Inactivation of the CB 2 receptor accelerated the neuropathological deterioration in TDP-43 transgenic mice, a model of amyotrophic lateral sclerosis

The activation of the cannabinoid receptor type-2 (CB₂ ) afforded neuroprotection in amyotrophic lateral sclerosis (ALS) models. The objective of this study was to further investigate the relevance of the CB₂ receptor through investigating the consequences of its inactivation. TDP-43(A315T) transgenic mice were crossed with CB₂ receptor knock-out mice to generate double mutants. Temporal and qualitative aspects of the pathological phenotype of the double mutants were compared to TDP-43 transgenic mice expressing the CB₂ receptor. The double mutants exhibited significantly accelerated neurological decline, such that deteriorated rotarod performance was visible at 7 weeks, whereas rotarod performance was normal up to 11 weeks in transgenic mice with intact expression of the CB₂ receptor. A morphological analysis of spinal cords confirmed an earlier death (visible at 65 days) of motor neurons labelled with Nissl staining and ChAT immunofluorescence in double mutants compared to TDP-43 transgenic mice expressing the CB₂ receptor. Evidence of glial reactivity, measured using GFAP and Iba-1 immunostaining, was seen in double mutants at 65 days, but not in TDP-43 transgenic mice expressing the CB₂ receptor. However, at 90 days, both genotypes exhibited similar changes for all these markers, although surviving motor neurons of transgenic mice presented some morphological abnormalities in absence of the CB₂ receptor that were not as evident in the presence of this receptor. This faster deterioration seen in double mutants led to premature mortality compared with TDP-43 transgenic mice expressing the CB₂ receptor. We also investigated the consequences of a pharmacological inactivation of the CB₂ receptor using the selective antagonist AM630 in TDP-43 transgenic mice, but results showed only subtle trends towards a greater deterioration. In summary, our results confirmed the potential of the CB₂ receptor agonists as a neuroprotective therapy in ALS and strongly support the need to progress towards an evaluation of this potential in patients.