(596t) Exploring Beta-Amyloid's Alteration of Signaling Cascades Associated with Learning and Memory of Neuron-Like Cells and the Subsequent Implication in the Mechanism of Alzheimer's Disease

Alzheimer's disease (AD) is a form of dementia that affects memory, thinking and behavior, and gradually worsens with time. Memory impairment, as well as problems with language, decision-making ability, judgment and personality are symptoms of this disease. AD primarily affects people over 65 years of age. In view of the United States’ ageing population, it is especially important to discover new therapeutic approaches to combat this disease. 

Neurofibrillary tangles and extracellular plaques are observed in the brains of AD patients at the histopathological level. Beta-amyloid (Aβ) peptide is a major component of these senile plaques and is believed to play a role in the pathology of the disease. Aβ is generated from the Amyloid precursor protein (APP) by enzymatic digestion involving β and γ-secretases. The formation of amyloid fibrils, proto-fibrils and oligomers from the Aβ peptide represent a hallmark of Alzheimer's disease. Aggregated Aβ is known to be toxic to neurons and is implicated in Alzheimer’s disease. However, the mechanism by which Aβ causes the apoptosis of cells is unclear and methods to prevent its toxicity are yet to be confirmed.  Moreover, the role that Aβ plays in changes in neuron function in this chronic neurodegenerative disease that leads to changes in learning and memory is yet to be elucidated. 

The first symptoms of Alzheimer’s disease are loss of learning and memory and not cell death; hence we look at how Aβ specifically influences this early phenomenon in cells.Learning and memory in neurons is linked directly to LTP, which is a candidate for synaptic mechanism, i.e., how neurons communicate with one another. There are a number of factors that play a major role in influencing LTP. Its induction and expression is mediated by N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4 isoxazolepropionic acid (AMPA) receptors respectively.

Glutamate is one of the two major excitatory neurotransmitters in the cerebral cortex (along with acetylcholine).  LTP is induced by glutamate receptor release and simultaneous depolarization, resulting in a postsynaptic calcium influx through the NMDA receptor. This Calcium influx through the NMDA receptor (NMDAR) is crucial for the induction of LTP and activates an intra-cellular signaling cascade that includes several kinases, and that, among other things, leads to an increased number of AMPA receptor. Hence phenomena such as LTP occur through multiple mechanisms many of which may involve protein phosphorylation.

Tyrosine protein phosphorylation is also believed to have an important role in the regulation of neuronal function. This is substantiated by previous work which has shown that protein tyrosine kinase (PTK) inhibitors can inhibit the induction of LTP. The tyrosine kinase Src was shown to regulate the activity of NMDA channels. Low levels of phosphorylation of the NMDAR appear to increase channel activity, while phosphorylation also targets the channel for ubiquitination and degradation.  Protein kinase C (PKC) is one of the protein kinase groups believed to be important for maintaining LTP. In particular PKM-zeta, a constitutively active form of an atypical protein kinase C (PKC) isozyme, has been implicated in long term memory maintenance in recent research.

Calcium is a key signaling ion involved in many different intracellular and extracellular processes ranging from synaptic activity to cell-cell communication and adhesion.  In the majority of synapses that support LTP, the postsynaptic increase in calcium is mediated by NMDAR. Excitotoxicity appears to play an important role in the pathogenesis of AD. The intricate relationship between Aβ, LTP and AD has been researched only to a certain extent and a number of questions remain unanswered. Observations from studies that Aβ reduces LTP and facilitates LTD is suggestive of a role for it in regulating glutamate receptors and postsynaptic phosphorylation , and playing an important role in modulating learning and memory during AD. Metabotropic glutamate reports are also reported to be involved in the Aβ induced toxicity pathway. Aβ increases the vulnerability of neurons to excitotoxicity mediated by NMDAR. Aβ oligomers have been found to cause an increase in NMDAR activity, which might require direct association between the oligomers and the NR1 subunit of NMDAR. Alternately, Aβ may lead to activation of a tyrosine kinase in the Src family which phosphorylates the NR2B subunit of the NMDAR, leading to its activation.  Following that deranged calcium signaling in AD is probable. The exact mechanism of alteration of LTP, the concentration and time scale effects of Aβ are as of yet unknown.

Cholesterol or lipid rich regions known as lipid rafts in the cell membrane are favorable for Aβ binding. Cholesterol has been found to control certain interactions between Aβ and neuronal membranes that are regarded as decisive in the initiation of a neurotoxic cascade.  

We hypothesize that Aβ , upon interaction with cells, influences a series of cell signaling events that eventually affect learning and memory capacity in the neurons. We will explore this alternate theory, wherein Aβ binds to cell receptors and facilitates pathways in the cell. To test this hypothesis, we investigate if Aβ interacts with cells leading to an increased phosphorylation of tyrosine proteins. We investigate if Aβ alters the receptor expression on the cell surface in this process. We determine if specific tyrosine proteins, the Src family of tyrosine kinases, PKM-zeta and the Protein Kinase C are targeted. We further investigate if activation of the NMDA receptor (via tyrosine phosphorylation) and activation of the AMPA receptor (via serine phosphorylation) are events that are induced by Aβ.    We study if Aβ perturbs intracellular calcium signaling by affecting calcium homeostasis, and if this phenomenon occurs in tandem with the other events induced by Aβ. We investigate if lipid rafts have a role in facilitating Aβ’s influence on these signaling pathways.

We will develop an Aβ time and concentration dependent model of the kinetics and mechanism of this signaling cascade, and with this model explore the complex regulatory/control mechanisms that Aβ alters, and how this may impact stability of signaling within the cell.