Glutamate-dependent calcium influx through NMDA Receptors (NMDA R) is essential for neuronal development and function. Typically composed of two NR1 and two NR2 subunits, heteromeric NMDA R proteins localize to excitatory synapses, where their actions are fundamental to the process of synaptic plasticity. Studies have shown that Eph receptors, a family of transmembrane tyrosine kinases that bind Ephrin ligands, play an important role in controlling NMDA R physiology. In response to EphB1-3 knockout, NMDA R expression is homeostatically upregulated in mouse brain but targeting to the synapse is lost, suggesting Eph receptors regulate synaptic localization and/or retention of NMDA R.1 More specifically, EphB2 has been shown to regulate NMDA R trafficking and function, through direct interaction and via Src-mediated tyrosine phosphorylation.1, 2, 3, 4
Previous research has investigated the possibility that disrupted EphB2-NMDA R binding is relevant to the development of Alzheimer’s disease (AD), a condition that is characterized by severe synaptic impairment. Biochemical analysis of human brain tissue revealed that the expression of both proteins is decreased in AD patients, when compared to age-matched non-demented controls.5 Amyloid-beta protein (A beta), the hypothesized molecular cause of AD, is known to impair NMDA R trafficking and synaptic plasticity.6 Moreover, in transgenic mouse models of AD that overexpress A beta, a reduction in EphB2 expression was shown to precede the onset of spatial memory deficits.5
A recent study by Cissé et al. tested the hypothesis that Abeta-induced disruption of EphB2-NMDA R biology contributes to the impairments in synaptic plasticity and cognitive function that characterize AD.7 Although AD is pathologically defined by the presence of senile plaques in the brain that are composed of insoluble Abeta, neuronal dysfunction is believed to be induced by soluble oligomeric forms of A beta.8, 9 Preliminary experiments showed that synthetic A beta oligomers and EphB2 co-immunoprecipitated from homogenates of rat primary hippocampal neurons and that this interaction was dependent on the extracellular fibronectin type III repeat domain of EphB2. Consistent with an involvement in NMDA R dysfunction, treatment of cultured neurons with A beta oligomers significantly decreased EphB2 expression, an effect that was blocked by the proteasome inhibitor Lactacystin.
To investigate the functional consequence of EphB2 depletion, the authors created lentiviral vectors that express anti-EphB2 short hairpin RNA (Lenti-sh-EphB2), and conducted electrophysiological studies using acute hippocampal slices from Lenti-sh-EphB2 injected mice. Whole-cell patch-clamp recordings from hippocampal granule cells in the dentate gyrus revealed robust impairments in long-term potentiation (LTP) following EphB2 knockdown, supporting a functional role for EphB2 in synaptic plasticity. To explore the therapeutic potential of augmenting EphB2 levels, a lentivirus expressing EphB2 was injected into the dentate gyri of the hAPP transgenic mouse model of AD. hAPP mice express reduced levels of EphB2 and display selective NMDA Rdependent impairments in synaptic strength and plasticity.5 Encouragingly, increasing the expression of EphB2 reversed the LTP deficits observed in hAPP mice and had no effect on synaptic plasticity in non-transgenic control littermates. Furthermore, behavioral studies confirmed that restoring EphB2 expression to normal endogenous levels reversed age-dependent learning and memory deficits in hAPP mice. Collectively, these findings suggest that A beta oligomers compromise synaptic transmission by inducing proteasomal degradation of EphB2, and internalization of NMDA R. These data suggest that increasing EphB2 expression levels may represent a novel therapeutic strategy for the treatment of AD.
However, other studies suggest that enhancing EphB2-NMDA R interactions, might have detrimental effects. A report by Attwood et al. indicated that an aberrant increase in EphB2-NMDA R signaling in the amygdala may underlie the development of stress-induced anxiety disorders.10 Attwood and colleagues present a model in which stress promotes the cleavage of EphB2 in the amygdala by the serine protease Neuropsin. EphB2 proteolysis at the plasma membrane disrupts EphB2- NMDA R interactions, resulting in enhanced EphB2 turnover, and further binding of NMDA R to new EphB2 molecules. This dynamic EphB2-NMDA R interaction leads to augmented NMDA R activity, increased Fkbp5 gene expression, and manifestation of the behavioral signatures of anxiety. A central role for Neuropsin-dependent EphB2 cleavage was supported by in vivo experiments. In Neuropsin knockout mice, injection of recombinant Neuropsin was required for the development of stressinduced anxiety, an effect that was blocked by the co-administration of an anti-EphB2 antibody.
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|Disruption of EphB2-NMDA Receptor Dynamics Promotes Memory Impairment and Anxiety Disorders. The Ephrin ligand receptor EphB2 is known to physiologically regulate NMDA Receptor (NMDA R) function. Recent studies indicate that modulations to this relationship may underlie the development of two prevalent neurological conditions. Binding of the Alzheimer’s disease toxin, Amyloid-beta protein (A beta), to EphB2 prevents EphB2-NMDA R interactions, leading to NMDA R internalization and memory impairment (left). In contrast, stress-induced cleavage of EphB2 by Neuropsin causes dissociation of EphB2 from NMDA Receptor subunit NR1 and increased turnover of EphB2 (right).1 Further EphB2-NR1 binding enhances NMDA R activity and promotes the development of anxiety disorders.2|
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