A Role in Learning for SRF: Deletion in the Adult Forebrain Disrupts LTD and the Formation of an Immediate Memory of a Novel Context

Brain-derived neurotrophic factor induces matrix metalloproteinase 9 expression in neurons via the serum response factor/c-Fos pathway

Brain-derived neurotrophic factor (BDNF) plays a pivotal role in the regulation of the transcription of genes that encode proplasticity proteins. In the present study, we provide evidence that stimulation of rat primary cortical neurons with BDNF upregulates matrix metalloproteinase 9 (MMP-9) mRNA and protein levels and increases enzymatic activity. The BDNF-induced MMP-9 transcription was dependent on extracellular signal-regulated kinase 1/2 (ERK1/2) pathway and c-Fos expression. Overexpression of AP-1 dimers in neurons led to MMP-9 promoter activation, with the most potent being those that contained c-Fos, whereas knockdown of endogenous c-Fos by small hairpin RNA (shRNA) reduced BDNF-mediated MMP-9 transcription. Additionally, mutation of the proximal AP-1 binding site in the MMP-9 promoter inhibited the activation of MMP-9 transcription. BDNF stimulation of neurons induced binding of endogenous c-Fos to the proximal MMP-9 promoter region. Furthermore, as the c-Fos gene is a known target of serum response factor (SRF), we investigated whether SRF contributes to MMP-9 transcription. Inhibition of SRF and its cofactors by either overexpression of dominant negative mutants or shRNA decreased MMP-9 promoter activation. In contrast, MMP-9 transcription was not dependent on CREB activity. Finally, we showed that neuronal activity stimulates MMP-9 transcription in a tyrosine kinase receptor B (TrkB)-dependent manner.

Synaptic depression in the CA1 region of freely behaving mice is highly dependent on afferent stimulation parameters

Persistent synaptic plasticity has been subjected to intense study in the decades since it was first described. Occurring in the form of long-term potentiation (LTP) and long-term depression (LTD), it shares many cellular and molecular properties with hippocampus-dependent forms of persistent memory. Recent reports of both LTP and LTD occurring endogenously under specific learning conditions provide further support that these forms of synaptic plasticity may comprise the cellular correlates of memory. Most studies of synaptic plasticity are performed using in vitro or in vivo preparations where patterned electrical stimulation of afferent fibers is implemented to induce changes in synaptic strength. This strategy has proven very effective in inducing LTP, even under in vivo conditions. LTD in vivo has proven more elusive: although LTD occurs endogenously under specific learning conditions in both rats and mice, its induction has not been successfully demonstrated with afferent electrical stimulation alone. In this study we screened a large spectrum of protocols that are known to induce LTD either in hippocampal slices or in the intact rat hippocampus, to clarify if LTD can be induced by sole afferent stimulation in the mouse CA1 region in vivo. Low frequency stimulation at 1, 2, 3, 5, 7, or 10 Hz given in the range of 100 through 1800 pulses produced, at best, short-term depression (STD) that lasted for up to 60 min. Varying the administration pattern of the stimuli (e.g., 900 pulses given twice at 5 min intervals), or changing the stimulation intensity did not improve the persistency of synaptic depression. LTD that lasts for at least 24 h occurs under learning conditions in mice. We conclude that a coincidence of factors, such as afferent activity together with neuromodulatory inputs, play a decisive role in the enablement of LTD under more naturalistic (e.g., learning) conditions.

Increased NR2A:NR2B ratio compresses long-term depression range and constrains long-term memory

The NR2A:NR2B subunit ratio of the NMDA receptors is widely known to increase in the brain from postnatal development to sexual maturity and to aging, yet its impact on memory function remains speculative. We have generated forebrain-specific NR2A overexpression transgenic mice and show that these mice had normal basic behaviors and short-term memory, but exhibited broad long-term memory deficits as revealed by several behavioral paradigms. Surprisingly, increased NR2A expression did not affect 1-Hz-induced long-term depression (LTD) or 100 Hz-induced long-term potentiation (LTP) in the CA1 region of the hippocampus, but selectively abolished LTD responses in the 3–5 Hz frequency range. Our results demonstrate that the increased NR2A:NR2B ratio is a critical genetic factor in constraining long-term memory in the adult brain. We postulate that LTD-like process underlies post-learning information sculpting, a novel and essential consolidation step in transforming new information into long-term memory.

Actin isoforms in neuronal development and function

The actin cytoskeleton contributes directly or indirectly to nearly every aspect of neuronal development and function. This diversity of functions is often attributed to actin regulatory proteins, although how the composition of the actin cytoskeleton itself may influence its function is often overlooked. In neurons, the actin cytoskeleton is composed of two distinct isoforms, β- and γ-actin. Functions for β-actin have been investigated in axon guidance, synaptogenesis, and disease. Insight from loss-of-function in vivo studies has also revealed novel roles for β-actin in select brain structures and behaviors. Conversely, very little is known regarding functions of γ-actin in neurons. The dysregulation or mutation of both β- and γ-actin has been implicated in multiple human neurological disorders, however, demonstrating the critical importance of these still poorly understood proteins. This chapter highlights what is currently known regarding potential distinct functions for β- and γ-actin in neurons as well as the significant areas that remain unexplored.

Elimination of the vesicular acetylcholine transporter in the forebrain causes hyperactivity and deficits in spatial memory and long-term potentiation

Basal forebrain cholinergic neurons, which innervate the hippocampus and cortex, have been implicated in many forms of cognitive function. Immunolesion-based methods in animal models have been widely used to study the role of acetylcholine (ACh) neurotransmission in these processes, with variable results. Cholinergic neurons have been shown to release both glutamate and ACh, making it difficult to deduce the specific contribution of each neurotransmitter on cognition when neurons are eliminated. Understanding the precise roles of ACh in learning and memory is critical because drugs that preserve ACh are used as treatment for cognitive deficits. It is therefore important to define which cholinergic-dependent behaviors could be improved pharmacologically. Here we investigate the contributions of forebrain ACh on hippocampal synaptic plasticity and cognitive behavior by selective elimination of the vesicular ACh transporter, which interferes with synaptic storage and release of ACh. We show that elimination of vesicular ACh transporter in the hippocampus results in deficits in long-term potentiation and causes selective deficits in spatial memory. Moreover, decreased cholinergic tone in the forebrain is linked to hyperactivity, without changes in anxiety or depression-related behavior. These data uncover the specific contribution of forebrain cholinergic tone for synaptic plasticity and behavior. Moreover, these experiments define specific cognitive functions that could be targeted by cholinergic replacement therapy.

A critical cell-intrinsic role for serum response factor in glial specification in the CNS

Astrocytes and oligodendrocytes play crucial roles in nearly every facet of nervous system development and function, including neuronal migration, synaptogenesis, synaptic plasticity, and myelination. Previous studies have widely characterized the signaling pathways important for astrocyte differentiation and unveiled a number of transcription factors that guide oligodendrocyte differentiation in the CNS. However, the identities of the transcription factors critical for astrocyte specification in the brain remain unknown. Here we show that deletion of the stimulus-dependent transcription factor, serum response factor (SRF), in neural precursor cells (NPCs) (Srf-Nestin-cKO) results in nearly 60% loss in astrocytes and 50% loss in oligodendrocyte precursors at birth. Cultured SRF-deficient NPCs exhibited normal growth rate and capacity to self-renew. However, SRF-deficient NPCs generated fewer astrocytes and oligodendrocytes in response to several lineage-specific differentiation factors. These deficits in glial differentiation were rescued by ectopic expression of wild-type SRF in SRF-deficient NPCs. Interestingly, ectopic expression of a constitutively active SRF (SRF-VP16) in NPCs augmented astrocyte differentiation in the presence of pro-astrocytic factors. However, SRF-VP16 expression in NPCs had an inhibitory effect on oligodendrocyte differentiation. In contrast, mice carrying conditional deletion of SRF in developing forebrain neurons (Srf-NEX-cKO) did not exhibit any deficits in astrocytes in the brain. Together, our observations suggest that SRF plays a critical cell-autonomous role in NPCs to regulate astrocyte and oligodendrocyte specification in vivo and in vitro.

Serum response factor regulates bone formation via IGF-1 and Runx2 signals

Serum response factor (SRF) plays vital roles in numerous cellular processes; however, the physiological function of SRF in skeletal tissue remains unknown. In several organ systems, SRF regulates the expression of insulin-like growth factor-1 (IGF-1), which is crucial for normal development of mineralized skeleton and bone remodeling throughout life. Here, we show that conditional deletion of SRF in osteoblasts by osteocalcin-Cre generated viable mice with normal body size and body weight. Compared with normal siblings, osteoblast-specific SRF-deficient adult mice exhibited a marked decrease in bone mineral density and bone formation rate. Deletion of SRF in primary mouse calvarial osteoblasts reduced cell differentiation and mineralization in vitro. This was accompanied by a decrease in IGF-1 expression and secretion. Addition of IGF-1 in the culture media enhanced osteoblast differentiation in control cells and partially restored the mineralization defect of SRF-deficient cells, supporting an important role of SRF in regulating IGF-1 and IGF-1-mediated osteoblast differentiation. IGF-1-induced Akt activation was inhibited in SRF-deficient calvarial cells and enhanced in the SRF overexpressed cells. In addition, SRF deficiency decreased the transcriptional activity of Runx2, the key transcription factor for osteogenesis. Overexpression of SRF induced Runx2 transactivity in control cells and restored Runx2 transactivity in the SRF-deficient cells. Taken together, we conclude that SRF is important for IGF-1-induced osteoblast differentiation and mineralization via regulating IGF-1 expression and Runx2 transactivity.

Overexpression of serum response factor in astrocytes improves neuronal plasticity in a model of early alcohol exposure

Neuronal plasticity deficits underlie many of the cognitive problems seen in fetal alcohol spectrum disorders (FASD). We have developed a ferret model showing that early alcohol exposure leads to a persistent disruption in ocular dominance (OD) plasticity. Recently, we showed that this deficit could be reversed by overexpression of serum response factor (SRF) in the primary visual cortex during the period of monocular deprivation (MD). Surprisingly, this restoration was observed throughout the extent of visual cortex and most of the cells transfected by the virus were positive for the astrocytic marker GFAP rather than the neuronal marker NeuN. Here we test whether overexpression of SRF exclusively in astrocytes is sufficient to restore OD plasticity in alcohol-exposed ferrets. To accomplish that, first we exposed cultured astrocytes to Sindbis viruses carrying either a constitutively active form of SRF (SRF+), a dominant negative (SRF-) or control Green Fluorescent Protein (GFP). After 24h, these astrocytes were implanted in the visual cortex of alcohol-exposed animals or saline controls one day before MD. Optical imaging of intrinsic signals showed that alcohol-exposed animals that were implanted with astrocytes expressing SRF, but not SRF- or GFP, showed robust restoration of OD plasticity in all visual cortex. These findings suggest that overexpression of SRF exclusively in astrocytes can improve neuronal plasticity in FASD.

Early alcohol exposure disrupts visual cortex plasticity in mice

There is growing evidence that deficits in neuronal plasticity underlie the cognitive problems seen in fetal alcohol spectrum disorders (FASD). However, the mechanisms behind these deficits are not clear. Here we test the effects of early alcohol exposure on ocular dominance plasticity (ODP) in mice and the reversibility of these effects by phosphodiesterase (PDE) inhibitors. Mouse pups were exposed to 5 g/kg of 25% ethanol i.p. on postnatal days (P) 5, 7 and 9. This type of alcohol exposure mimics binge drinking during the third trimester equivalent of human gestation. To assess ocular dominance plasticity animals were monocularly deprived at P21 for 10 days, and tested using optical imaging of intrinsic signals. During the period of monocular deprivation animals were treated with vinpocetine (20mg/kg; PDE1 inhibitor), rolipram (1.25mg/kg; PDE4 inhibitor), vardenafil (3mg/kg; PDE5 inhibitor) or vehicle solution. Monocular deprivation resulted in the expected shift in ocular dominance of the binocular zone in saline controls but not in the ethanol group. While vinpocetine successfully restored ODP in the ethanol group, rolipram and vardenafil did not. However, when rolipram and vardenafil were given simultaneously ODP was restored. PDE4 and PDE5 are specific to cAMP and cGMP respectively, while PDE1 acts on both of these nucleotides. Our findings suggest that the combined activation of the cAMP and cGMP cascades may be a good approach to improve neuronal plasticity in FASD models.

cAMP response element-binding protein is a primary hub of activity-driven neuronal gene expression

Long-lasting forms of neuronal plasticity require de novo gene expression, but relatively little is known about the events that occur genome-wide in response to activity in a neuronal network. Here, we unveil the gene expression programs initiated in mouse hippocampal neurons in response to different stimuli and explore the contribution of four prominent plasticity-related transcription factors (CREB, SRF, EGR1, and FOS) to these programs. Our study provides a comprehensive view of the intricate genetic networks and interactions elicited by neuronal stimulation identifying hundreds of novel downstream targets, including novel stimulus-associated miRNAs and candidate genes that may be differentially regulated at the exon/promoter level. Our analyses indicate that these four transcription factors impinge on similar biological processes through primarily non-overlapping gene-expression programs. Meta-analysis of the datasets generated in our study and comparison with publicly available transcriptomics data revealed the individual and collective contribution of these transcription factors to different activity-driven genetic programs. In addition, both gain- and loss-of-function experiments support a pivotal role for CREB in membrane-to-nucleus signal transduction in neurons. Our data provide a novel resource for researchers wanting to explore the genetic pathways associated with activity-regulated neuronal functions.

Serum response factor is required for cortical axon growth but is dispensable for neurogenesis and neocortical lamination

Previous studies have shown that neuron-specific deletion of serum response factor (SRF) results in deficits in tangential cell migration, guidance-dependent circuit assembly, activity-dependent gene expression, and synaptic plasticity in the hippocampus. Furthermore, SRF deletion in mouse embryonic stem cells causes cell death in vitro. However, the requirement of SRF for early neuronal development including neural stem cell homeostasis, neurogenesis, and axonal innervations remains unknown. Here, we report that SRF is critical for development of major axonal tracts in the forebrain. Conditional mutant mice lacking SRF in neural progenitor cells (Srf-Nestin-cKO) exhibit striking deficits in cortical axonal projections including corticostriatal, corticospinal, and corticothalamic tracts, and they show a variable loss of the corpus callosum. Neurogenesis and interneuron specification occur normally in the absence of SRF and the deficits in axonal projections were not due to a decrease or loss in cell numbers. Radial migration of neurons and neocortical lamination were also not affected. No aberrant cell death was observed during development, whereas there was an increase in the number of proliferative cells in the ventricular zone from embryonic day 14 to day 18. Similar axonal tract deficits were also observed in mutant mice lacking SRF in the developing excitatory neurons of neocortex and hippocampus (Srf-NEX-cKO). Together, these findings suggest distinct roles for SRF during neuronal development; SRF is specifically required in a cell-autonomous manner for axonal tract development but is dispensable for cell survival, neurogenesis, neocortical lamination, and neuronal differentiation.

Calcium signaling in synapse-to-nucleus communication

Changes in the intracellular concentration of calcium ions in neurons are involved in neurite growth, development, and remodeling, regulation of neuronal excitability, increases and decreases in the strength of synaptic connections, and the activation of survival and programmed cell death pathways. An important aspect of the signals that trigger these processes is that they are frequently initiated in the form of glutamatergic neurotransmission within dendritic trees, while their completion involves specific changes in the patterns of genes expressed within neuronal nuclei. Accordingly, two prominent aims of research concerned with calcium signaling in neurons are determination of the mechanisms governing information conveyance between synapse and nucleus, and discovery of the rules dictating translation of specific patterns of inputs into appropriate and specific transcriptional responses. In this article, we present an overview of the avenues by which glutamatergic excitation of dendrites may be communicated to the neuronal nucleus and the primary calcium-dependent signaling pathways by which synaptic activity can invoke changes in neuronal gene expression programs.

PI3Kγ is required for NMDA receptor-dependent long-term depression and behavioral flexibility

Phosphatidylinositol 3-kinase (PI3K) has been implicated in synaptic plasticity and other neural functions in the brain. However, the role of individual PI3K isoforms in the brain is unclear. We investigated the role of PI3Kγ in hippocampal-dependent synaptic plasticity and cognitive functions. We found that PI3Kγ has a crucial and specific role in NMDA receptor (NMDAR)-mediated synaptic plasticity at mouse Schaffer collateral-commissural synapses. Both genetic deletion and pharmacological inhibition of PI3Kγ disrupted NMDAR long-term depression (LTD) while leaving other forms of synaptic plasticity intact. Accompanying this physiological deficit, the impairment of NMDAR LTD by PI3Kγ blockade was specifically correlated with deficits in behavioral flexibility. These findings suggest that a specific PI3K isoform, PI3Kγ, is critical for NMDAR LTD and some forms of cognitive function. Thus, individual isoforms of PI3Ks may have distinct roles in different types of synaptic plasticity and may therefore influence various kinds of behavior.

Ephrin-A5 suppresses neurotrophin evoked neuronal motility, ERK activation and gene expression

During brain development, growth cones respond to attractive and repulsive axon guidance cues. How growth cones integrate guidance instructions is poorly understood. Here, we demonstrate a link between BDNF (brain derived neurotrophic factor), promoting axonal branching and ephrin-A5, mediating axonal repulsion via Eph receptor tyrosine kinase activation. BDNF enhanced growth cone filopodial dynamics and neurite branching of primary neurons. We show that ephrin-A5 antagonized this BDNF-evoked neuronal motility. BDNF increased ERK phosphorylation (P-ERK) and nuclear ERK entry. Ephrin-A5 suppressed BDNF-induced ERK activity and might sequester P-ERK in the cytoplasm. Neurotrophins are well established stimulators of a neuronal immediate early gene (IEG) response. This is confirmed in this study by e.g. c-fos, Egr1 and Arc upregulation upon BDNF application. This BDNF-evoked IEG response required the transcription factor SRF (serum response factor). Notably, ephrin-A5 suppressed a BDNF-evoked neuronal IEG response, suggesting a role of Eph receptors in modulating gene expression. In opposite to IEGs, long-term ephrin-A5 application induced cytoskeletal gene expression of tropomyosin and actinin. To uncover specific Eph receptors mediating ephrin-As impact on neurotrophin signaling, EphA7 deficient mice were analyzed. In EphA7 deficient neurons alterations in growth cone morphology were observed. However, ephrin-A5 still counteracted neurotrophin signaling suggesting that EphA7 is not required for ephrin and BDNF crosstalk. In sum, our data suggest an interaction of ephrin-As and neurotrophin signaling pathways converging at ERK signaling and nuclear gene activity. As ephrins are involved in development and function of many organs, such modulation of receptor tyrosine kinase signaling and gene expression by Ephs might not be limited to the nervous system.

Mechanisms of specificity in neuronal activity-regulated gene transcription

The brain is a highly adaptable organ that is capable of converting sensory information into changes in neuronal function. This plasticity allows behavior to be accommodated to the environment, providing an important evolutionary advantage. Neurons convert environmental stimuli into long-lasting changes in their physiology in part through the synaptic activity-regulated transcription of new gene products. Since the neurotransmitter-dependent regulation of Fos transcription was first discovered nearly 25 years ago, a wealth of studies have enriched our understanding of the molecular pathways that mediate activity-regulated changes in gene transcription. These findings show that a broad range of signaling pathways and transcriptional regulators can be engaged by neuronal activity to sculpt complex programs of stimulus-regulated gene transcription. However, the shear scope of the transcriptional pathways engaged by neuronal activity raises the question of how specificity in the nature of the transcriptional response is achieved in order to encode physiologically relevant responses to divergent stimuli. Here we summarize the general paradigms by which neuronal activity regulates transcription while focusing on the molecular mechanisms that confer differential stimulus-, cell-type-, and developmental-specificity upon activity-regulated programs of neuronal gene transcription. In addition, we preview some of the new technologies that will advance our future understanding of the mechanisms and consequences of activity-regulated gene transcription in the brain.

The AAA+ ATPase Thorase regulates AMPA receptor-dependent synaptic plasticity and behavior

The synaptic insertion or removal of AMPA receptors (AMPAR) plays critical roles in the regulation of synaptic activity reflected in the expression of long-term potentiation (LTP) and long-term depression (LTD). The cellular events underlying this important process in learning and memory are still being revealed. Here we describe and characterize the AAA+ ATPase Thorase, which regulates the expression of surface AMPAR. In an ATPase-dependent manner Thorase mediates the internalization of AMPAR by disassembling the AMPAR-GRIP1 complex. Following genetic deletion of Thorase, the internalization of AMPAR is substantially reduced, leading to increased amplitudes of miniature excitatory postsynaptic currents, enhancement of LTP, and elimination of LTD. These molecular events are expressed as deficits in learning and memory in Thorase null mice. This study identifies an AAA+ ATPase that plays a critical role in regulating the surface expression of AMPAR and thereby regulates synaptic plasticity and learning and memory.

Fetal alcohol spectrum disorders and abnormal neuronal plasticity

The ingestion of alcohol during pregnancy can result in a group of neurobehavioral abnormalities collectively known as fetal alcohol spectrum disorders (FASD). During the past decade, studies using animal models indicated that early alcohol exposure can dramatically affect neuronal plasticity, an essential property of the central nervous system responsible for the normal wiring of the brain and involved in processes such as learning and memory. The abnormalities in neuronal plasticity caused by alcohol can explain many of the neurobehavioral deficits observed in FASD. Conversely, improving neuronal plasticity may have important therapeutic benefits. In this review, the author discuss the mechanisms that lead to these abnormalities and comment on recent pharmacological approaches that have been showing promising results in improving neuronal plasticity in FASD.

Strain-dependent variations in spatial learning and in hippocampal synaptic plasticity in the dentate gyrus of freely behaving rats

Hippocampal synaptic plasticity is believed to comprise the cellular basis for spatial learning. Strain-dependent differences in synaptic plasticity in the CA1 region have been reported. However, it is not known whether these differences extend to other synapses within the trisynaptic circuit, although there is evidence for morphological variations within that path. We investigated whether Wistar and Hooded Lister (HL) rat strains express differences in synaptic plasticity in the dentate gyrus in vivo. We also explored whether they exhibit differences in the ability to engage in spatial learning in an eight-arm radial maze. Basal synaptic transmission was stable over a 24-h period in both rat strains, and the input–output relationship of both strains was not significantly different. Paired-pulse analysis revealed significantly less paired-pulse facilitation in the HL strain when pulses were given 40–100 ms apart. Low frequency stimulation at 1 Hz evoked long-term depression (>24 h) in Wistar and short-term depression (<2 h) in HL rats; 200 Hz stimulation induced long-term potentiation (>24 h) in Wistar, and a transient, significantly smaller potentiation (<1 h) in HL rats, suggesting that HL rats have higher thresholds for expression of persistent synaptic plasticity. Training for 10 days in an eight-arm radial maze revealed that HL rats master the working memory task faster than Wistar rats, although both strains show an equivalent performance by the end of the trial period. HL rats also perform more efficiently in a double working and reference memory task. On the other hand, Wistar rats show better reference memory performance on the final (8–10) days of training. Wistar rats were less active and more anxious than HL rats. These data suggest that strain-dependent variations in hippocampal synaptic plasticity occur in different hippocampal synapses. A clear correlation with differences in spatial learning is not evident however.

Substitution pattern of the CArG element in human and mouse genomes

cis-Elements CArG bound by serum response factor (SRF) are presently being intensively studied, but little is known about the substitution pattern of functional CArG elements. Here, we have performed the first evolutionary analysis of CArGome in the human and mouse genome through bioinformatic methods and statistical tests. We calculated the substitution rate at each site of the functional CArG elements. The results showed that the core sites of the functional CArG elements evolved faster than did the background DNA, indicating that these sites were likely to evolve under positive selection. Moreover, a strong TATA "motif" was evident in the core region within the functional CArG elements in both human and mouse promoters. This motif could probably be a major contribution to the formation of the spatial structure, which was important for CArG-SRF recognition. Thus, the study further revealed the sequence character and substitution pattern of CArG elements and provided useful information for the study of the SRF-binding efficiencies of CArG promoters in functional assays.

Therapeutic utility of phosphodiesterase type I inhibitors in neurological conditions

Neuronal plasticity is an essential property of the brain that is impaired in different neurological conditions. Phosphodiesterase type 1 (PDE1) inhibitors can enhance levels of the second messengers cAMP/cGMP leading to the expression of neuronal plasticity-related genes, neurotrophic factors, and neuroprotective molecules. These neuronal plasticity enhancement properties make PDE1 inhibitors good candidates as therapeutic agents in many neurological conditions. However, the lack of specificity of the drugs currently available poses a challenge to the systematic evaluation of the beneficial effect of these agents. The development of more specific drugs may pave the way for the use of PDE1 inhibitors as therapeutic agents in cases of neurodevelopmental conditions such as fetal alcohol spectrum disorders and in degenerative disorders such as Alzheimer's and Parkinson's.

Effects of serum response factor (SRF) deletion on conditioned reinforcement

Serum response factor (SRF) is a ubiquitously expressed stimulus-dependent transcription factor that regulates gene expression by binding to serum response element in the promoter region of target genes. Recent studies in mice have shown that SRF is important for activity-dependent gene expression and synaptic plasticity in the adult brain but is dispensable for neuronal survival. Given these important functions of SRF in the CNS, it is expected to play a critical role in several aspects of learning and memory. Here we evaluated the role of SRF in conditioned reinforcement using two lines of conditional SRF mutant mice. These SRF mutant mice exhibited different spatial patterns of SRF deletion in the post-natal forebrain and notably within the hippocampus. SRF deletion was more widespread in SRF-CKCre mutants than in SRF-SynCre mutants, particularly in areas of the cortex and striatum. Mutant and wild-type mice were trained to associate one auditory cue (CS+) with reward, whereas a second cue remained relatively neutral (CS-). All mice readily acquired this discrimination, entering the food cup during CS+ but not during CS-. In a subsequent test of conditioned reinforcement, in the absence of food, wild-type control mice and SRF-SynCre mice learned to selectively perform an instrumental response that yielded CS+ presentation rather than another response that produced CS-. SRF-CKCre mutants failed to show this preferential responding for CS+. These results suggest a role for SRF in conditioned reinforcement, a manifestation of incentive learning that has been implicated in many aspects of adaptive and maladaptive behavior, such as substance abuse and eating disorders.

Serum response factor promotes resilience to chronic social stress through the induction of DeltaFosB

The molecular mechanisms underlying stress- and drug-induced neuronal adaptations are incompletely understood. One molecule implicated in such adaptations is ΔFosB, a transcription factor that accumulates in the rodent nucleus accumbens (NAc), a key brain reward region, in response to either chronic stress or repeated exposure to drugs of abuse. The upstream transcriptional mechanisms controlling ΔFosB induction by these environmental stimuli remain elusive. Here, we identify the activity-dependent transcription factor, serum response factor (SRF), as a novel upstream mediator of stress-, but not cocaine-, induced ΔFosB. SRF is downregulated in NAc of both depressed human patients and in mice chronically exposed to social defeat stress. This downregulation of SRF is absent in resilient animals. Through the use of inducible mutagenesis, we show that stress-mediated induction of ΔFosB, which occurs predominantly in resilient mice, is dependent on SRF expression in this brain region. Furthermore, NAc-specific genetic deletion of SRF promotes a variety of prodepressant- and proanxiety-like phenotypes and renders animals more sensitive to the deleterious effects of chronic stress. In contrast, we demonstrate that SRF does not play a role in ΔFosB accumulation in NAc in response to chronic cocaine exposure. Furthermore, NAc-specific knock-out of SRF has no effect on cocaine-induced behaviors, indicating that chronic social defeat stress and repeated cocaine exposure regulate ΔFosB accumulation and behavioral sensitivity through independent mechanisms.

Hippocampal gene profiling: toward a systems biology of the hippocampus

Transcriptomics and proteomics approaches give a unique perspective for understanding brain and hippocampal functions but also pose unique challenges because of the singular complexity of the nervous system. The proliferation of genome-wide expression studies during the last decade has provided important insight into the molecular underpinnings of brain anatomy, neural plasticity, and neurological diseases. Microarray technology has dominated transcriptomics research, but this situation is rapidly changing with the recent technological advances in high-throughput sequencing. The full potential of transcriptomics in the neurosciences will be achieved as a result of its integration with other "-omics" disciplines as well as the development of novel analytical bioinformatics and systems biology tools for meta-analysis. Here, we review some of the most relevant advances in the gene profiling of the hippocampus, its relationship with proteomics approaches, and the promising perspectives for the future.

Actin-mediated gene expression in neurons: the MRTF-SRF connection

The traditional view of cellular actin is a rather autarkic cytoskeletal framework function confined to the cytoplasm. However, there is now evidence that alterations in actin dynamics are sensed by the nucleus and subsequently modulate gene expression. In communicating with the nucleus, cytoplasmic, and most likely also nucleus-resident actin, provides a further (gene) regulatory loop to cell motility. A transcription module composed of MRTF (myocardin-related transcription factor) and SRF (serum response factor) emerges as prime target of such actin signaling. Here, I focus on the nervous system, where the actin-MRTF-SRF entity governs multiple aspects of neuronal motility.

Hippocampal long-term depression is required for the consolidation of spatial memory

Although NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) and long-term depression (LTD) of glutamatergic transmission are candidate mechanisms for long-term spatial memory, the precise contributions of LTP and LTD remain poorly understood. Here, we report that LTP and LTD in the hippocampal CA1 region of freely moving adult rats were prevented by NMDAR 2A (GluN2A) and 2B subunit (GluN2B) preferential antagonists, respectively. These results strongly suggest that NMDAR subtype preferential antagonists are appropriate tools to probe the roles of LTP and LTD in spatial memory. Using a Morris water maze task, the LTP-blocking GluN2A antagonist had no significant effect on any aspect of performance, whereas the LTD-blocking GluN2B antagonist impaired spatial memory consolidation. Moreover, similar spatial memory deficits were induced by inhibiting the expression of LTD with intrahippocampal infusion of a short peptide that specifically interferes with AMPA receptor endocytosis. Taken together, our findings support a functional requirement of hippocampal CA1 LTD in the consolidation of long-term spatial memory.

Role of serum response factor in the pathogenesis of disease

Serum response factor (SRF) is a ubiquitously expressed transcription factor that binds to a DNA cis element known as the CArG box, which is found in the proximal regulatory regions of over 200 experimentally validated target genes. Genetic deletion of SRF is incompatible with life in a variety of animals from different phyla. In mice, loss of SRF throughout the early embryo results in gastrulation defects precluding analyses in individual organ systems. Genetic inactivation studies using conditional or inducible promoters directing the expression of the bacteriophage Cre recombinase have shown a vital role for SRF in such cellular processes as contractility, cell migration, synaptic activity, inflammation, and cell survival. A growing number of experimental and human diseases are associated with changes in SRF expression, suggesting that SRF has a role in the pathogenesis of disease. This review summarizes data from experimental model systems and human pathology where SRF expression is either deliberately or naturally altered.

Involvement of the serum response factor coactivator megakaryoblastic leukemia (MKL) in the activin-regulated dendritic complexity of rat cortical neurons

Dynamic changes in neuronal morphology and transcriptional regulation play crucial roles in the neuronal network and function. Accumulating evidence suggests that the megakaryoblastic leukemia (MKL) family members, which function not only as actin-binding proteins but also as serum response factor (SRF) transcriptional coactivators, regulate neuronal morphology. However, the extracellular ligands and signaling pathways, which activate MKL-mediated morphological changes in neurons, remain unresolved. Here, we demonstrate that in addition to MKL1, MKL2, highly enriched in the forebrain, strongly contributes to the dendritic complexity, and this process is triggered by stimulation with activin, a member of the transforming growth factor β (TGF-β) superfamily. Activin promoted dendritic complexity in a SRF- and MKL-dependent manner without drastically affecting MKL localization and protein levels. In contrast, activin promoted the nuclear export of suppressor of cancer cell invasion (SCAI), which is a corepressor for SRF and MKL. Furthermore, overexpression of SCAI blocked activin-induced SRF transcriptional responses and dendritic complexity. Collectively, these results strongly suggest that activin-SCAI-MKL signaling is a novel pathway that regulates the dendritic morphology of rat cortical neurons by excluding SCAI from the nucleus and activating MKL/SRF-mediated gene expression.

Myocardin-related transcription factors regulate the Cdk5/Pctaire1 kinase cascade to control neurite outgrowth, neuronal migration and brain development

Numerous motile cell functions depend on signaling from the cytoskeleton to the nucleus. Myocardin-related transcription factors (MRTFs) translocate to the nucleus in response to actin polymerization and cooperate with serum response factor (Srf) to regulate the expression of genes encoding actin and other components of the cytoskeleton. Here, we show that MRTF-A (Mkl1) and MRTF-B (Mkl2) redundantly control neuronal migration and neurite outgrowth during mouse brain development. Conditional deletion of the genes encoding these Srf coactivators disrupts the formation of multiple brain structures, reflecting a failure in neuronal actin polymerization and cytoskeletal assembly. These abnormalities were accompanied by dysregulation of the actin-severing protein gelsolin and Pctaire1 (Cdk16) kinase, which cooperates with Cdk5 to initiate a kinase cascade that governs cytoskeletal rearrangements essential for neuron migration and neurite outgrowth. Thus, the MRTF/Srf partnership interlinks two key signaling pathways that control actin treadmilling and neuronal maturation, thereby fulfilling a regulatory loop that couples cytoskeletal dynamics to nuclear gene transcription during brain development.

Involvement of the metabotropic glutamate receptor mGluR5 in NMDA receptor-dependent, learning-facilitated long-term depression in CA1 synapses

Learning-facilitated synaptic plasticity describes the ability of hippocampal synapses to respond with persistent synaptic plasticity to the coupling of weak afferent stimulation, which is subthreshold for the induction of plasticity, with a spatial learning experience. The metabotropic glutamate receptor subtype 5 (mGluR5) is critically involved in enabling the persistency of multiple forms of hippocampal synaptic plasticity. We compared the effects of pharmacological allosteric antagonism of mGluR5 in learning-facilitated plasticity with plasticity that had been induced solely by patterned afferent stimulation of the Schaffer collateral pathway to the CA1 stratum radiatum of adult freely behaving rats. Intracerebroventricular injection of the selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) had no effect on basal synaptic transmission but significantly prevented both long-term depression (LTD) elicited by electrical stimulation and LTD facilitated by novel object-place configuration learning. NMDA receptor antagonism also prevented learning-facilitated LTD. Habituation to the objects was prevented by MPEP application. Whereas reexposure to the object-place configuration (after 7 days) failed to facilitate LTD in control animals, those who had been treated previously with MPEP expressed LTD, suggesting that inhibition of learning contributed to the initial prevention of LTD. These data support a pivotal role for mGluR5 in both hippocampal LTD and the acquisition of object-place configurations.

Bidirectional regulation of hippocampal long-term synaptic plasticity and its influence on opposing forms of memory

Reference memory characterizes the long-term storage of information acquired through numerous trials. In contrast, working memory represents the short-term acquisition of trial-unique information. A number of studies in the rodent hippocampus have focused on the contribution of long-term synaptic potentiation (LTP) to long-term reference memory. In contrast, little is known about the synaptic plasticity correlates of hippocampal-based components of working memory. Here, we described a mouse with selective expression of a dominant-negative mutant of the regulatory subunit of protein kinase A (PKA) only in two regions of the hippocampus, the dentate gyrus and area CA1. This mouse showed a deficit in several forms of LTP in both hippocampal subregions and a lowered threshold for the consolidation of long-term synaptic depression (LTD). When trained with one trial per day in a water maze task, mutant mice displayed a deficit in consolidation of long-term memory. In contrast, these mice proved to be more flexible after a transfer test and also showed a delay-dependent increased performance in working memory, when repetitive information (proactive interference) was presented. We suggest that through its bidirectional control over synaptic plasticity PKA can regulate opposing forms of memory. The defect in L-LTP disrupts long-term memory consolidation. The persistence of LTD may allow acquisition of new information by restricting the body of previously stored information and suppressing interference.

Overexpression of serum response factor restores ocular dominance plasticity in a model of fetal alcohol spectrum disorders

Neuronal plasticity deficits underlie many of the neurobehavioral problems seen in fetal alcohol spectrum disorders (FASD). Recently, we showed that third trimester alcohol exposure leads to a persistent disruption in ocular dominance (OD) plasticity. For instance, a few days of monocular deprivation results in a robust reduction of cortical regions responsive to the deprived eye in normal animals, but not in ferrets exposed early to alcohol. This plasticity deficit can be reversed if alcohol-exposed animals are treated with a phosphodiesterase type 1 (PDE1) inhibitor during the period of monocular deprivation. PDE1 inhibition can increase cAMP and cGMP levels, activating transcription factors such as the cAMP response element binding protein (CREB) and the serum response factor (SRF). SRF is important for many plasticity processes such as LTP, LTD, spine motility, and axonal pathfinding. Here we attempt to rescue OD plasticity in alcohol-treated ferrets using a Sindbis viral vector to express a constitutively active form of SRF during the period of monocular deprivation. Using optical imaging of intrinsic signals and single-unit recordings, we observed that overexpression of a constitutively active form of SRF, but neither its dominant-negative nor GFP, restored OD plasticity in alcohol-treated animals. Surprisingly, this restoration was observed throughout the extent of the primary visual cortex and most cells infected by the virus were positive for GFAP rather than NeuN. This finding suggests that overexpression of SRF in astrocytes may reduce the deficits in neuronal plasticity seen in models of FASD.

Loss of the serum response factor in the dopamine system leads to hyperactivity

The serum response factor (SRF) is a key regulator of neural development and cellular plasticity, which enables it to act as a regulator of long-term adaptations in neurons. Here we performed a comprehensive analysis of SRF function in the murine dopamine system. We found that loss of SRF in dopaminoceptive, but not dopaminergic, neurons is responsible for the development of a hyperactivity syndrome, characterized by reduced body weight into adulthood, enhanced motor activity, and deficits in habituation processes. Most important, the hyperactivity also develops when the ablation of SRF is induced in adult animals. On the molecular level, the loss of SRF in dopaminoceptive cells is associated with altered expression of neuronal plasticity-related genes, in particular transcripts involved in calcium ion binding, formation of the cytoskeleton, and transcripts encoding neuropeptide precursors. Furthermore, abrogation of SRF causes specific deficits in activity-dependent transcription, especially a complete lack of psychostimulant-induced expression of the Egr genes. We inferred that alterations in SRF-dependent gene expression underlie the observed hyperactive behavior. Thus, SRF depletion in dopaminoceptive neurons might trigger molecular mechanisms responsible for development of psychopathological conditions involving hyperactivity.

Post-training reversible inactivation of the hippocampus enhances novel object recognition memory

Research on the role of the hippocampus in object recognition memory has produced conflicting results. Previous studies have used permanent hippocampal lesions to assess the requirement for the hippocampus in the object recognition task. However, permanent hippocampal lesions may impact performance through effects on processes besides memory consolidation including acquisition, retrieval, and performance. To overcome this limitation, we used an intrahippocampal injection of the GABA agonist muscimol to reversibly inactivate the hippocampus immediately after training mice in two versions of an object recognition task. We found that the inactivation of the dorsal hippocampus after training impairs object-place recognition memory but enhances novel object recognition (NOR) memory. However, inactivation of the dorsal hippocampus after repeated exposure to the training context did not affect object recognition memory. Our findings suggest that object recognition memory formation does not require the hippocampus and, moreover, that activity in the hippocampus can interfere with the consolidation of object recognition memory when object information encoding occurs in an unfamiliar environment.

Serum response factor regulates hippocampal lamination and dendrite development and is connected with reelin signaling

During brain development, neurons and their nerve fibers are often segregated in specific layers. The hippocampus is a well-suited model system to study lamination in health and aberrant cell/fiber lamination associated with neurological disorders. SRF (serum response factor), a transcription factor, regulates synaptic-activity-induced immediate-early gene (IEG) induction and cytoskeleton-based neuronal motility. Using early postnatal conditional SRF ablation, we uncovered distorted hippocampal lamination, including malpositioning of granule cell neurons and disruption of layer-restricted termination of commissural-associational and mossy fiber axons. Besides axons, dendrite branching and spine morphogenesis in Srf mutants were impaired, offering a first morphological basis for SRF's reported role in learning and memory. Srf mutants resemble mice lacking components of the reelin signaling cascade, a fundamental signaling entity in brain lamination. Our data indicate that reelin signaling and SRF-mediated gene transcription might be connected: reelin induces IEG and cytoskeletal genes in an SRF-dependent manner. Further, reelin-induced neurite motility is blocked in Srf mutants and constitutively active SRF rescues impaired neurite extension in reeler mouse mutants in vitro. In sum, data provided in this report show that SRF contributes to hippocampal layer and nerve fiber organization and point at a link between Srf gene transcription and reelin signaling.

Sleeping together using social interactions to understand the role of sleep in plasticity

Social experience alters the expression of genes related to synaptic function and plasticity, induces elaborations in the morphology of neural structures throughout the brain (Volkmar and Greenough, 1972; Greenough et al., 1978; Technau, 2007), improves cognitive and behavioral performance (Pham et al., 1999a; Toscano et al., 2006) and alters subsequent sleep (Ganguly-Fitzgerald et al., 2006). In this review, we discuss the plastic mechanisms that are induced in response to social experience and how social enrichment can provide insight into the biological functions of sleep.

Functional versatility of transcription factors in the nervous system: the SRF paradigm

Individual transcription factors in the brain frequently display broad functional versatility, thereby controlling multiple cellular outputs. In accordance, neuron-restricted mutagenesis of the murine Srf gene, encoding the transcription factor serum response factor (SRF), revealed numerous SRF functions in the nervous system. First, SRF controls immediate early gene (IEG) activation associated with perception of synaptic activity, learning and memory. Second, processes linked to actin cytoskeletal dynamics are mediated by SRF, such as developmental neuronal migration, outgrowth and pathfinding of neurites, as well as synaptic targeting. Therefore, SRF seems to be instrumental in converting synaptic activity into plasticity-associated structural changes in neuronal connectivities. This highlights the decisive role of SRF in integrating cytoskeletal actin dynamics and nuclear gene expression. Finally, we relate SRF to the multi-functional transcription factor CREB and point out overlapping, distinct and concerted functions of these two transcriptional regulators in the brain.

Locus coeruleus activation facilitates memory encoding and induces hippocampal LTD that depends on beta-adrenergic receptor activation

Spatial memory formation is enabled through synaptic information processing, in the form of persistent strengthening and weakening of synapses, within the hippocampus. It is, however, unclear how relevant spatial information is selected for encoding, in preference to less pertinent information. As the noradrenergic locus coeruleus (LC) becomes active in response to novel experiences, we hypothesized that the LC may provide the saliency signal required to promote hippocampal encoding of relevant information through changes in synaptic strength. Test pulse stimulation evoked stable basal synaptic transmission at Schaffer collateral (SC)-CA1 stratum radiatum synapses in freely behaving adult rats. Coupling of these test pulses with electrical stimulation of the LC induced long-term depression (LTD) at SC-CA1 synapses and induced a transient suppression of theta-frequency oscillations. Effects were N-methyl-D-aspartate and beta-adrenergic receptor dependent. Activation of the LC also increased CA1 noradrenalin levels and facilitated the encoding of spatial memory for a single episode via a beta-adrenoceptor-dependent mechanism. Our results demonstrate that the LC plays a key role in the induction of hippocampal LTD and in promoting the encoding of spatial information. This LC-hippocampal interaction may reflect a means by which salient information is distinguished for subsequent synaptic processing.

A nuclear actin function regulates neuronal motility by serum response factor-dependent gene transcription

Neuronal motility relies on actin treadmilling. In addition to regulating cytoskeletal dynamics in the cytoplasm, actin modulates nuclear gene expression. We present a hitherto unappreciated cross talk of actin signaling with gene expression governing neuronal motility. Toward this end, we used a novel approach using mutant actins either favoring (G15S) or inhibiting (R62D) F-actin assembly. Overexpressing these mutant actins in mouse hippocampal neurons not only modulated growth-cone function but also neurite elongation, which was ambiguous by traditional pharmacological interference. G15S actin enhanced neurite outgrowth and filopodia number. In contrast, R62D reduced neurite length and impaired growth-cone filopodia formation. Growth-cone collapse induced by ephrin-As, a family of repulsive axon guidance molecules, is impaired upon R62D expression, resulting in perseverance of ring-shaped F-actin filaments. R62D-induced phenotypes strongly resemble neurons lacking SRF (Serum Response Factor). SRF controls gene transcription of various actin isoforms (e.g., Actb, Acta1) and actin-binding proteins (e.g., Gsn) and is the archetypical transcription factor to study actin interplay with transcription. We show that neuronal motility evoked by these actin mutants requires SRF activity. Further, constitutively active SRF partially rescues R62D-induced phenotypes. Conversely, actin signaling regulates neuronal SRF-mediated gene expression. Notably, a nucleus-resident actin (R62D(NLS)) also regulates SRF's transcriptional activity. Moreover, R62D(NLS) decreases neuronal motility similar to the cytoplasmic R62D actin mutant although R62D(NLS) has no access to cytoplasmic actin dynamics. Thus, herein we provide first evidence that neuronal motility not only depends on cytoplasmic actin dynamics but also on the availability of actin to modulate nuclear functions such as gene transcription.

Use-dependent plasticity in clock neurons regulates sleep need in Drosophila

Sleep is important for memory consolidation and is responsive to waking experience. Clock circuitry is uniquely positioned to coordinate interactions between processes underlying memory and sleep need. Flies increase sleep both after exposure to an enriched social environment and after protocols that induce long-term memory. We found that flies mutant for rutabaga, period, and blistered were deficient for experience-dependent increases in sleep. Rescue of each of these genes within the ventral lateral neurons (LNVs) restores increased sleep after social enrichment. Social experiences that induce increased sleep were associated with an increase in the number of synaptic terminals in the LNV projections into the medulla. The number of synaptic terminals was reduced during sleep and this decline was prevented by sleep deprivation.

Genome-wide analysis of MEF2 transcriptional program reveals synaptic target genes and neuronal activity-dependent polyadenylation site selection

Although many transcription factors are known to control important aspects of neural development, the genome-wide programs that are directly regulated by these factors are not known. We have characterized the genetic program that is activated by MEF2, a key regulator of activity-dependent synapse development. These MEF2 target genes have diverse functions at synapses, revealing a broad role for MEF2 in synapse development. Several of the MEF2 targets are mutated in human neurological disorders including epilepsy and autism spectrum disorders, suggesting that these disorders may be caused by disruption of an activity-dependent gene program that controls synapse development. Our analyses also reveal that neuronal activity promotes alternative polyadenylation site usage at many of the MEF2 target genes, leading to the production of truncated mRNAs that may have different functions than their full-length counterparts. Taken together, these analyses suggest that the ubiquitously expressed transcription factor MEF2 regulates an intricate transcriptional program in neurons that controls synapse development.

The serum response factor and a putative novel transcription factor regulate expression of the immediate-early gene Arc/Arg3.1 in neurons

The immediate-early effector gene Arc/Arg3.1 is robustly upregulated by synaptic activity associated with learning and memory. Here we show in primary cortical neuron culture that diverse stimuli induce Arc expression through new transcription. Searching for regulatory regions important for Arc transcription, we found nine DNaseI-sensitive nucleosome-depleted sites at this genomic locus. A reporter gene encompassing these sites responded to synaptic activity in an NMDA receptor-dependent manner, consistent with endogenous Arc mRNA. Responsiveness mapped to two enhancer regions approximately 6.5 kb and approximately 1.4 kb upstream of Arc. We dissected these regions further and found that the proximal enhancer contains a functional and conserved "Zeste-like" response element that binds a putative novel nuclear protein in neurons. Therefore, activity regulates Arc transcription partly by a novel signaling pathway. We also found that the distal enhancer has a functional and highly conserved serum response element. This element binds serum response factor, which is recruited by synaptic activity to regulate Arc. Thus, Arc is the first target of serum response factor that functions at synapses to mediate plasticity.