Astrocytes Stimulate Microglial Proliferation and M2 Polarization In Vitro through Crosstalk between Astrocytes and Microglia

Microglia are resident immune cells of the central nervous system that act as brain-specific macrophages and are also known to regulate the innate immune functions of astrocytes through secretory molecules. This communication plays an important role in brain functions and homeostasis as well as in neuropathologic disease. In this study, we aimed to elucidate whether astrocytes and microglia could crosstalk to induce microglial polarization and proliferation, which can be further regulated under a microenvironment mimicking that of brain stroke. Microglia in a mixed glial culture showed increased survival and proliferation and were altered to M2 microglia; CD11b−GFAP+ astrocytes resulted in an approximately tenfold increase in microglial cell proliferation after the reconstitution of astrocytes. Furthermore, GM-CSF stimulated microglial proliferation approximately tenfold and induced them to become CCR7+ M1 microglia, which have a phenotype that could be suppressed by anti-inflammatory cytokines such as IL-4, IL-10, and substance P. In addition, the astrocytes in the microglial co-culture showed an A2 phenotype; they could be activated to A1 astrocytes by TNF-α and IFN-γ under the stroke-mimicking condition. Altogether, astrocytes in the mixed glial culture stimulated the proliferation of the microglia and M2 polarization, possibly through the acquisition of the A2 phenotype; both could be converted to M1 microglia and A1 astrocytes under the inflammatory stroke-mimicking environment. This study demonstrated that microglia and astrocytes could be polarized to M2 microglia and A2 astrocytes, respectively, through crosstalk in vitro and provides a system with which to explore how microglia and astrocytes may behave in the inflammatory disease milieu after in vivo transplantation.

Astrocyte Stimulates Microglial Proliferation and M2 Polarization in vitro through Cross-Talk between Astrocyte and Microglia

Microglia are resident immune cells of the central nervous system such as brain-specific macrophages and also known to regulate the innate immune functions of astrocytes through secretory molecules. This conversation plays an important role in brain functions and homeostasis as well as in neuropathologic disease. In this study, we aimed to elucidate whether astrocytes and microglia can cross-talk to induce microglial polarization and proliferation, which can be further regulated under the brain stroke-mimic microenvironment. Microglia in mixed glial culture increased their survival and proliferation and altered to the M2 microglia, whose role was provided by CD11b-GFAP+ astrocytes by showing approximately tenfold increase in microglia cell proliferation after the astrocyte reconstitution. Furthermore, GM-CSF stimulated microglial proliferation approximately tenfold and induced to CCR7+ M1 microglia, whose phenotype could be suppressed by anti-inflammatory cytokines such as IL-4, IL-10, and Substance-P. Also, astrocyte in the microglia co-culture revealed A2 phenotype, which could be activated to A1 astrocyte by TNFα and IFNγ under the stroke-mimic condition. Altogether, astrocyte in the mixed glial culture stimulated the microglia proliferation and M2 polarization possibly through its acquisition of A2 phenotype, both of which could be converted to M1 microglia and A1 astrocytes under the inflammatory stroke-mimic environment. This study demonstrated that microglia and astrocyte can be polarized to M2 microglia and A2 astrocytes respectively through the cross-talk in vitro and provided a system to explore how microglia and astrocyte may behave in the inflammatory disease milieu after in vivo transplantation.

518 New In Vitro Model of Traumatic Brain Injury to Assess Biomaterial Based Regenerative Strategies

Introduction Penetrating traumatic brain injury (pTBI) management is largely supportive, with no clinically established regenerative therapies. Neurocompatible biomaterials offer a high potential to promote regenerative mechanisms but facile, high throughput, pathomimetic in vitro pTBI models for the developmental testing of neuro-materials is lacking. Method A mouse mixed glial culture system was utilised within which penetrating injuries could be induced. DuraGen PlusTM – an FDA approved neurosurgical grade biomaterial could be implanted into lesions to assess cell-biomaterial responses. Reactive gliosis (astrocytic morphological responses/GFAP expression) and microglial infiltration (Iba1 expression) were assessed/quantified. Results Key pathological features of pTBI were observed in the model, with the ability to (i) introduce reproducible lesions (diameter 949 ± 26 μm) and (ii) for DuraGen PlusTM to be implanted into lesions. Peri-lesional astrocytes displayed hypertrophic palisading morphologies and GFAP upregulation, analogous to gliosis in vivo. Significant microglial numbers infiltrated the DuraGen PlusTM implant at 7 days post-lesion (132.41 ± 15.83 cells/mm2) versus lesion only (82.04 ± 5.11 cells/mm2), p < 0.05). Conclusions We have developed a novel, neuropathomimetic pTBI model, wherein biomaterial implantation enables investigation of neural cell-biomaterial responses. This model can facilitate early-stage evaluation of novel biomaterials as high throughput, inexpensive and facile screening tool.

Generation of CSF1-Independent Ramified Microglia-Like Cells from Leptomeninges In Vitro

Although del Río-Hortega originally reported that leptomeningeal cells are the source of ramified microglia in the developing brain, recent views do not seem to pay much attention to this notion. In this study, in vitro experiments were conducted to determine whether leptomeninges generate ramified microglia. The leptomeninges of neonatal rats containing Iba1+ macrophages were peeled off the brain surface. Leptomeningeal macrophages strongly expressed CD68 and CD163, but microglia in the brain parenchyma did not. Leptomeningeal macrophages expressed epidermal growth factor receptor (EGFR) as revealed by RT-PCR and immunohistochemical staining. Cells obtained from the peeled-off leptomeninges were cultured in a serum-free medium containing EGF, resulting in the formation of large cell aggregates in which many proliferating macrophages were present. In contrast, colony-stimulating factor 1 (CSF1) did not enhance the generation of Iba1+ cells from the leptomeningeal culture. The cell aggregates generated ramified Iba1+ cells in the presence of serum, which express CD68 and CD163 at much lower levels than primary microglia isolated from a mixed glial culture. Therefore, the leptomeningeal-derived cells resembled parenchymal microglia better than primary microglia. This study suggests that microglial progenitors expressing EGFR reside in the leptomeninges and that there is a population of microglia-like cells that grow independently of CSF1.

Hypoxic Preconditioning Suppresses Glial Activation and Neuroinflammation in Neonatal Brain Insults

Perinatal insults and subsequent neuroinflammation are the major mechanisms of neonatal brain injury, but there have been only scarce reports on the associations between hypoxic preconditioning and glial activation. Here we use neonatal hypoxia-ischemia brain injury model in 7-day-old rats andin vitrohypoxia model with primary mixed glial culture and the BV-2 microglial cell line to assess the effects of hypoxia and hypoxic preconditioning on glial activation. Hypoxia-ischemia brain insult induced significant brain weight reduction, profound cell loss, and reactive gliosis in the damaged hemisphere. Hypoxic preconditioning significantly attenuated glial activation and resulted in robust neuroprotection. As early as 2 h after the hypoxia-ischemia insult, proinflammatory gene upregulation was suppressed in the hypoxic preconditioning group.In vitroexperiments showed that exposure to 0.5% oxygen for 4 h induced a glial inflammatory response. Exposure to brief hypoxia (0.5 h) 24 h before the hypoxic insult significantly ameliorated this response. In conclusion, hypoxic preconditioning confers strong neuroprotection, possibly through suppression of glial activation and subsequent inflammatory responses after hypoxia-ischemia insults in neonatal rats. This might therefore be a promising therapeutic approach for rescuing neonatal brain injury.

Enhanced Prostacyclin Synthesis by Adenoviral Gene Transfer Reduced Glial Activation and Ameliorated Dopaminergic Dysfunction in Hemiparkinsonian Rats

Prostacyclin (PGI2), a potent vasodilator and platelet antiaggregatory eicosanoid, is cytoprotective in cerebral circulation. It is synthesized from arachidonic acid (AA) by the sequential action of cyclooxygenase- (COX-) 1 or 2 and prostacyclin synthase (PGIS). Because prostacyclin is unstablein vivo, PGI2analogs have been developed and demonstrated to protect against brain ischemia. This work attempts to selectively augment PGI2synthesis in mixed glial culture or in a model of Parkinson’s disease (PD) by direct adenoviral gene transfer of prostacyclin biosynthetic enzymes and examines whether it confers protection in cultures orin vivo. Confluent mixed glial cultures actively metabolized exogenous AA into PGE2and PGD2. These PGs were largely NS398 sensitive and considered as COX-2 products. Gene transfer of AdPGIS to the cultures effectively shunted the AA catabolism to prostacyclin synthesis and concurrently reduced cell proliferation. Furthermore, PGIS overexpression significantly reduced LPS stimulation in cultures.In vivo, adenoviral gene transfer of bicistronic COX-1/PGIS to substantia nigra protected 6-OHDA- induced dopamine depletion and ameliorated behavioral deficits. Taken together, this study shows that enhanced prostacyclin synthesis reduced glial activation and ameliorated motor dysfunction in hemiparkinsonian rats. Prostacyclin may have a neuroprotective role in modulating the inflammatory response in degenerating nigra-striatal pathway.