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Low density lipoprotein receptor related protein 1 (LRP1) is a type I transmembrane receptor that mediates the endocytosis of various ligands, including apolipoproteins and tau. Both proteins are genetically and pathologically linked to Alzheimer's disease (AD) (1,2). Human apolipoprotein E (ApoE) is a component of circulating lipoproteins when three human genetic ApoE variants, ApoE2, ApoE3, and ApoE4, exhibit distinct receptor-binding properties and differentially contribute to AD progression through a cellular mechanism that is poorly understood (2). Altered synaptic signaling is one proposed mechanism that contributes to altered neuronal function, which correlates with disease (3-5).

Postsynaptic Density protein 95 (PSD95) is a member of the membrane-associated guanylate kinase (MAGUK) family of proteins that functions as a scaffolding protein to promote assembly and function of the postsynaptic density complex (6,7). At the presynapse, synapsins function to regulate neurotransmitter release (8,9). Dynamic phosphorylation of PSD95 at Ser295 reflects synaptic signaling that may alter synaptic function (10). In addition to PSD95, postsynaptic glutamate receptors, including AMPA-(α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors, can be directly phosphorylated. Phosphorylation of AMPA Receptor 1 (GluA1) at either Ser831 or Ser845 alters AMPA receptor ion channel function to change synaptic efficacy (11). CREB is a bZIP transcription factor that activates target genes through cAMP response elements. CREB is activated by phosphorylation at Ser133 by various signaling pathways including Erk, Ca2+,stress signaling, as well as synaptic signaling (3).