Neonatal annulus fibrosus regeneration occurs via recruitment and proliferation of Scleraxis-lineage cells

Olivia M Torre,Anthony R Martinez Benitez,James C Iatridis,Victoria Mroz,Alice H Huang

doi:10.1038/s41536-019-0085-4

Olivia M Torre, Anthony R Martinez Benitez + Show 3 more

Open Access

https://doi.org/10.1038/s41536-019-0085-4

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Abstract

Intervertebral disc (IVD) injuries are a cause of degenerative changes in adults which can lead to back pain, a leading cause of disability. We developed a model of neonatal IVD regeneration with full functional restoration and investigate the cellular dynamics underlying this unique healing response. We employed genetic lineage tracing in mice using Scleraxis (Scx) and Sonic hedgehog (Shh) to fate-map annulus fibrosus (AF) and nucleus pulposus (NP) cells, respectively. Results indicate functional AF regeneration after severe herniation injury occurs in neonates and not adults. AF regeneration is mediated by Scx-lineage cells that lose ScxGFP expression and adopt a stem/progenitor phenotype (Sca-1, days 3–14), proliferate, and then redifferentiate towards type I collagen producing, ScxGFP+ annulocytes at day 56. Non Scx-lineage cells were also transiently observed during neonatal repair, including Shh-lineage cells, macrophages, and myofibroblasts; however, these populations were no longer detected by day 56 when annulocytes redifferentiate. Overall, repair did not occur in adults. These results identify an exciting cellular mechanism of neonatal AF regeneration that is predominantly driven by Scx-lineage annulocytes.

Highlights

Injured adult intervertebral discs (IVDs) heal poorly with the formation of a superficial, fibrous cap at the outermost layers of the annulus fibrosus (AF) and in some cases do not heal at all.[1,2,3] Regeneration of the Intervertebral disc (IVD) remains an elusive goal due to its complex structure consisting of multiple, developmentally distinct tissues, challenging microenvironment, and continuous mechanical loading at high forces
Acute injury or degenerative changes to the AF are commonly associated with IVD herniation, which is defined as extrusion of nucleus pulposus (NP) tissue through an AF defect
We identify the cellular players and their dynamics during neonatal and adult AF healing using inducible genetic lineage tracing of Scx-lineage (AF-derived, Scxlin) and Sonic hedgehog (Shh)-lineage (NP-derived, Shh-lin) cells

Summary

INTRODUCTION

Injured adult intervertebral discs (IVDs) heal poorly with the formation of a superficial, fibrous cap at the outermost layers of the annulus fibrosus (AF) and in some cases do not heal at all.[1,2,3] Regeneration of the IVD remains an elusive goal due to its complex structure consisting of multiple, developmentally distinct tissues, challenging microenvironment, and continuous mechanical loading at high forces. The adult IVD is normally non-regenerative since AF injury leads to several pathological changes, including decreased IVD cellularity, matrix degeneration, innervation, inflammation, and formation of granulation tissue.[20,21] Poor healing is generally thought to be a consequence of limited vascularity and the mechanically challenging microenvironment. Annulocyte differentiation in neonates was further assessed by identify a key cellular mechanism underlying neonatal AF AF-specific gene expression of Scx, Tnmd, Mkx, and Col1a1 at d3 regeneration and may inform regenerative strategies for improv- and d56 in uninjured, control IVDs and injured IVDs. In neonates, ing adult AF healing. Mkx, and Col1a1 gene expression were unchanged in injured IVDs compared to controls at d3 or d56, while Scx

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