Abstract
Recapitulation of developmental signals represents a promising strategy for treating intervertebral disc degeneration. During development, embryonic notochord-derived cells (NDCs) are the direct progenitors of cells that populate the adult nucleus pulposus (NP) and are an important source of secreted signaling molecules. The objective of this study was to define global gene expression profiles of NDCs at key stages of embryonic disc formation. NDCs were isolated from Shh-cre;ROSA:YFP mice at embryonic day 12.5 and postnatal day 0, representing opposite ends of the notochord to NP transformation. Differences in global mRNA abundance across this developmental window were established using RNA-Seq. Protein expression of selected molecules was confirmed using immunohistochemistry. Principal component analysis revealed clustering of gene expression at each developmental stage with more than 5000 genes significantly differentially expressed between E12.5 and P0. There was significantly lower mRNA abundance of sonic hedgehog pathway elements at P0 vs E12.5, while abundance of elements of the transforming growth factor-beta and insulin-like growth factors pathways, and extracellular matrix components including collagen 6 and aggrecan, were significantly higher at P0. This study represents the first transcriptome-wide analysis of embryonic NDCs. Results suggest signaling and biosynthesis of NDCs change dramatically as a function of developmental stage.
Highlights
Characteristics that more closely resemble those of articular cartilage chondrocytes[21]
The earliest manifestations of disc degeneration typically occur in the nucleus pulposus (NP), where reduced proteoglycan content compromises mechanical function leading to progressive structural deterioration of the entire intervertebral joint
As the notochord is a transient structure and disappears in early development, harvesting notochordal cells for direct therapeutic use for disc regeneration is not feasible; improved understanding of embryonic NP formation may enable recapitulation of important developmental signals that are necessary for the formation of the NP and its proteoglycan-rich extracellular matrix (ECM)
Summary
Characteristics that more closely resemble those of articular cartilage chondrocytes[21]. Current treatments for low back pain include physical therapy, steroid injections, or where surgery is warranted, spinal fusion or artificial total disc replacement[28] These treatments, seek to manage symptoms without maintaining or restoring native disc structure or biomechanical function. Given the established function of the notochord as a source of secreted signaling molecules that regulate embryonic disc formation, there is intense interest in identifying notochordal cell-secreted factors and applying them to develop improved therapeutic strategies for disc regeneration[31]. As the notochord is a transient structure and disappears in early development, harvesting notochordal cells for direct therapeutic use for disc regeneration is not feasible; improved understanding of embryonic NP formation may enable recapitulation of important developmental signals that are necessary for the formation of the NP and its proteoglycan-rich extracellular matrix (ECM). We examined changing expression levels of key ECM molecules and putative NP cell-specific markers as previously defined in the literature[37]
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