Induction of platelet-derived growth factor beta-receptor in focal ischemia of rat brain

In the Rat Hippocampus, Pilocarpine-Induced Status Epilepticus Is Associated with Reactive Glia and Concomitant Increased Expression of CD31, PDGFRβ, and Collagen IV in Endothelial Cells and Pericytes of the Blood-Brain Barrier

In humans and animal models, temporal lobe epilepsy (TLE) is associated with reorganization of hippocampal neuronal networks, gliosis, neuroinflammation, and loss of integrity of the blood-brain barrier (BBB). More than 30% of epilepsies remain intractable, and characterization of the molecular mechanisms involved in BBB dysfunction is essential to the identification of new therapeutic strategies. In this work, we induced status epilepticus in rats through injection of the proconvulsant drug pilocarpine, which leads to TLE. Using RT-qPCR, double immunohistochemistry, and confocal imaging, we studied the regulation of reactive glia and vascular markers at different time points of epileptogenesis (latent phase-3, 7, and 14 days; chronic phase-1 and 3 months). In the hippocampus, increased expression of mRNA encoding the glial proteins GFAP and Iba1 confirmed neuroinflammatory status. We report for the first time the concomitant induction of the specific proteins CD31, PDGFRβ, and ColIV-which peak at the same time points as inflammation-in the endothelial cells, pericytes, and basement membrane of the BBB. The altered expression of these proteins occurs early in TLE, during the latent phase, suggesting that they could be associated with the early rupture and pathogenicity of the BBB that will contribute to the chronic phase of epilepsy.

PDGFR-beta signaling mediates endogenous neurogenesis after postischemic neural stem/progenitor cell transplantation in mice

OBJECTIVE

Although platelet-derived growth factor receptor (PDGFR)-β mediates the self-renewal and multipotency of neural stem/progenitor cells (NSPCs) in vitro and in vivo, its mechanisms of activating endogenous NSPCs following ischemic stroke still remain unproven.

METHODS

The exogenous NSPCs were transplanted into the ischemic striatum of PDGFR-β conditionally neuroepithelial knockout (KO) mice at 24 h after transient middle cerebral artery occlusion (tMCAO). 5-Bromo-2'-deoxyuridine (BrdU) was intraperitoneally injected to label the newly formed endogenous NSPCs. Infarction volume was measured, and behavioral tests were performed. In the subventricular zone (SVZ), proliferation of endogenous NSPCs was tested, and synapse formation and expression of nutritional factors were measured.

RESULTS

Compared with control mice, KO mice showed larger infarction volume, delayed neurological recovery, reduced numbers of BrdU positive cells, decreased expression of neurogenic factors (including neurofilament, synaptophysin, and brain-derived neurotrophic factor), and decreased synaptic regeneration in SVZ after tMCAO. Moreover, exogenous NSPC transplantation significantly alleviated neurologic dysfunction, promoted neurogenesis, increased expression of neurologic factors, and diminished synaptic deformation in SVZ of FL mice after tMCAO but had no beneficial effect in KO mice.

CONCLUSION

PDGFR-β signaling may promote activation of endogenous NSPCs after postischemic NSPC transplantation, and thus represents a novel potential regeneration-based therapeutic target.

Fibrosis in the central nervous system: from the meninges to the vasculature

Formation of a collagenous connective tissue scar after penetrating injuries to the brain or spinal cord has been described and investigated for well over 100 years. However, it was studied almost exclusively in the context of penetrating injuries that resulted in infiltration of meningeal fibroblasts, which raised doubts about translational applicability to most CNS injuries where the meninges remain intact. Recent studies demonstrating the perivascular niche as a source of fibroblasts have debunked the traditional view that a fibrotic scar only forms after penetrating lesions that tear the meninges. These studies have led to a renewed interest in CNS fibrosis not only in the context of axon regeneration after spinal cord injury, but also across a spectrum of CNS disorders. Arising with this renewed interest is some discrepancy about which perivascular cell gives rise to the fibrotic scar, but additional studies are beginning to provide some clarity. Although mechanistic studies on CNS fibrosis are still lacking, the similarities to fibrosis of other organs should provide important insight into how CNS fibrosis can be therapeutically targeted to promote functional recovery.

Genetic deletion of endothelial microRNA-15a/16-1 promotes cerebral angiogenesis and neurological recovery in ischemic stroke through Src signaling pathway

Cerebral angiogenesis is tightly controlled by specific microRNAs (miRs), including the miR-15a/16-1 cluster. Recently, we reported that endothelium-specific conditional knockout of the miR-15a/16-1 cluster (EC-miR-15a/16-1 cKO) promotes post-stroke angiogenesis and improves long-term neurological recovery by increasing protein levels of VEGFA, FGF2, and their respective receptors VEGFR2 and FGFR1. Herein, we further investigated the underlying signaling mechanism of these pro-angiogenic factors after ischemic stroke using a selective Src family inhibitor AZD0530. EC-miR-15a/16-1 cKO and age- and sex-matched wild-type littermate (WT) mice were subjected to 1 h middle cerebral artery occlusion (MCAO) and 28d reperfusion. AZD0530 was administered daily by oral gavage to both genotypes of mice 3-21d after MCAO. Compared to WT, AZD0530 administration exacerbated spatial cognitive impairments and brain atrophy in EC-miR-15a/16-1 cKO mice following MCAO. AZD0530 also attenuated long-term recovery of blood flow and inhibited the formation of new microvessels, including functional vessels with blood circulation, in the penumbra of stroked cKO mice. Moreover, AZD0530 blocked the Src signaling pathway by downregulating phospho-Src and its downstream mediators (p-Stat3, p-Akt, p-FAK, p-p44/42 MAPK, p-p38 MAPK) in post-ischemic brains. Collectively, our data demonstrated that endothelium-targeted deletion of the miR-15a/16-1 cluster promotes post-stroke angiogenesis and improves long-term neurological recovery via activating Src signaling pathway.

Ultrastructural and Molecular Characterization of Platelet-derived growth factor Beta-Positive Leptomeningeal Cells in the Adult Rat Brain

The leptomeninges, referring to the arachnoid and pia mater and their projections into the perivascular compartments in the central nervous system, actively participate in diverse biological processes including fluid homeostasis, immune cell infiltrations, and neurogenesis, yet their detailed cellular and molecular identities remain elusive. This study aimed to characterize platelet-derived growth factor beta (PDGFR-β)-expressing cells in the leptomeninges in the adult rat brain using light and electron microscopy. PDGFR-β⁺ cells were observed in the inner arachnoid, arachnoid trabeculae, pia mater, and leptomeningeal sheath of the subarachnoid vessels, thereby forming a cellular network throughout the leptomeninges. Leptomeningeal PDGFR-β⁺ cells were commonly characterized by large euchromatic nuclei, thin branching processes forming web-like network, and the expression of the intermediate filaments nestin and vimentin. These cells were typical of active fibroblasts with a well-developed rough endoplasmic reticulum and close spatial correlation with collagen fibrils. Leptomeningeal PDGFR-β⁺ cells ensheathing the vasculature in the subarachnoid space joined with pial PDGFR-β⁺ cells upon entering the cortical parenchyma, yet perivascular PDGFR-β⁺ cells in these penetrating vessels underwent abrupt changes in their morphological and molecular characteristics: they became more flattened with loss of immunoreactivity for nestin and vimentin and deficient collagen deposition, which was indicative of inactive fibroblasts termed fibrocytes. In the cortical parenchyma, PDGFR-β immunoreactivity was almost exclusively localized to larger caliber vessels, and significantly decreased in capillary-like microvessels. Collectively, our data identify PDGFR-β as a novel cellular marker for leptomeningeal fibroblasts comprising the leptomeninges and perivascular adventitial cells of the subarachnoid and penetrating large-sized cortical vasculatures.

Therapeutically Targeting Platelet-Derived Growth Factor-Mediated Signaling Underlying the Pathogenesis of Subarachnoid Hemorrhage-Related Vasospasm

INTRODUCTION

Vasospasm accounts for a large fraction of the morbidity and mortality burden in patients sustaining subarachnoid hemorrhage (SAH). Platelet-derived growth factor (PDGF)-β levels rise following SAH and correlate with incidence and severity of vasospasm.

METHODS

The literature was reviewed for studies investigating the role of PDGF in the pathogenesis of SAH-related vasospasm and efficacy of pharmacological interventions targeting the PDGF pathway in ameliorating the same and improving clinical outcomes.

RESULTS

Release of blood under high pressure into the subarachnoid space activates the complement cascade, which results in release of PDGF. Abluminal contact of blood with cerebral vessels increases their contractile response to PDGF-β and thrombin, with the latter upregulating PDGF-β receptors and augmenting effects of PDGF-β. PDGF-β figures prominently in the early and late phases of post-SAH vasospasm. PDGF-β binding to the PDGF receptor-β results in receptor tyrosine kinase domain activation and consequent stimulation of intracellular signaling pathways, including p38 mitogen-activated protein kinase, phosphatidylinositol-3-kinase, Rho-associated protein kinase, and extracellular regulated kinase 1 and 2. Consequent increases in intracellular calcium and increased expression of genes mediating cellular growth and proliferation mediate PDGF-induced augmentation of vascular smooth muscle cell contractility, hypertrophy, and proliferation.

CONCLUSION

Treatments with statins, serine protease inhibitors, and small molecular pathway inhibitors have demonstrated varying degrees of efficacy in prevention of cerebral vasospasm, which is improved with earlier institution.

Fibrotic scarring following lesions to the central nervous system

Following lesions to the central nervous system, scar tissue forms at the lesion site. Injury often severs axons and scar tissue is thought to block axonal regeneration, resulting in permanent functional deficits. While scar-forming astrocytes have been extensively studied, much less attention has been given to the fibrotic, non-glial component of the scar. We here review recent progress in understanding fibrotic scar formation following different lesions to the brain and spinal cord. We specifically highlight recent evidence for pericyte-derived fibrotic scar tissue formation, discussing the origin, recruitment, function and therapeutic relevance of fibrotic scarring.

Effects of ischemic preconditioning on PDGF-BB expression in the gerbil hippocampal CA1 region following transient cerebral ischemia

Ischemic preconditioning (IPC) is induced by exposure to brief durations of transient ischemia, which results in ischemic tolerance to a subsequent longer or lethal period of ischemia. In the present study, the effects of IPC (2 min of transient cerebral ischemia) were examined on immunoreactivity of platelet‑derived growth factor (PDGF)‑BB and on neuroprotection in the gerbil hippocampal CA1 region following lethal transient cerebral ischemia (LTCI; 5 min of transient cerebral ischemia). IPC was subjected to a 2‑min sublethal ischemia and a LTCI was given 5‑min transient ischemia. The animals in all of the groups were given recovery times of 1, 2 and 5 days and change in PDGF‑BB immunoreactivity was examined as was the neuronal damage/death in the hippocampus induced by LTCI. LTCI induced a significant loss of pyramidal neurons in the hippocampal CA1 region 5 days after LTCI, and significantly decreased PDGF‑BB immunoreactivity in the CA1 pyramidal neurons from day 1 after LTCI. Conversely, IPC effectively protected the CA1 pyramidal neurons from LTCI and increased PDGF‑BB immunoreactivity in the CA1 pyramidal neurons post‑LTCI. In conclusion, the results demonstrated that LTCI significantly altered PDGF‑BB immunoreactivity in pyramidal neurons in the hippocampal CA1 region, whereas IPC increased the immunoreactivity. These findings indicated that PDGF‑BB may be associated with IPC‑mediated neuroprotection.

Angiogenesis-regulating microRNAs and Ischemic Stroke

Stroke is a leading cause of death and disability worldwide. Ischemic stroke is the dominant subtype of stroke and results from focal cerebral ischemia due to occlusion of major cerebral arteries. Thus, the restoration or improvement of reduced regional cerebral blood supply in a timely manner is very critical for improving stroke outcomes and poststroke functional recovery. The recovery from ischemic stroke largely relies on appropriate restoration of blood flow via angiogenesis. Newly formed vessels would allow increased cerebral blood flow, thus increasing the amount of oxygen and nutrients delivered to affected brain tissue. Angiogenesis is strictly controlled by many key angiogenic factors in the central nervous system, and these molecules have been well-documented to play an important role in the development of angiogenesis in response to various pathological conditions. Promoting angiogenesis via various approaches that target angiogenic factors appears to be a useful treatment for experimental ischemic stroke. Most recently, microRNAs (miRs) have been identified as negative regulators of gene expression in a post-transcriptional manner. Accumulating studies have demonstrated that miRs are essential determinants of vascular endothelial cell biology/angiogenesis as well as contributors to stroke pathogenesis. In this review, we summarize the knowledge of stroke-associated angiogenic modulators, as well as the role and molecular mechanisms of stroke-associated miRs with a focus on angiogenesis-regulating miRs. Moreover, we further discuss their potential impact on miR-based therapeutics in stroke through targeting and enhancing post-ischemic angiogenesis.

The roles of PDGF in development and during neurogenesis in the normal and diseased nervous system

The four platelet-derived growth factor (PDGF) ligands and PDGF receptors (PDGFRs), α and β (PDGFRA, PDGFRB), are essential proteins that are expressed during embryonic and mature nervous systems, i.e., in neural progenitors, neurons, astrocytes, oligodendrocytes, and vascular cells. PDGF exerts essential roles from the gastrulation period to adult neuronal maintenance by contributing to the regulation of development of preplacodal progenitors, placodal ectoderm, and neural crest cells to adult neural progenitors, in coordinating with other factors. In adulthood, PDGF plays critical roles for maintenance of many specific cell types in the nervous system together with vascular cells through controlling the blood brain barrier homeostasis. At injury or various stresses, PDGF modulates neuronal excitability through adjusting various ion channels, and affecting synaptic plasticity and function. Furthermore, PDGF stimulates survival signals, majorly PI3-K/Akt pathway but also other ways, rescuing cells from apoptosis. Studies imply an involvement of PDGF in dendrite spine morphology, being critical for memory in the developing brain. Recent studies suggest association of PDGF genes with neuropsychiatric disorders. In this review, we will describe the roles of PDGF in the nervous system, from the discovery to recent findings, in order to understand the broad spectrum of PDGF in the nervous system. Recent development of pharmacological and replacement therapies targeting the PDGF system is discussed.

5-Hydroxytryptamine type 7 receptor neuroprotection against NMDA-induced excitotoxicity is PDGFβ receptor dependent

The serotonin (5-HT) type 7 receptor is expressed throughout the CNS including the hippocampus. Long-term (2-24 h) activation of 5-HT7 receptors regulates growth factor receptor expression, including the expression of platelet-derived growth factor (PDGF) β receptors. Direct activation of PDGFβ receptors in primary hippocampal and cortical neurons inhibits NMDA receptor activity and attenuates NMDA receptor-induced neurotoxicity. Our objective was to investigate whether the 5-HT7 receptor-induced increase in PDGFβ receptor expression would be similarly neuroprotective. We demonstrate that 5-HT7 receptor agonist treatment in primary hippocampal neurons also increases the expression of phospholipase C (PLC) γ, a downstream effector of PDGFβ receptors associated with the inhibition of NMDA receptor activity. To determine if the up-regulation of PDGFβ receptors is neuroprotective, primary hippocampal neurons were incubated with the 5-HT7 receptor agonist, LP 12, for 24 h. Indeed, LP 12 treatment prevented NMDA-induced neurotoxicity and this effect was dependent on PDGFβ receptor kinase activity. Treatment of primary neurons with LP 12 also differentially altered NMDA receptor subunit expression, reducing the expression of NR1 and NR2B, but not NR2A. These findings demonstrate the potential for providing growth factor receptor-dependent neuroprotective effects using small-molecule ligands of G protein-coupled receptors.

PDGFR-β as a positive regulator of tissue repair in a mouse model of focal cerebral ischemia

Although platelet-derived growth factors (PDGFs) and receptors (PDGFRs) are abundantly expressed in the central nervous system, their functions largely remain elusive. We investigated the role of PDGFR-β in tissue responses and functional recovery after photothrombolic middle cerebral artery occlusion (MCAO). In the normal adult mouse brain, PDGFR-β was mainly localized in neurons and in pericyte/vascular smooth muscle cells (PC/vSMCs). From 3 to 28 days after MCAO, postnatally induced systemic PDGFR-β knockout mice (Esr-KO) exhibited the delayed recovery of body weight and behavior, and larger infarction volume than controls. In Esr-KO, PC/vSMC coverage was decreased and vascular leakage of infused fluorescent-labeled albumin was extensive within the ischemic lesion, but not in the uninjured cerebral cortex. Angiogenesis levels were comparable between Esr-KO and controls. In another PDGFR-β conditional KO mouse (Nestin-KO), PDGFR-β was deleted in neurons and astrocytes from embryonic day 10.5, but was preserved in PC/vSMCs. After MCAO, vascular leakage and infarction volume in Nestin-KO were worse than controls, but partly improved compared with Esr-KO. Astroglial scar formation in both Esr-KO and Nestin-KO was similarly reduced compared with controls after MCAO. These data suggested that PDGFR-β signaling is crucial for neuroprotection, endogenous tissue repair, and functional recovery after stroke by targeting neurons, PC/vSMCs, and astrocytes.

Neuroprotective effects of PDGF against oxidative stress and the signaling pathway involved

The neuroprotective effects of platelet-derived growth factor (PDGF) and the major signaling pathways involved in these were examined using primary cultured mouse cortical neurons subjected to H(2)O(2)-induced oxidative stress. The specific function of the PDGF beta-receptor (PDGFR-beta) was examined by the selective deletion of the corresponding gene using the Cre-loxP system in vitro. In wild-type neurons, PDGF-BB enhanced the survival of these neurons and suppressed H(2)O(2)-induced caspase-3 activation. The prosurvival effect of PDGF-AA was less than that of PDGF-BB. PDGF-BB highly activated Akt, extracellular signal-regulated kinase (ERK), c-jun amino-terminal kinase (JNK) and p38. PDGF-AA activated these molecules at lesser extent than PDGF-BB. In particular, PDGF-AA induced activation of Akt was at very low level. The neuroprotective effects of PDGF-BB were antagonized by inhibitors of phosphatidylinositol 3-kinase (PI3-K), mitogen-activated protein kinase kinase (MEK), JNK and p38. The PDGFR-beta-depleted neurons showed increased vulnerability to oxidative stress, and less responsiveness to PDGF-BB-induced cytoprotection and signal activation, in which Akt activation was most strongly suppressed. After all, these results demonstrated the neuroprotective effects of PDGF and the signaling pathways involved against oxidative stress. The effects of PDGF-BB were more potent than those of PDGF-AA. This might be due to the activation and additive effects of two PDGFRs after PDGF-BB stimulation. Furthermore, the PI3-K/Akt pathway that was deduced to be preferentially activated by PDGFR-beta may explain the potent effects of PDGF-BB.

Angiogenesis after cerebral ischemia

Though the vascular system of the adult brain is extremely stable under normal baseline conditions, endothelial cells start to proliferate in response to brain ischemia. The induction of angiogenesis, primarily in the ischemic boundary zone, enhances oxygen and nutrient supply to the affected tissue. Additionally, the generation of new blood vessels facilitates highly coupled neurorestorative processes including neurogenesis and synaptogenesis which in turn lead to improved functional recovery. To take advantage of angiogenesis as a therapeutic concept for stroke treatment, the knowledge of the precise molecular mechanisms is mandatory. Especially, since a couple of growth factors involved in post-ischemic angiogenesis may have detrimental adverse effects in the brain by increasing vascular permeability. This article summarizes the knowledge of molecular mechanisms of angiogenesis following cerebral ischemia. Finally, experimental pharmacological and cellular approaches to stimulate and enhance post-ischemic angiogenesis are discussed.

Growth factors, stem cells, and stroke

Postischemic neurogenesis has been identified as a compensatory mechanism to repair the damaged brain after stroke. Several factors are released by the ischemic tissue that are responsible for proliferation, differentiation, and migration of neural stem cells. An understanding of their roles may allow future therapies based on treatment with such factors. Although damaged cells release a variety of factors, some of them are stimulatory whereas some are inhibitory for neurogenesis. It is interesting to note that factors like insulin-like growth factor-I can induce proliferation in the presence of fibroblast growth factor-2 (FGF-2), and promote differentiation in the absence of FGF-2. Meanwhile, factors like transforming growth factor-beta can induce the differentiation of neurons while inhibiting the proliferation of neural stem cells. Therefore, understanding the role of each factor in the process of neurogenesis will help physicians to enhance the endogenous response and improve the clinical outcome after stroke. In this article the authors discuss the role of growth factors and stem cells following stroke.

Ischemia-Induced Neurogenesis: Role of Growth Factors

The neurogenic response in ischemic brain to growth factors is the net result of cell division and cell survival in specific regions of the brain. To increase the cell number, these physiologic processes should be active. Hence, when growth factors are infused into the brain, they might stimulate survival, cell division, or both to enhance neurogenesis. The end result is the interplay of all the endogenous factors with the infused exogenous factors. It is essential to understand the growth factors and their regulators that are expressed after ischemia if one is to pharmacologically enhance neurogenesis. It seems that a combinational therapy of factors or their inhibitors may provide powerful therapeutic potential for enhancing stroke-induced neurogenesis and restoring the damaged tissue to function.

Mouse brains deficient in neuronal PDGF receptor-beta develop normally but are vulnerable to injury

Platelet-derived growth factors (PDGFs) and PDGF receptors (PDGFRs) are widely expressed in the mammalian CNS, though their functional significance remains unclear. The corresponding null-knockout mutations are lethal. Here, we developed novel mutant mice in which the gene encoding the beta subunit of PDGFR (PDGFR-beta) was genetically deleted in CNS neurons to elucidate the role of PDGFR-beta, particularly in the post-natal stage. Our mutant mice reached adulthood without apparent anatomical defects. In the mutant brain, immunohistochemical analyses showed that PDGFR-beta detected in neurons and in the cells in the subventricular zone of the lateral ventricle in wild-type mice was depleted, but PDGFR-beta detected in blood vessels remained unaffected. The cerebral damage after cryogenic injury was severely exacerbated in the mutants compared with controls. Furthermore, TdT-mediated dUTP-biotin nick end labeling (TUNEL)-positive neuronal cell death and lesion formation in the cerebral hemisphere were extensively exacerbated in our mutant mice after direct injection of NMDA without altered NMDA receptor expression. Our results clearly demonstrate that PDGFR-beta expressed in neurons protects them from cryogenic injury and NMDA-induced excitotoxicity.

Mitochondrial dysfunction in schizophrenia: evidence for compromised brain metabolism and oxidative stress

The etiology and pathophysiology of schizophrenia remain unknown. A parallel transcriptomics, proteomics and metabolomics approach was employed on human brain tissue to explore the molecular disease signatures. Almost half the altered proteins identified by proteomics were associated with mitochondrial function and oxidative stress responses. This was mirrored by transcriptional and metabolite perturbations. Cluster analysis of transcriptional alterations showed that genes related to energy metabolism and oxidative stress differentiated almost 90% of schizophrenia patients from controls, while confounding drug effects could be ruled out. We propose that oxidative stress and the ensuing cellular adaptations are linked to the schizophrenia disease process and hope that this new disease concept may advance the approach to treatment, diagnosis and disease prevention of schizophrenia and related syndromes.

Hypoxia induces an autocrine-paracrine survival pathway via platelet-derived growth factor (PDGF)-B/PDGF-beta receptor/phosphatidylinositol 3-kinase/Akt signaling in RN46A neuronal cells

In neurons, hypoxia activates intracellular death-related pathways, yet the antiapoptotic mechanisms triggered by hypoxia remain unclear. In RN46A neuronal cells, minimum media growth conditions induced cell death as early as 12 h after the cells were placed in these conditions (i.e., after removal of B-27 supplement). However, apoptosis occurred in hypoxia (1% O2) only after 48 h, and in fact hypoxia reduced the apoptosis associated with trophic factor withdrawal. Furthermore, hypoxia induced time-dependent increases in expression of platelet-derived growth factor (PDGF) B mRNA and protein, as well as PDGF-beta receptor phosphorylation. Although exogenous PDGF-BB induced only transient Akt activation, hypoxia triggered persistent activation of Akt for up to 24 h. Inhibition of phosphatidylinositol 3-kinase (PI3K) or of PDGF-beta receptor phosphorylation abrogated both hypoxia-induced and exogenous PDGF-BB-induced Akt phosphorylation, and it completely abolished hypoxia-induced protection from media supplement deprivation, which suggests that the long-lasting activation of Akt during hypoxia and the prosurvival induction were due to endogenously generated PDGF-BB. Furthermore, these inhibitors decreased hypoxia-inducible factor 1alpha (HIF-1alpha) DNA binding, which suggests that the PDGF/PDGF-beta receptor/Akt pathway induces downstream HIF-1alpha gene transcription. We conclude that in RN46A neuronal cells, hypoxia activates an autocrine-paracrine antiapoptotic mechanism that involves up-regulation of PDGF-B and PDGF-beta receptor-dependent activation of the PI3K/Akt signaling pathway to induce downstream transcription of survival genes.

Time- and cell type-specific induction of platelet-derived growth factor receptor-beta during cerebral ischemia

During cerebral ischemia, angiogenesis occurs inside and around the infarcted area. The growth of new blood vessels may contribute to a better outcome after stroke due to accelerated and increased delivery of nutrients and oxygen to the ischemic tissue. The platelet-derived growth factor (PDGF)-B/PDGF receptor (PDGFR)-beta system, hitherto thought to contribute mainly to neuroprotection, may also support angiogenesis and vascular remodeling by mediating interactions of endothelial cells with pericytes after cerebral ischemia. While platelet-derived growth factor (PDGF)-B and its receptor PDGFR-beta are essential factors for the recruitment of pericytes to brain capillaries during embryonic development, their role in blood vessel maturation during cerebral ischemia is not clear. The aim of the present study was to investigate the time course and location of PDGF-B and PDGFR-beta expression in a mouse model of focal cerebral ischemia. In contrast to the early and continuous induction of PDGF-B, PDGFR-beta mRNA was specifically upregulated in vascular structures in the infarcted area 48 h after occlusion of the middle cerebral artery. Immunohistology and confocal microscopy analysis revealed the specific upregulation of PDGFR-beta on blood vessels and suggested expression mainly on pericytes. Our results imply PDGFR-beta as a key factor in vascular remodeling during stroke and suggest that the pleiotropic functions of PDGF-B may be regulated via the expression of its receptor. Influencing the PDGF system therapeutically might improve angiogenesis, cellular protection, and edema inhibition.

The polysaccharide fraction AIP1 from Artemisia iwayomogi suppresses apoptotic death of the mouse spleen cells in culture

A polysaccharide fraction, AIP1, purified from Artemisia iwayomogi was shown to have immunomodulating and anti-tumor activities in mice. In order to determine how the AIP1 fraction exhibits the immunomodulating activity, the effect of the fraction on the apoptosis of mouse spleen cells was investigated. Treatment of the mouse spleen cells with the AIP1 fraction resulted in the suppression of apoptotic death and an extension of cell survival in culture, indicating that the fraction might modulate the death of spleen cells. Treatment of the mice with the AIP1 fraction in vivo also resulted in less apoptosis of the spleen cells, which indicates the physiological relevance of the anti-apoptosis effect of the fraction in vitro. A mouse gene array was used to determine the profile of the gene expression change showing a pattern of up- and down-regulated genes by the AIP1 treatment. This study provides preliminary information regarding the immunomodulatory mechanism of the AIP1 fraction.

Inhibitors of inducible nitric oxide synthase expression from Artemisia iwayomogi

Nitric oxide (NO) is an important bioactive agent that mediates a wide variety of physiological and pathophysiological events. NO overproduction by inducible nitric oxide synthase (iNOS) results in severe hypotension and inflammation. This investigation is part of a study to discover new iNOS inhibitors from medicinal plants using a macrophage cell culture system. Two sesquiterpenes (1 and 2) were isolated from Artemisia iwayomogi (Compositae) and were found to inhibit NO synthesis (IC50 3.64 microg/mL and 2.81 microg/mL, respectively) in lipopolysaccharide (LPS)-activated RAW 264.7 cells. Their structures were identified as 3-O-methyl-isosecotanapartholide (1) and iso-secotanapartholide (2). Compounds 1 and 2 inhibited the LPS-induced expression of the iNOS enzyme in the RAW 264.7 cells. The inhibition of NO production via the down regulation of iNOS expression may substantially modulate the inflammatory responses.

In vivo PDGF beta receptor activation in the dorsocaudal brainstem of the rat prevents hypoxia-induced apoptosis via activation of Akt and BAD

Activation of platelet-derived growth factor receptor beta (PDGFR) within the caudal brainstem modulates the hypoxic ventilatory response. Since hypoxia does not induce apoptosis in the caudal brainstem, PDGFR could underlie such protective mechanism via a PI3 kinase-dependent phosphorylation of both Akt and BAD pathways. To further study this issue, caudal brainstem lysates were harvested from Sprague--Dawley rats during hypoxia (10% O(2)) after treatment with either vehicle or CGP 57148B (100 mg/kg), a selective blood-brain barrier-permeable PDGFR antagonist. Time-dependent increases in phosphorylated Akt occurred during hypoxia, peaking at 45' and lasting for up to 6 h, without parallel changes in total Akt protein. CGP 57148B attenuated Akt activation at all time points. Similarly, phosphorylation of BAD at serine136 but not at serine 112 occurred in the caudal brainstem as early as 15' of hypoxia, and was completely blocked by CGP 57148B. Furthermore, CGP 57148B treatment elicited significant increases in single-stranded DNA, caspase-like activity, and cleaved caspase 3 after 24 h of hypoxia that were absent in the caudal brainstem of hypoxic vehicle-treated animals. We conclude that PDGFR-dependent in vivo activation of both Akt and BAD during hypoxia prevents induction of apoptosis, and may contribute to the increased hypoxic tolerance of brainstem neurons.

PDGF-beta receptor expression and ventilatory acclimatization to hypoxia in the rat

Activation of platelet-derived growth factor-beta (PDGF-beta) receptors in the nucleus of the solitary tract (nTS) modulates the late phase of the acute hypoxic ventilatory response (HVR) in the rat. We hypothesized that temporal changes in PDGF-beta receptor expression could underlie the ventilatory acclimatization to hypoxia (VAH). Normoxic ventilation was examined in adult Sprague-Dawley rats chronically exposed to 10% O(2), and at 0, 1, 2, 7, and 14 days, Northern and Western blots of the dorsocaudal brain stem were performed for assessment of PDGF-beta receptor expression. Although no significant changes in PDGF-beta receptor mRNA occurred over time, marked attenuation of PDGF-beta receptor protein became apparent after day 7 of hypoxic exposure. Such changes were significantly correlated with concomitant increases in normoxic ventilation, i.e., with VAH (r: -0.56, P < 0.005). In addition, long-term administration of PDGF-BB in the nTS via osmotic pumps loaded with either PDGF-BB (n = 8) or vehicle (Veh; n = 8) showed that although no significant changes in the magnitude of acute HVR occurred in Veh over time, the typical attenuation of HVR by PDGF-BB decreased over time. Furthermore, PDGF-BB microinjections did not attenuate HVR in acclimatized rats at 7 and 14 days of hypoxia (n = 10). We conclude that decreased expression of PDGF-beta receptors in the dorsocaudal brain stem correlates with the magnitude of VAH. We speculate that the decreased expression of PDGF-beta receptors is mediated via internalization and degradation of the receptor rather than by transcriptional regulation.

Neovascularization with blood-brain barrier breakdown in delayed neuronal death

Various kinds of acute pathological events in the central nervous system, such as ischemia, hemorrhage, and trauma, often cause brain edema. The edema may advance for days or weeks while inducing extensive damage in neural function, regardless of the extent of the original damage, and often results in death. Delayed edema is thought to be vasogenic; however, the mechanism underlying edema induction remains unknown. We found delayed vascular cell proliferation with a blood-brain barrier breakdown in and around the gerbil CA1 hippocampus, a region known to be involved in delayed apoptotic neuronal death 2-6 days after transient ischemia. Vascular cell proliferation, assessed by (3)H-thymidine incorporation, was most prominent 4-6 days after ischemia, and extravasation of exogenously applied dye or endogenous serum albumin from blood vessels was observed concomitantly. We propose neovascularization in delayed neuronal death as a cause of brain edema advancing days after neurological events.

Platelet-derived growth factor rapidly increases activity and cell surface expression of the EAAC1 subtype of glutamate transporter through activation of phosphatidylinositol 3-kinase

Na(+)-dependent glutamate transporters are the primary mechanism for removal of excitatory amino acids (EAAs) from the extracellular space of the central nervous system and influence both physiologic and pathologic effects of these compounds. Recent evidence suggests that the activity and cell surface expression of a neuronal subtype of glutamate transporter, EAAC1, are rapidly increased by direct activation of protein kinase C and are decreased by wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-K). We hypothesized that this regulation could be analogous to insulin-induced stimulation of the GLUT4 subtype of glucose transporter, which is dependent upon activation of PI3-K. Using C6 glioma, a cell line that endogenously and selectively expresses EAAC1, we report that platelet-derived growth factor (PDGF) increased Na(+)-dependent L-[(3)H]-glutamate transport activity within 30 min. This effect of PDGF was not due to a change in total cellular EAAC1 immunoreactivity but was instead correlated with an increase cell surface expression of EAAC1, as measured using a membrane impermeant biotinylation reagent combined with Western blotting. A decrease in nonbiotinylated intracellular EAAC1 was also observed. These studies suggest that PDGF causes a redistribution of EAAC1 from an intracellular compartment to the cell surface. These effects of PDGF were accompanied by a 35-fold increase in PI3-K activity and were blocked by the PI3-K inhibitors, wortmannin and LY 294002, but not by an inhibitor of protein kinase C. Other growth factors, including insulin, nerve growth factor, and epidermal growth factor had no effect on glutamate transport nor did they increase PI3-K activity. These studies suggest that, as is observed for insulin-mediated translocation of GLUT4, EAAC1 cell surface expression can be rapidly increased by PDGF through activation of PI3-K. It is possible that this PDGF-mediated increase in EAAC1 activity may contribute to the previously demonstrated neuroprotective effects of PDGF.

Brainstem activation of platelet-derived growth factor-beta receptor modulates the late phase of the hypoxic ventilatory response

The early phase of the biphasic ventilatory response to hypoxia in mammals is critically dependent on NMDA glutamate receptor activation within the nucleus of the solitary tract. However, the mechanisms underlying the subsequent development of the typical ventilatory roll-off are unclear and could underlie important roles in the functional and molecular adaptation to oxygen deprivation. Because the growth factor platelet-derived growth factor (PDGF)-BB can modulate the open channel probability of NMDA receptors by activating PDGF-beta receptors, its contribution to hypoxic ventilatory roll-off was examined. Administration of PDGF-BB, but not PDGF-AA, in the nucleus of the solitary tract was associated with significant attenuations of the early hypoxic ventilatory response in conscious rats. Furthermore, marked reductions in the magnitude of hypoxic ventilatory roll-off occurred in mice heterozygous for a mutation in the PDGF-beta receptor. Administration of a PDGF-beta receptor antagonist to wild-type littermates elicited similar declines in hypoxic ventilatory roll-off. The relative abundance of PDGF-beta receptors was confirmed in the nucleus of the solitary tract and other nuclei implicated in the hypoxic ventilatory response. In nucleus of the solitary tract lysates, PDGF-beta receptor tyrosine phosphorylation was temporally correlated with hypoxic ventilatory roll-off formation. Increased PDGF-B chain mRNA expression was induced by hypoxia in the nucleus of the solitary tract, and PDGF-B chain immunoreactivity colocalized with approximately 40% of nucleus of the solitary tract neurons, demonstrating hypoxia-induced c-Fos enhancements. Thus, PDGF-BB release and PDGF-beta receptor activation in the nucleus of the solitary tract are critical components of hypoxic ventilatory roll-off and may have important functional implications in processes underlying survival and acclimatization to hypoxic environments.

Role of PDGF B-chain and PDGF receptors in rat tubular regeneration after acute injury

Various polypeptide growth factors are generally considered to be involved in the regulation of the nephrogenic process both after acute renal injury and during renal development. Because platelet-derived growth factor B-chain (PDGF-B) has been reported to be expressed in immature tubulus of the developing kidney, PDGF-B could play a role in the process of tubulogenesis. We examined the expression of PDGF-B and PDGF receptors alpha and beta and their localization in kidneys after ischemia/reperfusion injury. The mRNA expressions of PDGF-B, PDGFR-alpha, and PDGFR-beta were enhanced after injury. In the immunohistochemical analysis and/or in situ hybridization, PDGF-B and PDGFR-alpha, beta were expressed after reperfusion in the S3 segment of the proximal tubuli, where they were not expressed normally. The expressions of proliferating cell nuclear antigen and vimentin were concomitantly observed with PDGF-B and PDGFRs in the tubular cells of injured S3 segment at 48 hours after injury. Next, the inhibition of the PDGF-B/PDGFRs axis with either Trapidil or Ki6896, which was found to inhibit the phosphorylation of PDGFR-beta selectively, resulted in a rise of serum creatinine, higher mortality rate, abnormal regenerating process, and suppressed proliferation of tubular epithelial cells. These findings suggest that the PDGF-B/PDGFRs axis is involved in the proliferation of injured tubular cells and plays an important role in the regeneration of tubular cells from acute ischemic injury.

Spreading depression induces expression of calcium-independent protein kinase C subspecies in ischaemia-sensitive cortical layers: regulation by N-methyl-D-aspartate receptors and glucocorticoids

Spreading depression is a wave of sustained depolarization challenging the energy metabolism of the cells without causing irreversible damage. In the ischaemic brain, sreading depression-like depolarization contributes to the evolution of ischaemia to infarction. The depolarization is propagated by activation of N-methyl-D-aspartate receptors, but changes in signal transduction downstream of the receptors are not known. Because protein phosphorylation is a general mechanism whereby most cellular processes are regulated, and inhibition of N-methyl-D-aspartate receptors or protein kinase C is neuroprotective, the expression of protein kinase C subspecies in spreading depression was examined. Cortical treatment with KCl induced an upregulation of protein kinase Cdelta and zeta messenger RNA at 4 and 8 h, whereas protein kinase Calpha, beta, gamma and epsilon did not show significant changes. The gene induction was the strongest in layers 2 and 3, and was followed by an increased number of protein kinase Cdelta-immunoreactive neurons. Protein kinase Cdelta and zeta inductions were inhibited by pretreatment with an N-methyl-D-aspartate receptor antagonist, dizocilpine maleate, which also blocked spreading depression propagation, and with dexamethasone, which acted without blocking the propagation. Quinacrine, a phospholipase A2 inhibitor, reduced only protein kinase C5 induction. In addition, N(G)(-nitro-L-arginine methyl ester, a nitric oxide synthase inhibitor, did not influence protein kinase Cdelta or zeta induction, whereas 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione, an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate/kainate receptor antagonist, and the cyclo-oxygenase inhibitors indomethacin and diclophenac tended to increase gene expression. The data show that cortical spreading depression induces Ca2(+)-independent protein kinase C subspecies delta and zeta, but not Ca(2+)-dependent subspecies, through activation of N-methyl-D-aspartate receptors and phospholipase A2. Even though the signal pathway is similar to the induction described previously in ischaemia for genes implicated in delayed neuronal death, the gene inductions observed here are not necessarily pathogenetic, but may represent a general reaction to metabolic stress.

Expression of platelet-derived growth factor B-chain and beta-receptor in hypoxic/ischemic encephalopathy of neonatal rats

Expression of platelet-derived growth factor B-chain and of its specific receptor (beta-receptor) was investigated in immature brains with hypoxic/ischemic injury. After the left common carotid arteries of seven-day-old rats were ligated and pups were placed in a hypoxic chamber, the protein and messenger RNA of both B-chain and beta-receptor were assessed using immunocytochemistry and northern analysis, respectively. Transcripts for B-chain were localized by in situ hybridization. Faint but definite expression of B-chain and beta-receptor was seen in the brains of untreated neonatal controls. Three to 48 h after hypoxia B-chain protein was generally increased above control levels, but focally decreased expression was seen in infarcted areas. Enhanced induction of messenger RNA of B-chain was seen in the both sides of cerebral cortices and hippocampi at 3 h. Strongly increased positivity for B-chain protein and mRNA occurred in the neurons surrounding the infarct. In situ hybridization still showed this up-regulation seven days after hypoxia. Beta-receptor protein expression was enhanced in some neurons immediately surrounding the infarct at 3 h of hypoxia, and marked up-regulation was seen at 16 h. Beta-receptor messenger RNA remained at control levels. Immunocytochemistry showed strong immunoreactivity for the beta-receptor on the neurons surrounding the infarct at 72 h. These results indicate that a neonatal hypoxic/ischemic insult induces neuronal up-regulation of the platelet-derived growth factor B-chain as well as beta-receptor immediately after hypoxia. While this up-regulation is relatively transient in most neurons, sublethal damage to neurons immediately surrounding an infarct induces sustained up-regulation. Through autocrine and paracrine mechanisms, platelet-derived growth factor B-chain molecules may act as a neuroprotective factor in immature brain experiencing with hypoxic/ischemic injury.

Induction of infarct tolerance by platelet-derived growth factor against reversible focal ischemia

Nerve growth factor, brain-derived neurotrophic factor, and other neurotrophic factors have been reported to have neuroprotective effects against global ischemia. To investigate whether the homodimer of platelet-derived growth factor B-chain (PDGF-BB) can protect neurons against focal temporary ischemia, PDGF-BB was administered to the rat brain for a prolonged period prior to, during, and after ischemia, since PDGF-BB protected rat neurons from global ischemia in our previous study. A total of 82 male Sprague-Dawley rats were used. Recombinant PDGF-BB, or saline was administered into the left neocortex via an implanted osmotic pump for 3 days (1.2 microg in total), 7 days (2 microgram or 4 microgram in total), or 14 days (4 microgram in total) pre-ischemia and 2 days post-ischemia. In an additional group, PDGF-BB (4 microgram in total) was administered for 14 days by osmotic pump and focal ischemia was induced after an additional 7-day interval following removal of the pump. Focal temporary ischemia was induced in the left MCA territory by bilateral CCA and MCA occlusion for 2 h. All rats were sacrificed 2 days after ischemia and the volume of cerebral infarct was analyzed using TTC staining. In a separate set of animals, regional cerebral blood flow (rCBF) was monitored by the hydrogen clearance method and laser Doppler flowmetry (LDF) of the neocortex after 14 days of intracerebral administration of PDGF-BB or saline. In the group receiving PDGF-BB (4 microgram in total) for 7 or 14 days pre-ischemia, there was a significant reduction of neocortical infarction compared to that in the control or saline-infused group. The size of cerebral infarct was smallest in the group that received PDGF-BB for 14 days, when ischemia was induced 7 days after removal of the pump. Regarding rCBF measurement, there were no significant differences in groups receiving PDGF-BB or saline infusion for 14 days. The potent neuroprotective effect of PDGF-BB on global ischemia was also demonstrated in the focal ischemia model. However, prolonged intracerebral infusion for 7 to 14 days was necessary to achieve a significant reduction of infarct volume. Neuroprotection was not due to increased collateral flow during ischemia.

Platelet-derived growth factor prevents ischemia-induced neuronal injuries in vivo

Platelet-derived growth factor (PDGF) has been considered to be a neuroprotective factor candidate on the basis of several in vitro studies. However, the in vivo effect of PDGF on ischemic neurons has not been determined. In the present study, the effect of PDGF-BB on the ischemia-induced disability of passive avoidance task and hippocampal CA1 neuron death in normothermic gerbils, whose the brain temperature was kept at 37.0 +/- 0.2 degrees C during 3-min occlusion of the common carotid arteries was investigated. When PDGF-BB was continuously infused for 7 days into the cerebral ventricles of gerbils with transient forebrain ischemia, response latency time in a passive avoidance task was significantly prolonged. Subsequent histological examinations showed that PDGF-BB effectively increased the number of viable pyramidal neurons in the hippocampal CA1 region as well as synapses within the strata moleculare, radiatum and oriens of the region in comparison with the numbers of neurons and synapses in vehicle-treated ischemic gerbils. In situ detection of DNA fragmentation (TUNEL staining) revealed that TUNEL-positive neurons in the hippocampal CA1 field of vehicle-treated ischemic gerbils were much more numerous than those in the field of PDGF-BB-treated ischemic animals after 7 days ischemia. These findings suggest that the present ischemic animal model exhibits a more delayed neuronal degeneration of the hippocampal CA1 field than the conventional 5-min ischemic model and that the 7-day infusion of PDGF-BB, starting 2 h before ischemic insult, not only prevents delayed neuronal death in the hippocampal CA1 field at 7 days after forebrain ischemia but also inhibits a slowly progressive neuronal degeneration occurring thereafter.

Platelet-derived growth factor-BB, but not -AA, prevents delayed neuronal death after forebrain ischemia in rats

Our previous studies demonstrated coordinate expression of platelet-derived growth factor (PDGF) -B chain and beta-receptor in neurons at risk in the rat brain with focal ischemia. To clarify a role of the -B chain in the brain further, we examined whether PDGF-A or -B chain protects CA1 pyramidal neurons from delayed neuronal death after forebrain ischemia in rats. Pretreatment with PDGF-BB, but not -AA, at 120 ng/d for 2 days until forebrain ischemia was performed markedly ameliorated delayed neuronal death in CA1 pyramidal neurons on day 7 after ischemia. This neuroprotective effect of PDGF-BB was dose-dependent, and pretreatment with PDGF-BB at 240 ng/d showed almost complete inhibition of delayed neuronal death. In contrast, posttreatment with PDGF-BB at 120 ng/d starting 20 minutes after ischemia demonstrated no significant neuroprotective effect. The current study established marked neuroprotective actions of PDGF-BB in ischemic neuronal damage.

Roles of platelet-derived growth factor in the developing and mature nervous systems

In spite of its association by history and name to platelets, platelet-derived growth factor (PDGF) exerts important actions in a myriad of tissues, including the nervous system. PDGF and PDGF receptors are widely expressed in neuronal and glial cells of many regions of both the central and peripheral nervous systems. In this topical review, the roles played by PDGF in the development and maintenance of the nervous system are discussed. We also discuss the modulatory effects of PDGF on synaptic transmission, its role in neoplastic and non-neoplastic conditions of the central nervous system, and the neuroprotective effects of this growth factor.