Signaling by hepatocyte growth factor in neurons is induced by pharmacological stimulation of synaptic activity

C-Met Receptors Deficiency Was Involved in Absence Seizures Development in WAG/Rij Rats

Background: A variety of receptors may be involved in the pathogenesis of absence seizures. The c-Met receptors have a critical role in modulating the GABAergic interneurons and creating a balance between excitatory and inhibitory neurotransmission, sensorimotor gating, and normal synaptic plasticity. Objectives: This study aimed to assess the changes of the c-Met receptor during the appearance of absence attacks in the experimental model of absence epilepsy. Methods: A total of 48 animals were divided into four groups of two- and six-month-old WAG/Rij and Wistar rats. Epileptic WAG/Rij rats showing SWP in electrocorticogram (ECoG) were included in the epileptic group. The two-month-old WAG/Rij rats as well as two- and six-month-old Wistar rats not exhibiting SWP in ECoG were selected as the non-epileptic. Gene (RT-PCR) and protein expression (western blotting) of c-Met receptors as well as c-Met protein distribution (immunohistochemistry) in the somatosensory cortex and hippocampus were assessed during seizure development of the absence attacks. Results: According to the study findings, a lower c-Met gene and protein expression, as well as a lower protein distribution, were observed in the hippocampus (P < 0.001, P < 0.05, and P < 0.001, respectively) and cortex (P < 0.01, P < 0.001 and P < 0.001, respectively) of the two-month-old WAG/Rij rats compared to the same-age Wistar rats. Moreover, the data revealed a reduction of hippocampal and cortical c-Met protein expression (P < 0.001, for both) in six-month-old WAG/Rij rats compared to two-month-old ones. Six-month-old WAG/Rij rats had a lower cortical c-Met gene (P < 0.05) and protein expression (P < 0.001) as well as lower hippocampal and cortical protein distribution (P < 0.05 and P < 0.001) than the same-age Wistar rats. Conclusions: In sum, the c-Met receptor was found to play a significant role in the development of absence epilepsy. This receptor, therefore, may have been considered as an effective goal for absence seizure inhibition.

MET Oncogene Controls Invasive Growth by Coupling with NMDA Receptor

Simple Summary

The MET oncogene, encoding the tyrosine kinase receptor for a hepatocyte growth factor (HGF), plays a key role in the onset and progression of aggressive forms of breast cancer. Recently, it was found that the glutamate receptor, which has a well-known role in the nervous system, is expressed in many types of tumors outside the nervous system and contributes to metastatic behavior in breast cancer cells. Here, we highlight that MET protein physically interacts with glutamate receptors in two highly metastatic breast cancer cell lines. HGF, which creates a supportive proinvasive microenvironment for the tumor cells, stabilizes this interaction. Pharmacological inhibition of glutamate receptors blunts the migration and invasion elicited by HGF, suggesting drug repurposing of glutamate receptor antagonists for anticancer therapy.

Abstract

The N-methyl-D-aspartate receptor (NMDAR) is a glutamate-gated ion channel involved in excitatory synaptic transmission. Outside the nervous system, the NMDAR is expressed in a variety of tissues and in cancers, notably in the highly invasive and metastatic triple-negative breast carcinoma. MET encodes the tyrosine kinase receptor for HGF and is a master regulator gene for “invasive growth”. In silico analysis shows that high expression of the NMDAR2B subunit is a negative prognostic factor in human invasive breast carcinoma. Here, we show that in triple-negative breast cancer cell lines NMDAR2B and MET proteins are coexpressed. HGF stimulation of these cells is followed by autophosphorylation of the MET kinase and phosphorylation of the NMDAR2B subunit at tyrosines 1252 and 1474. MET and phosphorylated NMDAR2B are physically associated, as demonstrated by co-immunoprecipitation, confocal immunofluorescence, and proximity ligation assays. Notably, pharmacological inhibition of NMDAR by MK801 and ifenprodil blunts the biological response to HGF. These results demonstrate the existence of a MET-NMDAR crosstalk driving the invasive program, paving the way for a new combinatorial therapy.

Two-Chains Tissue Plasminogen Activator Unifies Met and NMDA Receptor Signalling to Control Neuronal Survival

Tissue-type plasminogen activator (tPA) plays roles in the development and the plasticity of the nervous system. Here, we demonstrate in neurons, that by opposition to the single chain form (sc-tPA), the two-chains form of tPA (tc-tPA) activates the MET receptor, leading to the recruitment of N-Methyl-d-Aspartate receptors (NMDARs) and to the endocytosis and proteasome-dependent degradation of NMDARs containing the GluN2B subunit. Accordingly, tc-tPA down-regulated GluN2B-NMDAR-driven signalling, a process prevented by blockers of HGFR/MET and mimicked by its agonists, leading to a modulation of neuronal death. Thus, our present study unmasks a new mechanism of action of tPA, with its two-chains form mediating a crosstalk between MET and the GluN2B subunit of NMDARs to control neuronal survival.

HGF and MET: From Brain Development to Neurological Disorders

Hepatocyte growth factor (HGF) and its tyrosine kinase receptor, encoded by the MET cellular proto-oncogene, are expressed in the nervous system from pre-natal development to adult life, where they are involved in neuronal growth and survival. In this review, we highlight, beyond the neurotrophic action, novel roles of HGF-MET in synaptogenesis during post-natal brain development and the connection between deregulation of MET expression and developmental disorders such as autism spectrum disorder (ASD). On the pharmacology side, HGF-induced MET activation exerts beneficial neuroprotective effects also in adulthood, specifically in neurodegenerative disease, and in preclinical models of cerebral ischemia, spinal cord injuries, and neurological pathologies, such as Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS). HGF is a key factor preventing neuronal death and promoting survival through pro-angiogenic, anti-inflammatory, and immune-modulatory mechanisms. Recent evidence suggests that HGF acts on neural stem cells to enhance neuroregeneration. The possible therapeutic application of HGF and HGF mimetics for the treatment of neurological disorders is discussed.

Brain Renin-Angiotensin System at the Intersect of Physical and Cognitive Frailty

The renin–angiotensin system (RAS) was initially considered to be part of the endocrine system regulating water and electrolyte balance, systemic vascular resistance, blood pressure, and cardiovascular homeostasis. It was later discovered that intracrine and local forms of RAS exist in the brain apart from the endocrine RAS. This brain-specific RAS plays essential roles in brain homeostasis by acting mainly through four angiotensin receptor subtypes; AT₁R, AT₂R, MasR, and AT₄R. These receptors have opposing effects; AT₁R promotes vasoconstriction, proliferation, inflammation, and oxidative stress while AT₂R and MasR counteract the effects of AT₁R. AT₄R is critical for dopamine and acetylcholine release and mediates learning and memory consolidation. Consequently, aging-associated dysregulation of the angiotensin receptor subtypes may lead to adverse clinical outcomes such as Alzheimer’s disease and frailty via excessive oxidative stress, neuroinflammation, endothelial dysfunction, microglial polarization, and alterations in neurotransmitter secretion. In this article, we review the brain RAS from this standpoint. After discussing the functions of individual brain RAS components and their intracellular and intracranial locations, we focus on the relationships among brain RAS, aging, frailty, and specific neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and vascular cognitive impairment, through oxidative stress, neuroinflammation, and vascular dysfunction. Finally, we discuss the effects of RAS-modulating drugs on the brain RAS and their use in novel treatment approaches.

Cellular diversity of the somatosensory cortical map plasticity

Sensory maps are representations of the sensory epithelia in the brain. Despite the intuitive explanatory power behind sensory maps as being neuronal precursors to sensory perception, and sensory cortical plasticity as a neural correlate of perceptual learning, molecular mechanisms that regulate map plasticity are not well understood. Here we perform a meta-analysis of transcriptional and translational changes during altered whisker use to nominate the major molecular correlates of experience-dependent map plasticity in the barrel cortex. We argue that brain plasticity is a systems level response, involving all cell classes, from neuron and glia to non-neuronal cells including endothelia. Using molecular pathway analysis, we further propose a gene regulatory network that could couple activity dependent changes in neurons to adaptive changes in neurovasculature, and finally we show that transcriptional regulations observed in major brain disorders target genes that are modulated by altered sensory experience. Thus, understanding the molecular mechanisms of experience-dependent plasticity of sensory maps might help to unravel the cellular events that shape brain plasticity in health and disease.

The renin angiotensin system and the brain: New developments

The traditional renin-angiotensin system (RAS) is indispensable system in adjusting sodium homeostasis, body fluid volume, and controlling arterial blood pressure. The key elements are renin splitting inactive angiotensinogen to yield angiotensin (Ang-I). Ang-1 is then changed by angiotensin-1 converting enzyme (ACE) into angiotensin II (Ang-II). Using PubMed, Google Scholar, and other means, we searched the peer-reviewed literature from 1990 to 2013 for articles on newly discovered findings related to the RAS, especially focusing on how the system influences the central nervous system (CNS). The classical RAS is now considered to be only part of the picture; the discovery of additional RAS pathways in the brain and elsewhere has yielded a vastly improved understanding of how the RAS influences the CNS. Newly discovered effects of the RAS on brain tissue include neuroprotection, cognition, and cerebral vasodilation. A number of brain biochemical pathways are influenced by the brain RAS. Within various pathways, there are potential opportunities for classical pharmacologic interventions as well as the possibility of controlling gene expression.

Open label study to assess the safety of VM202 in subjects with amyotrophic lateral sclerosis

OBJECTIVE

To assess safety and define efficacy measures of hepatocyte growth factor (HGF) DNA plasmid, VM202, administered by intramuscular injections in patients with amyotrophic lateral sclerosis (ALS).

METHODS

Eighteen participants were treated with VM202 administered in divided doses by injections alternating between the upper and lower limbs on d 0, 7, 14, and 21. Subjects were followed for nine months to evaluate possible adverse events. Functional outcome was assessed using the ALS Functional Rating Scale-Revised (ALSFRS-R) as well as by serially measuring muscle strength, muscle circumference, and forced vital capacity.

RESULTS

Seventeen of 18 participants completed the study. All participants tolerated 64 mg of VM202 well with no serious adverse events (SAE) related to the drug. Twelve participants reported 26 mild or moderate injection site reactions. Three participants experienced five SAEs unrelated to VM202. One subject died from respiratory insufficiency secondary to ALS progression.

CONCLUSIONS

Multiple intramuscular injection of VM202 into the limbs appears safe in ALS subjects. Future trials with retreatment after three months will determine whether VM202 treatment alters the long-term course of ALS.

The development of small molecule angiotensin IV analogs to treat Alzheimer's and Parkinson's diseases

Alzheimer's (AD) and Parkinson's (PD) diseases are neurodegenerative diseases presently without effective drug treatments. AD is characterized by general cognitive impairment, difficulties with memory consolidation and retrieval, and with advanced stages episodes of agitation and anger. AD is increasing in frequency as life expectancy increases. Present FDA approved medications do little to slow disease progression and none address the underlying progressive loss of synaptic connections and neurons. New drug design approaches are needed beyond cholinesterase inhibitors and N-methyl-d-aspartate receptor antagonists. Patients with PD experience the symptomatic triad of bradykinesis, tremor-at-rest, and rigidity with the possibility of additional non-motor symptoms including sleep disturbances, depression, dementia, and autonomic nervous system failure. This review summarizes available information regarding the role of the brain renin-angiotensin system (RAS) in learning and memory and motor functions, with particular emphasis on research results suggesting a link between angiotensin IV (AngIV) interacting with the AT4 receptor subtype. Currently there is controversy over the identity of this AT4 receptor protein. Albiston and colleagues have offered convincing evidence that it is the insulin-regulated aminopeptidase (IRAP). Recently members of our laboratory have presented evidence that the brain AngIV/AT4 receptor system coincides with the brain hepatocyte growth factor/c-Met receptor system. In an effort to resolve this issue we have synthesized a number of small molecule AngIV-based compounds that are metabolically stable, penetrate the blood-brain barrier, and facilitate compromised memory and motor systems. These research efforts are described along with details concerning a recently synthesized molecule, Dihexa that shows promise in overcoming memory and motor dysfunctions by augmenting synaptic connectivity via the formation of new functional synapses.

Synaptic and extrasynaptic location of the receptor tyrosine kinase met during postnatal development in the mouse neocortex and hippocampus

MET, a replicated autism risk gene, encodes a pleiotropic receptor tyrosine kinase implicated in multiple cellular processes during development and following injury. Previous studies suggest that Met modulates excitatory synapse development in the neocortex and hippocampus, although the underlying mechanism is unknown. The peak of Met expression corresponds to the period of process outgrowth and synaptogenesis, with robust expression in hippocampal and neocortical neuropil. Resolving whether neuropil expression represents presynaptic, postsynaptic or glial localization provides insight into potential mechanisms of Met action. The subcellular distribution of Met was characterized using complementary ultrastructural, in situ proximity ligation assay (PLA), and biochemical approaches. At postnatal day (P) 7, immunoelectron microscopy revealed near-equivalent proportions of Met-immunoreactive pre- (axons and terminals) and postsynaptic (dendritic shafts and spines) profiles in the stratum radiatum in the hippocampal CA1 region. Staining was typically in elements in which the corresponding pre- or postsynaptic apposition was unlabeled. By P21, Met-immunoreactive presynaptic profiles predominated and ~20% of Met-expressing profiles were glial. A different distribution of Met-immunoreactive profiles was observed in layer V of somatosensory cortex: Met-labeled spines were rare and a smaller proportion of glial profiles expressed Met. Strikingly, Met-immunoreactive presynaptic profiles predominated over postsynaptic profiles as early as P7. PLA analysis of neurons in vitro and biochemical analysis of tissue subsynaptic fractions confirmed the localization of Met in specific synaptic subcompartments. The study demonstrates that Met is enriched at synapses during development and its activation may modulate synapse formation and stability through both pre- and postsynaptic mechanisms.

Importance of the brain Angiotensin system in Parkinson's disease

Parkinson's disease (PD) has become a major health problem affecting 1.5% of the world's population over 65 years of age. As life expectancy has increased so has the occurrence of PD. The primary direct consequence of this disease is the loss of dopaminergic (DA) neurons in the substantia nigra and striatum. As the intensity of motor dysfunction increases, the symptomatic triad of bradykinesia, tremors-at-rest, and rigidity occur. Progressive neurodegeneration may also impact non-DA neurotransmitter systems including cholinergic, noradrenergic, and serotonergic, often leading to the development of depression, sleep disturbances, dementia, and autonomic nervous system failure. L-DOPA is the most efficacious oral delivery treatment for controlling motor symptoms; however, this approach is ineffective regarding nonmotor symptoms. New treatment strategies are needed designed to provide neuroprotection and encourage neurogenesis and synaptogenesis to slow or reverse this disease process. The hepatocyte growth factor (HGF)/c-Met receptor system is a member of the growth factor family and has been shown to protect against degeneration of DA neurons in animal models. Recently, small angiotensin-based blood-brain barrier penetrant mimetics have been developed that activate this HGF/c-Met system. These compounds may offer a new and novel approach to the treatment of Parkinson's disease.

Nanoscale mapping of the Met receptor on hippocampal neurons by AFM and confocal microscopy

Hepatocyte growth factor (HGF), a neurotrophic protein, acting through its tyrosine kinase receptor, Met, facilitates learning and synaptic plasticity. In concert with the role of the HGF/Met system in synaptic plasticity, we demonstrate that Met is localized to brain regions which undergo extensive synaptic remodeling. We demonstrate that Met activation results in an increase in dendritic spine density and functional synapses. Based on these observations, we hypothesized that Met should be associated with post-synaptic elements found on dendritic spines. Thus, the goal of this study was to determine the sub-cellular localization of Met on hippocampal neurons. Using an atomic force microscopy tip decorated with a specific Met antibody, the location of Met was mapped to different cellular compartments of hippocampal pyramidal neurons. Our results indicated that multimeric activated Met was found to be concentrated in the dendritic compartment while the inactivated monomeric form of Met was prominent on the soma.

FROM THE CLINICAL EDITOR

The goal of this study was to determine the sub-cellular localization of Met on hippocampal neurons using nanotechnology-based techniques, using an atomic force microscopy tip decorated with a specific Met antibody. The authors demonstrate that multimeric activated Met was found to be concentrated in the dendritic compartment while the inactivated monomeric form of Met was prominent in the soma of hippocampal pyramidal neurons.

Differential regulation of axonal growth and neuromuscular junction assembly by HGF/c-Met signaling

BACKGROUND

During vertebrate neuromuscular junction (NMJ) development, contact between motor axons and muscle fibers is followed by pre- and post-synaptic specialization. Using Xenopus nerve-muscle cocultures, we recently showed that spinal neurons initially contacted muscle cells by means of filopodial processes, and that muscle-derived basic fibroblast growth factor induced axonal filopodia and slowed axonal advance to promote nerve-muscle interaction and NMJ establishment. In contrast, neurotrophins enhanced axonal growth but suppressed the extension of axonal filopodia and blocked NMJ formation.

RESULTS

Here we report that hepatocyte growth factor (HGF), which also supports motor neuron survival, was expressed by Xenopus muscle cells, and that forced expression of HGF in Xenopus spinal neurons inhibited the extension of axonal filopodia. Overexpression of the HGF-receptor c-Met in neurons also blocked the formation of axonal filopodia and furthermore sped up axonal growth, but a kinase-dead form of c-Met was unable to effect these changes. Importantly, treatment of nerve-muscle cocultures with recombinant HGF or the expression of HGF or active c-Met in neurons, or that of excess HGF in muscle, inhibited nerve-induced AChR clustering in muscle.

CONCLUSIONS

Our results suggest that HGF/c-Met signaling in neurons promotes axonal growth but suppresses filopodial assembly in neurons and hinders NMJ establishment.

Hepatocyte growth factor overexpression in the nervous system enhances learning and memory performance in mice

Hepatocyte growth factor (HGF) and its receptor, c-Met, play pivotal roles in the nervous system during development and in disease states. However, the physiological roles of HGF in the adult brain are not well understood. In the present study, to assess its role in learning and memory function, we used transgenic mice that overexpress HGF in a neuron-specific manner (HGF-Tg) to deliver HGF into the brain without injury. HGF-Tg mice displayed increased alternation rates in the Y-maze test compared with age-matched wild-type (WT) controls. In the Morris water maze (MWM) test, HGF-Tg mice took less time to find the platform on the first day, whereas the latency to escape to the hidden platform was decreased over training days compared with WT mice. A transfer test revealed that the incidence of arrival at the exact location of the platform was higher for HGF-Tg mice compared with WT mice. These results demonstrate that overexpression of HGF leads to an enhancement of both short- and long-term memory. Western blot analyses revealed that the levels of N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR2B, but not NR1, were increased in the hippocampus of HGF-Tg mice compared with WT controls, suggesting that an upregulation of NR2A and NR2B could represent one mechanism by which HGF enhances learning and memory performance. These results demonstrate that modulation of learning and memory performance is an important physiological function of HGF that contributes to normal CNS plasticity, and we propose HGF as a novel regulator of higher brain functions.

Regulation of MET by FOXP2, genes implicated in higher cognitive dysfunction and autism risk

Autism spectrum disorder (ASD) is a highly heritable, behaviorally defined, heterogeneous disorder of unknown pathogenesis. Several genetic risk genes have been identified, including the gene encoding the receptor tyrosine kinase MET, which regulates neuronal differentiation and growth. An ASD-associated polymorphism disrupts MET gene transcription, and there are reduced levels of MET protein expression in the mature temporal cortex of subjects with ASD. To address the possible neurodevelopmental contribution of MET to ASD pathogenesis, we examined the expression and transcriptional regulation of MET by a transcription factor, FOXP2, which is implicated in regulation of cognition and language, two functions altered in ASD. MET mRNA expression in the midgestation human fetal cerebral cortex is strikingly restricted, localized to portions of the temporal and occipital lobes. Within the cortical plate of the temporal lobe, the pattern of MET expression is highly complementary to the expression pattern of FOXP2, suggesting the latter may play a role in repression of gene expression. Consistent with this, MET and FOXP2 also are reciprocally expressed by differentiating normal human neuronal progenitor cells (NHNPs) in vitro, leading us to assess whether FOXP2 transcriptionally regulates MET. Indeed, FOXP2 binds directly to the 5' regulatory region of MET, and overexpression of FOXP2 results in transcriptional repression of MET. The expression of MET in restricted human neocortical regions, and its regulation in part by FOXP2, is consistent with genetic evidence for MET contributing to ASD risk.

Cellular mechanisms underlying the regulation of dendritic development by hepatocyte growth factor

Acquisition of a mature dendritic morphology is critical for neural information processing. In particular, hepatocyte growth factor (HGF) controls dendritic arborization during brain development. However, the cellular mechanisms underlying the effects of HGF on dendritic growth remain elusive. Here, we show that HGF increases dendritic length and branching of rat cortical neurons through activation of the mitogen-activated protein kinase (MAPK) signaling pathway. Activation of MAPK by HGF leads to the rapid and transient phosphorylation of cAMP response element-binding protein (CREB), a key step necessary for the control of dendritic development by HGF. In addition to CREB phosphorylation, regulation of dendritic growth by HGF requires the interaction between CREB and CREB-regulated transcription coactivator 1 (CRTC1), as expression of a mutated form of CREB unable to bind CRTC1 completely abolished the effects of HGF on dendritic morphology. Treatment of cortical neurons with HGF in combination with brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family that regulates dendritic development via similar mechanisms, showed additive effects on MAPK activation, CREB phosphorylation and dendritic growth. Collectively, these results support the conclusion that regulation of cortical dendritic morphology by HGF is mediated by activation of the MAPK pathway, phosphorylation of CREB and interaction of CREB with CRTC1.

A new synaptic player leading to autism risk: Met receptor tyrosine kinase

The validity for assigning disorder risk to an autism spectrum disorder (ASD) candidate gene comes from convergent genetic, clinical, and developmental neurobiology data. Here, we review these lines of evidence from multiple human genetic studies, and non-human primate and mouse experiments that support the conclusion that the MET receptor tyrosine kinase (RTK) functions to influence synapse development in circuits relevant to certain core behavioral domains of ASD. There is association of both common functional alleles and rare copy number variants that impact levels of MET expression in the human cortex. The timing of Met expression is linked to axon terminal outgrowth and synaptogenesis in the developing rodent and primate forebrain, and both in vitro and in vivo studies implicate this RTK in dendritic branching, spine maturation, and excitatory connectivity in the neocortex. This impact can occur in a cell-nonautonomous fashion, emphasizing the unique role that Met plays in specific circuits relevant to ASD.

Distinct genetic risk based on association of MET in families with co-occurring autism and gastrointestinal conditions

OBJECTIVE

In addition to the core behavioral symptoms of autism spectrum disorder, many patients present with complex medical conditions including gastrointestinal dysfunction. A functional variant in the promoter of the gene encoding the MET receptor tyrosine kinase is associated with autism spectrum disorder, and MET protein expression is decreased in the temporal cortex of subjects with autism spectrum disorder. MET is a pleiotropic receptor that functions in both brain development and gastrointestinal repair. On the basis of these functions, we hypothesized that association of the autism spectrum disorder-associated MET promoter variant may be enriched in a subset of individuals with co-occurring autism spectrum disorder and gastrointestinal conditions.

PATIENTS AND METHODS

Subjects were 918 individuals from 214 Autism Genetics Resource Exchange families with a complete medical history including gastrointestinal condition report. Genotypes at the autism spectrum disorder-associated MET promoter variant rs1858830 were determined. Family-based association test and chi(2) analyses were used to determine the association of MET rs1858830 alleles with autism spectrum disorder and the presence of gastrointestinal conditions.

RESULTS

In the entire 214-family sample, the MET rs1858830 C allele was associated with both autism spectrum disorder and gastrointestinal conditions. Stratification by the presence of gastrointestinal conditions revealed that the MET C allele was associated with both autism spectrum disorder and gastrointestinal conditions in 118 families containing at least 1 child with co-occurring autism spectrum disorder and gastrointestinal conditions. In contrast, there was no association of the MET polymorphism with autism spectrum disorder in the 96 families lacking a child with co-occurring autism spectrum disorder and gastrointestinal conditions. chi(2) analyses of MET rs1858830 genotypes indicated over-representation of the C allele in individuals with co-occurring autism spectrum disorder and gastrointestinal conditions compared with non-autism spectrum disorder siblings, parents, and unrelated controls.

CONCLUSION

These results suggest that disrupted MET signaling may contribute to increased risk for autism spectrum disorder that includes familial gastrointestinal dysfunction.

Ang II and Ang IV: unraveling the mechanism of action on synaptic plasticity, memory, and epilepsy

The central angiotensin system plays a crucial role in cardiovascular regulation. More recently, angiotensin peptides have been implicated in stress, anxiety, depression, cognition, and epilepsy. Angiotensin II (Ang II) exerts its actions through AT(1) and AT(2) receptors, while most actions of its metabolite Ang IV were believed to be independent of AT(1) or AT(2) receptor activation. A specific binding site with high affinity for Ang IV was discovered and denominated "AT(4) receptor". The beneficiary effects of AT(4) ligands in animal models for cognitive impairment and epileptic seizures initiated the search for their mechanism of action. This proved to be a challenging task, and after 20 years of research, the nature of the "AT(4) receptor" remains controversial. Insulin-regulated aminopeptidase (IRAP) was first identified as the high-affinity binding site for AT(4) ligands. Recently, the hepatocyte growth factor receptor c-MET was also proposed as a receptor for AT(4) ligands. The present review focuses on the effects of Ang II and Ang IV on synaptic transmission and plasticity, learning, memory, and epileptic seizure activity. Possible interactions of Ang IV with the classical AT(1) and AT(2) receptor subtypes are evaluated, and other potential mechanisms by which AT(4) ligands may exert their effects are discussed. Identification of these mechanisms may provide a valuable target in the development in novel drugs for the treatment of cognitive disorders and epilepsy.

Divergence of the vertebrate sp1A/ryanodine receptor domain and SOCS box-containing (Spsb) gene family and its expression and regulation within the mouse brain

The Spsb family of genes encode well-conserved proteins of unknown function. Mammalian Spsb genes are likely the result of three separate duplication and divergence events during vertebrate evolution. The phylogenetic relationship along with expression and regulation of Spsb genes may offer insight into the evolution and function of this gene family in vertebrates. We have established that Spsb genes are expressed in numerous tissues, however their pattern and level of expression is tissue-dependent. Further, only Spsb1 is responsive to stress caused by ethanol exposure in the mouse brain, which suggests that Spsb genes have acquired different regulatory mechanisms. Analysis of cis-regulatory elements supports this, but also reveals some common regulatory modules involved in cell proliferation and stress response. Our results contribute to the growing body of data on the expression and function of Spsb genes, which serve as a model for studies on the origin, divergence and specialization of eukaryotic gene families.

Hepatocyte growth factor and c-Met promote dendritic maturation during hippocampal neuron differentiation via the Akt pathway

During central nervous system development, growth factors and their associated receptor protein tyrosine kinases regulate many neuronal functions such as neurite extension and dendrite maturation. Hepatocyte growth factor (HGF) and its receptor, c-Met, can promote formation of neurites and enhance elaboration of dendrites in mature neurons, but their effects on the early stages of dendrite maturation in hippocampal neurons and the signaling pathways by which they promote dendrite formation have not been studied. Exogenous HGF treatment effectively enhanced the phosphorylation and activation of c-Met in cultured hippocampal neurons at 4 days in vitro. HGF treatment increased the number of dendrites and promoted dendrite elongation in these neurons. Consistent with these results, HGF activated Akt, which phosphorylates glycogen synthase kinase-3beta (GSK-3beta) to inactivate it, and reduced phosphorylation of microtubule-associated protein 2 (MAP2), which can promote microtubule polymerization and dendrite elongation when dephosphorylated. Conversely, pharmacological inhibition of c-Met with its specific inhibitor, PHA-665752, or genetic knock-down of c-Met with short hairpin RNAs (shRNAs) suppressed HGF-induced phosphorylation of Akt and GSK-3beta, increased phosphorylation of MAP2, and reduced dendrite number and length in cultured hippocampal neurons. Moreover, suppressing c-Met with PHA-665752 or by shRNA decreased MAP2 expression. Inhibiting Akt activity with the phosphoinositide-3-kinase inhibitor LY294002 or Akt inhibitor X suppressed HGF-induced phosphorylation of GSK-3beta, increased MAP2 phosphorylation, and blocked the ability of HGF to enhance dendritic length. These observations indicate that HGF and c-Met can regulate the early stages of dendrite maturation via activation of the Akt/GSK-3beta pathway.

Hepatocyte growth factor-induced enhancement of dendritic branching is blocked by inhibitors of N-methyl-D-aspartate receptors and calcium/calmodulin-dependent kinases

Hepatocyte growth factor (HGF) and its receptor, Met, are clustered at excitatory synapses and can enhance N-methyl-D-aspartate (NMDA) receptor current and promote formation of neurites and dendrites. In this study, we examine the effects of HGF on dendritic arborization in mature cultures of dissociated hippocampal neurons. Exogenous HGF treatment caused a dose-dependent increase in total dendritic branch tip number, total dendritic branch length, and dendritic complexity in these neurons. NMDA receptor activity has been linked to changes in dendritic structure, so we tested the effects of HGF on the dendritic arbor in the presence of DL-2-amino-5-phosphonopentanoic acid (APV), an NMDA receptor inhibitor. APV blocked the HGF-induced enhancement of the dendritic arbor in a dose-dependent manner. Similarly, pretreatment of neurons with KN62, an inhibitor of calcium-dependent kinases, suppressed changes in dendritic branching induced by HGF. These results suggest that HGF initiates Ca2+-dependent processes, so we examined the effect of HGF on intracellular calcium levels and autophosphorylation of the calcium/calmodulin-dependent protein kinase II (CaMKII). HGF caused a persistent increase in fluorescence in clusters along dendrites of neurons preloaded with the Ca2+ indicator Fluo-4. HGF treatment also enhanced autophosphorylation of CaMKII. The increases in Fluo-4 fluorescence and autophosphorylation of CaMKII were blocked by pretreatment of neurons with APV. These results indicate that HGF stimulates Ca2+ influx into dendrites through the NMDA receptor and that this effect is necessary for the changes in dendritic morphology induced by HGF.

Disruption of cerebral cortex MET signaling in autism spectrum disorder

OBJECTIVE

Multiple genes contribute to autism spectrum disorder (ASD) susceptibility. One particularly promising candidate is the MET gene, which encodes a receptor tyrosine kinase that mediates hepatocyte growth factor (HGF) signaling in brain circuit formation, immune function, and gastrointestinal repair. The MET promoter variant rs1858830 allele "C" is strongly associated with ASD and results in reduced gene transcription. Here we examined expression levels of MET and members of the MET signaling pathway in postmortem cerebral cortex from ASD cases and healthy control subjects.

METHODS

Protein, total RNA, and DNA were extracted from postmortem temporal cortex gray matter samples (BA 41/42, 52, or 22) belonging to eight pairs of ASD cases and matched control subjects. MET protein expression was determined by Western blotting; messenger RNA expression of MET and other related transcripts was assayed by microarray and quantitative reverse transcriptase polymerase chain reaction.

RESULTS

MET protein levels were significantly decreased in ASD cases compared with control subjects. This was accompanied in ASD brains by increased messenger RNA expression for proteins involved in regulating MET signaling activity. Analyses of coexpression of MET and HGF demonstrated a positive correlation in control subjects that was disrupted in ASD cases.

INTERPRETATION

Altered expression of MET and related molecules suggests dysregulation of signaling that may contribute to altered circuit formation and function in ASD. The complement of genes that encode proteins involved in MET activation appears to undergo long-term compensatory changes in expression that may be a hallmark contribution to the pathophysiology of ASD.