Feedback control of selected biosynthetic activities of chondrocytes in culture

Assessment of TGF-β3 on production of aggrecan by human articular chondrocytes in pellet culture system

Background:

The Autologous Chondrocytes Transplantation (ACT) method is being studied for repair of cartilage diseases. As the chondrocytes dedifferentiated during monolayer culture, three-dimensional cultures are suggested to redifferentiate them. The aim of this study was investigation of the effect of TGF-β3 growth factor on chondrocytes in pellet culture system.

Materials and Methods:

The chondrocytes were isolated from three human articular cartilages by enzymatic digestion. The cells of the second passage were transferred to pellet culture system. We determined the chondrogenic medium with TGF-β3 as the experimental group and without it as the control group. After 2 weeks, the aggrecan production was investigated using histological and immunohistochemical (IHC) methods.

Results:

The presence of glycosaminoglycans was proved through Toluiden blue staining. Comparison of IHC results using MATLAB software showed that aggrecan in the experimental group was significantly higher than in the control group (P ≤ 0.05).

Conclusion:

The presence of TGF-β3 in the chondrogenic medium could lead to the production of more aggrecan in chondrocytes cultivated in pellet culture system.

Static compression of articular cartilage can reduce solute diffusivity and partitioning: implications for the chondrocyte biological response

Chondrocytes depend upon solute transport within the avascular extracellular matrix of adult articular cartilage for many of their biological activities. Alterations to bioactive solute transport may, therefore, represent a mechanism by which cartilage compression is transduced into cellular metabolic responses. We investigated the effects of cartilage static compression on diffusivity and partitioning of a range of model solutes including dextrans of molecular weights 3 and 40 kDa, and tetramethylrhodamine (a 430 Da fluorophore). New fluorescence methods were developed for real-time visualization and measurement of transport within compressed cartilage explants. Experimental design allowed for multiple measurements on individual explants at different compression levels in order to minimize confounding influences of compositional variations. Results demonstrate that physiological levels of static compression may significantly decrease solute diffusivity and partitioning in cartilage. Effects of compression were most dramatic for the relatively high molecular weight solutes. For 40 kDa dextran, diffusivity decreased significantly (p<0.01) between 8% and 23% compression, while partitioning of 3 and 40 kDa dextran decreased significantly (p<0.01) between free-swelling conditions and 8% compression. Since diffusivity and partitioning can influence pericellular concentrations of bioactive solutes, these observations support a role for perturbations to solute transport in mediating the cartilage biological response to compression.

Mechanical compression alters proteoglycan deposition and matrix deformation around individual cells in cartilage explants

We have used new techniques of cell-length scale quantitative autoradiography to assess matrix synthesis, deposition, and deformation around individual chondrocytes in mechanically compressed cartilage explants. Our objectives were to: (1) quantify the effects of static and dynamic compression on the deposition of newly synthesized proteoglycans into cell-associated and further-removed matrices; (2) measure cell-length scale matrix strains and morphological changes of the cell and matrix associated with tissue compression; and (3) relate microscopic physical stimuli to changes in proteoglycan synthesis as functions of compression level and position within mechanically compressed explants. Results indicate a high degree of structural organization in the extracellular matrix, with the pericellular matrix associated with the most rapid rates of proteoglycan deposition, and greatest sensitivity to mechanical compression. Static compression could stimulate directional deposition of secreted proteoglycans around chondrocytes, superimposed on an inhibition of proteoglycan synthesis; these events followed trends for compressive strain in the cell-associated matrix. Conversely, proteoglycan synthesis and pericellular deposition was stimulated by dynamic compression. Results suggest that cell-matrix interactions in the cell-associated matrix may be a particularly important aspect of the chondrocyte response to mechanical compression, possibly involving macromolecular transport limitations and morphological changes associated with fluid flow and local compaction of the matrix around cells.

Exogenous glycosaminoglycans (GAG) differentially modulate GAG synthesis by anchorage-independent cultures of the outer cells from neonatal rat calvaria in the absence and presence of TGF-beta

In anchorage-dependent (AD) cultures of the outer cell population (OCP) from neonatal rat calvaria, transforming growth factor-beta 1 (TGF-beta) specifically upregulated the synthesis of chondroitin sulfate (CS) proteoglycan (PG) and uncoupled the inhibitory effect of increasing cell density on CS PG synthesis (reference #30). Utilizing the same cell population, we have further examined the possibility that glycosaminoglycans (GAG) known to be synthesized and secreted by bone cells might exert feedback effects on GAG synthesis and/or its stimulation by TGF-beta. Although addition of TGF-beta alone stimulated net synthesis of HA and CS in both AD and anchorage-independent (AI) cultures, significant alterations of basal and TGF-beta-stimulated GAG synthesis by exogenous GAGs were observed only in AI cultures. In AI cultures exogenously added hyaluronic acid (HA) markedly enhanced the basal synthesis of HA and CS while heparin (H) suppressed the basal synthesis of HA, CS as well as dermatan sulfate (DS). Also, the addition of HA markedly potentiated the stimulation by TGF-beta of HA and CS synthesis as did heparan sulfate (HS) for CS and DS synthesis. H suppressed the stimulation of the synthesis of HA, CS and DS by TGF-beta. Overall, our results indicate specific effects of individual GAGs on basal and TGF-beta-stimulated GAG synthesis in OCP cultures. We suggest that some of the GAGs in the OCP microenvironment (which with the exception of HA are covalently linked to protein cores of secreted PGs), acting in concert with TGF-beta, may serve as an amplification system for upregulating GAG synthesis in the rapidly growing neonatal calvarium.

Characterization of proteoglycan accumulation during formation of cartilagenous tissue in vitro

In order to study proteoglycan retention and accumulation, we optimized a chondrocyte cell culture system in which isolated bovine articular chondrocytes accumulate extracellular matrix to form a continuous layer of cartilagenous tissue. The tissue can attain a thickness of up to 110 microns by 35 days. The cells synthesize large keratan sulfate containing proteoglycans and type II collagen indicating that the chondrocytes maintain their phenotype in these culture conditions. Matrix accumulation is enhanced by increased cell density and the presence of serum and ascorbic acid. The amount of proteoglycans synthesized by the chondrocytes increases up to day 21 and then decreases to the same levels as are synthesized during the first week of culture. The percentage of newly synthesized proteoglycans retained in the matrix increases from 20% on day 6 to a maximum of 85% by day 35. The proteoglycan and collagen content in the tissue increases with time in culture. The changes in the percentage of proteoglycans retained parallels the increase in proteoglycan content. After day 35, there is no further increase in the amount of proteoglycans and collagen nor in the percentage of newly synthesized proteoglycans retained in the extracellular matrix. These studies demonstrate that the cultures are going through two phases: one of matrix accumulation and then one of maintaining the existing matrix. The period of matrix accumulation occurs between days 10-21 whereas matrix maintenance is observed after day 35. Using this culture system to study proteoglycan accumulation and maintenance during these culture periods may prove useful in identifying the mechanisms regulating these processes.

Interaction of human leukocyte elastase with soluble and insoluble protein substrates. A practical kinetic approach

A progress-curve kinetic method was developed to investigate the interaction between human leukocyte elastase and macromolecular substrates, such as insoluble elastin and soluble plasma proteins. A fluorogenic, synthetic peptide (reporter substrate) was incubated in the presence of finely powdered elastin and enzyme under continuous stirring. The progress curves, which corresponded to the release of product from the reporter substrate, were very sensitive to the presence of various amounts of the macromolecular substrate. The kinetic parameters for the interaction between elastase and elastin were calculated using a pre-steady-state approach characteristic of slow-binding inhibitors. The interaction of elastase with the soluble protein substrates was studied with similar techniques, but formally treating the substrates as classical, fully competitive inhibitors. The adsorption of elastase on insoluble elastin was a time-dependent process consisting of at least three observable phases: The first step was a rapid formation of an encounter complex followed by a very slow step lasting several minutes, and the third step consisted of a steady-state release of products. On the contrary, elastase very rapidly formed productive complexes with bovine serum albumin and a human monoclonal immunoglobulin G. The progress-curve method was also suitable for analyzing the behavior of inhibitors in the presence of protein substrates. The kinetic parameters which characterize the interaction between elastase and protein substrates represent a practical tool to formulate hypotheses on the efficiency of inhibitors in vivo.