α-Synuclein accumulates in huntingtin inclusions but forms independent filaments and its deficiency attenuates early phenotype in a mouse model of Huntington's disease.

Huntington's disease (HD) is the most common of nine inherited neurological disorders caused by expanded polyglutamine (polyQ) sequences which confer propensity to self-aggregate and toxicity to their corresponding mutant proteins. It has been postulated that polyQ expression compromises the folding capacity of the cell which might affect other misfolding-prone proteins. α-Synuclein (α-syn) is a small neural-specific protein with propensity to self-aggregate that forms Parkinson's disease (PD) Lewy bodies. Point mutations in α-syn that favor self-aggregation or α-syn gene duplications lead to familial PD, thus indicating that increased α-syn aggregation or levels are sufficient to induce neurodegeneration. Since polyQ inclusions in HD and other polyQ disorders are immunopositive for α-syn, we speculated that α-syn might be recruited as an additional mediator of polyQ toxicity. Here, we confirm in HD postmortem brains and in the R6/1 mouse model of HD the accumulation of α-syn in polyQ inclusions. By isolating the characteristic filaments formed by aggregation-prone proteins, we found that N-terminal mutant huntingtin (N-mutHtt) and α-syn form independent filamentous microaggregates in R6/1 mouse brain as well as in the inducible HD94 mouse model and that N-mutHtt expression increases the load of α-syn filaments. Accordingly, α-syn knockout results in a diminished number of N-mutHtt inclusions in transfected neurons and also in vivo in the brain of HD mice. Finally, α-syn knockout attenuates body weight loss and early motor phenotype of HD mice. This study therefore demonstrates that α-syn is a modifier of polyQ toxicity in vivo and raises the possibility that potential PD-related therapies aimed to counteract α-syn toxicity might help to slow HD.

Loss of striatal type 1 cannabinoid receptors is a key pathogenic factor in Huntington's disease.

Endocannabinoids act as neuromodulatory and neuroprotective cues by engaging type 1 cannabinoid receptors. These receptors are highly abundant in the basal ganglia and play a pivotal role in the control of motor behaviour. An early downregulation of type 1 cannabinoid receptors has been documented in the basal ganglia of patients with Huntington's disease and animal models. However, the pathophysiological impact of this loss of receptors in Huntington's disease is as yet unknown. Here, we generated a double-mutant mouse model that expresses human mutant huntingtin exon 1 in a type 1 cannabinoid receptor-null background, and found that receptor deletion aggravates the symptoms, neuropathology and molecular pathology of the disease. Moreover, pharmacological administration of the cannabinoid Δ(9)-tetrahydrocannabinol to mice expressing human mutant huntingtin exon 1 exerted a therapeutic effect and ameliorated those parameters. Experiments conducted in striatal cells show that the mutant huntingtin-dependent downregulation of the receptors involves the control of the type 1 cannabinoid receptor gene promoter by repressor element 1 silencing transcription factor and sensitizes cells to excitotoxic damage. We also provide in vitro and in vivo evidence that supports type 1 cannabinoid receptor control of striatal brain-derived neurotrophic factor expression and the decrease in brain-derived neurotrophic factor levels concomitant with type 1 cannabinoid receptor loss, which may contribute significantly to striatal damage in Huntington's disease. Altogether, these results support the notion that downregulation of type 1 cannabinoid receptors is a key pathogenic event in Huntington's disease, and suggest that activation of these receptors in patients with Huntington's disease may attenuate disease progression.

Altered P2X7-receptor level and function in mouse models of Huntington's disease and therapeutic efficacy of antagonist administration.

The precise mechanism by which mutant huntingtin elicits its toxicity remains unknown. However, synaptic alterations and increased susceptibility to neuronal death are known contributors to Huntington's disease (HD) symptomatology. While decreased metabolism has long been associated with HD, recent findings have surprisingly demonstrated reduced neuronal apoptosis in Caenorhabditis elegans and Drosophila models of HD by drugs that diminish ATP production. Interestingly, extracellular ATP has been recently reported to elicit neuronal death through stimulation of P2X7 receptors. These are ATP-gated cation channels known to modulate neurotransmitter release from neuronal presynaptic terminals and to regulate cytokine production and release from microglia. We hypothesized that alteration in P2X7-mediated calcium permeability may contribute to HD synaptic dysfunction and increased neuronal apoptosis. Using mouse and cellular models of HD, we demonstrate increased P2X7-receptor level and altered P2X7-mediated calcium permeability in somata and terminals of HD neurons. Furthermore, cultured neurons expressing mutant huntingtin showed increased susceptibility to apoptosis triggered by P2X7-receptor stimulation. Finally, in vivo administration of the P2X7-antagonist Brilliant Blue-G (BBG) to HD mice prevented neuronal apoptosis and attenuated body weight loss and motor-coordination deficits. These in vivo data strongly suggest that altered P2X7-receptor level and function contribute to HD pathogenesis and highlight the therapeutic potential of P2X7 receptor antagonists.

Inhibition of the ATP-gated P2X7 receptor promotes axonal growth and branching in cultured hippocampal neurons.

During the establishment of neural circuits, the axons of neurons grow towards their target regions in response to both positive and negative stimuli. Because recent reports show that Ca2+ transients in growth cones negatively regulate axonal growth, we studied how ionotropic ATP receptors (P2X) might participate in this process. Our results show that exposing cultured hippocampal neurons to ATP induces Ca2+ transients in the distal domain of the axon and the concomitant inhibition of axonal growth. This effect is mediated by the P2X7 receptor, which is present in the growth cone of the axon. Pharmacological inhibition of P2X7 or its silencing by shRNA interference induces longer and more-branched axons, coupled with morphological changes to the growth cone. Our data suggest that these morphological changes are induced by a signalling cascade in which CaMKII and FAK activity activates PI3-kinase and modifies the activity of its downstream targets. Thus, in the absence or inactivation of P2X7 receptor, axons grow more rapidly and form more branches in cultured hippocampal neurons, indicative that ATP exerts a negative influence on axonal growth. These data suggest that P2X7 antagonists have therapeutic potential to promote axonal regeneration.

The CB2 cannabinoid receptor signals apoptosis via ceramide-dependent activation of the mitochondrial intrinsic pathway.

Delta9-tetrahydrocannabinol and other cannabinoids exert pro-apoptotic actions in tumor cells via the CB2 cannabinoid receptor. However, the molecular mechanism involved in this effect has remained elusive. Here we used the human leukemia cell line Jurkat-that expresses CB2 as the unique CB receptor-to investigate this mechanism. Our results show that incubation with the selective CB2 antagonist SR144528 abrogated the pro-apoptotic effect of Delta9-tetrahydrocannabinol. Cannabinoid treatment led to a CB2 receptor-dependent stimulation of ceramide biosynthesis and inhibition of this pathway prevented Delta9-tetrahydrocannabinol-induced mitochondrial hypopolarization and cytochrome c release, indicating that ceramide acts at a pre-mitochondrial level. Inhibition of ceramide synthesis de novo also prevented caspase activation and apoptosis. Caspase 8 activation-an event typically related with the extrinsic apoptotic pathway-was also evident in this model. However, activation of this protease was post-mitochondrial since (i) a pan-caspase inhibitor as well as a selective caspase 8 inhibitor were unable to prevent Delta9-tetrahydrocannabinol-induced loss of mitochondrial-membrane transmembrane potential, and (ii) cannabinoid-induced caspase 8 activation was not observed in Bcl-xL over-expressing cells. In summary, results presented here show that CB2 receptor activation signals apoptosis via a ceramide-dependent stimulation of the mitochondrial intrinsic pathway.

p38 MAPK is involved in CB2 receptor-induced apoptosis of human leukaemia cells.

Cannabinoids have been shown to inhibit the growth of a broad spectrum of tumour cells. However, the molecular mechanisms involved in that effect have not been completely elucidated. Here, we investigated the possible involvement of mitogen-activated protein kinases (MAPKs) in CB2 receptor-induced apoptosis of human leukaemia cells. Results show that stimulation of the CB2 receptor leads to p38 MAPK activation and that inhibition of this kinase attenuates CB2 receptor-induced caspase activation and apoptosis. These findings support a role for p38 MAPK in CB2 receptor-induced apoptosis of human leukaemia cells.