Breakdown of the blood-brain barrier: A mediator of increased Alzheimer's risk in patients with metabolic disorders?

Dynamic changes in key factors of the blood-brain barrier in early diabetic mice

Chronic hyperglycemia can result in damage to the hippocampus and dysfunction of the blood-brain barrier (BBB), potentially leading to neurological disorders. This study examined the histological structure of the hippocampus and the expression of critical genes associated with the BBB at 2 early stage time points in a streptozotocin-induced diabetes mellitus (DM) mouse model. Routine histology revealed vascular congestion and dilation of Virchow-Robin spaces in the hippocampal CA1 region of the DM group. Neuronal alterations included rounding and swelling and reduction in Nissl bodies and increased apoptosis. Compared to the control group, TJP1 mRNA expression in the DM group was significantly lower (P < .05 or P < .01), while mRNA levels of JAM3, TJP3, CLDN5, CLDN3, and OCLN initially increased and then decreased. At 7, 14, and 21 days, mRNA levels of the receptor for advanced glycation end products (AGER) were greater in the DM group than in the control group (P < .05 or P < .01). These findings indicate that early-stage diabetes may cause structural and functional impairments in hippocampal CA1 in mice. These abnormalities may parallel alterations in the expression of key BBB tight junction molecules and elevated AGER expression in early DM patients.

Distinct Roles for Brain and Pancreas in Basal and Postprandial Glucose Homeostasis

The glucose homeostasis system ensures that the circulating glucose level is maintained within narrow physiological limits both in the fasting (or basal) state and following a nutrient challenge. Although glucose homeostasis is traditionally conceptualized as a single overarching system, evidence reviewed here suggests that basal glycemia and glucose tolerance are governed by distinct control systems. Specifically, whereas glucose tolerance appears to be determined largely by interactions between insulin secretion and insulin sensitivity, basal-state glucose homeostasis is predominated by insulin-independent mechanisms governed largely by the brain. In addition to a new perspective on how glucose homeostasis is achieved, this "dual control system" hypothesis offers a feasible and testable explanation for observations that are otherwise difficult to reconcile and sheds new light on the integration of central and peripheral metabolic control mechanisms. The implications of this model for the pathogenesis and treatment of impaired fasting glucose, impaired glucose tolerance, and type 2 diabetes are also discussed.

GSK3α phosphorylates dynamin-2 to promote GLUT4 endocytosis in muscle cells

Insulin-stimulated translocation of glucose transporter 4 (GLUT4) to plasma membrane of skeletal muscle is critical for postprandial glucose uptake; however, whether the internalization of GLUT4 is also regulated by insulin signaling remains unclear. Here, we discover that the activity of dynamin-2 (Dyn2) in catalyzing GLUT4 endocytosis is negatively regulated by insulin signaling in muscle cells. Mechanistically, the fission activity of Dyn2 is inhibited by binding with the SH3 domain of Bin1. In the absence of insulin, GSK3α phosphorylates Dyn2 to relieve the inhibition of Bin1 and promotes endocytosis. Conversely, insulin signaling inactivates GSK3α and leads to attenuated GLUT4 internalization. Furthermore, the isoform-specific pharmacological inhibition of GSK3α significantly improves insulin sensitivity and glucose tolerance in diet-induced insulin-resistant mice. Together, we identify a new role of GSK3α in insulin-stimulated glucose disposal by regulating Dyn2-mediated GLUT4 endocytosis in muscle cells. These results highlight the isoform-specific function of GSK3α on membrane trafficking and its potential as a therapeutic target for metabolic disorders.