bims-tubesc 2023-01-08 papers

Issue of 2023–01–08
three papers selected by

Front Mol Neurosci. 2022 ;15 1019343

Spine morphogenesis and synapse formation in tubular sclerosis complex models.

Tuberous sclerosis complex (TSC) is caused by mutations in the Tsc1 or Tsc2 genes, whose products form a complex and inactivate the small G-protein Rheb1. The activation of Rheb1 may cause refractory epilepsy, intellectual disability, and autism, which are the major neuropsychiatric manifestations of TSC. Abnormalities in dendritic spines and altered synaptic structure are hallmarks of epilepsy, intellectual disability, and autism. In addition, spine dysmorphology and aberrant synapse formation are observed in TSC animal models. Therefore, it is important to investigate the molecular mechanism underlying the regulation of spine morphology and synapse formation in neurons to identify therapeutic targets for TSC. In this review, we focus on the representative proteins regulated by Rheb1 activity, mTORC1 and syntenin, which are pivotal downstream factors of Rheb1 in the alteration of spine formation and synapse function in TSC neurons.

Keywords: rapamycin; spine morphology; synapse formation; syntenin; tuberous sclerosis complex

DOI: https://doi.org/10.3389/fnmol.2022.1019343

Cell Commun Signal. 2023 Jan 05. 21(1): 4

mTORC2: a multifaceted regulator of autophagy.

Yanan Sun, Huihui Wang, Taiqi Qu, Junjie Luo, Peng An, Fazheng Ren, Yongting Luo, Yixuan Li.

Autophagy is a multi-step catabolic process that delivers cellular components to lysosomes for degradation and recycling. The dysregulation of this precisely controlled process disrupts cellular homeostasis and leads to many pathophysiological conditions. The mechanistic target of rapamycin (mTOR) is a central nutrient sensor that integrates growth signals with anabolism to fulfil biosynthetic and bioenergetic requirements. mTOR nucleates two distinct evolutionarily conserved complexes (mTORC1 and mTORC2). However, only mTORC1 is acutely inhibited by rapamycin. Consequently, mTORC1 is a well characterized regulator of autophagy. While less is known about mTORC2, the availability of acute small molecule inhibitors and multiple genetic models has led to increased understanding about the role of mTORC2 in autophagy. Emerging evidence suggests that the regulation of mTORC2 in autophagy is mainly through its downstream effector proteins, and is variable under different conditions and cellular contexts. Here, we review recent advances that describe a role for mTORC2 in this catabolic process, and propose that mTORC2 could be a potential clinical target for the treatment of autophagy-related diseases. Video abstract.

Keywords: AKT; Autophagy; PKC; SGK-1; mTORC2

DOI: https://doi.org/10.1186/s12964-022-00859-7

Mol Metab. 2022 Dec 28. pii: S2212-8778(22)00233-2. [Epub ahead of print] 101664

Knockout of TSC2 in Nav1.8+ neurons predisposes to the onset of normal weight obesity.

Jennifer M Brazill, David Shin, Kristann Magee, Anurag Majumdar, Ivana R Shen, Valeria Cavalli, Erica L Scheller.

OBJECTIVE: Obesity and nutrient oversupply increase mammalian target of rapamycin (mTOR) signaling in multiple cell types and organs, contributing to the onset of insulin resistance and complications of metabolic disease. However, it remains unclear when and where mTOR activation mediates these effects, limiting options for therapeutic intervention. The objective of this study was to isolate the role of constitutive mTOR activation in Nav1.8-expressing peripheral neurons in the onset of diet-induced obesity, bone loss, and metabolic disease.
METHODS: In humans, loss of function mutations in tuberous sclerosis complex 2 (TSC2) lead to maximal constitutive activation of mTOR. To mirror this in mice, we bred Nav1.8-Cre with TSC2fl/fl animals to conditionally delete TSC2 in Nav1.8-expressing neurons. Male and female mice were studied from 4- to 34-weeks of age and a subset of animals were fed a high-fat diet (HFD) for 24-weeks. We then performed assays of metabolism, body composition, bone morphology, and behavior.
RESULTS: By lineage tracing, Nav1.8-Cre targeted peripheral sensory neurons, a subpopulation of postganglionic sympathetics, and several regions of the brain. Conditional knockout of TSC2 in Nav1.8-expressing neurons (Nav1.8-TSC2KO) selectively upregulated neuronal mTORC1 signaling. Male, but not female, Nav1.8-TSC2KO mice had a 4-10% decrease in body size at baseline. When challenged with HFD, both male and female Nav1.8-TSC2KO mice resisted diet-induced gains in body mass. However, this did not protect against HFD-induced metabolic dysfunction and bone loss. In addition, despite not gaining weight, Nav1.8-TSC2KO mice fed HFD still developed high body fat, a unique phenotype previously referred to as 'normal weight obesity'. Nav1.8-TSC2KO mice also had signs of chronic itch, mild increases in anxiety-like behavior, and sex-specific alterations in HFD-induced fat distribution that led to enhanced visceral obesity in males and preferential deposition of subcutaneous fat in females.
CONCLUSIONS: Knockout of TSC2 in Nav1.8+ neurons substantially modifies the distribution of adipose tissues and the metabolic responses to HFD. Though it prevents HFD-induced weight gain, this masks persistent detrimental effects on metabolic health and peripheral organs such as bone, mimicking the 'normal weight obesity' phenotype that is of growing concern. This supports a mechanism by which increased neuronal mTOR signaling can predispose to altered adipose tissue distribution, adipose tissue expansion, impaired peripheral metabolism, and detrimental changes to skeletal health with HFD - despite resistance to weight gain.

Keywords: Bone; High fat diet; Normal weight obesity; Sensory neuron; Skinny fat; mTOR

DOI: https://doi.org/10.1016/j.molmet.2022.101664