The Yenari lab focuses on stroke as it pertains to stress proteins and inflammation, using both in vivo and in vitro models of ischemic brain injury. The first major area of research involves the study of the inflammatory response and how it potentiates injury from stroke. It is known that neutrophils infiltrate the brain within hours of stroke onset, and that cells of the monocyte lineage invade the brain a few days later. These cells were largely thought to remove necrotic debris from the brain and promote healing. It is now known that leukocytes and microglia (the brain's resident inflammatory cell) produce reactive oxygen species and proteases which may also exacerbate injury by breaking down the blood brain barrier and causing cerebral edema formation. Over the past few years, my laboratory has studied the inflammatory response following stroke, and how altering it might attenuate the extent of ischemic injury. Within the first 6 to 12 hours after a stroke, the first leukocyte populations found in the brain are the neutrophils. Neutrophils enter the brain because they express an integrin, CD11/CD18 that recognizes the intercellular adhesion molecule (ICAM-1) on the endothelial cell surface. ICAM-1 expression occurs when endothelium becomes activated, and this has been documented to occur following ischemia. Binding of CD11/CD18 to ICAM-1 permits neutrophil infiltration into damaged tissue. We found that by using a monoclonal antibody to block CD11/CD18, we could attenuate infarct size and neutrophil infiltration in a rabbit stroke model. We have also explored the significance of microglial activation in the brain and found that the addition of microglia to cultures of neurons, astrocytes and brain endothelial cells increases the extent of cell death compared to cultures lacking microglia. At the in vivo level, similar observations were made as blocking microglial activation reduced cerebral ischemic injury and also prevented disruption of the blood brain barrier and reduced the incidence of cerebral hemorrhage.

We then studied whether inhibition of the inflammatory response might explain the robust neuroprotective effect of mild hypothermia (cooling the brain by 4C). Although the neuroprotective effect of deep hypothermia (cooling the brain by more than 10C) has been attributed to reduction in metabolic rate, this cannot fully explain the equally robust effect of only modest reductions in brain temperature, nor can it explain why hypothermia can protect the brain when applied 2 hours after stroke onset, a time when most metabolic stores have already been depleted, and the peak of excitatory acid release has passed. We found that cooling the brain not only resulted in marked neuroprotection, but also reduced the number of infiltrating neutrophils. Over the past few years, my lab has found that mild hypothermia does indeed attenuates monocyte/macrophage infiltration
and microglial activation whether in a model of stroke or pure brain inflammation not associated with cell death. Mild hypothermia also attenuates generation of other inflammatory proteins such as adhesion molecules, inflammatory cytokines and nitric oxide synthase. Since many of these proteins are under the control of the inflammatory transcription factor, nuclear factor kappa B (NFkB), we found that hypothermia can regulate inflammatory gene expression by inhibiting activity of the kinase which activates NFkB's inhibitor protein. Therefore, the NF-kB system may
be an important target for improving outcome from stroke.

Recent data also indicate that heat shock proteins may also regulate NFkB, and thus, HSPs might inhibit inflammation at the transcriptional level. We have focused on the study of the 70 kDa heat shock protein (HSP70) which is highly upregulated following cerebral ischemia. Whether HSP70 served a protective or damaging role has been debated, and the existing data had been conflicting. However, we previously showed that overexpression of HSP70 was neuroprotective in various experimental models of stroke and brain ischemia. Overexpression of HSP70 in transgenic mice have decreased microglial activation, resulted in less inflammatory protein expression such as inflammatory cytokines, nitric oxide and the matrix metalloproteinases (MMP), enzymes expressed by inflammatory cells and thought to disrupt the extracellular matrix.

Together, these various areas of investigation should lead to an enhanced understanding of the pathogenesis of cerebral ischemia, and the identification of novel therapeutic targets that may someday be translated to the clinical level. Ongoing research in the lab focuses on further exploring the unique contribution of inflammatory cells to ischemia by applying bone marrow chimeras (mice transplanted with genetically altered bone marrow), mixed primary central nervous system culture, and gain/loss of function studies applying a variety of molecular techniques such as gene transfer or gene knockdown. We are also exploring anti-inflammatory and neuroprotective properties of potential therapeutic agents such as minocycline, statins and pyruvate.