In mammals, inter- and intraspecies differences in consumption of sweeteners largely depend on allelic variation of the Tas1r3 gene (locus Sac) encoding the T1R3 protein, a sweet taste receptor subunit. To assess the influence of Tas1r3 polymorphisms on feeding behavior and metabolism, we examined the phenotype of F₁ male hybrids obtained from crosses between the following inbred mouse strains: females from 129SvPasCrl (129S2) bearing the recessive Tas1r3 allele and males from either C57BL/6J (B6), carrying the dominant allele, or the Tas1r3-gene knockout strain C57BL/6J-Tas1r3^tm1Rfm (B6-Tas1r3-/-). The hybrids 129S2B6F1 and 129S2B6-Tas1r3-/-F1 had identical background genotypes and different sets of Tas1r3 alleles. The effect of Tas1r3 hemizygosity was analyzed by comparing the parental strain B6 (Tas1r3 homozygote) and hemizygous F₁ hybrids B6 × B6-Tas1r3-/-. Data showed that, in 129S2B6-Tas1r3-/-F1 hybrids, the reduction of glucose tolerance, along with lower consumption of and lower preference for sweeteners during the initial licking responses, is due to expression of the recessive Tas1r3 allele. Hemizygosity of Tas1r3 did not influence these behavioral and metabolic traits. However, the loss of the functional Tas1r3 allele was associated with a small decline in the long-term intake and preference for sweeteners and reduction of plasma insulin and body, liver, and fat mass.

A commercial strain of Hafnia alvei (H. alvei) 4597 bacteria was shown to reduce food intake and promote weight loss, effects possibly induced by the bacterial protein ClpB, an antigen-mimetic of the anorexigenic α-melanocyte-stimulating hormone. A decrease in the basal plasma glucose levels was also observed in overweight fasted humans and mice receiving H. alvei. However, it is not known whether H. alvei influences sweet taste preference and whether its protein extract or ClpB are sufficient to increase glucose tolerance; these are the objectives tested in the present study. C57BL/6J male mice were kept under standard diet and were gavaged daily for 17 days with a suspension of H. alvei (4.5 × 10⁷ CFU/animal) or with H. alvei total protein extract (5 μg/animal) or saline as a control. Sweet taste preference was analyzed via a brief-access licking test with sucrose solution. Glucose tolerance tests (GTT) were performed after the intraperitoneal (IP) or intragastric (IG) glucose administration at the 9th and 15th days of gavage, respectively. The expression of regulatory peptides’ mRNA levels was assayed in the hypothalamus. In another experiment performed in non-treated C57BL/6J male mice, effects of acute IP administration of recombinant ClpB protein on glucose tolerance were studied by both IP- and IG-GTT. Mice treated with the H. alvei protein extract showed an improved glucose tolerance in IP-GTT but not in IG-GTT. Both groups treated with H. alvei bacteria or protein extract showed a reduction of pancreatic tissue weight but without significant changes to basal plasma insulin. No significant effects of H. alvei bacteria or its total protein extract administration were observed on the sweet taste preference, insulin tolerance and expression of regulatory peptides’ mRNA in the hypothalamus. Acute administration of ClpB in non-treated mice increased glucose tolerance during the IP-GTT but not the IG-GTT, and reduced basal plasma glucose levels. We conclude that both the H. alvei protein extract introduced orally and the ClpB protein administered via IP improve glucose tolerance probably by acting at the glucose postabsorptive level. Moreover, H. alvei probiotic does not seem to influence the sweet taste preference. These results justify future testing of both the H. alvei protein extract and ClpB protein in animal models of diabetes.

The present study aimed to elucidate whether the metabolic effects of the probiotic bacterium Hafnia alvei 4597 depend on melanocortin receptor (MCR) signaling. The response to a 3-week intragastric treatment with the H. alvei bacterial suspension or total protein extract was compared between genetically similar mouse sub-strains but with different MCR sensitivity: KK and KK.Cg-Ay/a (KK-Ay)—the latter overproducing Agouti protein. Treatment of KK mice with H. alvei protein extract stimulated energy expenditure and carbohydrate oxidation but reduced lipid oxidation, leptin level, pancreatic weight, and hypothalamic insulin and leptin receptor mRNA expression. Live bacteria in KK mice reduced food intake and stimulated hypothalamic mRNA expression of proopiomelanocortin, Agouti-related peptide, and insulin receptors. In the sub-strain KK-Ay, probiotics had no effect on the aforementioned metabolic parameters. H. alvei-based probiotics improved glucose tolerance and decreased body fat and liver glycogen in both mouse sub-strains. Activation of MC4R by H. alvei protein extract was revealed by in vitro study using β-arrestin recruitment. These findings confirm beneficial effects of the H. alvei bacteria and show that, for several parameters, the bacterial protein extract may be more effective than bacterial suspension. Differential responses to the treatment between the mouse sub-strains, particularly in energy metabolism, as well as in vitro data, indicate that the effects of H. alvei are mediated by MCR signaling.

Objective. The present study aimed to elucidate if the metabolic effects of probiotic bacterium Hafnia alvei 4597 depend on the melanocortin receptors (MCRs) signaling. Methods. The response to a 3-week intragastric treatment with the H. alvei bacterial suspension or total protein extract was compared between genetically similar mouse sub-strains but with different sensitivity of MCRs, KK and KK.Cg-Ay/a (KK-Ay), the latter overproducing agouti protein. Results. Treatment of KK mice with H. alvei protein extract stimulated energy expenditure and carbohydrate oxidation but reduced lipid oxidation, leptin level, pancreatic weight, and hypothalamic insulin and leptin receptor mRNA expression. Live bacteria in KK mice reduced food intake and stimulated hypothalamic mRNA expression of proopiomelanocortin, agouti-related peptide, and insulin receptors. In the sub-strain KK-Ay, probiotics had no effect on the aforementioned metabolic parameters. H. alvei-based probiotics improved glucose tolerance and deceased body fat and liver glycogen in both mouse sub-strains. Activation of MC4R by H. alvei protein extract was revealed by in vitro study showing of β-arrestin recruitment. Conclusion. These findings confirm beneficial effects of the H. alvei bacteria and show that, for several parameters, the bacterial protein extract may be a more efficacious than bacterial suspension. Differential responses to the treatment between the mouse sub-strains, particularly in energy metabolism, as well as in vitro data, indicate that the effects of H. alvei are mediated by MCRs signaling.