The presence and distribution of lubricin in the caprine intervertebral disc

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

The mechano-response of murine annulus fibrosus cells to cyclic tensile strain is frequency dependent

The intervertebral disk (IVD) is a composite structure essential for spine stabilization, load bearing, and movement. Biomechanical factors are important contributors to the IVD microenvironment regulating joint homeostasis; however, the cell type-specific effectors of mechanotransduction in the IVD are not fully understood. The current study aimed to determine the effects of cyclic tensile strain (CTS) on annulus fibrosus (AF) cells and identify mechano-sensitive pathways. Using a cell-type specific reporter mouse to differentiation NP and AF cells from the murine IVD, we characterized AF cells in dynamic culture exposed to CTS (6% strain) at specific frequencies (0.1 Hz, 1.0 Hz, or 2.0 Hz). We demonstrate that our culture model maintains the phenotype of primary AF cells and that the bioreactor system delivers uniform biaxial strain across the cell culture surface. We show that exposure of AF cells to CTS induces cytoskeleton reorganization resulting in stress fiber formation, with acute exposure to CTS at 2.0 Hz inducing a significant yet transient increase ERK1/2 pathway activation. Using SYBPR-based qPCR to assess the expression of extracellular matrix (ECM) genes, ECM-remodeling genes, candidate mechano-sensitive genes, inflammatory cytokines and cell surface receptors, we demonstrated that exposure of AF cells to CTS at 0.1 Hz increased Acan, Prg4, Col1a1 and Mmp3 expression. AF cells exposed to CTS at 1.0 Hz showed a significant increase in the expression of Acan, Myc, and Tnfα. Exposure of AF cells to CTS at 2.0 Hz induced a significant increase in Acan, Prg4, Cox2, Myc, Fos, and Tnfα expression. Among the cell surface receptors assessed, AF cells exposed to CTS at 2.0 Hz showed a significant increase in Itgβ1, Itgα5, and Trpv4 expression. Our findings demonstrate that the response of AF cells to CTS is frequency dependent and suggest that mechanical loading may directly contribute to matrix remodeling and the onset of local tissue inflammation in the murine IVD.

Lubricin/proteoglycan 4 increases in both experimental and naturally occurring equine osteoarthritis

OBJECTIVE

The goals of this study were (1) to quantify proteoglycan 4 (PRG4) gene expression; (2) to assess lubricin immunostaining; and (3) to measure synovial fluid lubricin concentrations in clinical and experimental models of equine carpal osteoarthritis (OA).

DESIGN

Lubricin synovial fluid concentrations and cartilage and synovial membrane PRG4 expression were analyzed in research horses undergoing experimental OA induction (n = 8) and in equine clinical patients with carpal OA (n = 58). Lubricin concentrations were measured using a custom sandwich enzyme-linked immunosorbent assay, and PRG4 expression was quantified using qRT-PCR. Lubricin immunostaining was assessed in synovial membrane and osteochondral sections in the experimental model.

RESULTS

Lubricin concentrations increased in synovial fluid following induction of OA, peaking at 21 days post-operatively in OA joints vs sham-operated controls (331 ± 69 μg/mL vs 110 ± 19 μg/mL, P = 0.001). Lubricin concentrations also increased in horses with naturally occurring OA as compared to control joints (152 ± 32 μg/mL vs 68 ± 4 μg/mL, P = 0.003). Synovial membrane PRG4 expression increased nearly 2-fold in naturally occurring OA (P = 0.003), whereas cartilage PRG4 expression decreased 2.5-fold (P = 0.025). Lubricin immunostaining was more pronounced in synovial membrane from OA joints as compared to controls, with intense lubricin localization to sites of cartilage damage.

CONCLUSIONS

Although PRG4 gene expression decreases in OA cartilage, synovial membrane PRG4 expression, synovial fluid lubricin concentrations and lubricin immunostaining all increase in an equine OA model. Lubricin may be elevated to protect joints from post-traumatic OA.

Structure and mechanical function of the inter-lamellar matrix of the annulus fibrosus in the disc

The inter-lamellar matrix (ILM) has an average thickness of less than 30 µm and lies between adjacent lamellae in the annulus fibrosus (AF). The microstructure and composition of the ILM have been studied in various anatomic regions of the disc; however, their contribution to AF mechanical properties and structural integrity is unknown. It was suggested that the ILM components, mainly elastic fibers and cross-bridges, play a role in providing mechanical integrity of the AF. Therefore, the manner in which they respond to different loadings and stabilize adjacent lamellae structure will influence AF tear formation and subsequent herniation. This review paper summarizes the composition, microstructure, and potential role of the ILM in the progression of disc herniation, clarifies the micromechanical properties of the ILM, and proposes critical areas for future studies. There are a number of unknown characteristics of the ILM, such as its mechanical role, impact on AF integrity, and ultrastructure of elastic fibers at the ILM-lamella boundary. Determining these characteristics will provide important information for tissue engineering, repair strategies, and the development of more-physiological computational models to study the initiation and propagation of AF tears that lead to herniation and degeneration. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1307-1315, 2016.

Distribution of proteins within different compartments of tendon varies according to tendon type

Although the predominant function of all tendons is to transfer force from muscle to bone and position the limbs, some tendons additionally function as energy stores, reducing the energetic cost of locomotion. To maximise energy storage and return, energy‐storing tendons need to be more extensible and elastic than tendons with a purely positional function. These properties are conferred in part by a specialisation of a specific compartment of the tendon, the interfascicular matrix, which enables sliding and recoil between adjacent fascicles. However, the composition of the interfascicular matrix is poorly characterised and we therefore tested the hypothesis that the distribution of elastin and proteoglycans differs between energy‐storing and positional tendons, and that protein distribution varies between the fascicular matrix and the interfascicular matrix, with localisation of elastin and lubricin to the interfascicular matrix. Protein distribution in the energy‐storing equine superficial digital flexor tendon and positional common digital extensor tendon was assessed using histology and immunohistochemistry. The results support the hypothesis, demonstrating enrichment of lubricin in the interfascicular matrix in both tendon types, where it is likely to facilitate interfascicular sliding. Elastin was also localised to the interfascicular matrix, specifically in the energy‐storing superficial digital flexor tendon, which may account for the greater elasticity of the interfascicular matrix in this tendon. A differential distribution of proteoglycans was identified between tendon types and regions, which may indicate a distinct role for each of these proteins in tendon. These data provide important advances into fully characterising structure–function relationships within tendon.

Lubricin in human achilles tendon: The evidence of intratendinous sliding motion and shear force in achilles tendon

Achilles tendon is one of the most commonly injured tendons. Mechanical force is regarded as a major causative factor. However, the biomechanics of Achilles tendon and mechanical mechanism of the injuries are unclear. Lubricin expresses at regions exposed to sliding motion and shear force in a number of tissues. This study investigated the distribution and concentration of lubricin in human Achilles tendons for better understanding the biomechanics of Achilles tendon. Achilles tendons were harvested from nine cadavers. Lubricin was extracted from various locations proximal to the calcaneal insertion and quantified with ELISA. The distribution of lubricin was investigated with immunohistochemistry. Lubricin was mainly identified at the interfaces of tendon fascicles, especially in the mid-portion of the tendon. The concentration of lubricin in Achilles tendons varied by individual and the distance from its calcaneal insertion. The distal portion of the tendon had a higher concentration of lubricin than the proximal regions of the tendon. This study suggests the presence of intratendinous sliding motion of fascicles and shear force at interfaces of fascicles in human Achilles tendon. Shear force could be an important mechanical factor for the development of Achilles tendinopathy and rupture.

Lubricin deficiency in the murine lumbar intervertebral disc results in elevated torsional apparent modulus

The purpose of this study was to investigate the mechanical consequences of proteoglycan 4 (Prg4) deficiency on intervertebral disc mechanics using a Prg4 knockout mouse model. Prg4, also called lubricin, was first identified as the boundary lubricant in synovial fluid but has subsequently been localized within a number of musculoskeletal tissues in areas subjected to shear and tensile stresses, including the intervertebral disc. The function of lubricin in the intervertebral disc has not been determined. Lumbar level 1-2 vertebral body-disc-vertebral body motion segments were isolated from Prg4 null mice and wild type (WT) litter mate controls. Disc dimensions were measured and motion segments were tested in axial loading and torsion. Torque measurements and disc dimensions were used to calculate the torsional apparent modulus for discs from Prg4 null and WT discs. Discs from Prg4 null mice had a significantly smaller mean transverse disc area (p=0.0057), with a significantly larger proportion of this area occupied by the nucleus pulposus (p<0.0001), compared to WT specimens. Apparent torsional moduli were found to be elevated in Prg4 null lumbar discs compared to WT controls at 10-10° (p=0.0048) and 10-30° (p=0.0127) rotation. This study suggests a functional role for Prg4 in the murine intervertebral disc. The absence of Prg4 was associated with an increased apparent torsional modulus and the structural consequences of Prg4 deficiency in the intervertebral disc, with expansion of the area of the nucleus pulposus relative to the transverse disc area in Prg4 null specimens.

Basement membrane molecule expression attendant to chondrogenesis by nucleus pulposus cells and mesenchymal stem cells

Bone marrow-derived mesenchymal stem cells (MSCs) represent an autologous cell source for nucleus pulposus (NP) tissue engineering and regeneration. Although studies have demonstrated the ability of MSCs to differentiate to NP-like chondrocytic cells, few have comparatively studied the matrix synthesis and composition of the cartilaginous tissue formed in vitro from both cell types, particularly with respect to the expression of basement membrane (BM) molecules. The objective of this study was to evaluate chondrogenesis and expression of BM molecules, laminin and type IV collagen, in monolayer and in pellet cultures of caprine NP cells and MSCs. Both cell types demonstrated comparable levels of chondrogenesis, indicated by the percentage of chondrocytic cells, and the amounts of glycosaminoglycan and type II collagen. Laminin and type IV collagen were expressed intracellularly by NP cells and MSCs cultured in monolayer. During chondrogenesis in pellet cultures, the deposition of BM molecules in NP and MSC pellets followed an orderly spatiotemporal shift in pattern from a diffuse territorial and interterritorial distribution to a defined pericellular localization, as seen in normal adult NP. These results inform the use of MSCs for NP regeneration and suggest the possible involvement of certain BM molecules in chondrogenesis and cartilage regeneration.

Effects of stress deprivation on lubricin synthesis and gliding of flexor tendons in a canine model in vivo

BACKGROUND

Lubricin facilitates boundary lubrication of cartilage. The synthesis of lubricin in cartilage is regulated by mechanical stimuli, especially shear force. Lubricin is also found in flexor tendons. However, little is known about the effect of mechanical loading on lubricin synthesis in tendons or about the function of lubricin in flexor tendons. The purpose of this study was to investigate the relationship of mechanical loading to lubricin expression and gliding resistance of flexor tendons.

METHODS

Flexor tendons were harvested from canine forepaws that had been suspended without weight-bearing for twenty-one days and from the contralateral forepaws that had been allowed free motion. Lubricin expression in each flexor tendon was investigated with real-time RT-PCR (reverse transcription polymerase chain reaction) and immunohistochemistry. Lubricin in the flexor tendon was extracted and quantified with ELISA (enzyme-linked immunosorbent assay). The friction between the flexor tendon and the proximal pulley was measured.

RESULTS

The non-weight-bearing flexor tendons had a 40% reduction of lubricin expression (p < 0.01) and content (p < 0.01) compared with the flexor tendons in the contralateral limb. However, the gliding resistance of the tendons in the non-weight-bearing limb was the same as that of the tendons on the contralateral, weight-bearing side.

CONCLUSIONS

Mechanical loading affected lubricin expression in flexor tendons, resulting in a 40% reduction of lubricin content, but these changes did not affect the gliding resistance of the flexor tendons.

Acute injury affects lubricin expression in knee menisci: an immunohistochemical study

The aim of this study was to investigate for the first time lubricin expression in intact menisci and in menisci from patients with recent knee joint injury using histology, immunohistochemistry, Western blotting and gene expression analysis, to provide insights into pathological processes affecting meniscal tissue. Lubricin expression was studied in vivo in 20 patients (14 males and 6 females) with recent joint injury subjected to arthroscopic partial meniscectomy and in vitro in fibroblast-like cells from meniscus tissue to establish whether it is down-regulated following acute traumatic knee injury. The control group consisted of cadaver donors with normal menisci. Histology demonstrated a normal tissue without structural changes in control samples and structural alterations and clefts in injured menisci. Very strong lubricin immunohistochemical staining was observed in intact menisci; in contrast weak staining was seen in injured menisci. Western blot and mRNA expression analysis also demonstrated strong lubricin expression in control cells and a negligible amount of lubricin in injured fibroblast-like cells. Our data provide information concerning the immediate in vivo response to injury of human knee menisci by documenting early changes in the boundary-lubricating ability of synovial fluid and articular cartilage integrity. These findings may provide the biological basis for developing novel medical therapies to be applied before surgical treatment to preserve tissue function and prevent cartilage damage.

Intervertebral disc cell- and hydrogel-supported and spontaneous intervertebral disc repair in nucleotomized sheep

Purpose

Regenerative repair is a promising new approach in treating damaged intervertebral discs. An experimental scheme was established for autologous and/or allogenic repair after massive disc injury.

Methods

Disc healing was promoted in 11 animals by injecting in vitro expanded autologous/homologous disc cells 2 weeks after stab injury of lumbar discs L1-2. The following control discs were used in our sheep injury model: L2-3, vehicle only; L3-4, injury only; L4-5, undamaged; and lumbar discs from four non-experimental animals. Disc cells were suspended in a biologically supportive albumin/hyaluronan two-component hydrogel solution that polymerizes when inserted in order to anchor cells at the injection site. The parameters studied were MRI, DNA, glycosaminoglycan, collagen content, histology, immunohistology for collagens type I, II and aggrecan, and mRNA expression of GAPDH, β-actin, collagen type I, II, X, aggrecan, lubricin, and IL-1β.

Results

All parameters demonstrated almost complete healing of the injured discs after 6 months, when compared with data from both the endogenous non-injured controls as well as from the healthy animals.

Conclusion

Sheep experience spontaneous recovery from disc injury. The process of endogenous repair can be enhanced by means of hydrogel-supported cells.

Lubricin Distribution in the Menisci and Labra of Human Osteoarthritic Joints

OBJECTIVE

Lubricin is the principal boundary lubricant on articular cartilage. We aimed to describe the distribution of lubricin in the other articulating structures in the human knee and hip-menisci and labra-and to relate this distribution to the degree of tissue degeneration.

METHODS

Eighteen menisci and 6 labra were obtained from patients with osteoarthritis undergoing total knee and total hip replacements, respectively. Macroscopically intact specimens were fixed in formalin and processed for H&E staining and immunohistochemical evaluation with an antilubricin monoclonal antibody.

RESULTS

Lubricin was found in all tissues as a discrete layer on the tissue surface, within the extracellular matrix, and intracellularly, indicating that it plays a role in the tribology of these tissues in human subjects, and can be synthesized by cells within the tissues. While none of the samples displayed macroscopic tears, approximately 40% of the surface of the menisci and 80% of the surface of the labra displayed microscopic fibrillations and slight fraying. There was no effect of the degenerative changes on the distribution of lubricin.

CONCLUSIONS

Lubricin coats nearly the entirety of the surfaces of menisci and labra, including microfibrillations and tears, with possible implications towards the tribology of the tissues and healing of tissue damage.

Modulation of mesenchymal stem cell chondrogenesis in a tunable hyaluronic acid hydrogel microenvironment

An injectable and biodegradable hydrogel system comprising hyaluronic acid-tyramine (HA-Tyr) conjugates can safely undergo covalent cross-linking in vivo by the addition of small amounts of peroxidase and hydrogen peroxide (H(2)O(2)), with the independent tuning of the gelation rate and degree of cross-linking. Such hydrogel networks with tunable mechanical and degradation properties may provide the additional level of control needed to enhance chondrogenesis and overall cartilage tissue formation in vitro and in vivo. In this study, HA-Tyr hydrogels were explored as biomimetic matrices for caprine mesenchymal stem cells (MSCs) in cartilage tissue engineering. The compressive modulus, equilibrium swelling and degradation rate could be controlled by varying the concentration of H(2)O(2) as the oxidant in the oxidative coupling reaction. Cellular condensation reflected by the increase in effective number density of rounded cells in lacunae was greater in softer hydrogel matrices with lower cross-linking that displayed enhanced scaffold contracture. Conversely, within higher cross-linked matrices, cells adopted a more elongated morphology, with a reduced degree of cellular condensation. Furthermore, the degree of hydrogel cross-linking also modulated matrix biosynthesis and cartilage tissue histogenesis. Lower cross-linked matrix enhanced chondrogenesis with increases in the percentage of cells with chondrocytic morphology; biosynthetic rates of glycosaminoglycan and type II collagen; and hyaline cartilage tissue formation. With increasing cross-linking degree and matrix stiffness, a shift in MSC differentiation toward fibrous phenotypes with the formation of fibrocartilage and fibrous tissues was observed. These findings suggest that the tunable three-dimensional microenvironment of the HA-Tyr hydrogels modulates cellular condensation during chondrogenesis and has a dramatic impact on spatial organization of cells, matrix biosynthesis, and overall cartilage tissue histogenesis.

Effect of functional shift of the mandible on lubrication of the temporomandibular joint

Lubrication of synovial joints reduces the coefficient of friction of the articular cartilage surface. To investigate the effect of malocclusion on the lubrication of the temporomandibular joint (TMJ), we evaluated lubricin expression in the rat TMJ immunohistochemically, under conditions of functional lateral shift of the mandible, during period of growth. Thirty 5-week-old male Wistar rats were divided into experimental, recovery, and control groups. Each rt in the experimental and recovery groups was fitted with an acrylic-plate guiding appliance. The rats in the experimental and control groups were killed at 14 and 28 days after the appliance was attached. Each rat in the recovery group was detached from the appliance at 14 days, and was killed 14 days after the appliance was removed. In the experimental group, the expression of lubricin staining in TMJ cartilage was significantly decreased during the experimental period. In the recovery group, the expression of lubricin staining in TMJ cartilage was significantly greater than in the experimental group, and there was no significant difference at 28 days between the control and recovery groups. Analysis of these data suggests that a functional lateral shift of the mandible during the growth period influences lubrication of the TMJ.

Lubricin in human temporomandibular joint disc: an immunohistochemical study

AIMS

To evaluate, immunohistochemically, the presence and distribution of lubricin in human temporomandibular joint (TMJ) discs without any degenerative changes, obtained from autopsies, in order to elucidate the TMJ lubrication system and disc tribology.

METHODS

Immunohistochemistry for lubricin detection was carried out on 34 TMJ discs. Any disc had signs of degenerative or inflammatory joint disease nor disc were displaced. Sections were incubated with diluted rabbit polyclonal anti-lubricin antibody and scored according to the percentage of lubricin immunopositive cells. Three different TMJ disc tissue compartments taken from the intermediate zone were analysed, namely: the central region as well as the temporal (superior) and condylar (inferior) disc surfaces. The Friedman test, was used to compare lubricin at a protein level expression, amongst the regions of disc specimens.

RESULTS

Staining was noted within the TMJ disc cell populations in every disc tissue sample, however, the number of disc cells immunolabelled varied according to disc tissue regions. The percentage of immunostained cells, was statistically significant lower in the central region than in each disc surface (p<0.0001), whilst any statistically significant difference was found when comparing the two surfaces one another.

CONCLUSIONS

Lubricin is present in several location of TMJ disc being significantly more expressed at disc surfaces than in the central part.

Immunolocalization and expression of lubricin in the bilaminar zone of the human temporomandibular joint disc

Lubricin, which is a boundary joint lubricant, was investigated immunohistochemically in the bilaminar zone (BZ) of the human temporomandibular joint (TMJ), without any degenerative changes. Immunohistochemistry for lubricin detection was carried out on 33 TMJ discs obtained from 17 cadavers. Sections were incubated with diluted rabbit polyclonal anti-lubricin antibody and scored according to the percentage of lubricin immunopositive cells. Three different TMJ disc tissue compartments were analyzed, namely: the upper lamina, the inferior lamina and the loose connective tissue in the space between the laminae. The Mann-Whitney U test was used to compare protein expression (lubricin) among disc specimens' regions. Staining was noted within the TMJ disc cell populations in every disc tissue sample, with almost every cell immunolabeled by the lubricin antibody. The number of disc cells immunolabeled was almost the same in the 3 bilaminar zone regions. Positive extracellular matrix staining was also seen. The results of the present study suggest that lubricin is expressed in the TMJ disc bilaminar zone. Lubricin may have a role in normal disc posterior attachment physiology through the prevention of cellular adhesion as well as providing lubrication during normal bilaminar zone function.

Lubricin immunohistochemical expression in human temporomandibular joint disc with internal derangement

Lubricin is a chondroprotective, mucinous glycoprotein which contribute to joint lubrication, especially to boundary lubrication and maintains joint integrity. The present investigation aimed to study the immunolocalization of lubricin in TMJ discs from patients affected by anterior disc displacement with reduction (ADDwR) ADDwoR. Eighteen TMJ displaced disc affected by ADDwoR were processed immunohistochemically, with a polyclonal anti-lubricin antibody, used at 1:50 working dilution. The percentage of lubricin immunopositive cells (extent score = ES) and the extent of lubricin staining of the disc extracellular matrix (ECM), were evaluated. Each sample was scored for histopathological changes. Percentage of immunostained surface disc cells was the same (ES = 4) in both control and ADDwOR cells, being this data not statistically significant (P < 0.05). In pathological specimens the percentages of lubricin-stained cells was very high with an ES of 4 respect to control specimen, and this difference was statistically significant different (P > 0.05). The extracellular matrix (ECM) of discs at the disc surfaces of both pathological and normal specimens was very heavily stained (++++). Both the ES and ECM staining were not statistically correlated to the TMJ degeneration score according to the Spearman's rank correlation coefficient. According to our findings, a longstanding TMJ disc injury, affects lubricin expression in the TMJ disc tissue and not its surfaces, moreover, lubricin immunostaining is not correlated to TMJ disc histopathological changes.

Update on the pathophysiology of degenerative disc disease and new developments in treatment strategies

Degenerative disc disease (DDD) continues to be a prevalent condition that afflicts populations on a global scale. The economic impact and decreased quality of life primarily stem from back pain and neurological deficits associated with intervertebral disc degeneration. Although much effort has been invested into understanding the etiology of DDD and its relationship to the onset of back pain, this endeavor is a work in progress. The purpose of this review is to provide focused discussion on several areas in which recent advances have been made. Specifically, we have categorized these advances into early, middle, and late phases of age-related or degenerative changes in the disc and into promising minimally invasive treatments, which aim to restore mechanical and biological functions to the disc.

Lubricin: Its Presence in Repair Cartilage following Treatment with Autologous Chondrocyte Implantation

OBJECTIVE

To determine if lubricin was present in the surface layer of repair cartilage formed after autologous chondrocyte implantation (ACI).

DESIGN

Forty-three biopsies of repair tissue were taken from patients who had been treated with ACI 8 to 68 months previously (mean of 18.0 ± 14.4 months); 30 had flaps of periosteum and 13 of Chondro-Gide(®). Cryopreserved sections were stained with hematoxylin and eosin, toluidine blue, and immunostained for lubricin and type II collagen. The quality of repair tissue was scored via OsScore, and clinical improvement in patients was assessed via change in Lysholm score. Normal/control cartilage was studied for comparison (n = 5).

RESULTS

Patients' Lysholm scores improved from 48.1 ± 17 preoperatively to 69.5 ± 21.5 posttreatment. The thickness of repair tissue was 2.9 ± 1.7 mm compared with 2.3 ± 0.6 mm for control cartilage, with an OsScore of 6.7 ± 1.6 (8.9 ± 1.2 for controls). Ninety-eight percent of biopsies had staining for lubricin, with 84% having some in the surface layer (60% of periosteal treated and 100% of Chondro-Gide treated). The improvement in Lysholm score was not significantly different in patients with lubricin present at the surface compared with those without.

CONCLUSION

Lubricin was present in almost all samples of repair tissue formed post ACI, often in the surface layer, resembling the distribution that is seen in normal cartilage. The presence of lubricin in the upper layer is likely to have implications for the functioning of the tissue because, via its mucin-like repeats, it appears capable of reducing the friction that could arise in articulating joints.

Nanofibrous biologic laminates replicate the form and function of the annulus fibrosus

Successful engineering of load-bearing tissues requires recapitulation of their complex mechanical functions. Given the intimate relationship between function and form, biomimetic materials that replicate anatomic form are of great interest for tissue engineering applications. However, for complex tissues such as the annulus fibrosus, scaffolds have failed to capture their multi-scale structural hierarchy. Consequently, engineered tissues have yet to reach functional equivalence with their native counterparts. Here, we present a novel strategy for annulus fibrosus tissue engineering that replicates this hierarchy with anisotropic nanofibrous laminates seeded with mesenchymal stem cells. These scaffolds directed the deposition of an organized, collagen-rich extracellular matrix that mimicked the angle-ply, multi-lamellar architecture and achieved mechanical parity with native tissue after 10 weeks of in vitro culture. Furthermore, we identified a novel role for inter-lamellar shearing in reinforcing the tensile response of biologic laminates, a mechanism that has not previously been considered for these tissues.

Cellular mechanobiology of the intervertebral disc: new directions and approaches

The more we learn about the intervertebral disc (IVD), the more we come to appreciate the intricacies involved in transmission of forces through the ECM to the cell, and in the biological determinants of its response to mechanical stress. This review highlights recent developments in our knowledge of IVD physiology and examines their impact on cellular mechanobiology. Discussion centers around the continually evolving cellular and microstructural anatomy of the nucleus pulposus (NP) and the annulus fibrosus (AF) in response to complex stresses generated in support of axial load and spinal motion. Particular attention has been given to cells from the immature NP and the interlamellar AF, and assessment of their potential mechanobiologic contributions to the health and function of the IVD. In addition, several innovative approaches that have been brought to bear on studying the interplay between disc cells and their micromechanical environment are discussed. Techniques for "engineering" cellular function and technologies for fabricating more structurally defined biomaterial scaffolds have recently been employed in disc research. Such tools can be used to elucidate the biological and physical mechanisms by which different IVD cell populations are regulated by mechanical stress, and contribute to advancement of preventative and therapeutic measures.

Lubricin distribution in the human intervertebral disc

BACKGROUND

Previous studies have identified lubricin (also known as superficial zone protein) as a lubricating glycoprotein present in several musculoskeletal tissues including articular cartilage, meniscus, and tendon. In this immunohistochemical study, we determined the presence and distribution of lubricin in the cells, extracellular matrix, and tissue surfaces of human nucleus pulposus and anulus fibrosus tissues.

METHODS

Twenty-eight human intervertebral discs were resected at autopsy from fourteen cadavers. Disc specimens were fixed in formalin, processed, and paraffin-embedded prior to sectioning. Tissue sections were immunohistochemically stained for lubricin, the extent of extracellular matrix staining was evaluated semiquantitatively, and cellular staining was assessed quantitatively with use of a survey method.

RESULTS

Lubricin staining was evident in the extracellular matrix and at select surfaces of the nucleus pulposus and anulus fibrosus tissues. The extent of lubricin staining of the extracellular matrix was contingent on the disc region (nucleus pulposus, inner anulus fibrosus, or outer anulus fibrosus), with the greatest extent of matrix staining found in the nucleus pulposus, but it was not contingent on the Thompson grade. A subset of disc cells within the nucleus, inner anulus, and outer anulus also stained positively for lubricin, suggesting intrinsic cell synthesis of the glycoprotein. The disc region significantly affected the percentage of lubricin-staining cells, with the greatest percentage of cells staining for lubricin (nearly 10%) found in the nucleus pulposus. The percentage of cells staining for lubricin correlated with the extent of extracellular matrix staining for lubricin.

CONCLUSIONS

The results of this study confirm the presence of lubricin in the human intervertebral disc and demonstrate a unique distribution compared with that in the goat. The presence of lubricin in asymptomatic discs provides a foundation for future research regarding the role of lubricin in pathological disc conditions.

Measurement of local strains in intervertebral disc anulus fibrosus tissue under dynamic shear: contributions of matrix fiber orientation and elastin content

Shear strain has been implicated as an initiator of intervertebral disc anulus failure, however a clear, multi-scale picture of how shear strain affects the tissue microstructure has been lacking. The purposes of this study were to measure microscale deformations in anulus tissue under dynamic shear in two orientations, and to determine the role of elastin in regulating these deformations. Bovine AF tissue was simultaneously shear loaded and imaged using confocal microscopy following either a buffer or elastase treatment. Digital image analysis was used to track through time local shear strains in specimens sheared transversely, and stretch and rotation of collagen fiber bundles in specimens sheared circumferentially. The results of this study suggest that sliding does not occur between AF plies under shear, and that interlamellar connections are governed by collagen and fibrilin rather than elastin. The transverse shear modulus was found to be approximately 1.6 times as high in plies the direction of the collagen fibers as in plies across them. Under physiological levels of in-plane shear, fiber bundles stretched and re-oriented linearly. Elastin was found to primarily stiffen plies transversely. We conclude that alterations in the elastic fiber network, as found with IVD herniation and degeneration, can therefore be expected to significantly influence the AF response to shear making it more susceptible to micro failure under bending or torsion loading.