Cerebellar demyelination and neurodegeneration associated with mTORC1 hyperactivity may contribute to the developmental onset of autism‐like neurobehavioral phenotype in a rat model

Abstract

The cerebellum hosts more than half of all neurons of the human brain, with their organized activity playing a key role in coordinating motor functions. Cerebellar activity has also been implicated in the control of speech, communication, and social behavior, which are compromised in autism spectrum disorders (ASD). Despite major research advances, there is a shortage of mechanistic data relating cellular and molecular changes in the cerebellum to autistic behavior. We studied the impact of tuberous sclerosis complex 2 haploinsufficiency (Tsc2+/À) with downstream mTORC1 hyperactivity on cerebellar morphology and cellular organization in 1, 9, and 18 m.o. Eker rats, to determine possible structural correlates of an autism-like behavioural phenotype in this model. We report a greater developmental expansion of the cerebellar vermis, owing to enlarged white matter and thickened molecular layer. Histochemical and immunofluorescence data suggest age-related demyelination of central tract of the vermis, as evident from reduced level of myelin-basic protein in the arbora vitae. We also observed a higher number of astrocytes in Tsc2+/À rats of older age while the number of Purkinje cells (PCs) in these animals was lower than in wild-type controls. Unlike astrocytes and PCs, Bergmann glia remained unaltered at all ages in both genotypes, while the number of microglia was higher in Tsc2+/À rats of older age. The convergent evidence for a variety of age-dependent cellular changes in the cerebellum of rats associated with mTORC1 hyperactivity, thus, predicts an array of functional impairments, which may contribute to the developmental onset of an autism-like behavioral phenotype in this model. Lay SummaryThis study elucidates the impact of constitutive mTORC1 hyperactivity on cerebellar morphology and cellular organization in a rat model of autism and epilepsy. It describes age-dependent degeneration of Purkinje neurons, with demyelination of central tract as well as activation of microglia, and discusses the implications of these changes for neuro-behavioral phenotypes. The described changes provide new indications for the putative mechanisms underlying cerebellar impairments with their age-related onset, which may contribute to the pathobiology of autism, epilepsy, and related disorders.

Developmental effects of constitutive mTORC1 hyperactivity and environmental enrichment on structural synaptic plasticity and behaviour in a rat model of autism spectrum disorder

“…Interestingly, the decrease in anxiety levels by environmental enrichment was most pronounced in the older rats. These findings are in support of the overall anxiolytic effects of environmental enrichment reported in animal studies (Lopes et al, 2019;Sztainberg et al, 2010) and agree with the developmental progression of neurobehavioural phenotypes in Tsc2+/À animal models (Feliciano, 2020;Kutna et al, 2020Kutna et al, , 2021Kutna et al, , 2022 as well as ASD in humans (Davidovitch et al, 2013;Ozonoff et al, 2008). Of note, in the OF, the environmental enrichment had the opposite effect in older age groups, reducing the exploration of the central part of the arena (PND90 and PND365), which is usually taken as a sign of elevated anxiety.…”

“…It is generally assumed that behavioural abnormalities in ASD humans and model animals share cellular and molecular pathology. Indeed, multiple reports suggest specific histochemical and functional changes in the hippocampus, cerebellum and cerebral cortex in rat and mouse models, reminiscent of those in human ASD autopsies (Gadad et al, 2013;Kutna et al, 2021Kutna et al, , 2022. These changes have been demonstrated to affect glutamatergic and GABAergic neurons and synapses, cause immune response impairments and disrupt the mTORC1 signalling (de la Torre-Ubieta et al, 2016;Gadad et al, 2013;Won et al, 2013).…”

“…On the other hand, in the same test, we observed no genotype‐dependent differences in centre duration. The causes for decreased locomotion of Eker rats in the OF and amelioration of this difference in the aged rats are not clear but might be related to an overall reduction of motor activity reported in human ASD (Peier et al, 2000; Stins & Emck, 2018), as well as to changes in structures controlling locomotor activity and coordination such as the cerebellum and brain stem (Gadad et al, 2013; Kutna et al, 2022).…”

“…Mean signal intensity values were pulled from 14 to 26 optical sections using a dedicated macro of ImageJ. For analysis of fluorescence areas, data were acquired using a dedicated function of ImageJ, as specified elsewhere (Kutna et al, 2022). The mean values of each bar in the bar graphs in Figure 5 present the calculated average of nine individual measurements per rat from three rats per group.…”

“…Remarkably, autism-like behavioural phenotype could be also induced by silencing of the Tsc1 gene in Purkinje cells (Tsai et al, 2012(Tsai et al, , 2018. Detailed analysis of the impact of Tsc2 haploinsufficiency on the morphology and cellular organization of the rat cerebellum in different age groups (Kutna et al, 2022) has shown subtle developmental impairments with demyelination of the central tract and loss of Purkinje neurons.…”