Inhibiting amyloid beta (1-42) peptide-induced mitochondrial dysfunction prevents the degradation of synaptic proteins in the entorhinal cortex.

Increasing evidence suggests that mitochondrial dysfunction and aberrant release of mitochondrial reactive oxygen species (ROS) play crucial roles in early synaptic perturbations and neuropathology that drive memory deficits in Alzheimer's disease (AD). We recently showed that solubilized human amyloid beta peptide 1-42 (hAß1-42) causes rapid alterations at glutamatergic synapses in the entorhinal cortex (EC) through the activation of both GluN2A- and GluN2B-containing NMDA receptors. However, whether disruption of mitochondrial dynamics and increased ROS contributes to mechanisms mediating hAß1-42-induced synaptic perturbations in the EC is unknown. Here we assessed the impact of hAß1-42 on mitochondrial respiratory functions, and the expression of key mitochondrial and synaptic proteins in the EC. Measurements of mitochondrial respiratory function in wild-type EC slices exposed to 1 µM hAß1-42 revealed marked reductions in tissue oxygen consumption and energy production efficiency relative to control. hAß1-42 also markedly reduced the immunoexpression of both mitochondrial superoxide dismutase (SOD2) and mitochondrial-cytochrome c protein but had no significant impact on cytosolic-cytochrome c expression, voltage-dependent anion channel protein (a marker for mitochondrial density/integrity), and the immunoexpression of protein markers for all five mitochondrial complexes. The rapid impairments in mitochondrial functions induced by hAß1-42 were accompanied by reductions in the presynaptic marker synaptophysin, postsynaptic density protein (PSD95), and the vesicular acetylcholine transporter, with no significant changes in the degradative enzyme acetylcholinesterase. We then assessed whether reducing hAß1-42-induced increases in ROS could prevent dysregulation of entorhinal synaptic proteins, and found that synaptic impairments induced by hAß1-42 were prevented by the mitochondria-targeted antioxidant drug mitoquinone mesylate, and by the SOD and catalase mimetic EUK134. These findings indicate that hAß1-2 can rapidly disrupt mitochondrial functions and increase ROS in the entorhinal, and that this may contribute to synaptic dysfunctions that may promote early AD-related neuropathology.

G protein-coupled estrogen receptor-1 enhances excitatory synaptic responses in the entorhinal cortex.

Activation of estrogen receptors is thought to modulate cognitive function in the hippocampus, prefrontal cortex, and striatum by affecting both excitatory and inhibitory synaptic transmission. The entorhinal cortex is a major source of cortical sensory and associational input to the hippocampus, but it is unclear whether either estrogens or progestogens may modulate cognitive function through effects on synaptic transmission in the entorhinal cortex. This study assessed the effects of the brief application of either 17-ß estradiol (E2) or progesterone on excitatory glutamatergic synaptic transmission in the female rat entorhinal cortex in vitro. Rats were ovariectomized on postnatal day (PD) 63 and also received subdermal E2 implants to maintain constant low levels of circulating E2 on par with estrus. Electrophysiological recordings from brain slices were obtained between PD70 and PD86, and field excitatory postsynaptic potentials (fEPSPs) reflecting the activation of the superficial layers of the entorhinal cortex were evoked by the stimulation of layer I afferents. The application of E2 (10 nM) for 20 min resulted in a small increase in the amplitude of fEPSPs that reversed during the 30-min washout period. The application of the ERα agonist propylpyrazoletriol (PPT) (100 nM) or the ß agonist DPN (1 µM) did not significantly affect synaptic responses. However, the application of the G protein-coupled estrogen receptor-1 (GPER1) agonist G1 (100 nM) induced a reversible increase in fEPSP amplitude similar to that induced by E2. Furthermore, the potentiation of responses induced by G1 was blocked by the GPER1 antagonist G15 (1 µM). Application of progesterone (100 nM) or its metabolite allopregnanolone (1 µM) did not significantly affect synaptic responses. The potentiation of synaptic transmission in the entorhinal cortex induced by the activation of GPER1 receptors may contribute to the modulation of cognitive function in female rats.

Amyloid-ß (1-42) peptide induces rapid NMDA receptor-dependent alterations at glutamatergic synapses in the entorhinal cortex.

The hippocampus and entorhinal cortex (EC) accumulate amyloid beta peptides (Aß) that promote neuropathology in Alzheimer's disease, but the early effects of Aß on excitatory synaptic transmission in the EC have not been well characterized. To assess the acute effects of Aß1-42 on glutamatergic synapses, acute brain slices from wildtype rats were exposed to Aß1-42 or control solution for 3 hours, and tissue was analyzed using protein immunoblotting and quantitative PCR. Presynaptically, Aß1-42 induced marked reductions in synaptophysin, synapsin-2a mRNA, and mGluR3 mRNA, and increased both VGluT2 protein and Ca2+-activated channel KCa2.2 mRNA levels. Postsynaptically, Aß1-42 reduced PSD95 and GluN2B protein, and also downregulated GluN2B and GluN2A mRNA, without affecting scaffolding elements SAP97 and PICK1. mGluR5 mRNA was strongly increased, while mGluR1 mRNA was unaffected. Blocking either GluN2A- or GluN2B-containing NMDA receptors did not significantly prevent synaptic changes induced by Aß1-42, but combined blockade did prevent synaptic alterations. These findings demonstrate that Aß1-42 rapidly disrupts glutamatergic transmission in the EC through mechanisms involving concurrent activation of GluN2A- and GluN2B-containing NMDA receptors.

Molecular mechanisms of neurodegeneration in the entorhinal cortex that underlie its selective vulnerability during the pathogenesis of Alzheimer's disease.

The entorhinal cortex (EC) is a vital component of the medial temporal lobe, and its contributions to cognitive processes and memory formation are supported through its extensive interconnections with the hippocampal formation. During the pathogenesis of Alzheimer's disease (AD), many of the earliest degenerative changes are seen within the EC. Neurodegeneration in the EC and hippocampus during AD has been clearly linked to impairments in memory and cognitive function, and a growing body of evidence indicates that molecular and functional neurodegeneration within the EC may play a primary role in cognitive decline in the early phases of AD. Defining the mechanisms underlying molecular neurodegeneration in the EC is crucial to determining its contributions to the pathogenesis of AD. Surprisingly few studies have focused on understanding the mechanisms of molecular neurodegeneration and selective vulnerability within the EC. However, there have been advancements indicating that early dysregulation of cellular and molecular signaling pathways in the EC involve neurodegenerative cascades including oxidative stress, neuroinflammation, glia activation, stress kinases activation, and neuronal loss. Dysfunction within the EC can impact the function of the hippocampus, which relies on entorhinal inputs, and further degeneration within the hippocampus can compound this effect, leading to severe cognitive disruption. This review assesses the molecular and cellular mechanisms underlying early degeneration in the EC during AD. These mechanisms may underlie the selective vulnerability of neuronal subpopulations in this brain region to the disease development and contribute both directly and indirectly to cognitive loss.This paper has an associated Future Leader to Watch interview with the first author of the article.