Mutations in presenilins are the major cause of familial Alzheimer's disease, but the pathogenic mechanism by which presenilin mutations cause memory loss and neurodegeneration remains unclear. Here we demonstrate that conditional double knockout mice lacking both presenilins in the postnatal forebrain exhibit impairments in hippocampal memory and synaptic plasticity. These deficits are associated with specific reductions in NMDA receptor-mediated responses and synaptic levels of NMDA receptors and alphaCaMKII. Furthermore, loss of presenilins causes reduced expression of CBP and CREB/CBP target genes, such as c-fos and BDNF. With increasing age, mutant mice develop striking neurodegeneration of the cerebral cortex and worsening impairments of memory and synaptic function. Neurodegeneration is accompanied by increased levels of the Cdk5 activator p25 and hyperphosphorylated tau. These results define essential roles and molecular targets of presenilins in synaptic plasticity, learning and memory, and neuronal survival in the adult cerebral cortex.

Enduring forms of synaptic plasticity and memory require new protein synthesis, but little is known about the underlying regulatory mechanisms. Here, we investigate the role of MAPK signaling in these processes. Conditional expression of a dominant-negative form of MEK1 in the postnatal murine forebrain inhibited ERK activation and caused selective deficits in hippocampal memory retention and the translation-dependent, transcription-independent phase of hippocampal L-LTP. In hippocampal neurons, ERK inhibition blocked neuronal activity-induced translation as well as phosphorylation of the translation factors eIF4E, 4EBP1, and ribosomal protein S6. Correspondingly, protein synthesis and translation factor phosphorylation induced in control hippocampal slices by L-LTP-generating tetanization were significantly reduced in mutant slices. Translation factor phosphorylation induced in the control hippocampus by memory formation was similarly diminished in the mutant hippocampus. These results suggest a crucial role for translational control by MAPK signaling in long-lasting forms of synaptic plasticity and memory.

Autism is a complex genetic disorder, but single-gene disorders with a high prevalence of autism offer insight into its pathogenesis. Recent evidence suggests that some molecular defects in autism may interfere with the mechanisms of synaptic protein synthesis. We propose that aberrant synaptic protein synthesis may represent one possible pathway leading to autistic phenotypes, including cognitive impairment and savant abilities.

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson's disease. LRRK2 is a large protein containing a small GTPase domain and a kinase domain, but its physiological role is unknown. To identify the normal function of LRRK2 in vivo, we generated two independent lines of germ-line deletion mice. The dopaminergic system of LRRK2 −/− mice appears normal, and numbers of dopaminergic neurons and levels of striatal dopamine are unchanged. However, LRRK2 −/− kidneys, which suffer the greatest loss of LRRK compared with other organs, develop striking accumulation and aggregation of α-synuclein and ubiquitinated proteins at 20 months of age. The autophagy-lysosomal pathway is also impaired in the absence of LRRK2, as indicated by accumulation of lipofuscin granules as well as altered levels of LC3-II and p62. Furthermore, loss of LRRK2 dramatically increases apoptotic cell death, inflammatory responses, and oxidative damage. Collectively, our findings show that LRRK2 plays an essential and unexpected role in the regulation of protein homeostasis during aging, and suggest that LRRK2 mutations may cause Parkinson's disease and cell death via impairment of protein degradation pathways, leading to α-synuclein accumulation and aggregation over time.Parkinson's disease | knockout | autophagy | ubiquitin-proteasome system | aging P arkinson's disease (PD) is the most common movement disorder. The neuropathological hallmarks of PD are progressive degeneration of dopaminergic (DA) neurons and the presence of intraneuronal cytoplasmic inclusions known as Lewy bodies, of which α-synuclein is a major constituent (1). Dominantly inherited mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial PD (2, 3), highlighting the importance of LRRK2 in PD pathogenesis; however, the normal physiological role of LRRK2 is unknown. LRRK2 is a large protein containing a Ras-like small GTPase domain and a MAPKKK-like kinase domain, and has a functional homolog LRRK1, which shares similar domain structures (4). Crystal structural and biochemical studies showed that the GTPase domain forms a dimer; the pathogenic mutations destabilize the dimer and reduce GTPase activity (5-7). A recent in vitro study suggested that LRRK2 and LRRK1 can interact with each other and form a heterodimer (8). Although no physiological substrate of the LRRK2 kinase activity has been reported, studies in cultured cells have suggested that some pathogenic mutations in LRRK2 cause increases in LRRK2 kinase activity (9, 10).Protein aggregation is thought to play a major role in neurodegeneration and PD pathogenesis (11). The strongest evidence came from studies of α-synuclein. Gene multiplication and missense mutations in α-synuclein have been identified in early-onset familial PD with dominant inheritance (12). α-Synuclein is a major constituent of Lewy bodies (1). Overexpression of either WT or mutant α-synuclein in transgenic mice causes age-related neurodegeneration (13-15). Although patients carryi...

Memory and synaptic plasticity exhibit distinct temporal phases, with long-lasting forms distinguished by their dependence on macromolecular synthesis. Prevailing models for the molecular mechanisms underlying long-lasting synaptic plasticity have largely focused on transcriptional regulation. However, a growing body of evidence now supports a crucial role for neuronal activity-dependent mRNA translation, which may occur in dendrites for a subset of neuronal mRNAs. Recent work has begun to define the signaling mechanisms coupling synaptic activation to the protein synthesis machinery. The ERK and mTOR signaling pathways have been shown to regulate the activity of the general translational machinery, while the translation of particular classes of mRNAs is additionally controlled by gene-specific mechanisms. Rapid enhancement of the synthesis of a diverse array of neuronal proteins through such mechanisms provides the components necessary for persistent forms of LTP and LTD. These findings have important implications for the synapse specificity and associativity of protein synthesis-dependent changes in synaptic strength.

Dominantly inherited mutations in the genes encoding presenilins (PS) and the amyloid precursor protein (APP) are the major causes of familial Alzheimer's disease (AD). The prevailing view of AD pathogenesis posits that accumulation of ␤-amyloid (A␤) peptides, particularly A␤42, is the central event triggering neurodegeneration. Emerging evidence, however, suggests that loss of essential functions of PS could better explain dementia and neurodegeneration in AD. First, conditional inactivation of PS in the adult mouse brain causes progressive memory loss and neurodegeneration resembling AD, whereas mouse models based on overproduction of A␤ have failed to produce neurodegeneration. Second, whereas pathogenic PS mutations enhance A␤42 production, they typically reduce A␤40 generation and impair other PS-dependent activities. Third, ␥-secretase inhibitors can enhance the production of A␤42 while blocking other ␥-secretase activities, thus mimicking the effects of PS mutations. Finally, PS mutations have been identified in frontotemporal dementia, which lacks amyloid pathology. Based on these and other observations, we propose that partial loss of PS function may underlie memory impairment and neurodegeneration in the pathogenesis of AD. We also speculate that A␤42 may act primarily to antagonize PS-dependent functions, possibly by operating as an active site-directed inhibitor of ␥-secretase.A lzheimer's disease (AD) is an age-related neurodegenerative dementia and is the most common cause of both neurodegeneration and dementia. Neurodegenerative dementias are characterized clinically by progressive impairment of cognitive abilities, which most prominently affects memory in AD. Neuronal and synaptic loss is the essential neuropathological feature common to different forms of neurodegenerative dementias, including AD, frontotemporal dementia (FTD) and Lewy body dementia (LBD). These diseases are distinguished neuropathologically by characteristic patterns of abnormal protein aggregation, such as the presence in the AD brain of cerebral cortical amyloid plaques and neurofibrillary tangles (NFTs). Extracellular amyloid plaques consist primarily of 40-to 42-residue ␤-amyloid (A␤) peptides (A␤40 and A␤42) derived from proteolytic processing of the amyloid precursor protein (APP). NFTs are intraneuronal inclusions composed of hyperphosphorylated forms of the microtubule-associated protein tau.Research on AD has been greatly stimulated by the identification of causative mutations in the genes encoding APP and presenilins (PS1 and PS2). Dominantly inherited missense mutations in APP increase the production of A␤ peptides and account for Ϸ10% of mutations identified in familial AD (FAD). PSs harbor Ϸ90% of identified FAD mutations, and many of these mutations increase the relative production of A␤42 peptides. The prevailing amyloid hypothesis posits that accumulation of A␤ peptides, particularly the more hydrophobic and aggregation-prone A␤42, triggers a pathogenic cascade, leading to neurodegeneration in AD (1). However, a...

Long-term memory and its putative synaptic correlates the late phases of both long-term potentiation and long-term depression require enhanced protein synthesis. On the basis of recent data on translation-dependent synaptic plasticity and on the supralinear effect of activation of nearby synapses on action potential generation, we propose a model for the formation of long-term memory engrams at the single neuron level. In this model, which we call clustered plasticity, local translational enhancement, along with synaptic tagging and capture, facilitates the formation of long-term memory engrams through bidirectional synaptic weight changes among synapses within a dendritic branch.

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