Pro-inflammatory interleukin-6 signaling links cognitive impairments and peripheral metabolic alterations in Alzheimer's disease.

Alzheimer's disease (AD) is associated with memory impairment and altered peripheral metabolism. Mounting evidence indicates that abnormal signaling in a brain-periphery metabolic axis plays a role in AD pathophysiology. The activation of pro-inflammatory pathways in the brain, including the interleukin-6 (IL-6) pathway, comprises a potential point of convergence between memory dysfunction and metabolic alterations in AD that remains to be better explored. Using T2-weighted magnetic resonance imaging (MRI), we observed signs of probable inflammation in the hypothalamus and in the hippocampus of AD patients when compared to cognitively healthy control subjects. Pathological examination of post-mortem AD hypothalamus revealed the presence of hyperphosphorylated tau and tangle-like structures, as well as parenchymal and vascular amyloid deposits surrounded by astrocytes. T2 hyperintensities on MRI positively correlated with plasma IL-6, and both correlated inversely with cognitive performance and hypothalamic/hippocampal volumes in AD patients. Increased IL-6 and suppressor of cytokine signaling 3 (SOCS3) were observed in post-mortem AD brains. Moreover, activation of the IL-6 pathway was observed in the hypothalamus and hippocampus of AD mice. Neutralization of IL-6 and inhibition of the signal transducer and activator of transcription 3 (STAT3) signaling in the brains of AD mouse models alleviated memory impairment and peripheral glucose intolerance, and normalized plasma IL-6 levels. Collectively, these results point to IL-6 as a link between cognitive impairment and peripheral metabolic alterations in AD. Targeting pro-inflammatory IL-6 signaling may be a strategy to alleviate memory impairment and metabolic alterations in the disease.

Insulin and leptin as potential cognitive enhancers in metabolic disorders and Alzheimer's disease.

Recent clinical and epidemiological observations point to a correlation between disorders of energy metabolism, such as obesity and diabetes, and cognitive decline and dementia. Many studies indicate that these age-related conditions closely interact with each other, but the underlying molecular and physiological mechanisms for such correlations are largely unknown. Insulin and leptin, hormones classically implicated in diabetes and obesity, are gaining increasing attention for their participation in cognitive processes and memory. Disrupted signaling by those hormones is associated with impaired brain function. The current review discusses how restoration of insulin and leptin signaling in the brain may attenuate neuronal damage and promote cognition. We further discuss potential therapeutic approaches involving the use of insulin and leptin as cognitive enhancers in the context of metabolic disorders and Alzheimer's disease. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.