A More Holistic Perspective of Alzheimer's Disease: Roles of Gut Microbiome, Adipocytes, HPA Axis, Melatonergic Pathway and Astrocyte Mitochondria in the Emergence of Autoimmunity.

Alzheimer's disease is widely regarded as poorly treated due to poor conceptualization. For 40 years, Alzheimer's disease pathophysiology has focused on two culprits, amyloid-β induced plaques and hyperphosphorylated tau associated tangles, with no significant treatment advance. This is confounded by data showing amyloid-β to be an endogenous antimicrobial that is increased in a wide array of diverse medical conditions associated with heightened inflammation. This article reviews the wider bodies of data pertaining to Alzheimer's disease pathophysiology, highlighting the role of suppressed astrocyte mitochondrial function and mitochondrial melatonergic pathway as a core hub in driving neuronal loss in dementia. It is proposed that astrocyte function over aging becomes dysregulated, at least partly mediated by systemic processes involving the 10-fold decrease in pineal melatonin leading to the attenuated capacity of night-time melatonin to dampen residual daytime inflammation. Suppressed pineal melatonin also attenuates melatonin's inhibition of glucocorticoid receptor nuclear translocation, thereby changing not only stress/hypothalamus-pituitary-adrenal (HPA) axis consequences but also the consequences of the cortisol awakening response, which 'primes the body for the coming day'. Gut microbiome-derived butyrate also inhibits glucocorticoid receptor nuclear translocation, as well as inducing the mitochondrial melatonergic pathway. It is proposed that the loss of astrocyte melatonin prevents the autocrine and paracrine effects of melatonin in limiting amyloid-β levels and effects. Suppressed astrocyte melatonin production also attenuates the melatonin induction of astrocyte lactate, thereby decreasing neuronal mitochondrial metabolism and the neuronal mitochondrial melatonergic pathway. The loss of astrocyte lactate and melatonin, coupled to the suppression of neuronal mitochondrial metabolism and melatonin production decreases mitophagy, leading to the induction of the major histocompatibility complex (MHC)-1. MHC-1 initiates the chemoattraction of CD8+ t cells, leading to neuronal destruction in Alzheimer's disease being driven by 'autoimmune'/'immune-mediated' processes. Alzheimer's disease may therefore be conceptualized as being initiated by systemic processes that act on astrocytes as a core hub, with the suppression of the astrocyte melatonergic pathway leaving neurons deplete of appropriate metabolic substrates and co-ordinated antioxidants. This culminates in an 'immune-mediated' cell death. Future research and treatment/prevention implications are indicated.