains interactions with its usual partners. Here we show that the expanded polyglutamine tract differentially affects the function of the host protein in the context of different endogenous protein complexes. Polyglutamine expansion in ATXN1 favours the formation of a particular protein complex containing RBM17, contributing to SCA1 neuropathology by means of a gain-of-function mechanism. Concomitantly, polyglutamine expansion attenuates the formation and function of another protein complex containing ATXN1 and capicua, contributing to SCA1 through a partial loss-of-function mechanism. This model provides mechanistic insight into the molecular pathogenesis of SCA1 as well as other polyglutamine diseases.

The role of the polyglutamine tract in androgen receptor

The androgen receptor (AR) is a ligand-activated transcription factor which is responsible for the androgen responsiveness of target cells. Several types of mutations have been found in the AR and linked to endocrine dysfunctions. Surprisingly, the polymorphism involving the CAG triplet repeat expansion of the AR gene, coding for a polyglutamine (PolyGln) tract in the N-terminal transactivation domain of the AR protein, has been involved either in endocrine or neurological disorders. For example, among endocrine-related-diseases, the PolyGln size has been proposed to be associated to prostate cancer susceptibility, hirsutism, male infertility, cryptorchidism (in conjunction with polyglycine stretches polymorphism), etc.; the molecular mechanisms of these alterations are thought to involve a modulation of AR transcriptional competence, which inversely correlates with the PolyGln length. Among neurological alterations, a decreased AR function seems to be also involved in depression. Moreover, when the polymorphic PolyGln becomes longer than 35-40 contiguous glutamines (ARPolyGln), the ARPolyGln acquires neurotoxicity, because of an unknown gain-of-function. This mutation has been linked to a rare inherited X-linked motor neuronal disorder, the Spinal and Bulbar Muscular Atrophy, or Kennedy's disease. The disorder is characterized by death of motor neurons expressing high levels of AR. The degenerating motor neurons are mainly located in the anterior horns of the spinal cord and in the bulbar region; some neurons of the dorsal root ganglia may also be involved. Interestingly, the same type of PolyGln elongation has been found in other totally unrelated proteins responsible for different neurodegenerative diseases. A common feature of all these disorders is the formation of intracellular aggregates containing the mutated proteins; at present, but their role in the disease is largely debated. This review will discuss how the PolyGln neurotoxicity of SBMA AR may be either mediated or decreased by aggregates, and will present data on the dual role played by testosterone on motor neuronal functions and dysfunctions.

Evidence that androgens regulate early developmental events, prior to sexual differentiation

Androgen signaling is critical for normal fetal development but is not thought to regulate events in early embryogenesis. Given the interest in factors controlling the differentiation of embryonic stem (ES) cells, we have explored the possibility that androgens may play a role. This study demonstrates expression of androgen receptor (AR) RNA and protein in four independent mouse ES (mES) cell lines, and shows that the AR is functional and can interact with transfected androgen response elements to promote green fluorescent protein expression. AR mRNA was detected throughout 10-d differentiation in embryoid bodies (EBs). Exposure of EBs to testosterone (T) or dihydrotestosterone, at doses of 1 and 0.1 mum, respectively, promoted formation of beating cardiomyocytes. Flow cytometric analyses demonstrated a significant increase in the number of alpha-actinin and tropomyosin (cardiac markers) positive cells after these treatments. Addition of flutamide (1 microM) to T-treated EBs inhibited the T-induced proliferation of cardiomyocytes, confirming that, in this instance, androgens act via the classical AR-mediated genomic pathway. We also report that mES cells express key steroidogenic enzymes, as detected by RT-PCR, and during 24-h incubations secrete T at concentrations of 1.38 +/- 0.22 nM, levels comparable to those secreted by cultured Leydig cells. These novel data demonstrate the capacity of androgens to stimulate increased differentiation of mouse ES cells to cardiomyocytes, and are in keeping with recent observations that AR-deficient mice exhibit cardiac impairment in adulthood.

Overexpression of wild-type androgen receptor in muscle recapitulates polyglutamine disease

We created transgenic mice that overexpress WT androgen receptor (AR) exclusively in their skeletal muscle fibers. Unexpectedly, these mice display androgen-dependent muscle weakness and early death, show changes in muscle morphology and gene expression consistent with neurogenic atrophy, and exhibit a loss of motor axons. These features reproduce those seen in models of Kennedy disease, a polyglutamine expansion disorder caused by a CAG repeat expansion in the AR gene. These findings demonstrate that toxicity in skeletal muscles is sufficient to cause motoneuron disease and indicate that overexpression of the WT AR can exert toxicity comparable with the polyglutamine expanded protein. This model has two clear implications for Kennedy disease: (i) mechanisms affecting AR gene expression may cause neuromuscular symptoms similar to those of Kennedy disease and (ii) therapeutic approaches targeting skeletal muscle may provide effective treatments for this disease.

REVIEW ARTILCE: Cell therapy and stem cells in animal models of motor neuron disorders

Amyotrophic lateral sclerosis (ALS), spinal bulbar muscular atrophy (or Kennedy's disease), spinal muscular atrophy and spinal muscular atrophy with respiratory distress 1 are neurodegenerative disorders mainly affecting motor neurons and which currently lack effective therapies. Recent studies in animal models as well as primary and embryonic stem cell models of ALS, utilizing over-expression of mutated forms of Cu/Zn superoxide dismutase 1, have shown that motor neuron degeneration in these models is in part a non cell-autonomous event and that by providing genetically non-compromised supporting cells such as microglia or growth factor-excreting cells, onset can be delayed and survival increased. Using models of acute motor neuron injury it has been shown that embryonic stem cell-derived motor neurons implanted into the spinal cord can innervate muscle targets and improve functional recovery. Thus, a rationale exists for the development of cell therapies in motor neuron diseases aimed at either protecting and/or replacing lost motor neurons, interneurons as well as non-neuronal cells. This review evaluates approaches used in animal models of motor neuron disorders and their therapeutic relevance.

Motor and cognitive deficits in the heterozygous leaner mouse, a Cav2.1 voltage-gated Ca2+ channel mutant

The leaner mutation in mice affects the Ca(v)2.1 voltage-gated calcium channel alpha(1A)-subunit gene (Cacna1a), causing a reduction in calcium currents predominantly in Purkinje cells. This reduction in calcium currents causes severe progressive cerebellar ataxia, beginning around postnatal day 10, in homozygous leaner mice (tg(la)/tg(la)), while their heterozygous littermates (tg(la)/+) present no obvious behavioral deficits. In humans, heterozygous mutations in the Cacna1a orthologous gene produce a broad range of neurological manifestations. To evaluate the phenotypic status of the tg(la)/+ animals, we assessed motor performance and cognition, at different ages, in these mutant mice. We were able to observe age-dependent impairment in motor and cognitive tasks; balance and motor learning deficits were found in demanding tasks on the rotarod and on the hanging wire test, while spatial learning and memory impairment was observed in the Morris water maze. Progressive dysfunction in escape reflexes, indicative of neurological impairment, was also present in tg(la)/+ animals. Although not presenting major motor alterations, tg(la)/+ mice show age-dependent motor and cognitive deficits.

ATAXIN-1 Interacts with the Repressor Capicua in Its Native Complex to Cause SCA1 Neuropathology

Spinocerebellar ataxia type 1 (SCA1) is one of several neurodegenerative diseases caused by expansion of a polyglutamine tract in the disease protein, in this case, ATAXIN-1 (ATXN1). A key question in the field is whether neurotoxicity is mediated by aberrant, novel interactions with the expanded protein or whether its wild-type functions are augmented to a deleterious degree. We examined soluble protein complexes from mouse cerebellum and found that the majority of wild-type and expanded ATXN1 assembles into large stable complexes containing the transcriptional repressor Capicua. ATXN1 directly binds Capicua and modulates Capicua repressor activity in Drosophila and mammalian cells, and its loss decreases the steady-state level of Capicua. Interestingly, the S776A mutation, which abrogates the neurotoxicity of expanded ATXN1, substantially reduces the association of mutant ATXN1 with Capicua in vivo. These data provide insight into the function of ATXN1 and suggest that SCA1 neuropathology depends on native, not novel, protein interactions.

Androgen-dependent pathology demonstrates myopathic contribution to the Kennedy disease phenotype in a mouse knock-in model

Kennedy disease, a degenerative disorder characterized by androgen-dependent neuromuscular weakness, is caused by a CAG/glutamine tract expansion in the androgen receptor (Ar) gene. We developed a mouse model of Kennedy disease, using gene targeting to convert mouse androgen receptor (AR) to human sequence while introducing 113 glutamines. AR113Q mice developed hormone and glutamine length-dependent neuromuscular weakness characterized by the early occurrence of myopathic and neurogenic skeletal muscle pathology and by the late development of neuronal intranuclear inclusions in spinal neurons. AR113Q males unexpectedly died at 2-4 months. We show that this androgen-dependent death reflects decreased expression of skeletal muscle chloride channel 1 (CLCN1) and the skeletal muscle sodium channel alpha-subunit, resulting in myotonic discharges in skeletal muscle of the lower urinary tract. AR113Q limb muscles show similar myopathic features and express decreased levels of mRNAs encoding neurotrophin-4 and glial cell line-derived neurotrophic factor. These data define an important myopathic contribution to the Kennedy disease phenotype and suggest a role for muscle in non-cell autonomous toxicity of lower motor neurons.

JNK mediates pathogenic effects of polyglutamine-expanded androgen receptor on fast axonal transport

Expansion of the polyglutamine (polyQ) stretch in the androgen receptor (AR) protein leads to spinal and bulbar muscular atrophy (SBMA), a neurodegenerative disease characterized by lower motor neuron degeneration. The pathogenic mechanisms underlying SBMA remain unknown, but recent experiments show that inhibition of fast axonal transport (FAT) by polyQ-expanded proteins, including polyQ-AR, represents a new cytoplasmic pathogenic lesion. Using pharmacological, biochemical and cell biological experiments, we found a new pathogenic pathway that is affected in SBMA and results in compromised FAT. PolyQ-AR inhibits FAT in a human cell line and in squid axoplasm through a pathway that involves activation of cJun N-terminal kinase (JNK) activity. Active JNK phosphorylated kinesin-1 heavy chains and inhibited kinesin-1 microtubule-binding activity. JNK inhibitors prevented polyQ-AR-mediated inhibition of FAT and reversed suppression of neurite formation by polyQ-AR. We propose that JNK represents a promising target for therapeutic interventions in SBMA.

Genomic and Evolutionary Insights into Genes Encoding Proteins with Single Amino Acid Repeats

Mutations causing expansion of amino acid repeats are responsible for 19 hereditary disorders. Repeats in several other proteins also show length variations. These observations prompted us to identify single amino acid repeat-containing proteins (SARPs) in humans and to understand their functional and evolutionary significance. We identified 8812 SARPs containing 17 146 repeat domains, each harboring 4 or more residues. In all, 5% of SARPs (471) showed repeat length variations, and nearly 84% of them (394) have repeats of 10 residues or less. We find that SARPs are involved in functions that require formation of multiprotein complexes. Nearly 78% (6859) of the SARPs did not find a paralogue in the human proteome, and such proteins are considered as orphan SARPs. Orphan SARPs show longer repeat stretches, longer peptide length, and lower expression levels as compared with SARPs belonging to protein family. Because the intensity of gene expression is known to relate inversely with the rate of protein sequence evolution, our results suggest that the orphan SARPs evolve faster than the familial forms and therefore are under a weaker selection pressure. We also find that while GC-rich codons are favored for coding the repeat tracts of SARPs, specific codons and not nucleotide motifs per se are selected, suggesting functional constraints placed on the usage of codons. One of the constraints could be the mRNA stability as clustering of rare codons is known to destabilize the transcripts and rare codons are not favored for coding repeat tracts. Genes encoding polymorphic SARPs show preferential localization toward the telomeric segments. Further, the sex-specific recombination rates of the chromosomal locus strongly correlate with the parental gender that influence the repeat instability in disorder caused by dynamic mutation. Therefore, instability associated with repeats might be driven by processes that are specific to sperm or oocyte development, and the recombination frequency might play a positive role in this process.

Abnormalities of germ cell maturation and sertoli cell cytoskeleton in androgen receptor 113 CAG knock-in mice reveal toxic effects of the mutant protein

An unresolved question in the study of the polyglutamine neurodegenerative disorders is the extent to which partial loss of normal function of the mutant protein contributes to the disease phenotype. To address this, we studied Kennedy disease, a degenerative disorder of lower motor neurons caused by a CAG/glutamine expansion in the androgen receptor (Ar) gene. Signs of partial androgen insensitivity, including testicular atrophy and decreased fertility, are common in affected males, although the underlying mechanisms are not well understood. Here, we describe a knock-in mouse model that reproduces the testicular atrophy, diminished fertility, and systemic signs of partial androgen insensitivity that occur in Kennedy disease patients. Using this model, we demonstrate that the testicular pathology in this disorder is distinct from that mediated by loss of AR function. Testes pathology in 113 CAG knock-in mice was characterized by morphological abnormalities of germ cell maturation, decreased solubility of the mutant AR protein, and alterations of the Sertoli cell cytoskeleton, changes that are distinct from those produced by AR loss-of-function mutation in testicular feminization mutant mice. Our data demonstrate that toxic effects of the mutant protein mediate aspects of the Kennedy disease phenotype previously attributed to a loss of AR function.

Animal models of Kennedy disease

Since the identification of the polyglutamine repeat expansion responsible for Kennedy disease (KD) more than a decade ago, several laboratories have created animal models for KD. The slowly progressive nature of KD, its X-linked dominant mode of inheritance, and its recently elucidated hormone dependence have made the modeling of this lower motor neuron disease uniquely challenging. Several models have been generated in which variations in specificity, age of onset, and rate of progression have been achieved. Animal models that precisely reproduce the motor neuron specificity, delayed onset, and slow progression of disease may not support preclinical therapeutics testing, whereas models with rapidly progressing symptoms may preclude the ability to fully elucidate pathogenic pathways. Drosophila models of KD provide unique opportunities to use the power of genetics to identify pathogenic pathways at work in KD. This paper reviews the new wealth of transgenic mouse and Drosophila models for KD. Whereas differences, primarily in neuropathological findings, exist in these models, these differences may be exploited to begin to elucidate the most relevant pathological features of KD.

CAG repeat length in an infertile male population of Irish origin

The androgen receptor (AR) gene, located on the X chromosome, is an important regulator of human spermatogenesis. In the past decade, the link between the CAG polyglutamine tract, situated on exon one of the AR gene, and reduced spermatogenesis has become a controversial one. Alterations in the length of the CAG polyglutamine tract have been associated with prostate cancer at a reduced intrinsic length and neuromuscular diseases at a CAG repeat length of > or = 40. Minimal intermediate increases have been linked with depressed spermatogenesis in infertile males. Asian and Australian groups have published an association between increased CAG repeat length and reduced spermatogenesis while many European studies have found no such association. The aim of this study was to document the association between increased CAG repeat length and reduced spermatogenesis in a group of Irish infertile males and controls known to have fathered at least one child. The study employed the ABI 377 DNA sequencer to size the CAG repeat region of exon one of the AR gene in each group. Statistical analysis revealed no actual link between the length of the CAG tract and a reduction of spermatogenesis in a cohort of infertile patients (n = 66) of Irish ethnic origin when compared to a fertile control group (n = 77) (p = 0.599).

The Unfolded Protein Response Modulates Toxicity of the Expanded Glutamine Androgen Receptor

Kennedy disease, a degenerative disorder caused by an expanded glutamine tract, is mediated by misfolding of the mutant androgen receptor (AR) protein, a process that may disrupt proteasome function. We hypothesized that this might lead to endoplasmic reticulum (ER) stress and induction of the unfolded protein response (UPR), a complex physiologic pathway that regulates cell survival. To test this hypothesis, we used aminoterminal fragments of wild type (AR16Q) or mutant (AR112Q) AR that triggered glutamine length-dependent cell death and activated an ER stress-inducible promoter. To evaluate the role of the UPR, we examined the contributions of three proximal sensors of ER stress: activating transcription factor 6 (ATF6), inositol requiring 1 (IRE1), and PKR-like endoplasmic reticulum kinase (PERK). AR112Q toxicity was significantly increased by a dominant negative ATF6 mutant and significantly decreased by a constitutively active ATF6 mutant, indicating that ATF6 promoted cell survival. In contrast, co-transfection with three separate IRE1alpha dominant negative mutants failed to alter glutamine length-dependent toxicity, suggesting that this arm of the UPR did not significantly affect AR112Q induced cell death. Activation of PERK, an ER transmembrane protein that functions as the third proximal UPR sensor, promoted glutamine length-dependent toxicity. Although nuclear localization sequence- and nuclear export sequence-targeted proteins both activated the UPR, this pathway more potently influenced toxicity when proteins were targeted to the cytoplasm. Taken together, our data demonstrate that the UPR is activated in cells expressing long glutamine tracts and that this pathway modulates polyglutamine toxicity.

Progress in Spinobulbar muscular atrophy research: insights into neuronal dysfunction caused by the polyglutamine-expanded androgen receptor

Spinobulbar muscular atrophy (SBMA, Kennedy's disease) results from the dysfunction and degeneration of specific motor and sensory neurons. The underlying cause of this ligand-dependent neurodegenerative disease is expansion of the CAG trinucleotide repeat in the androgen receptor (AR) gene which leads to lengthening of the polyglutamine tract in the AR protein. Recently, the effects of the polyglutamine-expanded AR have been explored in a number of cellular and animal models. Common themes include research on polyglutamine-containing nuclear inclusions and the effect of molecular chaperone overexpression on their formation. In addition, investigations have highlighted the role that abnormal transcriptional regulation, proteasome dysfunction and altered axonal transport may play in disease pathogenesis. These studies suggest a number of potential treatments for restoring neuronal function. One of the most interesting advances in SBMA research has been the creation of mouse models that recapitulate the key features of SBMA progression in men. Lowering testosterone levels in affected transgenic male mice rescued, and even reversed the polyglutamine-induced neuromuscular phenotype, indicating that manipulating androgen levels in men could be of therapeutic benefit. Although the question of why only a distinct subset of neurons is affected by polyglutamine expansion of the AR remains unsolved, future research will provide further insights into the mechanisms contributing to disease progression in SBMA.

Castration restores function and neurofilament alterations of aged symptomatic males in a transgenic mouse model of spinal and bulbar muscular atrophy

Transgenic models of neurodegenerative disease have proved uniquely powerful for delineating pathways of neuronal dysfunction and cell death. We have developed a transgenic model of the polyglutamine disease spinal and bulbar muscular atrophy (SBMA), an adult-onset, slowly progressive motor neuron disease caused by polyglutamine expansion in the androgen receptor (AR). Mice bearing a human AR with 112 glutamines reproduce many aspects of SBMA, including slowly progressive, gender-specific motor deficits, and neuronal intranuclear inclusions. Despite substantial motor deficits in male AR112Q mice, no motor neuron loss was observed, indicating that neuronal dysfunction, rather than neuronal death, is central to disease. Moreover, reduced levels of unphosphorylated neurofilament heavy chain (NF-H) were observed in motor neurons, suggesting a role for NF-H in SBMA neuronal dysfunction. The elimination of androgens by surgical castration of severely affected, aged 112Q male mice partially restored motor function as well as NF-H levels. These data suggest that hormone-based therapies designed to treat SBMA patients, even with advanced disease, are likely to be effective.

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.

Genetic and environmental factors in the pathogenesis of Huntington's disease

Huntington's disease is a fatal inherited disorder in which there is progressive neurodegeneration in specific brain areas, mainly the striatum and cerebral cortex, producing motor, cognitive, and psychiatric symptoms. The trinucleotide repeat mutation involved is common to many other brain diseases, which may therefore involve similar mechanisms of pathogenesis. We are beginning to understand how a CAG trinucleotide repeat expansion in the disease gene, encoding an expanded polyglutamine tract, induces neuronal dysfunction and symptomatology in Huntington's disease. Recent evidence that environmental factors modify the onset and progression of neurodegeneration has shed new light on Huntington's disease and other devastating brain diseases. This review focuses on genetic mediators, environmental modulators, and associated gene-environment interactions in the pathogenesis of Huntington's disease.

Protein aggregation in motor neurone disorders

Toxicity associated with abnormal protein folding and protein aggregation are major hypotheses for neurodegeneration. This article comparatively reviews the experimental and human tissue-based evidence for the involvement of such mechanisms in neuronal death associated with the motor system disorders of X-linked spinobulbar muscular atrophy (SBMA; Kennedy's disease) and amyotrophic lateral sclerosis (ALS), especially disease related to mutations in the superoxide dismutase (SOD1) gene. Evidence from transgenic mouse, Drosophila and cell culture models of SBMA, in common with other trinucleotide repeat expansion disorders, show protein aggregation of the mutated androgen receptor, and intraneuronal accumulation of aggregated protein, to be obligate mechanisms. Strong experimental data link these phenomena with downstream biochemical events involving gene transcription pathways (CREB-binding protein) and interactions with protein chaperone systems. Manipulations of these pathways are already established in experimental systems of trinucleotide repeat disorders as potential beneficial targets for therapeutic activity. In contrast, the evidence for the role of protein aggregation in models of SOD1-linked familial ALS is less clear-cut. Several classes of intraneuronal inclusion body have been described, some of which are invariably present. However, the lack of understanding of the biochemical basis of the most frequent inclusion in sporadic ALS, the ubiquitinated inclusion, has hampered research. The toxicity associated with expression of mutant SOD1 has been intensively studied however. Abnormal protein aggregation and folding is the only one of the four major hypotheses for the mechanism of neuronal degeneration in this disorder currently under investigation (the others comprise oxidative stress, axonal transport and cytoskeletal dysfunctions, and glutamatergic excitotoxicity). Whilst hyaline inclusions, which are strongly immunoreactive to SOD1, are linked to degeneration in SOD1 mutant mouse models, the evidence from human tissue is less consistent and convincing. A role for mutant SOD1 aggregation in the mitochondrial dysfunction associated with ALS, and in potentially toxic interactions with heat shock proteins, both leading to apoptosis, are supported by some experimental data. Direct in vitro data on mutant SOD1 show evidence for spontaneous oligomerization, but the role of such oligomers remains to be elucidated, and therapeutic strategies are less well developed for this familial variant of ALS.

Pathogenesis of polyglutamine disorders: aggregation revisited

Expansion of CAG trinucleotide repeats coding for polyglutamine in unrelated proteins causes at least nine late-onset progressive neurodegenerative disorders, including Huntington's disease and a number of spinocerebellar ataxias. Expanded polyglutamine provokes a dominant gain-of-function neurotoxicity, regardless of the specific protein context within which it resides. Nevertheless, the protein context does modulate polyglutamine toxicity, as evidenced by the distinct clinical and pathological features of the various disorders. Importantly, polyglutamine toxicity might derive from its ability to aggregate. Indeed, aggregation probably underlies some defining attributes of the polyglutamine disorders, such as their late onset, progressive nature, and the dependence of onset age on polyglutamine length. However, the central role of aggregation in polyglutamine pathogenesis has been challenged by several studies, which instead argued that the soluble form of the disease proteins is responsible for neuronal damage. Thus, the question whether polyglutamine aggregates are deleterious, harmless or protective remains the most passionately disputed issue in the study of these diseases. In this review, we attempt to reconcile some of these controversies.

Modelling brain diseases in mice: the challenges of design and analysis

Genetically engineered mice have been generated to model a variety of neurological disorders. Several of these models have provided valuable insights into the pathogenesis of the relevant diseases; however, they have rarely reproduced all, or even most, of the features observed in the corresponding human conditions. Here, we review the challenges that must be faced when attempting to accurately reproduce human brain disorders in mice, and discuss some of the ways to overcome them. Building on the knowledge gathered from the study of existing mutants, and on recent progress in phenotyping mutant mice, we anticipate better methods for preclinical interventional trials and significant advances in the understanding and treatment of neurological diseases.