Distinctive roles of tumor necrosis factor receptor type 1 and type 2 in a mouse disc degeneration model

TNFR1-mediated senescence and lack of TNFR2-signaling limit human intervertebral disc cell repair potential in degenerative conditions

OBJECTIVE

To identify mechanisms and treatment targets in painful intervertebral disc (IVD) degeneration (IVDD) progression with a focus on pro-inflammatory tumor necrosis factor-alpha (TNFα)-receptor-1 (TNFR1) and pro-reparative TNFα receptor-2 (TNFR2) signaling.

DESIGN

IVDD tissues and cells from IVDD and autopsy subjects were analyzed with single-cell RNA-sequencing to identify cell populations expressing TNFR1 and TNFR2, and multiplexed array to identify inflammatory proteins in IVDD conditioned media (CM). Bulk RNA-seq evaluated inflammatory and cell cycle states of human annulus fibrosus (hAF) cells challenged with CM. hAF cell responses to TNFR1 and TNFR2 modulation were evaluated by treatment with TNFR1- and TNFR2-blocking antibodies and TNFR2-activator Atsttrin.

RESULTS

IVDD CM chemokines and cytokines were expressed primarily by a small macrophage population and at low levels by native IVD cells. CM-treated hAF cells exhibited TNFα-signaling responses with reduced metabolic rates (MTT: 0.75 [95%CI:0.67 to 0.82]), limited inflammatory responses (inferred from heatmap of 50 differentially expressed genes), and senescence (10.4% SA-β-Gal+ cells [95%CI:6.99 to 13.8]). TNFR1-inhibition sufficiently restored hAF cell metabolism to enable robust pro-inflammatory responses to the complex IVDD CM cytokine mixture (multiple assays,). TNFR2-staining was limited on human IVD cell membranes and TNFR2 modulation had no effect on hAF cells, together suggesting a lack of TNFR2-signaling in native IVD cells.

CONCLUSIONS

Secreted proteins from IVDD CM caused hAF cells to have reduced metabolic rates, attenuated inflammatory responses, and senescence indicating a TNFR1-dominated response with metabolic impairment. Meanwhile, human IVD cells lacked reparative TNFR2-signaling since its modulation caused no effects, to suggest enhanced TNFR2-signaling in IVD repair may need recruitment or delivery of macrophages or other TNFR2-expressing cells.

Therapeutic CRISPR epigenome editing of inflammatory receptors in the intervertebral disc

Low back pain (LBP) ranks among the leading causes of disability worldwide and generates a tremendous socioeconomic cost. Disc degeneration, a leading contributor to LBP, can be characterized by the breakdown of the extracellular matrix of the intervertebral disc (IVD), disc height loss, and inflammation. The inflammatory cytokine tumor necrosis factor α (TNF-α) has multiple signaling pathways, including proinflammatory signaling through tumor necrosis factor receptor 1 superfamily, member 1a (TNFR1 or TNFRSF1A), and has been implicated as a primary mediator of disc degeneration. We tested our ability to regulate the TNFR1 signaling pathway in vivo, utilizing CRISPR epigenome editing to slow the progression of disc degeneration in rats. Sprague-Dawley rats were treated with TNF-α and CRISPR interference (CRISPRi)-based epigenome-editing therapeutics targeting TNFR1, showing decreased behavioral pain in a disc degeneration model. Surprisingly, while treatment with the vectors alone was therapeutic, the TNF-α injection became therapeutic after TNFR1 modulation. These results suggest direct inflammatory receptor modulation as a potent strategy for treating disc degeneration.

TNFR1-mediated senescence and lack of TNFR2-signaling limit human intervertebral disc cell repair in back pain conditions

Poor intervertebral disc (IVD) healing causes IVD degeneration (IVDD) and progression to herniation and back pain. This study identified distinct roles of TNFα-receptors (TNFRs) in contributing to poor healing in painful IVDD. We first isolated IVDD tissue of back pain subjects and determined the complex pro-inflammatory mixture contained many chemokines for recruiting inflammatory cells. Single-cell RNA-sequencing of human IVDD tissues revealed these pro-inflammatory cytokines were dominantly expressed by a small macrophage-population. Human annulus fibrosus (hAF) cells treated with IVDD-conditioned media (CM) underwent senescence with greatly reduced metabolic rates and limited inflammatory responses. TNFR1 inhibition partially restored hAF cell metabolism sufficiently to enable a robust chemokine and cytokine response to CM. We showed that the pro-reparative TNFR2 was very limited on hIVD cell membranes so that TNFR2 inhibition with blocking antibodies or activation using Atsttrin had no effect on hAF cells with CM challenge. However, TNFR2 was expressed in high levels on macrophages identified in scRNA-seq analyses, suggesting their role in repair responses. Results therefore point to therapeutic strategies for painful IVDD involving immunomodulation of TNFR1 signaling in IVD cells to enhance metabolism and enable a more robust inflammatory response including recruitment or delivery of TNFR2 expressing immune cells to enhance IVD repair.

Mesenchymal stromal cell response to intervertebral disc-like pH is tissue source dependent

Intervertebral disc (IVD) degeneration (IVDD) has become increasingly prevalent and is a common contributing factor to low back pain. Current treatment options are limited to either symptom management or surgery. A promising treatment option being explored is intradiscal administration of mesenchymal stromal cells (MSCs). However, there remains a gap in knowledge as to whether MSCs from different tissue sources have similar responses to the low pH microenvironment of the IVD and the possible mechanisms governing these responses. To study this, MSCs from three different tissue sources: adipose (adipose-derived mesenchymal stem cell), bone marrow (bone marrow mesenchymal stem cells), and amnion (amniotic membrane mesenchymal stem cell) were cultured at low pHs representative of IVDD. MSCs were assessed for survival, senescence, apoptosis, metabolic activity, and cytokine release profile. Additionally, western blot was utilized to assess acid sensing ion channel 1 and 3 expression. The results of this study indicated that MSC viability, cell proliferation, senescence, and metabolic activity is negatively affected by low pH and alters MSC cytokine production. This study also demonstrated that MSCs behavior is dependent on tissue source. Understanding how MSC behavior is altered by pH will allow further research aimed at increasing the efficacy of MSC therapy to promote in situ IVD tissue regeneration to combat IVDD.

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.

Gene Expression Profiles Perturbed by Injury to the Mouse Intervertebral Disc

OBJECTIVES

Back pain subsequent to intervertebral disc injury is a common clinical problem. Previous work examining early molecular changes post injury mainly used a candidate marker approach. In this study, gene expression in the injured and intact mouse tail intervertebral discs was determined with a nonbiased whole transcriptome approach.

DESIGN

Mouse tail intervertebral disc injury was induced by a needle puncture. Whole murine transcriptome was determined by RNASeq. Transcriptomes of injured intervertebral discs were compared with those of intact controls by bioinformatic methods.

RESULTS

Among the 18,078 murine genes examined, 592 genes were differentially expressed ( P.adj < 0.01). Novel genes upregulated in injured compared with intact intervertebral discs included Chl1, Lum , etc. Ontology study of upregulated genes revealed that leukocyte migration was the most enriched biological process, and network analysis showed that Tnfa had the most protein-protein interactions. Novel downregulated genes in the injured intervertebral discs included 4833412C05Rik , Myoc , etc . The most enriched downregulated pathways were related to cytoskeletal organization.

CONCLUSIONS

Novel genes highly regulated after disc injury were identified with an unbiased approach; they may serve as biomarkers of injury and response to treatments in future experiments. Enriched biological pathways and molecules with high numbers of connections may be targets for treatments after injury.

Loss of Pinch Proteins Causes Severe Degenerative Disc Disease-Like Lesions in Mice

Degenerative disc disease (DDD) is one of the most common skeletal disorders affecting aged populations. DDD is the leading cause of low back/neck pain, resulting in disability and huge socioeconomic burdens. However, the molecular mechanisms underlying DDD initiation and progression remain poorly understood. Pinch1 and Pinch2 are LIM-domain-containing proteins with crucial functions in mediating multiple fundamental biological processes, such as focal adhesion, cytoskeletal organization, cell proliferation, migration, and survival. In this study, we found that Pinch1 and Pinch2 were both highly expressed in healthy intervertebral discs (IVDs) and dramatically downregulated in degenerative IVDs in mice. Deleting Pinch1 in aggrecan-expressing cells and Pinch2 globally (AggrecanCreERT2; Pinch1fl/fl; Pinch2-/-) caused striking spontaneous DDD-like lesions in lumbar IVDs in mice. Pinch loss inhibited cell proliferation and promoted extracellular matrix (ECM) degradation and apoptosis in lumbar IVDs. Pinch loss markedly enhanced the production of pro-inflammatory cytokines, especially TNFα, in lumbar IVDs and exacerbated instability-induced DDD defects in mice. Pharmacological inhibition of TNFα signaling mitigated the DDD-like lesions caused by Pinch loss. In human degenerative NP samples, reduced expression of Pinch proteins was correlated with severe DDD progression and a markedly upregulated expression of TNFα. Collectively, we demonstrate the crucial role of Pinch proteins in maintaining IVD homeostasis and define a potential therapeutic target for DDD.

Regulated cell death: Implications for intervertebral disc degeneration and therapy

As a controllable biological process, regulated cell death (RCD) extensively participates in cellular homeostasis, organismal development, and the pathogenesis of diseases. This review addresses the research gaps by synthesising the findings on the complexity of RCD modes and their role in disc degeneration, and summarises the preclinical strategies to alleviate disc degeneration and promote disc repair by regulating RCD.

BACKGROUND

Intervertebral disc degeneration (IDD) is the major source of chronic low back pain. As a controllable biological process, regulated cell death (RCD) extensively participates in the pathogenesis of IDD. Nevertheless, the initiation and progression of RCD remain unclear, and more importantly, the interaction between different RCD modes during IDD and therapy is far from well understood.

METHODS

Literature search was performed using "regulated cell death AND intervertebral disc degeneration" in PubMed, Embase, and Web of Science. Meanwhile, relevant findings have been reviewed and quoted.

RESULTS

In this review, we discuss the inducing factors of IDD, various modes of RCD in intervertebral disc, the interactions between different RCD modes, as well as the obstacles to achieve disc regeneration. Meanwhile, the research gaps and perspective in studies that targeting RCD are also presented.

CONCLUSION

Increasing evidence demonstrated the presence of different RCD modes in intervertebral disc during the progression of IDD. RCD in the resident disc cells is probably induced by the multiple factors such as abnormal mechanical loading, nutritional imbalance, inflammation microenvironment, circadian rhythm changes, withdraw of hormones, and other biomechanical factors. A better understanding of the fundamental mechanisms and the interactions between different RCD modes might contribute to the rescuing of disc degeneration and development of promising therapeutics.

TRANSLATIONAL POTENTIAL STATEMENT

The Translational potential of this article. This review aims to demonstrate a better understanding of the fundamental mechanisms governing RCD, which might contribute to the rescuing of disc degeneration and to the development of promising therapeutics in a clinical setting.

Role of Caspase Family in Intervertebral Disc Degeneration and Its Therapeutic Prospects

Intervertebral disc degeneration (IVDD) is a common musculoskeletal degenerative disease worldwide, of which the main clinical manifestation is low back pain (LBP); approximately, 80% of people suffer from it in their lifetime. Currently, the pathogenesis of IVDD is unclear, and modern treatments can only alleviate its symptoms but cannot inhibit or reverse its progression. However, in recent years, targeted therapy has led to new therapeutic strategies. Cysteine-containing aspartate proteolytic enzymes (caspases) are a family of proteases present in the cytoplasm. They are evolutionarily conserved and are involved in cell growth, differentiation, and apoptotic death of eukaryotic cells. In recent years, it has been confirmed to be involved in the pathogenesis of various diseases, mainly by regulating cell apoptosis and inflammatory response. With continuous research on the pathogenesis and pathological process of IVDD, an increasing number of studies have shown that caspases are closely related to the IVDD process, especially in the intervertebral disc (IVD) cell apoptosis and inflammatory response. Therefore, herein we study the role of caspases in IVDD with respect to the structure of caspases and the related signaling pathways involved. This would help explore the strategy of regulating the activity of the caspases involved and develop caspase inhibitors to prevent and treat IVDD. The aim of this review was to identify the caspases involved in IVDD which could be potential targets for the treatment of IVDD.