In This Issue of Diabetes Free

The antinarcolepsy drug modafinil appears to be able to reverse hypoglycemia unawareness, at least in mice, according to Patel et al. (p. 1144). Specifically, it seems that the drug can restore normal glucose sensitivity in perifornical hypothalamus orexin glucose–inhibited neurons, which in usual circumstances facilitate arousal but are suspected to have a role in hypoglycemia unawareness as well. The findings come from a combination of experiments that involved electrophysiology characterization of the effects of glucose exposure on the neurons and a modified animal behavior test, called “conditioned place preference,” to model hypoglycemia awareness. The authors found that conditions that mimic recurrent hypoglycemia enhanced glucose inhibition of the activity of the neurons, blunting their activation in low-glucose conditions. They also found that the antinarcolepsy drug modafinil restored normal glucose sensitivity in the neurons and restored hypoglycemia awareness in their mouse model. Based on the overall findings, they conclude that the orexin glucose–inhibited neurons of the lateral hypothalamus likely play a crucial role in hypoglycemia awareness and that drugs, specifically modafinil, that normalize glucose sensitivity of these neurons following recurrent hypoglycemia might have therapeutic value. They note that the findings are correlative and do not (yet) address causality and that the mechanisms underlying the effects of modafinil in this context remain to be investigated. They also point out that the drug is currently only in clinical use for narcolepsy and is part of the Schedule IV listed class of psychomotor stimulants, meaning further studies will be needed for safety and effectiveness in the context of use in diabetes. “Using the only mouse model that does not rely on glucoprivic feeding, we have shown that modafinil restores both the normal glucose sensitivity in orexin glucose–inhibited neurons and hypoglycemia awareness after recurrent hypoglycemia,” said author Vanessa H. Routh. “These observations pave the way for mechanistic studies leading to development of safe pharmacological therapies to treat hypoglycemia unawareness.”

Glycogen located in astrocytes in the brain is the major source of lactate in the ventromedial hypothalamus (VHM) in rats exposed to recurrent hypoglycemia, according to Su et al. (p. 1154). The finding appears to partly explain how glucose-responsive neurons fail to sense falling glucose levels, which can lead to the development of hypoglycemia unawareness. Lactate is considered a major metabolic fuel substrate, particularly when glucose is low, and the buildup in the VHM might explain how unawareness of hypoglycemia develops. In a series of experiments with rats exposed to recurrent hypoglycemia, the authors investigate whether glycogen is the source of elevated VHM lactate and glycogen turnover following recovery from hypoglycemia. Using an adeno-associated viral vector approach to target a key lactate transporter in VHM astrocytes, the authors found they could reduce lactate levels back to normal in recurrently hypoglycemic rats, which suggests that lactate is produced in astrocytes. To determine if the source in astrocytes is glycogen, the authors used a direct pharmacological approach to inhibit glycogen turnover, finding no rise in lactate in the VHM following recurrent episodes of hypoglycemia. Recurrently hypoglycemic controls, meanwhile, experienced increased levels of lactate, effectively indicating that lactate is derived from glycogen. The authors go on to examine glycogen turnover during and following hypoglycemia, noting that it appears to enhance glycogen shunt activity in the VHM. They also show that sustained elevations in glycogen phosphorylase activity in rats with recurrent hypoglycemia appear to help sustain elevated levels of lactate in the VHM. “This study furthers our understanding of the central mechanisms underlying the development of hypoglycemia unawareness and begins to unite some of the long-held theories that were put forth to explain the cause(s) of hypoglycemia unawareness,” said author Gong Su. “It is our hope that this research will help identify suitable therapeutic targets that can reduce the risk of hypoglycemia by improving hypoglycemia awareness in people with type 1 diabetes.”

A protein called growth differentiation factor 15 (GDF15) appears to enhance glucose-stimulated insulin secretion and is induced by muscle contractions during exercise, according to Zhang et al. (p. 1070). Their findings not only underscore the importance of exercise as a first-line therapy in type 2 diabetes but also raise the prospect that GDF15 is a therapeutic target for type 2 diabetes. The findings come from a series of experiments with cells and mice and three human studies that center on the underlying mechanisms involved in the improvements in insulin secretion seen following exercise. Using a technique called electric pulse stimulation to induce contractions in myotubes, they found that the resulting conditioned media applied to β-cells led to enhanced glucose-stimulated insulin secretion—indicating the presence of factors that can potentiate insulin secretion. Using transcriptomics and subsequent validation, the authors then identified GDF15 as a leading candidate to mediate the effect. Follow-up experiments with recombinant GDF15 demonstrated enhanced insulin secretion via the canonical insulin secretion pathway in β-cells, an effect that was canceled by GDF15 neutralizing antibody. Of note, they also found the same effects in GFRAL-deficient mice, ruling out mediation via that receptor and suggesting direct effects of GDF15. To contextualize the finding from the cell and mouse experiments, the authors also noted increased expression of the GDF15 gene in adults with obesity after 12 weeks of daily aerobic exercise. GDF15 was also incrementally increased in individuals with prediabetes or type 2 diabetes, and there was a positive association with C-peptide in individuals with obesity or overweight. Finally, 6 weeks of high-intensity training resulted in improved β-cell function that correlated with increasing levels of GDF15. The authors describe their findings as comprehensive evidence of a role of GDF15 in improving insulin secretion following exercise. However, they note that tracer studies in humans will be needed to confirm the same actions.

Methyl piperidine-3-carboxylate, which is a small-molecule agonist of the METTL3-METTL14-WATP complex, appears to promote thermogenesis in adipose tissue in mice, according to Xie et al. (p. 1083). Specifically, obese mice on a high-fat diet that received the agonist showed activation of beige adipose tissue, reduced body weight, and improved glucose metabolism and energy expenditure. As a result, the authors propose the agonist and the pathways it targets are therapeutic options for obesity and related issues. The findings come from an investigation into the posttranscriptional regulation of white adipose tissue beiging, a key process known to improve glucose and lipid metabolism. The studies focus on METTL3 (methyltransferase of N6-methyladenosine [m⁶A] mRNA modification), which is known to regulate energy expenditure in brown adipose tissue. The authors found that depletion of the Mettl3 gene specifically in adipose tissue of mice resulted in white adipose tissue beiging being restricted, and it impaired the metabolic capacity of mice on a high-fat diet. They also found that METTL3-catalyzed installation of m⁶A on thermogenic mRNAs promotes the stability of Krüppel-like factor 9, and in the process, it rescues impaired beiging due to Mettl3 gene depletion. They also found that the small-molecule agonist resulted in activation of the METTL3 complex and the promotion of white adipose tissue beiging; in mice, this resulted in a reduction of body weight and improved glucose metabolism. Most significantly, they found improved energy consumption and expenditure, including upregulated oxygen consumption, CO₂ generation, and heat generation. The authors conclude that the smallmolecule agonist and the combination of METTL3 and m⁶A might have potential as therapeutic targets for obesity and related issues. Commenting further, author Xiangwei Gao said, “The study highlights an important role of m⁶A writer METTL3 in promoting white adipose tissue beiging and improving energy expenditure. These findings create more possibility for combating obesity by targeting m⁶A.”