The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 88, Issue suppl 2

Workshop Articles | April 01, 2006

Biologic Modification of Animal Models of Intervertebral Disc Degeneration

James W. LarsonIII, MD; Eric A. Levicoff, MD; Lars G. Gilbertson, PhD; James D. Kang, MD

In support of their research for or preparation of this manuscript, one or more of the authors received grants or outside funding from Medtronic Sofamor Danek. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2006 Apr 01;88(suppl 2):83-87. doi: 10.2106/JBJS.F.00043

5 Recommendations (Recommend) | 3 Comments | Saved by 3 Users Save Case

Article

Comments (0)

text A A A

Abstract

Intervertebral disc degeneration is a chronic process that can become manifest in clinical disorders such as idiopathic low back pain, sciatica, disc herniation, spinal stenosis, and myelopathy. The limited available treatment options (including discectomy and spinal fusion) for these and other disabling conditions that arise from intervertebral disc degeneration are highly invasive, achieve limited success, and only address acute symptoms while doing nothing to halt the process of degeneration. Although the precise pathophysiology of intervertebral disc degeneration has yet to be clearly delineated, the progressive decline in aggrecan, the primary proteoglycan of the nucleus pulposus, appears to be a final common pathway. Animal models as well as in vitro studies of the process of disc degeneration have yielded many potentially useful targets for the reversal of disc degeneration. One current research trend is the use of established animal models of disc degeneration to study the role of therapeutic modalities in reversing the process of degeneration, often with use of the delivery of genes or gene products that influence the anabolic and catabolic pathways of the disc. This article reviews the ability of gene-product delivery systems and gene therapy to alter biologic processes in animal models of disc degeneration and examines future trends in this field.

Figures in this Article

Back pain is one of the most common chronic pain conditions in adults in the United States, with the majority of people reporting back pain at least once during adulthood. Back pain has enormous personal, social, and economic ramifications. Most chronic back pain is due to disorders of the spine. Most notable among these is degeneration of the intervertebral disc, which can manifest as and lead to many different clinical conditions, including discogenic pain, disc herniation, facet joint osteoarthritis, spinal stenosis, and instability. Current treatments for axial back pain include both nonoperative modalities (e.g., nonsteroidal anti-inflammatory medication, physical therapy, and pain management) and operative methods (e.g., fusion, intradiscal electrothermal therapy, and total disc replacement). However, all of these methods have limited efficacy. These treatment modalities do not produce reliable outcomes, nor do they result in a complete return to the preinjury state, because they are directed toward the symptoms rather than toward the degeneration that is the root of the problem.

Recent advances in biotechnology coupled with our expanded understanding of the process of degeneration present the possibility of developing novel treatments aimed directly at disc preservation during the multiple stages of degeneration. Many genes have been found to have a major impact on matrix synthesis and catabolism within the disc, providing targets for scientists seeking to alter the balance between these two processes. In parallel with the biologic approach to disc repair, many researchers have been working toward a better understanding of how the mechanical changes affect the process of degeneration and how they interact with the biologic changes. To these ends, much attention over the past several years has centered on both mechanical and biologic approaches to disc repair. The early results of these efforts have been promising and have formed the basis for a new approach to the treatment of intervertebral disc disease.

The Degenerative Cascade

The Degenerative Cascade | In Vivo Biologic Manipulation | References

Degeneration of the intervertebral disc is a multifactorial process involving genetic, mechanical, and biologic factors. Although the changes that occur in the disc during the process of degeneration have been well described, the exact pathophysiologic pathway has yet to be fully understood. Normal discs consist of a gelatinous nucleus pulposus surrounded by a tough peripheral structure called the anulus fibrosus. Proper biomechanical and biologic interaction between these two structures enables the disc to perform various functions, including bending, load distribution, and shock absorption. This mechanism is highly dependent on the integrity of these two structures on a molecular level as well as on the ability of normal disc tissue to imbibe and release water. As the water content of the disc decreases, abnormalities of function increase.

Disc hydration is maintained primarily via the extracellular matrix produced by the chondrocytic cells of the nucleus pulposus. These cells produce and maintain a matrix consisting of a type-II collagen scaffold supporting a network of proteoglycans, which attract and hold water within the disc by means of multiple negatively charged glycosaminoglycan side chains. One of the most important of these is aggrecan. This makeup is fairly similar to articular cartilage, and the ability of the matrix to imbibe and release water in relation to the stresses placed on it allows the disc to cushion compressive loads.

View Large | Download Slide(.ppt)
Add to My JBJS

Fig. 1: The degeneration of intervertebral discs is a multifactorial process. While the proximate cause of this cascade has yet to be fully defined, the resulting process is characterized by the upregulation of proinflammatory mediators (IL-6) and degradative enzymes (MMP, ADAMTS) without concurrent increases in the natural inhibitors of these factors (TIMPs).

As the disc degenerates, there is a net loss of proteoglycans and water from the nucleus. This reduction in proteoglycan content and water is hypothesized to lead to the clinical evidence of degeneration, including decreased disc height and decreased signal intensity of the disc on T2-weighted magnetic resonance imaging. It also leads to increased load on the structures surrounding the spine and alters the mechanics within the spine itself. Abnormal distribution of forces across the disc results in cracking and fissuring of the anulus fibrosus, further decreasing its ability to distribute stresses properly during loading. Fibrosis and lamination of the normally amorphous nucleus and herniation of the nucleus pulposus are commonly associated with a degenerating disc. Vertebral body pathology is also a hallmark of late-stage degeneration and includes subchondral sclerosis, end-plate ossification, and osteophyte formation.

Although the biochemical mechanism that initiates these changes has yet to be elucidated, research suggests that an imbalance between degradation and anabolism of this matrix is the main pathway. Investigation of herniated discs reveal an increased production of catabolic proteins and inflammatory cytokines¹. Upregulation of matrix metalloproteinases (MMPs), nitric oxide, interleukin-6, and prostaglandin E₂ correlates with disc degeneration in humans². Some of the inflammatory mediators are also associated with back pain and sciatica in the degenerating disc. Degradative enzymes collectively known as MMPs are produced at levels that begin to overwhelm production of their inhibitors, the tissue inhibitors of metalloproteinases (TIMPs). Degenerating discs have shown an increase in MMP-3 (stromelysin) and a relatively low level of its natural inhibitor TIMP-1. Similarly, aggrecanase (ADAMTS-4 [a disintegrin and metalloproteinase with thrombospondin motifs]) and its inhibitor TIMP-3 show a relative imbalance in levels (Fig. 1).

In Vivo Biologic Manipulation

The Degenerative Cascade | In Vivo Biologic Manipulation | References

Armed with an expanding number of growth factors and cytokines now known to influence the existing balance between synthesis and catabolism of proteoglycans within the nucleus pulposus, scientists and clinicians have begun to develop a biologic approach to the treatment of disc degeneration. Following in the footsteps of largely successful studies demonstrating the feasibility of biologic manipulation in vitro, studies examining its potential in vivo have increased dramatically in recent years. Many of these protocols incorporate the use of newly developed animal models of disc degeneration in order to study both the progression of the disease as well as the therapeutic potential of biologic interventions. These animal models, although surgical in origin and therefore not identical in origin to idiopathic disc degeneration, are characterized by many of the same biochemical and pathologic changes as seen in human disease³. Growth factors such as transforming growth factor-ß (TGF-ß), platelet-derived growth factor, Sry-type high-mobility-group box transcription factor-9 (Sox-9), and several of the proteins in the bone morphogenetic protein (BMP) family have been shown not only to increase the synthesis of structural and organic proteins within the disc but also to enhance cellular proliferation, decrease cell apoptosis in the nucleus pulposus, and drive cells of the nucleus toward a chondrogenic phenotype⁴. LIM mineralization protein-1 has also been a subject of recent research and advances. This molecule controls signaling on a broad scale as it increases matrix production as well as the transcription of both BMP-2 and BMP-7 in the intervertebral disc⁵.

An important issue separating the in vivo study of biologic manipulation from that done in vitro is the mechanism of protein delivery. While in vitro studies are essential in demonstrating the efficacy of potential therapeutic molecules, they are largely free of concerns regarding drug delivery, pharmacokinetics, and safety. The successful application of biologic treatments in the clinical setting will depend on a reasonable system of drug delivery. Many in vivo studies have employed simple protein injection as a means of achieving therapeutic concentrations of protein within the disc. Relative to current surgical interventions such as discectomy and fusion, protein injection is minimally invasive and safe and would likely result in less overall patient morbidity. Experiments that have made use of this method of dosing in animals have been successful, with injections of both osteogenic protein-1 and BMP-2 leading to a state of increased disc health⁶. However, while outcomes have been generally successful, application of protein injections to the problem of disc degeneration in humans may be limited by the short half-life of therapeutic growth factors. Although disc degeneration is usually a chronic phenomenon that spans a number of years, studies have shown the half-life of many growth factors to be more on the order of minutes. Despite this short half-life, the therapeutic effect of BMP-7 in the disc was found to be maintained over several months because of the slow turnover of the cells and newly synthesized matrices in the confined space of the disc⁷. Whether protein injection alone can provide therapeutic effects of matrix synthesis and pain relief in humans over the long term has yet to be determined.

View Large | Download Slide(.ppt)
Add to My JBJS

Fig. 2: Schematic representation of gene-therapy technology. Foreign genetic material is transferred to a host cell, in this case via viral vectors, and the cell then uses the information to become an in situ factory for protein product. GF = growth factors.

As opposed to growth-factor injection, gene therapy seeks to sustain a high level of therapeutic protein within the intervertebral disc without the need for multiple injections. Gene-therapy technology involves the transfer of actual genetic material to the cells of a target tissue, which then become in situ factories for the continuous production of the protein product (Fig. 2). Studies examining gene therapy in the disc have demonstrated an important potential to affect therapeutic change⁸. Cells of degenerated discs not only are amenable to gene transfer, but they also have the ability to upregulate production of extracellular matrix in response to transduction with therapeutic genes. In large part due to the unique biologic properties of the nucleus pulposus, gene therapy has been shown to have perhaps even greater potential in this site than in many other tissues in the body. Transduction of nucleus pulposus cells with foreign deoxyribonucleic acid (DNA) leads to long-term, undiminished expression of protein product, and the host immune response to this foreign material is limited, with influences on the production and quality of the extracellular matrix of the nucleus pulposus⁹.

Although the method of drug delivery is essential to optimizing the efficacy of biologic therapy, safety is of the utmost importance. Before intradiscal biologic manipulation can be applied to human subjects, it must be shown to be safe as well as efficacious. Particularly because degenerative disc disease is a nonlethal condition, rigorous studies must be conducted to fully profile the associated morbidity of biologic manipulation. Literature to date regarding this topic is somewhat limited; however, recently there has been a push toward developing technology aimed at increasing the therapeutic index of potentially therapeutic biologic molecules¹⁰. With respect to gene therapy, both viral and nonviral vectors are being developed that will dramatically decrease the risks associated with drug delivery. Researchers are beginning to incorporate the use of naturally less pathogenic viruses such as adeno-associated virus (Fig. 3), and new technology such as electroporation is dramatically improving the efficacy of nonviral vectors. In addition, methods for exogenous control of injected proteins are under development. Technology such as the Tet-ON and Tet-OFF tetracycline-inducible gene expression systems (TET Systems Holding, Heidelberg, Germany) will allow the physician to control dosing in a noninvasive manner. The continued development of these systems will be essential to the future application of biologic therapy to the problem of disc degeneration. With any gene therapy, the primary concerns are safety, efficiency of transduction or transfection, and duration of gene expression. Additional challenges of gene therapy for the treatment of the degenerated disc are the practicality of ex vivo methods, patient acceptance of viral-mediated methods, and the choice of genes. Future research will determine if gene therapy will be safe and efficacious over the long term.

Finally, cell-based therapy is gaining rapid acceptance as another biologic approach to the treatment of disc degeneration¹¹. While the success of gene therapy and growth-factor injection depends on a critical mass of cells within the disc, cell-based therapy does not share this requirement and may therefore be appropriate for a wide range of disease states, including severe degenerative disease in which the fibrotic nucleus is largely acellular. Repopulation of the nucleus with disc cells, chondrocytes, or stem cells may someday be used to help not only with early-stage disc disease, but also to regenerate or rescue discs from a severely degenerated state, provided that nutritional requirements are met in order for the cells to survive. Although proponents of stem-cell therapy will certainly have to combat their own set of medicolegal and safety issues, cell-based therapy certainly shows promise with respect to biologic disc repair.

View Large | Download Slide(.ppt)
Add to My JBJS

Fig. 3: In vivo demonstration of successful adeno-associated viral (AAV) transduction of nucleus pulposus cells. The nucleus pulposus of an intervertebral disc from a New Zealand white rabbit was transduced with ß galactosidase (AAV-LacZ) by means of a surgical procedure. Histologic sections were obtained after the animal was killed, demonstrating successful production of marker protein within cells of the nucleus pulposus as shown by the blue LacZ staining of these cells.

Our understanding of the biology of the intervertebral disc has grown tremendously in recent years. Numerous therapeutic proteins have been identified, and they have been used in largely successful studies examining the potential of biologic manipulation in in vivo models of disc degeneration. Improvements are being made in the areas of protein delivery and regulation of gene therapy, and cell therapy has come to the fore as a potentially powerful therapeutic tool. In vitro successes have directed in vivo protocols, and improved animal models of disc degeneration have provided a means to test not only the efficacy but also the safety of these biologic interventions. Although there is still much to be learned, we now have a better understanding of the final common pathway of degeneration. With continued dedication, the biologic technologies that we have begun to develop will someday play a major role in the treatment of intervertebral disc disease. ?

References

The Degenerative Cascade | In Vivo Biologic Manipulation | References

Le Maitre CL, Freemont AJ, Hoyland JA. Localization of degradative enzymes and their inhibitors in the degenerate human intervertebral disc. J Pathol. 2004;204: 47-54.20447 2004 [PubMed][CrossRef]

Burke JG, G Watson RW, Conhyea D, McCormack D, Dowling FE, Walsh MG, Fitzpatrick JM. Human nucleus pulposis can respond to a pro-inflammatory stimulus. Spine. 2003;28: 2685-93.282685 2003 [PubMed][CrossRef]

Sobajima S, Shimer AL, Chadderdon RC, Kompel JF, Kim JS, Gilbertson LG, Kang JD. Quantitative analysis of gene expression in a rabbit model of intervertebral disc degeneration by real-time polymerase chain reaction. Spine J. 2005;5: 14-23.514 2005 [PubMed][CrossRef]

Paul R, Haydon RC, Cheng H, Ishikawa A, Nenadovich N, Jiang W, Zhou L, Breyer B, Feng T, Gupta P, He TC, Phillips FM. Potential use of Sox9 gene therapy for intervertebral degenerative disc disease. Spine. 2003;28: 755-63.28755 2003 [PubMed]

Yoon ST, Park JS, Kim KS, Li J, Attallah-Wasif ES, Hutton WC, Boden SD. LMP-1 upregulates intervertebral disc cell production of proteoglycans and BMPs in vitro and in vivo. Spine. 2004;29: 2603-11.292603 2004 [PubMed][CrossRef]

An HS, Takegami K, Kamada H, Nguyen CM, Thonar EJ, Singh K, Andersson GB, Masuda K. Intradiscal administration of osteogenic protein-1 increases intervertebral disc height and proteoglycan content in the nucleus pulposus in normal adolescent rabbits. Spine. 2005;30: 25-31.3025 2005 [PubMed]

Masuda K, Imai Y, Okuma M, Muehlman C, Nakagawa K, Akeda K, Thonar E, Andersson G, An H. Osteogenic protein-1 (OP-1) injection into a degenerated disc induces the restoration of disc height and structural changes in the rabbit annular puncture model. Spine. In press.

Moon SH, Gilbertson LG, Nishida K, Knaub M, Muzzonigro T, Robbins PD, Evans CH, Kang JD. Human intervertebral disc cells are genetically modifiable by adenovirus-mediated gene transfer: implications for the clinical management of intervertebral disc disorders. Spine. 2000;25: 2573-9.252573 2000 [PubMed][CrossRef]

Wallach CJ, Sobajima S, Watanabe Y, Kim JS, Georgescu HI, Robbins P, Gilbertson LG, Kang JD. Gene transfer of the catabolic inhibitor TIMP-1 increases measured proteoglycans in cells from degenerated human intervertebral discs. Spine. 2003;28: 2331-7.282331 2003 [PubMed][CrossRef]

Yang Y, Nunes FA, Berencsi K, Furth EE, Gonczol E, Wilson JM. Cellular immunity to viral antigens limits E1-deleted adenoviruses for gene therapy. Proc Natl Acad Sci USA. 1994;91: 4407-11.914407 1994 [PubMed][CrossRef]

Risbud MV, Albert TJ, Guttapalli A, Vresilovic EJ, Hillibrand AS, Vaccaro AR, Shapiro IM. Differentiation of mesenchymal stem cells towards a nucleus pulposus-like phenotype in vitro: implications for cell-based transplantation therapy. Spine. 2004;29: 2627-32.292627 2004 [PubMed][CrossRef]

Topics

gene therapy ; intervertebral disc ; animal model ; degenerative disc disease ; tissue degeneration

Username

Password

Access for Patients

Forgot Password?

Have a subscription to the print edition?

Not a Subscriber?

Buy this Article

Get online access for 30 days for $35

New to JBJS?

Subscribe to the online edition for 30 days for $75

Register for a FREE limited account to get full access to all CME activities, to comment on public articles, or to sign up for alerts.

Have a subscription to the print edition?

Current subscribers to The Journal of Bone & Joint Surgery in either the print or quarterly DVD formats receive free online access to JBJS.org.

Activate your online account now

Forgot your password?

Enter your username and email address. We'll send you a reminder to the email address on record.

Username:*

Email Address:*

Forgot your username or need assistance? Please contact customer service at subs@jbjs.org. If your access is provided
by your institution, please contact you librarian or administrator for username and password information. Institutional
administrators, to reset your institution's master username or password, please contact subs@jbjs.org

References

Accreditation Statement

These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.

CME Activities Associated with This Article

Submit a Comment

Please read the other comments before you post yours. Contributors must reveal any conflict of interest.
Comments are moderated and will appear on the site at the discretion of JBJS editorial staff.

* = Required Field

Comment Author(s) * (if multiple authors, separate names by comma)

Example: John Doe

Affiliation & Institution*

Comment Title*

Comment*

Cancel

Print
PDF
Email

Recipient(s) will receive an email with a link (good for 5 days) to 'Biologic Modification of Animal Models of Intervertebral Disc Degeneration'.
Your Name:*
Example: John Doe

Email Address:* CC Me:
Enter your valid email address. Example: jdoe@example.com

Recipient's Email Address:*

Separate multiple email address with semi-colons (up to 5).

Subject:* 's The Journal of Bone & Joint Surgery: 'Biologic Modification of Animal Models of Intervertebral Disc Degeneration'
Subject for your email.

Message:

(Optional, message will truncate at 1000 characters)

Processing your request... Please Wait...

Copyright © in the material you requested is held by The Journal of Bone & Joint Surgery, Inc. (unless otherwise noted). This email ability is provided as a courtesy, and by using it you agree that that you are requesting the material solely for personal, non-commercial use, and that it is subject to Journal of Bone & Joint Surgery, Inc.'s Terms of Use. The information provided in order to email this topic will not be used to send unsolicited email, nor will it be furnished to third parties. Please refer to The Journal of Bone & Joint Surgery Inc.'s Privacy Policy for further information.

Copyright © The Journal of Bone & Joint Surgery Inc. All rights reserved.
Share
Get Citation

James W. LarsonIII, Eric A. Levicoff, Lars G. Gilbertson, James D. Kang; Biologic Modification of Animal Models of Intervertebral Disc Degeneration. The Journal of Bone & Joint Surgery. 2006 Apr;88(suppl_2):83-87.

Download citation file:

RIS (Zotero)

EndNote

BibTex

Medlars

ProCite

RefWorks

Reference Manager

Copyright © The Journal of Bone and Joint Surgery, Inc.
Rights & Permissions
Article Alerts

Processing your request... Please Wait.
Submit a Comment