Control of the Na+/Ca2+ exchanger 3 promoter by cyclic adenosine monophosphate and Ca2+ in differentiating neurons

Neurotoxicity of mycotoxin citrinin: Novel evidence in developing zebrafish and underlying mechanisms in human neuron cells

Citrinin (CTN) is a mycotoxin that is found as a contaminant in various types of food/feed grains and fermented food supplements. Previous studies have already established the nephrotoxicity and hepatotoxicity of CTN, but the neurotoxicity of CTN has not been clearly examined. In this study, CTN at 2-20 μM was first found to interfere with the neural ganglia formation and locomotive behavior of embryonic zebrafish, a vertebrate animal model, at 24 hpf and 6 dpf, respectively. Further exposure of human neuroblastoma SH-SY5Y cells to 10 and 20 μM CTN for 72 h indicated that pathways responsible for neuron differentiation and projection guidance were down-regulated while oxidative stress and electron transport chain pathways were up-regulated based on the enrichment results of GSEA in the transcriptomic profiling. PCR analysis verified that CTN significantly down-regulated the expression of marker genes involved in neuron differentiation and synaptic signaling. CTN at the doses impairing cellular neurite outgrowth did not trigger mitochondrial oxidative stress and dysfunction. The neurotoxic mechanisms of CTN provide new information that is valuable in the assessment of CTN-related health risk for the general public.

The Cardiac Na+ -Ca2+ Exchanger: From Structure to Function

Ca²⁺ homeostasis is essential for cell function and survival. As such, the cytosolic Ca²⁺ concentration is tightly controlled by a wide number of specialized Ca²⁺ handling proteins. One among them is the Na⁺ -Ca²⁺ exchanger (NCX), a ubiquitous plasma membrane transporter that exploits the electrochemical gradient of Na⁺ to drive Ca²⁺ out of the cell, against its concentration gradient. In this critical role, this secondary transporter guides vital physiological processes such as Ca²⁺ homeostasis, muscle contraction, bone formation, and memory to name a few. Herein, we review the progress made in recent years about the structure of the mammalian NCX and how it relates to function. Particular emphasis will be given to the mammalian cardiac isoform, NCX1.1, due to the extensive studies conducted on this protein. Given the degree of conservation among the eukaryotic exchangers, the information highlighted herein will provide a foundation for our understanding of this transporter family. We will discuss gene structure, alternative splicing, topology, regulatory mechanisms, and NCX's functional role on cardiac physiology. Throughout this article, we will attempt to highlight important milestones in the field and controversial topics where future studies are required. © 2021 American Physiological Society. Compr Physiol 12:1-37, 2021.

GATA3 (GATA-Binding Protein 3)/KMT2A (Lysine-Methyltransferase-2A) Complex by Increasing H3K4-3me (Trimethylated Lysine-4 of Histone-3) Upregulates NCX3 (Na+-Ca2+ Exchanger 3) Transcription and Contributes to Ischemic Preconditioning Neuroprotection

Background and Purpose

NCX3 (Na+-Ca2+ exchanger 3) plays a relevant role in stroke; indeed its pharmacological blockade or its genetic ablation exacerbates brain ischemic damage, whereas its upregulation takes part in the neuroprotection elicited by ischemic preconditioning. To identify an effective strategy to induce an overexpression of NCX3, we examined transcription factors and epigenetic mechanisms potentially involved in NCX3 gene regulation.

Methods

Brain ischemia and ischemic preconditioning were induced in vitro by exposure of cortical neurons to oxygen and glucose deprivation plus reoxygenation (OGD/Reoxy) and in vivo by transient middle cerebral artery occlusion. Western blot and quantitative real-time polymerase chain reaction were used to evaluate transcripts and proteins of GATA3 (GATA-binding protein 3), KMT2A (lysine-methyltransferase-2A), and NCX3. GATA3 and KMT2A binding on NCX3 gene was evaluated by chromatin immunoprecipitation and Rechromatin immunoprecipitation experiments.

Results

Among the putative transcription factors sharing a consensus sequence on the ncx3 brain promoter region, GATA3 was the only able to up-regulate ncx3. Interestingly, GATA3 physically interacted with KMT2A, and their overexpression or knocking-down increased or downregulated NCX3 mRNA and protein, respectively. Notably, site-direct mutagenesis of GATA site on ncx3 brain promoter region counteracted GATA3 and KMT2A binding on NCX3 gene. More importantly, we found that in the perischemic cortical regions of preconditioned rats GATA3 recruited KMT2A and the complex H3K4-3me (trimethylated lysine-4 of histone-3) on ncx3 brain promoter region, thus reducing transient middle cerebral artery occlusion–induced damage. Consistently, in vivo silencing of either GATA3 or KMT2A prevented NCX3 upregulation and consequently the neuroprotective effect of preconditioning stimulus. The involvement of GATA3/KMT2A complex in neuroprotection elicited by ischemic preconditioning was further confirmed by in vitro experiments in which the knocking-down of GATA3 and KMT2A reverted the neuroprotection induced by NCX3 overexpression in cortical neurons exposed to anoxic preconditioning followed by oxygen and glucose deprivation plus reoxygenation.

Conclusions

Collectively, our results revealed that GATA3/KMT2A complex epigenetically activates NCX3 gene transcription during ischemic preconditioning.

Calmodulin/CaMKII-γ mediates prosurvival capability in apicidin-persistent hepatocellular carcinoma cells via ERK1/2/CREB/c-fos signaling pathway

Calmodulin (CaM), a Ca²⁺ binding protein, plays a critical role in cancer initiation and progression through binding and activating numerous target proteins, including Ca²⁺ /calmodulin-dependent protein kinase (CaMK) family proteins. However, the mechanisms underlying the effects of CaM/CaMKs on the survival capability of liver cancer cells is unclear, and this study investigates this mechanism in apicidin-persistent HA22T cells. CaM level was upregulated, especially in the cytosol, in apicidin-persistent HA22T cells than in parental HA22T cells and was positively associated with cell proliferation and migration capacity of apicidin-persistent HA22T cells. Further, the expression of CaM-activated CaMKs-dependent signaling cascades, including CaMKK2, CaMKIV, CaMKII-γ, and p-CaMKII was observed in apicidin-persistent HA22T cells, which were transiently activated by mitogen-activated protein kinase oncogenic signaling, such as CREB, ERK1/2, and c-fos. Furthermore, a specific CaM inhibitor trifluoperazine reduced the levels of p-CREB, p-ERK1/2, and c-fos in apicidin-persistent HA22T cells than in parental HA22T cells. Additionally, inhibition of CaM also suppressed CaM-induced Bcl-XL (an antiapoptotic protein) expression in apicidin-persistent HA22T cells. Our finding emphasizes an essential role of CaM/CaMKs in augmentation of the survival capability of apicidin-persistent liver cancer cells and suggests that CaM inhibition significantly attenuates CaM-induced tumor growth and abrogates antiapoptotic function and also offers a promising therapeutic target for cancer treatment.

HDAC4 and HDAC5 form a complex with DREAM that epigenetically down-regulates NCX3 gene and its pharmacological inhibition reduces neuronal stroke damage

The histone deacetylases (HDACs)-dependent mechanisms regulating gene transcription of the Na⁺/Ca⁺ exchanger isoform 3 (ncx3) after stroke are still unknown. Overexpression or knocking-down of HDAC4/HDAC5 down-regulates or increases, respectively, NCX3 mRNA and protein. Likewise, MC1568 (class IIa HDACs inhibitor), but not MS-275 (class I HDACs inhibitor) increased NCX3 promoter activity, gene and protein expression. Furthermore, HDAC4 and HDAC5 physically interacted with the transcription factor downstream regulatory element antagonist modulator (DREAM). As MC1568, DREAM knocking-down prevented HDAC4 and HDAC5 recruitment to the ncx3 promoter. Importantly, DREAM, HDAC4, and HDAC5 recruitment to the ncx3 gene was increased in the temporoparietal cortex of rats subjected to transient middle cerebral artery occlusion (tMCAO), with a consequent histone-deacetylation of ncx3 promoter. Conversely, the tMCAO-induced NCX3 reduction was prevented by intracerebroventricular injection of siDREAM, siHDAC4, and siHDAC5. Notably, MC1568 prevented oxygen glucose deprivation plus reoxygenation and tMCAO-induced neuronal damage, whereas its neuroprotective effect was abolished by ncx3 knockdown. Collectively, we found that: (1) DREAM/HDAC4/HDAC5 complex epigenetically down-regulates ncx3 gene transcription after stroke, and (2) pharmacological inhibition of class IIa HDACs reduces stroke-induced neurodetrimental effects.

TGF-β Signaling Regulates SLC8A3 Expression and Prevents Oxidative Stress in Developing Midbrain Dopaminergic and Dorsal Raphe Serotonergic Neurons

Calcium homeostasis is a cellular process required for proper cell function and survival, maintained by the coordinated action of several transporters, among them members of the Na⁺/Ca²⁺-exchanger family, such as SLC8A3. Transforming growth factor beta (TGF-β) signaling defines neuronal development and survival and may regulate the expression of channels and transporters. We investigated the regulation of SLC8A3 by TGF-β in a conditional knockout mouse with deletion of TGF-β signaling from Engrailed 1-expressing cells, i.e., in cells from the midbrain and rhombomere 1, and elucidated the underlying molecular mechanisms. The results show that SLC8A3 is significantly downregulated in developing dopaminergic and dorsal raphe serotonergic neurons in mutants and that low SLC8A3 abundance prevents the expression of the anti-apoptotic protein Bcl-xL. TGF-β signaling affects SLC8A3 via the canonical and p38 signaling pathway and may increase the binding of Smad4 to the Slc8a3 promoter. Expression of the lipid peroxidation marker malondialdehyde (MDA) was increased following knockdown of Slc8a3 expression in vitro. In neurons lacking TGF-β signaling, the number of MDA- and 4-hydroxynonenal (4-HNE)-positive cells was significantly increased, accompanied with increased cellular 4-HNE abundance. These results suggest that TGF-β contributes to the regulation of SLC8A3 expression in developing dopaminergic and dorsal raphe serotonergic neurons, thereby preventing oxidative stress.

Sodium/calcium exchanger as main effector of endogenous neuroprotection elicited by ischemic tolerance

The ischemic tolerance (IT) paradigm represents a fundamental cell response to certain types or injury able to render an organ more "tolerant" to a subsequent, stronger, insult. During the 16th century, the toxicologist Paracelsus described for the first time the possibility that a noxious event might determine a state of tolerance. This finding was summarized in one of his most important mentions: "The dose makes the poison". In more recent years, ischemic tolerance in the brain was first described in 1991, when it was demonstrated by Kirino and collaborators that two minutes of subthreshold brain ischemia in gerbils produced tolerance against global brain ischemia. Based on the time in which the conditioning stimulus is applied, it is possible to define preconditioning, perconditioning and postconditioning, when the subthreshold insult is applied before, during or after the ischemic event, respectively. Furthermore, depending on the temporal delay from the ischemic event, two different modalities are distinguished: rapid or delayed preconditioning and postconditioning. Finally, the circumstance in which the conditioning stimulus is applied on an organ distant from the brain is referred as remote conditioning. Over the years the "conditioning" paradigm has been applied to several brain disorders and a number of molecular mechanisms taking part to these protective processes have been described. The mechanisms are usually classified in three distinct categories identified as triggers, mediators and effectors. As concerns the putative effectors, it has been hypothesized that brain cells appear to have the ability to adapt to hypoxia by reducing their energy demand through modulation of ion channels and transporters, which delays anoxic depolarization. The purpose of the present review is to summarize the role played by plasmamembrane proteins able to control ionic homeostasis in mediating protection elicited by brain conditioning, particular attention will be deserved to the role played by Na⁺/Ca²⁺ exchanger.

Acute and long-term NCX activation reduces brain injury and restores behavioral functions in mice subjected to neonatal brain ischemia

Hypoxic-ischemic encephalopathy (HI) accounts for the majority of developmental, motor and cognitive deficits in children, leading to life-long neurological impairments. Since the plasmamembrane sodium/calcium exchanger (NCX) plays a fundamental role in maintaining ionic homeostasis during adult brain ischemia, in the present work we aimed to demonstrate (1)the involvement of NCX in the pathophysiology of neonatal HI and (2)a possible NCX-based pharmacological intervention. HI was induced in neonatal mice at postnatal day 7(P7) by unilateral cut of the right common carotid artery, followed by 60 min exposure to 8%O₂. Expression profiles of NCX isoforms from embryos stage to adulthood was evaluated in the hippocampus of hypoxic-ischemic and control mice. To assess the effect of NCX pharmacological stimulation, brain infarct volume was evaluated in brain sections, obtained at several time intervals after systemic administration of the newly synthesized NCX activator neurounina. Moreover, the long term effect of NCX activation was evaluated in adult mice (P60) subjected to neonatal HI and daily treated with neurounina for three weeks. Hypoxic-ischemic insult induced a reduction of NCX1 and NCX3 expression starting from day 7 until day 60. Notably, 8 weeks after HI induction in P7 mice, NCX pharmacological stimulation not only reduced infarct volume but improved also motor behaviour, spatial and visual memory. The present study highlights the significant role of NCX in the evolution of neonatal brain injury and in the learning and memory processes that are impaired in mice injured in the neonatal period.

Ionic homeostasis in brain conditioning

Most of the current focus on developing neuroprotective therapies is aimed at preventing neuronal death. However, these approaches have not been successful despite many years of clinical trials mainly because the numerous side effects observed in humans and absent in animals used at preclinical level. Recently, the research in this field aims to overcome this problem by developing strategies which induce, mimic, or boost endogenous protective responses and thus do not interfere with physiological neurotransmission. Preconditioning is a protective strategy in which a subliminal stimulus is applied before a subsequent harmful stimulus, thus inducing a state of tolerance in which the injury inflicted by the challenge is mitigated. Tolerance may be observed in ischemia, seizure, and infection. Since it requires protein synthesis, it confers delayed and temporary neuroprotection, taking hours to develop, with a pick at 1–3 days. A new promising approach for neuroprotection derives from post-conditioning, in which neuroprotection is achieved by a modified reperfusion subsequent to a prolonged ischemic episode. Many pathways have been proposed as plausible mechanisms to explain the neuroprotection offered by preconditioning and post-conditioning. Although the mechanisms through which these two endogenous protective strategies exert their effects are not yet fully understood, recent evidence highlights that the maintenance of ionic homeostasis plays a key role in propagating these neuroprotective phenomena. The present article will review the role of protein transporters and ionic channels involved in the control of ionic homeostasis in the neuroprotective effect of ischemic preconditioning and post-conditioning in adult brain, with particular regards to the Na⁺/Ca2⁺ exchangers (NCX), the plasma membrane Ca2⁺-ATPase (PMCA), the Na⁺/H⁺ exchange (NHE), the Na⁺/K⁺/2Cl⁻ cotransport (NKCC) and the acid-sensing cation channels (ASIC). Ischemic stroke is the third leading cause of death and disability. Up until now, all clinical trials testing potential stroke neuroprotectants failed. For this reason attention of researchers has been focusing on the identification of brain endogenous neuroprotective mechanisms activated after cerebral ischemia. In this context, ischemic preconditioning and ischemic post-conditioning represent two neuroprotecive strategies to investigate in order to identify new molecular target to reduce the ischemic damage.

Transcriptional regulation of ncx1 gene in the brain

The ubiquitous sodium-calcium exchanger isoform 1 (NCX1) is a -bidirectional transporter that plays a relevant role under physiological and pathophysiological conditions including brain ischemia by regulating intraneuronal Ca(2+) and Na(+) homeostasis. Although changes in ncx1 protein and transcript expression have been detected during stroke, its transcriptional regulation is still largely unexplored. Here, we reviewed our recent findings on several transcription factors including cAMP response element-binding protein (CREB), nuclear factor kappa B (NF-κB), and hypoxia-inducible factor-1 (HIF-1) in the control of the ncx1 gene expression in neuronal cells.

The NCX3 isoform of the Na+/Ca2+ exchanger contributes to neuroprotection elicited by ischemic postconditioning

It has been recently shown that a short sublethal brain ischemia subsequent to a prolonged harmful ischemic episode may confer ischemic neuroprotection, a phenomenon termed ischemic postconditioning. Na(+)/Ca(2+) exchanger (NCX) isoforms, NCX1, NCX2, and NCX3, are plasma membrane ionic transporters widely distributed in the brain and involved in the control of Na(+) and Ca(2+) homeostasis and in the progression of stroke damage. The objective of this study was to evaluate the role of these three proteins in the postconditioning-induced neuroprotection. The NCX protein and mRNA expression was evaluated at different time points in the ischemic temporoparietal cortex of rats subjected to tMCAO alone or to tMCAO plus ischemic postconditioning. The results of this study showed that NCX3 protein and ncx3 mRNA were upregulated in those brain regions protected by postconditioning treatment. These changes in NCX3 expression were mediated by the phosphorylated form of the ubiquitously expressed serine/threonine protein kinase p-AKT, as the p-AKT inhibition prevented NCX3 upregulation. The relevant role of NCX3 during postconditioning was further confirmed by results showing that NCX3 silencing, induced by intracerebroventricular infusion of small interfering RNA (siRNA), partially reverted the postconditioning-induced neuroprotection. The results of this study support the idea that the enhancement of NCX3 expression and activity might represent a reasonable strategy to reduce the infarct extension after stroke.

Effect of +36T>C in intron 1 on the glutamine: fructose-6-phosphate amidotransferase 1 gene and its contribution to type 2 diabetes in different populations

Glutamine: fructose-6-phosphate amidotransferase 1 (GFPT1) acts as a rate-limiting enzyme in the hexosamine biosynthetic pathway, which is an alternative branch of glucose metabolism. To evaluate GFPT1 as a susceptibility gene to type 2 diabetes, we surveyed the polymorphisms related with the gene function of GFPT1 and assessed its contribution to type 2 diabetes with a case-control association study. Screening of the 5'-flanking and all coding regions of GFPT1 revealed eight polymorphisms, one in the 5'-flanking region, one synonymous polymorphism in exon 8, five in introns and one in 3'-UTR, but no mis-sense or non-sense polymorphism. With in silico simulation, a putative promoter region was apparently predicted between 1 kb upstream and 1 kb downstream of the start codon. In this region, +36T>C polymorphism was located on the GC box sequence in intron 1, and its functional effect on promoter activity was confirmed by luciferase reporter assay, introducing a new functional polymorphism of the GFPT1 gene. To examine its association with type 2 diabetes, we analyzed 2,763 Japanese (1,461 controls and 1,302 cases) and 330 Caucasians (190 controls and 140 cases). One possible association of +36T>C was observed in Caucasians, but no association of polymorphisms including +36T>C in intron 1 or haplotypes was observed in Japanese. Although we could not completely rule out a contribution to specific sub-groups or other populations, genetic variation of GFPT1 is unlikely to have a major role in the susceptibility to type 2 diabetes in Japanese.

Downstream regulatory element antagonist modulator regulates Ca2+ homeostasis and viability in cerebellar neurons

The Na+/Ca2+ exchangers NCX1, NCX2, and NCX3 are vital for the control of cellular Ca2+ homeostasis. Here, we show that a doublet of downstream regulatory element sites in the promoter of the NCX3 gene mediates transcriptional repression of NCX3 by the Ca2+-modulated transcriptional repressor downstream regulatory element antagonist modulator (DREAM). Overexpression of a DREAM EF-hand mutant insensitive to Ca2+ (EFmDREAM) in hippocampus and cerebellum of transgenic mice significantly reduced NCX3 mRNA and protein levels without modifying NCX1 and NCX2 expression. Cerebellar granules from EFmDREAM transgenic mice showed increased levels of cytosolic Ca2+ and were more vulnerable to increased Ca2+ influx after partial opening of voltage-gated plasma membrane Ca2+ channels induced by increasing K+ in the culture medium but survived better in the conditions of reduced Ca2+ influx prevailing in low extracellular K+. Overexpression of NCX3 in EFmDREAM transgenic granules using a lentiviral vector restored the normal survival response to high K+ observed in wild-type granules. Thus, the downregulation of the regulator of Ca2+ homeostasis NCX3 by Ca2+-regulated DREAM is a striking example of the autoregulatory property of the Ca2+ signal in neurons.

Calmodulin and Calmodulin-dependent Kinase IIα Regulate Osteoblast Differentiation by Controlling c-fos Expression

Ca(2+)/calmodulin-dependent protein kinase IIalpha (alpha-CaMKII) was once thought to be exclusively expressed in neuronal tissue, but it is becoming increasingly evident that CaMKII is also expressed in various extraneural cells. CaMKII plays a critical role in regulating various signaling pathways leading to modulation of several aspects of cellular functions, including proliferation, differentiation, cytoskeletal structure, and gene expression. The purpose of this study was to examine the expression of CaMKII in osteoblast-like cells (MC4) and to elucidate its role in osteoblast differentiation. We demonstrated that CaMKII, specifically the alpha isoform, is expressed in osteoblasts both in vitro and in vivo. Inhibition of CaMKII by the calmodulin antagonist trifluoperazine or the CaMKII antagonist KN93 reduces alkaline phosphatase activity and mineralization, as well as causes 85 and 56% decreases in alkaline phosphatase and osteocalcin gene expression, respectively. CaM and CaMKII antagonists, using the newborn mouse calvaria in vivo model, cause a 50% decrease in osteoblast number (N.Ob-BS) and a 32% decrease in mineralization (BV/TV). Pharmacologic and genetic inhibition of alpha-CaMKII by using trifluoperazine, KN93, and alpha-CaMKII small interfering RNA decreases the phosphorylation of ERK and of cAMP-response element-binding protein, leading to a significant decrease in the transactivation of serum response element and cAMP-response element. Inhibition of alpha-CaMKII decreases the expression of c-fos, AP-1 transactivation, and AP-1 DNA binding activity. Our findings demonstrated that alpha-CaMKII is expressed in osteoblasts and is involved in c-fos expression via regulation of serum response element and cAMP-response element. Inhibition of alpha-CaMKII results in a decrease in c-fos expression and AP-1 activation, leading to inhibition of osteoblast differentiation.

Transcriptional regulation by cAMP and Ca2+ links the Na+/Ca2+ exchanger 3 to memory and sensory pathways

The signaling cascades triggered by neurotrophins such as BDNF and by several neurotransmitters and hormones lead to the rapid induction of gene transcription by increasing the intracellular concentration of cAMP and Ca2+. This review examines the mechanisms by which these second messengers control transcriptional initiation at CRE promoters via transcription factor CREB, as well as at DRE sites via transcriptional repressor DREAM. The regulation of the SLC8A3 gene encoding the Na+/Ca2+ exchanger 3 (NCX3) is taken as an example to illustrate both mechanisms since it includes a CRE site in the promoter and several DRE sites in the exon 1 sequence. The upregulation of the NCX3 by Ca2+ signals may be specifically required to establish the Ca2+ balance that regulates several physiological and pathological processes in neurons. The regulatory features and the expression pattern of SLC8A3 gene suggest that NCX3 activity could be crucial in neuronal functions such as memory formation and sensory processing.

The human SLC8A3 gene and the tissue-specific Na+/Ca2+ exchanger 3 isoforms

We have identified the human gene for member 3 of Solute Carrier family 8 (SLC8A3) by bioinformatic analysis of human genomic sequences. The gene is located on chromosome 14q24.2, and spans a region of about 150 kb. The full-length DNA complementary to RNA encoding the Na(+)/Ca(2+) exchanger isoform 3 (NCX3), amplified by reverse transcriptase-polymerase chain reaction (RT-PCR) from the human neuroblastoma SH-SY5Y RNA, includes seven exons and encodes a protein of about 100 kDa. RT-PCR analysis was performed in different tissues to determine the exon composition in the region encoding the large intracellular loop of the protein. The region underwent modifications by alternative tissue-specific splicing. NCX3.2, including exon 4 but not exon 5, was found in human brain and in the neuroblastoma cell line. In human skeletal muscle two additional isoforms were identified: NCX3.3, including exons 4 and 5, and a truncated isoform (NCX3.4) produced by the skipping of both exons 3 and 4. The skipping causes a frame shift downstream of the exon 2 sequence. The new coding sequence of 25 amino acids terminates with a stop codon in exon 6. The NCX3.4 isoform (68 kDa) is truncated in the C-terminal portion of the domain first found in Drosophila Na(+)/Ca(2+) exchanger domain (Calxbeta) and lacks the C-terminal hydrophobic segments.