Genetic interrogation of central circuit regulating blood pressure

Nearly two-thirds of the UK population is now overweight or obese. This leads to a multitude of related diseases, including high blood pressure, which can increase the chances of suffering a heart attack or stroke. The costs of obesity and its related diseases also put extreme pressure on the NHS.
There is a direct relationship between body weight and blood pressure. Thus, the higher your weight the more likely you will suffer from high blood pressure. However, currently, it is unknown what the major factors are which lead to this direct relationship. One suggestion is that obese people produce a lot of a hormone, called leptin, which normally acts on the brain to try and moderate increasing weight. However, in doing this, leptin may also indirectly cause increased blood pressure. We wish to understand where in the brain leptin can have this effect, as we have done previously to show where leptin acts to affect body weight. For example, leptin can increase the number of calories we burn by activating nerve cells (neurones) which lie in a tiny region of the brain called the dorsomedial nucleus of the hypothalamus (or DMH). These neurones contain a messenger called PrRP.
We now have evidence from humans for links between PrRP and its receptor, and with obesity. A number of obese patients have been identified who have mutations in the gene that encodes for the PrRP receptor. Interestingly, these patients have lower than expected blood pressure. Furthermore, we have shown that mice that have mutations in their PrRP receptor also have low blood pressure. So, could PrRP neurones in the DMH respond to leptin in obese patients and be responsible for their high blood pressure? To start with we will generate mice that have the same mutations as found in humans, so that we can study the functioning of this gene in an experimental animal. Then we can study the complex circuits in the brain to determine how leptin and PrRP have their effects.
We are able to study the extremely complex network of neurones in the brain because we can see the different types, such as the PrRP neurones, in mice because they have been made to shine with fluorescent light. We can record the activity of these neurones while stimulating the other cells that connect with them - all in a petri dish! However, what is also very new is that we can stimulate specific types of neurone in mice while they are behaving perfectly normally. This can be done by either giving the mice an injection of a "designer drug" or by shining light into the mouse's brain using an optic fibre. We can also inhibit the activity of the same neurones and see how the mice respond to us experimentally lowering their blood pressure. By doing this, we can see whether mice without functioning PrRP neurones find it more difficult to counteract low blood pressure when compared with normal mice. This will allow us to study the role of PrRP in the healthy control of blood pressure, as well as the consequences of mutations in both mice and humans. Importantly, it will help us determine how obesity is linked with high blood pressure, which potentially could lead to tailored therapies for obese patients.

There is a direct relationship between body mass index and blood pressure (BP). About two-thirds of cases of hypertension in are caused directly by obesity, with leptin acting as a major mediator that links weight with BP. Some of these effects of leptin are dependent on melanocortin signalling in the hypothalamic paraventricular nucleus, but some are independent and are, instead, mediated by the dorsomedial nucleus (DMH). Interestingly, we have demonstrated previously that leptin's effects on thermogenesis in mice are dependent on neurones in the DMH which contain prolactin-releasing peptide (PrRPDMH). Severely obese patients recruited to the Genetics of Obesity (GOOS) cohort contain rare heterozygous mutations in the PrRP receptor, GPR10, and preliminary physiological studies show that, in addition to their obesity, carriers have low systolic BP and heart rate suggesting effects on autonomic tone. We will demonstrate that the most penetrant mutations found in the human GPR10 (and PrRP genes) are functional in vivo using transgenic "humanised mice." These will be compared with mice we have already generated with null mutations in the two genes.
Although PrRPDMH neurones may have a role in mediating leptin's effects on BP, brainstem PrRPNTS neurones are positioned to affect baroreceptor input, while another population lies entirely within the caudal pressor area of the ventrolateral medulla. Using our previously generated PrRP-cre transgenic mouse line, we will investigate the relative importance of PrRP populations, using chemogenetic activation or inactivation with designer receptors. Then using both chemo- and optogenetic manipulations, we will map this novel brain-cardiovascular signalling system. This will allow us to study normal, healthy physiology as well as the consequences of mutations in mouse models, to determine the potential link between obesity, leptin and hypertension.

It is estimated that by 2025 approximately 1.56 billion people worldwide will suffer from high blood pressure (BP). This 60% increase from 2000 is attributed largely to the increasing prevalence of obesity: in the UK, 66% of men and 55% of women are either overweight or obese (contributing to 1.1 billion adults worldwide). Hypertension increases cardiovascular disease, because high BP damages the heart and blood vessels in other organs (for example, leading to heart attack, stroke, kidney failure and eye damage). Heart disease and other metabolic co-morbidities related to obesity, such as diabetes and kidney disorders, create a massive public health problem, and are projected to cost the NHS £9.7 billion per annum by 2050. Our laboratories are well placed to make a major contribution to the basic understanding of how obesity is linked with hypertension. Our findings will be disseminated at international conferences and by publication in high-impact journals during the grant's duration. Following publication, each of the mouse models we develop will be made freely available.
A conservative commercial estimate of the annual market opportunity for anti-obesity and anti-hypertensive drugs is over $100 bn. This project will guide future development of drugs and provide a sound knowledge environment to understand the mechanisms that affect behaviour. Both applicants have excellent track records of collaborating with a number of industrial partners, providing evidence for several novel targets for drug development.
During the lifetime of the grant, the basic research will be discussed at meetings organised by the Child Health Research Network, the Diabetes and Obesity Research Network and the Association for the Study of Obesity. These annual meetings are forums for basic researchers, psychologists, clinicians, community nurses and other health professionals, patient group representatives and policy makers. Outreach work will be encouraged at all levels within the laboratory. Over the three years, the applicants will lecture at two local schools and at a local Café Scientifique-type meeting. The Researcher Co-Investigator will be strongly encouraged to follow the example set by previous lab members, to tutor for the Manchester Access and STEM programmes (aimed at helping under-privileged children into further education), and to complete both a Wellcome Trust Researchers in Residence Scheme and a UK GRADschool.
This project will provide strong training in both in vivo skills and specialist techniques in electrophysiology, chemogenetics and optogenetics, metabolic and behavioural research. In the last twelve years, the applicants have supervised thirteen PhD students, nineteen masters students and eleven post-doctoral research associates, all of whom have remained in science (some have their own independent research groups and others have moved into the commercial sector). The PI is external examiners on integrative masters courses. He also directs a cross-University IMB initiative to promote and expand research and training in in vivo biology. This problematic area is crucial to the UK economy and to the ambitions of Manchester to be a world-leading university.