Microglial involvement in neuroplastic changes following focal brain ischemia in rats

Histopathological Investigation of Different MCAO Modalities and Impact of Autologous Bone Marrow Mononuclear Cell Administration in an Ovine Stroke Model

Translational researchers and clinicians recommend the use of large animal models in preclinical stroke research. This represents an important part of a strategy aiming to prevent past translational failures in future therapeutic developments. Thirty-five Merino rams were subjected to sham surgery (n = 3), one-branch middle cerebral artery occlusion (MCAO, n = 8) or total MCAO (n = 24). Twelve animals from the latter group received intravenous administration of 4 × 10⁶ autologous mononuclear bone marrow cells (BM MNC) per kilogram 24 h after total MCAO. Animals were sacrificed at day 49 post MCAO. Histological investigations were performed to reveal (1) the impact of different MCAO modalities on a cellular level and (2) the influence of BM MNC therapy following stroke. Clear differences between one-branch and total MCAO were observed histologically with results being comparable to those seen in human patients. BM MNC treatment reduced final lesion extension, lymphocytic infiltration and axonal degeneration after MCAO. The sheep model may represent a feasible tool for translational stroke research as pathohistological findings mimic the situation in humans. Histological evidence was found for beneficial impact of autologous BM MNC therapy. Further studies are needed to assess the neurofunctional impact of the approach in the gyrencephalic brain.

Capillary blood flow around microglial somata determines dynamics of microglial processes in ischemic conditions

Microglia are the resident immune cells in the brain. Under normal conditions, resting ramified microglia constantly extend and retract fine processes while performing immunological surveillance. In ischemia, microglia become activated as demonstrated by morphological changes during deramification leading to transformation from ramified to amoeboid form. In vivo two-photon microscopy of enhanced green fluorescent protein (EGFP)-expressing microglia in mouse neocortex was used to examine microglial dynamics during the early periods of focal and global ischemia. A penumbra-like "area-at-risk" surrounded by a square-shaped area of severely hypoperfused tissue was created by laser-induced photothrombosis. The dynamics of microglial processes in the area-at-risk was strongly correlated with capillary blood flow (BF) measured within 10 μm of microglial somata. Changes in BF around distal microglial processes (>30 μm from somata) had no effect on microglial dynamics. A severe reduction of capillary BF near somata by 84% ± 6% resulted in initiation of microglial deramification, suggesting activation. A moderate decrease in BF near somata by 22% ± 5% or increase by 87% ± 10%, reflecting a redistribution of capillary BF, had no effect on microglial morphology. Complete BF loss during cardiac arrest (CA) or transient bilateral common carotid artery occlusion (BCCAO) entirely stalled all microglial processes without structural changes. Reperfusion after BCCAO induced recovery of microglial dynamics to preocclusion values. These findings suggest that during ischemia, the severe drop in BF around microglial somata coincides with morphological activation. However, this activation requires some residual BF, because complete perfusion loss (as during BCCAO and CA) did not support microglial deramification.

Neuroinflammation modulates distinct regional and temporal clinical responses in ALS mice

An inflammatory response is a pathological hallmark of amyotrophic lateral sclerosis (ALS), a relentless and devastating degenerative disease of motoneurons. This response is not simply a late consequence of motoneuron degeneration, but actively contributes to the balance between neuroprotection and neurotoxicity; initially infiltrating lymphocytes and microglia slow disease progression, while later, they contribute to the acceleration of disease. Since motor weakness begins in the hindlimbs of ALS mice and only later involves the forelimbs, we determined whether differential protective versus injurious inflammatory responses in the cervical and lumbar spinal cords explained the temporally distinct clinical disease courses between the limbs of these mice. Densitometric evaluation of immunohistochemical sections and quantitative RT-PCR (qRT-PCR) demonstrated that CD68 and CD11c were differentially increased in their spinals cords. qRT-PCR revealed that protective and anti-inflammatory factors, including BDNF, GDNF, and IL-4, were increased in the cervical region compared with the lumbar region. In contrast, the toxic markers TNF-α, IL-1β and NOX2 were not different between ALS mice cervical and lumbar regions. T lymphocytes were observed infiltrating lumbar spinal cords of ALS mice prior to the cervical region; mRNA levels of the transcription factor gata-3 (Th2 response) were differentially elevated in the cervical cord of ALS mice whereas t-bet (Th1 response) was increased in the lumbar cord. These results reinforce the important balance between specific protective/injurious inflammatory immune responses in modulating clinical outcomes and suggest that the delayed forelimb motor weakness in ALS mice is partially explained by augmented protective responses in the cervical spinal cords.

BDNF and GAP43 contribute to dynamic transhemispheric functional reorganization in rat brain after contralateral C7 root transfer following brachial plexus avulsion injuries

It is known that contralateral seventh cervical nerve (C7) root transfer after brachial plexus avulsion injuries (BPAI) causes interhemispheric cortical functional reorganization. However, the potential mechanisms and the role of neurotrophic factors and/or growth-associated protein expression in the process of cerebral reorganization are not well understood. The present study identified the expression of brain-derived neurotrophic factor (BDNF) and growth-associated protein 43 (GAP43) mRNA in primary motor cortex after contralateral C7 root transfer following BPAI. BDNF and GAP43 mRNA levels were significantly increased in brain samples at both 6 and 9 months after contralateral C7 root transfer following BPAI, in comparison with the samples from the rats with BPAI only. These findings indicate that BDNF and GAP43 may play an important role during the dynamic transhemispheric functional reorganization.

Stroke and the immune system: from pathophysiology to new therapeutic strategies

Stroke is the second most common cause of death worldwide and a major cause of acquired disability in adults. Despite tremendous progress in understanding the pathophysiology of stroke, translation of this knowledge into effective therapies has largely failed, with the exception of thrombolysis, which only benefits a small proportion of patients. Systemic and local immune responses have important roles in causing stroke and are implicated in the primary and secondary progression of ischaemic lesions, as well as in repair, recovery, and overall outcome after a stroke. However, potential therapeutic targets in the immune system and inflammatory responses have not been well characterised. Development of novel and effective therapeutic strategies for stroke will require further investigation of these pathways in terms of their temporal profile (before, during, and after stroke) and risk-to-benefit therapeutic ratio of modulating them.

Neurotoxic (+)-methamphetamine treatment in rats increases brain-derived neurotrophic factor and tropomyosin receptor kinase B expression in multiple brain regions

Methamphetamine (MA) is an abused stimulant which can result in cognitive deficits and monoamine depletions. Animal models of neurotoxic MA exposure show reductions in dopamine, serotonin, and their associated transporters. MA abuse can result in long-term attention, working memory, and executive function deficits in humans and deficits in route-based egocentric learning, novel object recognition, and novel odor preference in rodents. MA has also been shown to affect brain-derived neurotrophic factor (BDNF) in humans and rodents. This experiment examined the effects of a MA binge dosing regimen (10 mg/kg x 4 at 2 h intervals, s.c.) in Sprague-Dawley rats on BDNF, tropomyosin receptor kinase B (TrkB), and tyrosine hydroxylase (TH) mRNA expression, and plasma corticosterone. Tissues were collected 1, 7, and 24 h following the last MA dose. Expression of BDNF and TrkB mRNA was analyzed using in situ hybridization with cRNA probes. Frontal, parietal, and entorhinal cortical BDNF mRNA expression were increased by MA exposure at all time-points. Increases in BDNF mRNA were also seen in the hippocampal CA1, prefrontal cortex (PFC), piriform cortex, and locus coeruleus but only at specific times. TrkB mRNA expression was modified in several subregions of the hippocampus as well as in PFC and striatum. TH mRNA was increased at the 1 h time-point in the substantia nigra pars compacta with no differences noted at the other times. Corticosterone levels were increased at all three time-points. The findings suggest that BDNF and its receptor may be upregulated as a compensatory mechanism after MA exposure.

Pattern of invasion of the embryonic mouse spinal cord by microglial cells at the time of the onset of functional neuronal networks

Microglial cells invade the central nervous system during embryonic development, but their developmental functional roles in vivo remain largely unknown. Accordingly, their invasion pattern during early embryonic development is still poorly understood. To address this issue, we analyzed the initial developmental pattern of microglial cell invasion in the spinal cord of CX3CR1-eGFP mouse embryos using immunohistochemistry. Microglial cells began to invade the mouse embryonic spinal cord at a developmental period corresponding to the onset of spontaneous electrical activity and of synaptogenesis. Microglial cells reached the spinal cord through the peripheral vasculature and began to invade the parenchyma at 11.5 days of embryonic age (E11.5). Remarkably, at E12.5, activated microglial cells aggregated in the dorsolateral region close to terminals of dying dorsal root ganglia neurons. At E13.5, microglial cells in the ventral marginal zone interacted with radial glial cells, whereas ramified microglial cells within the parenchyma interacted with growing capillaries. At this age, activated microglial cells (Mac-2 staining) also accumulated within the lateral motor columns at the onset of the developmental cell death of motoneurons. This cell aggregation was still observed at E14.5, but microglial cells no longer expressed Mac-2. At E15.5, microglial cells were randomly distributed within the parenchyma. Our results provide the essential basis for further studies on the role of microglial cells in the early development of spinal cord neuronal networks in vivo.

Depression: a repair response to stress-induced neuronal microdamage that can grade into a chronic neuroinflammatory condition?

Depression is a major contributor to the global burden of disease and disability, yet it is poorly understood. Here we review data supporting a novel theoretical model for the biology of depression. In this model, a stressful life event leads to microdamage in the brain. This damage triggers an injury repair response consisting of a neuroinflammatory phase to clear cellular debris and a spontaneous tissue regeneration phase involving neurotrophins and neurogenesis. During healing, released inflammatory mediators trigger sickness behavior and psychological pain via mechanisms similar to those that produce physical pain during wound healing. The depression remits if the neuronal injury repair process resolves successfully. Importantly, however, the acute psychological pain and neuroinflammation often transition to chronicity and develop into pathological depressive states. This hypothesis for depression explains substantially more data than alternative models, including why emerging data show that analgesic, anti-inflammatory, pro-neurogenic and pro-neurotrophic treatments have antidepressant effects. Thus, an acute depressive episode can be conceptualized as a normally self-limiting but highly error-prone process of recuperation from stress-triggered neuronal microdamage.

Wnt1, FoxO3a, and NF-kappaB oversee microglial integrity and activation during oxidant stress

Elucidating the underlying mechanisms that govern microglial activation and survival is essential for the development of new treatment strategies for neurodegenerative disorders, since microglia serve not only as guardian sentries of the nervous system, but also play a significant role in determining neuronal and vascular cell fate. Here we show that endogenous and exogenous Wnt1 in inflammatory microglial cells is necessary for the prevention of apoptotic early membrane phosphatidylserine exposure and later DNA degradation, since blockade of Wnt1 signaling abrogates cell survival during oxidative stress. Wnt1 prevents apoptotic demise through the post-translational phosphorylation and maintenance of FoxO3a in the cytoplasm to inhibit an apoptotic cascade that relies upon the loss of mitochondrial membrane permeability, cytochrome c release, Bad phosphorylation, and activation of caspase 3 and caspase 1 as demonstrated by complimentary gene knockdown studies of FoxO3a. Furthermore, subcellular trafficking and gene knockdown studies of NF-kappaB p65 illustrate that microglial cell survival determined by Wnt1 during oxidative stress requires NF-kappaB p65. Our work highlights Wnt1 and the control of novel downstream transcriptional pathways as critical components for the oversight of nervous system microglial cells.

The temporal dynamics of poststroke neuroinflammation: a longitudinal diffusion tensor imaging-guided PET study with 11C-PK11195 in acute subcortical stroke

Animal experiments suggest that 2 different types of activated microglia (AMG) cells occur in the brain after a stroke: local AMG in the area of the infarct and remote AMG, which occurs along affected fiber tracts. We used (11)C-PK11195 PET to image AMG in vivo after stroke in humans in a prospective longitudinal study to investigate the temporal dynamics of AMG and relate local and remote AMG activity to pyramidal tract (PT) damage using diffusion tensor imaging (DTI).

METHODS

Eighteen patients underwent DTI-MRI, (11)C-PK11195 PET, and behavioral testing within 2 wk and 6 mo of acute subcortical stroke. In 12 patients, the PT was affected by the stroke (PT group), and in 6 patients it was not (non-PT group). Standardized volumes of interest (VOIs) were placed along the PT at the level of the brain stem, semioval center, and infarct. Tracer uptake ratios (ipsilateral to contralateral) were calculated for each VOI and related to tract damage (measured as fractional anisotropy ratio) and clinical outcome. Six controls underwent the same protocol but only once.

RESULTS

In both patient groups, local AMG activity in the infarct was increased initially and significantly decreased over the follow-up period. In contrast, remote AMG was detected only in the PT group in the brain stem along the affected tract and persisted during follow-up. No AMG was observed retrograde to the lesion at any time. Remote AMG activity along the affected PT in the brain stem correlated with initial PT damage as measured by DTI in the same tract portion. Local AMG activity in the infarct correlated with anterograde PT damage only at follow-up. After controlling for PT damage, initial AMG activity in the brain stem showed a positive correlation with clinical outcome, whereas persisting AMG activity in the infarct tended to be negatively correlated.

CONCLUSION

DTI-guided (11)C-PK11195 PET in acute subcortical stroke demonstrates differential temporal dynamics of local and remote AMG. Activity of both types related to anterograde PT damage as measured by DTI and might contribute differently to clinical outcome.

Microglia and neuroprotection: from in vitro studies to therapeutic applications

Microglia are the main immune cells in the brain, playing a role in both physiological and pathological conditions. Microglial involvement in neurodegenerative diseases is well-established, being microglial activation and neuroinflammation common features of these neuropathologies. Microglial activation has been considered harmful for neurons, but inflammatory state is not only associated with neurotoxic consequences, but also with neuroprotective effects, such as phagocytosis of dead neurons and clearance of debris. This brought to the idea of protective autoimmunity in the brain and to devise immunomodulatory therapies, aimed to specifically increase neuroprotective aspects of microglia. During the last years, several data supported the intrinsic neuroprotective function of microglia through the release of neuroprotective molecules. These data led to change the traditional view of microglia in neurodegenerative diseases: from the idea that these cells play an detrimental role for neurons due to a gain of their inflammatory function, to the proposal of a loss of microglial neuroprotective function as a causing factor in neuropathologies. This "microglial dysfunction hypothesis" points at the importance of understanding the mechanisms of microglial-mediated neuroprotection to develop new therapies for neurodegenerative diseases. In vitro models are very important to clarify the basic mechanisms of microglial-mediated neuroprotection, mainly for the identification of potentially effective neuroprotective molecules, and to design new approaches in a gene therapy set-up. Microglia could act as both a target and a vehicle for CNS gene delivery of neuroprotective factors, endogenously produced by microglia in physiological conditions, thus strengthening the microglial neuroprotective phenotype, even in a pathological situation.

Protection by D609 through cell-cycle regulation after stroke

Expressions of cell-cycle regulating proteins are altered after stroke. Cell-cycle inhibition has shown dramatic reduction in infarction after stroke. Ceramide can induce cell-cycle arrest by up-regulation of cyclin-dependent kinase (Cdk) inhibitors p21 and p27 through activation of protein phosphatase 2A (PP2A). Tricyclodecan-9-yl-xanthogenate (D609)-increased ceramide levels after transient middle cerebral artery occlusion (tMCAO) in spontaneously hypertensive rat (SHR) probably by inhibiting sphingomyelin synthase (SMS). D609 significantly reduced cerebral infarction and up-regulated Cdk inhibitor p21 and down-regulated phospho-retinoblastoma (pRb) expression after tMCAO in rat. Others have suggested bFGF-induced astrocyte proliferation is attenuated by D609 due to an increase in ceramide by SMS inhibition. D609 also reduced the formation of oxidized phosphatidylcholine (OxPC) protein adducts. D609 may attenuate generation of reactive oxygen species and formation of OxPC by inhibiting microglia/macrophage proliferation after tMCAO (please also see note added in proof: D609 may prevent mature neurons from entering the cell cycle at the early reperfusion, however may not interfere with later proliferation of microglia/ macrophages that are the source of brain derived neurotrophic factor (BDNF) and insulin-like growth factor (IGF-1) in offering protection). It has been proposed that D609 provides benefit after tMCAO by attenuating hypoxia-inducible factor-1alpha and Bcl2/adenovirus E1B 19 kDa interacting protein 3 expressions. Our data suggest that D609 provides benefit after stoke through inhibition of SMS, increased ceramide levels, and induction of cell-cycle arrest by up-regulating p21 and causing hypophosphorylation of Rb (through increased protein phosphatase activity and/or Cdk inhibition).