Background: Cell-cell communication within the “neurovascular unit”—brain endothelium, neurons, glia, and extracellular matrix—and the cross-talk between the systemic circulation and the brain largely determines stroke outcome. Maintenance of the microenvironment—keeping a balance between cytokine and growth factor production and removal of debris—are essential functions of microglial cells, resident macrophages of the C NS . In response to injury, microglial cells undergo a graded process of activation, including morphological transformation and acquisition of one or more functions: upregulation of surface antigens and receptors necessary for production of inflammatory cytokines and cytotoxic mediators, migratory activity, proliferation, and phagocytosis. In vivo data from acute neurodegeneration models (cerebral ischemia and trauma) have suggested that more severe brain injury is associated with higher macrophage densities and that inhibiting synthesis of toxic molecules produced in microglia can be neuroprotective. Microglial cells may act as “double-edged sword” in brain repair. Depending on microenvironment, they can either suppress or facilitate neurogenesis in models of C NS injury, but the ways to protect or repair the brain by changing the microglial phenotype remain essentially unknown.

My lab was the first to create a model clinically relevant to arterial stroke in term babies, a focal transient middle cerebral artery occlusion model in the postnatal day 7 rat. Using this model and a similar model in adult rodents, we have obtained evidence that immaturity affects both brain injury and recovery after stroke by identifying major differences between adults and neonates in the post-ischemic status of the blood-brain barrier (BBB) and leukocyte behavior. Our data in both adult and neonatal stroke have also shown that while a potential for brain self-repair exists, endogenous neurogenesis is short-lived and ineffective .

Major Goals: 1) determine the mechanisms by which scavenger receptors on microglial cells affect removal of apoptotic neurons after stroke; 2) identify signaling pathways in microglial cells that enhance angiogenesis and neurogenesis after stroke; and 3) determine the role of neutrophil chemoattractant proteins in modulating the blood-brain barrier after stroke .

On-going Research:

Microglial cells as modulators of acute stroke injury.
Our recent studies have shown that microglial activation and accumulation of several inflammatory cytokines occur rapidly after ischemia-reperfusion. Using a number of pharmacological approaches, we have demonstrated that the inability to attenuate the increased levels of inflammatory cytokines in the brain after ischemia-reperfusion in neonates is associated with only limited/transient neuroprotection. Disruption of monocyte trafficking via inhibition of the MCP-1/CCR2 signaling is also not sufficient for protection. Considering that inflammation adversely affects the ability of microglial cells to phagocytose apoptotic cells, thus allowing dying cells to undergo post-apoptotic necrosis, we are interested in the role of the interaction between dying neurons and microglial scavenger receptors. These receptors are important for recognition, uptake, and processing of apoptotic debris. To address this question, we use a number of in vivo and in vitro approaches in mice deficient of scavenger receptors or caspase-3.

Microglial cells as modulators of neurogenesis and repair after stroke.
The reparative potential of microglia in the newborn brain after stroke is not known. We are exploring whether neurogenesis and long-term functional outcomes of neonatal stroke can be improved by ablation or alteration of the microglial phenotype, which is achieved by using nanotechnology approaches to selectively target receptor-mediated signalling in microglia. We use an array of biochemical techniques to define the role of microglia in migration and maturation of neuroprogenitor cells. To obtain a comprehensive view of how manipulated microglial phenotypes affect injury evolution and long-term outcome of stroke during the early postnatal period , we use a multi-modality approach that consists of conventional non-invasive imaging, MRI and DTI (diffusion tensor imaging), combined with an array of biomarkers .

Neutrophil chemoattractant proteins as modulators of the blood-brain barrier after stroke.
Functional failure of the BBB is believed to contribute to injury after stroke. At the same time, drugs that otherwise do not penetrate through the BBB, can enter the brain through the disrupted BBB. Our recent data show that the patterns of BBB disruption and neutrophil extravasation differ acutely after stroke in adult and neonatal rodents, with an increased permeability in adult, but not in neonatal, rats. Minimal neutrophil extravasation into injured brain tissue in neonates after stroke is seen despite a marked transient increase of a major neutrophil chemoattractant protein CINC-1 in the brain. In on-going studies, we explore the mechanisms of the disconnect between elevated, CINC-1, levels and the lack of neutrophil extravasation and the preserved BBB integrity. In particular, we are interested in neutrophil maturation and factors that limit migration of these cells as modulators of BBB leakiness.