[Hyaluronic acid and extracellular matrix: a primitive molecule?]. / Acide hyaluronique et matrice extracellulaire : une molécule primitive ?

Hyaluronic acid, or hyaluronan, is a polymer made of the repetition of a unique disaccharidic unit, D-glucuronic acid and D-N-acetylglucosamine, that can reach a molecular mass of 10(7) daltons. This primitive polymer has emerged as a remarkable extracellular matrix component by its viscoelastic properties, its hygroscopic capacities and the diversity of cell processes it controls. Identified in all vertebrate tissues, more than 50% of acid hyaluronic of the organism is present in skin. Having no protein core, its synthesis is performed through a unique process, depending on enzymatic activity of hyaluronan synthases acting at the internal face of the plasmatic membrane and extruding the nascent polymer to the extracellular medium. This polymer constitutes a scaffold on which a large number of sulfated proteoglycans, up to one hundred, can be linked. These supramolecular structures of considerable size are able to entrap large amounts of water and ions to provide tissues with hydration and turgescence. Hyaluronic acid is recognized by cell membrane receptors, notably CD44 which is the best known. Interaction of hyaluronic acid with its receptors triggers several intracellular signaling pathways regulating proliferation, migration and differentiation. Cell response is largely influenced by the size of the polymer and by that of the fragments generated upon degradation by hyaluronidases or free radicals. Hyaluronic acid is metabolically very active, as, for example, its half-life in skin is less than one day. Detected in epidermis where it could play a role in the control of proliferation and differentiation of basal cells, it is however prominent in dermis in association with versican. The remarkable physicochemical properties of hyaluronic acid as well as the diversity of biological processes it controls largely surpass the primitive character of this polymer.

Altered expression of angiogenesis and lymphangiogenesis markers in the uninvolved skin of plaque-type psoriasis.

BACKGROUND: Vascular alterations are significant events in the pathomechanism of psoriasis. A disorder in the mechanisms regulating skin angiogenesis and lymphangiogenesis could participate in the pathogenesis of the disease. OBJECTIVES: To quantify differences in the expression of angiogenesis and lymphangiogenesis growth factors, receptors, coreceptors as well as their antagonists in the uninvolved skin of patients with psoriasis compared with the skin of nonpsoriatic volunteers. METHODS: Skin biopsies were collected from the involved skin of 13 patients with untreated plaque-type psoriasis, from their nonlesional skin at distance from the lesions and from the skin of 16 healthy volunteers. The mRNA steady-state level of keratins 10, 14 and 16, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), vimentin, collagen I and IV, proliferating cell nuclear antigen, the various splice variants of vascular endothelial growth factor, VEGF-A, VEGF-C and VEGF-D, their receptors VEGFR1, VEGFR2 and VEGFR3, neuropilin (NRP)-1 and its soluble forms, NRP-2, semaphorin 3A and prox-1, was measured by reverse transcription-polymerase chain reaction. Immunohistochemistry was performed for Ki-67, von Willebrand factor and D2-40. Blood and lymphatic vessel density, area and distance from epidermis were estimated by morphological analysis coupled to an original computer-assisted method of quantification. RESULTS: Skin from healthy volunteers and nonlesional skin from patients with psoriasis displayed similar histological, morphometric and proliferative features. However, a significant overexpression of VEGFR3, the VEGF-A isoform VEGF121, soluble 12 NRP-1 and GAPDH was observed in the nonlesional psoriatic skin as compared with that of normal volunteers. CONCLUSIONS: These data point to significant differences in the blood and lymphatic vascular transcriptome between the clinically normal-appearing skin of patients with psoriasis and the skin of volunteers without psoriasis.

Establishment of stable human fibroblast cell lines constitutively expressing active Rho-GTPases.

Small GTP-binding proteins of the Rho family (RhoA, Cdc42, Rac1) regulate the organisation and the turnover of the cell's cytoskeleton and adhesion structures. A significant function of these cellular structures is to translate and counterbalance forces applied to, or generated by, cells in order to maintain homeostasis and control cell movement. We therefore hypothesised that Rho-GTPases are directly involved in cellular gravity perception and may participate in the alterations induced in microgravity. To define an adequate cellular model allowing to investigate this issue, we have established stable cell lines constitutively expressing active forms of either RhoA, Cdc42, or Rac1. The three cell lines differ by morphology and by their ability to form filopodia, lamellipodia, and bundles of actin stress fibers. Overexpression of the active form of either RhoA, Cdc42, or Rac1 is compatible with cell viability and does not affect cell population doubling time. Thus, our series of mutant cells appear well suited to gain further knowledge on the molecular mechanisms of cellular gravity perception.

Effects of constitutively active GTPases on fibroblast behavior.

The GTP-binding proteins RhoA, Cdc42 and Rac1 regulate the organization and turnover of the cytoskeleton and cell-matrix adhesions, structures bridging cells to their support, and translating forces, external or generated within the cell. To investigate the specific requirements of Rho GTPases for biomechanical activities of clonal cell populations, we compared side-by-side stable lines of human fibroblasts expressing constitutively active (CA) RhoA, Cdc42 or Rac1. There was no marked effect of any CA GTPase on cell adhesion to different extracellular matrix proteins. Cell spreading was CA Rho GTPase specific and independent of the extracellular matrix proteins allowing adhesion. Mechanical properties were dramatically restricted by CA RhoA on bi- and in tri-dimensional surroundings, were boosted by CA Rac1 on bi-dimensional surroundings only, and were not or marginally affected by CA Cdc42. In conclusion, the action of Rho GTPases appears to depend on the task cells are performing.

RGDS and DGEA-induced [Ca2+]i signalling in human dermal fibroblasts.

A pulse of short peptides, RGDS and DGEA in the millimolar range, immediately elicits in normal human fibroblasts a transient increase of intracellular Ca2+ ([Ca2+]i). In the present study, we show that this [Ca2+]i occurs in an increasing number of cells as a function of peptides concentration. It is specific of each peptide and inhibited at saturating concentration of the peptide in the culture medium. The [Ca2+]i transient depends on signalling pathways slightly different for DGEA and RGDS involving tyrosine kinase(s) and phosphatase(s), phospholipase C, production of inositol-trisphosphate and release of Ca2+ from the cellular stores. GFOGER, the classical collagen binding peptide of alpha1- alpha2- and alpha11-beta1 integrins, in triple helical or denatured form, does not produce any Ca2+ signal. The [Ca2+]i signalling induced by RGDS and DGEA is inhibited by antibodies against beta1 integrin subunit while that mediated by RGDS is also inhibited by antibodies against the alpha3 integrin. Delay in the acquisition of responsiveness is observed during cell adhesion and spreading on a coat of fibronectin for RGDS or collagen for DGEA or on a coat of the specific integrin-inhibiting antibodies but not by seeding cells on GFOGER or laminin-5. This delay is suppressed specifically by collagenase acting on the collagen coat or trypsin on the fibronectin coat. Our results suggest that free integrins and associated focal complexes generate a Ca2+ signal upon recognition of DGEA and RGDS by different cellular pathways.

Distinct pathways in the over-expression of matrix metalloproteinases in human fibroblasts by relaxation of mechanical tension.

The aim of the work was to analyze, on a comparative basis, the signaling pathways operating in the regulation of a panel of matrix metalloproteinases (MMP) expressed by human dermal fibroblasts submitted to mechanical stress relaxation by cytochalasin D (CD) and in a retracting collagen gel (RCG). The mRNA steady-state level of MMPs was measured by a quantitative RT-PCR procedure using a synthetic RNA as internal standard. In monolayer, most MMPs were barely detected, except MMP-2. Disruption of the actin stress fibers by CD induced a moderate increase of MMP-2 mRNA and a much larger stimulation of MMP-3, -9, -13 and -14 mRNAs. In RCG, a significant up-regulation of these MMPs was also observed although to a lower extent than in CD-treated monolayers. Among the investigated MMPs, the MMP-8 and -11 were not reproducibly detected. MMP-2 was processed to its active form both by CD and in RCG. The CD-induced up-regulation of gene expression was largely repressed by blocking protein synthesis by cycloheximide for all the MMPs, by inhibiting the tyrosine-kinases of the src family by herbimycin A for all MMPs, except MMP-2, and by inhibiting the TPA-inducible PKC isoforms by bisindoyl maleimide for all MMPs, except MMP-14. The up-regulation induced by stress relaxation in RCG was protein synthesis-dependent for MMP-2 and MMP-13, tyrosine kinases-dependent for MMP-3 and MMP-13, as previously described for MMP-1. Inhibiting TPA-inducible PKC did not affect any MMP in RCG except MMP-13, which was strongly induced. The processing of MMP-2 was tyrosine kinases-dependent but PKC-independent. Inhibitors of the ERK1,2 and p38 MAP kinases pathways diversely affected the MMPs expression. Inhibiting the Rho-kinase activity by Y-27632 was inactive. These results point to the potent regulation operated by the status of the cytoskeleton on the cell phenotype, and to distinct regulatory pathways involved in the control of different MMPs expression.

Apoptosis in v-myc-transfected MSU-1.1 fibroblasts is induced by cell-matrix contact and differs from that of normal dermal fibroblasts.

In order to characterize a fibroblast cell line representing normal human skin fibroblasts in three-dimensional cultures, we compared the fibroblast line MSU-1.1, derived from human foreskin and immortalized by v-myc, to primary human dermal fibroblasts (NDF). Our results demonstrate that in contrast to NDF, all MSU-1.1 fibroblasts die within 3-4 d when cultured within three-dimensional contractile collagen matrices. Also, in contrast to NDF. MSU-1.1 cells die markedly in anchored collagen gels as well. Death is due to apoptosis and is attenuated by addition of antibodies against collagen-recognizing receptors alpha1beta1 and alpha2beta1. Apoptosis of NDF in collagen lattices was repressed by an inhibitor of caspase-1, which was ineffective on apoptosis of MSU-1.1. Further, apoptosis by MSU-1.l fibroblasts was also observed in anchored, i.e., restrained collagen lattices, an environment that supports proliferation of NDF.

Coordinated regulation of procollagens I and III and their post-translational enzymes by dissipation of mechanical tension in human dermal fibroblasts.

Mechanical tension governs fibroblast proliferation and survival and the homeostasis of the extracellular matrix to adapt its resistance to the mechanical requirements of the organs. To consolidate this view, we analysed the effect of tension release on the expression of molecules involved in the architecture and stabilisation of the collagen fibres, namely the procollagens type I and III, the amino- and carboxy-procollagen peptidases (N-pCP and C-pCP) and lysyl oxidase. Cells were cultured in conditions of high mechanical stress in monolayer on a collagen coat and under reduced tension by disruption of the cytoskeleton upon treatment with cytochalasin D in monolayer on a collagen coat or by integrin-mediated stress relaxation in a freely retracting collagen gel. The mRNAs were measured by quantitative RT-PCR monitored by simultaneous reverse-transcription and amplification of an original internal standard. Tension relaxation resulted in a decreased expression of the procollagens type I and III, of the two expressed forms of C-pCP, of the two forms of N-pCP and of lysyl oxidase. Type III collagen, known to control diameter of the fibres, was less down-regulated than type I collagen. Interestingly, the expression of the two alternatively spliced forms of the N-pCP was dissimilarly regulated. These data suggest that mechanical tension may modulate the stiffness of the extracellular matrix by controlling not only the level of expression of its fibrillar constituents but also that of the enzymes participating in their extracellular processing and mechanical stabilisation.

Topically applied vitamin C enhances the mRNA level of collagens I and III, their processing enzymes and tissue inhibitor of matrix metalloproteinase 1 in the human dermis.

Ascorbic acid (vitamin C) is a cofactor required for the function of several hydroxylases and monooxygenases. It is not synthesized in humans and some other animal species and has to be provided by diet or pharmacologic means. Its absence is responsible for scurvy, a condition related in its initial phases to a defective synthesis of collagen by the reduced function of prolylhydroxylase and production of collagen polypeptides lacking hydroxyproline, therefore, they are unable to assemble into stable triple-helical collagen molecules. In fibroblast cultures, vitamin C also stimulates collagen production by increasing the steady-state level of mRNA of collagen types I and III through enhanced transcription and prolonged half-life of the transcripts. The aim of the experimental work has been to evaluate the effect on dermal cells of a preparation of vitamin C topically applied on one side vs placebo on the other side of the dorsal face of the upper forearm of postmenopausal women. Biopsies were collected on both sides and the level of mRNA measured by non competitive reverse transcription-polymerase chain reaction made quantitative by the simultaneous transcription and amplification of synthetic RNA used as internal standards. The mRNA of collagen type I and type III were increased to a similar extent by vitamin C and that of three post-translational enzymes, the carboxy- and amino-procollagen proteinases and lysyloxidase similarly increased. The mRNA of decorin was also stimulated, but elastin, and fibrillin 1 and 2 were not modified by the vitamin. The expression of matrix metalloproteinases 1, 2, and 9 was not significantly changed, but an increased level of tissue inhibitor of matrix metalloproteinase 1 mRNA was observed without modification of tissue inhibitor of matrix metalloproteinase 2 mRNA. The stimulating activity of topical vitamin C was most conspicuous in the women with the lowest dietary intake of the vitamin and unrelated to the level of actinic damage. The results indicate that the functional activity of the dermal cells is not maximal in postmenopausal women and can be increased.

Down-regulation of vascular endothelial growth factor by tissue inhibitor of metalloproteinase-2: effect on in vivo mammary tumor growth and angiogenesis.

The tissue inhibitor of metalloproteinases-2 (TIMP-2) has at least two independent functions, i.e., regulation of matrix metalloproteinases and growth promoting activity. We investigated the effects of TIMP-2 overexpression, induced by retroviral mediated gene transfer, on the in vivo development of mammary tumors in syngeneic mice inoculated with EF43.fgf-4 cells. The EF43.fgf-4 cells established by stably infecting the normal mouse mammary EF43 cells with a retroviral expression vector for the fgf-4 oncogene, are highly tumorigenic and overproduce vascular endothelial growth factor (VEGF). Despite a promotion of the in vitro growth rate of EF43.fgf-4 cells overexpressing timp-2, the in vivo tumor growth was delayed. At day 17 post-cell injection, the volume of tumor derived from TIMP-2-overexpressing cells was reduced by 80% as compared with that obtained with control cells. Overexpression of TIMP-2 was associated with a down-regulation of VEGF expression in vitro and in vivo, a reduction of vessel size, density, and blood supply in the induced tumors. In addition, TIMP-2 completely inhibited the angiogenic activity of EF43.fgf-4 cell-conditioned medium in vitro using a rat aortic ring model. Our findings suggest that overexpression of TIMP-2 delays growth and angiogenesis of mammary carcinoma in vivo and that down-regulation of VEGF expression may play an important role in this TIMP-2-mediated antitumoral and antiangiogenic effects. Finally the in vivo delivery of TIMP-2, as assessed by i.v. injection of recombinant adenoviruses vectors, significantly reduced the growth of the EF43.fgf-4-induced tumors. This effect of TIMP-2 was shown to be equally comparable with that of angiostatin, a known potent inhibitor of angiogenesis.

In vitro tubulogenesis of endothelial cells by relaxation of the coupling extracellular matrix-cytoskeleton.

OBJECTIVE: This investigation aimed at determining the importance of the rigidity of the adhesive support and the participation of the cytoskeleton in tubulogenesis of endothelial cells in vitro. METHODS: The morphotype, biosynthetic phenotype and cytoskeleton organization of human umbilical vein endothelial cells (HUVEC) were analyzed on supports of variable mechanical resistance. RESULTS: Western blot analysis revealed a strong reduction of the expression of actin and focal-adhesion plaque (FAP) proteins in HUVEC organized in tube-like structures (TLS) on soft matrigel or on matrigel co-polymerized with heat-denatured collagen as compared to HUVEC remaining in a monolayer pattern on rigid matrigel-coat or on matrigel co-polymerized with type I collagen. Human skin fibroblasts morphotype was not altered in these culture conditions and the pattern of FAP proteins and actin was not modulated. By using polyacrylamide gels polymerized with various concentrations of bis-acrylamide to modulate the mechanical resistance of the support and cross-linked to a constant amount of gelatin to provide an equal density of attachment sites, it was shown that the less rigid the support, the more endothelial cells switched to a tube-like pattern. Collagen type I-induced tubulogenesis was accompanied by a profound and reversible remodeling of the actin-FAP complex suggesting a weakening of the bridging between extracellular matrix (ECM) and the cytoskeleton. Human skin fibroblasts and smooth muscle cells, used as control cells, adhered strongly to the collagen, did not form TLS and their network of actin stress fibers was not remodeled. The inhibition of collagen type I-induced tubulogenesis by agents altering the actin cytoskeleton-FAP complex including calpain type I inhibitor, orthovanadate, KT5720 and jasplakinolide, further supports the determinant role of mechanical coupling between the cells and the matrix in tubulogenesis. CONCLUSIONS: A reduced tension between the endothelial cells and the extracellular matrix, originating in the support or within the cells is sufficient to trigger an intracellular signaling cascade leading to tubulogenesis, an event mimicking one of the last steps of angiogenesis.

Comparative caustic and biological activity of trichloroacetic and glycolic acids on keratinocytes and fibroblasts in vitro.

OBJECTIVES: Beside their causticity, the biological mechanism by which trichloroacetic acid (TCA) and glycolic acid (GA), two agents extensively used for chemical peeling, might act remains unknown. The purpose of this study was to examine in vitro the effect of TCA and GA on human keratinocytes and the influence of the released epithelial mediators on collagen and matrix metalloproteinases (MMPs) production by human dermal fibroblasts. METHOD: Cultured keratinocytes were treated by TCA and GA at 10 mg/ml brought to pH 3, 5 and 7, and the conditioned media neutralized to pH 7 were added to human normal skin fibroblasts. RESULTS: TCA was cytotoxic for keratinocytes at each tested pH. The conditioned medium depressed protein and collagen synthesis and the expression of MMPs when added to fibroblasts as did also TCA when added directly to fibroblasts. GA was not cytotoxic for keratinocytes at neutral pH and the conditioned medium obtained at each pH applied to fibroblasts did not alter protein, collagen nor MMPs production while causing an elevated secretion of IL-6. CONCLUSION: TCA exerts a toxic effect on keratinocytes and fibroblasts while GA does not alter the metabolism of fibroblasts but induces the secretion of IL-6.

Human Ehlers-Danlos syndrome type VII C and bovine dermatosparaxis are caused by mutations in the procollagen I N-proteinase gene.

Ehlers-Danlos syndrome (EDS) type VIIC is a recessively inherited connective-tissue disorder, characterized by extreme skin fragility, characteristic facies, joint laxity, droopy skin, umbilical hernia, and blue sclera. Like the animal model dermatosparaxis, EDS type VIIC results from the absence of activity of procollagen I N-proteinase (pNPI), the enzyme that excises the N-propeptide of type I and type II procollagens. The pNPI enzyme is a metalloproteinase containing properdin repeats and a cysteine-rich domain with similarities to the disintegrin domain of reprolysins. We used bovine cDNA to isolate human pNPI. The human enzyme exists in two forms: a long version similar to the bovine enzyme and a short version that contains the Zn++-binding catalytic site but lacks the entire C-terminal domain in which the properdin repeats are located. We have identified the mutations that cause EDS type VIIC in the six known affected human individuals and also in one strain of dermatosparactic calf. Five of the individuals with EDS type VIIC were homozygous for a C-->T transition that results in a premature termination codon, Q225X. Four of these five patients were homozygous at three downstream polymorphic sites. The sixth patient was homozygous for a different transition that results in a premature termination codon, W795X. In the dermatosparactic calf, the mutation is a 17-bp deletion that changes the reading frame of the message. These data provide direct evidence that EDS type VIIC and dermatosparaxis result from mutations in the pNPI gene.

Gelatinases in drug-induced toxic epidermal necrolysis.

BACKGROUND: The matrix metalloproteinases (MMPs) MMP2 and MMP9 play a significant role in epidermal detachment, inflammation and re-epithelialization. We have evaluated their activity in toxic epidermal necrolysis (TEN). DESIGN: The level and pattern of activity of MMP2 and MMP9 were investigated by measuring the degradation of 3H-labelled gelatin and by zymography in blister fluid from six TEN patients and compared the results with three other blistering conditions: bullous pemphigoid (n = 6), second-degree burn (n = 13) or suction blister (n = 3). RESULTS: A higher amount of MMP2 was found in TEN blister fluid with the constant presence of a significantly larger proportion of the activated forms of MMP2, a particular feature of TEN, than the other blistering diseases studied. CONCLUSION: This study emphasizes the potential role of MMP2 in the specific inflammatory reaction and reparation process in TEN skin.

An interleukin-1 loop is induced in human skin fibroblasts upon stress relaxation in a three-dimensional collagen gel but is not involved in the up-regulation of matrix metalloproteinase 1.

The integrin-mediated stress relaxation as it occurs in a retracting three-dimensional collagen gel (RCG) is accompanied by a large up-regulation of the interstitial collagenase, matrix metalloproteinase 1 ((MMP-1), EC 3.4.24.7), regulated notably by interleukin-1 (IL-1), phorbol esters, and cytoskeleton-disrupting drugs as cytochalasin D (CD). The repression of MMP-1 up-regulation in RCG by cycloheximide suggested the participation in the regulation process of a de novo synthesized intermediary component. We demonstrate here that culture of human skin fibroblasts in RCG or in CD- and 12-O-tetradecanoylphorbol-13-acetate (TPA)-treated monolayers resulted in the activation of an IL-1 autocrine feedback loop that was switched off by the naturally occurring IL-1 receptor antagonist (IL-1RA), a blocker of the common IL-1 receptor. The IL-1RA did not suppress the MMP-1 up-regulation induced in RCG nor in CD-treated cells, indicating that the up-regulation of MMP-1 and the IL-1 autocrine loop occurred in an independent way, while the TPA-induced MMP-1 expression was suppressed by the receptor antagonist. The RCG- as well as the TPA-, IL-1-, and CD-induced up-regulation of both MMP-1 and IL-1 was totally suppressed by protein tyrosine kinases inhibitors. In contrast bisindoylmaleimide, at a concentration (5 microM) that inhibits the TPA-induced protein kinase C activity, suppressed the CD-induced MMP-1 expression but did not or barely altered that induced in RCG or by IL-1. None of the other tested inhibitors of a variety of signaling pathways including those used by integrins was able to suppress the RCG or CD-induced MMP-1. These results point to a potent regulation of MMP-1 by mechanical stress relaxation, a process depending on de novo protein synthesis and occurring independently of the activation of an IL-1 autocrine feedback loop.

Surface located procollagen N-propeptides on dermatosparactic collagen fibrils are not cleaved by procollagen N-proteinase and do not inhibit binding of decorin to the fibril surface.

Dermatosparaxis is a recessive disorder of animals (including man) which is caused by mutations in the gene for the enzyme procollagen N-proteinase and is characterised by extreme skin fragility. Partial loss of enzyme activity results in accumulation of pNcollagen (collagen with N-propeptides) and abnormal collagen fibrils in the fragile skin. How the N-propeptides persist in the tissue and how abnormal fibril morphology results in fragile skin is poorly understood. Using biochemical and quantitative mass mapping electron microscopy we showed that the collagen fibrils in the skin of a dermatosparactic calf contained 57% type I pNcollagen and 43% type I collagen and the fibrils were irregularly arranged in bundles and hieroglyphic in cross-section. Image analysis of the fibril cross-sections suggested that the deviation from circularity of dermatosparactic fibrils was caused by N-propeptides of pNcollagen being located at the fibril surface. Comparison of experimental and theoretical axial mass distributions of the fibrils showed that the N-propeptides were located to the overlap zone of the fibril D-period (where D=67 nm, the characteristic axial periodicity of collagen fibrils). Treatment of the dermatosparactic fibrils with N-proteinase did not remove the N-propeptides from the fibrils, although the N-propeptides were efficiently removed by trypsin and chymotrypsin. However, the N-propeptides were efficiently cleaved by the N-proteinase when the pNcollagen molecules were extracted from the fibrils. These results are consistent with close packing of N-propeptides at the fibril surface which prevented cleavage by the N-proteinase. Long-range axial mass determination along the fibril length showed gross non-uniformity with multiple mass bulges. Of note is the skin fragility in dermatosparaxis, and also the appearance of mass bulges along the fibril long axis symptomatic of the fragile skin of mice which lack decorin. Western blot analysis showed that the dermatosparactic fibrils bound elevated levels of the proteoglycan, compared with normal skin fibrils. The results showed that N-propeptides can distort the morphology of fibrils, that they do not inhibit binding of gap-associated macromolecules (such as decorin) and that the normal mechanical properties of skin are strongly dependent on the close association of near-cylindrical fibrils, thereby enabling maximal fibril-fibril interactions.

Mechano-sensing and mechano-reaction of soft connective tissue cells.

One main function of the connective tissues is to provide cells with a mechanically resistant attachment support required for survival, division and differentiation. All cells contain membrane-anchored attachment proteins able to recognize specific chemical motifs in the extracellular macromolecules forming the supporting scaffold, made of various types of collagen, adhesive glycoproteins, elastin, proteoglycans, etc... These cell-matrix interactions are mainly mediated by receptors of the integrins family, heterodimeric molecules made of an extracellular domain connected through a transmembrane sequence to an intracytoplasmic tail. Upon recognition of the extracellular ligand, the clustering and activation of the integrins result in the recruitment of a complex of proteins and formation of the focal adhesion plaque, containing both cytoskeletal and catalytic signaling molecules. Activation results in polymerization of actin and formation of stress fibers. These structures establish a physical link between the extracellular matrix components and the cytoskeleton through the integrins providing a continuous path acting as a mechanotransducer. This connection is used by the cells to perform their mechanical functions as adhesion, migration and traction. In vitro experimental models using fibroblasts in a collagen gel demonstrate that cells are in mechanical equilibrium with their support which regulates their replicative and biosynthetic phenotype. The present review discusses the molecular structures operating in the transmission of the mechanical messages from the support to the connective tissue cells, and their effect on the cellular machinery. We present arguments for investigating these mechanisms in understanding the perception of reduced gravity and the resulting reaction leading to microgravity induced pathologies.