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Intervertebral Disk Degeneration
and Emerging Biologic
Treatments
Abstract
Although understanding of the biologic basis of intervertebral disk
(IVD) degeneration is rapidly advancing, the unique IVD
environment presents challenges to the development and delivery
of biologic treatments. Acceleration of cellular senescence and
apoptosis in degenerative IVDs and the depletion of matrix proteins
have prompted the development of treatments based on replacing
IVD cells using various cell sources. However, this strategy has not
been tested in animal models. IVD degeneration and associated
pain have led to interest in pathologic innervation of the IVD and
ultimately to the development of percutaneous devices to ablate
afferent nerve endings in the posterior annulus. Degeneration leads
to changes in the expression of matrix protein, cytokines, and
proteinases. Injection of growth factors and mitogens may help
overcome these degenerative changes in IVD phenotype, and
these potential treatments are being explored in animal studies.
Gene therapy is an elegant method to address changes in protein
expression, but efforts to apply this technology to IVD degeneration
are still at early stages.
Intervertebral disk (IVD) degenerationis a complex process that was recog-
nized as a clinical entity prior to the elu-
cidation of the underlying molecular
pathophysiology. Traditional surgical
management of low back pain, which
is often attributed to IVD degeneration,
has focused on eliminating motion
through arthrodesis. More recent sur-
gical alternatives include nucleus re-
placement and motion-sparing im-
plants. The clinical results of these
procedures are suboptimal, however.
Often, despite technical success, clini-
cal failure is noted related to difficulty
in determining the source of pain and/or
the morbidity associated with spinal
surgery.
Research into the molecular basis
of degenerative disk disease has
greatly increased understanding of
the biology underlying this complex
process. Much remains to be learned,
but the degenerative cascade is
known to encompass disruption of
normal extracellular matrix (ECM)
protein synthesis as well as increased
production of inflammatory cyto-
kines and degradative enzymes.
These molecular degenerative path-
ways may offer targets for therapeu-
tic intervention to slow or reverse
disk degeneration. Potential biologic
options for the management of IVD
degeneration include disk cell reim-
plantation, stem cell implantation,
disk denervation, protein injection,
and gene therapy (Table 1).
Christopher K. Kepler, MD,
MBA
D. Greg Anderson, MD
Chadi Tannoury, MD
Ravi K. Ponnappan, MD
From the Department of Orthopedic
Surgery, Thomas Jefferson
University Hospital, Philadelphia,
PA.
Dr. Anderson or an immediate family
member has received royalties from
and is a member of a speakers’
bureau or has made paid
presentations on behalf of DePuy
and Medtronic; serves as a paid
consultant to DePuy, Medtronic,
Synthes, Seaspine, and Globus
Medical; has received research or
institutional support from DePuy;
and serves as a board member,
owner, officer, or committee member
of the Cervical Spine Research
Society. Dr. Ponnappan or an
immediate family member serves as
a paid consultant to DePuy and
serves as an unpaid consultant to
Biomet. Neither of the following
authors nor any immediate family
member has received anything of
value from or owns stock in a
commercial company or institution
related directly or indirectly to the
subject of this article: Dr. Kepler and
Dr. Tannoury.
J Am Acad Orthop Surg 2011;19:
543-553
Copyright 2011 by the American
Academy of Orthopaedic Surgeons.
Review Article
September 2011, Vol 19, No 9 543
Anatomy and
Biomechanics
The IVD is a two-part structure com-
posed of a tough outer portion (ie,
anulus fibrosus [AF]) and an amor-
phous central core (ie, nucleus pul-
posus [NP]) (Figure 1). Cells in the
AF are fibroblast-like, with elon-
gated nuclei aligned with robust col-
lagen fibrils. The ECM in the AF is
dominated by type I collagen and
contains relatively low amounts of
proteoglycan and water. Moving
centrally within the AF, cells become
similar to those in the NP, with a
rounded chondrocyte-like shape, and
they produce ECM with abundant
type II collagen and proteoglycan.
Collagen content in the AF makes up
approximately 60% of dry weight,
and proteoglycans account for ap-
proximately 25%.14,15 ECM in the
NP is an amorphous arrangement of
type II collagen. It has a high proteo-
glycan concentration with roughly
inverse proportions of collagen and
proteoglycan compared with AF. Ag-
grecan is the most common proteo-
glycan and accounts for approxi-
mately 70% of the dry weight of
NP.14 Lower concentrations are seen
of type XI collagen, which assembles
type II collagen fibers, and type IX
collagen, which cross-links collagen
fibrils.
The biomechanical properties of
the normal IVD are closely related to
disk hydration. Aggrecan produces
an osmotic gradient within the disk
through the negative molecular
charge associated with disaccharide
side chains, which draw and retain
water molecules. The hydrated disk
maintains a swelling pressure, pro-
viding its characteristic viscoelastic-
ity and resistance to compressive
loads. IVD height reflects a balance
between NP swelling pressure and
AF tensile strength, which converts
the axial compression of the NP into
AF hoop stresses. Over time, the disk
loses water content, resulting in des-
iccation and inferior load-bearing
characteristics; this, in turn, may ac-
celerate degenerative changes on
MRI (Figure 2) and may result in
symptoms of IVD degeneration.
Cell-based Therapies to
Address Cell Senescence
and Decreased
Extracellular Matrix
Production
Cellular senescence was originally
defined as the period during which a
cell stops dividing. Although cellular
senescence is a natural part of cellu-
lar aging both in vivo and in vitro,
studies of age-matched cells from
healthy and degenerative disks have
provided evidence to suggest that se-
nescence is accelerated in degenera-
tive disks.17 The failure of a cell to
replicate may be caused by either
replicative senescence (RS) or stress-
induced premature senescence (SIPS).
Telomeres are repetitive sequences
of DNA at the end of each chromo-
some, portions of which are lost with
replication. Telomeres do not encode
Photomicrograph of a rabbit
intervertebral disk showing the four
regions of the disk: outer anulus
fibrosus (AF), inner AF (*),
transition zone (arrowhead), and
nucleus pulposus (NP). (Adapted
with permission from Walker MH,
Anderson DG: Molecular basis of
intervertebral disc degeneration.
Spine J 2004;4[6 suppl]:158S-
166S.)
Figure 1
Table 1
Summary of Prospects for Biologic Treatment of Disk Degeneration
Treatment Strategy Strength of Scientific Basis Progress Toward Clinical Application
Disk cell reimplantation Strong: Survival >1 y in canine models with nor-
mal extracellular matrix production1,2
Euro Disc Trial: Expansion and reimplantation of
cells harvested during diskectomy2
Stem cell implantation Weak: Mixed results in trials in larger animals3,4 No systematic trials to date
Painful disk denervation Weak: Unclear if devices generate enough heat
to denervate the IVD5,6
Poor evidence of efficacy. RCTs have shown little or
no benefit over placebo.
Injection of therapeutic
proteins
Moderate: BMP-7 and GDF-5 have shown
promise in rodent models7-9
Separate clinical trials are under way to test the ef-
fect of single BMP-7 and GDF-5 injections on IVD
degeneration
Gene therapy Moderate: Some success in rodent models, but
vector not yet optimized10-13
Past gene therapy attempts have failed, and con-
cern for infection has precluded human trials
BMP = bone morphogenetic protein, GDF = growthdifferentiation factor, IVD = intervertebral disk, RCT = randomized clinical trial
Intervertebral Disk Degeneration and Emerging Biologic Treatments
544 Journal of the American Academy of Orthopaedic Surgeons
genes; rather, they ensure that the se-
quence lost during replication is a
nonessential, noncoding sequence.
Once the safety margin provided by
a telomere is consumed, gene coding
regions of the chromosome are no
longer protected during replication,
leading to progressive damage to the
chromosome. Loss of telomeres re-
sults in RS. If enough damage accu-
mulates and the cell cannot make re-
pairs to vital genes, the cell may stop
replicating but remain metabolically
active.
The etiology of SIPS involves the ac-
cumulation of irreparable damage to
DNA sequences from stimuli such as
mechanical injury and/or inflammatory
cytokines, which disrupts the replica-
tion process. The harsh biomechanical
and biochemical environment of the
IVD has led to speculation that SIPS,
not RS, is responsible for most senes-
cence within human disk cells.
Aging and degeneration are associ-
ated with changes in cellular activity,
including alterations in the makeup
and levels of ECM proteins and
proteoglycans18-22 and increasing
acidity. In early degeneration, mito-
gens are upregulated as the cell at-
tempts to combat progressive ECM
loss before synthesis of collagen
types II and IX decreases at an accel-
erating rate. The declining matrix
synthesis is compounded by reduced
collagen cross-linking, which may
further weaken the structural integ-
rity of the nucleus ECM.23 Con-
versely, collagen types I and X are
upregulated in more advanced stages
of disk degeneration (Figure 3). This
may account for NP fibrosis and the
loss of delineation between the AF
and the NP observed in the latter
stages of degeneration.
The normal IVD is hypocellular,
which amplifies the effect of in-
creased senescence and declining ma-
trix production associated with disk
degeneration. Methods devised to
enhance disk cellularity as a thera-
Progressive appearance of disk degeneration as demonstrated on T2-
weighted MRI and categorized according to Pfirrmann et al.16 The high disk
signal found in grades I and II (A) begins to decrease as disk desiccation
occurs and becomes heterogeneous (grade III [B]). C, Grade IV is character-
ized by low T2-weighted signal with or without loss of disk space height.
D, Grade V. The distinction between the NP and AF has been lost, the disk
space has collapsed, and end plate changes are evident.
Figure 2
Schematic illustration demonstrating characteristic molecular changes of
intervertebral disk degeneration accompanied by neurovascular ingrowth into
the posterior anulus. A, Healthy disk. B, Degenerated disk with increased
collagen type I, decreased collagen type II, decreased aggrecan content,
decreased water content, increased matrix metalloproteinase, increased
interleukin (IL)-1 and -8, and increased transforming growth factor-α.
Figure 3
Christopher K. Kepler, MD, MBA, et al
September 2011, Vol 19, No 9 545
peutic approach for disk degenera-
tion include in vitro expansion and
subsequent reimplantation of disk
tissue, mesenchymal stem cell (MSC)
implantation, and injection of
platelet-rich plasma (PRP) into the
disk. Success with these strategies is
challenged by the harsh avascular
nature of the disk, which limits nu-
tritional support and clearance of
waste products from transplanted
cells, creating an acidic environment
(Figure 4) (Table 2).
Disk Tissue Reimplantation
Animal Studies
Nishimura and Mochida24 were the
first to report the reimplantation of
autologous disk tissue in vivo. They
harvested disk tissue from rats via
percutaneous nucleotomy. This tissue
was later reimplanted into adjacent
degenerative disks induced by needle
puncture. Compared with control
disks injected with silicon jelly, disks
treated with autologous disk tissue
demonstrated less radiographic and
histologic evidence of degeneration.
Greater synthesis of type II collagen
was seen with early cell reimplanta-
tion after nucleotomy. The same
group of investigators reported suc-
cess with allograft cells in a rabbit
model.25 These rodent models em-
phasize the importance of cell reim-
plantation accompanied by native
disk ECM.25 In another study, im-
proved results were noted when cells
were implanted along with IVD ma-
trix; cells were viable for ≤8
months.26 Cell survival >1 year after
reimplantation and continued pro-
duction of a normal-appearing ECM
have been reported in canine mod-
els.1,2 Recent characterization of
novel hydrogels may lead to the de-
velopment of a carrier that could be
used to reintroduce cells to the disk
after in vivo expansion, thereby pro-
viding a scaffold on which the cells
would grow until they produced a
functional ECM.27
Human Studies
In 2002, a clinical trial was begun to
explore the feasibility of ex vivo IVD
cell expansion in humans.2 In the
Euro Disc Randomized Trial, disk
cells were harvested at the time of
single-level diskectomy for symptom-
atic herniated nucleus pulposus. The
Graphic representation and photograph of nutrient gradients across the disk
nucleus end plate–end plate. A, Oxygen and glucose concentrations fall and
lactic acid concentrations rise toward the center of the nucleus, creating an
acidic environment. B, Sagittal section through a human lumbar disk
demonstrating the dimensions of the disk and the direction of the gradient
shown in panel A. (Adapted with permission from Urban JP, Smith S,
Fairbank JC: Nutrition of the intervertebral disc. Spine [Phila Pa 1976]
2004;29[23]:2700-2709.)
Figure 4
Table 2
Challenging Biochemical Characteristics of the Degenerative Intervertebral Disk
IVD Characteristic
IVD Components
Affected Therapeutic Challenge
Avascular Disk cells Tenuous nutritional support for therapies that rely on implantation of cells.
Implantation of cells would increase lactic acid, which is cleared slowly, and it
would decrease pH.
ECM Cannot use intravascular delivery of agents to modulate cytokine or growth
factor signaling
Relatively acellular ECM Synthetic burden of ECM maintenance is susceptible to increases in apoptosis
rate
Acidic Disk cells Acidity decreases production of ECM and would decrease the effectiveness of
implanted cells
ECM Acidity does not affect MMP activity but does enhance the activity of
cathepsins
ECM = extracellular matrix, IVD = intervertebral disk, MMP = matrix metalloproteinase
Intervertebral Disk Degeneration and Emerging Biologic Treatments
546 Journal of the American Academy of Orthopaedic Surgeons
investigators assumed that removal
of disk tissue compromises the bio-
mechanical function of the disk.
Thus, they sought to “accentuate
disc metabolism” by ex vivo cell cul-
ture and expansion with reimplanta-
tion at 12 weeks after the primary
procedure to allow healing of the AF
herniation defect. The methods used
to accentuate metabolism and the
matrix in which the cells were rein-
troduced into the disk were not pub-
lished in early reports. Data from the
study were most recently published
in 2008.2 Twenty-eight patients were
evaluated at 2-year follow-up. The
16 control patients were treated with
sequestrectomy alone, and 12 pa-
tients were treated with sequestrec-
tomy and autologous IVD cell re-
implantation. Patients treated with
sequestrectomy and autologous IVD
cell reimplantation demonstrated a
trend toward greater improvement in
Oswestry Disability Index (ODI)
scores and other clinical indicators as
well as degree of disk desiccation on
MRI. These data must be cautiously
interpreted because of the small sam-
ple size and the lack of patient blind-
ing in the trial.
Stem Cell Implantation
Stem cells are increasingly used as a
source of multipotent cells in bio-medical applications, and they have
been considered to be a possible ther-
apeutic solution in persons with IVD
degeneration. Promising findings re-
lated to cell viability and ECM pro-
duction following implantation have
been reported in animal models. One
study documented the differentiation
of MSCs into cells similar to IVD
cells after exposure to a hypoxic,
disk-like environment.28 Others have
investigated the viability of MSCs as
a cell source for IVD repopula-
tion.29,30 However, intradiscal im-
plantation of MSCs must be studied
in larger animal models to confirm
that longer nutrient diffusion dis-
tances are not a barrier to cell sur-
vival.
Recent experiments exploring
MSC viability and metabolism in
Göttingen minipigs have shown
mixed results. One study reported
early loss of nearly all of the trans-
planted MSCs.3 Another reported
cell survival and IVD-like matrix
production by human xenograft
MSCs at 6 months postimplanta-
tion.4 The authors of the first study
suggested that the nutritional re-
quirements of the transplanted cells
were not adequately met by the
harsh degenerative environment of
the disk.3 They recommended trans-
plantation of fewer cells to decrease
the nutritional demands. Although
Göttingen minipigs are larger than
rabbits, they may not be large
enough to satisfy questions about
nutritional support in a larger IVD,
especially given the variable results.
One case report has been published
describing the use of MSCs in the
management of human IVD degener-
ation.31 The authors of the report
pointed to improvement in disk sig-
nal and vacuum phenomenon on se-
rial magnetic resonance images as in-
dicators of successful treatment in
two patients. However, they did not
report any objective measures of
clinical outcome.
Platelet-rich Plasma
PRP has received attention recently
as a readily available source of autol-
ogous multipotent stem cells. Al-
though there is little clinical evidence
of success, PRP has been employed
in several musculoskeletal applica-
tions with the goal of using the stem
cells to differentiate into and repair
the tissue in question. In a rabbit
model, intradiscal injection of PRP
with a hydrogel carrier demonstrated
less severe degenerative changes on
histology and increased staining for
proteoglycan compared with control
disks at 8-week follow-up.32 Subse-
quent investigation using the same
model found improved disk charac-
teristics on MRI, upregulated expres-
sion of collagen II and proteoglycan
core protein, and a decreased pro-
portion of apoptotic cells compared
with control disks.33
Disk Innervation
Innervation of the normal disk is
limited to the outermost layers of the
AF, which is supplied dorsally by the
sinuvertebral nerve, a branch off the
ventral ramus. Neurovascular in-
growth into degenerative disks is
consistently noted on histology.34 Ex-
pression of pain mediator substance
P suggests that these nerves are noci-
ceptors (Figure 3). Aggrecan acts as a
nerve ingrowth inhibitor in vitro.
Aggrecan from the AF slows neurite
growth more than does aggrecan
taken from the NP.35 Vascular in-
growth accompanies neural in-
growth, and endothelial cells interact
with aggrecan in a manner similar to
that of neurites; aggrecan from
healthy IVDs strongly inhibits en-
dothelial growth, whereas deglyco-
sylated aggrecan loses inhibitory po-
tency. Neural and vascular ingrowth
are also linked through the expres-
sion of nerve growth factor (NGF)
by the capillaries that accompany
neurites. Neurites express trk-A (ty-
rosine kinase receptor type 1), a
high-affinity receptor for NGF that
triggers intracellular signals to en-
courage nerve growth and survival.36
The expression of NGF exclusively
in painful disks despite histologic ev-
idence of vascular ingrowth in both
painful and nonpainful degenerative
disks suggests a potential association
between expression of NGF and the
development of symptoms.36 Brain-
derived growth factor also encour-
ages neuronal ingrowth; it plays a
Christopher K. Kepler, MD, MBA, et al
September 2011, Vol 19, No 9 547
role similar to that of NGF in the
differentiation and survival of noci-
ceptors within the disk. Brain-
derived growth factor is produced by
disk cells themselves, and elevated
levels have been correlated with in-
creasing disk degeneration.37
The relationship between radio-
graphic evidence of IVD degenera-
tion and associated symptoms is not
well understood, as demonstrated by
a high prevalence of degenerative
changes in the asymptomatic popula-
tion. Thus, a successful clinical out-
come might be achieved by reducing
or eliminating the source of pain,
without addressing disk degenera-
tion. Intradiscal electrothermal ther-
apy (IDET) and intradiscal radiofre-
quency thermocoagulation (IRFT)
are used in the management of symp-
tomatic IVD degeneration. Probes
are placed percutaneously to pro-
duce a controlled thermal injury.
Proposed mechanisms of action in-
clude denervation of a painful IVD
or thermal shrinkage of collagen fi-
bers and resultant stiffening of the
IVD. The probe is positioned to pri-
marily affect the posterior portion of
the IVD, where the posterior annulus
is innervated. Discrepant informa-
tion regarding whether these devices
can achieve the high temperatures
necessary to ablate nociceptive
nerves and denature collagen has led
to skepticism regarding their efficacy.
Intradiscal Electrothermic
Therapy
Two randomized clinical trials of
IDET have been performed. Both
used diskography to identify the
treated disk as a primary pain gener-
ator. Freeman et al38 found no differ-
ence in clinical outcomes between
patients treated with IDET and those
treated with a sham procedure (38
and 19, respectively). Neither group
achieved significant benefit. Pauza et
al39 reported modest improvement in
ODI, Medical Outcomes Study 36-
Item Short Form, and visual analog
scale (VAS) scores at 6-month
follow-up in patients who underwent
IDET compared with those treated
with a sham procedure (37 and 27,
respectively). This study has been
criticized for its highly selective in-
clusion criteria, which limited the
study to 64 patients from a pool of
1,360 eligible patients.
Intradiscal Radiofrequency
Thermocoagulation
IRFT also has been studied in two
randomized controlled trials. Bar-
endse et al5 compared ablation and a
sham procedure and found no differ-
ence based on ODI and VAS scores.
Only one patient in the treatment
group had a successful outcome. Er-
çelen et al6 compared two durations
of treatment (120 and 360 seconds).
Significant improvement in VAS was
noted at early time points (P < 0.05).
However, at 6-month follow-up no
differences were found between
groups, and scores returned to base-
line levels. The authors of both stud-
ies questioned whether IRFT can
generate sufficiently high tempera-
tures to ablate neurons and eliminate
pain. Based on the best available
studies to date, IDET and IRFT seem
to have little clinical advantage over
sham procedures.
Changes in Protein
Expression in the
Degenerative
Intervertebral Disk
Degradative Enzyme
Upregulation
Degenerative IVD cells decrease
growth factor production and increase
synthesis of degradative enzymes. Mi-
togenic factors such as platelet-derived
growth factor,40 insulin-like growth
factor-1,41 and basic fibroblast
growth factor42,43 decrease with pro-
gressive degeneration. Matrix metal-
loproteinases (MMPs) are extracel-
lular zinc-dependent degradative
proteinases produced in a latent
form and later activated, often by
other MMPs. MMP expression has
been well-characterized in other or-
gans in which these molecules regu-
late ECM turnover. MMP-1, -8, and
-13 (collagenases); MMP-2 and -9,
which degrade denatured collagen;
and MMP-3, which degrades non-
collagen matrix proteins,are all up-
regulated in disk degeneration.44-46
This suggests that MMPs contribute
to ECM degradation and accelera-
tion of degeneration. MMPs directly
damage the ECM and indirectly con-
tribute to degeneration via activation
of latent degradative enzymes.
The ADAMTS (a disintegrin and
metalloprotease with thrombospondin-
like repeat) enzyme family (ie, aggreca-
nase) and cathepsins are two other
classes of proteases that are upregu-
lated in degenerative disks.47 Al-
though MMPs have garnered more
attention for their association with
the degenerative ECM in IVDs,
cathepsins function ideally in this mi-
lieu because their peak enzymatic ac-
tivity coincides with the slightly
acidic environment. This is another
feature of the IVD that complicates
efforts to reverse or slow degenera-
tive changes through bolstering
ECM production.
Inflammatory Cytokine
Upregulation
Cytokines are small protein-signaling
molecules found throughout the
body that are involved in inflamma-
tion and pain pathways. This inflam-
matory role is important within the
degenerative disk because cytokines
contribute to the acceleration of disk
degeneration by promoting the de-
generative phenotype (Figure 2).
Interleukin-1 (IL-1) is the proto-
Intervertebral Disk Degeneration and Emerging Biologic Treatments
548 Journal of the American Academy of Orthopaedic Surgeons
typic inflammatory cytokine within
the disk. IL-1 inhibits ECM pro-
duction,48 upregulates degradative
enzymes,48-50 contributes to a positive
feedback loop that increases cy-
tokine production,50 and sensitizes
IVD cells to other apoptosis triggers;
however, it does not trigger apopto-
sis itself51,52 (Figure 5). IL-1 is ex-
pressed in normal disks, but balance
between an activating receptor (IL-1
receptor type 1) and an inhibiting re-
ceptor (IL-1 receptor antagonist) cur-
tail its actions in the normal disk;
this mechanism becomes unbalanced
during degeneration.48
Although tissue necrosis factor-α
(TNF-α) has catabolic actions simi-
lar to IL-1,53 its larger role within the
disk is likely related to its participa-
tion in nociception.54 An association
between TNF-α and spine pain was
deduced through study of the irritat-
ing effect of herniated NP on nerve
roots.55 This effect was later found to
be mediated in part by TNF-α.
TNF-α upregulation in the degenera-
tive IVD46 may contribute to disco-
genic pain, given the evidence of
nerve ingrowth into degenerative
IVDs34 and the noxious stimulus to
afferent nerves provided by TNF-α.
The role of other cytokines (eg,
IL-6, IL-8) has been investigated in
the degenerative IVD,56-58 but the bi-
ologic function of these cytokines
and their contribution to disk degen-
eration are not well understood. Al-
though much remains to be learned
about their function in the IVD, cy-
tokines are an attractive therapeutic
target because their established role
in nociception suggests that blocking
the cytokine cascades may directly
improve symptoms associated with
IVD degeneration.
Management
Protein-based Therapy
Growth factors, mitogens, and cy-
tokines play known roles in disk de-
generation. In persons with degener-
ative IVD, preparations containing
these agents or their inhibitors can
be injected directly into the disk.
This strategy is limited by the short-
lived nature of proteins in the body.
However, the simplicity of this ap-
proach is attractive, and many in vi-
tro and in vivo experiments have
been done in animal models.
The mitogenic capabilities of bone
morphogenetic proteins (BMPs) in
stimulation of IVD cells has been
studied both in vitro and in vivo.
Early investigation by An et al7 using
an in vivo rabbit model established
the ability of BMP-7 (ie, osteogenic
protein-1) to preserve intervertebral
height and increase production of
proteoglycan 2 weeks after injection.
Subsequent studies confirmed main-
tenance of disk height in rat59 and
rabbit models.60,61 BMP-7 injection
resulted in substantial increases in
proteoglycan content61 and biome-
chanical disk function compared
with control disks.60 Chubinskaya
et al8 studied the effect of BMP-7 on
expression of a wide variety of rele-
vant gene products in an in vivo rat
model. In addition to upregulating
matrix proteins and triggering devel-
opment of a more normal disk mor-
phology compared with degenerative
controls, BMP-7 decreased produc-
tion of aggrecanase, MMP-13, sub-
stance P, TNF-α, and IL-1β. These
results strengthen the case for the
clinical use of BMP-7 to halt or re-
verse IVD degeneration. Further
study of these factors is required.
Growth and differentiation
factor-5 (GDF-5) was first shown to
upregulate collagen type II and ag-
Schematic illustration of a positive feedback loop of interleukin (IL)-1β in
intervertebral disk herniation (IDH) cells. Endogenous IL-1β was produced
from the IDH cells in response to exogenous IL-1β, which affected the cells
of IDH in a way similar to exogenous IL-1β. This positive feedback loop of
IL-1β upregulated the production of IL-6, cyclooxygenase (COX)-2, and
matrix metalloproteinase (MMP)-1 and MMP-3, resulting in inflammation,
neurologic pain, and disk degeneration. PG E2 = prostaglandin E2. (Adapted
with permission from Jimbo K, Park JS, Yokosuka K, Sato K, Nagata K:
Positive feedback loop of interleukin-1beta upregulating production of
inflammatory mediators in human intervertebral disc cells in vitro. J
Neurosurg Spine 2005;2[5]:589-595.)
Figure 5
Christopher K. Kepler, MD, MBA, et al
September 2011, Vol 19, No 9 549
grecan and to downregulate MMP-3
in in vitro models.10 Management of
IVDs with a single GDF-5 injection
in a murine model resulted in cellular
proliferation and increased disk
height.62 Disks managed with multi-
ple injections did not benefit. In-
stead, they demonstrated an inflam-
matory reaction with connective
tissue infiltration and disk collapse;
this reaction was suspected to be re-
lated to multiple annular punctures.
Chujo et al9 found that rabbit disks
treated with recombinant GDP-5
maintained disk height and signal on
MRI and had improved histologic
grading compared with control disks
16 weeks after annular puncture.
Inflammatory cytokine cascades
may be used to block key proinflam-
matory mediators. Cytokine antago-
nists that disrupt inflammatory cas-
cades may prevent disk degeneration
and reduce associated pain by block-
ing nociceptive actions of cytokines
such as TNF-α. This strategy was in-
vestigated in vitro by Genevay et al,63
using recombinant IL-1ra and a
monoclonal antibody to TNF. Both
treatments decreased production of
MMP-3 in IVD cells isolated from
herniated human disks. Although
this line of investigation shows
promise, agents that block cytokine
function have not yet been suffi-
ciently characterized for use in hu-
mans.
No human trial of the management
of IVDs using proteins related to the
degenerative process has been pub-
lished. One clinical trial is under way
to define the therapeutic benefits of
BMP-7 in patients with IVD degener-
ation, but no data have been re-
leased. A phase I/II trial has been ini-
tiated to explore the use of GDF-5
injections at eight sites around the
United States. These studies will be
carefully watched to discern the po-
tential success of protein-based IVD
injections.
Gene Therapy
In theory, gene therapy is an elegant
solution to the ephemeral nature of
injected proteins. Whereas proteins
induce changes in cellular activity
during a few half-lives before becom-
ing degraded, gene therapy adds the
nucleic acid sequence coding for the
gene of interest along with a pro-
moter sequence into the cellular ge-
nome, thereby continuing protein
production far beyond the half-life of
the protein alone.
A gene can be introduced into a
cellular genome in several ways, in-
cluding via aviral vector that infects
the cell and inserts its own genome,
which carries the exogenous gene
and its promoter, into the cell ge-
nome. Viruses such as retrovirus,
adeno-associated virus, baculovirus,
and adenovirus, have been used. Ad-
enovirus is the most common, but
it is also the most virulent/
immunogenic. Although these vi-
ruses are relatively benign with re-
spect to the potential for causing
disease, cells with genes introduced
via a viral vector typically have de-
clining protein expression over time
as the immune system detects the
presence of the virus. Alternative
gene transfer methods include the
use of plasmids carrying the gene
and use of a gene gun. Although
these nonviral methods are safer be-
cause they eliminate risk of infection,
they typically have low transfection
rates and insufficient gene incorpora-
tion.
In vitro studies of viral vectors es-
tablished the feasibility of transfer-
ring reporter genes to animal cells in
culture. However, transfection rates
varied widely depending on the vec-
tor used, from 1% with retrovirus to
nearly 100% with adenovirus.64
Early studies have shown that ex-
pression of gene products such as
GDF-510 and BMPs11 can trigger ele-
vated expression of type II collagen
and aggrecan. Using a rabbit model
with adenovirus-mediated transfec-
tion of human transforming growth
factor-β1 (TGF-β1), Nishida et al12
induced a fivefold IVD increase in
TGF-β1 expression and a 100% in-
crease in proteoglycan expression
compared with control disks by 1
week after transfection. Liang et al13
used a mouse IVD degeneration
model to show a protective effect of
overexpression of GDF-5 in the
maintenance of normal disk signal
on MRI and expression of gly-
cosaminoglycans compared with un-
treated degenerating disks.
No in vivo experiments have been
performed in humans. Le Maitre
et al65 delivered IL-1 receptor antag-
onist via an adenovirus vector to hu-
man IVD cells in vitro and reported
significant decreases in MMP expres-
sion at 48 hours resulting from dis-
ruption of the IL-1 cascade. Moon
et al66 transfected human cells with
combinations of three genes sus-
pected of having a role in IVD regen-
eration (TGF-β1, insulin-like growth
factor-1, BMP-2). Increased proteo-
glycan synthesis was noted at 18
hours after viral transduction. When
all three molecules were overex-
pressed simultaneously, proteoglycan
synthesis reached 4.7 times the rate
seen in control cells. Before these
techniques can be used in therapeutic
investigations in humans, many
questions must be addressed with
regard to viral vector safety, the
method of viral introduction (in
vivo versus ex vivo with subsequent
transfected cell implantation), and
whether it may be safer to optimize
less effective but less virulent vectors.
Summary
An understanding of the molecular
basis of IVD degeneration is neces-
sary to design biologic therapies.
Several avenues of treatment appear
promising based on experiments in
Intervertebral Disk Degeneration and Emerging Biologic Treatments
550 Journal of the American Academy of Orthopaedic Surgeons
animal models. Early human trials
have already begun. Considerable
challenges remain, however. The
avascularity of the IVD complicates
delivery of therapeutic proteins
which otherwise could be trans-
ported via the bloodstream. It is not
clear whether a single injection of
therapeutic protein can provide a
stimulus that is sufficiently robust to
rescue a degenerative disk over the
long term. The large, metabolically
challenged, acidic environment of
the human disk provides a limited
supply of nutrients, making it a hos-
tile environment for transplanted
cells. It is also unclear at what stage
in the cascade of degeneration thera-
pies should be applied. Disks in
late-stage degeneration may lack re-
generative potential. Early-stage de-
generation may be asymptomatic
and go untreated. Knowledge re-
garding the complex environment of
the disk is growing rapidly, and
modern cell and molecular biology
are now capable of providing the
tools and techniques to produce tis-
sue repair. Biologic therapies appear
promising as a future treatment op-
tion for persons with IVD degenera-
tion.
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