Ex vivo intervertebral disc cultures: degeneration-induction methods and their implications for clinical translation

Because low back pain is frequently a result of intervertebral disc degeneration (IVDD), strategies to regenerate or repair the IVD are currently being investigated. Often, ex vivo disc cultures of non-human IVD organs or tissue explants are used that usually do not exhibit natural IVDD. Therefore, degenerative changes mimicking those reported in human IVDD need to be induced. To support researchers in selecting ex vivo disc cultures, a systematic search was performed for them and their potential use for studying human IVDD reviewed. Five degeneration induction categories (proinflammatory cytokines, injury/damage, degenerative loading, enzyme, and other) were identified in 129 studies across 7 species. Methods to induce degeneration are diverse and can induce mild to severe degenerative changes that progress over time, as described for human IVDD. The induced degenerative changes are model-specific and there is no "one-fits-all" IVDD induction method. Nevertheless, specific aspects of human IVDD can be well mimicked. Currently, spontaneously degenerated disc cultures from large animals capture human IVDD in most aspects. Combinatorial approaches of several induction methods using discs derived from large animals are promising to recapitulate pathological changes on several levels, such as cellular behaviour, extracellular matrix composition, and biomechanical function, and therefore better mimic human IVDD. Future disc culture setups might increase in complexity, and mimic human IVDD even better. As ex vivo disc cultures have the potential to reduce and even replace animal trials, especially during preclinical development, advancement of such models is highly relevant for more efficient and cost-effective clinical translation from bench-to-bedside.

Osteochondral explants for diarthrodial joint diseases: bridging the gap between bench and bedside

Diarthrodial joint diseases, affecting hundreds of millions of people worldwide, mainly include osteoarthritis and cartilage injuries. No consensus on joint disease models has been achieved so far owing to the complex aetiologies, pathophysiological mechanisms and heterogeneity of disorders. The disease models established using isolated chondrocytes or small animals have the weaknesses of lacking native extracellular matrix and inter-species differences in anatomical and biomechanical cartilage properties. Osteochondral explants (OCEs) from large-animal or human joints present characteristics of native articular cartilage, showing promising potential for application in research on joint diseases. The present review focuses on OCEs and highlights the OCE sources, harvesting techniques, culture systems, applications and future developments. The OCE-centred ex vivo system has the potential to develop into preclinical models mimicking human joint diseases to help elucidate disease mechanisms, prompt therapeutic strategies and facilitate the clinical translation of findings in basic research.

Editorial - Disc Biology Special Issue

The intervertebral disc (IVD) has long been known as a mechanical structure responsible for spinal flexibility and load distribution, while its dysfunction is a frequent source of pain and disability. In recent years, multiple signaling pathways contributing to the regulation of the IVD homeostasis in health and disease have been discovered. At the same time, crosstalk of the IVD with adjacent tissues, immune cells, nerve cells and systemic mediators has been identified as an essential mechanism of degeneration and repair. Such discoveries open the door for the design of new therapeutic and diagnostic targets. This Disc Biology Special Issue provides an abstract of cutting-edge findings in terms of tissue regulation, therapeutic intervention and preclinical models, which will help to improve the management of IVD disorders.

Vertebral osteomyelitis is characterised by increased RANK/OPG and RANKL/OPG expression ratios in vertebral bodies and intervertebral discs

Vertebral osteomyelitis (VO) is an infection of the spine mainly caused by bacterial pathogens. The pathogenesis leading to destruction of intervertebral discs (IVDs) and adjacent vertebral bodies (VBs) is poorly described. The present study aimed at investigating the connection between infection and bone/disc metabolism in VO patients. 14 patients with VO (infection group) and 14 patients with burst fractures of the spine (fracture group; control) were included prospectively. Tissue biopsies from affected IVDs and adjacent VBs were analysed by RT-qPCR for mRNA-expression levels of 18 target genes including chemokines, adipokines and genes involved in bone metabolism. Most importantly, the receptor activator of NF-κB/osteoprotegerin (RANK/OPG) expression ratio was drastically elevated in both VBs and IVDs of the infection group. In parallel, expression of genes of the prostaglandin-E2-dependent prostanoid system was induced. Such genes regulate tissue degradation processes via the triad OPG/RANK/RANKL as well as via the chemokines IL-8 and CCL-20, whose expression was also found to be increased upon infection. The gene expression of the adipokine leptin, which promotes inflammatory tissue degradation, was higher in IVD tissue of the infection group, whereas the transcription of omentin and resistin genes, whose functions are largely unknown in the context of infectious diseases, was lower in infected VBs. In summary, similar expression patterns of pro-inflammatory cytokines and pro-osteoclastogenic factors were identified in VBs and IVDs of patients suffering from VO. This suggests that common immuno-metabolic pathways are involved in the mechanisms leading to tissue degradation in VBs and IVDs during VO.

Evaluation of the influence of platelet-rich plasma (PRP), platelet lysate (PL) and mechanical loading on chondrogenesis in vitro

The aim of this work is to investigate the capability of PRP as an adjuvant therapy to autologous chondrocyte implantation (ACI) in combination with multi-axial load with respect to cartilage regeneration. Articular cartilage shows poor repair capacity and therapies for cartilage defects are still lacking. Well-established operative treatments include ACI, and growing evidence shows the beneficial effects of PRP. Platelets contain numerous growth factors, among them transforming growth factor beta (TGF-β). Dynamic mechanical loading is known to be essential for tissue formation, improving extracellular matrix (ECM) production. For our ACI model monolayer expanded human chondrocytes were seeded into polyurethane scaffolds and embedded in fibrin (hChondro), in PRP-Gel (PRP), or in fibrin with platelet lysate (PL), which was added to the media once a week with a concentration of 50 vol%. The groups were either exposed to static conditions or multi-axial forces in a ball-joint bioreactor for 1 h per day over 2 weeks, mimicking ACI under physiological load. The culture medium was collected and analyzed for glycosaminoglycan (GAG), nitrite and transforming growth factor beta 1 (TGF-β1) content. The cell-scaffold constructs were collected for DNA and GAG quantification; the expression of chondrogenic genes, TGF-β and related receptors, as well as inflammatory genes, were analyzed using qPCR. Loading conditions showed superior chondrogenic differentiation (upregulation of COL2A1, ACAN, COMP and PRG4 expression) than static conditions. PRP and PL groups combined with mechanical loading showed upregulation of COL2A1, ACAN and COMP. The highest amount of total TGF-β1 was quantified in the PL group. Latent TGF-β1 was activated in all loaded groups, while the highest amount was found in the PL group. Load increased TGFBR1/TGFBR2 mRNA ratio, with further increases in response to supplements. In general, loading increased nitrite release into the media. However, over time, the media nitrite content was lower in the PL group compared to the control group. Based on these experiments, we conclude that chondrogenic differentiation is strongest when simulated ACI is performed in combination with dynamic mechanical loading and PRP-gel or PL supplementation. An inflammatory reaction was reduced by PRP and PL, which could be one of the major therapeutic effects. Loading presumably can enhance the action of TGF-β1, which was predominantly activated in loaded PL groups. The combination of load and PRP represents an effective and promising synergy concerning chondrocyte-based cartilage repair.

The effect of hyaluronic acid on nucleus pulposus extracellular matrix production through hypoxia-inducible factor-1α transcriptional activation of CD44 under hypoxia

Intervertebral disc degeneration (IDD) is the leading cause of low-back pain. Implantation of hyaluronic acid (HA) is potentially a therapeutic strategy for IDD, but its pharmacological effects and mechanism under hypoxic conditions remain unclear. In this study, the expression of extracellular matrix genes and proteins were enhanced in nucleus pulposus cells (NPCs) in the presence of HA under hypoxic condition, as shown by real-time reverse transcription-polymerase chain reaction, immunofluorescence staining, and dimethylmethylene blue assays. Moreover, the expression of CD44 was increased in the presence of both HA and hypoxia compared to either alone. Using a bioinformatic database, hypoxia inducible factor-1α (HIF-1α), a key transcription factor in the hypoxic condition, was found to have 4 predicted binding sites on the CD44 promoter. CD44 expression was significantly increased by treatment with cobalt chloride or dimethyloxalylglycine. Over-expression of HIF-1α in NPCs significantly up-regulated the expression of CD44. The binding site of HIF-1α in the CD44 promoter region, was identified by promoter truncation experiments and chromatin immunoprecipitation assays. Taken together, these results indicated that hypoxic conditions positively potentiated the ability of NPCs matrix synthesis in the presence of HA, which correlated with the increasing CD44 expression by HIF-1α transcriptional activation.

The tissue-renin-angiotensin-system of the human intervertebral disc

Symptomatic intervertebral disc (IVD) degeneration accounts for significant socioeconomic burden. Recently, the expression of the tissue renin-angiotensin system (tRAS) in rat and bovine IVD was demonstrated. The major effector of tRAS is angiotensin II (AngII), which participates in proinflammatory pathways. The present study investigated the expression of tRAS in human IVDs, and the correlation between tRAS, inflammation and IVD degeneration. Human IVD tissue was collected during spine surgery and distributed according to principal diagnosis. Gene expression of tRAS components, proinflammatory and catabolic markers in the IVD tissue was assessed. Hydroxyproline (OHP) and glycosaminoglycan (GAG) content in the IVD tissue were determined. Tissue distribution of tRAS components was investigated by immunohistochemistry. Gene expression of tRAS components such as angiotensin-converting enzyme (ACE), Ang II receptor type 2 (AGTR2), angiotensinogen (AGT) and cathepsin D (CTSD) was confirmed in human IVDs. IVD samples that expressed tRAS components (n = 21) revealed significantly higher expression levels of interleukin 6 (IL-6), tumour necrosis factor α (TNF-α), a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) 4 and 5 compared to tRAS-negative samples (n = 37). Within tRAS-positive samples, AGT, matrix-metalloproteinases 13 and 3, IL-1, IL-6 and IL-8 were more highly expressed in traumatic compared to degenerated IVDs. Total GAG/DNA content of non-tRAS expressing IVD tissue was significantly higher compared to tRAS positive tissue. Immunohistochemistry confirmed the presence of AngII in the human IVD. The present study identified the existence of tRAS in the human IVD and suggested a correlation between tRAS expression, inflammation and ultimately IVD degeneration.

Functional cell phenotype induction with TGF-β1 and collagen-polyurethane scaffold for annulus fibrosus rupture repair

Appropriate cell sources, bioactive factors and biomaterials for generation of functional and integrated annulus fibrosus (AF) tissue analogues are still an unmet need. In the present study, the AF cell markers, collagen type I, cluster of differentiation 146 (CD146), mohawk (MKX) and smooth muscle protein 22α (SM22α) were found to be suitable indicators of functional AF cell induction. In vitro 2D culture of human AF cells showed that transforming growth factor β1 (TGF-β1) upregulated the expression of the functional AF markers and increased cell contractility, indicating that TGF-β1-pre-treated AF cells were an appropriate cell source for AF tissue regeneration. Furthermore, a tissue engineered construct, composed of polyurethane (PU) scaffold with a TGF-β1-supplemented collagen type I hydrogel and human AF cells, was evaluated with in vitro 3D culture and ex vivo preclinical bioreactor-loaded organ culture models. The collagen type I hydrogel helped maintaining the AF functional phenotype. TGF-β1 supplement within the collagen I hydrogel further promoted cell proliferation and matrix production of AF cells within in vitro 3D culture. In the ex vivo IVD organ culture model with physiologically relevant mechanical loading, TGF-β1 supplement in the transplanted constructs induced the functional AF cell phenotype and enhanced collagen matrix synthesis. In conclusion, TGF-β1-containing collagen-PU constructs can induce the functional cell phenotype of human AF cells in vitro and in situ. This combined cellular, biomaterial and bioactive agent therapy has a great potential for AF tissue regeneration and rupture repair.

Evaluation of biomimetic hyaluronic-based hydrogels with enhanced endogenous cell recruitment and cartilage matrix formation

Biomaterials play a pivotal role in cell-free cartilage repair approaches, where cells must migrate through the scaffold, fill the defect, and then proliferate and differentiate facilitating tissue remodeling. Here we used multiple assays to test the influence of chemokines and growth factors on cell migration and cartilage repair in two different hyaluronan (HA)-based hydrogels. We first investigated bone marrow Mesenchymal Stromal Cells (BMSC) migration in vitro, in response to different concentrations of platelet-derived growth factor-BB (PDGF-BB), chemokine ligand 5 (CCL5/RANTES) and stromal cell-derived factor 1 (SDF-1), using a 3D spheroid-based assay. PDGF-BB was selected as most favourable chemotactic agent, and MSC migration was assessed in the context of physical impediment to cell recruitment by testing Fibrin-HA and HA-Tyramine hydrogels of different cross-linking densities. Supplementation of PDGF-BB stimulated progressive migration of MSC through the gels over time. We then investigated in situ cell migration into the hydrogels with and without PDGF-BB, using a cartilage-bone explant model implanted subcutaneously in athymic mice. In vivo studies show that when placed into an osteochondral defect, both hydrogels supported endogenous cell infiltration and provided an amenable microenvironment for cartilage production. These processes were best supported in Fibrin-HA hydrogel in the absence of PDGF-BB. This study used an advanced preclinical testing platform to select an appropriate microenvironment provided by implanted hydrogels, demonstrating that HA-based hydrogels can promote the initial and critical step of endogenous cell recruitment and circumvent some of the clinical challenges in cartilage tissue repair. STATEMENT OF SIGNIFICANCE: The challenge of articular cartilage repair arises from its complex structure and architecture, which confers the unique mechanical behavior of the extracellular matrix. The aim of our research is to identify biomaterials for implants that can support migration of endogenous stem and progenitor cell populations from cartilage and bone tissue, in order to permanently replace damaged cartilage with the original hyaline structure. Here, we present an in vitro 3D spheroid-based migration assay and an osteochondral defect model, which provide the opportunity to assess biomaterials and biomolecules, and to get stronger experimental evidence of the not well-characterized dynamic process of endogenous cells colonization in an osteochondral defect. Furthermore, the delicate step of early cell migration into biomaterials towards functional tissue engineering is reproduced. These tests can be used for pre-clinical testing of newly developed material designs in the field of scaffold engineering.

CD146/MCAM distinguishes stem cell subpopulations with distinct migration and regenerative potential in degenerative intervertebral discs

OBJECTIVE

This study aimed to characterize the mesenchymal stem cell (MSC) subpopulation migrating towards a degenerated intervertebral disc (IVD) and to assess its regenerative potential.

DESIGN

Based on initial screening for migration towards C-C motif chemokine ligand 5 (CCL5), the migration potential of CD146+ and CD146- mesenchymal stem cells (MSCs) was evaluated in vitro and in a degenerated organ culture model (degeneration by high-frequency loading in a bioreactor). Discogenic differentiation potential of CD146+ and CD146- MSCs was investigated by in vitro pellet culture assay with supplementation of growth and differentiation factor-6 (GDF6). Furthermore, trypsin degenerated IVDs were treated by either homing or injection of CD146+ or CD146- MSCs and glycosaminoglycan synthesis was evaluated by Sulphur 35 incorporation after 35 days of culture.

RESULTS

Surface expression of CD146 led to a higher number of migrated MSCs both in vitro and in organ culture. CD146+ and CD146- pellets responded with a similar up-regulation of anabolic markers. A higher production of sulfated glycosaminoglycans (sGAG)/DNA was observed for CD146+ pellets, while in organ cultures, sGAG synthesis rate was higher for IVDs treated with CD146- MSCs by either homing or injection.

CONCLUSIONS

The CD146+ MSC subpopulation held greater migration potential towards degenerative IVDs, while the CD146- cells induced a stronger regenerative response in the resident IVD cells. These findings were independent of the application route (injection vs migration). From a translational point of view, our data suggests that CD146+ MSCs may be suitable for re-population, while CD146- MSCs may represent the primary choice for stimulation of endogenous IVD cells.

Mechanically stimulated osteochondral organ culture for evaluation of biomaterials in cartilage repair studies

Surgical procedures such as microfracture or autologous chondrocyte implantation have been used to treat articular cartilage lesions; however, repair often fails in terms of matrix organization and mechanical behaviour. Advanced biomaterials and tissue engineered constructs have been developed to improve cartilage repair; nevertheless, their clinical translation has been hampered by the lack of reliable in vitro models suitable for pre-clinical screening of new implants and compounds. In this study, an osteochondral defect model in a bioreactor that mimics the multi-axial motion of an articulating joint, was developed. Osteochondral explants were obtained from bovine stifle joints, and cartilage defects of 4 mm diameter were created. The explants were used as an interface against a ceramic ball applying dynamic compressive and shear loading. Osteochondral defects were filled with chondrocytes-seeded fibrin-polyurethane constructs and subjected to mechanical stimulation. Cartilage viability, proteoglycan accumulation and gene expression of seeded chondrocytes were compared to free swelling controls. Cells within both cartilage and bone remained viable throughout the 10-day culture period. Loading did not wear the cartilage, as indicated by histological evaluation and glycosaminoglycan release. The gene expression of seeded chondrocytes indicated a chondrogenic response to the mechanical stimulation. Proteoglycan 4 and cartilage oligomeric matrix protein were markedly increased, while mRNA ratios of collagen type II to type I and aggrecan to versican were also enhanced. This mechanically stimulated osteochondral defect culture model provides a viable microenvironment and will be a useful pre-clinical tool to screen new biomaterials and biological regenerative therapies under relevant complex mechanical stimuli. STATEMENT OF SIGNIFICANCE: Articular cartilage lesions have a poor healing capacity and reflect one of the most challenging problems in orthopedic clinical practice. The aim of current research is to develop a testing system to assess biomaterials for implants, that can permanently replace damaged cartilage with the original hyaline structure and can withstand the mechanical forces long term. Here, we present an osteochondral ex vivo culture model within a cartilage bioreactor, which mimics the complex motion of an articulating joint in vivo. The implementation of mechanical forces is essential for pre-clinical testing of novel technologies in the field of cartilage repair, biomaterial engineering and regenerative medicine. Our model provides a unique opportunity to investigate healing of articular cartilage defects in a physiological joint-like environment.

Mechanical loading of intervertebral disc modulates microglia proliferation, activation, and chemotaxis

OBJECTIVE

The aim of the study is to assess the effects of the neuroinflammatory microenvironment of a mechanically-induced degenerating intervertebral disc (IVD) on neuroinflammatory like cells such as microglia, in order to comprehend the role of microglial cells in degenerative disc disease.

METHODS

Bovine caudal IVDs were kept in culture in an ex vivo bioreactor under high frequency loading and limited nutrition or in free swelling conditions as control samples. Conditioned media (CM) were collected, analysed for cytokine and neurotrophin content and applied to microglial cells for neuroinflammatory activation assessment.

RESULTS

Degenerative conditioned medium (D-CM) induced a higher production of interleukin (IL)-8, nerve growth factor (NGF), interferon (IFN)-γ, IL-17 from IVD cells than unloaded control conditioned medium (U-CM). Upon 48 h of co-incubation with microglia, D-CM stimulated microglia proliferation, activation, with increased expression of ionized calcium binding adaptor molecule 1 (IBA1) and CD68, and chemotaxis. Moreover, an increment of nitrite production was observed. Interestingly, D-CM caused an upregulation of IL-1β, IL-6, tumour necrosis factor α (TNFα), inducible NO synthase (iNOS), IBA1, and vascular endothelial growth factor (VEGF) genes in microglia. Similar results were obtained when microglia were treated with the combination of the measured cytokines.

CONCLUSIONS

Our findings show that in IVD degenerative microenvironment, IL-8, NGF, IFN-γ, IL-17 drive activation of microglia in the spinal cord and increase upregulation of neuroinflammatory markers. This, in turn, enhances the inflammatory milieu within IVD tissues and in the peridiscal space, aggravating the cascade of degenerative events. This study provides evidence for an important role of microglia in maintaining IVD neuroinflammatory microenvironment and probably inducing low back pain.

Systemic blood plasma CCL5 and CXCL6: Potential biomarkers for human lumbar disc degeneration

Lumbar disc degeneration severity on magnetic resonance imaging (MRI) is associated with low back pain. Pro-inflammatory chemokines CCL5 and CXCL6 are released by induced degenerative discs, and CCL5 has been associated with discogenic back pain. A case-control study was performed, based on the Hong Kong Disc Degeneration Population-Based Cohort of Southern Chinese, to investigate if systemic levels of CCL5 and CXCL6 were elevated in subjects with disc degeneration compared to non-degenerated individuals. Eighty subjects were selected, 40 with no disc degeneration (control group; DDD score 0) and 40 with moderate/severe disc degeneration (disc degeneration group; DDD score ≥5) as noted on MRI. Subjects were matched for age, sex, body mass index and workload. Blood plasma samples were obtained from each individual, and levels of CCL5 and CXCL6 were measured. Secondary phenotypes of lumbar disc displacement and cervical disc changes were also assessed. CCL5 concentrations were significantly increased in the disc degeneration (mean: 19.8 ng/mL) compared to the control group (mean: 12.8 ng/mL) (p = 0.015). The degeneration group demonstrated higher levels of CXCL6 (mean: 56.9 pg/mL) compared to the control group (mean: 43.4 pg/mL) (p = 0.010). There was a trend towards elevated CCL5 levels with disc displacement in the degeneration group (p = 0.073). Cervical disc degeneration was not associated with elevated chemokine levels (p > 0.05). This is the first study to note that elevated systemic CCL5 and CXCL6 were associated with moderate/severe lumbar disc degeneration, further corroborating tissue studies of painful discs. These chemokines may be systemic biomarkers for the diagnosis and monitoring of disc degeneration.

Cell therapy for intervertebral disc repair: advancing cell therapy from bench to clinics

Intervertebral disc (IVD) degeneration is a major cause of pain and disability; yet therapeutic options are limited and treatment often remains unsatisfactory. In recent years, research activities have intensified in tissue engineering and regenerative medicine, and pre-clinical studies have demonstrated encouraging results. Nonetheless, the translation of new biological therapies into clinical practice faces substantial barriers. During the symposium "Where Science meets Clinics", sponsored by the AO Foundation and held in Davos, Switzerland, from September 5-7, 2013, hurdles for translation were outlined, and ways to overcome them were discussed. With respect to cell therapy for IVD repair, it is obvious that regenerative treatment is indicated at early stages of disc degeneration, before structural changes have occurred. It is envisaged that in the near future, screening techniques and non-invasive imaging methods will be available to detect early degenerative changes. The promises of cell therapy include a sustained effect on matrix synthesis, inflammation control, and prevention of angio- and neuro-genesis. Discogenic pain, originating from "black discs" or annular injury, prevention of adjacent segment disease, and prevention of post-discectomy syndrome were identified as prospective indications for cell therapy. Before such therapy can safely and effectively be introduced into clinics, the identification of the patient population and proper standardisation of diagnostic parameters and outcome measurements are indispensable. Furthermore, open questions regarding the optimal cell type and delivery method need to be resolved in order to overcome the safety concerns implied with certain procedures. Finally, appropriate large animal models and well-designed clinical studies will be required, particularly addressing safety aspects.

CCL5/RANTES is a key chemoattractant released by degenerative intervertebral discs in organ culture

Release of chemotactic factors in response to tissue damage has been described for different musculoskeletal tissues, including the intervertebral disc (IVD). This study investigated the chemoattractants that are released by induced degenerative IVDs and may be involved in recruiting mesenchymal stem cells (MSCs). Bovine caudal discs were cultured within a bioreactor and loaded under conditions that mimicked physiological or degenerative settings. Between days 4-6, medium was replaced by PBS, which was subsequently used for proteomic, ELISA and immunoprecipitation analyses of secreted chemokines and cytokines. A Boyden chamber assay was used to observe human MSC migration towards native and chemokine depleted media. Gene expression levels of chemokine receptors in human MSCs were analysed, and CCL5 was localised in bovine and human IVD by immunohistochemistry. Proteomic analysis revealed the presence of CCL5 and CXCL6 within conditioned media. Higher concentrations of CCL5 were found in the degenerative media, and a relationship was found between interleukin-1β and CCL5 concentration. Chemokine immunoprecipitation showed that MSCs had a significantly reduced chemotactic migration towards CCL5-immunoprecipitated and CCL5/CXCL6 co-immunoprecipitated media, whilst CXCL6 depletion did not change MSC chemotaxis. MSCs showed a significant increase in mRNA expression of the CCL5 receptors, CCR1 and CCR4, upon culture in degenerative media. Furthermore, CCL5 was identified in bovine and human disc tissue by immunohistochemistry. Hence, CCL5 may be a key chemoattractant that is produced and released by the intervertebral disc cells. Therefore, these factors could be used to enhance stem/progenitor cell mobilisation in regenerative therapies for early stages of disc degeneration.

Sliding motion modulates stiffness and friction coefficient at the surface of tissue engineered cartilage

OBJECTIVE

Functional cartilage tissue engineering aims to generate grafts with a functional surface, similar to that of authentic cartilage. Bioreactors that stimulate cell-scaffold constructs by simulating natural joint movements hold great potential to generate cartilage with adequate surface properties. In this study two methods based on atomic force microscopy (AFM) were applied to obtain information about the quality of engineered graft surfaces. For better understanding of the molecule-function relationships, AFM was complemented with immunohistochemistry.

METHODS

Bovine chondrocytes were seeded into polyurethane scaffolds and subjected to dynamic compression, applied by a ceramic ball, for 1h daily [loading group 1 (LG1)]. In loading group 2 (LG2), the ball additionally oscillated over the scaffold, generating sliding surface motion. After 3 weeks, the surfaces of the engineered constructs were analyzed by friction force and indentation-type AFM (IT-AFM). Results were complemented and compared to immunohistochemical analyses.

RESULTS

The loading type significantly influenced the mechanical and histological outcomes. Constructs of LG2 exhibited lowest friction coefficient and highest micro- and nanostiffness. Collagen type II and aggrecan staining were readily observed in all constructs and appeared to reach deeper areas in loaded (LG1, LG2) compared to unloaded scaffolds. Lubricin was specifically detected at the top surface of LG2.

CONCLUSIONS

This study proposes a quantitative AFM-based functional analysis at the micrometer- and nanometer scale to evaluate the quality of cartilage surfaces. Mechanical testing (load-bearing) combined with friction analysis (gliding) can provide important information. Notably, sliding-type biomechanical stimuli may favor (re-)generation and maintenance of functional articular surfaces and support the development of mechanically competent engineered cartilage.

A combination of shear and dynamic compression leads to mechanically induced chondrogenesis of human mesenchymal stem cells

There is great interest in how bone marrow derived stem cells make fate decisions. Numerous studies have investigated the role of individual growth factors on mesenchymal stem cell differentiation, leading to protocols for cartilage, bone and adipose tissue. However, these protocols overlook the role of biomechanics on stem cell differentiation. There have been various studies that have applied mechanical stimulation to constructs containing mesenchymal stem cells, with varying degrees of success. One critical fate decision is that between cartilage and bone. Articular motion is a combination of compressive, tensile and shear deformations; therefore, one can presume that compression alone is unlikely to be a sufficient mechanical signal to generate a cartilage-like tissue in vitro. Within this study, we aimed to determine the role of shear on the fate of stem cell differentiation. Specifically, we investigated the potential enhancing effect of surface shear, superimposed on cyclic axial compression, on chondrogenic differentiation of human bone marrow-derived stem cells. Using a custom built loading device we applied compression, shear or a combination of both stimuli onto fibrin/polyurethane composites in which human mesenchymal stem cells were embedded, while no exogenous growth-factors were added to the culture medium. Both compression or shear alone was insufficient for the chondrogenic induction of human mesenchymal stem cells. However, the application of shear superimposed upon dynamic compression led to significant increases in chondrogenic gene expression. Histological analysis detected sulphated glycosaminoglycan and collagen II only in the compression and shear group. The results obtained may provide insight into post-operative care after cell therapy involving mesenchymal stromal cells.

Role of hypoxia and growth and differentiation factor-5 on differentiation of human mesenchymal stem cells towards intervertebral nucleus pulposus-like cells

There is evidence that mesenchymal stem cells (MSCs) can differentiate towards an intervertebral disc (IVD)-like phenotype. We compared the standard chondrogenic protocol using transforming growth factor beta-1 (TGFß) to the effects of hypoxia, growth and differentiation factor-5 (GDF5), and coculture with bovine nucleus pulposus cells (bNPC). The efficacy of molecules recently discovered as possible nucleus pulposus (NP) markers to differentiate between chondrogenic and IVD-like differentiation was evaluated. MSCs were isolated from human bone marrow and encapsulated in alginate beads. Beads were cultured in DMEM (control) supplemented with TGFß or GDF5 or under indirect coculture with bNPC. All groups were incubated at low (2 %) or normal (20 %) oxygen tension for 28 days. Hypoxia increased aggrecan and collagen II gene expression in all groups. The hypoxic GDF5 and TGFß groups demonstrated most increased aggrecan and collagen II mRNA levels and glycosaminoglycan accumulation. Collagen I and X were most up-regulated in the TGFß groups. From the NP markers, cytokeratin-19 was expressed to highest extent in the hypoxic GDF5 groups; lowest expression was observed in the TGFß group. Levels of forkhead box F1 were down-regulated by TGFß and up-regulated by coculture with bNPC. Carbonic anhydrase 12 was also down-regulated in the TGFß group and showed highest expression in the GDF5 group cocultured with bNPC under hypoxia. Trends in gene expression regulation were confirmed on the protein level using immunohistochemistry. We conclude that hypoxia and GDF5 may be suitable for directing MSCs towards the IVD-like phenotype.

Variations in gene and protein expression in human nucleus pulposus in comparison with annulus fibrosus and cartilage cells: potential associations with aging and degeneration

OBJECTIVE

Regardless of recent progress in the elucidation of intervertebral disc (IVD) degeneration, the basic molecular characteristics that define a healthy human IVD are largely unknown. Although work in different animal species revealed distinct molecules that might be used as characteristic markers for IVD or specifically nucleus pulposus (NP) cells, the validity of these markers for characterization of human IVD cells remains unknown.

DESIGN

Eleven potential marker molecules were characterized with respect to their occurrence in human IVD cells. Gene expression levels of NP were compared with annulus fibrosus (AF) and articular cartilage (AC) cells, and potential correlations with aging were assessed.

RESULTS

Higher mRNA levels of cytokeratin-19 (KRT19) and of neural cell adhesion molecule-1 were noted in NP compared to AF and AC cells. Compared to NP cytokeratin-18 expression was lower in AC, and alpha-2-macroglobulin and desmocollin-2 lower in AF. Cartilage oligomeric matrix protein (COMP) and glypican-3 expression was higher in AF, while COMP, matrix gla protein (MGP) and pleiotrophin expression was higher in AC cells. Furthermore, an age-related decrease in KRT19 and increase in MGP expression were observed in NP cells. The age-dependent expression pattern of KRT19 was confirmed by immunohistochemistry, showing the most prominent KRT19 immunoreaction in the notochordal-like cells in juvenile NP, whereas MGP immunoreactivity was not restricted to NP cells and was found in all age groups.

CONCLUSIONS

The gene expression of KRT19 has the potential to characterize human NP cells, whereas MGP cannot serve as a characteristic marker. KRT19 protein expression was only detected in NP cells of donors younger than 54 years.

Different response of articular chondrocyte subpopulations to surface motion

OBJECTIVE

To investigate the effect of surface motion on the gene expression of proteoglycan 4 (PRG4), hyaluronan synthases (HAS1, HAS2) and on the hyaluronan (HA) and proteoglycan 4 (PRG4) release of chondrocytes from different zones of bovine articular cartilage.

DESIGN

Superficial zone, deep zone, full thickness, and superficial/deep 1:1 mixed chondrocytes were seeded into 3D polyurethane scaffolds and stimulated using our bioreactor that approximates kinematics and surface motion characteristics of natural joints. One hour of surface motion superimposed on cyclic compression was applied twice a day over 3 consecutive days. Scaffolds were cut into top and bottom sections and analyzed for gene expression of PRG4, HAS1, and HAS2.

RESULTS

Depending on the cell population, the gene expression levels increased within 8 days of culture in unloaded scaffolds, with a stronger increase in the top compared to the bottom sections. Mechanical loading further enhanced the messenger RNA (mRNA) levels in all cell types, with most pronounced up-regulations observed for the PRG4 expression in deep zone and the HAS2 expression in superficial zone cells. The effect of the biochemical and biomechanical environment appeared to be additive, resulting in highest mRNA levels in the top sections of loaded constructs. Bioreactor stimulation also enhanced the HA release in all cell populations. Full thickness chondrocytes experienced the greatest effect on HAS1 mRNA expression and HA release, indicating that the interaction between cell populations may promote HA synthesis compared to subpopulations alone.

CONCLUSIONS

Reciprocating sliding can be an efficient tool for generating tissue-engineered constructs from various chondrocyte populations by providing a functional cartilage-synovial interface.

Effects of hypobaric hypoxia on vascular endothelial growth factor and the acute phase response in subjects who are susceptible to high-altitude pulmonary oedema

In order to investigate whether vascular endothelial growth factor (VEGF) and inflammatory pathways are activated during acute hypobaric hypoxia in subjects who are susceptible to high-altitude pulmonary oedema (HAPE-S), seven HAPE-S and five control subjects were exposed to simulated altitude corresponding to 4000 m in a hypobaric chamber for 1 day. Peripheral venous blood was taken at 450 m (Zürich level) and at 4000 m, and levels of erythropoietin (EPO), VEGF, interleukin-6 (IL-6) and the acute-phase proteins complement C3 (C3), alpha1-antitrypsin (alpha1AT), transferrin (Tf) and C-reactive protein (CRP) were measured. Peripheral arterial oxygen saturation (SaO2) was recorded. Chest radiography was performed before and immediately after the experiment. EPO increased during altitude exposure, correlating with SaO2, in both groups (r = -0.86, P < 0.001). Venous serum VEGF did not show any elevation despite a marked decrease in SaO2 in the HAPE-S subjects [mean (SD) HAPE-S: 69.6 (9.1)%; controls: 78.7 (5.2)%]. C3 and alpha1AT levels increased in HAPE-S during hypobaric hypoxia [from 0.94 (0.11) g/l to 1.07 (0.13) g/l, and from 1.16 (0.08) g/l to 1.49 (0.27) g/l, respectively; P < 0.05], but remained within the clinical reference ranges. No significant elevations of IL-6, Tf or CRP were observed in either group. The post-exposure chest radiography revealed no signs of oedema. We conclude that VEGF is not up-regulated in HAPE-S and thus does not seem to increase critically pulmonary vascular permeability during the 1st day at high altitude. Furthermore, our data provide evidence against a clinically relevant inflammation in the initial phase of exposure to hypoxia in HAPE-S, although C3 and alpha1AT are mildly induced.

VEGF levels in asthmatic airways do not correlate with plasma extravasation

BACKGROUND

Vascular permeability/vascular endothelial growth factor (VEGF) is a multifunctional cytokine which plays a role in chronic inflammation and angiogenesis. Its expression in bronchoalveolar lavage (BAL) has not been determined although VEGF may be relevant to the pathophysiology of asthma in which oedema is an important feature.

METHODS

We studied VEGF, albumin and IgA immunoreactive levels in the BAL fluids obtained from 27 chronic stable asthmatics, nine untreated chronic bronchitis patients and 15 control subjects.

RESULTS

BAL fluid levels of VEGF and VEGF normalized to IgA were not significantly different in any patient group. Both asthmatic steroid- and non-steroid-treated groups had significantly lower albumin levels in their BAL fluids explaining most of the 179% increased VEGF normalized to albumin ratios in non-steroid treated asthmatics. Moreover, VEGF and albumin markers correlated in control subjects (r = 0.73, P = 0.006) and in chronic bronchitics (r = 0.75, P = 0.03, Spearman test), but not in asthmatics. VEGF was inversely correlated with asthma severity (GINA/NHLBI scores) in non-steroid treated asthmatics (tau = - 0.52, P = 0.009, Kendall test).

CONCLUSIONS

Thus, the potential role of VEGF in asthma requires further studies on bronchial biopsies and induced sputum.

Strongly enhanced serum levels of vascular endothelial growth factor (VEGF) after polytrauma and burn

Angiogenesis is a key component of the repair mechanisms triggered by tissue injury. Vascular endothelial growth factor (VEGF) is an important mediator of angiogenesis, as it acts directly and specifically on endothelial cells. VEGF produced locally in regenerating tissue may spill over into the systemic circulation, and measuring levels of circulating VEGF may allow monitoring of angiogenesis. To determine whether circulating VEGF is increased after severe injury, we measured concentrations of VEGF in serial serum samples of 23 mechanical burn patients, 55 patients with multiple trauma and 56 healthy normal controls, using a newly established ELISA assay. In burn patients, serum VEGF was increased on day 1 (369.4 +/- 88.0 pg/ml) and on day 3 (452.0 +/- 65.3 pg/ml), reached highest levels on day 14 (1809.5 +/- 239.7 pg/ml) and was still elevated on day 21 post-burn (1339.8 +/- 208.7 pg/ml) (mean +/- SEM, p < 0.01), when compared with healthy controls (82.2 +/- 10.8 pg/ml (mean +/- SEM)). Likewise, in trauma patients, serum VEGF showed a trend towards elevated values on the day of admission (186.9 +/- 43.9 pg/ml) and on day 3 after injury (193.2 +/- 62.1 pg/ml). Thereafter, serum VEGF increased further (day 7,507.0 +/- 114.7 pg/ml), peaked on day 14 (742.4 +/- 151.8 pg/ml) and was still elevated on day 21 after injury (693.1 +/- 218.6 pg/ml (mean +/- SEM, p < 0.01)). No significant correlation was observed between peak serum VEGF and initial severity of mechanical (Injury Severity Score) or burn injury (percentage of body surface burned). However, in both burn and trauma patients, the subgroup of patients with uncomplicated healing showed significantly higher increases of serum VEGF than the subgroup who developed severe complications during the post-traumatic course, such as sepsis, adult respiratory distress syndrome or multiple organ failure (p < 0.05). Thus, markedly enhanced levels of serum VEGF are present one to three weeks after trauma or burn injury. Further, occurrence of severe complications during the post-traumatic period is associated with lesser increases of serum VEGF.

Income change at retirement

Less than one-fourth of SIPP retirees made the transition from full-time work and no benefit receipt to benefit receipt and no work during the 32 months of the 1984 SIPP panel, and these full retirees appear to differ economically from those who had not fully retired during the 32 months. Income declined to 60 percent of preretirement levels for those who received Social Security benefits and employer pensions, and to 46 percent for those who received only one type of benefit after ceasing full-time work. Family income cushioned the decline. About one-third of retirees had larger changes in income shortly before or after observed retirement transitions than right at the retirement transitions. Larger changes in income were associated with changing one's work effort. Asset income was quite variable across the entire panel. A monthly view of persons in various stages of retirement has brought into sharper focus than has previously been recognized the many paths into retirement, often over an extended period of time, during which status may change back and forth. A one-time or occasional extension of the SIPP to perhaps a 5-year period for the near-retirement-age population would increase the proportion of retirees for whom the full process is observed in such great detail. Alternatively, information on the timing of changes in labor-force and pension status may be collected in a proposed new Retirement History Study that calls for interviewing persons at 2-year intervals over a 10-year period for a more accurate picture of the retirement process.

Income and assets of Social Security beneficiaries by type of benefit

The SIPP data have provided a first look at the relative economic status of various types of Social Security beneficiaries. They have shown that the different types of Social Security beneficiaries face very different economic circumstances. Retired workers and wife beneficiaries have the highest family incomes adjusted for family size. Aged widows and minor children have the lowest family incomes, with high proportions of poor or near poor. And disabled workers are in between, but also have high proportions of poor or near poor. Retired-worker and wife beneficiary households also have considerably more asset holdings than disabled-worker or widow beneficiary households. Beneficiaries with high family incomes are very likely to live with relatives and to rely heavily on the relatives' income. The high-income families tend to have non-means-tested sources of family income other than Social Security amounting to substantial proportions of their total income and to have high asset holdings. Conversely, beneficiaries with low family incomes are very likely to live alone or with nonrelatives, to rely heavily on Social Security and means-tested benefits, and to have low asset holdings. A majority of ever-poor beneficiaries (with the exception of widow beneficiaries) are poor in only some months of a year. This situation is not consistent with the stereotype of beneficiaries living on fixed incomes. But the change in poverty status is often due to a change in the income of other family members rather than of the beneficiary. And in some cases, a change in poverty status occurs with little or no change in income as the cost of living rises.

Income of retirement-aged persons in the United States

This article reviews the composition and level of retirement income in the United States and how this has changed over time, focusing on two overlapping but distinct groups--the entire population aged 65 or older, and recent retirees. Changes in the composition of income of the aged over the past 20-30 years, including greatly expanded Social Security and pension coverage and an increasing number of persons with retirement savings, have improved the economic status of the aged not only in comparison with the aged in earlier years, but also in comparison with younger adults who derive most of their income from earnings. New retired workers are better off than the total aged population in several respects. The younger cohorts now in the labor force will spend more of their working lives in the more favorable conditions now present than was true of past new beneficiaries or the aged as a whole. It is, therefore, not unreasonable to expect that today's workers will enjoy more and larger pensions and increased income from savings to supplement their Social Security benefits when they retire.

Economic security, 1935-85

The economic well-being of both working and retired persons has improved significantly since the Social Security Act was passed in 1935. More people are employed now than at any time since then, despite declining employment among the aged and more years of school attendance among the young. The ratio of non-workers to workers--a broad measure of dependency--is lower now than at any time since the 1930's. Social security has grown and matured to become a strong foundation of retirement income, and other work-related employee benefits have grown in tandem with social security. Employer contributions for social insurance and related employee benefits have grown from being about a 1-percent supplement to aggregate wages and salaries in 1929 to nearly 20 percent today. Social security and Medicare account for just over a fourth of employer contributions, while other public and private retirement systems represent just over another fourth. The balance of benefits for active workers includes group health and life insurance, unemployment insurance, workers' compensation, temporary disability insurance, and related benefits. Pay for holidays, vacations, and sick leave is estimated to have increased from less than 1 percent of aggregate pay in 1929 to about 10 percent today. The improved economic status of the aged has been documented by a series of surveys beginning in 1941-42 and carried out from time to time until 1972 and biennially since 1976. The earlier surveys were supplemented with estimates from record data and tables from the Bureau of the Census. The income of the aged as a whole has grown by about 75 percent over the past 2 decades after taking inflation into account. The income of the aged as a whole grew faster than that of the nonaged in the 1970's and early 1980's when real social security benefits increased faster than inflation and wages lagged behind it. New beneficiaries in 1982 were in better health and were more likely to retire because they wanted to than was true of their counterparts in the early 1940's. Not only have benefits continued to be the main component of income of the aged as total incomes have grown, but also benefits have become much larger in relation to average earnings than used to be the case. Retired workers are much more likely now than in the early 1940's to have other pensions or income from assets to supplement benefits.(ABSTRACT TRUNCATED AT 400 WORDS)

Incomes of the aged and nonaged, 1950-82

This article discusses the results of previous studies, both supportive and nonsupportive of the conclusion that the aged are as well-off as the nonaged, and then presents a range of figures from Bureau of the Census reports over the period 1950-82 that measure both the incomes of the aged and nonaged and those of subgroups within these populations. Census figures indicate that the aged and nonaged have about equal levels of average per capita family income and that about the same proportions of these groups have incomes below the poverty line. However, aged unrelated individuals, who account for about a third of all aged persons, have less than three-fifths the income of nonaged unrelated individuals. When the per capita family income of the aged is compared separately with that of families headed by persons aged 25-44 and 45-64, aged persons receive more than those under age 45 but less than those aged 45-64. Trends in the economic status of the aged and nonaged over the period 1950-82 indicate numerous fluctuations rather than a consistent improvement in the income of either group in relation to the other.

Civil Service Retirement System annuitants and Social Security

This article examines the extent to which annuitants of the Federal employee Civil Service Retirement System (CSRS) are entitled to Social Security benefits. It is based on linked administrative data files from the two systems. Ninety-one percent of all those receiving CSRS annuities in 1979 had worked at some point in their careers in jobs that were covered by Social Security. Almost 80 percent of the annuitants aged 65 or older were entitled to Medicare benefits. Of those aged 62 or older, 73 percent were entitled to Social Security cash benefits, including 10 percent who were entitled only as spouses or survivors of workers covered under Social Security. About 39 percent of the female annuitants aged 62 or older were entitled to benefits as wives or widows of workers, including some who were also insured for benefits based on their own earnings. The average combined benefit for those with Social Security was +845 among annuitants aged 62 or older. For those receiving only CSRS annuities, the average was +959. Those not entitled to a Social Security benefit tended to have more years of Federal service and higher annuities than did those entitled to both a Social Security benefit and a CSRS annuity.

Impact on widows of proposed changes in OASI mother's benefits

This article examines the impact of a proposal that would terminate mother's and father's benefits when the youngest child in the care of a surviving spouse reaches age 16, rather than age 18, as in current law. Losses to families and savings to the social security program are estimated by taking into account 1977 benefit receipts and provisions in the Social Security Act that would have affected benefit size in that year. The study reveals that very few widowers would have lost any income if such a provision had been in effect at that time. Two-thirds of the families headed by widows with a youngest child aged 16-17, however, would have lost some income. Sixteen percent would have had family incomes below the poverty line without mother's benefits-10 percent already below the line and 6 percent that would have fallen below it because of the change. Fewer families with children under age 16 than with youngest children aged 16-17 would have lost benefits, but more would have been in poverty. An analysis of the characteristics of widows who might use a short-term training benefit if one were made available to them suggests that such a benefit would not be used by those widows who are neediest economically and educationally.

Income of the population aged 55 and older, 1976

This article inaugurates a biennial series of reports on the income of persons aged 55 and older. The survey on which it is based updates information on the broad economic picture for a cross-section of this population and forms the basis for an analysis of trends in the financial status of the group as a whole. In 1976, persons aged 55--61 were generally working full-time, not receiving income from pensions, and married. Those aged 73 and older were predominantly not working, mostly recipients of retirement pensions, and likely to be widows. Most persons aged 65 and older were receiving social security benefits; those with neither earnings nor a second pension constituted the largest subgroup and had the lowest median income. Married couples, as a group, were substantially better off financially than nonmarried persons because they were younger, two-person rather than one-person units, and had one member who was a man. Blacks were less likely than whites to receive pension income other than social security benefits, and their median benefits and earnings were both lower.

New retirees and the stability of the retirement decision

Examination of the benefit awards data under the social security program may furnish at least partial answers to two questions: What is the estimated number retiring each year? How long do they stay retired? The calculation presented here of the number retiring in 1971 takes into account the fact that the date of the award is not necessarily the actual date of retirement. The difference has been accentuated since Medicare began and individuals have been filing for benefits solely to obtain hospital insurance protection, continuing to earn at a level that precludes receipt of benefits under the earnings test. This study of the data on payable benefits indicates that only a small proportion of retirees lose benefits because of their return to work within 3 years after award.