Positively correlated miRNA-mRNA regulatory networks in mouse frontal cortex during early stages of alcohol dependence.

BACKGROUND: Although the study of gene regulation via the action of specific microRNAs (miRNAs) has experienced a boom in recent years, the analysis of genome-wide interaction networks among miRNAs and respective targeted mRNAs has lagged behind. MicroRNAs simultaneously target many transcripts and fine-tune the expression of genes through cooperative/combinatorial targeting. Therefore, they have a large regulatory potential that could widely impact development and progression of diseases, as well as contribute unpredicted collateral effects due to their natural, pathophysiological, or treatment-induced modulation. We support the viewpoint that whole mirnome-transcriptome interaction analysis is required to better understand the mechanisms and potential consequences of miRNA regulation and/or deregulation in relevant biological models. In this study, we tested the hypotheses that ethanol consumption induces changes in miRNA-mRNA interaction networks in the mouse frontal cortex and that some of the changes observed in the mouse are equivalent to changes in similar brain regions from human alcoholics. RESULTS: miRNA-mRNA interaction networks responding to ethanol insult were identified by differential expression analysis and weighted gene coexpression network analysis (WGCNA). Important pathways (coexpressed modular networks detected by WGCNA) and hub genes central to the neuronal response to ethanol are highlighted, as well as key miRNAs that regulate these processes and therefore represent potential therapeutic targets for treating alcohol addiction. Importantly, we discovered a conserved signature of changing miRNAs between ethanol-treated mice and human alcoholics, which provides a valuable tool for future biomarker/diagnostic studies in humans. We report positively correlated miRNA-mRNA expression networks that suggest an adaptive, targeted miRNA response due to binge ethanol drinking. CONCLUSIONS: This study provides new evidence for the role of miRNA regulation in brain homeostasis and sheds new light on current understanding of the development of alcohol dependence. To our knowledge this is the first report that activated expression of miRNAs correlates with activated expression of mRNAs rather than with mRNA downregulation in an in vivo model. We speculate that early activation of miRNAs designed to limit the effects of alcohol-induced genes may be an essential adaptive response during disease progression.

Evidence for muscle-dependent neuromuscular synaptic site determination in mammals.

Recent evidence challenges the prevalent view that neural factors induce the formation of a de novo postsynaptic apparatus during development of the vertebrate neuromuscular junction. The latest experiments suggest an alternative model in which the muscle fiber induces a nascent postsynaptic apparatus and sets the location of the future synapse. On axonal contact, these sites, laid out in a prepattern in the central area of developing muscle fibers, mature into synapses by the combined action of neural factors such as agrin and ACh. We sought to test in mammals these two models of neuromuscular synaptogenesis. Previously, we showed that continuous prenatal muscle expression of constitutively active ErbB2 (CAErbB2) led to synaptic loss, exuberant axonal sprouting, and lethality at birth. Here, we transiently induced CAErbB2 during midgestation and examined synapse restoration after inducer withdrawal. Centrally enriched ACh receptor (AChR) transcription and clustering were abolished after transient CAErbB2 induction. After inducer withdrawal, synapses were restored but were distributed widely over the entire diaphragm muscle. Under the nerve-dependent model, this distribution is explained by the wide pattern of axonal sprouting triggered by CAErbB2. Yet, in the absence of the nerve, introduced in our animals by mating to Hb9(+/-) mice, a very similar, wide distribution of aneural AChR clusters resulted. Thus, transient expression of CAErbB2 in skeletal muscles leads to reprogramming of the endogenous muscle AChR prepattern. This, and not the nerve, seems primarily responsible for the widely distributed pattern of synapses in our experimental animals.

Schwann cell-derived neuregulin-2alpha can function as a cell-attached activator of muscle acetylcholine receptor expression.

Here we show that neuregulin-2 (Nrg-2) alpha- and beta-isoforms can activate acetylcholine receptor (AChR) transcription as surface-attached ligands. More importantly, we demonstrate that Schwann cells that express Nrg-2alpha on their cell surface, the same Nrg-2 isoform expressed by terminal Schwann cells at the neuromuscular junction, can induce AChR expression if brought into cell-to-cell contact with myotubes specifically expressing ErbB4. These Schwann cells, the D6P2T cell line, induce AChR expression apparently as well as 293T cells transfected with Nrg-2beta, the isoform with the highest AChR-inducing activity when presented in a soluble form. These results provide a potential role for the previously reported, paradoxical perisynaptic accumulation of Nrg-2alpha, the isoform with the least AChR-inducing activity when presented in a soluble form. They also raise the possibility that Schwann cell-derived Nrg-2 could activate ErbB receptors on the synaptic sarcolemma and that this could account, at least in part, for the Nrg-mediated regulation of AChR expression.

High frequency induction of mitotic recombination by ionizing radiation in Mlh1 null mouse cells.

Mitotic recombination in somatic cells involves crossover events between homologous autosomal chromosomes. This process can convert a cell with a heterozygous deficiency to one with a homozygous deficiency if a mutant allele is present on one of the two homologous autosomes. Thus mitotic recombination often represents the second mutational step in tumor suppressor gene inactivation. In this study we examined the frequency and spectrum of ionizing radiation (IR)-induced autosomal mutations affecting Aprt expression in a mouse kidney cell line null for the Mlh1 mismatch repair (MMR) gene. The mutant frequency results demonstrated high frequency induction of mutations by IR exposure and the spectral analysis revealed that most of this response was due to the induction of mitotic recombinational events. High frequency induction of mitotic recombination was not observed in a DNA repair-proficient cell line or in a cell line with an MMR-independent mutator phenotype. These results demonstrate that IR exposure can initiate a process leading to mitotic recombinational events and that MMR function suppresses these events from occurring.

Defective neuromuscular synaptogenesis in mice expressing constitutively active ErbB2 in skeletal muscle fibers.

We overexpressed a constitutively active form of the neuregulin receptor ErbB2 (CAErbB2) in skeletal muscle fibers in vivo and in vitro by tetracycline-inducible expression. Surprisingly, CAErbB2 expression during embryonic development was lethal and impaired synaptogenesis yielding a phenotype with loss of synaptic contacts, extensive axonal sprouting, and diffuse distribution of acetylcholine receptor (AChR) transcripts, reminiscent of agrin-deficient mice. CAErbB2 expression in cultured myotubes inhibited the formation and maintenance of agrin-induced AChR clusters, suggesting a muscle- and not a nerve-origin for the defect in CAErbB2-expressing mice. Levels of tyrosine phosphorylated MuSK, the signaling component of the agrin receptor, were similar, while tyrosine phosphorylation of AChRbeta subunits was dramatically reduced in CAErbB2-expressing embryos relative to controls. Thus, a gain-of-function manipulation of ErbB2 signaling pathways renders an agrin-deficient-like phenotype that uncouples MuSK and AChR tyrosine phosphorylation.

Persistence of chromatid aberrations in the cells of solid mouse tissues exposed to 137Cs gamma radiation.

Primary mouse ear and kidney cultures were established for determination of cytogenetic aberrations at short (3 days to 1 month) and long (12-23 months) times after exposure of their right sides to 7.5 Gy of (137)Cs gamma radiation. In every case, higher levels of aberrations were observed in primary cultures established from the irradiated tissues than in those established from the contralateral tissues. The most common aberrations in the contralateral tissues and those from nonirradiated mice were chromatid and isochromatid breaks and small chromatid fragments. Primary cells from irradiated tissues removed from animals within a month of exposure displayed a variety of unstable chromosome-type aberrations characteristic of recent exposure to ionizing radiation including rings, dicentrics, double minutes, and large acentric fragments. The percentages of cells exhibiting chromatid breaks and small chromatid fragments were also markedly elevated. Although the levels of chromosome-type aberrations found in primary cells from irradiated tissues dropped to near background levels a year or more after exposure, chromatid-type aberrations remained elevated. These results are consistent with long-term persistence of damage in the genomes of ionizing radiation-exposed cells in solid tissues and the induction of genomic instability in vivo.

A mouse kidney cell line with a G:C --> C:G transversion mutator phenotype.

We report the identification of a mouse kidney epithelial cell line (K435) in which G:C-->C:G transversion mutations occur at an elevated rate and are the predominant spontaneous events observed at the selectable Aprt locus. Of three genotoxins tested, ultraviolet radiation (UV), ionizing radiation, and hydrogen peroxide, only UV exposure was able to alter the spectrum of small mutational events. To determine if the G:C-->C:G mutator phenotype was due to a deficiency in the mismatch repair pathway, the K435 cells were tested for resistance to 6-thioguanine, cisplatin, and MNNG. Although the K435 cells were as resistant to 6-thioguanine and cisplatin as Pms2 and Mlh1 null kidney cells, they were hypersensitive to MNNG. Moreover, the K435 cells do not exhibit microsatellite instability, a hallmark of mismatch repair deficiency. These results suggest that a novel mechanism, which does not include a classical deficiency in mismatch repair, accounts for the G:C-->C:G mutator phenotype.

Tissue-specific deletion and discontinuous loss of heterozygosity are signatures for the mutagenic effects of ionizing radiation in solid tissues.

The mouse Aprt locus on chromosome 8 was used as the selectable target for the study of spontaneous and ionizing radiation-induced mutations in kidney epithelia and ear fibroblasts. Fifty-two Aprt heterozygous mice were exposed to 7.5 Gy of (137)Cs-gamma radiation on their right sides, and Aprt-deficient clones were isolated from enzymatically digested tissues at times ranging from 1 day to 14 months after irradiation. A statistically significant increase in the mutant frequencies for the irradiated tissues was observed when compared with the spontaneous mutant frequencies for the nonirradiated tissues. A molecular analysis of spontaneous mutations observed for the nonirradiated tissues revealed tissue-specific differences; apparent chromosome loss was common in kidney mutants but infrequent in the ear mutants, whereas apparent deletions were common in the ear mutants but not detected in the kidney mutants. For the irradiated kidneys, apparent deletions were observed commonly demonstrating that these events are markers for ionizing radiation mutagenesis in this tissue. All of the loss of heterozygosity (LOH) tracts observed in the spontaneous mutants were continuous, but discontinuous LOH patterns were observed in 6--8% of ionizing radiation-induced ear and kidney cell mutants. Work with kidney-derived cell lines showed that discontinuous LOH is a novel signature for delayed ionizing radiation mutagenesis. Considered together, these results suggest that ionizing radiation-induced mutations in vivo can result from both direct and delayed mutagenic effects.