Hyperglycemia reduces functional expression of astrocytic Kir4.1 channels and glial glutamate uptake

Abstract

Diabetics are at risk for a number of serious health complications including an increased incidence of epilepsy and poorer recovery after ischemic stroke. Astrocytes play a critical role in protecting neurons by maintaining extracellular homeostasis and preventing neurotoxicity through glutamate uptake and potassium buffering. These functions are aided by the presence of potassium channels, such as Kir4.1 inwardly rectifying potassium channels, in the membranes of astrocytic glial cells. The purpose of the present study was to determine if hyperglycemia alters Kir4.1 potassium channel expression and homeostatic functions of astrocytes. We used q-PCR, Western blot, patch-clamp electrophysiology studying voltage and potassium step responses and a colorimetric glutamate clearance assay to assess Kir4.1 channel levels and homeostatic functions of astrocytes grown in normal and high glucose conditions. We found that astrocytes grown in high glucose (25 mM) had an approximately 50% reduction in Kir4.1 mRNA and protein expression as compared with those grown in normal glucose (5 mM). These reductions occurred within 4 to 7 days of exposure to hyperglycemia, whereas reversal occurred between 7 to 14 days after return to normal glucose. The decrease in functional Kir channels in the astrocytic membrane was confirmed using barium to block Kir channels. In the presence of 100 μm barium, the currents recorded from astrocytes in response to voltage steps were reduced by 45%. Furthermore, inward currents induced by stepping extracellular [K+]o from 3 to 10 mM (reflecting potassium uptake) were 50% reduced in astrocytes grown in high glucose. In addition, glutamate clearance by astrocytes grown in high glucose was significantly impaired. Taken together, our results suggest that down-regulation of astrocytic Kir4.1 channels by elevated glucose may contribute to the underlying pathophysiology of diabetes-induced CNS disorders and contribute to the poor prognosis after stroke.

Metabolomic Analysis Identifies Lactate as an Important Pathogenic Factor in Diabetes-associated Cognitive Decline Rats

“…Astrocytes were seeded in 24 culture flasks (10 6 cells/flask) and five 24-well plates (10 5 cells/well) for examination of intracellular and extracellular lactate levels, respectively. Cells were washed once with HBSS 24 h after seeding, and the culture medium was replaced by fresh DMEM containing 5.5 mM glucose for the control group or 25 mM for the high-glucose group (31)(32)(33).…”

Downregulation of Astrocytic Kir4.1 Potassium Channels Is Associated with Hippocampal Neuronal Hyperexcitability in Type 2 Diabetic Mice

“…Moreover, in humans, seizure susceptibility is increased by hyperosmolarity, hyponatremia and hyperglycemia, caused by both type 1 and type 2 diabetes [24,25]. In cell culture, we have shown that Kir4.1 mRNA and protein levels are downregulated in astrocytes cultured in high glucose medium (simulating uncontrolled diabetes) and down-regulation is associated with impaired K + uptake and glutamate clearance by these astrocytes [26].…”

Oral lactate intensifies insulin toxicity during severe insulin-induced hypoglycemia in mice

“…In other words, there is a disruption of the harmonic interaction between astrocytes (glycolysis, lactate release, and glutamate uptake) and neurons (lactate uptake and its use as a fuel source in oxidative metabolism). But, it must be emphasized that the interactions between astrocytes and neurons involve not only glucose/lactate metabolism, but also neuroprotective mechanisms similar than that triggered during ischemia (Gouix et al, 2014), chronic hyperglycemia (Rivera-Aponte et al, 2015), and traumatic brain injury (Crupi et al, 2013).…”