The CNS has a key role in energy balance and body weight regulation, and further understanding of how the CNS regulates energy balance can aid in the development of effective treatments for obesity. Decades of research have demonstrated that the brain integrates hormonal, metabolic, cognitive and emotional signals to maintain energy balance, and dysregulation of these neural circuits can lead to conditions such as obesity or eating disorders. For example, the hindbrain is responsible for integrating information necessary for energy homeostasis from diverse systems, including vagally mediated gastrointestinal signals, alterations in glucose and other circulating metabolites and descending neuroendocrine signals from the midbrain and forebrain. In tandem, the arcuate nucleus of the hypothalamus, which contains both anorexigenic proopiomelanocortin-expressing neurons and orexigenic agouti-related protein/neuropeptide Y-co-expressing neurons, has a key role in the homeostatic control of food intake. However, energy balance is not exclusively regulated by homeostatic mechanisms. The CNS also contains ‘reward systems’ that influence motivational processes related to eating behaviour. The ventral tegmental area provides dopaminergic input to the nucleus accumbens to encode the rewarding properties of food. Moreover, the consumption of palatable food increases extracellular dopamine levels in the nucleus accumbens to regulate food-seeking behaviour. Given all these functions, identifying the mechanisms for energy balance in the CNS is important for understanding obesity as a disease and for developing effective pharmacotherapies for obesity.
Glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are gut-derived incretin hormones that facilitate glucose-stimulated insulin secretion. In addition, both have extra-pancreatic functions that impact appetite regulation in the CNS. GLP-1 binds to receptors (GLP-1R) in the hindbrain, hypothalamus, lateral septum and cortex, engaging circuits that inhibit food intake (extensively reviewed in previous work). Similarly, GIP receptor (GIPR) and GIP binding have been detected in multiple brain regions, including the hippocampus, olfactory bulb, brainstem, lateral septum and hypothalamus. Within the hypothalamus, Gipr- expressing cells are localized in the paraventricular, dorsomedial and arcuate nuclei and colocalize with markers for neurons, glia and vascular cells. Activation of the hypothalamic GIPR suppresses food intake. Moreover, in recent studies, chemogenetic stimulation of brainstem GIPR was also shown to suppress food intake while activating conditioned taste avoidance responses.
GLP-1R and GIPR have crucial roles in metabolic homeostasis and energy balance, making them promising targets for obesity treatment. This potential is exemplified by the therapeutic efficacy of the dual agonist tirzepatide. Additionally, novel peptide–antibody conjugates blocking GIPR and activating the GLP-1R have recently been shown to be an effective and tolerable pharmacotherapy for obesity in mice, monkeys and humans. The peptide–antibody conjugate GIPR-Ab/GLP-1 is a fully human monoclonal anti-human GIPR antagonist antibody (GIPR-Ab) conjugated by amino acid linkers to two GLP-1 analogues. The mechanisms engaged by this peptide–antibody conjugate leading to weight loss and metabolic improvement remain unclear. Here, we identify CNS sites activated by GIPR-Ab/GLP-1 and reveal key roles for both CNS GLP-1Rs and GIPRs to enable the full extent of weight loss and metabolic benefit.
To investigate the role of GIPR in energy balance in the CNS, male mice that were fed a high-fat diet (HFD) were administered intracerebroventricular (ICV) injections of varying doses of a fully human anti-human GIPR antibody that cross-reacts with mouse GIPR (mGIPR-Ab). Mice were treated every other day for a total of ten doses, with daily body weight and food intake measurements (schematic of study design Fig. 1a). The dosing rationale was determined from the pharmacokinetic profile of mGIPR-Ab following a single ICV-administered dose (Extended Data Fig. 1a,b). Mice treated with 7.5, 15 and 30 µg of mGIPR-Ab lost an average of 5.1%, 9.3% and 8.2% of their initial body weight, respectively (Fig. 1b,c), whereas control mice treated with either artificial cerebrospinal fluid or two different doses of IgG1 (a non-binding control antibody) maintained a stable body weight (Fig. 1c). Cumulative food intake was significantly reduced with mGIPR-Ab treatment (Fig. 1d). Hence, medium and high doses of CNS mGIPR-Ab reduce body weight in obese mice.
We next compared ICV versus intraperitoneal (IP) administration of mGIPR-Ab in diet-induced obese (DIO) mice. Both ICV and IP administration of mGIPR-Ab led to similar magnitudes of weight loss relative to their respective controls (Fig. 1e,f and Extended Data Fig. 1c), which was accompanied by comparable reductions in cumulative food intake (Fig. 1g). Pharmacokinetic analysis revealed that mGIPR-Ab exposure in the forebrain was significantly higher with ICV administration (ICV avg, 3,469 ng g−1; IP avg, 923.3 ng g−1; Fig. 1h). On the other hand, mGIPR-Ab exposure in the hindbrain was significantly higher with IP administration (ICV avg, 186.5 ng g−1; IP avg, 884.0 ng g−1; Fig. 1i). Overall, mGIPR-Ab exposure in the whole brain was not significantly different between ICV and IP routes of administration (ICV avg, 3,655 ng g−1; IP avg, 1,807 ng g−1; Fig. 1j). As expected, mGIPR-Ab exposure in the plasma (ICV avg, 14,383 ng g−1, IP avg, 374,067 ng g−1), inguinal white adipose tissue (WAT) (ICV avg, 158.5 ng g−1, IP avg, 5,496 ng g−1) and epididymal WAT (ICV avg, 268.3 ng g−1, IP avg, 15,423 ng g−1) was higher with IP administration (Fig. 1k–m). To determine whether these exposure levels result in significant changes in GIPR function, a cAMP assay was used to determine the IC 50 of mGIPR-Ab in mouse Neuro2A cells (Extended Data Fig. 1d) and then compared to in vivo exposure levels. The results indicate that low exposure levels (~100 ng g−1) in the hindbrain and peripheral tissues are probably meaningful and may contribute to the effects of mGIPR-Ab on body weight. Altogether, these results demonstrate that CNS GIPR antagonism leads to body weight loss, and both central and systemic dosing of mGIPR-Ab can target the CNS to regulate body weight. Moreover, following systemic administration, mGIPR-Ab has access to the brain and results in weight loss comparable to central administration of mGIPR-Ab.
We next examined the effect of mGIPR-Ab/GLP-1 on body weight using two previously characterized molecules of mGIPR-Ab/GLP-1 (mGIPR-Ab/P1 (ref. 16) and mGIPR-Ab/P3 (ref. 15)).
