Inorganic polyphosphates enhances nucleus pulposus tissue formation in vitro

Engineering Intervertebral Disc Regeneration: Biomaterials, Cell Sources and Animal Models

Intervertebral disc (IVD) degeneration is an age-related problem triggering chronic spinal issues, such as low back pain and IVD herniation. Standard surgical treatment for such spinal issues is the removal of the degenerated or herniated IVD and fusion of adjacent vertebrae to stabilise the joint and locally decompress the spinal cord and/or nerve roots to relieve pain. However, a key challenge of current surgical strategies is the increasing risk of adjacent segment degeneration due to the disruption of native biomechanics of the functional spinal unit, dominated by the loss of the IVD. In the past two decades, research has focused on developing a number of bioengineering approaches to repair and regenerate the IVD; in particular, tissue engineering of the IVD, using bioscaffolds and stem cells represents a promising area. This review highlights the current tissue engineering approaches utilising biomaterials, animal models and cell sources for IVD regeneration and discusses future opportunities.

Development of 2-D and 3-D culture platforms derived from decellularized nucleus pulposus

Bioscaffolds derived from the extracellular matrix (ECM) have shown the capacity to promote regeneration by providing tissue-specific biological instructive cues that can enhance cell survival and direct lineage-specific differentiation. This study focused on the development and characterization of two-dimensional (2-D) and three-dimensional (3-D) cell culture platforms incorporating decellularized nucleus pulposus (DNP). First, a detergent-free protocol was developed for decellularizing bovine nucleus pulposus (NP) tissues that was effective at removing cellular content while preserving key ECM constituents including collagens, glycosaminoglycans, and the cell-adhesive glycoproteins laminin and fibronectin. Next, novel 2-D coatings were generated using the DNP or commercially-sourced bovine collagen type I (COL) as a non-tissue-specific control. In addition, cryo-milled DNP or COL particles were incorporated within methacrylated chondroitin sulphate (MCS) hydrogels as a 3-D cell culture platform for exploring the effects of ECM particle composition. Culture studies showed that the 2-D coatings derived from the DNP could support cell attachment and growth, but did not maintain or rescue the phenotype of primary bovine NP cells, which de-differentiated when serially passaged in monolayer culture. Similarly, while bovine NP cells remained highly viable following encapsulation and 14 days of culture within the hydrogel composites, the incorporation of DNP particles within the MCS hydrogels was insufficient to maintain or rescue changes in NP phenotype associated with extended in vitro culture based on gene expression patterns. Overall, DNP produced with our new decellularization protocol was successfully applied to generate both 2-D and 3-D bioscaffolds; however, further studies are required to assess if these platforms can be combined with additional components of the endogenous NP microenvironment to stimulate regeneration or lineage-specific cell differentiation.

Induced senescence of healthy nucleus pulposus cells is mediated by paracrine signaling from TNF-α-activated cells

Intervertebral disc degeneration is an irreversible process associated with accumulation of senescent nucleus pulposus (NP) cells. This study investigates the hypothesis that Tumor necrosis factor-α (TNF-α)-treated senescent NP cells propagate senescence of neighboring healthy cells via a paracrine effect that involves p-Stat3 signaling and the cytokine interleukin-6 (IL-6). NP cells isolated from bovine caudal intervertebral disc (IVD) were treated with TNF-α to induce senescence which was confirmed by demonstrating upregulation of senescence-associated β-galactosidase and p16. This was correlated with downregulation of NP-associated markers, Aggrecan, Col2A1, and Sox9. Direct contact and non-contact co-culture of healthy and senescent cells showed that TNF-α-treated cells increased the senescence in healthy cells via a paracrine effect. The senescent cells have a secretory phenotype as indicated by increased gene and protein levels of IL-6. Phosphorylated Signal Transducer and Activator of Transcription 3 (pStat3) levels were also high in treated cells and appeared to upregulate IL-6 as inhibition of Stat3 phosphorylation by StatticV downregulated IL-6 mRNA expression in cells and protein levels in the culture media. All trans retinoic acid, an IL-6 inhibitor, also decreased the secretion of IL-6 and reduced the paracrine effect of senescent cells on healthy cells. Decreased pStat3 levels and inhibition of IL-6 secretion did not fully restore NP gene expression of Col2A1 but importantly, appeared to cause senescent cells to undergo apoptosis and cell death. This study demonstrated the paracrine effect of senescent NP cells which involves Stat3 and IL-6 and may explain why senescent NP cells accumulate in IVD with age. The role of pSTAT3 and IL-6 in mediating NP senescence requires further study as it may be a novel strategy for modulating the senescent-inducing effects of TNF-α.

Inorganic polyphosphates stimulates matrix production in human annulus fibrosus cells

INTRODUCTION

Ubiquitously found in all life forms, inorganic polyphosphates (polyP) are linear polymers of repeated orthophosphate units. Present in intervertebral disc tissue, polyP was previously shown to increase extracellular matrix production in nucleus pulposus (NP) cells. However, the effects of polyP on human annulus fibrosus (hAF) cell metabolism is not known.

METHODS AND RESULTS

Here, hAF cells cultured in the presence of 0.5 to 1 mM polyP, chain length 22 (polyP-22), showed an increase in glycosaminoglycan content, proteoglycan and collagen synthesis, and aggrecan and collagen type 1 gene expression. Gene expression level of matrix metalloproteinases 1 was reduced while matrix metalloproteinases 3 level was increased in hAF cells treated with 1 mM polyP. Adenosine triphosphate (ATP) synthesis was also significantly increased in hAF cell culture 72 hours after the exposure to 1 mM polyP-22.

CONCLUSIONS

PolyP thus has both anabolic and bioenergetic effects in AF cells, similar to that observed in NP cells. Together, these results suggest polyP as a potential energy source and a metabolic regulator of disc cells.

Transforming Growth Factor β Enhances Tissue Formation by Passaged Nucleus Pulposus Cells In Vitro

The nucleus pulposus (NP) is composed of NP and notochord cell. It is a paucicellular tissue and if it is to be used as a source of cells for tissue engineering the cell number will have to be expanded by cell passaging. The hypothesis of this study is that passaged NP and notochordal cells grown in three-dimensional (3D) culture in the presence of transforming growth factor β (TGFβ) will show enhanced NP tissue formation compared with cells grown in the absence of this growth factor. Bovine NP cells isolated by sequential enzymatic digestion from caudal intervertebral discs were either placed directly in 3D culture (P0) or serially passaged up to passage 3 (P3) prior to placement in 3D culture. Serial cell passage in monolayer culture led to de-differentiation, increased senescence and oxidative stress and decreases in the gene expression of NP and notochordal associated markers and increases in de-differentiation markers. The NP tissue regeneration capacity of cells in 3D culture decreases with passaging as indicated by diminished tissue thickness and total collagen content when compared with tissues formed by P0 cells. Immunohistochemical studies showed that type II collagen accumulation appeared to decrease. TGFβ1 or TGFβ3 treatment enhanced the ability of cells at each passage to form tissue, in part by decreasing cell death. However, neither TGFβ1 nor TGFβ3 were able to restore the notochordal phenotype. Although TGFβ1/3 recovered NP tissue formation by passaged cells, to generate NP in vitro that resembles the native tissue will require identification of conditions facilitating retention of notochordal cell differentiation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:438-449, 2020.

Molecular Mechanisms of Intervertebral Disc Degeneration

Intervertebral disc degeneration is a well-known cause of disability, the result of which includes neck and back pain with associated mobility limitations. The purpose of this article is to provide an overview of the known molecular mechanisms through which intervertebral disc degeneration occurs as a result of complex interactions of exogenous and endogenous stressors. This review will focus on some of the identified molecular changes leading to the deterioration of the extracellular matrix of both the annulus fibrosus and nucleus pulposus. In addition, we will provide a summation of our current knowledge supporting the role of associated DNA and intracellular damage, cellular senescence's catabolic effects, oxidative stress, and the cell's inappropriate response to damage in contributing to intervertebral disc degeneration. Our current understanding of the molecular mechanisms through which intervertebral disc degeneration occurs provides us with abundant insight into how physical and chemical changes exacerbate the degenerative process of the entire spine. Furthermore, we will describe some of the related molecular targets and therapies that may contribute to intervertebral repair and regeneration.

Simple Silica Column-Based Method to Quantify Inorganic Polyphosphates in Cartilage and Other Tissues

OBJECTIVE

Inorganic polyphosphates (polyP) play a multitude of roles in mammalian biology. PolyP research is hindered by the lack of a simple and sensitive quantification method. The aim of this study was to develop a robust method for quantifying the low levels of polyP in mammalian tissue such as cartilage, which is rich in macromolecules that interfere with its determination.

DESIGN

Native and in vitro formed tissues were digested with proteinase K to release sequestrated polyP. The tissue digest was loaded on to silica spin columns, followed by elution of bound polyP and various treatments were assessed to minimize non-polyP fluorescence. The eluent was then quantified for polyP content using fluorometry based on DAPI (4',6-diamidino-2-phenylindole) fluorescence shift occurring with polyP.

RESULTS

Proteinase K pretreatment reduced the inhibitory effect of proteins on polyP recovery. The eluent was contaminated with nucleic acids and glycosaminoglycans, which cause extraneous fluorescence signals. These were then effectively eliminated by nucleases treatment and addition of concentrated Tris buffer. PolyP levels were quantified and recovery ratio determined using samples spiked with a known amount of polyP. This silica spin column method was able to recover at least 80% of initially loaded polyP, and detect as little as 10^-10 mol.

CONCLUSIONS

This sensitive, reproducible, easy to do method of quantifying polyP will be a useful tool for investigation of polyP biology in mammalian cells and tissues. Although the protocol was developed for mammalian tissues, this method should be able to quantify polyP in most biological sources, including fluid samples such as blood and serum.