Rat oligodendroglia express c-met and focal adhesion kinase, protein tyrosine kinases implicated in regulating epithelial cell motility

Mechanotransduction: Exploring New Therapeutic Avenues in Central Nervous System Pathology

Cells are continuously exposed to physical forces and the central nervous system (CNS) is no exception. Cells dynamically adapt their behavior and remodel the surrounding environment in response to forces. The importance of mechanotransduction in the CNS is illustrated by exploring its role in CNS pathology development and progression. The crosstalk between the biochemical and biophysical components of the extracellular matrix (ECM) are here described, considering the recent explosion of literature demonstrating the powerful influence of biophysical stimuli like density, rigidity and geometry of the ECM on cell behavior. This review aims at integrating mechanical properties into our understanding of the molecular basis of CNS disease. The mechanisms that mediate mechanotransduction events, like integrin, Rho/ROCK and matrix metalloproteinases signaling pathways are revised. Analysis of CNS pathologies in this context has revealed that a wide range of neurological diseases share as hallmarks alterations of the tissue mechanical properties. Therefore, it is our belief that the understanding of CNS mechanotransduction pathways may lead to the development of improved medical devices and diagnostic methods as well as new therapeutic targets and strategies for CNS repair.

Mechanisms of oligodendrocyte progenitor developmental migration

Oligodendrocytes, the myelinating cells of the central nervous system (CNS), develop from oligodendrocyte progenitor cells (OPCs) that must first migrate extensively throughout the developing brain and spinal cord. Specified at particular times from discrete regions in the developing CNS, OPCs are one of the most migratory of cell types and disperse rapidly. A variety of factors act on OPCs to trigger intracellular changes that regulate their migration. We will discuss factors that act as long-range guidance cues, those that act to regulate cellular motility, and those that are critical in determining the final positioning of OPCs. In addition, recent evidence has identified the vasculature as the physical substrate used by OPCs for their migration. Several new findings relating to this oligodendroglial-vascular signaling axis reveal new insight on the relationship between OPCs and blood vessels in the developing and adult brain.

Met is required for oligodendrocyte progenitor cell migration in Danio rerio

During vertebrate central nervous system development, most oligodendrocyte progenitor cells (OPCs) are specified in the ventral spinal cord and must migrate throughout the neural tube until they become evenly distributed, occupying non-overlapping domains. While this process of developmental OPC migration is well characterized, the nature of the molecular mediators that govern it remain largely unknown. Here, using zebrafish as a model, we demonstrate that Met signaling is required for initial developmental migration of OPCs, and, using cell-specific knock-down of Met signaling, show that Met acts cell-autonomously in OPCs. Taken together, these findings demonstrate in vivo, the role of Met signaling in OPC migration and provide new insight into how OPC migration is regulated during development.

The Protein Tyrosine Phosphatase Shp2 Regulates Oligodendrocyte Differentiation and Early Myelination and Contributes to Timely Remyelination

Shp2 is a nonreceptor protein tyrosine phosphatase that has been shown to influence neurogenesis, oligodendrogenesis, and oligodendrocyte differentiation. Furthermore, Shp2 is a known regulator of the Akt/mammalian target of rapamycin and ERK signaling pathways in multiple cellular contexts, including oligodendrocytes. Its role during later postnatal CNS development or in response to demyelination injury has not been examined. Based on the current studies, we hypothesize that Shp2 is a negative regulator of CNS myelination. Using transgenic mouse technology, we show that Shp2 is involved in oligodendrocyte differentiation and early myelination, but is not necessary for myelin maintenance. We also show that Shp2 regulates the timely differentiation of oligodendrocytes following lysolecithin-induced demyelination, although apparently normal remyelination occurs at a delayed time point. These data suggest that Shp2 is a relevant therapeutic target in demyelinating diseases such as multiple sclerosis.SIGNIFICANCE STATEMENT In the present study, we show that the protein phosphatase Shp2 is an important mediator of oligodendrocyte differentiation and myelination, both during developmental myelination as well as during myelin regeneration. We provide important insight into the signaling mechanisms regulating myelination and propose that Shp2 acts as a transient brake to the developmental myelination process. Furthermore, we show that Shp2 regulates oligodendrocyte differentiation following demyelination and therefore has important therapeutic implications in diseases such as multiple sclerosis.

TAM receptor tyrosine kinases: expression, disease and oncogenesis in the central nervous system

Receptor tyrosine kinases (RTKs) are cell surface proteins that tightly regulate a variety of downstream intra-cellular processes; ligand-receptor interactions result in cascades of signaling events leading to growth, proliferation, differentiation and migration. There are 58 described RTKs, which are further categorized into 20 different RTK families. When dysregulated or overexpressed, these RTKs are implicated in disordered growth, development, and oncogenesis. The TAM family of RTKs, consisting of Tyro3, Axl, and MerTK, is prominently expressed during the development and function of the central nervous system (CNS). Aberrant expression and dysregulated activation of TAM family members has been demonstrated in a variety of CNS-related disorders and diseases, including the most common but least treatable brain cancer in children and adults: glioblastoma multiforme.

Signaling mechanisms regulating myelination in the central nervous system

The precise and coordinated production of myelin is essential for proper development and function of the nervous system. Diseases that disrupt myelin, including multiple sclerosis, cause significant functional disability. Current treatment aims to reduce the inflammatory component of the disease, thereby preventing damage resulting from demyelination. However, therapies are not yet available to improve natural repair processes after damage has already occurred. A thorough understanding of the signaling mechanisms that regulate myelin generation will improve our ability to enhance repair. in this review, we summarize the positive and negative regulators of myelination, focusing primarily on central nervous system myelination. Axon-derived signals, extracellular signals from both diffusible factors and the extracellular matrix, and intracellular signaling pathways within myelinating oligodendrocytes are discussed. Much is known about the positive regulators that drive myelination, while less is known about the negative regulators that shift active myelination to myelin maintenance at the appropriate time. Therefore, we also provide new data on potential negative regulators of CNS myelination.

Focal adhesion kinase can play unique and opposing roles in regulating the morphology of differentiating oligodendrocytes

During development cells of the oligodendrocyte lineage undergo significant changes in morphology when they differentiate from migratory oligodendrocyte progenitors, which are mostly bipolar, into post-migratory pre-myelinating oligodendrocytes, which extend complex and expanded process networks, and then finally into mature oligodendrocytes, which generate myelin sheaths required for efficient signal propagation within the nervous system. This extensive morphological remodeling occurs in the context of a complex extracellular environment and requires significant rearrangement of the cell's cytoskeleton. The molecular mechanisms underlying this intricate integration of signals, however, remain poorly understood. A key regulator of extracellular matrix to cytoskeleton signaling is the non-receptor tyrosine kinase FAK (focal adhesion kinase). Here, we report that FAK can regulate the morphology of differentiating post-migratory pre-myelinating oligodendrocytes in a unique and opposing fashion that is dependent on the nature of the extracellular matrix and mediated largely by FAK's catalytic activity. More specifically, FAK was found to restrict process network expansion in the presence of fibronectin but to promote morphological maturation in the presence of laminin-2. In addition, FAK's restraining role predominated for postnatal day 3-derived cells, while its maturation promoting role prevailed for postnatal day 5-derived cells. Taken together, our findings reveal a complex role of FAK in regulating the morphology of post-migratory pre-myelinating oligodendrocytes.

Focal adhesion kinase (FAK): A regulator of CNS myelination

The formation of the myelin sheath is a crucial step during development because it enables fast and efficient propagation of signals within the limited space of the mammalian central nervous system (CNS). During the process of myelination, oligodendrocytes actively interact with the extracellular matrix (ECM). These interactions are considered crucial for proper and timely completion of the myelin sheath. However, the exact regulatory circuits involved in the signaling events that occur between the ECM and oligodendrocytes are currently not fully understood. Therefore, in the present study we investigated the role of a known integrator of cell-ECM signaling, namely, focal adhesion kinase (FAK), in CNS myelination via the use of conditional (oligodendrocyte-specific) and inducible FAK-knockout mice (Fak(flox/flox): PLP/CreER(T) mice). When inducing FAK knockout just prior to and during active myelination of the optic nerve, we observed a significant reduction in the number of myelinated fibers on postnatal day 14. In addition, our data revealed a decreased number of primary processes extending from oligodendrocyte cell bodies at this postnatal age and on induction of FAK knockout. In contrast, myelination appeared normal on postnatal day 28. Thus, our data suggest that FAK controls the efficiency and timing of CNS myelination during its initial stages, at least in part, by regulating oligodendrocyte process outgrowth and/or remodeling.

Focal adhesion kinase regulates laminin-induced oligodendroglial process outgrowth

In the central nervous system (CNS), myelination of axons occurs when oligodendrocyte progenitor cells undergo terminal differentiation, and initiate process formation and axonal ensheathment. Although Fyn, a member of the Src-family kinases (SFKs), plays an important role in this differentiation process, the substrates of Fyn in oligodendrocytes are largely unknown. Using mass spectrometric analysis, we identified focal adhesion kinase (FAK) as a tyrosine-phosphorylated protein in the rat-derived CG4 oligodendrocyte cell line. Tyrosine phosphorylation of FAK was enhanced during differentiation of CG4 cells in a Fyn-dependent manner. In addition, phosphorylation of FAK was stimulated by laminin, one of the ligands for integrin. Knockdown of FAK expression in CG4 cells suppressed process outgrowth on laminin. Rac1 and Cdc42 activities, which are required for oligodendrocyte process formation, were down-regulated in FAK-knockdown cells. Expression of wild-type (WT) FAK in FAK-knockdown CG4 cells restored outgrowth of processes, but the Y397F mutant lacking the autophosphorylation site did not. These results suggest that FAK/Fyn-mediated activation of Rac1 and Cdc42 is critical for laminin-induced outgrowth of oligodendrocyte processes.

Phosphodiesterase-Ialpha/autotaxin's MORFO domain regulates oligodendroglial process network formation and focal adhesion organization

Development of a complex process network by maturing oligodendrocytes is a critical but currently poorly characterized step toward myelination. Here, we demonstrate that the matricellular oligodendrocyte-derived protein phosphodiesterase-Ialpha/autotaxin (PD-Ialpha/ATX) and especially its MORFO domain are able to promote this developmental step. In particular, the single EF hand-like motif located within PD-Ialpha/ATX's MORFO domain was found to stimulate the outgrowth of higher order branches but not process elongation. This motif was also observed to be critical for the stimulatory effect of PD-Ialpha/ATX's MORFO domain on the reorganization of focal adhesions located at the leading edge of oligodendroglial protrusions. Collectively, our data suggest that PD-Ialpha/ATX promotes oligodendroglial process network formation and expansion via the cooperative action of multiple functional sites located within the MORFO domain and more specifically, a novel signaling pathway mediated by the single EF hand-like motif and regulating the correlated events of process outgrowth and focal adhesion organization.

Focal adhesion kinase and p53 signaling in cancer cells

The progression of human cancer is characterized by a process of tumor cell motility, invasion, and metastasis to distant sites, requiring the cancer cells to be able to survive the apoptotic pressures of anchorage-independent conditions. One of the critical tyrosine kinases linked to these processes of tumor invasion and survival is the focal adhesion kinase (FAK). FAK was first isolated from human tumors, and FAK mRNA was found to be upregulated in invasive and metastatic human breast and colon cancer samples. Recently, the FAK promoter was cloned, and it has been found to contain p53-binding sites. p53 inhibits FAK transcription, and recent data show direct binding of FAK and p53 proteins in vitro and in vivo. The structure of FAK and p53, proteins interacting with FAK, and the role of FAK in tumorigenesis and FAK-p53-related therapy are reviewed. This review focuses on FAK signal transduction pathways, particularly on FAK and p53 signaling, revealing a new paradigm in cell biology, linking signaling from the extracellular matrix to the nucleus.

Phosphodiesterase-I alpha/autotaxin controls cytoskeletal organization and FAK phosphorylation during myelination

Myelination within the central nervous system (CNS) involves substantial morphogenesis of oligodendrocytes requiring plastic changes in oligodendrocyte-extracellular matrix (ECM) interactions, that is, adhesion. Our previous studies indicated that a regulator of such adhesive plasticity is oligodendrocyte-released phosphodiesterase-I alpha/autotaxin (PD-I alpha/ATX). We report here, that PD-I alpha/ATX's adhesion antagonism is mediated by a protein fragment different from the one that stimulates tumor cell motility. Furthermore, PD-I alpha/ATX's adhesion-antagonizing fragment causes a reorganized distribution of the focal adhesion components vinculin and paxillin and an integrin-dependent reduction in focal adhesion kinase (FAK) phosphorylation at tyrosine residue 925 (pFAK-925). In vivo, a similar reduction in pFAK-925 occurs at the onset of myelination when PD-I alpha/ATX expression is significantly upregulated. Most importantly, it can also be induced by the application of exogenous PD-I alpha/ATX. Our data, therefore, suggest that PD-I alpha/ATX participates in the regulation of myelination via a novel signaling pathway leading to changes in integrin-dependent focal adhesion assembly and consequently oligodendrocyte-ECM interactions.

A mechanism of impaired mobility of oligodendrocyte progenitor cells by tenascin C through modification of wnt signaling

In demyelinating diseases, the mechanisms of how oligodendrocyte (OLG) progenitor cells are affected in the demyelinated area remain to be elucidated. To investigate one aspect of the mechanisms, we focused on the role of tenascin C in regulating the migratory mobility of the progenitor cells via beta-catenin. By cDNA subtraction screening, we found tenascin C expression to be increased in OLG progenitors (rat primary O2A cells). Tenascin C inhibited the migration of OLG progenitors and CG-4 cells, and beta-catenin accumulated at focal adhesions in these cells. These changes were associated with the inactivation of canonical wnt signaling. Overexpression of the wnt-signaling antagonist Dapper prevented the migration of CG-4 cells. This suggests that inactivation of the wnt signal is responsible for impaired migration of OLG caused by tenascin C. Our results suggest that tenascin C is involved in the impaired mobility of OLG progenitor cells through increased amounts of adhesion complex as well as the prevention of wnt signaling.

Oligodendrocyte precursors on the move: mechanisms directing migration

Oligodendrocyte precursor cells traverse long distances to reach their axonal targets. The molecules that influence their migration include a combination of short-range attractants and repellents and long-range chemoattractants and chemorepellents. Here, the authors review mechanisms that direct oligodendrocyte precursor cells as they migrate throughout the developing CNS.

Transforming growth factor beta receptor family ligands inhibit hepatocyte growth factor synthesis and secretion from astrocytoma cells

Transforming growth factor beta (TGFbeta) and hepatocyte growth factor (HGF) promote glioma progression. Using U87human astrocytoma cells, which express TGFbeta receptors (TbetaRs), we show (1) mRNA expression of Smads (2, 3, 4), bone morphogenetic protein (BMP)- and activin-A receptors; (2) TGFbeta1 inhibits and HGF induces proliferation; (3) TGFbeta1 and activin-A equipotently inhibit HGF secretion more than BMP-2, but none alters c-Met expression. Because interfering with TbetaR signaling might nullify the beneficial inhibition of HGF secretion, activin-A should instead be considered for combination glioma therapy.

Hepatocyte growth factor stimulates the proliferation and migration of oligodendrocyte precursor cells

Hepatocyte growth factor (HGF) was initially identified as a potent mitogen for mature hepatocytes and has since been found to affect a variety of cells. Evidence suggests that HGF may also influence the nervous system, in that HGF stimulates the proliferation of myelin-forming Schwann cells and olfactory ensheathing cells. However, it is not known whether HGF influences oligodendrocytes. To address this issue, oligodendrocyte precursors were obtained from neonatal rat cerebra and cultured. Immunostaining and Western blotting revealed expression of both HGF and the HGF receptor (c-Met) by cultured oligodendrocytes. When the ability of HGF to stimulate oligodendrocyte division and migration was examined, we observed that treatment with HGF (10-50 ng/ml) elicited twofold increases in oligodendrocyte precursor proliferation. HGF also enhanced oligodendrocyte precursor migration, with 2.5-fold increases in rates of migration seen after treatment for 8 hr. HGF also influenced inducing the oligodendrocyte cytoskeleton by altering patterns of F-actin and beta-tubulin distribution and enhanced the expression of actin and beta-tubulin. These observations show that a functional HGF/c-Met system is present in oligodendrocytes, which can influence the growth, development, and cytoskeletal organization of oligodendrocytes.

Neurotrophin receptor expression and responsiveness by postnatal cerebral oligodendroglia

The low affinity neurotrophin receptor (p75(NTR)) is implicated in promoting oligodendrocytic death after nerve growth factor (NGF) stimulation but NGF and neurotrophin-3 (NT-3) can also potentiate oligodendrocytic survival. We show regional variability in p75(NTR) expression within the central nervous system of the postnatal rat; expression is readily detectable by immunohistochemistry upon a subset of CNPase-positive oligodendroglia in optic nerve but not within the cerebrum. Nevertheless, oligodendroglia isolated from the cerebrum and cultured for 16 hours express p75(NTR) as well as the trkC but not the TrkA gene. Viability was not, however, influenced by exposure to either NGF or NT-3. Cells overexpressing p75(NTR) remained unresponsive to NGF but exhibited potentiated survival with NT-3, correlating with the differential expression profile of their high affinity receptors.

Hepatocyte growth factor is a mitogen for olfactory ensheathing cells

Hepatocyte growth factor (HGF) is a potent mitogen for mature hepatocytes, and it has multi-functional effects in a variety of cells in various organs. HGF stimulates DNA synthesis and promotes cell migration and morphogenesis in several cell types including the olfactory system. To characterize the potential mitogenic activity of HGF that might contribute to olfactory ensheathing cell (OEC) proliferation, we tested the ability of HGF to stimulate OEC division in vitro. OECs were obtained from adult rat olfactory bulbs and cultured in serum-free medium, and were identified by double immunostaining for p75 and S-100 antibodies. DNA synthesis assayed by pulsing BrdU for 24 hr showed that HGF at the concentration of 5-100 ng/ml elicited a 5-10-fold increase of OEC proliferation. By immunocytochemical analysis, we demonstrated that c-Met-immunoreactivity was present in cultured OECs, and c-Met anti-serum significantly sequestered the activity of HGF on OECs proliferation, suggesting that HGF-induced proliferation of OECs is mediated by the c-Met receptor. The mitogenic activity of HGF was potentiated by addition of heregulin (HRG), but inhibited by addition of forskolin. These results demonstrate that HGF is a novel mitogen for rat OECs in vitro, and HGF/c-Met system is involved in regulating OECs growth and development.

Agonists of the retinoic acid- and retinoid X-receptors inhibit hepatocyte growth factor secretion and expression in U87 human astrocytoma cells

Retinoids participate in the onset of differentiation, apoptosis and the inhibition of growth in a wide variety of normal and cancerous cells. Several recent reports have shown that hepatocyte growth factor (HGF), and its receptor, c-Met, are expressed at abnormally high levels in various human malignant gliomas and exert a strong proliferative action in an autocrine fashion. These results, consequently, imply that HGF and its receptor may represent a major contributor to the progression of such malignancies. Since astrocytomas are the most frequently occurring glioma, we have shown here that U87 cells - a well-established, human astrocytoma cell line - express both HGF and c-Met, thereby providing a suitable astrocytic tumor model for studying the potential role of HGF, functioning in an autocrine mode, in astrocytic tumorigenesis. Furthermore, we demonstrated the expression of the retinoic acid receptor (RAR) isoforms, RARalpha, -beta and -gamma, as well as the retinoid x-receptor (RxR) isoforms, RxRalpha and -beta, by RT-PCR and western blot analysis in these cells. Since ligands of the RARs and RxRs are known to exert growth inhibitory effects on various tumor cells which include some astrocytomas, we speculated that such effect of retinoids might be mediated via inhibition of HGF secretion in human astrocytoma cells. Indeed, we have shown that the RAR agonists, all-trans retinoic acid (ATRA) and (E)-4-[2-(5,6,7,8-Tetrahydro-5,5,8,8-tetramethyl-2-naphthylenyl)-1-propenyl] benzoic acid (TTNPB), inhibited HGF secretion with half maximal inhibition occurring at 3.0 microM and 15 nM, respectively, as did the RxR agonists, 9-cis- and 13-cis retinoic acid (9cRA and 13cRA, respectively), which exerted half-maximal inhibitory effects at 40 and 25 nM, respectively. These actions of the RAR and RxR agonists appear to be exerted at the transcriptional level as assessed by Northern blot analysis. Taken together, our results show for the first time that retinoids, acting via the RAR and RxRs, significantly inhibit both the secretion and expression of HGF, thereby interrupting a potentially highly tumorigenic autocrine loop in astrocytoma cells.