Role of Trk Receptors in the Development and Function of Non-neuronal Structures

Division Of Basic Sciences - Nci

Investigators

Linked publications, trials & patents

Abstract

Our past accomplishments include the discovery of a number of neurotrophin functions outside the nervous system. For example, we were the first to show that NT-3 and its receptor TrkC are important for normal development of the heart. Through a novel reverse conditional gene targeting approach, we also showed that NGF activation of TrkA has only limited effects on the development of the immune system. This was a surprising finding because it contradicted a large body of work, obtained mainly from in vitro studies, that suggested a major role for NGF in this organ. Importantly, our data provided essential information to pharmaceutical companies that are developing anti-NGF therapy for the management of chronic pain. As another example to illustrate the power of in vivo studies to understand the contribution of neurotrophin signaling in organism homeostasis, we also reported that BDNF plays a critical role in the regulation of energy balance. We demonstrated that disruption of the normal level of BDNF causes hyperphagia and obesity in mice, a finding that has led to the identification of a similar phenotype in humans. Since the last Site Visit, we further reported that the role of BDNF/TrkB signaling on energy homeostasis goes beyond regulation of food intake since we have found that muscle-derived BDNF signals directly in pancreatic b-cells expressing a specific TrkB isoform to regulate insulin secretion. Therefore, our studies are expanding the field of investigation into the role of neurotrophins on energy homeostasis with wide implications for the many diseases where obesity and metabolism play a role. Within the neurotrophin field, a unique focus of my laboratory has been on understanding the role of truncated Trk receptor isoforms in vivo. The TrkB and TrkC genes, by alternative splicing, can generate not only the full-length tyrosine kinase receptors but also receptors with an alternative intracellular domain that lacks kinase activity (i.e. TrkB.T1 and TrkC.T1). These receptors are extremely conserved among species and are the most highly expressed receptor isoforms in both the mature nervous system and in non-neuronal structures. Though they were first discovered about 30 years ago, only recently have we begun to learn about the function of these types of truncated receptors. Outside the nervous system, by a conditional deletion approach, we have identified an unexpected role of BDNF in adult cardiac physiology. This function is mediated by truncated TrkB.T1 receptor expression in cardiomyocytes and is independent of BDNF actions in the nervous system. We have reported by deletion and in vivo tagging of endogenous TrkB.T1 that expression of this TrkB isoform in pancreatic b-cells increases glucose-induced insulin secretion. Importantly, we showed that this pathway is conserved in humans. Altogether, these studies were key to finally providing strong and definitive physiological functions mediated by TrkB.T1 signaling that are independent of its classic dominant/negative role on TrkB.FL or BDNF sequestering activity. Moreover, they pave the way for the next phase aimed at identifying the still uncharacterized molecular pathway/s activated by TrkB.T1. So far, we and others have shown that TrkB.T1 regulates Ca++ signaling in astrocytes, cardiomyocytes and pancreatic b-cells but it is unclear as to which proteins it binds to induce signaling. This is still one of the major open questions in neurotrophin biology. It is likely that the small TrkB.T1 intracellular domain is unable to form stable interactions with proteins precluding their isolation and identification by mass-spectrometry. Therefore, to identify putative molecular partner/s of BDNF activated TrkB.T1we have decided to pursue a comprehensive unbiased approach including different techniques such as Yeast-two hybrid; BioID and APEX2 screenings followed by mass-spec analysis, and the CRISPR/Cas9 mediated transcriptional activation (CRISPRa) system. The identification of these pathways may shed new light into how BDNF regulates the functions of such important cell types as glia cells, cardiomyocytes and pancreatic b-cells. Most importantly, these pathways may identify new targets for the treatment of neurodegenerative disorders, cardiac pathological conditions and diabetes. For these reasons we are also expanding our future work to investigate whether TrkB.T1 signaling plays a role in cardiac injury and situations of metabolic stress.