Periodic Reporting for period 1 - TANYFEEDNEURONS (Gliotransmission and shuttling of metabolic signals to feeding neuronal circuits by tanycytes)
Reporting period: 2015-10-01 to 2017-09-30
Given the strong electrophysiological components of the project, my work in the first months of the funding period focused on setting up an electrophysiology station in the host laboratory to appropriately execute the initially planned experiments. In the first aim, by combining electrophysiology in living brain slices with DREADDs and Cre/LoxP strategies, I demonstrated that activation of calcium signaling in tanycytes reduced the activity of neurons that control food intake, likely through the release of neuroactive substances. These data suggest that an active communication between tanycytes and neurons may exist. My current work carried out during the third year thanks to funding from an ANR Grant acquired by the host group aims to i) identify ATP as the neuroactive substance, ii) determine the release mechanisms of ATP and iii) evaluate the consequences of obesity on ATP release.
In the second aim, I tested whether the intercellular channels gap junctions in tanycytes allow the trafficking of the energy substrates glucose and its derived metabolite lactate towards neurons that control food intake. Immunohistochemistry showed that tanycyte cell bodies highly expressed the connexin 43 (Cx43) gap junction proteins. Using patch-clamp technique and intracellular filling in living brain slices, I found that tanycytes communicated to each other through gap junctions and this permitted the passage of a fluorescent glucose analog. Interestingly, diffusion of glucose was completely blocked when I selectively knocked out Cx43 gene in tanycytes and this led to perturbations in energy balance towards an increase in food intake and a decrease in energy expenditure. Together, these results reveal that tanycytes form a functional network of highly interconnected cells made up of Cx43 gap junctions through which energy substrates can diffuse to regulate energy balance. I also found that the electrical activity of the neurons that inhibit hunger relied on the entry of the lactate into the cells, suggesting that these neurons could use glucose-derived lactate shuttled from the tanycyte networks as energy fuel to sustain their activity and drive satiety. My current work carried out during the third year thanks to funding from an ANR Grant acquired by the host group intends to determine i) whether tanycytes and neurons that control food intake are metabolically coupled through the lactate shuttle and ii) whether obesity influences trafficking of energy substrates through tanycyte networks.
In the third aim, I tested whether tanycytes can shuttle the blood-born leptin from the cerebrospinal fluid to neurons that control food intake. In the early stage of this study, my work focused on setting up a protocol in living brain slices to visualize a fluorescent bioactive leptin transported by tanycytes towards neurons that control food intake. The protocol consisted in applying a fluorescent bioactive leptin locally at the apical surface of the cell body of tanycytes. However, after adjusting multiple parameters to optimize the local application, it was not possible to clearly visualize the uptake of the fluorescent bioactive leptin. Consequently, this method was not reliable enough to pursue the third aim of this project. Instead, I tested the new hypothesis that tanycytes could sense leptin from the cerebrospinal fluid and respond to it by releasing neuroactive substances to mediate the inhibitory effect of leptin on hunger. Interestingly, I found that local application of leptin at the apical surface of the cell body of tanycytes stimulated intracellular calcium signaling which was blocked by the mutated recombinant leptin antagonist, suggesting that the effect was mediated by functional leptin receptors. These compelling results led us to define a new aim in which a series of experiments will be designed to determine i) whether leptin-induced calcium increases in tanycytes trigger the release of ATP, ii) whether this ATP release influences the activity of nearby neurons that control food intake and iii) whether this signaling is impacted by obesity. The full completion of these experiments will be carried out by myself during the third year thanks to funding from an ANR Grant acquired by the host group.