TY  - GEN
AV  - public
N2  - Stroke is the number two cause of death in the western world and responsible for about 6
billion euros economic costs caused by illness in Germany annually. Approx. 80% of all
strokes are ischemic strokes and often develop from a focal cerebral hypoperfusion caused
by an occlusion of a major cerebral artery. Despite intensive research, the pathophysiological
and regenerative molecular mechanisms of stroke are only partially understood and no
causative therapy dealing with the consequences of the ischemic insult is available.
The blood-brain barrier (BBB) is a structure built up by endothelial cells, astrocytes and
perizytes. It separates the brain from the periphery while still allowing essential metabolites
to cross. During cerebral ischemia transendothelial connections are degraded and the BBB
permeability increases. This eventually causes vasogenic edema formation.
The vessel occlusion-induced local hypoperfusion unleashes a series of events called ischemic
cascade. Within minutes, hypoxia- and anoxia-induced intracellular mechanisms
lead to excitotoxicity, a process which describes neuronal cell death induction caused by
overstimulation of ionotropic glutamate receptors, such as NMDA receptors. Overactivation
of NMDA receptors leads to massive Ca2+ influx into neurons and triggers signaling cascades,
which among others cause mitochondrial dysfunction and eventually cell death.
Among others, the receptor tyrosine kinase EphB2 interacts with the NMDA receptor and
regulates NMDA receptor functions which lead to increased Ca2+ influx through the NMDA
receptor in the neuron.
The EphB/ephrin-B system is a receptor/ligand system, in which both, receptor and ligands
are membrane-bound requiring direct cell-to-cell contact in order to induce signaling processes.
Additionally, it had been shown that interaction of monocytic EphB2 with endothelial
ephrin-B ligands influences transendothelial migration and induces a pro-inflammatory phenotype
in endothelial cells. Therefore, this study aims to unravel the involvement of
EphB2/ephrin-B signaling during cerebral ischemia.
Cerebral ischemia was induced in Ephb2-deficient mice by transient middle cerebral artery
occlusion (MCAo) followed by different times of reperfusion. Histological, neurofunctional
and transcriptome analyses demonstrated that progression of stroke after 12-48 h of reperfusion
is attenuated in Ephb2-deficient mice as compared to wild-type (WT) mice. However,
multiple analyses (CT scans, immunofluorescence staining techniques, BBB permeability
assays) showed that this observation was not caused by an impact on vascular function.
On the other hand, further evaluations based on magnetic resonance imaging revealed that
cytotoxic edema formation during the first 6 h after reperfusion was diminished in Ephb2-
deficient mice as compared to WT mice pointing towards a neuronal cause.
In this context, analysis of brain lysates from WT animals revealed that activation (shown
by phosphorylation) of EphB2 was increased after 6 h of reperfusion in the ischemic hemisphere.
Next, the influence of EphB2 on neuronal NMDA receptor function was investigated.
It turned out that NMDA receptor-mediated Ca2+ transients and mitochondrial membrane
depolarization was decreased in Ephb2-deficient neurons as compared to WT neurons. As
mitochondrial dysfunction is a hallmark of stroke pathogenesis and a crucial step during
induction of excitotoxicity, cell death assays were performed. Widespread exposure to excitotoxic
stimuli did not reveal differences in the fraction of dead cells between WT and
Ephb2-deficient mice though.
Collectively, this is the first study to show that EphB2 is activated within 6 h after stroke and
decisively contributes to cytotoxic edema formation while BBB permeability is not influenced
by EphB2. In the absence of EphB2, progression of stroke is attenuated with respect to
infarct and edema volume as well as neurological function. NMDA receptor-mediated mitochondrial
responses are decreased in Ephb2-deficient neurons highlighting its importance
for the function of the NMDA receptor. Hence, EphB2-dependent mechanisms contribute to
signaling events that eventually worsen stroke progression.
A1  - Ernst, Anne-Sophie
ID  - heidok25152
UR  - https://archiv.ub.uni-heidelberg.de/volltextserver/25152/
Y1  - 2018///
TI  - Impact of the receptor tyrosine kinase EphB2 on cerebral ischemia in mice
ER  -