Hedgehog proteins and parathyroid hormone-related protein are involved in intervertebral disc maturation, degeneration, and calcification

Roles of organokines in intervertebral disc homeostasis and degeneration

The intervertebral disc is not isolated from other tissues. Recently, abundant research has linked intervertebral disc homeostasis and degeneration to various systemic diseases, including obesity, metabolic syndrome, and diabetes. Organokines are a group of diverse factors named for the tissue of origin, including adipokines, osteokines, myokines, cardiokines, gastrointestinal hormones, and hepatokines. Through endocrine, paracrine, and autocrine mechanisms, organokines modulate energy homeostasis, oxidative stress, and metabolic balance in various tissues to mediate cross-organ communication. These molecules are involved in the regulation of cellular behavior, inflammation, and matrix metabolism under physiological and pathological conditions. In this review, we aimed to summarize the impact of organokines on disc homeostasis and degeneration and the underlying signaling mechanism. We focused on the regulatory mechanisms of organokines to provide a basis for the development of early diagnostic and therapeutic strategies for disc degeneration.

Cartilaginous endplates: A comprehensive review on a neglected structure in intervertebral disc research

The cartilaginous endplates (CEP) are key components of the intervertebral disc (IVD) necessary for sustaining the nutrition of the disc while distributing mechanical loads and preventing the disc from bulging into the adjacent vertebral body. The size, shape, and composition of the CEP are essential in maintaining its function, and degeneration of the CEP is considered a contributor to early IVD degeneration. In addition, the CEP is implicated in Modic changes, which are often associated with low back pain. This review aims to tackle the current knowledge of the CEP regarding its structure, composition, permeability, and mechanical role in a healthy disc, how they change with degeneration, and how they connect to IVD degeneration and low back pain. Additionally, the authors suggest a standardized naming convention regarding the CEP and bony endplate and suggest avoiding the term vertebral endplate. Currently, there is limited data on the CEP itself as reported data is often a combination of CEP and bony endplate, or the CEP is considered as articular cartilage. However, it is clear the CEP is a unique tissue type that differs from articular cartilage, bony endplate, and other IVD tissues. Thus, future research should investigate the CEP separately to fully understand its role in healthy and degenerated IVDs. Further, most IVD regeneration therapies in development failed to address, or even considered the CEP, despite its key role in nutrition and mechanical stability within the IVD. Thus, the CEP should be considered and potentially targeted for future sustainable treatments.

Signaling Mechanisms of Stem Cell Therapy for Intervertebral Disc Degeneration

Low back pain is the leading cause of disability worldwide. Intervertebral disc degeneration (IDD) is the primary clinical risk factor for low back pain and the pathological cause of disc herniation, spinal stenosis, and spinal deformity. A possible approach to improve the clinical practice of IDD-related diseases is to incorporate biomarkers in diagnosis, therapeutic intervention, and prognosis prediction. IDD pathology is still unclear. Regarding molecular mechanisms, cellular signaling pathways constitute a complex network of signaling pathways that coordinate cell survival, proliferation, differentiation, and metabolism. Recently, stem cells have shown great potential in clinical applications for IDD. In this review, the roles of multiple signaling pathways and related stem cell treatment in IDD are summarized and described. This review seeks to investigate the mechanisms and potential therapeutic effects of stem cells in IDD and identify new therapeutic treatments for IDD-related disorders.

Recommendations for intervertebral disc notochordal cell investigation: From isolation to characterization

Background

Lineage-tracing experiments have established that the central region of the mature intervertebral disc, the nucleus pulposus (NP), develops from the embryonic structure called "the notochord". However, changes in the cells derived from the notochord which form the NP (i.e., notochordal cells [NCs]), in terms of their phenotype and functional identity from early developmental stages to skeletal maturation are less understood. These key issues require further investigation to better comprehend the role of NCs in homeostasis and degeneration as well as their potential for regeneration. Progress in utilizing NCs is currently hampered due to poor consistency and lack of consensus methodology for in vitro NC extraction, manipulation, and characterization.

Methods

Here, an international group has come together to provide key recommendations and methodologies for NC isolation within key species, numeration, in vitro manipulation and culture, and characterization.

Results

Recommeded protocols are provided for isolation and culture of NCs. Experimental testing provided recommended methodology for numeration of NCs. The issues of cryopreservation are demonstrated, and a pannel of immunohistochemical markers are provided to inform NC characterization.

Conclusions

Together we hope this article provides a road map for in vitro studies of NCs to support advances in research into NC physiology and their potential in regenerative therapies.

Sonic Hedgehog Signaling Pathway: A Role in Pain Processing

Pain, as one of the most prevalent clinical symptoms, is a complex physiological and psychological activity. Long-term severe pain can become unbearable to the body. However, existing treatments do not provide satisfactory results. Therefore, new mechanisms and therapeutic targets need to be urgently explored for pain management. The Sonic hedgehog (Shh) signaling pathway is crucial in embryonic development, cell differentiation and proliferation, and nervous system regulation. Here, we review the recent studies on the Shh signaling pathway and its action in multiple pain-related diseases. The Shh signaling pathway is dysregulated under various pain conditions, such as pancreatic cancer pain, bone cancer pain, chronic post-thoracotomy pain, pain caused by degenerative lumbar disc disease, and toothache. Further studies on the Shh signaling pathway may provide new therapeutic options for pain patients.

L-type Voltage-Gated calcium channels partly mediate Mechanotransduction in the intervertebral disc

Background

Intervertebral disc (IVD) degeneration continues to be a major global health challenge, with strong links to lower back pain, while the pathogenesis of this disease is poorly understood. In cartilage, much more is known about mechanotransduction pathways involving the strain-generated potential (SGP) and function of voltage-gated ion channels (VGICs) in health and disease. This evidence implicates a similar important role for VGICs in IVD matrix turnover. However, the field of VGICs, and to a lesser extent the SGP, remains unexplored in the IVD.

Methods

A two-step process was utilized to investigate the role of VGICs in the IVD. First, immunohistochemical staining was used to identify and localize several different VGICs in bovine and human IVDs. Second, a pilot study was conducted on the function of L-type voltage gated calcium channels (VGCCs) by inhibiting these channels with nifedipine (Nf) and measuring calcium influx in monolayer or gene expression from 3D cell-embedded alginate constructs subject to dynamic compression.

Results

Several VGICs were identified at the protein level, one of which, Cav2.2, appears to be upregulated with the onset of human IVD degeneration. Inhibiting L-type VGCCs with Nf supplementation led to an altered cell calcium influx in response to osmotic loading as well as downregulation of col 1a, aggrecan and ADAMTS-4 during dynamic compression.

Conclusions

This study demonstrates the presence of several VGICs in the IVD, with evidence supporting a role for L-type VGCCs in mechanotransduction. These findings highlight the importance of future detailed studies in this area to fully elucidate IVD mechanotransduction pathways and better inform treatment strategies for IVD degeneration.

The role of nerve fibers and their neurotransmitters in regulating intervertebral disc degeneration

Intervertebral disc degeneration (IVDD) has been the major contributor to chronic lower back pain (LBP). Abnormal apoptosis, senescence, and pyroptosis of IVD cells, extracellular matrix (ECM) degradation, and infiltration of immune cells are the major molecular alternations during IVDD. Changes at tissue level frequently occur at advanced IVD tissue. Ectopic ingrowth of nerves within inner annulus fibrosus (AF) and nucleus pulposus (NP) tissue has been considered as the primary cause for LBP. Innervation at IVD tissue mainly included sensory and sympathetic nerves, and many markers for these two types of nerves have been detected since 1940. In fact, in osteoarthritis (OA), beyond pain transmission, the direct regulation of neuropeptides on functions of chondrocytes have attracted researchers' great attention recently. Many physical and pathological similarities between joint and IVD have shed us the light on the neurogenic mechanism involved in IVDD. Here, an overview of the advances in the nervous system within IVD tissue will be performed, with a discussion on in the role of nerve fibers and their neurotransmitters in regulating IVDD. We hope this review can attract more research interest to address neuromodulation and IVDD itself, which will enhance our understanding of the contribution of neuromodulation to the structural changes within IVD tissue and inflammatory responses and will help identify novel therapeutic targets and enable the effective treatment of IVDD disease.

Pentosan Polysulphate (PPS), a Semi-Synthetic Heparinoid DMOAD With Roles in Intervertebral Disc Repair Biology emulating The Stem Cell Instructive and Tissue Reparative Properties of Heparan Sulphate

This review highlights the attributes of pentosan polysulphate (PPS) in the promotion of intervertebral disc (IVD) repair processes. PPS has been classified as a disease modifying osteoarthritic drug (DMOAD) and many studies have demonstrated its positive attributes in the countering of degenerative changes occurring in cartilaginous tissues during the development of osteoarthritis (OA). Degenerative changes in the IVD also involve inflammatory cytokines, degradative proteases and cell signalling pathways similar to those operative in the development of OA in articular cartilage. PPS acts as a heparan sulphate (HS) mimetic to effect its beneficial effects in cartilage. The IVD contains small cell membrane HS-proteoglycans (HSPGs) such as syndecan, and glypican and a large multifunctional HS/chondroitin sulphate (CS) hybrid proteoglycan (HSPG2/perlecan) that have important matrix stabilising properties and sequester, control and present growth factors from the FGF, VEGF, PDGF and BMP families to cellular receptors to promote cell proliferation, differentiation and matrix synthesis. HSPG2 also has chondrogenic properties and stimulates the synthesis of extracellular matrix (ECM) components, expansion of cartilaginous rudiments and has roles in matrix stabilisation and repair. Perlecan is a perinuclear and nuclear proteoglycan in IVD cells with roles in chromatin organisation and control of transcription factor activity, immunolocalises to stem cell niches in cartilage, promotes escape of stem cells from quiescent recycling, differentiation and attainment of pluripotency and migratory properties. These participate in tissue development and morphogenesis, ECM remodelling and repair. PPS also localises in the nucleus of stromal stem cells, promotes development of chondroprogenitor cell lineages, ECM synthesis and repair and discal repair by resident disc cells. The availability of recombinant perlecan and PPS offer new opportunities in repair biology. These multifunctional agents offer welcome new developments in repair strategies for the IVD.

Circular RNAs in Intervertebral Disc Degeneration: An Updated Review

Low back pain, a common medical condition, could result in severe disability and inflict huge economical and public health burden. Its pathogenesis is attributed to multiple etiological factors, including intervertebral disc degeneration (IDD). Emerging evidence suggests that circular RNAs (circRNAs), a major type of regulatory non-coding RNA, play critical roles in cellular processes that are pertinent to IDD development, including nucleus pulposus cell proliferation and apoptosis as well as extracellular matrix deposition. Increasing number of translational studies also indicated that circRNAs could serve as novel biomarkers for the diagnosis of IDD and/or predicting its clinical outcomes. Our review aims to discuss the recent progress in the functions and mechanisms of newly discovered IDD-related circRNAs.

hsa-miR-508-5p as a New Potential Player in Intervertebral Disc Degeneration

Intervertebral disc degeneration (IDD) is widely known as the principal cause of low back pain, diminishing patients' quality of life and imposing a huge economic burden on healthcare systems worldwide. However, the underlying mechanisms of IDD remain to be determined. This study aimed to scrutinize data sets via bioinformatics to identify microRNAs (miRNAs)/genes and pathways associated with IDD. The array profiling of patients with IDD and individuals without IDD was acquired from the Gene Expression Omnibus (GEO) database (viz., GSE19943, GSE63492, and GSE34095). The expression profiles of miRNAs and genes with differential patterns were analyzed using GEO2R. The target genes of the chosen miRNA were then examined, and in silico functional analyses were performed on the signaling pathways and biological processes of the differentially expressed genes. Three human miRNAs were up and downregulated in IDD patients in the examined data sets. Among them, hsa-miR-508-5p had a significant differential expression in the IDD group, and SEC11A, IPO5, FN1, and MRPS10, as the targets of hsa-miR-508-5p, were upregulated in the IDD group. Furthermore, extracellular matrix-receptor interactions, focal adhesion, and actin cytoskeleton regulation were important pathways involved in IDD. Our analysis identified hsa-miR-508-5p as a novel miRNA involved in IDD pathogenies. Our findings not only further confirmed the significant role of miRNAs in IDD pathogenesis but also extended the spectrum of the miRNAs and genes involved in IDD. Though, still, further experimental investigations are needed to confirm our findings.

Pharmacological Disruption of Phosphorylated Eukaryotic Initiation Factor-2α/Activating Transcription Factor 4/Indian Hedgehog Protects Intervertebral Disc Degeneration via Reducing the Reactive Oxygen Species and Apoptosis of Nucleus Pulposus Cells

Excessive reactive oxygen species (ROS) and apoptosis in nucleus pulposus (NP) cells accelerate the process of intervertebral disc degeneration (IDD). Here, we integrated pathological samples and in vitro and in vivo framework to investigate the impact of phosphorylation of eukaryotic initiation factor-2α (eIF2α)/activating transcription factor 4 (ATF4)/Indian hedgehog (Ihh) signaling in the IDD. From the specimen analysis of the IDD patients, we found phosphorylated eIF2α (p-eIF2α), ATF4 and Ihh protein levels were positively related while the NP tissue went degenerative. In vitro, tumor necrosis factor (TNF)-α caused the NP cell degeneration and induced a cascade of upregulation of p-eIF2α, ATF4, and Ihh. Interestingly, ATF4 could enhance Ihh expression through binding its promoter region, and silencing of ATF4 decreased Ihh and protected the NP cells from degeneration. Moreover, ISRIB inhibited the p-eIF2α, which resulted in a suppression of ATF4/Ihh, and alleviated the TNF-α-induced ROS production and apoptosis of NP cells. On the contrary, further activating p-eIF2α aggravated the NP cell degeneration, with amplification of ATF4/Ihh and a higher level of ROS and apoptosis. Additionally, applying cyclopamine (CPE) to suppress Ihh was efficient to prevent NP cell apoptosis but did not decrease the ROS level. In an instability-induced IDD model in mice, ISRIB suppressed p-eIF2α/ATF4/Ihh and prevented IDD via protecting the anti-oxidative enzymes and decreased the NP cell apoptosis. CPE prevented NP cell apoptosis but did not affect anti-oxidative enzyme expression. Taken together, p-eIF2α/ATF4/Ihh signaling involves the ROS level and apoptosis in NP cells, the pharmacological disruption of which may provide promising methods in preventing IDD.

Deletion at an 1q24 locus reveals a critical role of long noncoding RNA DNM3OS in skeletal development

BACKGROUND

Skeletal development and maintenance are complex processes known to be coordinated by multiple genetic and epigenetic signaling pathways. However, the role of long non-coding RNAs (lncRNAs), a class of crucial epigenetic regulatory molecules, has been under explored in skeletal biology.

RESULTS

Here we report a young patient with short stature, hypothalamic dysfunction and mild macrocephaly, who carries a maternally inherited 690 kb deletion at Chr.1q24.2 encompassing a noncoding RNA gene, DNM3OS, embedded on the opposite strand in an intron of the DYNAMIN 3 (DNM3) gene. We show that lncRNA DNM3OS sustains the proliferation of chondrocytes independent of two co-cistronic microRNAs miR-199a and miR-214. We further show that nerve growth factor (NGF), a known factor of chondrocyte growth, is a key target of DNM3OS-mediated control of chondrocyte proliferation.

CONCLUSIONS

This work demonstrates that DNM3OS is essential for preventing premature differentiation of chondrocytes required for bone growth through endochondral ossification.

Immunohistochemical analysis of protein expression in formalin fixed paraffin embedded human intervertebral disc tissues

Immunohistochemistry (IHC) is a useful technique for the localization and semiquantification of protein expression within tissues. Adult human intervertebral disc (IVD) tissues contain a large amount of auto-fluorescence which often makes immunofluorescence techniques inappropriate on tissue samples but can be applied to isolated cell samples. Thus, IHC remains one of, if not the most common application for protein detection within IVD tissue. Immunostaining localizes antigen expression through specific epitope-antibody interactions. Within the field of IVD research, IHC is commonly used on fresh frozen and paraffin embedded tissues to elucidate the expression of antigens. Here, we discuss the principles of IHC applied to formalin fixed paraffin embedded IVD tissue and supply optimized protocols for antibodies used within our group to guide research within the IVD field.