Molecular Pathways of Memory

BRNI scientists have for many years been investigating the sequence of molecular events responsible for learning, memory consolidation and permanent memory storage in the brain.  Using a multidisciplinary approach to define the synaptic connections between neurons in brain structures involved in learning, Institute scientists have uncovered molecular pathways that control these synaptic connections.  BRNI research has demonstrated that different forms of the enzyme Protein Kinase C (PKC) activate multiple targets during memory acquisition and storage.  In past research they demonstrated that a number of models of learning and memory such as Pavlovian condition of the snail Hermissenda, rabbit nictating membrane conditioning, rat spatial maze learning and rat olfactory discrimination learning, all activate PKC by causing it to move from the cell body to the membrane walls of neurons critical for the memory task involved.  Synaptic signals within specific brain areas were found to be amplified by associative learning paradigms that engage these PKC pathways.  Some of these synaptic changes involved in memory could then be explained by changes of specific ion channels within the relevant connections between neurons in these specific areas.  Remarkably, at very different levels of animal evolution, similar molecular pathways involving PKC, similar changes of synaptic networks involving similar ion channels and similar associations (such as Pavlovian conditioning) were observed at the more primitive (involving sea snails) as well as the more advanced (involving mammals such as rabbits) levels of evolution.  These similarities illustrated a principle in evolution called “conservation”.  Mechanisms of synaptic change to store memory were found by BRNI scientists to be conserved across evolution.

This memory mechanism conservation encouraged the prediction that these same conserved mechanisms would also be conserved in humans – the highest level of evolution.  In fact, BRNI basic and clinical research on Alzheimer’s disease which starts with loss of recent memory has provided strong support for exactly this conservation.  Namely, Alzheimer’s disease appears to involve early deficits of PKC that could explain the characteristic early symptom of recent memory loss and therapeutics, now scheduled for their first Phase II trial, have successfully reversed the loss of synapses, cognitive deficits and toxic protein elevation in mice with human Alzheimer’s disease genes.

Many of the molecular events in these PKC-mediated memory pathways were also shown to be activated specifically when learning and memory storage occurred.  A number of different techniques including genomic microarray analysis, RT-PCR, in situ hybridization, and immunohistochemical, biochemical fractionation, electrophysiologic and electronmicroscopic measures demonstrated memory-specific changes of specific proteins and their substrates within exactly those brain structures previously implicated as storage sites for particular memory tasks.

In recent years, molecular steps both upstream and downstream from PKC isozymes within the PKC pathways have been implicated at BRNI.  These involve upstream factors such as the insulin receptor and growth factors such as BDNF, the fibroblast growth factor 18 and the insulin growth factor and downstream factors such the mRNA stabilizing proteins (Hud, HuC, HuR), the ryanodine receptor, and indirectly, the cbl-b protein regulator of proteasome degradation.  The mRNA stabilizing proteins (also called the ELAV protein class) are a direct target of PKC and regulate a group of key protein factors that control synaptic development and remodeling.  This ELAV-target group includes GAP-43 (growth associated protein), fos, jun, the insulin receptor, and the amyloid precursor protein (or APP).

BRNI scientists also investigate the structural basis of memory acquisition and storage in brain regions critical for mammalian memory such as the hippocampus.  Using confocal microscopic visualization of individual hippocampal pyramidal cells and their sites of synaptic contacts, they are measuring memory specific changes of synaptic structure.  These changes were found to be enhanced by the PKC activating drug bryostatin and impaired in animal models of Alzheimer’s disease, stroke and traumatic brain injury (TBI), as well as aged animals with decreased learning ability.  Other structurally-specific changes that occur with PKC activation involve movement of the PKC substrate mRNA stabilizing proteins (or ELAV proteins) from the nucleus into the surrounding cell body and into the dendritic tree.


Application to Alzheimer's Disease and Other Neurological Disorders

The molecular pathways of memory described above have been shown by BRNI scientists to converge with molecular pathways important for processing of proteins critical for Alzheimer’s disease pathophysiology.  The toxic protein known as beta amyloid or Abeta (Aβ), for example, is generated by enzymatic breakdown by beta-secretase of the Amyloid Precursor Protein (APP). PKC isozymes (α, ε), on the other hand, activate another secretase known as the alpha-secretase to generate the normal breakdown product known as the soluble APP or sAPP.  These same PKC isozymes also activate MAP Kinase Erk1/2 that is within the pathways that activate alpha-secretase.  Furthermore, PKC α and ε activate an enzyme (ECE) that degrades A beta and inhibit the enzyme known as glycogen synthase 3-β kinase which is important for phosphorylating the tau protein that contributes to the pathologic Alzheimer’s changes known as “neurofibrillary tangles”.  The role of PKC-isozyme pathway dysfunction in the initial causes of Alzheimer’s disease received important support when BRNI scientists demonstrated that the key toxic Alzheimer’s protein Aβ directly inhibits PKC functions.  This finding was further confirmed by another laboratory’s demonstration of the structural location on Aβ that directly combines with PKC isozymes.

The direct relevance of these Aβ inhibitory effects on PKC was suggested by BRNI scientists when they first discovered some time ago that Aβ blocked specific potassium channels that were found to be defective in Alzheimer's disease and that were regulated by PKC activators such as bryostatin.  In a series of other studies, BRNI research has demonstrated that PKC alpha and epsilon activators such as Bryostatin, and PKC epsilon-specific activators can prevent the earliest stages of Alzheimer’s disease elevation of soluble A Beta in mouse models of Alzheimer’s disease such as the single-mutation model (Tg2576) and the 5-mutation model (5xFAD).  In these models, the PKC activators not only prevented the A Beta elevation, cognitive deficits, and synaptic loss, they also reversed some of these changes (in the 5xFAD) model  - after they had already occurred.

In another related approach, molecular analogues of soluble forms of Aβ activate glutamate receptors in the absence of conventional activators (agonists and co-agonists). These analogues reduced glutamate evoked receptor responses. Furthermore, these effects of soluble oligomer analogues were not observed for other forms such as Aβ monomers or fibrils.   other recent BRNI studies showed that Dopamine D1 ligands, agonists, and antagonists, directly block NMDA receptors as channel pore blockers.  These ligands appear to act like memantine, a NMDA receptor channel blocker, drug currently used clinically for the treatment of Alzheimer's disease.

Memory and synaptic loss occurs in a variety of neurologic disorders that include not only Alzheimer’s disease, stroke and TBI, but also depression and mental retardation.  On this basis, BRNI scientists will continue to explore the generality of the memory restoration drugs with a variety of brain disease models as well as the potential benefits of such drugs in treating such diseases in future clinical trials.