HMGB2–RAD21 Axis Promotes Fibro/Adipogenic Progenitor Proliferation and Regulates Fat Infiltration

This study constructed the first developmental atlas of embryonic skeletal muscle fibro/adipogenic progenitors (FAPs) and identified an HMGB2+ FAPs subpopulation that regulates FAP pool size and muscle homeostasis. HMGB2 directly targets the RAD21 promoter, and its knockout significantly reduces FAP numbers, thereby lowering the potential for intermuscular adipocyte deposition. These findings provide novel insights into the mechanisms of fat infiltration. ABSTRACT Intermuscular fat (IMF) infiltration is not only associated with myopathies and insulin resistance, but also serves as a key determinant of meat quality in the livestock industry. However, the molecular and cellular mechanisms influencing the intermuscular adipocyte abundance remain poorly understood. Based on porcine samples, we confirmed that the differentiation of intermuscular preadipocytes begins after birth, which prompted us to focus on the changes in the number of fibro/adipogenic progenitors (FAPs) during the embryonic stage. Using single‐cell sequencing (ScRNA‐seq) analysis of pig embryonic muscle, we constructed the first developmental atlas of embryonic FAPs and identified a distinct HMGB2+ subpopulation (FAPsHMGB2+) as a key determinant of FAP pool size. When HMGB2 is knocked out, both heterozygous and homozygous mice exhibit a remarkable reduction in the number of FAPs and impaired adipogenic potential. Correspondingly, the FAPsHMGB2+ were also found during muscle regeneration in mice. Unlike targeting C/EBPβ in vitro, HMGB2 governs FAP proliferation in vivo through targeting RAD21, a gene involved in DNA replication. Collectively, these findings provide novel insights into analyzing differences in IMF content and highlight potential targets for enhancing pork quality and mitigating pathological fat infiltration in skeletal muscles. This study constructed the first developmental atlas of embryonic skeletal muscle fibro/adipogenic progenitors (FAPs) and identified an HMGB2 + FAPs subpopulation that regulates FAP pool size and muscle homeostasis. HMGB2 directly targets the RAD21 promoter, and its knockout significantly reduces FAP numbers, thereby lowering the potential for intermuscular adipocyte deposition. These findings provide novel insights into the mechanisms of fat infiltration. ABSTRACT Intermuscular fat (IMF) infiltration is not only associated with myopathies and insulin resistance, but also serves as a key determinant of meat quality in the livestock industry. However, the molecular and cellular mechanisms influencing the intermuscular adipocyte abundance remain poorly understood. Based on porcine samples, we confirmed that the differentiation of intermuscular preadipocytes begins after birth, which prompted us to focus on the changes in the number of fibro/adipogenic progenitors (FAPs) during the embryonic stage. Using single-cell sequencing (ScRNA-seq) analysis of pig embryonic muscle, we constructed the first developmental atlas of embryonic FAPs and identified a distinct HMGB2 + subpopulation (FAPs HMGB2+ ) as a key determinant of FAP pool size. When HMGB2 is knocked out, both heterozygous and homozygous mice exhibit a remarkable reduction in the number of FAPs and impaired adipogenic potential. Correspondingly, the FAPs HMGB2+ were also found during muscle regeneration in mice. Unlike targeting C/EBPβ in vitro, HMGB2 governs FAP proliferation in vivo through targeting RAD21, a gene involved in DNA replication. Collectively, these findings provide novel insights into analyzing differences in IMF content and highlight potential targets for enhancing pork quality and mitigating pathological fat infiltration in skeletal muscles. Advanced Science, EarlyView.