Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Pharmacological Rescue of Long-Term Potentiation in Alzheimer Diseased Synapses

Long-term potentiation (LTP) is an activity-dependent and persistent increase in synaptic transmission. Currently available techniques to measure LTP are time-intensive and require highly specialized expertise and equipment, and thus are not well suited for screening of multiple candidate treatments, even in animal models. To expand and facilitate the analysis of LTP, here we use a flow cytometry-based method to track chemically induced LTP by detecting surface AMPA receptors in isolated synaptosomes: fluorescence analysis of single-synapse long-term potentiation (FASS-LTP). First, we demonstrate that FASS-LTP is simple, sensitive, and models electrically induced LTP recorded in intact circuitries. Second, we conducted FASS-LTP analysis in two well-characterized Alzheimer's disease (AD) mouse models (3xTg and Tg2576) and, importantly, in cryopreserved human AD brain samples. By profiling hundreds of synaptosomes, our data provide the first direct evidence to support the idea that synapses from AD brain are intrinsically defective in LTP. Third, we used FASS-LTP for drug evaluation in human synaptosomes. Testing a panel of modulators of cAMP and cGMP signaling pathways, FASS-LTP identified vardenafil and Bay-73-6691 (phosphodiesterase-5 and -9 inhibitors, respectively) as potent enhancers of LTP in synaptosomes from AD cases. These results indicate that our approach could provide the basis for protocols to study LTP in both healthy and diseased human brains, a previously unattainable goal.

SIGNIFICANCE STATEMENT

Learning and memory depend on the ability of synapses to strengthen in response to activity. Long-term potentiation (LTP) is a rapid and persistent increase in synaptic transmission that is thought to be affected in Alzheimer's disease (AD). However, direct evidence of LTP deficits in human AD brain has been elusive, primarily due to methodological limitations. Here, we analyze LTP in isolated synapses from AD brain using a novel approach that allows testing LTP in cryopreserved brain. Our analysis of hundreds of synapses supports the idea that AD-diseased synapses are intrinsically defective in LTP. Further, we identified pharmacological agents that rescue LTP in AD, thus opening up a new avenue for drug screening and evaluation of strategies for alleviating memory impairments.

The Contribution of Spatial and Temporal Molecular Networks in the Induction of Long-term Memory and Its Underlying Synaptic Plasticity

The ability to form long-lasting memories is critical to survival and thus is highly conserved across the animal kingdom. By virtue of its complexity, this same ability is vulnerable to disruption by a wide variety of neuronal traumas and pathologies. To identify effective therapies with which to treat memory disorders, it is critical to have a clear understanding of the cellular and molecular mechanisms which subserve normal learning and memory. A significant challenge to achieving this level of understanding is posed by the wide range of distinct temporal and spatial profiles of molecular signaling induced by learning-related stimuli. In this review we propose that a useful framework within which to address this challenge is to view the molecular foundation of long-lasting plasticity as composed of unique spatial and temporal molecular networks that mediate signaling both within neurons (such as via kinase signaling) as well as between neurons (such as via growth factor signaling). We propose that evaluating how cells integrate and interpret these concurrent and interacting molecular networks has the potential to significantly advance our understanding of the mechanisms underlying learning and memory formation.

Genetic Dissection of Aversive Associative Olfactory Learning and Memory in Drosophila Larvae

Memory formation is a highly complex and dynamic process. It consists of different phases, which depend on various neuronal and molecular mechanisms. In adult Drosophila it was shown that memory formation after aversive Pavlovian conditioning includes—besides other forms—a labile short-term component that consolidates within hours to a longer-lasting memory. Accordingly, memory formation requires the timely controlled action of different neuronal circuits, neurotransmitters, neuromodulators and molecules that were initially identified by classical forward genetic approaches. Compared to adult Drosophila, memory formation was only sporadically analyzed at its larval stage. Here we deconstruct the larval mnemonic organization after aversive olfactory conditioning. We show that after odor-high salt conditioning larvae form two parallel memory phases; a short lasting component that depends on cyclic adenosine 3’5’-monophosphate (cAMP) signaling and synapsin gene function. In addition, we show for the first time for Drosophila larvae an anesthesia resistant component, which relies on radish and bruchpilot gene function, protein kinase C activity, requires presynaptic output of mushroom body Kenyon cells and dopamine function. Given the numerical simplicity of the larval nervous system this work offers a unique prospect for studying memory formation of defined specifications, at full-brain scope with single-cell, and single-synapse resolution.

Learning and memory helps organisms to predict and adapt to events in their environment. Gained experience leaves traces of memory in the nervous system. Yet, memory formation in vertebrates and invertebrates is a highly complex and dynamic process that consists of different phases, which depend on various neuronal and molecular mechanisms. To understand which changes occur in a brain when it learns, we applied a reductionist approach. Instead of studying complex cases, we analyzed learning and memory in Drosophila larvae that have a simple brain that is genetically and behaviorally accessible and consists of only about 10,000 neurons. Drosophila larvae are able to learn to associate an odor with punishing high salt concentrations. It is therefore possible to correlate changes in larval behavior with molecular events in identifiable neurons after classical olfactory conditioning. We show that under these circumstances larvae form two parallel memory phases; a short lasting component (lSTM) that is molecularly conserved throughout the animal kingdom as it depends on the classical cAMP pathway. In parallel they establish a larval anesthesia resistant memory (lARM) that relies on a different molecular signal. lARM has not been described in larvae before.

Genealogical correspondence of a forebrain centre implies an executive brain in the protostome-deuterostome bilaterian ancestor

Orthologous genes involved in the formation of proteins associated with memory acquisition are similarly expressed in forebrain centres that exhibit similar cognitive properties. These proteins include cAMP-dependent protein kinase A catalytic subunit (PKA-Cα) and phosphorylated Ca(2+)/calmodulin-dependent protein kinase II (pCaMKII), both required for long-term memory formation which is enriched in rodent hippocampus and insect mushroom bodies, both implicated in allocentric memory and both possessing corresponding neuronal architectures. Antibodies against these proteins resolve forebrain centres, or their equivalents, having the same ground pattern of neuronal organization in species across five phyla. The ground pattern is defined by olfactory or chemosensory afferents supplying systems of parallel fibres of intrinsic neurons intersected by orthogonal domains of afferent and efferent arborizations with local interneurons providing feedback loops. The totality of shared characters implies a deep origin in the protostome-deuterostome bilaterian ancestor of elements of a learning and memory circuit. Proxies for such an ancestral taxon are simple extant bilaterians, particularly acoels that express PKA-Cα and pCaMKII in discrete anterior domains that can be properly referred to as brains.

An integrated proteomics approach shows synaptic plasticity changes in an APP/PS1 Alzheimer's mouse model

The aim of this study was to elucidate the molecular signature of Alzheimer's disease-associated amyloid pathology.We used the double APPswe/PS1ΔE9 mouse, a widely used model of cerebral amyloidosis, to compare changes in proteome, including global phosphorylation and sialylated N-linked glycosylation patterns, pathway-focused transcriptome and neurological disease-associated miRNAome with age-matched controls in neocortex, hippocampus, olfactory bulb and brainstem. We report that signalling pathways related to synaptic functions associated with dendritic spine morphology, neurite outgrowth, long-term potentiation, CREB signalling and cytoskeletal dynamics were altered in 12 month old APPswe/PS1ΔE9 mice, particularly in the neocortex and olfactory bulb. This was associated with cerebral amyloidosis as well as formation of argyrophilic tangle-like structures and microglial clustering in all brain regions, except for brainstem. These responses may be epigenetically modulated by the interaction with a number of miRNAs regulating spine restructuring, Aβ expression and neuroinflammation.We suggest that these changes could be associated with development of cognitive dysfunction in early disease states in patients with Alzheimer's disease.

The Mammalian Orthologs of Drosophila Lgd, CC2D1A and CC2D1B, Function in the Endocytic Pathway, but Their Individual Loss of Function Does Not Affect Notch Signalling

CC2D1A and CC2D1B belong to the evolutionary conserved Lgd protein family with members in all multi-cellular animals. Several functions such as centrosomal cleavage, involvement in signalling pathways, immune response and synapse maturation have been described for CC2D1A. Moreover, the Drosophila melanogaster ortholog Lgd was shown to be involved in the endosomal trafficking of the Notch receptor and other transmembrane receptors and physically interacts with the ESCRT-III component Shrub/CHMP4. To determine if this function is conserved in mammals we generated and characterized Cc2d1a and Cc2d1b conditional knockout mice. While Cc2d1b deficient mice displayed no obvious phenotype, we found that Cc2d1a deficient mice as well as conditional mutants that lack CC2D1A only in the nervous system die shortly after birth due to respiratory distress. This finding confirms the suspicion that the breathing defect is caused by the central nervous system. However, an involvement in centrosomal function could not be confirmed in Cc2d1a deficient MEF cells. To analyse an influence on Notch signalling, we generated intestine specific Cc2d1a mutant mice. These mice did not display any alterations in goblet cell number, proliferating cell number or expression of the Notch reporter Hes1-emGFP, suggesting that CC2D1A is not required for Notch signalling. However, our EM analysis revealed that the average size of endosomes of Cc2d1a mutant cells, but not Cc2d1b mutant cells, is increased, indicating a defect in endosomal morphogenesis. We could show that CC2D1A and its interaction partner CHMP4B are localised on endosomes in MEF cells, when the activity of the endosomal protein VPS4 is reduced. This indicates that CC2D1A cycles between the cytosol and the endosomal membrane. Additionally, in rescue experiments in D. melanogaster, CC2D1A and CC2D1B were able to functionally replace Lgd. Altogether our data suggest a functional conservation of the Lgd protein family in the ESCRT-III mediated process in metazoans.

The proteins of the Lgd/CC2D1 family are conserved in all multicellular animals. The Drosophila melanogaster ortholog Lgd is involved in the regulation of signalling receptor degradation via the endosomal pathway. Loss of lgd function causes ectopic ligand-independent activation of the Notch signalling pathway due to a defect in the endosomal pathway. For the mammalian proteins no endosomal function has been defined so far. Here, we asked whether the function of Lgd is conserved in mammals with the focus on the question whether its orthologs are also involved in the endosomal pathway and regulation of Notch pathway activity. Therefore, we generated and characterised Cc2d1a and Cc2d1b conditional knockout mice. We found that the loss of Cc2d1b does not lead to an obvious phenotype, while the known lethality of Cc2d1a deficient newborns is nervous system dependent. In experiments with MEFs isolated from knockout animals we provide evidence that both CC2D1 proteins are involved in the function of the ESCRT-III complex in a similar manner as Lgd in D. melanogaster. Moreover, we found that the loss of one CC2D1 protein is not sufficient to cause ectopic activation of Notch signalling.

Nicotinic modulation of hippocampal cell signaling and associated effects on learning and memory

The hippocampus is a key brain structure involved in synaptic plasticity associated with long-term declarative memory formation. Importantly, nicotine and activation of nicotinic acetylcholine receptors (nAChRs) can alter hippocampal plasticity and these changes may occur through modulation of hippocampal kinases and transcription factors. Hippocampal kinases such as cAMP-dependent protein kinase (PKA), calcium/calmodulin-dependent protein kinases (CAMKs), extracellular signal-regulated kinases 1 and 2 (ERK1/2), and c-jun N-terminal kinase 1 (JNK1), and the transcription factor cAMP-response element-binding protein (CREB) that are activated either directly or indirectly by nicotine may modulate hippocampal plasticity and in parallel hippocampus-dependent learning and memory. Evidence suggests that nicotine may alter hippocampus-dependent learning by changing the time and magnitude of activation of kinases and transcription factors normally involved in learning and by recruiting additional cell signaling molecules. Understanding how nicotine alters learning and memory will advance basic understanding of the neural substrates of learning and aid in understanding mental disorders that involve cognitive and learning deficits.

β-Adrenergic receptor signaling and modulation of long-term potentiation in the mammalian hippocampus

Encoding new information in the brain requires changes in synaptic strength. Neuromodulatory transmitters can facilitate synaptic plasticity by modifying the actions and expression of specific signaling cascades, transmitter receptors and their associated signaling complexes, genes, and effector proteins. One critical neuromodulator in the mammalian brain is norepinephrine (NE), which regulates multiple brain functions such as attention, perception, arousal, sleep, learning, and memory. The mammalian hippocampus receives noradrenergic innervation and hippocampal neurons express β-adrenergic receptors, which are known to play important roles in gating the induction of long-lasting forms of synaptic potentiation. These forms of long-term potentiation (LTP) are believed to importantly contribute to long-term storage of spatial and contextual memories in the brain. In this review, we highlight the contributions of noradrenergic signaling in general and β-adrenergic receptors in particular, toward modulating hippocampal LTP. We focus on the roles of NE and β-adrenergic receptors in altering the efficacies of specific signaling molecules such as NMDA and AMPA receptors, protein phosphatases, and translation initiation factors. Also, the roles of β-adrenergic receptors in regulating synaptic "tagging" and "capture" of LTP within synaptic networks of the hippocampus are reviewed. Understanding the molecular and cellular bases of noradrenergic signaling will enrich our grasp of how the brain makes new, enduring memories, and may shed light on credible strategies for improving mental health through treatment of specific disorders linked to perturbed memory processing and dysfunctional noradrenergic synaptic transmission.

Molecular mechanisms underlying the memory-enhancing effects of estradiol

This article is part of a Special Issue "Estradiol and cognition". Since the publication of the 1998 special issue of Hormones and Behavior on estrogens and cognition, substantial progress has been made towards understanding the molecular mechanisms through which 17β-estradiol (E2) regulates hippocampal plasticity and memory. Recent research has demonstrated that rapid effects of E2 on hippocampal cell signaling, epigenetic processes, and local protein synthesis are necessary for E2 to facilitate the consolidation of object recognition and spatial memories in ovariectomized female rodents. These effects appear to be mediated by non-classical actions of the intracellular estrogen receptors ERα and ERβ, and possibly by membrane-bound ERs such as the G-protein-coupled estrogen receptor (GPER). New findings also suggest a key role of hippocampally-synthesized E2 in regulating hippocampal memory formation. The present review discusses these findings in detail and suggests avenues for future study.

Diverse impact of neuronal activity at θ frequency on hippocampal long-term plasticity

Brain oscillatory activity is considered an essential aspect of brain function, and its frequency can vary from <1 Hz to >200 Hz, depending on the brain states and projection. Episodes of rhythmic activity accompany hippocampus-dependent learning and memory in vivo. Therefore, long-term synaptic potentiation (LTP) and long-term depression, which are considered viable substrates of learning and memory, are often experimentally studied in paradigms of patterned high-frequency (>50 Hz) and low-frequency (<5 Hz) stimulation. However, the impact of intermediate frequencies on neuronal plasticity remains less well understood. In particular, hippocampal neurons are specifically tuned for activity at θ frequency (4-8 Hz); this band contributes significantly to electroencephalographic signals, and it is likely to be involved in shaping synaptic strength in hippocampal circuits. Here, we review in vitro and in vivo studies showing that variation of θ-activity duration may affect long-term modification of synaptic strength and neuronal excitability in the hippocampus. Such θ-pulse-induced neuronal plasticity 1) is long-lasting, 2) may be built on previously stabilized potentiation in the synapse, 3) may produce opposite changes in synaptic strength, and 4) requires complex molecular machinery. Apparently innocuous episodes of low-frequency synaptic activity may have a profound impact on network signaling, thereby contributing to information processing in the hippocampus and beyond. In addition, θ-pulse-induced LTP might be an advantageous protocol in studies of specific molecular mechanisms of synaptic plasticity.

Noradrenergic regulation of fear and drug-associated memory reconsolidation

Emotional and traumatic experiences lead to the development of particularly strong memories that can drive neuropsychiatric disorders, such as posttraumatic stress disorder (PTSD) and drug addiction. Disruption of these memories would therefore serve as a powerful treatment option, and targeting the pathologic emotional, but not declarative, component of a memory would be ideal for clinical intervention. Research reveals that after retrieval of a consolidated memory, the memory can be destabilized, and must then be reconsolidated through synaptic plasticity to allow subsequent retrieval. Disruption of reconsolidation-related plasticity would therefore impair specific, reactivated memories. Noradrenergic signaling strengthens synaptic plasticity and is essential for encoding the emotional components of memory. Consistent with this, investigations have now revealed that noradrenergic signaling is a critical mechanism for reconsolidation of emotional memories in rodent and human models. Here, we discuss these investigations and promising clinical trials indicating that disruption of noradrenergic signaling during reconsolidation may abolish the pathologic emotional, but not declarative, component of memories allowing alleviation of neuropsychiatric disorders including PTSD and drug addiction.

Impaired presynaptic long-term potentiation in the anterior cingulate cortex of Fmr1 knock-out mice

Fragile X syndrome is a common inherited form of mental impairment. Fragile X mental retardation protein (FMRP) plays important roles in the regulation of synaptic protein synthesis, and loss of FMRP leads to deficits in learning-related synaptic plasticity and behavioral disability. Previous studies mostly focus on postsynaptic long-term potentiation (LTP) in Fmr1 knock-out (KO) mice. Here, we investigate the role of FMRP in presynaptic LTP (pre-LTP) in the adult mouse anterior cingulate cortex (ACC). Low-frequency stimulation induced LTP in layer II/III pyramidal neurons under the voltage-clamp mode. Paired-pulse ratio, which is a parameter for presynaptic changes, was decreased after the low-frequency stimulation in Fmr1 wild-type (WT) mice. Cingulate pre-LTP was abolished in Fmr1 KO mice. We also used a 64-electrode array system for field EPSP recording and found that the combination of low-frequency stimulation paired with a GluK1-containing kainate receptor agonist induced NMDA receptor-independent and metabotropic glutamate receptor-dependent pre-LTP in the WT mice. This potentiation was blocked in Fmr1 KO mice. Biochemical experiments showed that Fmr1 KO mice displayed altered translocation of protein kinase A subunits in the ACC. Our results demonstrate that FMRP plays an important role in pre-LTP in the adult mouse ACC, and loss of this pre-LTP may explain some of the behavioral deficits in Fmr1 KO mice.

Multiple cellular cascades participate in long-term potentiation and in hippocampus-dependent learning

Since its discovery by Bliss and Lomo, the phenomenon of long-term potentiation (LTP) has been extensively studied, as it was viewed as a potential cellular mechanism of learning and memory. Over the years, many signaling cascades have been implicated in its induction, consolidation and maintenance, raising questions regarding its real significance. Here, we review several of the most commonly studie signaling cascades and discuss how they converge on a common set of mechanisms likely to be involved in the maintenance of LTP. We further argue that the existence of cross-talks between these different signaling cascades can not only account for several discrepancies in the literature, but also account for the existence of different forms of LTP, which can be engaged by different types of stimulus parameters under different experimental conditions. Finally, we discuss how the understanding of the diversity of LTP mechanisms can help us understand the diversity of the types of learning and memory. This article is part of a Special Issue entitled SI: Brain and Memory.

Cyclophilin D deficiency rescues Aβ-impaired PKA/CREB signaling and alleviates synaptic degeneration

The coexistence of neuronal mitochondrial pathology and synaptic dysfunction is an early pathological feature of Alzheimer's disease (AD). Cyclophilin D (CypD), an integral part of mitochondrial permeability transition pore (mPTP), is involved in amyloid beta (Aβ)-instigated mitochondrial dysfunction. Blockade of CypD prevents Aβ-induced mitochondrial malfunction and the consequent cognitive impairments. Here, we showed the elimination of reactive oxygen species (ROS) by antioxidants probucol or superoxide dismutase (SOD)/catalase blocks Aβ-mediated inactivation of protein kinase A (PKA)/cAMP regulatory-element-binding (CREB) signal transduction pathway and loss of synapse, suggesting the detrimental effects of oxidative stress on neuronal PKA/CREB activity. Notably, neurons lacking CypD significantly attenuate Aβ-induced ROS. Consequently, CypD-deficient neurons are resistant to Aβ-disrupted PKA/CREB signaling by increased PKA activity, phosphorylation of PKA catalytic subunit (PKA C), and CREB. In parallel, lack of CypD protects neurons from Aβ-induced loss of synapses and synaptic dysfunction. Furthermore, compared to the mAPP mice, CypD-deficient mAPP mice reveal less inactivation of PKA-CREB activity and increased synaptic density, attenuate abnormalities in dendritic spine maturation, and improve spontaneous synaptic activity. These findings provide new insights into a mechanism in the crosstalk between the CypD-dependent mitochondrial oxidative stress and signaling cascade, leading to synaptic injury, functioning through the PKA/CREB signal transduction pathway.

C. elegans positive olfactory associative memory is a molecularly conserved behavioral paradigm

While it is thought that short-term memory arises from changes in protein dynamics that increase the strength of synaptic signaling, many of the underlying fundamental molecular mechanisms remain unknown.Our lab developed a Caenorhabditis elegans assay of positive olfactory short-term associative memory (STAM), in which worms learn to associate food with an odor and can remember this association for over 1h. Here we use this massed olfactory associative assay to identify regulators of C. elegans short-term and intermediate-term associative memory (ITAM) processes. We show that there are unique molecular characteristics for different temporal phases of STAM, which include: learning, which is tested immediately after training, short-term memory, tested 30min after training, intermediate-term memory, tested 1h after training, and forgetting, tested 2h after training. We find that, as in higher organisms, C. elegans STAM requires calcium and cAMP signaling, and ITAM requires protein translation. Additionally, we found that STAM and ITAM are distinct from olfactory adaptation, an associative paradigm in which worms learn to disregard an inherently attractive odor after starvation in the presence of that odor. Adaptation mutants show variable responses to short-term associative memory training. Our data distinguish between shorter forms of a positive associative memory in C. elegans that require canonical memory pathways. Study of STAM and ITAM in C. elegans could lead to a more general understanding of the distinctions between these important processes and also to the discovery of novel conserved memory regulators.

A novel mechanism for cyclic adenosine monophosphate-mediated memory formation: Role of amyloid beta

Cyclic adenosine monophosphate (cAMP) regulates long-term potentiation (LTP) and ameliorates memory in healthy and diseased brain. Increasing evidence shows that, under physiological conditions, low concentrations of amyloid β (Aβ) are necessary for LTP expression and memory formation. Here, we report that cAMP controls amyloid precursor protein (APP) translation and Aβ levels, and that the modulatory effects of cAMP on LTP occur through the stimulation of APP synthesis and Aβ production.

The differential hippocampal phosphoproteome of Apodemus sylvaticus paralleling spatial memory retrieval in the Barnes maze

Protein phosphorylation is a well-known and well-documented mechanism in memory processes. Although a large series of protein kinases involved in memory processes have been reported, information on phosphoproteins is limited. It was therefore the aim of the study to determine a partial and differential phosphoproteome along with the corresponding network in hippocampus of a wild caught mouse strain with excellent performance in several paradigms of spatial memory. Apodemus sylvaticus mice were trained in the Barnes maze, a non-invasive test system for spatial memory and untrained mice served as controls. Animals were sacrificed 6h following memory retrieval, hippocampi were taken, proteins extracted and in-solution digestion was carried out with subsequent iTRAQ double labelling. Phosphopeptides were enriched by a TiO2-based method and semi-quantified using two fragmentation principles on the LTQ-orbitrap Velos. In hippocampi of trained animals phosphopeptide levels representing signalling, neuronal, synaptosomal, cytoskeletal and metabolism proteins were at least twofold reduced or increased. Furthermore, a network revealing a link to pathways of ubiquitination, the androgen receptor, small GTPase Rab5 and MAPK signaling as well as synucleins was constructed. This work is relevant for interpretation of previous work and the design of future studies on protein phosphorylation in spatial memory.

Parkinson-related LRRK2 mutation R1441C/G/H impairs PKA phosphorylation of LRRK2 and disrupts its interaction with 14-3-3

Leucine-rich repeat kinase 2 (LRRK2) is a multidomain protein implicated in Parkinson disease (PD); however, the molecular mechanism and mode of action of this protein remain elusive. cAMP-dependent protein kinase (PKA), along with other kinases, has been suggested to be an upstream kinase regulating LRRK2 function. Using MS, we detected several sites phosphorylated by PKA, including phosphorylation sites within the Ras of complex proteins (ROC) GTPase domain as well as some previously described sites (S910 and S935). We systematically mapped those sites within LRRK2 and investigated their functional consequences. S1444 in the ROC domain was confirmed as a target for PKA phosphorylation using ROC single-domain constructs and through site-directed mutagenesis. Phosphorylation at S1444 is strikingly reduced in the major PD-related LRRK2 mutations R1441C/G/H, which are part of a consensus PKA recognition site ((1441)RASpS(1444)). Furthermore, our work establishes S1444 as a PKA-regulated 14-3-3 docking site. Experiments of direct binding to the three 14-3-3 isotypes gamma, theta, and zeta with phosphopeptides encompassing pS910, pS935, or pS1444 demonstrated the highest affinities to phospho-S1444. Strikingly, 14-3-3 binding to phospho-S1444 decreased LRRK2 kinase activity in vitro. Moreover, substitution of S1444 by alanine or by introducing the mutations R1441C/G/H, abrogating PKA phosphorylation and 14-3-3 binding, resulted in increased LRRK2 kinase activity. In conclusion, these data clearly demonstrate that LRRK2 kinase activity is modulated by PKA-mediated binding of 14-3-3 to S1444 and suggest that 14-3-3 interaction with LRRK2 is hampered in R1441C/G/H-mediated PD pathogenesis.

NMDA receptor-dependent long-term potentiation comprises a family of temporally overlapping forms of synaptic plasticity that are induced by different patterns of stimulation

N-methyl-d-aspartate receptor (NMDAR)-dependent long-term potentiation (LTP) is extensively studied since it is believed to use the same molecular mechanisms that are required for many forms of learning and memory. Unfortunately, many controversies exist, not least the seemingly simple issue concerning the locus of expression of LTP. Here, we review our recent work and some of the extensive literature on this topic and present new data that collectively suggest that LTP can be explained, during its first few hours, by the coexistence of at least three mechanistically distinct processes that are all triggered by the synaptic activation of NMDARs.

Essential role for synaptopodin in dendritic spine plasticity of the developing hippocampus

Dendritic spines are a major substrate of brain plasticity. Although many studies have focused on Ca(2+)/calmodulin-dependent protein kinase II (CaMKII)-mediated regulation of spine dynamics and synaptic function in adult brain, much less is know about protein kinase A (PKA)-dependent regulation of spine shape dynamics during postnatal brain development. Synaptopodin is a dendritic spine associated modulator of actin dynamics and a substrate of PKA. Here we show that NMDA and cAMP-induced dendritic spine expansion is impaired in hippocampal slices from 15- and 21-d-old synaptopodin-deficient mice. We further show that synaptopodin is required for full expression of PKA-dependent hippocampal long-term potentiation in 15- and 21-d-old, but not adult, mice. PKA-induced cAMP response element-binding phosphorylation is normal in the hippocampus of synaptopodin-deficient mice, suggesting that synaptopodin functions independently of cAMP response element-binding. Our results identify synaptopodin as a substrate of PKA in hippocampal neurons and point to an essential role for synaptopodin in activity-dependent regulation of dendritic spine dynamics and synaptic plasticity in postnatal brain development.

Distinct set of kinases induced after retrieval of spatial memory discriminate memory modulation processes in the mouse hippocampus

Protein phosphorylation and dephosphorylation events play a key role in memory formation and various protein kinases and phosphatases have been firmly associated with memory performance. Here, we determined expression changes of protein kinases and phosphatases following retrieval of spatial memory in CD1 mice in a Morris Water Maze task, using antibody microarrays and confirmatory Western blot. Comparing changes following single and consecutive retrieval, we identified stably and differentially expressed kinases, some of which have never been implicated before in memory functions. On the basis of these findings we define a small signaling network associated with spatial memory retrieval. Moreover, we describe differential regulation and correlation of expression levels with behavioral performance of polo-like kinase 1. Together with its recently observed genetic association to autism-spectrum disorders our data suggest a role of this kinase in balancing preservation and flexibility of learned behavior.

Transient domoic acid excitotoxicity increases BDNF expression and activates both MEK- and PKA-dependent neurogenesis in organotypic hippocampal slices

Background

We have previously reported evidence of cell proliferation and increased neurogenesis in rat organotypic hippocampal slice cultures (OHSC) after a transient excitotoxic injury to the hippocampal CA1 area induced by low concentrations of the AMPA/kainate agonist domoic acid (DOM). An increased baseline rate of neurogenesis may contribute to recovery from DOM-induced mild injury but the intracellular mechanism(s) responsible for neuronal proliferation remain unclear. The current study investigated the key intracellular pathways responsible for DOM-induced neurogenesis in OHSC including the effects of transient excitotoxicity on the expression of brain-derived neurotrophic factor (BDNF), a well-known regulator of progenitor cell mitosis.

Results

Application of a low concentration of DOM (2 μM) for 24 h followed by recovery induced a significant and long lasting increase in BDNF protein levels expressed by both neurons and microglial cells. Furthermore, the mild DOM toxicity stimulated both PKA and MEK-dependent intracellular signaling cascades and induced a significant increase in BDNF- transcription factor CREB activation and BDNF-receptor TrkB expression. Coexposure to specific inhibitors of PKA and MEK phosphorylation resulted in a significant decrease in the neurogenic marker doublecortin.

Conclusions

Our results suggest that transient excitotoxic insult induced by DOM produces BDNF and CREB overexpression via MEK and PKA pathways and that both pathways mediate, at least in part, the increased neural proliferation resulting from mild excitotoxicity.

Mice overexpressing type 1 adenylyl cyclase show enhanced spatial memory flexibility in the absence of intact synaptic long-term depression

There is significant interest in understanding the contribution of intracellular signaling and synaptic substrates to memory flexibility, which involves new learning and suppression of obsolete memory. Here, we report that enhancement of Ca²⁺-stimulated cAMP signaling by overexpressing type 1 adenylyl cyclase (AC1) facilitated long-term potentiation (LTP) but impaired long-term depression (LTD) at the hippocampal Shaffer collateral-CA1 synapses. However, following the induction of LTP, low-frequency stimulation caused comparable synaptic depotentiation in both wild type and AC1 transgenic (AC1 TG) mice. Although previous studies have suggested the function of LTD in spatial memory flexibility, AC1 TG mice showed not only better initial learning in the Morris water maze, but also faster acquisition and increased ratio of new memory formation to old memory retention during the reversal platform training. In the memory extinction test, which requires suppression of old memory without involving the acquisition of the new platform information, AC1 TG and wild type mice showed comparable performance. Our results demonstrate new functions of Ca²⁺-stimulated AC1, and also suggest that certain aspects of hippocampus-dependent behavioral flexibility may not require intact LTD.

Concomitant docosahexaenoic acid administration ameliorates stress-induced cognitive impairment in rats

Long chain n-3 fatty acids, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) may slow cognitive decline. DHA plays an important role in neural function and decreased plasma DHA are associated with cognitive decline in healthy elderly adults and in patients with Alzheimer's disease. In this study we tested a hypothesis that DHA protects cognitive functions of male Wistar rats against negative impact of prolonged restraint stress. Specifically, we attempted to characterize the preventive action of prolonged treatment with DHA enriched preparation (daily dose of DHA: 300mg/kg, p.o. for 21days) in comparison with positive control (fluoxetine: 10mg/kg daily, p.o. for 21days) against an impairment caused by chronic restraint stress (2h daily for 21days) on recognition memory tested in a object recognition task and on the spatial working memory tested in Morris water maze. We found that administration of DHA enriched preparation prevented deleterious effects of chronic restraint stress both on recognition (p<0.01) and on the working spatial memory (p<0.001).

Dysregulation of synaptic plasticity precedes appearance of morphological defects in a Pten conditional knockout mouse model of autism

The phosphoinositide signaling system is a crucial regulator of neural development, cell survival, and plasticity. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) negatively regulates phosphatidylinositol 3-kinase signaling and downstream targets. Nse-Cre Pten conditional knockout mice, in which Pten is ablated in granule cells of the dentate gyrus and pyramidal neurons of the hippocampal CA3, but not CA1, recapitulate many of the symptoms of humans with inactivating PTEN mutations, including progressive hypertrophy of the dentate gyrus and deficits in hippocampus-based social and cognitive behaviors. However, the impact of Pten loss on activity-dependent synaptic plasticity in this clinically relevant mouse model of Pten inactivation remains unclear. Here, we show that two phosphatidylinositol 3-kinase- and protein synthesis-dependent forms of synaptic plasticity, theta burst-induced long-term potentiation and metabotropic glutamate receptor (mGluR)-dependent long-term depression, are dysregulated at medial perforant path-to-dentate gyrus synapses of young Nse-Cre Pten conditional knockout mice before the onset of visible morphological abnormalities. In contrast, long-term potentiation and mGluR-dependent long-term depression are normal at CA3-CA1 pyramidal cell synapses at this age. Our results reveal that deletion of Pten in dentate granule cells dysregulates synaptic plasticity, a defect that may underlie abnormal social and cognitive behaviors observed in humans with Pten inactivating mutations and potentially other autism spectrum disorders.

The role of phosphodiesterases in hippocampal synaptic plasticity

Phosphodiesterases (PDEs) degrade cyclic nucleotides, signalling molecules that play important roles in synaptic plasticity and memory. Inhibition of PDEs may therefore enhance synaptic plasticity and memory as a result of elevated levels of these signalling molecules, and this has led to interest in PDE inhibitors as cognitive enhancers. The development of new mouse models in which PDE subtypes have been selectively knocked out and increasing selectivity of PDE antagonists means that this field is currently expanding. Roles for PDE2, 4, 5 and 9 in synaptic plasticity have so far been demonstrated and we review these studies here in the context of cyclic nucleotide signalling more generally. The role of other PDE families in synaptic plasticity has not yet been investigated, and this area promises to advance our understanding of cyclic nucleotide signalling in synaptic plasticity in the future. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

Brief environmental enrichment elicits metaplasticity of hippocampal synaptic potentiation in vivo

Long-term environmental enrichment (EE) elicits enduring effects on the adult brain, including altered synaptic plasticity. Synaptic plasticity may underlie memory formation and includes robust (>24 h) and weak (<2 h) forms of long-term potentiation (LTP) and long-term depression (LTD). Most studies of the effect of EE on synaptic efficacy have examined the consequences of very prolonged EE-exposure. It is unclear whether brief exposure to EE can alter synaptic plasticity. Clarifying this issue could help develop strategies to address cognitive deficits arising from neglect in children or adults. We assessed whether short-term EE elicits alterations in hippocampal synaptic plasticity and if social context may play a role. Adult mice were exposed to EE for 14 consecutive days. We found that robust late-LTP (>24 h) and short-term depression (<2 h) at Schaffer-collateral-CA1 synapses in freely behaving mice were unaltered, whereas early-LTP (E-LTP, <2 h) was significantly enhanced by EE. Effects were transient: E-LTP returned to control levels 1 week after cessation of EE. Six weeks later, animals were re-exposed to EE for 14 days. Under these conditions, E-LTP was facilitated into L-LTP (>24 h), suggesting that metaplasticity was induced during the first EE experience and that EE-mediated modifications are cumulative. Effects were absent in mice that underwent solitary enrichment or were group-housed without EE. These data suggest that EE in naïve animals strengthens E-LTP, and also promotes L-LTP in animals that underwent EE in the past. This indicates that brief exposure to EE, particularly under social conditions can elicit lasting positive effects on synaptic strength that may have beneficial consequences for cognition that depends on synaptic plasticity.

Synaptic effects induced by alcohol

Ethanol (EtOH) has effects on numerous cellular molecular targets, and alterations in synaptic function are prominent among these effects. Acute exposure to EtOH activates or inhibits the function of proteins involved in synaptic transmission, while chronic exposure often produces opposing and/or compensatory/homeostatic effects on the expression, localization, and function of these proteins. Interactions between different neurotransmitters (e.g., neuropeptide effects on release of small molecule transmitters) can also influence both acute and chronic EtOH actions. Studies in intact animals indicate that the proteins affected by EtOH also play roles in the neural actions of the drug, including acute intoxication, tolerance, dependence, and the seeking and drinking of EtOH. This chapter reviews the literature describing these acute and chronic synaptic effects of EtOH and their relevance for synaptic transmission, plasticity, and behavior.

Molecular constraints on synaptic tagging and maintenance of long-term potentiation: a predictive model

Protein synthesis-dependent, late long-term potentiation (LTP) and depression (LTD) at glutamatergic hippocampal synapses are well characterized examples of long-term synaptic plasticity. Persistent increased activity of protein kinase M ζ (PKMζ) is thought essential for maintaining LTP. Additional spatial and temporal features that govern LTP and LTD induction are embodied in the synaptic tagging and capture (STC) and cross capture hypotheses. Only synapses that have been “tagged” by a stimulus sufficient for LTP and learning can “capture” PKMζ. A model was developed to simulate the dynamics of key molecules required for LTP and LTD. The model concisely represents relationships between tagging, capture, LTD, and LTP maintenance. The model successfully simulated LTP maintained by persistent synaptic PKMζ, STC, LTD, and cross capture, and makes testable predictions concerning the dynamics of PKMζ. The maintenance of LTP, and consequently of at least some forms of long-term memory, is predicted to require continual positive feedback in which PKMζ enhances its own synthesis only at potentiated synapses. This feedback underlies bistability in the activity of PKMζ. Second, cross capture requires the induction of LTD to induce dendritic PKMζ synthesis, although this may require tagging of a nearby synapse for LTP. The model also simulates the effects of PKMζ inhibition, and makes additional predictions for the dynamics of CaM kinases. Experiments testing the above predictions would significantly advance the understanding of memory maintenance.

A fundamental problem in neurobiology is to understand how memories are maintained for up to years. Long-term potentiation (LTP), an enduring increase in the strength of specific connections (synapses) between neurons, is thought to comprise, at least in part, the substrate of learning and memory. What processes transduce brief stimuli into persistent LTP? Persistent increased activity of an enzyme denoted protein kinase M ζ (PKMζ) is thought essential for maintaining LTP. Only synapses that have been “tagged” by a stimulus, such as stimuli needed for LTP and learning, can “capture” PKMζ. We developed a model simulating dynamics of key molecules required for LTP and its opposite, long-term depression (LTD). The model concisely represents relationships between tagging, capture, LTD, and LTP maintenance. It makes testable predictions concerning the dynamics of PKMζ. The maintenance of LTP and memory is predicted to require positive feedback in which PKMζ enhances its own synthesis at potentiated synapses. Without synaptic capture of PKMζ, no positive feedback would occur. LTD induction is also predicted to increase PKMζ synthesis. The model also makes predictions about regulation of PKMζ synthesis. Experiments testing the above predictions would advance the understanding of memory maintenance.

Proteolytic regulation of the mitochondrial cAMP-dependent protein kinase

The mitochondrial cAMP-dependent protein kinase (PKA) is activatable in a cAMP-independent fashion. The regulatory (R) subunits of the PKA holoenzyme (R(2)C(2)), but not the catalytic (C) subunits, suffer proteolysis upon exposure of bovine heart mitochondria to digitonin, Ca(2+), and a myriad of electron transport inhibitors. Selective loss of both the RI- and RII-type subunits was demonstrated via Western blot analysis, and activation of the C subunit was revealed by phosphorylation of a validated PKA peptide substrate. Selective proteolysis transpires in a calpain-dependent fashion as demonstrated by exposure of the R and C subunits of PKA to calpain and by attenuation of R and C subunit proteolysis in the presence of calpain inhibitor I. By contrast, exposure of mitochondria to cAMP fails to promote R subunit degradation, although it does result in enhanced C subunit catalytic activity. Treatment of mitochondria with electron transport chain inhibitors rotenone, antimycin A, sodium azide, and oligomycin, as well as an uncoupler of oxidative phosphorylation, also elicits enhanced C subunit activity. These results are consistent with the notion that signals, originating from cAMP-independent sources, elicit enhanced mitochondrial PKA activity.

Protein implicated in nonsyndromic mental retardation regulates protein kinase A (PKA) activity

Mutation of the coiled-coil and C2 domain-containing 1A (CC2D1A) gene, which encodes a C2 domain and DM14 domain-containing protein, has been linked to severe autosomal recessive nonsyndromic mental retardation. Using a mouse model that produces a truncated form of CC2D1A that lacks the C2 domain and three of the four DM14 domains, we show that CC2D1A is important for neuronal differentiation and brain development. CC2D1A mutant neurons are hypersensitive to stress and have a reduced capacity to form dendrites and synapses in culture. At the biochemical level, CC2D1A transduces signals to the cyclic adenosine 3′,5′-monophosphate (cAMP)-protein kinase A (PKA) pathway during neuronal cell differentiation. PKA activity is compromised, and the translocation of its catalytic subunit to the nucleus is also defective in CC2D1A mutant cells. Consistently, phosphorylation of the PKA target cAMP-responsive element-binding protein, at serine 133, is nearly abolished in CC2D1A mutant cells. The defects in cAMP/PKA signaling were observed in fibroblast, macrophage, and neuronal primary cells derived from the CC2D1A KO mice. CC2D1A associates with the cAMP-PKA complex following forskolin treatment and accumulates in vesicles or on the plasma membrane in wild-type cells, suggesting that CC2D1A may recruit the PKA complex to the membrane to facilitate signal transduction. Together, our data show that CC2D1A is an important regulator of the cAMP/PKA signaling pathway, which may be the underlying cause for impaired mental function in nonsyndromic mental retardation patients with CC2D1A mutation.

Multi-faced neuroprotective effects of Ginsenoside Rg1 in an Alzheimer mouse model

There has been no extensive characterization of the effects of Ginsenoside Rg1, a pharmacological active component purified from the nature product ginseng, in an Alzheimer's disease mouse model. The well-characterized transgenic Alzheimer disease (AD) mice over expressing amyloid precursor protein (APP)/Aβ (Tg mAPP) and nontransgenic (nonTg) littermates at age of 6 and 9 months were treated with Rg 1 for three months via intraperitoneal injection. Mice were then evaluated for changes in amyloid pathology, neuropathology and behavior. Tg mAPP treated with Rg1 showed a significant reduction of cerebral Aβ levels, reversal of certain neuropathological changes, and preservation of spatial learning and memory, as compared to vehicle-treated mice. Rg1 treatment inhibited activity of γ-secretase in both Tg mAPP mice and B103-APP cells, indicating the involvement of Rg1 in APP regulation pathway. Furthermore, administration of Rg1 enhanced PKA/CREB pathway activation in mAPP mice and in cultured cortical neurons exposed to Aβ or glutamate-mediated synaptic stress. Most importantly, the beneficial effects on attenuation of cerebral Aβ accumulation, improvement in neuropathological and behavioral changes can be extended to the aged mAPP mice, even to 12-13 months old mice that had extensive amyloid pathology and severe neuropathological and cognitive malfunction. These studies indicate that Rg1 has profound multi-faced and neuroprotective effects in an AD mouse model. Rg1 induces neuroprotection through ameliorating amyloid pathology, modulating APP process, improving cognition, and activating PKA/CREB signaling. These findings provide a new perspective for the treatment of AD and demonstrate potential for a new class of drugs for AD treatment.

PGE2 promotes angiogenesis through EP4 and PKA Cγ pathway

Inflammation is increasingly recognized as a critical mediator of angiogenesis, and unregulated angiogenic response is involved in human diseases, including cancer. Proinflammatory prostaglandin E2 (PGE2) is secreted by many cell types and plays important roles in the process of angiogenesis via activation of cognate EP1-4 receptors. Here, we provide evidence that PGE2 promotes the in vitro tube formation of human microvascular endothelial cells, ex vivo vessel outgrowth of aortic rings, and actual in vivo angiogenesis. Use of EP subtype-selective agonists and antagonists suggested EP4 mediates the prostaglandin-induced tube formation, and this conclusion was substantiated with small interfering RNA to specifically knockdown the EP4 expression. EP4 couples to Gαs, leading to activation of protein kinase A (PKA). Inhibition of PKA activity or knockdown of PKA catalytic subunit γ with RNAi attenuates the PGE2-induced tube formation. Further, knocking down the expression of Rap1A, HSPB6, or endothelial NO synthase, which serve as PKA-activatable substrates, inhibits the tube formation, whereas knockdown of RhoA or glycogen synthase kinase 3β that are inactivated after phosphorylation by PKA increases the tube formation. These results support the existence of EP4-to-PKA angiogenic signal and provide rationale for use of selective EP4 signal inhibitors as a probable strategy to control pathologic angiogenesis.

Bidirectional synaptic plasticity and spatial memory flexibility require Ca2+-stimulated adenylyl cyclases

When certain memory becomes obsolete, effective suppression of the previously established memory is essential for animals to adapt to the changing environment. At the cellular level, reversal of synaptic potentiation may be important for neurons to acquire new information and to prevent synaptic saturation. Here, we investigated the function of Ca(2+)-stimulated cAMP signaling in the regulation of bidirectional synaptic plasticity and spatial memory formation in double knock-out mice (DKO) lacking both type 1 and 8 adenylyl cyclases (ACs). In anesthetized animals, the DKO mutants showed defective long-term potentiation (LTP) after a single high-frequency stimulation (HFS) or two spaced HFSs at 100 Hz. However, DKO mice showed normal LTP after a single HFS at 200 Hz or two compressed HFSs at 100 Hz. Interestingly, reversal of synaptic potentiation as well as de novo synaptic depression was impaired in DKO mice. In the Morris water maze, DKO mice showed defective acquisition and memory retention, although the deficits could be attenuated by overtraining or compressed trainings with a shorter intertrial interval. In the reversal platform test, DKO animals were impaired in both relearning and old memory suppression. Furthermore, the extinction of the old spatial memory was not efficient in DKO mice. These data demonstrate that Ca(2+)-stimulated AC activity is important not only for LTP and spatial memory formation but also for the suppression of both previously established synaptic potentiation and old spatial memory.

Brain region-specific decrease in the activity and expression of protein kinase A in the frontal cortex of regressive autism

Autism is a severe neurodevelopmental disorder that is characterized by impaired language, communication, and social skills. In regressive autism, affected children first show signs of normal social and language development but eventually lose these skills and develop autistic behavior. Protein kinases are essential in G-protein-coupled, receptor-mediated signal transduction and are involved in neuronal functions, gene expression, memory, and cell differentiation. We studied the activity and expression of protein kinase A (PKA), a cyclic AMP–dependent protein kinase, in postmortem brain tissue samples from the frontal, temporal, parietal, and occipital cortices, and the cerebellum of individuals with regressive autism; autistic subjects without a clinical history of regression; and age-matched developmentally normal control subjects. The activity of PKA and the expression of PKA (C-α), a catalytic subunit of PKA, were significantly decreased in the frontal cortex of individuals with regressive autism compared to control subjects and individuals with non-regressive autism. Such changes were not observed in the cerebellum, or the cortices from the temporal, parietal, and occipital regions of the brain in subjects with regressive autism. In addition, there was no significant difference in PKA activity or expression of PKA (C-α) between non-regressive autism and control groups. These results suggest that regression in autism may be associated, in part, with decreased PKA-mediated phosphorylation of proteins and abnormalities in cellular signaling.

G-protein-associated signal transduction processes are restored after postweaning environmental enrichment in Ts65Dn, a Down syndrome mouse model

Individuals with Down syndrome (DS) present cognitive deficits that can be improved by early implementation of special care programs. However, they showed limited and temporary cognitive effects. We previously demonstrated that postnatal environmental enrichment (EE) improved clearly, though temporarily, the execution of visuospatial memory tasks in Ts65Dn mice, a DS model bearing a partial trisomy of murine chromosome 16; but in contrast to wild-type littermates, there was a lack of structural plasticity in pyramidal cell structure in the trisomic cerebral cortex. In the present study, we have investigated the impact of EE on the function of adenylyl cyclase and phospholipase C as a possible mechanism underlying the time-limited improvements observed. Basal production of cyclic adenosine monophosphate (cAMP) was not affected, but responses to GTPγS, isoprenaline, noradrenaline, SKF 38393 and forskolin were depressed in the Ts65Dn hippocampus. In EE conditions, cAMP accumulation was not significantly modified in control animals with respect to nonenriched controls. However, EE had a marked effect in Ts65Dn mice, in which cAMP production was significantly increased. Similarly, EE increased phospholipase C activity in Ts65Dn mice, in response to carbachol and calcium. We conclude that EE restores the G-protein-associated signal transduction systems that are altered in Ts65Dn mice.

Increased PKA signaling disrupts recognition memory and spatial memory: role in Huntington's disease

Huntington's disease (HD) patients and mouse models show learning and memory impairment even before the onset of motor symptoms. However, the molecular events involved in this cognitive decline are still poorly understood. Here, using three different paradigms, the novel object recognition test, the T-maze spontaneous alternation task and the Morris water maze, we detected severe cognitive deficits in the R6/1 mouse model of HD before the onset of motor symptoms. When we examined the putative molecular pathways involved in these alterations, we observed hippocampal cAMP-dependent protein kinase (PKA) hyper-activation in naïve R6/1 mice compared with wild-type (WT) mice, whereas extracellular signal-regulated kinase 1/2 and calcineurin activities were not modified. Increased PKA activity resulted in hyper-phosphorylation of its substrates N-methyl-D-aspartate receptor subunit 1, Ras-guanine nucleotide releasing factor-1 and striatal-enriched protein tyrosine phosphatase, but not cAMP-responsive element binding protein or the microtubule-associated protein tau. In correlation with the over-activation of the PKA pathway, we found a down-regulation of the protein levels of some phosphodiesterase (PDE) 4 family members. Similar molecular changes were found in the hippocampus of R6/2 mice and HD patients. Furthermore, chronic treatment of WT mice with the PDE4 inhibitor rolipram up-regulated PKA activity, and induced learning and memory deficits similar to those seen in R6 mice, but had no effect on R6/1 mice cognitive impairment. Importantly, hippocampal PKA inhibition by infusion of Rp-cAMPS restored long-term memory in R6/2 mice. Thus, our results suggest that occlusion of PKA-dependent processes is one of the molecular mechanisms underlying cognitive decline in R6 animals.

Influence of late-life exposure to environmental enrichment or exercise on hippocampal function and CA1 senescent physiology

Aged (20-22 months) male Fischer 344 rats were randomly assigned to sedentary (A-SED), environmentally-enriched (A-ENR), or exercise (A-EX) conditions. After 10-12 weeks of differential experience, the 3 groups of aged rats and young sedentary controls were tested for physical and cognitive function. Spatial discrimination learning and memory consolidation, tested on the water maze, were enhanced in environmentally-enriched compared with sedentary. A-EX exhibited improved and impaired performance on the cue and spatial task, respectively. Impaired spatial learning in A-EX was likely due to a bias in response selection associated with exercise training, as object recognition memory improved for A-EX rats. An examination of senescent hippocampal physiology revealed that enrichment and exercise reversed age-related changes in long-term depression (LTD) and long-term potentiation (LTP). Rats in the enrichment group exhibited an increase in cell excitability compared with the other 2 groups of aged animals. The results indicate that differential experience biased the selection of a spatial or a response strategy and factors common across the 2 conditions, such as increased hippocampal activity associated with locomotion, contribute to reversal of senescent synaptic plasticity.