The brainstem BBSome regulates glucose homeostasis and lean mass in a state-dependent manner

Obesity disrupts metabolic homeostasis through changes in brain function. Hypothalamic cilia and associated proteins, such as the BBSome, a protein complex composed of eight Bardet-Biedl syndrome (BBS) proteins, have been implicated in metabolic regulation and disorders. Here, we investigated the significance of brainstem cilia and the BBSome for energy balance and glucose homeostasis.

Primary cilia were assessed by immunofluorescence and confocal imaging, and brainstem neuron transcriptomes were analyzed using single-cell RNA sequencing. Mice with Phox2b-specific deletion of Ift88 or Bbs1 were studied under control or high-fat diets. Metabolic tests, insulin signaling, nerve recordings, and viral techniques were used to evaluate the impact of cilia or Bbs1 disruption.

We found that diet-induced obese mice display increased primary cilia length in the nucleus tractus solitarius. Single cell RNAseq revealed that cilia related genes are enriched in glutamatergic dorsal vagal complex (DVC) neurons expressing Phox2b. Primary cilia deletion in Phox2b neurons (Phox2b^Cre/Ift88 ^fl/fl) caused a mild weight reduction during adolescence without altering metabolic homeostasis during adulthood. We next investigated the brainstem BBSome using Phox2b^Cre/Bbs1^fl/fl mice, which exhibited reduced adolescent lean mass gain but normal adult body weight. Surprisingly, these mice developed glucose intolerance and elevated fasting glucose associated with contrasting changes in hepatic sympathetic and parasympathetic activity, pointing to autonomic imbalance as a cause of glucose dysregulation. Targeted BBSome disruption in the DVC replicated elevations in fasting glucose and chemogenetic DVC Phox2b neuron activation attenuated hyperglycemia during glucose tolerance test and suppressed hepatic sympathetic nerve activity. Interestingly, diet-induced obese Phox2b^Cre/Bbs1^fl/fl mice exhibited lower lean mass and a paradoxical improvement in glucose tolerance despite insulin resistance, suggesting a complex role for the brainstem BBSome in obesity-associated metabolic dysfunction.

Our findings highlight novel brainstem mechanisms regulating metabolic homeostasis and distinct roles for primary cilia and the BBSome in glucose regulation and lean mass.