Loading-Induced Heat-Shock Response in Bovine Intervertebral Disc Organ Culture

Mechanically sensitive HSF1 is a key regulator of left-right symmetry breaking in zebrafish embryos

The left-right symmetry breaking of vertebrate embryos requires nodal flow. However, the molecular mechanisms that mediate the asymmetric gene expression regulation under nodal flow remain elusive. Here, we report that heat shock factor 1 (HSF1) is asymmetrically activated in the Kupffer's vesicle of zebrafish embryos in the presence of nodal flow. Deficiency in HSF1 expression caused a significant situs inversus and disrupted gene expression asymmetry of nodal signaling proteins in zebrafish embryos. Further studies demonstrated that HSF1 is a mechanosensitive protein. The mechanical sensation ability of HSF1 is conserved in a variety of mechanical stimuli in different cell types. Moreover, cilia and Ca2+-Akt signaling axis are essential for the activation of HSF1 under mechanical stress in vitro and in vivo. Considering the conserved expression of HSF1 in organisms, these findings unveil a fundamental mechanism of gene expression regulation by mechanical clues during embryonic development and other physiological and pathological transformations.

The role of biomechanical factors in models of intervertebral disc degeneration across multiple length scales

Low back pain is the leading cause of disability, producing a substantial socio-economic burden on healthcare systems worldwide. Intervertebral disc (IVD) degeneration is a primary cause of lower back pain, and while regenerative therapies aimed at full functional recovery of the disc have been developed in recent years, no commercially available, approved devices or therapies for the regeneration of the IVD currently exist. In the development of these new approaches, numerous models for mechanical stimulation and preclinical assessment, including in vitro cell studies using microfluidics, ex vivo organ studies coupled with bioreactors and mechanical testing rigs, and in vivo testing in a variety of large and small animals, have emerged. These approaches have provided different capabilities, certainly improving the preclinical evaluation of these regenerative therapies, but challenges within the research environment, and compromises relating to non-representative mechanical stimulation and unrealistic test conditions, remain to be resolved. In this review, insights into the ideal characteristics of a disc model for the testing of IVD regenerative approaches are first assessed. Key learnings from in vivo, ex vivo, and in vitro IVD models under mechanical loading stimulation to date are presented alongside the merits and limitations of each model based on the physiological resemblance to the human IVD environment (biological and mechanical) as well as the possible feedback and output measurements for each approach. When moving from simplified in vitro models to ex vivo and in vivo approaches, the complexity increases resulting in less controllable models but providing a better representation of the physiological environment. Although cost, time, and ethical constraints are dependent on each approach, they escalate with the model complexity. These constraints are discussed and weighted as part of the characteristics of each model.

Controversies in spine research: Organ culture versus in vivo models for studies of the intervertebral disc

Intervertebral disc degeneration is a common cause of low back pain, the leading cause of disability worldwide. Appropriate preclinical models for intervertebral disc research are essential to achieving a better understanding of underlying pathophysiology and for the development, evaluation, and translation of more effective treatments. To this end, in vivo animal and ex vivo organ culture models are both widely used by spine researchers; however, the relative strengths and weaknesses of these two approaches are a source of ongoing controversy. In this article, members from the Spine and Preclinical Models Sections of the Orthopedic Research Society, including experts in both basic and translational spine research, present contrasting arguments in support of in vivo animal models versus ex vivo organ culture models for studies of the disc, supported by a comprehensive review of the relevant literature. The objective is to provide a deeper understanding of the respective advantages and limitations of these approaches, and advance the field toward a consensus with respect to appropriate model selection and implementation. We conclude that complementary use of several model types and leveraging the unique advantages of each is likely to result in the highest impact research in most instances.

Suppression of Sost/Sclerostin and Dickkopf-1 Augment Intervertebral Disc Structure in Mice

Intervertebral disc (IVD) degeneration is a leading cause of low back pain, characterized by accelerated extracellular matrix breakdown and IVD height loss, but there is no approved pharmacological therapeutic. Deletion of Wnt ligand competitor Lrp5 induces IVD degeneration, suggesting that Wnt signaling is essential for IVD homeostasis. Therefore, the IVD may respond to neutralization of Wnt ligand competitors sost(gene)/sclerostin(protein) and/or dickkopf-1 (dkk1). Anti-sclerostin antibody (scl-Ab) is an FDA-approved bone therapeutic that activates Wnt signaling. We aimed to (i) determine if pharmacological neutralization of sclerostin, dkk1, or their combination would stimulate Wnt signaling and augment IVD structure and (ii) determine the prolonged adaptation of the IVD to global, persistent deletion of sost. Nine-week-old C57Bl/6J female mice (n = 6-7/group) were subcutaneously injected 2×/week for 5.5 weeks with scl-Ab (25 mg/kg), dkk1-Ab (25 mg/kg), 3:1 scl-Ab/dkk1-Ab (18.75:6.25 mg/kg), or vehicle (veh). Separately, IVD of sost KO and wild-type (WT) mice (n = 8/group) were harvested at 16 weeks of age. First, compared with vehicle, injection of scl-Ab, dkk1-Ab, and 3:1 scl-Ab/dkk1-Ab similarly increased lumbar IVD height and β-catenin gene expression. Despite these similarities, only injection of scl-Ab alone strengthened IVD mechanical properties and decreased heat shock protein gene expressions. Genetically and compared with WT, sost KO enlarged IVD height, increased proteoglycan staining, and imbibed IVD hydration. Notably, persistent deletion of sost was compensated by upregulation of dkk1, which consequently reduced the cell nuclear fraction for Wnt signaling co-transcription factor β-catenin in the IVD. Lastly, RNA-sequencing pathway analysis confirmed the compensatory suppression of Wnt signaling and revealed a reduction of cellular stress-related pathways. Together, suppression of sost/sclerostin or dkk1 each augmented IVD structure by stimulating Wnt signaling, but scl-Ab outperformed dkk1-Ab in strengthening the IVD. Ultimately, postmenopausal women prescribed scl-Ab injections to prevent vertebral fracture may also benefit from a restoration of IVD height and health. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Mechanobiology of annulus fibrosus and nucleus pulposus cells in intervertebral discs

Low back pain (LBP) is a chronic condition that can affect up to 80% of the global population. It is the number one cause of disability worldwide and has enormous socioeconomic consequences. One of the main causes of this condition is intervertebral disc (IVD) degeneration. IVD degenerative processes and inflammation associated with it has been the subject of many studies in both tissue and cell level. It is believed that the phenotype of the resident cells within the IVD directly affects homeostasis of the tissue. At the same time, IVDs located between vertebral bodies of spine are under various mechanical loading conditions in vivo. Therefore, investigating how mechanical loading can affect the behaviour of IVD cells has been a subject of many research articles. In this review paper, following a brief explanation of the anatomy of the IVD and its resident cells, we compiled mechanobiological studies of IVD cells (specifically, annulus fibrosus and nucleus pulposus cells) and synthesized and discussed the key findings of the field.