Increase in activin beta A mRNA in rat hippocampus during long-term potentiation

Reactivation of herpesvirus type 6 and IgA/IgM-mediated responses to activin-A underpin long COVID, including affective symptoms and chronic fatigue syndrome

BACKGROUND

Persistent infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), reactivation of dormant viruses, and immune-oxidative responses are involved in long COVID.

OBJECTIVES

To investigate whether long COVID and depressive, anxiety, and chronic fatigue syndrome (CFS) symptoms are associated with IgA/IgM/IgG to SARS-CoV-2, human herpesvirus type 6 (HHV-6), Epstein-Barr Virus (EBV), and immune-oxidative biomarkers.

METHODS

We examined 90 long COVID patients and ninety healthy controls. We measured serum IgA/IgM/IgG against HHV-6 and EBV and their deoxyuridine 5′-triphosphate nucleotidohydrolase (duTPase), SARS-CoV-2, and activin-A, C-reactive protein (CRP), advanced oxidation protein products (AOPP), and insulin resistance (HOMA2-IR).

RESULTS

Long COVID patients showed significant elevations in IgG/IgM-SARS-CoV-2, IgG/IgM-HHV-6, and HHV-6-duTPase, IgA/IgM-activin-A, CRP, AOPP, and HOMA2-IR. Neural network analysis yielded a highly significant predictive accuracy of 80.6% for the long COVID diagnosis (sensitivity: 78.9%, specificity: 81.8%, area under the ROC curve = 0.876); the topmost predictors were as follows: IGA-activin-A, IgG-HHV-6, IgM-HHV-6-duTPase, IgG-SARS-CoV-2, and IgM-HHV-6 (all positively) and a factor extracted from all IgA levels to all viral antigens (inversely). The top 5 predictors of affective symptoms due to long COVID were IgM-HHV-6-duTPase, IgG-HHV-6, CRP, education, IgA-activin-A (predictive accuracy of r = 0.636). The top 5 predictors of CFS due to long COVID were in descending order: CRP, IgG-HHV-6-duTPase, IgM-activin-A, IgM-SARS-CoV-2, and IgA-activin-A (predictive accuracy: r = 0.709).

CONCLUSION

Reactivation of HHV-6, SARS-CoV-2 persistence, and autoimmune reactions to activin-A combined with activated immune-oxidative pathways play a major role in the pathophysiology of long COVID as well as the severity of its affective symptoms and CFS.

Specific exercise patterns generate an epigenetic molecular memory window that drives long-term memory formation and identifies ACVR1C as a bidirectional regulator of memory in mice

Exercise has beneficial effects on cognition throughout the lifespan. Here, we demonstrate that specific exercise patterns transform insufficient, subthreshold training into long-term memory in mice. Our findings reveal a potential molecular memory window such that subthreshold training within this window enables long-term memory formation. We performed RNA-seq on dorsal hippocampus and identify genes whose expression correlate with conditions in which exercise enables long-term memory formation. Among these genes we found Acvr1c, a member of the TGF ß family. We find that exercise, in any amount, alleviates epigenetic repression at the Acvr1c promoter during consolidation. Additionally, we find that ACVR1C can bidirectionally regulate synaptic plasticity and long-term memory in mice. Furthermore, Acvr1c expression is impaired in the aging human and mouse brain, as well as in the 5xFAD mouse model, and over-expression of Acvr1c enables learning and facilitates plasticity in mice. These data suggest that promoting ACVR1C may protect against cognitive impairment.

Increase in activin A may counteract decline in synaptic plasticity with age

Activin A, a member of the transforming growth factor β (TGF-β) family, is widely recognized for its neurotrophic and neuroprotective function in the developing and injured brain, respectively. Moreover, in the healthy adult brain, activin A has been shown to tune signal processing at excitatory synapses in a fashion that improves cognitive performance. Because its level in human cerebrospinal fluid rises with age, we wondered whether activin A has a role in mitigating the gradual cognitive decline that healthy individuals experience in late-life. To interrogate the role of activin A in synaptic plasticity in the aging brain, we used an established transgenic mouse line, in which expression of a dominant-negative mutant of activin receptor IB (dnActRIB) serves to disrupt activin receptor signaling in a forebrain-specific fashion. In brain slices of young adult dnActRIB mice (2-4 months old), the NMDA receptor-dependent and -independent forms of long-term potentiation (LTP) at the Schaffer collateral-CA1 pyramidal cell synapse of the hippocampus were equally impaired relative to the extent of LTP measured in the wild-type preparation. Unexpectedly, the difference between the genotypes disappeared when the two forms of LTP were re-examined in slices from middle-aged mice (13-16 months old). Since the level of activin A and endogenous ActRIB both displayed a significant elevation in middle-aged hippocampus, we reasoned that with such a rise, the dominant-negative effect of the mutant receptors could be overcome. Substantiating this idea, we found that administration of recombinant activin A was indeed capable of restoring full-blown LTP in slices from young dnActRIB mice. Our data suggest that, beginning in the middle-aged brain, endogenous activin receptor signaling appears to become strengthened in an attempt to stave off cognitive decline. If further corroborated, this concept would also hold promise for new therapeutic venues to preserve cognitive functions in the aged brain.

Dorsal-Ventral Gradient of Activin Regulates Strength of GABAergic Inhibition along Longitudinal Axis of Mouse Hippocampus in an Activity-Dependent Fashion

The functional and neurophysiological distinction between the dorsal and ventral hippocampus affects also GABAergic inhibition. In line with this notion, ventral CA1 pyramidal cells displayed a more dynamic and effective response to inhibitory input compared to their dorsal counterparts. We posit that this difference is effected by the dorsal-ventral gradient of activin A, a member of the transforming growth factor-β family, which is increasingly recognized for its modulatory role in brain regions involved in cognitive functions and affective behavior. Lending credence to this hypothesis, we found that in slices from transgenic mice expressing a dominant-negative mutant of activin receptor IB (dnActRIB), inhibitory transmission was enhanced only in CA1 neurons of the dorsal hippocampus, where the basal activin A level is much higher than in the ventral hippocampus. We next asked how a rise in endogenous activin A would affect GABAergic inhibition along the longitudinal axis of the hippocampus. We performed ex vivo recordings in wild-type and dnActRIB mice after overnight exposure to an enriched environment (EE), which engenders a robust increase in activin A levels in both dorsal and ventral hippocampi. Compared to control mice from standard cages, the behaviorally induced surge in activin A produced a decline in ventral inhibition, an effect that was absent in slices from dnActRIB mice. Underscoring the essential role of activin in the EE-associated modulation of ventral inhibition, this effect was mimicked by acute application of recombinant activin A in control slices. In summary, both genetic and behavioral manipulations of activin receptor signaling affected the dorsal-ventral difference in synaptic inhibition, suggesting that activin A regulates the strength of GABAergic inhibition in a region-specific fashion.

Activin A Reduces GIRK Current to Excite Dentate Gyrus Granule Cells

Activin A, a member of the TGF-β family, is recognized as a multifunctional protein in the adult brain with a particular impact on neuronal circuits associated with cognitive and affective functions. Activin receptor signaling in mouse hippocampus is strongly enhanced by the exploration of an enriched environment (EE), a behavioral paradigm known to improve performance in learning and memory tasks and to ameliorate depression-like behaviors. To interrogate the relationship between EE, activin signaling, and cellular excitability in the hippocampus, we performed ex vivo whole-cell recordings from dentate gyrus (DG) granule cells (GCs) of wild type mice and transgenic mice expressing a dominant-negative mutant of activin receptor IB (dnActRIB), which disrupts activin signaling in a forebrain-specific fashion. We found that, after overnight EE housing, GC excitability was strongly enhanced in an activin-dependent fashion. Moreover, the effect of EE on GC firing was mimicked by pre-treatment of hippocampal slices from control mice with recombinant activin A for several hours. The excitatory effect of activin A was preserved when canonical SMAD-dependent signaling was pharmacologically suppressed but was blocked by inhibitors of ERK-MAPK and PKA signaling. The involvement of a non-genomic signaling cascade was supported by the fact that the excitatory effect of activin A was already achieved within minutes of application. With respect to the ionic mechanism underlying the increase in intrinsic excitability, voltage-clamp recordings revealed that activin A induced an apparent inward current, which resulted from the suppression of a standing G protein-gated inwardly rectifying K+ (GIRK) current. The link between EE, enhanced activin signaling, and inhibition of GIRK current was strengthened by the following findings: (i) The specific GIRK channel blocker tertiapin Q (TQ) occluded the characteristic electrophysiological effects of activin A in both current- and voltage-clamp recordings. (ii) The outward current evoked by the GIRK channel activator adenosine was significantly reduced by preceding EE exploration as well as by recombinant activin A in control slices. In conclusion, our study identifies GIRK current suppression via non-canonical activin signaling as a mechanism that might at least in part contribute to the beneficial effects of EE on cognitive performance and affective behavior.

Activin A regulates the excitability of hippocampal mossy cells

Mossy cells (MCs) in the hilus of the dentate gyrus (DG) receive increasing attention as a major player controlling information processing in the DG network. Furthermore, disturbed MC activity has been implicated in widespread neuropsychiatric disorders such as epilepsy and major depression. Using whole-cell patch-clamp recordings from MCs in acute hippocampal slices from wild type and transgenic mice, we demonstrate that activin, a member of the transforming growth factor-β (TGF-β) family, has a strong neuromodulatory effect on MC activity. Disruption of activin receptor signaling reduced MC firing, dampened their excitatory input and augmented their inhibitory input. By contrast, acute application of recombinant activin A strongly increased MC activity and promoted excitatory synaptic drive. Notably, similar changes of MC activity have been observed in a rodent model of depression and after antidepressant drug therapy, respectively. Given that a rise in activin signaling particularly in the DG has been proposed as a mechanism of antidepressant action, our data suggest that the effect of activin on MC excitability might make a considerable contribution in this regard.

Activin A in Mammalian Physiology

Activins are dimeric glycoproteins belonging to the transforming growth factor beta superfamily and resulting from the assembly of two beta subunits, which may also be combined with alpha subunits to form inhibins. Activins were discovered in 1986 following the isolation of inhibins from porcine follicular fluid, and were characterized as ovarian hormones that stimulate follicle stimulating hormone (FSH) release by the pituitary gland. In particular, activin A was shown to be the isoform of greater physiological importance in humans. The current understanding of activin A surpasses the reproductive system and allows its classification as a hormone, a growth factor, and a cytokine. In more than 30 yr of intense research, activin A was localized in female and male reproductive organs but also in other organs and systems as diverse as the brain, liver, lung, bone, and gut. Moreover, its roles include embryonic differentiation, trophoblast invasion of the uterine wall in early pregnancy, and fetal/neonate brain protection in hypoxic conditions. It is now recognized that activin A overexpression may be either cytostatic or mitogenic, depending on the cell type, with important implications for tumor biology. Activin A also regulates bone formation and regeneration, enhances joint inflammation in rheumatoid arthritis, and triggers pathogenic mechanisms in the respiratory system. In this 30-yr review, we analyze the evidence for physiological roles of activin A and the potential use of activin agonists and antagonists as therapeutic agents.

Adult Neurogenesis Conserves Hippocampal Memory Capacity

The hippocampus is crucial for declarative memories in humans and encodes episodic and spatial memories in animals. Memory coding strengthens synaptic efficacy via an LTP-like mechanism. Given that animals store memories of everyday experiences, the hippocampal circuit must have a mechanism that prevents saturation of overall synaptic weight for the preservation of learning capacity. LTD works to balance plasticity and prevent saturation. In addition, adult neurogenesis in the hippocampus is proposed to be involved in the down-scaling of synaptic efficacy. Here, we show that adult neurogenesis in male rats plays a crucial role in the maintenance of hippocampal capacity for memory (learning and/or memory formation). Neurogenesis regulated the maintenance of LTP, with decreases and increases in neurogenesis prolonging or shortening LTP persistence, respectively. Artificial saturation of hippocampal LTP impaired memory capacity in contextual fear conditioning, which completely recovered after 14 d, which was the time required for LTP to decay to the basal level. Memory capacity gradually recovered in parallel with neurogenesis-mediated gradual decay of LTP. Ablation of neurogenesis by x-ray irradiation delayed the recovery of memory capacity, whereas enhancement of neurogenesis using a running wheel sped up recovery. Therefore, one benefit of ongoing adult neurogenesis is the maintenance of hippocampal memory capacity through homeostatic renewing of hippocampal memory circuits. Decreased neurogenesis in aged animals may be responsible for the decline in cognitive function with age.SIGNIFICANCE STATEMENT Learning many events each day increases synaptic efficacy via LTP, which can prevent the storage of new memories in the hippocampal circuit. In this study, we demonstrate that hippocampal capacity for the storage of new memories is maintained by ongoing adult neurogenesis through homoeostatic renewing of hippocampal circuits in rats. A decrease or an increase in neurogenesis, respectively, delayed or sped up the recovery of memory capacity, suggesting that hippocampal adult neurogenesis plays a critical role in reducing LTP saturation and keeps the gate open for new memories by clearing out the old memories from the hippocampal memory circuit.

Neuronal Activity, TGFβ-Signaling and Unpredictable Chronic Stress Modulate Transcription of Gadd45 Family Members and DNA Methylation in the Hippocampus

Neuronal activity is altered in several neurological and psychiatric diseases. Upon depolarization not only neurotransmitters are released but also cytokines and other activators of signaling cascades. Unraveling their complex implication in transcriptional control in receiving cells will contribute to understand specific central nervous system (CNS) pathologies and will be of therapeutically interest. In this study we depolarized mature hippocampal neurons in vitro using KCl and revealed increased release not only of brain-derived neurotrophic factor (BDNF) but also of transforming growth factor beta (TGFB). Neuronal activity together with BDNF and TGFB controls transcription of DNA modifying enzymes specifically members of the DNA-damage-inducible (Gadd) family, Gadd45a, Gadd45b, and Gadd45g. MeDIP followed by massive parallel sequencing and transcriptome analyses revealed less DNA methylation upon KCl treatment. Psychiatric disorder-related genes, namely Tshz1, Foxn3, Jarid2, Per1, Map3k5, and Arc are transcriptionally activated and demethylated upon neuronal activation. To analyze whether misexpression of Gadd45 family members are associated with psychiatric diseases, we applied unpredictable chronic mild stress (UCMS) as established model for depression to mice. UCMS led to reduced expression of Gadd45 family members. Taken together, our data demonstrate that Gadd45 family members are new putative targets for UCMS treatments.

Learning induces the translin/trax RNase complex to express activin receptors for persistent memory

Long-lasting forms of synaptic plasticity and memory require de novo protein synthesis. Yet, how learning triggers this process to form memory is unclear. Translin/trax is a candidate to drive this learning-induced memory mechanism by suppressing microRNA-mediated translational silencing at activated synapses. We find that mice lacking translin/trax display defects in synaptic tagging, which requires protein synthesis at activated synapses, and long-term memory. Hippocampal samples harvested from these mice following learning show increases in several disease-related microRNAs targeting the activin A receptor type 1C (ACVR1C), a component of the transforming growth factor-β receptor superfamily. Furthermore, the absence of translin/trax abolishes synaptic upregulation of ACVR1C protein after learning. Finally, synaptic tagging and long-term memory deficits in mice lacking translin/trax are mimicked by ACVR1C inhibition. Thus, we define a new memory mechanism by which learning reverses microRNA-mediated silencing of the novel plasticity protein ACVR1C via translin/trax.

Regulation of Drosophila hematopoietic sites by Activin-β from active sensory neurons

An outstanding question in animal development, tissue homeostasis and disease is how cell populations adapt to sensory inputs. During Drosophila larval development, hematopoietic sites are in direct contact with sensory neuron clusters of the peripheral nervous system (PNS), and blood cells (hemocytes) require the PNS for their survival and recruitment to these microenvironments, known as Hematopoietic Pockets. Here we report that Activin-β, a TGF-β family ligand, is expressed by sensory neurons of the PNS and regulates the proliferation and adhesion of hemocytes. These hemocyte responses depend on PNS activity, as shown by agonist treatment and transient silencing of sensory neurons. Activin-β has a key role in this regulation, which is apparent from reporter expression and mutant analyses. This mechanism of local sensory neurons controlling blood cell adaptation invites evolutionary parallels with vertebrate hematopoietic progenitors and the independent myeloid system of tissue macrophages, whose regulation by local microenvironments remain undefined.

Modulation of the NMDA Receptor Through Secreted Soluble Factors

Synaptic activity is a critical determinant in the formation and development of excitatory synapses in the central nervous system (CNS). The excitatory current is produced and regulated by several ionotropic receptors, including those that respond to glutamate. These channels are in turn regulated through several secreted factors that function as synaptic organizers. Specifically, Wnt, brain-derived neurotrophic factor (BDNF), fibroblast growth factor (FGF), and transforming growth factor (TGF) particularly regulate the N-methyl-D-aspartate receptor (NMDAR) glutamatergic channel. These factors likely regulate early embryonic development and directly control key proteins in the function of important glutamatergic channels. Here, we review the secreted molecules that participate in synaptic organization and discuss the cell signaling behind of this fine regulation. Additionally, we discuss how these factors are dysregulated in some neuropathologies associated with glutamatergic synaptic transmission in the CNS.

Acute modulation of synaptic plasticity of pyramidal neurons by activin in adult hippocampus

Activin A is known as a neuroprotective factor produced upon acute excitotoxic injury of the hippocampus (in pathological states). We attempt to reveal the role of activin as a neuromodulator in the adult male hippocampus under physiological conditions (in healthy states), which remains largely unknown. We showed endogenous/basal expression of activin in the hippocampal neurons. Localization of activin receptors in dendritic spines (=postsynapses) was demonstrated by immunoelectron microscopy. The incubation of hippocampal acute slices with activin A (10 ng/mL, 0.4 nM) for 2 h altered the density and morphology of spines in CA1 pyramidal neurons. The total spine density increased by 1.2-fold upon activin treatments. Activin selectively increased the density of large-head spines, without affecting middle-head and small-head spines. Blocking Erk/MAPK, PKA, or PKC prevented the activin-induced spinogenesis by reducing the density of large-head spines, independent of Smad-induced gene transcription which usually takes more than several hours. Incubation of acute slices with activin for 2 h induced the moderate early long-term potentiation (moderate LTP) upon weak theta burst stimuli. This moderate LTP induction was blocked by follistatin, MAPK inhibitor (PD98059) and inhibitor of NR2B subunit of NMDA receptors (Ro25-6981). It should be noted that the weak theta burst stimuli alone cannot induce moderate LTP. These results suggest that MAPK-induced phosphorylation of NMDA receptors (including NR2B) may play an important role for activin-induced moderate LTP. Taken together, the current results reveal interesting physiological roles of endogenous activin as a rapid synaptic modulator in the adult hippocampus.

Growth factors in synaptic function

Synapses are increasingly recognized as key structures that malfunction in disorders like schizophrenia, mental retardation, and neurodegenerative diseases. The importance and complexity of the synapse has fuelled research into the molecular mechanisms underlying synaptogenesis, synaptic transmission, and plasticity. In this regard, neurotrophic factors such as netrin, Wnt, transforming growth factor-β (TGF-β), tumor necrosis factor-α (TNF-α), and others have gained prominence for their ability to regulate synaptic function. Several of these factors were first implicated in neuroprotection, neuronal growth, and axon guidance. However, their roles in synaptic development and function have become increasingly clear, and the downstream signaling pathways employed by these factors have begun to be elucidated. In this review, we will address the role of these factors and their downstream effectors in synaptic function in vivo and in cultured neurons.

Mg(2+) block of Drosophila NMDA receptors is required for long-term memory formation and CREB-dependent gene expression

NMDA receptor (NMDAR) channels allow Ca(2+) influx only during correlated activation of both pre- and postsynaptic cells; a Mg(2+) block mechanism suppresses NMDAR activity when the postsynaptic cell is inactive. Although the importance of NMDARs in associative learning and long-term memory (LTM) formation has been demonstrated, the role of Mg(2+) block in these processes remains unclear. Using transgenic flies expressing NMDARs defective for Mg(2+) block, we found that Mg(2+) block mutants are defective for LTM formation but not associative learning. We demonstrate that LTM-dependent increases in expression of synaptic genes, including homer, staufen, and activin, are abolished in flies expressing Mg(2+) block defective NMDARs. Furthermore, we show that genetic and pharmacological reduction of Mg(2+) block significantly increases expression of a CREB repressor isoform. Our results suggest that Mg(2+) block of NMDARs functions to suppress basal expression of a CREB repressor, thus permitting CREB-dependent gene expression upon LTM induction.

More than being protective: functional roles for TGF-β/activin signaling pathways at central synapses

It is becoming increasingly clear that members of the transforming growth factor-β (TGF-β) family have roles in the central nervous system that extend beyond their well-established roles as neurotrophic and neuroprotective factors. Recent findings have indicated that the TGF-β signaling pathways are involved in the modulation of both excitatory and inhibitory synaptic transmission in the adult mammalian brain. In this review, we discuss how TGF-β, bone morphogenetic protein and activin signaling at central synapses modulate synaptic plasticity, cognition and affective behavior. We also discuss the implications of these findings for the molecular understanding and potential treatment of neuropsychiatric diseases, such as anxiety, depression and other neurological disorders.

Multifunctional roles of activins in the brain

Activins, which are members of the TGF-β superfamily, were initially isolated from gonads and served as modulators of follicle-stimulating hormone secretion. Activins regulate various biological functions, including induction of the dorsal mesoderm, craniofacial development, and differentiation of numerous cell types. Activin receptors are highly expressed in neuronal cells, and activin mRNA expression is upregulated by neuronal activity. Activins also exhibit neuroprotective action during excitotoxic brain injury. However, very little is known about the functional roles of activins in the brain. We recently generated various types of transgenic mice, demonstrating that activins regulate spine formation, behavioral activity, anxiety, adult neurogenesis, late-phase long-term potentiation, and maintenance of long-term memory. The present chapter describes recent progress in the study of the role of activin in the brain.

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.

LTP induction within a narrow critical period of immature stages enhances the survival of newly generated neurons in the adult rat dentate gyrus

Neurogenesis occurs in the adult hippocampus of various animal species. A substantial fraction of newly generated neurons die before they mature, and the survival rate of new neurons are regulated in an experience-dependent manner. Previous study showed that high-frequency stimulation (HFS) of perforant path fibers to the hippocampal dentate gyrus (DG) induces the long-term potentiation (LTP) in the DG, and enhances the survival of newly generated neurons in the DG. In this study, we addressed whether a time period exists during which the survival of new neurons is maximally sensitive to the HFS. We found that the enhancement of cell survival by HFS was exclusively restricted to the specific narrow period during immature stages of new neurons (7-10 days after birth). Furthermore, the pharmacological blockade of LTP induction suppressed the enhancement of cell survival by the HFS. These results suggest that the LTP induction within a narrow critical period of immature stages enhances the survival of newly generated neurons in rat DG.

Activin plays a key role in the maintenance of long-term memory and late-LTP

A recent study has revealed that fear memory may be vulnerable following retrieval, and is then reconsolidated in a protein synthesis-dependent manner. However, little is known about the molecular mechanisms of these processes. Activin betaA, a member of the TGF-beta superfamily, is increased in activated neuronal circuits and regulates dendritic spine morphology. To clarify the role of activin in the synaptic plasticity of the adult brain, we examined the effect of inhibiting or enhancing activin function on hippocampal long-term potentiation (LTP). We found that follistatin, a specific inhibitor of activin, blocked the maintenance of late LTP (L-LTP) in the hippocampus. In contrast, administration of activin facilitated the maintenance of early LTP (E-LTP). We generated forebrain-specific activin- or follistatin-transgenic mice in which transgene expression is under the control of the Tet-OFF system. Maintenance of hippocampal L-LTP was blocked in the follistatin-transgenic mice. In the contextual fear-conditioning test, we found that follistatin blocked the formation of long-term memory (LTM) without affecting short-term memory (STM). Furthermore, consolidated memory was selectively weakened by the expression of follistatin during retrieval, but not during the maintenance phase. On the other hand, the maintenance of memory was also influenced by activin overexpression during the retrieval phase. Thus, the level of activin in the brain during the retrieval phase plays a key role in the maintenance of long-term memory.

Levetiracetam attenuates hippocampal expression of synaptic plasticity-related immediate early and late response genes in amygdala-kindled rats

BACKGROUND

The amygdala-kindled rat is a model for human temporal lobe epilepsy and activity-dependent synaptic plasticity. Hippocampal RNA isolated from amygdala-kindled rats at different kindling stages was analyzed to identify kindling-induced genes. Furthermore, effects of the anti-epileptic drug levetiracetam on kindling-induced gene expression were examined.

RESULTS

Cyclooxygenase-2 (Cox-2), Protocadherin-8 (Pcdh8) and TGF-beta-inducible early response gene-1 (TIEG1) were identified and verified as differentially expressed transcripts in the hippocampus of kindled rats by in situ hybridization and quantitative RT-PCR. In addition, we identified a panel of 16 additional transcripts which included Arc, Egr3/Pilot, Homer1a, Ania-3, MMP9, Narp, c-fos, NGF, BDNF, NT-3, Synaptopodin, Pim1 kinase, TNF-alpha, RGS2, Egr2/krox-20 and beta-A activin that were differentially expressed in the hippocampus of amygdala-kindled rats. The list consists of many synaptic plasticity-related immediate early genes (IEGs) as well as some late response genes encoding transcription factors, neurotrophic factors and proteins that are known to regulate synaptic remodelling. In the hippocampus, induction of IEG expression was dependent on the afterdischarge (AD) duration. Levetiracetam, 40 mg/kg, suppressed the development of kindling measured as severity of seizures and AD duration. In addition, single animal profiling also showed that levetiracetam attenuated the observed kindling-induced IEG expression; an effect that paralleled the anti-epileptic effect of the drug on AD duration.

CONCLUSIONS

The present study provides mRNA expression data that suggest that levetiracetam attenuates expression of genes known to regulate synaptic remodelling. In the kindled rat, levetiracetam does so by shortening the AD duration thereby reducing the seizure-induced changes in mRNA expression in the hippocampus.

Adult neurogenesis modulates the hippocampus-dependent period of associative fear memory

Acquired memory initially depends on the hippocampus (HPC) for the process of cortical permanent memory formation. The mechanisms through which memory becomes progressively independent from the HPC remain unknown. In the HPC, adult neurogenesis has been described in many mammalian species, even at old ages. Using two mouse models in which hippocampal neurogenesis is physically or genetically suppressed, we show that decreased neurogenesis is accompanied by a prolonged HPC-dependent period of associative fear memory. Inversely, enhanced neurogenesis by voluntary exercise sped up the decay rate of HPC dependency of memory, without loss of memory. Consistently, decreased neurogenesis facilitated the long-lasting maintenance of rat hippocampal long-term potentiation in vivo. These independent lines of evidence strongly suggest that the level of hippocampal neurogenesis play a role in determination of the HPC-dependent period of memory in adult rodents. These observations provide a framework for understanding the mechanisms of the hippocampal-cortical complementary learning systems.

Activin signaling as an emerging target for therapeutic interventions

After the initial discovery of activins as important regulators of reproduction, novel and diverse roles have been unraveled for them. Activins are expressed in various tissues and have a broad range of activities including the regulation of gonadal function, hormonal homeostasis, growth and differentiation of musculoskeletal tissues, regulation of growth and metastasis of cancer cells, proliferation and differentiation of embryonic stem cells, and even higher brain functions. Activins signal through a combination of type I and II transmembrane serine/threonine kinase receptors. Activin receptors are shared by multiple transforming growth factor-β (TGF-β) ligands such as myostatin, growth and differentiation factor-11 and nodal. Thus, although the activity of each ligand is distinct, they are also redundant, both physiologically and pathologically in vivo. Activin receptors activated by ligands phosphorylate the receptor-regulated Smads for TGF-β, Smad2 and 3. The Smad proteins then undergo multimerization with the co-mediator Smad4, and translocate into the nucleus to regulate the transcription of target genes in cooperation with nuclear cofactors. Signaling through receptors and Smads is controlled by multiple mechanisms including phosphorylation and other posttranslational modifications such as sumoylation, which affect potein localization, stability and transcriptional activity. Non-Smad signaling also plays an important role in activin signaling. Extracellularly, follistatin and related proteins bind to activins and related TGF-β ligands, and control the signaling and availability of ligands.

The functions of activins through activin receptors are pleiotrophic, cell type-specific and contextual, and they are involved in the etiology and pathogenesis of a variety of diseases. Accordingly, activin signaling may be a target for therapeutic interventions. In this review, we summarize the current knowledge on activin signaling and discuss the potential roles of this pathway as a molecular target of therapy for metabolic diseases, musculoskeletal disorders, cancers and neural damages.

Requirement of the immediate early gene vesl-1S/homer-1a for fear memory formation

Background

The formation of long-term memory (LTM) and the late phase of long-term potentiation (L-LTP) depend on macromolecule synthesis, translation, and transcription in neurons. vesl-1S (VASP/Ena-related gene upregulated during seizure and LTP, also known as homer-1a) is an LTP-induced immediate early gene. The short form of Vesl (Vesl-1S) is an alternatively spliced isoform of the vesl-1 gene, which also encodes the long form of the Vesl protein (Vesl-1L). Vesl-1L is a postsynaptic scaffolding protein that binds to and modulates the metabotropic glutamate receptor 1/5 (mGluR1/5), the IP₃ receptor, and the ryanodine receptor. Vesl-1 null mutant mice show abnormal behavior, which includes anxiety- and depression-related behaviors, and an increase in cocaine-induced locomotion; however, the function of the short form of Vesl in behavior is poorly understood because of the lack of short-form-specific knockout mice.

Results

In this study, we generated short-form-specific gene targeting (KO) mice by knocking in part of vesl-1L/homer-1c cDNA. Homozygous KO mice exhibited normal spine number and morphology. Using the contextual fear conditioning test, we demonstrated that memory acquisition and short-term memory were normal in homozygous KO mice. In contrast, these mice showed impairment in fear memory consolidation. Furthermore, the process from recent to remote memory was affected in homozygous KO mice. Interestingly, reactivation of previously consolidated fear memory attenuated the conditioning-induced freezing response in homozygous KO mice, which suggests that the short form plays a role in fear memory reconsolidation. General activity, emotional performance, and sensitivity to electrofootshock were normal in homozygous KO mice.

Conclusion

These results indicate that the short form of the Vesl family of proteins plays a role in multiple steps of long-term, but not short-term, fear memory formation.

N-acetyltransferase ARD1-NAT1 regulates neuronal dendritic development

ARD1 and NAT1 constitute an N-acetyltransferase complex where ARD1 holds the enzymatic activity of the complex. The ARD1-NAT1 complex mediates N-terminal acetylation of nascent polypeptides that emerge from ribosomes after translation. ARD1 may also acetylate the internal lysine residues of proteins. Although ARD1 and NAT1 have been found in the brain, the physiological role and substrates of the ARD1-NAT1 complex in neurons remain unclear. Here we investigated role of N-acetyltransferase activity in the process of neuronal development. Expression of ARD1 and NAT1 increased during dendritic development, and both proteins colocalized with microtubules in dendrites. The ARD1-NAT1 complex displayed acetyltransferase activity against a purified microtubule fraction in vitro. Inhibition of the complex limited the dendritic extension of cultured neurons. These findings suggest that the ARD1-NAT1 complex has acetyltransferase activity against microtubules in dendrites. Regulation by acetyltransferase activity is a novel mechanism that is required for dendritic arborization during neuronal development.

Neuron type-selective effects of activin on development of the hippocampus

Activin is a member of the transforming growth factor-beta superfamily and affects the viability of hippocampal neurons during postnatal neurogenesis. We used primary hippocampal neuron to study the actions of activin on developing neurons. Continuous treatment of hippocampal cultures with activin suppressed the emergence of GAD67(+) neurons, which are a subtype of GABAergic interneurons, and increased the percentage of Prox1(+) neurons, which are dentate granule cells. The effects of activin were abolished by co-treatment with follistatin, which is a direct inhibitor of activin. In contrast, follistatin treatment alone increased the percentage of GAD67(+) neurons and decreased the percentage of Prox1(+) neurons. These results indicate that changes in activin signaling during postnatal neural development alter the composition of the neural circuitry and suggest that alterations in the ratio of excitatory to inhibitory neurons may be responsible for changes in the spontaneous and evoked-reactivity of these neurons to other neural inputs.

Activin in the brain modulates anxiety-related behavior and adult neurogenesis

Activin, a member of the transforming growth factor-beta superfamily, is an endocrine hormone that regulates differentiation and proliferation of a wide variety of cells. In the brain, activin protects neurons from ischemic damage. In this study, we demonstrate that activin modulates anxiety-related behavior by analyzing ACM4 and FSM transgenic mice in which activin and follistatin (which antagonizes the activin signal), respectively, were overexpressed in a forebrain-specific manner under the control of the alphaCaMKII promoter. Behavioral analyses revealed that FSM mice exhibited enhanced anxiety compared to wild-type littermates, while ACM4 mice showed reduced anxiety. Importantly, survival of newly formed neurons in the subgranular zone of adult hippocampus was significantly decreased in FSM mice, which was partially rescued in ACM4/FSM double transgenic mice. Our findings demonstrate that the level of activin in the adult brain bi-directionally influences anxiety-related behavior. These results further suggest that decreases in postnatal neurogenesis caused by activin inhibition affect an anxiety-related behavior in adulthood. Activin and its signaling pathway may represent novel therapeutic targets for anxiety disorder as well as ischemic brain injury.

Activin increases the number of synaptic contacts and the length of dendritic spine necks by modulating spinal actin dynamics

Long-lasting modifications in synaptic transmission depend on de novo gene expression in neurons. The expression of activin, a member of the transforming growth factor beta (TGF-beta) superfamily, is upregulated during hippocampal long-term potentiation (LTP). Here, we show that activin increased the average number of presynaptic contacts on dendritic spines by increasing the population of spines that were contacted by multiple presynaptic terminals in cultured neurons. Activin also induced spine lengthening, primarily by elongating the neck, resulting in longer mushroom-shaped spines. The number of spines and spine head size were not significantly affected by activin treatment. The effects of activin on spinal filamentous actin (F-actin) morphology were independent of protein and RNA synthesis. Inhibition of cytoskeletal actin dynamics or of the mitogen-activated protein (MAP) kinase pathway blocked not only the activin-induced increase in the number of terminals contacting a spine but also the activin-induced lengthening of spines. These results strongly suggest that activin increases the number of synaptic contacts by modulating actin dynamics in spines, a process that might contribute to the establishment of late-phase LTP.

Local production of sex hormones and their modulation of hippocampal synaptic plasticity

It is believed that sex hormones are synthesized in the gonads and reach the brain via the blood circulation. In contrast with this view, the authors have demonstrated that sex hormones are also synthesized locally in the hippocampus and that these steroids act rapidly to modulate neuronal synaptic plasticity. The authors demonstrated that estrogens are locally synthesized from cholesterol through dehydroepiandrosterone and testosterone in adult hippocampal neurons. Significant expression of mRNA for P450(17alpha), P450arom, and other steroidogenic enzymes was demonstrated. Localization of P450(17alpha) and P450arom was observed in synapses of principal neurons. In contrast to the slow action of gonadal estradiol, hippocampal neuron-derived estradiol may act locally and rapidly within the neurons. For example, 1 to 10 nM estradiol rapidly enhances long-term depression (LTD). The density of thin spines is selectively increased within two hours upon application of estradiol in pyramidal neurons. Estrogen receptor ERalpha agonist has the same enhancing effect as estradiol on both LTD and spinogenesis. Localization of ERalpha in spines in addition to nuclei of principal neurons implies that synaptic ERalpha is responsible for rapid modulation of synaptic plasticity by endogenous estradiol. Activin A, a peptide sex hormone, may also play a role as a local endogenous modulator of synaptic plasticity.

Activin A In Brain Injury

Activin A is a growth factor composed of two betaA subunits belonging to the transforming growth factor beta (TGF-beta) superfamily of dimeric proteins. The biological activity of activin A is mediated by two different types of receptors, the type I (ARI and ARIB) and the type II receptors (ARII and ARIIB), and by two activin-binding proteins, follistatin and follistatin-related gene. These factors bind to activin A and thereby inhibit its biological effects. Activin A, its receptors, and binding proteins are widely distributed throughout the brain. Studies employing models of acute brain injury strongly implicate enhanced activin A expression as a common response to acute neuronal damage of various origins. Hypoxic/ischemic injury, mechanical irritation, and chemical damage of brain evoke a strong upregulation of activin A. Subsequent experimental studies have shown that activin A has a beneficial role to neuronal recovery and that, by activating different pathways, activin A has robust neuroprotective activities. Because activin A induction occurs early after brain injury, its measurement may provide a potential biochemical index of the presence, location, and extent of brain injury. This approach may also facilitate the diagnosis of subclinical lesions at stages when monitoring procedures are unable to detect brain lesion and furthermore establish a prognosis.

Alternative poly(A) site-selection regulates the production of alternatively spliced vesl-1/homer1 isoforms that encode postsynaptic scaffolding proteins

The vesl-1/homer1 gene encodes a scaffold protein that interacts with several receptors to modulate synaptic functions. The gene also encodes two shorter forms that counteract the functions of the long form of Vesl. Expression of the shorter forms is driven by neural activities such as long-term potentiation. Here we analyzed the mechanism regulating vesl-1 alternative splicing. Each functional poly(A) site was in a different part of the 3'-terminal exon, with promoter-proximal and promoter-distal sites at the end of exons corresponding to the short and long form Vesl-1, respectively. 3'-End-processing at proximal poly(A) site, specifically at the vesl-1M poly(A) site, was enhanced by extracellular stimuli, thereby switching transcription termination from promoter-distal to -proximal poly(A) site. This switch was not specifically coupled to the vesl-1 promoter and was independent of de novo protein synthesis. Analysis of transcripts from mini-genes that mimic the structure of endogenous vesl-1 revealed that the vesl-1M poly(A) region plays a crucial role in switching to the alternative pre-mRNA splicing that is triggered by extracellular stimuli. Therefore, a 3'-end-processing event regulates the neural activity-dependent alternative splicing of vesl-1. This is the first report of a gene in which alternative poly(A) site-selection regulates alternative splicing in a protein synthesis-independent manner.

Transgenic mice expressing dominant-negative activin receptor IB in forebrain neurons reveal novel functions of activin at glutamatergic synapses

The transforming growth factor beta family member activin is an important regulator of development and tissue repair. It is strongly up-regulated after acute injury to the adult brain, and application of exogenous activin protects neurons in several lesion models. To explore the role of endogenous activin in the normal and acutely damaged brain, we generated transgenic mice expressing a dominant-negative activin receptor IB (dnActRIB) mutant in forebrain neurons. The functionality of the transgene was verified in vivo. Hippocampal neurons from dnActRIB mice were significantly more vulnerable to intracerebroventricular injection of the excitotoxin kainic acid than those from control littermates, indicating a crucial role of endogenous activin in the rescue of neurons from excitotoxic insult. Because dnActRIB is only expressed in neurons, but not in glial cells, activin affords protection at least in part through a direct action on endangered neurons. Unexpectedly, the transgenic mice also revealed a prominent novel role of activin in glutamatergic neurotransmission in the intact adult brain. Electrophysiologic examination of excitatory synapses onto CA1 pyramidal cells in hippocampal slices of dnActRIB mice showed a reduced NMDA current response, which was associated with impaired long term potentiation. This is the first demonstration that activin receptor signaling is essential to optimize the performance of neuronal circuits in the mature brain under physiological conditions.

Increased Plasma Concentrations of Activin A Predict Intraventricular Hemorrhage in Preterm Newborns

Background: Intraventricular hemorrhage (IVH) is a major cause of neurologic disabilities in preterm newborns. We evaluated the use of plasma activin A concentrations to predict the development of perinatal IVH.

Methods: We measured nucleated erythrocyte (NRBC) counts, plasma activin A, hypoxanthine (Hyp), and xanthine (Xan) in arterial blood samples obtained from 53 preterm infants during the first hour after birth. Cerebral ultrasound was performed within 48 h of birth and repeated at 5- or 6-day intervals until the age of 4 weeks.

Results: Grade I or II IVH was detected during the first 10 days of life in 11 of 53 patients (21%). Activin A, Hyp, and Xan concentrations and NRBC counts were higher in preterm newborns who subsequently developed IVH than in those who did not (P <0.0001, except P = 0.019 for Xan). Neonatal activin A was correlated (P <0.0001) with Hyp (r = 0.95), Xan (r = 0.90), and NRBC count (r = 0.90) in newborns without later IVH and in those who developed IVH (Hyp, r = 0.89, P = 0.0002; Xan, r = 0.95, P <0.0001; NRBC count, r = 0.90, P = 0.0002). At a cutoff of 0.8 μg/L activin A, the sensitivity and specificity were 100% [11 of 11; 95% confidence interval (CI), 71%–100%] and 93% (39 of 42; 95% CI, 81%–98%), and positive and negative predictive values were 79% (95% CI, 61%–100%) and 0% (95% CI, 0%–2%), respectively. The area under the ROC curve was 0.98.

Conclusions: Activin A concentrations at birth are increased in preterm newborns who later develop IVH and may be useful for early identification of infants with hypoxic-ischemic brain insults who are at high risk for IVH.

Regulation of activin mRNA and Smad2 phosphorylation by antidepressant treatment in the rat brain: effects in behavioral models

Activin is a member of the transforming growth factor-beta family that is involved in cell differentiation, hormone secretion, and regulation of neuron survival. The cellular responses to activin are mediated by phosphorylation of a downstream target, Smad2. The current study examines the influence of chronic electroconvulsive seizures (ECSs), as well as chemical antidepressants, on the expression of activin betaA and the phosphorylation of Smad2 in the rat hippocampus and frontal cortex. Chronic ECSs (10 d) resulted in a significant increase in activin betaA mRNA expression and Smad2 phosphorylation in both the hippocampus and frontal cortex. Chronic fluoxetine did not influence activin betaA expression, but fluoxetine as well as desipramine did increase Smad2 phosphorylation in the frontal cortex. The functional significance of increased activin was further tested by examining the effects of activin infusions into the hippocampus on a behavioral model of depression, the forced swim test (FST). A single bilateral infusion of activin A or activin B into the dentate gyrus of the hippocampus produced an antidepressant-like effect in the FST that was comparable in magnitude with fluoxetine. In contrast, infusion of the activin antagonist inhibin A did not influence behavior but blocked the effect of activin A. The results suggest that regulation of activin and Smad signaling may contribute to the actions of antidepressant treatment and may represent novel targets for antidepressant drug development.

The role of myostatin and bone morphogenetic proteins in muscular disorders

Skeletal muscle is the largest organ in the human body, and plays an important role in body movement and metabolism. Skeletal muscle mass is lost in genetic disorders such as muscular dystrophy, muscle wasting and ageing. Chemicals and proteins that restore muscle mass and function are potential drugs that can improve human health and could be used in the clinic. Myostatin is a muscle-specific member of the transforming growth factor (TGF)-beta superfamily that plays an essential role in the negative regulation of muscle growth. Inhibition of myostatin activity is a promising therapeutic method for restoring muscle mass and strength. Potential inhibitors of myostatin include follistatin domain-containing proteins, myostatin propeptide, myostatin antibodies and chemical compounds. These inhibitors could be beneficial for the development of clinical drugs for the treatment of muscular disorders. Bone morphogenetic protein (BMP) plays a significant role in the development of neuromuscular architecture and its proper functions. Modulation of BMP activity could be beneficial for muscle function in muscular disorders. This review will describe the current progress in therapy for muscular disorders, emphasising the importance of myostatin as a drug target.

Hippocampal gene expression is modulated by hypergravity

We used the cDNA microarray technique to monitor simultaneously possible changes induced by hypergravity in the expression level of thousands of hippocampal genes. We tested the mRNA level of about 5000 genes in the hippocampus of mice subjected to 1.09 g (1g) or to 1.85 g (2g) for five repeated 1-h daily rotations in a centrifuge (g = 9.81 m/s2). Data were compared with those obtained for mice kept stationary (C). The ratios 1g/C and 2g/C identified genes affected by rotation and rotation + hypergravity, respectively, whereas 2g/1g ratio identified those affected by hypergravity. We found that about 200 genes were affected by rotation and/or rotation + hypergravity. Almost all the genes affected by rotation + hypergravity were up-regulated, only five being down-regulated. The modulated genes code for proteins involved in a wide range of cellular functions (DNA/RNA metabolism, protein processing, intermediate metabolism, cytoskeleton and motility, cell cycle and apoptosis, signal transduction, neuronal structure/function), suggesting that rotation + hypergravity may affect several aspects of the hippocampal function in order to compensate for environmental changes. Six genes directly or indirectly involved in synaptic transmission and plasticity (proSAAS, neuroblastoma ras oncogene, ESTs moderately similar to thymosin beta-10, syndet, inhibin beta E and Ngfi-A binding protein 2) were found to be significantly modulated by hypergravity and unaffected or only slightly affected by rotation. The modulation by hypergravity of these genes suggests that this stimulus might induce plastic remodelling of the hippocampal circuits, possibly both at structural and functional level.

Alivin 1, a novel neuronal activity-dependent gene, inhibits apoptosis and promotes survival of cerebellar granule neurons

Neurons require Ca2+-dependent gene transcription for their activity-dependent survival, the mechanisms of which have not been fully elucidated yet. Here, we demonstrate that a novel primary response gene, alivin 1 (ali1), is an activity-dependent gene and promotes survival of neurons. Sequence analyses reveal that rat, mouse, and human Ali1 proteins contain seven leucine-rich repeats, one IgC2-like loop and a transmembrane domain, and display homology to Kek and Trk families. Expression of ali1 mRNA in cultured cerebellar granule neurons is rigidly regulated by KCl and/or NMDA concentrations in the culture medium and tightly correlated to depolarization-dependent survival and/or NMDA-dependent survival of the granule neuron. ali1 mRNA expression was regulated at the transcriptional step by the Ca2+ influx through voltage-dependent L-type Ca2+ channels when the cells were stimulated by 25 mm KCl. Expression of ali1 mRNA in cultured cortical neurons was inhibited when their spontaneous electrical activity was blocked by tetrodotoxin. Thus, the expression is neuronal activity dependent. Overexpression of Ali1 in cerebellar granule neurons inhibited apoptosis that was induced by the medium containing 5 mm KCl. The addition of anti-Ali1 antiserum or the soluble putative extracellular Ali1 domain to the 25 mm KCl-supported culture inhibited the survival of the granule neuron. These results suggest that expression of ali1 promotes depolarization-dependent survival of the granule neuron. Mouse ali1 was mapped to a locus approximately 55.3 cM from the centromere on chromosome 15 that is syntenic to positional candidate loci for familial Alzheimer's disease type 5 and Parkinson's disease 8 on human chromosome 12.

Calcium-regulated expression of activin A in RBL-2H3 mast cells

The present study examined the regulatory expression of activin A, a potent growth and differentiation factor, in rat basophilic leukemia (RBL-2H3) mast cells. Treatment of RBL-2H3 cells sensitized with anti-dinitrophenyl IgE with multivalent dinitrophenyl led to a clear increase in RT-PCR products of inhibin/activin beta(A). The steady-state mRNA of inhibin/activin beta(A) was also induced by increasing cytosolic Ca(2+) concentration with ionomycin, which required de novo protein synthesis, and was regulated at the transcriptional level. Pretreatment of RBL-2H3 cells with antagonists or inhibitors for the calmodulin pathway blocked ionomycin-dependent inhibin/activin beta(A) transcription and mRNA induction, suggesting the involvement of calmodulin-dependent kinase (CaMK) and calcineurin. The ionomycin-dependent inhibin/activin beta(A) induction was also partially blocked by preincubation with c-Jun NH(2)-terminal kinase (JNK) and p38 kinase inhibitors, but not with MEK1 inhibitor. These results suggest that inhibin/activin beta(A) gene activation is achieved by the JNK and p38 kinase activation through the calmodulin pathway in mast cells.

Hippocampal LTP is accompanied by enhanced F-actin content within the dendritic spine that is essential for late LTP maintenance in vivo

The dendritic spine is an important site of neuronal plasticity and contains extremely high levels of cytoskeletal actin. However, the dynamics of the actin cytoskeleton during synaptic plasticity and its in vivo function remain unclear. Here we used an in vivo dentate gyrus LTP model to show that LTP induction is associated with actin cytoskeletal reorganization characterized by a long-lasting increase in F-actin content within dendritic spines. This increase in F-actin content is dependent on NMDA receptor activation and involves the inactivation of actin depolymerizing factor/cofilin. Inhibition of actin polymerization with latrunculin A impaired late phase of LTP without affecting the initial amplitude and early maintenance of LTP. These observations suggest that mechanisms regulating the spine actin cytoskeleton contribute to the persistence of LTP.

LTP but not seizure is associated with up-regulation of AKAP-150

We have used differential display to profile and compare the mRNAs expressed in the hippocampus of freely moving animals after the induction of long-term potentiation (LTP) at the perforant path-dentate gyrus synapse with control rats receiving low-frequency stimulation. We have combined this with in situ hybridization and have identified A-kinase anchoring protein of 150 kDa (AKAP-150) as a gene selectively up-regulated during the maintenance phase of LTP. AKAP-150 mRNA has a biphasic modulation in the dentate gyrus following the induction of LTP. The expression of AKAP-150 was 29% lower than stimulated controls 1 h after the induction of LTP. Its expression was enhanced 3 (50%), 6 (239%) and 12 h (210%) after induction, returning to control levels by 24 h postinduction. The NMDA receptor antagonist CPP blocked the tetanus-induced modulation of AKAP-150 expression. Interestingly, strong generalized stimulation produced by electroconvulsive shock did not increase the expression of AKAP-150. This implies that the AKAP-150 harbours a novel property of selective responsiveness to the stimulation patterns that trigger NMDA-dependent LTP in vivo. Its selective up-regulation during LTP and its identified functions as a scaffold for protein kinase A, protein kinase C, calmodulin, calcineurin and ionotropic glutamate receptors suggest that AKAP-150 encodes is an important effector protein in the expression of late LTP.

Transgenic calmodulin-dependent protein kinase II activation: dose-dependent effects on synaptic plasticity, learning, and memory

Genetic disruption of calmodulin-dependent protein kinase II (CaMKII) function alters hippocampal synaptic plasticity and memory in mice. We used transgenic mice carrying a tetracycline-regulated, calcium-independent form of CaMKII (CaMKII-Asp286) to investigate the role of CaMKII activation on synaptic plasticity and behavior. Mice expressing low levels of a CaMKII-Asp286 transgene have facilitated low-frequency (5 Hz)-induced long-term potentiation (LTP), whereas mice with high levels of transgene expression have a deficit in this form of plasticity. Behavioral impairments on fear-conditioned memory and visible water maze correlate with the level of CaMKII-Asp286 expression. Mice with high levels of CaMKII-Asp286 have reversible, compensatory changes in the expression of genes associated with inhibitory neurotransmission. These results demonstrate that in the hippocampus, CaMKII activation facilitates the induction of low-frequency LTP, but at high levels of expression, compensatory mechanisms act to inhibit the induction of this form of LTP. The most severe behavioral impairments are associated with activation of this compensatory mechanism.

mRNA differential display identification of thyroid hormone-responsive protein (THRP) gene in association with early phase of long-term potentiation

The process of long-term potentiation (LTP) consists of the early induction and late maintenance phases. Few studies have examined the cellular mechanisms underlying these two phases; their respective mRNA expression profiles have not yet been elucidated. Here we used the technique of PCR differential display to identify genes that are differentially expressed between the early and late phases of LTP in vivo. Our results indicated that the cDNA fragment corresponding to one mRNA with preferentially increased expression during the early, but not late, phase of LTP encodes the rat thyroid hormone-responsive protein (THRP) gene. In situ hybridization analysis confirmed the results obtained from the PCR differential display. Prior NMDA receptor blockade with MK801 prevented induction of LTP and decreased THRP mRNA expression in the dentate gyrus, as assayed by quantitative RT-PCR analysis. THRP antisense oligonucleotide treatment before tetanic stimulation also prevented induction of LTP. However, when THRP antisense oligonucleotide was administered after induction of LTP, it did not affect expression and maintenance of LTP. THRP is known to be responsive to thyroid hormone. Our results indicate that direct thyroid hormone (T3) injection into the dentate gyrus produces a long-lasting enhancement of synaptic efficacy of these neurons. T3 injection also markedly increased THRP mRNA expression in the dentate gyrus. Taken together, our results suggest that THRP mRNA expression plays an important role in the early phase, but not the late phase, of LTP and that both THRP and thyroid hormone are involved in synaptic plasticity in hippocampal neurons.

Functional genomics in neuropsychiatric disorders and in neuropharmacology

The rapidly accumulating amount of information concerning gene and protein expression patterns produced by functional genomics, proteomics and bioinformatics is presently providing new targets for drug development. Furthermore, the analysis of gene expression in cells and tissues affected by a disease may reveal the underlying metabolic pathways and cellular processes affected. Finally, changes in gene expression may be used in either diagnostics or the monitoring of drug responses. This review focuses on advances in the use of functional genomics in neurological and neuropsychiatric diseases and neuropsychopharmacology. Although the number of published studies in this field is still limited, it already appears that this strategy may become a fruitful means in the analysis of the aetiology of neuropsychiatric disorders and the search for novel neuropharmacological drugs.

Stimulation of gene expression of NeuroD-related factor in the mouse brain following pentylenetetrazol-induced seizures

Various genes for transcription factors are induced in neurons involving long-lasting synaptic plasticity that is accompanied by de novo protein synthesis. In this study, we analyzed the gene expression of NeuroD-related factor (NDRF/neuroD2), a neural basic helix-loop-helix transcription factor, in the mouse hippocampus following pentylenetetrazol (PTZ)-induced seizures. Both the levels of mRNA and protein of NDRF were elevated by PTZ injection. In contrast to c-fos, a representative neuronal activation-related immediate-early gene that was induced within 1 h after PTZ administration, induction of the NDRF gene expression reached a maximum level at 7-8 h after PTZ injection and was inhibited by pretreatment with cycloheximide and MK801. In situ hybridization of the mouse hippocampus revealed that NDRF mRNA was significantly induced in the dentate gyrus. During hippocampal development, NDRF transcripts were found to be highly expressed in a juvenile period, when extensive synaptogenesis occurs. Our present results demonstrate that NDRF is a new member of the family of activation-induced transcription factors, whose expression is probably regulated by immediate-early transcription factors. NDRF is thought to be involved in long-lasting neuronal activation.

LIRF, a gene induced during hippocampal long-term potentiation as an immediate-early gene, encodes a novel RING finger protein

We describe here an LTP-induced gene, LIRF, which encodes a novel protein with RING finger and B30.2 domains in its N- and C-terminal portions, respectively. Each domain is encoded by one exon, suggesting that the organization of the gene was generated by exon shuffling. The amino acid sequences of the mouse, rat, and human LIRF proteins are highly conserved and contain a putative PEST sequence. LIRF is an immediate-early gene in hippocampal granule cells, and its expression is upregulated immediately after the induction of long-lasting long-term potentiation at perforant pathway-dentate gyrus synapses and returns to the basal level within 150 min. A heterologously expressed LIRF protein fused to EGFP localizes specifically to the cytoplasm in COS-7 cells. These findings suggest a possible involvement of LIRF in a limited, early phase of synaptic plasticity.

Expression of vesl-1S/homer-1a, a gene associated with long-term potentiation, in the brain of the epileptic EI mouse

The El mouse is an animal model for hereditary epilepsy. It undergoes a convulsive seizure lasting 10-30 s after vestibular stimulation, such as "tossing-up". To better characterize the effects of seizure activity in the El mouse, we examined the expression profile of vesl-1S/homer-1a mRNA in the brain. It has been shown that this mRNA is expressed after long-term potentiation or seizures induced by electrical or pharmacological stimulations. The basal level of vesl-1S mRNA was very low even in seizure-susceptible El mice. The El mouse hippocampus, but not that of ddY mice, showed a remarkable increase in vesl-1S mRNA expression in the dentate granule cell layer after seizure induced by "tossing-up". The mRNA remained for about 3 h after the seizure and disappeared after 8 h.

Traumatic brain injury-induced changes in gene expression and functional activity of mitochondrial cytochrome C oxidase

Traumatic brain injury (TBI) is documented to have detrimental effects on CNS metabolism, including alterations in glucose utilization and the depression of mitochondrial oxidative phosphorylation. Studies on mitochondrial metabolism have also provided evidence for reduced activity of the cytochrome oxidase complex of the electron transport chain (complex IV) after TBI and an immediate (lhr) reduction in mitochondrial state 3 respiratory rate, which can persist for up to 14 days postinjury. Using differential display methods to screen for differences in gene expression, we have found that cytochrome c oxidase II (COII), a mitochondrial encoded subunit of complex IV, is upregulated following TBI. Since COII carries a binding site for cytochrome c in the respiratory chain, and since it is required for the passage of chain electrons to molecular oxygen, driving the production of ATP, we hypothesized that metabolic dysfunction resulting from TBI alters COII gene expression directly, perhaps influencing the synaptic plasticity that occurs during postinjury recovery processes. To test this hypothesis, we documented COII mRNA expression and complex IV (cytochrome c oxidase) functional activity at 7 days postinjury, focusing on the long-term postinjury period most closely associated with synaptic reorganization. Both central fluid percussion TBI and combined TBI and bilateral entorhinal cortical lesion were examined. At 7 days survival, differential display, RT-PCR, and Northern blot analysis of hippocampal RNA from both TBI and combined insult models showed a significant induction of COII mRNA. This long-term elevation in COII gene expression was supported by increases in COII immunobinding. By contrast, cytochrome oxidase histochemical activity within tissue sections from injured brains suggested a reduction of complex IV activity within the TBI cases, but not within animals subjected to the combined insult. These differences in cytochrome c oxidase activity were supported by in vitro assay of complex IV using cerebral cortical and hippocampal tissues. Our present results support the hypothesis that COII is selectively vulnerable to TBI and that COII differences may indicate the degree of metabolic dysfunction induced by different pathologies. Taken together, such data will better define the role of metabolic function in long-term recovery after TBI.