Novel Fish Ghrelin Receptor GHS-R3a Is Activated by Endogenous Ghrelin But Unresponsive to Synthetic Hexarelin
Background
The ghrelin system is critical for regulating growth hormone secretion, appetite, and energy balance. It primarily acts through the growth hormone secretagogue receptor, GHS-R1a, which is activated by the endogenous hormone ghrelin and synthetic compounds like hexarelin. However, the full complexity of ghrelin signaling and its evolutionary history are not completely understood. Prior research in channel catfish identified two receptors, GHS-R1a and GHS-R2a. This study investigates a newly discovered third receptor, GHS-R3a, to understand its unique structure, function, and regulation, particularly in response to nutritional state, which could reveal novel aspects of metabolic control and the evolution of this peptide system.
Study Design
Researchers characterized a novel ghrelin receptor gene, GHS-R3a, from channel catfish. Its amino acid sequence was compared to GHS-R1a and GHS-R2a, and a phylogenetic analysis was conducted. For functional analysis, human embryonic kidney (HEK 293) cells stably expressing catfish GHS-R3a were treated with homologous catfish ghrelin, GHRP-6, and hexarelin, and intracellular Ca2+ concentration was measured as the primary endpoint. Gene expression was quantified using Realtime PCR in brain, gonads, pituitary, stomach, and Brockmann bodies of catfish under fasted and refed conditions.
Results
The novel GHS-R3a receptor shared only 48% and 52% sequence identity with catfish GHS-R1a and GHS-R2a, respectively. In functional assays, homologous catfish ghrelin successfully increased intracellular Ca2+ concentration in cells expressing GHS-R3a. In contrast, the synthetic GHSs, GHRP-6 and hexarelin, had no effect. GHS-R3a expression was highest in the brain and gonads. Fasting significantly altered receptor expression. Most notably, GHS-R3a was the only receptor to increase in multiple tissues during fasting.
GHS-R3aexpression significantly increased by 2.9- to 6.3-fold in the brain, pituitary, and stomach within 4 days of fasting (P < .05).
Stomach GHS-R1a also increased after 4 days of fasting (P < .05), while GHS-R2a was only elevated in the brain and pituitary after 1 week of refeeding. All three receptors were elevated in the Brockmann bodies after 2 weeks of fasting (P < .05).
Key Findings
- The synthetic secretagogues hexarelin and GHRP-6 did not activate the novel
GHS-R3areceptor. - Fasting for 4 days increased
GHS-R3aexpression by 2.9- to 6.3-fold in the brain, pituitary, and stomach (P < .05). - The
GHS-R3areceptor shares only 48% and 52% amino acid sequence identity with catfishGHS-R1aandGHS-R2a. - Expression of all three ghrelin receptors increased in the Brockmann bodies after 2 weeks of fasting (P < .05).
- Catfish ghrelin increased intracellular Ca2+ in HEK 293 cells expressing
GHS-R3a, confirming its function.
Why It Matters
The discovery of a third ghrelin receptor with distinct properties highlights the greater complexity of the ghrelin system, at least in fish. For researchers and biohackers, this implies that synthetic GHS peptides like hexarelin may not engage the full spectrum of ghrelin-related pathways. The finding that GHS-R3a is specifically unresponsive to synthetic agonists but highly regulated by fasting suggests it may play a unique, conserved role in energy sensing that is separate from the canonical growth hormone axis targeted by many peptides. This challenges the assumption that synthetic GHSs fully mimic all of ghrelin's endogenous actions and suggests that novel, more selective compounds could be developed to target these different receptor subtypes for metabolic or gonadal functions. Clinical translation is distant, as this is fish-based research, but it opens new avenues for understanding metabolic regulation.
