Dysregulation of the endocannabinoid signaling system in the cerebellum and brainstem in a transgenic mouse model of spinocerebellar ataxia type-3

Abstract

Spinocerebellar ataxia type-3 (SCA-3) is a rare disease but it is the most frequent type within the autosomal dominant inherited ataxias. The disease lacks an effective treatment to alleviate major symptoms and to modify disease progression. Our recent findings that endocannabinoid receptors and enzymes are significantly altered in the post-mortem cerebellum of patients affected by autosomal-dominant hereditary ataxias suggest that targeting the endocannabinoid signaling system may be a promising therapeutic option. Our goal was to investigate the status of the endocannabinoid signaling system in a transgenic mouse model of SCA-3, in the two CNS structures most affected in this disease - cerebellum and brainstem. These animals exhibited progressive motor incoordination, imbalance, abnormal gait, muscle weakness, and dystonia, in parallel to reduced in vivo brain glucose metabolism, deterioration of specific neuron subsets located in the dentate nucleus and pontine nuclei, small changes in microglial morphology, and reduction in glial glutamate transporters. Concerning the endocannabinoid signaling, our data indicated no changes in CB receptors. By contrast, CB receptors increased in the Purkinje cell layer, in particular in terminals of basket cells, but they were reduced in the dentate nucleus. We also measured the levels of endocannabinoid lipids and found reductions in anandamide and oleoylethanolamide in the brainstem. These changes correlated with an increase in the FAAH enzyme in the brainstem, which also occurred in some cerebellar areas, whereas other endocannabinoid-related enzymes were not altered. Collectively, our results in SCA-3 mutant mice confirm a possible dysregulation in the endocannabinoid system in the most important brain structures affected in this type of ataxia, suggesting that a pharmacological manipulation addressed to correct these changes could be a promising option in SCA-3.

Altered striatal endocannabinoid signaling in a transgenic mouse model of spinocerebellar ataxia type-3

“…Such alterations were also found recently in a transgenic mouse model of SCA-3 [17], which reproduces many of the neurological and neuropathological signs of the disease [28]. This earlier study [17] concentrated on two key CNS structures (cerebellum and brainstem) closely related to the classic symptoms (motor incoordination, imbalance, gait anomalies) and neuropathological lesions (the loss of specific neuronal subpopulations in the dentate nucleus, Purkinje layer and pontine nuclei) observed in SCA-3 [25,26,29,30]. Hence, the pharmacological correction of such changes in endocannabinoid signaling may represent a promising therapeutic option for this disease.…”

“…We recently described the progressive motor deterioration experienced by SCA-3 transgenic mice, as evident in the hanging wire, balance beam and rotarod tests [17,28]. In these studies, we also detected the presence of hindlimb clasping, a common marker of dystonia in mouse models of neurological disorders (including certain cerebellar ataxias [21]) and an extremely frequent response in SCA-3 mouse models [17,33].…”

“…Subsequently possible alterations in the endocannabinoid signaling were evaluated in this structure. These animals were characterized 7–55 weeks after birth [17], detecting dystonic-like behavior (limb clasping) and establishing three different disease states: (i) an early symptomatic stage, up to 16 weeks after birth, characterized by a loss of muscle strength in the hanging wire test [28]; (ii) a stable symptomatic stage, up to 32 weeks after birth, involving impaired performance on the balance beam and rotarod indicative of impaired balance and motor coordination [28]; and (iii) an advanced stage, up to 56 weeks after birth, in which marked deficiencies in gait and dystonia were reflected by the animal performance in footprinting tests and through their clasping behavior, respectively [17]. …”

“…Such responses have also been seen in autosomal-dominant spinocerebellar ataxias (SCAs; [15–17]), a group of inherited neurodegenerative disorders that mainly affect the cerebellum and its afferent/efferent structures, in which motor discoordination (“ataxia”) is the key symptom [18–21]. However, in most SCAs, and in particular SCA-3 or Machado-Joseph disease, the most prevalent of these rare disorders ([22] for review), neuronal malfunction/degeneration also occurs in extracerebellar structures like the brainstem, optic nerve, cortical areas and the basal ganglia ([23–25] for review).…”

“…Hence, the pharmacological correction of such changes in endocannabinoid signaling may represent a promising therapeutic option for this disease. However, the neuropathology of SCA-3 is not restricted to these two CNS areas and it is also affects other important structures like the basal ganglia, which explains the occurrence of extrapyramidal defects in SCA-3 patients [31] and in our SCA-3 transgenic mouse model (e.g., dystonia, rigidity: [17]). Therefore, the goal of this follow-up study was to investigate the changes to the endocannabinoid signaling system in the striatum of these transgenic mice using different biochemical and histological approaches.…”