Characterization of an expanded glutamine repeat androgen receptor in a neuronal cell culture system

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.

Neuropathy-causing TRPV4 mutations disrupt TRPV4-RhoA interactions and impair neurite extension

TRPV4 is a cell surface-expressed calcium-permeable cation channel that mediates cell-specific effects on cellular morphology and function. Dominant missense mutations of TRPV4 cause distinct, tissue-specific diseases, but the pathogenic mechanisms are unknown. Mutations causing peripheral neuropathy localize to the intracellular N-terminal domain whereas skeletal dysplasia mutations are in multiple domains. Using an unbiased screen, we identified the cytoskeletal remodeling GTPase RhoA as a TRPV4 interactor. TRPV4-RhoA binding occurs via the TRPV4 N-terminal domain, resulting in suppression of TRPV4 channel activity, inhibition of RhoA activation, and extension of neurites in vitro. Neuropathy but not skeletal dysplasia mutations disrupt TRPV4-RhoA binding and cytoskeletal outgrowth. However, inhibition of RhoA restores neurite length in vitro and in a fly model of TRPV4 neuropathy. Together these results identify RhoA as a critical mediator of TRPV4-induced cell structure changes and suggest that disruption of TRPV4-RhoA binding may contribute to tissue-specific toxicity of TRPV4 neuropathy mutations.

TRPV4 dominant mutations cause neuropathy. Here, the authors show that TRPV4 binds and interacts with RhoA, modulating the actin cytoskeleton. Neuropathy-causing mutations of TRPV4 disrupt this complex, leading to RhoA activation and impairment of neurite extension in cultured cells and flies.

A Crucial Role for the Protein Quality Control System in Motor Neuron Diseases

Motor neuron diseases (MNDs) are fatal diseases characterized by loss of motor neurons in the brain cortex, in the bulbar region, and/or in the anterior horns of the spinal cord. While generally sporadic, inherited forms linked to mutant genes encoding altered RNA/protein products have also been described. Several different mechanisms have been found altered or dysfunctional in MNDs, like the protein quality control (PQC) system. In this review, we will discuss how the PQC system is affected in two MNDs—spinal and bulbar muscular atrophy (SBMA) and amyotrophic lateral sclerosis (ALS)—and how this affects the clearance of aberrantly folded proteins, which accumulate in motor neurons, inducing dysfunctions and their death. In addition, we will discuss how the PQC system can be targeted to restore proper cell function, enhancing the survival of affected cells in MNDs.

Molecular Mechanisms and Therapeutics for SBMA/Kennedy's Disease

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by a polyglutamine (polyQ) expansion in the androgen receptor (AR). Despite the fact that the monogenic cause of SBMA has been known for nearly 3 decades, there is no effective treatment for this disease, underscoring the complexity of the pathogenic mechanisms that lead to a loss of motor neurons and muscle in SBMA patients. In the current review, we provide an overview of the system-wide clinical features of SBMA, summarize the structure and function of the AR, discuss both gain-of-function and loss-of-function mechanisms of toxicity caused by polyQ-expanded AR, and describe the cell and animal models utilized in the study of SBMA. Additionally, we summarize previously conducted clinical trials which, despite being based on positive results from preclinical studies, proved to be largely ineffective in the treatment of SBMA; nonetheless, these studies provide important insights as researchers develop the next generation of therapies.

Gene expression analysis reveals early dysregulation of disease pathways and links Chmp7 to pathogenesis of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) results from a CAG repeat expansion within the androgen receptor gene (AR). It is unclear why motor neurons selectively degenerate and there are currently no treatments for this debilitating disease. To uncover the causative genes and pathways involved in motor neuron dysfunction, we undertook transcriptomic profiling of primary embryonic motor neurons from SBMA mice. We show that transcriptional dysregulation occurs early during development in SBMA motor neurons. One gene found to be dysregulated, Chmp7, was also altered in vivo in spinal cord before symptom onset in SBMA mice, and crucially in motor neuron precursor cells derived from SBMA patient stem cells, suggesting that Chmp7 may play a causal role in disease pathogenesis by disrupting the endosome-lysosome system. Furthermore, genes were enriched in SBMA motor neurons in several key pathways including p53, DNA repair, WNT and mitochondrial function. SBMA embryonic motor neurons also displayed dysfunctional mitochondria along with DNA damage, possibly resulting from DNA repair gene dysregulation and/or mitochondrial dysfunction. This indicates that a coordinated dysregulation of multiple pathways leads to development of SBMA. Importantly, our findings suggest that the identified pathways and genes, in particular Chmp7, may serve as potential therapeutic targets in SBMA.

Selective modulation of the androgen receptor AF2 domain rescues degeneration in spinal bulbar muscular atrophy

Spinal bulbar muscular atrophy (SBMA) is a motor neuron disease caused by toxic gain of function of the androgen receptor (AR). Previously, we found that co-regulator binding through the activation function-2 (AF2) domain of AR is essential for pathogenesis, suggesting that AF2 may be a potential drug target for selective modulation of toxic AR activity. We screened previously identified AF2 modulators for their ability to rescue toxicity in a Drosophila model of SBMA. We identified two compounds, tolfenamic acid (TA) and 1-[2-(4-methylphenoxy)ethyl]-2-[(2-phenoxyethyl)sulfanyl]-1H-benzimidazole (MEPB), as top candidates for rescuing lethality, locomotor function and neuromuscular junction defects in SBMA flies. Pharmacokinetic analyses in mice revealed a more favorable bioavailability and tissue retention of MEPB compared with TA in muscle, brain and spinal cord. In a preclinical trial in a new mouse model of SBMA, MEPB treatment yielded a dose-dependent rescue from loss of body weight, rotarod activity and grip strength. In addition, MEPB ameliorated neuronal loss, neurogenic atrophy and testicular atrophy, validating AF2 modulation as a potent androgen-sparing strategy for SBMA therapy.

Androgenic mechanisms of sexual differentiation of the nervous system and behavior

Testicular androgens are the major endocrine factor promoting masculine phenotypes in vertebrates, but androgen signaling is complex and operates via multiple signaling pathways and sites of action. Recently, selective androgen receptor mutants have been engineered to study androgenic mechanisms of sexual differentiation of the nervous system and behavior. The focus of these studies has been to evaluate androgenic mechanisms within the nervous system by manipulating androgen receptor conditionally in neural tissues. Here we review both the effects of neural loss of AR function as well as the effects of neural overexpression of AR in relation to global AR mutants. Although some studies have conformed to our expectations, others have proved challenging to assumptions underlying the dominant hypotheses. Notably, these studies have called into question both the primacy of direct, neural mechanisms and also the linearity of the relationship between androgenic dose and sexual differentiation of brain and behavior.

MiR-298 Counteracts Mutant Androgen Receptor Toxicity in Spinal and Bulbar Muscular Atrophy

Spinal and bulbar muscular atrophy (SBMA) is a currently untreatable adult-onset neuromuscular disease caused by expansion of a polyglutamine repeat in the androgen receptor (AR). In SBMA, as in other polyglutamine diseases, a toxic gain of function in the mutant protein is an important factor in the disease mechanism; therefore, reducing the mutant protein holds promise as an effective treatment strategy. In this work, we evaluated a microRNA (miRNA) to reduce AR expression. From a list of predicted miRNAs that target human AR, we selected microRNA-298 (miR-298) for its ability to downregulate AR mRNA and protein levels when transfected in cells overexpressing wild-type and mutant AR and in SBMA patient-derived fibroblasts. We showed that miR-298 directly binds to the 3'-untranslated region of the human AR transcript, and counteracts AR toxicity in vitro. Intravenous delivery of miR-298 with adeno-associated virus serotype 9 vector resulted in efficient transduction of muscle and spinal cord and amelioration of the disease phenotype in SBMA mice. Our findings support the development of miRNAs as a therapeutic strategy for SBMA and other neurodegenerative disorders caused by toxic proteins.

In Vitro and In Vivo Modeling of Spinal and Bulbar Muscular Atrophy

Spinal and bulbar muscular atrophy (SBMA) is an X-linked neuromuscular disease characterized by late-onset, progressive degeneration of lower motor neurons and skeletal muscle atrophy. SBMA is caused by expansions of a CAG trinucleotide repeat in the gene encoding the androgen receptor (AR). One striking feature of SBMA is sex specificity: SBMA fully manifests only in males, whereas females show subclinical or mild disease manifestations even when homozygous for the mutation. Since the identification of the mutation responsible for SBMA in 1991, several cell and animal models have been developed to recapitulate the main features of disease in vitro and in vivo. In this review, we describe the most widely used cellular and animal models of SBMA, highlighting advantages and disadvantages in the use of these models to gain mechanistic and therapeutic insights into SBMA.

Protein arginine methyltransferase 6 enhances polyglutamine-expanded androgen receptor function and toxicity in spinal and bulbar muscular atrophy

Polyglutamine expansion in androgen receptor (AR) is responsible for spinobulbar muscular atrophy (SBMA) that leads to selective loss of lower motor neurons. Using SBMA as a model, we explored the relationship between protein structure/function and neurodegeneration in polyglutamine diseases. We show here that protein arginine methyltransferase 6 (PRMT6) is a specific co-activator of normal and mutant AR and that the interaction of PRMT6 with AR is significantly enhanced in the AR mutant. AR and PRMT6 interaction occurs through the PRMT6 steroid receptor interaction motif, LXXLL, and the AR activating function 2 surface. AR transactivation requires PRMT6 catalytic activity and involves methylation of arginine residues at Akt consensus site motifs, which is mutually exclusive with serine phosphorylation by Akt. The enhanced interaction of PRMT6 and mutant AR leads to neurodegeneration in cell and fly models of SBMA. These findings demonstrate a direct role of arginine methylation in polyglutamine disease pathogenesis.

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PRMT6 is a coactivator of AR whose function is enhanced by polyglutamine expansion

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PRMT6 methylates the AR at the two Akt consensus site motifs RXRXXS

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AR arginine methylation by PRMT6 and phosphorylation by Akt are mutually exclusive

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PRMT6 enhances mutant AR toxicity in spinobulbar muscular atrophy cells and flies

Proceedings of the fourth international conference on central hypoventilation

Central hypoventilation syndromes (CHS) are rare diseases of central autonomic respiratory control associated with autonomous nervous dysfunction. Severe central hypoventilation is the hallmark and the most life-threatening feature. CHS is a group of not-fully defined disorders. Congenital CHS (CCHS) (ORPHA661) is clinically and genetically well-characterized, with the disease-causing gene identified in 2003. CCHS presents at birth in most cases, and associated with Hirschsprung's disease (ORPHA99803) and neural crest tumours in 20% and 5% of cases, respectively. The incidence of CCHS is estimated to be 1 of 200,000 live births in France, yet remains unknown for the rest of the world. In contrast, late-onset CHS includes a group of not yet fully delineated diseases. Overlap with CCHS is likely, as a subset of patients harbours PHOX2B mutations. Another subset of patients present with associated hypothalamic dysfunction. The number of these patients is unknown (less than 60 cases reported worldwide). Treatment of CHS is palliative using advanced techniques of ventilation support during lifetime. Research is ongoing to better understand physiopathological mechanisms and identify potential treatment pathways.The Fourth International Conference on Central Hypoventilation was organised in Warsaw, Poland, April 13-15, 2012, under the patronage of the European Agency for Health and Consumers and Public Health European Agency of European Community. The conference provided a state-of-the-art update of knowledge on all the genetic, molecular, cellular, and clinical aspects of these rare diseases.

Human adipose-derived mesenchymal stem cells as a new model of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) or Kennedy's disease is an X-linked CAG/polyglutamine expansion motoneuron disease, in which an elongated polyglutamine tract (polyQ) in the N-terminal androgen receptor (ARpolyQ) confers toxicity to this protein. Typical markers of SBMA disease are ARpolyQ intranuclear inclusions. These are generated after the ARpolyQ binds to its endogenous ligands, which promotes AR release from chaperones, activation and nuclear translocation, but also cell toxicity. The SBMA mouse models developed so far, and used in preclinical studies, all contain an expanded CAG repeat significantly longer than that of SBMA patients. Here, we propose the use of SBMA patients adipose-derived mesenchymal stem cells (MSCs) as a new human in vitro model to study ARpolyQ toxicity. These cells have the advantage to express only ARpolyQ, and not the wild type AR allele. Therefore, we isolated and characterized adipose-derived MSCs from three SBMA patients (ADSC from Kennedy's patients, ADSCK) and three control volunteers (ADSCs). We found that both ADSCs and ADSCKs express mesenchymal antigens, even if only ADSCs can differentiate into the three typical cell lineages (adipocytes, chondrocytes and osteocytes), whereas ADSCKs, from SBMA patients, showed a lower growth potential and differentiated only into adipocyte. Moreover, analysing AR expression on our mesenchymal cultures we found lower levels in all ADSCKs than ADSCs, possibly related to negative pressures exerted by toxic ARpolyQ in ADSCKs. In addition, with proteasome inhibition the ARpolyQ levels increased specifically in ADSCKs, inducing the formation of HSP70 and ubiquitin positive nuclear ARpolyQ inclusions. Considering all of this evidence, SBMA patients adipose-derived MSCs cultures should be considered an innovative in vitro human model to understand the molecular mechanisms of ARpolyQ toxicity and to test novel therapeutic approaches in SBMA.

Polyglutamine-expanded androgen receptor interferes with TFEB to elicit autophagy defects in SBMA

Macroautophagy (hereafter autophagy) is a key pathway in neurodegeneration. Despite protective actions, autophagy may contribute to neuron demise when dysregulated. Here we consider X-linked spinal and bulbar muscular atrophy (SBMA), a repeat disorder caused by polyglutamine-expanded androgen receptor (polyQ-AR). We found that polyQ-AR reduced long-term protein turnover and impaired autophagic flux in motor neuron-like cells. Ultrastructural analysis of SBMA mice revealed a block in autophagy pathway progression. We examined the transcriptional regulation of autophagy and observed a functionally significant physical interaction between transcription factor EB (TFEB) and AR. Normal AR promoted, but polyQ-AR interfered with, TFEB transactivation. To evaluate physiological relevance, we reprogrammed patient fibroblasts to induced pluripotent stem cells and then to neuronal precursor cells (NPCs). We compared multiple SBMA NPC lines and documented the metabolic and autophagic flux defects that could be rescued by TFEB. Our results indicate that polyQ-AR diminishes TFEB function to impair autophagy and promote SBMA pathogenesis.

Stem cell-derived motor neurons from spinal and bulbar muscular atrophy patients

Spinal and bulbar muscular atrophy (SBMA, Kennedy's disease) is a motor neuron disease caused by polyglutamine repeat expansion in the androgen receptor. Although degeneration occurs in the spinal cord and muscle, the exact mechanism is not clear. Induced pluripotent stem cells from spinal and bulbar muscular atrophy patients provide a useful model for understanding the disease mechanism and designing effective therapy. Stem cells were generated from six patients and compared to control lines from three healthy individuals. Motor neurons from four patients were differentiated from stem cells and characterized to understand disease-relevant phenotypes. Stem cells created from patient fibroblasts express less androgen receptor than control cells, but show androgen-dependent stabilization and nuclear translocation. The expanded repeat in several stem cell clones was unstable, with either expansion or contraction. Patient stem cell clones produced a similar number of motor neurons compared to controls, with or without androgen treatment. The stem cell-derived motor neurons had immunoreactivity for HB9, Isl1, ChAT, and SMI-32, and those with the largest repeat expansions were found to have increased acetylated α-tubulin and reduced HDAC6. Reduced HDAC6 was also found in motor neuron cultures from two other patients with shorter repeats. Evaluation of stably transfected mouse cells and SBMA spinal cord showed similar changes in acetylated α-tubulin and HDAC6. Perinuclear lysosomal enrichment, an HDAC6 dependent process, was disrupted in motor neurons from two patients with the longest repeats. SBMA stem cells present new insights into the disease, and the observations of reduced androgen receptor levels, repeat instability, and reduced HDAC6 provide avenues for further investigation of the disease mechanism and development of effective therapy.

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.

B2 attenuates polyglutamine-expanded androgen receptor toxicity in cell and fly models of spinal and bulbar muscular atrophy

Expanded polyglutamine tracts cause neurodegeneration through a toxic gain-of-function mechanism. Generation of inclusions is a common feature of polyglutamine diseases and other protein misfolding disorders. Inclusion formation is likely to be a defensive response of the cell to the presence of unfolded protein. Recently, the compound B2 has been shown to increase inclusion formation and decrease toxicity of polyglutamine-expanded huntingtin in cultured cells. We explored the effect of B2 on spinal and bulbar muscular atrophy (SBMA). SBMA is caused by expansion of polyglutamine in the androgen receptor (AR) and is characterized by the loss of motor neurons in the brainstem and spinal cord. We found that B2 increases the deposition of mutant AR into nuclear inclusions, without altering the ligand-induced aggregation, expression, or subcellular distribution of the mutant protein. The effect of B2 on inclusions was associated with a decrease in AR transactivation function. We show that B2 reduces mutant AR toxicity in cell and fly models of SBMA, further supporting the idea that accumulation of polyglutamine-expanded protein into inclusions is protective. Our findings suggest B2 as a novel approach to therapy for SBMA.

N-terminal polyglutamine-containing fragments inhibit androgen receptor transactivation function

Several neurodegenerative diseases, including Kennedy's disease (KD), are associated with misfolding and aggregation of polyglutamine (polyQ)-expansion proteins. KD is caused by a polyQ-expansion in the androgen receptor (AR), a key player in male sexual differentiation. Interestingly, KD patients often show signs of mild-to-moderate androgen insensitivity syndrome (AIS) resulting from AR dysfunction. Here, we used the yeastSaccharomyces cerevisiaeto investigate the molecular mechanism behind AIS in KD. Upon expression in yeast, polyQ-expanded N-terminal fragments of AR lacking the hormone binding domain caused a polyQ length-dependent growth defect. Interestingly, while AR fragments with 67 Q formed large, SDS-resistant inclusions, the most pronounced toxicity was observed upon expression of 102 Q fragments which accumulated exclusively as soluble oligomers in the 100–600 kDa range. Analysis using a hormone-dependent luciferase reporter revealed that full-length polyQ-expanded AR is fully functional in transactivation, but becomes inactivated in the presence of the corresponding polyQ-expanded N-terminal fragment. Furthermore, the greatest impairment of AR activity was observed upon interaction of full-length AR with soluble AR fragments. Taken together, our results suggest that soluble polyQ-containing fragments bind to full-length AR and inactivate it, thus providing insight into the mechanism behind AIS in KD and possibly other polyglutamine diseases, such as Huntington's disease.

Mitochondrial abnormalities in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a motor neuron disease caused by polyglutamine expansion mutation in the androgen receptor (AR). We investigated whether the mutant protein alters mitochondrial function. We found that constitutive and doxycycline-induced expression of the mutant AR in MN-1 and PC12 cells, respectively, are associated with depolarization of the mitochondrial membrane. This was mitigated by cyclosporine A, which inhibits opening of the mitochondrial permeability transition pore. We also found that the expression of the mutant protein in the presence of ligand results in an elevated level of reactive oxygen species, which is blocked by the treatment with the antioxidants co-enzyme Q10 and idebenone. The mutant protein in MN-1 cells also resulted in increased Bax, caspase 9 and caspase 3. We assessed the effects of mutant AR on the transcription of mitochondrial proteins and found altered expression of the peroxisome proliferator-activated receptor γ coactivator 1 and the mitochondrial specific antioxidant superoxide dismutase-2 in affected tissues of SBMA knock-in mice. In addition, we found that the AR associates with mitochondria in cultured cells. This study thus provides evidence for mitochondrial dysfunction in SBMA cell and animal models, either through indirect effects on the transcription of nuclear-encoded mitochondrial genes or through direct effects of the mutant protein on mitochondria or both. These findings indicate possible benefit from mitochondrial therapy for SBMA.

A human spinal cord cell promotes motoneuron survival and maturation in vitro

Primary cultures of motoneurons represent a good experimental model for studying mechanisms underlying certain spinal cord pathologies, such as amyotrophic lateral sclerosis and spinal bulbar muscular atrophy (Kennedy's disease). However, a major problem with such culture systems is the relatively short cell survival times, which limits the extent of motoneuronal maturation. In spite of supplementing culture media with various growth factors, it remains difficult to maintain motoneurons viable longer than 10 days in vitro. This study employs a new approach, in which rat motoneurons are plated on a layer of cultured cells derived from newborn human spinal cord. For all culture periods, more motoneurons remain viable in such cocultures compared with control monocultures. Moreover, although no motoneurons survive in control cultures after 22 days, viable motoneurons were observed in cocultures even after 7 weeks. Although no significant difference in neurite length was observed between 8-day mono- and cocultures, after 22 and 50 days in coculture motoneurons had a very mature morphology. They extended extremely robust, very long neurites, which formed impressive branched networks. Data obtained using a system in which the spinal cord cultures were separated from motoneurons by a porous polycarbonate filter suggest that soluble factors released from the supporting cells are in part responsible for the beneficial effects on motoneurons. Several approaches, including immunocytochemistry, immunoblotting, and electron microscopy, indicated that these supporting cells, capable of extending motoneuron survival and enhancing neurite growth, had an undifferentiated or poorly differentiated, possibly mesenchymal phenotype.

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.

Cellular localization of androgen and estrogen receptors in mouse-derived motoneuron hybrid cells and mouse facial motoneurons

The ability of gonadal steroid hormones to augment axonal regeneration after peripheral nerve injury has been well established in rat and hamster motoneuron systems, and provides a foundation for the use of these agents as neurotherapeutics. With the advent of mouse genetics and the availability of transgenic and knockout mice, the use of mice in studies of neuroprotection is growing. It has recently been demonstrated that both androgens and estrogens rescue motoneurons (MN) from injury in mouse-derived motoneuron hybrid cells in vitro and mouse facial motoneurons (FMN) in vivo (Tetzlaff et al. [2006] J Mol Neurosci 28:53-64). To elucidate the molecular mechanisms of these effects, the present study examined the cellular localization of androgen and estrogen receptors in mouse MN in vitro and in vivo. Immunoblotting and immunocytochemistry studies established the presence of androgen receptor (AR) and estrogen receptor alpha/beta in immortalized mouse motoneuron hybrid cells and AR and estrogen receptor alpha in mouse FMN.

Proteolytic cleavage of ataxin-7 by caspase-7 modulates cellular toxicity and transcriptional dysregulation

Spinocerebellar ataxia type 7 (SCA7) is a polyglutamine (polyQ) disorder characterized by specific degeneration of cerebellar, brainstem, and retinal neurons. Although they share little sequence homology, proteins implicated in polyQ disorders have common properties beyond their characteristic polyQ tract. These include the production of proteolytic fragments, nuclear accumulation, and processing by caspases. Here we report that ataxin-7 is cleaved by caspase-7, and we map two putative caspase-7 cleavage sites to Asp residues at positions 266 and 344 of the ataxin-7 protein. Site-directed mutagenesis of these two caspase-7 cleavage sites in the polyQ-expanded form of ataxin-7 produces an ataxin-7 D266N/D344N protein that is resistant to caspase cleavage. Although ataxin-7 displays toxicity, forms nuclear aggregates, and represses transcription in human embryonic kidney 293T cells in a polyQ length-dependent manner, expression of the non-cleavable D266N/D344N form of polyQ-expanded ataxin-7 attenuated cell death, aggregate formation, and transcriptional interference. Expression of the caspase-7 truncation product of ataxin-7-69Q or -92Q, which removes the putative nuclear export signal and nuclear localization signals of ataxin-7, showed increased cellular toxicity. We also detected N-terminal polyQ-expanded ataxin-7 cleavage products in SCA7 transgenic mice similar in size to those generated by caspase-7 cleavage. In a SCA7 transgenic mouse model, recruitment of caspase-7 into the nucleus by polyQ-expanded ataxin-7 correlated with its activation. Our results, thus, suggest that proteolytic processing of ataxin-7 by caspase-7 may contribute to SCA7 disease pathogenesis.

Akt blocks ligand binding and protects against expanded polyglutamine androgen receptor toxicity

Spinal and bulbar muscular atrophy (SBMA) is a progressive neurodegenerative disease caused by an expansion of the polyglutamine tract in the androgen receptor (AR). Here, we investigated the regulation of AR phosphorylation in order to understand factors that may modify SBMA disease progression. We show that expanded polyglutamine AR is phosphorylated by Akt. Substitution of the AR at two Akt consensus sites, S215 and S792, with aspartate, which mimics phosphorylation, reduces ligand binding, ligand-dependent nuclear translocation, transcriptional activation and toxicity of expanded polyglutamine AR. Co-expression of constitutively active Akt and the AR has similar consequences, which are blocked by alanine substitutions at residues 215 and 792. Furthermore, in motor neuron-derived MN-1 cells toxicity associated with polyglutamine-expanded AR is rescued by co-expression with Akt. Insulin-like growth factor-1 (IGF-1) stimulation, which activates several cell survival promoting pathways, also reduces toxicity of the expanded polyglutamine AR in MN-1 cells, in a manner dependent upon phospho-inositol-3-kinase. IGF-1 rescue of AR toxicity is diminished by alanine substitutions at the Akt consensus sites. These results highlight potential targets for therapeutic intervention in SBMA.

Loss of endogenous androgen receptor protein accelerates motor neuron degeneration and accentuates androgen insensitivity in a mouse model of X-linked spinal and bulbar muscular atrophy

X-linked spinal and bulbar muscular atrophy (SBMA; Kennedy's disease) is a polyglutamine (polyQ) disease in which the affected males suffer progressive motor neuron degeneration accompanied by signs of androgen insensitivity, such as gynecomastia and reduced fertility. SBMA is caused by CAG repeat expansions in the androgen receptor (AR) gene resulting in the production of AR protein with an extended glutamine tract. SBMA is one of nine polyQ diseases in which polyQ expansion is believed to impart a toxic gain-of-function effect upon the mutant protein, and initiate a cascade of events that culminate in neurodegeneration. However, whether loss of a disease protein's normal function concomitantly contributes to the neurodegeneration remains unanswered. To address this, we examined the role of normal AR function in SBMA by crossing a highly representative AR YAC transgenic mouse model with 100 glutamines (AR100) and a corresponding control (AR20) onto an AR null (testicular feminization; Tfm) background. Absence of endogenous AR protein in AR100Tfm mice had profound effects upon neuromuscular and endocrine-reproductive features of this SBMA mouse model, as AR100Tfm mice displayed accelerated neurodegeneration and severe androgen insensitivity in comparison to AR100 littermates. Reduction in size and number of androgen-sensitive motor neurons in the spinal cord of AR100Tfm mice underscored the importance of AR action for neuronal health and survival. Promoter-reporter assays confirmed that AR transactivation competence diminishes in a polyQ length-dependent fashion. Our studies indicate that SBMA disease pathogenesis, both in the nervous system and the periphery, involves two simultaneous pathways: gain-of-function misfolded protein toxicity and loss of normal protein 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.

Molecular cloning of Clock cDNA from the prawn, Macrobrachium rosenbergii

CLOCK, which belongs to the basic helix-loop-helix (bHLH)/PER-ARNT-SIM (PAS) superfamily of transcription factors, is one of the most essential proteins involved in circadian systems of animals. Clock genes have been cloned from several species, including mammals, insects, birds, fish, and amphibians. In the present study, we successfully isolated a Clock homolog (termed Mar-Clock) from the giant prawn, Macrobrachium rosenbergii. The 2949-bp cDNA contained a 2115 bp open reading frame that encoded a putative CLOCK protein of 704 amino acids (termed Mar-CLOCK) exhibiting high identities with CLOCK homologs in other species (30-35%). This is the first report of a circadian clock gene from crustaceans. Mar-CLOCK possessed an exceptionally long glutamine-rich domain (140 amino acids) in its C-terminus, which usually ranges from 14 to 57 amino acids in other known CLOCKs and is supposed to function in transcriptional activation. Using RT-PCR, we observed that Mar-Clock was expressed in all tested tissues. Semiquantitative RT-PCR was performed to investigate the gene expression profile during the light-dark cycle. The results indicated that the expression of the Mar-Clock gene had no significant rhythmicity in central nervous tissues (thoracic ganglia and eyestalk) or peripheral tissues (gill, ovary, hepatopancreas, and muscle). Furthermore, gene expression tended to increase in the central nervous system (brain, thoracic, and abdominal ganglia) of eyestalk-ablated or constant dark (DD) prawns, and in the eyestalk-ablated gill. No expression change was found under constant light (LL) or in heart and muscle.

Androgen-induced neurite outgrowth is mediated by neuritin in motor neurones

In the brain, the spinal cord motor neurones express the highest levels of the androgen receptor (AR). Experimental data have suggested that neurite outgrowth in these neurones may be regulated by testosterone or its derivative 5alpha-dihydrotestosterone (DHT), formed by the 5alpha-reductase type 2 enzyme. In this study we have produced and characterized a model of immortalized motor neuronal cells expressing the mouse AR (mAR) [neuroblastoma-spinal cord (NSC) 34/mAR] and analysed the role of androgens in motor neurones. Androgens either activated or repressed several genes; one has been identified as the mouse neuritin, a protein responsible for neurite elongation. Real-time PCR analysis has shown that the neuritin gene is expressed in the basal condition in immortalized motor neurones and is selectively up-regulated by androgens in NSC34/mAR cells; the DHT effect is counteracted by the anti-androgen Casodex. Moreover, DHT induced neurite outgrowth in NSC34/mAR, while testosterone was less effective and its action was counteracted by the 5alpha-reductase type 2 enzyme inhibitor finasteride. Finally, the androgenic effect on neurite outgrowth was abolished by silencing neuritin with siRNA. Therefore, the trophic effects of androgens in motor neurones may be explained by the androgenic regulation of neuritin, a protein linked to neurone development, elongation and regeneration.

Effect of CAT or AGG Interruptions and CpG Methylation on Nucleosome Assembly upon Trinucleotide Repeats on Spinocerebellar Ataxia, Type 1 and Fragile X Syndrome*

Nucleosome packaging regulates many aspects of DNA metabolism and is thought to mediate genetic instability and transcription of expanded trinucleotide repeats. Both instability and transcription are sensitive to repeat length, tract purity, and CpG methylation. CAT or AGG interruptions within the (CAG)n or (CGG)n tracts of spinocerebellar ataxia, type 1 or fragile X syndrome, respectively, confer increased genetic stability to the repeats. We report the formation of nucleosomes on sequences containing pure and interrupted (CAG)n and (CGG)n repeats having lengths above and below the genetic stability thresholds. Increased lengths of pure repeats led to increased and decreased propensities for nucleosome assembly on the (CAG)n and (CGG)n repeats, respectively. CpG methylation of the CGG repeat further reduced assembly. CAT interruptions in (CAG)n tracts decreased nucleosome assembly. In contrast, AGG interruptions in (CGG)n tracts did not affect assembly by hypoacetylated histones. The latter observation was unaltered by CpG methylation of the repeats. However, nucleosome assembly by hyperacetylated histones on interrupted CGG tracts was increased relative to pure tracts and this effect was abolished by CpG methylation. Thus, CAT or AGG interruptions can modulate the ability of (CAG)n and (CGG) tracts to assemble into chromatin and the effect of the AGG interruptions is dependent upon both the methylation status of the DNA and the acetylation status of the histones. Compared with the genetically unstable pure repeats, both interruptions permit a propensity of nucleosome assembly closer to that of random (genetically stable) sequences, suggesting an association of nucleosome assembly of trinucleotide repeats and genetic instability.

Transgenic mouse models of spinal and bulbar muscular atrophy (SBMA)

Spinal and bulbar muscular atrophy (SBMA) is a late-onset motor neuron disease characterized by proximal muscle atrophy, weakness, contraction fasciculations, and bulbar involvement. Only males develop symptoms, while female carriers usually are asymptomatic. A specific treatment for SBMA has not been established. The molecular basis of SBMA is the expansion of a trinucleotide CAG repeat, which encodes the polyglutamine (polyQ) tract, in the first exon of the androgen receptor (AR) gene. The pathologic hallmark is nuclear inclusions (NIs) containing the mutant and truncated AR with expanded polyQ in the residual motor neurons in the brainstem and spinal cord as well as in some other visceral organs. Several transgenic (Tg) mouse models have been created for studying the pathogenesis of SBMA. The Tg mouse model carrying pure 239 CAGs under human AR promoter and another model carrying truncated AR with expanded CAGs show motor impairment and nuclear NIs in spinal motor neurons. Interestingly, Tg mice carrying full-length human AR with expanded polyQ demonstrate progressive motor impairment and neurogenic pathology as well as sexual difference of phenotypes. These models recapitulate the phenotypic expression observed in SBMA. The ligand-dependent nuclear localization of the mutant AR is found to be involved in the disease mechanism, and hormonal therapy is suggested to be a therapeutic approach applicable to SBMA.

Effect of a short CAG (glutamine) repeat on human androgen receptor function

BACKGROUND

The human androgen receptor (AR) gene contains an uninterrupted CAG repeat that is polymorphic in length in the general population (range, 11-31 CAG's; median, 21). The CAG repeat encodes a glutamine repeat in the N-terminal transactivation domain of the AR protein. We previously reported that a 17-CAG AR gene was much more common in a cohort of men with prostate cancer (8.5%) than in the general European American population (1.3%). This suggested that a 17-CAG repeat may have pathophysiological consequences. The goal of the present study was to directly test the hypothesis that a 17-CAG repeat might uniquely affect androgen action in human prostate cancer cells.

METHODS

DU145 cells, lacking endogenous AR, were transiently transfected with an AR expression plasmid (with a CAG repeat ranging in length from 14 to 25) and an androgen-responsive reporter plasmid (PSA-luciferase).

RESULTS

We found a significant effect of CAG repeat length on AR protein levels per unit amount of DNA transfected (one-way ANOVA, P = 0.02), indicating the need to express transactivation data per unit amount of AR protein. CAG17 AR had 40% more transactivation activity per unit amount of AR protein than CAG21 AR (P < 0.01).

CONCLUSIONS

Thus, an AR with a 17-CAG repeat may mediate more efficacious growth stimulation of androgen-dependent prostate epithelial cells, and thereby increase the risk that prostate cancer cells develop more efficiently into a clinically significant cancer.

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.

Role of proteolysis in polyglutamine disorders

To date, nine polyglutamine disorders have been characterised, including Huntington's disease (HD), spinobulbar muscular atrophy (SBMA), dentatorubral-pallidoluysian atrophy (DRPLA), and spinocerebellar ataxias 1, 2, 3, 6, 7 and 17 (SCAs). Although knockout and transgenic mouse experiments suggest that a toxic gain of function is central to neuronal death in these diseases (with the probable exception of SCA6), the exact mechanisms of neurotoxicity remain contentious. A further conundrum is the characteristic distribution of neuronal damage in each disease, despite ubiquitous expression of the abnormal proteins. One mechanism that could possibly underlie the specific distribution of neuronal toxicity is proteolytic cleavage of the full-length expanded polyglutamine tract-containing proteins. There is evidence found in vitro or in vivo (or both) of proteolytic cleavage in HD, SBMA, DRPLA, and SCAs 2, 3, and 7. In HD, cleavage has been demonstrated to be regionally specific, occurring as a result of caspase activation. These diseases are also characterised by development of intraneuronal aggregates of the abnormal protein that co-localise with components of the ubiquitin-proteasome pathway. It remains unclear whether these aggregates are pathogenic or merely disease markers; however, at least in the case of ataxin-3, cleavage promotes aggregation. Inhibition of specific proteases constitutes a potential therapeutic approach in these diseases.

Androgen receptors containing expanded polyglutamine tracts exhibit progressive toxicity when stably expressed in the neuroblastoma cell line, SH-SY 5Y

The pathogenesis of X-linked spinal and bulbar muscular atrophy (SBMA) has been traced to an expansion of repeated glutamine (Gln) residues within the amino terminus of the human androgen receptor (AR). To examine the mechanisms by which these expanded repeat ARs (Exp-ARs) are toxic to neurons, we have established and characterized a cell culture model by stably transfecting SH-SY 5Y neuroblastoma cells with cDNAs containing either normal AR (81 series; 23 Glns) or Exp-AR (902 series; 56 Glns). At a low passage number, no differences in cell morphology, growth properties, or susceptibility to toxic insults were observed between clones expressing normal AR or Exp-AR. Initially, both types of cultures were found to express similar levels of specific hormone binding in monolayer binding assays. Immunohistochemical studies demonstrated the vast majority of both the normal AR and Exp-AR were localized to the nucleus in the absence and presence of androgen. As the 902 series of clones were propagated, the Exp-AR content in the cells appeared to decline progressively. However, this decrease actually reflects a gradual disappearance of the Exp-AR cell population. No such selection occurred during the propagation of cells expressing the normal AR. This selection against cells expressing physiological levels of Exp-AR occurs in the absence of intracellular aggregates and suggests that mechanisms other than those involving the formation of aggregates underlie the observed toxicity of Exp-ARs.

Androgen 5-alpha-reductase type 2 is highly expressed and active in rat spinal cord motor neurones

Spinal cord motoneurones express high levels of androgen receptor. However, in responsive tissue, the effects of testosterone is often mediated by the more potent androgenic derivative 5-alpha-dihydrotestosterone (DHT). This compound is formed in androgen target cells by the enzyme 5-alpha-reductase. Two isoforms of the 5-alpha-reductase, with limited degree of homology, have been cloned, type 1 and type 2. The low affinity-constitutive type 1 isoenzyme is widely distributed in the body; the high affinity-androgen regulated 5-alpha-reductase type 2 is confined to androgen-dependent structures and shows a peculiar pH optimum at acidic values. We have previously shown that high levels of 5-alpha-reductase activity are detectable in rat spinal cord. Here, using reverse transcriptase-polymerase chain reaction, we show that both isoforms are expressed in the whole spinal cord of the rat. The enzymatic pH optimum measured in immortalized spinal cord motoneurones (NSC34) is typical of the type 2 isoenzyme. Using in situ hybridization technique, we found that 5-alpha-reductase type 2 is confined to the motoneuronal cells of the anterior horns of the rat spinal cord, the cells that also are known to express high levels of androgen receptor. Because of the close association of androgen receptor and 5-alpha alpha-reductase type 2, motoneuronal cells should be considered as target cells for androgens.

Numerous microRNPs in neuronal cells containing novel microRNAs

Spinal muscular atrophy (SMA) is a common neurodegenerative disease that is caused by deletions or loss-of-function mutations in the Survival of Motor Neuron (SMN) protein. SMN is part of a large complex that functions in the assembly/restructuring of ribonucleoprotein (RNP) complexes. We recently showed in HeLa cells that two components of the SMN complex, Gemin3 and Gemin4, together with the argonaute protein eIF2C2, also associate with microRNAs (miRNAs) as part of a novel class of RNPs termed miRNPs. Here we report on miRNPs isolated from neuronal cell lines of mouse and human, and describe 53 novel miRNAs. Several of these miRNAs are conserved in divergent organisms, including rat, zebrafish, pufferfish, and the nematode Caenorhabditis elegans. The chromosomal locations of most of the novel miRNAs were identified and indicate some phylogenetic conservation of the likely precursor structures. Interestingly the gene locus of one miRNA, miR-175, is a candidate region for two neurologic diseases: early-onset parkinsonism (Waisman syndrome) and X-linked mental retardation (MRX3). Also, several miRNAs identified as part of miRNPs in these cells appear to constitute two distinct subfamilies. These subfamilies comprise multiple copies of miRNAs on different chromosomes, suggesting an important function in the regulation of gene expression.

Trinucleotide repeat disease. The androgen receptor in spinal and bulbar muscular atrophy

It has been more than 10 years since the discovery that the expansion of a simple CAG trinucleotide repeat within the coding region of the androgen receptor gene leads to the motor neuronopathy spinal and bulbar muscular atrophy (SBMA). A flurry of investigation into this and the other, more recently discovered, polyglutamine diseases has led to an understanding of many aspects of the molecular pathogenesis of this family of diseases. A characteristics pathological feature of the polyglutamine diseases is the occurrence in affected neurons of ubiquitinated aggregates; such aggregates also contain, among others, proteins involved in the folding and degradation of the mutant proteins. Aggregates themselves are likely not directly cytotoxic, but rather mark the accumulation of all or part of the mutant protein. Furthermore, aggregation occurs because of the inefficient clearance of the mutant protein by the ubiquitin-proteasome pathway for protein degradation. These findings are common to the polyglutamine diseases and reflect the general problem of folding/degrading expanded polyglutamines. In SBMA, the altered metabolism of the androgen receptor is ligand dependent. How the accumulation of the mutant protein causes neuronal dysfunction and disease is not well understood, but several cellular processes have been implicated. Although these findings provide insight into the toxic function of the expanded polyglutamine protein, additional investigations have led to the finding that intrinsic AR transactivational function is somewhat diminished in the presence of the expanded polyglutamine; this likely leads to the partial androgen insensitivity that characterizes patients with SBMA. The recent development of useful animal and cell models of SBMA will lead to increased understanding of disease pathogenesis, as well as to the development of new and better therapeutic strategies.

Ligand promotes intranuclear inclusions in a novel cell model of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA, Kennedy's disease) is one of a group of progressive neurodegenerative diseases resulting from a polyglutamine repeat expansion. In SBMA the polymorphic trinucleotide CAG repeat in exon 1 of the androgen receptor (AR) gene is increased, resulting in expansion of a polyglutamine tract. Patient autopsy material reveals neuronal intranuclear inclusions (NII) in affected regions that contain only amino-terminal epitopes of the AR. Cell models have previously been unable to produce intranuclear inclusions containing only a portion of the AR. We report here the creation of an inducible cell model of SBMA that reproduces this important characteristic of disease pathology. PC12 cells expressing highly expanded AR form ubiquitinated intranuclear inclusions containing amino-terminal epitopes of the AR as well as heat shock proteins. Inclusions appear as distinct granular electron-dense structures in the nucleus by immunoelectron microscopy. Dihydrotestosterone treatment of mutant AR-expressing cells results in increased inclusion load. This model mimics the formation of ubiquitinated intranuclear inclusions containing the amino-terminal portion of AR observed in patient tissue and reveals a role for ligand in the pathogenesis of SBMA.

Androgen receptor with elongated polyglutamine tract forms aggregates that alter axonal trafficking and mitochondrial distribution in motor neuronal processes

The CAG/polyglutamine (polyGln)-related diseases include nine different members that together form the most common class of inherited neurodegenerative disorders; neurodegeneration is linked to the same type of mutation, found in unrelated genes, consisting of an abnormal expansion of a polyGln tract normally present in the wild-type proteins. Nuclear, cytoplasmic, or neuropil aggregates are detectable in CAG/polyGln-related diseases, but their role is still debated. Alteration of the androgen receptor (AR), one of these proteins, has been linked to spinal and bulbar muscular atrophy, an X-linked recessive disease characterized by motoneuronal death. By using immortalized motoneuronal cells (the neuroblastoma-spinal cord cell line NSC34), we analyzed neuropil aggregate formation and toxicity: green fluorescent protein-tagged wild-type or mutated ARs were cotransfected into NSC34 cells with a blue fluorescent protein tagged to mitochondria. Altered mitochondrial distribution was observed in neuronal processes containing aggregates; occasionally, neuropil aggregates and mitochondrial concentration corresponded to axonal swelling. Neuropil aggregates also impaired the distribution of the motor protein kinesin. These data suggest that neuropil aggregates may physically alter neurite transport and thus deprive neuronal processes of factors or components that are important for axonal and dendritic functions. The soma may then be affected, leading to neuronal dysfunctions and possibly to cell death.

Molecular pathogenesis of spinal and bulbar muscular atrophy

Studies of the molecular pathogenesis of spinal and bulbar muscular atrophy, as well as of the other polyglutamine repeat diseases, has led to an understanding of the role of protein accumulation in disease pathogenesis. Aggregation of the expanded repeat androgen receptor (AR), rather than playing a pathogenic role, likely reflects the insoluble nature of the misfolded AR. Proteolytic processing of the expanded AR at various stages of its metabolism may contribute to cellular toxicity through the enhancement of AR insolubility, and potentially through the disruption of normal proteolytic degradation processes. The finding that molecular chaperones not only promote solubility, but also enhance the degradation of expanded polyglutamines as well, make them promising targets for therapeutic development. Understanding the role of ligand binding in expanded AR metabolism may provide additional avenues of therapeutic manipulation as well.

Polyglutamine tract expansion of the androgen receptor in a motoneuronal model of spinal and bulbar muscular atrophy

Spinobulbar muscular atrophy (SBMA) is a late-onset disorder characterized by progressive muscle loss, degeneration of motoneurons in the spinal cord and brainstem, and partial androgen insensitivity. SBMA is directly correlated with the expansion of CAG repeats encoding a polyglutamine tract (polyQ) of extended length. The identification of polyQ expansion in SBMA led to the discovery of an entire class of neurodegenerative disorders. In fact, at least eight different diseases, including Huntington's disease, share a common molecular mechanism involving an expansion of a polyQ tract within different proteins. The elongated polyQ tract causes a toxic gain of function in the mutant protein and is associated with the formation of intracellular aggregates, whose pathogenetic role has not been fully established yet. Our observations in a motoneuron cell line (NSC34), indicate that the expression of the androgen receptor (AR) carrying the elongated polyQ tract (AR-Q48) has a toxic effect in aggregate-independent manner. In fact, in basal condition, AR-Q48 shows a cytoplasmic diffuse distribution, yet it reduces the viability of transfected NSC34. In contrast, testosterone treatment, while inducing aggregation of the mutant AR, also increases cell viability. Aggregates in NSC34 are localized mainly in the perinuclear region and occasionally in the neuropil, whereas no nuclear aggregate has ever been found. Further observations of the minor subset of cells showing neuropil aggregates, reveal an alteration of the neurite morphology, suggesting a different role of the two types of cytoplasmic aggregates.

Characterization of intracellular aggregates using fluorescently-tagged polyglutamine-expanded androgen receptor

Spinal bulbar muscular atrophy (SBMA) is a classic CAG-repeat neurodegenerative disease. It is caused by expansion of a polyglutamine (polyGln) tract in the androgen receptor (AR). Recent evidence has indicated a potential role for nuclear and cytoplasmic inclusions in the pathogenesis of these diseases. We have used blue and green fluorescently-tagged AR to show that both wild-type (WT) and poly-Gln-expanded full-length AR can form aggregates and that aggregation is not related to cytotoxicity. Twenty to thirty-five percent of all cell types transfected into COS cells showed aggregation containing both amino- and carboxy-terminal fluorescent tags. The aggregates reacted with (F39.4.1), an anti-AR antibody and with IC2, an expanded polyGln tract antibody. Western analysis of protein extracts revealed little evidence of proteolysis although some cleavage of the fusion proteins was seen. The general caspase inhibitor, Z-DEVD-FMK, did not affect aggregation in either wild type or polyGln-expanded GFP-AR transfected cells. Surprisingly, addition of Mibolerone a synthetic androgen significantly decreased inclusion formation in both WT and polyGln-expanded AR-transfected cells. Overall, we show that both WT and polyGln expanded full-length AR are found in aggregates and that proteolysis is not a requirement for aggregation. Our results also suggest that toxicity is not related to intracellular aggregation of polyGln expanded AR.

Androgen induced cell death in SHSY5Y neuroblastoma cells expressing wild-type and spinal bulbar muscular atrophy mutant androgen receptors

Spinal bulbar muscular atrophy (SBMA) is one of a family of inherited neurodegenerative diseases caused by expansion of CAG encoding polyglutamine repeats; in SBMA the affected gene is the androgen receptor. To understand further the mechanisms that lead to neuronal cell death in SBMA, we generated SHSY5Y neuroblastoma cell lines that stably express identical levels of wild-type (19 polyglutamine repeat) or SBMA (52 polyglutamine repeat) androgen receptor. Parental SHSY5Y cells do not express detectable levels of the androgen receptor. In the absence of androgen, the transfected cell lines have similar phenotypes and growth characteristics to parental SHSY5Y cells. However, upon treatment with androgen, both cell lines undergo a marked dose-dependent loss of viability; this loss was significantly greater in cells expressing the SBMA receptor. Morphological analyses of the androgen treated cells revealed that cell death bore hallmarks of apoptosis involving altered nuclear morphology and cleavage of poly(ADP-ribose) polymerase and of caspase 3 in both wild-type and SBMA cell lines. The caspase inhibitor VAD-fmk was able to decrease loss of viability of both cell lines on exposure to androgen.

Androgens regulate the mammalian homologues of invertebrate sex determination genes tra-2 and fox-1

Androgens, like other steroid hormones, exert profound effects on cell growth and survival by modulating the expression of target genes. In vertebrates, androgens play a critical role downstream of the testis determination pathway, influencing the expression of sexually dimorphic traits. Among cells of the nervous system, motor neurons respond to trophic effects of androgen stimulation, with a subpopulation of spinal motor neurons exhibiting sexually dimorphic survival. To study the mechanisms of androgen action in these cells, we performed a subtractive screen for genes upregulated by androgen in a motor neuron cell line. We show androgen-inducible expression of two RNA-binding proteins that are the mammalian homologues of invertebrate sex determination genes. Androgens upregulate the expression of tra-2alpha, an enhancer of RNA splicing homologous to Drosophila tra-2, and promote redistribution of the protein from a diffuse to a speckled pattern within the nucleus. Similarly, androgens upregulate the expression of a novel gene homologous to Caenorhabditis elegans fox-1. These data indicate that androgens exert their effects, in part, by modulating the expression and function of genes involved in RNA processing, and identify homologues of invertebrate sex determination genes as androgen-responsive genes in mammals.

Triplet repeat expansion in neuromuscular disease

Expansions of unstable trinucleotide repeats cause at least 15 inherited neurologic diseases. Here we review what has been learned of three neuromuscular diseases caused by this type of mutation. X-linked spinal and bulbar muscular atrophy is a motor neuronopathy caused by a CAG repeat expansion in the androgen receptor gene. The mutated protein has an expanded polyglutamine tract, forms intranuclear aggregates, and mediates neurodegeneration through a toxic gain-of-function mechanism. Oculopharyngeal muscular dystrophy is a dominantly inherited myopathy caused by a GCG/polyalanine expansion in the gene encoding poly(A)-binding protein 2. Myotonic dystrophy is a clinically variable multisystem disease caused by a CTG expansion in the 3' untranslated region of the myotonin gene. For each of these disorders, we summarize the clinical and pathologic features and review current understanding of the molecular mechanisms underlying their pathogenesis.

Increased T-type Ca2+ channel activity as a determinant of cellular toxicity in neuronal cell lines expressing polyglutamine-expanded human androgen receptors

We have analyzed Ca2+ currents in two neuroblastoma-motor neuron hybrid cell lines that expressed normal or glutamine-expanded human androgen receptors (polyGln-expanded AR) either transiently or stably. The cell lines express a unique, low-threshold, transient type of Ca2+ current that is not affected by L-type Ca2+ channel blocker (PN 200-110), N-type Ca2+ channel blocker (omega-conotoxin GVIA) or P-type Ca2+ channel blocker (Agatoxin IVA) but is blocked by either Cd2+ or Ni2+. This pharmacological profile most closely resembles that of T-type Ca2+ channels [1-3]. Exposure to androgen had no effect on control cell lines or cells transfected with normal AR but significantly changed the steady-state activation in cells transfected with expanded AR. The observed negative shift in steady-state activation results in a large increase in the T-type Ca2+ channel window current. We suggest that Ca2+ overload due to abnormal voltage-dependence of transient Ca2+ channel activation may contribute to motor neuron toxicity in spinobulbar muscular atrophy (SBMA). This hypothesis is supported by the additional finding that, at concentrations that selectively block T-type Ca2+ channel currents, Ni2+ significantly reduced cell death in cell lines transfected with polyGln-expanded AR.

Androgen insensitivity

The androgen receptor (AR) protein regulates transcription of certain genes. Usually, this activity depends upon a central DNA-binding domain that permits the binding of androgen-AR complexes to regulatory DNA sequences near or in a target gene. The AR also has a C-terminal androgen-binding domain (ABD) and an N-terminal modulatory domain. These domains interact among themselves and with coregulatory, nonreceptor proteins to determine vector control over a gene's transcription rate. The precise roles of these proteins are active research areas. Severe X-linked androgen receptor gene (AR) mutations cause complete androgen insensitivity, mild ones impair virilization with or without infertility, and moderate ones sometimes yield a wide phenotypic spectrum among sibs. Different expressivity may reflect variability of AR-interactive proteins. The family history must identify heterozygous XX females with sparse, delayed, or asymmetric pubic/axillary hair or delayed menarche and infertile XY maternal aunts or uncles. Mutation type and density vary along the length of the AR. N-terminal polyglutamine tract expansion limits AR transactivation, causing a form of mild androgen insensitivity. Analysis of ABD mutations that do not impair androgen binding or impair it selectively will illuminate its intradomain properties. For partial androgen insensitivity and mild androgen insensitivity, pharmacotherapy with certain androgens or other steroids may overcome some dysfunction of certain mutant ARs. Experience with this approach is limited; outcomes have been generally disappointing.

Polyglutamine pathogenesis

An increasing number of neurodegenerative disorders have been found to be caused by expanding CAG triplet repeats that code for polyglutamine. Huntington's disease (HD) is the most common of these disorders and dentatorubral-pallidoluysian atrophy (DRPLA) is very similar to HD, but is caused by mutation in a different gene, making them good models to study. In this review, we will concentrate on the roles of protein aggregation, nuclear localization and proteolytic processing in disease pathogenesis. In cell model studies of HD, we have found that truncated N-terminal portions of huntingtin (the HD gene product) with expanded repeats form more aggregates than longer or full length huntingtin polypeptides. These shorter fragments are also more prone to aggregate in the nucleus and cause more cell toxicity. Further experiments with huntingtin constructs harbouring exogenous nuclear import and nuclear export signals have implicated the nucleus in direct cell toxicity. We have made mouse models of HD and DRPLA using an N-terminal truncation of huntingtin (N171) and full-length atrophin-1 (the DRPLA gene product), respectively. In both models, diffuse neuronal nuclear staining and nuclear inclusion bodies are observed in animals expressing the expanded glutamine repeat protein, further implicating the nucleus as a primary site of neuronal dysfunction. Neuritic pathology is also observed in the HD mice. In the DRPLA mouse model, we have found that truncated fragments of atrophin-1 containing the glutamine repeat accumulate in the nucleus, suggesting that proteolysis may be critical for disease progression. Taken together, these data lead towards a model whereby proteolytic processing, nuclear localization and protein aggregation all contribute to pathogenesis.