Synaptic plasticity, AMPA-R trafficking, and Ras-MAPK signaling

Identification of mapk genes, and their expression profiles in response to low salinity stress, in cobia (Rachycentron canadum)

Mitogen-activated protein kinases (MAPKs) are a class of protein kinases that regulate various physiological processes, and play a crucial role in maintaining the osmotic equilibrium of fish. The objective of this study was to identify and characterize the mapk family genes in cobia (Rachycentron canadum) and examine their expression profiles under different low salinity stress regimes (acute: from 30‰ to 10‰ in 1 h, sub-chronic: from 30‰ to 10‰ over 4 d). A total of 12 cobia mapk genes (Rcmapks) were identified and cloned, including six erk subfamily genes (Rcmapk1/3/4/6/7/15), three jnk subfamily genes (Rcmapk8/9/10) and three p38 mapk subfamily genes (Rcmapk 11/13/14). Domain analysis indicated that the RcMAPKs possessed the typical domains including S_TKc and PKc_like domain. Phylogenetic analysis revealed that the Rcmapks were most closely related to those of the turbot (Scophthalmus maximus). The tissue distribution of mapk genes in adult cobia and the expression patterns of Rcmapks under different low salinity stress regimes were investigated using quantitative real-time PCR (qRT-PCR). The results revealed that Rcmapk3/9/10/11/13/14 exhibited a relatively broad expression distribution across 14 different tissues. For all these genes the highest expression level was in the brain, except for Rcmapk14 (highly expressed in the stomach, gill, and skin). The genes Rcmapk1/6/15 showed significantly higher expression in the testis. Under acute low salinity stress, expression of Rcmapk1/3/6/7/9/11/13/14 was significantly altered in the gill, intestine, and trunk kidney, however, the aforementioned genes exhibited very different expression patterns among the three tissues. In the gill, most of the genes from the erk (Rcmapk3/6/7) and p38 mapk subfamily (Rcmapk11/13/14) were significantly up-regulated at almost all the time points (P < 0.05); Similarly, the expression of Rcmapk3/9/11/13/14 genes were significantly increased in the trunk kidney; while in the intestine, most of the altered genes (Rcmapk6/7/9/11/13/14) were significantly down-regulated at 1 h. Following the sub-chronic low salinity stress, expression of Rcmapk1/3/6/7/9/11/13/14 genes were significantly altered in all three tissues. These findings provide important reference data for elucidating the roles of cobia mapk family genes in response to low salinity stress.

Novel crosstalk mechanisms between GluA3 and Epac2 in synaptic plasticity and memory in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease which accounts for the most cases of dementia worldwide. Impaired memory, including acquisition, consolidation, and retrieval, is one of the hallmarks in AD. At the cellular level, dysregulated synaptic plasticity partly due to reduced long-term potentiation (LTP) and enhanced long-term depression (LTD) underlies the memory deficits in AD. GluA3 containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are one of key receptors involved in rapid neurotransmission and synaptic plasticity. Recent studies revealed a novel form of GluA3 involved in neuronal plasticity that is dependent on cyclic adenosine monophosphate (cAMP), rather than N-methyl-d-aspartate (NMDA). However, this cAMP-dependent GluA3 pathway is specifically and significantly impaired by amyloid beta (Aβ), a pathological marker of AD. cAMP is a key second messenger that plays an important role in modulating memory and synaptic plasticity. We previously reported that exchange protein directly activated by cAMP 2 (Epac2), acting as a main cAMP effector, plays a specific and time-limited role in memory retrieval. From electrophysiological perspective, Epac2 facilities the maintenance of LTP, a cellular event closely associated with memory retrieval. Additionally, Epac2 was found to be involved in the GluA3-mediated plasticity. In this review, we comprehensively summarize current knowledge regarding the specific roles of GluA3 and Epac2 in synaptic plasticity and memory, and their potential association with AD.

RAGE signaling is required for AMPA receptor dysfunction in the hippocampus of hyperglycemic mice

Diabetes in humans has been associated for a long time with cognitive dysfunction. In rodent animal models, cognitive dysfunction can manifest as impaired hippocampal synaptic plasticity. Particular attention has been concentrated on the receptor for advanced glycation end products (RAGE), which is implicated in multiple diabetic complications involving the development of vascular and peripheral nerve abnormalities. In this study, we hypothesize that RAGE signaling alters glutamate receptor function and expression, impairing synaptic transmission in the hippocampus. Using preparations of hippocampal slices from male mice, we show a RAGE-dependent decrease in long-term potentiation (LTP) and an increase in paired-pulse facilitation (PPF) following streptozotocin (STZ)-induced diabetes. Consistently, in hippocampal cultures from male and female neonatal mice, high glucose caused a RAGE-dependent reduction of AMPA- but not NMDA-evoked currents, and an increase in cytosolic reactive oxygen species (ROS). Consistently, when cultures were co-treated with high glucose and the RAGE antagonist FPS-ZM1, AMPA-evoked currents were unchanged. Hippocampi from STZ-induced hyperglycemic wild type (WT) mice showed increased RAGE expression concomitant with a decrease of both expression and phosphorylation (Ser 831 and 845) of the AMPA GluA1 subunit. We found these changes correlated to activation of the MAPK pathway, consistent with decreased pJNK/JNK ratio and the JNK kinase, pMEK7. As no changes in expression or phosphorylation of regulatory proteins were observed in hippocampi from STZ-induced hyperglycemic RAGE-KO mice, we report a RAGE-dependent impairment in the hippocampi of hyperglycemic WT mice, with reduced AMPA receptor expression/function and LTP deficits.

ALWPs Improve Cognitive Function and Regulate Aβ Plaque and Tau Hyperphosphorylation in a Mouse Model of Alzheimer's Disease

Recently, we reported that ALWPs, which we developed by combining Liuwei Dihuang pills (LWPs) with antler, regulate the LPS-induced neuroinflammatory response and rescue LPS-induced short- and long-term memory impairment in wild-type (WT) mice. In the present study, we examined the effects of ALWPs on Alzheimer’s disease (AD) pathology and cognitive function in WT mice as well as 5x FAD mice (a mouse model of AD). We found that administration of ALWPs significantly reduced amyloid plaque levels in 5x FAD mice and significantly decreased amyloid β (Aβ) levels in amyloid precursor protein (APP)-overexpressing H4 cells. In addition, ALWPs administration significantly suppressed tau hyperphosphorylation in 5x FAD mice. Oral administration of ALWPs significantly improved long-term memory in scopolamine (SCO)-injected WT mice and 5x FAD mice by altering dendritic spine density. Importantly, ALWPs promoted spinogenesis in primary hippocampal neurons and WT mice and modulated the dendritic spine number in an extracellular signal-regulated kinase (ERK)-dependent manner. Taken together, our results suggest that ALWPs are a candidate therapeutic drug for AD that can modulate amyloid plaque load, tau phosphorylation, and synaptic/cognitive function.

The implication of AMPA receptor in synaptic plasticity impairment and intellectual disability in fragile X syndrome

Fragile X syndrome (FXS) is the most frequently inherited form of intellectual disability and prevalent single-gene cause of autism. A priority of FXS research is to determine the molecular mechanisms underlying the cognitive and social functioning impairments in humans and the FXS mouse model. Glutamate ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) mediate a majority of fast excitatory neurotransmission in the central nervous system and are critically important for nearly all aspects of brain function, including neuronal development, synaptic plasticity, and learning and memory. Both preclinical and clinical studies have indicated that expression, trafficking, and functions of AMPARs are altered and result in altered synapse development and plasticity, cognitive impairment, and poor mental health in FXS. In this review, we discuss the contribution of AMPARs to disorders of FXS by highlighting recent research advances with a specific focus on change in AMPARs expression, trafficking, and dependent synaptic plasticity. Since changes in synaptic strength underlie the basis of learning, development, and disease, we suggest that the current knowledge base of AMPARs has reached a unique point to permit a comprehensive re-evaluation of their roles in FXS.

Role of astrocyte-synapse interactions in CNS disorders

Astrocytes comprise half of the cells in the brain. Although astrocytes have traditionally been described as playing a supportive role for neurons, they have recently been recognized as active participants in the development and plasticity of dendritic spines and synapses. Astrocytes can eliminate dendritic spines, induce synapse formation, and regulate neurotransmission and plasticity. Dendritic spine and synapse impairments are features of many neurological disorders, including autism spectrum disorder, schizophrenia, and Alzheimer's disease. In this review we will present evidence from multiple neurological disorders demonstrating that changes in astrocyte-synapse interaction contribute to the pathologies. Genomic analysis has connected altered astrocytic gene expression with synaptic deficits in a number of neurological disorders. Alterations in astrocyte-secreted factors have been implicated in the neuronal morphology and synaptic changes seen in neurodevelopmental disorders, while alteration in astrocytic glutamate uptake is a core feature of multiple neurodegenerative disorders. This evidence clearly demonstrates that maintaining astrocyte-synapse interaction is crucial for normal central nervous system functioning. Obtaining a better understanding of the role of astrocytes at synapses in health and disease will provide a new avenue for future therapeutic targeting.

The developmental pattern of the RAS/RAF/Erk1/2 pathway in the BTBR autism mouse model

BTBR mice exhibit several autistic-like behaviors and are currently used as a model for understanding mechanisms that may be responsible for the pathogenesis of autism. Ras/Raf/ERK1/2 signaling has been suggested to play an important role in neural development, learning, memory, and cognition. Two studies reported that a deletion of a locus on chromosome 16 containing the mitogen-activated protein kinase 3 (MAPK3) gene, which encodes ERK1, is associated with autism. In the present study, Ras/Raf/ERK1/2 signaling was found to be up-regulated in BTBR mice relative to matched control B6 mice, to further suggest involvement in the pathogenesis of autism. To further characterize the developmental pattern of Ras/Raf/ERK1/2 signaling, varying stages during development were sampled to reveal an up-regulation in newborn and 2-week old BTBR mice relative to age-matched B6 mice. By the age of 3-week, Ras/Raf/ERK1/2 signaling in the brain of BTBR mice was unaltered relative to B6 mice, with this trend maintained in 6-week samples. These results suggest that the alteration of Ras/Raf/ERK signaling in the early developmental stages in mice could contribute to the noted autistic phenotype. Furthermore, these findings support the value of BTBR mice to serve as a human analog for autistic etiological research and aid in a better understanding of the developmental mechanisms of autism.

A tetra(ethylene glycol) derivative of benzothiazole aniline ameliorates dendritic spine density and cognitive function in a mouse model of Alzheimer's disease

We recently reported that the tetra(ethylene glycol) derivative of benzothiazole aniline, BTA-EG4, acts as an amyloid-binding small molecule that promotes dendritic spine density and cognitive function in wild-type mice. This raised the possibility that BTA-EG4 may benefit the functional decline seen in Alzheimer's disease (AD). In the present study, we directly tested whether BTA-EG4 improves dendritic spine density and cognitive function in a well-established mouse model of AD carrying mutations in APP, PS1 and tau (APPswe;PS1M146V;tauP301L, 3xTg AD mice). We found that daily injections of BTA-EG4 for 2 weeks improved dendritic spine density and cognitive function of 3xTg AD mice in an age-dependent manner. Specifically, BTA-EG4 promoted both dendritic spine density and morphology alterations in cortical layers II/III and in the hippocampus at 6-10 months of age compared to vehicle-injected mice. However, at 13-16 months of age, only cortical spine density was improved without changes in spine morphology. The changes in dendritic spine density correlated with Ras activity, such that 6-10 month old BTA-EG4 injected 3xTg AD mice had increased Ras activity in the cortex and hippocampus, while 13-16 month old mice only trended toward an increase in Ras activity in the cortex. Finally, BTA-EG4 injected 3xTg AD mice at 6-10 months of age showed improved learning and memory; however, only minimal improvement was observed at 13-16 months of age. This behavioral improvement corresponds to a decrease in soluble Aβ 40 levels. Taken together, these findings suggest that BTA-EG4 may be beneficial in ameliorating the synaptic loss seen in early AD.

Up-regulation of Ras/Raf/ERK1/2 signaling impairs cultured neuronal cell migration, neurogenesis, synapse formation, and dendritic spine development

The Ras/Raf/ERK1/2 signaling pathway controls many cellular responses such as cell proliferation, migration, differentiation, and death. In the nervous system, emerging evidence also points to a death-promoting role for ERK1/2 in both in vitro and in vivo models of neuronal death. Recent studies have suggested that abnormal apoptosis in the central nervous system may be involved in the pathogenesis of autism. Two studies reported that both a microdeletion and microduplication on chromosome 16, which includes the MAPK3 gene that encodes ERK1, are associated with autism. In addition, our recent work showed that Ras/Raf/ERK1/2 signaling activities were significantly up-regulated in the frontal cortex of autistic individuals and in the BTBR murine model of autism. To further investigate how Ras/Raf/ERK1/2 up-regulation may lead to the development of autism, we developed a cellular model of Raf/ERK up-regulation by over-expressing c-Raf in cultured cortical neurons (CNs) and cerebellar granule cells (CGCs). We found that Raf/ERK up-regulation stimulates the migration of both CNs and CGCs, and impairs the formation of excitatory synapses in CNs. In addition, we found that Raf/ERK up-regulation inhibits the development of mature dendritic spines in CNs. Investigating the possible mechanisms through which Raf/ERK up-regulation affects excitatory synapse formation and dendritic spine development, we discovered that Raf/ERK up-regulation suppresses the development and maturation of CNs. Together, these results suggest that the up-regulation of the Raf/ERK signaling pathway may contribute to the pathogenesis of autism through both its impairment of cortical neuron development and causing neural circuit imbalances.

Synaptic dysfunction and intellectual disability

Intellectual disability (ID) is a common and highly heterogeneous paediatric disorder with a very severe social impact. Intellectual disability can be caused by environmental and/or genetic factors. Although in the last two decades a number of genes have been discovered whose mutations cause mental retardation, we are still far from identifying the impact of these mutations on brain functions. Many of the genes mutated in ID code for several proteins with a variety of functions: chromatin remodelling, pre-/post-synaptic activity, and intracellular trafficking. The prevailing hypothesis suggests that the ID phenotype could emerge from abnormal cellular processing leading to pre- and/or post-synaptic dysfunction. In this chapter, we focus on the role of small GTPases and adhesion molecules, and we discuss the mechanisms through which they lead to synaptic network dysfunction.

Ras and Rap signaling in synaptic plasticity and mental disorders

The Ras family GTPases (Ras, Rap1, and Rap2) and their downstream mitogen-activated protein kinases (ERK, JNK, and p38MAPK) and PI3K signaling cascades control various physiological processes. In neuronal cells, recent studies have shown that these parallel cascades signal distinct forms of AMPA-sensitive glutamate receptor trafficking during experience-dependent synaptic plasticity and adaptive behavior. Interestingly, both hypo- and hyperactivation of Ras/ Rap signaling impair the capacity of synaptic plasticity, underscoring the importance of a "happy-medium" dynamic regulation of the signaling. Moreover, accumulating reports have linked various genetic defects that either up- or down-regulate Ras/Rap signaling with several mental disorders associated with learning disability (e.g., Alzheimer's disease, Angelman syndrome, autism, cardio-facio-cutaneous syndrome, Coffin-Lowry syndrome, Costello syndrome, Cowden and Bannayan-Riley-Ruvalcaba syndromes, fragile X syndrome, neurofibromatosis type 1, Noonan syndrome, schizophrenia, tuberous sclerosis, and X-linked mental retardation), highlighting the necessity of happy-medium dynamic regulation of Ras/Rap signaling in learning behavior. Thus, the recent advances in understanding of neuronal Ras/Rap signaling provide a useful guide for developing novel treatments for mental diseases.

Gene expression of conditioned locomotion and context-specific locomotor sensitization controlled by morphine-associated environment

The nucleus accumbens (NAc) is involved in contextual drug associations, which might be particularly important for environmental cue-induced relapse to drug seeking. In the present study, rats were first administered repeated morphine for 5 days (5 mg/kg, i.p.) in a contextually paired and unpaired design. After reexposure to the morphine-associated environment, which induced conditioned locomotor activity in the morphine-paired group, we performed a rat 27k 70-mer oligo array to profile gene expression in the NAc. One hundred fifty-five upregulated and 88 downregulated genes were found in the paired group compared with the unpaired group. Eight gene transcripts were then selected to confirm their alterations by quantitative real-time polymerase chain reaction (qRT-PCR). The identified genes generally play important roles in neuroactive receptor-ligand interactions, synapse plasticity, ion transport, and protein phosphorylation. Furthermore, the expression of the eight selected genes that were identified and confirmed to show significant fold changes in the first microarray experiment were again measured with qRT-PCR after morphine challenge (2 mg/kg, i.p.). As expected, 2 mg/kg morphine-induced context-specific sensitization. Meanwhile, mRNA expression of the selected genes showed marked upregulation in the morphine-paired group compared with the unpaired and acute groups. These results suggest that alterations in the expression of the identified genes in the NAc may contribute to the neuroplasticity underlying contextual cue-induced relapse to drug use.

Activity level-dependent synapse-specific AMPA receptor trafficking regulates transmission kinetics

Central glutamatergic synapses may express AMPA-sensitive glutamate receptors (AMPA-Rs) with distinct gating properties and exhibit different transmission dynamics, which are important for computing various synaptic inputs received at different populations of synapses. However, how glutamatergic synapses acquire AMPA-Rs with distinct kinetics to influence synaptic integration remains poorly understood. Here I report synapse-specific trafficking of distinct AMPA-Rs in rat cortical layer 4 stellate and layer 5 pyramidal neurons. The analysis indicates that in single layer 4 stellate neurons thalamocortical synapses generate faster synaptic responses than intracortical synapses. Moreover, GluR1-containing AMPA-Rs traffic selectively into intracortical synapses, and this process requires sensory experience-dependent activity and slows down transmission kinetics. GluR4-containing AMPA-Rs traffic more heavily into thalamocortical synapses than intracortical synapses, and this process requires spontaneous synaptic activity and speeds up transmission kinetics. GluR2-containing AMPA-Rs traffic equally into both thalamocortical and intracortical synapses, and this process requires no synaptic activity and resets transmission kinetics. Notably, synaptic trafficking of distinct AMPA-Rs differentially regulates synaptic integration. Thus, synapse-specific AMPA-R trafficking coarsely sets and synaptic activity finely tunes transmission kinetics and integration properties at different synapses in central neurons.

Activity patterns govern synapse-specific AMPA receptor trafficking between deliverable and synaptic pools

In single neurons, glutamatergic synapses receiving distinct afferent inputs may contain AMPA receptors (-Rs) with unique subunit compositions. However, the cellular mechanisms by which differential receptor transport achieves this synaptic diversity remain poorly understood. In lateral geniculate neurons, we show that retinogeniculate and corticogeniculate synapses have distinct AMPA-R subunit compositions. Under basal conditions at both synapses, GluR1-containing AMPA-Rs are transported from an anatomically defined reserve pool to a deliverable pool near the postsynaptic density (PSD), but further incorporate into the PSD or functional synaptic pool only at retinogeniculate synapses. Vision-dependent activity, stimulation mimicking retinal input, or activation of CaMKII or Ras signaling regulated forward GluR1 trafficking from the deliverable pool to the synaptic pool at both synapses, whereas Rap2 signals reverse GluR1 transport at retinogeniculate synapses. These findings suggest that synapse-specific AMPA-R delivery involves constitutive and activity-regulated transport steps between morphological pools, a mechanism that may extend to the site-specific delivery of other membrane protein complexes.

Controlled aquaporin-2 expression in the hypertonic environment

The corticomedullary osmolality gradient is the driving force for water reabsorption occurring in the kidney. In the collecting duct, this gradient allows luminal water to move across aquaporin (AQP) water channels, thereby increasing urine concentration. However, this same gradient exposes renal cells to great osmotic challenges. These cells must constantly adapt to fluctuations of environmental osmolality that challenge cell volume and incite functional change. This implies profound alterations of cell phenotype regarding water permeability. AQP2 is an essential component of the urine concentration mechanism whose controlled expression dictates apical water permeability of collecting duct principal cells. This review focuses on changes of AQP2 abundance and trafficking in hypertonicity-challenged cells. Intracellular mechanisms governing these events are discussed and the biological relevance of altered AQP2 expression by hypertonicity is outlined.

Ras signaling mechanisms underlying impaired GluR1-dependent plasticity associated with fragile X syndrome

Fragile X syndrome, caused by the loss of FMR1 gene function and loss of fragile X mental retardation protein (FMRP), is the most commonly inherited form of mental retardation. The syndrome is characterized by associative learning deficits, reduced risk of cancer, dendritic spine dysmorphogenesis, and facial dysmorphism. However, the molecular mechanism that links loss of function of FMR1 to the learning disability remains unclear. Here, we report an examination of small GTPase Ras signaling and synaptic AMPA receptor (AMPA-R) trafficking in cultured slices and intact brains of wild-type and FMR1 knock-out mice. In FMR1 knock-out mice, synaptic delivery of GluR1-, but not GluR2L- and GluR4-containing AMPA-Rs is impaired, resulting in a selective loss of GluR1-dependent long-term synaptic potentiation (LTP). Although Ras activity is upregulated, its downstream MEK (extracellular signal-regulated kinase kinase)-ERK (extracellular signal-regulated kinase) signaling appears normal, and phosphoinositide 3-kinase (PI3K)-protein kinase B (PKB; or Akt) signaling is compromised in FMR1 knock-out mice. Enhancing Ras-PI3K-PKB signaling restores synaptic delivery of GluR1-containing AMPA-Rs and normal LTP in FMR1 knock-out mice. These results suggest aberrant Ras signaling as a novel mechanism for fragile X syndrome and indicate manipulating Ras-PI3K-PKB signaling to be a potentially effective approach for treating patients with fragile X syndrome.

Acute hypertonicity alters aquaporin-2 trafficking and induces a MAPK-dependent accumulation at the plasma membrane of renal epithelial cells

The unique phenotype of renal medullary cells allows them to survive and functionally adapt to changes of interstitial osmolality/tonicity. We investigated the effects of acute hypertonic challenge on AQP2 (aquaporin-2) water channel trafficking. In the absence of vasopressin, hypertonicity alone induced rapid (<10 min) plasma membrane accumulation of AQP2 in rat kidney collecting duct principal cells in situ, and in several kidney epithelial lines. Confocal microscopy revealed that AQP2 also accumulated in the trans-Golgi network (TGN) following hypertonic challenge. AQP2 mutants that mimic the Ser(256)-phosphorylated and -nonphosphorylated state accumulated at the cell surface and TGN, respectively. Hypertonicity did not induce a change in cytosolic cAMP concentration, but inhibition of either calmodulin or cAMP-dependent protein kinase A activity blunted the hypertonicity-induced increase of AQP2 cell surface expression. Hypertonicity increased p38, ERK1/2, and JNK MAPK activity. Inhibiting MAPK activity abolished hypertonicity-induced accumulation of AQP2 at the cell surface but did not affect either vasopressin-dependent AQP2 trafficking or hypertonicity-induced AQP2 accumulation in the TGN. Finally, increased AQP2 cell surface expression induced by hypertonicity largely resulted from a reduction in endocytosis but not from an increase in exocytosis. These data indicate that acute hypertonicity profoundly alters AQP2 trafficking and that hypertonicity-induced AQP2 accumulation at the cell surface depends on MAP kinase activity. This may have important implications on adaptational processes governing transcellular water flux and/or cell survival under extreme conditions of hypertonicity.