Feedback regulation of collagen gene expression: a Trojan horse approach

SPARC and the N-propeptide of collagen I influence fibroblast proliferation and collagen assembly in the periodontal ligament

The periodontal ligament (PDL) is a fibrous connective tissue that anchors tooth cementum into alveolar bone. Secreted protein acidic and rich in cysteine (SPARC) is a collagen-binding matricellular protein known to influence collagen fiber assembly in the PDL. In contrast, functional properties of the N-propeptide of collagen I, encoded in exon 2 of the COL1A1 gene, are poorly understood. In this study, the PDL of collagen I exon 2-deleted (wt/ko), SPARC-null (ko/wt), and double transgenic (ko/ko) mice were evaluated in terms of cellularity, collagen area, fiber morphology, and extraction force and compared to WT (wt/wt) mice. Picro sirius red staining indicated a decrease in total PDL collagen content in each of the transgenic mice compared to WT at 1 and 3 month age points. At 12 months, only SPARC-null (ko/wt) and double-null PDL demonstrated less total collagen versus WT. Likewise, an increase in thin PDL collagen fibers was observed at 1 and 3 months in each transgenic, with increases only in SPARC-null and double-null mice at 12 months. The force required for tooth extraction was significantly reduced in SPARC-null versus exon 2-deleted and WT mice, whereas double-null mice demonstrated further decreases in force required for tooth extraction. The number of proliferating fibroblasts and number and size of epithelial rests of Malassez were increased in each transgenic versus WT with double-null PDL exhibiting highest levels of proliferation and rests of Malassez at 1 month of age. Consistent with increases in PDL collagen in exon-2 deleted mice, with age, numbers of rests decreased at 12 months in this genotype. These results demonstrate for the first time a functional role of the N-propeptide in regulating collagen fiber assembly and cell behavior and suggest that SPARC and the N-propeptide of collagen I have distinct activities in regulating collagen fiber assembly and fibroblast function.

Decreased oxygen tension lowers reactive oxygen species and apoptosis and inhibits osteoblast matrix mineralization through changes in early osteoblast differentiation

Accumulating data show that oxygen tension can have an important effect on cell function and fate. We used the human pre-osteoblastic cell line SV-HFO, which forms a mineralizing extracellular matrix, to study the effect of low oxygen tension (2%) on osteoblast differentiation and mineralization. Mineralization was significantly reduced by 60-70% under 2% oxygen, which was paralleled by lower intracellular levels of reactive oxygen species (ROS) and apoptosis. Following this reduction in ROS the cells switched to a lower level of protection by down-regulating their antioxidant enzyme expression. The downside of this is that it left the cells more vulnerable to a subsequent oxidative challenge. Total collagen content was reduced in the 2% oxygen cultures and expression of matrix genes and matrix-metabolizing enzymes was significantly affected. Alkaline phosphatase activity and RNA expression as well as RUNX2 expression were significantly reduced under 2% oxygen. Time phase studies showed that high oxygen in the first phase of osteoblast differentiation and prior to mineralization is crucial for optimal differentiation and mineralization. Switching to 2% or 20% oxygen only during mineralization phase did not change the eventual level of mineralization. In conclusion, this study shows the significance of oxygen tension for proper osteoblast differentiation, extra cellular matrix (ECM) formation, and eventual mineralization. We demonstrated that the major impact of oxygen tension is in the early phase of osteoblast differentiation. Low oxygen in this phase leaves the cells in a premature differentiation state that cannot provide the correct signals for matrix maturation and mineralization.

Differential roles of endothelin-1 in angiotensin II-induced atherosclerosis and aortic aneurysms in apolipoprotein E-null mice

Because both endothelin-1 (ET-1) and angiotensin II (AngII) are independent mediators of arterial remodeling, we sought to determine the role of ET receptor inhibition in AngII-accelerated atherosclerosis and aortic aneurysm formation. We administered saline or AngII and/or bosentan, an endothelin receptor antagonist (ERA) for 7, 14, or 28 days to 6-week- and 6-month-old apolipoprotein E-knockout mice. AngII treatment increased aortic atherosclerosis, which was reduced by ERA. ET-1 immunostaining was localized to macrophage-rich regions in aneurysmal vessels. ERA did not prevent AngII-induced aneurysm formation but instead may have increased aneurysm incidence. In AngII-treated animals with aneurysms, ERA had a profound effect on the non-aneurysmal thoracic aorta via increasing wall thickness, collagen/elastin ratio, wall stiffness, and viscous responses. These observations were confirmed in acute in vitro collagen sheet production models in which ERA inhibited AngII's dose-dependent effect on collagen type 1 α 1 (COL1A1) gene transcription. However, chronic treatment reduced matrix metalloproteinase 2 mRNA expression but enhanced COL3A1, tissue inhibitor of metalloproteinase 1 (TIMP-1), and TIMP-2 mRNA expressions. These data confirm a role for the ET system in AngII-accelerated atherosclerosis but suggest that ERA therapy is not protective against the formation of AngII-induced aneurysms and can paradoxically stimulate a chronic arterial matrix remodeling response.

Expression in SPARC-null mice of collagen type I lacking the globular domain of the α1(I) N-propeptide results in abdominal hernias and loss of dermal collagen

The sequence encoding the N-propeptide of collagen I is characterized by significant conservation of amino acids across species; however, the function of the N-propeptide remains poorly defined. Studies in vitro have suggested that one activity of this propeptide might be to act as a feedback inhibitor of collagen I synthesis. To determine whether the N-propeptide contributed to decreased collagen content in SPARC-null mice, mice carrying a deletion of exon 2, which encodes the globular domain of the N-propeptide of collagen I, were crossed to SPARC-null animals. Mice lacking SPARC and expressing collagen I without the globular domain of the N-propeptide were viable and fertile. However, a significant number of animals developed abdominal hernias within the first 2 months of life with an approximate 20% penetrance (~35% of males). The dermis of SPARC-null/exon 2-deleted mice was thinner and contained fewer large collagen fibers in comparison with wild-type or in either single transgenic animal. The average collagen fibril diameter of exon 2-deleted mice did not significantly differ from wild-type mice (WT: 87.9 nm versus exon 2-deleted: 88.2 nm), whereas SPARC-null/exon 2-deleted fibrils were smaller than that of SPARC-null dermis (SPARC-null: 60.2 nm, SPARC-null/exon 2-deleted: 40.8 nm). As measured by hydroxyproline analysis, double transgenic skin biopsies contained significantly less collagen than those of wild-type, those of exon 2-deleted, and those of SPARC-null biopsies. Acetic acid extraction of collagen from skin biopsies revealed an increase in the proportion of soluble collagen in the SPARC-null/exon 2-deleted mice. These results support a function of the N-propeptide of collagen I in facilitating incorporation and stabilization of collagen I into the insoluble ECM and argue against a primary function of the N-propeptide as a negative regulator of collagen synthesis.

The pentapeptide KTTKS promoting the expressions of type I collagen and transforming growth factor-beta of tendon cells

Pentapeptide KTTKS, a subfragment of type I collagen propeptide, has been demonstrated to promote the extracellular release of collagen in fibroblasts. The present study was designed to investigate the effect and molecular mechanism of KTTKS on type I collagen expression of rat Achilles tendon cells. Genes related to the upregulation of collagen expression such as transforming growth factor-beta (TGF-beta), and mRNA-binding proteins (mRBP) E1 and K were also examined. MTT assay revealed cell viability was not affected by KTTKS treatment. Protein expression of type I collagen was determined by immunocytochemistry and Western blot analysis. Results showed that KTTKS induced type I collagen expression of tendon cells. The mRNA expressions of alpha1(I) procollagen, TGF-beta, mRBP K, as determined by reverse transcription-polymerase chain reaction (RT-PCR), were also enhanced after KTTKS treatment. The stability of preexisting alpha1(I) procollagen mRNA after the addition of alpha-amanitin was analyzed by RT-PCR at 24 and 48 h. Results showed that KTTKS slowed down the degradation of alpha1(I) procollagen mRNA (p = 0.021). Furthermore, the concentration of TGF-beta in conditioned medium, as determined by enzyme-linked immunosorbent assay, increased dose dependently in cells treated with KTTKS (p = 0.001). In conclusion, KTTKS promotes the expression of type I collagen and maintains its mRNA stability in a process associated with the upregulation of TGF-beta.

Novel collagen glomerulopathy in a homotrimeric type I collagen mouse (oim)

BACKGROUND

Oim/oim mice [osteogenesis imperfecta model; homozygous null for the proalpha2(I) collagen gene] synthesize exclusively the homotrimeric type I collagen isotype, alpha1(I)3, and are unable to synthesize the normal heterotrimeric type I collagen isotype, alpha1(I)2alpha2(I). Previous studies of the oim/oim mouse have focused on the musculoskeletal system, with no systematic evaluation of other organ systems.

METHODS

Multiple tissues from oim/oim, oim/+ (heterozygous) and +/+ (wild-type) mice were examined for gross and histological abnormalities. Tissues were stained with (1) hematoxylin and eosin (to assess lesion formation), (2) picrosirius red (collagen content), and (3) periodic acid methenamine silver (basement membrane). Kidneys were further evaluated ultrastructurally by electron microscopy and immunohistochemically with anti-alpha1(I) and anti-alpha1(III) collagen antibodies.

RESULTS

Histological analyses revealed accumulations of picrosirius red-positive material, consistent with collagen, in glomeruli of 28/29 oim/oim mice, with no evidence of mesangial cell proliferation. Only the most severely affected animals had evidence of increased capillary basement membrane thickening or mild inflammation around the affected glomeruli. Electron microscopy confirmed the presence of fibrillar collagen. Immunohistochemistry with anti-alpha1(I) collagen antibodies confirmed accumulation of type I collagen in the oim/oim glomeruli. The +/+ and oim/+ kidneys had normal mesangium with no evidence of infiltration of collagenous material.

CONCLUSIONS

This study demonstrates the first evidence, to our knowledge, of abnormal glomerular collagen deposition associated with a type I collagen defect. Further in vivo and in vitro studies are necessary to elucidate the mechanistic, functional, and pathological significance of the oim/oim collagen glomerulopathy.

The NH(2)-terminal propeptides of fibrillar collagens: highly conserved domains with poorly understood functions

The impetus for this review comes from the recent finding that the absence of the majority of the non-triple-helical sequence in the NH(2)-terminal propeptide (N-propeptide) of the pro alpha 1(I) collagen chain fails to generate a significant phenotype in the mouse (Bornstein et al., J. Biol. Chem., 277:2605-2613, 2002). This result is in apparent conflict with those of numerous studies in vitro that have implicated the N-propeptide in a number of processes that are involved in the biogenesis, maturation and function of type 1 collagen. To seek an explanation for this discrepancy, the sequences of the highly conserved, 55-57-amino acid, cysteine-rich repeats (CRR), which constitute the majority of the globular domains in the N-propeptides, were compared among 13 vertebrate species. Surprisingly, the CRR in mice and rats differs substantially from those in other mammalian species. Indeed, the CRR in birds, fish and amphibia are more similar to those of other mammals than are the CRR in rodents. This finding raises the possibility that the mutant mouse, which lacks exon 2 that encodes the CRR in the N-propeptide, might not be an appropriate model in which to study the function of the N-propeptide in other mammals. Alternatively, compensation, possibly by procollagens II or III, could account for the mild phenotype of the exon 2-deleted mouse. Yet another possibility is that the CRR plays a developmental role in the mouse, akin to that recently proposed for the N-propeptide in type IIA procollagen, rather than a function in collagen biogenesis. Some support for the latter possibility is provided by the observation that, on one background, the breeding of heterozygous exon 2-deleted mice generated homozygous mutants at less than the expected frequency. Experiments to examine these possibilities are proposed.

The globular domain of the proalpha 1(I) N-propeptide is not required for secretion, processing by procollagen N-proteinase, or fibrillogenesis of type I collagen in mice

The globular domain in the NH(2)-terminal propeptide (N-propeptide) of the proalpha1(I) chain is largely encoded by exon 2 of the Col1a1 gene and has been implicated in a number of processes that are involved in the biogenesis, maturation, and function of type I collagen. These include intracellular chain association, transcellular transport and secretion, proteolytic processing of the precursor, feedback regulation of synthesis, and control of fibrillogenesis. However, none of these proposed functions has been firmly established. To evaluate the function of this procollagen domain we have used a targeted mutagenesis approach to generate mice that lack exon 2 in the Col1a1 gene. Mouse lines were established on both a mixed 129 OlaHsd/Sv and C57BL/6 background and a pure 129 OlaHsd/Sv background. Adult mice on the mixed background are normal in appearance and are fertile. To the extent that they have been studied, procollagen synthesis, secretion, and proteolytic processing are normal in these mice, and collagen fibrillogenesis is only slightly altered. However, breeding of heterozygous mutant mice on the 129 background generated homozygous mutants at only 64% of the expected frequency. These findings suggest that although the N-propeptide is not essential for collagen biogenesis in mice it may play some essential role during embryonic development.

Type IIA procollagen: expression in developing chicken limb cartilage and human osteoarthritic articular cartilage

Type IIA procollagen is an alternatively spliced product of the type II collagen gene and uniquely contains the cysteine (cys)-rich globular domain in its amino (N)-propeptide. To understand the function of type IIA procollagen in cartilage development under normal and pathologic conditions, the detailed expression pattern of type IIA procollagen was determined in progressive stages of development in embryonic chicken limb cartilages (days 5-19) and in human adult articular cartilage. Utilizing the antibodies specific for the cys-rich domain of the type IIA procollagen N-propeptide, we localized type IIA procollagen in the pericellular and interterritorial matrix of condensing pre-chondrogenic mesenchyme (day 5) and early cartilage (days 7-9). The intensity of immunostaining was gradually lost with cartilage development, and staining became restricted to the inner layer of perichondrium and the articular cap (day 12). Later in development, type IIA procollagen was re-expressed at the onset of cartilage hypertrophy (day 19). Different from type X collagen, which is expressed throughout hypertrophic cartilage, type IIA procollagen expression was transient and restricted to the zone of early hypertrophy. Immunoelectron microscopic and immunoblot analyses showed that a significant amount of the type IIA procollagen N-propeptide, but not the carboxyl (C)-propeptide, was retained in matrix collagen fibrils of embryonic limb cartilage. This suggests that the type IIA procollagen N-propeptide plays previously unrecognized roles in fibrillogenesis and chondrogenesis. We did not detect type IIA procollagen in healthy human adult articular cartilage. Expression of type IIA procollagen, together with that of type X collagen, was activated by articular chondrocytes in the upper zone of moderately and severely affected human osteoarthritic cartilage, suggesting that articular chondrocytes, which normally maintain a stable phenotype, undergo hypertrophic changes in osteoarthritic cartilage. Based on our data, we propose that type IIA procollagen plays a significant role in chondrocyte differentiation and hypertrophy during normal cartilage development as well as in the pathogenesis of osteoarthritis.

Physical characterization of the procollagen module of human thrombospondin 1 expressed in insect cells

Thrombospondin 1 (TSP1) is a homotrimeric glycoprotein composed of 150-kDa subunits connected by disulfide bridges. The procollagen module of thrombospondin 1 has been implicated in antiangiogenic activity. Procollagen modules are found in a number of extracellular proteins and are identifiable by 10 cysteines with characteristic spacing. We expressed and studied the procollagen module (C) of human TSP1, both by itself and in the context of the adjoining oligomerization sequence (o) and N-terminal module (N). The coding sequences were introduced into baculoviruses along with an N-terminal signal sequence and C-terminal polyhistidine tag. Proteins were purified from conditioned medium of infected insect cells by nickel-chelate chromatography. NoC is a disulfide bonded trimer and cleaves readily at a site of preferential proteolysis to yield monomeric N and trimeric oC. These are known properties of full-length TSP1. Mass spectroscopy indicated that C is N-glycosylated, and all 10 cysteine residues of C are in disulfides. By equilibrium ultracentrifugation, C is a monomer in physiological salt solution. Circular dichroism, intrinsic fluorescence, and differential scanning calorimetry experiments suggest that the stability of C is determined by the disulfides. The two tryptophans of C are in a polar, exposed environment as assessed by iodide fluorescence quenching and solvent perturbation. The oC far UV circular dichroism spectrum could be modeled as the sum of C and a coiled-coil oligomerization domain. The results indicate that the recombinant C folds autonomously into its native structure, and trimerization of the modules in TSP1 does not perturb their structures.

Increase in expression of Hsp47 and collagen in hereditary gingival fibromatosis is modulated by stress and terminal procollagen N-propeptides

HGF is a rare oral condition characterized by a slow, progressive enlargement of the gingiva, involving both the maxilla and mandible. HGF provides a model for the study of regulatory features of conditions characterized by connective tissue hyperplasia. In this study, the culture characteristics of gingival fibroblasts derived from patients of the same family with HGF (n = 4) were similar with regard to cell cycle analysis. Flow cytometric DNA content analysis revealed uniform DNA diploidy for fibroblasts cultured from NG and HGF. NG cells showed a low S-phase fraction (19.8%) and G2/M fraction (5.8%) and a relatively high G1 phase fraction (74%). In contrast, HGF cells from all members of the tested kindred, exhibited diploid cells with a higher S-phase (40.9%) and G2/M (10.1%) fraction and a relatively low G1 phase fraction (40.9%). Furthermore, we demonstrated that the expression and production of Hsp47 parallels the increased levels of collagen secretion observed in HGF. In addition, we show that Hsp47 and collagen are coordinately regulated following stress via a feedback mechanism mediated by N-terminal procollagen propeptides. Utilizing confocal microscopy and antibodies directed against GST-fusion proteins encompassing the pro alpha1(I) N-propeptide globular domain (NP1) (residues 23-108), it was apparent that this regulatory mechanism does not involve significant interaction with Hsp47's chaperoning of procollagen.

An immunocytochemical approach to the study of beta-endorphin production in human keratinocytes using confocal microscopy

Proopiomelanocortin (POMC) is a protein that is posttranslationally processed to yield POMC peptides. The main site of POMC expression is the anterior pituitary lobe but many other sources have been identified. There is evidence that the skin produces POMC peptides, although their roles have not yet been defined. In the skin, regulation of POMC gene expression is known to be hair-cycle dependent, and it is localized to the sebaceous gland. In particular, beta-endorphin, a POMC peptide, has been shown to be modulated by TPA, IL-1 alpha, and ultraviolet radiation in keratinocytes. These results were obtained by examination of POMC mRNA levels using the Northern blot method; beta-endorphin protein production by the Western blot method on cultured cells; and immunocytochemistry for tissue preparations. This report represents an approach to use immunocytochemistry to quantify beta-endorphin production in cultured human keratinocytes. Additionally, we examined whether exposure to 20 mJ ultraviolet B radiation (UVB) and/or UVA could influence beta-endorphin production in these cells. Keratinocytes were grown in monolayers, in serum-free medium, fixed, and incubated with antiserum to whole synthetic beta-endorphin. Fluorescence microscopy was performed with a confocal laser scanning microscope. The integrated level of fluorescence was evaluated in n = 18 +/- 8 individual cells, and this was assumed to be proportional to beta-endorphin content. High variability was observed in the fluorescence intensity among cells. No significant differences between control and UVB- or UVA + UVB-treated cells was found. Similar results were produced by using brefeldin A, a compound that disrupts the secretory pathway, eliminating the possibility that the absence of a difference between beta-endorphin content in the treated and control cells was due to secretion of the peptide into the medium. We conclude that: (1) beta-endorphin or beta-endorphin-like peptides are produced in human keratinocytes and are readily detected by immunocytochemistry; (2) under the conditions tested, UVA and/or UVB did not increase beta-endorphin-like immunoreactivity in these cells.

Type IIA procollagen containing the cysteine-rich amino propeptide is deposited in the extracellular matrix of prechondrogenic tissue and binds to TGF-beta1 and BMP-2

Type II procollagen is expressed as two splice forms. One form, type IIB, is synthesized by chondrocytes and is the major extracellular matrix component of cartilage. The other form, type IIA, contains an additional 69 amino acid cysteine-rich domain in the NH2-propeptide and is synthesized by chondrogenic mesenchyme and perichondrium. We have hypothesized that the additional protein domain of type IIA procollagen plays a role in chondrogenesis. The present study was designed to determine the localization of the type IIA NH2-propeptide and its function during chondrogenesis. Immunofluorescence histochemistry using antibodies to three domains of the type IIA procollagen molecule was used to localize the NH2-propeptide, fibrillar domain, and COOH-propeptides of the type IIA procollagen molecule during chondrogenesis in a developing human long bone (stage XXI). Before chondrogenesis, type IIA procollagen was synthesized by chondroprogenitor cells and deposited in the extracellular matrix. Immunoelectron microscopy revealed type IIA procollagen fibrils labeled with antibodies to NH2-propeptide at approximately 70 nm interval suggesting that the NH2-propeptide remains attached to the collagen molecule in the extracellular matrix. As differentiation proceeds, the cells switch synthesis from type IIA to IIB procollagen, and the newly synthesized type IIB collagen displaces the type IIA procollagen into the interterritorial matrix. To initiate studies on the function of type IIA procollagen, binding was tested between recombinant NH2-propeptide and various growth factors known to be involved in chondrogenesis. A solid phase binding assay showed no reaction with bFGF or IGF-1, however, binding was observed with TGF-beta1 and BMP-2, both known to induce endochondral bone formation. BMP-2, but not IGF-1, coimmunoprecipitated with type IIA NH2-propeptide. Recombinant type IIA NH2-propeptide and type IIA procollagen from media coimmunoprecipitated with BMP-2 while recombinant type IIB NH2-propeptide and all other forms of type II procollagens and mature collagen did not react with BMP-2. Taken together, these results suggest that the NH2-propeptide of type IIA procollagen could function in the extracellular matrix distribution of bone morphogenetic proteins in chondrogenic tissue.

Collagen synthesis by liver stellate cells is released from its normal feedback regulation by acetaldehyde-induced modification of the carboxyl-terminal propeptide of procollagen

Acetaldehyde stimulates collagen synthesis in stellate cells and forms adducts with procollagen in the liver of alcoholics. To assess the possibility that modification of the carboxyl-terminal propeptide by acetaldehyde affects its capacity to exert a feedback inhibition of collagen synthesis after splitting from procollagen, the propeptide was prepared by gel filtration of the bacterial collagenase digests of procollagen type I (obtained from 10(9) calvaria fibroblasts of newborn rats) and reacted with either 250 mM acetaldehyde and 100 mM CNBH3 or with 170 microM acetaldehyde without reducing agents, to mimick in vivo conditions. The unmodified propeptide produced a concentration-dependent inhibition of collagen synthesis by Ito cells. By contrast, the acetaldehyde-modified propeptide produced a lesser inhibition of procollagen synthesis in the cells, associated with a greater accumulation of collagen in the media. The incubation with 170 microM acetaldehyde and, to a lesser extent, 50 mM ethanol produced collagenase-digestible adducts in stellate cells. Thus, the formation of acetaldehyde adducts with the carboxyl-terminal propeptide of procollagen may account, at least in part, for the stimulatory effect of acetaldehyde on collagen synthesis by stellate cells and may lead to collagen accumulation through a decrease of the normal feedback regulation of collagen synthesis by the propeptide.

Type IIA procollagen amino propeptide is localized in human embryonic tissues

Type II procollagen is synthesized in two forms generated by the alternative splicing of its precursor mRNA. The alternatively spliced domain, exon 2, encodes the 69-amino-acid cysteine-rich region of the NH2 propeptide. Studies of mRNA expression have shown that the longer form, designated Type IIA procollagen, is synthesized by chondroprogenitor cells and various noncartilaginous tissues. The shorter form, Type IIB procollagen, is synthesized by differentiated chondrocytes. As the initial step in our investigations of the function of the Type IIA procollagen, the protein domain corresponding to exon 2 was created as a recombinant fusion protein and used to raise antibodies in rabbits. The resulting antiserum was specific for Type IIA procollagen NH2 propeptide as shown by ELISA, Western blotting, and immunofluorescent co-localization with the triple-helical domain of Type II collagen. Type IIA procollagen was identified in tissue culture medium of 54-day human fetal ribs. Confocal microscopy was used to localize the Type IIA NH2 propeptide in Day 50 and 53 human embryos. In the digital rays of the developing hand, where only Type IIA procollagen mRNA was detected, Type IIA procollagen NH2 propeptide was observed in the extracellular matrix. The presence of Type IIA procollagen NH2 propeptide was observed in the cartilage of the developing long bones of the lower arm and vertebral bodies even though these tissues synthesize Type IIB mRNA at this developmental stage. Type IIA procollagen NH2 propeptide was localized in the developing trachea, a cartilage that does not undergo endochondral bone formation. Type IIA NH2 propeptide was also localized in noncartilaginous tissues known to synthesize Type IIA mRNA, such as the intervertebral area, perichondrium, notochordal sheath, and neuroepithelium of the otic vesicle. In most tissues, co-localization with antiserum against the triple-helical domain of Type II collagen was observed. Positive immunoreactivity with the Type IIA NH2 propeptide antiserum indicates, for the first time, that this propeptide is present in the tissue. Co-localization of NH2 propeptide antibodies with the triple-helical domain of the collagen molecule suggests that Type IIA procollagen is intact in the extracellular matrix of these tissues. Taken together, these results strongly suggest that around cells that synthesize Type IIA procollagen mRNA, Type IIA procollagen NH2 propeptide is secreted and deposited into the extracellular matrix. In light of these results, we predict that Type IIA procollagen plays a role in differentiation of tissues that augments its purely architectural function.

Pathogenesis of scleroderma. Collagen

It is now evident that persistent overproduction of collagen and other connective tissue macromolecules results in excessive tissue deposition, and is responsible for the progressive nature of fibrosis in SSc. Up-regulation of collagen gene expression in SSc fibroblasts appears to be a critical event in the development of tissue fibrosis. The coordinate transcriptional activation of a number of extracellular matrix genes suggests a fundamental alteration in the regulatory control of gene expression in SSc fibroblasts. Trans-acting nuclear factors that bind to cis-acting elements in enhancer and promoter regions of the genes modulate the basal and inducible transcriptional activity of the collagen genes. The identity of the nuclear transcriptional factors that regulate normal collagen gene expression remains to be firmly established, and to date, no alterations in the level or in the activity of such DNA binding factors has been demonstrated in SSc fibroblasts. In addition to important interactions between fibroblasts and the extracellular matrix, cytokines and other cellular mediators can positively and negatively influence fibroblast collagen synthesis. Some of these signaling molecules may have physiologic roles, and their aberrant expression, or altered responsiveness of SSc fibroblasts to them, may result in the acquisition of the activated phenotype. The rapid expansion of knowledge regarding the effects of cytokines on extracellular matrix synthesis has led to an appreciation of the enormous complexity of regulatory networks that operate in the physiologic maintenance of connective tissue and which may be responsible for the occurrence of pathologic fibrosis. The ubiquitous growth factor TGF beta is the most potent inducer of collagen gene expression and connective tissue accumulation yet discovered. The expression of TGF beta in activated infiltrating mononuclear cells suggests a role for this cytokine as a mediator of fibroblast activation in SSc. Furthermore, the recognition that TGF beta is capable of inducing its own expression in a variety of cell types, coupled with the demonstration that a subpopulation of SSc dermal fibroblasts produces TGF beta, indicates the existence of a possible autocrine loop whereby lymphocyte-derived TGF beta in early SSc not only signals biosynthetic activation of fibroblasts in a paracrine manner, but autoinduces endogenous TGF beta production by the target fibroblasts themselves. Such an autocrine loop involving TGF beta may explain the persistent activation of collagen gene expression in SSc fibroblasts, and could be responsible for the progressive nature of fibrosis in SSc. Numerous other cytokines, as well as cell-matrix interactions, also modify collagen gene expression and can significantly influence the effects of TGF beta. Although their physiologic function in tissue remodeling or their involvement in abnormal fibrogenesis has not yet been conclusively demonstrated, the study of the biologic effects of these cytokines may provide important clues to understanding the pathogenesis of SSc, and to the development of rational drug therapy aimed at interrupting the abnormal fibrogenic process in this disease.

Aortic smooth muscle cells in a three-dimensional collagen lattice culture. Evidence for posttranslational regulation of collagen synthesis

Aortic smooth muscle cells were cultivated as monolayers on plastic or within collagen lattices with low- and high-serum supplementation, and the expression of mRNAs specific for pro alpha 1 (I) and pro alpha 1 (III) collagen were studied by slot blot hybridization. The steady-state levels of pro alpha 1 (I) and pro alpha 1 (III) collagen mRNA of cells within collagen lattices were found to be higher than those grown on plastic, although the production of collagen was lower. The degradation of pro alpha 1 (I) and pro alpha 1 (III) collagen mRNAs as revealed in the presence of actinomycin D was not affected by culturing the cells within a collagen lattice. In vitro translation assays of mRNAs of monolayer- and lattice-cultured cells showed no differences in translatability. These data suggest the involvement of posttranslational control of collagen production in collagen lattice-cultured smooth muscle cells.

Alternate exon usage is a commonly used mechanism for increasing coding diversity within genes coding for extracellular matrix proteins

Extracellular matrix proteins are a diverse family of secreted proteins and glycoproteins that are responsible for a variety of critical functions in different tissues. A large number of multiexon genes encode these proteins of the extracellular matrix. Over the last few years, it has become evident that the processing of the pre-mRNA from several of these genes involves alternative splicing. This review summarizes the known examples of alternative splicing in genes coding for the extracellular matrix and attempts to relate the increase in coding diversity generated by alternate exon usage to the function(s) of individual extracellular matrix proteins.

Transinhibition of C1 inhibitor synthesis in type I hereditary angioneurotic edema

To ascertain the mechanism for decreased synthesis of C1 inhibitor (C1 INH) in certain patients with the autosomal dominant disorder hereditary angioneurotic edema, we studied expression of C1 INH in fibroblasts in which the mutant and wild type mRNA and protein could be distinguished because of deletion of exon 7 (delta Ex7). In the HANE delta Ex7 cells, the amount of wild type mRNA (2.1 kb) was expressed at 52 +/- 2% (n = 5) of normal, whereas the mutant mRNA was 17 +/- 1% (n = 5) of normal. Rates of synthesis of both wild type and mutant proteins (11 +/- 3 and 3 +/- 1% of normal, respectively) were lower than predicted from the mRNA levels. There was no evidence of increased C1 INH protein catabolism. These data indicate that there are multiple levels of control of C1 INH synthesis in type I hereditary angioneurotic edema. Pretranslational regulation results in < 50% of the mutant truncated 1.9-kb mRNA. In addition, translational regulation results in decreased synthesis of both wild type and mutatn C1 INH proteins. These data suggest a transinhibition of wild type C1 INH translation by mutant mRNA and/or protein.

Osteoporosis in lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is an autosomal recessive disease characterized by defective transport of cationic amino acids. Patients have an increased incidence of fractures and their skeletal radiographs show osteoporosis. The aim of the study was to characterize the osteopenia in LPI. Twenty-nine Finnish LPI patients (age range 3.7-44.4 years) were screened for parameters of bone metabolism. Morphometric analysis of bone was carried out in specimens of 9 patients. Collagen synthesis was studied with cultured skin fibroblasts (4 patients) and collagen fibril sizes (3 patients) were measured using electron microscopy. Most histological bone specimens (8/9) showed osteoporosis. Osteomalacia was excluded. Routine clinical laboratory tests were unrevealing. The concentrations of free hydroxyproline and type III procollagen N-propeptide in serum and the urinary excretion of hydroxyproline were increased in almost all patients during their growth and in about half of adult patients. Collagen synthesis in LPI fibroblast cultures was significantly decreased compared with that in age-matched controls at 5 (p < 0.01), 14 (p < 0.01) and still at 30 years (p < 0.01), whereas no difference was observed at the age of 44 years (p = N.S.). Osteoporosis in LPI might reflect defective matrix protein synthesis caused by protein deprivation and deficiency of cationic amino acids. Increased collagen turnover can also contribute to the osteoporosis.