Decreased rates of cerebral protein synthesis measured in vivo in a mouse model of Tuberous Sclerosis Complex: unexpected consequences of reduced tuberin

Physiological and molecular impairment of PV circuit homeostasis in mouse models of autism

Circuit dysfunction in autism may involve a failure of homeostatic plasticity. To test this, we studied parvalbumin (PV) interneurons which exhibit rapid homeostatic plasticity of intrinsic excitability following whisker deprivation in mouse somatosensory cortex. Brief deprivation reduces PV excitability by increasing Kv1 current to increase PV spike threshold. We found that PV homeostatic plasticity is disrupted in Tsc2 +/- and Fmr1 -/- models of autism. In wildtype mice, deprivation elevates the transcription factor ER81 which drives Kcna1 transcription, increasing Kv1.1 protein in the axon initial segment and soma. These molecular signatures of homeostasis were absent in Tsc2 +/- and Fmr1 -/- . Whisker enrichment increased PV excitability, but not in Tsc2 +/- , indicating that homeostasis is lost bidirectionally. Deprivation reduced feedforward L4-L2/3 inhibition in wildtype but not Tsc2 +/- mice. Thus, two autism models show a convergent loss of PV circuit homeostasis at physiological and molecular levels, potentially contributing to sensory processing impairments.

The role of TSC1 and TSC2 proteins in neuronal axons

Tuberous Sclerosis Complex 1 and 2 proteins, TSC1 and TSC2 respectively, participate in a multiprotein complex with a crucial role for the proper development and function of the nervous system. This complex primarily acts as an inhibitor of the mechanistic target of rapamycin (mTOR) kinase, and mutations in either TSC1 or TSC2 cause a neurodevelopmental disorder called Tuberous Sclerosis Complex (TSC). Neurological manifestations of TSC include brain lesions, epilepsy, autism, and intellectual disability. On the cellular level, the TSC/mTOR signaling axis regulates multiple anabolic and catabolic processes, but it is not clear how these processes contribute to specific neurologic phenotypes. Hence, several studies have aimed to elucidate the role of this signaling pathway in neurons. Of particular interest are axons, as axonal defects are associated with severe neurocognitive impairments. Here, we review findings regarding the role of the TSC1/2 protein complex in axons. Specifically, we will discuss how TSC1/2 canonical and non-canonical functions contribute to the formation and integrity of axonal structure and function.

Confirmation of Decreased Rates of Cerebral Protein Synthesis In Vivo in a Mouse Model of Tuberous Sclerosis Complex

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder that results in intellectual disability and, in ∼50% of patients, autism spectrum disorder. The protein products that are altered in TSC (TSC1 and TSC2) form a complex to inhibit the mammalian target of rapamycin [mTOR; mTOR complex 1 (mTORC1)] pathway. This pathway has been shown to affect the process of mRNA translation through its action on ribosomal protein S6 and 4-elongation binding protein 1. It is thought that mutations in the TSC proteins lead to upregulation of the mTORC1 pathway and consequently an increase in protein synthesis. Unexpectedly, our previous study of a mouse model of TSC (Tsc2Djk +/) demonstrated decreased in vivo rates of protein synthesis throughout the brain. In the present study, we confirm those results in another Tsc2+/- mouse model, one with a different mutation locus and on a mixed background (Tsc2Mjg +/-). We also examine mTORC1 signaling and possible effects of prior isoflurane anesthesia. Because measurements of protein synthesis rates in vivo require surgical preparation of the animal and anesthesia, we examine mTORC1 signaling pathways both under baseline conditions and following recovery from anesthesia. Our results demonstrate regionally selective effects of prior anesthesia. Overall, our results in both in vivo models suggest differences to the central hypothesis regarding TSC and show the importance of studying protein synthesis in vivo Significance StatementProtein synthesis is an important process for brain function. In the disorder, tuberous sclerosis complex (TSC), the inhibition of the mammalian target of rapamycin (mTOR) pathway is reduced and this is thought to lead to excessive protein synthesis. Most studies of protein synthesis in models of TSC have been conducted in vitro We report here confirmation of our previous in vivo study showing decreased brain protein synthesis rates in a second mouse model of TSC, results counter to the central hypothesis regarding TSC. We also explore the possible influence of prior isoflurane exposure on signaling pathways involved in regulation of protein synthesis. This study highlights a novel aspect of TSC and the importance of studying cellular processes in vivo.

Genetic and Environmental Contributions to Autism Spectrum Disorder Through Mechanistic Target of Rapamycin

Autism spectrum disorder (ASD) is a neurodevelopmental disorder that affects an individual’s reciprocal social interaction and communication ability. Numerous genetic and environmental conditions are associated with ASD, including tuberous sclerosis complex, phosphatase and tensin homolog hamartoma tumor syndrome, fragile X syndrome, and neurofibromatosis 1. The pathogenic molecular mechanisms of these diseases are integrated into the hyperactivation of mTORC1 (mechanistic target of rapamycin complex 1). Rodent models of these diseases have shown high mTORC1 activity in the brain and ASD-related behavioral deficits, which were reversed by the mTORC1 inhibitor rapamycin. Environmental stress can also affect this signaling pathway. In utero exposure to valproate caused ASD in offspring and enhanced mTORC1 activity in the brain, which was sensitive to mTORC1 inhibition. mTORC1 is a signaling hub for diverse cellular functions, including protein synthesis, through the phosphorylation of its targets, such as ribosomal protein S6 kinases. Metabotropic glutamate receptor 5–mediated synaptic function is also affected by the dysregulation of mTORC1 activity, such as in fragile X syndrome and tuberous sclerosis complex. Reversing these downstream changes that are associated with mTORC1 activation normalizes behavioral defects in rodents. Despite abundant preclinical evidence, few clinical studies have investigated the treatment of ASD and cognitive deficits. Therapeutics other than mTORC1 inhibitors failed to show efficacy in fragile X syndrome and neurofibromatosis 1. mTORC1 inhibitors have been tested mainly in tuberous sclerosis complex, and their effects on ASD and neuropsychological deficits are promising. mTORC1 is a promising target for the pharmacological treatment of ASD associated with mTORC1 activation.

Inhibition of mTOR signaling by genetic removal of p70 S6 kinase 1 increases anxiety-like behavior in mice

The mechanistic target of rapamycin (mTOR) is a ubiquitously expressed kinase that acts through two complexes, mTORC1 and mTORC2, to regulate protein homeostasis, as well as long lasting forms of synaptic and behavioral plasticity. Alteration of the mTOR pathway is classically involved in neurodegenerative disorders, and it has been linked to dysregulation of cognitive functions and affective states. However, information concerning the specific involvement of the p70 S6 kinase 1 (S6K1), a downstream target of the mTORC1 pathway, in learning and memory processes and in the regulation of affective states remains scant. To fill this gap, we exposed adult male mice lacking S6K1 to a battery of behavioral tests aimed at measuring their learning and memory capabilities by evaluating reference memory and flexibility with the Morris water maze, and associative memory using the contextual fear conditioning task. We also studied their anxiety-like and depression-like behaviors by, respectively, performing elevated plus maze, open field, light-dark emergence tests, and sucrose preference and forced swim tests. We found that deleting S6K1 leads to a robust anxious phenotype concomitant with associative learning deficits; these symptoms are associated with a reduction of adult neurogenesis and neuronal atrophy in the hippocampus. Collectively, these results provide grounds for the understanding of anxiety reports after treatments with mTOR inhibitors and will be critical for developing novel compounds targeting anxiety.

Increased rates of cerebral protein synthesis in Shank3 knockout mice: Implications for a link between synaptic protein deficit and dysregulated protein synthesis in autism spectrum disorder/intellectual disability

SHANK3 is a postsynaptic scaffolding protein that plays a critical role in synaptic development and brain function. Mutations in SHANK3 are implicated in Phelan-McDermid syndrome (PMS), a neurodevelopmental disorder characterized by autistic-like behavior, delayed speech, hypotonia, and intellectual disability (ID). Moreover, mutations in SHANK3 occur in 1-2% of cases of idiopathic autism spectrum disorder (ASD). In fragile X syndrome (FXS), a syndromic form of autism, SHANK3 is one of the 842 targets of fragile X mental retardation protein (FMRP), the protein product of the silenced FMR1 gene. FXS is likely a primary disorder of the regulation of translation, whereas other syndromic forms of ASD/ID, e.g. PMS, appear to be primary disorders of synaptic structure. In this study, we asked if a knockout of the synaptic protein, Shank3, is linked to an effect on translation. Specifically, we measured the effect of Shank3 loss on rates of cerebral protein synthesis (rCPS) in vivo by means of the L-[1-¹⁴C]leucine quantitative autoradiographic method. We found that Shank3 knockout mice had significantly increased rCPS in every brain region examined. Our results suggest a link in ASD/ID between synaptic structure and regulation of translation.