Skeletal progenitor LRP1-mediated endocytosis is critical for synovial joint formation and bone growth

The layer of stem cells surrounding developing limbs is essential for bone formation and regeneration. Our work addresses the critical question of how these stem cells and bone template communicate to ensure that limbs form correctly. Low-density lipoprotein receptor-related protein 1 (LRP1) is a multifunctional endocytic receptor whose mutations are linked to bone and joint pathologies. Here, we show the abundant expression of LRP1 in skeletal progenitor cells, especially in the perichondrium – the dense layer of fibrous connective tissue enveloping the cartilage of the developing limb bud. Our mouse models reveal that LRP1 deficiency in these stem cells ( Lrp1 flox/flox / Prrx1 Cre ) but not in chondrocytes ( Lrp1 flox/flox / Acan CreERT2 ) causes disrupted articulation and cavitation starting at as early as embryonic stage 16.5. LRP1 deficiency is also associated with aberrant accumulation of LRP1 ligands including tissue-inhibitor of metalloproteinase 3 and CCN2. These early abnormalities result in severe defects in multiple joints, plus markedly deformed and low-density long bones leading to dwarfism and impaired mobility. Our in vitro exploration shows unique regulation of non-canonical WNT components by LRP1 that may explain the malformation of long bones. Mechanistically, we found that LRP1 facilitates cell-association, endocytic recycling but not degradation, and graded distribution of Wnt5a in the developing limbs. We propose that LRP1-mediated endocytic regulation of availability and distribution of extracellular signalling molecules play a critical role in limb development. This provides a novel mechanism for crosstalk among skeletal elements.