Approximately 29.1 million patients in the United States suffer from diabetes (9.3% of the population) costing 245 billion dollars a year, which have increased 40% in the last five years. These staggering figures underscore the importance of developing more effective and potentially cheaper therapeutics for diabetes treatment. While the goal of diabetes treatment is normoglycemia, most therapeutics (especially insulin and insulin secretagogues) carry with them a significant risk of potentially life-threatening hypoglycemia; this risk increases with the intensity of therapy. Hypoglycemic shock is the cause of 282,000 emergency room visits a year and the death of about 1 in every 20 patients with type 1 diabetes. In addition to the financial burden and risk associated with hypoglycemic episodes, the risk of hypoglycemia limits diabetes treatment. Low glucose in the brain during hypoglycemia triggers the counter-regulatory response (CRR), activating the sympathetic nervous system (SNS) and hormonal response that stimulate glucose production. Repeated exposure to hypoglycemia promotes hypoglycemia-associated autonomic failure (HAAF) and hypoglycemia unawareness, increasing the risk and severity of hypoglycemic events. Hence, it will be crucial to understand the systems that mediate the CRR to hypoglycemia so that we may identify potential targets for therapeutic intervention. My goal is to become an independent investigator studying the neurocircuits that initiate the counterregulatory responses to hypoglycemia. I plan to use the techniques outlined in this proposal to reveal the neurocircuitry of counterregulation, as well as novel factors within this circuitry that can regulate these responses. These techniques (TRAP-seq, optogenetics, and in vivo microdialysis) in conjunction with previously gathered tools, will provide me with a strong set of skills for direct circuit analysis. In addition to the technical development outlined in this proposal, significant professional development will take place as I transition into an independent faculty position. The University of Michigan offers a host of resources, in the form of workshops, seminars, and short courses provided by the Postdoctoral Association, Center for Research Learning and Teaching, Office of Faculty Affairs and Faculty Development, among others. I will also receive guidance from my career advisory committee, who will assist me in finding a position that I will be successful at as an independent investigator. As the foundation of postdoctoral training (T32 and F32 support), I have been studying leptin action via brainstem sites (primarily the parabrachial nucleus (PBN)). These areas contain significant populations of leptin responsive neurons that, until recently, have been essentially unstudied. We found that CCK- expressing, leptin and glucose-inhibited PBN neurons project to the ventromedial hypothalamic nucleus to initiate counterregulatory responses to hypoglycemia, revealed by responses to insulin, 2-deoxyglucose, and during a hyperinsulinemic/ hypoglycemic clamp. Remotely activating these neurons revealed that these neurons can sufficiently drive hyperglycemia and associated homonal changes and remotely inhibiting these neurons blunt counterregulation. Following these studies that yielded exciting results published in a Nature Neuroscience paper, we followed the circuit down to the ventromedial hypothalamic nucleus (VMN) to study the downstream neurons and decipher this neurocircuit that induces pronounced hyperglycemia. The VMN links a variety of metabolic parameters (including glucose concentrations) to SNS outflow and other parameters of the CRR. Many VMN neurons connect to brain areas essential to glucose homeostasis. Distinct, but intermingled, subsets of VMN neurons mediate different effects, however. Indeed, not only do neurons that increase glucose production as part of the CRR reside in the VMN, but the VMN also contains neurons that promote glucose disposal and thus decrease circulating glucose. The intermingling of essentially oppositely-acting neurons within the VMN and the lack of molecular markers to define CRR-specific subtypes of VMN neurons has hampered our ability to study and understand the VMN neurons that contribute to the CRR. Since the VMN contains substantial CCK receptor B (Cckbr) (aka CCK-2-receptor) and the PBN neurons that drive hyperglycemia express CCK, we generated Cckbrcre mice to study CCK-responsive VMN cells. Our preliminary data demonstrate that Cckbrcre identifies a subpopulation of VMN neurons, and that pharmacogenetic activation of these VMNCckbr neurons increases blood glucose. The proposed experiments test the hypothesis that low glucose activates VMNCckbr neurons, these neurons project to regions that mediate SNS outflow and glucose production, and VMNCckbr neurons play a central role in the CRR to hypoglycemia and the onset/appearance of HAAF. In addition to potentially revealing a neural system underlying the CRR, these studies will define the regulation, circuitry and function of a novel subpopulation of neurons in the metabolically-important VMN, thereby increasing our understanding of brain systems that control metabolism. By understanding the underlying mechanisms of both the CRR and HAAF, we will potentially determine patients who are susceptible to HAAF and carefully administer treatment, while also aggressively treating patients without such a risk. We may also be able to develop therapeutics to co-administer with insulin and insulin secretagogues or design better therapeutics that will lower blood glucose without inducing diabetic shock and HAAF.
Dysfunctional responses to hypoglycemia following recurrent insulin treatments plague diabetic patients, making them susceptible to diabetic shock and can result in death. This project aims to decipher the neurocircuits and components thereof that regulate these responses. Once the mechanisms of the responses have been uncovered, we will be able to develop therapeutics to co-administer with insulin and insulin secretagogues or design better therapeutics that will lower blood glucose without inducing diabetic shock
| Flak, Jonathan N (2017) A role for leptin-regulated neurocircuitry in subordination stress. Physiol Behav 178:144-150 |
