Germ Cell Nuclear Factor Relieves cAMP-response Element Modulator τ-mediated Activation of the Testis-specific Promoter of Human Mitochondrial Glycerol-3-phosphate Dehydrogenase

Hippocampal NR6A1 impairs CREB-BDNF signaling and leads to the development of depression-like behaviors in mice

Chronic stress exposure is a risk factor that can induce the development of depression-like behaviors by impairing the hippocampal cyclic adenosine monophosphate-response element binding protein (CREB)-brain-derived neurotrophic factor (BDNF) signaling, but its underlying mechanisms remain largely unknown. We identified an orphan receptor that can suppress the activity of CREB, nuclear receptor sub-family 6, group A, member 1 (NR6A1), in mouse brain neurons. Given the critical role of the impaired CREB-BDNF signaling in depression, we speculate that the neuronal NR6A1 may mediate the pathogenesis of depression. Results showed that chronic unpredictable stress (CUS) markedly increased the expression levels of hippocampal NR6A1 protein, which reduced hippocampal CREB phosphorylation and BDNF protein expression. Overexpression of hippocampal NR6A1 in stress-naïve mice simulated chronic stress, inducing depression-like behaviors in the tail suspension test, forced swimming test, and sucrose preference test, and impairing the hippocampal CREB-BDNF signaling cascade. Genetic knockdown of hippocampal NR6A1 did not affect mouse behaviors but prevented the CUS-induced depression-like behaviors in mice and impairment in hippocampal CREB-BDNF signaling. Furthermore, genetic knockdown of hippocampal CREB or BDNF abrogated the preventive effect of hippocampal NR6A1 down-regulation on CUS-induced depression-like behaviors in mice. Collectively, these results for the first time identified a nuclear expression of NR6A1 in mouse brain neurons, and showed that the abnormally increased NR6A1 protein in the hippocampus in mice treated with or without chronic stress can impair the CREB-BDNF signaling cascade and lead to the development of depression-like behaviors. Hippocampal NR6A1 could be a novel target for the development of antidepressants.

Metformin's Therapeutic Efficacy in the Treatment of Diabetes Does Not Involve Inhibition of Mitochondrial Glycerol Phosphate Dehydrogenase

Mitochondrial glycerol phosphate dehydrogenase (mGPD) is the rate-limiting enzyme of the glycerol phosphate redox shuttle. It was recently claimed that metformin, a first-line drug used for the treatment of type 2 diabetes, inhibits liver mGPD 30-50%, suppressing gluconeogenesis through a redox mechanism. Various factors cast doubt on this idea. Total-body knockout of mGPD in mice has adverse effects in several tissues where the mGPD level is high but has little or no effect in liver, where the mGPD level is the lowest of 10 tissues. Metformin has beneficial effects in humans in tissues with high levels of mGPD, such as pancreatic β-cells, where the mGPD level is much higher than that in liver. Insulin secretion in mGPD knockout mouse β-cells is normal because, like liver, β-cells possess the malate aspartate redox shuttle whose redox action is redundant to the glycerol phosphate shuttle. For these and other reasons, we used four different enzyme assays to reassess whether metformin inhibited mGPD. Metformin did not inhibit mGPD in homogenates or mitochondria from insulin cells or liver cells. If metformin actually inhibited mGPD, adverse effects in tissues where the level of mGPD is much higher than that in the liver could prevent the use of metformin as a diabetes medicine.

NR6A1 regulates lipid metabolism through mammalian target of rapamycin complex 1 in HepG2 cells

Background

Lipogenesis is required for the optimal growth of many types of cancer cells, it is shown to control the biosynthesis of the lipid bilayer membrane during rapid proliferation and metastasis, provides cancer cells with signaling lipid molecules to support cancer development and make cancer cells more resistant to oxidative stress-induced cell death. Though multiple lipogenic enzymes have been identified to mediate this metabolic change, how the expression of these lipogenic enzymes are transcriptionally regulated remains unclear.

Methods

Gain- and loss-of-function experiments were conducted to assess the role of transcriptional repressor, nuclear receptor sub-family 6, group A, member 1 (NR6A1) in HepG2 cells. RT-qPCR method was performed to investigate target gene of NR6A1. Western blot was employed to determine the mechanisms by which NR6A1 regulates lipid accumulation in HepG2 cells.

Results

We provide evidence that NR6A1 is a novel regulator of lipid metabolism in HepG2 cells. NR6A1 knockdown can increase lipid accumulation as well as insulin-induced proliferation and migration of HepG2 cells. The lipogenic effect correlated well with the expression of lipogenic genes, including fatty acid synthase (FAS), diglyceride acyltransferase-2 (DGAT2), malic enzyme 1 (ME1), microsomal triglyceride transfer protein (MTTP) and phosphoenolpyruvate carboxykinase (PEPCK). NR6A1 knockdown also increased the expression of carnitine palmitoyltransferase 1A (CPT1a), the rate-limiting enzyme in fatty acid oxidation. Furthermore, NR6A1 knockdown induced lipid accumulation through mammalian target of rapamycin complex 1 (mTORC1), but not mTORC2. Moreover, siRNA-mediated knockdown of NR6A1 increased expression of insulin receptor (INSR) and potentitated insulin-induced phosphorylation of mTOR and AKT partly via miR-205-5p in HepG2 cells.

Conclusions

These findings provide important new insights into the role of NR6A1 in the lipogenesis in HepG2 cells.

Graphical abstract

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Electronic supplementary material

The online version of this article (10.1186/s12964-019-0389-4) contains supplementary material, which is available to authorized users.

NR6A1 couples with cAMP response element binding protein and regulates vascular smooth muscle cell migration

Vascular smooth muscle cell (VSMC) migration is implicated in atherosclerosis and restenosis. Nuclear receptor subfamily 6, group A, member 1 (NR6A1) is involved in regulating embryonic stem cell differentiation, reproduction, neuronal differentiation. Functional cooperation between cAMP response element modulator tau (CREMtau) and NR6A1 can direct gene expression in cells. cAMP response element binding protein (CREB) plays a key role in VSMC migration. In this study, we sought to determine whether CREB involved in NR6A1-modulated VSMC migration. VSMCs treated with platelet-derived growth factor-BB (PDGF-BB) displayed reduced mRNA and protein levels of NR6A1. Adenovirus-mediated expression of NR6A1 (Ad-NR6A1) could inhibit PDGF-BB- and serum-induced VSMC migration. The mRNA and protein expressions of secreted phosphoprotein 1 (SPP1) were down-regulated by NR6A1 overexpression. SPP1 promoter reporter activity was repressed by NR6A1. NR6A1 was found to physically couple with nuclear actin and the large subunit of RNA polymerase II. Furthermore, we showed that CREB interacted with NR6A1 in VSMCs. NR6A1 overexpression repressed cAMP response element (CRE) activity. ChIP assay revealed that NR6A1 bind to SPP1 promoter. Luciferase reporter assay showed that NR6A1 regulated SPP1 promoter activity via a putative CRE site. Adenovirus mediated local NR6A1 gene transfer attenuated stenosis after balloon-induced arterial injury in Sprague-Dawley rats. Taken together, this study provided experimental evidence that NR6A1 modulated SPP1 expression via its binding with CREB protein in VSMCs. We also revealed a NR6A1-CREB-SPP1 axis that serves as a regulatory mechanism for atherosclerosis and restenosis.

Revisiting the role of GCNF in embryonic development

GCNF (NR6A1) is essential for embryonic development. GCNF belongs to the nuclear receptor (NR) gene family, it is distantly related to other NRs and is the only member of subfamily 6. As the ligand for GCNF has not been identified, GCNF is designated an orphan nuclear receptor. GCNF has been found to be a transcriptional repressor, through specific binding to DR0 response elements, which is found in the Oct4 proximal promoter for example. GCNF is expressed widely in early mouse embryos, and later in the developing nervous system. GCNF knockout mouse embryos die around E10.5. GCNF is required for the restriction of Oct4 expression to primordial germ cells after gastrulation. GCNF is expressed in ES/EC cells and during their differentiation, and has been reported to be required for pluripotency gene repression during retinoic acid (RA)-induced mES cell differentiation. GCNF can interact with DNA methylation proteins, and is suggested to recruit DNA methylation complexes to repress and silence Oct4 expression. Nuclear receptor regulation in embryonic development is a complex process, as different nuclear receptors have overlapping and distinct functions. In-depth exploration of GCNF function and mechanism of action will help to comprehensively understand the nuclear receptor regulation in embryonic development.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 3: developmental changes in spermatid flagellum and cytoplasmic droplet and interaction of sperm with the zona pellucida and egg plasma membrane

Spermiogenesis constitutes the steps involved in the metamorphosis of spermatids into spermatozoa. It involves modification of several organelles in addition to the formation of several structures including the flagellum and cytoplasmic droplet. The flagellum is composed of a neck region and middle, principal, and end pieces. The axoneme composed of nine outer microtubular doublets circularly arranged to form a cylinder around a central pair of microtubules is present throughout the flagellum. The middle and principal pieces each contain specific components such as the mitochondrial sheath and fibrous sheath, respectively, while outer dense fibers are common to both. A plethora of proteins are constituents of each of these structures, with each playing key roles in functions related to the fertility of spermatozoa. At the end of spermiogenesis, a portion of spermatid cytoplasm remains associated with the released spermatozoa, referred to as the cytoplasmic droplet. The latter has as its main feature Golgi saccules, which appear to modify the plasma membrane of spermatozoa as they move down the epididymal duct and hence may be partly involved in male gamete maturation. The end product of spermatogenesis is highly streamlined and motile spermatozoa having a condensed nucleus equipped with an acrosome. Spermatozoa move through the female reproductive tract and eventually penetrate the zona pellucida and bind to the egg plasma membrane. Many proteins have been implicated in the process of fertilization as well as a plethora of proteins involved in the development of spermatids and sperm, and these are high lighted in this review.

Functional cooperation between CREM and GCNF directs gene expression in haploid male germ cells

Cellular differentiation and development of germ cells critically depend on a coordinated activation and repression of specific genes. The underlying regulation mechanisms, however, still lack a lot of understanding. Here, we describe that both the testis-specific transcriptional activator CREMτ (cAMP response element modulator tau) and the repressor GCNF (germ cell nuclear factor) have an overlapping binding site which alone is sufficient to direct cell type-specific expression in vivo in a heterologous promoter context. Expression of the transgene driven by the CREM/GCNF site is detectable in spermatids, but not in any somatic tissue or at any other stages during germ cell differentiation. CREMτ acts as an activator of gene transcription whereas GCNF suppresses this activity. Both factors compete for binding to the same DNA response element. Effective binding of CREM and GCNF highly depends on composition and epigenetic modification of the binding site. We also discovered that CREM and GCNF bind to each other via their DNA binding domains, indicating a complex interaction between the two factors. There are several testis-specific target genes that are regulated by CREM and GCNF in a reciprocal manner, showing a similar activation pattern as during spermatogenesis. Our data indicate that a single common binding site for CREM and GCNF is sufficient to specifically direct gene transcription in a tissue-, cell type- and differentiation-specific manner.

Haploinsufficiency of the GPD2 gene in a patient with nonsyndromic mental retardation

We have investigated the chromosome abnormalities in a female patient exhibiting mild nonsyndromic mental retardation. The patient carries a de novo balanced reciprocal translocation 46,XX,t(2;7)(q24.1;q36.1). Physical mapping of the breakpoints by fluorescent in situ hybridization experiments revealed the disruption of the GPD2 gene at the 2q24.1 region. This gene encodes the mitochondrial glycerophosphate dehydrogenase (mGPDH), which is located on the outer surface of the inner mitochondrial membrane, and catalyzes the unidirectional conversion of glycerol-3-phosphate (G3P) to dihydroxyacetone phosphate with concomitant reduction of the enzyme-bound FAD. Molecular and functional studies showed approximately a twofold decrease of GPD2 transcript level as well as decreased activity of the coded mGPDH protein in lymphoblastoid cell lines of the patient compared to controls. Bioinformatics analysis allowed us to confirm the existence of a novel transcript of the GPD2 gene, designated GPD2c, which is directly disrupted by the 2q breakpoint. To validate GPD2 as a new candidate gene for mental retardation, we performed mutation screening of the GPD2 gene in 100 mentally retarded patients; however, no mutations have been identified. Nevertheless, our results propose that a functional defect of the mGPDH protein could be associated with mental retardation, suggesting that GPD2 gene could be involved in mental retardation in some cases.

Germ cell nuclear factor directly represses the transcription of peroxisome proliferator-activated receptor delta gene

Germ cell nuclear factor (GCNF) is a transcription factor that can repress gene transcription and plays an important role during spermatogenesis. Peroxisome proliferator-activated receptor delta (PPARdelta) is a nuclear hormone receptor belonging to the steroid receptor superfamily. It can activate the expression of many genes, including those involved in lipid metabolism. In this report, we showed that GCNF specifically interacts with PPARdelta promoter. Overexpression of GCNF in African green monkey SV40-transformed kidney fibroblast COS7 cells and mouse embryo fibroblast NIH 3T3 cells represses the activity of PPARdelta promoter. The mutation of GCNF response element in PPARdelta promoter relieves the repression in NIH 3T3 cells and mouse testis. Moreover, we showed that GCNF in nuclear extracts of mouse testis is able to bind to PPARdelta promoter directly. We also found that GCNF and PPARdelta mRNA were expressed with different patterns in mouse testis by in situ hybridization. These results suggested that GCNF might be a negative regulator of PPARdelta gene expression through its direct interaction with PPARdelta promoter in mouse testis.

The transcription factor CREMtau and cAMP regulate promoter activity of the Na,K-ATPase alpha4 isoform

The Na,K-ATPase is an essential enzyme of the plasma membrane that plays a key role in numerous cell processes that depend on the transcellular gradients of Na(+) and K(+). Among the various isoforms of the catalytic subunit of the Na,K-ATPase, alpha4 exhibits the most limited pattern of expression, being restricted to male germ cells. Activity of alpha4 is essential for sperm function, and alpha4 is upregulated during spermatogenesis. The present study addressed the transcriptional control of the human Na,K-ATPase alpha4 gene, ATP1A4. We describe that a 5' untranslated region of the ATP1A4 gene (designated -339/+480 based on the ATP1A4 transcription initiation site) has promoter activity in luciferase reporter assays. Computer analysis of this promoter region revealed consensus sites (CRE) for the cyclic AMP (cAMP) response element modulator (CREM). Accordingly, dibutyryl cAMP (db-cAMP) and ectopic expression of CREMtau, a testis specific splice variant of CREM were able to activate the ATP1A4 promoter driven expression of luciferase in HEK 293 T, JEG-3 and GC-1 cells. Further characterization of the effect of db-cAMP and CREMtau on deleted constructs of the ATP1A4 promoter (-339/+80, and +25/+480), and on the -339/+480 region carrying mutations in the CRE sites showed that db-cAMP and CREMtau effect required the CRE motif located 263 bp upstream the transcription initiation site. EMSA experiments confirmed the CRE sequence as a bonafide CREMtau binding site. These results constitute the first demonstration of the transcriptional control of ATP1A4 gene expression by cAMP and by CREMtau, a transcription factor essential for male germ cell gene expression.

Germ cell nuclear factor is a repressor of CRIPTO-1 and CRIPTO-3

The pluripotency of embryonic stem and embryonic carcinoma cells is maintained by the expression of a set of "stemness" genes. Whereas these genes are down-regulated upon induction of differentiation, the germ cell nuclear factor (GCNF) is transiently up-regulated and represses several pluripotency genes. CRIPTO-1, a co-receptor for the morphogen nodal, is strongly expressed in undifferentiated cells and is rapidly down-regulated during retinoic acid-induced differentiation. Although CRIPTO-1 is expressed at very low levels in adult tissues under normal conditions, it is found highly expressed in a broad range of tumors, where it acts as a potent oncogene. We show that expression of CRIPTO-1 is directly repressed by GCNF during differentiation of the human teratocarcinoma cell line, NT2. GCNF bound to a DR0 element of the CRIPTO-1 promoter in vitro, as shown by electrophoretic mobility shift assays, and in vivo, as demonstrated by chromatin immunoprecipitation. Reporter gene assays demonstrated that GCNF-mediated repression of the CRIPTO-1 promoter is dependent upon the DR0 site. Overexpression of GCNF in NT2 cells resulted in repression of CRIPTO-1 transcription, whereas expression of the transcription-activating fusion construct GCNF-VP16 led to an induction of the CRIPTO-1 gene and prevented its retinoic acid-induced down-regulation. Furthermore, we demonstrated that CRIPTO-3, a processed pseudogene of CRIPTO-1 on the X chromosome, is expressed in undifferentiated NT2 cells and is regulated by GCNF in parallel to CRIPTO-1. Thus, our study supports the hypothesis of GCNF playing a central role during differentiation of stem cells by repression of stem cell-specific genes.

Four promoters direct expression of the calpastatin gene

Calpastatin is a specific endogenous protein inhibitor of the ubiquitous calcium dependent proteinases mu- and m-calpain. The calpain-calpastatin system is involved in various physiological and pathological processes. In the present study, we determined the bovine calpastatin gene structure and demonstrated that four promoters direct its expression. The gene harbours 35 exons spanning at least 130kb on genomic DNA. Its structure is similar to that of mouse, pig, and human gene. Transient transfection assays in both C2C12 and COS7 cell lines demonstrated that the putative promoter regions situated 5' to exon 1xa, 1xb, 1u, and 14t were functional. We also established that the region situated upstream exon 14t is subjected to a tissue specific regulation. The implication of numerous high-scoring cis acting transcriptional motifs which are present in these regions will need to be determined. The existence of four promoters suggests differential expression patterns which must have a physiological significance.