Deletion of SREBF1, a functional bone-muscle pleiotropic gene, alters bone mass & lipid signaling in zebrafish

In the older persons, bone mineral density (BMD) and lean (muscle) mass usually decline, both serving as proxies for musculoskeletal (MSK) disease. Recent bivariate genome-wide analysis for BMD and lean mass identified Sterol Regulatory Element-Binding Factor 1 (SREBF1) as having an effect on both traits. SREBF1 codes for SREBP-1 protein, a transcription factor which role in lipid homeostasis could be essential for understanding its pleiotropic functions on both muscle and bone. We established a zebrafish SREBF1 knockout (KO) model by CRISPR-Cas9 (S23fs allele). Dorsal muscles of adult KO were quantified for Lipid Mediators (LMs) by liquid chromatography-tandem mass spectrometry using targeted Lipidomics Profiling method. After adjusting for sex and weight, we found that the levels of 11,12 epoxyeicosatrienoic acid (11,12-EET) were linearly associated with the number of S23fs alleles. Total-body BMD was evaluated in adult KO by micro-computed tomography. Lower BMD was found in KO compared with sex-matched WT.

We also compared gene expression between KO and WT zebrafish by RNA-sequencing. Significantly enriched pathways included Fatty acid elongation, Linoleic acid metabolism, Arachidonic acid metabolism, Adipocytokine signaling, and DNA replication.

In conclusion, KO of SREBF1 decreases BMD and affects lipid-signaling mediators rather specifically. EETs, metabolites of arachidonic acid through cytochrome P450/epoxygenase, play an important role in insulin-mediated augmentation of microvascular blood flow in skeletal muscle. EETs attenuate inflammatory signaling, while enhancing adipocytokine signaling, implying that their modulation could have major therapeutic potential in the treatment of MSK disease. These findings could lead to new understanding of SREBF1 biology for MSK homeostasis.