Liver X Receptor (LXR) activation is associated with behavioural and neuropathological improvements in Huntington's disease

La Maladie de Huntington (MH) est une maladie neurodégénérative héréditaire à transmission autosomique dominante, associée à des symptômes moteurs, cognitifs et psychiatriques. La MH est due à une expansion anormale du triplet de nucléotides CAG dans le gène codant la Huntingtine, conduisant à la forme mutée de la protéine (mHTT). La MH a été très largement décrite pour sa dégénérescence striatale, associée aux symptômes moteurs, toutefois de nombreuses autres régions cérébrales sont touchées, tel que l’hippocampe, le cortex, la substance blanche. Parmi les nombreuses altérations cellulaires consécutives à l'expression de la mHTT, une dérégulation du métabolisme du cholestérol a été décrite chez les patients et chez les modèles murins MH. Le catabolisme du cholestérol en 24S-OHC (24S-hydroxycholestérol) par l’enzyme neuronale CYP46A1 (24S-hydroxylase) est notamment réduit dans la MH. Le 24S-OHC est un ligand naturel des récepteurs nucléaires LXR (Liver X Receptor), qui agissant comme facteurs de transcription. La restauration de l’expression de CYP46A1 dans le striatum de souris MH induit des effets bénéfiques chez les souris MH, associés à une augmentation de l'expression des gènes cibles LXR. Il existe deux isoformes des LXR, les LXRalpha exprimés majoritairement dans les tissus présentant une activité métabolique élevée, comme le foie, et les LXRbeta avec une expression ubiquitaire et enrichis dans le cerveau. Les LXR sont impliqués dans de nombreuses fonctions cellulaires, dont la synthèse d’acide gras, la régulation du métabolisme du cholestérol ou encore de l’inflammation. L’intérêt thérapeutique des LXR a ainsi été soulevé dans plusieurs maladies neurodégénératives, mais ils ont été, jusqu’à présent, peu étudiés dans la MH. Le but de ce projet est d’étudier l’effet de l’activation des LXR sur la physiopathologie d'un modèle pré-clinique murin de la MH, les souris knock-in zQ175. Dans une première partie, une étude intégrée, cellulaire et moléculaire a été réalisée à l'aide d'un ligand synthétique commercial non sélectif des deux isoformes, le T0901317 (T0). L’activation des LXR sur des cultures primaires de neurones striataux MH induit une neuroprotection associée à une réduction des agrégats de mHTT et une restauration de la survie neuronale. Le traitement des souris zQ175 symptomatiques avec le T0 réduit la perte de poids et restaure les déficits cognitifs, d’interaction sociale et le comportement de type dépressif des souris MH. En revanche, les altérations motrices ne sont pas améliorées par le T0. Les effets comportementaux du T0 sont associés à une signature transcriptionnelle dans l’hippocampe reliée à une régulation des voies de transmission synaptique et du métabolisme du cholestérol. En particulier, les transcrits du BDNF, diminués dans la MH, sont restaurés spécifiquement dans les neurones et non les astrocytes, associés à une restauration de la prolifération cellulaire dans l'hippocampe adulte. L’utilisation répétée de ce type de ligands synthétiques non sélectifs des isoformes alpha ou beta est limitée de part leurs effets sur la lipogenèse, ce qui bloque leur accès en clinique. La caractérisation des effets d'un ligand plus spécifique de l'isoforme beta a été initiée dans cette étude afin de proposer de nouvelles cibles thérapeutiques. L’ensemble de ces données montrent l’intérêt tout particulier de cibler les LXR dans la MH et offre de nouvelles perspectives pour le traitement des symptômes cognitifs et psychiatriques.Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disorder associated with motor, cognitive and psychiatric symptoms. HD is caused by an abnormal expansion of the CAG nucleotide triplet in the gene encoding Huntingtin, leading to the mutated form of the protein (mHTT). Although HD has been widely described for its striatal degeneration, associated with motor symptoms, many other brain regions are affected, such as the hippocampus, cortex and white matter. Among the many cellular alterations resulting from mHTT expression, deregulation of cholesterol metabolism has been described in HD patients and in HD mouse models. The catabolism of cholesterol to 24S-OHC (24S-hydroxycholesterol) by the neuronal enzyme CYP46A1 (24S-hydroxylase) is notably reduced in HD. 24S-OHC is a natural ligand for the nuclear receptor LXR (Liver X Receptor) acting as a transcription factor. Restoration of CYP46A1 expression in the striatum of HD mice induces beneficial effects associated with regulation of LXR target gene expression. There are two LXRs isoforms, LXRalpha expressed predominantly in tissues with high metabolic activity, such as the liver, and LXRbeta with ubiquitous expression, enriched in the brain. LXRs are involved in numerous cellular functions, including fatty acid synthesis, regulation of cholesterol metabolism and inflammation, and have been shown to be of therapeutic interest in several neurodegenerative diseases, but have so far received little attention in HD. The aim of this project is to study the effect of LXR activation on the pathophysiology of a pre-clinical mouse model of HD: zQ175 knock-in mice. In the first part, an integrated study from mouse behaviour to cellular and molecular features, was carried out using a non-selective commercial synthetic ligand for both isoforms (T0901317 (T0)). LXR activation in primary cultures of HD striatal neurons induced neuroprotection associated with a reduction in mHTT aggregates and restoration of neuronal survival. Treatment of symptomatic zQ175 mice with T0 alleviates weight loss in HD mice and restores cognitive alterations, social interaction deficits and depressive-like behaviour in HD mice. However, T0 did not improve mouse motor behaviour. The behavioural effects of T0 are associated with a transcriptional signature in the hippocampus with regulation of pathways related to synaptic transmission and cholesterol metabolism. In particular, BDNF transcripts, decreased in HD, are restored specifically in neurons, not astrocytes, associated with restoration of cell proliferation in adult hippocampus. Repeated use of these non-isoform-selective synthetic ligands is limited due to their effects on lipogenesis, blocking their access to the clinic. The characterization of a ligand more specific to the beta isoform was initiated in this study in order to propose new therapeutic targets. Taken together, these data demonstrate the particular interest of targeting LXRs in HD and proposes new perspectives for treating cognitive and psychiatric symptoms

Altered Cholesterol Homeostasis in Huntington’s Disease

International audienceHuntington'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-brainbarrier, 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

CYP46A1 gene therapy deciphers the role of brain cholesterol metabolism in Huntington’s disease

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Acadesine Circumvents Azacitidine Resistance in Myelodysplastic Syndrome and Acute Myeloid Leukemia

Myelodysplastic syndrome (MDS) defines a group of heterogeneous hematologic malignancies that often progresses to acute myeloid leukemia (AML). The leading treatment for high-risk MDS patients is azacitidine (Aza, Vidaza®), but a significant proportion of patients are refractory and all patients eventually relapse after an undefined time period. Therefore, new therapies for MDS are urgently needed. We present here evidence that acadesine (Aca, Acadra®), a nucleoside analog exerts potent anti-leukemic effects in both Aza-sensitive (OCI-M2S) and resistant (OCI-M2R) MDS/AML cell lines in vitro. Aca also exerts potent anti-leukemic effect on bone marrow cells from MDS/AML patients ex-vivo. The effect of Aca on MDS/AML cell line proliferation does not rely on apoptosis induction. It is also noteworthy that Aca is efficient to kill MDS cells in a co-culture model with human medullary stromal cell lines, that mimics better the interaction occurring in the bone marrow. These initial findings led us to initiate a phase I/II clinical trial using Acadra® in 12 Aza refractory MDS/AML patients. Despite a very good response in one out 4 patients, we stopped this trial because the highest Aca dose (210 mg/kg) caused serious renal side effects in several patients. In conclusion, the side effects of high Aca doses preclude its use in patients with strong comorbidities