Most mutations that cause spinocerebellar ataxia autosomal recessive type 16 (SCAR16) destabilize the protein quality-control E3 ligase CHIP

A dimer-monomer switch controls CHIP-dependent substrate ubiquitylation and processing

The high substrate selectivity of the ubiquitin/proteasome system is mediated by a large group of E3 ubiquitin ligases. The ubiquitin ligase CHIP regulates the degradation of chaperone-controlled and chaperone-independent proteins. To understand how CHIP mediates substrate selection and processing, we performed a structure-function analysis of CHIP and addressed its physiological role in Caenorhabditis elegans and human cells. The conserved function of CHIP in chaperone-assisted degradation requires dimer formation to mediate proteotoxic stress resistance and to prevent protein aggregation. The CHIP monomer, however, promotes the turnover of the membrane-bound insulin receptor and longevity. The dimer-monomer transition is regulated by CHIP autoubiquitylation and chaperone binding, which provides a feedback loop that controls CHIP activity in response to cellular stress. Because CHIP also binds other E3 ligases, such as Parkin, the molecular switch mechanism described here could be a general concept for the regulation of substrate selectivity and ubiquitylation by combining different E3s.

The molecular basis of spinocerebellar ataxia type 48 caused by a de novo mutation in the ubiquitin ligase CHIP

The spinocerebellar ataxias (SCAs) are a class of incurable diseases characterized by degeneration of the cerebellum that results in movement disorder. Recently, a new heritable form of SCA, spinocerebellar ataxia type 48 (SCA48), was attributed to dominant mutations in STIP1 homology and U box-containing 1 (STUB1); however, little is known about how these mutations cause SCA48. STUB1 encodes for the protein C terminus of Hsc70 interacting protein (CHIP), an E3 ubiquitin ligase. CHIP is known to regulate proteostasis by recruiting chaperones via a N-terminal tetratricopeptide repeat domain and recruiting E2 ubiquitin-conjugating enzymes via a C-terminal U-box domain. These interactions allow CHIP to mediate the ubiquitination of chaperone-bound, misfolded proteins to promote their degradation via the proteasome. Here we have identified a novel, de novo mutation in STUB1 in a patient with SCA48 encoding for an A52G point mutation in the tetratricopeptide repeat domain of CHIP. Utilizing an array of biophysical, biochemical, and cellular assays, we demonstrate that the CHIPA52G point mutant retains E3-ligase activity but has decreased affinity for chaperones. We further show that this mutant decreases cellular fitness in response to certain cellular stressors and induces neurodegeneration in a transgenic Caenorhabditis elegans model of SCA48. Together, our data identify the A52G mutant as a cause of SCA48 and provide molecular insight into how mutations in STUB1 cause SCA48.

Chemical Regulation of the Protein Quality Control E3 Ubiquitin Ligase C-Terminus of Hsc70 Interacting Protein (CHIP)

The ubiquitin ligase C-terminus of Hsc70 interacting protein (CHIP) is an important regulator of proteostasis. Despite playing an important role in maintaining proteostasis, little progress has been made in developing small molecules that regulate ubiquitin transfer by CHIP. Here we used differential scanning fluorimetry to identify compounds that bound CHIP. Compounds that bound CHIP were then analyzed by quantitative ubiquitination assays to identify those that altered CHIP function. One compound, MS.001, inhibited both the chaperone binding and ubiquitin ligase activity of CHIP at low micromolar concentrations. Interestingly, we found that MS.001 did not have activity against isolated U-box or tetratricopeptide (TPR) domains, but instead only inhibited full-length CHIP. Using in silico docking we identified a potential MS.001 binding site on the linker domain of CHIP and mutation of this site rendered CHIP resistant to MS.001. Together our data identify an inhibitor of the E3 ligase CHIP and provides insight into the development of compounds that regulate CHIP activity.

The UBE2D ubiquitin conjugating enzymes: Potential regulatory hubs in development, disease and evolution

Ubiquitination of cellular proteins plays critical roles in key signalling pathways and in the regulation of protein turnover in eukaryotic cells. E2 ubiquitin conjugating enzymes function as essential intermediates in ubiquitination reactions by acting as ubiquitin donors for the E3 ubiquitin ligase enzymes that confer substrate specificity. The members of the UBE2D family of E2 enzymes are involved in regulating signalling cascades through ubiquitination of target proteins that include receptor tyrosine kinases (RTKs) and components of the Hedgehog, TGFβ and NFκB pathways. UBE2D enzymes also function in transcriptional control by acting as donors for ubiquitination of histone tails by the Polycomb protein Ring1B and the DNA methylation regulator UHRF1 as well as having roles in DNA repair and regulation of the level of the tumour suppressor p53. Here we review the functional roles and mechanisms of regulation of the UBE2D proteins including recent evidence that regulation of the level of UBE2D3 is critical for controlling ubiquitination of specific targets during development. Cellular levels of UBE2D3 have been shown to be regulated by phosphorylation, which affects folding of the protein, reducing its stability. Specific variations in the otherwise highly conserved UBE2D3 protein sequence in amniotes and in a subgroup of teleost fishes, the Acanthomorpha, suggest that the enzyme has had important roles during vertebrate evolution.

Chaperone-assisted E3 ligase CHIP: A double agent in cancer

The carboxy-terminus of Hsp70-interacting protein (CHIP) is a ubiquitin ligase and co-chaperone belonging to Ubox family that plays a crucial role in the maintenance of cellular homeostasis by switching the equilibrium of the folding-refolding mechanism towards the proteasomal or lysosomal degradation pathway. It links molecular chaperones viz. HSC70, HSP70 and HSP90 with ubiquitin proteasome system (UPS), acting as a quality control system. CHIP contains charged domain in between N-terminal tetratricopeptide repeat (TPR) and C-terminal Ubox domain. TPR domain interacts with the aberrant client proteins via chaperones while Ubox domain facilitates the ubiquitin transfer to the client proteins for ubiquitination. Thus, CHIP is a classic molecule that executes ubiquitination for degradation of client proteins. Further, CHIP has been found to be indulged in cellular differentiation, proliferation, metastasis and tumorigenesis. Additionally, CHIP can play its dual role as a tumor suppressor as well as an oncogene in numerous malignancies, thus acting as a double agent. Here, in this review, we have reported almost all substrates of CHIP established till date and classified them according to the hallmarks of cancer. In addition, we discussed about its architectural alignment, tissue specific expression, sub-cellular localization, folding-refolding mechanisms of client proteins, E4 ligase activity, normal physiological roles, as well as involvement in various diseases and tumor biology. Further, we aim to discuss its importance in HSP90 inhibitors mediated cancer therapy. Thus, this report concludes that CHIP may be a promising and worthy drug target towards pharmaceutical industry for drug development.

Genetic Dominant Variants in STUB1, Segregating in Families with SCA48, Display In Vitro Functional Impairments Indistinctive from Recessive Variants Associated with SCAR16

Variants in STUB1 cause both autosomal recessive (SCAR16) and dominant (SCA48) spinocerebellar ataxia. Reports from 18 STUB1 variants causing SCA48 show that the clinical picture includes later-onset ataxia with a cerebellar cognitive affective syndrome and varying clinical overlap with SCAR16. However, little is known about the molecular properties of dominant STUB1 variants. Here, we describe three SCA48 families with novel, dominantly inherited STUB1 variants (p.Arg51_Ile53delinsProAla, p.Lys143_Trp147del, and p.Gly249Val). All the patients developed symptoms from 30 years of age or later, all had cerebellar atrophy, and 4 had cognitive/psychiatric phenotypes. Investigation of the structural and functional consequences of the recombinant C-terminus of HSC70-interacting protein (CHIP) variants was performed in vitro using ubiquitin ligase activity assay, circular dichroism assay and native polyacrylamide gel electrophoresis. These studies revealed that dominantly and recessively inherited STUB1 variants showed similar biochemical defects, including impaired ubiquitin ligase activity and altered oligomerization properties of the CHIP. Our findings expand the molecular understanding of SCA48 but also mean that assumptions concerning unaffected carriers of recessive STUB1 variants in SCAR16 families must be re-evaluated. More investigations are needed to verify the disease status of SCAR16 heterozygotes and elucidate the molecular relationship between SCA48 and SCAR16 diseases.

TTC3-Mediated Protein Quality Control, A Potential Mechanism for Cognitive Impairment

The tetrapeptide repeat domain 3 (TTC3) gene falls within Down's syndrome (DS) critical region. Cognitive impairment is a common phenotype of DS and Alzheimer’s disease (AD), and overexpression of TTC3 can accelerate cognitive decline, but the specific mechanism is unknown. The TTC3-mediated protein quality control (PQC) mechanism, similar to the PQC system, is divided into three parts: it acts as a cochaperone to assist proteins in folding correctly; it acts as an E3 ubiquitin ligase (E3s) involved in protein degradation processes through the ubiquitin–proteasome system (UPS); and it may also eventually cause autophagy by affecting mitochondrial function. Thus, this article reviews the research progress on the structure, function, and metabolism of TTC3, including the recent research progress on TTC3 in DS and AD; the role of TTC3 in cognitive impairment through PQC in combination with the abovementioned attributes of TTC3; and the potential targets of TTC3 in the treatment of such diseases.

Functional analysis of tomato CHIP ubiquitin E3 ligase in heat tolerance

Plants have evolved genetic and physiological mechanisms to mitigate the adverse effects of high temperature. CARBOXYL TERMINUS OF THE HSC70-INTERACTING PROTEINS (CHIP) is a conserved chaperone-dependent ubiquitin E3 ligase that targets misfolded proteins. Here, we report functional analysis of the SlCHIP gene from tomato (Solanum lycopersicum) in heat tolerance. SlCHIP encodes a CHIP protein with three tandem tetracopeptide repeat (TPR) motifs and a C-terminal U box domain. Phylogenetic analysis of CHIP homologs from animals, spore-bearing and seed plants revealed a tree topology similar to the evolutionary tree of the organisms. Expression of SlCHIP was induced under high temperature and was also responsive to plant stress hormones. Silencing of SlCHIP in tomato reduced heat tolerance based on increased heat stress symptoms, reduced photosynthetic activity, elevated electrolyte leakage and accumulation of insoluble protein aggregates. The accumulated protein aggregates in SlCHIP-silenced plants were still highly ubiquitinated, suggesting involvement of other E3 ligases in ubiquitination. SlCHIP restored the heat tolerance of Arabidopsis chip mutant to the wild type levels. These results indicate that tomato SlCHIP plays a critical role in heat stress responses most likely by targeting degradation of misfolded proteins that are generated during heat stress.

CHIP mutations affect the heat shock response differently in human fibroblasts and iPSC-derived neurons

C-terminus of HSC70-interacting protein (CHIP) encoded by the gene STUB1 is a co-chaperone and E3 ligase that acts as a key regulator of cellular protein homeostasis. Mutations in STUB1 cause autosomal recessive spinocerebellar ataxia type 16 (SCAR16) with widespread neurodegeneration manifesting as spastic-ataxic gait disorder, dementia and epilepsy. CHIP^-/- mice display severe cerebellar atrophy, show high perinatal lethality and impaired heat stress tolerance. To decipher the pathomechanism underlying SCAR16, we investigated the heat shock response (HSR) in primary fibroblasts of three SCAR16 patients. We found impaired HSR induction and recovery compared to healthy controls. HSPA1A/B transcript levels (coding for HSP70) were reduced upon heat shock but HSP70 remained higher upon recovery in patient- compared to control-fibroblasts. As SCAR16 primarily affects the central nervous system we next investigated the HSR in cortical neurons (CNs) derived from induced pluripotent stem cells of SCAR16 patients. We found CNs of patients and controls to be surprisingly resistant to heat stress with high basal levels of HSP70 compared to fibroblasts. Although heat stress resulted in strong transcript level increases of many HSPs, this did not translate into higher HSP70 protein levels upon heat shock, independent of STUB1 mutations. Furthermore, STUB1(-/-) neurons generated by CRISPR/Cas9-mediated genome editing from an isogenic healthy control line showed a similar HSR to patients. Proteomic analysis of CNs showed dysfunctional protein (re)folding and higher basal oxidative stress levels in patients. Our results question the role of impaired HSR in SCAR16 neuropathology and highlight the need for careful selection of proper cell types for modeling human diseases.

CHIP as a therapeutic target for neurological diseases

Carboxy-terminus of Hsc70-interacting protein (CHIP) functions both as a molecular co-chaperone and ubiquitin E3 ligase playing a critical role in modulating the degradation of numerous chaperone-bound proteins. To date, it has been implicated in the regulation of numerous biological functions, including misfolded-protein refolding, autophagy, immunity, and necroptosis. Moreover, the ubiquitous expression of CHIP in the central nervous system suggests that it may be implicated in a wide range of functions in neurological diseases. Several recent studies of our laboratory and other groups have highlighted the beneficial role of CHIP in the pathogenesis of several neurological diseases. The objective of this review is to discuss the possible molecular mechanisms that contribute to the pathogenesis of neurological diseases in which CHIP has a pivotal role, such as stroke, intracerebral hemorrhage, Alzheimer's disease, Parkinson's disease, and polyglutamine diseases; furthermore, CHIP mutations could also cause neurodegenerative diseases. Based on the available literature, CHIP overexpression could serve as a promising therapeutic target for several neurological diseases.

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.

UbcH5 Interacts with Substrates to Participate in Lysine Selection with the E3 Ubiquitin Ligase CHIP

The E3 ubiquitin ligase C-terminus of Hsc70 interacting protein (CHIP) plays a critical role in regulating the ubiquitin-dependent degradation of misfolded proteins. CHIP mediates the ubiquitination of the α-amino-terminus of substrates with the E2 Ube2w and facilitates the ubiquitination of lysine residues with the E2 UbcH5. While it is known that Ube2w directly interacts with the disordered regions at the N-terminus of its substrates, it is unclear how CHIP and UbcH5 mediate substrate lysine selection. Here, we have decoupled the contributions of the E2, UbcH5, and the E3, CHIP, in ubiquitin transfer. We show that UbcH5 selects substrate lysine residues independent of CHIP, and that CHIP participates in lysine selection by fine-tuning the subset of substrate lysines that are ubiquitinated. We also identify lysine 128 near the C-terminus of UbcH5 as a critical residue for the efficient ubiquitin transfer by UbcH5 in both the presence and absence of CHIP. Together, these data demonstrate an important role of the UbcH5/substrate interactions in mediating the efficient ubiquitin transfer by the CHIP/UbcH5 complex.

Guilt-by-Association - Functional Insights Gained From Studying the LRRK2 Interactome

The Parkinson's disease-associated Leucine-rich repeat kinase 2 (LRRK2) is a complex multi-domain protein belonging to the Roco protein family, a unique group of G-proteins. Variants of this gene are associated with an increased risk of Parkinson's disease. Besides its well-characterized enzymatic activities, conferred by its GTPase and kinase domains, and a central dimerization domain, it contains four predicted repeat domains, which are, based on their structure, commonly involved in protein-protein interactions (PPIs). In the past decades, tremendous progress has been made in determining comprehensive interactome maps for the human proteome. Knowledge of PPIs has been instrumental in assigning functions to proteins involved in human disease and helped to understand the connectivity between different disease pathways and also significantly contributed to the functional understanding of LRRK2. In addition to an increased kinase activity observed for proteins containing PD-associated variants, various studies helped to establish LRRK2 as a large scaffold protein in the interface between cytoskeletal dynamics and the vesicular transport. This review first discusses a number of specific LRRK2-associated PPIs for which a functional consequence can at least be speculated upon, and then considers the representation of LRRK2 protein interactions in public repositories, providing an outlook on open research questions and challenges in this field.

Spinocerebellar ataxia type 48: last but not least

INTRODUCTION

Biallelic mutations in STUB1, which encodes the E3 ubiquitin ligase CHIP, were originally described in association with SCAR16, a rare autosomal recessive spinocerebellar ataxia, so far reported in 16 kindreds. In the last 2 years, a new form of spinocerebellar ataxia (SCA48), associated with heterozygous mutations in the same gene, has been described in 12 kindreds with autosomal dominant inheritance.

METHODS

We reviewed molecular and clinical findings of both SCAR16 and SCA48 described patients.

RESULTS AND CONCLUSION

SCAR16 is characterized by early onset spastic ataxia and a wide disease spectrum, including cognitive dysfunction, hyperkinetic disorders, epilepsy, peripheral neuropathy, and hypogonadism. SCA48 is an adult-onset syndrome characterized by ataxia and cognitive-psychiatric features, variably associated with chorea, parkinsonism, dystonia, and urinary symptoms. SCA48, the last dominant ataxia to be described, could emerge as the most frequent among the SCAs due to conventional mutations. The overlap of several clinical signs between SCAR16 and SCA48 indicates the presence of a continuous clinical spectrum among recessively and dominantly inherited mutations of STUB1. Different kinds of mutations, scattered over the three gene domains, have been found in both disorders. Their pathogenesis and the relationship between SCA48 and SCAR16 remain to be clarified.

Heterozygous STUB1 missense variants cause ataxia, cognitive decline, and STUB1 mislocalization

OBJECTIVE

To identify the genetic cause of autosomal dominant ataxia complicated by behavioral abnormalities, cognitive decline, and autism in 2 families and to characterize brain neuropathologic signatures of dominant STUB1-related ataxia and investigate the effects of pathogenic variants on STUB1 localization.

METHODS

Clinical and research-based exome sequencing was used to identify the causative variants for autosomal dominant ataxia in 2 families. Gross and microscopic neuropathologic evaluations were performed on the brains of 4 affected individuals in these families.

RESULTS

Mutations in STUB1 have been primarily associated with childhood-onset autosomal recessive ataxia, but here we report heterozygous missense variants in STUB1 (p.Ile53Thr and p.The37Leu) confirming the recent reports of autosomal dominant inheritance. Cerebellar atrophy on imaging and cognitive deficits often preceded ataxia. Unique neuropathologic examination of the 4 brains showed the marked loss of Purkinje cells (PCs) without microscopic evidence of significant pathology outside the cerebellum. The normal pattern of polarized somatodendritic STUB1 protein expression in PCs was lost, resulting in aberrant STUB1 localization in the distal PC dendritic arbors.

CONCLUSIONS

This study confirms a dominant inheritance pattern in STUB1-ataxia in addition to a recessive one and documents its association with cognitive and behavioral disability, including autism. In the most extensive analysis of cerebellar pathology in this disease, we demonstrate disruption of STUB1 protein in PCs as part of the underlying pathogenesis.

Clinical and pathologic phenotype of a large family with heterozygous STUB1 mutation

Objective

To describe the clinical and pathologic features of a novel pedigree with heterozygous STUB1 mutation causing SCA48.

Methods

We report a large pedigree of Dutch decent. Clinical and pathologic data were reviewed, and genetic analyses (whole-exome sequencing, whole-genome sequencing, and linkage analysis) were performed on multiple family members.

Results

Patients presented with adult-onset gait disturbance (ataxia or parkinsonism), combined with prominent cognitive decline and behavioral changes. Whole-exome sequencing identified a novel heterozygous frameshift variant c.731_732delGC (p.C244Yfs*24) in STUB1 segregating with the disease. This variant was present in a linkage peak on chromosome 16p13.3. Neuropathologic examination of 3 cases revealed a consistent pattern of ubiquitin/p62-positive neuronal inclusions in the cerebellum, neocortex, and brainstem. In addition, tau pathology was present in 1 case.

Conclusions

This study confirms previous findings of heterozygous STUB1 mutations as the cause of SCA48 and highlights its prominent cognitive involvement, besides cerebellar ataxia and movement disorders as cardinal features. The presence of intranuclear inclusions is a pathologic hallmark of the disease. Future studies will provide more insight into its pathologic heterogeneity.

Changes in protein function underlie the disease spectrum in patients with CHIP mutations

Monogenetic disorders that cause cerebellar ataxia are characterized by defects in gait and atrophy of the cerebellum; however, patients often suffer from a spectrum of disease, complicating treatment options. Spinocerebellar ataxia autosomal recessive 16 (SCAR16) is caused by coding mutations in STUB1, a gene that encodes the multifunctional enzyme CHIP (C terminus of HSC70-interacting protein). The disease spectrum of SCAR16 includes a varying age of disease onset, cognitive dysfunction, increased tendon reflex, and hypogonadism. Although SCAR16 mutations span the multiple functional domains of CHIP, it is unclear whether the location of the mutation and the change in the biochemical properties of CHIP contributes to the clinical spectrum of SCAR16. In this study, we examined relationships between the clinical phenotypes of SCAR16 patients and the changes in biophysical, biochemical, and functional properties of the corresponding mutated protein. We found that the severity of ataxia did not correlate with age of onset; however, cognitive dysfunction, increased tendon reflex, and ancestry were able to predict 54% of the variation in ataxia severity. We further identified domain-specific relationships between biochemical changes in CHIP and clinical phenotypes and specific biochemical activities that associate selectively with either increased tendon reflex or cognitive dysfunction, suggesting that specific changes to CHIP–HSC70 dynamics contribute to the clinical spectrum of SCAR16. Finally, linear models of SCAR16 as a function of the biochemical properties of CHIP support the concept that further inhibiting mutant CHIP activity lessens disease severity and may be useful in the design of patient-specific targeted approaches to treat SCAR16.

The E3 ubiquitin ligase CHIP in normal cell function and in disease conditions

In eukaryotic cells, ubiquitination and proteasomal degradation is an essential mechanism for regulating protein functions. For example, critical signaling proteins play their roles by controlling different cellular functions. Once a signaling protein has been activated, its activity needs to be quickly downregulated by different mechanisms, including ubiquitination/proteasome regulation. Failure to regulate the activity or expression levels of these proteins may cause human diseases. Protein ubiquitination involves a cascade of biochemical processes and requires three types of ubiquitin enzymes: E1 activating enzyme, E2 conjugating enzyme, and E3 ligase. Among these enzymes, E3 ubiquitin ligases play a specific role in recognizing specific protein substrates. There are several structurally diverse groups of E3 ubiquitin ligases in eukaryotic cells, and one type of these E3 ligases is the U-box ubiquitin ligases. Carboxyl terminus of HSP70-interacting protein (CHIP) is a member of a family of U-box E3 ligases. It plays critical roles in multiple organs and tissues in the body. In this review article, we provide an update on some of the most recent discoveries about CHIP in normal physiological function and in disease.

The Regulation of the Small Heat Shock Protein B8 in Misfolding Protein Diseases Causing Motoneuronal and Muscle Cell Death

Misfolding protein diseases are a wide class of disorders in which the aberrantly folded protein aggregates accumulate in affected cells. In the brain and in the skeletal muscle, misfolded protein accumulation induces a variety of cell dysfunctions that frequently lead to cell death. In motoneuron diseases (MNDs), misfolded proteins accumulate primarily in motoneurons, glial cells and/or skeletal muscle cells, altering motor function. The deleterious effects of misfolded proteins can be counteracted by the activity of the protein quality control (PQC) system, composed of chaperone proteins and degradative systems. Here, we focus on a PQC system component: heat shock protein family B (small) member 8 (HSPB8), a chaperone induced by harmful stressful events, including proteotoxicity. In motoneuron and muscle cells, misfolded proteins activate HSPB8 transcription and enhance HSPB8 levels, which contributes to prevent aggregate formation and their harmful effects. HSPB8 acts not only as a chaperone, but also facilitates the autophagy process, to enable the efficient clearance of the misfolded proteins. HSPB8 acts as a dimer bound to the HSP70 co-chaperone BAG3, a scaffold protein that is also capable of binding to HSP70 (associated with the E3-ligase CHIP) and dynein. When this complex is formed, it is transported by dynein to the microtubule organization center (MTOC), where aggresomes are formed. Here, misfolded proteins are engulfed into nascent autophagosomes to be degraded via the chaperone-assisted selective autophagy (CASA). When CASA is insufficient or impaired, HSP70 and CHIP associate with an alternative co-chaperone, BAG1, which routes misfolded proteins to the proteasome for degradation. The finely tuned equilibrium between proteasome and CASA activity is thought to be crucial for maintaining the functional cell homeostasis during proteotoxic stresses, which in turn is essential for cell survival. This fine equilibrium seems to be altered in MNDs, like Amyotrophic lateral sclerosis (ALS) and spinal and bulbar muscular atrophy (SBMA), contributing to the onset and the progression of disease. Here, we will review how misfolded proteins may affect the PQC system and how the proper activity of this system can be restored by boosting or regulating HSPB8 activity, with the aim to ameliorate disease progression in these two fatal MNDs.

Feedback inhibition of cAMP effector signaling by a chaperone-assisted ubiquitin system

Activation of G-protein coupled receptors elevates cAMP levels promoting dissociation of protein kinase A (PKA) holoenzymes and release of catalytic subunits (PKAc). This results in PKAc-mediated phosphorylation of compartmentalized substrates that control central aspects of cell physiology. The mechanism of PKAc activation and signaling have been largely characterized. However, the modes of PKAc inactivation by regulated proteolysis were unknown. Here, we identify a regulatory mechanism that precisely tunes PKAc stability and downstream signaling. Following agonist stimulation, the recruitment of the chaperone-bound E3 ligase CHIP promotes ubiquitylation and proteolysis of PKAc, thus attenuating cAMP signaling. Genetic inactivation of CHIP or pharmacological inhibition of HSP70 enhances PKAc signaling and sustains hippocampal long-term potentiation. Interestingly, primary fibroblasts from autosomal recessive spinocerebellar ataxia 16 (SCAR16) patients carrying germline inactivating mutations of CHIP show a dramatic dysregulation of PKA signaling. This suggests the existence of a negative feedback mechanism for restricting hormonally controlled PKA activities.

Disrupted structure and aberrant function of CHIP mediates the loss of motor and cognitive function in preclinical models of SCAR16

CHIP (carboxyl terminus of heat shock 70-interacting protein) has long been recognized as an active member of the cellular protein quality control system given the ability of CHIP to function as both a co-chaperone and ubiquitin ligase. We discovered a genetic disease, now known as spinocerebellar autosomal recessive 16 (SCAR16), resulting from a coding mutation that caused a loss of CHIP ubiquitin ligase function. The initial mutation describing SCAR16 was a missense mutation in the ubiquitin ligase domain of CHIP (p.T246M). Using multiple biophysical and cellular approaches, we demonstrated that T246M mutation results in structural disorganization and misfolding of the CHIP U-box domain, promoting oligomerization, and increased proteasome-dependent turnover. CHIP-T246M has no ligase activity, but maintains interactions with chaperones and chaperone-related functions. To establish preclinical models of SCAR16, we engineered T246M at the endogenous locus in both mice and rats. Animals homozygous for T246M had both cognitive and motor cerebellar dysfunction distinct from those observed in the CHIP null animal model, as well as deficits in learning and memory, reflective of the cognitive deficits reported in SCAR16 patients. We conclude that the T246M mutation is not equivalent to the total loss of CHIP, supporting the concept that disease-causing CHIP mutations have different biophysical and functional repercussions on CHIP function that may directly correlate to the spectrum of clinical phenotypes observed in SCAR16 patients. Our findings both further expand our basic understanding of CHIP biology and provide meaningful mechanistic insight underlying the molecular drivers of SCAR16 disease pathology, which may be used to inform the development of novel therapeutics for this devastating disease.

CHIP is a multi-functional protein that bridges two opposing cellular processes: protein refolding and protein degradation. Mutations in CHIP are drivers of a debilitating and fatal disease, called spinocerebellar ataxia autosomal recessive 16 (SCAR16). Patients with CHIP mutations suffer from pathologies in both the brain, neuroendocrine, and muscle systems. Why or how CHIP mutations drive disease is unclear. At this early stage in understanding SCAR16, it is imperative to establish preclinical models to help understand the pathophysiology and mechanism of the disease, as well as to use as a platform to design and test therapies. In this manuscript we identified the structural, biochemical, cellular, and in vivo repercussions of the first mutation described in SCAR16 patients using two rodent models engineered with CRISPR/Cas9 editing to mimic a disease-causing human mutation. We established a new framework to better understand diseases involving the loss of CHIP function, the spectrum of disease-causing mutations, and the affected pathways that, in turn, will allow precision medicine approaches to treat this disease.

Adverse Effects of Fenofibrate in Mice Deficient in the Protein Quality Control Regulator, CHIP

We previously reported how the loss of CHIP expression (Carboxyl terminus of Hsc70-Interacting Protein) during pressure overload resulted in robust cardiac dysfunction, which was accompanied by a failure to maintain ATP levels in the face of increased energy demand. In this study, we analyzed the cardiac metabolome after seven days of pressure overload and found an increase in long-chain and medium-chain fatty acid metabolites in wild-type hearts. This response was attenuated in mice that lack expression of CHIP (CHIP^−/−). These findings suggest that CHIP may play an essential role in regulating oxidative metabolism pathways that are regulated, in part, by the nuclear receptor PPARα (Peroxisome Proliferator-Activated Receptor alpha). Next, we challenged CHIP^−/− mice with the PPARα agonist called fenofibrate. We found that treating CHIP^−/− mice with fenofibrate for five weeks under non-pressure overload conditions resulted in decreased skeletal muscle mass, compared to wild-type mice, and a marked increase in cardiac fibrosis accompanied by a decrease in cardiac function. Fenofibrate resulted in decreased mitochondrial cristae density in CHIP^−/− hearts as well as decreased expression of genes involved in the initiation of autophagy and mitophagy, which suggests that a metabolic challenge, in the absence of CHIP expression, impacts pathways that contribute to mitochondrial quality control. In conclusion, in the absence of functional CHIP expression, fenofibrate results in unexpected skeletal muscle and cardiac pathologies. These findings are particularly relevant to patients harboring loss-of-function mutations in CHIP and are consistent with a prominent role for CHIP in regulating cardiac metabolism.