Activin induces long-lasting N-methyl-d-aspartate receptor activation via scaffolding PDZ protein activin receptor interacting protein 1

Tonic activin signaling shapes cellular and synaptic properties of CA1 neurons mainly in dorsal hippocampus

Dorsal and ventral hippocampus serve different functions in cognition and affective behavior, but the underpinnings of this diversity at the cellular and synaptic level are not well understood. We found that the basal level of activin A, a member of the TGF-β family, which regulates hippocampal circuits in a behaviorally relevant fashion, is much higher in dorsal than in ventral hippocampus. Using transgenic mice with a forebrain-specific disruption of activin receptor signaling, we identified the pronounced dorsal-ventral gradient of activin A as a major factor determining the distinct neurophysiologic signatures of dorsal and ventral hippocampus, ranging from pyramidal cell firing, tuning of frequency-dependent synaptic facilitation, to long-term potentiation (LTP), long-term depression (LTD), and de-potentiation. Thus, the strong activin A tone in dorsal hippocampus appears crucial to establish cellular and synaptic phenotypes that are tailored specifically to the respective network operations in dorsal and ventral hippocampus.

Environmental enrichment recruits activin A to recalibrate neural activity in mouse hippocampus

The TGF-β family member activin A modulates neural underpinnings of cognitive and affective functions in an activity-dependent fashion. We have previously shown that exploration of a novel and enriched environment (EE) strongly enhanced activin signaling. Whereas the many beneficial effects of EE are amply documented, the underlying mechanisms remain largely elusive. Here, we examined the hypothesis that EE recruits activin to regulate synaptic plasticity in a coordinated, cognition-promoting manner. Elevated activin levels after EE enhanced CA1 pyramidal cell excitability, facilitated synaptic transmission, and promoted long-term potentiation. These EE-induced changes were largely absent in mice expressing a dominant-negative mutant of activin receptor IB. We then interrogated the impact of activin on network oscillations and functional connectivity, using high-speed Ca 2+ imaging to study spike routing within networks formed by dissociated primary hippocampal cultures. Activin facilitated Ca2+ signaling, enhanced the network strength, and shortened the weighted characteristic path length. In the slice preparation, activin promoted theta oscillations during cholinergic stimulation. Thus, we advance activin as an activity-dependent and very early molecular effector that translates behavioral stimuli experienced during EE exposure into a set of synchronized changes in neuronal excitability, synaptic plasticity, and network activity that are all tuned to improve cognitive functions.

The transforming growth factor beta ligand TIG-2 modulates the function of neuromuscular junction and muscle energy metabolism in Caenorhabditis elegans

Deciphering the physiological function of TGF-β (the transforming growth factor beta) family ligands is import for understanding the role of TGF-β in animals' development and aging. Here, we investigate the function of TIG-2, one of the ligands in Caenorhabditis elegans TGF-β family, in animals' behavioral modulation. Our results show that a loss-of-function mutation in tig-2 gene result in slower locomotion speed in the early adulthood and an increased density of cholinergic synapses, but a decreased neurotransmitter release at neuromuscular junctions (NMJs). Further tissue-specific rescue results reveal that neuronal and intestinal TIG-2 are essential for the formation of cholinergic synapses at NMJs. Interestingly, tig-2(ok3416) mutant is characterized with reduced muscle mitochondria content and adenosine triphosphate (ATP) production, although the function of muscle acetylcholine receptors and the morphology muscle fibers in the mutant are comparable to that in wild-type animals. Our result suggests that TIG-2 from different neuron and intestine regulates worm locomotion by modulating synaptogenesis and neurotransmission at NMJs, as well as energy metabolism in postsynaptic muscle cells.

Enriched environment remedies cognitive dysfunctions and synaptic plasticity through NMDAR-Ca2+-Activin A circuit in chronic cerebral hypoperfusion rats

Chronic cerebral ischemia (CCI) is one of the critical factors in the occurrence and development of vascular cognitive impairment (VCI). Apoptosis of nerve cells and changes in synaptic activity after CCI are the key factors to induce VCI. Synaptic stimulation up-regulates intraneuronal Ca2+ level through N-methyl-D-aspartic acid receptor (NMDAR) via induction of the activity-regulated inhibitor of death (AID) expression to produce active-dependent neuroprotection. Moreover, the regulation of synaptic plasticity could improve cognition and learning ability. Activin A (ActA), an exocrine protein of AID, can promote NMDAR phosphorylation and participate in the regulation of synaptic plasticity. We previously found that exogenous ActA can improve the cognitive function of rats with chronic cerebral ischemia and enhance the oxygenated glucose deprivation of intracellular Ca2+ level. In addition to NMDAR, the Wnt pathway is critical in the positive regulation of LTP through activation or inhibition. It plays an essential role in synaptic transmission and activity-dependent synaptic plasticity. The enriched environment can increase ActA expression during CCI injury. We speculated that the NMDAR-Ca2+-ActA signal pathway has a loop-acting mode, and the environmental enrichment could improve chronic cerebral ischemia cognitive impairment via NMDAR-Ca2+-ActA, Wnt/β-catenin pathway is involved in this process. For the hypothesis verification, this study intends to establish chronic cerebral hypoperfusion (CCH) rat model, explore the improvement effect of enriched environment on VCI, detect the changes in plasticity of synaptic morphology and investigate the regulatory mechanism NMDAR-Ca2+-ActA-Wnt/β-catenin signaling loop, providing a therapeutic method for the treatment of CCH.

Neuroadaptations in the dorsal hippocampus underlie cocaine seeking during prolonged abstinence

Relapse vulnerability in substance use disorder is attributed to persistent cue-induced drug seeking that intensifies (or "incubates") during drug abstinence. Incubated cocaine seeking has been observed in both humans with cocaine use disorder and in preclinical relapse models. This persistent relapse vulnerability is mediated by neuroadaptations in brain regions involved in reward and motivation. The dorsal hippocampus (DH) is involved in context-induced reinstatement of cocaine seeking but the role of the DH in cocaine seeking during prolonged abstinence has not been investigated. Here we found that transforming growth factor-β (TGF-β) superfamily member activin A is increased in the DH on abstinence day (AD) 30 but not AD1 following extended-access cocaine self-administration compared to saline controls. Moreover, activin A does not affect cocaine seeking on AD1 but regulates cocaine seeking on AD30 in a bidirectional manner. Next, we found that activin A regulates phosphorylation of NMDA receptor (NMDAR) subunit GluN2B and that GluN2B-containing NMDARs also regulate expression of cocaine seeking on AD30. Activin A and GluN2B-containing NMDARs have both previously been implicated in hippocampal synaptic plasticity. Therefore, we examined synaptic strength in the DH during prolonged abstinence and observed an increase in moderate long-term potentiation (LTP) in cocaine-treated rats compared to saline controls. Lastly, we examined the role of DH projections to the lateral septum (LS), a brain region implicated in cocaine seeking and found that DH projections to the LS govern cocaine seeking on AD30. Taken together, this study demonstrates a role for the DH in relapse behavior following prolonged abstinence from cocaine self-administration.

Perspectives of small molecule inhibitors of activin receptor‑like kinase in anti‑tumor treatment and stem cell differentiation (Review)

Activin receptor‑like kinases (ALKs), members of the type I activin receptor family, belong to the serine/threonine kinase receptors of the transforming growth factor‑β (TGF‑β) superfamily. ALKs mediate the roles of activin/TGF‑β in a wide variety of physiological and pathological processes, ranging from cell differentiation and proliferation to apoptosis. For example, the activities of ALKs are associated with an advanced tumor stage in prostate cancer and the chondrogenic differentiation of mesenchymal stem cells. Therefore, potent and selective small molecule inhibitors of ALKs would not only aid in investigating the function of activin/TGF‑β, but also in developing treatments for these diseases via the disruption of activin/TGF‑β. In recent studies, several ALK inhibitors, including LY‑2157299, SB‑431542 and A‑83‑01, have been identified and have been confirmed to affect stem cell differentiation and tumor progression in animal models. This review discusses the therapeutic perspective of small molecule inhibitors of ALKs as drug targets in tumor and stem cells.

Activin A increases arterial pressure in the hypothalamic paraventricular nucleus in rats by angiotension II

Activin A, a member of the transforming growth factor β superfamily, plays an important role in the central nervous system as a neurotrophic and neuroprotective factor. The hypothalamic paraventricular nucleus (PVN) in the central nervous system is characterized as an important integrative site to regulate arterial pressure (AP). However, whether activin A in the PVN is involved in the regulation of AP is not well characterized. This study aimed to determine the effect of activin A on AP in the PVN in rats. The results showed that activin βA, activin type IIA and IIB receptors (ActRIIA and ActRIIB), and Smad2 and Smad3 mRNA expressions were detectable in the PVN of WKY rats by reverse-transcription PCR, and the expression of ActRIIA protein in the PVN was further confirmed by immunohistochemical staining. A microinjection of angiotensin II (AngII) (0.1 nmol/100 nl) or activin A (2 ng/100 nl) into the PVN increased AP significantly in WKY rats (P<0.05). Moreover, activin A (5 ng/ml) promoted AngII release from the primary cultured PVN neurons that can increase AP and upregulated the expressions of ActRIIA and Smad3 mRNA in the primary cultured PVN neurons (P<0.05). These data suggest that activin A can regulate AP in the PVN in an autocrine or a paracrine manner, which is related to AngII release and the ActRIIA-Smad3 signal pathway.

TNFα drives pulmonary arterial hypertension by suppressing the BMP type-II receptor and altering NOTCH signalling

Heterozygous germ-line mutations in the bone morphogenetic protein type-II receptor (BMPR-II) gene underlie heritable pulmonary arterial hypertension (HPAH). Although inflammation promotes PAH, the mechanisms by which inflammation and BMPR-II dysfunction conspire to cause disease remain unknown. Here we identify that tumour necrosis factor-α (TNFα) selectively reduces BMPR-II transcription and mediates post-translational BMPR-II cleavage via the sheddases, ADAM10 and ADAM17 in pulmonary artery smooth muscle cells (PASMCs). TNFα-mediated suppression of BMPR-II subverts BMP signalling, leading to BMP6-mediated PASMC proliferation via preferential activation of an ALK2/ACTR-IIA signalling axis. Furthermore, TNFα, via SRC family kinases, increases pro-proliferative NOTCH2 signalling in HPAH PASMCs with reduced BMPR-II expression. We confirm this signalling switch in rodent models of PAH and demonstrate that anti-TNFα immunotherapy reverses disease progression, restoring normal BMP/NOTCH signalling. Collectively, these findings identify mechanisms by which BMP and TNFα signalling contribute to disease, and suggest a tractable approach for therapeutic intervention in PAH.

Reduced BMP receptor II signalling underlies pulmonary arterial hypertension (PAH). Here, Hurst et al. show that TNFα subverts BMP signalling by increasing BMP6 expression and signalling via an alternative BMP receptor, ALK2, in pulmonary artery smooth muscle cells to drive abnormal proliferation and PAH.

Activin Signaling in the Pathogenesis and Therapy of Neuropsychiatric Diseases

Activins are members of the transforming growth factor β (TGFβ) family and serve as multifunctional regulatory proteins in many tissues and organs. In the brain, activin A, which is formed by two disulfide-linked βA subunits, is recognized as the predominant player in activin signaling. Over the last years, considerable progress has been made in elucidating novel and unexpected functions of activin in the normal and diseased brain and in deciphering the underlying molecular mechanisms. Initially identified as a neurotrophic and protective factor during development and in several forms of acute injury, the scope of effects of activin A in the adult central nervous system (CNS) has been considerably broadened by now. Here, we will highlight recent findings that bear significance for a better understanding of the pathogenesis of various neuropsychiatric diseases and might hold promise for novel therapeutic strategies. While the basal level of activin A in the adult brain is low, significant short-term up-regulation occurs in response to increased neuronal activity. In fact, brief exposure to an enriched environment (EE) is already sufficient to considerably strengthen activin signaling. Enhancement of this pathway tunes the performance of glutamatergic and GABAergic synapses in a fashion that impacts on cognitive functions and affective behavior, counteracts death-inducing signals through extrasynaptic NMDA receptors (NMDARs), and stimulates adult neurogenesis in the hippocampus. We will discuss how impaired activin signaling is involved in anxiety disorders, depression, drug dependence, and neurodegenerative diseases such as Alzheimer’s and Parkinson’s, and how reinforcement of activin signaling might be exploited for therapeutic interventions.

Ligand trap for the activin type IIA receptor protects against vascular disease and renal fibrosis in mice with chronic kidney disease

The causes of cardiovascular mortality associated with chronic kidney disease (CKD) are partly attributed to the CKD-mineral bone disorder (CKD-MBD). The causes of the early CKD-MBD are not well known. Our discovery of Wnt (portmanteau of wingless and int) inhibitors, especially Dickkopf 1, produced during renal repair as participating in the pathogenesis of the vascular and skeletal components of the CKD-MBD implied that additional pathogenic factors are critical. In the search for such factors, we studied the effects of activin receptor type IIA (ActRIIA) signaling by using a ligand trap for the receptor, RAP-011 (a soluble extracellular domain of ActRIIA fused to a murine IgG-Fc fragment). In a mouse model of CKD that stimulated atherosclerotic calcification, RAP-011 significantly increased aortic ActRIIA signaling assessed by the levels of phosphorylated Smad2/3. Furthermore, RAP-011 treatment significantly reversed CKD-induced vascular smooth muscle dedifferentiation as assessed by smooth muscle 22α levels, osteoblastic transition, and neointimal plaque calcification. In the diseased kidneys, RAP-011 significantly stimulated αklotho levels and it inhibited ActRIIA signaling and decreased renal fibrosis and proteinuria. RAP-011 treatment significantly decreased both renal and circulating Dickkopf 1 levels, showing that Wnt activation was downstream of ActRIIA. Thus, ActRIIA signaling in CKD contributes to the CKD-MBD and renal fibrosis. ActRIIA signaling may be a potential therapeutic target in CKD.

The DAF-7/TGF-β signaling pathway regulates abundance of the Caenorhabditis elegans glutamate receptor GLR-1

Transforming growth factor-β (TGF-β) family signaling pathways have roles in both neuronal development and the regulation of synaptic function. Here we identify a novel role for the Caenorhabditis elegans DAF-7/TGF-β signaling pathway in the regulation of the AMPA-type glutamate receptor GLR-1. We found that the abundance of GLR-1 increases at synapses in the ventral nerve cord (VNC) of animals with loss-of-function mutations in multiple DAF-7/TGF-β pathway components including the TGF-β ligand DAF-7, the type I receptor DAF-1, and the Smads DAF-8 and DAF-14. The GLR-1 defect can be rescued by expression of daf-8 specifically in glr-1-expressing interneurons. The effect on GLR-1 was specific for the DAF-7 pathway because mutations in the DBL-1/TGF-β family pathway did not increase GLR-1 levels in the VNC. Immunoblot analysis indicates that total levels of GLR-1 protein are increased in neurons of DAF-7/TGF-β pathway mutants. The increased abundance of GLR-1 in the VNC of daf-7 pathway mutants is dependent on the transcriptional regulator DAF-3/Smad suggesting that DAF-3-dependent transcription controls GLR-1 levels. Furthermore, we found that glr-1 transcription is increased in daf-7 mutants based on a glr-1 transcriptional reporter. Together these results suggest that the DAF-7/TGF-β signaling pathway functions in neurons and negatively regulates the abundance of GLR-1, in part, by controlling transcription of the receptor itself. Finally, DAF-7/TGF-β pathway mutants exhibit changes in spontaneous locomotion that are dependent on endogenous GLR-1 and consistent with increased glutamatergic signaling. These results reveal a novel mechanism by which TGF-β signaling functions in the nervous system to regulate behavior.

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.

Activin Signaling Disruption in the Cochlea Does Not Influence Hearing in Adult Mice

Activin, a member of the TGF-F superfamily, was found to play an important role in the development, repair and apoptosis of different tissues and organs. Accordingly, activin signaling is involved in the development of the cochlea. Activin binds to its receptor ActRII, then dimerizes with ActRI and induces a signaling pathway resulting in gene expression. A study reported a case of fibrodysplasia ossificans progressiva with an unusual mutation in the ActRI gene leading to sensorineural hearing loss. This draws attention to the role of activin and its receptors in the developed cochlea. To date, only the expression of ActRII is known in the adult mammalian cochlea. In this study, we present for the first time the presence of activin A and ActRIB in the adult cochlea. Transgenic mice with postnatal dominant-negative ActRIB expression causing disruption of activin signaling in vivo were used for assessing cochlear morphology and hearing ability through the auditory brainstem response (ABR) threshold. Nonfunctioning ActRIB did not affect the ABR thresholds and did not alter the microscopic anatomy of the cochlea. We conclude, therefore, that activin signaling is not necessary for hearing in adult mice under physiological conditions but may be important during and after damaging events in the inner ear. i 2014 S. Karger AG, Basel

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.

TGF-beta superfamily member activin A acts with BDNF and erythropoietin to improve survival of spiral ganglion neurons in vitro

Activins are regulators of embryogenesis, osteogenesis, hormones and neuronal survival. Even though activin receptor type II has been detected in spiral ganglion neurons (SGN), little is known about the role of activins in the inner ear. An activin-mediated neuroprotection is of considerable clinical interest since SGN are targets of electrical stimulation with cochlear implants in hearing impaired patients. Thus, the presence of activin type-I and type-II receptors was demonstrated immunocytochemically and the individual and combined effects of activin A, erythropoietin (EPO) and brain-derived neurotrophic factor (BDNF) on SGN were examined in vitro. SGN isolated from neonatal rats (P 3-5) were cultured in serum-free medium supplemented with activin A, BDNF and EPO. Compared to the negative control, survival rates of SGN were significantly improved when cultivated individually with activin A (p<0.001) and in combination with BDNF (p<0.001). Neither neurite outgrowth nor neuronal survival was influenced by the addition of EPO to activin A-treated neurons. However, when all three factors were added, a significantly (p<0.001) improved neuronal survival was observed (61.2±3.6%) compared to activin A (25.4±2.1%), BDNF (22.8±3.3%) and BDNF+EPO (19.2±1.5%). Under the influence of the EPO-inhibitors, this increase in neuronal survival was blocked. Acting with BDNF and EPO to promote neuronal survival in vitro, activin A presents an interesting factor for pharmacological intervention in the inner ear. The present study demonstrates a synergetic effect of a combined therapy with several trophic factors.

Localisation and role of activin receptor-interacting protein 1 in mouse brain

Activin A, a stimulator of follicle-stimulating hormone secretion from the pituitary, acts as a neurotrophic and neuroprotective factor in the central nervous system. Activin receptor-interacting protein 1 (ARIP1) has been identified as a cytoplasmic protein that interacts with the type II receptor of activin (ActRII). However, the distribution pattern and function of ARIP1 are not well characterised in the brain. In the present study, we confirmed the existence of mRNA and protein of ARIP1 in the mouse brain, and found that ARIP1 was mainly localised at the hippocampus and hypothalamus in the cerebrum, granular layers in the cerebellum (especially in Purkinje cells of the cerebellum) and choroid epithelial cells by immunohistochemical staining. Furthermore, in contrast to the significant increase of activin A mRNA, ARIP1 mRNA and protein expression decreased in the mechanically lesioned brain of the mouse. Using neuroblastoma-derived Neuro-2a cells to investigate the function of ARIP1, we found that overexpression of ARIP1 down-regulated the activin A-induced signal transduction and significantly decreased the voltage-gated Na(+) current (I(Na) ). These data indicate that ARIP1 is a key molecule for the regulation of the action of activin in neurones, and also that decreased ARIP1 expression in the lesioned brain may be beneficial to the neurotrophic and neuroprotective roles of activin A in recovery after brain injury.

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.

Canonical TGF-beta signaling is required for the balance of excitatory/inhibitory transmission within the hippocampus and prepulse inhibition of acoustic startle

Smad4 is a unique nuclear transducer for all TGF-beta signaling pathways and regulates gene transcription during development and tissue homeostasis. To elucidate the postnatal role of TGF-beta signaling in the mammalian brain, we generated forebrain-specific Smad4 knock-out mice. Surprisingly, the mutants showed no alteration in long-term potentiation and water maze, suggesting that Smad4 is not required for spatial learning and memory. However, these mutant mice did show enhancement of paired-pulse facilitation in excitatory synaptic transmission and stronger paired-pulse depression of GABA(A) currents in the hippocampus. The alteration of hippocampal electrophysiology correlated with mouse hyperactivity in homecage and open field tests. Mutant mice also showed overgrooming as well as deficits of prepulse inhibition, a widely used endophenotype of schizophrenia. With a specific real-time PCR array focused on TGF-beta signaling pathway, we identified a novel regulation mechanism of the pathway in the hippocampal neurons, in which Smad4-mediated signaling suppresses the level of extracellular antagonism of TGF-beta ligands through transcriptional regulation of follistatin, a selective inhibitor to activin/TGF-beta signaling in the hippocampus. In summary, we suggest that the canonical TGF-beta signaling pathway is critical for use-dependent modulation of GABA(A) synaptic transmission and dendritic homeostasis; furthermore, a disruption in the balance of the excitatory and inhibitory hippocampal network can result in psychiatric-like behavior.

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.

Fyn kinase controls FcepsilonRI receptor-operated calcium entry necessary for full degranulation in mast cells

IgE-antigen-dependent crosslinking of the high affinity IgE receptor (FcepsilonRI) on mast cells leads to degranulation, leukotriene synthesis and cytokine production. Calcium (Ca(2+)) mobilization is a sine qua non requisite for degranulation, allowing the rapid secretion of stored pro-inflammatory mediators responsible for allergy symptoms. Fyn is a Src-family kinase that positively controls FcepsilonRI-induced mast cell degranulation. However, our understanding of the mechanism connecting Fyn activation to secretion of pre-synthesized mediators is very limited. We analyzed FcepsilonRI-dependent Ca(2+) mobilization in bone marrow-derived mast cells (BMMCs) differentiated from WT and Fyn -/- knock out mice. Fyn -/- BMMCs showed a marked defect in extracellular Ca(2+) influx after FcepsilonRI crosslinking but not after thapsigargin addition. High concentrations of Gadolinium (Gd(3+)) partially blocked FcepsilonRI-induced Ca(2+) influx in WT cells but, in contrast, completely inhibited Ca(2+) mobilization in Fyn -/- cells. Low concentrations of an inhibitor of the canonical transient receptor potential (TRPC) Ca(2+) channels (2-aminoethoxyphenyl-borane, 2-APB) blocked FcepsilonRI-induced maximal Ca(2+) rise in WT but not in Fyn -/- cells. Ca(2+) entry through Fyn-controlled, 2-APB sensitive channels was found to be important for full degranulation and IL-2 mRNA accumulation in WT cells. Immunoprecipitation assays showed that Fyn kinase interacts with TRPC 3/6/7 channels after IgE-antigen stimulation, but its association is not related to protein tyrosine phosphorylation. Results indicate Fyn kinase mediates the receptor-dependent activation of TRPC channels that contribute to degranulation in FcepsilonRI-stimulated mast cells.

Essential involvement of the NMDA receptor in ethanol preconditioning-dependent neuroprotection from amyloid-betain vitro

In several epidemiological studies, moderate ethanol consumption has been associated with reduced risks of cognitive decline or Alzheimer's dementia. Of potential relevance is that brain cultures preconditioned with moderate ethanol concentrations are resistant to neurotoxic Alzheimer's amyloid-beta (Abeta) peptides. Using rat cerebellar mixed cultures we investigated whether certain membrane receptors were early 'sensors' in moderate ethanol preconditioning (MEP). In a 6-day MEP protocol (30 mM ethanol), neuroprotection from Abeta25-35 was undiminished by antagonism during the first 3 days of either adenosine A(1) or Galpha(i/o) protein-coupled receptors. However, similar cotreatment with memantine or DL-2-amino-5-phosphono-pentanoic acid (AP-5), antagonists of NMDA receptors (NMDAR), abolished neuroprotection, indicating key early involvement of this ionotropic glutamate receptor. Also in these cultures, directly activating NMDAR using subexcitotoxic NMDA preconditioning prevented Abeta neurotoxicity. By day 2 of MEP, we observed increased levels of NMDAR subunits NR1, NR2B, and NR2C that persisted through day 6. Interestingly, memantine co-exposure blocked elevations in the obligatory NR1 subunit. Furthermore, 2 days of MEP significantly increased two indicators of synaptic NMDAR localization, NR2B phospho-Tyr1472, and post-synaptic density 95 scaffolding protein. The results indicate that ethanol preconditioning-dependent neuroprotection is associated with early increases in NR subunits concomitant with enhancement of synaptic localization and activity of NMDAR.

Differential expression of metabotropic glutamate receptors 2 and 3 in schizophrenia: a mechanism for antipsychotic drug action?

OBJECTIVE

Preclinical and clinical data implicate the group II metabotropic glutamate receptors mGluR2 and mGluR3 in the pathophysiology of schizophrenia. Moreover, a recent phase II clinical trial demonstrated the antipsychotic efficacy of a mGluR2/mGluR3 agonist. The purpose of the present study was to distinguish the expression of mGluR2 and mGluR3 receptor proteins in schizophrenia and to quantify glutamate carboxypeptidase II (GCP II) in order to explore a role for the metabotropic receptors in schizophrenia therapeutics. GCP II is an enzyme that metabolizes N-acetyl-aspartyl-glutamate (NAAG), which is the only known specific endogenous agonist of mGluR3 in the mammalian brain.

METHOD

The normal expression levels of mGluR2, mGluR3, and GCP II were determined for 10 regions of the postmortem human brain using specific antibodies. Differences in expression levels of each protein were examined in the dorsolateral prefrontal cortex, temporal cortex, and motor cortex in 15 postmortem schizophrenia subjects and 15 postmortem matched normal comparison subjects. Chronic antipsychotic treatment in rodents was conducted to examine the potential effect of antipsychotic drugs on expression of the three proteins.

RESULTS

Findings revealed a significant increase in GCP II protein and a reduction in mGluR3 protein in the dorsolateral prefrontal cortex in schizophrenia subjects, with mGluR2 protein levels unchanged. Chronic antipsychotic treatment in rodents did not influence GCP II or mGluR3 levels.

CONCLUSIONS

Increased GCP II expression and low mGluR3 expression in the dorsolateral prefrontal cortex suggest that NAAG-mediated signaling is impaired in this brain region in schizophrenia. Further, these data implicate the mGluR3 receptor in the antipsychotic action of mGluR2/mGluR3 agonists.

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.

Expression and localization of activin receptor-interacting protein 2 in mouse tissues

Activin plays important roles in reproductive tissues as a stimulator of follicle-stimulating hormone (FSH) secretion. Activin receptor-interacting protein 2 (ARIP2) has been recently identified in mouse tissues as a regulatory protein of activin signal transduction. However, the localization and function of ARIP2 are not well characterized. In this study, we found that ARIP2 mRNA and protein were widely expressed in mouse tissues by reverse transcription-PCR (RT-PCR) and Western blotting. The immunoreactivities of ARIP2 were mainly localized at myocardial cells of heart, Leydig cells in testis, macrophages and epithelial cells of bronchus in lung, renal tubule and collecting tubule, pancreatic islet, adrenal gland, adenohypophysis and hypothalamus by immunohistochemical staining. Furthermore, ARIP2 overexpression down-regulated signal transduction induced by activin A in pituitary gonadotroph LbetaT2 cells and inhibited FSH secretion from primary cultured anterior pituitary cells induced by activin A. These findings suggest that ARIP2 is widely distributed in various tissues and may be a negative regulator of activin action in pituitary cells.

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.