Remodeling of the rough endoplasmic reticulum during stimulation of procollagen secretion by ascorbic acid in cultured chondrocytes. A biochemical and morphological study

Visualization of ER-to-Golgi trafficking of procollagen X

Collagen is the most abundant protein in the extracellular matrix of animals, and 28 types of collagen have been reported in humans. We previously analyzed the endoplasmic reticulum (ER)-to-Golgi transport of fibril-forming type III collagen (Hirata et al., 2022) and network-forming type IV collagen (Matsui et al., 2020), both of which have long collagenous triple-helical regions. To understand the ER-to-Golgi trafficking of various types of collagens, we analyzed the transport of short-chain type X collagen in this study. We fused cysteine-free GFP to the N-telopeptide region of procollagen X (GFP-COL10A1), as employed in our previous analysis of procollagens III and IV, and analyzed its transport by live-cell imaging. Procollagen X was transported to the Golgi apparatus via vesicular and tubular carriers containing ERGIC53 and RAB1B, similar to those used for procollagen III. Carriers containing procollagen X probably used the same transport processes as those containing conventional cargoes such as α1-antitrypsin. SAR1, TANGO1, SLY1/SCFD1, and BET3/TRAPPC3 were required for trafficking of procollagen X, which are different from the factors required for trafficking of procollagens III (SAR1, TANGO1, and CUL3) and IV (SAR1 and SLY1/SCFD1). These findings reveal that accommodation of various types of collagens with different shapes into carriers may require fine-tuning of the ER-to-Golgi transport machinery.Key words: collagen, GFP-procollagen X, ER-to-Golgi trafficking, export from ER, TANGO1.

Genetically encoded fluorescent tools: Shining a little light on ER-to-Golgi transport

Since the first fluorescent proteins (FPs) were identified and isolated over fifty years ago, FPs have become commonplace yet indispensable tools for studying the constitutive secretory pathway in live cells. At the same time, genetically encoded chemical tags have provided a new use for much older fluorescent dyes. Innovation has also produced several specialized methods to allow synchronous release of cargo proteins from the endoplasmic reticulum (ER), enabling precise characterization of sequential trafficking steps in the secretory pathway. Without the constant innovation of the researchers who design these tools to control, image, and quantitate protein secretion, major discoveries about ER-to-Golgi transport and later stages of the constitutive secretory pathway would not have been possible. We review many of the tools and tricks, some 25 years old and others brand new, that have been successfully implemented to study ER-to-Golgi transport in intact and living cells.

ER-to-Golgi trafficking of procollagen III via conventional vesicular and tubular carriers

Collagen is the major protein component of the extracellular matrix. Synthesis of procollagens starts in the endoplasmic reticulum (ER), and three ⍺ chains form a rigid triple helix 300-400 nm in length. It remains unclear how such a large cargo is transported from the ER to the Golgi apparatus. In this study, to elucidate the intracellular transport of fibril-forming collagens, we fused cysteine-free GFP to the N-telopeptide region of procollagen III (GFP-COL3A1) and analyzed transport by live-cell imaging. We found that the maturation dynamics of procollagen III were largely different from those of network-forming procollagen IV (Matsui et al. 2020). Proline hydroxylation of procollagen III uniquely triggered the formation of intralumenal droplet-like structures similar to events caused by liquid-liquid phase separation, and ER exit sites surrounded large droplets containing chaperones. Procollagen III was transported to the Golgi apparatus via vesicular and tubular carriers containing ERGIC53 and RAB1B; this process required TANGO1 and CUL3, which we previously reported were dispensable for procollagen IV. GFP-COL3A1 and mCherry-⍺1AT were co-transported in the same vesicle. Based on these findings, we propose that shortly after ER exit, enlarged carriers containing procollagen III fuse to ERGIC for transport to the Golgi apparatus by conventional cargo carriers. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].

Magnetic Nanoparticles and Magnetic Field Exposure Enhances Chondrogenesis of Human Adipose Derived Mesenchymal Stem Cells But Not of Wharton Jelly Mesenchymal Stem Cells

Purpose: Iron oxide based magnetic nanoparticles (MNP) are versatile tools in biology and medicine. Adipose derived mesenchymal stem cells (ADSC) and Wharton Jelly mesenchymal stem cells (WJMSC) are currently tested in different strategies for regenerative regenerative medicine (RM) purposes. Their superiority compared to other mesenchymal stem cell consists in larger availability, and superior proliferative and differentiation potential. Magnetic field (MF) exposure of MNP-loaded ADSC has been proposed as a method to deliver mechanical stimulation for increasing conversion to musculoskeletal lineages. In this study, we investigated comparatively chondrogenic conversion of ADSC-MNP and WJMSC with or without MF exposure in order to identify the most appropriate cell source and differentiation protocol for future cartilage engineering strategies. Methods: Human primary ADSC and WJMSC from various donors were loaded with proprietary uncoated MNP. The in vitro effect on proliferation and cellular senescence (beta galactosidase assay) in long term culture was assessed. In vitro chondrogenic differentiation in pellet culture system, with or without MF exposure, was assessed using pellet histology (Safranin O staining) as well as quantitative evaluation of glycosaminoglycan (GAG) deposition per cell. Results: ADSC-MNP complexes displayed superior proliferative capability and decreased senescence after long term (28 days) culture in vitro compared to non-loaded ADSC and to WJMSC-MNP. Significant increase in chondrogenesis conversion in terms of GAG/cell ratio could be observed in ADSC-MNP. MF exposure increased glycosaminoglycan deposition in MNP-loaded ADSC, but not in WJMSC. Conclusion: ADSC-MNP display decreased cellular senescence and superior chondrogenic capability in vitro compared to non-loaded cells as well as to WJMSC-MNP. MF exposure further increases ADSC-MNP chondrogenesis in ADSC, but not in WJMSC. Loading ADSC with MNP can derive a successful procedure for obtaining improved chondrogenesis in ADSC. Further in vivo studies are needed to confirm the utility of ADSC-MNP complexes for cartilage engineering.

Profound architectural and functional readjustments of the secretory pathway in decidualization of endometrial stromal cells

Certain cell types must expand their exocytic pathway to guarantee efficiency and fidelity of protein secretion. A spectacular case is offered by decidualizing human endometrial stromal cells (EnSCs). In the midluteal phase of the menstrual cycle, progesterone stimulation induces proliferating EnSCs to differentiate into professional secretors releasing proteins essential for efficient blastocyst implantation. Here, we describe the architectural rearrangements of the secretory pathway of a human EnSC line (TERT-immortalized human endometrial stromal cells (T-HESC)). As in primary cells, decidualization entails proliferation arrest and the coordinated expansion of the entire secretory pathway without detectable activation of unfolded protein response (UPR) pathways. Decidualization proceeds also in the absence of ascorbic acid, an essential cofactor for collagen biogenesis, despite also the secretion of some proteins whose folding does not depend on vitamin C is impaired. However, even in these conditions, no overt UPR induction can be detected. Morphometric analyses reveal that the exocytic pathway does not increase relatively to the volume of the cell. Thus, differently from other cell types, abundant production is guaranteed by a coordinated increase of the cell size following arrest of proliferation.

Ascorbic Acid and Iron Supplement Treatment Improves Stem Cell-Mediated Cartilage Regeneration in a Minipig Model

BACKGROUND

The transplantation of mesenchymal stem cells (MSCs) into cartilage defects has led to variable cartilage repair outcomes. Previous in vitro studies have shown that ascorbic acid and reduced iron independently can improve the chondrogenic differentiation of MSCs. However, the combined effect of ascorbic acid and iron supplementation on MSC differentiation has not been investigated.

PURPOSE

To investigate the combined in vivo effects of ascorbic acid and a US Food and Drug Administration (FDA)-approved iron supplement on MSC-mediated cartilage repair in mature Göttingen minipigs.

STUDY DESIGN

Controlled laboratory study.

METHODS

We pretreated bone marrow-derived MSCs with ascorbic acid and the FDA-approved iron supplement ferumoxytol and then transplanted the MSCs into full-thickness cartilage defects in the distal femurs of Göttingen minipigs. Untreated cartilage defects served as negative controls. We evaluated the cartilage repair site with magnetic resonance imaging at 4 and 12 weeks after MSC implantation, followed by histological examination and immunofluorescence staining at 12 weeks.

RESULTS

Ascorbic acid plus iron-pretreated MSCs demonstrated a significantly better MOCART (magnetic resonance observation of cartilage repair tissue) score (73.8 ± 15.5), better macroscopic cartilage regeneration score according to the International Cartilage Repair Society (8.6 ± 2.0), better Pineda score (2.9 ± 0.8), and larger amount of collagen type II (28,469 ± 21,313) compared with untreated controls (41.3 ± 2.5, 1.8 ± 2.9, 12.8 ± 1.9, and 905 ± 1326, respectively). The obtained scores were also better than scores previously reported in the same animal model for MSC implants without ascorbic acid.

CONCLUSION

Pretreatment of MSCs with ascorbic acid and an FDA-approved iron supplement improved the chondrogenesis of MSCs and led to hyaline-like cartilage regeneration in the knee joints of minipigs.

CLINICAL RELEVANCE

Ascorbic acid and iron supplements are immediately clinically applicable. Thus, these results, in principle, could be translated into clinical applications.

XBP1 signalling is essential for alleviating mutant protein aggregation in ER-stress related skeletal disease

The unfolded protein response (UPR) is a conserved cellular response to the accumulation of proteinaceous material in endoplasmic reticulum (ER), active both in health and disease to alleviate cellular stress and improve protein folding. Multiple epiphyseal dysplasia (EDM5) is a genetic skeletal condition and a classic example of an intracellular protein aggregation disease, whereby mutant matrilin-3 forms large insoluble aggregates in the ER lumen, resulting in a specific 'disease signature' of increased expression of chaperones and foldases, and alternative splicing of the UPR effector XBP1. Matrilin-3 is expressed exclusively by chondrocytes thereby making EDM5 a perfect model system to study the role of protein aggregation in disease. In order to dissect the role of XBP1 signalling in aggregation-related conditions we crossed a p.V194D Matn3 knock-in mouse model of EDM5 with a mouse line carrying a cartilage specific deletion of XBP1 and analysed the resulting phenotype. Interestingly, the growth of mice carrying the Matn3 p.V194D mutation compounded with the cartilage specific deletion of XBP1 was severely retarded. Further phenotyping revealed increased intracellular retention of amyloid-like aggregates of mutant matrilin-3 coupled with dramatically decreased cell proliferation and increased apoptosis, suggesting a role of XBP1 signalling in protein accumulation and/or degradation. Transcriptomic analysis of chondrocytes extracted from wild type, EDM5, Xbp1-null and compound mutant lines revealed that the alternative splicing of Xbp1 is crucial in modulating levels of protein aggregation. Moreover, through detailed transcriptomic comparison with a model of metaphyseal chondrodysplasia type Schmid (MCDS), an UPR-related skeletal condition in which XBP1 was removed without overt consequences, we show for the first time that the differentiation-state of cells within the cartilage growth plate influences the UPR resulting from retention of a misfolded mutant protein and postulate that modulation of XBP1 signalling pathway presents a therapeutic target for aggregation related conditions in cells undergoing proliferation.

How early studies on secreted and membrane protein quality control gave rise to the ER associated degradation (ERAD) pathway: the early history of ERAD

All newly synthesized proteins are subject to quality control check-points, which prevent aberrant polypeptides from harming the cell. For proteins that ultimately reside in the cytoplasm, components that also reside in the cytoplasm were known for many years to mediate quality control. Early biochemical and genetic data indicated that misfolded proteins were selected by molecular chaperones and then targeted to the proteasome (in eukaryotes) or to proteasome-like particles (in bacteria) for degradation. What was less clear was how secreted and integral membrane proteins, which in eukaryotes enter the endoplasmic reticulum (ER), were subject to quality control decisions. In this review, we highlight early studies that ultimately led to the discovery that secreted and integral membrane proteins also utilize several components that constitute the cytoplasmic quality control machinery. This component of the cellular quality control pathway is known as ER associated degradation, or ERAD. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Laserterapia de baixa intensidade na expressão de colágeno após lesão muscular cirúrgica

A laserterapia é um procedimento utilizado em larga escala nas lesões musculoesqueléticas, devido as suas diversas propriedades, antiinflamatórias, cicatrizantes entre outras. Além disso, há tipos distintos de aparelhos de laser. Mesmo com os diversos modelos experimentais existentes na literatura, não há um consenso sobre a faixa de utilização, bem como o tipo de laser que promove melhor reparo no tecido muscular. Este estudo visa analisar os efeitos da laserterapia de baixa intensidade na expressão de colágeno após lesão muscular. Camundongos Swiss albinos (n=18) foram submetidos à lesão muscular cirúrgica e divididos em dois grupos, controle (C) e teste (T). Os animais foram submetidos a uma irradiação diária de 5 J/cm² pelos lasers AsGaAl 830 nm e AsGa 904 nm e, em diferentes tempos de sacrifício (7 e 14 dias). Os resultados não demonstraram diferença estatística significativa na expressão de colágeno em ambos os grupos analisados. Contudo, os dados apontam que a dose de 5 J/cm² do laser AsGa 904 nm promoveu maior deposição de fibras colágenas após 14 dias de tratamento, sugerindo que a terapia seja efetiva na síntese de colágeno. Outros estudos experimentais, em humanos, devem ser propostos para maiores inferências sobre os resultados do laser no tratamento da lesão muscular.

Extracellular matrix genes as hypoxia-inducible targets

Low oxygen tension, i.e., hypoxia, is a pathophysiological component involved in many human disorders but is also a critically important phenomenon in normal development and differentiation. The ability of cells to survive under hypoxia or to adapt to it depends on a family of hypoxia-inducible transcription factors (HIFs) that induce the expression of a number of genes involved in hematopoiesis, angiogenesis, iron transport, glucose utilization, resistance to oxidative stress, cell proliferation, survival and apoptosis, and extracellular matrix homeostasis. We introduce here the recently identified molecular mechanisms responsible for the oxygen-dependent stability and activity of HIF, after which we focus on extracellular matrix genes as HIF targets. The vital role of the hypoxia response pathway in chondrogenesis and joint development is then discussed.

Low-level laser irradiation modulates matrix metalloproteinase activity and gene expression in porcine aortic smooth muscle cells

BACKGROUND AND OBJECTIVES

The vascular extracellular matrix is maintained by a dynamic balance between matrix synthesis and degradation. This equilibrium is disrupted in arterial pathologies such as abdominal aortic aneurysm. Low-level laser irradiation (LLLI) promotes wound healing. However, its effect on smooth muscle cells (SMCs), a central player in these responses, has not been established. The current study was designed to determine the effects of LLLI on arterial SMC proliferation, inflammatory markers, and matrix proteins.

STUDY DESIGN/MATERIALS AND METHODS

Porcine primary aortic SMCs were irradiated with a 780 nm laser diode (1 and 2 J/cm(2)). Trypan blue exclusion assay, immunofluorescent staining for collagen I and III, Sircol assay, gelatin zymography, and RT-PCR were used to monitor proliferation; collagen trihelix formation; collagen synthesis; matrix metalloproteinase-2 (MMP-2) activity, and gene expression of MMP-1, MMP-2, tissue inhibitor of MMP-1 (TIMP-1), TIMP-2, and IL-1-beta, respectively.

RESULTS

LLLI-increased SMC proliferation by 16 and 22% (1 and 2 J/cm(2), respectively) compared to non-irradiated cells (P<0.01 and P<0.0005). Immediately after LLLI, trihelices of collagen I and III appeared as perinuclear fluorescent rings. Collagen synthesis was increased twofold (2 days after LLLI: 14.3+/-3.5 microg, non-irradiated control: 6.6+/-0.7 microg, and TGF-beta stimulated control: 7.1+/-1.2 microg, P<0.05), MMP-2 activity after LLLI was augmented (over non-irradiated control) by 66+/-18% (2 J/cm(2); P<0.05), and MMP-1 gene expression upregulated. However, TIMP-2 was upregulated, and MMP-2 gene expression downregulated. IL-1-beta gene expression was reduced.

CONCLUSIONS

LLLI stimulates SMC proliferation, stimulates collagen synthesis, modulates the equilibrium between regulatory matrix remodeling enzymes, and inhibits pro-inflammatory IL-1-beta gene expression. These findings may be of therapeutic relevance for arterial diseases such as aneurysm where SMC depletion, weakened extracellular matrix, and an increase in pro-inflammatory markers are major pathologic components.

Steroid hormones strongly support bovine articular cartilage integration in the absence of interleukin-1β

OBJECTIVE

Posttraumatic integration of articular cartilage at fracture sites is essential for mechanical stability of cartilage, and ruptured cartilage is a prerequisite for early osteoarthritis. This study was undertaken to investigate effects on articular cartilage integration mediated by steroid hormones, interleukin-1beta (IL-1beta), and combinations thereof.

METHODS

Articular cartilage blocks were cultured in partial apposition for 2 weeks with ascorbic acid, testosterone, 17beta-estradiol, and dehydroepiandrosterone (DHEA), with or without IL-1beta. Mechanical integration was measured as adhesive strength, i.e., the maximum force at rupture of integrated cartilage blocks divided by the overlap area. Glycosaminoglycan content was used to study synthesized extracellular matrix.

RESULTS

Culture in medium without supplements did not lead to integration (adhesive strength 0 kPa). With administration of ascorbic acid (100 microg/ml), the median adhesive strength was 49 kPa. In comparison with ascorbic acid alone, all steroid hormones induced a strong, concentration-dependent stimulation of integration (with maximum values observed with DHEA at 3 x 10(-5)M, testosterone at 10(-8)M, and 17beta-estradiol at 10(-11)M). For testosterone and 17beta-estradiol, this was also reflected by an increase of glycosaminoglycan content. Adhesive strength was increased with IL-1beta at 10 pg/ml, but not at 1 pg/ml or 100 pg/ml. In the presence of both IL-1beta and sex hormones, integration of articular cartilage was reduced.

CONCLUSION

This is the first study to demonstrate that steroid hormones such as 17beta-estradiol, DHEA, and testosterone stimulate articular cartilage integration. This effect is abrogated by low concentrations of IL-1beta. In the absence of IL-1beta or after neutralization of IL-1beta, steroid hormones might be favorable adjuvant compounds to optimize cartilage integration.

Nutraceutical Therapies for Degenerative Joint Diseases: A Critical Review

There is growing recognition of the importance of nutritional factors in the maintenance of bone and joint health, and that nutritional imbalance combined with endocrine abnormalities may be involved in the pathogenesis of osteoarthritis (OA) and osteochondritis dissecans (OCD). Despite this, dietary programs have played a secondary role in the management of these connective tissue disorders. Articular cartilage is critically dependent upon the regular provision of nutrients (glucose and amino acids), vitamins (particularly vitamin C), and essential trace elements (zinc, magnesium, and copper). Therefore, dietary supplementation programs and nutraceuticals used in conjunction with non-steroidal, anti-inflammatory drugs (NSAIDs) may offer significant benefits to patients with joint disorders, such as OA and OCD. This article examines the available clinical evidence for the efficacy of nutraceuticals, antioxidant vitamin C, polyphenols, essential fatty acids, and mineral cofactors in the treatment of OA and related joint disorders in humans and veterinary species. This article also attempts to clarify the current state of knowledge. It also highlights the need for additional targeted research to elucidate the changes in nutritional status and potential alterations to the expression of plasma membrane transport systems in synovial structures in pathophysiological states, so that current therapy and future treatments may be better focused.

Prefibrillar amyloid protein aggregates share common features of cytotoxicity

The intracellular free Ca(2+) concentration and redox status of murine fibroblasts exposed to prefibrillar aggregates of the HypF N-terminal domain have been investigated in vitro and in vivo using a range of fluorescent probes. Aggregate entrance into the cytoplasm is followed by an early rise of reactive oxygen species and free Ca(2+) levels and eventually by cell death. Such changes correlate directly with the viability of the cells and are not observed when cell are cultured in the presence of reducing agents or in Ca(2+)-free media. In addition, moderate cell stress following exposure to the aggregates was found to be fully reversible. The results show that the cytotoxicity of prefibrillar aggregates of HypF-N, a protein not associated with clinical disease, has the same fundamental origin as that produced by similar types of aggregates of proteins linked with specific amyloidoses. These findings suggest that misfolded proteinaceous aggregates stimulate generic cellular responses as a result of the exposure of regions of the structure (such as hydrophobic residues and the polypeptide main chain) that are buried in the normally folded proteins. They also support the idea that a higher number of degenerative pathologies than previously known might be considered as protein deposition diseases.

The effects of ascorbic acid on cartilage metabolism in guinea pig articular cartilage explants

Ascorbic acid has been associated with the slowing of osteoarthritis progression in guinea pig and man. The goal of this study was to evaluate transcriptional and translational regulation of cartilage matrix components by ascorbic acid. Guinea pig articular cartilage explants were grown in the presence of L-ascorbic acid (L-Asc), D-isoascorbic acid (D-Asc), sodium L-ascorbate (Na L-Asc), sodium D-isoascorbate (Na D-Asc), or ascorbyl-2-phosphate (A2P) to isolate and analyze the acidic and nutrient effects of ascorbic acid. Transcription of type II collagen, prolyl 4-hydroxylase (alpha subunit), and aggrecan increased in response to the antiscorbutic forms of ascorbic acid (L-Asc, Na L-Asc, and A2P) and was stereospecific to the L-forms. Collagen and aggrecan synthesis also increased in response to the antiscorbutic forms but only in the absence of acidity. All ascorbic acid forms tended to increase oxidative damage over control. This was especially true for the non-nutrient D-forms and the high dose L-Asc. Finally, we investigated the ability of chondrocytes to express the newly described sodium-dependent vitamin C transporters (SVCTs). We identified transcripts for SVCT2 but not SVCT1 in guinea pig cartilage explants. This represents the first characterization of SVCTs in chondrocytes. This study confirms that ascorbic acid stimulates collagen synthesis and in addition modestly stimulates aggrecan synthesis. These effects are exerted at both transcriptional and post-transcriptional levels. The stereospecificity of these effects is consistent with chondrocyte expression of SVCT2, shown previously to transport L-Asc more efficiently than D-Asc. Therefore, this transporter may be the primary mechanism by which the L-forms of ascorbic acid enter the chondrocyte to control matrix gene activity.

Axonal secretion of Reelin by Cajal-Retzius cells: evidence from comparison of normal and Reln(Orl) mutant mice

A novel secretory pathway has been identified in the study of mice homozygous for the Reln(Orl) mutation, a line characterised by the defective secretion of the large extracellular matrix glycoprotein Reelin. By using both light and electron microscopy, immunohistochemical studies for Reelin in these mutants identified morphological changes in their Cajal-Retzius cells (CR cells). The CR cells of the mutant displayed the characteristic features of bipolar, tangentially elongated neurons with a dendritic proximal pole and an axonal cone at the opposite end of the soma. At either pole, cisterns of prominent rough endoplasmic reticulum (RER) were found to be rich in Reelin. However, the Reelin-positive RER cisterns of the axonal cones were hugely dilated in homozygous Reln(Orl) mice as compared with their wild type counterparts. CR cell axons displayed beads throughout their length, each contained a smooth spheroidal cistern filled with Reelin-immunoreactive fibrillar material, and were increased in number and size in Reln(Orl) mice. RER phenotype was rescued in the Reln(Alb2) mice, a mutation in which no Reelin protein is produced. We propose that the RER dilations viewed in the Reln(Orl) mutation are due to the accumulation of the defective Reelin protein, and the large axonal beads in Reln(Orl) mice reflect the accumulation of truncated Reelin as the result of defects in its secretion. These observations point to an original, hitherto unrecognised, mechanism of secretion by bulk transport in smooth cisterns from the axonal cone into the axon, followed by secretion in the cortical marginal zone from the axonal cisterns that we have named axonal reelin reservoirs.

Aggresomes and Russell bodies. Symptoms of cellular indigestion?

All cells are equipped with a proteolytic apparatus that eliminates damaged, misfolded and incorrectly assembled proteins. The principal engine of cytoplasmic proteolysis, the 26S proteasome, requires that substrates be unfolded to gain access to the active site; consequently, it is relatively ineffective at degrading aggregated proteins. Cellular indigestion occurs when the production of aggregation-prone proteins exceeds the cell's (or organelle's) capacity to eliminate them. Cellular pathways that resolve this indigestion exist, but appear to have limited capacities. Russell bodies and aggresomes are manifestations of cellular indigestion in the endoplasmic reticulum and cytoplasmic compartments, respectively, and are often associated with disease.

Microscopic and histochemical manifestations of hyaline cartilage dynamics

Structure and function of hyaline cartilages has been the focus of many correlative studies for over a hundred years. Much of what is known regarding dynamics and function of cartilage constituents has been derived or inferred from biochemical and electron microscopic investigations. Here we show that in conjunction with ultrastructural, and high-magnification transmission light and polarization microscopy, the well-developed histochemical methods are indispensable for the analysis of cartilage dynamics. Microscopically demonstrable aspects of cartilage dynamics include, but are not limited to, formation of the intracellular liquid crystals, phase transitions of the extracellular matrix and tubular connections between chondrocytes. The role of the interchondrocytic liquid crystals is considered in terms of the tensegrity hypothesis and non-apoptotic cell death. Phase transitions of the extracellular matrix are discussed in terms of self-alignment of chondrons, matrix guidance pathways and cartilage growth in the absence of mitosis. The possible role of nonenzymatic glycation reactions in cartilage dynamics is also reviewed.

Synthesis of a high-mannose-type glycopeptide analog containing a glucose-asparagine linkage

The title compound was prepared by enzymatic transfer of oligosaccharide to a synthetic pentapeptide containing the Glc-Asn linkage. The compound was not hydrolyzed by glycoamidases from plant and bacterial sources, but it inhibited both enzymes in the micromolar range. Its activity is compared to other potential inhibitors.

Articular chondrocytes and synoviocytes in culture: influence of antioxidants on lipid peroxidation and proliferation

Chondrocytes and synoviocytes are the main cell types in articular joints. Articular cartilage is fed by synoviocytes via synovial fluid and has a low partial oxygen pressure. Thus, chondrocytes show oxygen radical protective mechanisms in vivo and are unprotected against these factors under common culture conditions. We investigated the influence of ascorbic acid, Fe2+, glutathione and alpha-tocopherol on lipid peroxidation and proliferation of rat articular chondrocytes and rabbit synoviocytes (HIG-82) in vitro. A combination of ascorbic acid and Fe2+ induced the production of thiobarbituric acid-reactive material as a marker of radical-mediated lipid peroxidation in homogenates and/or supernatants of cultured chondrocytes and synoviocytes. The amount of lipid peroxidation of chondrocytes was about 3-fold higher than that of synoviocytes. Ascorbic acid or Fe2+ alone had no significant influence on the production of thiobarbituric acid-reactive material. Lipid peroxidation could be abolished by addition of the radical scavenger alpha-tocopherol, whereas glutathione had no effect. 25-50 microM alpha-tocopherol decreased the ascorbic acid-(100 micrograms/ml) and Fe(2+)-(3 microM) induced lipid peroxidation to a basal level. Moreover, ascorbic acid inhibited the proliferation of rat chondrocytes and rabbit synoviocytes measured by [3H]-thymidine incorporation. Alpha-tocopherol and glutathione had no influence on the proliferation of chondrocytes but alpha-tocopherol decreased the growth of synoviocytes and increased the anti-proliferative effect of ascorbic acid on these cells. The importance of these findings for the use of ascorbic acid, glutathione and alpha-tocopherol in chondrocyte and synoviocyte cultures, or the influence of these molecules on the etiology and treatment of articular diseases will be discussed.

Ascorbic acid alters collagen integrins in bone culture

The effects of ascorbic acid on collagen synthesis, mineralization, and integrins were investigated in a mineralizing organ culture system derived from 20-day fetal rat parietal bones. A significant dose-dependent decrease in calcification at 96 h was demonstrated with decreasing concentrations of ascorbic acid (100-0 microg/ml). No effect on DNA content, [3H]thymidine incorporation, or dry weight was found in control (100 microg/ml ascorbic acid) bones compared with bones treated with decreased ascorbic acid concentrations (10, 1, and 0 microg/ml). Collagen synthesis, measured by [3H]proline incorporation, and alpha1(I) procollagen messenger RNA levels were also unaffected. However, ascorbic acid produced a dose-dependent decrease in the hydroxyproline content, with a maximal 76.8% decrease in bones without ascorbic acid compared with the control bones with 100 microg/ml ascorbic acid. Light microscopy of the ascorbic acid-deficient bones revealed a disruption of the osteoblast layer with misshapen osteoblasts and a decrease in the osteoid seam. The loss of osteoblast organization was also confirmed by analyzing the integrins for collagen by Northern and Western blot and immunofluorescence microscopy. A dose-dependent decrease in alpha2 and beta1 integrin messenger RNA levels and in alpha1, alpha2, and beta1 protein were found in 96-h bone cultures deficient in ascorbic acid. These integrin subunits mediate the binding of osteoblasts to collagen. Immunofluorescence microscopy also demonstrated a dose-dependent decrease in alpha2 and beta1 staining of the osteoblast layer. However, the protein levels of alpha3 and alpha5 subunits were not affected. No beta5 was detected, whereas only bones cultured without ascorbic acid demonstrated a small decrease in alpha(v) and beta3 protein levels. The alpha3, alpha5, alpha(v), and beta3 subunits are involved in cell binding to extracellular matrix proteins other than collagen. Thus, the integrins for collagen are down-regulated, probably in response to the underhydroxylated collagen fibrils, which causes a disruption of osteoblast organization leading to a decrease in mineralization of bone. Integrin assays for specific extracellular proteins may be useful tools in detecting matrix defects in various metabolic bone diseases.

Analysis of types I, II, III, IX and XI collagens synthesized by fetal bovine chondrocytes in high-density culture

OBJECTIVE

This study was undertaken in order to determine phenotypic modulation of the chondrocytes more closely in high-density culture conditions and to clarify the role of ascorbate. Levels of five collagen types were analyzed qualitatively and quantitatively, and their distribution was observed in the cell layer and the culture medium.

DESIGN

Types I, II, III, IX and XI collagens, synthesized by fetal bovine chondrocytes in high-density culture, were analyzed qualitatively and quantitatively by direct measurement of radiolabeled collagens separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and by specific radioimmunoassays.

RESULTS

Under the experimental conditions used in this study (0.6 x 10(6) cells/cm2), chondrocytes did not proliferate in the absence of ascorbate, whereas a twofold increase in cell number was observed in the presence of ascorbate at day 14. Cartilage-specific collagens (types II, IX and XI) were synthesized throughout the culture period (up to 47 days), as was type III collagen, which appeared as early as day 1 and was essentially present in the culture medium. Partial dedifferentiation of chondrocytes was demonstrated by the synthesis of type I collagen, which was detected by day 2 in culture medium containing ascorbate, and by day 6 without ascorbate. After 33 days of culture, a threefold increase in type I collagen synthesis was observed in culture medium with ascorbate, reaching 66% of the type II collagen content of the cell layer. One month of culture marked the onset of a progressive decrease in the synthesis of all collagen types.

CONCLUSIONS

Under these high-density culture conditions, fetal bovine chondrocytes undergo a time and ascorbate-dependent program of partial dedifferentiation. This system provides a simple model for studying the initial mechanisms of chondrocytes dedifferentiation.

Cell-free assembly of rough and smooth endoplasmic reticulum

Smooth endoplasmic reticulum assembly was studied in a cell-free system using thin-section and freeze-fracture electron microscopy. Incubation of rat hepatocyte rough and smooth microsomes in the presence of ATP, GTP, cytosol (Xenopus egg) and an ATP-regenerating system led to assembly of membrane networks comprising a central core of interconnecting smooth tubules continuous with peripherally located rough membrane cisternae. Glucose-6-phosphatase cytochemistry confirmed the endoplasmic reticulum origin of the reconstituted membranes. When both ATP and GTP were omitted from the incubation medium, or when GTP was replaced by a variety of nucleotide analogues, including GTP gamma S, membrane aggregates contained only unfused microsomes. The presence of GTP alone stimulated assembly of rough membrane cisternae but had no effect on smooth membranes. Smooth tubule formation occurred independent of cytosol and an ATP-regenerating system, but did require GTP and ATP. Omission of ATP, or replacement of this nucleotide with a variety of analogues, including ATP gamma S, prevented tubule formation but did not affect the assembly of the rough membrane cisternae. Morphometric studies revealed sequential formation of rough membrane cisternae (0-60 minutes) followed by appearance of interconnecting smooth tubules (&gt; 60 minutes). The amount of rough membrane cisternae per membrane network diminished with time after 60 minutes; that of smooth tubules increased. Thus GTP is required for reconstitution of rough membrane cisternae, both GTP and ATP are required for smooth tubule formation, and assembly of smooth tubules occurs as an outgrowth (i.e. via tubulation) from rough membranes.

Quality control in the secretory pathway

Whereas newly synthesized proteins that have acquired a properly folded and assembled structure are transported from the endoplasmic reticulum to their final destinations, incompletely folded and assembled proteins are, as a rule, retained and eventually degraded. The molecular mechanisms of this unique molecular sorting phenomenon, called 'quality control', have been illuminated by recent studies.

Intracellular degradation of type I collagen and fibronectin in human lung fibroblasts: evidence against degradation in pre-lysosomal compartments

Fibroblasts degrade about 15% of newly synthesised collagen within the cell before it can be secreted. When the helical structure of collagen is disrupted, about 30% is degraded intracellularly. To determine if collagen degradation occurs in a pre-lysosomal compartment, the passage of type 1 collagen out of the endoplasmic reticulum or Golgi was inhibited by incubating human lung fibroblasts with brefeldin A or monensin. In both cases, the type I collagen retained within the cell was stable over a 20 h period. Disrupting the helical structure of collagen with cis-hydroxyproline, 2,2'-bipyridyl or ethyl 3,4-dihydroxybenzoate did not alter the stability of type I collagen in brefeldin or monensin-treated cells. Incubating permeabilised cells in the presence of GTP gamma S (guanosine 5'-(3-O-thio)triphosphate), which blocks transport out of the endoplasmic reticulum, also resulted in the stable retention of type I collagen. Addition of dithiothreitol to permeabilised cells failed to initiate intracellular degradation. Similar results were obtained with fibronectin. Both normal fibronectin and fibronectin in which canavanine replaced arginine were stable for 20 h in cells treated with brefeldin A or monensin. The degradation of native collagen is sensitive to inhibition by a cell-permeable cysteine proteinase inhibitor (ALLN) but is insensitive to chloroquine (which raises the pH of acidic intracellular compartments), whereas the degradation of abnormal collagen was sensitive to both ALLN and chloroquine. These results argue against the intracellular degradation of collagen or fibronectin in a pre-lysosomal compartment.

A rapid and ultrasensitive method for measurement of DNA, calcium and protein content, and alkaline phosphatase activity of chondrocyte cultures

Most investigators are cognizant of the problems inherent in counting cells embedded in a complex and abundant extracellular matrix. To overcome these obstacles, we developed a new method of isolating nucleic acids from chondrocytes which facilitates measurement of cell number by DNA analysis. Chondrocytes were isolated from chick embryo sterna and grown continuously without subculturing for 2-3 weeks in monolayer. The cells were treated with triton X-100 and the nucleic acid content of the extract was determined by measuring DNA fluorescence in the presence of Hoechst dye 33258. To minimize background fluorescence due to the triton, we precipitated the DNA with alcohol and then solubilized the nucleic acids in EDTA. This simple procedure removed the detergent and substantially increased the sensitivity of the method. Thus, we could measure with high precision and high recovery, the DNA content of cultures of 10,000-50,000 cells. In a single well containing 0.5-1.0 million cells, sufficient material remained for subsequent measurements of alkaline phosphatase activity and protein and calcium content. As the mineral present in the triton-treated samples was soluble in EDTA, we experienced no problems in measuring the calcium content of the culture. In addition, as triton X-100 is a nonionic detergent, we were able to measure cell and matrix proteins; moreover, the presence of the triton maintained the catalytic state of alkaline phosphatase. We conclude that this procedure provides a simple and rapid approach to measuring major indicators of chondrocyte maturation and function.

Proteoglycan synthesis by cultured human chondrocytes

Iliac crest biopsies are important in the detection of human skeletal dysplasias. Therefore, culture of these cells may serve as a valuable method for studying proteoglycan metabolism in chondrocytes of individuals with skeletal abnormalities. Morphological and biochemical studies were performed on human iliac crest chondrocytes grown in monolayer and in agarose gels. Two proteoglycan populations of different hydrodynamic size and glycosaminoglycan composition were synthesized by cells grown in monolayer. Chondrocytes cultured in an agarose gel for 2 weeks synthesized proteoglycans identical to those of the native tissue with respect to hydrodynamic size and glycosaminoglycan chain length. However, the ratio of chondroitin-6-sulfate to chondroitin-4-sulfate was higher than in the native tissue. This ratio was not influenced by different sulfate concentrations in the medium. Moreover, treatment with ascorbic acid did not influence proteoglycan synthesis; however, there was a pericellular accumulation of proteoglycans.

Retinoic acid is a major regulator of chondrocyte maturation and matrix mineralization

During the process of endochondral bone formation, chondrocytes undergo a series of complex maturational changes. Our recent studies indicate that this maturational process is influenced by the vitamin A derivative retinoic acid (RA). To learn how this agent regulates chondrocyte development, we characterized matrix gene expression during maturation of cartilage cells in chick sternum. RNAs were isolated from the cephalic portion of day 13, 14, 16, 18, and 20 chick embryo sternum and analyzed via northern blots. Type II collagen RNA levels remained fairly constant during this developmental period. In contrast, expression of type X collagen and alkaline phosphatase (APase) genes was first detected at day 16, followed by that of osteonectin (ON) and osteopontin (OP). To explore the mechanisms triggering these changes, chondrocytes were isolated from the cephalic portion of day 17-18 sternum (US cells) and grown in monolayer in standard serum-containing medium. After 3 weeks in culture, most of the cells enlarged and became type X collagen-positive, but they exhibited low APase activity and contained only trace amounts of ON and OP mRNAs. Treatment of parallel 3-week-old cultures with RA (10-100 nM) rapidly increased expression of the APase, ON, and OP genes severalfold. In concert with a significant increase in APase activity, there was abundant calcium accumulation in the RA-treated cultures. Electron microscopy confirmed the formation of large matrix-associated mineral crystals and the presence of numerous matrix vesicles. The effects of RA were also studied in cultures of immature chondrocytes isolated from the caudal portion of sternum (LS cells).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ascorbic acid on collagen matrix formation and osteoblast differentiation in murine MC3T3-E1 cells

Treatment of mouse MC3T3-E1 cells with ascorbic acid initiates the formation of a collagenous extracellular matrix and synthesis of several osteoblast-related proteins. We recently showed that ascorbic acid dramatically increases alkaline phosphatase and osteocalcin mRNAs and that this induction is blocked by inhibitors of collagen triple-helix formation (Franceschi and Iyer, J Bone Miner Res 7:235). In the present study, the relationship between collagen matrix formation and osteoblast-specific gene expression is explored in greater detail. Kinetic studies revealed that ascorbic acid increased proline hydroxylation in the intracellular procollagen pool within 1 h and stimulated the cleavage of type I collagen propeptides beginning at 2.5 h. Mature alpha 1(I) and alpha 2(I) collagen components were first detected at 10 h and continued to increase in both cell layer and culture medium for up to 72 h. Ascorbic acid also increased the rate of procollagen secretion from cell layers to culture medium. The secretion of another matrix protein, fibronectin, was only slightly affected. Alkaline phosphatase or its mRNA was first detected 2-3 days after ascorbic acid addition, but osteocalcin mRNA was not seen until day 6. Two inhibitors of collagen triple-helix formation, ethyl-3,4-dihydroxybenzoate and 3,4-dehydroproline, inhibited procollagen hydroxylation and alkaline phosphatase induction. 3,4-Dehydroproline also inhibited the induction of alkaline phosphatase and osteocalcin mRNAs. Surprisingly, induction was not blocked if cells were exposed to ascorbic acid before inhibitor addition. Alkaline phosphatase was also partially inhibited if cells were grown in the presence of purified bacterial collagenase. These results indicate that the induction of osteoblast markers by ascorbic acid does not require the continuous hydroxylation and processing of procollagens and suggest that a stable, possibly matrix-associated signal is generated at early times after ascorbic acid addition that allows subsequent induction of osteoblast-related genes.

Modulation of GTP-dependent fusion by linoleic and arachidonic acid in derivatives of rough endoplasmic reticulum from rat liver

The effect of modulation of the content of unsaturated free fatty acids on GTP-dependent fusion of stripped rough microsomes from rat liver was determined. Cytidine monophosphate, CDP and CTP were all observed to be able to stimulate free fatty acid accumulation and coincident membrane fusion. GTP was required for membrane fusion in the presence of cytidine nucleotide but was not required for free fatty acid accumulation. In the presence of GTP and cytidine nucleotide, the addition of ATP and CoA led to the synthesis of triacyglycerol and marked inhibition of both free fatty acid accumulation and membrane fusion. Delipidated bovine serum albumin also inhibited both free fatty acid accumulation and membrane fusion. Analysis by gas chromatography indicated that linoleic acid and arachidonic acid were the most actively fluctuating of the accumulated free fatty acids. Comparison by quantitation indicated a high correlation between GTP-dependent membrane fusion and changes in amount of unesterified linoleic acid and arachidonic acid. The results suggest that polyunsaturated free fatty acids may be required for GTP-dependent membrane fusion.

Assembly and transport of seed storage proteins

Plant seeds store nitrogen by accumulating storage proteins in protein bodies within various compartments of the endomembrane system. The prolamin storage proteins of some cereal species are normally retained and assembled into protein bodies within the ER. Yet, these proteins lack a C-terminal KDEL/HDEL signal, suggesting that their retention is regulated by novel mechanisms. Furthermore, in other cereal species, such protein bodies formed within the ER may be subsequently internalized into vacuoles by a special route that does not utilize the Golgi complex. Thus, studies of the routing of seed storage proteins are revealing novel mechanisms of protein assembly and transport in the endomembrane system.

Structural reorganization of the rough endoplasmic reticulum without size expansion accounts for dexamethasone-induced secretory activity in AR42J cells

A striking reorganization of the rough endoplasmic reticulum (RER) from a tubulo-vesicular (TV-RER) to a stacked cisternal (SC-RER) configuration was observed when the secretory activity of AR42J cells, a cell line derived from a rat pancreatic acinar carcinoma, was induced by dexamethasone. Treatment with 10 nM dexamethasone resulted in a 6.6-fold increase in the intracellular and a 4.6-fold increase in the secreted amylase activity, respectively. On the basis of the morphometric analysis of thin-section electron micrographs it has been previously reported that this increase in secretory activity is accompanied by a 2.4-fold or 30-fold increase in the size of the RER. We have developed a new biochemical method to determine the size of the RER by quantifying the membrane-bound ribosomes. Using this procedure we did not detect any change in the size of the RER after induction of an active secretory state in AR42J cells. Electron microscopic observation showed the predominance of SC-RER in dexamethasone-treated cells compared to the abundance of TV-RER in control cells. Laser scanning confocal microscopy showed a patchy distribution of ER staining in dexamethasone-treated cells compared to more basal localization in control cells. On the basis of our observations we conclude that in AR42J cells the increase in secretory activity induced by dexamethasone is accompanied by a reorganization of the RER rather than by an increase in ER surface area, as reported by others. Our results suggest that SC-RER is a biosynthetically more efficient form of the RER, which is found predominantly in actively secreting cells.

Coordinate inhibition of alkaline phosphatase and type X collagen syntheses by 1,25-dihydroxyvitamin D3 in primary cultured hypertrophic chondrocytes

The effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) (2.3 x 10(-12) - 1.4 x 10(-6) [M]) on alkaline phosphatase, collagen, and cell proliferation were examined in primary cultured hypertrophic chondrocytes prepared from the distal epiphyseal growth plate of the tibias of 12-day chick embryos. 1,25(OH)2D3 showed time- and dose-dependent inhibitory effects on the alkaline phosphatase and collagen levels. The inhibition of alkaline phosphatase activity became detectable at 2 x 10(-11) [M] and reached 10% of control at 10(-7) [M]. The concentration of 1,25(OH)2D3 giving a 50% inhibition of the enzyme level was approximately 3 x 10(-10) [M]. Of the two extracellular collagen pools, a cell-associated matrix pool showed a more dramatic decrease (to 10% of control) than a culture medium pool (to 50% of control) at increased 1,25(OH)2D3 concentrations. The degree of inhibition was different for each type of chondrocyte-specific collagen (types II, IX, X, and XI). Types II and IX were inhibited in a parallel manner to only 60-80% of control. On the other hand, types X and XI were more greatly reduced up to 10% of control, and their dose-dependent inhibitory curves were similar to that of alkaline phosphatase. On cell proliferation, 1,25(OH)2D3 had a biphasic effect: stimulation at 10(-10)-10(-8) [M] and inhibition at higher levels. The results revealed the significant involvement of 1,25(OH)2D3 in the metabolism of two probable calcification-related products, alkaline phosphatase and type X collagen.

The role of ascorbic acid in mesenchymal differentiation

Survival of all higher vertebrates requires that they either synthesize vitamin C (ascorbic acid) or obtain it from their diet. The role of ascorbic acid as a reductant for the iron prosthetic group of hydroxylase enzymes involved in collagen biosynthesis is well established. In contrast, the relationship between the biochemical functions of ascorbic acid and the broad defects in connective tissue formation associated with vitamin C deficiency is less obvious. This review will develop the hypothesis that vitamin C is required for the differentiation of mesenchyme-derived connective tissues such as muscle, cartilage, and bone. It is proposed that the collagen matrix produced by ascorbic acid-treated cells provides a permissive environment for tissue-specific gene expression.

Effects of glutathione depletion on the synthesis of proteoglycan and collagen in cultured chondrocytes

We studied the effect of the depletion of glutathione on the synthesis of proteoglycan and collagen in cultured chick chondrocytes. When the cultured chondrocytes were incubated with 1 mM buthionine sulfoximine (BSO), a specific inhibitor of gamma-glutamyl-cysteine synthetase, the intracellular glutathione level markedly dropped within 12 h with no loss of cell viability. Incorporation of 35SO2-4 into proteoglycan was lowered in the presence of BSO. When the 35S-labeled proteoglycans were separated into two fractions by glycerol density gradient centrifugation, the inhibitory effect of BSO on the synthesis of proteoglycan was greater in the fast-sedimenting proteoglycan fraction, which consisted mainly of cartilage specific large proteoglycan (PG-H), than in the slowly sedimenting proteoglycan fraction. The inhibition by BSO of the synthesis of core protein-free glycosaminoglycan chains primed by p-nitrophenyl-beta-D-xyloside was smaller than the inhibition of the synthesis of proteoglycan. Analysis of glycosaminoglycans labeled with [3H]glucosamine indicated that the treatment of chondrocytes with BSO resulted in a small increase in the proportion of synthesis of hyaluronic acid to the synthesis of total glycosaminoglycan. The incorporation of [3H]proline into collagen was also inhibited by BSO. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the 3H-labeled collagen showed that, in the presence of BSO, processing of Type II collagen appeared to slow down and the proportion of Type X collagen synthesis was reduced.

Rapid induction of type X collagen gene expression in cultured chick vertebral chondrocytes

During endochondral ossification, small rapidly proliferating chondrocytes mature into flattened disc-shaped cells and then into large round hypertrophic cells. These morphological changes are accompanied by a decrease in the rate of cell proliferation. Type X collagen synthesis is initiated during chondrocyte maturation and reaches very high levels in the hypertrophic cells. We have analyzed type X collagen gene expression in chick embryo vertebral chondrocytes that were allowed to mature in monolayer culture and were then switched to suspension culture. The resuspended chondrocytes changed in shape from flat to round and decreased the proliferation rate as they do in vivo. These events were accompanied by a rapid, dramatic increase in type X collagen gene expression at the levels of transcription, steady-state mRNA and protein synthesis, as well as an increase in the number of cells producing type X collagen. The amount of type X collagen gene expression in resuspended chondrocytes was comparable to that in mineralizing cartilage in vivo. These results indicate that events accompanying the switch from monolayer to suspension culture (for example, the change from a flat to a round shape and/or the decrease in proliferation rate) may play a role in the induction of type X collagen gene expression during chondrocyte maturation. Thus we have developed an in vitro system that appears to mimic the events occurring during in vivo chondrocyte maturation. This in vitro model may provide an ideal system for further examination of the parameters regulating chondrocyte maturation and type X collagen gene expression.

Cell hypertrophy and type X collagen synthesis in cultured articular chondrocytes

Articular cartilage is a permanent tissue whose cells do not normally take part in the endochondral ossification process. To determine whether articular chondrocytes possess the potential to express traits associated with this process such as cell hypertrophy and type X collagen, chondrocytes were isolated from adult chicken tibial articular cartilage and maintained in long-term suspension cultures. As a positive control in these experiments, we used parallel cultures of chondrocytes from the caudal portion of chick embryo sternum. Both articular and sternal chondrocytes readily proliferated and progressively increased in size with time in culture. Many had undergone hypertrophy by 4-5 weeks. Analysis of medium-released collagenous proteins revealed that both articular and sternal chondrocytes initiated type X collagen synthesis between 3 and 4 weeks of culture; synthesis of this macromolecule increased with further growth. Immunofluorescence analysis of 5-week-old cultures showed that about 15% of articular chondrocytes and 30% of sternal chondrocytes produced type X collagen; strikingly, there appeared to be no obvious relationship between type X collagen production and cell size. The results of this study show that articular chondrocytes from adult chicken tibia possess the ability to express traits associated with endochondral ossification when exposed to a permissive environment. They suggest also that the process of cell hypertrophy and initiation of type X collagen synthesis are independently regulated both in articular and sternal chondrocytes.

Type X collagen gene expression is transiently up-regulated by retinoic acid treatment in chick chondrocyte cultures

During endochondral ossification, resting and proliferating chondrocytes mature into hypertrophic chondrocytes that initiate synthesis of type X collagen. The mechanisms regulating the differential expression of type X collagen gene were examined in confluent Day 12 secondary cultures of chick vertebral chondrocytes in monolayer treated with the vitamin A analog retinoic acid (RA). Preliminary results showed that major effects of RA on chondrocyte gene expression occurred between 24 and 48 h of treatment. Thus in subsequent experiments cultures were treated for 24, 30, 36, 42, 48, 72, 96, and 120 h. Total RNAs were isolated and analyzed by hybridization with 32P-labeled plasmid probes coding for five matrix macromolecules including type X collagen. We found that the steady-state levels of mRNAs for the large keratan sulfate/chondroitin sulfate proteoglycan (KS:CS-PG) core protein and type II collagen decreased several fold between 24 and 48 h of treatment compared to untreated cells, and remained low with further treatment. In sharp contrast, the level of type X collagen mRNA increased threefold by 42 h of treatment; thereafter it began to decrease and reached minimal levels by 72-120 h of treatment. The changes in steady-state mRNA levels during RA regimen paralleled similar changes in relative rates of protein synthesis. The transient up-regulation of type X collagen gene expression at 42 h of treatment was preceded by a five-fold increase in fibronectin gene expression, was followed by a several fold increase in type I collagen gene expression, and was accompanied by cell flattening and loss of the pericellular proteoglycan matrix. Thus, RA treatment leads to a unique biphasic modulation of type X collagen gene expression in maturing chondrocyte cultures. The underlying, RA-sensitive mechanisms effecting this modulation may reflect those normally regulating the differential expression of this collagen gene during endochondral ossification.

Morphogenesis of endoplasmic reticulum in Xenopus oocytes after microinjection of rat liver smooth microsomes

We have determined the kinetics of endoplasmic reticulum (ER) reconstitution following insertion of rat-liver smooth microsomes (SM) into Xenopus oocyte cytoplasm using electron microscopy as well as cytochemistry and thick-section 3-dimensional reconstruction. Oocytes were fixed 0, 10, 20, 40, 80, and 120 min after microinjection with SM and processed for thin- and thick-section electron microscopy. At 0 min postinjection, rat liver SM were observed as small vesicles and were loosely dispersed amongst oocyte organelles. At 10 min, tubules were discerned among many elongate vesicles; and these structures comprised large cytoplasmic regions delimited by mitochondria and yolk platelets. By 20 min, segregation of transplanted organelles yielded yolk-platelet-free regions composed of few vesicles but increasingly numerous, long and anastomosing tubules. By 40 min, a network with numerous tubular branches and fenestrations was observed among the few remaining vesicles. By 80 min, transformation of rat liver SM into a complex network of branching and anastomosing tubules was complete. Three-dimensional reconstruction revealed the network to be composed of interconnecting elements consisting of anastomosing tubules. The reconstituted network of anastomosing tubules in Xenopus oocytes was compared to the network of anastomosing tubules in rat liver hepatocytes and was found to be essentially identical. Network formation occurred in oocytes pretreated with either vinblastine (40 microM) or nocodazole (0.166 microM), and network organization was maintained in oocytes treated with the same drugs after microinjection and reconstitution. We conclude that SM retain sufficient molecular information for rapid self-assembly into structures resembling those in the cells from which they were derived. Both the assembly and maintenance of ER structure in oocyte cytoplasm are microtubule-independent. The formation of such structures following microinjection of SM into living cells provides a unique assay for this type of membrane subfraction.