ApoE-directed therapeutics rapidly clear ß-amyloid and reverse deficits in AD mouse models.

Alzheimer's disease (AD) is associated with impaired clearance of ß-amyloid (Aß) from the brain, a process normally facilitated by apolipoprotein E (apoE). ApoE expression is transcriptionally induced through the action of the nuclear receptors peroxisome proliferator-activated receptor gamma and liver X receptors in coordination with retinoid X receptors (RXRs). Oral administration of the RXR agonist bexarotene to a mouse model of AD resulted in enhanced clearance of soluble Aß within hours in an apoE-dependent manner. Aß plaque area was reduced more than 50% within just 72 hours. Furthermore, bexarotene stimulated the rapid reversal of cognitive, social, and olfactory deficits and improved neural circuit function. Thus, RXR activation stimulates physiological Aß clearance mechanisms, resulting in the rapid reversal of a broad range of Aß-induced deficits.

Opposing synaptic regulation of amyloid-ß metabolism by NMDA receptors in vivo.

The concentration of amyloid-ß (Aß) within the brain extracellular space is one determinant of whether the peptide will aggregate into toxic species that are important in Alzheimer's disease (AD) pathogenesis. Some types of synaptic activity can regulate Aß levels. Here we demonstrate two distinct mechanisms that are simultaneously activated by NMDA receptors and regulate brain interstitial fluid (ISF) Aß levels in opposite directions in the living mouse. Depending on the dose of NMDA administered locally to the brain, ISF Aß levels either increase or decrease. Low doses of NMDA increase action potentials and synaptic transmission which leads to an elevation in synaptic Aß generation. In contrast, high doses of NMDA activate signaling pathways that lead to ERK (extracellular-regulated kinase) activation, which reduces processing of APP into Aß. This depression in Aß via APP processing occurs despite dramatically elevated synaptic activity. Both of these synaptic mechanisms are simultaneously active, with the balance between them determining whether ISF Aß levels will increase or decrease. NMDA receptor antagonists increase ISF Aß levels, suggesting that basal activity at these receptors normally suppresses Aß levels in vivo. This has implications for understanding normal Aß metabolism as well as AD pathogenesis.

Serotonin signaling is associated with lower amyloid-ß levels and plaques in transgenic mice and humans.

Aggregation of amyloid-ß (Aß) as toxic oligomers and amyloid plaques within the brain appears to be the pathogenic event that initiates Alzheimer's disease (AD) lesions. One therapeutic strategy has been to reduce Aß levels to limit its accumulation. Activation of certain neurotransmitter receptors can regulate Aß metabolism. We assessed the ability of serotonin signaling to alter brain Aß levels and plaques in a mouse model of AD and in humans. In mice, brain interstitial fluid (ISF) Aß levels were decreased by 25% following administration of several selective serotonin reuptake inhibitor (SSRI) antidepressant drugs. Similarly, direct infusion of serotonin into the hippocampus reduced ISF Aß levels. Serotonin-dependent reductions in Aß were reversed if mice were pretreated with inhibitors of the extracellular regulated kinase (ERK) signaling cascade. Chronic treatment with an SSRI, citalopram, caused a 50% reduction in brain plaque load in mice. To test whether serotonin signaling could impact Aß plaques in humans, we retrospectively compared brain amyloid load in cognitively normal elderly participants who were exposed to antidepressant drugs within the past 5 y to participants who were not. Antidepressant-treated participants had significantly less amyloid load as quantified by positron emission tomography (PET) imaging with Pittsburgh Compound B (PIB). Cumulative time of antidepressant use within the 5-y period preceding the scan correlated with less plaque load. These data suggest that serotonin signaling was associated with less Aß accumulation in cognitively normal individuals.