Pathways for insulin access to the brain: the role of the microvascular endothelial cell

Abstract

Insulin affects multiple important central nervous system (CNS) functions including memory and appetite, yet the pathway(s) by which insulin reaches brain interstitial fluid (bISF) has not been clarified. Recent studies demonstrate that to reach bISF, subarachnoid cerebrospinal fluid (CSF) courses through the Virchow-Robin space (VRS) which sheaths penetrating pial vessels down to the capillary level. Whether insulin predominantly enters the VRS and bISF by local transport through the blood-brain barrier, or by being secreted into the CSF by the choroid plexus, is unknown. We injected I-TyrA14-insulin or regular insulin intravenously and compared the rates of insulin reaching subarachnoid CSF with its plasma clearance by brain tissue samples (an index of microvascular endothelial cell binding/uptake/transport). The latter process was more than 40-fold more rapid. We then showed that selective insulin receptor blockade or 4 wk of high-fat feeding each inhibited microvascular brainI-TyrA14-insulin clearance. We further confirmed that I-TyrA14-insulin was internalized by brain microvascular endothelial cells, indicating that the in vivo tissue association reflected cellular transport, not simply microvascular tracer binding.

Unravelling the regulation of insulin transport across the brain endothelial cell

“…Uptake was rapid and approximately linear between 0 and 15 min; therefore 15 min incubations were used for subsequent experiments. We previously reported that the IR-specific antagonist S-961 blocked 125 I-TyrA14-insulin uptake by RBMVECs, while IGF-I receptor blockade did not [25]. Figure 1a shows similar findings in hBECs.…”

“…However, increased oxidative stress, acting via increased NF-κB binding activity, may contribute. While we [25] and others [3, 4, 6, 47] found a delay of insulin entry into or action in the brains of HFD-fed animals, this is the first study to our knowledge to report impaired insulin transport at the level of the BEC. Our co-culture findings, as well as recent work regarding insulin signalling in astrocytes [48, 49], may suggest a role for astrocytes in the effect of an HFD on BBB insulin transport.…”

“…In vivo, 4 weeks of HFD feeding decreased brain insulin clearance [25], so we purified iBECs from male Sprague Dawley rats fed a ND or an HFD (60% lipid) ad lib for 4 weeks. iBECs were cultured for 5–7 days before experiments.…”

“…We recently compared the rate of 125 I-TyrA14-insulin transport into brain tissue with its movement into CSF and found its appearance in brain tissue preceded its appearance in CSF and that this was mediated by the insulin receptor (IR) and blunted by HFD feeding [25]. However, these studies were unable to differentiate insulin appearance in brain tissue from its binding and potential uptake into the BECs.…”

Insulin exits skeletal muscle capillaries by fluid-phase transport

“…One hormone whose activity is controlled by the endothelium is insulin (3,4). The ability of insulin to stimulate glucose uptake into SkM, the major site of insulin-stimulated glucose disposal, depends on the rate at which insulin traverses the endothelium (5).…”