Dysregulation of Insulin Receptor in Fragile X Mouse Model

Background: Fragile X syndrome (FXS) is the most common monogenic cause of autism spectrum disorders (ASD) and hereditary intellectual disability (ID). FXS is caused by silencing of the FMR1 gene and subsequent loss of the fragile X mental retardation protein (FMRP). Fmr1 knockout mice (Fmr1^-/y) recapitulate many of FXS phenotypes, such as increased repetitive behaviors, impairment in social novelty, macroorchidism, increased general protein synthesis (mRNA translation), exaggerated LTD and abnormal dendritic spine morphology. Impaired translational mechanisms represent one of the features of FXS. Insulin, a pancreatic hormone, is the main regulator of glucose levels in the blood. Insulin acts by binding to insulin receptors (IR), stimulating their autophosphorylation and activation. Dysregulated insulin signaling has been shown in diabetes and cancer, and has recently been implicated in cognitive dysfunctions in neurodegenerative disorders such as Alzheimer’s disease. Recently, dysregulated insulin signaling has been shown to underlie cognitive deficits in Drosophila FXS model.

Objectives: The objective of this study is to investigate aberrant insulin and insulin receptor β in the brain and periphery of FXS mice in order to understand its role in the pathophysiology of the disease.

Methods: We have collected tissues of the brain areas implicated in autism etiology (prefrontal cortex and hippocampus), as well as peripheral tissues (liver, white adipose tissue and serum) from adult male wild-type (WT) and Fmr1^-/y mice. We measured the protein levels of IRβ in four above-mentioned tissues by Western blot (N= 6 per group). FMRP and GAPDH were used as a genotype and loading controls, respectively. To measure gene expression, the levels of IR mRNA in the prefrontal cortex and hippocampus were measured by quantitative RT-PCR (N= 6 per group). Insulin levels in the serum were measured by ELISA (N= 8 per group).

Results: We discovered that the expression of IRβ protein is upregulated in the Fmr1^-/y mice compared to WT, not only in the brain (hippocampus and prefrontal cortex), but also in peripheral tissues (liver and white adipose tissue). The levels of IR mRNA are not changed in Fmr1^-/y compared to WT mice. Insulin levels in the serum of Fmr1^-/y are decreased compared to WT mice.

Conclusions: The results from this study will provide knowledge in the dysregulated insulin receptor signaling that contribute to translational control in FXS, and might introduce new pharmaco-therapeutical directions for the prevention of phenotypic impairments in FXS.