Neonatal mouse intervertebral discs heal with restored function following herniation injury

Abstract

Adult intervertebral discs (IVDs) have poor endogenous healing capacity, because of their challenging microenvironment and complex mechanical demands, which can result in painful IVD herniation. There are no regenerative strategies available to improve IVD healing and restore its function. Neonatal mice are excellent models of mammalian regeneration, but there are no studies of the regenerative capacity of neonatal IVDs. In this study, we developed a neonatal model of improved IVD healing to inform repair strategies after herniation. In vivo puncture injuries were performed to simulate herniation with complete annulus fibrosus (AF) tears in caudal IVDs of neonatal (postnatal d 5) and adult (4-6 mo) Scleraxis green fluorescent protein ( ScxGFP) mice. Acute and long-term healing responses were assessed with histologic, radiologic, and biomechanical measurements. Neonates underwent accelerated IVD healing compared to adults with functional restoration and enhanced structural repair after herniation. A population of ScxGFP cells identified in the neonatal repair site may be associated with this improved healing and warrants future investigation. In summary, function of neonatal IVDs was restored after herniation injury, whereas that of adult discs was not. This improved healing response is likely driven by multiple mechanisms that may include differences in mechanical loading and available repair cells during growth.-Torre, O. M., Das, R., Berenblum, R. E., Huang, A. H., Iatridis, J. C. Neonatal mouse intervertebral discs heal with restored function following herniation injury.

Annulus fibrosus cell phenotypes in homeostasis and injury: implications for regenerative strategies

“…Collectively, these neonatal murine models identify potential cellular mechanisms and provide a roadmap toward adult mammalian regeneration. In the context of the IVD, neonates demonstrated excellent healing of severe IVD injuries involving large AF defects and complete loss of NP . After 8 weeks of healing, neonatal mouse IVDs restored full functional biomechanics and improved collagen organization surrounding the puncture tract.…”

“…SFRP2 , a modulator of Wnt signaling, is a putative AF marker specific to outer annulocytes in human and bovine tissue . Outer annulocytes share similar markers to other dense, fibrous connective tissue cells, such as tenocytes and ligamentocytes, including the genes Tnmd, Mkx , and Scx in human, bovine, and murine AF …”

“…78,79 Outer annulocytes share similar markers to other dense, fibrous connective tissue cells, such as tenocytes and ligamentocytes, including the genes Tnmd, Mkx, and Scx in human, bovine, and murine AF. [79][80][81][82] From the outer AF region toward the inner AF and NP, there is a marked change in cell morphology and phenotype from that of the classically defined outer annulocyte to a more rounded cell shape. These cells have been variously termed "chondrocyte-like cell," "inner-AF cell," or "discoidal chondrocytes."…”

“…Although true AF regeneration has yet to be achieved, the native AF has the potential for improved healing within certain contexts, such as in the neonatal mouse where IVDs heal via restored function following herniation injury. 81 Delivery of cells and factors to promote regeneration shows promise in animal models, such as delivery of MSCs overexpressing Mohawk, a transcription factor important in regulating outer AF growth and homeostasis. 82 However, several challenges regarding clinically translatable and efficacious delivery remain unresolved.…”

Aging aggravates intervertebral disc degeneration by regulating transcription factors toward chondrogenesis

“…The utility of such a Cre has yet to be fully explored, but the only other inducible-Cre drivers that target multiple regions of the IVD are the AcanCreERT2 (NP and AF) 31 and the Scleraxis-Cre (IAF and OAF). 32 The majority of the rest target one of the three regions of the IVD. [33][34][35][36][37] The consequences of mutations in OSX on the IVD are unknown, but are associated with osteogenesis imperfect in global mutations 38…”