Analysis of the brain GLP-1 circuitry at cellular level to characterise its roles in the control of food intake

Obesity, diabetes, and associated diseases such as hypertension, are a serious health burden for patients and a strain on healthcare services. A class of drugs that are increasingly being used clinically to treat obesity (and diabetes) are glucagon-like peptide-1 (GLP-1) receptor agonists (GLP-1RAs). GLP-1 is a hormone that is produced by our gut and released into the blood after a meal. Its main role is to help keeping sugars low in the blood, but it also generates the feeling of fullness, i.e. satiation. GLP-1RAs reproduce the effect of the hormone GLP-1 and that is how they suppress eating.
Interestingly, GLP-1 is also produced in the brain, and acts there to suppress eating. It has been widely assumed that GLP-1RAs also mimic the action of GLP-1 released by the brain and that this contributes to their anti-obesity effect. However, our laboratory has now shown that this is not the case, but that activation of the nerve cells in the brain that produce GLP-1, the PPG neurons, suppress eating independently from and in addition to clinically-used GLP-1RAs.
Whilst this is a highly exciting finding PPG neurons fulfil a variety of roles in our brain; they reduce food intake, but they also raise heart rate, reduce alcohol consumption, change body temperature and play a role in our response to stress. We hypothesise that different subgroups of these cells govern these different functions. Thus, the major aim of this research is to identify those different subgroups of PPG neurons that fulfil the different functions, and then selectively activate only those that suppress food intake for obesity treatment, and possibly even inhibit another group of these neurons that raises heart rate.
Understanding in detail how these PPG neuron groups fulfil their functions and revealing their individual properties will then facilitate the design a novel treatment strategy that could work in patients.

The proposal derives from recent findings that brainstem preproglucagon neurons (PPGs) are the predominant source of glucagon-like peptide 1 (GLP-1) in the brain, that chemogenetic activation of PPGs produces marked hypophagia, and that their hypophagic effect persists in the presence of an anti-obesity GLP-1 receptor agonist (GLP-1RA). This suggests that PPG neuronal activation reduces food intake via pathways separate from, and in addition to, the action of GLP-1RAs. The programme of research is designed to characterise in detail the pharmacological pathways to achieve this hypophagic effect, to discover the circuitry underlying the multitude of effects of PPG neuronal activation, and to delineate this circuitry from that activated by clinically-used GLP-1RAs.
These aims will be achieved by a combination of in vivo and in vitro studies using genetically modified mouse models expressing cre-recombinase in GLP-1 producing cells. We will identify and manipulate PPG subgroups to interrogate their function. We will transduce PPGs by projection target using retrograde AAVs encoding cre-dependent DREADDs and fluorescent reporters, to neuroanatomically map and functionally characterise these subpopulations using various behavioural assays. We will use single-nucleus transcriptomics to generate 'fingerprints' of the neuronal subgroups with translationally desirable behavioural outputs (e.g. satiation without nausea) to identify pharmacological targets that can be used to selectively activate these PPG subgroups in lean and obese mice. We will validate those identified targets in vitro, using imaging and electrophysiology in brain slice preparations from mice expressing genetically encoded Ca2+ sensors in PPGs. This research will provide us with deep understanding of the cellular-level architecture of the entire brain GLP-1 system and how the physiological functions of central GLP-1 signalling are organised to modulate diverse aspects of energy homeostasis and behaviour.