Loss of caveolin-1 expression in knock-in mouse model of Huntington's disease suppresses pathophysiology in vivo

Identification and Benchmarking of Myokinasib-II as a Selective and Potent Chemical Probe for Exploring MLCK1 Inhibition

Deciphering the functional relevance of every protein is crucial to developing a better (patho)physiological understanding of human biology. The discovery and use of quality chemical probes propel exciting developments for developing drugs in therapeutic areas with unmet clinical needs. Myosin light-chain kinase (MLCK) serves as a possible therapeutic target in a plethora of diseases, including inflammatory diseases, cancer, etc. Recent years have seen a substantial increase in interest in exploring MLCK biology. However, there is only one widely used MLCK modulator, namely, ML-7, that too with a narrow working concentration window and high toxicity profile leading to limited insights. Herein, we report the identification of a potent and highly selective chemical probe, Myokinasib-II, from the synthesis and structure-activity relationship studies of a focused indotropane-based compound collection. Notably, it is structurally distinct from ML-7 and hence meets the need for an alternative inhibitor to study MLCK biology as per the recommended best practices. Moreover, our extensive benchmarking studies demonstrate that Myokinasib-II displays better potency, better selectivity profile, and no nonspecific interference in relevant assays as compared to other known MLCK inhibitors.

Physiological and pathological roles of caveolins in the central nervous system

Caveolins are a family of transmembrane proteins located in caveolae, small lipid raft invaginations of the plasma membrane. The roles of caveolin-enriched lipid rafts are diverse, and include mechano-protection, lipid homeostasis, metabolism, transport, and cell signaling. Caveolin-1 (Cav-1) and other caveolins were described in endothelial cells and later in other cell types of the central nervous system (CNS), including neurons, astrocytes, oligodendrocytes, microglia, and pericytes. This pancellular presence of caveolins demands a better understanding of their functional roles in each cell type. In this review we describe the various functions of Cav-1 in the cells of normal and pathological brains. Several emerging preclinical findings suggest that Cav-1 could represent a potential therapeutic target in brain disorders.

Ribosome Profiling and Mass Spectrometry Reveal Widespread Mitochondrial Translation Defects in a Striatal Cell Model of Huntington Disease

Huntington disease (HD) is caused by an expanded polyglutamine mutation in huntingtin (mHTT) that promotes prominent atrophy in the striatum and subsequent psychiatric, cognitive deficits, and choreiform movements. Multiple lines of evidence point to an association between HD and aberrant striatal mitochondrial functions; however, the present knowledge about whether (or how) mitochondrial mRNA translation is differentially regulated in HD remains unclear. We found that protein synthesis is diminished in HD mitochondria compared to healthy control striatal cell models. We utilized ribosome profiling (Ribo-Seq) to analyze detailed snapshots of ribosome occupancy of the mitochondrial mRNA transcripts in control and HD striatal cell models. The Ribo-Seq data revealed almost unaltered ribosome occupancy on the nuclear-encoded mitochondrial transcripts involved in oxidative phosphorylation (SDHA, Ndufv1, Timm23, Tomm5, Mrps22) in HD cells. By contrast, ribosome occupancy was dramatically increased for mitochondrially encoded oxidative phosphorylation mRNAs (mt-Nd1, mt-Nd2, mt-Nd4, mt-Nd4l, mt-Nd5, mt-Nd6, mt-Co1, mt-Cytb, and mt-ATP8). We also applied tandem mass tag-based mass spectrometry identification of mitochondrial proteins to derive correlations between ribosome occupancy and actual mature mitochondrial protein products. We found many mitochondrial transcripts with comparable or higher ribosome occupancy, but diminished mitochondrial protein products, in HD. Thus, our study provides the first evidence of a widespread dichotomous effect on ribosome occupancy and protein abundance of mitochondria-related genes in HD.

Are There Lipid Membrane-Domain Subtypes in Neurons with Different Roles in Calcium Signaling?

Lipid membrane nanodomains or lipid rafts are 10-200 nm diameter size cholesterol- and sphingolipid-enriched domains of the plasma membrane, gathering many proteins with different roles. Isolation and characterization of plasma membrane proteins by differential centrifugation and proteomic studies have revealed a remarkable diversity of proteins in these domains. The limited size of the lipid membrane nanodomain challenges the simple possibility that all of them can coexist within the same lipid membrane domain. As caveolin-1, flotillin isoforms and gangliosides are currently used as neuronal lipid membrane nanodomain markers, we first analyzed the structural features of these components forming nanodomains at the plasma membrane since they are relevant for building supramolecular complexes constituted by these molecular signatures. Among the proteins associated with neuronal lipid membrane nanodomains, there are a large number of proteins that play major roles in calcium signaling, such as ionotropic and metabotropic receptors for neurotransmitters, calcium channels, and calcium pumps. This review highlights a large variation between the calcium signaling proteins that have been reported to be associated with isolated caveolin-1 and flotillin-lipid membrane nanodomains. Since these calcium signaling proteins are scattered in different locations of the neuronal plasma membrane, i.e., in presynapses, postsynapses, axonal or dendritic trees, or in the neuronal soma, our analysis suggests that different lipid membrane-domain subtypes should exist in neurons. Furthermore, we conclude that classification of lipid membrane domains by their content in calcium signaling proteins sheds light on the roles of these domains for neuronal activities that are dependent upon the intracellular calcium concentration. Some examples described in this review include the synaptic and metabolic activity, secretion of neurotransmitters and neuromodulators, neuronal excitability (long-term potentiation and long-term depression), axonal and dendritic growth but also neuronal cell survival and death.

Early proteostasis of caveolins synchronizes trafficking, degradation, and oligomerization to prevent toxic aggregation

Caveolin-1 (CAV1) and CAV3 are membrane-sculpting proteins driving the formation of the plasma membrane (PM) caveolae. Within the PM mosaic environment, caveola assembly is unique as it requires progressive oligomerization of newly synthesized caveolins while trafficking through the biosynthetic-secretory pathway. Here, we have investigated these early events by combining structural, biochemical, and microscopy studies. We uncover striking trafficking differences between caveolins, with CAV1 rapidly exported to the Golgi and PM while CAV3 is initially retained in the endoplasmic reticulum and laterally moves into lipid droplets. The levels of caveolins in the endoplasmic reticulum are controlled by proteasomal degradation, and only monomeric/low oligomeric caveolins are exported into the cis-Golgi with higher-order oligomers assembling beyond this compartment. When any of those early proteostatic mechanisms are compromised, chemically or genetically, caveolins tend to accumulate along the secretory pathway forming non-functional aggregates, causing organelle damage and triggering cellular stress. Accordingly, we propose a model in which disrupted proteostasis of newly synthesized caveolins contributes to pathogenesis.

Altered Cholesterol Homeostasis in Huntington’s Disease

Huntington’s disease (HD) is an autosomal dominant genetic disorder caused by an expansion of the CAG repeat in the first exon of Huntingtin’s gene. The associated neurodegeneration mainly affects the striatum and the cortex at early stages and progressively spreads to other brain structures. Targeting HD at its earlier stages is under intense investigation. Numerous drugs were tested, with a rate of success of only 3.5% approved molecules used as symptomatic treatment. The restoration of cholesterol metabolism, which is central to the brain homeostasis and strongly altered in HD, could be an interesting disease-modifying strategy. Cholesterol is an essential membrane component in the central nervous system (CNS); alterations of its homeostasis have deleterious consequences on neuronal functions. The levels of several sterols, upstream of cholesterol, are markedly decreased within the striatum of HD mouse model. Transcription of cholesterol biosynthetic genes is reduced in HD cell and mouse models as well as post-mortem striatal and cortical tissues from HD patients. Since the dynamic of brain cholesterol metabolism is complex, it is essential to establish the best method to target it in HD. Cholesterol, which does not cross the blood-brain-barrier, is locally synthesized and renewed within the brain. All cell types in the CNS synthesize cholesterol during development but as they progress through adulthood, neurons down-regulate their cholesterol synthesis and turn to astrocytes for their full supply. Cellular levels of cholesterol reflect the dynamic balance between synthesis, uptake and export, all integrated into the context of the cross talk between neurons and glial cells. In this review, we describe the latest advances regarding the role of cholesterol deregulation in neuronal functions and how this could be a determinant factor in neuronal degeneration and HD progression. The pathways and major mechanisms by which cholesterol and sterols are regulated in the CNS will be described. From this overview, we discuss the main clinical strategies for manipulating cholesterol metabolism in the CNS, and how to reinstate a proper balance in HD.

An infrared spectral biomarker accurately predicts neurodegenerative disease class in the absence of overt symptoms

Although some neurodegenerative diseases can be identified by behavioral characteristics relatively late in disease progression, we currently lack methods to predict who has developed disease before the onset of symptoms, when onset will occur, or the outcome of therapeutics. New biomarkers are needed. Here we describe spectral phenotyping, a new kind of biomarker that makes disease predictions based on chemical rather than biological endpoints in cells. Spectral phenotyping uses Fourier Transform Infrared (FTIR) spectromicroscopy to produce an absorbance signature as a rapid physiological indicator of disease state. FTIR spectromicroscopy has over the past been used in differential diagnoses of manifest disease. Here, we report that the unique FTIR chemical signature accurately predicts disease class in mouse with high probability in the absence of brain pathology. In human cells, the FTIR biomarker accurately predicts neurodegenerative disease class using fibroblasts as surrogate cells.

The Discovery of Highly Potent THP Derivatives as OCTN2 Inhibitors: From Structure-Based Virtual Screening to In Vivo Biological Activity

A mismatch between β-oxidation and the tricarboxylic acid cycle (TCA) cycle flux in mitochondria produces an accumulation of lipid metabolic intermediates, resulting in both blunted metabolic flexibility and decreased glucose utilization in the affected cells. The ability of the cell to switch to glucose as an energy substrate can be restored by reducing the reliance of the cell on fatty acid oxidation. The inhibition of the carnitine system, limiting the carnitine shuttle to the oxidation of lipids in the mitochondria, allows cells to develop a high plasticity to metabolic rewiring with a decrease in fatty acid oxidation and a parallel increase in glucose oxidation. We found that 3-(2,2,2-trimethylhydrazine)propionate (THP), which is able to reduce cellular carnitine levels by blocking both carnitine biosynthesis and the cell membrane carnitine/organic cation transporter (OCTN2), was reported to improve mitochondrial dysfunction in several diseases, such as Huntington's disease (HD). Here, new THP-derived carnitine-lowering agents (TCL), characterized by a high affinity for the OCTN2 with a minimal effect on carnitine synthesis, were developed, and their biological activities were evaluated in both in vitro and in vivo HD models. Certain compounds showed promising biological activities: reducing protein aggregates in HD cells, ameliorating motility defects, and increasing the lifespan of HD Drosophila melanogaster.

Non-caveolar caveolins – duties outside the caves

Caveolae are invaginations of the plasma membrane that are remarkably abundant in adipocytes, endothelial cells and muscle. Caveolae provide cells with resources for mechanoprotection, can undergo fission from the plasma membrane and can regulate a variety of signaling pathways. Caveolins are fundamental components of caveolae, but many cells, such as hepatocytes and many neurons, express caveolins without forming distinguishable caveolae. Thus, the function of caveolins goes beyond their roles as caveolar components. The membrane-organizing and -sculpting capacities of caveolins, in combination with their complex intracellular trafficking, might contribute to these additional roles. Furthermore, non-caveolar caveolins can potentially interact with proteins normally excluded from caveolae. Here, we revisit the non-canonical roles of caveolins in a variety of cellular contexts including liver, brain, lymphocytes, cilia and cancer cells, as well as consider insights from invertebrate systems. Non-caveolar caveolins can determine the intracellular fluxes of active lipids, including cholesterol and sphingolipids. Accordingly, caveolins directly or remotely control a plethora of lipid-dependent processes such as the endocytosis of specific cargoes, sorting and transport in endocytic compartments, or different signaling pathways. Indeed, loss-of-function of non-caveolar caveolins might contribute to the common phenotypes and pathologies of caveolin-deficient cells and animals.

Mutant huntingtin fails to directly impair brain mitochondria

Although the mechanisms by which mutant huntingtin (mHtt) results in Huntington's disease (HD) remain unclear, mHtt-induced mitochondrial defects were implicated in HD pathogenesis. The effect of mHtt could be mediated by transcriptional alterations, by direct interaction with mitochondria, or by both. In the present study, we tested a hypothesis that mHtt directly damages mitochondria. To test this hypothesis, we applied brain cytosolic fraction from YAC128 mice, containing mHtt, to brain non-synaptic and synaptic mitochondria from wild-type mice and assessed mitochondrial respiration with a Clark-type oxygen electrode, membrane potential and Ca2+ uptake capacity with tetraphenylphosphonium (TPP+ )- and Ca2+ -sensitive electrodes, respectively, and, reactive oxygen species production with Amplex Red assay. The amount of mHtt bound to mitochondria following incubation with mHtt-containing cytosolic fraction was greater than the amount of mHtt bound to brain mitochondria isolated from YAC128 mice. Despite mHtt binding to wild-type mitochondria, no abnormalities in mitochondrial functions were detected. This is consistent with our previous results demonstrating the lack of defects in brain mitochondria isolated from R6/2 and YAC128 mice. This, however, could be because of partial loss of mitochondrially bound mHtt during the isolation procedure. Consequently, we increased the amount of mitochondrially bound mHtt by incubating brain non-synaptic and synaptic mitochondria isolated from YAC128 mice with mHtt-containing cytosolic fraction. Despite the enrichment of YAC128 brain mitochondria with mHtt, mitochondrial functions (respiration, membrane potential, reactive oxygen species production, Ca2+ uptake capacity) remained unchanged. Overall, our results suggest that mHtt does not directly impair mitochondrial functions, arguing against the involvement of this mechanism in HD pathogenesis. Open Science: This manuscript was awarded with the Open Materials Badge For more information see: https://cos.io/our-services/open-science-badges/.

A common pathomechanism in GMAP-210- and LBR-related diseases

Biallelic loss-of-function mutations in TRIP11, encoding the golgin GMAP-210, cause the lethal human chondrodysplasia achondrogenesis 1A (ACG1A). We now find that a homozygous splice-site mutation of the lamin B receptor (LBR) gene results in the same phenotype. Intrigued by the genetic heterogeneity, we compared GMAP-210- and LBR-deficient primary cells to unravel how particular mutations in LBR cause a phenocopy of ACG1A. We could exclude a regulatory interaction between LBR and GMAP-210 in patients' cells. However, we discovered a common disruption of Golgi apparatus architecture that was accompanied by decreased secretory trafficking in both cases. Deficiency of Golgi-dependent glycan processing indicated a similar downstream effect of the disease-causing mutations upon Golgi function. Unexpectedly, our results thus point to a common pathogenic mechanism in GMAP-210- and LBR-related diseases attributable to defective secretory trafficking at the Golgi apparatus.

XJB-5-131-mediated improvement in physiology and behaviour of the R6/2 mouse model of Huntington's disease is age- and sex- dependent

We have reported that the radical scavenger XJB-5-131 attenuates or reverses progression of the disease phenotype in the HdhQ(150/150) mouse, a slow onset model of HD. Here, we tested whether XJB-5-131 has beneficial effects in R6/2 mice, a severe early onset model of HD. We found that XJB-5-131 has beneficial effects in R6/2 mice, by delaying features of the motor and histological phenotype. The impact was sex-dependent, with a stronger effect in male mice. XJB-5-131 treatment improved some locomotor deficits in female R6/2 mice, but the effects were, in general, greater in male mice. Chronic treatment of male R6/2 mice with XJB-5-1-131 reduced weight loss, and improved the motor and temperature regulation deficits, especially in male mice. Treatment with XJB-5-131 had no effect on the lifespan of R6/2 mice. Nevertheless, it significantly slowed somatic expansion at 90 days, and reduced the density of inclusions. Our data show that while treatment with XJB-5-131 had complex effects on the phenotype of R6/2 mice, it produced a number of significant improvements in this severe model of HD.

Alteration in Fluidity of Cell Plasma Membrane in Huntington Disease Revealed by Spectral Phasor Analysis

Huntington disease (HD) is a late-onset genetic neurodegenerative disorder caused by expansion of cytosine-adenine-guanine (CAG) trinucleotide in the exon 1 of the gene encoding the polyglutamine (polyQ). It has been shown that protein degradation and lipid metabolism is altered in HD. In many neurodegenerative disorders, impaired lipid homeostasis is one of the early events in the disease onset. Yet, little is known about how mutant huntingtin may affect phospholipids membrane fluidity. Here, we investigated how membrane fluidity in the living cells (differentiated PC12 and HEK293 cell lines) are affected using a hyperspectral imaging of widely used probes, LAURDAN. Using phasor approach, we characterized the fluorescence of LAURDAN that is sensitive to the polarity of the immediate environment. LAURDAN is affected by the physical order of phospholipids (lipid order) and reports the membrane fluidity. We also validated our results using a different fluorescent membrane probe, Nile Red (NR). The plasma membrane in the cells expressing expanded polyQ shows a shift toward increased membrane fluidity revealed by both LAURDAN and NR spectral phasors. This finding brings a new perspective in the understanding of the early stages of HD that can be used as a target for drug screening.

Cholesterol-loaded nanoparticles ameliorate synaptic and cognitive function in Huntington's disease mice

Brain cholesterol biosynthesis and cholesterol levels are reduced in mouse models of Huntington's disease (HD), suggesting that locally synthesized, newly formed cholesterol is less available to neurons. This may be detrimental for neuronal function, especially given that locally synthesized cholesterol is implicated in synapse integrity and remodeling. Here, we used biodegradable and biocompatible polymeric nanoparticles (NPs) modified with glycopeptides (g7) and loaded with cholesterol (g7‐NPs‐Chol), which per se is not blood–brain barrier (BBB) permeable, to obtain high‐rate cholesterol delivery into the brain after intraperitoneal injection in HD mice. We report that g7‐NPs, in contrast to unmodified NPs, efficiently crossed the BBB and localized in glial and neuronal cells in different brain regions. We also found that repeated systemic delivery of g7‐NPs‐Chol rescued synaptic and cognitive dysfunction and partially improved global activity in HD mice. These results demonstrate that cholesterol supplementation to the HD brain reverses functional alterations associated with HD and highlight the potential of this new drug‐administration route to the diseased brain.

Oxidative metabolism and Ca2+ handling in isolated brain mitochondria and striatal neurons from R6/2 mice, a model of Huntington's disease

Alterations in oxidative metabolism and defects in mitochondrial Ca²⁺ handling have been implicated in the pathology of Huntington's disease (HD), but existing data are contradictory. We investigated the effect of human mHtt fragments on oxidative metabolism and Ca²⁺ handling in isolated brain mitochondria and cultured striatal neurons from the R6/2 mouse model of HD. Non-synaptic and synaptic mitochondria isolated from the brains of R6/2 mice had similar respiratory rates and Ca²⁺ uptake capacity compared with mitochondria from wild-type (WT) mice. Respiratory activity of cultured striatal neurons measured with Seahorse XF24 flux analyzer revealed unaltered cellular respiration in neurons derived from R6/2 mice compared with neurons from WT animals. Consistent with the lack of respiratory dysfunction, ATP content of cultured striatal neurons from R6/2 and WT mice was similar. Mitochondrial Ca²⁺ accumulation was also evaluated in cultured striatal neurons from R6/2 and WT animals. Our data obtained with striatal neurons derived from R6/2 and WT mice show that both glutamate-induced increases in cytosolic Ca²⁺ and subsequent carbonilcyanide p-triflouromethoxyphenylhydrazone-induced increases in cytosolic Ca²⁺ were similar between WT and R6/2, suggesting that mitochondria in neurons derived from both types of animals accumulated comparable amounts of Ca²⁺ Overall, our data argue against respiratory deficiency and impaired Ca²⁺ handling induced by human mHtt fragments in both isolated brain mitochondria and cultured striatal neurons from transgenic R6/2 mice.

Uptake Mechanism and Direct Translocation of a New CPP for siRNA Delivery

Since their development, cell-penetrating peptides (CPPs) have been used as delivery vehicles for various genetic or therapeutic agents; however, the uptake mechanisms of CPPs and the delivery details are still unclear. Understanding the mechanisms of cellular internalization of CPPs facilitate their development of CPPs as gene delivery vectors. In the present study, we evaluated the internalization process of a previously designed CPP, STR-KV, complexed with small interference RNA (siRNA) targeting at the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene. Using heparin treatment and chemical endocytic inhibitors, we elucidated that the electrostatic interaction of STR-KV/siRNA complex with heparin sulfate proteoglycans at the cell membrane surface triggered the energy-independent uptake of the majority of the complexes, which most likely through a direct translocation pathway. The intracellular trafficking and internalization kinetics observed by confocal microscopy also confirmed that the complex was uptaken through a nonendocytic pathway.

CYP46A1, the rate-limiting enzyme for cholesterol degradation, is neuroprotective in Huntington's disease

Huntington’s disease is an autosomal dominant neurodegenerative disease caused by abnormal polyglutamine expansion in huntingtin (Exp-HTT) leading to degeneration of striatal neurons. Altered brain cholesterol homeostasis has been implicated in Huntington’s disease, with increased accumulation of cholesterol in striatal neurons yet reduced levels of cholesterol metabolic precursors. To elucidate these two seemingly opposing dysregulations, we investigated the expression of cholesterol 24-hydroxylase (CYP46A1), the neuronal-specific and rate-limiting enzyme for cholesterol conversion to 24S-hydroxycholesterol (24S-OHC). CYP46A1 protein levels were decreased in the putamen, but not cerebral cortex samples, of post-mortem Huntington’s disease patients when compared to controls. Cyp46A1 mRNA and CYP46A1 protein levels were also decreased in the striatum of the R6/2 Huntington’s disease mouse model and in ST hdh Q111 cell lines. In vivo , in a wild-type context, knocking down CYP46A1 expression in the striatum, via an adeno-associated virus-mediated delivery of selective shCYP46A1, reproduced the Huntington’s disease phenotype, with spontaneous striatal neuron degeneration and motor deficits, as assessed by rotarod. In vitro , CYP46A1 restoration protected ST hdh Q111 and Exp-HTT-expressing striatal neurons in culture from cell death. In the R6/2 Huntington’s disease mouse model, adeno-associated virus-mediated delivery of CYP46A1 into the striatum decreased neuronal atrophy, decreased the number, intensity level and size of Exp-HTT aggregates and improved motor deficits, as assessed by rotarod and clasping behavioural tests. Adeno-associated virus-CYP46A1 infection in R6/2 mice also restored levels of cholesterol and lanosterol and increased levels of desmosterol. In vitro , lanosterol and desmosterol were found to protect striatal neurons expressing Exp-HTT from death. We conclude that restoring CYP46A1 activity in the striatum promises a new therapeutic approach in Huntington’s disease.

Brain Cholesterol Metabolism and Its Defects: Linkage to Neurodegenerative Diseases and Synaptic Dysfunction

Cholesterol is an important constituent of cell membranes and plays a crucial role in the compartmentalization of the plasma membrane and signaling. Brain cholesterol accounts for a large proportion of the body's total cholesterol, existing in two pools: the plasma membranes of neurons and glial cells and the myelin membranes . Cholesterol has been recently shown to be important for synaptic transmission, and a link between cholesterol metabolism defects and neurodegenerative disorders is now recognized. Many neurodegenerative diseases are characterized by impaired cholesterol turnover in the brain. However, at which stage the cholesterol biosynthetic pathway is perturbed and how this contributes to pathogenesis remains unknown. Cognitive deficits and neurodegeneration may be associated with impaired synaptic transduction. Defects in cholesterol biosynthesis can trigger dysfunction of synaptic transmission. In this review, an overview of cholesterol turnover under physiological and pathological conditions is presented (Huntington's, Niemann-Pick type C diseases, Smith-Lemli-Opitz syndrome). We will discuss possible mechanisms by which cholesterol content in the plasma membrane influences synaptic processes. Changes in cholesterol metabolism in Alzheimer's disease, Parkinson's disease, and autistic disorders are beyond the scope of this review and will be summarized in our next paper.

Loss of parkin promotes lipid rafts-dependent endocytosis through accumulating caveolin-1: implications for Parkinson's disease

Background

Parkinson’s disease (PD) is characterized by progressive loss of midbrain dopaminergic neurons, resulting in motor dysfunctions. While most PD is sporadic in nature, a significant subset can be linked to either autosomal dominant or recessive mutations. PARK2, encoding the E3 ubiquitin ligase, parkin, is the most frequently mutated gene in autosomal recessive early onset PD. It has recently been reported that PD-associated gene products such as PINK1, α-synuclein, LRRK2, and DJ-1, as well as parkin associate with lipid rafts, suggesting that the dysfunction of these proteins in lipid rafts may be a causal factor of PD. Therefore here, we examined the relationship between lipid rafts-related proteins and parkin.

Results

We identified caveolin-1 (cav-1), which is one of the major constituents of lipid rafts at the plasma membrane, as a substrate of parkin. Loss of parkin function was found to disrupt the ubiquitination and degradation of cav-1, resulting in elevated cav-1 protein level in cells. Moreover, the total cholesterol level and membrane fluidity was altered by parkin deficiency, causing dysregulation of lipid rafts-dependent endocytosis. Further, cell-to-cell transmission of α-synuclein was facilitated by parkin deficiency.

Conclusions

Our results demonstrate that alterations in lipid rafts by the loss of parkin via cav-1 may be a causal factor of PD, and cav-1 may be a novel therapeutic target for PD.

Suppression of Somatic Expansion Delays the Onset of Pathophysiology in a Mouse Model of Huntington's Disease

Huntington’s Disease (HD) is caused by inheritance of a single disease-length allele harboring an expanded CAG repeat, which continues to expand in somatic tissues with age. The inherited disease allele expresses a toxic protein, and whether further somatic expansion adds to toxicity is unknown. We have created an HD mouse model that resolves the effects of the inherited and somatic expansions. We show here that suppressing somatic expansion substantially delays the onset of disease in littermates that inherit the same disease-length allele. Furthermore, a pharmacological inhibitor, XJB-5-131, inhibits the lengthening of the repeat tracks, and correlates with rescue of motor decline in these animals. The results provide evidence that pharmacological approaches to offset disease progression are possible.

Huntington’s Disease (HD) is caused by inheritance of a single disease-length allele harboring an expanded CAG repeat, which continues to expand in somatic tissues with age. There is no correction for the inherited mutation, but if somatic expansion contributes to disease, then a therapeutic approach is possible. The inherited disease allele expresses a toxic protein, and whether further somatic expansion adds to toxicity is unknown. Here we describe a mouse model of Huntington’s disease that allows us to separate out the effects of the inherited gene from the expansion that occurs during life. We find that blocking the continued expansion of the gene causes a delay in onset of symptoms. This result opens the doors to future therapeutics designed to shorten the repeat.

Oxidative metabolism in YAC128 mouse model of Huntington's disease

Alterations in oxidative metabolism are considered to be one of the major contributors to Huntington's disease (HD) pathogenesis. However, existing data about oxidative metabolism in HD are contradictory. Here, we investigated the effect of mutant huntingtin (mHtt) on oxidative metabolism in YAC128 mice. Both mHtt and wild-type huntingtin (Htt) were associated with mitochondria and the amount of bound Htt was four-times higher than the amount of bound mHtt. Percoll gradient-purified brain synaptic and non-synaptic mitochondria as well as unpurified brain, liver and heart mitochondria, isolated from 2- and 10-month-old YAC128 mice and age-matched WT littermates had similar respiratory rates. There was no difference in mitochondrial membrane potential or ADP and ATP levels. Expression of selected nuclear-encoded mitochondrial proteins in 2- and 10-month-old YAC128 and WT mice was similar. Cultured striatal and cortical neurons from YAC128 and WT mice had similar respiratory and glycolytic activities as measured with Seahorse XF24 analyzer in medium containing 10 mm glucose and 15 mm pyruvate. In the medium with 2.5 mm glucose, YAC128 striatal neurons had similar respiration, but slightly lower glycolytic activity. Striatal neurons had lower maximal respiration compared with cortical neurons. In vivo experiments with YAC128 and WT mice showed similar O2 consumption, CO2 release, physical activity, food consumption and fasted blood glucose. However, YAC128 mice were heavier and had more body fat compared with WT mice. Overall, our data argue against respiratory deficiency in YAC128 mice and, consequently, suggest that mitochondrial respiratory dysfunction is not essential for HD pathogenesis.