Neuronatin mRNA in PC12 cells: downregulation by nerve growth factor

Differences in the transcriptome of medullary thyroid cancer regarding the status and type of RET gene mutations

Medullary thyroid cancer (MTC) can be caused by germline mutations of the RET proto-oncogene or occurs as a sporadic form. It is well known that RET mutations affecting the cysteine-rich region of the protein (MEN2A-like mutations) are correlated with different phenotypes than those in the kinase domain (MEN2B-like mutations). Our aim was to analyse the whole-gene expression profile of MTC with regard to the type of RET gene mutation and the cancer genetic background (hereditary vs sporadic). We studied 86 MTC samples. We demonstrated that there were no distinct differences in the gene expression profiles of hereditary and sporadic MTCs. This suggests a homogeneous nature of MTC. We also noticed that the site of the RET gene mutation slightly influenced the gene expression profile of MTC. We found a significant association between the localization of RET mutations and the expression of three genes: NNAT (suggested to be a tumour suppressor gene), CDC14B (involved in cell cycle control) and NTRK3 (tyrosine receptor kinase that undergoes rearrangement in papillary thyroid cancer). This study suggests that these genes are significantly deregulated in tumours with MEN2A-like and MEN2B-like mutations; however, further investigations are necessary to demonstrate any clinical impact of these findings.

Temporal and spatial expression pattern of Nnat during mouse eye development

BACKGROUND

Neuronatin (Nnat) was initially identified as a highly expressed gene in neonatal mammalian brain. In this study, we analyze the spatial and temporal expression pattern of Nnat during mouse eye development as well as in the adult.

METHODS

The expression of Nnat was analyzed on mRNA as well as protein level. The presence of Nnat transcripts in the adult retina was examined using reverse transcription-polymerase chain reaction (RT-PCR). Nnat protein expression was evaluated by Western blot and immunohistochemistry during eye development at embryonic day (E) 12, 15, 16 and postnatal day (P) 7, 14, 30 and 175 (adult).

RESULTS

Immunohistochemical studies of the developing mouse eye revealed Nnat expression in embryonic and adult neuroretina as well as in corneal epithelial, stromal, endothelial cells and in lens epithelium. Expression of Nnat was detected from E12 onwards and was also present in adult eyes.

CONCLUSIONS

The expression pattern suggests that Nnat may play an important role during eye development and in the maintenance of mature eye.

Integrated transcriptome analysis of human iPS cells derived from a fragile X syndrome patient during neuronal differentiation

Fragile X syndrome (FXS) patients carry the expansion of over 200 CGG repeats at the promoter of fragile X mental retardation 1 (FMR1), leading to decreased or absent expression of its encoded fragile X mental retardation protein (FMRP). However, the global transcriptional alteration by FMRP deficiency has not been well characterized at single nucleotide resolution, i.e., RNA-seq. Here, we performed in-vitro neuronal differentiation of human induced pluripotent stem (iPS) cells that were derived from fibroblasts of a FXS patient (FXS-iPSC). We then performed RNA-seq and examined the transcriptional misregulation at each intermediate stage during in-vitro differentiation of FXS-iPSC into neurons. After thoroughly analyzing the transcriptomic data and integrating them with those from other platforms, we found up-regulation of many genes encoding TFs for neuronal differentiation (WNT1, BMP4, POU3F4, TFAP2C, and PAX3), down-regulation of potassium channels (KCNA1, KCNC3, KCNG2, KCNIP4, KCNJ3, KCNK9, and KCNT1) and altered temporal regulation of SHANK1 and NNAT in FXS-iPSC derived neurons, indicating impaired neuronal differentiation and function in FXS patients. In conclusion, we demonstrated that the FMRP deficiency in FXS patients has significant impact on the gene expression patterns during development, which will help to discover potential targeting candidates for the cure of FXS symptoms.

Metformin induces ER stress-dependent apoptosis through miR-708-5p/NNAT pathway in prostate cancer

Although the antitumor role of metformin has been widely reported, the molecular mechanism of this biguanide agent in the inhibition of tumor progression remains unclear. Here, we identified miR-708-5p as a novel target of metformin in prostate cancer cells. Metformin promotes increased expression of miR-708-5p, leading to suppression of endoplasmic reticulum (ER) membrane protein neuronatin (NNAT) expression and subsequently induces apoptosis of prostate cancer cells through the ER stress pathway. Further, miR-708-5p-induced knockdown of NNAT is associated with downregulated intracellular calcium levels and induced malformation of ER-ribosome structure revealed by electronic microscopy. Meanwhile, the unfolded protein response regulator CHOP, p-eIF2α, calreticulin, GRP78 and ATP2A1, all of which are also considered as ER stress markers, are upregulated by metformin and miR-708-5p. Taken together, our findings clearly demonstrate that metformin stimulates increased expression of miR-708-5p to target the NNAT-mediated response to ER stress and apoptosis. This novel regulatory mechanism of metformin in prostate cancer cells not only advances our knowledge on the molecular mechanism of metformin but also provides a promising therapeutic strategy by targeting miR-708-5p and NNAT for prostate cancer treatment.

Neuronatin gene: Imprinted and misfolded: Studies in Lafora disease, diabetes and cancer may implicate NNAT-aggregates as a common downstream participant in neuronal loss

Neuronatin (NNAT) is a ubiquitous and highly conserved mammalian gene involved in brain development. Its mRNA isoforms, chromosomal location, genomic DNA structure and regulation have been characterized. More recently there has been rapid progress in the understanding of its function in physiology and human disease. In particular there is fairly direct evidence implicating neuronatin in the causation of Lafora disease and diabetes. Neuronatin protein has a strong predisposition to misfold and form cellular aggregates that cause cell death by apoptosis. Aggregation of Neuronatin within cortical neurons and resulting cell death is the hallmark of Lafora disease, a progressive and fatal neurodegenerative disease. Under high glucose conditions simulating diabetes, neuronatin protein also accumulates and destroys pancreatic beta cells. The neuronatin gene is imprinted and only the paternal allele is normally expressed in the adult. However, changes in DNA methylation may cause the maternal allele to lose imprinting and trigger cell proliferation and metastasis. Neuronatin has also been shown to be translated peripherally within the dendrites of neurons, a finding of relevance in synaptic plasticity. The current understanding of the function of neuronatin raises the possibility that this gene may participate in the common downstream mechanisms associated with aberrant neuronal growth and death. A better understanding of these mechanisms may open new therapeutic targets to help modify the progression of devastating neurodegenerative conditions such as Alzheimer's and anterior horn cell disease.

Loss of neuronatin promotes "browning" of primary mouse adipocytes while reducing Glut1-mediated glucose disposal

Failure of white adipose tissue to appropriately store excess metabolic substrate seems to underpin obesity-associated type 2 diabetes. Encouraging "browning" of white adipose has been suggested as a therapeutic strategy to help dispose of excess stored lipid and ameliorate the resulting insulin resistance. Genetic variation at the DNA locus encoding the novel proteolipid neuronatin has been associated with obesity, and we recently observed that neuronatin expression is reduced in subcutaneous adipose tissue from obese humans. Thus, to explore the function of neuronatin further, we used RNAi to silence its expression in murine primary adipocyte cultures and examined the effects on adipocyte phenotype. We found that primary adipocytes express only the longer isoform of neuronatin. Loss of neuronatin led to increased mitochondrial biogenesis, indicated by greater intensity of MitoTracker Green staining. This was accompanied by increased expression of UCP1 and the key genes in mitochondrial oxidative phosphorylation, PGC-1α, Cox8b, and Cox4 in primary subcutaneous white adipocytes, indicative of a "browning" effect. In addition, phosphorylation of AMPK and ACC was increased, suggestive of increased fatty acid utilization. Similar, but less pronounced, effects of neuronatin silencing were also noted in primary brown adipocytes. In contrast, loss of neuronatin caused a reduction in both basal and insulin-stimulated glucose uptake and glycogen synthesis, likely mediated by a reduction in Glut1 protein upon silencing of neuronatin. In contrast, loss of neuronatin had no effect on insulin signaling. In conclusion, neuronatin appears to be a novel regulator of browning and metabolic substrate disposal in white adipocytes.

Differential pre-mRNA splicing regulates Nnat isoforms in the hypothalamus after gastric bypass surgery in mice

Background

Neuronatin (NNAT) is an endoplasmic reticulum proteolipid implicated in intracellular signalling. Nnat is highly-expressed in the hypothalamus, where it is acutely regulated by nutrients and leptin. Nnat pre-mRNA is differentially spliced to create Nnat-α and -β isoforms. Genetic variation of NNAT is associated with severe obesity. Currently, little is known about the long-term regulation of Nnat.

Methods

Expression of Nnat isoforms were examined in the hypothalamus of mice in response to acute fast/feed, chronic caloric restriction, diet-induced obesity and modified gastric bypass surgery. Nnat expression was assessed in the central nervous system and gastrointestinal tissues. RTqPCR was used to determine isoform-specific expression of Nnat mRNA.

Results

Hypothalamic expression of both Nnat isoforms was comparably decreased by overnight and 24-h fasting. Nnat expression was unaltered in diet-induced obesity, or subsequent switch to a calorie restricted diet. Nnat isoforms showed differential expression in the hypothalamus but not brainstem after bypass surgery. Hypothalamic Nnat-β expression was significantly reduced after bypass compared with sham surgery (P = 0.003), and was positively correlated with post-operative weight-loss (R² = 0.38, P = 0.01). In contrast, Nnat-α expression was not suppressed after bypass surgery (P = 0.19), and expression did not correlate with reduction in weight after surgery (R² = 0.06, P = 0.34). Hypothalamic expression of Nnat-β correlated weakly with circulating leptin, but neither isoform correlated with fasting gut hormone levels post- surgery. Nnat expression was detected in brainstem, brown-adipose tissue, stomach and small intestine.

Conclusions

Nnat expression in hypothalamus is regulated by short-term nutrient availability, but unaltered by diet-induced obesity or calorie restriction. While Nnat isoforms in the hypothalamus are co-ordinately regulated by acute nutrient supply, after modified gastric bypass surgery Nnat isoforms show differential expression. These results raise the possibility that in the radically altered nutrient and hormonal milieu created by bypass surgery, resultant differential splicing of Nnat pre-mRNA may contribute to weight-loss.

Gene expression profile in rat dorsal root ganglion following sciatic nerve injury and systemic neurotrophin-3 administration

Following sciatic nerve transection in adult rats, a proportion of injured dorsal root ganglion (DRG) neurons die, through apoptosis, over the following 6 months. Previous studies showed that axotomy and neurotrophin-3 administration may have effects on expression of neurotrophins and their receptors in DRG. In the current study, the fourth and fifth lumbar DRGs of rats were examined 2 weeks after right sciatic nerve transection and ligation. The effects of axotomy and systemic NT-3 treatment on neuronal genes were investigated by microarray. The results demonstrated that bone morphogenetic protein (BMP) and Janus protein tyrosine kinase signaling pathways are induced in axotomized DRG, and PI-3 kinase and BMP pathways and genes controlling various cellular functions were induced after axotomy and NT-3 administration.

Lafora disease ubiquitin ligase malin promotes proteasomal degradation of neuronatin and regulates glycogen synthesis

Lafora disease (LD) is the inherited progressive myoclonus epilepsy caused by mutations in either EPM2A gene, encoding the protein phosphatase laforin or the NHLRC1 gene, encoding the ubiquitin ligase malin. Since malin is an ubiquitin ligase and its mutations cause LD, it is hypothesized that improper clearance of its substrates might lead to LD pathogenesis. Here, we demonstrate for the first time that neuronatin is a novel substrate of malin. Malin interacts with neuronatin and enhances its degradation through proteasome. Interestingly, neuronatin is an aggregate prone protein, forms aggresome upon inhibition of cellular proteasome function and malin recruited to those aggresomes. Neuronatin is found to stimulate the glycogen synthesis through the activation of glycogen synthase and malin prevents neuronatin-induced glycogen synthesis. Several LD-associated mutants of malin are ineffective in the degradation of neuronatin and suppression of neuronatin-induced glycogen synthesis. Finally, we demonstrate the increased levels of neuronatin in the skin biopsy sample of LD patients. Overall, our results indicate that malin negatively regulates neuronatin and its loss of function in LD results in increased accumulation of neuronatin, which might be implicated in the formation of Lafora body or other aspect of disease pathogenesis.

The imprinted gene neuronatin is regulated by metabolic status and associated with obesity

Using restriction fragment differential display (RFDD) technology, we have identified the imprinted gene neuronatin (Nnat) as a hypothalamic target under the influence of leptin. Nnat mRNA expression is decreased in several key appetite regulatory hypothalamic nuclei in rodents with impaired leptin signaling and during fasting conditions. Furthermore, peripheral administration of leptin to ob/ob mice normalizes hypothalamic Nnat expression. Comparative immunohistochemical analysis of human and rat hypothalami demonstrates that NNAT protein is present in anatomically equivalent nuclei, suggesting human physiological relevance of the gene product(s). A putative role of Nnat in human energy homeostasis is further emphasized by a consistent association between single nucleotide polymorphisms (SNPs) in the human Nnat gene and severe childhood and adult obesity.

High expressions of neuronatin isoforms in favorable neuroblastoma

Neuroblastoma is a malignant solid tumor of children, which derives from the embryonal sympathoadrenal linage. Clinical cases can vary widely from a favorable to an unfavorable prognosis according to the presence of genetic aberrations, such as MYCN amplification. Our cDNA microarray analysis which compared the gene expressions between favorable and unfavorable neuroblastomas showed a high expression of the neuronatin (Nnat) gene in favorable neuroblastomas. Nnat is highly conserved in mammalian species, and its expression appears in nervous systems from the hindbrain to the peripherals during the prenatal periods. The Nnat mRNA expression, investigated in 63 of neuroblastoma samples by quantitative reverse-transcription polymerase chain reaction, was found to be significantly higher in the favorable prognosis groups than in the unfavorable groups. Nnat is an imprinted gene, and its expression in IMR32 neuroblastoma cell line was up-regulated by treatment with a demethylating agent. High expressions of Nnat isoforms induced in an IMR32 neuroblastoma cell line changed the cell morphology to the extension of the neural processes, which thus indicated the occurrence of cell differentiation. In conclusion, the high expressions of Nnat were found to be associated with good prognoses in neuroblastoma, which might indicate tumor differentiation, and its suppressions in unfavorable tumors are considered to be under epigenetic control.

Effects of a verbenachalcone derivative on neurite outgrowth, inhibition of caspase induction and gene expression

A verbenachalcone derivative was synthesized and shown to protect N2a cells from caspase induction caused by serum starvation and to enhance the effect of NGF on neurite outgrowth in PC12 cells. As an initial investigation of the compound's mechanism(s) of action, we performed differential gene expression profiling in PC12 cells using oligonucleotide ( approximately 10,000 gene probes) microarrays. Gene expression patterns were compared in the presence of NGF (2 and 50 ng/mL) and NGF (2 ng/mL) plus the verbenachalcone derivative. Ten genes were significantly (2-fold; p0.05) up-regulated and seven genes were significantly down-regulated in the presence of the compound. These results were independently validated by quantitative real-time PCR for a subset of genes (cathepsin L, sigma-1 receptor and protein tyrosine phosphatase receptor type R). These genes or their protein products may represent useful therapeutic targets for treating neurodegeneration, such as Alzheimer's disease.

Bcl-2 mediates induction of neural differentiation

Bcl-2 is an antiapoptotic protein expressed in a wide variety of cell types. We have found that overexpression of bcl-2 in PC12 neural crest tumor cells leads to increased expression of neural differentiation-associated genes and decreased expression of proliferation-related genes. Culture growth rate decreases as well. Overexpression of bcl-2 also leads to increased expression of TrkA and increased phosphorylation of signal transductants in, albeit not specific for, the TrkA-MEK-ERK pathway. Blocking of NGF-mediated signaling through TrkA prevents Bcl-2-associated expression changes in differentiation-associated genes, raising the possibility that Bcl-2 mediates induction of neural differentiation through TrkA/NGF signaling.

Physiological functions of imprinted genes

Genomic imprinting in gametogenesis marks a subset of mammalian genes for parent-of-origin-dependent monoallelic expression in the offspring. Embryological and classical genetic experiments in mice that uncovered the existence of genomic imprinting nearly two decades ago produced abnormalities of growth or behavior, without severe developmental malformations. Since then, the identification and manipulation of individual imprinted genes has continued to suggest that the diverse products of these genes are largely devoted to controlling pre- and post-natal growth, as well as brain function and behavior. Here, we review this evidence, and link our discussion to a website (http://www.otago.ac.nz/IGC) containing a comprehensive database of imprinted genes. Ultimately, these data will answer the long-debated question of whether there is a coherent biological rationale for imprinting.

The fetal and neonatal brain protein neuronatin protects PC12 cells against certain types of toxic insult

The protein neuronatin is expressed in the nervous system of the fetus and neonate at a much higher level than in the adult. Its function is unknown. As a result of variable splicing, neuronatin mRNA exists in two forms, alpha and beta. Wild type PC12 cells express neuronatin-alpha. We have isolated a PC12 variant, called 1.9, that retains many of the neuron-like properties of wild type PC12 cells, but it does not express neuronatin and it exhibits markedly increased sensitivity to the toxic effects of nigericin, rotenone and valinomycin. Pretreatment of the 1.9 cells with alpha-methyltyrosine, which inhibits dopamine synthesis, had little effect on the cells' sensitivity to nigericin, rotenone or valinomycin indicating that dopamine-induced oxidative stress was not involved in the toxicity of these compounds. However, flattened cell subvariants of the 1.9 cells, which do not have any neuron-specific characteristics, did not exhibit increased sensitivity to nigericin indicating that some neuronal characteristic of the 1.9 cells contributed to the toxicity of nigericin. After the neuronatin-beta gene was transfected into and expressed in the 1.9 cells, they regained wild type PC12 levels of resistance to nigericin, rotenone and valinomycin. These studies suggest that the function of neuronatin during development could be to protect developing cells from toxic insult occurring during that period.

Gene expression in Wilms' tumor mimics the earliest committed stage in the metanephric mesenchymal-epithelial transition

Wilms' tumor (WT) has been considered a prototype for arrested cellular differentiation in cancer, but previous studies have relied on selected markers. We have now performed an unbiased survey of gene expression in WTs using oligonucleotide microarrays. Statistical criteria identified 357 genes as differentially expressed between WTs and fetal kidneys. This set contained 124 matches to genes on a microarray used by Stuart and colleagues (Stuart RO, Bush KT, Nigam SK: Changes in global gene expression patterns during development and maturation of the rat kidney. Proc Natl Acad Sci USA 2001, 98:5649-5654) to establish genes with stage-specific expression in the developing rat kidney. Mapping between the two data sets showed that WTs systematically overexpressed genes corresponding to the earliest stage of metanephric development, and underexpressed genes corresponding to later stages. Automated clustering identified a smaller group of 27 genes that were highly expressed in WTs compared to fetal kidney and heterologous tumor and normal tissues. This signature set was enriched in genes encoding transcription factors. Four of these, PAX2, EYA1, HBF2, and HOXA11, are essential for cell survival and proliferation in early metanephric development, whereas others, including SIX1, MOX1, and SALL2, are predicted to act at this stage. SIX1 and SALL2 proteins were expressed in the condensing mesenchyme in normal human fetal kidneys, but were absent (SIX1) or reduced (SALL2) in cells at other developmental stages. These data imply that the blastema in WTs has progressed to the committed stage in the mesenchymal-epithelial transition, where it is partially arrested in differentiation. The WT-signature set also contained the Wnt receptor FZD7, the tumor antigen PRAME, the imprinted gene NNAT and the metastasis-associated transcription factor E1AF.

Activation by GDNF of a transcriptional program repressing neurite growth in dorsal root ganglia

Glial cell line-derived neurotrophic factor (GDNF) promotes the survival of postnatal-but not embryonic-mouse dorsal root ganglion cells in vitro, despite the fact that its receptors are expressed at both ages. To understand this difference, we have performed an oligonucleotide microarray experiment. We found that several hundred genes were regulated between embryonic and postnatal stages, and that several important classes of genes were differentially regulated by GDNF treatment, including genes related to translation and to phenotypic specification and maturation. Interestingly, a set of genes related to cell adhesion, cytoskeleton and cellular morphology were consistently down-regulated by GDNF, suggesting a previously uncharacterized role for GDNF in repressing neurite growth and/or branching. This nuclear program initiated by GDNF was functionally confirmed in cultures of embryonic wild-type neurons sustained with nerve growth factor and in bax(-/-) neurons that survive in the absence of trophic support.

p38 mitogen-activated protein kinase-independent induction of gadd45 expression in nerve growth factor-induced apoptosis in medulloblastomas

We describe a novel nerve growth factor (NGF)-signaling pathway leading to gadd45 induction that is independent of JNK and p38 MAPK. We used cDNA arrays representing 588 genes to investigate the role of differential gene expression in NGF-mediated pleiotropic responses. We compared the gene expression profiles obtained from MED283-TrkA cells undergoing NGF-induced apoptosis to PC12 cells undergoing NGF-induced differentiation. An early and specific transcriptional target of NGF in MED283-TrkA cells was the DNA-damage-inducible gene gadd45. Its magnitude of induction directly correlated with the magnitude of apoptosis in MED283 clones transfected with mutant TrkA receptors. Although gadd45 has been implicated in stress response signaling, in vitro kinase assays indicated that NGF neither activated c-Jun NH2-terminal kinase (JNK) nor p38 mitogen-activated protein kinase (MAPK). Furthermore, the p38 MAPK inhibitor SB203580 (20 microM) failed to prevent NGF-induced apoptosis and NGF-induced gadd45 expression. These results suggest that differential regulation of gadd45 expression possibly through BRCA1 may be a potential mechanism whereby NGF regulates pleiotropic responses.

cDNA cloning and mRNA expression analysis of the human neuronatin. High level expression in human pituitary gland and pituitary adenomas

The authors cloned the nearly complete cDNA of human neuronatin with the aid of an expressed sequence tag (EST) database, and analyzed its expression in various human tissues by Northern blot analysis. The nucleotide and deduced amino acid sequences of the human neuronatin showed a high similarity to those of rodents. The Northern blot analysis revealed that the human neuronatin message was expressed predominantly in the fetal brain in the brain-specific manner, but only faintly in the adult brain. Among the various adult human tissues examined, the anterior pituitary gland was shown to be the only place where the neuronatin mRNA was strongly expressed. Intense neuronatin expression was also observed in several human pituitary adenomas, including ACTH-producing, GH-producing, and nonfunctioning adenomas, but hardly detected in other brain tumors.