cAMP-dependent differential regulation of extracellular matrix (ECM) gene expression in cultured rat Schwann cells

cAMP level modulates scleral collagen remodeling, a critical step in the development of myopia

The development of myopia is associated with decreased ocular scleral collagen synthesis in humans and animal models. Collagen synthesis is, in part, under the influence of cyclic adenosine monophosphate (cAMP). We investigated the associations between cAMP, myopia development in guinea pigs, and collagen synthesis by human scleral fibroblasts (HSFs). Form-deprived myopia (FDM) was induced by unilateral masking of guinea pig eyes. Scleral cAMP levels increased selectively in the FDM eyes and returned to normal levels after unmasking and recovery. Unilateral subconjunctival treatment with the adenylyl cyclase (AC) activator forskolin resulted in a myopic shift accompanied by reduced collagen mRNA levels, but it did not affect retinal electroretinograms. The AC inhibitor SQ22536 attenuated the progression of FDM. Moreover, forskolin inhibited collagen mRNA levels and collagen secretion by HSFs. The inhibition was reversed by SQ22536. These results demonstrate a critical role of cAMP in control of myopia development. Selective regulation of cAMP to control scleral collagen synthesis may be a novel therapeutic strategy for preventing and treating myopia.

Sustained delivery of dibutyryl cyclic adenosine monophosphate to the transected spinal cord via oligo [(polyethylene glycol) fumarate] hydrogels

This study describes the use of oligo [(polyethylene glycol) fumarate] (OPF) hydrogel scaffolds as vehicles for sustained delivery of dibutyryl cyclic adenosine monophosphate (dbcAMP) to the transected spinal cord. dbcAMP was encapsulated in poly(lactic-co-glycolic acid) (PLGA) microspheres, which were embedded within the scaffolds architecture. Functionality of the released dbcAMP was assessed using neurite outgrowth assays in PC12 cells and by delivery to the transected spinal cord within OPF seven channel scaffolds, which had been loaded with Schwann cells or mesenchymal stem cells (MSCs). Our results showed that encapsulation of dbcAMP in microspheres lead to prolonged release and continued functionality in vitro. These microspheres were then successfully incorporated into OPF scaffolds and implanted in the transected thoracic spinal cord. Sustained delivery of dbcAMP inhibited axonal regeneration in the presence of Schwann cells but rescued MSC-induced inhibition of axonal regeneration. dbcAMP was also shown to reduce capillary formation in the presence of MSCs, which was coupled with significant functional improvements. Our findings demonstrate the feasibility of incorporating PLGA microsphere technology for spinal cord transection studies. It represents a novel sustained delivery mechanism within the transected spinal cord and provides a platform for potential delivery of other therapeutic agents.

Transcriptional regulation of laminin gene expression

Laminins are the most abundant structural non-collagenous glycoproteins ubiquitously present in basement membranes. They are multidomain molecules consisting of of alpha, beta, and gamma chains. Although the precise functional differences between the laminin variants are not well understood, the diversity of laminin isoforms may reflect the formation of distinct basement membranes. The laminins display a remarkable restricted expression profile, suggesting a fine regulation of their genes. In this review, we focus on the most recent developments of laminin biology, centering on transcriptional and posttranscriptional controls. We discuss only those laminin chains whose gene organization and promoter elements have been characterized and proved to be functional. When possible, we correlate the effects of growth factors, cytokines, retinoids, and transcription factors on laminin gene expression with the identity of cis-acting elements in their genomic control regions.

Glutamate receptor 5/6/7-like and glutamate transporter-1-like immunoreactivity in the leech central nervous system

Previous physiological and pharmacological evidence has suggested a neurotransmitter role for the excitatory amino acid glutamate in the leech central nervous system (CNS). In the present study, we sought to localize glutamate receptor (GluR) subunits (GluR 5/6/7, GluR 2/3 and N-methyl-D-aspartate receptor 1 [NMDAR 1]) and a glutamate transporter subtype [GLT-1] within the leech CNS using mono- and polyclonal antibodies. In whole-mounted tissue, small cells of the outer capsule and putative microglia labeled with both GluR 5/6/7 and GluR 2/3 but not NMDAR 1 subunit antisera. In general, GluR 5/6/7-like immunofluorescence was both more intense and more widespread than GluR 2/3-like immunolabeling. Cryostat-sectioned tissue revealed extensive GluR 5/6/7-like immunoreactivity throughout the neuropil as well as labeling within a few neuronal somata. GLT-1-like immunoreactivity localized to the inner capsule, which is the interface between neuronal somata and the neuropil and is deeply invested by processes of neuropil glia. These results complement previous physiological and pharmacological findings indicating that the leech CNS possesses the cellular machinery to respond to glutamate and to transport glutamate from extracellular spaces. Together, they provide further evidence for glutamate's role as a neurotransmitter within the leech CNS.

Differential signaling cascade of MAP kinase and S6 kinase depends on 3',5'-monophosphate concentration in schwann cells: correlation to cellular differentiation and proliferation

Schwann cells produce myelin in the peripheral nervous system (PNS) and play an important role in the maintenance of the normal function of PNS. Our previous studies have shown that derivatives of adenosine 3',5'-monophosphate (cAMP) can regulate the cell-fate (i.e., proliferation and differentiation into cell surface galactocerebroside-positive cells) depending on its concentration in vitro. Higher concentration of cAMP can induce the expression of cell surface galactocerebroside, while proliferation can be induced by lower concentration of cAMP. However, the detailed molecular mechanism of how the same second messenger yields different phenotypes of Schwann cells depending on its concentration remains to be elucidated. Here we show that low concentration of 8-bromo cAMP, a cell-permeable derivative of cAMP, activates S6 kinase activity with a short-lived activation of mitogen-activated protein kinase (MAPK), whereas high dose of the reagent activates S6 kinase much less than that of low dose with a small and prolonged activation of MAPK in Schwann cells. These data clearly demonstrated that a rise in the intracellular cAMP uses the MAPK-S6 kinase pathway as an intracellular sinaling cascade and different magnitude and duration of the activation of this pathway might underlie the different cellular fate depending on the intensity of the stimulation.

Platelet-derived growth factor and pentoxifylline modulation of collagen synthesis in myofibroblasts

Fibroblast proliferation and extracellular matrix accumulation are two major events occurring in fibrosis. Hepatic stellate cells are the major collagen-producing cells of the liver and are transformed into proliferative myofibroblasts following activation. Whether proliferation and extracellular matrix production are regulated by the same cytokines is not known. Monocyte-conditioned medium obtained from pigs with yellow phosphorus-induced hepatic fibrosis increased the collagen production by cultured procine myofibroblasts. Liver biopsies from these same fibrotic animals had increased levels of collagen alpha 1(I) and alpha 1(III) mRNA compared to control animals. Preincubation with platelet-derived growth factor (PDGF) B/B antibody significantly reduced the collagen-stimulating ability of the monocyte-conditioned medium. Recombinant PDGF stimulated proliferation in nonconfluent myofibroblasts and stimulated collagen production in confluent cultures of myofibroblasts without increasing cell number, suggesting that these events can occur independent of each other. Pentoxifylline and one of its active metabolites (metabolite-1) inhibited PDGF-stimulated collagen production in cultured porcine myofibroblasts. These results demonstrate the importance of PDGF in the pathogenesis of liver fibrosis and provide evidence that pentoxifylline interferes with PDGF-mediated events during experimental liver fibrosis.

G proteins are involved in the suppression of collagen alpha 1 (I) gene expression in cultured rat hepatic stellate cells

We analyse the role of the G proteins in regulating collagen gene expression by measuring collagen alpha 1(I) mRNA levels in cultured hepatic stellate cells in basal conditions and after stimulating or inhibiting the major intracellular signalling pathways. Stimulation of Gs protein and adenylyl cyclase or the addition of 8Br-cAMP to the cells led to a decrease in collagen alpha 1(I) mRNA levels, while blocking protein kinase A abolished this effect. Blocking Gi protein, phospholipase A2 and C, calcium channels and calmodulin resulted in a significant increase in collagen mRNA levels. PKC stimulation led to a marked decrease in these levels. These results suggest that collagen gene expression is inhibited by a number of intracellular pathways. A Gs and a pertussis toxin-sensitive G protein seem to initiate cellular response. Transcription factors, acting in these pathways, must be identified. However, it seems that they do not need to be synthesised.

Effect of pentoxifylline on the degradation of procollagen type I produced by human hepatic stellate cells in response to transforming growth factor-beta 1

1. Pentoxifylline (PTF) may act as a potential antifibrogenic agent by inhibiting cell proliferation and/or collagen deposition in cell type(s) responsible for the accumulation of extracellular matrix. The aim of the present study was to investigate at which level PTF may affect synthesis and degradation of type I collagen in human hepatic stellate cells (HSCs), a key source of connective tissue in fibrotic liver. 2. Procollagen type I synthesis and release were evaluated in cells maintained in serum free/insulin free medium for 48 h and then stimulated with transforming growth factor-beta 1 (TGF-beta 1) for different time periods in the presence or absence of PTF. TGF-beta 1 caused an upregulation of procollagen I mRNA levels with a peak increase after 3-6 h of stimulation. This effect was followed by an increase in both the cell associated and the extracellular levels of the corresponding protein, with a peak effect at 9-12 h after the addition of TGF-beta 1. Co-incubation with PTF slightly but consistently reduced basal as well as stimulated procollagen I mRNA levels, with negligible effects on the cell-associated expression of the corresponding protein. Conversely, PTF dose-dependently reduced procollagen type I levels detected in supernatants from unstimulated and stimulated cells. 3. Pulse-chase experiments employing L-[3H]-proline revealed that PTF was able to induce significantly the degradation of procollagen, mainly in the extracellular compartment. We next analysed the effect of PTF on the major pathway involved in type I collagen degradation. PTF did not affect the expression of metalloproteinase 1 (MMP-1) mRNA both in basal and stimulated conditions, whereas it markedly reduced the expression of tissue inhibitor of metalloproteinase 1 (TIMP-1) mRNA. Accordingly incubation with PTF increased the levels of 'activated MMP-1' in cell supernatants in both basal and stimulated conditions. 4. These results suggest that the antifibrogenic action of PTF on human HSCs is mainly mediated by extracellular collagen degradation rather than by a reduction of collagen synthesis.

Expression of laminin and its possible role in adrenal cortex homeostasis

The adult mammalian adrenal cortex undergoes permanent regeneration. This process implies a cellular proliferation step restricted to the external zone of the tissue, and a subsequent centripetal cell migration during which phenotypic transition from glomerulosa into fasciculata and reticularis cells and elimination of senescent cells through apoptosis occur. As the molecular mechanisms implied in adrenocortical cell migration are still generally unknown, we addressed that question in the present study. Of several extracellular matrix proteins tested, laminin was the most potent chemotactic and haptotactic factor for bovine fasciculata adrenocortical cells. The maximal chemotactic effect (3-fold stimulation) was observed with 50-75 micrograms/ml laminin, whereas the haptotactic effect (3.5-fold stimulation) plateaued for laminin concentrations in the coating solution over 25 micrograms/ml. Using an anti-Engelbreth-Holm-Swarm laminin antibody, we could demonstrate that adrenocortical cells actively synthesize and secrete Engelbreth-Holm-Swarm-laminin, with the A chain produced in limiting quantities. ACTH treatment of adrenocortical cells specifically induced a 2.7- to 4.5-fold increase in A chain synthesis, resulting in a corresponding increase in the amount of secreted laminin. The distribution of laminin in the adrenal cortex tissue was then evaluated by standard immunohistochemistry. The protein appeared to be uniformly expressed in the three zones of the cortex. This observation does not favor the hypothesis that laminin acts as an attractant driving centripetal cell migration. Laminin, which is synthesized under the control of the systemic hormone ACTH, appears as a permissive factor that facilitates proper homeostasis of the adrenocortical tissue.