Obesity prevalence has more than tripled in the last four decades with over 750 million people now living with obesity worldwide. The myriad health, social and economic costs of obesity have stimulated research into the physiological mechanisms that govern body weight and feeding behaviour with the aim of developing new treatment strategies. Insulin-like peptide 5 (INSL5), a peptide hormone secreted from the distal gut, and its cognate receptor, relaxin/insulin-like family peptide receptor 4 (RXFP4), have been implicated in feeding in animal models. Administration of INSL5 and chemogenetic manipulation of Rxfp4-expressing cells influence feeding behaviour in mice. As feeding is regulated, in part, by neuroendocrine signalling in the gut-brain axis (GBA), we aimed to examine INSL5/RXFP4 activity at each level of the GBA to determine the potential mechanisms by which this hormone-receptor pair modulate food intake.
Using an Rxfp4-Cre mouse model combined with immunohistochemistry and transcriptomic techniques, we identified Rxfp4 expression in enterochromaffin, L and tuft cells in the colon, in neurons of the dorsal root ganglia (DRG) and nodose ganglia (NG), and in multiple brain regions associated with feeding. By combining Rxfp4-Cre mice with a cyclic adenosine monophosphate (cAMP) imaging technique, INSL5 was found to reduce intracellular cAMP levels in Rxfp4-expressing cells in the colon, DRG, NG and the ventromedial hypothalamus (VMH), a known feeding centre in the brain. Transcriptomic analysis of Rxfp4-expressing cells in the hypothalamus revealed enriched expression of multiple feeding-related neuropeptides and receptors. Circuit mapping of hypothalamic Rxfp4-expressing neurons using viral tracing tools indicated that these neurons are part of feeding-, reward- and memory-related neurocircuits. Modulation of Rxfp4-expressing neurons within the VMH using intraparenchymal INSL5 infusions and chemogenetic tools increased and decreased intake of highly palatable meals, respectively.
Together, these data suggest that INSL5/RXFP4 signalling within the GBA can regulate feeding behaviour. RXFP4 is therefore a potential target for the development of pharmaceutical treatments for obesity and other feeding-related disorders. Increased understanding of neuroendocrine signalling within the GBA may also aid the development of more successful intervention strategies for obesity management
