Spotlight on the Pathophysiological Trajectories Linked to Alzheimer’s Disease (AD): Blood-Brain Barrier (BBB); Neuroinflammation; APOE4 Allele and AD- Insulin Resistance (IR) Pathogenic Link

Alzheimer’s disease (AD) is a predominant and incurable chronic debilitating neuro-degenerative disorder occupying more than 60% of all types of dementias. It is inaugurating by a cascade of events initiating from amnesic-type memory impairment and by gradually loss of cognitive and executive capacities, which is an intolerable problem for the patients and their familiars. Pathologically, there is overwhelming evidence that clumps of misfolded amyloid-β (Aβ) and hyperphosphorylated Tau protein aggregate in the brain. These pathological processes leading to synapse loss, neuronal loss, brain atrophy and gliosis culminating in neurodegeneration and fuelling AD. Thus, at a basic level, abnormality in brain proteins function is observed, causing disruption in the brain network and loss of neural connectivity. Nevertheless, AD is a multifactorial disorder, caused by a combination of age-related changes, genetic, environmental and lifestyle factors that affect the brain over time, its mysterious pathology seems non limited to senile plaques (Aβ) and neurofibrillary tangles (tau), but plethora of substantial and biological processes have been also emerged in its pathogenesis such as — breakdown and/or dysfunction of the blood–brain barrier (BBB); patients’ carriers of the gene variant APOE4; and the immunosenescence of the immune system. Furthermore, type 2 diabetes (T2DM) and metabolic syndrome (MS) whose have also observed as early markers that may provoke pathogenic pathways that lead or aggravate AD progression and pathology. Notwithstanding, of various pathological pathways, there are numerous substantial AD features that require shedding light on, such as chronic neuroinflammation, decrease glucose utilization and energy metabolism as well as brain insulin resistance (IR). Herein, we come to broadened our understanding & to connect the dots of the multiple comorbidities and their cumulative impact that may have synergistic effects on BBB dysfunction and AD pathology. We shed light on the path-physiological modifications in the cerebral vasculature that may contribute to AD pathology and cognitive decline prior to clinically detectable changes in amyloid beta (Aβ) and tau pathology, diagnostic biomarkers of AD, neuroimmune involvement and the role of APOE4 allele and AD-IR pathogenic link - the shared genetics and metabolomics biomarkers between AD and IR disorders. Investment in the future researches brings us closer to know the pathogenesis of AD, and paves the way to build preventive and treatment strategy.