Multiple novel transcription initiation sites for NRG1

Functional genomics and schizophrenia: endophenotypes and mutant models

This article summarizes the rationale, methods, and results of gene discovery programs in schizophrenia research and describes functional methods of investigating potential candidate genes. It focuses next on the most prominent current candidate genes and describes (1) evidence for their association with schizophrenia and research into the function of each gene; (2) investigation of the clinical phenotypes and endophenotypes associated with each gene, at the levels of psychopathologic, neurocognitive, electrophysiologic, neuroimaging, and neuropathologic findings; and (3) research into the ethologic, cognitive, social, and psychopharmacologic phenotype of mutants with targeted deletion of each gene. It examines gene-gene and gene-environment interactions. Finally, it looks at future directions for research.

Identification and characterization of novel spliced variants of neuregulin 4 in prostate cancer

PURPOSE

The neuregulin (NRG) 1, 2, and 3 genes undergo extensive alternative mRNA splicing, which results in variants that show structural and functional diversity. The aims of this study were to establish whether the fourth member of this family, NRG4, is expressed in prostate cancer, if it is alternatively spliced and whether any functional differences between the variants could be observed.

EXPERIMENTAL DESIGN

The expression of NRG4 was determined using immunohistochemical staining of 40 cases of primary prostate cancer. Bioinformatic analysis and reverse transcription-PCR (RT-PCR) using NRG4 isotype-specific primers on a panel of normal and prostate cancer cell lines were used to identify alternatively spliced NRG4 variants. Expression of these variants was determined using isotype-specific antibodies. Transfection into Cos-7 cells of two of these green fluorescent protein-tagged variants allowed analysis of their subcellular location. Four of the variants were chemically synthesized and tested for their ability to activate the ErbB4 receptor.

RESULTS

NRG4 was variably expressed in the cytoplasm in the majority of prostate cancer cases, and in a subset of cases in the membrane, high levels were associated with advanced disease stage. Four novel NRG4 splice variants (NRGA2, NRG4 B1-3) were characterized, where each seemed to have a different subcellular location and were also expressed in the cytoplasm of the prostate tumors. NRG4 B3 was also present in endothelial cells. In transfected cells, the A type variant (NRG4 A1) was localized to the membrane, whereas the B type variant (NRG4 B1), which lacks the predicted transmembrane region, had an intracellular localization. Only the variants with an intact epidermal growth factor-like domain activated ErbB4 signaling.

CONCLUSION

NRG4 overexpression is associated with advanced-stage prostate cancer. The alternative splice variants may have different roles in cell signaling, some acting as classic receptor ligands and some with as-yet unknown functions.

Neuregulins at the neuromuscular synapse: Past, present, and future

At the developing vertebrate neuromuscular junction, neuregulins are growth/differentiation factors essential for terminal Schwann cell survival. Neuregulins have also been thought as the critical signals responsible for the increased transcription of acetylcholine receptor subunit genes at the neuromuscular synapse. This latter role is now highly controversial. This article reviews the evidence that has shaped the views of the neuregulins and how these views have been challenged. The most recent experiments indicate that neuregulin signaling to postsynaptic muscle fibers may modulate, rather than determine, acetylcholine receptor expression at the neuromuscular junction. Based on findings from my lab and those of others, I propose that this modulation might involve novel posttranscriptional molecular mechanisms. Finally, I also suggest that neuregulin signaling may have an important role to play in mediating the response of adult terminal Schwann cells to denervation.

Molecular cloning of a brain-specific, developmentally regulated neuregulin 1 (NRG1) isoform and identification of a functional promoter variant associated with schizophrenia

Neuregulin 1 (NRG1) is essential for the development and function of multiple organ systems, and its dysregulation has been linked to diseases such as cancer and schizophrenia. Recently, altered expression of a novel isoform (type IV) in the brain has been associated with schizophrenia-related genetic variants, especially rs6994992 (SNP8NRG243177). Here we have isolated and characterized full-length NRG1 type IV cDNAs from the adult and fetal human brain and identified novel splice variants of NRG1. Full-length type IV spans 1.8 kb and encodes a putative protein of 590 amino acids with a predicted molecular mass of approximately 66 kDa. The transcript consists of 11 exons with an Ig-like domain, an epidermal growth factor-like (EGF) domain, a beta-stalk, a transmembrane domain, and a cytoplasmic "a-tail," placing it in the beta1a NRG1 subclass. NRG1 type IV was not detected in any tissues except brain and a putative type IV NRG1 protein of 66 kDa was similarly brain-specific. Type IV transcripts are more abundantly expressed in the fetal brain, where, in addition to the full-length structure, two novel type IV variants were identified. In vitro luciferase-reporter assays demonstrate that the 5' promoter region upstream of type IV is functional, with differential activity associated with genetic variation at rs6994992, and that promoter competition may impact on type IV expression. Our data suggest that type IV is a unique brain-specific NRG1 that is differentially expressed and processed during early development, is translated, and its expression regulated by a schizophrenia risk-associated functional promoter or single nucleotide polymorphism (SNP).

Phenotypic characterization of spatial cognition and social behavior in mice with 'knockout' of the schizophrenia risk gene neuregulin 1

Neuregulin-1 (NRG1) has been identified as a candidate susceptibility gene for schizophrenia. In the present study the functional role of the NRG1 gene, as it relates to cognitive and social processes known to be disrupted in schizophrenia, was assessed in mice with heterozygous deletion of transmembrane (TM)-domain NRG1 in comparison with wildtypes (WT). Social affiliative behavior was assessed using the sociability and preference for social novelty paradigm, in terms of time spent in: (i) a chamber containing an unfamiliar conspecific vs. an empty chamber (sociability), or (ii) a chamber containing an unfamiliar conspecific vs. a chamber containing a familiar conspecific (preference for social novelty). Social dominance and aggressive behavior were examined in the resident-intruder paradigm. Spatial learning and memory were assessed using the Barnes maze paradigm, while spatial working memory was measured using the continuous variant of the spontaneous alternation task. Barnes maze data revealed intact spatial learning in NRG1 mutants, with elevated baseline latency to enter the escape hole in male NRG1 mutants reflecting an increase in activity level. Similarly, although a greater number of overall arm entries were found, spontaneous alternation was unaffected in NRG1 mice. Social affiliation data revealed NRG1 mutants to evidence a specific loss of WT preference for spending time with an unfamiliar as opposed to a familiar conspecific. This suggests that NRG1 mutants show a selective impairment in response to social novelty. While spatial learning and working memory processes appear intact, heterozygous deletion of TM-domain NRG1 was associated with disruption to social novelty behavior. These data inform at a novel phenotypic level on the functional role of this gene in the context of its association with risk for schizophrenia.

Identification of Neuregulin as a Factor Required for Formation of Aligned Spermatogonia

In the absence of somatic cells, medium conditioned by the SNL fibroblast line (SNL-CM) is able to stimulate primary cultures of rat type-A single spermatogonia to develop into chains of aligned spermatogonia at the 8-, 16-, and 32-cell stages. By comparison, medium conditioned by an MSC-1 Sertoli cell line is ineffective. Glial cell line-derived neurotrophic factor (GDNF)-like molecules were identified in SNL-CM and recombinant forms of GDNF, neurturin, and artemin were shown to stimulate formation of aligned spermatogonia, but principally to only the 4- and 8-cell stages. Because SNL-CM and GDNF-like molecules stimulated the formation of spermatogonial chain length differently, we purified components of SNL-CM to identify the additional contributing factor(s). A fraction was isolated that was dependent on GDNF, but required for effective formation of 16- and 32-cell chain lengths. Sequence analysis identified the factor as mouse neuregulin-1. At picomolar concentrations, recombinant neuregulin-1 in combination with GDNF effectively stimulated formation of aligned spermatogonia up to the 32-cell stage. Neuregulin in the absence of GDNF was relatively ineffective. Soluble receptors for neuregulins blocked the effects of GDNF and SNL-CM, suggesting that both neuregulin and GDNF are required for effective formation of long spermatogonial chains. Addition of neuregulin-1 to cultures on MSC-1 feeder layers resulted in spermatogonial behavior similar to that seen on feeder layers of SNL fibroblasts. In fact, SNL cells were found to express 100-fold higher levels of neuregulin-1 transcripts than MSC-1 cells. Thus, we identify neuregulin as a factor required for spermatogonial amplification and differentiation in culture.

Association of Neuregulin 1 with schizophrenia and bipolar disorder in a second cohort from the Scottish population

Neuregulin 1 (NRG1) is a strong candidate for involvement in the aetiology of schizophrenia. A haplotype, initially identified as showing association in the Icelandic and Scottish populations, has shown a consistent effect size in multiple European populations. Additionally, NRG1 has been implicated in susceptibility to bipolar disorder. In this first study to select markers systematically on the basis of linkage disequilibrium across the entire NRG1 gene, we used haplotype-tagging single-nucleotide polymorphisms to identify single markers and haplotypes associated with schizophrenia and bipolar disorder in an independently ascertained Scottish population. Haplotypes in two regions met an experiment-wide significance threshold of P=0.0016 (Nyholt's SpD) and were permuted to correct for multiple testing. Region A overlaps with the Icelandic haplotype and shows nominal association with schizophrenia (P=0.00032), bipolar disorder (P=0.0011), and the combined case group (P=0.0017). This region includes the 5' exon of the NRG1 GGF2 isoform and overlaps the expressed sequence tag (EST) cluster Hs.97362. However, no haplotype in Region A remains significant after permutation analysis (P>0.05). Region B contains a haplotype associated with both schizophrenia (P=0.00014), and the combined case group (P=0.000062), although it does not meet Nyholt's threshold in bipolar disorder alone (P=0.0022). This haplotype remained significant after permutation analysis in both the schizophrenia and combined case groups (P=0.024 and P=0.016, respectively). It spans a approximately 136 kb region that includes the coding sequence of the sensory and motor neuron derived factor (SMDF) isoform and 3' exons of all other known NRG1 isoforms. Our study identifies a new of NRG1 region involved in schizophrenia and bipolar disorder in the Scottish population.

Axonal regulation of myelination by neuregulin 1

Neuregulins comprise a family of epidermal growth factor-like ligands that interact with ErbB receptor tyrosine kinases to control many aspects of neural development. One of the most dramatic effects of neuregulin-1 is on glial cell differentiation. The membrane-bound neuregulin-1 type III isoform is an axonal ligand for glial ErbB receptors that regulates the early Schwann cell lineage, including the generation of precursors. Recent studies have shown that the amount of neuregulin-1 type III expressed on axons also dictates the glial phenotype, with a threshold level triggering Schwann cell myelination. Remarkably, neuregulin-1 type III also regulates Schwann cell membrane growth to adjust myelin sheath thickness to match axon caliber precisely. Whether this signaling system operates in central nervous system myelination remains an open question of major importance for human demyelinating diseases.

Neuregulins: Versatile growth and differentiation factors in nervous system development and human disease

The neuregulins are a family of growth and differentiation factors with a wide range of functions in the nervous system. The power and diversity of the neuregulin signaling system comes in part from a large number of alternatively-spliced forms of the NRG1 gene that can produce both soluble and membrane-bound forms. The soluble forms of neuregulin are unique from other factors in that they have a structurally distinct heparin-binding domain that targets and potentiates its actions. In addition, a finely tuned, bidirectional mechanism regulates when and where neuregulin is released from neurons in response to neurotrophic factors produced by both neuronal targets and supporting glial cells. Together, this produces a balanced intercellular signaling system that can be localized to distinct regions for both normal development and maintenance of the mature nervous system. Recent evidence suggests that neuregulin signaling plays important roles in many neurological disorders including multiple sclerosis, traumatic brain and spinal cord injury, peripheral neuropathy, and schizophrenia. Here, we review the basic biology of neuregulins and relate this to research suggesting their involvement with and potential therapeutic uses for neurological disorders.

Neuregulin 1 and schizophrenia: genetics, gene expression, and neurobiology

Neuregulin 1 (NRG1) is a leading schizophrenia susceptibility gene. The NRG1 locus on chromosome 8p shows linkage to the disorder, and genetic association has been found between schizophrenia and various non-coding polymorphisms and haplotypes, especially at the 5' end of the NRG1 gene, in many but not all case-control and family studies. NRG1 is a pleiotropic growth factor, important in nervous system development and functioning; roles include the modulation of neuronal migration, synaptogenesis, gliogenesis, neuron-glia communication, myelination, and neurotransmission. Understanding the neurobiology of NRG1 and its involvement in schizophrenia is challenged by the complexity of the gene, which gives rise to multiple functionally distinct isoforms, including six "types" of NRG1 defined by 5' exon usage. Type IV and type I NRG1 may be particularly relevant to schizophrenia, with initial data showing altered expression of these isoforms in the disorder or in association with NRG1 risk alleles. We review the structure and functions of NRG1, consider the evidence for and against it being a schizophrenia susceptibility gene, and discuss mechanisms that might underlie the contribution of NRG1 to disease pathophysiology.

Meta-analysis shows strong positive association of the neuregulin 1 (NRG1) gene with schizophrenia

Chromosome 8p22-p11 has been identified as a locus for schizophrenia in several genome-wide scans and confirmed by meta-analysis of published linkage data. Systematic fine mapping using extended Icelandic pedigrees identified an associated haplotype in the gene neuregulin 1 (NRG1), also known as heuregulin, glial growth factor, NDF43 and ARIA. A 290 kb core at risk haplotype at the 5' end of the gene (HAP(ICE)), defined by five SNPs and two microsatellite polymorphisms was found to be associated with schizophrenia in the Icelandic and Scottish populations. A number of subsequent independent studies have attempted to replicate the association, and while some have been successful, the associated haplotype is not always HAP(ICE). Furthermore, no obviously functional or pathogenic variants have been identified, and the relationship between the gene and schizophrenia has remained inconclusive. To reconcile these conflicting findings and to give a comprehensive picture of the genetic architecture of this important gene, we performed a meta-analysis of 13 published population-based and family-based association studies up to November 2005. We analysed data from the SNP markers SNP8NRG241930, SNP8NRG243177, SNP8NRG221132 and SNP8NRG221533, and the microsatellite markers 478B14-848, 420M9-1395. Across these studies, strong positive association was found for all six polymorphisms. The haplotype analysis also showed significant association in the pooled international populations (OR=1.22, 95% CI 1.15-1.3, P=8 x 10(-10)). In Asian populations, the risk haplotype was focused around the two microsatellite markers, 478B14-848, 420M9-1395 (haplotype block B), and in Caucasian populations with the remaining four SNP markers (haplotype block A). This meta-analysis supports the involvement of NRG1 in the pathogenesis of schizophrenia, but with association between two different but adjacent haplotypes blocks in the Caucasian and Asian populations.

Neuregulin 1 transcripts are differentially expressed in schizophrenia and regulated by 5' SNPs associated with the disease

Genetic variation in neuregulin 1 (NRG1) is associated with schizophrenia. The disease-associated SNPs are noncoding, and their functional implications remain unknown. We hypothesized that differential expression of the NRG1 gene explains its association to the disease. We examined four of the disease-associated SNPs that make up the original risk haplotype in the 5' upstream region of the gene for their effects on mRNA abundance of NRG1 types I-IV in human postmortem hippocampus. Diagnostic comparisons revealed a 34% increase in type I mRNA in schizophrenia and an interaction of diagnosis and genotype (SNP8NRG221132) on this transcript. Of potentially greater interest, a single SNP within the risk haplotype (SNP8NRG243177) and a 22-kb block of this core haplotype are associated with mRNA expression for the novel type IV isoform in patients and controls. Bioinformatic promoter analyses indicate that both SNPs lead to a gain/loss of putative binding sites for three transcription factors, serum response factor, myelin transcription factor-1, and High Mobility Group Box Protein-1. These data implicate variation in isoform expression as a molecular mechanism for the genetic association of NRG1 with schizophrenia.

Characterization of a neural-specific splicing form of the human neuregulin 3 gene involved in oligodendrocyte survival

Neuregulins are a family of genes involved in key aspects of neural biology. Neuregulins 1, 2 and 3 (NRG1, NRG2 and NRG3) are expressed in the mammalian nervous system. It is well established that NRG1, with fifteen different splicing forms, is central for brain development and function. However, the biological relevance of NRG2 and NRG3 remains elusive. Here, we report the identification of a new isoform of NRG3 that is specifically expressed in the human embryonic central nervous system. Sequence alignment with the human genome suggests that this transcript is produced by alternative promoter usage. The encoded polypeptide is a type-I-glycosylated plasma membrane protein, which is shed into the extracellular space where it activates erbB4, a pivotal receptor for brain development. In addition, we show that the protein has a signal sequence that is cleaved after membrane insertion. Proteasome inhibition with Lactacystin enhances the expression of the protein, whereas impairment of ubiquitylation in the conditional mutant cell line ts20 protects the protein from degradation. These observations imply that the ubiquitin/proteasome pathway regulates biogenesis of the protein. We also show that recombinant neuregulin 3 acts as an oligodendrocyte survival factor by activating the phosphoinositide 3-kinase signalling pathway. Therefore, we report a new post-translationally regulated isoform of neuregulin 3 expressed in the developing human central nervous system with a role in oligodendrocyte survival.

Analysis of polymorphisms in AT-rich domains of neuregulin 1 gene in schizophrenia

Linkage analysis and association studies have pointed to neuregulin 1 (NRG1) as the prime candidate for 8p-linked schizophrenia (SZ). However, so far, no specific functional alleles in the gene's exons, intron-exon junctions and promoters have been identified that are unequivocally associated with SZ. In this study, we analyzed several NRG1 polymorphisms that affect ATTT motifs and AT-rich regions of the gene. We have previously identified a number of such polymorphisms in the promoters of other SZ and bipolar disorder (BD) candidate genes and found positive associations to several of them. In addition, allele specific differences in the binding of brain proteins have been found for many of the polymorphisms. A case control design was used to compare allele frequencies in Caucasian and African American patients with SZ and controls. In the African American group, a significant difference was found in the allele and genotype distribution for several of the markers and haplotype blocks located in the 5'- and 3'-ends of the gene. The most significant result was obtained for rs6150532, an insertion/deletion variant in a conserved region of an intron that separates two small, alternatively spliced exons. Allele-specific and developmental differences were detected in the binding of a brain protein using newborn rat pups when probes containing the two rs6150532 alleles were used in electromobility gel shift assays. There were no significant differences in allele or genotype distribution found for any of the markers in the Caucasian sample. Although the samples size is relatively small, the findings support a role for NRG1 in SZ in African Americans and suggest that polymorphic differences in regions of the gene that recognize AT-binding proteins may be a factor in disease pathogenesis.

Microarray analysis of gene expression in adult retinal ganglion cells

Retinal ganglion cells (RGCs) transfer visual information to the brain and are known to be susceptible to selective degeneration in various neuropathies such as glaucoma. This selective vulnerability suggests that these highly specialized neurons possess a distinct gene expression profile that becomes altered by neuropathy-associated stresses, which lead to the RGC death. In this study, to identify genes expressed predominantly in adult RGCs, a global transcriptional profile of purified primary RGCs has been compared to that of the whole retina. To avoid alterations of the original gene expression profile by cell culture conditions, we isolated RNA directly from adult RGCs purified by immunopanning without prior sub-cultivation. Genes expressed predominantly in RGCs included: Nrg1, Rgn, 14-3-3 family (Ywhah, Ywhaz, Ywhab), Nrn1, Gap43, Vsnl1, Rgs4. Some of these genes may serve as novel markers for these neurons. Our analysis revealed enrichment in genes controlling the pro-survival pathways in RGCs as compared to other retinal cells.

Neuregulin-1 polymorphism in late onset Alzheimer's disease families with psychoses

Probands with late onset Alzheimer's disease (LOAD) exhibit positive symptoms of psychosis, 30-60% of the time. Positive symptoms of psychosis have been shown to appear prior to the onset of dementia to be accompanied by greater cognitive deficits, and to predict a more rapid decline. A study of the distribution of AD with psychosis (ADP) in families from the NIMH Alzheimer's Disease Genetic Initiative sample indicates that the trait is heritable, and linkage studies of multiplex ADP families have found suggestive peaks on 2p, 6q, 8p, and 21q. A genome scan of idiopathic psychosis, schizophrenia, in the Icelandic population identified a risk haplotype within the 5' region of neuregulin-1 (NRG1) on 8p12. Associations with NRG1 SNPs have also been found in other schizophrenia populations from Scotland, Ireland, and China. Here, we report results demonstrating a significant linkage peak for ADP on 8p12 in the NIMH AD dataset, encompassing the NRG1 region. We also demonstrate that there is a significant association with a NRG1 SNP (single nucleotide polymorphism), rs392499, with ADP, chi2 = 7.0, P = 0.008. This same SNP is part of a 3-SNP haplotype preferentially transmitted to individuals with this phenotype. Our results suggest that NRG1 plays a role in increasing the genetic risk to positive symptoms of psychosis in a proportion of LOAD families.

Phylogenetic origin of LI-cadherin revealed by protein and gene structure analysis

The intestine specific LI-cadherin differs in its overall structure from classical and desmosomal cadherins by the presence of seven instead of five cadherin repeats and a short cytoplasmic domain. Despite the low sequence similarity, a comparative protein structure analysis revealed that LI-cadherin may have originated from a five-repeat predecessor cadherin by a duplication of the first two aminoterminal repeats. To test this hypothesis, we cloned the murine LI-cadherin gene and compared its structure to that of other cadherins. The intron-exon organization, including the intron positions and phases, is perfectly conserved between repeats 3-7 of LI-cadherin and 1-5 of classical cadherins. Moreover, the genomic structure of the repeats 1-2 and 3-4 is identical for LI-cadherin and highly similar to that of the repeats 1-2 of classical cadherins. These findings strengthen our assumption that LI-cadherin originated from an ancestral cadherin with five domains by a partial gene duplication event.