ESN Research Strategies

The 'translational roadblock'

Clinical strokes are of heterogeneous etiology (stroke subtypes). Ischemic stroke, which represents more than 80% of all strokes, is caused by the occlusion of one or more cerebral arteries and is often induced by thromboembolism. Ischemic stroke subtypes have different risk factor profiles, with consequences on pathogenesis and prognosis. The last 25 years have seen remarkable progress in the understanding of the pathophysiology of ischemic stroke to which members of this consortium have significantly contributed. However, translation of this knowledge into effective therapy showed only limited success: Early reperfusion is presently the only therapeutic intervention in ischemic stroke with proven efficacy. The reasons for the 'translational roadblock' in the treatment of stroke have to be analyzed in the light of our current understanding of stroke pathophysiology and what is known about regeneration and repair.

Mechanisms and therapeutic strategies

Once a brain vessel has become occluded, a complex series of cellular and molecular events is rapidly set in motion. Cells depolarize and swell, excitatory amino acids and K+ ions are released while intracellular Ca2+ levels soar ('excitotoxicity'). Numerous enzymes are activated, which together may acutely and terminally damage brain cells, leading to infarction within hours (overview see Dirnagl et al. 1999). It is this acute phase of focal cerebral ischemia which was the focus of the neuroprotection trials of recent decades. Although early mechanisms may have the largest impact on overall tissue destruction and neurological deficit, approaches to limit early damage are limited by logistical problems in minimizing the 'symptom to needle' time. Animal models as well as clinical observations have demonstrated that after a rather dramatic phase of early damage the lesion may indeed continue to grow many hours and even days after the onset of ischemia (secondary lesion propagation). Targeting the underlying mechanisms may widen the time window for treatment. The research of the ESN consortium will therefore focus on mechanisms and therapeutic strategies which will either prolong the therapeutic window, or act on mechanisms which are involved in the delayed development and propagation of damage.

Pathomechanisms of cerebral ischemia - a 'double-edged sword'

It is important to note that the pathomechanisms of cerebral ischemia and their interactions turned out to be exceedingly complex. Specifically, many mechanisms act as a double-edged sword, with both beneficial and deleterious actions: Reperfusion is the only effective pharmacological therapy, yet may contribute to damage via reperfusion injury (Benchenane et al. 2004). Glutamate is a major player in excitotoxicity, and blocking glutamate receptors is neuroprotective. However, glutamatergic neurotransmission is essential for normal brain function and a key driving force of reorganization and synaptogenesis after injury (Bernabeu and Sharp, 2000). Nitric oxide (NO) derived from endothelia may increase blood flow while neuronal and inducible NO synthase may contribute to the formation of peroxynitrite and hydroxyl anions (Iadecola et al., 1997; Endres et al., 2004). Inflammation promotes secondary expansion of the lesion, but is also responsible for clearing debris and, most importantly, provides the necessary environment for regeneration and repair (Kerschensteiner et al,. 1999; Manoonkitiwongsa et al., 2001). Formation of a glial scar may contain the lesion and impede its progression, but at the same time produces a barrier for axonal sprouting and thus regeneration of function (Nedergard and Dirnagl, 2005). Apoptosis or other forms of programmed death of neurons contribute to lesion growth, but are key negative regulators of inflammation (Zipfel et al., 2000). Stroke induces immunodepression, which may prevent the development of autoreactivity, but at the price of an increased susceptibility for infection (Meisel et al., 2005). This list of 'Janus-faced' mechanisms could be further extended with many items. Time, cellular context and stimulus intensity are important in determining whether the same molecule, signaling pathway, or cell will partake in destruction or repair. Some of the failures in the clinical stroke trials of the past may have resulted from wrong timing of treatment, and / or a cancellation of positive by negative effects of the drug tested. It is thus a key strategy of our proposed research to disentangle protective from destructive mechanisms, to establish therapies (and their appropriate timing), which specifically foster protective and/or inhibit destructive signaling.

ESN research goals for unraveling the complex pathophysiology of stroke

Based on the pioneering work of members of this consortium, and our thorough analysis of the current challenges and opportunities of improving stroke outcome (Meairs et al. 2006, Endres et al. 2008), the European Stroke Network will focus on reducing potential negative effects on brain tissue of therapeutic reperfusion by tPA, inducing endogenous neuroprotection, stroke-induced inflammation, the consequences on outcome of the interaction of the brain with the immune system after stroke, and on fostering repair and regeneration of lost brain function after stroke.

Pathobiology of cerebral vessels

As compared to research on the cellular and molecular events of neuronal injury in stroke, less attention has been devoted to the pathobiology of cerebral blood vessels, especially to the interaction between cerebrovascular cells (endothelial and smooth-muscle cells, pericytes and cells of the adventitia) and other brain cells. Consequently, there are large gaps in our understanding of the heterogeneity of cerebral blood vessels as compared with vessels in other vascular territories.

ESN research will focus on cerebral blood vessels and their relationships with other brain cells. This approach is justified by a growing body of evidence indicating that neurons, glia and vascular cells are closely related developmentally, structurally and functionally. The term “neurovascular unit” highlights the intimate functional relationships between these cells and their coordinated pattern of reaction to injury.

The neurovascular unit

The neurovascular unit (NVU) consists of a complex cellular system including circulating blood elements, highly specialized endothelial cells, a high number of pericytes embedded in the endothelial cell basement membrane, perivascular antigen presenting cells, astrocytic endfeet and associated parenchymal basement membrane and neurons Although the endothelial cells form the blood-brain barrier (BBB), i.e. the diffusion barrier, proper alone they are insufficient to account for the unique barrier properties of CNS microvessels, rather it is their interactions with extracellular matrix and cross-talk with adjacent cells that are pre-requisites for barrier function. These interactions in turn control/influence endothelial cell morphology, biochemistry and function that make BBB endothelial cells unique and distinguishable from any other endothelial cell in the body. Under physiological conditions, the BBB endothelial cells inhibit transcellular passage of molecules across the barrier by an extremely low pinocytotic activity and restrict the paracellular diffusion of hydrophilic molecules due to an elaborate network of complex tight junctions (TJ) between the endothelial cells. In contrast to epithelial TJs, BBB endothelial TJs are sensitive to molecular changes within the NVU, i.e. their function is impaired under pathological conditions such as stroke. In contrast, in order to meet the high metabolic needs of the CNS tissue, specific transport systems selectively expressed in the capillary BBB endothelial cell membranes mediate the directed transport of nutrients into the CNS or of toxic metabolites out of the CNS. Apart from the endothelial cell monolayer per se, disruptions or defects in the underlying basement membrane structures, pericyte layer or ensheathing astrocyte endfeet have all been shown to compromise the barrier function of CNS microvessels, emphasizing the importance of the cross talk between these cell and ECM layers in conferring the unique barrier properties of the CNS vessels.