Expression of expanded polyglutamine targets profilin for degradation and alters actin dynamics

Shared Gene Expression Dysregulation Across Subtypes of Sanfilippo and Morquio Diseases: The Role of PFN1 in Regulating Glycosaminoglycan Levels

BACKGROUND

Mucopolysaccharidosis (MPS) is a class of hereditary metabolic diseases that demonstrate itself by accumulating incompletely degraded glycosaminoglycans (GAGs). MPS are classified according to the kind(s) of stored GAG(s) and specific genetic/enzymatic defects. Despite the accumulation of the same type of GAG, two MPS diseases, Sanfilippo (MPS III) and Morquio (MPS IV), are further distinguished into subclasses based on different enzymes that are deficient. Although genetic defects in MPS are known, molecular mechanisms of particular MPS types are still incomplete. This work aimed to investigate gene expression patterns in MPS III and MPS IV subtypes to identify dysregulated genes that could indicate unidentified molecular mechanisms of the diseases.

METHODS

Transcriptomic analyses were conducted to assess gene expression patterns in MPS and control cells. Western blotting and immunohistochemistry determined selected protein levels (products of the most significantly dysregulated genes). Effects of decreased levels of gene expression were investigated using small interferring RNA (siRNA)-mediated gene silencing.

RESULTS

Transcriptomic analyses indicated 45 commonly dysregulated genes among all MPS III subtypes and as many as 150 commonly dysregulated genes among both MPS IV subtypes. A few genes revealed particularly high levels of dysregulation, including PFN1, MFAP5, and MMP12. Intriguingly, elevated levels of profilin-1 (product of the PFN1 gene) could be reduced by decreasing GAG levels in genistein-treated MPS III and MPS IV cells, while silencing of PFN1 caused a significant decrease in GAG accumulation in these cells, indicating an interdependent correlation between profilin-1 and GAG levels.

CONCLUSIONS

A plethora of commonly dysregulated genes were identified in MPS subtypes III and IV. Some of these genes, like PFN1, MFAP5, and MMP12, revealed highly pronounced changes in expression relative to control cells. An interdependent correlation between GAG levels and the expression of the PFN1 gene was identified. Thus, PFN1 could be suggested as a potential new therapeutic target for MPS III and IV.

Single-nuclei transcriptome analysis of Huntington disease iPSC and mouse astrocytes implicates maturation and functional deficits

Huntington disease (HD) is a neurodegenerative disorder caused by expanded CAG repeats in the huntingtin gene that alters cellular homeostasis, particularly in the striatum and cortex. Astrocyte signaling that establishes and maintains neuronal functions are often altered under pathological conditions. We performed single-nuclei RNA-sequencing on human HD patient-induced pluripotent stem cell (iPSC)-derived astrocytes and on striatal and cortical tissue from R6/2 HD mice to investigate high-resolution HD astrocyte cell state transitions. We observed altered maturation and glutamate signaling in HD human and mouse astrocytes. Human HD astrocytes also showed upregulated actin-mediated signaling, suggesting that some states may be cell-autonomous and human specific. In both species, astrogliogenesis transcription factors may drive HD astrocyte maturation deficits, which are supported by rescued climbing deficits in HD drosophila with NFIA knockdown. Thus, dysregulated HD astrocyte states may induce dysfunctional astrocytic properties, in part due to maturation deficits influenced by astrogliogenesis transcription factor dysregulation.

Profilin Isoforms in Health and Disease - All the Same but Different

Profilins are small actin binding proteins, which are structurally conserved throughout evolution. They are probably best known to promote and direct actin polymerization. However, they also participate in numerous cell biological processes beyond the roles typically ascribed to the actin cytoskeleton. Moreover, most complex organisms express several profilin isoforms. Their cellular functions are far from being understood, whereas a growing number of publications indicate that profilin isoforms are involved in the pathogenesis of various diseases. In this review, we will provide an overview of the profilin family and "typical" profilin properties including the control of actin dynamics. We will then discuss the profilin isoforms of higher animals in detail. In terms of cellular functions, we will focus on the role of Profilin 1 (PFN1) and Profilin 2a (PFN2a), which are co-expressed in the central nervous system. Finally, we will discuss recent findings that link PFN1 and PFN2a to neurological diseases, such as amyotrophic lateral sclerosis (ALS), Fragile X syndrome (FXS), Huntington's disease and spinal muscular atrophy (SMA).

Huntingtin fibrils with different toxicity, structure, and seeding potential can be interconverted

The first exon of the huntingtin protein (HTTex1) important in Huntington's disease (HD) can form cross-β fibrils of varying toxicity. We find that the difference between these fibrils is the degree of entanglement and dynamics of the C-terminal proline-rich domain (PRD) in a mechanism analogous to polyproline film formation. In contrast to fibril strains found for other cross-β fibrils, these HTTex1 fibril types can be interconverted. This is because the structure of their polyQ fibril core remains unchanged. Further, we find that more toxic fibrils of low entanglement have higher affinities for protein interactors and are more effective seeds for recombinant HTTex1 and HTTex1 in cells. Together these data show how the structure of a framing sequence at the surface of a fibril can modulate seeding, protein-protein interactions, and thereby toxicity in neurodegenerative disease.

Polyphasic linkage and the impact of ligand binding on the regulation of biomolecular condensates

Cellular matter can be spatially and temporally organized into membraneless biomolecular condensates. The current thinking is that these condensates form and dissolve via phase transitions driven by one or more condensate-specific multivalent macromolecules known as scaffolds. Cells likely regulate condensate formation and dissolution by exerting control over the concentrations of regulatory molecules, which we refer to as ligands. Wyman and Gill introduced the framework of polyphasic linkage to explain how ligands can exert thermodynamic control over phase transitions. This review focuses on describing the concepts of polyphasic linkage and the relevance of such a mechanism for controlling condensate formation and dissolution. We describe how ligand-mediated control over scaffold phase behavior can be quantified experimentally. Further, we build on recent studies to highlight features of ligands that make them suppressors vs drivers of phase separation. Finally, we highlight areas where advances are needed to further understand ligand-mediated control of condensates in complex cellular environments. These advances include understanding the effects of networks of ligands on condensate behavior and how ligands modulate phase transitions controlled by different combinations of homotypic and heterotypic interactions among scaffold macromolecules. Insights gained from the application of polyphasic linkage concepts should be useful for designing novel pharmaceutical ligands to regulate condensates.

Pluripotent stem cell-derived models of neurological diseases reveal early transcriptional heterogeneity

BACKGROUND

Many neurodegenerative diseases develop only later in life, when cells in the nervous system lose their structure or function. In many forms of neurodegenerative diseases, this late-onset phenomenon remains largely unexplained.

RESULTS

Analyzing single-cell RNA sequencing from Alzheimer's disease (AD) and Huntington's disease (HD) patients, we find increased transcriptional heterogeneity in disease-state neurons. We hypothesize that transcriptional heterogeneity precedes neurodegenerative disease pathologies. To test this idea experimentally, we use juvenile forms (72Q; 180Q) of HD iPSCs, differentiate them into committed neuronal progenitors, and obtain single-cell expression profiles. We show a global increase in gene expression variability in HD. Autophagy genes become more stable, while energy and actin-related genes become more variable in the mutant cells. Knocking down several differentially variable genes results in increased aggregate formation, a pathology associated with HD. We further validate the increased transcriptional heterogeneity in CHD8+/- cells, a model for autism spectrum disorder.

CONCLUSIONS

Overall, our results suggest that although neurodegenerative diseases develop over time, transcriptional regulation imbalance is present already at very early developmental stages. Therefore, an intervention aimed at this early phenotype may be of high diagnostic value.

A Progressive Loss of phosphoSer138-Profilin Aligns with Symptomatic Course in the R6/2 Mouse Model of Huntington's Disease: Possible Sex-Dependent Signaling

The R6/2 transgenic mouse model of Huntington’s disease (HD) carries several copies of exon1 of the huntingtin gene that contains a highly pathogenic 120 CAG-repeat expansion. We used kinome analysis to screen for kinase activity patterns in neural tissues from wildtype (WT) and R6/2 mice at a pre-symptomatic (e.g., embryonic) and symptomatic (e.g., between 3 and 10 weeks postnatal) time points. We identified changes in several signaling cascades, for example, the Akt/FoxO3/CDK2, mTOR/ULK1, and RAF/MEK/CREB pathways. We also identified the Rho-Rac GTPase cascade that contributes to cytoskeleton organization through modulation of the actin-binding proteins, cofilin and profilin. Immunoblotting revealed higher levels of phosphoSer138-profilin in embryonic R6/2 mouse samples (cf. WT mice) that diminish progressively and significantly over the postnatal, symptomatic course of the disease. We detected sex- and genotype-dependent patterns in the phosphorylation of actin-regulators such a ROCK2, PAK, LIMK1, cofilin, and SSH1L, yet none of these aligned consistently with the changing levels of phosphoSer138-profilin. This could be reflecting an imbalance in the sequential influences these regulators are known to exert on actin signaling. The translational potential of these observations was inferred from preliminary observations of changes in LIMK-cofilin signaling and loss of neurite integrity in neural stem cells derived from an HD patient (versus a healthy control). Our observations suggest that a pre-symptomatic, neurodevelopmental onset of change in the phosphorylation of Ser138-profilin, potentially downstream of distinct signaling changes in male and female mice, could be contributing to cytoskeletal phenotypes in the R6/2 mouse model of HD pathology.

Profilin choreographs actin and microtubules in cells and cancer

Actin and microtubules play essential roles in aberrant cell processes that define and converge in cancer including: signaling, morphology, motility, and division. Actin and microtubules do not directly interact, however shared regulators coordinate these polymers. While many of the individual proteins important for regulating and choreographing actin and microtubule behaviors have been identified, the way these molecules collaborate or fail in normal or disease contexts is not fully understood. Decades of research focus on Profilin as a signaling molecule, lipid-binding protein, and canonical regulator of actin assembly. Recent reports demonstrate that Profilin also regulates microtubule dynamics and polymerization. Thus, Profilin can coordinate both actin and microtubule polymer systems. Here we reconsider the biochemical and cellular roles for Profilin with a focus on the essential cytoskeletal-based cell processes that go awry in cancer. We also explore how the use of model organisms has helped to elucidate mechanisms that underlie the regulatory essence of Profilin in vivo and in the context of disease.

Neuronal Cell Death Mechanisms in Major Neurodegenerative Diseases

Neuronal cell death in the central nervous system has always been a challenging process to decipher. In normal physiological conditions, neuronal cell death is restricted in the adult brain, even in aged individuals. However, in the pathological conditions of various neurodegenerative diseases, cell death and shrinkage in a specific region of the brain represent a fundamental pathological feature across different neurodegenerative diseases. In this review, we will briefly go through the general pathways of cell death and describe evidence for cell death in the context of individual common neurodegenerative diseases, discussing our current understanding of cell death by connecting with renowned pathogenic proteins, including Tau, amyloid-beta, alpha-synuclein, huntingtin and TDP-43.

Cofilin and profilin: partners in cancer aggressiveness

This review covers aspects of cofilin and profilin regulations and their influence on actin polymerisation responsible for cell motility and metastasis. The regulation of their activity by phosphorylation and nitration, miRs, PI(4,5)P₂ binding, pH, oxidative stress and post-translational modification is described. In this review, we have highlighted selected similarities, complementarities and differences between the two proteins and how their interplay affects actin filament dynamics.

Pharmacological intervention of early neuropathy in neurodegenerative diseases

Extensive studies have reported the significant roles of numerous cellular features and processes in properly maintaining neuronal morphology and function throughout the lifespan of an animal. Any alterations in their homeostasis appear to be strongly associated with neuronal aging and the pathogenesis of various neurodegenerative diseases, even before the occurrence of prominent neuronal death. However, until recently, the primary focus of studies regarding many neurodegenerative diseases has been on the massive cell death occurring at the late stages of disease progression. Thus, our understanding on early neuropathy in these diseases remains relatively limited. The complicated nature of various neuropathic features manifested early in neurodegenerative diseases suggests the involvement of a system-wide transcriptional regulation and epigenetic control. Epigenetic alterations and consequent changes in the neuronal transcriptome are now begun to be extensively studied in various neurodegenerative diseases. Upon the catastrophic incident of neuronal death in disease progression, it is utterly difficult to reverse the deleterious defects by pharmacological treatments, and therefore, therapeutics targeting the system-wide transcriptional dysregulation associated with specific early neuropathy is considered a better option. Here, we review our current understanding on the system-wide transcriptional dysregulation that is likely associated with early neuropathy shown in various neurodegenerative diseases and discuss the possible future developments of pharmaceutical therapeutics.

Coupling of D2R Short but not D2R Long receptor isoform to the Rho/ROCK signaling pathway renders striatal neurons vulnerable to mutant huntingtin

Huntington's disease, an inherited neurodegenerative disorder, results from abnormal polyglutamine extension in the N-terminal region of the huntingtin protein. This mutation causes preferential degeneration of striatal projection neurons. We previously demonstrated, in vitro, that dopaminergic D2 receptor stimulation acted in synergy with expanded huntingtin to increase aggregates formation and striatal death through activation of the Rho/ROCK signaling pathway. In vivo, in a lentiviral-mediated model of expanded huntingtin expression in the rat striatum, we found that the D2 antagonist haloperidol protects striatal neurons against expanded huntingtin-mediated toxicity. Two variant transcripts are generated by alternative splicing of the of D2 receptor gene, the D2R-Long and the D2R-Short, which are thought to play different functional roles. We show herein that overexpression of D2R-Short, but not D2R-Long in cell lines is associated with activation of the RhoA/ROCK signaling pathway. In striatal neurons in culture, the selective D2 agonist Quinpirole triggers phosphorylation of cofilin, a downstream effector of ROCK, which is abrogated by siRNAs that knockdown both D2R-Long and D2R-Short, but not by siRNAs targeting D2R-Long alone. Aggregate formation and neuronal death induced by expanded huntingtin, were potentiated by Quinpirole. This D2 agonist-mediated effect was selectively inhibited by the siRNA targeting both D2R-Long and D2R-Short but not D2R-Long alone. Our data provide evidence for a specific coupling of D2R-Short to the RhoA/ROCK/cofilin pathway, and its involvement in striatal vulnerability to expanded huntingtin. A new route for targeting Rho-ROCK signaling in Huntington's disease is unraveled with our findings.

Proteasome activation delays aging in vitro and in vivo

Aging is a natural biological process that is characterized by a progressive accumulation of macromolecular damage. In the proteome, aging is accompanied by decreased protein homeostasis and function of the major cellular proteolytic systems, leading to the accumulation of unfolded, misfolded, or aggregated proteins. In particular, the proteasome is responsible for the removal of normal as well as damaged or misfolded proteins. Extensive work during the past several years has clearly demonstrated that proteasome activation by either genetic means or use of compounds significantly retards aging. Importantly, this represents a common feature across evolution, thereby suggesting proteasome activation to be an evolutionarily conserved mechanism of aging and longevity regulation. This review article reports on the means of function of these proteasome activators and how they regulate aging in various species.

The role of the cofilin-actin rod stress response in neurodegenerative diseases uncovers potential new drug targets

The cofilin-actin rod stress response is an actin cytoskeletal dynamic arrest that occurs in cells under a variety of stress conditions. Upon stress, the rapidly activated cofilin saturates actin filaments causing them to bundle into rod structures in either the nucleus or cytoplasm, halting actin polymerization and thus freeing ATP. Importantly, these rods dissociate quickly following relief of the transient stress. The rods form inappropriately in neurons involved in the progression of Alzheimer disease (AD) and we have linked dysfunctional dynamics of the nuclear rod response to Huntington disease (HD). Cofilin levels are also perturbed in Parkinson disease (PD), and profilin, an actin binding protein with opposite action to cofilin, is mutated in Amyotrophic Lateral Sclerosis (ALS). The persistence of the rods post-stress suggests that critical molecular switches to turn this response both on and off are being affected in neurodegeneration. We have recently shown that the cofilin protein is regulated by highly conserved nuclear import and export signals and that these signals are required to be functional for an appropriate rod formation during stress. The ability of cofilin to form rods is required in a cell culture model for cells to be resistant to apoptosis under stress conditions, indicating that a normal cofilin-actin rod response is likely integral to proper cell health in higher order organisms. Here we hypothesize on the potential physiological function of nuclear cofilin-actin rods and why the dysregulation of this response could lead to the selective vulnerability of the most susceptible populations of cells in HD. We further suggest that learning more about this cytoskeletal cell stress response will open up new avenues for drug target discovery in neurodegenerative disorders.

Live cell imaging and biophotonic methods reveal two types of mutant huntingtin inclusions

Huntington's disease (HD) is an autosomal dominant, neurodegenerative disorder that can be characterized by the presence of protein inclusions containing mutant huntingtin within a subset of neurons in the brain. Since their discovery, the relevance of inclusions to disease pathology has been controversial. We show using super-resolution fluorescence imaging and Förster resonance energy transfer (FRET) in live cells, that mutant huntingtin fragments can form two morphologically and conformationally distinct inclusion types. Using fluorescence recovery after photobleaching (FRAP), we demonstrate that the two huntingtin inclusion types have unique dynamic properties. The ability to form one or the other type of inclusion can be influenced by the phosphorylation state of serine residues at amino acid positions 13 and 16 within the huntingtin protein. We can define two types of inclusions: fibrillar, which are tightly packed, do not exchange protein with the soluble phase, and result from phospho-modification at serines 13 and 16 of the N17 domain, and globular, which are loosely packed, can readily exchange with the soluble phase, and are not phosphorylated in N17. We hypothesize that the protective effect of N17 phosphorylation or phospho-mimicry seen in animal models, at the level of protein inclusions with elevated huntingtin levels, is to induce a conformation of the huntingtin amino-terminus that causes fragments to form tightly packed inclusions that do not exit the insoluble phase, and hence exert less toxicity. The identification of these sub-types of huntingtin inclusions could allow for drug discovery to promote protective inclusions of mutant huntingtin protein in HD.

Caspase-2 promotes cytoskeleton protein degradation during apoptotic cell death

The caspase family of proteases cleaves large number of proteins resulting in major morphological and biochemical changes during apoptosis. Yet, only a few of these proteins have been reported to selectively cleaved by caspase-2. Numerous observations link caspase-2 to the disruption of the cytoskeleton, although it remains elusive whether any of the cytoskeleton proteins serve as bona fide substrates for caspase-2. Here, we undertook an unbiased proteomic approach to address this question. By differential proteome analysis using two-dimensional gel electrophoresis, we identified four cytoskeleton proteins that were degraded upon treatment with active recombinant caspase-2 in vitro. These proteins were degraded in a caspase-2-dependent manner during apoptosis induced by DNA damage, cytoskeleton disruption or endoplasmic reticulum stress. Hence, degradation of these cytoskeleton proteins was blunted by siRNA targeting of caspase-2 and when caspase-2 activity was pharmacologically inhibited. However, none of these proteins was cleaved directly by caspase-2. Instead, we provide evidence that in cells exposed to apoptotic stimuli, caspase-2 probed these proteins for proteasomal degradation. Taken together, our results depict a new role for caspase-2 in the regulation of the level of cytoskeleton proteins during apoptosis.

NeuroGeM, a knowledgebase of genetic modifiers in neurodegenerative diseases

Background

Neurodegenerative diseases (NDs) are characterized by the progressive loss of neurons in the human brain. Although the majority of NDs are sporadic, evidence is accumulating that they have a strong genetic component. Therefore, significant efforts have been made in recent years to not only identify disease-causing genes but also genes that modify the severity of NDs, so-called genetic modifiers. To date there exists no compendium that lists and cross-links genetic modifiers of different NDs.

Description

In order to address this need, we present NeuroGeM, the first comprehensive knowledgebase providing integrated information on genetic modifiers of nine different NDs in the model organisms D. melanogaster, C. elegans, and S. cerevisiae. NeuroGeM cross-links curated genetic modifier information from the different NDs and provides details on experimental conditions used for modifier identification, functional annotations, links to homologous proteins and color-coded protein-protein interaction networks to visualize modifier interactions. We demonstrate how this database can be used to generate new understanding through meta-analysis. For instance, we reveal that the Drosophila genes DnaJ-1, thread, Atx2, and mub are generic modifiers that affect multiple if not all NDs.

Conclusion

As the first compendium of genetic modifiers, NeuroGeM will assist experimental and computational scientists in their search for the pathophysiological mechanisms underlying NDs. http://chibi.ubc.ca/neurogem.

Polyglutamine domain flexibility mediates the proximity between flanking sequences in huntingtin

Huntington disease (HD) is a neurodegenerative disorder caused by a CAG expansion within the huntingtin gene that encodes a polymorphic glutamine tract at the amino terminus of the huntingtin protein. HD is one of nine polyglutamine expansion diseases. The clinical threshold of polyglutamine expansion for HD is near 37 repeats, but the mechanism of this pathogenic length is poorly understood. Using Förster resonance energy transfer, we describe an intramolecular proximity between the N17 domain and the downstream polyproline region that flanks the polyglutamine tract of huntingtin. Our data support the hypothesis that the polyglutamine tract can act as a flexible domain, allowing the flanking domains to come into close spatial proximity. This flexibility is impaired with expanded polyglutamine tracts, and we can detect changes in huntingtin conformation at the pathogenic threshold for HD. Altering the structure of N17, either via phosphomimicry or with small molecules, also affects the proximity between the flanking domains. The structural capacity of N17 to fold back toward distal regions within huntingtin requires an interacting protein, protein kinase C and casein kinase 2 substrate in neurons 1 (PACSIN1). This protein has the ability to bind both N17 and the polyproline region, stabilizing the interaction between these two domains. We also developed an antibody-based FRET assay that can detect conformational changes within endogenous huntingtin in wild-type versus HD fibroblasts. Therefore, we hypothesize that wild-type length polyglutamine tracts within huntingtin can form a flexible domain that is essential for proper functional intramolecular proximity, conformations, and dynamics.

Multiple phenotypes in Huntington disease mouse neural stem cells

Neural stem (NS) cells are a limitless resource, and thus superior to primary neurons for drug discovery provided they exhibit appropriate disease phenotypes. Here we established NS cells for cellular studies of Huntington's disease (HD). HD is a heritable neurodegenerative disease caused by a mutation resulting in an increased number of glutamines (Q) within a polyglutamine tract in Huntingtin (Htt). NS cells were isolated from embryonic wild-type (Htt(7Q/7Q)) and "knock-in" HD (Htt(140Q/140Q)) mice expressing full-length endogenous normal or mutant Htt. NS cells were also developed from mouse embryonic stem cells that were devoid of Htt (Htt(-/-)), or knock-in cells containing human exon1 with an N-terminal FLAG epitope tag and with 7Q or 140Q inserted into one of the mouse alleles (Htt(F7Q/7Q) and Htt(F140Q/7Q)). Compared to Htt(7Q/7Q) NS cells, HD Htt(140Q/140Q) NS cells showed significantly reduced levels of cholesterol, increased levels of reactive oxygen species (ROS), and impaired motility. The heterozygous Htt(F140Q/7Q) NS cells had increased ROS and decreased motility compared to Htt(F7Q/7Q). These phenotypes of HD NS cells replicate those seen in HD patients or in primary cell or in vivo models of HD. Huntingtin "knock-out" NS cells (Htt(-/-)) also had impaired motility, but in contrast to HD cells had increased cholesterol. In addition, Htt(140Q/140Q) NS cells had higher phospho-AKT/AKT ratios than Htt(7Q/7Q) NS cells in resting conditions and after BDNF stimulation, suggesting mutant htt affects AKT dependent growth factor signaling. Upon differentiation, the Htt(7Q/7Q) and Htt(140Q/140Q) generated numerous Beta(III)-Tubulin- and GABA-positive neurons; however, after 15 days the cellular architecture of the differentiated Htt(140Q/140Q) cultures changed compared to Htt(7Q/7Q) cultures and included a marked increase of GFAP-positive cells. Our findings suggest that NS cells expressing endogenous mutant Htt will be useful for study of mechanisms of HD and drug discovery.

Neurogenic and myogenic contributions to hereditary motor neuron disease

Spinal muscular atrophy and spinal and bulbar muscular atrophy are characterized by lower motor neuron loss and muscle atrophy. Although it is accepted that motor neuron loss is a primary event in disease pathogenesis, inherent defects in muscle may also contribute to the disease progression and severity. In this review, we discuss the relative contributions of primary pathological processes in the motor axons, neuromuscular junctions and muscle to disease manifestations. Characterizing these contributions helps us to better understand the disease mechanisms and to better target therapeutic intervention.

Polyglutamine aggregation in Huntington and related diseases

Polyglutamine (polyQ)-expansions in different proteins cause nine neurodegenerative diseases. While polyQ aggregation is a key pathological hallmark of these diseases, how aggregation relates to pathogenesis remains contentious. In this chapter, we review what is known about the aggregation process and how cells respond and interact with the polyQ-expanded proteins. We cover detailed biophysical and structural studies to uncover the intrinsic features of polyQ aggregates and concomitant effects in the cellular environment. We also examine the functional consequences ofpolyQ aggregation and how cells may attempt to intervene and guide the aggregation process.

Live cell imaging of cytoskeletal dynamics in neurons using fluorescence photoactivation

Alzheimer's disease and other neurodegenerative disorders share some common features at the cellular level, which are often associated with a change in cytoskeletal dynamics. Live cell imaging has been applied to study various aspects of cell physiology including cytoskeletal dynamics. Recently, fluorescence photoactivation (FPA) has been developed as a novel approach to analyze protein dynamics in living cells with high sensitivity. Here, we describe the application of FPA to determine interactions and mobility of the microtubule-associated protein tau in neurons as an example for a disease-related cytoskeletal protein and discuss further applications of FPA to study cytoskeletal dynamics in neurodegenerative diseases.

F-actin binding regions on the androgen receptor and huntingtin increase aggregation and alter aggregate characteristics

Protein aggregation is associated with neurodegeneration. Polyglutamine expansion diseases such as spinobulbar muscular atrophy and Huntington disease feature proteins that are destabilized by an expanded polyglutamine tract in their N-termini. It has previously been reported that intracellular aggregation of these target proteins, the androgen receptor (AR) and huntingtin (Htt), is modulated by actin-regulatory pathways. Sequences that flank the polyglutamine tract of AR and Htt might influence protein aggregation and toxicity through protein-protein interactions, but this has not been studied in detail. Here we have evaluated an N-terminal 127 amino acid fragment of AR and Htt exon 1. The first 50 amino acids of ARN127 and the first 14 amino acids of Htt exon 1 mediate binding to filamentous actin in vitro. Deletion of these actin-binding regions renders the polyglutamine-expanded forms of ARN127 and Htt exon 1 less aggregation-prone, and increases the SDS-solubility of aggregates that do form. These regions thus appear to alter the aggregation frequency and type of polyglutamine-induced aggregation. These findings highlight the importance of flanking sequences in determining the propensity of unstable proteins to misfold.

Animal models of polyglutamine diseases and therapeutic approaches

The dominant gain-of-function polyglutamine repeat diseases, in which the initiating mutation is known, allow development of models that recapitulate many aspects of human disease. To the extent that pathology is a consequence of disrupted fundamental cellular activities, one can effectively study strategies to ameliorate or protect against these cellular insults. Model organisms allow one to identify pathways that affect disease onset and progression, to test and screen for pharmacological agents that affect pathogenic processes, and to validate potential targets genetically as well as pharmacologically. Here, we describe polyglutamine repeat diseases that have been modeled in a variety of organisms, including worms, flies, mice, and non-human primates, and discuss examples of how they have broadened the therapeutic landscape.

Profilin, a multi-modal regulator of neuronal plasticity

Thirty years after its initial characterization and more than 1000 publications listed in PubMed describing its properties, the small (ca 15 kDa) protein profilin continues to surprise us with new, recently discovered functions. Originally described as an actin-binding protein, profilin has now been shown to interact with more than a dozen proteins in mammalian cells. Some of the more recently described and intriguing interactions are within neurons involving a neuronal profilin family member. Profilin is now regarded as a regulator of various cellular processes such as cytoskeletal dynamics, membrane trafficking and nuclear transport. Profilin is a necessary element in key steps of neuronal differentiation and synaptic plasticity, and embodies properties postulated for a synaptic tag. These findings identify profilin as an important factor linking cellular and behavioural plasticity in neural circuits.

Phosphorylation of profilin by ROCK1 regulates polyglutamine aggregation

Y-27632, an inhibitor of the Rho-associated kinase ROCK, is a therapeutic lead for Huntington disease (HD). The downstream targets that mediate its inhibitory effects on huntingtin (Htt) aggregation and toxicity are unknown. We have identified profilin, a small actin-binding factor that also interacts with Htt, as being a direct target of the ROCK1 isoform. The overexpression of profilin reduces the aggregation of polyglutamine-expanded Htt and androgen receptor (AR) peptides. This requires profilin's G-actin binding activity and its direct interaction with Htt, which are both inhibited by the ROCK1-mediated phosphorylation of profilin at Ser-137. Y-27632 blocks the phosphorylation of profilin in HEK293 cells and primary neurons, which maintains profilin in an active state. The knockdown of profilin blocks the inhibitory effect of Y-27632 on both AR and Htt aggregation. A signaling pathway from ROCK1 to profilin thus controls polyglutamine protein aggregation and is targeted by a promising therapeutic lead for HD.