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The intervertebral disc (IVD) acts as a fibrocartilaginous joint to anchor adjacent vertebrae. Although several studies have demonstrated the cellular heterogeneity of adult mature IVDs, a single‐cell transcriptomic atlas mapping early IVD formation is still lacking. Here, the authors generate a spatiotemporal and single cell‐based transcriptomic atlas of human IVD formation at the embryonic stage and a comparative mouse transcript landscape. They identify two novel human notochord (NC)/nucleus pulposus (NP) clusters, SRY‐box transcription factor 10 (SOX10)+ and cathepsin K (CTSK)+, that are distributed in the early and late stages of IVD formation and they are validated by lineage tracing experiments in mice. Matrisome NC/NP clusters, T‐box transcription factor T (TBXT)+ and CTSK+, are responsible for the extracellular matrix homeostasis. The IVD atlas suggests that a subcluster of the vertebral chondrocyte subcluster might give rise to an inner annulus fibrosus of chondrogenic origin, while the fibroblastic outer annulus fibrosus preferentially expresseds transgelin and fibromodulin. Through analyzing intercellular crosstalk, the authors further find that notochordal secreted phosphoprotein 1 (SPP1) is a novel cue in the IVD microenvironment, and it is associated with IVD development and degeneration. In conclusion, the single‐cell transcriptomic atlas will be leveraged to develop preventative and regenerative strategies for IVD degeneration. [ABSTRACT FROM AUTHOR]

Chordoma is an extremely rare malignant bone tumor with a high rate of relapse. While cancer stem cells (CSCs) are closely associated with tumor recurrence, which depend on its capacity to self‐renew and induce chemo‐/radioresistance, whether and how CSCs participate in chordoma recurrence remains unclear. The current study found that tumor cells in recurrent chordoma displayed more dedifferentiated CSC‐like properties than those in corresponding primary tumor tissues. Meanwhile, MTNR1B deletion along with melatonin receptor 1B (MTNR1B) down‐regulation was observed in recurrent chordoma. Further investigation revealed that activation of Gαi2 by MTNR1B upon melatonin stimulation could inhibit SRC kinase activity via recruiting CSK and SRC, increasing SRC Y530 phosphorylation, and decreasing SRC Y419 phosphorylation. This subsequently suppressed β‐catenin signaling and stemness via decreasing β‐catenin p‐Y86/Y333/Y654. However, MTNR1B loss in chordoma mediated increased CSC properties, chemoresistance, and tumor progression by releasing melatonin's repression of β‐catenin signaling. Clinically, MTNR1B deletion was found to correlate with patients' survival. Together, our study establishes a novel convergence between melatonin and β‐catenin signaling pathways and reveals the significance of this cross talk in chordoma recurrence. Besides, we propose that MTNR1B is a potential biomarker for prediction of chordoma prognosis and selection of treatment options, and chordoma patients might benefit from targeting MTNR1B/Gαi2/SRC/β‐catenin axis. [ABSTRACT FROM AUTHOR]

Arthrogryposis is a group of phenotypically and genetically heterogeneous disorders characterized by congenital contractures of two or more parts of the body; the pathogenesis and the causative genes of arthrogryposis remain undetermined. We examined a four‐generation arthrogryposis pedigree characterized by camptodactyly, limited forearm supination, and loss of myofibers in the forearms and hands. By using whole‐exome sequencing, we confirmed MET p.Y1234C mutation to be responsible for arthrogryposis in this pedigree. MET p.Y1234C mutation caused the failure of activation of MET tyrosine kinase. A Met p.Y1232C mutant mouse model was established. The phenotypes of homozygous mice included embryonic lethality and complete loss of muscles that originated from migratory precursors. Heterozygous mice were born alive and showed reduction of the number of myofibers in both appendicular and axial muscles. Defective migration of muscle progenitor cells and impaired proliferation of secondary myoblasts were proven to be responsible for the skeletal muscle dysplasia of mutant mice. Overall, our study shows MET to be a causative gene of arthrogryposis and MET mutation could cause skeletal muscle dysplasia in human beings. Synopsis: In this study, MET was identified as a causative gene for arthrogryposis, characterized by congenital contractures of two or more parts of the body. The mutant MET p.Y1234C impaired activation of MET tyrosine kinase, resulting in muscular dysplasia of the limbs. MET p.Y1234C mutation was identified in a four‐generation arthrogryposis pedigree.Muscular dysplasia of the upper arms and hands was observed in the arthrogryposis patients.In vitro, MET p.Y1234C was responsible for the failure in MET tyrosine kinase activation.In vivo, Met p.Y1232C led to defective migration of muscle progenitor cells and impaired proliferation of secondary myoblasts. In this study, MET was identified as a causative gene for arthrogryposis, characterized by congenital contractures of two or more parts of the body. The mutant MET p.Y1234C impaired activation of MET tyrosine kinase, resulting in muscular dysplasia of the limbs. [ABSTRACT FROM AUTHOR]