An international research team has identified signaling proteins that act in the brain’s hypothalamus to direct the development of neuronal circuits that are involved in weight control. Studies headed by Sadaf Farooqi, PhD, FRCP, FMedSci, a professor at the University of Cambridge, U.K., and Sebastien Bouret, PhD, investigator, endocrinology, diabetes and metabolism at the Children’s Hospital Los Angeles (CHLA) identified rare mutations in the genes for class 3 semaphorins (SEMA3) in severely obese people. Subsequent experiments in cultured cells, and investigations in genetically engineered zebrafish and mice indicated that SEMA3-mediated signaling drives the development of melanocortin circuits in the hypothalamus, which are involved in energy homeostasis.
“We have now discovered the genes that establish the precise neural connections that form these circuits,” commented Agatha van der Klaauw, PhD, who led the study in Farooqi’s lab and who is co-first author of the researchers’ published paper in Cell. “This work provides new insights into the development of hypothalamic circuits that regulate appetite and metabolism.” The team described its studies and findings in a paper titled, “Human Semaphorin 3 Variants Link Melanocortin Circuit Development and Energy Balance.”
Neural circuits in the hypothalamus play a critical role in the regulation of energy homeostasis, and melanocortin neurons play a “pivotal” role in weight regulation, the authors wrote. “We know that the brain, in particular an area called the hypothalamus, has a very important role in the regulation of food intake and blood sugar,” explained Bouret, who is also an associate professor of pediatrics at the Keck School of Medicine at the University of Southern California. Scientists have for years been studying the role of the hypothalamus in obesity, a condition that now affects nearly 14 million children and adolescents in the U.S. alone. What scientists don’t yet understand, Bouret noted, is how these circuits in the hypothalamus are being organized. “We want to know how the brain puts itself together and what exactly governs that process.” Understanding how brain cells in the hypothalamus form highly specific and complex connections and how this process can be disrupted, could feasibly provide insight into the development of childhood obesity, and hypothalamic disorders.
Class 3 semaphorins are involved in the development of specific subsets of neurons in the hypothalamus, and it is already known that rare variants in SEMA3 genes that disrupt signaling are associated with hypogonadotropic hypogonadism in humans, a condition in which the testes and ovaries produce very little, or no sex hormones. The team hypothesized that if the genes encoding SEMA3s and their receptors contribute to the development of neurons involved in regulating body weight in humans, then some people with severe early-onset obesity might carry functional variants of these genes.
When they analyzed sequencing data from an initial set of 573 individuals with severe early-onset obesity they found 40 rare variants in 13 genes involved in semaphorin signaling. These very rare functional variants were also found to be enriched in a larger cohort of 982 severely obese individuals (which included the initial 573 people) when compared with 4,449 healthy controls. The SEMA3 signaling gene variants acted to disrupt normal signaling through multiple molecular mechanisms. “Many of the SEMA3 variants reduced secretion and/or receptor-mediated signaling,” the scientists noted. They did acknowledge that, given the rarity of the variants, the associations didn’t reach statistical significance at the single gene level, so larger scale comparisons will be needed.
Semaphorins act as a communication system between brain cells that can be thought of as a sort of road map that guides cells towards or away from other cells. To see what would happen when this map was inactivated, Sophie Croizier, PhD, who led the work in Bouret’s lab, blocked semaphorin signaling in laboratory-grown hypothalamic cells, and found that the brain cells no longer grew the way they were supposed to, and that connections between cells failing to establish.
The team then used CRISPR technology to disrupt some of the previously identified semaphorin signaling-related genes in very early zebrafish embryos. The tests showed that deletion of seven of the genes was associated with the animals developing increased bodyweight and/or fat. Further experiments using engineered mice similarly showed that disrupting SEMA3 signaling-relevant genes also resulted in weight gain. “What we are seeing is that semaphorins are guiding and shaping the development of hypothalamic circuits that ultimately regulate calorie intake,” explained Bouret.
“In this study, we identified rare heterozygous variants in SEMA3s, their receptors, and co-receptors in individuals with severe early-onset obesity,” the authors concluded. “In zebrafish, we showed that deletion of several genes in this pathway increased weight-related phenotypes establishing a role for these molecules in energy homeostasis. These genes might modulate body weight and/or fat mass by several potential mechanisms … Cumulatively, these studies demonstrate that SEMA3-mediated signaling drives the development of hypothalamic melanocortin circuits involved in energy homeostasis.”