Initiation of oligodendrocyte progenitor cell migration by a PDGF-A activated extracellular regulated kinase (ERK) signaling pathway

Differential effects of growth factors on oligodendrocyte progenitor migration

Oligodendrocytes are myelinating cells of the CNS that originate as progenitor cells (OP) in discrete areas of the developing brain. During brain development, OP migrate significant distances prior to proliferating and myelinating the axons of the putative white matter tracts. Growth factors play a major regulatory role in the behavior of OP. Specifically, platelet-derived growth factor A (PDGF-A) and fibroblast growth factor 2 (FGF2) are two of the most well characterized regulators of OP development. Both growth factors interact with tyrosine kinase receptors, activating various intracellular signaling pathways. The current study advances our earlier research by comparing the effects of both PDGF-A and FGF2 on OP migration. Our results show that activation of ERK is required for OP migration. These findings correlate well with our previous demonstration of the ERK pathway mediating PDGF-A induced OP migration. We also demonstrate the significance of threshold levels of growth factors and temporal regulation for OP migration. In addition, ERK activation alone is not sufficient to induce OP migration. The current research supports the involvement of the non-ERK mediated signaling pathway in OP migration.

Genetic modeling of gliomas in mice: new tools to tackle old problems

The recently published comprehensive profiles of genomic alterations in glioma have led to a refinement in our understanding of the molecular events that underlie this cancer. Using state-of-the-art genomic tools, several laboratories have created and characterized accurate genetically engineered mouse models of glioma based on specific genetic alterations observed in human tumors. These in vivo brain tumor models faithfully recapitulate the histopathology, etiology, and biology of gliomas and provide an exceptional experimental system to discover novel therapeutic targets and test therapeutic agents. This review focuses on mouse models of glioma with a special emphasis on genetically engineered models developed around key genetic glioma signature mutations in the PDGFR, EGFR, and NF1 genes and pathways. The resulting animal models have provided insight into many fundamental and mechanistic facets of tumor initiation, maintenance and resistance to therapeutic intervention and will continue to do so in the future.

Imatinib targets PDGF signaling in melanoma and host smooth muscle neighboring cells

In previous in vitro studies, we showed that imatinib abrogated platelet-derived growth factor receptor α (PDGFRα) signaling, disrupting both breast cancer and smooth muscle cells (SMC). PDGF is also a powerful mitogen for neural crest origin cells like melanocytes. The purpose of the present study was to evaluate the effect of imatinib on melanoma growth and in angiogenesis, with emphasis to the involvement in PDGF signaling. B16 melanoma cells incubation with 5 µM (IC50) imatinib resulted in a significant reduction in cell proliferation and migration. Apoptosis, however, was not significantly affected. Phosphorylated-PDGFRα expression was decreased in B16 lysates. In a mouse model of B16 melanoma, intraperitoneal administration of imatinib at early day light significantly decreased tumor growth. These findings were corroborated by a highly significant reduction in cell proliferation and increase in apoptosis in melanoma tumors. This was accompanied by a decrease in microvessel density and in the number of SMC-presenting vessels. Imatinib further inhibited PDGFRα expression and activity, as confirmed by the down-regulation of downstream Erk signaling pathway. Altogether, this study demonstrates that besides targeting tumor cells, imatinib also prevents vascular integrity. The current study provides evidence that the paracrine crosstalk between tumor cells and host neighboring cells is crucial for the elucidation of imatinib effects. In addition, the fact that this molecule targets vascular support cells further enlarges its therapeutic purpose to a wide range of vasculoproliferative pathologies.

Persistent roles of signal transduction of platelet-derived growth factor B in genesis, growth, and anaplastic transformation of gliomas in an in-vivo serial transplantation model

We previously reported that retrovirally transduced platelet-derived growth factor-B (PDGFB) in glial progenitors of the rat cerebral white matter, subventricular zone, or brain stem induced malignant brain tumors closely resembling human glioblastoma (GBM). While human GBMs may progress over the period of several months to a few years, prospective, long-term in-vivo observation of histological changes of the tumor tissues is not feasible in these models, because the animals undergo rapid tumor progression and mortality within approximately 1 month. We thus performed successive, long-term in-vivo transplantation of the PDGFB-induced tumor cells into the rat cerebrum. Primary retroviral transduction of PDGFB in the glial progenitors of the rat basal ganglia induced malignant glioma resembling human GBM or anaplastic oligodendroglioma (AOL) consisting of relatively monomorphous tumor cells expressing markers for the oligodendrocyte lineage. In the course of long-term successive transplantation, tumor cells presented pleomorphism as well as focal GFAP expression. This suggests that secondary chromosomal aberration and dysregulation of gene expression following accelerated cell cycle by PDGFB stimulation would induce morphological and immunophenotypic changes in tumor cells. Furthermore, while the primary tumors contained only a minor fraction of proviral GFP-expressing or hemagglutinin-expressing cells, most tumor cells came to express these proviral genes in the course of serial transplantation suggesting a persistent role of PDGFB-expressing cells in maintenance and growth of the tumors. This model would be useful for investigation of the long-term effects of PDGFB stimulation in glioma tissues on anaplastic evolution.

Hypoxia-ischemia induces an endogenous reparative response by local neural progenitors in the postnatal mouse telencephalon

Perinatal hypoxia-ischemia in the preterm neonate commonly results in white matter injury for which there is no specific therapy. The subventricular zone (SVZ) of the brain harbors neural stem cells and more committed progenitors including oligodendroglial progenitor cells that might serve as replacement cells for treating white matter injury. Data from rodent models suggest limited replacement of mature oligodendroglia by endogenous cells. Rare newly born mature oligodendrocytes have been reported within the striatum, corpus callosum and infarcted cortex 1 month following hypoxia-ischemia. Whether these oligodendrocytes arise in situ or emigrate from the SVZ is unknown. We used a postnatal day 9 mouse model of hypoxia-ischemia, BrdU labeling of mitotic cells, immunofluorescence and time-lapse multiphoton microscopy to determine whether hypoxia-ischemia increases production of oligodendroglial progenitors within the SVZ with emigration toward injured areas. Although cells of the oligodendroglial lineage increased in the brain ipsilateral to hypoxic-ischemic injury, they did not originate from the SVZ but rather arose within the striatum and cortex. Furthermore, they resulted from proliferation within the striatum but not within the cortex. Thus, an endogenous regenerative oligodendroglial response to postnatal hypoxia-ischemia occurs locally, with minimal long-distance contribution by cells of the SVZ.

Multiple kinase pathways regulate voltage-dependent Ca2+ influx and migration in oligodendrocyte precursor cells

It is becoming increasingly clear that voltage-operated Ca(2+) channels (VOCCs) play a fundamental role in the development of oligodendrocyte progenitor cells (OPCs). Because direct phosphorylation by different kinases is one of the most important mechanisms involved in VOCC modulation, the aim of this study was to evaluate the participation of serine-threonine kinases and tyrosine kinases (TKs) on Ca(2+) influx mediated by VOCCs in OPCs. Calcium imaging revealed that OPCs exhibited Ca(2+) influx after plasma membrane depolarization via L-type VOCCs. Furthermore, VOCC-mediated Ca(2+) influx declined with OPC differentiation, indicating that VOCCs are developmentally regulated in OPCs. PKC activation significantly increased VOCC activity in OPCs, whereas PKA activation produced the opposite effect. The results also indicated that OPC morphological changes induced by PKC activation were partially mediated by VOCCs. Our data clearly suggest that TKs exert an activating influence on VOCC function in OPCs. Furthermore, using the PDGF response as a model to probe the role of TK receptors (TKr) on OPC Ca(2+) uptake, we found that TKr activation potentiated Ca(2+) influx after membrane depolarization. Interestingly, this TKr modulation of VOCCs appeared to be essential for the PDGF enhancement of OPC migration rate, because cell motility was completely blocked by TKr antagonists, as well as VOCC inhibitors, in migration assays. The present study strongly demonstrates that PKC and TKrs enhance Ca(2+) influx induced by depolarization in OPCs, whereas PKA has an inhibitory effect. These kinases modulate voltage-operated Ca(2+) uptake in OPCs and participate in the modulation of process extension and migration.

Adult human mesenchymal cells proliferate and migrate in response to chemokines expressed in demyelination

Systemic delivery of multipotent mesenchymal stem cells (MSC) may be of benefit in the treatment of neurological diseases, including multiple sclerosis (MS). Certainly, animal studies have demonstrated functional benefits following MSC transplantation, although the mechanisms by which MSCs migrate to lesions and stimulate repair remain unknown. Chemokines stimulate migration in other settings. In this study, we systematically explore the migratory and proliferative responses of human MSCs (hMSC) to chemokines expressed in MS lesions. We demonstrate that these chemokines trigger hMSC migration. In addition, we show that RANTES and IP-10 promote hMSC proliferation.

Directional cell migration and chemotaxis in wound healing response to PDGF-AA are coordinated by the primary cilium in fibroblasts

Cell motility and migration play pivotal roles in numerous physiological and pathophysiological processes including development and tissue repair. Cell migration is regulated through external stimuli such as platelet-derived growth factor-AA (PDGF-AA), a key regulator in directional cell migration during embryonic development and a chemoattractant during postnatal migratory responses including wound healing. We previously showed that PDGFRalpha signaling is coordinated by the primary cilium in quiescent cells. However, little is known about the function of the primary cilium in cell migration. Here we used micropipette analysis to show that a normal chemosensory response to PDGF-AA in fibroblasts requires the primary cilium. In vitro and in vivo wound healing assays revealed that in ORPK mouse (IFT88(Tg737Rpw)) fibroblasts, where ciliary assembly is defective, chemotaxis towards PDGF-AA is absent, leading to unregulated high speed and uncontrolled directional cell displacement during wound closure, with subsequent defects in wound healing. These data suggest that in coordination with cytoskeletal reorganization, the fibroblast primary cilium functions via ciliary PDGFRalpha signaling to monitor directional movement during wound healing.

Oligodendrocyte-spinal cord explant co-culture: an in vitro model for the study of myelination

The in vitro models developed to investigate the growth and myelination of axons, such as dorsal root ganglion (DRG)-Schwann cell co-culture, DRG-oligodendrocyte co-culture and central nervous system (CNS) neuron-oligodendrocyte co-culture, have provided an effective way to reveal the mechanisms that underlie the interaction between neurons and myelin-forming cells. In order to better understand the complex process of myelination during CNS development and spinal cord repair, we established a rat spinal cord neuron-oligodendrocyte co-culture model. In this co-culture system, the spinal cord explants were used as the source of neurons, and the oligodendrocytes were induced from GFP-oligodendrocyte precursor cells (GFP-OPCs). The results showed that the GFP-oligodendrocytes that differentiated from GFP-OPCs in co-culture attached to the neurites growing out from the spinal cord explants and formed myelin structures. As the oligodendrocytes expressed GFP, and the neuron somas remained in the explants, the interaction between oligodendrocytes and neurites in co-culture were observed clearly and dynamically without immunostaining.

The insulin-like growth factor (IGF) receptor type 1 (IGF1R) as an essential component of the signalling network regulating neurogenesis

The insulin-like growth factor receptor type 1 (IGF1R) signalling pathway is activated in the mammalian nervous system from early developmental stages. Its major effect on developing neural cells is to promote their growth and survival. This pathway can integrate its action with signalling pathways of growth and morphogenetic factors that induce cell fate specification and selective expansion of specified neural cell subsets. This suggests that during developmental and adult neurogenesis cellular responses to many signalling factors, including ligands of Notch, sonic hedgehog, fibroblast growth factor family members, ligands of the epidermal growth factor receptor, bone morphogenetic proteins and Wingless and Int-1, may be modified by co-activation of the IGF1R. Modulation of cell migration is another possible role that IGF1R activation may play in neurogenesis. Here, I briefly overview neurogenesis and discuss a role for IGF1R-mediated signalling in the developing and mature nervous system with emphasis on crosstalk between the signalling pathways of the IGF1R and other factors regulating neural cell development and migration. Studies on neural as well as on non-neural cells are highlighted because it may be interesting to test in neurogenic paradigms some of the models based on the information obtained in studies on non-neural cell types.

Toxic effect of blood components on perinatal rat subventricular zone cells and oligodendrocyte precursor cell proliferation, differentiation and migration in culture

The germinal matrix of human brain gives rise to oligodendrocytes and astrocytes after mid-gestation. Hemorrhage in the germinal matrix of premature infants is associated with suppressed cell proliferation. We hypothesize that soluble blood constituents have an adverse effect on the proliferation of cultured rat subventricular zone (SVZ) cells and the proliferation, migration, and differentiation of oligodendrocyte progenitor cells (OPC). Using caspase 3 activation and lactate dehydrogenase release assays, rat plasma, serum, thrombin, and kallikrein killed SVZ cells when grown in the presence (but not absence) of platelet derived growth factor. Plasma and serum killed OPC at 1:1 to 1:100 dilutions. Using a bromodeoxyuridine incorporation assay OPC proliferation was reduced by plasma, serum, thrombin and plasmin. Blood proteins also suppressed OPC migration in a concentration dependent manner. However, differentiation of OPC into myelin basic protein expressing cells was suppressed only by thrombin. We conclude that soluble blood components, particularly thrombin, have an adverse effect on maturing SVZ cells and OPC derived from newborn rat brain.

Essential function, sophisticated regulation and pathological impact of the selective RNA-binding protein QKI in CNS myelin development

The selective RNA-binding protein QKI play a key role in advancing oligodendrocyte-dependent myelination, which is essential for the function and development of the CNS. The emerging evidence that QKI abnormalities are associated with schizophrenia and may underlie myelin impairment in this devastating disease has greatly increased interest in understanding the function of QKI. Despite the discovery of the biochemical basis for QKI-RNA interaction, a comprehensive model is currently missing regarding how QKI regulates its mRNA ligands to promote normal myelinogenesis and how deficiency of the QKI pathway is involved in the pathogenesis of human diseases that affect CNS myelin. In this review, we will focus on the role of QKI in regulating distinct mRNA targets at critical developmental steps to promote oligodendrocyte differentiation and myelin formation. In addition, we will discuss molecular mechanisms that control QKI expression and activity during normal myelinogenesis as well as the pathological impact of QKI deficiency in dysmyelination mutant animals and in human myelin disorders.