Bilateral BBSRC-FAPESP: Behavioural and neuroendocrine mechanisms regulating hydromineral homeostasis - a lifelong perspective

Lead Research Organisation: University of Bristol
Department Name: Clinical Science at South Bristol

Abstract

The bodily fluids of the mammalian organism are in a constant state of flux. Even in the absence of challenges such as dehydration, haemorrhage or starvation, salt and water are constantly being lost as a consequence of normal, obligatory renal excretory functions, and by the processes of respiration and perspiration. The body has two mechanisms that function to control the consumption and the excretion of water and salt, in order to maintain the optimal bodily content required for good health. The first mechanism involves the production by a part of the brain paraventricular nucleus (PVN) of a hormone called vasopressin that tells the kidney to conserve water. The second mechanism is behavioural, and involves the instincts of thirst and salt appetite that emotionally drive the organism to correct its fluid balance. These mechanisms can go wrong resulting in ill-health. For example, disorders of fluid balance are evident in a substantial proportion of elderly patients admitted to hospital, and dehydration is a frequent cause of morbidity and mortality in old people. An age-related decline in the response to a variety of dehydrating challenges has been reported in humans, and this seems to involve a reduction in thirst and salt appetite, as well as dysregulation of vasopressin production. Another way that disorders of fluid balance can affect wellbeing is as a consequence of an excessive intake of dietary sodium, which is associated with the development of several chronic degenerative diseases, such as cardiovascular disorders, including hypertension. These medical conditions, which are becoming more prevalent as a result of increased life expectancy, progressively decrease life quality and increase the need for medical and social assistance. Epidemiological and experimental studies have suggested that events occurring in utero and during lactation can result in long-term consequences in adult life. Interestingly, both excessive salt intake and dehydration during pregnancy provoke increased salt appetite in adult offspring, which may then impact on long-term health and wellbeing.
We have recently produced exciting new evidence that suggests that the PVN is not only involved in the production of vasopressin, but also has a central role in generating the instincts that control the consumption of salt. We thus hypothesise:
i) that the PVN integrates hormonal and behavioural responses to salt and imbalance.
ii) that these integrative functions are perturbed in old age, resulting in decreased thirst perception, reduced sodium appetite, and altered activity of AVP neurones.
iii) that these integrative functions are perturbed by foetal exposure to high salt, resulting in a reprogramming of the set point for adult salt consumption.
Or aims are now to decipher the molecular mechanisms by which the PVN controls behavioural (thirst and sodium appetite) and hormonal (AVP synthesis and secretion) mechanisms of salt and water homeostasis. Further, we will find out how these mechanisms go wrong in old age and following foetal programming.

Technical Summary

We are interested in the mechanisms whereby the brain integrates the behavioural and neuroendocrine defence of salt and water balance. Transcriptome analysis has identified genes that are regulated by dehydration in key brain regions that controls salt and water homeostasis. We have found that shRNA-mediated inhibition of one of these genes, Gonadotophin inducible transcription factor 1 (Giot1), in the paraventricular nucleus (PVN) eliminates ad libitum salt appetite. This is reminiscent of the homeostatic deterioration seen in old age, when both thirst and salt appetite are much reduced. In contrast, RNAi mediated inhibition of another dehydration induced gene, small G-protein Rasd1, promoted the ingestion of salt. This is reminiscent of the effect of foetal programming, whereby the adult offspring of dams fed a high salt diet have increased salt appetite. These exciting new results suggest that the PVN is not only important for its neuroendocrine function, namely the elaboration of the antidiuretic hormone vasopressin, but is also a crucial component of the behavioural response to perturbation in salt and water balance. We will now test the hypothesis that these mechanism go wrong in old age and are disrupted by foetal programming. Or aims are now to:
i) decipher the molecular mechanisms by which Rasd1 and Giot1 affect the activity of neural circuitries that control behavioural (thirst and sodium appetite) and neuroendocrine mechanisms of hydromineral homeostasis, and to asses how these mechanisms are altered by old age and foetal programming.
ii) catalogue dynamic changes in the transcriptomes of sensory, integrative and effector components of the osmo-responsive circuitry as a consequence of both foetal programming and old age, and how these respond to an osmotic challenge.
iii) assess the physiological and behavioural roles of novel genes by altering their expression using in vivo somatic gene transfer followed by physiological and behavioural analyses.

Planned Impact

Academic Impact
In the short-term, the proposed studies will generate important new basic scientific knowledge about the way that the brain regulates water and salt homeostasis, about how these processes go wrong as we age, and about how a high slat diet during pregnancy can affect the health of the offspring. The new information gleaned from these studies will be published in high-impact, peer reviewed international journals, and will also be presented at national and international conferences. We will also train new scientific researchers in Brazil, whose integrative skills will be in great demand. Our collaborative activities will also contribute to scientific capacity building in Brazil, through the transfer of key technologies such as the use of viral vectors in physiological research. In contrast, the research will bolster system physiology in the UK, which has been in decline of late.
Economic Impact
It is unlikely that our results will be clinically or commercially relevant, at least in the short term. However, it is possible that genes will be uncovered that are possible targets for the treatment or amelioration of ageing disorders. If this is the case, we will seek to exploit our findings in collaboration with the BBSRC and the University of Bristol Research and Enterprise Development unit. We already have excellent links with industry (Source BioScience, DuPont, Merck Sharp and Dohme, Pfizer, Astellas, Eli Lilly, Ardian). In the long-term, it may well be that our findings will improve quality of life and functionality in old age. This would reduce welfare and health costs whilst boosting economic productivity. Given that our work will reveal the physiological effects of a high salt diet, we might anticipate interest from the food industry.
Social Impact
It is estimated that by 2050, 40% of the population of the UK will over 50 years of age, and 25% will be over 65. Whilst this dramatic increase in lifespan should be celebrated, the resultant demographic change represents a major challenge. This is because wellbeing and health in old age has not improved proportionally with longevity. This imbalance not only impacts on the individual, who would obviously wish to enjoy a happy and healthy old age and retirement, but it also imposes considerable economic and social burdens on families and society. However, the reasons why homeostatic systems deteriorate in the elderly is not well understood, and it remains a major challenge for the biomedical research community to address these issues, with a view to ultimately improving the quality of life of our senior citizens.
We hope that the knowledge gleaned from the proposed studies will ultimately enhance quality of life, health and well being as we get older. This has clear benefits for the individual, for families and for society in general. We also envisage a contribution towards evidence based policy-making and influencing public policies and legislation at a local, regional, national and
international level; these scientific advancement will have global impact, as the ageing population is a world-wide phenomenon.
Policy makers and governments are keen to encourage us to consume a healthy diet, and that includes imploring us to take less salt with our food. Food companies, by signing up to the Public Health Responsibility Deal Companies, are committed to reducing the amount of salt in their products in order to meet Government targets for individual consumption. However, these endeavours have not been based on too much solid empirical evidence of the benefits, particularly in relation to combating diseases of old age. Our studies will provide such evidence. As such we also expect an impact with nutritionists, dieticians and occupational and therapists.

Publications

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Freiria-Oliveira AH (2015) Catecholaminergic neurons in the comissural region of the nucleus of the solitary tract modulate hyperosmolality-induced responses. in American journal of physiology. Regulatory, integrative and comparative physiology

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Greenwood M (2014) Transcription factor CREB3L1 regulates vasopressin gene expression in the rat hypothalamus. in The Journal of neuroscience : the official journal of the Society for Neuroscience

 
Description We have clearly demonstrated that it is possible to progress from transcriptomic data gathering to successful and detailed in vivo functional studies of key prioritised target genes. Thus, we have identified novel genes involved in the biosynthesis of vasopressin in the hypothalamus (Creb3l1, Azin1, Caprin-2), the physiological response of vasopressin neurones to osmotic cues (Slc12a1), and the behavioural control of salt ingestion (Rasd1 and Giot1). Further, we are presently exploring the functions of a number of novel hypertension-related brain genes. It should be noted that none of these genes would have been studied in the context of these physiological systems without the guidance of the transcriptome data. A key aspect of this approach has been our collaborations with bioinformaticians on the application of these mathematical tools to transcriptomic data in order to address biological questions by pinpointing key, hub genes that are good candidates for physiological study.
Exploitation Route Unlikely in the short term.
Sectors Healthcare

 
Description Saudi Arabia collaboration 
Organisation King Saud University
Department Department of Zoology
Country Saudi Arabia 
Sector Academic/University 
PI Contribution Transcriptomic analysis of Jerboa tissues.
Collaborator Contribution Privision of Jerboa tissues.
Impact Not applicable as yet/
Start Year 2018