The Reversible Carnitine Palmitoyltransferase 1 Inhibitor (Teglicar) Ameliorates the Neurodegenerative Phenotype in a Drosophila Huntington's Disease Model by Acting on the Expression of Carnitine-Related Genes.

Huntington's disease (HD) is a dramatic neurodegenerative disorder caused by the abnormal expansion of a CAG triplet in the huntingtin gene, producing an abnormal protein. As it leads to the death of neurons in the cerebral cortex, the patients primarily present with neurological symptoms, but recently metabolic changes resulting from mitochondrial dysfunction have been identified as novel pathological features. The carnitine shuttle is a complex consisting of three enzymes whose function is to transport the long-chain fatty acids into the mitochondria. Here, its pharmacological modification was used to test the hypothesis that shifting metabolism to lipid oxidation exacerbates the HD symptoms. Behavioural and transcriptional analyses were carried out on HD Drosophila model, to evaluate the involvement of the carnitine cycle in this pathogenesis. Pharmacological inhibition of CPT1, the rate-limiting enzyme of the carnitine cycle, ameliorates the HD symptoms in Drosophila, likely acting on the expression of carnitine-related genes.

FAN1 exo- not endo-nuclease pausing on disease-associated slipped-DNA repeats: A mechanism of repeat instability.

Ongoing inchworm-like CAG and CGG repeat expansions in brains, arising by aberrant processing of slipped DNAs, may drive Huntington's disease, fragile X syndrome, and autism. FAN1 nuclease modifies hyper-expansion rates by unknown means. We show that FAN1, through iterative cycles, binds, dimerizes, and cleaves slipped DNAs, yielding striking exo-nuclease pauses along slip-outs: 5'-C↓A↓GC↓A↓G-3' and 5'-C↓T↓G↓C↓T↓G-3'. CAG excision is slower than CTG and requires intra-strand A·A and T·T mismatches. Fully paired hairpins arrested excision, whereas disease-delaying CAA interruptions further slowed excision. Endo-nucleolytic cleavage is insensitive to slip-outs. Rare FAN1 variants are found in individuals with autism with CGG/CCG expansions, and CGG/CCG slip-outs show exo-nuclease pauses. The slip-out-specific ligand, naphthyridine-azaquinolone, which induces contractions of expanded repeats in vivo, requires FAN1 for its effect, and protects slip-outs from FAN1 exo-, but not endo-, nucleolytic digestion. FAN1's inchworm pausing of slip-out excision rates is well suited to modify inchworm expansion rates, which modify disease onset and progression.

FAN1 controls mismatch repair complex assembly via MLH1 retention to stabilize CAG repeat expansion in Huntington's disease.

CAG repeat expansion in the HTT gene drives Huntington's disease (HD) pathogenesis and is modulated by DNA damage repair pathways. In this context, the interaction between FAN1, a DNA-structure-specific nuclease, and MLH1, member of the DNA mismatch repair pathway (MMR), is not defined. Here, we identify a highly conserved SPYF motif at the N terminus of FAN1 that binds to MLH1. Our data support a model where FAN1 has two distinct functions to stabilize CAG repeats. On one hand, it binds MLH1 to restrict its recruitment by MSH3, thus inhibiting the assembly of a functional MMR complex that would otherwise promote CAG repeat expansion. On the other hand, it promotes accurate repair via its nuclease activity. These data highlight a potential avenue for HD therapeutics in attenuating somatic expansion.

DNA polymerase θ promotes CAG•CTG repeat expansions in Huntington's disease via insertion sequences of its catalytic domain.

Huntington's disease (HD), a neurodegenerative disease characterized by progressive dementia, psychiatric problems, and chorea, is known to be caused by CAG repeat expansions in the HD gene HTT. However, the mechanism of this pathology is not fully understood. The translesion DNA polymerase θ (Polθ) carries a large insertion sequence in its catalytic domain, which has been shown to allow DNA loop-outs in the primer strand. As a result of high levels of oxidative DNA damage in neural cells and Polθ's subsequent involvement in base excision repair of oxidative DNA damage, we hypothesized that Polθ contributes to CAG repeat expansion while repairing oxidative damage within HTT. Here, we performed Polθ-catalyzed in vitro DNA synthesis using various CAG•CTG repeat DNA substrates that are similar to base excision repair intermediates. We show that Polθ efficiently extends (CAG)n•(CTG)n hairpin primers, resulting in hairpin retention and repeat expansion. Polθ also triggers repeat expansions to pass the threshold for HD when the DNA template contains 35 repeats upward. Strikingly, Polθ depleted of the catalytic insertion fails to induce repeat expansions regardless of primers and templates used, indicating that the insertion sequence is responsible for Polθ's error-causing activity. In addition, the level of chromatin-bound Polθ in HD cells is significantly higher than in non-HD cells and exactly correlates with the degree of CAG repeat expansion, implying Polθ's involvement in triplet repeat instability. Therefore, we have identified Polθ as a potent factor that promotes CAG•CTG repeat expansions in HD and other neurodegenerative disorders.

A Novel Pharmacological Protective Role for Safranal in an Animal Model of Huntington's Disease.

Huntington's disease (HD) is a progressive, neurodegenerative and inherited disease and recent years have witnessed the understanding of the cellular and molecular mechanisms related to HD. Safranal, an organic compound isolated from saffron, has been reported to have anti-apoptotic, anti-inflammatory and antioxidant activity and has studied in chronic and neurodegenerative disease. Therefore, this study was aimed to investigate the effect of safranal on 3-NP induced locomotor activity and biochemical alterations in rats. To this aim, 40 male Wistar rats weighting 250-300 g were divided into 5 groups (n = 8) including sham, 3-NP group (10 mg/kg) as control and treatment groups (3-NP + safranal 0.75, 1.5 and 3 mg/kg) in two weeks duration of treatment. Behavioral/movement assessments in addition to oxidant/antioxidant markers in rat cortex and striatum were evaluated in control and treatment groups. Here, we found that safranal significantly alleviated 3-NP-induced changes of body weight, rotarod activity, number of vacuous chewing movements (VCMs), and locomotor activity. In addition, brain tissue assessments in cortex and striatum revealed that safranal could prevent the elevation of nitrite and malondialdehyde (MDA) levels as well as decrease of superoxide dismutase (SOD), catalase activity and glutathione (GSH) induced by 3-NP. In conclusion our results showed that safranal prevented the motor dysfunction induced by 3-NP in animal model of Huntington's disease. This effect might be due to its modulating effect on oxidants-antioxidant balance.

Inhibition of p38 Mitogen-Activated Protein Kinase Ameliorates HAP40 Depletion-Induced Toxicity and Proteasomal Defect in Huntington's Disease Model.

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by an expansion of polyglutamine stretch (polyQ) at the N-terminus of huntingtin (Htt) protein. The abnormally expanded polyQ stretch of mutant Htt makes it prone to aggregate, leading to neuropathology. HAP40 is a 40-kDa huntingtin-associated protein with undefined functions. HAP40 protein has been shown to increase in HD patients and HD mouse model cells. However, recent proteomic analysis provides new evidence that HAP40 protein is decreased in the striatum of HD knockin model mice. In this study, we developed HAP40-specific antibody and showed that both HAP40 mRNA and its encoded protein were reduced in HD striatal neuronal STHDHQ111/Q111 cells. Depletion of endogenous HAP40 led to cytotoxicity that was linked to increased accumulation of aggregated and soluble forms of mutant Htt, which recapitulates HD pathology. Moreover, we found that HAP40 depletion reduced the proteasomal chymotrypsin-like activity and increased the autophagic flux. Importantly, inhibition of p38 MAPK pathway by PD169316 increased chymotrypsin-like activity and reduced accumulation of aggregated and soluble forms of mutant Htt in HAP40-depleted cells to alleviate HAP40-depletion induced cytotoxicity. Taken together, our results suggest that modulation of p38 MAPK-mediated proteasomal peptidase activity may provide a new therapeutic target to restore proteostasis in neurodegenerative diseases.

Purines and Pyrimidines: Metabolism, Function and Potential as Therapeutic Options in Neurodegenerative Diseases.

Various neurodegenerative disorders have various molecular origins but some common molecular mechanisms. In the current scenario, there are very few treatment regimens present for advanced neurodegenerative diseases. In this context, there is an urgent need for alternate options in the form of natural compounds with an ameliorating effect on patients. There have been individual scattered experiments trying to identify potential values of various intracellular metabolites. Purines and Pyrimidines, which are vital molecules governing various aspects of cellular biochemical reactions, have been long sought as crucial candidates for the same, but there are still many questions that go unanswered. Some critical functions of these molecules associated with neuromodulation activities have been identified. They are also known to play a role in foetal neurodevelopment, but there is a lacuna in understanding their mechanisms. In this review, we have tried to assemble and identify the importance of purines and pyrimidines, connecting them with the prevalence of neurodegenerative diseases. The leading cause of this class of diseases is protein misfolding and the formation of amyloids. A direct correlation between loss of balance in cellular homeostasis and amyloidosis is yet an unexplored area. This review aims at bringing the current literature available under one umbrella serving as a foundation for further extensive research in this field of drug development in neurodegenerative diseases.

Oxidative Stress Orchestrates MAPK and Nitric-Oxide Synthase Signal.

Reactive oxygen species (ROS) are not only harmful to cell survival but also essential to cell signaling through cysteine-based redox switches. In fact, ROS triggers the potential activation of mitogen-activated protein kinases (MAPKs). The 90 kDa ribosomal S6 kinase 1 (RSK1), one of the downstream mediators of the MAPK pathway, is implicated in various cellular processes through phosphorylating different substrates. As such, RSK1 associates with and phosphorylates neuronal nitric oxide (NO) synthase (nNOS) at Ser847, leading to a decrease in NO generation. In addition, the RSK1 activity is sensitive to inhibition by reversible cysteine-based redox modification of its Cys223 during oxidative stress. Aside from oxidative stress, nitrosative stress also contributes to cysteine-based redox modification. Thus, the protein kinases such as Ca2+/calmodulin (CaM)-dependent protein kinase I (CaMKI) and II (CaMKII) that phosphorylate nNOS could be potentially regulated by cysteine-based redox modification. In this review, we focus on the role of post-translational modifications in regulating nNOS and nNOS-phosphorylating protein kinases and communication among themselves.

Histone deacetylase knockouts modify transcription, CAG instability and nuclear pathology in Huntington disease mice.

Somatic expansion of the Huntington's disease (HD) CAG repeat drives the rate of a pathogenic process ultimately resulting in neuronal cell death. Although mechanisms of toxicity are poorly delineated, transcriptional dysregulation is a likely contributor. To identify modifiers that act at the level of CAG expansion and/or downstream pathogenic processes, we tested the impact of genetic knockout, in HttQ111 mice, of Hdac2 or Hdac3 in medium-spiny striatal neurons that exhibit extensive CAG expansion and exquisite disease vulnerability. Both knockouts moderately attenuated CAG expansion, with Hdac2 knockout decreasing nuclear huntingtin pathology. Hdac2 knockout resulted in a substantial transcriptional response that included modification of transcriptional dysregulation elicited by the HttQ111 allele, likely via mechanisms unrelated to instability suppression. Our results identify novel modifiers of different aspects of HD pathogenesis in medium-spiny neurons and highlight a complex relationship between the expanded Htt allele and Hdac2 with implications for targeting transcriptional dysregulation in HD.

The cholesterol 24-hydroxylase activates autophagy and decreases mutant huntingtin build-up in a neuroblastoma culture model of Huntington's disease.

OBJECTIVE: Compromised brain cholesterol turnover and altered regulation of brain cholesterol metabolism have been allied with some neurodegenerative diseases, including Huntington's disease (HD). Following our previous studies in HD, in this study we aim to investigate in vitro in a neuroblastoma cellular model of HD, the effect of CYP46A1 overexpression, an essential enzyme in cholesterol metabolism, on huntingtin aggregation and levels. RESULTS: We found that CYP46A1 reduces the quantity and size of mutant huntingtin aggregates in cells, as well as the levels of mutant huntingtin protein. Additionally, our results suggest that the observed beneficial effects of CYP46A1 in HD cells are linked to the activation of autophagy. Taken together, our results further demonstrate that CYP46A1 is a pertinent target to counteract HD progression.

A novel specific PERK activator reduces toxicity and extends survival in Huntington's disease models.

One of the pathways of the unfolded protein response, initiated by PKR-like endoplasmic reticulum kinase (PERK), is key to neuronal homeostasis in neurodegenerative diseases. PERK pathway activation is usually accomplished by inhibiting eIF2α-P dephosphorylation, after its phosphorylation by PERK. Less tried is an approach involving direct PERK activation without compromising long-term recovery of eIF2α function by dephosphorylation. Here we show major improvement in cellular (STHdhQ111/111) and mouse (R6/2) Huntington's disease (HD) models using a potent small molecule PERK activator that we developed, MK-28. MK-28 showed PERK selectivity in vitro on a 391-kinase panel and rescued cells (but not PERK-/- cells) from ER stress-induced apoptosis. Cells were also rescued by the commercial PERK activator CCT020312 but MK-28 was significantly more potent. Computational docking suggested MK-28 interaction with the PERK activation loop. MK-28 exhibited remarkable pharmacokinetic properties and high BBB penetration in mice. Transient subcutaneous delivery of MK-28 significantly improved motor and executive functions and delayed death onset in R6/2 mice, showing no toxicity. Therefore, PERK activation can treat a most aggressive HD model, suggesting a possible approach for HD therapy and worth exploring for other neurodegenerative disorders.

Cyclin-Dependent Kinase 5 Dysfunction Contributes to Depressive-like Behaviors in Huntington's Disease by Altering the DARPP-32 Phosphorylation Status in the Nucleus Accumbens.

BACKGROUND: Depression is the most common psychiatric condition in Huntington's disease (HD), with rates more than twice those found in the general population. At the present time, there is no established molecular evidence to use as a basis for depression treatment in HD. Indeed, in some patients, classic antidepressant drugs exacerbate chorea or anxiety. Cyclin-dependent kinase 5 (Cdk5) has been involved in processes associated with anxiety and depression. This study evaluated the involvement of Cdk5 in the development and prevalence of depressive-like behaviors in HD and aimed to validate Cdk5 as a target for depression treatment. METHODS: We evaluated the impact of pharmacological inhibition of Cdk5 in depressive-like and anxiety-like behaviors in Hdh+/Q111 knock-in mutant mice by using a battery of behavioral tests. Biochemical and morphological studies were performed to define the molecular mechanisms acting downstream of Cdk5 activation. A double huntingtin/DARPP-32 (dopamine- and cAMP-regulated phosphoprotein 32) knock-in mutant mouse was generated to analyze the role of DARPP-32 in HD depression. RESULTS: We found that Hdh+/Q111 mutant mice exhibited depressive-like, but not anxiety-like, behaviors starting at 2 months of age. Cdk5 inhibition by roscovitine infusion prevented depressive-like behavior and reduced DARPP-32 phosphorylation at Thr75 in the nucleus accumbens. Hdh+/Q111 mice heterozygous for DARPP-32 Thr75Ala point mutation were resistant to depressive-like behaviors. We identified ß-adducin phosphorylation as a Cdk5 downstream mechanism potentially mediating structural spine plasticity changes in the nucleus accumbens and depressive-like behavior. CONCLUSIONS: These results point to Cdk5 in the nucleus accumbens as a critical contributor to depressive-like behaviors in HD mice by altering DARPP-32/ß-adducin signaling and disrupting the dendritic spine cytoskeleton.

Inhibiting pathologically active ADAM10 rescues synaptic and cognitive decline in Huntington's disease.

A disintegrine and metalloproteinase 10 (ADAM10) is implicated in synaptic function through its interaction with postsynaptic receptors and adhesion molecules. Here, we report that levels of active ADAM10 are increased in Huntington's disease (HD) mouse cortices and striata and in human postmortem caudate. We show that, in the presence of polyglutamine-expanded (polyQ-expanded) huntingtin (HTT), ADAM10 accumulates at the postsynaptic densities (PSDs) and causes excessive cleavage of the synaptic protein N-cadherin (N-CAD). This aberrant phenotype is also detected in neurons from HD patients where it can be reverted by selective silencing of mutant HTT. Consistently, ex vivo delivery of an ADAM10 synthetic inhibitor reduces N-CAD proteolysis and corrects electrophysiological alterations in striatal medium-sized spiny neurons (MSNs) of 2 HD mouse models. Moreover, we show that heterozygous conditional deletion of ADAM10 or delivery of a competitive TAT-Pro-ADAM10709-729 peptide in R6/2 mice prevents N-CAD proteolysis and ameliorates cognitive deficits in the mice. Reduction in synapse loss was also found in R6/2 mice conditionally deleted for ADAM10. Taken together, these results point to a detrimental role of hyperactive ADAM10 at the HD synapse and provide preclinical evidence of the therapeutic potential of ADAM10 inhibition in HD.

Olfactory bulb atrophy and caspase activation observed in the BACHD rat models of Huntington disease.

Olfactory dysfunction is observed in several neurological disorders, including Huntington disease (HD), and correlates with global cognitive performance, depression and degeneration of olfactory regions in the brain. Despite clear evidence demonstrating olfactory dysfunction in HD patients, only limited details are available in murine models and the underlying mechanisms are unknown. In order to determine if alterations in the olfactory bulb (OB) are observed in HD we assessed OB weight or area from 3 to 12 months of age in the BACHD transgenic lines (TG5 and TG9). A significant decrease in the OB was observed at 6 and 12 months of age compared to WT. We also detected increased mRNA and protein expression of mutant huntingtin (mHTT) in the OB of TG5 compared to TG9 at specific ages. Despite the higher expression of mHTT in the TG5 OBs, there was increased nuclear accumulation of mHTT in the OB of TG9 compared to WT and TG5 rats. As we observed atrophy of the OB in the BACHD rats we assessed for caspase activation, a known mechanism underlying the cell death observed in HD. We characterized caspase-3, -6, -8 and - 9 mRNA and protein expression levels in the OB of the BACHD transgenic lines at 3, 6 and 12 months of age. Alterations in caspase mRNA and protein expression were detected in the TG5 and TG9 lines. However, the changes observed in the mRNA and protein levels are in some cases discordant, suggesting that the caspase protein modifications detected may be more attributable to post-translational modifications. The caspase activation studies support that cell death may be increased in the rodent HD OB and further our understanding of the olfactory dysfunction and the role of caspases in the pathogenesis of HD.

The S1P Axis: New Exciting Route for Treating Huntington's Disease.

Huntington's disease (HD) is a single-gene inheritable neurodegenerative disorder with an associated complex molecular pathogenic profile that renders it the most 'curable incurable' brain disorder. Continuous effort in the field has contributed to the recent discovery of novel potential pathogenic mechanisms. Findings in preclinical models of the disease as well as in human post-mortem brains from affected patients demonstrate that alteration of the sphingosine-1-phosphate (S1P) axis may represent a possible key player in the pathogenesis of the disease and may act as a potential actionable drug target for the development of more targeted and effective therapeutic approaches. The relevance of the path of this new 'therapeutic route' is underscored by the fact that some drugs targeting the S1P axis are currently in clinical trials for the treatment of other brain disorders.

Inhibition of phosphodiesterases as a strategy to achieve neuroprotection in Huntington's disease.

Huntington's disease (HD) is a fatal neurodegenerative condition, due to a mutation in the IT15 gene encoding for huntingtin. Currently, disease-modifying therapy is not available for HD, and only symptomatic drugs are administered for the management of symptoms. In the last few years, preclinical and clinical studies have indicated that pharmacological strategies aimed at inhibiting cyclic nucleotide phosphodiesterase (PDEs) may develop into a novel therapeutic approach in neurodegenerative disorders. PDEs are a family of enzymes that hydrolyze cyclic nucleotides into monophosphate isoforms. Cyclic nucleotides are second messengers that transduce the signal of hormones and neurotransmitters in many physiological processes, such as protein kinase cascades and synaptic transmission. An alteration in their balance results in the dysregulation of different biological mechanisms (transcriptional dysregulation, immune cell activation, inflammatory mechanisms, and regeneration) that are involved in neurological diseases. In this review, we discuss the action of phosphodiesterase inhibitors and their role as therapeutic agents in HD.

Glycogen synthase protects neurons from cytotoxicity of mutant huntingtin by enhancing the autophagy flux.

Healthy neurons do not store glycogen while they do possess the machinery for the glycogen synthesis albeit at an inactive state. Neurons in the degenerating brain, however, are known to accumulate glycogen, although its significance was not well understood. Emerging reports present contrasting views on neuronal glycogen synthesis; a few reports demonstrate a neurotoxic effect of glycogen while a few others suggest glycogen to be neuroprotective. Thus, the specific role of glycogen and glycogen synthase in neuronal physiology is largely unexplored. Using cellular and animal models of Huntington's disease, we show here that the overexpression of cytotoxic mutant huntingtin protein induces glycogen synthesis in the neurons by activating glycogen synthase and the overexpressed glycogen synthase protected neurons from the cytotoxicity of the mutant huntingtin. Exposure of neuronal cells to proteasomal blockade and oxidative stress also activate glycogen synthase to induce glycogen synthesis and to protect against stress-induced neuronal death. We show that the glycogen synthase plays an essential and inductive role in the neuronal autophagic flux, and helps in clearing the cytotoxic huntingtin aggregate. We also show that the increased neuronal glycogen inhibits the aggregation of mutant huntingtin, and thus could directly contribute to its clearance. Finally, we demonstrate that excessive autophagy flux is the molecular basis of cell death caused by the activation of glycogen synthase in unstressed neurons. Taken together, our results thus provide a novel function for glycogen synthase in proteolytic processes and offer insight into the role of glycogen synthase and glycogen in both survival and death of the neurons.

Differential susceptibility of striatal, hippocampal and cortical neurons to Caspase-6.

Active cysteinyl protease Caspase-6 is associated with early Alzheimer and Huntington diseases. Higher entorhinal cortex and hippocampal Caspase-6 levels correlate with lower cognitive performance in aged humans. Caspase-6 induces axonal degeneration in human primary neuron cultures and causes inflammation and neurodegeneration in mouse hippocampus, and age-dependent memory impairment. To assess whether Caspase-6 causes damage to another neuronal system, a transgenic knock-in mouse overexpressing a self-activated form of Caspase-6 five-fold in the striatum, the area affected in Huntington disease, and 2.5-fold in the hippocampus and cortex, was generated. Detection of Tubulin cleaved by Caspase-6 confirmed Caspase-6 activity. The Caspase-6 expressing mice and control littermates were subjected to behavioral tests to assess Huntington disease-relevant psychiatric, motor, and cognitive deficits. Depression was excluded with the forced swim and sucrose consumption tests. Motor deficits were absent in the nesting, clasping, rotarod, vertical pole, gait, and open field analyzes. However, Caspase-6 mice developed age-dependent episodic and spatial memory deficits identified by novel object recognition, Barnes maze and Morris water maze assays. Neuron numbers were maintained in the striatum, hippocampus, and cortex. Microglia and astrocytes were increased in the hippocampal stratum lacunosum molecular and in the cortex, but not in the striatum. Synaptic mRNA profiling identified two differentially expressed genes in transgenic hippocampus, but none in striatum. Caspase-6 impaired synaptic transmission and induced neurodegeneration in hippocampal CA1 neurons, but not in striatal medium spiny neurons. These data revealed that active Caspase-6 in the striatal medium spiny neurons failed to induce inflammation, neurodegeneration or behavioral abnormalities, whereas active Caspase-6 in the cortex and hippocampus impaired episodic and spatial memories, and induced inflammation, neuronal dysfunction, and neurodegeneration. The results indicate age and neuronal subtype-dependent Caspase-6 toxicity and highlight the importance of targeting the correct neuronal subtype to identify underlying molecular mechanisms of neurodegenerative diseases.

Cdk5 Contributes to Huntington's Disease Learning and Memory Deficits via Modulation of Brain Region-Specific Substrates.

Cognitive deficits are a major hallmark of Huntington's disease (HD) with a great impact on the quality of patient's life. Gaining a better understanding of the molecular mechanisms underlying learning and memory impairments in HD is, therefore, of critical importance. Cdk5 is a proline-directed Ser/Thr kinase involved in the regulation of synaptic plasticity and memory processes that has been associated with several neurodegenerative disorders. In this study, we aim to investigate the role of Cdk5 in learning and memory impairments in HD using a novel animal model that expresses mutant huntingtin (mHtt) and has genetically reduced Cdk5 levels. Genetic reduction of Cdk5 in mHtt knock-in mice attenuated both corticostriatal learning deficits as well as hippocampal-dependent memory decline. Moreover, the molecular mechanisms by which Cdk5 counteracts the mHtt-induced learning and memory impairments appeared to be differentially regulated in a brain region-specific manner. While the corticostriatal learning deficits are attenuated through compensatory regulation of NR2B surface levels, the rescue of hippocampal-dependent memory was likely due to restoration of hippocampal dendritic spine density along with an increase in Rac1 activity. This work identifies Cdk5 as a critical contributor to mHtt-induced learning and memory deficits. Furthermore, we show that the Cdk5 downstream targets involved in memory and learning decline differ depending on the brain region analyzed suggesting that distinct Cdk5 effectors could be involved in cognitive impairments in HD.

The ubiquitin conjugating enzyme Ube2W regulates solubility of the Huntington's disease protein, huntingtin.

Huntington's disease (HD) is caused by a CAG repeat expansion that encodes a polyglutamine (polyQ) expansion in the HD disease protein, huntingtin (HTT). PolyQ expansion promotes misfolding and aggregation of mutant HTT (mHTT) within neurons. The cellular pathways, including ubiquitin-dependent processes, by which mHTT is regulated remain incompletely understood. Ube2W is the only ubiquitin conjugating enzyme (E2) known to ubiquitinate substrates at their amino (N)-termini, likely favoring substrates with disordered N-termini. By virtue of its N-terminal polyQ domain, HTT has an intrinsically disordered amino terminus. In studies employing immortalized cells, primary neurons and a knock-in (KI) mouse model of HD, we tested the effect of Ube2W deficiency on mHTT levels, aggregation and neurotoxicity. In cultured cells, deficiency of Ube2W activity markedly decreases mHTT aggregate formation and increases the level of soluble monomers, while reducing mHTT-induced cytotoxicity. Consistent with this result, the absence of Ube2W in HdhQ200 KI mice significantly increases levels of soluble monomeric mHTT while reducing insoluble oligomeric species. This study sheds light on the potential function of the non-canonical ubiquitin-conjugating enzyme, Ube2W, in this polyQ neurodegenerative disease.