Impact of natriuretic peptide polymorphisms on diastolic and metabolic function in a populational cohort: insights from the STANISLAS cohort

AIMS

Elevated brain natriuretic peptide (BNP) and the N-terminal fragment of its pro-hormone (NT-proBNP) have become established biomarkers for heart failure and are associated with cardiovascular morbidity and mortality. Investigating sources of inter-individual heterogeneity, particularly genetic factors, could help better identify patients at risk of future cardiovascular disease. The aim of this study was to estimate the heritability of circulating NT-proBNP levels, to perform a genome-wide association study (GWAS) and gene-candidate analysis focused on NPPB-NPPA genes on these levels, and to examine their association with cardiovascular or metabolic outcomes.

METHODS AND RESULTS

A total of 1555 individuals from the STANISLAS study were included. The heritability of circulating NT-proBNP levels was estimated at 15%, with seven single nucleotide polymorphisms (SNPs) reaching the significant threshold in the GWAS. All above SNPs were located on the same gene cluster constituted of MTHFR, CLCN6, NPPA, NPPB, and C1orf167. NPPA gene expression was also associated with NT-proBNP levels. Moreover, six other SNPs from NPPA-NPPB genes were associated with diastolic function (lateral e' on echocardiography) and metabolic features (glycated haemoglobin).

CONCLUSIONS

The heritability of natriuretic peptides appears relatively low (15%) and mainly based on the same gene cluster constituted of MTHFR, CLCN6, NPPA, NPPB, and C1orf167. Natriuretic peptide polymorphisms are associated with natriuretic peptide levels and diastolic function. These results suggest that natriuretic peptide polymorphisms may have an impact in the early stages of cardiovascular and metabolic disease.

The Inability of the Choroid to Revascularize in Oxygen-Induced Retinopathy Results from Increased p53/miR-Let-7b Activity

Retinopathy of prematurity (ROP) is characterized by an initial retinal avascularization, followed by pathologic neovascularization. Recently, choroidal thinning has also been detected in children formerly diagnosed with ROP; a similar sustained choroidal thinning is observed in ROP models. But the mechanism underlying the lack of choroidal revascularization remains unclear and was investigated in an oxygen-induced retinopathy (OIR) model. In OIR, evidence of senescence was detected, preceded by oxidative stress in the choroid and the retinal pigment epithelium. This was associated with a global reduction of proangiogenic factors, including insulin-like growth factor 1 receptor (Igf1R). Coincidentally, tumor suppressor p53 was highly expressed in the OIR retinae. Curtailing p53 activity resulted in reversal of senescence, normalization of Igf1r expression, and preservation of choroidal integrity. OIR-induced down-regulation of Igf1r was mediated at least partly by miR-let-7b as i) let-7b expression was augmented throughout and beyond the period of oxygen exposure, ii) let-7b directly targeted Igf1r mRNA, and iii) p53 knock-down blunted let-7b expression, restored Igf1r expression, and elicited choroidal revascularization. Finally, restoration of Igf1r expression rescued choroid thickness. Altogether, this study uncovers a significant mechanism for defective choroidal revascularization in OIR, revealing a new role for p53/let-7b/IGF-1R axis in the retina. Future investigations on this (and connected) pathway could further our understanding of other degenerative choroidopathies, such as geographic atrophy.

Uterotonic Neuromedin U Receptor 2 and Its Ligands Are Upregulated by Inflammation in Mice and Humans, and Elicit Preterm Birth

Uterine labor requires the conversion of a quiescent (propregnancy) uterus into an activated (prolabor) uterus, with increased sensitivity to endogenous uterotonic molecules. This activation is induced by stressors, particularly inflammation in term and preterm labor. Neuromedin U (NmU) is a neuropeptide known for its uterocontractile effects in rodents. The objective of the study was to assess the expression and function of neuromedin U receptor 2 (NmU-R2) and its ligands NmU and the more potent neuromedin S (NmS) in gestational tissues, and the possible implication of inflammatory stressors in triggering this system. Our data show that NmU and NmS are uterotonic ex vivo in murine tissue, and they dose-dependently trigger labor by acting specifically via NmU-R2. Expression of NmU-R2, NmU, and NmS is detected in murine and human gestational tissues by immunoblot, and the expression of NmS in placenta and of NmU-R2 in uterus increases considerably with gestation age and labor, which is associated with amplified NmU-induced uterocontractile response in mice. NmU- and NmS-induced contraction is associated with increased NmU-R2-coupled Ca⁺⁺ transients, and Akt and Erk activation in murine primary myometrial smooth muscle cells (mSMCs), which are potentiated with gestational age. NmU-R2 is upregulated in vitro in mSMCs and in vivo in uterus in response to proinflammatory interleukin 1beta (IL1beta), which is associated with increased NmU-induced uterocontractile response and Ca⁺⁺ transients in murine and human mSMCs; additionally, placental NmS is markedly upregulated in vivo in response to IL1beta. In human placenta at term, immunohistological analysis revealed NmS expression primarily in cytotrophoblasts; furthermore, stimulation with lipopolysaccharide (LPS; Gram-negative endotoxin) markedly upregulates NmS expression in primary human cytotrophoblasts isolated from term placentas. Correspondingly, decidua of women with clinical signs of infection who delivered preterm display significantly higher expression of NmS compared with those without infection. Importantly, in vivo knockdown of NmU-R2 prevents LPS-triggered preterm birth in mice and the associated neonatal mortality. Altogether, our data suggest a critical role for NmU-R2 and its ligands NmU and NmS in preterm labor triggered by infection. We hereby identify NmU-R2 as a relevant target for preterm birth.

Cyclin-dependent kinase 5 phosphorylation of familial prion protein mutants exacerbates conversion into amyloid structure

Familial prion protein (PrP) mutants undergo conversion from soluble and protease-sensitive to insoluble and partially protease-resistant proteins. Cyclin-dependent kinase 5 (Cdk5) phosphorylation of wild type PrP (pPrP) at serine 43 induces a conversion of PrP into aggregates and fibrils. Here, we investigated whether familial PrP mutants are predisposed to Cdk5 phosphorylation and whether phosphorylation of familial PrP mutants increases conversion. PrP mutants representing three major familial PrP diseases and different PrP structural domains were studied. We developed a novel in vitro kinase reaction coupled with Thioflavin T binding to amyloid structure assay to monitor phosphorylation-dependent amyloid conversion. Although non-phosphorylated full-length wild type or PrP mutants did not convert into amyloid, Cdk5 phosphorylation rapidly converted these into Thioflavin T-positive structures following first order kinetics. Dephosphorylation partially reversed conversion. Phosphorylation-dependent conversion of PrP from α-helical structures into β-sheet structures was confirmed by circular dichroism. Relative to wild type pPrP, most PrP mutants showed increased rate constants of conversion. In contrast, non-phosphorylated truncated PrP Y145X (where X represents a stop codon) and Q160X mutants converted spontaneously into Thioflavin T-positive fibrils after a lag phase of over 20 h, indicating nucleation-dependent polymerization. Phosphorylation reduced the lag phase by over 50% and thus accelerated the formation of the nucleating event. Consistently, phosphorylated Y145X and phosphorylated Q160X exacerbated conversion in a homologous seeding reaction, whereas WT pPrP could not seed WT PrP. These results demonstrate an influence of both the N terminus and the C terminus of PrP on conversion. We conclude that post-translational modifications of the flexible N terminus of PrP can cause or exacerbate PrP mutant conversion.

Characterization of the survival motor neuron (SMN) promoter provides evidence for complex combinatorial regulation in undifferentiated and differentiated P19 cells

There exist two SMN (survival motor neuron) genes in humans, the result of a 500 kb duplication in chromosome 5q13. Deletions/mutations in the SMN1 gene are responsible for childhood spinal muscular atrophy, an autosomal recessive neurodegenerative disorder. While the SMN1 and SMN2 genes are not functionally equivalent, up-regulation of the SMN2 gene represents an important therapeutic target. Consequently, we exploited in silico, in vitro and in vivo approaches to characterize the core human and mouse promoters in undifferentiated and differentiated P19 cells. Phylogenetic comparison revealed four highly conserved regions that contained a number of cis-elements, only some of which were shown to activate/repress SMN promoter activity. Interestingly, the effect of two Sp1 cis-elements varied depending on the state of P19 cells and was only observed in combination with a neighbouring Ets cis-element. Electrophoretic mobility-shift assay and in vivo DNA footprinting provided evidence for DNA-protein interactions involving Sp, NF-IL6 and Ets cis-elements, whereas transient transfection experiments revealed complex interactions involving these recognition sites. SMN promoter activity was strongly regulated by an NF-IL6 response element and this regulation was potentiated by a downstream Ets element. In vivo results suggested that the NF-IL6 response must function either via a protein-tethered transactivation mechanism or a transcription factor binding an upstream element. Our results provide strong evidence for complex combinatorial regulation and suggest that the composition or state of the basal transcription complex binding to the SMN promoter is different between undifferentiated and differentiated P19 cells.

The SMN genes are subject to transcriptional regulation during cellular differentiation

Proximal spinal muscular atrophy (SMA) is an autosomal recessive disease characterized by degeneration of alpha-motor neurons and muscular atrophy. The causal survival motor neuron (SMN) gene maps to a complex region of chromosome 5q13 harbouring an inverted duplication. Thus, there are two SMN genes, SMN1 and SMN2, but SMN1-deficiency alone causes SMA. In this study we demonstrate, for the first time, down-regulation of SMN promoter activity during cellular differentiation. Specifically, the minimal SMN promoter is four times more active in undifferentiated embryonal carcinoma P19 cells compared to cells treated with retinoic acid (RA) to initiate neuronal differentiation. This effect is mediated by sequences contained within the minimal core promoter that we have confined to the 257 nucleotides upstream of exon 1. We have identified seven regions that are highly conserved between the mouse and human SMN core promoters and this region contains the consensus sequence for a number of transcription factors. Most notably, AhR, HNF-3 and N-Oct3 have already been shown to respond to RA treatment of EC cells, while E47, HNF-3, MAZ, N-Oct3 and Pit-1a have been implicated in embryonic, muscle or neural development. In addition, we have mapped two strong transcription initiation sites upstream of SMN exon 1. The novel -79 site identified in this study is preferentially utilized during human foetal development. Furthermore, analysis of RNA from SMA patients with deletions of the entire SMN1 gene or chimpanzees that lack SMN2 suggests that the level of transcription initiation at these sites may be different for the SMN1 and SMN2 genes. Taken together, this work provides the first demonstration of transcriptional regulation of these genes during cellular differentiation and development. Deciphering the underlying mechanisms responsible for regulating SMN transcription may provide important clues towards enhancing SMN2 gene expression, one target for the treatment of SMA.

Genotoxicity of 3-methylcholanthrene in liver of transgenic big Blue mice

Transgenic mice provide a unique tool for studying the tissue specificity and mutagenic potential of chemicals. Because 3-methylcholanthrene (3MC) was found mutagenic in bacteria, clastogenic in bone marrow, and induces DNA adducts in animals, we were interested to determinine whether this xenobiotic provokes (1) cell proliferation, (2) transcriptional activity changes, (3) DNA adducts, and (4) hepatic mutations in transgenic Big Blue mice carrying the lambdaLIZ phage shuttle vector. Big Blue C57/Bl male mice were treated with a single intraperitoneal dose of 80 mg/kg 3MC for 1, 3, 6, 14, or 30 days. Cell proliferation was checked by 5-bromo-2-deoxyuridine labeling and immunohistochemical detection. The maximal increase of the mitotic index was evidenced after 3 days (2.9 times the control value; P < 0.01). The relative nucleus area, reflecting the transcriptional activity, was also the highest in the treated group after 3 days: 1.86 times the control value, on average (P < 0.01). Four major DNA adducts, determined according to the [(32)P]-postlabeling method, were evidenced in liver DNA of treated mice, 6 days after the treatment: the spot intensities increased in a time-dependent manner. The mutant frequency of liver DNA was the highest after 14 days: 20.3 +/- 2.9 x 10(-5) in the treated vs. 7.6 +/- 2.7 x 10(-5) in the control mice (P < 0.01). Sequencing of the lambda lacI mutant plaques showed mainly G:C --> T:A and C:G --> A:T transversions. In conclusion, 3MC at first induced nuclear enlargement and a slight increase of cell proliferation in liver, followed by parallel formation of DNA adducts and mutations. This study shows how transgenic models allow in vivo evaluation of mechanistically simultaneous endpoints.