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Neurobiological Mechanisms of Memory Loss in Alzheimer's Disease

Final Report Summary - MEMOLOAD (Neurobiological Mechanisms of Memory Loss in Alzheimer's Disease)

Executive Summary:

The MEMOLOAD project focused on the molecular and biological mechanisms underlying memory loss that occurs in Alzheimer’s disease, the leading cause of dementia and an enormous medical, social and economic challenge to Europe.

Several lines of evidence point to accumulation of amyloid –beta peptide (Abeta) in the brain as the key pathologic event in the disease. There is growing evidence that Abeta causes memory loss by directly or directly interacting with the known key signalling pathways involved in memory consolidation.

However, at the time of the project beginning the data were fragmentary and consisted mainly of single observations in particular models (cell culture, brain slice, in vivo). Furthermore, Abeta peptide itself is a labile molecule, and it different conformers may have different biological effects. In most cases, we still lack the evidence that a clear molecular level interaction translates into memory impairment in vivo.

The objective of this proposal was to elucidate the molecular level mechanisms by which accumulation of Abeta in the brain results in impaired synaptic plasticity and memory loss. Moreover, the consortium developed new peptidomimetics that could neutralize the most harmful Abeta species and tested their effects on neuronal function and memory at the behavioural level. The MEMOLOAD consortium consists of a well-balanced mixture of the seven best available European research groups in terms of research experience on both the mechanisms of memory consolidation and the pathophysiology of Alzheimer’s disease. The current research topic is thus the primary research interest of all partners.

MEMOLOAD has significantly contributed to a better understanding of brain memory mechanisms at the behavioural, network, synaptic and molecular levels and of dysfunction at all these levels in Alzheimer’s disease (AD). The knowledge acquired during the course of MEMOLOAD will translate into new validated in vitro and in vivo models for the memory impairing effect of Abeta and will feed into industrial development leading to new therapies. The output of MEMOLOAD includes both identification of new drug targets and development of novel peptidomimetic compounds that neutralize the deleterious effects of most harmful Abeta species.

Project Context and Objectives:

The overall objectives of MEMOLOAD were twofold. First, to elucidate the molecular level mechanisms by which accumulation of Aβ in the brain results in impaired synaptic plasticity and memory loss. Second, to develop new peptidomimetic molecules that disrupt Aβ oligomerization and fibrillization, characterize their pharmacokinetic profile and to to evaluate these compounds in validated cellular and animal models.

The specific objectives were to:

1. Characterize the currently highly variable natural and synthetic Aβ preparations used by research groups in this field and to provide standardized oligomeric and fibrillar Aβ preparations for research.
2. Determine the relationship between acute effect of Aβ on synaptic plasticity models (LTP and LTD) and long-term changes in synaptic plasticity, neural network communication and behavioral expression of memory.
3. Specify the type of synapses (inhibitory or excitatory) and the synaptic elements that are first affected by accumulating Aβ concentrations.
4. Determine direct and indirect effects of Aβ on two key signaling pathways, the PI3K-Akt-GSK3 cascade connecting memory consolidation to other key pathological events of the disease, and the ERK1/2 – CREB pathway regulating gene transcription.
5. Identify potential new drug targets among protein kinases and phosphatases that regulate key molecules of the memory consolidation cascade.
6. Develop and synthesize novel peptidomimetic compounds counteracting Aβ effects and test their biological effect on synaptic plasticity and memory.

Technically MEMOLOAD divided into 8 work packages (WP), each of which with their specific tasks.

WP1 aimed at developing new behavioral tasks that are sensitive to small changes in synaptic signaling and plasticity in brain areas that are mostly affected in the human disease as well as in the transgenic mouse models, the cerebral cortex and the hippocampus. Once established these tasks were used in all participating laboratories for assessment the effects of exogenously applied or endogenously produces Aβ and the success of various drug treatments.

WP2 aimed at linking Aβ induced changes in individual synapses to memory loss. First, the focus was shifted from individual synapses to the function or dysfunction of large neuronal populations and the entire neural network. Second, acute effects of Aβ on synaptic plasticity were compared with its long-term effects in the intact brain with an aim to better understand how acute synaptic plasticity models to correspond to long-term functional changes. Third, recording of LTP, LTD and network oscillations in freely moving rodents were performed to allow assessment of changes in network communication in the real-life context of a memory task.

WP3 set out to determine the biological effects of various Aβ species or anti- Aβ peptidomimetic lead compounds in a validated model of synaptic plasticity. The primary tool to assess acute effects of Aβ on synaptic plasticity was LTP in the rat hippocampus. Characterized synthetic Aβ species were compared with ‘gold standard’, natural Aβ oligomers, in their ability to block LTP. Moreover, peptide drugs were tested first in this model for their ability to abrogate Aβ induced deficit in synaptic plasticity.

WP4 aimed at increasing understanding of the spatial and temporal dynamics and cellular signaling contexts of Aβ -induced synaptic failure in the cerebral cortex. Synapses are the key functional platforms transmitting information from one neuron to another and thus provide the cellular and signaling basis for memory consolidation (memory trace). To understand the spatial and temporal dynamics of Aβ -induced synaptic failure we evaluated the impact of Aβ on various forms of short- and long-term synaptic plasticity at identified cortical synapses, and the roles of postsynaptic regulatory mechanisms (receptor availability, retrograde signaling) in the progressive impairment of synaptic communication. Our identified cellular substrates of Aβ action together with available inhibitors of Aβ oligomerization were evaluated for their ability to reduce synaptic dysfunction.

WP5 aimed at determining the direct and indirect effects of Aβ on two signaling pathways that are central to memory consolidation and affected by Aβ in cell culture experiments. The signal for an event to be memorized needs to pass from the cell surface to the nucleus to initiate reading of the genetic code from new proteins. At the same time it needs to initiate protein synthesis locally at the dendrite to 'tag' the active synapse to receive newly synthesized proteins from the cell soma. This communication involves well characterized signaling cascades. This WP determined downstream effects of Aβ binding to the synapse on intracellular signaling and explored possibilities to counteract its deleterious effects through drug interaction with these signaling molecules. Two signalling pathways were in particular focus. First, is the PI3K-Akt-GSK3b that is involved in neuronal growth and survival vs. apoptotic cell death, neuronal plasticity, energy balance, and also directly links to main pathological events of AD, overproduction of Aβ and phosphorylation of tau. A second pathway of particular interest was the MEK - ERK1/2 – CREB pathway that is the main regulator of transcription and its primary output, immediate early genes.

WP6 aimed at providing other WPs with conformationally characterised synthetic Aβ species as research tool as well as to synthetise and validate novel peptidomimetic compounds preventing Aβ oligomerization. While other technical WPs dealt with measurement of biological effects, this WP constitutes of pure protein chemistry. This WP used new optic methods to determine details 3-dimensional structure of various Aβ species to help understand their reported biological effects. This WP developed procedures to produce standardized synthetic Aβ oligomers and fibrils to study their biological effects. In addition, it developed novel peptides inhibiting either oligomerization or fibrillization of Aβ. These peptides were then tested for their biological effects in other technical WPs.

WP7 was assigned to dissemination and exploitation of the results and WP 8 to project management.

Project Results:

See attached document ST_Results 25-9-2013.pdf

Potential Impact:

1. Increase in general knowledge base

MEMOLOAD has significantly contributed to a better understanding of brain memory mechanisms at the behavioral, network, synaptic and molecular levels and of dysfunction at all these levels in Alzheimer’s disease (AD). Focus areas in this regard included:

- Contribution of different Abeta conformers (monomer, oligomer, protofibril, fibril) to neurotoxicity and neuronal dysfunction
- Abeta induced changes in neuronal excitability, which may underlie AD-related epilepsy
- Changes in brain endocannabinoid system in AD and AD models, and their potential link to failed synaptic communication
- Alterations in the signaling of main neurotrophic factor BDNF in AD models, especially changes in the balance of signaling full-length TrkB.TK receptor and nonsignaling truncated TrkB.T1 receptor
- Alterations in adult neurogenesis in AD and AD models
- Mechanisms of Abeta induced blockade of LTP induction in brain slices and in vivo
- Abeta induced changes in neuronal energy balance
- Specificity of memory impairment in AD mouse models, in terms of memory encoding, consolidation and retrieval
- Role of two key signaling pathways for memory formation (PI3K-Akt and MEK-ERK-CREB) and their interaction with Abeta

2. New standardized laboratory protocols

MEMOLOAD has established new validated in vitro and in vivo models for the memory impairing effect of Abeta. These include:

- Standardized protocol to produce an Abeta isopeptide that is totally inert during the storage phase and can be triggered to starts a slow aggregation process so as to remain several hours in an oligomeric state.
- New assays to control for the physicochemical state of Abeta in a solution
- New validated behavioral tasks to assess potential long-term memory impairment in AD model mice
- New AD animal models through cross-breeding APP transgenic mice with another line that modifies the disease process or by combining genetic and pharmacological manipulations or genetic and environmental manipulations
- New imaging methods to screen for Abeta induced changes in synaptic structure and function
- New in vivo electrophysiological measured for long-term changes in neuronal networks after Abeta administration

3. New peptidomimetic drugs to counteract Abeta effects

MEMOLOAD has synthetized and screened a dozen different peptidomimetic compound for their potential to prevent Abeta aggregation from monomers to oligomers or to hasten their further confomeric change to inert fibrils. Two of these peptidomimetic compounds were taken to in vivo testing. Some evidence for their effect was obtained also in vivo, but in general their pharmacokinetic properties (short half-life, poor access through the blood-brain barrier) remain a limiting factor for further drug development.

4. Effective dissemination of knowledge

As evidence by the separate massive list of dissemination events, MEMOLOAD has

(1) Provided the scientific community a substantial knowledge base on basic research of Alzheimer’s disease
(2) Increased the public awareness of the need for better understanding of the basic disease mechanisms underlying Alzheimer’s disease and the current state and future perspectives of finding an effective therapy to halt the progression of the disease.

List of Websites: