Noa J.M. Mutsters, Afonso de Oliveira Santos Goulart, Patric J.D. Delhanty, Jenny A. Visser

Department of Internal Medicine, Erasmus MC, University Medical Center Rotterdam, Rotterdam, The Netherlands

Syndecans are a family of heparan sulfate proteoglycans, proposed to function as GPCR co-receptors. SDC3 is known to stimulate feeding behavior in mice. In this study, we explored whether SDC3 affects downstream MC4R signaling.

Analysis of human hypothalamic single-cell RNA sequencing data showed neuronal co-expression of MC4R and SDC3 at a ratio of 1:1. We assessed the impact of SDC3 at this ratio on α-MSH-induced MC4R cAMP responsiveness in HEK293 cells and found that SDC3 suppressed Emax by 24±1.5% (P<0.0001). A previous mouse study showed that fasting causes hypothalamic SDC3 protein levels to increase >4-fold. To mimic this hunger state, we investigated the effect of SDC3 on α-MSH-induced cAMP production using an MC4R:SDC3 ratio of 1:5. Here, we observed stronger suppression of Emax (36±1.5%; P<0.0001). Next, we investigated whether SDC3 modulates MC4R expression on the cell surface. We found that SDC3 had no significant effect at a ratio of 1:1, but a ratio of 1:5 markedly suppressed MC4R cell surface levels by 51±4.2% (P<0.0001), potentially explaining the reduction in cAMP production. Similarly, total expression levels of MC4R were not suppressed at a ratio of 1:1, but were suppressed by 33±5.8% (P<0.05) at a ratio of 1:5.

In conclusion, SDC3 suppresses downstream MC4R signaling. The lack of effect on cell surface MC4R levels at a 1:1 ratio suggests that SDC3 can modulate intracellular signaling. The dose-dependency of this effect could be paralleled in vivo upon fasting when hypothalamic SDC3 levels are increased.