Therapeutic potential for glial-derived octadecaneuropeptide in the treatment of obesity

1. The grand challenge

The contribution of non-neuronal cell populations to energy balance has been largely overlooked. This dearth in research represents a fundamental gap in our understanding of energy balance control given that glia (e.g. astrocytes and tanycytes) critically monitor and respond to nutrient-state indications that regulate bodyweight and metabolic homeostasis. Glia also play a pivotal role in mediating the food intake and bodyweight suppressive effects of current glucagon-like peptide-1 receptor (GLP-1 R) agonists [1]. Despite achieving weight loss, these medications have significant adverse effects that impact patient tolerability, tend to plateau in effect with chronic use, and upon discontinuation cause a rebounding of bodyweight gain back to or beyond pre-treatment levels [2,3]. As such, there remains a need for novel targets to advance the field of obesity treatment beyond GLP-1 R agonists. Understanding not only the contribution of glia to the normal homeostatic control of energy balance, but also in modulating pharmacological interventions on broader neural networks will be vital in developing new therapies for obesity. One exciting approach is that offered by the glial-derived neuropeptide octadecaneuropeptide (ODN; Figure 1) [4].

Figure 1.

Sequence of diazepam-binding inhibitor (DBI) and known proteolytic fragments including triakontatetraneuropeptide (TTN) and octadecaneuropeptide (ODN). Box highlights reported ODN phenotypes with therapeutic relevance.

2. Introduction

How glia contribute to GLP-1 R ligand effects are likely multi-faceted but undoubtedly involve engagement of gliotransmitters. To this end, endozepines, endogenous analogs of benzodiazepines derived from diazepam-binding inhibitor (DBI; Figure 1) are now emerging as a significant component of neuroglial-based energy balance control [5]. Tryptic digestion of DBI generates several major processing products, including DBI 33–50 (ODN; Figure 1) and DBI 17–50 (triakontatetraneuropeptide, TTN; Figure 1) These fragments exhibit distinct biological activities. TTN binds the mitochondrial translocator protein (TSPO), also called the peripheral-type benzodiazepine receptor, with over 100-fold higher affinity than ODN. While TSPO is involved in mitochondrial cholesterol transport, steroidogenesis, and redox homeostasis and may play a role in neurodegenerative diseases, the physiological impact of TTN as an endogenous ligand of TSPO remains unresolved [5]. In turn, ODN demonstrates a number of vital functions distinct from TTN. ODN stimulates astroglial antioxidant enzyme systems and protects against apoptosis via signaling at its unidentified orphan GPCR (oGPCR) across a range of neurodegenerative and cytotoxic models with underlying oxidative stress injuries [6]. Signal transduction of the ODN oGPCR may depend on cell type, as protection against toxin-induced apoptosis in astrocytes involves a PKA/MAPK mechanism [7], while in neurons it is mediated by PLC signaling [8]. Beyond the cytoprotective actions attributed to ODN signaling at its oGPCR, the anorexigenic effects of ODN (in rodents) are also mediated by signaling at the oGPCR, being unaffected by antagonizing central-type or peripheral-type benzodiazepine receptors [9,10].

The appetite suppressive effects of ODN signaling are mediated by brain regions that critically control energy balance homeostasis, including the dorsal vagal complex of the brainstem. Exogenous administration of ODN directly targeting the dorsal vagal complex suppresses food intake and bodyweight while improving glucose tolerance without eliciting malaise behaviors in rats [4]. The dorsal vagal complex comprises three unique nuclei: (1) the area postrema, a region outside the blood–brain barrier with access to circulating factors and therapeutics, (2) the nucleus tractus solitarius, the first site in the brain to receive and process vagally mediated signals arising from the gut, and (3) the dorsal motor nucleus of the vagus, the origin of parasympathetic efferent nerves that regulate peripheral metabolic function in response to integration of these metabolic-state cues [11]. Additionally, a border of specialized glia below the area postrema critically monitors systemic metabolic signals and gates access of nutrients, hormones, and pharmaceutical agents into the adjacent nucleus tractus solitarius (Figure 2). Transport across the subpostrema border is critical for energy balance regulating therapies, and indeed GLP-1 R ligand action in the nucleus tractus solitarius in large part mediates the therapeutic benefit of these compounds [12,13]. Within the dorsal vagal complex, DBI is most robustly expressed by tanycytes/epidomocytes and astrocytes in the subpostrema border [4]. Given the anatomical location and physiological role of glia in the dorsal vagal complex, ODN gliotransmission is of particular relevance for anti-obesity therapeutic development.

Figure 2.

Schematic representation of dorsal vagal complex octadecaneuropeptide (ODN) signaling stimulated by circulating metabolic signals in the area postrema (glucose and GLP-1) to engage neurocircuitry coordinating the postprandial metabolic response to suppress appetite, inhibit glucagon, and improve glucose clearance.

New findings suggest that ODN may partially mediate the energy balance effects GLP-1 R signaling. We report that DBI expression, especially in the subpostrema border, is upregulated by GLP-1 R ligand administration [4], suggesting ODN may be downstream of GLP-1 R ligand action on tanycytes and astrocytes (Figure 2). Antagonism of ODN signaling using AntOP, an ODN-oGPCR peptide antagonist, attenuates the intake and body weight suppressive effects of GLP-1 R agonism [4], providing in vivo evidence that part of the anorectic and bodyweight suppressive effects of GLP-1 R agonism may recruit a secondary signaling cascade mechanism involving gliotransmission. In line with the findings that ODN administration does not induce nausea, blocking ODN signaling with AntOP treatment does not affect the nausea induced by GLP-1 R agonism [4]. This suggests that ODN released from glia preferentially modulates a distinct subset of to-be-determined nucleus tractus solitarius neurons involved in processing satiation signals and does not contribute to the emetogenic neurocircuitry that partially overlaps with the GLP-1 R central system. Several nucleus tractus solitarius neuronal populations known to induce satiation, including nesfatin-1, catecholamine, and GLP-1 producing neurons, may be engaged by ODN signaling [14]. Whole-brain imaging studies could provide a future agnostic approach to identify the circuits and brain regions implicated in the energy balance effects of ODN or ODN-analogues. Importantly, the contribution of glia and ODN-specific signaling within the dorsal vagal complex to energy balance control has largely been under-investigated, despite being ripe for novel pathways that can putatively lead to pharmacological interventions targeting the ODN system.

Excitingly, ODN administration to obese rats and mice robustly suppresses appetite and induces weight loss [4], supporting that ODN signaling at its oGPCR is not resistant/impaired in obesity. We propose that instead production/processing of ODN from DBI may be impacted by metabolic disease. Accordingly, bypassing this point of dysregulation by administering ODN, or drug candidate ODN analogs, provides an exciting and novel anti-obesity strategy. Indeed, we recently showed that an artificial ODN analog, tridecaneuropeptide (TDN) with improved stability and proposed enhanced central penetrance compared to ODN, improves glycemia and reduces bodyweight in diet-induced obese mice upon peripheral administration [4]. Importantly, systemic delivery of TDN showed no signs of emesis in a musk shrew model [4]. This demonstrated for the first time that systemic administration of an ODN-analog could exert metabolic improvements mimicking central action.

3. Expert opinion on therapeutic potential

ODN is an emerging therapeutic target with considerable space for drug design and translation. ODN-based therapeutics hold the possibility as a new class of drugs for appetite and weight control, devoid of the gastrointestinal AE elicited by GLP-1 R-based drugs. This advantage is a consequence of the postulated downstream, or distinct, ODN site of action compared to GLP-1 R agonism, which does not engage known emetogenic neurons in the dorsal vagal complex. Moving forward, completing the picture of what circuits mediate ODN’s effects on energy balance will be critical in advancing ODN-based therapeutics for use as monotherapies or intandem with existing obesity targets such as GLP-1 R. Indeed, a tandem approach offers considerable scope for weight reduction while mitigating adverse events, given that ODN exerts additional food-intake reductions when administered acutely in combination with a GLP-1 R ligand [4], which also suggests that GLP-1 R agonists alone do not maximally recruit ODN signaling. A particular emphasis needs to be on the identification of the putative ODN-oGPCR, although it is clear that ODN, and other DBI-based proteolytic fragments, may interact with other binding proteins, transporters, or yet unidentified receptors and an extensive screening of ODN across the metabolome is warranted. Such studies may reveal hitherto unknown physiology and help avoid an overly narrow focus on a single (and still unknown) GPCR to better inform future drug discovery strategies – including the design of molecules based on alternative mechanisms of action.

Looking forward, deorphanizing the ODN-oGPCR (and others mechanistic investigations beyond) will be necessitated by the fact that ODN-based treatments for obesity will be taken chronically, and as such we will need to understand the potential long-term safety considerations. For example, given ODN is cytoprotective, could prolonged administration influence glial proliferative states, neuroinflammatory tone, or overall brain homeostasis? Questions on how chronic administration of ODN analogs may differ to current GLP-1 R agonists in terms of not just tolerability, but positive effects on fibrosis, heart, and (possibly) brain protection are all nascent as well. Such questions further highlight that many basic elements of endozepine physiological regulation are under-explored, with questions remaining as to what drives extracellular DBI export and what peptidases are responsible for subsequent DBI processing into ODN (and other active fragments?). Answers to the above questions will facilitate developing future ODN-based therapeutics through rational design of analogs that are long-acting (through lipidation and/or further sequence modifications), as well as potent and selective binders of ODN receptor targets. Such analogs may also be utilized as part of a polypharmacy approach to address metabolic disease, tackling multiple targets in a chimeric pharmaceutical. Overall, we view the pursuit of ODN physiology and pharmacology as at an early stage with an extremely interesting future ahead.