TRPP2 channels regulate apoptosis through the Ca2+ concentration in the endoplasmic reticulum

The membrane transporter SLC25A48 enables transport of choline into human mitochondria

Choline has important physiological functions as a precursor for essential cell components, signaling molecules, phospholipids, and the neurotransmitter acetylcholine. Choline is a water-soluble charged molecule requiring transport proteins to cross biological membranes. Although transporters continue to be identified, membrane transport of choline is incompletely understood and knowledge about choline transport into intracellular organelles such as mitochondria remains limited. Here we show that SLC25A48 imports choline into human mitochondria. Human loss-of-function mutations in SLC25A48 show impaired choline transport into mitochondria and are associated with elevated urine and plasma choline levels. Thus, our studies may have implications for understanding and treating conditions related to choline metabolism.

Construction of multileveled and oriented micro/nano channels in Mg doped hydroxyapitite bioceramics and their effect on mimicking mechanical property of cortical bone and biological performance of cancellous bone

Drawing on the structure and components of natural bone, this study developed Mg-doped hydroxyapatite (Mg-HA) bioceramics, characterized by multileveled and oriented micro/nano channels. These channels play a critical role in ensuring both mechanical and biological properties, making bioceramics suitable for various bone defects, particularly those bearing loads. Bioceramics feature uniformly distributed nanogrooves along the microchannels. The compressive strength or fracture toughness of the Mg-HA bioceramics with micro/nano channels formed by single carbon nanotube/carbon fiber (CNT/CF) (Mg-HA(05-CNT/CF)) are comparable to those of cortical bone, attributed to a combination of strengthened compact walls and microchannels, along with a toughening mechanism involving crack pinning and deflection at nanogroove intersections. The introduction of uniform nanogrooves also enhanced the porosity by 35.4 %, while maintaining high permeability owing to the capillary action in the oriented channels. This leads to superior degradation properties, protein adsorption, and in vivo osteogenesis compared with bioceramics with only microchannels. Mg-HA(05-CNT/CF) exhibited not only high strength and toughness comparable to cortical bone, but also permeability similar to cancellous bone, enhanced cell activity, and excellent osteogenic properties. This study presents a novel approach to address the global challenge of applying HA-based bioceramics to load-bearing bone defects, potentially revolutionizing their application in tissue engineering.

A new atypical splice mutation in PKD2 leading to autosomal dominant polycystic kidney disease in a Chinese family

INTRODUCTION

Autosomal dominant polycystic kidney disease (ADPKD) is a very common hereditary renal disorder. Mutations in PKD1 and PKD2 , identified as disease-causing genes, account for 85% and 15% of the ADPKD cases, respectively.

METHODS

In this study, the mutation analysis of polycystic kidney disease (PKD) genes was performed in a Chinese family with suspected ADPKD using targeted clinical exome sequencing (CES). The candidate pathogenic variants were further tested by using Sanger sequencing and validated for co-segregation. In addition, reverse transcription-polymerase chain reaction (RT-PCR) was performed to test for abnormal splicing and assess its potential pathogenicity.

RESULTS

A novel atypical splicing mutation that belongs to unclassified variants (UCVs), IVS6+5G>C, was identified in three family members by CES and was shown to co-segregate only with the affected individuals. The RT-PCR revealed the abnormal splicing of exon 6, which thus caused truncating mutation. These findings suggested that the atypical splice site alteration, IVS6+5G>C, in the PKD2 gene was the potential pathogenic mutation leading to ADPKD in this Chinese family.

CONCLUSION

The data available in this study provided strong evidence that IVS6+5G>C is the potential pathogenic mutation for ADPKD. In addition, our findings emphasised the significance of functional analysis of UCVs and genotype-phenotype correlation in ADPKD.

In vivo Polycystin-1 interactome using a novel Pkd1 knock-in mouse model

PKD1 is the most commonly mutated gene causing autosomal dominant polycystic kidney disease (ADPKD). It encodes Polycystin-1 (PC1), a putative membrane protein that undergoes a set of incompletely characterized post-transcriptional cleavage steps and has been reported to localize in multiple subcellular locations, including the primary cilium and mitochondria. However, direct visualization of PC1 and detailed characterization of its binding partners remain challenging. We now report a new mouse model with HA epitopes and eGFP knocked-in frame into the endogenous mouse Pkd1 gene by CRISPR/Cas9. Using this model, we sought to visualize endogenous PC1-eGFP and performed affinity-purification mass spectrometry (AP-MS) and network analyses. We show that the modified Pkd1 allele is fully functional but the eGFP-tagged protein cannot be detected without signal amplification by secondary antibodies. Using nanobody-coupled beads and large quantities of tissue, AP-MS identified an in vivo PC1 interactome, which is enriched for mitochondrial proteins and components of metabolic pathways. These studies suggest this mouse model and interactome data will be useful to understand PC1 function, but that new methods and brighter tags will be required to track endogenous PC1.

Calcium signaling in polycystic kidney disease- cell death and survival

Polycystic kidney disease is typified by cysts in the kidney and extra-renal manifestations including hypertension and heart failure. The main genetic underpinning this disease are loss-of function mutations to the two polycystin proteins, polycystin 1 and polycystin 2. Molecularly, the disease is characterized by changes in multiple signaling pathways including down regulation of calcium signaling, which, in part, is contributed by the calcium permeant properties of polycystin 2. These signaling pathways enable the cystic cells to survive and avoid cell death. This review focuses on the studies that have emerged in the past 5 years describing how the structural insights gained from PC-1 and PC-2 inform the calcium dependent molecular pathways of autophagy and the unfolded protein response that are regulated by the polycystin proteins and how it leads to cell survival and/or cell death.

Polycystin-2 Associates With Malignancy in Meningiomas

The involvement of polycystin-2 (PC2) in cell survival pathways raises questions about its role in carcinogenesis. Aberrant expression of PC2 has been associated with malignancy in various tumors. No evidence exists referring to PC2 expression in meningiomas. The aim of this study was to investigate the expression levels of PC2 in meningiomas and compare them with normal brain samples including leptomeninges. PC2 immunohistochemical expression was quantitatively analyzed in archival tissue from 60 patients with benign (WHO grade 1) and 22 patients with high-grade (21: WHO grade 2 and 1: grade 3) meningiomas. Specifically, the labeling index [the percentage of positive (labeled) cells out of the total number of tumor cells counted] was determined. PC2 mRNA levels were evaluated by quantitative real-time polymerase chain reaction. PC2 immunostaining was not detected in the leptomeninges. Gene expression analysis revealed increased levels of PC2 in WHO grade 1 ( P = 0.008) and WHO grade 2 ( P = 0.0007) meningiomas compared with that of normal brains. PC2 expression was significantly associated with an ascending grade of malignancy by both immunohistochemistry and quantitative real-time polymerase chain reaction ( P < 0.05). Recurrent meningiomas displayed higher levels of PC2 compared with primary meningiomas ( P = 0.008). Although no significant association of PC2 with the overall survival of the patients was found ( P > 0.05), it was noticed that the patients with WHO grade 2 meningiomas with low expression of PC2 survived longer compared with the patients with WHO grade 1 meningioma with high expression of PC2 (mean survival 49.5 and 28 months, respectively). The above results indicate a possible association of PC2 with malignancy in meningiomas. However, the mechanisms underlying PC2 implication in meningioma pathogenesis should be further elucidated.

Molecular Mechanisms of Epigenetic Regulation, Inflammation, and Cell Death in ADPKD

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disorder, which is caused by mutations in the PKD1 and PKD2 genes, characterizing by progressive growth of multiple cysts in the kidneys, eventually leading to end-stage kidney disease (ESKD) and requiring renal replacement therapy. In addition, studies indicate that disease progression is as a result of a combination of factors. Understanding the molecular mechanisms, therefore, should facilitate the development of precise therapeutic strategies for ADPKD treatment. The roles of epigenetic modulation, interstitial inflammation, and regulated cell death have recently become the focuses in ADPKD. Different epigenetic regulators, and the presence of inflammatory markers detectable even before cyst growth, have been linked to cyst progression. Moreover, the infiltration of inflammatory cells, such as macrophages and T cells, have been associated with cyst growth and deteriorating renal function in humans and PKD animal models. There is evidence supporting a direct role of the PKD gene mutations to the regulation of epigenetic mechanisms and inflammatory response in ADPKD. In addition, the role of regulated cell death, including apoptosis, autophagy and ferroptosis, have been investigated in ADPKD. However, there is no consensus whether cell death promotes or delays cyst growth in ADPKD. It is therefore necessary to develop an interactive picture between PKD gene mutations, the epigenome, inflammation, and cell death to understand why inherited PKD gene mutations in patients may result in the dysregulation of these processes that increase the progression of renal cyst formation.

Decoding the Effect of Hydrostatic Pressure on TRPV1 Lower-Gate Conformation by Molecular-Dynamics Simulation

In response to hydrostatic pressure, the cation channel transient receptor potential vanilloid 1 (TRPV1) is essential in signaling pathways linked to glaucoma. When activated, TRPV1 undergoes a gating transition from a closed to an open state that allows the influx of Ca2+ ions. However, the gating mechanism of TRPV1 in response to hydrostatic pressure at the molecular level is still lacking. To understand the effect of hydrostatic pressure on the activation of TRPV1, we conducted molecular-dynamics (MD) simulations on TRPV1 under different hydrostatic pressure configurations, with and without a cell membrane. The TRPV1 membrane-embedded model is more stable than the TPRV1-only model, indicating the importance of including the cell membrane in MD simulation. Under elevated pressure at 27.6 mmHg, we observed a more dynamic and outward motion of the TRPV1 domains in the lower-gate area than in the simulation under normal pressure at 12.6 mmHg. While a complete closed-to-open-gate transition was not evident in the limited course of our MD simulations, an increase in the channel radius at the lower gate was observed at 27.6 mmHg versus that at 12.6 mmHg. These findings provide novel information regarding the effect of hydrostatic pressure on TRPV1 channels.

Polycystin-2 (TRPP2): Ion channel properties and regulation

Polycystin-2 (TRPP2, PKD2, PC2) is the product of the PKD2 gene, whose mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD). PC2 belongs to the superfamily of TRP (Transient Receptor Potential) proteins that generally function as Ca²⁺-permeable nonselective cation channels implicated in Ca²⁺ signaling. PC2 localizes to various cell domains with distinct functions that likely depend on interactions with specific channel partners. Functions include receptor-operated, nonselective cation channel activity in the plasma membrane, intracellular Ca²⁺ release channel activity in the endoplasmic reticulum (ER), and mechanosensitive channel activity in the primary cilium of renal epithelial cells. Here we summarize our current understanding of the properties of PC2 and how other transmembrane and cytosolic proteins modulate this activity, providing functional diversity and selective regulatory mechanisms to its role in the control of cellular Ca²⁺ homeostasis.

Study on the mechanism of efficient extracellular expression of toxic streptomyces phospholipase D in Brevibacillus choshinensis under Mg2+ stress

Background

Phospholipase D (PLD) has significant advantages in the food and medicine industries due to its unique transphosphatidylation. However, the high heterologous expression of PLD is limited by its cytotoxicity. The present study sought to develop an efficient and extracellular expression system of PLD in the non-pathogenic Brevibacillus choshinensis (B. choshinensis).

Results

The extracellular PLD was effectively expressed by the strong promoter (P2) under Mg2+ stress, with the highest activity of 10 U/mL. The inductively coupled plasma–mass spectrometry (ICP-MS) results elucidated that the over-expression of PLD by P2 promoter without Mg2+ stress induced the ionic homeostasis perturbation caused by the highly enhanced Ca2+ influx, leading to cell injury or death. Under Mg2+ stress, Ca2+ influx was significantly inhibited, and the strengths of P2 promoter and HWP gene expression were weakened. The study results revealed that the mechanism of Mg2+ induced cell growth protection and PLD expression might be related to the lowered strength of PLD expression by P2 promoter repression to meet with the secretion efficiency of B. choshinensis, and the redistribution of intracellular ions accompanied by decreased Ca2+ influx.

Conclusions

The PLD production was highly improved under Mg2+ stress. By ICP-MS and qPCR analysis combined with other results, the mechanism of the efficient extracellular PLD expression under Mg2+ stress was demonstrated. The relatively low-speed PLD expression during cell growth alleviated cell growth inhibition and profoundly improved PLD production. These results provided a potential approach for the large-scale production of extracellular PLD and novel insights into PLD function.

Generation of flow and droplets with an ultra-long-range linear concentration gradient

In the chemical and biological fields, the creation of concentration gradient microenvironments is an important approach for many applications, such as crystal growth and drug screening. Although many concentration gradient generators have been demonstrated, current generators can hardly produce ultra-long linear concentration gradients. In this paper, we propose a concentration-gradient flow/droplet generator which consists of a microfluidic flow switch, a cavity array for stage-by-stage concentration dilution, and an optional T-junction for droplet formation. The generator can realize an ultra-long continuously-varying concentration gradient along the flow direction. Generation of a 38 mm concentration gradient was demonstrated. The length can be further extended by enlarging the capacity of the cavities and increasing the number of the stages. The concentration gradient showed high linearity in the range of 10% to 90%. Moreover, cyclic generation of a concentration gradient flow and droplets with different concentrations was realized by the generator. In a demonstration of drug screening, the generator was employed to produce paclitaxel in different concentrations. A negative correlation between the 4T1 cell viability and the paclitaxel concentration was observed after the treatment. We envision that the concentration gradient generator will be a promising candidate for various drug screening applications.

Loss of PKD1 and PKD2 share common effects on intracellular Ca2+ signaling

In polycystic kidney disease (PKD) multiple bilateral renal cysts gradually enlarge causing a decline in renal function. Transepithelial chloride secretion through cystic fibrosis transmembrane conductance regulator (CFTR) and TMEM16A (anoctamin 1) drive cyst enlargement. We demonstrated recently that a loss of PKD1 increases expression and function of TMEM16A in murine kidneys and in mouse M1 collecting duct cells. The data demonstrated that TMEM16A contributes essentially to cyst growth by upregulating intracellular Ca2+ signaling. Enhanced expression of TMEM16A and Ca2+ signaling increased both cell proliferation and fluid secretion, which suggested inhibition of TMEM16A as a novel therapy in ADPKD. About 15 % of all ADPKD cases are caused by mutations in PKD2. To analyze the effects of loss of function of PKD2 on Ca2+ signaling, we knocked-down Pkd2 in mouse primary renal epithelial cells in the present study, using viral transfection of shRNA. Unlike in Pkd1-/- cells, knockdown of PKD2 lowered basal Ca2+ and augmented store-operated Ca2+ entry, which was both independent of TMEM16A. However, disease causing purinergic Ca2+ store release was enhanced, similar to that observed in Pkd1-/- renal epithelial cells. The present data suggest pharmacological inhibition of TMEM16A as a treatment in ADPKD caused by mutations in both PKD1 and PKD2.

A comprehensive overview of the complex world of the endo- and sarcoplasmic reticulum Ca2+-leak channels

Inside cells, the endoplasmic reticulum (ER) forms the largest Ca²⁺ store. Ca²⁺ is actively pumped by the SERCA pumps in the ER, where intraluminal Ca²⁺-binding proteins enable the accumulation of large amount of Ca²⁺. IP₃ receptors and the ryanodine receptors mediate the release of Ca²⁺ in a controlled way, thereby evoking complex spatio-temporal signals in the cell. The steady state Ca²⁺ concentration in the ER of about 500 μM results from the balance between SERCA-mediated Ca²⁺ uptake and the passive leakage of Ca²⁺. The passive Ca²⁺ leak from the ER is often ignored, but can play an important physiological role, depending on the cellular context. Moreover, excessive Ca²⁺ leakage significantly lowers the amount of Ca²⁺ stored in the ER compared to normal conditions, thereby limiting the possibility to evoke Ca²⁺ signals and/or causing ER stress, leading to pathological consequences. The so-called Ca²⁺-leak channels responsible for Ca²⁺ leakage from the ER are however still not well understood, despite over 20 different proteins have been proposed to contribute to it. This review has the aim to critically evaluate the available evidence about the various channels potentially involved and to draw conclusions about their relative importance.

Unstructural Biology of TRP Ion Channels: The Role of Intrinsically Disordered Regions in Channel Function and Regulation

The first genuine high-resolution single particle cryo-electron microscopy structure of a membrane protein determined was a transient receptor potential (TRP) ion channel, TRPV1, in 2013. This methodical breakthrough opened up a whole new world for structural biology and ion channel aficionados alike. TRP channels capture the imagination due to the sheer endless number of tasks they carry out in all aspects of animal physiology. To date, structures of at least one representative member of each of the six mammalian TRP channel subfamilies as well as of a few non-mammalian families have been determined. These structures were instrumental for a better understanding of TRP channel function and regulation. However, all of the TRP channel structures solved so far are incomplete since they miss important information about highly flexible regions found mostly in the channel N- and C-termini. These intrinsically disordered regions (IDRs) can represent between a quarter to almost half of the entire protein sequence and act as important recruitment hubs for lipids and regulatory proteins. Here, we analyze the currently available TRP channel structures with regard to the extent of these "missing" regions and compare these findings to disorder predictions. We discuss select examples of intra- and intermolecular crosstalk of TRP channel IDRs with proteins and lipids as well as the effect of splicing and post-translational modifications, to illuminate their importance for channel function and to complement the prevalently discussed structural biology of these versatile and fascinating proteins with their equally relevant 'unstructural' biology.

Role of the AMPK/ACC Signaling Pathway in TRPP2-Mediated Head and Neck Cancer Cell Proliferation

Transient receptor potential polycystic 2 (TRPP2) exerts vital roles in various types of cancer; however, its underlying mechanisms remain largely unknown. This study is aimed at investigating whether knockdown of TRPP2 affected the AMP-activated protein kinase (AMPK)/acetyl-CoA carboxylase (ACC) signaling pathway and the proliferation of HN-4, cell line originating from human oral and hypopharyngeal squamous cell carcinoma. In addition, the interactions among AMPK/ACC, AMPK/protein kinase RNA-like endoplasmic reticulum kinase (PERK)/eukaryotic initiation factor 2α (eIF2α) and TRPP2/PERK/eIF2α signaling pathways, and their association with cell proliferation were also explored. The results showed that the relative expression levels of phosphorylated (p)-ACC, p-PERK, and p-eIF2α in HN-4 cells were significantly increased following treatment with 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) and significantly decreased in cells treated with compound C. Therefore, consistent with previous studies, the AMPK/ACC and AMPK/PERK/eIF2α signaling pathways were upregulated and downregulated following treatment with an AMPK agonist and inhibitor, respectively. Furthermore, TRPP2 knockdown decreased p-PERK and p-eIF2α expression levels and increased those of p-AMPK and p-ACC. Additionally, knockdown of TRPP2 increased HN-4 cell proliferation, while treatment with an AMPK inhibitor or agonist increased or inhibited TRPP2-specific siRNA-mediated cell proliferation, respectively. In conclusion, silencing of TRPP2 expression increased HN-4 cell proliferation via inhibiting the PERK/eIF2α signaling pathway, while the AMPK/ACC signaling pathway was possibly activated by a feedback mechanism to reduce enhanced cell proliferation.

TRPP2 and STIM1 form a microdomain to regulate store-operated Ca2+ entry and blood vessel tone

Background

Polycystin-2 (TRPP2) is a Ca²⁺ permeable nonselective cationic channel essential for maintaining physiological function in live cells. Stromal interaction molecule 1 (STIM1) is an important Ca²⁺ sensor in store-operated Ca²⁺ entry (SOCE). Both TRPP2 and STIM1 are expressed in endoplasmic reticular membrane and participate in Ca²⁺ signaling, suggesting a physical interaction and functional synergism.

Methods

We performed co-localization, co-immunoprecipitation, and fluorescence resonance energy transfer assay to identify the interactions of TRPP2 and STIM1 in transfected HEK293 cells and native vascular smooth muscle cells (VSMCs). The function of the TRPP2-STIM1 complex in thapsigargin (TG) or adenosine triphosphate (ATP)-induced SOCE was explored using specific small interfering RNA (siRNA). Further, we created TRPP2 conditional knockout (CKO) mouse to investigate the functional role of TRPP2 in agonist-induced vessel contraction.

Results

TRPP2 and STIM1 form a complex in transfected HEK293 cells and native VSMCs. Genetic manipulations with TRPP2 siRNA, dominant negative TRPP2 or STIM1 siRNA significantly suppressed ATP and TG-induced intracellular Ca²⁺ release and SOCE in HEK293 cells. Inositol triphosphate receptor inhibitor 2-aminoethyl diphenylborinate (2APB) abolished ATP-induced Ca²⁺ release and SOCE in HEK293 cells. In addition, TRPP2 and STIM1 knockdown significantly inhibited ATP- and TG-induced STIM1 puncta formation and SOCE in VSMCs. Importantly, knockdown of TRPP2 and STIM1 or conditional knockout TRPP2 markedly suppressed agonist-induced mouse aorta contraction.

Conclusions

Our data indicate that TRPP2 and STIM1 are physically associated and form a functional complex to regulate agonist-induced intracellular Ca²⁺ mobilization, SOCE and blood vessel tone.

Ion channels in capillary endothelium

Vascular beds are anatomically and functionally compartmentalized into arteries, capillaries, and veins. The bulk of the vasculature consists of the dense, anastomosing capillary network, composed of capillary endothelial cells (cECs) that are intimately associated with the parenchyma. Despite their abundance, the ion channel expression and function and Ca²⁺ signaling behaviors of capillaries have only recently begun to be explored in detail. Here, we discuss the established and emerging roles of ion channels and Ca²⁺ signaling in cECs. By mining a publicly available RNA-seq dataset, we outline the wide variety of ion channel genes that are expressed in these cells, which potentially imbue capillaries with a broad range of sensing and signal transduction capabilities. We also underscore subtle but critical differences between cEC and arteriolar EC ion channel expression that likely underlie key functional differences in ECs at these different levels of the vascular tree. We focus our discussion on the cerebral vasculature, but the findings and principles being elucidated in this area likely generalize to other vascular beds.

TMEM16A drives renal cyst growth by augmenting Ca2+ signaling in M1 cells

Abstract

Polycystic kidney disease (PKD) leads to continuous decline of renal function by growth of renal cysts. Enhanced proliferation and transepithelial chloride secretion through cystic fibrosis transmembrane conductance regulator (CFTR) and Ca²⁺-activated TMEM16A Cl⁻ channels is thought to cause an increase in cyst volume. Recent work shows the pro-proliferative role of the Ca²⁺ activated Cl⁻ channel TMEM16A (anoctamin 1), and demonstrates the essential contribution of TMEM16A to CFTR-dependent Cl⁻ secretion. The present data demonstrate an increase in intracellular Ca²⁺ ([Ca²⁺]i) signals and Cl⁻ secretion by TMEM16A, in renal collecting duct principle cells from dog (MDCK) and mouse (M1) as well as primary tubular epithelial cells from PKD1−/− knockout mice. M1 organoids proliferated, increased expression of TMEM16A, and secreted Cl⁻ upon knockdown of endogenous polycystin 1 or 2 (PKD1,2), by retroviral transfection with shPKD1 and shPKD2, respectively. Knockdown of PKD1 or PKD2 increased basal intracellular Ca²⁺ levels and enhanced purinergic Ca²⁺ release from endoplasmic reticulum. In contrast, ryanodine receptors were found not to be expressed in mouse renal epithelial cells and caffeine had no effects on [Ca²⁺]i. Ca²⁺ signals, proliferation, and Cl⁻ secretion were largely reduced by knockdown or blockade of TMEM16A. TMEM16A may be therefore important for enhanced Ca²⁺ release from IP₃-sensitive Ca²⁺ stores in polycystic kidney disease.

Key messages

• ADPKD leads to continuous decline of renal function by growth of renal cysts.

• Knockdown of PKD1 or PKD2 increases TMEM16A expression.

• TMEM16A enhanced intracellular Ca²⁺ signals, Cl⁻ secretion, and proliferation.

• TMEM16A contributes to cyst growth in ADPKD.

Electronic supplementary material

The online version of this article (10.1007/s00109-020-01894-y) contains supplementary material, which is available to authorized users.

Role of the TRPV Channels in the Endoplasmic Reticulum Calcium Homeostasis

It has been widely established that transient receptor potential vanilloid (TRPV) channels play a crucial role in calcium homeostasis in mammalian cells. Modulation of TRPV channels activity can modify their physiological function leading to some diseases and disorders like neurodegeneration, pain, cancer, skin disorders, etc. It should be noted that, despite TRPV channels importance, our knowledge of the TRPV channels functions in cells is mostly limited to their plasma membrane location. However, some TRPV channels were shown to be expressed in the endoplasmic reticulum where their modulation by activators and/or inhibitors was demonstrated to be crucial for intracellular signaling. In this review, we have intended to summarize the poorly studied roles and functions of these channels in the endoplasmic reticulum.

Polycystin 2 is increased in disease to protect against stress-induced cell death

Polycystin 2 (PC2 or TRPP1, formerly TRPP2) is a calcium-permeant Transient Receptor Potential (TRP) cation channel expressed primarily on the endoplasmic reticulum (ER) membrane and primary cilia of all cell and tissue types. Despite its ubiquitous expression throughout the body, studies of PC2 have focused primarily on its role in the kidney, as mutations in PC2 lead to the development of autosomal dominant polycystic kidney disease (ADPKD), a debilitating condition for which there is no cure. However, the endogenous role that PC2 plays in the regulation of general cellular homeostasis remains unclear. In this study, we measure how PC2 expression changes in different pathological states, determine that its abundance is increased under conditions of cellular stress in multiple tissues including human disease, and conclude that PC2-deficient cells have increased susceptibility to cell death induced by stress. Our results offer new insight into the normal function of PC2 as a ubiquitous stress-sensitive protein whose expression is up-regulated in response to cell stress to protect against pathological cell death in multiple diseases.

Roles for the Endoplasmic Reticulum in Regulation of Neuronal Calcium Homeostasis

By influencing Ca²⁺ homeostasis in spatially and architecturally distinct neuronal compartments, the endoplasmic reticulum (ER) illustrates the notion that form and function are intimately related. The contribution of ER to neuronal Ca²⁺ homeostasis is attributed to the organelle being the largest reservoir of intracellular Ca²⁺ and having a high density of Ca²⁺ channels and transporters. As such, ER Ca²⁺ has incontrovertible roles in the regulation of axodendritic growth and morphology, synaptic vesicle release, and neural activity dependent gene expression, synaptic plasticity, and mitochondrial bioenergetics. Not surprisingly, many neurological diseases arise from ER Ca²⁺ dyshomeostasis, either directly due to alterations in ER resident proteins, or indirectly via processes that are coupled to the regulators of ER Ca²⁺ dynamics. In this review, we describe the mechanisms involved in the establishment of ER Ca²⁺ homeostasis in neurons. We elaborate upon how changes in the spatiotemporal dynamics of Ca²⁺ exchange between the ER and other organelles sculpt neuronal function and provide examples that demonstrate the involvement of ER Ca²⁺ dyshomeostasis in a range of neurological and neurodegenerative diseases.

Nicotinic acid affects cytoskeleton remodeling via increasing the activity of gelsolin

Our previous research has demonstrated that nicotinic acid (NA) might suppress the angiogenesis by modulating the expression of angiogenesis factors and promoting the cytoskeleton remodeling. However, the underlying mechanism need to be further elucidated. The intracellular Ca²⁺ concentration was measured by a [Ca²⁺ ] detection kit. The F-actin depolymerization was shown by immunofluorescence staining. The protein levels of F-actin and G-actin were determined by Western blot. The effects of NA treatment on the gelsolin-PI3Kα (p110α) interaction were investigated by co-immunoprecipitation (Co-IP). NA treatment caused an initial drop and then induced a significant increase in [Ca²⁺ ] with a time and dose dependent manner. In addition, NA promoted the depolymerization of F-actin and knockdown of gelsolin substantially rescued the effects caused by NA treatment. NA treatment significantly inhibited the interaction between phosphoinositide 3-kinase (PI3K) α (p110α) and gelsolin and addition of phosphatidylinositol (3,4,5)-triphosphate (PIP3) increased the protein level of F-actin and rescued the F/G-actin ratio. In conclusion, our results indicated NA treatment could interfere with the ability of PI3Kα (p110α) to inhibit the activity of gelsolin by decomposing PIP2 to produce PIP3, thereby increasing the activity of gelsolin, which ultimately acted on the remodeling of the cytoskeleton and exerted an inhibitory effect on angiogenesis.

Polycystin 2 regulates mitochondrial Ca2+ signaling, bioenergetics, and dynamics through mitofusin 2

Mitochondria and the endoplasmic reticulum (ER) have an intimate functional relationship due to tethering proteins that bring their membranes in close (~30 nm) apposition. One function of this interorganellar junction is to increase the efficiency of Ca²⁺ transfer into mitochondria, thus stimulating mitochondrial respiration. Here, we showed that the ER cation-permeant channel polycystin 2 (PC2) functions to reduce mitochondria-ER contacts. In cell culture models, PC2 knockdown led to a 50% increase in mitofusin 2 (MFN2) expression, an outer mitochondrial membrane GTPase. Live-cell super-resolution and electron microscopy analyses revealed enhanced MFN2-dependent tethering between the ER and mitochondria in PC2 knockdown cells. PC2 knockdown also led to increased ER-mediated mitochondrial Ca²⁺ signaling, bioenergetic activation, and mitochondrial density. Mutation or deletion of the gene encoding for PC2 results in autosomal dominant polycystic kidney disease (ADPKD), a condition characterized by numerous fluid-filled cysts. In cell culture models and mice with kidney-specific PC2 knockout, knockdown of MFN2 rescued defective mitochondrial Ca²⁺ transfer and diminished cell proliferation in kidney cysts. Consistent with these results, cyst-lining epithelial cells from human ADPKD kidneys had a twofold increase in mitochondria and MFN2 expression. Our data suggest that PC2 normally serves to limit key mitochondrial proteins at the ER-mitochondrial interface and acts as a checkpoint for mitochondrial biogenesis and bioenergetics. Loss of this regulation may contribute to the increased oxidative metabolism and aberrant cell proliferation typical of kidney cysts in ADPKD.

TMEM33 regulates intracellular calcium homeostasis in renal tubular epithelial cells

Mutations in the polycystins cause autosomal dominant polycystic kidney disease (ADPKD). Here we show that transmembrane protein 33 (TMEM33) interacts with the ion channel polycystin-2 (PC2) at the endoplasmic reticulum (ER) membrane, enhancing its opening over the whole physiological calcium range in ER liposomes fused to planar bilayers. Consequently, TMEM33 reduces intracellular calcium content in a PC2-dependent manner, impairs lysosomal calcium refilling, causes cathepsins translocation, inhibition of autophagic flux upon ER stress, as well as sensitization to apoptosis. Invalidation of TMEM33 in the mouse exerts a potent protection against renal ER stress. By contrast, TMEM33 does not influence pkd2-dependent renal cystogenesis in the zebrafish. Together, our results identify a key role for TMEM33 in the regulation of intracellular calcium homeostasis of renal proximal convoluted tubule cells and establish a causal link between TMEM33 and acute kidney injury.

The aminoglycoside antibiotic gentamicin is able to alter metabolic activity and morphology of MDCK-C11 cells: a cell model of intercalated cells

It is well known that the aminoglycoside antibiotic gentamicin is capable of causing damage to kidney cells. Given the known involvement of Ca²⁺ in the nephrotoxic action of gentamicin, the purpose of this study was to establish a relationship between the concentration of intracellular Ca²⁺ ([Ca²⁺]_i) and cellular cytotoxicity using MDCK-C11 cells, a clone that has several properties that resemble those of intercalated cells of the distal nephron. Changes in [Ca²⁺]_i was determined using fluorescence microscopy. Cell viability was evaluated by the neutral red method, and cell cytotoxicity by the MTT method. The [Ca²⁺]_i gradually increased when cells were exposed to 0.1 mM gentamicin for 10, 20, and 30 min. The presence of extracellular Ca²⁺ was found to be necessary to stimulate the increase in [Ca²⁺]_i induced by gentamicin, since this stimulus disappeared by using 1.8 mM EGTA (a Ca²⁺ chelator). Morphological changes were observed with scanning electron microscopy in epithelial cells exposed to the antibiotic. Furthermore, with the MTT method, a decrease in metabolic activity induced by gentamicin was observed, which indicates a cytotoxic effect. In conclusion, gentamicin was able to alter [Ca²⁺]_i, change the morphology of MDCK-C11 cells, and promote cytotoxicity.

A cleavage product of Polycystin-1 is a mitochondrial matrix protein that affects mitochondria morphology and function when heterologously expressed

Recent studies have reported intrinsic metabolic reprogramming in Pkd1 knock-out cells, implicating dysregulated cellular metabolism in the pathogenesis of polycystic kidney disease. However, the exact nature of the metabolic changes and their underlying cause remains controversial. We show herein that Pkd1 k o /ko renal epithelial cells have impaired fatty acid utilization, abnormal mitochondrial morphology and function, and that mitochondria in kidneys of ADPKD patients have morphological alterations. We further show that a C-terminal cleavage product of polycystin-1 (CTT) translocates to the mitochondria matrix and that expression of CTT in Pkd1 ko/ko cells rescues some of the mitochondrial phenotypes. Using Drosophila to model in vivo effects, we find that transgenic expression of mouse CTT results in decreased viability and exercise endurance but increased CO2 production, consistent with altered mitochondrial function. Our results suggest that PC1 may play a direct role in regulating mitochondrial function and cellular metabolism and provide a framework to understand how impaired mitochondrial function could be linked to the regulation of tubular diameter in both physiological and pathological conditions.

Role of apoptosis in the development of autosomal dominant polycystic kidney disease (ADPKD)

Autosomal dominant polycystic kidney disease (ADPKD) is a widespread genetic disorder in the Western world and is characterized by cystogenesis that often leads to end-stage renal disease (ESRD). Mutations in the pkd1 gene, encoding for polycystin-1 (PC1) and its interaction partner pkd2, encoding for polycystin-2 (PC2), are the main drivers of this disease. PC1 and PC2 form a multiprotein membrane complex at cilia sites of the plasma membrane and at intracellular membranes. This complex mediates calcium influx and stimulates various signaling pathways regulating cell survival, proliferation and differentiation. The molecular consequences of pkd1 and pkd2 mutations are still a matter of debate. In particular, the ways in which the cysts are initially formed and progress throughout the disease are unknown. The mechanisms proposed to play a role include enhanced cell proliferation, increased apoptotic cell death and diminished autophagy. In this review, we summarize our current understanding about the contribution of apoptosis to cystogenesis and ADPKD. We present the animal models and the tools and methods that have been created to analyze this process. We also critically review the data that are in favor or against the involvement of apoptosis in disease generation. We argue that apoptosis is probably not the sole driver of cystogenesis but that a cooperative action of cell death, compensatory cell proliferation and perturbed autophagy gradually establish the disease. Finally, we propose novel strategies for uncovering the mode of action of PC1 and PC2 and suggest means by which their dysfunction or loss of expression lead to cystogenesis and ADPKD development.

On the Many Actions of Ouabain: Pro-Cystogenic Effects in Autosomal Dominant Polycystic Kidney Disease

Ouabain and other cardenolides are steroidal compounds originally discovered in plants. Cardenolides were first used as poisons, but after finding their beneficial cardiotonic effects, they were rapidly included in the medical pharmacopeia. The use of cardenolides to treat congestive heart failure remained empirical for centuries and only relatively recently, their mechanisms of action became better understood. A breakthrough came with the discovery that ouabain and other cardenolides exist as endogenous compounds that circulate in the bloodstream of mammals. This elevated these compounds to the category of hormones and opened new lines of investigation directed to further study their biological role. Another important discovery was the finding that the effect of ouabain was mediated not only by inhibition of the activity of the Na,K-ATPase (NKA), but by the unexpected role of NKA as a receptor and a signal transducer, which activates a complex cascade of intracellular second messengers in the cell. This broadened the interest for ouabain and showed that it exerts actions that go beyond its cardiotonic effect. It is now clear that ouabain regulates multiple cell functions, including cell proliferation and hypertrophy, apoptosis, cell adhesion, cell migration, and cell metabolism in a cell and tissue type specific manner. This review article focuses on the cardenolide ouabain and discusses its various in vitro and in vivo effects, its role as an endogenous compound, its mechanisms of action, and its potential use as a therapeutic agent; placing especial emphasis on our findings of ouabain as a pro-cystogenic agent in autosomal dominant polycystic kidney disease (ADPKD).

Intracellular Ca(2+) signaling and Ca(2+) microdomains in the control of cell survival, apoptosis and autophagy

The endoplasmic reticulum (ER), mitochondria and lysosomes are physically and/or functionally linked, establishing close contact sites between these organelles. As a consequence, Ca(2+) release events from the ER, the major intracellular Ca(2+)-storage organelle, have an immediate effect on the physiological function of mitochondria and lysosomes. Also, the lysosomes can act as a Ca(2+) source for Ca(2+) release into the cytosol, thereby influencing ER-based Ca(2+) signaling. Given the important role for mitochondria and lysosomes in cell survival, cell death and cell adaptation processes, it has become increasingly clear that Ca(2+) signals from or towards these organelles impact these processes. In this review, we discuss the most recent insights in the emerging role of Ca(2+) signaling in cellular survival by controlling basal mitochondrial bioenergetics and by regulating apoptosis, a mitochondrial process, and autophagy, a lysosomal process, in response to cell damage and cell stress.

Role of calcium in polycystic kidney disease: From signaling to pathology

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited monogenic kidney disease. Characterized by the development and growth of cysts that cause progressive kidney enlargement, it ultimately leads to end-stage renal disease. Approximately 85% of ADPKD cases are caused by mutations in the PKD1 gene, while mutations in the PKD2 gene account for the remaining 15% of cases. The PKD1 gene encodes for polycystin-1 (PC1), a large multi-functional membrane receptor protein able to regulate ion channel complexes, whereas polycystin-2 (PC2), encoded by the PKD2 gene, is an integral membrane protein that functions as a calcium-permeable cation channel, located mainly in the endoplasmic reticulum (ER). In the primary cilia of the epithelial cells, PC1 interacts with PC2 to form a polycystin complex that acts as a mechanosensor, regulating signaling pathways involved in the differentiation of kidney tubular epithelial cells. Despite progress in understanding the function of these proteins, the molecular mechanisms associated with the pathogenesis of ADPKD remain unclear. In this review we discuss how an imbalance between functional PC1 and PC2 proteins may disrupt calcium channel activities in the cilium, plasma membrane and ER, thereby altering intracellular calcium signaling and leading to the aberrant cell proliferation and apoptosis associated with the development and growth of renal cysts. Research in this field could lead to the discovery of new molecules able to rebalance intracellular calcium, thereby normalizing cell proliferation and reducing kidney cyst progression.

The functions of TRPP2 in the vascular system

TRPP2 (polycystin-2, PC2 or PKD2), encoded by the PKD2 gene, is a non-selective cation channel with a large single channel conductance and high Ca(2+) permeability. In cell membrane, TRPP2, along with polycystin-1, TRPV4 and TRPC1, functions as a mechanotransduction channel. In the endoplasmic reticulum, TRPP2 modulates intracellular Ca(2+) release associated with IP3 receptors and the ryanodine receptors. Noteworthily, TRPP2 is widely expressed in vascular endothelial and smooth muscle cells of all major vascular beds, and contributes to the regulation of vessel function. The mutation of the PKD2 gene is a major cause of autosomal dominant polycystic kidney disease (ADPKD), which is not only a common genetic disease of the kidney but also a systemic disorder associated with abnormalities in the vasculature; cardiovascular complications are the leading cause of mortality and morbidity in ADPKD patients. This review provides an overview of the current knowledge regarding the TRPP2 protein and its possible role in cardiovascular function and related diseases.

Short-hairpin RNA library: identification of therapeutic partners for gefitinib-resistant non-small cell lung cancer

Somatic mutations of the epidermal growth factor receptor often cause resistance to therapy with tyrosine kinase inhibitor in non-small cell lung cancer (NSCLC). In this study, we aimed to identify partner drugs and pathways that can induce cell death in combination with gefitinib in NSCLC cells. We undertook a genome-wide RNAi screen to identify synthetic lethality with gefitinib in tyrosine kinase inhibitor resistant cells. The screening data were utilized in different approaches. Firstly, we identified PRKCSH as a candidate gene, silencing of which induces apoptosis of NSCLC cells treated with gefitinib. Next, in an in silico gene signature pathway analysis of shRNA library data, a strong correlation of genes involved in the CD27 signaling cascade was observed. We showed that the combination of dasatinib (NF-κB pathway inhibitor) with gefitinib synergistically inhibited the growth of NSCLC cells. Lastly, utilizing the Connectivity Map, thioridazine was identified as a top pharmaceutical perturbagen. In our experiments, it synergized with gefitinib to reduce p-Akt levels and to induce apoptosis in NSCLC cells. Taken together, a pooled short-hairpin library screen identified several potential pathways and drugs that can be therapeutic targets for gefitinib resistant NSCLC.

Vasopressin and disruption of calcium signalling in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenic kidney disease and is responsible for 5-10% of cases of end-stage renal disease worldwide. ADPKD is characterized by the relentless development and growth of cysts, which cause progressive kidney enlargement associated with hypertension, pain, reduced quality of life and eventual kidney failure. Mutations in the PKD1 or PKD2 genes, which encode polycystin-1 (PC1) and polycystin-2 (PC2), respectively, cause ADPKD. However, neither the functions of these proteins nor the molecular mechanisms of ADPKD pathogenesis are well understood. Here, we review the literature that examines how reduced levels of functional PC1 or PC2 at the primary cilia and/or the endoplasmic reticulum directly disrupts intracellular calcium signalling and indirectly disrupts calcium-regulated cAMP and purinergic signalling. We propose a hypothetical model in which dysregulated metabolism of cAMP and purinergic signalling increases the sensitivity of principal cells in collecting ducts and of tubular epithelial cells in the distal nephron to the constant tonic action of vasopressin. The resulting magnified response to vasopressin further enhances the disruption of calcium signalling that is initiated by mutations in PC1 or PC2, and activates downstream signalling pathways that cause impaired tubulogenesis, increased cell proliferation, increased fluid secretion and interstitial inflammation.

Evidence for a KATP Channel in Rough Endoplasmic Reticulum (rerKATP Channel) of Rat Hepatocytes

We report in a previous study the presence of a large conductance K⁺ channel in the membrane of rough endoplasmic reticulum (RER) from rat hepatocytes incorporated into lipid bilayers. Channel activity in this case was found to decrease in presence of ATP 100 µM on the cytoplasmic side and was totally inhibited at ATP concentrations greater than 0.25 mM. Although such features would be compatible with the presence of a K_ATP channel in the RER, recent data obtained from a brain mitochondrial inner membrane preparation have provided evidence for a Maxi-K channel which could also be blocked by ATP within the mM concentration range. A series of channel incorporation experiments was thus undertaken to determine if the ATP-sensitive channel originally observed in the RER corresponds to K_ATP channel. Our results indicate that the gating and permeation properties of this channel are unaffected by the addition of 800 nM charybdotoxin and 1 µM iberiotoxin, but appeared sensitive to 10 mM TEA and 2.5 mM ATP. Furthermore, adding 100 µM glibenclamide at positive potentials and 400 µM tolbutamide at negative or positive voltages caused a strong inhibition of channel activity. Finally Western blot analyses provided evidence for Kir6.2, SUR1 and/or SUR2B, and SUR2A expression in our RER fractions. It was concluded on the basis of these observations that the channel previously characterized in RER membranes corresponds to K_ATP, suggesting that opening of this channel may enhance Ca²⁺ releases, alter the dynamics of the Ca²⁺ transient and prevent accumulation of Ca²⁺ in the ER during Ca²⁺ overload.

Plasma-activated medium induces A549 cell injury via a spiral apoptotic cascade involving the mitochondrial-nuclear network

Plasma medicine is a rapidly expanding new field of interdisciplinary research that combines physics, chemistry, biology, and medicine. Nonthermal atmospheric pressure plasma can be applied to living cells and tissues and has emerged as a novel technology for cancer therapy. Plasma has recently been shown to affect cells not only directly, but also by indirect treatment with previously prepared plasma-activated medium (PAM). The objective of this study was to demonstrate the inhibitory effects of PAM on A549 cell survival and elucidate the signaling mechanisms responsible for cell death. PAM maintained its ability to suppress cell viability for at least 1 week when stored at -80°C. The severity of PAM-triggered cell injury depended on the kind of culture medium used to prepare the PAM, especially that with or without pyruvate. Hydrogen peroxide (H2O2) and/or its derived or cooperating reactive oxygen species reduced the mitochondrial membrane potential, downregulated the expression of the antiapoptotic protein Bcl2, activated poly(ADP-ribose) polymerase-1, and released apoptosis-inducing factor from mitochondria with endoplasmic reticulum stress. However, the activation of caspase 3/7 and attenuation of cell viability by the addition of caspase inhibitor were not observed. The accumulation of adenine 5'-diphosphoribose as a product of the above reactions activated transient receptor potential melastatin 2, which elevated intracellular Ca(2+) levels and subsequently led to cell death. These results demonstrated that H2O2 and/or other reactive species in PAM disturbed the mitochondrial-nuclear network in cancer cells through a caspase-independent apoptotic pathway. Moreover, damage to the plasma membrane by H2O2-cooperating charged species not only induced apoptosis, but also increased its permeability to extracellular reactive species. These phenomena were also detected in PAM-treated HepG2 and MCF-7 cells.

Gating behavior of endoplasmic reticulum potassium channels of rat hepatocytes in diabetes

BACKGROUND

Defects in endoplasmic reticulum homeostasis are common occurrences in different diseases, such as diabetes, in which the function of endoplasmic reticulum is disrupted. It is now well established that ion channels of endoplasmic reticulum membrane have a critical role in endoplasmic reticulum luminal homeostasis. Our previous studies showed the presence of an ATP-sensitive cationic channel in endoplasmic reticulum. Therefore, in this study, we examined and compared the activities of this channel in control and diabetic rats using single-channel recording techniques.

METHOD

Male Wistar rats were made diabetic for 2 weeks with a single dose injection of streptozotocin (45 mg/kg). Ion channel incorporation of rough endoplasmic reticulum of diabetic hepatocytes into the bilayer lipid membrane allowed the characterization of K+ channel.

RESULTS

Ion channel incorporation of rough endoplasmic reticulum vesicles into the bilayer lipid revealed that the channel current-voltage (I-V) relation with a mean slope conductance of 520 ± 19 pS was unaffected in diabetes. Interestingly, the channel Po-voltage relation was significantly lower in diabetic rats at voltages above +30 mV.

CONCLUSION

We concluded that the endoplasmic reticulum cationic channel is involved in diabetes. Also, this finding could be considered as a goal for further therapeutic plans.

Exon sequencing of PKD1 gene in an Iranian patient with autosomal-dominant polycystic kidney disease

Introduction: Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common genetic kidney disorders with the incidence of 1 in 1,000 births. ADPKD is genetically heterogeneous with two genes identified: PKD1 (16p13.3, 46 exons) and PKD2 (4q21, 15 exons). Eighty five percent of the patients with ADPKD have at least one mutation in the PKD1 gene. Genetic studies have demonstrated an important allelic variability among patients, but very few data are known about the genetic variation among Iranian populations. Methods: In this study, exon direct sequencing of PKD1 was performed in a seven-year old boy with ADPKD and in his parents. The patient’s father was ADPKD who was affected without any kidney dysfunction, and the patient’s mother was congenitally missing one kidney. Results: Molecular genetic testing found a mutation in all three members of this family. It was a missense mutation GTG>ATG at position 3057 in exon 25 of PKD1. On the other hand, two novel missense mutations were reported just in the 7-year-old boy: ACA>GCA found in exon 15 at codon 2241 and CAC>AAC found in exon 38 at codon 3710. For checking the pathogenicity of these mutations, exons 15, 25, and 38 of 50 unrelated normal cases were sequenced. Conclusion: our findings suggested that GTG>ATG is a polymorphism with high frequency (60%) as well as ACA>GCA and CAC>AAC are polymorphisms with frequencies of 14% and 22%, respectively in the population of Southwest Iran.

New and notable ion-channels in the sarcoplasmic/endoplasmic reticulum: do they support the process of intracellular Ca²⁺ release?

Intracellular Ca(2+) release through ryanodine receptor (RyR) and inositol trisphosphate receptor (IP3 R) channels is supported by a complex network of additional proteins that are located in or near the Ca(2+) release sites. In this review, we focus, not on RyR/IP3 R, but on other ion-channels that are known to be present in the sarcoplasmic/endoplasmic reticulum (ER/SR) membranes. We review their putative physiological roles and the evidence suggesting that they may support the process of intracellular Ca(2+) release, either indirectly by manipulating ionic fluxes across the ER/SR membrane or by directly interacting with a Ca(2+) -release channel. These channels rarely receive scientific attention because of the general lack of information regarding their biochemical and/or electrophysiological characteristics makes it difficult to predict their physiological roles and their impact on SR Ca(2+) fluxes. We discuss the possible role of SR K(+) channels and, in parallel, detail the known biochemical and biophysical properties of the trimeric intracellular cation (TRIC) proteins and their possible biological and pathophysiological roles in ER/SR Ca(2+) release. We summarise what is known regarding Cl(-) channels in the ER/SR and the non-selective cation channels or putative 'Ca(2+) leak channels', including mitsugumin23 (MG23), pannexins, presenilins and the transient receptor potential (TRP) channels that are distributed across ER/SR membranes but which have not yet been fully characterised functionally.

A TRPV channel in Drosophila motor neurons regulates presynaptic resting Ca2+ levels, synapse growth, and synaptic transmission

Presynaptic resting Ca(2+) influences synaptic vesicle (SV) release probability. Here, we report that a TRPV channel, Inactive (Iav), maintains presynaptic resting [Ca(2+)] by promoting Ca(2+) release from the endoplasmic reticulum in Drosophila motor neurons, and is required for both synapse development and neurotransmission. We find that Iav activates the Ca(2+)/calmodulin-dependent protein phosphatase calcineurin, which is essential for presynaptic microtubule stabilization at the neuromuscular junction. Thus, loss of Iav induces destabilization of presynaptic microtubules, resulting in diminished synaptic growth. Interestingly, expression of human TRPV1 in Iav-deficient motor neurons rescues these defects. We also show that the absence of Iav causes lower SV release probability and diminished synaptic transmission, whereas Iav overexpression elevates these synaptic parameters. Together, our findings indicate that Iav acts as a key regulator of synaptic development and function by influencing presynaptic resting [Ca(2+)].

Ion channels in the regulation of apoptosis

Apoptosis, a type of genetically controlled cell death, is a fundamental cellular mechanism utilized by multicellular organisms for disposal of cells that are no longer needed or potentially detrimental. Given the crucial role of apoptosis in physiology, deregulation of apoptotic machinery is associated with various diseases as well as abnormalities in development. Acquired resistance to apoptosis represents the common feature of most and perhaps all types of cancer. Therefore, repairing and reactivating apoptosis represents a promising strategy to fight cancer. Accumulated evidence identifies ion channels as essential regulators of apoptosis. However, the contribution of specific ion channels to apoptosis varies greatly depending on cell type, ion channel type and intracellular localization, pathology as well as intracellular signaling pathways involved. Here we discuss the involvement of major types of ion channels in apoptosis regulation. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Cyst formation following disruption of intracellular calcium signaling

Mutations in polycystin 1 and 2 (PC1 and PC2) cause the common genetic kidney disorder autosomal dominant polycystic kidney disease (ADPKD). It is unknown how these mutations result in renal cysts, but dysregulation of calcium (Ca(2+)) signaling is a known consequence of PC2 mutations. PC2 functions as a Ca(2+)-activated Ca(2+) channel of the endoplasmic reticulum. We hypothesize that Ca(2+) signaling through PC2, or other intracellular Ca(2+) channels such as the inositol 1,4,5-trisphosphate receptor (InsP3R), is necessary to maintain renal epithelial cell function and that disruption of the Ca(2+) signaling leads to renal cyst development. The cell line LLC-PK1 has traditionally been used for studying PKD-causing mutations and Ca(2+) signaling in 2D culture systems. We demonstrate that this cell line can be used in long-term (8 wk) 3D tissue culture systems. In 2D systems, knockdown of InsP3R results in decreased Ca(2+) transient signals that are rescued by overexpression of PC2. In 3D systems, knockdown of either PC2 or InsP3R leads to cyst formation, but knockdown of InsP3R type 1 (InsP3R1) generated the largest cysts. InsP3R1 and InsP3R3 are differentially localized in both mouse and human kidney, suggesting that regional disruption of Ca(2+) signaling contributes to cystogenesis. All cysts had intact cilia 2 wk after starting 3D culture, but the cells with InsP3R1 knockdown lost cilia as the cysts grew. Studies combining 2D and 3D cell culture systems will assist in understanding how mutations in PC2 that confer altered Ca(2+) signaling lead to ADPKD cysts.

Nuclear BK channels regulate gene expression via the control of nuclear calcium signaling

Ion channels are essential for the regulation of neuronal functions. The significance of plasma membrane, mitochondrial, endoplasmic reticulum and lysosomal ion channels in the regulation of Ca(2+) is well established. In contrast, surprisingly little is known about the function of ion channels on the nuclear envelope (NE). Here we demonstrate the presence of functional large-conductance, calcium-activated potassium channels (BK channels) on the NE of rodent hippocampal neurons. Functionally, blockade of nuclear BK channels (nBK channels) induces NE-derived Ca(2+) release, nucleoplasmic Ca(2+) elevation and cyclic AMP response element binding protein (CREB)-dependent transcription. More importantly, blockade of nBK channels regulates nuclear Ca(2+)-sensitive gene expression and promotes dendritic arborization in a nuclear Ca(2+)-dependent manner. These results suggest that the nBK channel functions as a molecular link between neuronal activity and nuclear Ca(2+) to convey signals from synapse to nucleus and is a new modulator, operating at the NE, of synaptic activity-dependent neuronal functions.

Ion channels in cancer: future perspectives and clinical potential

Ion transport across the cell membrane mediated by channels and carriers participate in the regulation of tumour cell survival, death and motility. Moreover, the altered regulation of channels and carriers is part of neoplastic transformation. Experimental modification of channel and transporter activity impacts tumour cell survival, proliferation, malignant progression, invasive behaviour or therapy resistance of tumour cells. A wide variety of distinct Ca(2+) permeable channels, K(+) channels, Na(+) channels and anion channels have been implicated in tumour growth and metastasis. Further experimental information is, however, needed to define the specific role of individual channel isoforms critically important for malignancy. Compelling experimental evidence supports the assumption that the pharmacological inhibition of ion channels or their regulators may be attractive targets to counteract tumour growth, prevent metastasis and overcome therapy resistance of tumour cells. This short review discusses the role of Ca(2+) permeable channels, K(+) channels, Na(+) channels and anion channels in tumour growth and metastasis and the therapeutic potential of respective inhibitors.

The TRPP subfamily and polycystin-1 proteins

It has been exciting times since the identification of polycystic kidney disease 1 (PKD1) and PKD2 as the genes mutated in autosomal dominant polycystic kidney disease (ADPKD). Biological roles of the encoded proteins polycystin-1 and TRPP2 have been deduced from phenotypes in ADPKD patients, but recent insights from vertebrate and invertebrate model organisms have significantly expanded our understanding of the physiological functions of these proteins. The identification of additional TRPP (TRPP3 and TRPP5) and polycystin-1-like proteins (PKD1L1, PKD1L2, PKD1L3, and PKDREJ) has added yet another layer of complexity to these fascinating cellular signalling units. TRPP proteins assemble with polycystin-1 family members to form receptor-channel complexes. These protein modules have important biological roles ranging from tubular morphogenesis to determination of left-right asymmetry. The founding members of the polycystin family, TRPP2 and polycystin-1, are a prime example of how studying human disease genes can provide insights into fundamental biological mechanisms using a so-called "reverse translational" approach (from bedside to bench). Here, we discuss the current literature on TRPP ion channels and polycystin-1 family proteins including expression, structure, physical interactions, physiology, and lessons from animal model systems and human disease.

Role of ion transport in control of apoptotic cell death

Cell shrinkage is a hallmark and contributes to signaling of apoptosis. Apoptotic cell shrinkage requires ion transport across the cell membrane involving K(+) channels, Cl(-) or anion channels, Na(+)/H(+) exchange, Na(+),K(+),Cl(-) cotransport, and Na(+)/K(+)ATPase. Activation of K(+) channels fosters K(+) exit with decrease of cytosolic K(+) concentration, activation of anion channels triggers exit of Cl(-), organic osmolytes, and HCO3(-). Cellular loss of K(+) and organic osmolytes as well as cytosolic acidification favor apoptosis. Ca(2+) entry through Ca(2+)-permeable cation channels may result in apoptosis by affecting mitochondrial integrity, stimulating proteinases, inducing cell shrinkage due to activation of Ca(2+)-sensitive K(+) channels, and triggering cell-membrane scrambling. Signaling involved in the modification of cell-volume regulatory ion transport during apoptosis include mitogen-activated kinases p38, JNK, ERK1/2, MEKK1, MKK4, the small G proteins Cdc42, and/or Rac and the transcription factor p53. Osmosensing involves integrin receptors, focal adhesion kinases, and tyrosine kinase receptors. Hyperosmotic shock leads to vesicular acidification followed by activation of acid sphingomyelinase, ceramide formation, release of reactive oxygen species, activation of the tyrosine kinase Yes with subsequent stimulation of CD95 trafficking to the cell membrane. Apoptosis is counteracted by mechanisms involved in regulatory volume increase (RVI), by organic osmolytes, by focal adhesion kinase, and by heat-shock proteins. Clearly, our knowledge on the interplay between cell-volume regulatory mechanisms and suicidal cell death is still far from complete and substantial additional experimental effort is needed to elucidate the role of cell-volume regulatory mechanisms in suicidal cell death.

Polycystin-2 induces a conformational change in polycystin-1

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in the genes encoding either polycystin-1 (PC1) or polycystin-2 (PC2). PC2 acts as a nonselective cation channel and together with PC1 plays a role in intracellular Ca(2+) signaling. Using atomic force microscopy (AFM) imaging, we have shown previously that the N and C termini of PC1 appear as unequally sized particles connected by a "string" largely composed of tandem immunoglobulin-like, polycystic kidney disease (PKD) domains. Here, we show that coexpression of PC1 and PC2 causes an elongation of the PC1 string and a corresponding reduction in the size of the larger (C-terminal) particle. This change in the conformation of PC1 does not depend on its delivery to the plasma membrane. In addition, the use of the L3040H PC1 mutant showed that the conformational change does not require GPS cleavage. Coexpression of PC1 with PC2 mutants revealed that the conformational change in PC1 does not require either a stable interaction between PC1 and PC2 or PC2 channel function. Finally, we show that the tandem PKD repeats and to a lesser extent the receptor for egg jelly (REJ) domain both contribute to the extension of the PC1 string in the presence of PC2. We propose that the PKD repeats detach from the C-terminal fragment in response to PC2 activity. The resulting remodeling of PC1 may be responsible for enhancing GPS cleavage of PC1 and the separation of the PC1 N-terminal fragment from the C terminus during its maturation.