Targeting Synaptic Dysfunction in Alzheimer's Disease

Agenda

*Presentation times are subject to change.

Abstracts

Overview of Synaptic Pathology in Aging and Alzheimer's Disease

Effective neurotransmission requires an orchestrated series of interactions between various proteins that encapsulate neurotransmitters and exocytose them into the synaptic cleft. Since effective neurotransmission and neurotransmitter release must underlie normal cognitive function, impairments in the levels or functions of synaptic proteins are likely involved in the cognitive deficits of persons with dementia. Different proteins serve different functions within the synaptic boutons and include proteins associated with the vesicular membrane (e.g., synaptophysin, synaptotagmin); the presynaptic membranes, where they participate in the fusion of synaptic vesicles to the synaptic membrane promoting exocytosis (e.g., SNARE complex constituents SNAP-25, Syntaxin,Septin5, and Synaptobrevin);or reside in the presynaptic cytoplasm or other compartments, where they aid in fusion/secretory processes. The levels and distributions of some synaptic proteins have been investigated in AD and have been consistently shown to be reduced and associated with dementia. However, the relationship of functionally different synaptic proteins with dementia, dementia in young-old and oldest-old persons, and with the multiple neuropathological and biochemical deficits associated with Alzheimer's disease has not been studied extensively. Recent studies in our laboratories have focused on the interrelationships between synaptic proteins, their mRNA expression and different indices of AD neurobiology in both young-old and oldest-old persons. These studies have revealed that althoughother dementia-associated hallmarks of AD neuropathology (NPs and NFTs) become less prominent with increasing age, synaptic marker abnormalities in dementia remain constant with increasing age and may represent a neuropathology-independent substrate of dementia spanning all ages.

Synaptic Dysfunction in Aging and Disease

The organization of neuronal microcircuits that control information flow through all brain regions involved in cognitive processing consists of excitatory glutamatergic projection neurons and inhibitory GABAergic interneurons Activity of glutamatergic neurons is modulated by synaptic inputs from noradrenergic, serotonergic and cholinergic neurons. An excitatory imbalance resulting in cellular Ca2⁺ dysregulation may occur in normal aging and more so in Alzheimer's disease (AD) as the result of oxidative and metabolic stress. Synaptic dysfunction may result, in part, from amyloid β-peptide (Aβ)-associated redox reactions that simultaneously promote Aβ aggregation and cell membrane-associated oxidative stress. The function and structural integrity of synapses is compromised as the result of oxidative impairment of ion-motive ATPases, and glucose and glutamate transporters, thereby rendering the neurons vulnerable to energy depletion, Ca2⁺ overload and neurofibrillary degeneration. The good news is that the function of synapses involved in cognition can be preserved by reduction of energy intake and regular exercise which activate signaling pathways that increase the production of proteins that protect neurons against oxidative and metabolic stress (e.g., BDNF, protein chaperones, mitochondrial uncoupling proteins, and redox enzymes). Our recent findings suggest that signaling pathways that mediate health benefits of exercise and energy restriction in the periphery may also protect neurons and promote synaptic plasticity. One such pathway, involving PI3 kinase/Akt and the transcription factor CREB, can be activated by the peptide hormone GLP1, and a long-acting GLP1 analog called Exendin-4/Byetta developed for treatment of diabetes and now being tested in AD patients.

Targeting Neuronal Calcium Signaling to Halt AD-linked Synaptic Dysfunction

Neuronal calcium signaling is fundamental to a myriad of synaptic processes, including neurotransmission, long term plasticity, and vesicle release. Because of this, neurons dedicate substantial metabolic resources to maintain calcium homeostasis and protect network stability. In neurodegenerative diseases involving calcium disruptions, such as Alzheimer's disease (AD), alterations in calcium homeostasis have direct effects on synaptic transmission and plasticity expression, and by association, long-term memory encoding. In AD, proximal pathogenic alterations involve dysregulated ER calcium release via ryanodine receptor (RyR) and IP₃R channels, and we have previously shown increased RyR-evoked calcium responses in dendrites and spines of AD mice at young ages. The downstream implications are wide-ranging, but within synaptic compartments, the RyR-mediated calcium alterations introduce shifts in membrane excitability, synaptic transmission and synaptic plasticity thresholds.

Here we present the functional implications of early calcium signaling abnormalities for synaptic transmission and plasticity in AD mouse models, and offer novel pharmacological approaches to preserve functional synaptic properties in AD mice prior to late disease stages associated with synaptic and memory loss. Using 2-photon calcium imaging, patch clamp electrophysiology, and field potential recordings in hippocampal brain slice preparations from 6-12 week old AD and NonTg mice, we demonstrate sensitized RyR-evoked calcium responses in pre- and post-synaptic compartments of AD hippocampal neurons. Presynaptically, disrupted ER calcium alters neurotransmitter release properties and depletes vesicular stores, while postsynaptically, aberrant RyR-evoked calcium release interacts with calcium-dependent plasma membrane channels to alter membrane excitability. Manipulating RyR function revealed notable differences in synaptic plasticity between NonTg and AD mice as well. Acute RyR antagonism blocked LTP and LTD in NonTg mice, however, this same treatment converted LTP to mild LTD, and markedly enhanced LTD, in 3xTg-AD mice. Notably, chronic treatment with RyR blockers had little effect in NonTg mice, yet, this same treatment normalized calcium signaling in the AD mice back to within control conditions. This includes reversing the aberrant ER calcium responses, altered basal synaptic transmission, and impaired plasticity expression in AD mice back to patterns observed in NonTg mice. Therefore, this RyR-targeted approach to normalizing aberrant calcium signaling early in the disease process offers an optimistic therapeutic approach to preserving synaptic and cognitive function in AD.

Targeting CREB through Phosphodiesterase Inhibitors

Existing Alzheimer's disease (AD) therapies have limited efficacy. Given that Aβ, a protein that accumulates in the brain of AD patients, has been shown to impair synaptic plasticity and memory, major efforts are ongoing to decrease Aβ load. However, the normal physiological roles of amyloid-β precursor protein (APP), its fragments and processing enzymes might present a problem in providing effective and safe approaches to AD therapy. We have therefore explored a different strategy using targets at the downstream level of Aβ accumulation. In studies of the mechanisms underlying Aβ-induced dysfunction of synaptic plasticity and memory we have defined a central role for cAMP response element (CRE)-dependent gene expression, which is mediated by the transcription factor CRE binding protein (CREB). We have demonstrated that CREB phosphorylation is reduced during induction of synaptic plasticity and memory in the presence of high Aβ levels. Consistent with these observations, up-regulation of both the adenylyl cyclase-cAMP-PKA pathway and the NO synthase-guanylyl cyclase-cGMP-PKG pathway, two enzymatic cascades that are known to increase CREB phosphorylation, rescued the Aβ-induced defects in memory and its electrophysiological correlate, long-term potentiation (LTP). Most importantly, from a drug discovery point of view, two targets, the cAMP degrading enzyme phosphodiesterase- (PDE) 4, and the cGMP degrading enzyme PDE-5, have provided very encouraging data strongly supporting the possibility of using PDE4 and PDE5 inhibitors in order to enhance activation of PKA and PKG, two kinases phosphorylating CREB. Using a SAR approach, we have synthesized a novel compound, JFOA1, with high potency, excellent selectivity for PDE5 over other PDE isoforms, blood brain barrier permeable and safe up to 2 g/kg in acute toxicity test. The compound also showed both ex vivo and in vivo efficacy, as it ameliorated LTP in hippocampal slices treated with Aβ and memory in mice infused with Aβ. Because of these findings, we propose using PDE inhibitors as AD therapeutics.

Neuroprotective Strategies: BDNF and NGF

Neurotrophins, including NGF and BDNF are central to many facets of CNS function, with critical roles in neuronal differentiation, dendritic arborization, synaptogenesis and activity-dependent forms of synaptic plasticity. BDNF has pleiotropic effects on synaptic activity that underlie circuit formation and cognitive function. The regulatory mechanisms that permit rapid and dynamic refinement of BDNF action in neurons, particularly during aging or following pathological insult, will be considered. In addition, the distinct actions of NGF and its precursor form, proNGF will be discussed. NGF critically regulates the survival and morphology of cholinergic neurons, and strategies to augment NGF following CNS injury have been considered for neuroprotection. However, proNGF is induced following injury or in Alzheimer's disease, and may mediate neuronal death or dysfunction. Multiple strategies that may act to limit proNGF action, and promote NGF function are considered as potential therapeutic targets for future development.

Dysregulation of Lipid Signaling in Alzheimer's Disease

Lipid-mediated signaling regulates a plethora of physiological processes, including critical aspects of brain and synaptic function. In addition, dysregulation of lipid pathways has been implicated in a growing number of neurodegenerative disorders, such as Alzheimer's disease (AD). Although much attention has been given to the link between cholesterol and AD pathogenesis, growing evidence suggests that other lipids, such as phosphoinositides and phosphatidic acid, as well as the enzymes controlling their metabolism (e.g., synaptojanin and phospholipase D, resp.), play an important role. Because regulators of lipid metabolism (e.g. statins) are a highly successful class of marketed drugs, exploration of lipid dysregulation in AD and identification of novel therapeutic agents acting through relevant lipid pathways offers new and effective options for the treatment of synaptic dysfunction in this devastating disorder. In this respect, the use of "lipidomics," an unbiased systems based approach relying on mass spectrometry and allowing for the quantification of hundreds of lipids (including low abundance signaling lipids), is an important step forward, as highlighted in this presentation.

Aß Oligomer Binding to Post-Synaptic Prion Protein Activates Fyn to Mediate Neuronal Dysfunction

Amyloid-beta (Aß) peptide oligomers are thought to trigger Alzheimer’s disease (AD) pathophysiology. Cellular Prion protein (PrP-C) selectively binds oligomeric Aß assemblies and mediates many AD-related phenotypes. Here, we examined the specificity, the distribution and the signal transduction of Aß/PrP complexes. PrP-C is enriched in post-synaptic densities, and Aß/PrPC interaction leads to Fyn kinase activation. Particular Aß assemblies derived from human AD brain interact with PrP-C and induce Fyn activation. Aß engagement of PrP-C/Fyn signaling yields phosphorylation of the NR2B subunit of NMDA-receptors, which is coupled to an initial increase and then loss of surface NMDA-receptors. Aß-induced LDH release and dendritic spine loss require both PrP-C and Fyn, and AD transgene-induced epileptiform discharges do not occur in mice lacking PrP-C expression. These results delineate an Aß oligomer signal transduction pathway requiring PrP-C and Fyn to alter synaptic function with relevance to AD.

Targeting Mitochondrial-derived ROS to Reverse Synaptic Plasticity and Memory Impairments in Alzheimer's Disease

Generation of reactive oxygen species (ROS) causes cellular oxidative damage and has been implicated in the etiology of Alzheimer's disease (AD). We have shown that decreasing the level of superoxide through the overexpression of mitochondrial superoxide dismutase (SOD-2) prevents memory deficits in the Tg2576 mouse model of AD. Consistent with these findings, we recently demonstrated that AD-related LTP impairments could be prevented when ROS generation from mitochondria was diminished either pharmacologically or via genetic manipulation by the overexpression of SOD-2. In wild-type hippocampal slices treated with exogenous amyloid β peptide (Aβ1-42) and in slices from APP/PS1 mutant mice that model AD, LTP impairments were observed that were prevented by MitoQ, a mitochondria-targeted antioxidant, and EUK134, an SOD and catalase mimetic. Moreover, live staining with MitoSOX Red, a mitochondrial superoxide indicator, combined with confocal microscopy, revealed that Aβ-induced superoxide production could be blunted by MitoQ, in agreement with our electrophysiological findings. Finally, in transgenic mice overexpressing SOD-2, Aβ-induced LTP impairments and superoxide generation were prevented. Taken together, our findings suggest a causal relationship between mitochondrial ROS and Aβ-induced impairments in hippocampal synaptic plasticity and memory. Finally, our findings suggest that mitochondrial-derived ROS are a reasonable therapeutic target for the treatment of memory dysfunction in AD patients.

Targeting Gamma-Secretase Activating Protein for Alzheimer’s Disease

In collaboration with Paul Greengard and Rockefeller University, we reported (Nature 467: 95, 2010) the identification of a novel target for drug discovery Gamma-Secretase Activating Protein (GSAP). GSAP participates in the formation of amyloid beta through the interaction with amyloid precursor protein (APP) and gamma-secretase. It is able to impart substrate specificity to the promiscuous protease gamma-secretase at the level of substrate presentation of APP. Importantly, GSAP does not affect NOTCH cleavage. Intra-Cellular Therapies is developing novel therapeutic agents to treat disease progression of Alzheimer’s disease. Agents have been identified that interact with GSAP to limit amyloid beta formation. Novel therapeutic agents for Alzheimer’s disease are currently being developed that represent distinct chemical classes devoid of off-target activity and displaying good oral bioavailability.

Travel & Lodging

Our Location

The New York Academy of Sciences

7 World Trade Center
250 Greenwich Street, 40th floor
New York, NY 10007-2157
212.298.8600

Click here for directions.

Hotels Near 7 World Trade Center

Recommended partner hotel:

The New York Academy of Sciences is a part of the Club Quarters network. Please feel free to make accommodations with Club Quarters on-line to save significantly on hotel costs.

Club Quarters Reservation Password: NYAS

Club Quarters, World Trade Center
140 Washington Street
New York, NY 10006
Phone: (212) 577-1133

Located on the south side of the World Trade Center, opposite Memorial Plaza, Club Quarters, 140 Washington Street, is just a short walk to our location.

Other hotels located near 7 WTC: