The contribution of tanycyte signalling to the function of hypothalamic networks

An area of the brain called the hypothalamus contains key neural circuits involved in the control of appetite, feeding and energy expenditure (collectively "energy balance"). Stimulated by the inexorable rise of obesity and consequent health problems, there has been intense study of these neural circuits. However, the possible role of non-neuronal cells in the brain has not been extensively considered. We have previously demonstrated that hypothalamic tanycytes, cells that contact cerebrospinal fluid in the ventricles of the brain and send processes into the hypothalamus, respond to both neuron-derived and circulating agents that signal energy status and arousal. Our data therefore suggest that tanycytes have the potential to actively participate in the control of energy balance. There is also abundant data from other researchers showing, for example, that processes of tanycytes come into close contact with neurons (very suggestive of communication passing between them) that supports the idea that tanycytes may be active players in the hypothalamic network.

We propose to test directly the idea that tanycyte-neuron communication exists and to characterize where and how it occurs (which neurons receive signals from tanycytes and what are the chemical signals released by tanycytes to effect this communication). Our approach will employ and develop selective optical methods to allow us to activate only the tanycytes. We shall combine this with functional imaging methods to screen for consequent responses in individual neurons, giving an efficient and comprehensive way of mapping these interactions. While some selective approaches for selective tanycyte activation already exist (and we shall use them), an important aspect of our project is to develop new potentially more powerful genetic methods to accomplish this aim. We shall develop ways to target light sensitive ion channels so that they express only in tanycytes thereby allowing illumination of the correct wavelength of light to selectively activate them. We shall verify that our methods do indeed work and use this new approach, along with existing methods, to map the tanycyte-neuron interactions. A further important aspect of our approach will be to identify the types of neurons that tanycytes influence. The nature of these interactions, and the types of neurons that tanycytes communicate with is central to our being able to hypothesize the potential functional roles of tanycytes. Finally there is evidence that the way the hypothalamic network functions depends upon metabolic state (e.g. fasting, diet induced obesity). We shall therefore also examine whether tanycyte-neuron interactions also depends upon metabolic state.

Our work will be influential in the design of future experiments both by proposing specific hypotheses for tanycyte function and by developing the tools necessary to rigorously link tanycyte signalling to behaviour. This has the potential to radically alter our understanding of how the brain controls energy balance and may suggest new ways to regulate appetite, and hence help to reduce the burden to society of obesity related illnesses.

The unremitting rise of obesity has led to intense study of the hypothalamic networks that are central to the regulation of energy balance. The study of these networks has concentrated almost exclusively on the roles of neurons. We have recently shown that hypothalamic tanycytes, cells at the interface between the ventricular cerebrospinal fluid and the brain parenchyma, respond to both neuron-derived and circulating agents that signal energy status and arousal. Other researchers have evidence that processes of tanycytes come into close contact with hypothalamic neurons. An active role for tanycytes in the hypothalamic network is highly plausible. We shall therefore: 1) develop methods to target optogenetic tools to tanycytes to allow selective activation of these cells; 2) establish whether tanycytes communicate directly to neurons; and 3) examine whether tanycyte-neuron communication is depends on metabolic state.

We shall develop a lentiviral construct that efficiently drives tanycyte specific expression of a channel rhodopsin (ChR2). The selectivity of expression and efficacy of activation will be verified with cultured cells and tissue. We shall then map activate tanycyte-neuron communication in brain slices. We shall activate tanycytes by optical methods that include use of our new optogenetic tools but also involve multiphoton flash photolysis methods to release intracellular calcium ions within single tanycytes, or uncage ATP next to single tanycytes. Neuronal responses will be assessed via calcium imaging and their identity determined through use of mice with genetically labelled populations of hypothalamic neurons (NPY and POMC). We shall use pharmacological methods and biosensing to identify the transmitters released by tanycytes and the receptors activated on the neurons. Finally we shall examine the whether tanycyte-neuron communication exhibits state-dependent plasticity and can be altered by prior fasting, or diet-induced obesity.

The most obvious non-academic beneficiaries of this research will be in the pharmaceutical sector. Given the explosion of obesity and obesity related illness including diabetes, considerable research and development effort is being devoted to ways to control appetite and feeding and restore autonomic regulation to metabolically compromised patients. A slightly less tangible impact will be on the awareness of clinicians, health professionals and clinician researchers of the role of the brain in the regulation of body weight, the potentially complex and diverse mechanisms involved, that these mechanisms can be studied experimentally and the resulting potential for exciting developments in patient treatment. Awareness in this sector is important for engendering enthusiasm and understanding that underpins the collaboration between clinicians and researchers that is vital for the success of future translational research. Basic scientists depend very much on convincing clinicians (under considerable work pressures of their own) of the value of for example sample collection for metabolite measurement, genotyping etc.


It is unlikely that benefits and impact will arise quickly during the present project. However establishing the role of tanycytes in the hypothalamic networks has the potential to offer new ways to intervene and modify how these networks function. With luck this might offer a highly specific intervention by creating pharmaceutical tools to target specifically the tanycyte-neurons interaction to influence the body weight. This of course could only come through detailed knowledge of tanycyte signalling, how these cells influence neurons and the specific transmitters and receptors involved. A more immediate outcome is that we can establish our research and approach to studying the physiology of tanycyte-neuron interactions as being of sufficient interest to Pharma that we can start to collaborate with them. This may lead to significant impact in the longer term.