Neurobiology: new order for thought disorders

Kallikrein 8: A key sheddase to strengthen and stabilize neural plasticity

Neural networks are modified and reorganized throughout life, even in the matured brain. Synapses in the networks form, change, or disappear dynamically in the plasticity state. The pre- and postsynaptic signaling, transmission, and structural dynamics have been studied considerably well. However, not many studies have shed light on the events in the synaptic cleft and intercellular space. Neural activity-dependent protein shedding is a phenomenon in which (1) presynaptic excitation evokes secretion or activation of sheddases, (2) sheddases are involved not only in cleavage of membrane- or matrix-bound proteins but also in mechanical modulation of cell-to-cell connectivity, and (3) freed activity domains of protein factors play a role in receptor-mediated or non-mediated biological actions. Kallikrein 8/neuropsin (KLK8) is a kallikrein family serine protease rich in the mammalian limbic brain. Accumulated evidence has suggested that KLK8 is an important modulator of neural plasticity and consequently, cognition. Insufficiency, as well as excess of KLK8 may have detrimental effects on limbic functions.

Neuroprotective effects of dexmedetomidine against hyperoxia-induced injury in the developing rat brain

Dexmedetomidine (DEX) is a highly selective agonist of α2-receptors with sedative, anxiolytic, and analgesic properties. Neuroprotective effects of dexmedetomidine have been reported in various brain injury models. In the present study, we investigated the effects of dexmedetomidine on hippocampal neurogenesis, specifically the proliferation capacity and maturation of neurons and neuronal plasticity following the induction of hyperoxia in neonatal rats. Six-day old sex-matched Wistar rats were exposed to 80% oxygen or room air for 24 h and treated with 1, 5 or 10 μg/kg of dexmedetomidine or normal saline. A single pretreatment with DEX attenuated the hyperoxia-induced injury in terms of neurogenesis and plasticity. In detail, both the proliferation capacity (PCNA+ cells) as well as the expression of neuronal markers (Nestin+, PSA-NCAM+, NeuN+ cells) and transcription factors (SOX2, Tbr1/2, Prox1) were significantly reduced under hyperoxia compared to control. Furthermore, regulators of neuronal plasticity (Nrp1, Nrg1, Syp, and Sema3a/f) were also drastically decreased. A single administration of dexmedetomidine prior to oxygen exposure resulted in a significant up-regulation of expression-profiles compared to hyperoxia. Our results suggest that dexmedetomidine may have neuroprotective effects in an acute hyperoxic model of the neonatal rat.

Neuregulin 1 Prevents Phencyclidine-Induced Behavioral Impairments and Disruptions to GABAergic Signaling in Mice

BACKGROUND

Substantial evidence from human post-mortem and genetic studies has linked the neurotrophic factor neuregulin 1 (NRG1) to the pathophysiology of schizophrenia. Genetic animal models and in vitro experiments have suggested that altered NRG1 signaling, rather than protein changes, contributes to the symptomatology of schizophrenia. However, little is known about the effect of NRG1 on schizophrenia-relevant behavior and neurotransmission (particularly GABAergic and glutamatergic) in adult animals.

METHOD

To address this question, we treated adult mice with the extracellular signaling domain of NRG1 and assessed spontaneous locomotor activity and acoustic startle response, as well as extracellular GABA, glutamate, and glycine levels in the prefrontal cortex and hippocampus via microdialysis. Furthermore, we asked whether the effect of NRG1 would differ under schizophrenia-relevant impairments in mice and therefore co-treated mice with NRG1 and phencyclidine (PCP) (3 mg/kg).

RESULTS

Acute intraventricularly- or systemically-injected NRG1 did not affect spontaneous behavior, but prevented PCP induced hyperlocomotion and deficits of prepulse inhibition. NRG1 retrodialysis (10 nM) reduced extracellular glutamate and glycine levels in the prefrontal cortex and hippocampus, and prevented PCP-induced increase in extracellular GABA levels in the hippocampus.

CONCLUSION

With these results, we provide the first compelling in vivo evidence for the involvement of NRG1 signaling in schizophrenia-relevant behavior and neurotransmission in the adult nervous system, which highlight its treatment potential. Furthermore, the ability of NRG1 treatment to alter GABA, glutamate, and glycine levels in the presence of PCP also suggests that NRG1 signaling has the potential to alter disrupted neurotransmission in patients with schizophrenia.

Neuregulin-ERBB signaling in the nervous system and neuropsychiatric diseases

Neuregulins (NRGs) comprise a large family of growth factors that stimulate ERBB receptor tyrosine kinases. NRGs and their receptors, ERBBs, have been identified as susceptibility genes for diseases such as schizophrenia (SZ) and bipolar disorder. Recent studies have revealed complex Nrg/Erbb signaling networks that regulate the assembly of neural circuitry, myelination, neurotransmission, and synaptic plasticity. Evidence indicates there is an optimal level of NRG/ERBB signaling in the brain and deviation from it impairs brain functions. NRGs/ERBBs and downstream signaling pathways may provide therapeutic targets for specific neuropsychiatric symptoms.

Dysregulated expression of neuregulin-1 by cortical pyramidal neurons disrupts synaptic plasticity

Neuregulin-1 (NRG1) gene variants are associated with increased genetic risk for schizophrenia. It is unclear whether risk haplotypes cause elevated or decreased expression of NRG1 in the brains of schizophrenia patients, given that both findings have been reported from autopsy studies. To study NRG1 functions in vivo, we generated mouse mutants with reduced and elevated NRG1 levels and analyzed the impact on cortical functions. Loss of NRG1 from cortical projection neurons resulted in increased inhibitory neurotransmission, reduced synaptic plasticity, and hypoactivity. Neuronal overexpression of cysteine-rich domain (CRD)-NRG1, the major brain isoform, caused unbalanced excitatory-inhibitory neurotransmission, reduced synaptic plasticity, abnormal spine growth, altered steady-state levels of synaptic plasticity-related proteins, and impaired sensorimotor gating. We conclude that an "optimal" level of NRG1 signaling balances excitatory and inhibitory neurotransmission in the cortex. Our data provide a potential pathomechanism for impaired synaptic plasticity and suggest that human NRG1 risk haplotypes exert a gain-of-function effect.

Prolonged administration of pyridostigmine impairs neuromuscular function with and without down-regulation of acetylcholine receptors

BACKGROUND

The acetylcholinesterase inhibitor, pyridostigmine, is prophylactically administered to mitigate the toxic effects of nerve gas poisoning. The authors tested the hypothesis that prolonged pyridostigmine administration can lead to neuromuscular dysfunction and even down-regulation of acetylcholine receptors.

METHODS

Pyridostigmine (5 or 25 mg·kg·day) or saline was continuously administered via osmotic pumps to rats, and infused for either 14 or 28 days until the day of neuromuscular assessment (at day 14 or 28), or discontinued 24 h before neuromuscular assessment. Neurotransmission and muscle function were examined by single-twitch, train-of-four stimulation and 100-Hz tetanic stimulation. Sensitivity to atracurium and acetylcholine receptor number (quantitated by I-α-bungarotoxin) provided additional measures of neuromuscular integrity.

RESULTS

Specific tetanic tensions (Newton [N]/muscle weight [g]) were significantly (P < 0.05) decreased at 14 (10.3 N/g) and 28 (11.1 N/g) days of 25 mg·kg·day pyridostigmine compared with controls (13.1-13.6 N/g). Decreased effective dose (0.81-1.05 vs. 0.16-0.45 mg/kg; P < 0.05) and decreased plasma concentration (3.02-3.27 vs. 0.45-1.37 μg/ml; P < 0.05) of atracurium for 50% paralysis (controls vs. 25 mg·kg·day pyridostigmine, respectively), irrespective of discontinuation of pyridostigmine, confirmed the pyridostigmine-induced altered neurotransmission. Pyridostigmine (25 mg·kg·day) down-regulated acetylcholine receptors at 28 days.

CONCLUSIONS

Prolonged administration of pyridostigmine (25 mg·kg·day) leads to neuromuscular impairment, which can persist even when pyridostigmine is discontinued 24 h before assessment of neuromuscular function. Pyridostigmine has the potential to down-regulate acetylcholine receptors, but induces neuromuscular dysfunction even in the absence of receptor changes.

Neuregulin-1 signalling and antipsychotic treatment: potential therapeutic targets in a schizophrenia candidate signalling pathway

Identifying the signalling pathways underlying the pathophysiology of schizophrenia is an essential step in the rational development of new antipsychotic drugs for this devastating disease. Evidence from genetic, transgenic and post-mortem studies have strongly supported neuregulin-1 (NRG1)-ErbB4 signalling as a schizophrenia susceptibility pathway. NRG1-ErbB4 signalling plays crucial roles in regulating neurodevelopment and neurotransmission, with implications for the pathophysiology of schizophrenia. Post-mortem studies have demonstrated altered NRG1-ErbB4 signalling in the brain of schizophrenia patients. Antipsychotic drugs have different effects on NRG1-ErbB4 signalling depending on treatment duration. Abnormal behaviours relevant to certain features of schizophrenia are displayed in NRG1/ErbB4 knockout mice or those with NRG1/ErbB4 over-expression, some of these abnormalities can be improved by antipsychotic treatment. NRG1-ErbB4 signalling has extensive interactions with the GABAergic, glutamatergic and dopaminergic neurotransmission systems that are involved in the pathophysiology of schizophrenia. These interactions provide a number of targets for the development of new antipsychotic drugs. Furthermore, the key interaction points between NRG1-ErbB4 signalling and other schizophrenia susceptibility genes may also potentially provide specific targets for new antipsychotic drugs. In general, identification of these targets in NRG1-ErbB4 signalling and interacting pathways will provide unique opportunities for the development of new generation antipsychotics with specific efficacy and fewer side effects.

Processing of neuregulin-1 by neuropsin regulates GABAergic neuron to control neural plasticity of the mouse hippocampus

Protease-mediated signaling is an important modulator of the nervous system. However, identifying the specific signaling substrates of such proteases is limited by the rapidity with which intermediate substrate forms are cleaved and released. Here, a screening method to detect noncleaved enzyme-bound forms was developed and used to identify a novel neuropsin/neuregulin-1 (NRG-1) proteolytic signaling system, which is specifically localized in the microdomain of synaptic cleft, in the mouse hippocampus. The extracellular protease, neuropsin, cleaved mature NRG-1 (comprising the extracellular domain of the NRG-1) at three newly identified sites to remove the heparin-binding domain of NRG-1. This released the ligand moiety from the matrix-glycosaminoglycan pool and enabled it to trigger the phosphorylation of NRG-1 receptor, p185 (ErbB4). Proteolysis of mature NRG-1 by neuropsin led to colocalization of the processed NRG-1 with ErbB4 in parvalbumin-positive hippocampal interneurons and consequent phosphorylation of tyrosine residues of proteins in the cells. Moreover, neuropsin knock-out mice exhibited impairments in Schaffer collateral early phase long-term potentiation, and application of the recombinant NRG-1 lacking heparin-binding activity reversed the effects through the activation of ErbB4 and GABA(A) receptors. Thus, ErbB4 signaling induced by neuropsin-dependent processing of NRG-1 contributes to the modulation of synaptic plasticity via regulation of GABAergic transmission. This signaling system may be involved in human cognition and mental disorders, such as schizophrenia and bipolar disorder, by its dysfunction.

Sustained high levels of neuregulin-1 in the longest-lived rodents; a key determinant of rodent longevity

Naked mole-rats (Heterocephalus glaber), the longest-lived rodents, live 7-10 times longer than similarly sized mice and exhibit normal activities for approximately 75% of their lives. Little is known about the mechanisms that allow them to delay the aging process and live so long. Neuregulin-1 (NRG-1) signaling is critical for normal brain function during both development and adulthood. We hypothesized that long-lived species will maintain higher levels of NRG-1 and that this contributes to their sustained brain function and concomitant maintenance of normal activity. We monitored the levels of NRG-1 and its receptor ErbB4 in H. glaber at different ages ranging from 1 day to 26 years and found that levels of NRG-1 and ErbB4 were sustained throughout development and adulthood. In addition, we compared seven rodent species with widely divergent (4-32 year) maximum lifespan potential (MLSP) and found that at a physiologically equivalent age, the longer-lived rodents had higher levels of NRG-1 and ErbB4. Moreover, phylogenetic independent contrast analyses revealed that this significant strong correlation between MLSP and NRG-1 levels was independent of phylogeny. These results suggest that NRG-1 is an important factor contributing to divergent species MLSP through its role in maintaining neuronal integrity.

Transgenic overexpression of the type I isoform of neuregulin 1 affects working memory and hippocampal oscillations but not long-term potentiation

Neuregulin 1 (NRG1) is a growth factor involved in neurodevelopment and plasticity. It is a schizophrenia candidate gene, and hippocampal expression of the NRG1 type I isoform is increased in the disorder. We have studied transgenic mice overexpressing NRG1 type I (NRG1^{tg-type I}) and their wild-type littermates and measured hippocampal electrophysiological and behavioral phenotypes. Young NRG1^{tg-type I} mice showed normal memory performance, but in older NRG1^{tg-type I} mice, hippocampus-dependent spatial working memory was selectively impaired. Hippocampal slice preparations from NRG1^{tg-type I} mice exhibited a reduced frequency of carbachol-induced gamma oscillations and an increased tendency to epileptiform activity. Long-term potentiation in NRG1^{tg-type I} mice was normal. The results provide evidence that NRG1 type I impacts on hippocampal function and circuitry. The effects are likely mediated via inhibitory interneurons and may be relevant to the involvement of NRG1 in schizophrenia. However, the findings, in concert with those from other genetic and pharmacological manipulations of NRG1, emphasize the complex and pleiotropic nature of the gene, even with regard to a single isoform.

Behavioural characterization of neuregulin 1 type I overexpressing transgenic mice

Neuregulin 1 (NRG1) is a pleiotropic growth factor involved in diverse aspects of brain development and function. In schizophrenia, expression of the NRG1 type I isoform is selectively increased. However, virtually nothing is known about the roles of this isoform in brain. We have studied transgenic mice overexpressing type I NRG1(NRG1type 1-tg) using a series of behavioural tests. NRG1(type 1-tg) mice have a tremor, are impaired on the accelerating rotarod, and have reduced prepulse inhibition in the context of an increased baseline startle response. There is no overall anxiety or activity phenotype, although female NRG(1type 1-tg) mice show mild increases in anxiety on some measures. The pattern of results shows both similarities and differences to those reported in hypomorphic NRG1 mice, and may be relevant for interpreting the increased NRG1 type I expression observed in schizophrenia.

Bidirectional signaling of ErbB and Eph receptors at synapses

Synapse development and remodeling are regulated by a plethora of molecules such as receptor tyrosine kinases (RTKs), a family of cell surface receptors that play critical roles in neural development. Two families of RTKs implicated in synaptic functions, ErbBs and Ephs, share similar characteristics in terms of exhibiting forward and reverse signaling. In this review, we will discuss the latest advances in the functions of ErbBs and Ephs at the synapse, including dendritic spine morphogenesis, synapse formation and maturation, and synaptic transmission and plasticity. In addition to signaling at interneuronal synapses, communication between neuron and glia is increasingly implicated in the control of synaptic functions. Studies on RTKs and their cognate ligands in glial cells enhance our understanding on the nature of 'tripartite synapse'. Implications of these signaling events in human diseases will be discussed.

Neuregulin-1, a key axonal signal that drives Schwann cell growth and differentiation

Interactions between neuronal and glial cells are crucial for establishing a functional nervous system. Many aspects of Schwann cell development and physiology are regulated by neuronal signals; possibly the most spectacular is the elaboration of the myelin sheath. An extensive line of research has revealed that one neuronal factor, termed "neuregulin", promotes Schwann cell growth and survival, migration along the extending axon, and myelination. The versatility of glial responses elicited by this factor is thus clearly astounding.

Episode-specific differential gene expression of peripheral blood mononuclear cells in rapid cycling supports novel treatment approaches

Molecular mechanisms underlying bipolar affective disorders are unknown. Difficulties arise from genetic and phenotypic heterogeneity of patients and the lack of animal models. Thus, we focused on only one patient (n = 1) with an extreme form of rapid cycling. Ribonucleic acid (RNA) from peripheral blood mononuclear cells (PBMC) was analyzed in a three-tiered approach under widely standardized conditions. Firstly, RNA was extracted from PBMC of eight blood samples, obtained on two consecutive days within one particular episode, including two different consecutive depressive and two different consecutive manic episodes, and submitted to (1) screening by microarray hybridizations, followed by (2) detailed bioinformatic analysis, and (3) confirmation of episode-specific regulation of genes by quantitative real-time polymerase chain reaction (qRT-PCR).Secondly, results were validated in additional blood samples obtained one to two years later. Among gene transcripts elevated in depressed episodes were prostaglandin D synthetase (PTGDS) and prostaglandin D2 11-ketoreductase (AKR1C3), both involved in hibernation. We hypothesized them to account for some of the rapid cycling symptoms. A subsequent treatment approach over 5 months applying the cyclooxygenase inhibitor celecoxib (2 x 200 mg daily) resulted in reduced severity rating of both depressed and manic episodes. This case suggests that rapid cycling is a systemic disease, resembling hibernation, with prostaglandins playing a mediator role.

Support for neuregulin 1 as a susceptibility gene for bipolar disorder and schizophrenia

BACKGROUND

There is support that Neuregulin 1 (NRG1) plays a role in susceptibility to schizophrenia but limited evidence for its involvement in bipolar disorder. We wished to investigate further the involvement of NRG1 in schizophrenia and bipolar disorder.

METHODS

We used hierarchical association analysis in parent-offspring trios, 634 with schizophrenia/schizoaffective disorder (SZ/SA) and 243 with bipolar 1 disorder (BP1). The primary analysis was the markers defining the "core Icelandic haplotype" (HAP(ICE)). We undertook polymorphism discovery, additional genotyping, and also explored phenotypic associations, as a secondary analysis aimed at refining the signal.

RESULTS

The initial global haplotype test yielded significant evidence for association (p = .01) with SZ/SA and BP1 (p = .004), although HAP(ICE) was not overtransmitted. The marker showing strongest evidence for association in the deCODE studies, SNP8NRG221533, was associated with SZ/SA (p(corrected) = .039) and with BP1 (p(corrected) = .039), with BP1 showing association to the opposite allele as SZ/SA. The pattern of transmission at SNP8NRG221533 was significantly different in SZ/SA than in BP1 (p = .0004). Secondary analyses of markers and phenotypes provided no additional evidence for association to SZ/SA. However, a new marker, rs7014762, was associated with an a priori defined "typical" bipolar phenotype characterized by excellent recovery between episodes and no mood incongruent features (p(corrected) = .003).

CONCLUSIONS

Our data provide significant levels of support for NRG1 as a susceptibility gene for both major forms of psychosis, and this cannot be interpreted as being due to population stratification. More tentatively, they also might indicate the presence of multiple alleles that influence the psychosis phenotype.

Presynaptic type III neuregulin1-ErbB signaling targets {alpha}7 nicotinic acetylcholine receptors to axons

Type III Neuregulin1 (Nrg1) isoforms are membrane-tethered proteins capable of participating in bidirectional juxtacrine signaling. Neuronal nicotinic acetylcholine receptors (nAChRs), which can modulate the release of a rich array of neurotransmitters, are differentially targeted to presynaptic sites. We demonstrate that Type III Nrg1 back signaling regulates the surface expression of α7 nAChRs along axons of sensory neurons. Stimulation of Type III Nrg1 back signaling induces an increase in axonal surface α7 nAChRs, which results from a redistribution of preexisting intracellular pools of α7 rather than from increased protein synthesis. We also demonstrate that Type III Nrg1 back signaling activates a phosphatidylinositol 3-kinase signaling pathway and that activation of this pathway is required for the insertion of preexisting α7 nAChRs into the axonal plasma membrane. These findings, in conjunction with prior results establishing that Type III Nrg1 back signaling controls gene transcription, demonstrate that Type III Nrg1 back signaling can regulate both short-and long-term changes in neuronal function.

Neuregulin 1 in neural development, synaptic plasticity and schizophrenia

Schizophrenia is a highly debilitating mental disorder that affects approximately 1% of the general population, yet it continues to be poorly understood. Recent studies have identified variations in several genes that are associated with this disorder in diverse populations, including those that encode neuregulin 1 (NRG1) and its receptor ErbB4. The past few years have witnessed exciting progress in our knowledge of NRG1 and ErbB4 functions and the biological basis of the increased risk for schizophrenia that is potentially conferred by polymorphisms in the two genes. An improved understanding of the mechanisms by which altered function of NRG1 and ErbB4 contributes to schizophrenia might eventually lead to the development of more effective therapeutics.