P4‐031: Suppression of drebrin gene expression causes Alzheimer's‐like synapse loss in primary hippocampal neurons in culture

Abstract

Alzheimer's disease (AD) is characterized by the progressive neuronal and synapse loss in brain regions associated with cognitive dysfunction as well as the deposition of amyloid plaques and neurofibrillary tangles. Synapse loss occurs early in the pathogenesis of AD and is regarded as the basis of dementia as observed in both patients and related transgenic mouse models. Therefore, restoration, prevention and/or reduction of synapse loss represents a major therapeutic target for AD. Excitatory (glutamatergic) synapses are formed between presynaptic axon terminals and highly specialized protrusions on the postsynaptic dendrites known as dendritic spines. In AD, a significant reduction of spine density and number has been observed. Filamentous actin (F-actin) is the major structural element of dendritic spines. Drebrin, a neuron-specific F-actin binding protein, is concentrated in dendritic spines and is known to play a role in dendritic spine morphology via modulation of actin dynamics. In AD, a significant downregulation of drebrin has been observed. Loss of drebrin has been suggested to destabilize actin cytoskeleton leading to dendritic pathology. The mechanism responsible for drebrin loss remains to be elucidated. Western blotting analysis has been used to measure drebrin protein level in Egr-1 KO mice as well primary hippocampal nuerons overexpressing Egr-1 via lentivirus-mediated infection. Drebrin mRNA level in primary hippocampal neurons was measured using qRT-PCR. In this study, we have found significant elevation of drebrin protein level in mice brain deficient in Early Growth Response 1 (Egr-1) transcription factor known to be upregulated in AD brain. When overexpression in primary hippocampal neurons in culture, Egr-1 causes reduction in drebrin level and decrease in the number of dendritic spines. In silico RVista analysis revealed three putative Egr-1 binding sites within the drebrin promoter. In vitro qRT-PCR showed a dose-dependent decrease of drebrin mRNA level in Egr-1 overexpressing neurons. Our data demonstrate that Egr-1 is an in vivo transcription suppressor of drebrin gene expression and suggest that Egr-1 plays a role in AD synaptic pathology by reducing drebrin level in the brain.