Levin, B. E., Becker, T. C., Eiki, J.-I., Zhang, B. B. & Dunn-Meynell, A. A. Ventromedial hypothalamic glucokinase is an important mediator of the counterregulatory response to insulin-induced hypoglycemia. Diabetes 57, 1371-1379

Neurology Service, Department of Veterans Affairs New Jersey Health Care System, East Orange, New Jersey 07018-1095, USA.
Diabetes (Impact Factor: 8.1). 06/2008; 57(5):1371-9. DOI: 10.2337/db07-1755
Source: PubMed


The counterregulatory response to insulin-induced hypoglycemia is mediated by the ventromedial hypothalamus (VMH), which contains specialized glucosensing neurons, many of which use glucokinase (GK) as the rate-limiting step in glucose's regulation of neuronal activity. Since conditions associated with increased VMH GK expression are associated with a blunted counterregulatory response, we tested the hypothesis that increasing VMH GK activity would similarly attenuate, while decreasing GK activity would enhance the counterregulatory response to insulin-induced hypoglycemia.
The counterregulatory response to insulin-induced hypoglycemia was evaluated in Sprague-Dawley rats after bilateral VMH injections of 1) a GK activator drug (compound A) to increase VMH GK activity, 2) low-dose alloxan (4 mug) to acutely inhibit GK activity, 3) high-dose alloxan (24 microg), or 4) an adenovirus expressing GK short hairpin RNA (shRNA) to chronically reduce GK expression and activity.
Compound A increased VMH GK activity sixfold in vitro and reduced the epinephrine, norepinephrine, and glucagon responses to insulin-induced hypoglycemia by 40-62% when injected into the VMH in vivo. On the other hand, acute and chronic reductions of VMH GK mRNA or activity had a lesser and more selective effect on increasing primarily the epinephrine response to insulin-induced hypoglycemia by 23-50%.
These studies suggest that VMH GK activity is an important regulator of the counterregulatory response to insulin-induced hypoglycemia and that a drug that specifically inhibited the rise in hypothalamic GK activity after insulin-induced hypoglycemia might improve the dampened counterregulatory response seen in tightly controlled diabetic subjects.

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    • "Compartmentalization of GK by GKRP observed at 30 min of glucose may act as a molecular switch to arrest cellular signaling in response to increased glucose in the hypothalamus. When GK is inhibited by i.c.v injection of its pharmacological inhibitor, the feeding behavior was increased [57]; reduced activity of GK also generates an increase in the counterregulatory response to insulin-induced hypoglycemia in rats [68], which is in agreement with the inhibition of lactate flux from the glia to neurons. However, there is some controversy regarding the mechanism of tanycyte response and its function [13]–[14]. "
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    ABSTRACT: Glucokinase (GK), the hexokinase involved in glucose sensing in pancreatic β cells, is also expressed in hypothalamic tanycytes, which cover the ventricular walls of the basal hypothalamus and are implicated in an indirect control of neuronal activity by glucose. Previously, we demonstrated that GK was preferentially localized in tanycyte nuclei in euglycemic rats, which has been reported in hepatocytes and is suggestive of the presence of the GK regulatory protein, GKRP. In the present study, GK intracellular localization in hypothalamic and hepatic tissues of the same rats under several glycemic conditions was compared using confocal microscopy and Western blot analysis. In the hypothalamus, increased GK nuclear localization was observed in hyperglycemic conditions; however, it was primarily localized in the cytoplasm in hepatic tissue under the same conditions. Both GK and GKRP were next cloned from primary cultures of tanycytes. Expression of GK by Escherichia coli revealed a functional cooperative protein with a S0.5 of 10 mM. GKRP, expressed in Saccharomyces cerevisiae, inhibited GK activity in vitro with a Ki 0.2 µM. We also demonstrated increased nuclear reactivity of both GK and GKRP in response to high glucose concentrations in tanycyte cultures. These data were confirmed using Western blot analysis of nuclear extracts. Results indicate that GK undergoes short-term regulation by nuclear compartmentalization. Thus, in tanycytes, GK can act as a molecular switch to arrest cellular responses to increased glucose.
    PLoS ONE 04/2014; 9(4):e94035. DOI:10.1371/journal.pone.0094035 · 3.23 Impact Factor
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    • "Similar to its role in pancreatic beta-cells, glucokinase appears to act as a critical glucose sensor in a substantial proportion of these VMN neurons (Kang et al. 2006). Glucose-sensing neurons play a critical role in regulating release of counter-regulatory hormones, such as glucagon, in response to hypoglycemia (Borg et al. 1995, 1997; Levin et al. 2008). In situations such as diet-induced obesity, where there is defective central counter-regulatory response to hypoglycemia, VMN glucokinase mRNA expression is increased (Dunn-Meynell et al. 2002). "
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    11/2013; 1(6):e00162. DOI:10.1002/phy2.162
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    • "Cold-anesthetized, postnatal day 5 (P5) rats were stereotaxically injected bilaterally in the border between the VMN and ARC over 10 min with 0.4 μL of saline containing 2 × 108 genomic particles of AAV per side containing either CD36 short hairpin RNA (shRNA) + mCherry or a control sequence + mCherry. With level bregma and lambda, coordinates were as follows: 2.6 mm caudal to bregma, 0.40 mm lateral to midline, and 6.6 mm down from dural surface as previously described (3). "
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    ABSTRACT: Hypothalamic "metabolic sensing" neurons sense glucose, fatty acids (FA) and play an integral role in the regulation of glucose, energy homeostasis, the development of obesity and diabetes. Using pharmacologic agents, we previously found that ∼50% of these neurons responded to oleic acid, by utilizing the fatty acid (FA) translocator/receptor, FAT/CD36 (CD36). To further elucidate the role of CD36 in neuronal FA sensing, ventromedial hypothalamus (VMH) CD36 was depleted using adeno-associated viral (AAV) vector expressing CD36 shRNA in rats. While their neuronal glucosensing was unaffected by CD36 depletion, their percent of neurons that responded to oleic acid was decreased specifically in glucosensing neurons. A similar effect was seen in total body CD36 knockout mice. Next, weanling rats were injected in the VMH with CD36 AAV shRNA. Despite significant VMH CD36 depletion, there was no effect on food intake, body weight gain or total carcass adiposity on chow or 45% fat diets. However, VMH CD36 depleted rats did have increased plasma leptin and subcutaneous fat deposition and markedly abnormal glucose tolerance. These results demonstrate that CD36 is a critical factor in both VMH neuronal FA sensing and the regulation of energy and glucose homeostasis.
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