Synaptic hyperactivity and BDNF induction are early neuronal responses to sub-lethal beta-amyloid concentrations in organotypic hippocampal cultures

The β-amyloid peptide (Aβ) has been identified as a key molecular factor in the etiology of AD. Aβ overproduction leads to protein build-up and abnormal aggregation into toxic oligomers which progressively assemble into fibrils. This process is slow and the culminating events are neuronal death and formation of extensive extracellular brain deposits. Aβ toxicity is directed to synaptic function with significant synaptic loss when neuronal degeneration is ongoing. Little is yet known on the exact mechanisms preceding synaptic dysfunction coinciding with pre-symptomatic stages of AD. As these events likely represent a critical time span for preventive intervention, further studies are needed. To this end we used an in vitro model of organotypic hippocampal slice cultures (OHC) exposed to low, sub-lethal concentrations of Aβ in order to re-create a condition of slow maturation of neuronal damage in the hippocampus. Treatment of OHC with Aβ leads to early and transient (16-72 h for 2 μM A Aβ(25-35) and 7 d for 500 nM Aβ(1-42)) increase of synaptic proteins synaptophysin, synapsin and PSD95, followed by their decrease coincident with neuronal death (7d or 14d). At 72 h of Aβ(25-35) (2 μM) exposure, higher levels of released glutamate and increased loading and unloading of FM 1-43-labeled synaptic vesicles point to increased synaptic activity. All these effects are prevented when OHC are pre-exposed to neuroprotective agent 17-β-estradiol (10 nM). BDNF is an important neurotrophic factor expressed at high levels by neuronal and glial cells in the brain. Recently, a biphasic pattern of expression similar to that we observed for synaptic proteins has been shown for BDNF in AD, with an increase in early stages and reduction in advanced stages of disease. Also, BDNF up-regulation has been linked to sustained microglial activation and release of TNFalfa. In order to test the involvement of BDNF in early synaptic changes induced by Aβ, BDNF levels were evaluated by Western blot. A significant increase of BDNF was detected in OHC exposed to Aβ(25-35) for 72 h or Aβ(1-42) for 7 d. To assess the contribution of each cell type to BDNF increase, its levels were also assessed in primary neurons, astrocytes and microglia treated with Aβ(25-35) 25 μM for 18 h. Early increase of synaptic components and activity, likely mediated by BDNF, may represent an initial step that precedes synaptic loss and neuronal death and may be interpreted as an attempted neuronal response to Aβ toxicity appearing as a new potential target for intervention in AD.