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Well-Aging and the Tanycytic Control of Health

Periodic Reporting for period 2 - WATCH (Well-Aging and the Tanycytic Control of Health)

Okres sprawozdawczy: 2020-09-01 do 2022-02-28

The survival of an organism depends on the accurate and timely exchange of signals between the brain and peripheral organs and tissues. Among the processes that need to be regulated by such an exchange is energy homeostasis, that is, maintaining the balance between energy expenditure by bodily functions and energy intake in the form of food. Imbalances in this process are linked to metabolic diseases such as type-2 diabetes or obesity. On the other end of the spectrum, the breakdown of communication between the brain and the rest of the body is also a hallmark of neurodegenerative disorders such as Alzheimer’s, which occur with age and in which cognitive deficits are often preceded by a loss of weight. Understanding how the brain perceives and regulates bodily functions in response to peripheral signals is therefore essential to dealing with both metabolic disorders and neurodegenerative diseases, whose prevalence is dramatically increasing due to changes in lifestyle and the aging of the population in some parts of the world.

Energy homeostasis is maintained by neuroendocrine circuits – neurons and glia - located in the hypothalamus, a small and highly specialized part of the brain. These circuits can sense and integrate metabolic feedback in the form of molecules such as glucose or hormones from peripheral tissues that signal hunger or satiety, and thus adapt the response of the organism to physiological demands. Tanycytes are peculiar glial cells present in a part of the hypothalamus that is both exposed to blood-borne signals and secretes neuroendocrine signals from the brain into the circulation to control the function of peripheral tissues, thanks to the fact that the endothelial cells lining the blood vessels in this region are “fenestrated” (that is, having windows) instead of walling off the blood from the brain tissue as in most other brain regions. Among their characteristics, tanycytes act as linchpins of this two-way exchange process, forming a bridge between the blood contained in the fenestrated blood vessels and the cerebrospinal fluid contained in brain pockets known as ventricles, which connects this part of the hypothalamus with other brain regions. Tanycytes can thus ferry signals across the traditional barriers that separate the two compartments, allowing them to reach, notably, neurons that regulate food intake or other physiological processes. In addition to their shuttling properties, they are also capable of undergoing morphological and functional changes in coordination with the endothelial cells lining the fenestrated blood vessels, thereby controlling the entry or exit of signaling molecules and hormones in response to bodily needs.

The overarching goal of WATCH is to use a variety of techniques, animal models and patients with metabolic or neurodegenerative diseases to explore the role of these unique and versatile cells, and develop new therapeutic approaches based on improving or restoring their function for a variety of disorders that impair well-aging.
We have obtained a number of groundbreaking results, published in the top journals of our respective fields. Notably, we have shown that tanycytes control the entry of leptin into the brqin through a bouble shuttle, controlling metabolism and pancreatic function. In addition other tanycytes feed glucose to hypothalamic neurons that mediate energy balance. We have also shown that hypothalamic neurons that are involved in controlling the generation of heat from peripheral fat deposits influence the barrier properties of endothelial cells lining the blood vessels through which metabolic signals enter the brain. In addition, we have shown that a molecule named Sirtuin 3 appears to be involved in regulating the function of hypothalamic neurons that regulate feeding, and that this regulation is sex- and diet-dependent. And finally, we have shown that the anorexia and weight loss that accompanies certain inflammatory diseases is mediated by a molecule named NEMO, which is part of an inflammatory complex, in tanycytes, putting these cells front and center in a type of disease mechanism different from metabolic disorders.

Given the preoccupying nature of the COVID-19 pandemic and the intriguing observation that most COVID-19 patients shown symptoms of brain infection, we have also been working on whether and how SARS-CoV-2, the virus responsible for COVID-19, enters brain cells, why certain individuals are at higher risk for severe forms of the disease, and whether we can predict or limit the long-term consequences of infection.

Several other studies are in the pipelines.
The hypothalamus contains numerous populations of neurons with specialized functions, notably the control of food intake in response to hunger or satiety, the storage of excess energy in the form of fats or sugars, and the use of such stored energy to generate heat or meet bodily needs. Previous work from our teams has shown that specialized hypothalamic glial cells called tanycytes replace the blood-brain barrier in a particular part of the hypothalamus, and cooperate with endothelial cells in this region, which harbor “fenestrations” or windows, to allow blood-borne signals to enter the hypothalamus. When the entry of such molecules through the tanycytes is blocked, changes in feeding behavior and metabolic imbalances result, indicating that these seemingly insignificant cells play a primordial role in controlling the exchange of signals between the brain and peripheral tissues. However, among other barriers to understanding this process, the cell populations involved in these various functions are so small and heterogeneous that it is easy to miss changes in the proteins they express or their activity, making it difficult to say with certainty how they control the access of these signals to the brain or how the neurons involved in these functions modulate their response.

In the WATCH project, we would like to understand the process as a whole, as well as the contribution of tanycytes to the entry of metabolic signals from the blood into the brain, and how these signals are interpreted to change the activity of the neurons sensing them in order to affect the behavior or functional response of the individual. Specifically, we aim to go beyond the state of the art regarding this process first by developing new tools or adapting existing tools to allow us to distinguish between different subpopulations of these sparse cell types and to identify and block individual steps in the transport and regulatory mechanisms, and second, by studying which of these subpopulations react to a specific physiological or pathological condition, in conjunction with what other cells and in what manner.

We have developed a number of techniques that can used to separate, observe or interfere with these molecular processes in living cells or animals and made several novel observations. By the end of the project, once we have obtained a more detailed understanding of the mechanisms through which these cells perform their functions and how they are altered with aging or under disease conditions, for example due to obesity, diabetes, Alzheimer disease or other neurodegenerative disorders, we can design ways to interfere with these pathological changes or restore normal function, thus providing a road-map for well-aging.
Hypothalamic regions and cells studied by the WATCH consortium