Insulin Resistance and Inflammation in Neurodegeneration: Lessons from in vitro Models

Abstract

Type 2 diabetes (T2D) is a growing epidemic of the developed world and significantly increases the risk for neurodegenerative diseases such as Alzheimer’s disease (AD). Current evidence suggests that these two diseases are closely linked. Both share common etiologies such as insulin resistance (IR) and chronic inflammation. While it was once believed that the brain was not susceptible to the peripheral environment, it has now been demonstrated that systemic conditions impact the brain, especially so in metabolic conditions such as T2D, where blood brain barrier (BBB) permeability is prevalent. The exact mechanisms of how reduced insulin sensitivity and chronic inflammation are translated to the brain and the subsequent consequences in the central nervous system are not entirely understood. This thesis utilizes in vitro models of human brain cells to examine the effects of IR and chronic inflammation in neurons and pericytes that line the BBB. A chronic insulin model of insulin resistance in differentiated human neurons was developed to examine changes in gene expression associated with an insulin resistant state. Microarray analysis revealed altered expression of transcripts that regulate cholesterol and lipid metabolism. This is important because cholesterol homeostasis is required for efficient neuronal signalling and protein trafficking, and disruption of this balance results in AD-like pathologies. Pericytes are crucial components of the BBB and together with endothelial cells, astrocytes, and neurons, comprise the neurovascular unit. Brain pericytes are required for neurovascular regulation, and BBB integrity. Pericyte dysfunction is also an early pathology associated with diabetic retinopathy indicating these cells are exposed to systemic conditions. For the first time, this study demonstrated that cultured human brain pericytes exposed to cytokines associated with systemic inflammation responded by secreting chemokines IP-10 and MCP-1. Microarray analysis of pericytes treated with proinflammatory cytokines IFNγ and IL-1β revealed an extensive immune signature, identifying these cells as dynamic participants in the immune response. Pericytes were also exposed to chronic cytokine treatment in order to model systemic inflammation, typified by diabetic conditions. Chronic inflammation was shown to affect protein and gene expression of α smooth muscle actin (SMA) and platelet-derived growth factor receptor beta (PDGFRβ), two critical proteins involved in pericyte biology. Recent studies have demonstrated that PDGFRβ is required for pericyte function and survival in animal models. Indeed knockdown of PDGFRβ in human brain pericytes resulted in reduced proliferation in response to PDGF treatment, however, had no effect on overall cell numbers. Interestingly, chronic inflammatory conditions mimicked the PDGFRβ knockdown phenotype with blocked PDGF induced proliferation in pericytes. Taken together these data indicate that systemic conditions such as chronic high insulin and inflammation can have deleterious effects on human brain cells, and offers mechanistic explanations for the relationship between T2D and AD at a cellular level.