Molecular cellular and neuronal circuit basis of torpor regulation

Abstract

The advent of endothermy is a defining feature of mammalian and avian evolution, achieved through finely tuned homeostatic regulation of body temperature and metabolism. However, when faced with food deprivation or harsh environmental conditions, many mammalian species initiate adaptive energy-conserving survival strategies, such as torpor and hibernation, during which body temperature drops far below its homeostatic setpoint. Despite their significance, the mechanisms by which homeothermic mammals initiate and regulate these extraordinary hypothermic states remain poorly understood. In Chapter 2, we show that mouse torpor, a fasting-induced state characterized by a significantly decreased metabolic rate and body temperature as low as 20°C, is regulated by neurons in the medial and lateral preoptic area of the hypothalamus. We demonstrate that re-stimulation of neurons activated during a previous bout of torpor is sufficient to initiate key features of torpor, even in animals that are not calorically restricted. Among these neurons, we identify a population of glutamatergic Adcyap1+ cells whose activity reliably determines the timing of torpor initiation and arousal, and whose inhibition disrupts the natural process of torpor entry, maintenance, and recovery. Collectively, this work uncovers a specific neuronal population in the hypothalamus that serves as a core regulator of torpor in mice. Next, in Chapter 3, we confirm that the Etruscan shrew, a mammalian species with the smallest body mass and an unfavorable surface-to-volume ratio, readily enters torpor upon fasting. Compared to the rat, a non-torpid species, the Etruscan shrew exhibits a relatively enlarged median preoptic area, the putative torpor-regulating region. This finding suggests that the expansion of this region in the Etruscan shrew might be a specialized adaptation to orchestrate torpor in a mammal with an exceptionally high metabolic rate. Finally, in Chapter 4, we investigate how fasting-associated molecular changes influence avMLPA torpor-regulating neurons during the fasting-induced torpor process. Using single-nucleus multiome sequencing, we profile gene expression and chromatin accessibility changes in the anterior and ventral portions of the medial and lateral preoptic area, where torpor-regulating neurons reside in mice. Notably, the majority of fasting-induced gene expression changes prior to torpor entry occurred in non-neuronal cells rather than neurons. In astrocytes and oligodendrocytes, we identify upregulated genes associated with glucocorticoid receptor signaling. Moreover, glucocorticoid receptor-binding sites, represented by the nuclear receptor family 3 sequence motif, are among the most differentially accessible regions during fasting compared to fed states. These findings suggest that fasting-induced changes in glial cells may prime avMLPA torpor-regulating neurons for activation. Together, this work provides a foundation for future exploration of the molecular, cellular, and neuronal mechanisms regulating extreme hypothermic and hypometabolic states. Our findings enable genetic access to monitor, initiate, and manipulate these ancient adaptations of homeothermic biology, with potential applications in developing novel therapeutic strategies, advancing critical medical care, and facilitating adaptation to long-distance space travel.