Spatiotemporal Characterization of Human Early Intervertebral Disc Formation at Single‐Cell Resolution

Abstract

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.

In vitro and in vivo models define a molecular signature reference for human embryonic notochordal cells

“…Interestingly, some have been previously implicated in the regulation of notochord formation in various animal models (mouse, zebrafish, Xenopus, chicken) or the maintenance of IVD homeostasis in humans, dogs or bovine. 15 , 25 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 From this analysis, no lysosome-related organelles, nor vacuole markers were found except for CAV1 (manually curated gene list from the following ref. 72 , 83 , 84 is available on the Gitlink ).…”

Single cell RNA sequencing reveals emergent notochord‐derived cell subpopulations in the postnatal nucleus pulposus

“…46 Additional roles regulating cell migration and adhesion, and ECM organization, 47 with Spp1 knockout mice exhibiting aberrant wound healing. 48 Interestingly, Spp1 was previously shown to be expressed in the notochord, 49,50 and a very recent study used single cell sequencing analysis to identify Spp1 as playing a key role in intercellular crosstalk during early human disc formation. 50 We also identified two surface markers, Cd9 and Cd109, that were significantly elevated in their expression in mature versus progenitor NP cells.…”

“…48 Interestingly, Spp1 was previously shown to be expressed in the notochord, 49,50 and a very recent study used single cell sequencing analysis to identify Spp1 as playing a key role in intercellular crosstalk during early human disc formation. 50 We also identified two surface markers, Cd9 and Cd109, that were significantly elevated in their expression in mature versus progenitor NP cells. Two recent studies used proteomics to identify Cd109 as a novel NP-specific marker in both mice and Cd109 is a glycosylphosphatidylinositol-anchored glyco- 53 and while its function in the disc is unknown, in other tissues such as skin it functions as a negative regulator of TGF-β signaling.…”

Association of the FGF4L2 retrogene with fibrocartilaginous embolic myelopathy in dogs

“…17,52 The exact mechanism relating FGF4 overexpression to premature IVDD is not defined, however recent single cell RNASeq studies defining the heterogenous cellular populations in the embryonic and adult intervertebral disc might provide a framework for potential mechanisms. [66][67][68][69] Whatever the mechanism is determined to entail, premature degeneration of the IVD in chondrodystrophic dogs does not preclude other mechanisms for FCE pathogenesis since FCE is frequently documented in older non chondrodystrophic breeds where similar age-related NP degeneration is also likely to be present. 70,71 Developmental differences in vascular anatomy of the disc, vertebra, and spinal cord in chondrodystrophic dogs might also be nonpermissive to entry of fibrocartilage, and/or subsequent passage of the material to clinically relevant areas.…”