Acidosis-induced V-ATPase trafficking in salivary ducts is initiated by cAMP/PKA/CREB pathway via regulation of Rab11b expression

Regulation of electrogenic Na+ /HCO3 - cotransporter 1 (NBCe1) function and its dependence on m-TOR mediated phosphorylation of Ser245

Astrocytes are pivotal responders to alterations of extracellular pH, primarily by regulation of their principal acid-base transporter, the membrane-bound electrogenic Na⁺ /bicarbonate cotransporter 1 (NBCe1). Here, we describe amammalian target of rapamycin (mTOR)-dependent and NBCe1-mediated astroglial response to extracellular acidosis. Using primary mouse cortical astrocytes, we investigated the effect of long-term extracellular metabolic acidosis on regulation of NBCe1 and elucidated the underlying molecular mechanisms by immunoblotting, biotinylation of surface proteins, intracellular H⁺ recording using the H⁺ -sensitive dye 2',7'-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein, and phosphoproteomic analysis. The results showed significant increase of NBCe1-mediated recovery of intracellular pH from acidification in WT astrocytes, but not in cortical astrocytes from NBCe1-deficient mice. Acidosis-induced upregulation of NBCe1 activity was prevented following inhibition of mTOR signaling by rapamycin. Yet, during acidosis or following exposure of astrocytes to rapamycin, surface protein abundance of NBCe1 remained -unchanged. Mutational analysis in HeLa cells suggested that NBCe1 activity was dependent on phosphorylation state of Ser²⁴⁵ , a residue conserved in all NBCe1 variants. Moreover, phosphorylation state of Ser²⁴⁵ is regulated by mTOR and is inversely correlated with NBCe1 transport activity. Our results identify pSer²⁴⁵ as a novel regulator of NBCe1 functional expression. We propose that context-dependent and mTOR-mediated multisite phosphorylation of serine residues of NBCe1 is likely to be a potent mechanism contributing to the response of astrocytes to acid/base challenges during pathophysiological conditions.

Systematical analysis reveals a strong cancer relevance of CREB1-regulated genes

The transcription factor cyclic-AMP response element-binding protein 1 (CREB1) responds to cAMP level and controls the expression of target genes, which regulates nutrition partitioning. The promoters of CREB1-targeted genes responsive to cAMP have been extensively investigated and characterized with the presence of both cAMP response element and TATA box. Compelling evidence demonstrates that CREB1 also plays an essential role in promoting tumor development. However, only very few genes required for cell survival, proliferation and migration are known to be constitutively regulated by CREB1 in tumors. Their promoters mostly do not harbor any cAMP response element. Thus, it is very likely that CREB1 regulates the expressions of distinct sets of target genes in normal tissues and tumors. The whole gene network constitutively regulated by CREB1 in tumors has remained unrevealed. Here, we employ a systematical and integrative approach to decipher this gene network in the context of both tissue cultured cancer cells and patient samples. We combine transcriptomic, Rank-Rank Hypergeometric Overlap, and Chipseq analysis, to define and characterize CREB1-regulated genes in a multidimensional fashion. A strong cancer relevance of those top-ranked targets, which meet the most stringent criteria, is eventually verified by overall survival analysis of cancer patients. These findings strongly suggest the importance of genes constitutively regulated by CREB1 for their implicative involvement in promoting tumorigenesis.

Cimetidine-induced androgenic failure causes cell death and changes in actin, EGF and V-ATPase immunoexpression in rat submandibular glands

Submandibular gland (SMG) is responsive to androgens via androgen receptor (AR). We verified whether cimetidine induces androgenic dysfunction in SMG, and evaluated the structural integrity, cell death and immunoexpression of actin, EGF and V-ATPase in androgen-deficient SMG. Male rats received cimetidine (CMTG) and control animals (CG) received saline. Granular convoluted tubules (GCTs) diameter and number of acinar cell nuclei were evaluated. TUNEL and immunofluorescence reactions for detection of AR, testosterone, actin, EGF and V-ATPase were quantitatively analysed. In CG, testosterone immunolabelling was detected in acinar and ductal cells cytoplasm. AR-immunolabelled nuclei were observed in acinar cells whereas ductal cells showed AR-immunostained cytoplasm, indicating a non-genomic AR action. In CMTG, the weak testosterone and AR immunoexpression confirmed cimetidine-induced androgenic failure. A high cell death index was correlated with decreased number of acinar cells, GCTs diameter and EGF immunoexpression under androgenic dysfunction. Actin immunofluorescence decreased in the SMG cells, but an increased and diffuse cytoplasmic V-ATPase immunolabelling was observed in striated ducts, suggesting a disruption in the actin-dependent V-ATPase recycling due to androgenic failure. Our findings reinforce the androgenic role in the maintenance of SMG histophysiology, and point to a potential clinical use of cimetidine against androgen-dependent glandular tumour cells.

Rab11B participates in erythrocyte storage lesion of under-collected whole blood

BACKGROUND AND OBJECTIVES

The storage lesion of the red blood cell affects the life span of RBC and the quality of blood component. The elucidation of this mechanism is helpful to reduce the storage damage of RBC and improve the efficacy and safety of blood transfusion. The aim of this study was to discover the potential molecular mechanism of erythrocyte storage lesion with Under-collected whole blood (UC-WB) model.

METHODS

The label-free MS/MS quantitative method was used to identify the differential proteins of erythrocyte membrane proteins and the difference of Rab11B, V-ATPase and plasma GDI2 protein expression were further verified by western blot at the end of blood storage.

RESULTS

A total of 12 Rab proteins and 3 interacting effector proteins were identified among the membrane protein of normal WB and UC-WB, including 5 differential Rab proteins and 2 interacting effector proteins. Compared with normal WB, the expression of membrane Rab11B protein and ATP6V1B1/2 subunit of V-ATPases protein as well as the plasma GDI2 protein of UC-WB increased at the end of storage period.

CONCLUSION

Rab protein might be related to RBC storage lesions, Rab11B participates in the RBC storage lesion through Rab11B/V-ATPases pathways.

Crustacean hyperglycemic hormone (CHH) regulates the ammonia excretion and metabolism in white shrimp, Litopenaeus vannamei under ammonia-N stress

To understand the adaptation of Litopenaeus vannamei to high environmental ammonia-N, RNA interference was used to investigate the function of crustacean hyperglycemic hormone (CHH) in the physiological process of neuroendocrine signaling transduction, and ammonia excretion and metabolism. The shrimp were exposed to 25 mg/L NH₄Cl and injected with 20 μg/shrimp CHH dsRNA for 72 h. The results showed that hemolymph ammonia content increased under ammonia-N stress and further increased after CHH knockdown, suggesting that CHH can promote ammonia excretion. Moreover, after CHH knockdown, the levels of CHH, DA, and Wnts decreased significantly, the expression of receptor GC, DA1R, Frizzled and LRP 5/6 also decreased, while DA4R increased remarkably. Moreover, PKA and PKG decreased, while PKC markedly increased, and nuclear transcription factors (CREB and TCF) as well as effector proteins (β-catenin, FXYD2, and 14-3-3) were significantly downregulated. Furthermore, ammonia transporters Na⁺/K⁺-ATPase (NKA), K⁺channel, Rh protein, AQP, V-ATPase, and VAMP decreased significantly, while Na⁺/H⁺ exchangers (NHE) and Na⁺/K⁺/2Cl^- cotransporter (NKCC) increased significantly. These results suggest that CHH regulates ammonia excretion in three ways: 1) by mainly regulating ion channels via PKA, PKC, and PKG signaling pathways; 2) by activating related proteins primarily through Wnt signaling pathway; and 3) by exocytosis, mostly induced by the PKA signaling pathway. In addition, the levels of Gln, uric acid, and urea increased in accordance with the activities of GDH/GS, XDH, and arginase, respectively, suggesting that ammonia excretion was inhibited but ammonia metabolism was slightly enhanced. This study deepens our understanding of the mechanism by which crustaceans respond to high environmental ammonia-N.

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.

Differential regulation of vacuolar H+ -ATPase subunits by transforming growth factor-β1 in salivary ducts

Bicarbonate concentration in saliva is controlled by the action of acid-base transporters in salivary duct cells. We show for the first time expression of ATP6V1B1 in submandibular gland and introduce transforming growth factor-beta (TGF-β) as a novel regulator of V-ATPase subunits. Using QRT-PCR, immunoblotting, biotinylation of surface proteins, immunofluorescence, chromatin immunoprecipitation, and intracellular H(⁺ ) recording with H(⁺ )-sensitive dye 2',7'-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein we show that in the human submandibular gland (HSG) cell line, activation of TGF-β signaling upregulates ATP6V1E1 and ATP6V1B2, downregulates ATP6V1B1, and has no effect on ATP6V1A. TGF-β1 effects on ATP6V1B1 are mediated through the canonical, the soluble adenylate cyclase, and ERK signaling. A CREB binding sequence was identified in the ATP6V1B1 promoter and CREB binding decreased after TGF-β1 treatment. Following acidosis, a bafilomycin-sensitive and Na⁺ -independent cell pH recovery was observed in HSG cells, an effect that was not influenced after disruption of acidic lysosomes. Moreover, neutralization of TGF-βs, inhibition of TGF-β receptor, or inhibition of the canonical pathway decreased membrane expression of ATP6V1A and prevented the acidosis-induced increased V-ATPase activity. The results suggest multiple modes of action of TGF-β1 on V-ATPase subunits in HSG cells: TGF-β1 may regulate transcription or protein synthesis of certain subunits and trafficking of other subunits in a context-dependent manner. Moreover, surface V-ATPase is active in salivary duct cells and involved in intracellular pH regulation following acidosis.

K+-Cl- co-transporter 2 (KCC2) - a membrane trafficking perspective

K⁺-Cl^- co-transporter 2 (KCC2/SLC12A5) is a neuronal specific cation chloride co-transporter which is active under isotonic conditions, and thus a key regulator of intracellular Cl^- levels. It also has an ion transporter-independent structural role in modulating the maturation and regulation of excitatory glutamatergic synapses. KCC2 levels are developmentally regulated, and a postnatal upregulation of KCC2 generates a low intracellular chloride concentration that allows the neurotransmitters γ-aminobutyric acid (GABA) and glycine to exert inhibitory neurotransmission through its Cl^- permeating channel. Functional expression of KCC2 at the neuronal cell surface is necessary for its activity, and impairment in KCC2 cell surface transport and/or internalization may underlie a range of neuropathological conditions. Although recent advances have shed light on a range of cellular mechanisms regulating KCC2 activity, little is known about its membrane trafficking itinerary and regulatory proteins. In this review, known membrane trafficking signals, pathways and mechanisms pertaining to KCC2's functional surface expression are discussed.

TGF-β signaling directly regulates transcription and functional expression of the electrogenic sodium bicarbonate cotransporter 1, NBCe1 (SLC4A4), via Smad4 in mouse astrocytes

The electrogenic sodium bicarbonate cotransporter NBCe1 (SLC4A4) expressed in astrocytes regulates intracellular and extracellular pH. Here, we introduce transforming growth factor beta (TGF-β) as a novel regulator of NBCe1 transcription and functional expression. Using hippocampal slices and primary hippocampal and cortical astrocyte cultures, we investigated regulation of NBCe1 and elucidated the underlying signaling pathways by RT-PCR, immunoblotting, immunofluorescence, intracellular H(+ ) recording using the H(+ ) -sensitive dye 2',7'-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein, mink lung epithelial cell (MLEC) assay, and chromatin immunoprecipitation. Activation of TGF-β signaling significantly upregulated transcript, protein, and surface expression of NBCe1. These effects were TGF-β receptor-mediated and suppressed following inhibition of JNK and Smad signaling. Moreover, 4-aminopyridine (4AP)-dependent NBCe1 regulation requires TGF-β. TGF-β increased the rate and amplitude of intracellular H+ changes upon challenging NBCe1 in wild-type astrocytes but not in cortical astrocytes from Slc4a4-deficient mice. A Smad4 binding sequence was identified in the NBCe1 promoter and Smad4 binding increased after activation of TGF-β signaling. The data show for the first time that NBCe1 is a direct target of TGF-β/Smad4 signaling. Through activation of the canonical pathway TGF-β acts directly on NBCe1 by binding of Smad4 to the NBCe1 promoter and regulating its transcription, followed by increased protein expression and transport activity.

The membrane trafficking and functionality of the K+-Cl- co-transporter KCC2 is regulated by TGF-β2

Functional activation of the neuronal K(+)-Cl(-) co-transporter KCC2 (also known as SLC12A5) is a prerequisite for shifting GABAA responses from depolarizing to hyperpolarizing during development. Here, we introduce transforming growth factor β2 (TGF-β2) as a new regulator of KCC2 membrane trafficking and functional activation. TGF-β2 controls membrane trafficking, surface expression and activity of KCC2 in developing and mature mouse primary hippocampal neurons, as determined by immunoblotting, immunofluorescence, biotinylation of surface proteins and KCC2-mediated Cl(-) extrusion. We also identify the signaling pathway from TGF-β2 to cAMP-response-element-binding protein (CREB) and Ras-associated binding protein 11b (Rab11b) as the underlying mechanism for TGF-β2-mediated KCC2 trafficking and functional activation. TGF-β2 increases colocalization and interaction of KCC2 with Rab11b, as determined by 3D stimulated emission depletion (STED) microscopy and co-immunoprecipitation, respectively, induces CREB phosphorylation, and enhances Rab11b gene expression. Loss of function of either CREB1 or Rab11b suppressed TGF-β2-dependent KCC2 trafficking, surface expression and functionality. Thus, TGF-β2 is a new regulatory factor for KCC2 functional activation and membrane trafficking, and a putative indispensable molecular determinant for the developmental shift of GABAergic transmission.

sAC from aquatic organisms as a model to study the evolution of acid/base sensing

Soluble adenylyl cyclase (sAC) is poised to play multiple physiological roles as an acid/base (A/B) sensor in aquatic organisms. Many of these roles are probably similar to those in mammals; a striking example is the evolutionary conservation of a mechanism involving sAC, carbonic anhydrase and vacuolar H⁺-ATPase that acts as a sensor system and regulator of extracellular A/B in shark gills and mammalian epididymis and kidney. Additionally, the aquatic environment presents unique A/B and physiological challenges; therefore, sACs from aquatic organisms have likely evolved distinct kinetic properties as well as distinct physiological roles. sACs from aquatic organisms offer an excellent opportunity for studying the evolution of A/B sensing at both the molecular and whole organism levels. Moreover, this information could help understand and predict organismal responses to environmental stress based on mechanistic models.This article is part of a Special Issue entitled "The Role of Soluble Adenylyl Cyclase in Health and Disease," guest edited by J. Buck and L. R. Levin.

Vacuolar H+-ATPase: An Essential Multitasking Enzyme in Physiology and Pathophysiology

Vacuolar H+-ATPases (V-ATPases) are large multisubunit proton pumps that are required for housekeeping acidification of membrane-bound compartments in eukaryotic cells. Mammalian V-ATPases are composed of 13 different subunits. Their housekeeping functions include acidifying endosomes, lysosomes, phagosomes, compartments for uncoupling receptors and ligands, autophagosomes, and elements of the Golgi apparatus. Specialized cells, including osteoclasts, intercalated cells in the kidney and pancreatic beta cells, contain both the housekeeping V-ATPases and an additional subset of V-ATPases, which plays a cell type specific role. The specialized V-ATPases are typically marked by the inclusion of cell type specific isoforms of one or more of the subunits. Three human diseases caused by mutations of isoforms of subunits have been identified. Cancer cells utilize V-ATPases in unusual ways; characterization of V-ATPases may lead to new therapeutic modalities for the treatment of cancer. Two accessory proteins to the V-ATPase have been identified that regulate the proton pump. One is the (pro)renin receptor and data is emerging that indicates that V-ATPase may be intimately linked to renin/angiotensin signaling both systemically and locally. In summary, V-ATPases play vital housekeeping roles in eukaryotic cells. Specialized versions of the pump are required by specific organ systems and are involved in diseases.

Rab11 as a modulator of synaptic transmission

Many neurodegenerative disorders are characterized by synaptic dysfunction preceding general neuronal loss and subsequent cognitive or behavioral anomalies. Much recent research has been aimed at understanding the early underlying processes leading to dysfunction at the synapse, as this knowledge would likely inform interventions that could potentially slow progression and delay onset of disease. We have recently reported that synaptic dysfunction in a Drosophila melanogaster model of Huntington's disease (HD) can be prevented by enhanced neuronal expression of Rab11, a Rab family GTPase involved in endosomal recycling, which complements studies that have found disrupted Rab11 activity in several models of this disorder. Indeed, inhibition of Rab11 function in fibroblasts of HD patients has been observed to perturb vesicle formation from recycling endosomes. Therefore, our study investigated a potential role of Rab11 in synaptic dysfunction prior to the onset of HD symptoms, with the aim of finding a possible early intervention to disease progression. We found that Rab11 ameliorates synaptic dysfunction due to expression of mutant huntingtin-the causative protein in HD-by normalizing synaptic vesicle size, which consequently ameliorates locomotor deficits in Drosophila larvae. Here we further consider these results and the implications this work has on potential therapeutic intervention in HD and other neurodegenerative disorders.

Functional vacuolar ATPase (V-ATPase) proton pumps traffic to the enterocyte brush border membrane and require CFTR

Vacuolar ATPases (V-ATPases) are highly conserved proton pumps that regulate organelle pH. Epithelial luminal pH is also regulated by cAMP-dependent traffic of specific subunits of the V-ATPase complex from endosomes into the apical membrane. In the intestine, cAMP-dependent traffic of cystic fibrosis transmembrane conductance regulator (CFTR) channels and the sodium hydrogen exchanger (NHE3) in the brush border regulate luminal pH. V-ATPase was found to colocalize with CFTR in intestinal CFTR high expresser (CHE) cells recently. Moreover, apical traffic of V-ATPase and CFTR in rat Brunner's glands was shown to be dependent on cAMP/PKA. These observations support a functional relationship between V-ATPase and CFTR in the intestine. The current study examined V-ATPase and CFTR distribution in intestines from wild-type, CFTR(-/-) mice and polarized intestinal CaCo-2BBe cells following cAMP stimulation and inhibition of CFTR/V-ATPase function. Coimmunoprecipitation studies examined V-ATPase interaction with CFTR. The pH-sensitive dye BCECF determined proton efflux and its dependence on V-ATPase/CFTR in intestinal cells. cAMP increased V-ATPase/CFTR colocalization in the apical domain of intestinal cells and redistributed the V-ATPase Voa1 and Voa2 trafficking subunits from the basolateral membrane to the brush border membrane. Voa1 and Voa2 subunits were localized to endosomes beneath the terminal web in untreated CFTR(-/-) intestine but redistributed to the subapical cytoplasm following cAMP treatment. Inhibition of CFTR or V-ATPase significantly decreased pHi in cells, confirming their functional interdependence. These data establish that V-ATPase traffics into the brush border membrane to regulate proton efflux and this activity is dependent on CFTR in the intestine.

Variants of the electrogenic sodium bicarbonate cotransporter 1 (NBCe1) in mouse hippocampal neurons are regulated by extracellular pH changes: evidence for a Rab8a-dependent mechanism

Changes in extracellular pH are common events in both pathological conditions and during normal brain function. In organs other than the brain, cells may respond to pH changes by trafficking of acid-base transporters. However, regulation of neuronal acid-base transporters during pH shifts is not understood. The aim of this study was to investigate regulatory mechanisms of the variants of the electrogenic sodium/bicarbonate cotransporter 1, NBCe1-A and NBCe1-B/C, in neurons following changes of extracellular pH. Therefore, primary mouse hippocampal neurons were exposed to extracellular acidosis or alkalosis. We show that acid-base changes regulated trafficking and membrane expression of neuronal NBCe1 but the underlying molecular cues were distinct for individual NBCe1 variants. Following extracellular acidosis NBCe1-A was recruited from intracellular pools to the plasma membrane, followed by increased membrane expression, whereas NBCe1-B/C was retrieved from the membrane. Extracellular alkalosis had no impact on NBCe1-A, but caused translocation of NBCe1-B/C toward the dendrites. We also show that acidosis-induced NBCe1-A, but not NBCe1-B/C, trafficking is mediated by Rab8a. Rab8a is expressed in hippocampal neurons, co-localizes, and interacts with NBCe1-A. Loss-of-function of Rab8a using specific siRNA prevented acidosis-induced redistribution of NBCe1-A. These data propose opposite recruitment pattern for NBCe1 variants in neurons following extracellular acid-base changes, implicating distinct physiological functions of individual NBCe1 variants, and introduce Rab8a as a novel molecular determinant and crucial mediator of acidosis-induced NBCe1 trafficking in neurons.