Astrocytes attenuate oligodendrocyte death in vitro through an alpha(6) integrin-laminin-dependent mechanism

Monomeric Amyloid Peptide-induced Toxicity in Human Oligodendrocyte Cell Line and Mouse Brain Primary Mixed-glial Cell Cultures: Evidence for a Neuroprotective Effect of Neurosteroid 3α-O-allyl-allopregnanolone

Amyloid-peptide (Aβ) monomeric forms (ABM) occurring in presymptomatic Alzheimer's disease (AD) brain are thought to be devoid of neurotoxicity while the transition/aggregation of ABM into oligomers is determinant for Aβ-induced toxicity since Aβ is predominantly monomeric up to 3 µM and aggregates over this concentration. However, recent imaging and/or histopathological investigations revealed alterations of myelin in prodromal AD brain in absence of aggregated Aβ oligomers, suggesting that ABM may induce toxicity in myelin-producing cells in early AD-stages. To check this hypothesis, here we studied ABM effects on the viability of the Human oligodendrocyte cell line (HOG), a reliable oligodendrocyte model producing myelin proteins. Furthermore, to mimic closely interactions between oligodendrocytes and other glial cells regulating myelination, we investigated also ABM effects on mouse brain primary mixed-glial cell cultures. Various methods were combined to show that ABM concentrations (600 nM-1 µM), extremely lower than 3 µM, significantly decreased HOG cell and mouse brain primary mixed-glial cell survival. Interestingly, flow-cytometry studies using specific cell-type markers demonstrated that oligodendrocytes represent the most vulnerable glial cell population affected by ABM toxicity. Our work also shows that the neurosteroid 3α-O-allyl-allopregnanolone BR351 (250 and 500 nM) efficiently prevented ABM-induced HOG and brain primary glial cell toxicity. Bicuculline (50-100 nM), the GABA-A-receptor antagonist, was unable to block/reduce BR351 effect against ABM-induced HOG and primary glial cell toxicity, suggesting that BR351-evoked neuroprotection of these cells may not depend on GABA-A-receptor allosterically modulated by neurosteroids. Altogether, our results suggest that further exploration of BR351 therapeutic potential may offer interesting perspectives to develop effective neuroprotective strategies.

The role of glial cells in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) has traditionally been considered a neuron-centric disease. This view is now outdated, with increasing recognition of cell autonomous and non-cell autonomous contributions of central and peripheral nervous system glia to ALS pathomechanisms. With glial research rapidly accelerating, we comprehensively interrogate the roles of astrocytes, microglia, oligodendrocytes, ependymal cells, Schwann cells and satellite glia in nervous system physiology and ALS-associated pathology. Moreover, we highlight the inter-glial, glial-neuronal and inter-system polylogue which constitutes the healthy nervous system and destabilises in disease. We also propose classification based on function for complex glial reactive phenotypes and discuss the pre-requisite for integrative modelling to advance translation. Given the paucity of life-enhancing therapies currently available for ALS patients, we discuss the promising potential of harnessing glia in driving ALS therapeutic discovery.

Neuroprotective astroglial response to neural damage and its relevance to affective disorders

Astrocytes not only support neuronal function with essential roles in synaptic neurotransmission, action potential propagation, metabolic support, or neuroplastic and developmental adaptations. They also respond to damage or dysfunction in surrounding neurons and oligodendrocytes by releasing neurotrophic factors and other molecules that increase the survival of the supported cells or contribute to mechanisms of structural and molecular restoration. The neuroprotective responsiveness of astrocytes is based on their ability to sense signals of degeneration, metabolic jeopardy and structural damage, and on their aptitude to locally deliver specific molecules to remedy threats to the molecular and structural features of their cellular partners. To the extent that neuronal and other glial cell disturbances are known to occur in affective disorders, astrocyte responsiveness to those disturbances may help to better understand the roles astrocytes play in affective disorders. The astrocytic sensing apparatus supporting those responses involves receptors for neurotransmitters, purines, cell adhesion molecules and growth factors. Astrocytes also share with the immune system the capacity of responding to cytokines released upon neuronal damage. In addition, in responses to specific signals astrocytes release unique factors such as clusterin or humanin that have been shown to exert potent neuroprotective effects. Astrocytes integrate the signals above to further deliver structural lipids, removing toxic metabolites, stabilizing the osmotic environment, normalizing neurotransmitters, providing anti-oxidant protection, facilitating synaptogenesis and acting as barriers to contain varied deleterious signals, some of which have been described in brain regions relevant to affective disorders and related animal models. Since various of the injurious signals that activate astrocytes have been implicated in different aspects of the etiopathology of affective disorders, particularly in relation to the diagnosis of depression, potentiating the corresponding astrocyte neuroprotective responses may provide additional opportunities to improve or complement available pharmacological and behavioral therapies for affective disorders.

Emerging roles of oligodendrocyte precursor cells in neural circuit development and remodeling

Oligodendrocyte precursor cells (OPCs) are non-neuronal brain cells that give rise to oligodendrocytes, glia that myelinate the axons of neurons in the brain. Classically known for their contributions to myelination via oligodendrogenesis, OPCs are increasingly appreciated to play diverse roles in the nervous system, ranging from blood vessel formation to antigen presentation. Here, we review emerging literature suggesting that OPCs may be essential for the establishment and remodeling of neural circuits in the developing and adult brain via mechanisms that are distinct from the production of oligodendrocytes. We discuss the specialized features of OPCs that position these cells to integrate activity-dependent and molecular cues to shape brain wiring. Finally, we place OPCs within the context of a growing field focused on understanding the importance of communication between neurons and glia in the contexts of both health and disease.

Laminin regulates oligodendrocyte development and myelination

Oligodendrocytes are the cells that myelinate axons and provide trophic support to neurons in the CNS. Their dysfunction has been associated with a group of disorders known as demyelinating diseases, such as multiple sclerosis. Oligodendrocytes are derived from oligodendrocyte precursor cells, which differentiate into premyelinating oligodendrocytes and eventually mature oligodendrocytes. The development and function of oligodendrocytes are tightly regulated by a variety of molecules, including laminin, a major protein of the extracellular matrix. Accumulating evidence suggests that laminin actively regulates every aspect of oligodendrocyte biology, including survival, migration, proliferation, differentiation, and myelination. How can laminin exert such diverse functions in oligodendrocytes? It is speculated that the distinct laminin isoforms, laminin receptors, and/or key signaling molecules expressed in oligodendrocytes at different developmental stages are the reasons. Understanding molecular targets and signaling pathways unique to each aspect of oligodendrocyte biology will enable more accurate manipulation of oligodendrocyte development and function, which may have implications in the therapies of demyelinating diseases. Here in this review, we first introduce oligodendrocyte biology, followed by the expression of laminin and laminin receptors in oligodendrocytes and other CNS cells. Next, the functions of laminin in oligodendrocyte biology, including survival, migration, proliferation, differentiation, and myelination, are discussed in detail. Last, key questions and challenges in the field are discussed. By providing a comprehensive review on laminin's roles in OL lineage cells, we hope to stimulate novel hypotheses and encourage new research in the field.

Astrocytes and Microglia as Major Players of Myelin Production in Normal and Pathological Conditions

Myelination is an essential process that consists of the ensheathment of axons by myelin. In the central nervous system (CNS), myelin is synthesized by oligodendrocytes. The proliferation, migration, and differentiation of oligodendrocyte precursor cells constitute a prerequisite before mature oligodendrocytes extend their processes around the axons and progressively generate a multilamellar lipidic sheath. Although myelination is predominately driven by oligodendrocytes, the other glial cells including astrocytes and microglia, also contribute to this process. The present review is an update of the most recent emerging mechanisms involving astrocyte and microglia in myelin production. The contribution of these cells will be first described during developmental myelination that occurs in the early postnatal period and is critical for the proper development of cognition and behavior. Then, we will report the novel findings regarding the beneficial or deleterious effects of astroglia and microglia, which respectively promote or impair the endogenous capacity of oligodendrocyte progenitor cells (OPCs) to induce spontaneous remyelination after myelin loss. Acute delineation of astrocyte and microglia activities and cross-talk should uncover the way towards novel therapeutic perspectives aimed at recovering proper myelination during development or at breaking down the barriers impeding the regeneration of the damaged myelin that occurs in CNS demyelinating diseases.

Myelin Plasticity and Repair: Neuro-Glial Choir Sets the Tuning

The plasticity of the central nervous system (CNS) in response to neuronal activity has been suggested as early as 1894 by Cajal (1894). CNS plasticity has first been studied with a focus on neuronal structures. However, in the last decade, myelin plasticity has been unraveled as an adaptive mechanism of importance, in addition to the previously described processes of myelin repair. Indeed, it is now clear that myelin remodeling occurs along with life and adapts to the activity of neuronal networks. Until now, it has been considered as a two-part dialog between the neuron and the oligodendroglial lineage. However, other glial cell types might be at play in myelin plasticity. In the present review, we first summarize the key structural parameters for myelination, we then describe how neuronal activity modulates myelination and finally discuss how other glial cells could participate in myelinic adaptivity.

Mechanical regulation of oligodendrocyte biology

Oligodendrocytes (OL) are a subset of glial cells in the central nervous system (CNS) comprising the brain and spinal cord. The CNS environment is defined by complex biochemical and biophysical cues during development and response to injury or disease. In the last decade, significant progress has been made in understanding some of the key biophysical factors in the CNS that modulate OL biology, including their key role in myelination of neurons. Taken together, those studies offer translational implications for remyelination therapies, pharmacological research, identification of novel drug targets, and improvements in methods to generate human oligodendrocyte progenitor cells (OPCs) and OLs from donor stem cells in vitro. This review summarizes current knowledge of how various physical and mechanical cues affect OL biology and its implications for disease, therapeutic approaches, and generation of human OPCs and OLs.

Dysfunction of Cerebrovascular Endothelial Cells: Prelude to Vascular Dementia

Vascular dementia (VaD) is the second most common type of dementia after Alzheimer's disease (AD), characterized by progressive cognitive impairment, memory loss, and thinking or speech problems. VaD is usually caused by cerebrovascular disease, during which, cerebrovascular endothelial cells (CECs) are vulnerable. CEC dysfunction occurs before the onset of VaD and can eventually lead to dysregulation of cerebral blood flow and blood-brain barrier damage, followed by the activation of glia and inflammatory environment in the brain. White matter, neuronal axons, and synapses are compromised in this process, leading to cognitive impairment. The present review summarizes the mechanisms underlying CEC impairment during hypoperfusion and pathological role of CECs in VaD. Through the comprehensive examination and summarization, endothelial nitric oxide synthase (eNOS)/nitric oxide (NO) signaling pathway, Ras homolog gene family member A (RhoA) signaling pathway, and CEC-derived caveolin-1 (CAV-1) are proposed to serve as targets of new drugs for the treatment of VaD.

The vascular basement membrane in the healthy and pathological brain

The vascular basement membrane contributes to the integrity of the blood-brain barrier (BBB), which is formed by brain capillary endothelial cells (BCECs). The BCECs receive support from pericytes embedded in the vascular basement membrane and from astrocyte endfeet. The vascular basement membrane forms a three-dimensional protein network predominantly composed of laminin, collagen IV, nidogen, and heparan sulfate proteoglycans that mutually support interactions between BCECs, pericytes, and astrocytes. Major changes in the molecular composition of the vascular basement membrane are observed in acute and chronic neuropathological settings. In the present review, we cover the significance of the vascular basement membrane in the healthy and pathological brain. In stroke, loss of BBB integrity is accompanied by upregulation of proteolytic enzymes and degradation of vascular basement membrane proteins. There is yet no causal relationship between expression or activity of matrix proteases and the degradation of vascular matrix proteins in vivo. In Alzheimer's disease, changes in the vascular basement membrane include accumulation of Aβ, composite changes, and thickening. The physical properties of the vascular basement membrane carry the potential of obstructing drug delivery to the brain, e.g. thickening of the basement membrane can affect drug delivery to the brain, especially the delivery of nanoparticles.

Is There Evidence for Myelin Modeling by Astrocytes in the Normal Adult Brain?

A set of astrocytic process associated with altered myelinated axons is described in the forebrain of normal adult rodents with confocal, electron microscopy, and 3D reconstructions. Each process consists of a protuberance that contains secretory organelles including numerous lysosomes which polarize and open next to disrupted myelinated axons. Because of the distinctive asymmetric organelle distribution and ubiquity throughout the forebrain neuropil, this enlargement is named paraxial process (PAP). The myelin envelope contiguous to the PAP displays focal disruption or disintegration. In routine electron microscopy clusters of large, confluent, lysosomes proved to be an effective landmark for PAP identification. In 3D assemblies lysosomes organize a series of interconnected saccules that open up to the plasmalemma next to the disrupted myelin envelope(s). Activity for acid hydrolases was visualized in lysosomes, and extracellularly at the PAP-myelin interface and/or between the glial and neuronal outer aspects. Organelles in astrocytic processes involved in digesting pyknotic cells and debris resemble those encountered in PAPs supporting a likewise lytic function of the later. Conversely, processes entangling tripartite synapses and glomeruli were devoid of lysosomes. Both oligodendrocytic and microglial processes were not associated with altered myelin envelopes. The possible roles of the PAP in myelin remodeling in the context of the oligodendrocyte-astrocyte interactions and in the astrocyte's secretory pathways are discussed.

Mesenchymal stromal cells from umbilical cord Wharton's jelly trigger oligodendroglial differentiation in neural progenitor cells through cell-to-cell contact

BACKGROUND AIMS

Wharton's jelly mesenchymal stromal cells (WJ-MSCs) might be ideal candidates to treat perinatal brain damage. Their secretome has been shown to have beneficial effects on neuroregeneration, in part through interaction with neural progenitor cells (NPCs). However, it remains unclear whether cell-to-cell contact decisively contributes to this positive effect. The objective of this study was to elucidate the mechanism through which differentiation in NPCs is triggered after exposure to WJ-MSCs. Furthermore, given that WJ-MSCs can be derived from term (tWJ-MSCs) or preterm (ptWJ-MSCs) deliveries and that WJ-MSCs might be used for transplantations independent of gestational age, the influence of tWJ-MSCs versus ptWJ-MSCs on the differentiation capacities of NPCs was studied.

METHODS

The effect of tWJ-MSCs and ptWJ-MSCs on the expression of neuroglial markers in NPCs was assessed in co-culture (CC), conditioned medium (CM) or transwell CC experiments by immunocytochemistry, real-time polymerase chain reaction and Western blot. Additionally, mass spectrometry was used to study their secretomes.

RESULTS

NPCs showed an increased expression of glial markers after CC with WJ-MSCs or exposure to WJ-MSC-CMs. CC had a more prominent effect on the expression of glial markers compared with CM or transwell CCs. tWJ-MSCs more strongly induced the expression of mature oligodendroglial markers compared with ptWJ-MSCs. A possible role in enhancing this maturation could be attributed to the laminin α2-subunit.

CONCLUSIONS

Cell-to-cell contact between WJ-MSCs and NPCs induces oligodendrogenesis on NPCs, whereas trophic factor secretion is sufficient to promote astrogenesis. Thus, transplanting WJ-MSCs may promote endogenous neuroregeneration in perinatal brain damage.

Oligodendrocyte, Astrocyte, and Microglia Crosstalk in Myelin Development, Damage, and Repair

Oligodendrocytes are the myelinating glia of the central nervous system. Myelination of axons allows rapid saltatory conduction of nerve impulses and contributes to axonal integrity. Devastating neurological deficits caused by demyelinating diseases, such as multiple sclerosis, illustrate well the importance of the process. In this review, we focus on the positive and negative interactions between oligodendrocytes, astrocytes, and microglia during developmental myelination and remyelination. Even though many lines of evidence support a crucial role for glia crosstalk during these processes, the nature of such interactions is often neglected when designing therapeutics for repair of demyelinated lesions. Understanding the cellular and molecular mechanisms underlying glial cell communication and how they influence oligodendrocyte differentiation and myelination is fundamental to uncover novel therapeutic strategies for myelin repair.

Oligodendrocyte, Astrocyte, and Microglia Crosstalk in Myelin Development, Damage, and Repair

Oligodendrocytes are the myelinating glia of the central nervous system. Myelination of axons allows rapid saltatory conduction of nerve impulses and contributes to axonal integrity. Devastating neurological deficits caused by demyelinating diseases, such as multiple sclerosis, illustrate well the importance of the process. In this review, we focus on the positive and negative interactions between oligodendrocytes, astrocytes, and microglia during developmental myelination and remyelination. Even though many lines of evidence support a crucial role for glia crosstalk during these processes, the nature of such interactions is often neglected when designing therapeutics for repair of demyelinated lesions. Understanding the cellular and molecular mechanisms underlying glial cell communication and how they influence oligodendrocyte differentiation and myelination is fundamental to uncover novel therapeutic strategies for myelin repair.

Subcortical ischemic vascular disease: Roles of oligodendrocyte function in experimental models of subcortical white-matter injury

Oligodendrocytes are one of the major cell types in cerebral white matter. Under normal conditions, they form myelin sheaths that encircle axons to support fast nerve conduction. Under conditions of cerebral ischemia, oligodendrocytes tend to die, resulting in white-matter dysfunction. Repair of white matter involves the ability of oligodendrocyte precursors to proliferate and mature. However, replacement of lost oligodendrocytes may not be the only mechanism for white-matter recovery. Emerging data now suggest that coordinated signaling between neural, glial, and vascular cells in the entire neurovascular unit may be required. In this mini-review, we discuss how oligodendrocyte lineage cells participate in signaling and crosstalk with other cell types to underlie function and recovery in various experimental models of subcortical white-matter injury.

Adrenocorticotropin hormone 1-39 promotes proliferation and differentiation of oligodendroglial progenitor cells and protects from excitotoxic and inflammation-related damage

Oligodendroglia (OL) are highly susceptible to damage and, like neurons, are terminally differentiated. It is important to protect OL precursors (OPC) because they are reservoirs of differentiating cells capable of myelination following perinatal insult and remyelination in white matter diseases, including multiple sclerosis (MS). Patients with relapsing-remitting MS are commonly treated with high-dose corticosteroids (CS) when experiencing an exacerbation. Adrenocorticotropin hormone (ACTH), a primary component of another approved MS exacerbation treatment, is a melanocortin peptide that stimulates production of CS by the adrenals. Melanocortin receptors are also found in the central nervous system (CNS) and on immune cells. ACTH is produced within the CNS and may have CS-independent effects on glia. We found that ACTH 1-39 stimulated proliferation of OPC, and to a lesser extent astroglia (AS) and microglia (MG), in rat glial cultures. ACTH accelerated differentiation of PDGFRα(+) OPC to a later stage marked by galactolipid expression and caused greater expansion of OL myelin-like sheets compared with untreated cells. Protective effects of ACTH on OPC were assessed by treating cultures with selected toxic agents, with or without ACTH. At 200 nM, ACTH protected OPC from death induced by staurosporine, glutamate, NMDA, AMPA, kainate, quinolinic acid, H2 O2 , and slow NO release, but not against kynurenic acid or rapid NO release. These agents and ACTH were not toxic to AS or MG. Our findings indicate that ACTH 1-39 provides benefits by increasing the number of OPC, accelerating their development into mature OL, and reducing OPC death from toxic insults.

The effect of glia-glia interactions on oligodendrocyte precursor cell biology during development and in demyelinating diseases

Oligodendrocyte precursor cells (OPCs) originate in specific areas of the developing central nervous system (CNS). Once generated, they migrate towards their destinations where they differentiate into mature oligodendrocytes. In the adult, 5-8% of all cells in the CNS are OPCs, cells that retain the capacity to proliferate, migrate, and differentiate into oligodendrocytes. Indeed, these endogenous OPCs react to damage in demyelinating diseases, like multiple sclerosis (MS), representing a key element in spontaneous remyelination. In the present work, we review the specific interactions between OPCs and other glial cells (astrocytes, microglia) during CNS development and in the pathological scenario of MS. We focus on: (i) the role of astrocytes in maintaining the homeostasis and spatial distribution of different secreted cues that determine OPC proliferation, migration, and differentiation during CNS development; (ii) the role of microglia and astrocytes in the redistribution of iron, which is crucial for myelin synthesis during CNS development and for myelin repair in MS; (iii) how microglia secrete different molecules, e.g., growth factors, that favor the recruitment of OPCs in acute phases of MS lesions; and (iv) how astrocytes modify the extracellular matrix in MS lesions, affecting the ability of OPCs to attempt spontaneous remyelination. Together, these issues demonstrate how both astroglia and microglia influence OPCs in physiological and pathological situations, reinforcing the concept that both development and neural repair are complex and global phenomena. Understanding the molecular and cellular mechanisms that control OPC survival, proliferation, migration, and differentiation during development, as well as in the mature CNS, may open new opportunities in the search for reparative therapies in demyelinating diseases like MS.

Extracellular proteolytic pathophysiology in the neurovascular unit after stroke

The NINDS Stroke Progress Review Group recommended a shift in emphasis from a purely neurocentric view of cell death towards a more integrative approach whereby responses in all brain cells and matrix are considered. The neurovascular unit (fundamentally comprising endothelium, astrocyte, and neuron) provides a conceptual framework where cell-cell and cell-matrix signaling underlies the overall tissue response to stroke and its treatments. Here, we briefly review recent data on extracellular proteolytic dysfunction in the neurovascular unit after a stroke. The breakdown of neurovascular matrix initiates blood-brain barrier disruption with edema and/or hemorrhage. Endothelial dysfunction amplifies inflammatory responses. Perturbation of cell-matrix homeostasis triggers multiple cell death pathways. Interactions between the major classes of extracellular proteases from the plasminogen and matrix metalloprotease families may underlie processes responsible for some of the hemorrhagic complications of thrombolytic stroke therapy. Targeting the proteolytic imbalance within the neurovascular unit may provide new approaches for improving the safety and efficacy of thrombolytic reperfusion therapy for stroke.

Inhibition of a novel specific neuroglial integrin signaling pathway increases STAT3-mediated CNTF expression

Background

Ciliary neurotrophic factor (CNTF) expression is repressed in astrocytes by neuronal contact in the CNS and is rapidly induced by injury. Here, we defined an inhibitory integrin signaling pathway.

Results

The integrin substrates laminin, fibronectin and vitronectin, but not collagen, thrombospondin or fibrinogen, reduced CNTF expression in C6 astroglioma cells. Antibodies against αv and β5, but not α6 or β1, integrin induced CNTF. Together, the ligand and antibody specificity suggests that CNTF is repressed by αvβ5 integrin. Antibodies against Thy1, an abundant neuronal surface protein whose function is unclear, induced CNTF in neuron-astrocyte co-cultures indicating that it is a neuroglial CNTF repressor. Inhibition of the integrin signaling molecule Focal Adhesion Kinase (FAK) or the downstream c-Jun N-terminal kinase (JNK), but not extracellular regulated kinase (ERK) or p38 MAPK, greatly induced CNTF mRNA and protein expression within 4 hours. This selective inhibitory pathway phosphorylated STAT3 on its inhibitory ser-727 residue interfering with activity of the pro-transcription Tyr-705 residue. STAT3 can activate CNTF transcription because it bound to its promoter and FAK antagonist-induced CNTF was reduced by blocking STAT3. Microinjection of FAK inhibitor directly into the brain or spinal cord in adult mice rapidly induced CNTF mRNA and protein expression. Importantly, systemic treatment with FAK inhibitors over 3 days induced CNTF in the subventricular zone and increased neurogenesis.

Conclusions

Neuron-astroglia contact mediated by integrins serves as a sensor to enable rapid neurotrophic responses and provides a new pharmacological avenue to exploit the neuroprotective properties of endogenous CNTF.

Pathophysiologic cascades in ischemic stroke

Many advances have been achieved in terms of understanding the molecular and cellular mechanisms of ischemic stroke. But thus far, clinically effective neuroprotectants remain elusive. In this minireview, we summarize the basics of ischemic cascades after stroke, covering neuronal death mechanisms, white matter pathophysiology, and inflammation with an emphasis on microglia. Translating promising mechanistic knowledge into clinically meaningful stroke drugs is very challenging. An integrative approach that encompasses the multimodal and multicell signaling phenomenon of stroke will be required to move forward.

Crosstalk between oligodendrocytes and cerebral endothelium contributes to vascular remodeling after white matter injury

After stroke and brain injury, cortical gray matter recovery involves mechanisms of neurovascular matrix remodeling. In white matter, however, the mechanisms of recovery remain unclear. In this study, we demonstrate that oligodendrocytes secrete matrix metalloproteinase-9 (MMP-9), which accelerates the angiogenic response after white matter injury. In primary oligodendrocyte cultures, treatment with the proinflammatory cytokine interleukin-1β (IL-1β) induced an upregulation and secretion of MMP-9. Conditioned media from IL-1β-stimulated oligodendrocytes significantly amplified matrigel tube formation in brain endothelial cells, indicating that MMP-9 from oligodendrocytes can promote angiogenesis in vitro. Next, we asked whether similar signals and substrates operate after white matter injury in vivo. Focal white matter injury and demyelination was induced in mice via stereotactic injection of lysophosphatidylcholine into corpus callosum. Western blot analysis showed that IL-1β expression was increased in damaged white matter. Immunostaining demonstrated MMP-9 signals in myelin-associated oligodendrocytic basic protein-positive oligodendrocytes. Treatment with an IL-1β-neutralizing antibody suppressed the MMP-9 response in oligodendrocytes. Finally, we confirmed that the broad spectrum MMP inhibitor GM6001 inhibited angiogenesis around the injury area in this white matter injury model. In gray matter, a neurovascular niche promotes cortical recovery after brain injury. Our study suggests that an analogous oligovascular niche may mediate recovery in white matter.

Vascular endothelial growth factor regulates the migration of oligodendrocyte precursor cells

Originally identified as an angiogenic factor, vascular endothelial growth factor (VEGF-A) is now known to play multiple roles in the CNS, including the direct regulation of neuronal and astrocytic functions. Here, we ask whether VEGF-A can also have a novel role in white matter by modulating oligodendrocyte precursor cells (OPCs). OPCs were cultured from rat neonatal cortex. Expression of VEGF-receptor2/KDR/Flk-1 was confirmed with Western blot and immunostaining. VEGF-A did not affect proliferation or differentiation in OPC cultures, but VEGF-A promoted OPC migration in a concentration-dependent manner. Consistent with this migration phenotype, VEGF-A-treated OPCs showed reorganization of actin cytoskeleton in leading-edge processes. VEGF-A-induced migration and actin reorganization were inhibited by an anti-Flk-1 receptor-blocking antibody. Mechanistically, VEGF-A induced binding of focal adhesion kinase (FAK) with paxillin. The FAK inhibitor PF573228 reduced VEGF-A-induced OPC migration. VEGF-A signaling also evoked a transient rise in reactive oxygen species (ROS), and OPC migration was increased when antioxidants were removed from the culture media. Our findings demonstrate that VEGF-A can induce OPC migration via an ROS- and FAK-dependent mechanism, and suggest a novel role for VEGF-A in white-matter maintenance and homeostasis.

Cellular mechanisms of neurovascular damage and repair after stroke

The biological processes underlying stroke are complex, and patients have a narrow repertoire of therapeutic opportunities. After the National Institutes of Health (NIH) convened the Stroke Progress Review Group in 2001, stroke research shifted from having a purely neurocentric focus to adopting a more integrated view wherein dynamic interactions between all cell types contribute to function and dysfunction in the brain. This so-called "neurovascular unit" provides a conceptual framework that emphasizes cell-cell interactions between neuronal, glial, and vascular elements. Under normal conditions, signaling within the neurovascular unit helps maintain homeostasis. After stroke, cell-cell signaling is disturbed, leading to pathophysiology. More recently, emerging data now suggest that these cell-cell signaling mechanisms may also mediate parallel processes of neurovascular remodeling during stroke recovery. Because plasticity is a signature feature of the young and developing brain, these concepts may have special relevance to how the pediatric brain responds after stroke.

Integrin signaling in oligodendrocytes and its importance in CNS myelination

Multiple sclerosis is characterized by repeated demyelinating attacks of the central nervous system (CNS) white matter tracts. To tailor novel therapeutics to halt or reverse disease process, we require a better understanding of oligodendrocyte biology and of the molecular mechanisms that initiate myelination. Cell extrinsic mechanisms regulate CNS myelination through the interaction of extracellular matrix proteins and their transmembrane receptors. The engagement of one such receptor family, the integrins, initiates intracellular signaling cascades that lead to changes in cell phenotype. Oligodendrocytes express a diverse array of integrins, and the expression of these receptors is developmentally regulated. Integrin-mediated signaling is crucial to the proliferation, survival, and maturation of oligodendrocytes through the activation of downstream signaling pathways involved in cytoskeletal remodeling. Here, we review the current understanding of this important signaling axis and its role in oligodendrocyte biology and ultimately in the myelination of axons within the CNS.

Astrocytes protect oligodendrocyte precursor cells via MEK/ERK and PI3K/Akt signaling

Accumulating evidence suggest that trophic coupling among different cell types in the brain is required to maintain normal CNS function. Here we show that astrocytes secrete soluble factors that can be oligodendrocyte-supportive. Oligodendrocyte precursor cells (OPCs) and astrocytes were prepared from neonatal rat brain and cultured separately. We conducted cell culture medium-transfer experiments to examine whether astrocytes secrete OPC-protective factors. Conditioned media from astrocytes protected OPCs against H(2)O(2)-induced oxidative stress, starvation, and oxygen-glucose deprivation. This protective effect may be mediated in part via ERK and Akt signaling pathways. Astrocyte-conditioned media upregulated the phosphorylation levels of ERK and Akt in OPC cultures. Blockade of ERK or Akt signaling with U0126 or LY294002 cancelled the OPC-protective effects of astrocyte-conditioned media. Taken together, these data suggest that astrocytes are an important source for oligodendrocyte-supportive factors. Coupling between these two major glial components in brain may be vital for sustaining white matter homeostasis.

Disruption of astrocyte STAT3 signaling decreases mitochondrial function and increases oxidative stress in vitro

BACKGROUND

Astrocytes exert a wide variety of functions in health and disease and respond to a wide range of signaling pathways, including members of the Janus-kinase signal transducers and activators of transcription (Jak-STAT) family. We have recently shown that STAT3 is an important regulator of astrocyte reactivity after spinal cord injury in vivo[1].

METHODOLOGY/PRINCIPAL FINDINGS

Here, we used both a conditional gene deletion strategy that targets the deletion of STAT3 selectively to astrocytes (STAT3-CKO), and a pharmacological inhibitor of JAK-2, AG490, in cultured astrocytes in vitro, to investigate potential functions and molecules influenced by STAT3 signaling in relation to mitochondrial function and oxidative stress. Our findings show that the absence of STAT3 signaling in astrocytes leads to (i) increased production of superoxide anion and other reactive oxygen species and decreased level of glutathione, (ii) decreased mitochondrial membrane potential and decreased ATP production, and (iii) decreased rate of cell proliferation. Many of the differences observed in STAT3-CKO astrocytes were distinctly altered by exposure to rotenone, suggesting a role for complex I of the mitochondrial electron transport chain. Gene expression microarray studies identified numerous changes in STAT3-CKO cells that may have contributed to the identified deficits in cell function.

CONCLUSIONS/SIGNIFICANCE

Taken together, these STAT3-dependent alterations in cell function and gene expression have relevance to both reactive gliosis and to the support and protection of surrounding cells in neural tissue.

Oligovascular signaling in white matter stroke

Stroke is one of the leading causes of death and disability in developed countries. Since protecting neurons alone is not sufficient for stroke therapy, research has shifted to the rescue of multiple cell types in the brain. In particular, attention has focused on the study of how cerebral blood vessels and brain cells communicate with each other. Recent findings suggest that cerebral endothelial cells may secrete trophic factors that nourish neighboring cells. Although data are strongest in terms of supporting endothelial-neuronal interactions, it is likely that similar interactions occur in white matter as well. In this mini-review, we summarize recent advances in the dissection of cell-cell interactions in white matter. We examine two key concepts. First, trophic interactions between vessels and oligodendrocytes (OLGs) and oligodendrocyte precursor cells (OPCs) play critical roles in white matter homeostasis. Second, cell-cell trophic coupling is disturbed under diseased conditions that incur oxidative stress. White matter pathophysiology is very important in stroke. A deeper understanding of the mechanisms of oligovascular signaling in normal and pathologic conditions may lead us to new therapeutic targets for stroke and other neurodegenerative diseases.

Maintenance of the relative proportion of oligodendrocytes to axons even in the absence of BAX and BAK

Highly purified oligodendroglial lineage cells from mice lacking functional bax and bak genes were resistant to apoptosis after in-vitro differentiation, indicating an essential role of the intrinsic apoptotic pathway in apoptosis of oligodendrocytes in the absence of neurons (axons) and other glial cells. These mice therefore provide a valuable tool with which to evaluate the significance of the intrinsic apoptotic pathway in regulating the population sizes of oligodendrocytes and oligodendroglial progenitor cells. Quantitative analysis of the optic nerves and the dorsal columns of the spinal cord revealed that the absolute numbers of mature oligodendrocytes immunolabeled for aspartoacylase and adult glial progenitor cells expressing NG2 chondroitin sulfate proteoglycan were increased in both white matter tracts of adult bax/bak-deficient mice and, to a lesser extent, bax-deficient mice, except that there was no increase in NG2-positive progenitor cells in the dorsal columns of these strains of mutant mice. These increases in mature oligodendrocytes and progenitor cells in bax/bak-deficient mice were unexpectedly proportional to increases in numbers of axons in these white matter tracts, thus retaining the oligodendroglial lineage to axon ratios of at most 1.3-fold of the physiological numbers. This is in contrast to the prominent expansion in numbers of neural precursor cells in the subventricular zones of these adult mutant mice. Our study indicates that homeostatic control of cell number is different for progenitors of the oligodendroglial and neuronal lineages. Furthermore, regulatory mechanism(s) operating in addition to apoptotic elimination through the intrinsic pathway, appear to prevent the overproduction of highly mitotic oligodendroglial progenitor cells.

Beta1 integrins are required for normal CNS myelination and promote AKT-dependent myelin outgrowth

Oligodendrocytes in the central nervous system (CNS) produce myelin sheaths that insulate axons to ensure fast propagation of action potentials. beta1 integrins regulate the myelination of peripheral nerves, but their function during the myelination of axonal tracts in the CNS is unclear. Here we show that genetically modified mice lacking beta1 integrins in the CNS present a deficit in myelination but no defects in the development of the oligodendroglial lineage. Instead, in vitro data show that beta1 integrins regulate the outgrowth of myelin sheaths. Oligodendrocytes derived from mutant mice are unable to efficiently extend myelin sheets and fail to activate AKT (also known as AKT1), a kinase that is crucial for axonal ensheathment. The inhibition of PTEN, a negative regulator of AKT, or the expression of a constitutively active form of AKT restores myelin outgrowth in cultured beta1-deficient oligodendrocytes. Our data suggest that beta1 integrins play an instructive role in CNS myelination by promoting myelin wrapping in a process that depends on AKT.

Integrins in lens development and disease

Integrins are the major cell surface receptors for proteins in the extracellular matrix. These receptors form major cell signaling centers that are bidirectional, communicating messages between the cell and its environment. They are a large receptor family, with members well-known to regulate cellular processes essential to both development and disease. In this review we examine the literature regarding integrins in the lens. Here we cover integrin function in lens cell differentiation, in the development of the lens and in protection of the lens epithelial cell phenotype. In addition, we analyze the role of integrins in the progression of lens fibrotic diseases, focusing particularly on integrin regulation of TGFbeta signaling pathways in posterior capsule opacification (PCO) and anterior subcapsular cataract (ASC).

Proteinase-activated receptor-2 exerts protective and pathogenic cell type-specific effects in Alzheimer's disease

The proteinase-activated receptors (PARs) are a novel family of G protein-coupled receptors, and their effects in neurodegenerative diseases remain uncertain. Alzheimer's disease (AD) is a neurodegenerative disorder defined by misfolded protein accumulation with concurrent neuroinflammation and neuronal death. We report suppression of proteinase-activated receptor-2 (PAR2) expression in neurons of brains from AD patients, whereas PAR2 expression was increased in proximate glial cells, together with up-regulation of proinflammatory cytokines and chemokines and reduced IL-4 expression (p < 0.05). Glial PAR2 activation increased expression of formyl peptide receptor-2 (p < 0.01), a cognate receptor for a fibrillar 42-aa form of beta-amyloid (Abeta(1-42)), enhanced microglia-mediated proinflammatory responses, and suppressed astrocytic IL-4 expression, resulting in neuronal death (p < 0.05). Conversely, neuronal PAR2 activation protected human neurons against the toxic effects of Abeta(1-42) (p < 0.05), a key component of AD neuropathogenesis. Amyloid precursor protein-transgenic mice, displayed glial fibrillary acidic protein and IL-4 induction (p < 0.05) in the absence of proinflammatory gene up-regulation and neuronal injury, whereas PAR2 was up-regulated at this early stage of disease progression. PAR2-deficient mice, after hippocampal Abeta(1-42) implantation, exhibited enhanced IL-4 induction and less neuroinflammation (p < 0.05), together with improved neurobehavioral outcomes (p < 0.05). Thus, PAR2 exerted protective properties in neurons, but its activation in glia was pathogenic with secretion of neurotoxic factors and suppression of astrocytic anti-inflammatory mechanisms contributing to Abeta(1-42)-mediated neurodegeneration.

Astrocytes--friends or foes in multiple sclerosis?

In multiple sclerosis (MS), the presence of demyelinating plaques has concentrated researchers' minds on the role of the oligodendrocyte in its pathophysiology. Recently, with the rediscovery of early and widespread loss of axons in the disease, new emphasis has been put on the role of axons and axon-oligodendrocyte interactions in MS. Despite the fact that, in 1904, Müller claimed that MS was a disease of astrocytes, more recently, astrocytes have taken a back seat, except as the cells that form the final glial scar after all hope of demyelination is over. However, perhaps it is time for the return of the astrocyte to popularity in the pathogenesis of MS, with recent reports on the dual role of astrocytes in aiding degeneration and demyelination, by promoting inflammation, damage of oligodendrocytes and axons, and glial scarring, but also in creating a permissive environment for remyelination by their action on oligodendrocyte precursor migration, oligodendrocyte proliferation, and differentiation. We review these findings to try to provide a cogent view of astrocytes in the pathology of MS.

Identification of dystroglycan as a second laminin receptor in oligodendrocytes, with a role in myelination

Developmental abnormalities of myelination are observed in the brains of laminin-deficient humans and mice. The mechanisms by which these defects occur remain unknown. It has been proposed that, given their central role in mediating extracellular matrix (ECM) interactions, integrin receptors are likely to be involved. However, it is a non-integrin ECM receptor, dystroglycan, that provides the key linkage between the dystrophin-glycoprotein complex (DGC) and laminin in skeletal muscle basal lamina, such that disruption of this bridge results in muscular dystrophy. In addition, the loss of dystroglycan from Schwann cells causes myelin instability and disorganization of the nodes of Ranvier. To date, it is unknown whether dystroglycan plays a role during central nervous system (CNS) myelination. Here, we report that the myelinating glia of the CNS, oligodendrocytes, express and use dystroglycan receptors to regulate myelin formation. In the absence of normal dystroglycan expression, primary oligodendrocytes showed substantial deficits in their ability to differentiate and to produce normal levels of myelin-specific proteins. After blocking the function of dystroglycan receptors, oligodendrocytes failed both to produce complex myelin membrane sheets and to initiate myelinating segments when co-cultured with dorsal root ganglion neurons. By contrast, enhanced oligodendrocyte survival in response to the ECM, in conjunction with growth factors, was dependent on interactions with beta-1 integrins and did not require dystroglycan. Together, these results indicate that laminins are likely to regulate CNS myelination by interacting with both integrin receptors and dystroglycan receptors, and that oligodendrocyte dystroglycan receptors may have a specific role in regulating terminal stages of myelination, such as myelin membrane production, growth, or stability.

Beta1-integrin signaling mediates premyelinating oligodendrocyte survival but is not required for CNS myelination and remyelination

Previous reports, including transplantation experiments using dominant-negative inhibition of beta1-integrin signaling in oligodendrocyte progenitor cells, suggested that beta1-integrin signaling is required for myelination. Here, we test this hypothesis using conditional ablation of the beta1-integrin gene in oligodendroglial cells during the development of the CNS. This approach allowed us to study oligodendroglial beta1-integrin signaling in the physiological environment of the CNS, circumventing the potential drawbacks of a dominant-negative approach. We found that beta1-integrin signaling has a much more limited role than previously expected. Although it was involved in stage-specific oligodendrocyte cell survival, beta1-integrin signaling was not required for axon ensheathment and myelination per se. We also found that, in the spinal cord, remyelination occurred normally in the absence of beta1-integrin. We conclude that, although beta1-integrin may still contribute to other aspects of oligodendrocyte biology, it is not essential for myelination and remyelination in the CNS.

Human astrocytes are resistant to Fas ligand and tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis

Human astrocytes express Fas yet are resistant to Fas-induced apoptosis. Here, we report that calcium/calmodulin-dependent protein kinase II (CaMKII) is constitutively activated in human astrocytes and protects the cells from apoptotic stimulation by Fas agonist. Once stimulated, Fas recruits Fas-associated death domain and caspase-8 for the assembly of the death-inducing signaling complex (DISC); however, caspase-8 cleavage is inhibited in the DISC. Inhibition of CaMKII kinase activity inhibits the expression of phosphoprotein enriched astrocytes-15 kDa/phosphoprotein enriched in diabetes (PEA-15/PED) and cellular Fas-associated death domain-like interleukin-1beta-converting enzyme-inhibitory protein (c-FLIP), thus releasing their inhibition of caspase-8 cleavage. Inhibition of PEA-15/PED or c-FLIP by small interfering RNA sensitizes human astrocytes to Fas-induced apoptosis. In contrast, inhibition of CaMKII, PEA-15, or c-FLIP does not affect the sensitivity of human astrocytes to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). TRAIL death receptors (DR4, DR5) are weakly expressed at mRNA, protein, and cell surface levels and thus fail to mediate the assembly of the DISC in human astrocytes. Overexpression of DR5 restores TRAIL signaling pathways and sensitizes the human astrocytes to TRAIL-induced apoptosis if CaMKII kinase activity or expression of PEA-15 and c-FLIP is inhibited; the results suggest that CaMKII-mediated pathways prevent TRAIL-induced apoptosis in human astrocytes under conditions in which TRAIL death receptors are upregulated. This study has therefore identified the molecular mechanisms that protect normal human astrocytes from apoptosis induced by Fas ligand and TRAIL.

Differential effects of chemokines on oligodendrocyte precursor proliferation and myelin formation in vitro

Chemokines have recently been postulated to have important functions in the central nervous system (CNS) in addition to their principal role of directional migration of leukocytes. In particular, it has been shown that chemokines may play a role in the regulation of oligodendrocyte biology. Here, we have chosen to study the role of certain chemokines in regulating myelination. We have used the murine oligodendrocyte precursor-like cell line, Oli-neu, and primary mixed cortical cultures as experimental systems to assess their activities on oligodendrocyte precursor proliferation and developmental in vitro myelination. GRO-alpha, IL-8, SDF-1alpha and RANTES dose-dependently increased proliferation of this mouse A2B5 precursor-like cell line, while MCP-1 did not. Furthermore, the CXC chemokines GRO-alpha, IL-8 and SDF-1alpha stimulated myelin basic protein synthesis in a dose-dependent manner in primary myelinating cultures and enhanced myelin segment formation in this system, while the CC chemokines MCP-1 and RANTES did not. We also demonstrate that the receptor for SDF-1alpha, CXCR4, is expressed in mixed cortical cultures by PDGFalphaR positive oligodendrocyte precursors (OLPs) as well as by Oli-neu cells. SDF-1alpha induced proliferation in primary mixed cultures and the Oli-neu cell line was mediated through this receptor. We propose, therefore, that CXC chemokines and in particular SDF-1alpha regulates CNS myelination via their effects on cells of the oligodendrocyte lineage, specifically stimulation of OLP proliferation.

Proteinase-activated receptor 2 modulates neuroinflammation in experimental autoimmune encephalomyelitis and multiple sclerosis

The proteinase-activated receptors (PARs) are widely recognized for their modulatory properties of inflammation and neurodegeneration. We investigated the role of PAR2 in the pathogenesis of multiple sclerosis (MS) in humans and experimental autoimmune encephalomyelitis (EAE) in mice. PAR2 expression was increased on astrocytes and infiltrating macrophages in human MS and murine EAE central nervous system (CNS) white matter (P < 0.05). Macrophages and astrocytes from PAR2 wild-type (WT) and knockout (KO) mice exhibited differential immune gene expression with PAR2 KO macrophages showing significantly higher interleukin 10 production after lipopolysaccharide stimulation (P < 0.001). PAR2 activation in macrophages resulted in the release of soluble oligodendrocyte cytotoxins (P < 0.01). Myelin oligodendrocyte glycoprotein-induced EAE caused more severe inflammatory gene expression in the CNS of PAR2 WT animals (P < 0.05), together with enhanced T cell proliferation and interferon gamma production (P < 0.05), compared with KO littermates. Indeed, PAR2 WT animals showed markedly greater microglial activation and T lymphocyte infiltration accompanied by worsened demyelination and axonal injury in the CNS compared with their PAR2 KO littermates. Enhanced neuropathological changes were associated with a more severe progressive relapsing disease phenotype (P < 0.001) in WT animals. These findings reveal previously unreported pathogenic interactions between CNS PAR2 expression and neuroinflammation with ensuing demyelination and axonal injury.

Aberrant cortical neurogenesis in a pediatric neuroAIDS model: neurotrophic effects of growth hormone

OBJECTIVE

To study the effects of HIV-1 and feline immunodeficiency virus (FIV) on neural stem cell viability, together with the neurotrophic properties of growth hormone (GH) in models of pediatric neuroAIDS.

DESIGN AND METHODS

Mouse neural stem cells were infected in vitro with a Sindbis virus vector (SIN-HIVenv) expressing the envelope protein from the brain-derived HIV-1 strain JR-FL using a vector expressing enhanced green fluorescent protein (SIN-EGFP) as control. Cell survival and alterations in expression of neural stem cell markers upon GH treatment was assessed. Neonatal cats were infected with a neurovirulent FIV strain and 6 weeks after infection treated with GH for 6 weeks. Twelve weeks post-infection, neural progenitor cell marker expression, neuronal loss and neuroinflammation in brain were examined using real time reverse transcription-PCR and immunohistochemical analyses.

RESULTS

HIV-1 envelope expression in neural stem cells reduced nestin expression (P < 0.05) and induced cell death (P < 0.001), which was blocked by GH. In the frontal cortex of FIV-infected cats neuroinflammation, loss of differentiated neurons (P < 0.01) and aberrant neuronal progenitor cell gene expression (P < 0.05) were observed. FIV envelope expression was detected in neural progenitor and monocytoid cells. GH treatment of FIV-infected animals induced insulin-like growth factor-1 expression in neurons (P < 0.01), enhanced neuronal survival (P < 0.01) and increased nestin expression (P < 0.05). Moreover, improved neurobehavioral performance (P < 0.01) and immunological status (P < 0.001) were observed, among GH-treated animals infected with FIV.

CONCLUSION

GH protects neural stem cells that are susceptible to lentivirus-mediated injury. Thus, GH may be a potential treatment for pediatric neuroAIDS because of its neurotrophic actions.

Human diseases reveal novel roles for neural laminins

Extracellular matrix molecules such as laminins have a central role in regulating cell behaviour. However, our understanding of their functions in the mammalian nervous system is incomplete. It is important to establish these functions, both for an understanding of normal development and to devise strategies to enhance repair. Here, we review how insights gained from human diseases caused by genetic mutations in laminins or their receptors have revealed significant and sometimes unexpected roles for laminins in neural stem cells, migrating neurons and myelinating glia, in both the PNS and CNS.

Selective down-regulation of the alpha6-integrin subunit in melanocytes by UVB light

In vivo, melanocytes bind to laminin (LM) molecules of the basement membrane (BM) via the integrins alpha3beta1 and alpha6beta1, and they adhere to neighbouring keratinocytes via E-cadherin. Only few studies have addressed the impact of ultraviolet (UV) light on the interaction of melanocytes with their microenvironment. In this report, we examined the influence of UVB irradiation on the expression of the most important melanocyte-adhesion molecules (E-, N-cadherin, alpha2-, alpha3-, alpha5-, alpha6-, alphaV-, beta1-, beta3-integrins and ICAM-1) in vitro by flow cytometry. We were able to demonstrate that the alpha6-integrin subunit is selectively and reversibly down-regulated by UVB in a dwzm 150ose-dependent manner. In comparison, keratinocytes lacked UVB-inducible alterations in the expression of alpha6-integrin. In the presence of LM-1, the UVB-induced down-regulation of alpha6-integrin in melanocytes was significantly reduced. Moreover, LM-1 increased the resistance of melanocytes to UVB-induced cell death, as measured by annexinV-binding analysis. This effect was reversed by preincubation with an alpha6-integrin-blocking antibody. By immunofluorescence, we could demonstrate that UVB leads to a dose-dependent internalization of alpha6-integrin, providing an obvious explanation for the down-regulation on the outer cell surface observed by flow cytometry. We suggest that adhesion to LM-1 through alpha6-integrin represents a protective mechanism for melanocytes to withstand UVB damage. Through alpha6-integrin internalization, sunburns might alter the interaction between melanocytes and the BM, resulting in apoptosis induced by loss of anchorage (anoikis). Repeated sunburns may then lead to the selection of a population of melanocytes which are capable of anchorage-independent survival, culminating in solar nevogenesis and melanoma development.

Integrin-growth factor interactions as regulators of oligodendroglial development and function

Central nervous system (CNS) development requires mechanisms for the regulation of cell number. Although growth factors are essential determinants of the proliferation and apoptosis that determine final numbers, the long-range nature of signals from diffusible growth factors makes them insufficient for the provision of the precise and localized signals required. Integration of integrin and growth factor receptor signaling in controlling cell behavior has been an important theme of research over the past several years. The focus of this review is on the mechanisms by which integrin-growth factor interactions regulate the development of oligodendrocytes and provide a mechanism for controlling, both in space and in time, oligodendrocyte numbers in the developing CNS.

The expression of matrix metalloproteinase-12 by oligodendrocytes regulates their maturation and morphological differentiation

Matrix metalloproteinases (MMPs) have important roles in many processes of the developing CNS requiring proteolytic activity such as the migration of neuronal precursors, axonal outgrowth, and vascularization. Another developmental event involving proteolysis is myelin formation, whereby the extensive processes elaborated from oligodendrocytes (OLs) enwrap axons. Here we find MMP-12 transcripts to be produced by OLs in much higher levels than other MMP members examined. MMP-12 activity correlated with the ability of OLs to extend processes in vitro, suggesting a role for MMP-12 in the morphological differentiation of OLs. This was corroborated by results that OL lineage cells from MMP-12 null mice were retarded in their ability to differentiate morphologically and that this deficiency was overcome by the exogenous addition of active MMP-12. Finally, the maturation of oligodendrocyte precursor cells (OPCs) to OLs was significantly reduced in cultures from MMP-12 null mice compared with wild-type controls. We conclude that OL lineage cells express MMP-12 during their maturation and that MMP-12 activity has functional involvement both in maturation of OPCs and in the ability of OPCs and OLs to extend processes.

Expression of T-type calcium channel splice variants in human glioma

In humans, three isoforms of the T-type (Ca(v)3.1) calcium-channel alpha(1) subunit have been reported as a result of alternate splicing of exons 25 and 26 in the III-IV linker region (Ca(v)3.1a, Ca(v)3.1b or Ca(v)3.1bc). In the present study, we report that human glioma express Ca(v)3.1 channels in situ, that splicing of these exons is uniquely regulated and that there is expression of a glioma-specific novel T-type variant (Ca(v)3.1ac). Seven human glioma samples were collected at surgery, RNA was extracted, and cDNA was produced for RT-PCR analysis. In addition, three glioma cell lines (U87, U563, and U251N), primary cultures of human fetal astrocytes, as well as adult and fetal human brain cDNA were used. Previously described Ca(v)3.1 splice variants were present in glioma samples, cultured cells and whole brain. Consistent with the literature, our results reveal that in the normal adult brain, Ca(v)3.1a transcripts predominate, while Ca(v)3.1b is mostly fetal-specific. RT-PCR results on glioma and glioma cell lines showed that Ca(v)3.1 expression in tumor cells resemble fetal brain expression pattern as Ca(v)3.1bc is predominantly expressed. In addition, we identified a novel splice variant, Ca(v)3.1ac, expressed in three glioma biopsies and one glioma cell line, but not in normal brain or fetal astrocytes. Transient expression of this variant demonstrates that Ca(v)3.1ac displays similar current-voltage and steady-state inactivation properties compared with Ca(v)3.1b, but a slower recovery from inactivation. Taken together, our data suggest glioma-specific Ca(v)3.1 gene regulation, which could possibly contribute to tumor pathogenesis.

Osteopontin is upregulated during in vivo demyelination and remyelination and enhances myelin formation in vitro

We have used in vitro oligodendrocyte differentiation and the in vivo remyelination model, the cuprizone model, to identify genes regulating oligodendrocyte function and remyelination. One of the genes we identified, osteopontin (opn), is a secreted glycoprotein with cytokine-like, chemotactic, and anti-apoptotic properties that contains an Arg-Gly-Asp (RGD) cell adhesion motif-mediating interactions with several integrins. Both microglia and astrocytes in demyelinating brain regions of cuprizone-fed mice expressed OPN protein. Recombinant OPN protein produced in a baculovirus expression system induced proliferation of both the rat CG-4 and the mouse Oli-neu oligodendrocyte precursor (OLP)-like cell lines in a dose-dependent manner. In addition, recombinant OPN treatment stimulated both myelin basic protein (MBP) synthesis and myelin sheath formation in mixed cortical cultures from embryonic mouse brain, an in vitro primary culture model of myelination. Interestingly, myelinating mixed cultures prepared from OPN(-/-) mice contained significantly less MBP compared to wild-type cultures after 17 days in culture. We propose that in the central nervous system, OPN may act as a novel regulator of myelination and remyelination.