SLIT2 regulates the skeletal stem cell niche through sympathetic innervation

Sympathetic tone has long been known as a central signaling axis inhibiting osteogenesis. However, the mechanism of this nerve to bone influence remains elusive, especially regarding the specific cellular targets of sympathetic activity. Recently, a bona fide tissue-resident stem cell giving rise to skeletal cell types, skeletal stem cells (SSCs), have been identified. To explore if and how nerves impact SSCs, we utilized mice with conditional deletion of SLIT2, a classic axonal repellent, finding that neural (Slit2syn1 mice) and sympathetic (Slit2th mice) but not bone stem/progenitor (Slit2prx1 mice) or sensory (Slit2adv mice) deletion of Slit2, led to osteopenia due to impaired bone formation associated with an increase in sympathetic innervation and a decrease in the numbers of SSCs. Consistent with this, pharmacological or surgical sympathectomy caused expansion of the SSC pool. More directly, wild type (WT) SSCs transplanted orthotopically into the bones of Slit2th mice with increased sympathetic innervation displayed impaired osteogenic capability, demonstrating that sympathetic nerves functionally contribute to the SSC niche. In line with these findings, the increased sympathetic innervation in Slit2th mice disrupted bone regeneration and bone fracture healing by reducing SSC expansion. Transcriptomic profiling in sympathetic neurons identified Follistatin-like 1 (FSTL1) as a SLIT2-regulated soluble factor that suppressed SSC self-renewal and osteogenic capacity. Accordingly, ablation of Fstl1 in sympathetic neurons enhanced SSC-driven osteogenesis and attenuated the bone loss seen in Slit2th mice in vivo. Altogether, our study both newly establishes SLIT2 as a regulator of skeletal sympathetic innervation and establishes sympathetic nerves as a key component of the SSC niche, providing new therapeutic opportunities to augment SSC function to treat skeletal disorders.