The brain serotonin system regulates several physiological functions such as mood, sleep, and food intake by binding to their cognate receptors (Jonnakuty and Gragnoli, 2008). Particularly, the serotonin 2C receptor (Htr2c) has been considered to mediate the anorexigenic (appetite-suppressing) effect of serotonin (Yao et al., 2021). Belviq^TM (lorcaserin), an Htr2c agonist, was approved by the U.S. Food and Drug Administration as an anti-obesity drug; although, it was later withdrawn from the market due to its potential carcinogenic effect (Gumpper and Roth, 2023). While Htr2c-deficient mice exhibited obesity and hyperphagia (increased food intake) (Nonogaki et al., 1998), it remained unknown whether Htr2c mutations or variants caused hyperphagia and obesity in humans.

Recently, the Farooqi and Xu laboratories identified 13 rare variants of Htr2c in severely obese individuals with maladaptive behavior (He et al., 2022). They transfected these variants into HEK 293T cells and examined their expression through cell surface ELISA and high-content confocal microscopy. Interestingly, the F327L Htr2c variant was mostly expressed in the endoplasmic reticulum rather than on the cell surface whereas the locations of other variants were similar to those of the wild type (WT). It was observed that the IP₃ turnover rate was remarkably reduced in the F327L Htr2c variant, which was not observed in any of the other variants. These included 9 and 8 Htr2c variants which showed reduced recruitment of β-arrestin-1 and β-arrestin-2, respectively, and 11 of the 13 Htr2c variants induced loss-of-function in least in one assay. Typically, the F327 residue is a binding site for Htr2c agonists, but the F327L variant preferentially binds to mesulergine, an Htr2c antagonist, which suggests that the F327L substitution stabilizes the inactive conformation of Htr2c. It was confirmed that these Htr2c variants are associated with an increased body mass index and obesity in a population-based cohort study.

To further examine whether the F327L Htr2c variant is able to induce obesity, they generated a male hemizygous Htr2c^F327L mouse line utilizing the CRISPR-Cas9 system. The authors discovered that male Htr2c^F327L mice showed increased body weight and increased fat mass with hyperphagia on a chow diet. There was no change in the volume of oxygen consumption (VO₂) or carbon dioxide production (VCO₂), while physical activities were augmented in the Htr2c^F327L mice. Moreover, the anorexigenic effect of lorcaserin was attenuated in the Htr2c^F327L mice. When fed a high-fat diet (HFD), the Htr2c^F327L mice showed a drastic body weight gain with increased fat and lean mass as well as marked hyperphagia, impaired glucose tolerance, and impaired insulin sensitivity. VO₂ and VCO₂ remained unchanged but physical activities were increased in the Htr2c^F327L mice on HFD. Because the Htr2c was located on the X chromosome, the authors generated female heterozygous Htr2c^F327L/+ mice with the variant gene on one X chromosome and the WT gene on the other. The female Htr2c^F327L/+ mice showed hyperphagic obesity only when they were fed on HFD. Previous studies demonstrated that Htr2c expressed by the anorexigenic pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus is both necessary and sufficient for the anti-obesity effects of Htr2c stimulation (Berglund et al., 2013; Xu et al., 2008), and that Htr2c agonists activate the arcuate POMC neurons (Gao et al., 2017; Sohn et al., 2011). Consistent with the compromised function of Htr2c^F327L, the authors observed significantly attenuated responsiveness of the POMC neurons to lorcaserin in Htr2c^F327L mice.

Given the role of serotonin in coordinating the defensive behaviors in response to threats (Haller et al., 2005), the authors performed several behavioral assessments. In the resident–intruder tests, the “resident” Htr2c^F327L mice showed an increase in defensive behaviors and augmented offensive attempts toward the “intruder” mice. Moreover, the Htr2c^F327L mice demonstrated reduced social activity in the three-chamber social interaction test. Paradoxically, the mice spent less time assessing the risk before entering the open arm and spent more time in the open arms of the elevated plus-maze test. The female Htr2c^F327L/+ mice largely recapitulated the social behavior of the male Htr2c^F327L mice. The authors suggested that these results were due to learning difficulties and cognitive deficits of the Htr2c^F327L mice.

The World Health Organization declared obesity as a disease because of its prevalence and comorbidities such as type 2 diabetes, hypertension, and dyslipidemia (James, 2008). Obesity associated with mutations of a few genes such as those coding for leptin (Lep), leptin receptor (Lepr), melanocortin-4 receptor (Mc4r), and single-minded 1 (Sim1) is usually more severe and begins early in childhood (Loos and Yeo, 2022). This study adds Htr2c to the list of genes that need to be screened for the genetic causes of obesity.