CREB regulates the expression of type 1 inositol 1,4,5-trisphosphate receptors

Functional investigation of a putative calcium-binding site involved in the inhibition of inositol 1,4,5-trisphosphate receptor activity

A wide variety of factors influence inositol 1,4,5-trisphosphate (IP 3 ) receptor (IP 3 R) activity resulting in modulation of intracellular Ca 2+ release. This regulation is thought to define the spatio-temporal patterns of Ca 2+ signals necessary for the appropriate activation of downstream effectors. The binding of both IP 3 and Ca 2+ are obligatory for IP 3 R channel opening, however, Ca 2+ regulates IP 3 R activity in a biphasic manner. Mutational studies have revealed that Ca 2+ binding to a high-affinity pocket formed by the ARM3 domain and linker domain promotes IP 3 R channel opening without altering the Ca 2+ dependency for channel inactivation. These data suggest a distinct low-affinity Ca 2+ binding site is responsible for the reduction in IP 3 R activity at higher [Ca 2+ ]. We determined the consequences of mutating a cluster of acidic residues in the ARM2 and central linker domain reported to coordinate Ca 2+ in cryo-EM structures of the IP 3 R type 3. This site is termed the "CD Ca 2+ binding site" and is well-conserved in all IP 3 R sub-types. We show that the CD site Ca 2+ binding mutants where the negatively charged glutamic acid residues are mutated to alanine exhibited enhanced sensitivity to IP 3 -generating agonists. Ca 2+ binding mutants displayed spontaneous elemental Ca 2+ events (Ca 2+ puffs) and the number of IP 3 -induced Ca 2+ puffs was significantly augmented in cells stably expressing Ca 2+ binding site mutants. When measured with "on-nucleus" patch clamp, the inhibitory effect of high [Ca 2+ ] on single channel-open probability (P o ) was reduced in mutant channels and this effect was dependent on [ATP]. These results indicate that Ca 2+ binding to the putative CD Ca 2+ inhibitory site facilitates the reduction in IP 3 R channel activation when cytosolic [ATP] is reduced and suggest that at higher [ATP], additional Ca 2+ binding motifs may contribute to the biphasic regulation of IP 3 -induced Ca 2+ release.

Molecular characterization of positively selected genes contributing aquatic adaptation in marine mammals

BACKGROUND

Marine mammals, which have evolved independently into three distinct lineages, share common physiological features that contribute to their adaptation to the marine environment.

OBJECTIVE

To identify positively selected genes (PSGs) for adaptation to the marine environment using available genomic data from three taxonomic orders: cetaceans, pinnipeds, and sirenians.

METHODS

Based on the genomes within each group of Artiodactyla, Carnivora and Afrotheria, we performed selection analysis using the branch-site model in CODEML.

RESULTS

Based on the branch-site model, 460, 614, and 359 PSGs were predicted for the cetaceans, pinnipeds, and sirenians, respectively. Functional enrichment analysis indicated that genes associated with hemostasis were positively selected across all lineages of marine mammals. We observed positive selection signals for the hemostasis and coagulation-related genes plasminogen activator, urokinase (PLAU), multimerin 1 (MMRN1), gamma-glutamyl carboxylase (GGCX), and platelet endothelial aggregation receptor 1 (PEAR1). Additionally, we found out that the sodium voltage-gated channel alpha subunit 9 (SCN9A), serine/arginine repetitive matrix 4 (SRRM4), and Ki-ras-induced actin-interacting protein (KRAP) are under positive selection pressure and are associated with cognition, neurite outgrowth, and IP3-mediated Ca2 + release, respectively.

CONCLUSION

This study will contribute to our understanding of the adaptive evolution of marine mammals by providing information on a group of candidate genes that are predicted to influence adaptation to aquatic environments, as well as their functional characteristics.

Two-pore channel-2 and inositol trisphosphate receptors coordinate Ca2+ signals between lysosomes and the endoplasmic reticulum

Lysosomes and the endoplasmic reticulum (ER) are Ca2+ stores mobilized by the second messengers NAADP and IP3, respectively. Here, we establish Ca2+ signals between the two sources as fundamental building blocks that couple local release to global changes in Ca2+. Cell-wide Ca2+ signals evoked by activation of endogenous NAADP-sensitive channels on lysosomes comprise both local and global components and exhibit a major dependence on ER Ca2+ despite their lysosomal origin. Knockout of ER IP3 receptor channels delays these signals, whereas expression of lysosomal TPC2 channels accelerates them. High-resolution Ca2+ imaging reveals elementary events upon TPC2 opening and signals coupled to IP3 receptors. Biasing TPC2 activation to a Ca2+-permeable state sensitizes local Ca2+ signals to IP3. This increases the potency of a physiological agonist to evoke global Ca2+ signals and activate a downstream target. Our data provide a conceptual framework to understand how Ca2+ release from physically separated stores is coordinated.

Mitigating transcriptional bottleneck using a constitutively active transcription factor, VP16-CREB, in mammalian cells

High-level expression of recombinant proteins in mammalian cells has long been an area of interest. Inefficient transcription machinery is often an obstacle in achieving high-level expression of recombinant proteins in mammalian cells. Synthetic promoters have been developed to improve the transcription efficiency, but have achieved limited success due to the limited availability of transcription factors (TFs). Here, we present a TF-engineering approach to mitigate the transcriptional bottlenecks of recombinant proteins. This includes: (i) identification of cAMP response element binding protein (CREB) as a candidate TF by searching for TFs enriched in the cytomegalovirus (CMV) promoter-driven high-producing recombinant Chinese hamster ovary (rCHO) cell lines via transcriptome analysis, (ii) confirmation of transcriptional limitation of active CREB in rCHO cell lines, and (iii) direct activation of the transgene promoter by expressing constitutively active CREB at non-cytotoxic levels in rCHO cell lines. With the expression of constitutively active VP16-CREB, the production of therapeutic proteins, such as monoclonal antibody and etanercept, in CMV promoter-driven rCHO cell lines was increased up to 3.9-fold. VP16-CREB was also used successfully with synthetic promoters containing cAMP response elements. Taken together, this strategy to introduce constitutively active TFs into cells is a useful means of overcoming the transcriptional limitations in recombinant mammalian cells.

Cyclic-AMP response element binding protein (CREB) and microRNA miR-29b regulate renalase gene expression under catecholamine excess conditions

AIMS

Renalase, a key mediator of cross-talk between kidneys and sympathetic nervous system, exerts protective roles in various cardiovascular/renal disease states. However, molecular mechanisms underpinning renalase gene expression remain incompletely understood. Here, we sought to identify the key molecular regulators of renalase under basal/catecholamine-excess conditions.

MATERIALS AND METHODS

Identification of the core promoter domain of renalase was carried out by promoter-reporter assays in N2a/HEK-293/H9c2 cells. Computational analysis of the renalase core promoter domain, over-expression of cyclic-AMP-response-element-binding-protein (CREB)/dominant negative mutant of CREB, ChIP assays were performed to determine the role of CREB in transcription regulation. Role of the miR-29b-mediated-suppression of renalase was validated in-vivo by using locked-nucleic-acid-inhibitors of miR-29. qRT-PCR and Western-blot analyses measured the expression of renalase, CREB, miR-29b and normalization controls in cell lysates/ tissue samples under basal/epinephrine-treated conditions.

KEY FINDINGS

CREB, a downstream effector in epinephrine signaling, activated renalase expression via its binding to the renalase-promoter. Physiological doses of epinephrine and isoproteronol enhanced renalase-promoter activity and endogenous renalase protein level while propranolol diminished the promoter activity and endogenous renalase protein level indicating a potential role of beta-adrenergic receptor in renalase gene regulation. Multiple animal models (acute exercise, genetically hypertensive/stroke-prone mice/rat) displayed directionally-concordant expression of CREB and renalase. Administration of miR-29b inhibitor in mice upregulated endogenous renalase expression. Moreover, epinephrine treatment down-regulated miR-29b promoter-activity/transcript levels.

SIGNIFICANCE

This study provides evidence for renalase gene regulation by concomitant transcriptional activation via CREB and post-transcriptional attenuation via miR-29b under excess epinephrine conditions. These findings have implications for disease states with dysregulated catecholamines.

IP3R at ER-Mitochondrial Contact Sites: Beyond the IP3R-GRP75-VDAC1 Ca2+ Funnel

Membrane contact sites (MCS) circumvent the topological constraints of functional coupling between different membrane-bound organelles by providing a means of communication and exchange of materials. One of the most characterised contact sites in the cell is that between the endoplasmic reticulum and the mitochondrial (ERMCS) whose function is to couple cellular Ca2+ homeostasis and mitochondrial function. Inositol 1,4,5-trisphosphate receptors (IP3Rs) on the ER, glucose-regulated protein 75 (GRP 75) and voltage-dependent anion channel 1 (VDAC1) on the outer mitochondrial membrane are the canonical component of the Ca2+ transfer unit at ERMCS. These are often reported to form a Ca2+ funnel that fuels the mitochondrial low-affinity Ca2+ uptake system. We assess the available evidence on the IP3R subtype selectivity at the ERMCS and consider if IP3Rs have other roles at the ERMCS beyond providing Ca2+. Growing evidence suggests that all three IP3R subtypes can localise and regulate Ca2+ signalling at ERMCS. Furthermore, IP3Rs may be structurally important for assembly of the ERMCS in addition to their role in providing Ca2+ at these sites. Evidence that various binding partners regulate the assembly and Ca2+ transfer at ERMCS populated by IP3R-GRP75-VDAC1, suggesting that cells have evolved mechanisms that stabilise these junctions forming a Ca2+ microdomain that is required to fuel mitochondrial Ca2+ uptake.

Functional determination of calcium-binding sites required for the activation of inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate receptors (IP3Rs) initiate a diverse array of physiological responses by carefully orchestrating intracellular calcium (Ca2+) signals in response to various external cues. Notably, IP3R channel activity is determined by several obligatory factors, including IP3, Ca2+, and ATP. The critical basic amino acid residues in the N-terminal IP3-binding core (IBC) region that facilitate IP3 binding are well characterized. In contrast, the residues conferring regulation by Ca2+ have yet to be ascertained. Using comparative structural analysis of Ca2+-binding sites identified in two main families of intracellular Ca2+-release channels, ryanodine receptors (RyRs) and IP3Rs, we identified putative acidic residues coordinating Ca2+ in the cytosolic calcium sensor region in IP3Rs. We determined the consequences of substituting putative Ca2+ binding, acidic residues in IP3R family members. We show that the agonist-induced Ca2+ release, single-channel open probability (P0), and Ca2+ sensitivities are markedly altered when the negative charge on the conserved acidic side chain residues is neutralized. Remarkably, neutralizing the negatively charged side chain on two of the residues individually in the putative Ca2+-binding pocket shifted the Ca2+ required to activate IP3R to higher concentrations, indicating that these residues likely are a component of the Ca2+ activation site in IP3R. Taken together, our findings indicate that Ca2+ binding to a well-conserved activation site is a common underlying mechanism resulting in increased channel activity shared by IP3Rs and RyRs.

Segregated cation flux by TPC2 biases Ca2+ signaling through lysosomes

Two-pore channels are endo-lysosomal cation channels with malleable selectivity filters that drive endocytic ion flux and membrane traffic. Here we show that TPC2 can differentially regulate its cation permeability when co-activated by its endogenous ligands, NAADP and PI(3,5)P₂. Whereas NAADP rendered the channel Ca²⁺-permeable and PI(3,5)P₂ rendered the channel Na⁺-selective, a combination of the two increased Ca²⁺ but not Na⁺ flux. Mechanistically, this was due to an increase in Ca²⁺ permeability independent of changes in ion selectivity. Functionally, we show that cell permeable NAADP and PI(3,5)P₂ mimetics synergistically activate native TPC2 channels in live cells, globalizing cytosolic Ca²⁺ signals and regulating lysosomal pH and motility. Our data reveal that flux of different ions through the same pore can be independently controlled and identify TPC2 as a likely coincidence detector that optimizes lysosomal Ca²⁺ signaling.

TPC2 is a lysosomal ion channel permeable to both calcium and sodium ions. Here, the authors show that TPC2 can selectively increase its calcium permeability when simultaneously challenged by both its natural activators- NAADP and PI(3,5)P₂.

First person – Vikas Arige

First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping early-career researchers promote themselves alongside their papers. Vikas Arige is first author on ‘ CREB regulates the expression of type 1 inositol 1,4,5-trisphosphate receptors’, published in JCS. Vikas is a postdoctoral associate in the lab of Dr David I. Yule at the University of Rochester, NY, USA, investigating the role of CREB in the regulation of type 1 inositol 1,4,5-trisphosphate receptors.