Hypothalamic mechanisms of thermal homeostasis and adaptation

Project Information

Acclimatize

Grant agreement ID: 772395

DOI

10.3030/772395

Project closed

EC signature date 22 February 2018

Start date 1 September 2018

End date 28 February 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 902 500,00

EU contribution

€ 1 902 500,00

Coordinated by

UNIVERSITATSKLINIKUM HEIDELBERG
Germany

Periodic Reporting for period 4 - Acclimatize (Hypothalamic mechanisms of thermal homeostasis and adaptation)

Reporting period: 2023-03-01 to 2025-02-28

Summary of the context and overall objectives of the project

The overarching goal of the ERC project ACCLIMATIZE was to identify and understand molecular sensors, neuronal circuits, and plasticity mechanisms that allow organisms—including humans and laboratory mice—to adapt to elevated environmental temperatures. Our research focused on the preoptic area (POA) of the hypothalamus, the brain’s central thermoregulatory center. Although acute thermoregulation has been relatively well studied, the neural basis for long-term heat acclimation remained largely unexplored.
The project addressed three main aims:
(i) to identify molecules responsible for deep brain temperature sensing,
(ii) to define the architecture and connectivity of thermoregulatory neurons, and
(iii) to uncover the mechanisms of neuronal plasticity that underlie heat acclimation.
Body temperature homeostasis is essential to survival and influences numerous physiological processes—including circadian rhythms, immunity, hormone secretion, and metabolism. Understanding the neural regulation of body temperature thus has broad biomedical relevance.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Early Phase and Technological Setup
In the initial project phase (from September 2018), we recruited personnel, purchased key equipment, and built the necessary experimental infrastructure. Notably, we established an advanced optogenetics-capable electrophysiology setup for ex vivo and in vivo recordings and trained the team in thermoregulatory physiology, tracing, and behavioral assays.
Aim 1: Molecular Sensors of Brain Temperature
We focused on the TRPM2 ion channel, previously implicated in thermoregulation (Song et al., Science 2016). Our ERC-funded work demonstrated that TRPM2 functions as a synaptic temperature sensor within deep brain circuits. This is a novel finding, shifting the paradigm from whole-cell thermosensitivity to temperature-dependent synaptic plasticity. The manuscript is currently under consideration at Neuron.
In parallel, we initiated a project to identify thermoregulation-related neuropeptides. A graduate student implemented a microdialysis system in freely moving mice, enabling the collection of hypothalamic interstitial fluid and CSF. Using ELISA and targeted mass spectrometry (in collaboration with the Haefeli group), we identified several candidate peptides with heat-responsive release profiles. These are currently undergoing validation.
Aim 2: Thermoregulatory Circuit Mapping
To dissect output pathways of hypothalamic thermoregulatory neurons, we focused on a POA cell population we now term Acclimation-Activated Neurons (AANs). Using histological tracing and optogenetics, we mapped their projections to distinct downstream areas, including the paraventricular hypothalamic nucleus (PVH)—a region not previously linked to thermoregulation. In vivo stimulation of defined AAN outputs selectively activated peripheral thermal effectors, including brown adipose tissue (BAT) and tail vasculature. These data demonstrate functional pathway selectivity, a feature that had been hypothesized but not previously demonstrated.
Aim 3: Mechanisms of Heat Acclimation
We hypothesized that the same POA circuits involved in acute thermoregulation might also orchestrate long-term adaptation. Using in vivo imaging and genetic perturbation, we found that AANs exhibit increased activity after repeated heat exposure, whereas neighboring non-AAN neurons do not—indicating cell-type-specific plasticity. Inhibition of AANs blocked the development of heat acclimation. Conversely, optogenetic or chemogenetic activation of AANs was sufficient to induce heat tolerance without prior heat exposure. These findings reveal, for the first time, a central neuronal mechanism underlying heat acclimation. A manuscript is currently in preparation.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Key Conceptual Advances
A. A Synaptic Temperature Sensor
We introduced the concept that TRP channels can mediate temperature detection at synapses—not just at the membrane level—thereby modulating synaptic strength and contributing to adaptive responses such as long-lasting hypothermia. This proposes a new role for interoceptive thermosensation in synaptic plasticity.
B. Discovery of Acclimation-Activated Neurons (AANs)
The identification of neurons that actively drive heat acclimation represents a breakthrough in thermoadaptation research. AANs likely serve as integrators of multiple homeostatic signals and may emerge as promising targets for modulating energy metabolism, thermotolerance, and cardiovascular function.
C. Development of a Miniature Deep Brain Thermode
We engineered a head-mounted thermode to precisely heat deep brain regions in behaving mice. This novel tool is already in use in our lab and may have future applications in studying energy balance, obesity, and thermal resilience.
Challenges and COVID-Related Delays
The COVID-19 pandemic significantly impacted experimental progress, including delayed access to animal facilities and interruptions in mouse breeding. Nevertheless, all major objectives have been addressed, and publication of key findings is underway.
Ongoing work will continue to explore how deep brain temperature manipulation may be used to modulate energy metabolism and thermoregulatory behavior, with potential future applications in climate resilience and metabolic disease.

Fig. 8 from: Thermally induced neuronal plasticity in the hypothalamus mediates heat tolerance