Protein 4.1N is required for translocation of inositol 1,4,5-trisphosphate receptor type 1 to the basolateral membrane domain in polarized Madin-Darby canine kidney cells

Molecular determinants of peri-apical targeting of inositol 1,4,5-trisphosphate receptor type 3 in cholangiocytes

Fluid and bicarbonate secretion is a principal function of cholangiocytes, and impaired secretion results in cholestasis. Cholangiocyte secretion depends on peri-apical expression of the type 3 inositol trisphosphate receptor (ITPR3), and loss of this intracellular Ca2+ release channel is a final common event in most cholangiopathies. Here we investigated the mechanism by which ITPR3 localizes to the apical region to regulate secretion. Isolated bile duct units, primary mouse cholangiocytes, and polarized Madin-Darby canine kidney (MDCK) cells were examined using a combination of biochemical and fluorescence microscopy techniques to investigate the mechanism of ITPR3 targeting to the apical region. Apical localization of ITPR3 depended on the presence of intact lipid rafts as well as interactions with both caveolin 1 (CAV1) and myosin heavy chain 9 (MYH9). Chemical disruption of lipid rafts or knockdown of CAV1 or MYH9 redistributed ITPR3 away from the apical region. MYH9 interacted with the five c-terminal amino acids of the ITPR3 peptide. Disruption of lipid rafts impaired Ca2+ signaling, and absence of CAV1 impaired both Ca2+ signaling and fluid secretion. Conclusion: A cooperative mechanism involving MYH9, CAV1, and apical lipid rafts localize ITPR3 to the apical region to regulate Ca2+ signaling and secretion in cholangiocytes.

4.1N-Mediated Interactions and Functions in Nerve System and Cancer

Scaffolding protein 4.1N is a neuron-enriched 4.1 homologue. 4.1N contains three conserved domains, including the N-terminal 4.1-ezrin-radixin-moesin (FERM) domain, internal spectrin–actin–binding (SAB) domain, and C-terminal domain (CTD). Interspersed between the three domains are nonconserved domains, including U1, U2, and U3. The role of 4.1N was first reported in the nerve system. Then, extensive studies reported the role of 4.1N in cancers and other diseases. 4.1N performs numerous vital functions in signaling transduction by interacting, locating, supporting, and coordinating different partners and is involved in the molecular pathogenesis of various diseases. In this review, recent studies on the interactions between 4.1N and its contactors (including the α7AChr, IP3R1, GluR1/4, GluK1/2/3, mGluR8, KCC2, D2/3Rs, CASK, NuMA, PIKE, IP6K2, CAM 1/3, βII spectrin, flotillin-1, pp1, and 14-3-3) and the 4.1N-related biological functions in the nerve system and cancers are specifically and comprehensively discussed. This review provides critical detailed mechanistic insights into the role of 4.1N in disease relationships.

Selective effects of protein 4.1N deficiency on neuroendocrine and reproductive systems

Protein 4.1N, a member of the protein 4.1 family, is highly expressed in the brain. But its function remains to be fully defined. Using 4.1N^−/− mice, we explored the function of 4.1N in vivo. We show that 4.1N^−/− mice were born at a significantly reduced Mendelian ratio and exhibited high mortality between 3 to 5 weeks of age. Live 4.1N^−/− mice were smaller than 4.1N^+/+ mice. Notably, while there were no significant differences in organ/body weight ratio for most of the organs, the testis/body and ovary/body ratio were dramatically decreased in 4.1N^−/− mice, demonstrating selective effects of 4.1N deficiency on the development of the reproductive systems. Histopathology of the reproductive organs showed atrophy of both testis and ovary. Specifically, in the testis there is a lack of spermatogenesis, lack of leydig cells and lack of mature sperm. Similarly, in the ovary there is a lack of follicular development and lack of corpora lutea formation, as well as lack of secretory changes in the endometrium. Examination of pituitary glands revealed that the secretory granules were significantly decreased in pituitary glands of 4.1N^−/− compared to 4.1N^+/+. Moreover, while GnRH was expressed in both neuronal cell body and axons in the hypothalamus of 4.1N^+/+ mice, it was only expressed in the cell body but not the axons of 4.1N^-/- mice. Our findings uncover a novel role for 4.1N in the axis of hypothalamus-pituitary gland-reproductive system.

Profound analgesia is associated with a truncated peptide resulting from tissue specific alternative splicing of DRG CA8-204 regulated by an exon-level cis-eQTL

Carbonic anhydrase-8 (CA8) is an intracellular protein that functions as an allosteric inhibitor of inositol trisphosphate receptor-1 (ITPR1) critical to intracellular Ca⁺⁺ release, synaptic functions and neuronal excitability. We showed previously that murine nociception and analgesic responses are regulated by the expression of this gene in dorsal root ganglion (DRG) associated with a cis-eQTL. In this report, we identify an exon-level cis-eQTL (rs6471859) that regulates human DRG CA8 alternative splicing, producing a truncated 1,697bp transcript (e.g., CA8-204). Our functional genomic studies show the “G” allele at rs6471859 produces a cryptic 3’UTR splice site regulating expression of CA8-204. We developed constructs to study the expression and function of the naturally occurring CA8-204^G transcript (G allele at rs6471859), CA8-204^C (C allele at rs6471859 reversion mutation) and CA8-201 (full length transcript). CA8-204^G transcript expression occurred predominantly in non-neuronal cells (HEK293), while CA8-204^C expression was restricted to neuronal derived cells (NBL) in vitro. CA8-204^G produced a stable truncated transcript in HEK293 cells that was barely detectable in NBL cells. We also show CA8-204 produces a stable peptide that inhibits pITPR1 and Ca⁺⁺ release in HEK293 cells. These results imply homozygous G/G individuals at rs6471859, which are common in the general population, produce exclusively CA8-204^G that is barely detectable in neuronal cells. CA8 null mutations that greatly impact neuronal functions are associated with severe forms of spinal cerebellar ataxia, and our data suggest G/G homozygotes should display a similar phenotype. To address this question, we show in vivo using AAV8-FLAG-CA8-204^G and AAV8-V5-CA8-201 gene transfer delivered via intra-neural sciatic nerve injection (SN), that these viral constructs are able to transduce DRG cells and produce similar analgesic and anti-hyperalgesic responses to inflammatory pain. Immunohistochemistry (IHC) examinations of DRG tissues further show CA8-204^G peptide is expressed in advillin expressing neuronal cells, but to a lesser extent compared to glial cells. These findings explain why G/G homozygotes that exclusively produce this truncated functional peptide in DRG evade a severe phenotype. These genomic studies significantly advance the literature regarding structure-function studies on CA8-ITPR1 critical to calcium signaling pathways, synaptic functioning, neuronal excitability and analgesic responses.

Carbonic anhydrase-8 (CA8) inhibits IP3 binding to the inositol trisphosphate receptor-1(ITPR1), which regulates intracellular calcium signaling critical to neuronal functions. Recessive CA8-null mutants are associated with spinocerebellar ataxia and neurodegenerative disorders. We have previously demonstrated that nociception and analgesic responses are associated with a DRG cis-eQTL that regulates murine expression of this gene. This study focuses on a human DRG exon-level cis-eQTL (rs6471859) that regulates CA8-204 alternative splicing producing a truncated 1,697bp transcript. Herein, we demonstrate the “G” allele at rs6471859 produces a cryptic 3’UTR splice site regulating tissue-specific CA8-204 expression. In vitro studies show the “G” allele (CA8-204^G) produces a stable peptide that inhibits ITPR1 activation and Ca⁺⁺ release in non-neuronal cells, but not in neuronal cells. However, using AAV8 gene transfer in vivo we show CA8-204^G peptide is expressed in both glial and to a lesser extent in neuronal cells, producing profound analgesia and anti-hyperalgesia using inflammatory pain models, similar to the full length CA8-201 positive control. These data significantly extend our understanding of CA8 structure-function, demonstrating the truncated peptide may represent a novel therapeutic candidate.

Consensus report of the 8 and 9th Weinman Symposia on Gene x Environment Interaction in carcinogenesis: novel opportunities for precision medicine

The relative contribution of intrinsic genetic factors and extrinsic environmental ones to cancer aetiology and natural history is a lengthy and debated issue. Gene-environment interactions (G x E) arise when the combined presence of both a germline genetic variant and a known environmental factor modulates the risk of disease more than either one alone. A panel of experts discussed our current understanding of cancer aetiology, known examples of G × E interactions in cancer, and the expanded concept of G × E interactions to include somatic cancer mutations and iatrogenic environmental factors such as anti-cancer treatment. Specific genetic polymorphisms and genetic mutations increase susceptibility to certain carcinogens and may be targeted in the near future for prevention and treatment of cancer patients with novel molecularly based therapies. There was general consensus that a better understanding of the complexity and numerosity of G × E interactions, supported by adequate technological, epidemiological, modelling and statistical resources, will further promote our understanding of cancer and lead to novel preventive and therapeutic approaches.

Remodeling of Ca2+ signaling in cancer: Regulation of inositol 1,4,5-trisphosphate receptors through oncogenes and tumor suppressors

The calcium ion (Ca²⁺) is a ubiquitous intracellular signaling molecule that regulates diverse physiological and pathological processes, including cancer. Increasing evidence indicates that oncogenes and tumor suppressors regulate the Ca²⁺ transport systems. Inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃Rs) are IP₃-activated Ca²⁺ release channels located on the endoplasmic reticulum (ER). They play pivotal roles in the regulation of cell death and survival by controlling Ca²⁺ transfer from the ER to mitochondria through mitochondria-associated ER membranes (MAMs). Optimal levels of Ca²⁺ mobilization to mitochondria are necessary for mitochondrial bioenergetics, whereas excessive Ca²⁺ flux into mitochondria causes loss of mitochondrial membrane integrity and apoptotic cell death. In addition to well-known functions on outer mitochondrial membranes, B-cell lymphoma 2 (Bcl-2) family proteins are localized on the ER and regulate IP₃Rs to control Ca²⁺ transfer into mitochondria. Another regulatory protein of IP₃R, IP₃R-binding protein released with IP₃ (IRBIT), cooperates with or counteracts the Bcl-2 family member depending on cellular states. Furthermore, several oncogenes and tumor suppressors, including Akt, K-Ras, phosphatase and tensin homolog (PTEN), promyelocytic leukemia protein (PML), BRCA1, and BRCA1 associated protein 1 (BAP1), are localized on the ER or at MAMs and negatively or positively regulate apoptotic cell death through interactions with IP₃Rs and regulation of Ca²⁺ dynamics. The remodeling of Ca²⁺ signaling by oncogenes and tumor suppressors that interact with IP₃Rs has fundamental roles in the pathology of cancers.

Dissection of local Ca(2+) signals inside cytosol by ER-targeted Ca(2+) indicator

Calcium (Ca(2+)) is a versatile intracellular second messenger that operates in various signaling pathways leading to multiple biological outputs. The diversity of spatiotemporal patterns of Ca(2+) signals, generated by the coordination of Ca(2+) influx from the extracellular space and Ca(2+) release from the intracellular Ca(2+) store the endoplasmic reticulum (ER), is considered to underlie the diversity of biological outputs caused by a single signaling molecule. However, such Ca(2+) signaling diversity has not been well described because of technical limitations. Here, we describe a new method to report Ca(2+) signals at subcellular resolution. We report that OER-GCaMP6f, a genetically encoded Ca(2+) indicator (GECI) targeted to the outer ER membrane, can monitor Ca(2+) release from the ER at higher spatiotemporal resolution than conventional GCaMP6f. OER-GCaMP6f was used for in vivo Ca(2+) imaging of C. elegans. We also found that the spontaneous Ca(2+) elevation in cultured astrocytes reported by OER-GCaMP6f showed a distinct spatiotemporal pattern from that monitored by plasma membrane-targeted GCaMP6f (Lck-GCaMP6f); less frequent Ca(2+) signal was detected by OER-GCaMP6f, in spite of the fact that Ca(2+) release from the ER plays important roles in astrocytes. These findings suggest that targeting of GECIs to the ER outer membrane enables sensitive detection of Ca(2+) release from the ER at subcellular resolution, avoiding the diffusion of GECI and Ca(2+). Our results indicate that Ca(2+) imaging with OER-GCaMP6f in combination with Lck-GCaMP6f can contribute to describing the diversity of Ca(2+) signals, by enabling dissection of Ca(2+) signals at subcellular resolution.

Calcium signaling and secretion in cholangiocytes

Alcoholic hepatitis affects up to one-third of individuals who abuse alcohol and can be associated with high mortality. Although this disorder is characterized by hepatocellular damage, steatosis and neutrophil infiltration, recent evidence suggests that cholestasis or impaired bile secretion may be a frequent occurrence as well. Bile secretion results from the concerted activity of hepatocytes and cholangiocytes, the epithelial cells that line the bile ducts. Hepatocytes secrete bile acids and conjugated products into the bile canaliculi, which then are modified by cholangiocytes through secretion of bicarbonate and water to give rise to the final secreted bile. Here the molecular mechanisms regulating bile secretion in cholangiocytes are reviewed. Moreover, we discuss how the expression of intracellular Ca(2+) channels might be regulated in cholangiocytes, plus evidence that components of the Ca(2+) signaling machinery are altered in a range of cholestatic diseases of the bile ducts.

Dynamic regulation of a cell adhesion protein complex including CADM1 by combinatorial analysis of FRAP with exponential curve-fitting

Protein components of cell adhesion machinery show continuous renewal even in the static state of epithelial cells and participate in the formation and maintenance of normal epithelial architecture and tumor suppression. CADM1 is a tumor suppressor belonging to the immunoglobulin superfamily of cell adhesion molecule and forms a cell adhesion complex with an actin-binding protein, 4.1B, and a scaffold protein, MPP3, in the cytoplasm. Here, we investigate dynamic regulation of the CADM1-4.1B-MPP3 complex in mature cell adhesion by fluorescence recovery after photobleaching (FRAP) analysis. Traditional FRAP analysis were performed for relatively short period of around 10 min. Here, thanks to recent advances in the sensitive laser detector systems, we examine FRAP of CADM1 complex for longer period of 60 min and analyze the recovery with exponential curve-fitting to distinguish the fractions with different diffusion constants. This approach reveals that the fluorescence recovery of CADM1 is fitted to a single exponential function with a time constant (τ) of approximately 16 min, whereas 4.1B and MPP3 are fitted to a double exponential function with two τs of approximately 40-60 sec and 16 min. The longer τ is similar to that of CADM1, suggesting that 4.1B and MPP3 have two distinct fractions, one forming a complex with CADM1 and the other present as a free pool. Fluorescence loss in photobleaching analysis supports the presence of a free pool of these proteins near the plasma membrane. Furthermore, double exponential fitting makes it possible to estimate the ratio of 4.1B and MPP3 present as a free pool and as a complex with CADM1 as approximately 3:2 and 3:1, respectively. Our analyses reveal a central role of CADM1 in stabilizing the complex with 4.1B and MPP3 and provide insight in the dynamics of adhesion complex formation.

Comprehensive characterization of protein 4.1 expression in epithelium of large intestine

The protein 4.1 family consists of four members, 4.1R, 4.1N, 4.1B and 4.1G, each encoded by a distinct gene. All 4.1 mRNAs undergo extensive alternative splicing. Functionally, they usually serve as adapters that link actin-based cytoskeleton to plasma membrane proteins. It has been reported that 4.1 proteins are expressed in most animal cell types and tissues including epithelial cells and epithelial tissues. However, the expression of 4.1 proteins in large intestine has not been well characterized. In the present study, we performed RT-PCR, western blot and immunohistochemistry analysis to characterize the transcripts, the protein expression and cellular localization of 4.1 proteins in the epithelia of mouse large intestine. We show that multiple transcripts derive from each gene, including eight 4.1R isoforms, four 4.1N isoforms, four 4.1B isoforms and six 4.1G isoforms. However, at the protein level, only one or two major bands were detected, implying that not all transcripts are translated and/or the proteins do not accumulate at detectable levels. Immunohistochemistry revealed that 4.1R, 4.1N and 4.1B are all expressed at the lateral membrane as well as cytoplasm of epithelial cells, suggesting a potentially redundant role of these proteins. Our findings not only provide new insights into the structure of protein 4.1 genes but also lay the foundation for future functional studies.

The Protein 4.1 family: hub proteins in animals for organizing membrane proteins

Proteins of the 4.1 family are characteristic of eumetazoan organisms. Invertebrates contain single 4.1 genes and the Drosophila model suggests that 4.1 is essential for animal life. Vertebrates have four paralogues, known as 4.1R, 4.1N, 4.1G and 4.1B, which are additionally duplicated in the ray-finned fish. Protein 4.1R was the first to be discovered: it is a major mammalian erythrocyte cytoskeletal protein, essential to the mechanochemical properties of red cell membranes because it promotes the interaction between spectrin and actin in the membrane cytoskeleton. 4.1R also binds certain phospholipids and is required for the stable cell surface accumulation of a number of erythrocyte transmembrane proteins that span multiple functional classes; these include cell adhesion molecules, transporters and a chemokine receptor. The vertebrate 4.1 proteins are expressed in most tissues, and they are required for the correct cell surface accumulation of a very wide variety of membrane proteins including G-Protein coupled receptors, voltage-gated and ligand-gated channels, as well as the classes identified in erythrocytes. Indeed, such large numbers of protein interactions have been mapped for mammalian 4.1 proteins, most especially 4.1R, that it appears that they can act as hubs for membrane protein organization. The range of critical interactions of 4.1 proteins is reflected in disease relationships that include hereditary anaemias, tumour suppression, control of heartbeat and nervous system function. The 4.1 proteins are defined by their domain structure: apart from the spectrin/actin-binding domain they have FERM and FERM-adjacent domains and a unique C-terminal domain. Both the FERM and C-terminal domains can bind transmembrane proteins, thus they have the potential to be cross-linkers for membrane proteins. The activity of the FERM domain is subject to multiple modes of regulation via binding of regulatory ligands, phosphorylation of the FERM associated domain and differential mRNA splicing. Finally, the spectrum of interactions of the 4.1 proteins overlaps with that of another membrane-cytoskeleton linker, ankyrin. Both ankyrin and 4.1 link to the actin cytoskeleton via spectrin, and we hypothesize that differential regulation of 4.1 proteins and ankyrins allows highly selective control of cell surface protein accumulation and, hence, function. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé

Shear stress-induced Ca²⁺ mobilization in MDCK cells is ATP dependent, no matter the primary cilium

Primary cilium has emerged as mechanosensor to subtle flow variations in epithelial cells, but its role in shear stress detection remains controversial. To probe the function of this non-motile organelle in shear stress detection by cells, we compared calcium signalling responses induced by shear stress in ciliated and unciliated MDCK cells. Cytosolic free Ca²⁺ ([Ca²⁺]i) was measured using Fura-PE3 video imaging fluorescence microscopy in response to shear stress due to laminar flow (385 μl s⁻¹). Our results show that both unciliated and ciliated MDCK cells are shear stress sensitive via ATP release and autocrine feedback through purinergic receptors. However, purinergic calcium signals differed in response intensity and receptor subtypes. In unciliated cells, shear stress-induced elevation in [Ca²⁺]i was predominantly mediated through P2X receptors (P2XR). In contrast, calcium mobilization in ciliated MDCK cells resulted from P2YRs and store-operated Ca²⁺-permeable channels besides P2XRs. These findings lend support to the hypothesis that ATP release in response to shear stress is independent of the primary cilium and that transduction of mechanical strain into a specific biochemical responses stems on the mobilization of different sets of purinergic receptors.

Uncoupled IP3 receptor can function as a Ca2+-leak channel: cell biological and pathological consequences

Ca(2+) release via intracellular release channels, IP(3)Rs (inositol 1,4,5-trisphosphate receptors) and RyRs (ryanodine receptors), is perhaps the most ubiquitous and versatile cellular signalling mechanism, and is involved in a vast number of cellular processes. In addition to this classical release pathway there is limited, but yet persistent, information about less well-defined Ca(2+)-leak pathways that may play an important role in the control of the Ca(2+) load of the endo(sarco)plasmic reticulum. The mechanisms responsible for this 'basal' leak are not known, but recent data suggest that both IP(3)Rs and RyRs may also operate as Ca(2+)-leak channels, particularly in pathological conditions. Proteolytic cleavage or biochemical modification (such as hyperphosphorylation or nitrosylation), for example, occurring during conditions of cell stress or apoptosis, can functionally uncouple the cytoplasmic control domains from the channel domain of the receptor. Highly significant information has been obtained from studies of malfunctioning channels in various disorders; for example, RyRs in cardiac malfunction or genetic muscle diseases and IP(3)Rs in neurodegenerative diseases. In this review we aim to summarize the existing information about functionally uncoupled IP(3)R and RyR channels, and to discuss the concept that those channels can participate in Ca(2+)-leak pathways.

Determinants of AQP6 trafficking to intracellular sites versus the plasma membrane in transfected mammalian cells

BACKGROUND INFORMATION

Most AQPs (aquaporins) function at the plasma membrane, however AQP6 is exclusively localized to membranes of intracellular vesicles in acid-secreting type-A intercalated cells of renal collecting ducts. The intracellular distribution indicates that AQP6 has a function distinct from trans-epithelial water movement.

RESULTS

We show by mutational analyses and immunofluorescence that the N-terminus of AQP6 is a determinant for its intracellular localization. Presence or absence at the plasma membrane of AQP6 constructs was confirmed by electrophysiological methods. Addition of a GFP (green fluorescent protein) or a HA (haemagglutinin) epitope tag (GFP-AQP6 or HA-AQP6) to the N-terminus of AQP6, directed AQP6 to the plasma membranes of transfected Madin-Darby canine kidney cells. In contrast, addition of a GFP tag to the C-terminus (AQP6-GFP) caused the protein to remain intracellular, similar to untagged wild-type AQP6. Replacement of the N-terminus of AQP6 by that of AQP1 also directed AQP6 to the plasma membranes, whereas the N-terminus of AQP6 retained AQP1 in cytosolic sites.

CONCLUSION

Our results suggest that the N-terminus of AQP6 is critical for trafficking of the protein to the intracellular sites. Moreover, our studies provide an approach for future identification of proteins involved in vesicle sorting in the acid-secreting type-A intercalated cells.

Analysis of IP3 receptors in and out of cells

BACKGROUND

Inositol 1,4,5-trisphosphate receptors (IP3R) are expressed in almost all animal cells. Three mammalian genes encode closely related IP3R subunits, which assemble into homo- or hetero-tetramers to form intracellular Ca2+ channels.

SCOPE OF THE REVIEW

In this brief review, we first consider a variety of complementary methods that allow the links between IP3 binding and channel gating to be defined. How does IP3 binding to the IP3-binding core in each IP3R subunit cause opening of a cation-selective pore formed by residues towards the C-terminal? We then describe methods that allow IP3, Ca2+ signals and IP3R mobility to be examined in intact cells. A final section briefly considers genetic analyses of IP3R signalling.

MAJOR CONCLUSIONS

All IP3R are regulated by both IP3 and Ca2+. This allows them to initiate and regeneratively propagate intracellular Ca2+ signals. The elementary Ca2+ release events evoked by IP3 in intact cells are mediated by very small numbers of active IP3R and the Ca2+-mediated interactions between them. The spatial organization of these Ca2+ signals and their stochastic dependence on so few IP3Rs highlight the need for methods that allow the spatial organization of IP3R signalling to be addressed with single-molecule resolution.

GENERAL SIGNIFICANCE

A variety of complementary methods provide insight into the structural basis of IP3R activation and the contributions of IP3-evoked Ca2+ signals to cellular physiology. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Inositol 1, 4, 5-trisphosphate receptor interacts with the SNARE domain of syntaxin 1B

Inositol 1, 4, 5-trisphosphate receptors (IP(3)Rs) are intracellular ligand-gated Ca(2+) channels that mediate Ca(2+) release from the endoplasmic reticulum (ER) into the cytosol and function in diverse cellular processes including fertilization, muscle contraction, apoptosis, secretion, and synaptic plasticity. The Ca(2+) release activity of IP(3)Rs is tightly regulated by many factors including IP(3)R-binding proteins. We show that IP(3)Rs interact with syntaxin 1 (Syx1), a membrane trafficking protein that regulates various plasma-membrane ion channels including N-, P/Q, and L-type voltage-gated Ca(2+) channels, voltage-gated potassium channels, and an epithelial sodium channel. We found that a SNARE-domain of Syx1B, one of the two Syx1 isoforms, directly interacted with the type1 IP(3)R (IP(3)R1) internal coupling domain, a known modulator for channel opening. These results indicate that Syx1B is an IP(3)R-interacting protein and that its interaction may play a crucial role in regulating the channel activity of IP(3)Rs in Syx1B-expressing cells.

The type I inositol 1,4,5-trisphosphate receptor interacts with protein 4.1N to mediate neurite formation through intracellular Ca waves

Ca²⁺ waves are an important mechanism for encoding Ca²⁺ signaling information, but the molecular basis for wave formation and how this regulates neuronal function is not entirely understood. Using nerve growth factor-differentiated PC12 cells as a model system, we investigated the interaction between the type I inositol 1,4,5-trisphosphate receptor (IP3R1) and the cytoskeletal linker, protein 4.1N, to examine the relationship between Ca²⁺ wave formation and neurite development. This was examined using RNAi and overexpressed dominant negative binding regions of each protein. Confocal microscopy was used to monitor neurite formation and Ca²⁺ waves. Knockdown of IP3R1 or 4.1N attenuated neurite formation, as did binding regions of IP3R1 and 4.1N, which colocalized with endogenous 4.1N and IP3R1, respectively. Upon stimulation with the IP3-producing agonist carbachol, both RNAi and dominant negative molecules shifted signaling events from waves to homogeneous patterns of Ca²⁺ release. These findings provide evidence that IP3R1 localization, via protein 4.1N, is necessary for Ca²⁺ wave formation, which in turn mediates neurite formation.

IRBIT: it is everywhere

IRBIT (IP(3)Rs binding protein released with IP(3)) is a protein originally identified by the Mikoshiba group as an inhibitor of IP(3) receptors function. Subsequently it was found to have multiple functions and regulate the activity of diverse proteins, including regulation of HCO(3)(-) transporters to coordinate epithelial HCO(3)(-) secretion and to determine localization of the Fip1 subunit of the CPSF complex to regulate mRNA processing. This review highlights the remarkably divers functions of IRBIT that are likely only a fraction of all the potential functions of this protein.

Lateral diffusion of inositol 1,4,5-trisphosphate receptor type 1 in Purkinje cells is regulated by calcium and actin filaments

Inositol 1,4,5-trisphosphate receptor type 1 (IP(3) R1) is an intracellular Ca(2+) release channel that plays crucial roles in the functions of Purkinje cells. The dynamics of IP(3) R1 on the endoplasmic reticulum membrane and the distribution of IP(3) R1 in neurons are thought to be important for the spatial regulation of Ca(2+) release. In this study, we analyzed the lateral diffusion of IP(3) R1 in Purkinje cells in cerebellar slice cultures using fluorescence recovery after photobleaching. In the dendrites of Purkinje cells, IP(3) R1 showed lateral diffusion with an effective diffusion constant of approximately 0.30 μm(2) /s, and the diffusion of IP(3) R1 was negatively regulated by actin filaments. We found that actin filaments were also involved in the regulation of IP(3) R1 diffusion in the spine of Purkinje cells. Glutamate or quisqualic acid stimulation, which activates glutamate receptors and leads to a Ca(2+) transient in Purkinje cells, decreased the diffusion of IP(3) R1 and increased the density of actin in spines. These findings indicate that the neuronal activity-dependent augmentation of actin contributes to the stabilization of IP(3) R1 in spines.

Band 4.1 proteins are expressed in the retina and interact with both isoforms of the metabotropic glutamate receptor type 8

The function of the CNS depends on the correct regulation of neurotransmitter receptors by interacting proteins. Here, we screened a retinal cDNA library for proteins interacting with the intracellular C-terminus of the metabotropic glutamate receptor isoform 8a (mGluR8a). The band 4.1B protein binds to the C-termini of mGluR8a and mGluR8b, co-localizes with these glutamate receptors in transfected mammalian cells, facilitates their cell surface expression and inhibits the mGluR8 mediated reduction of intracellular cAMP concentrations. In contrast, no interaction with 4.1B was observed for other mGluRs tested. Amino acids encoded by exons 19 and 20 of 4.1B and a stretch of four basic amino acids present in the mGluR8 C-termini mediate the protein interaction. Besides binding to 4.1B, mGluR8 isoforms interact with 4.1G, 4.1N, and 4.1R. Because band 4.1 transcripts undergo extensive alternative splicing, we analyzed the splicing pattern of interacting regions and detected a 4.1B isoform expressed specifically in the retina. Within this tissue, mGluR8 and 4.1B, 4.1G, 4.1N, and 4.1R show a comparable distribution, being expressed in both synaptic layers and in somata of the ganglion cell layer. In summary, our studies identified band 4.1 proteins as new players for the mGluR8 mediated signal transduction.

The N-terminal domain of the type 1 Ins(1,4,5)P3 receptor stably expressed in MDCK cells interacts with myosin IIA and alters epithelial cell morphology

Cytosolic Ca(2+) controls a wide range of cellular events. The versatility of this second messenger depends on its ability to form diverse spatial and temporal patterns, including waves and oscillations. Ca(2+)-signaling patterns are thought to be determined in part by the subcellular distribution of inositol (1,4,5)-trisphosphate receptors [Ins(1,4,5)P(3)Rs] but little is currently known about how the localization of the Ins(1,4,5)P(3)R itself is regulated. Here, we report that the recruitment of GFP-tagged Ins(1,4,5)P(3)Rs in the vicinity of tight junctions in Madin-Darby canine kidney (MDCK) cells requires the N-terminal domain. Stable expression of this domain in polarized MDCK cells induced a flattened morphology, affected cytokinesis, accelerated cell migration in response to monolayer wounding and interfered with the cortical targeting of myosin IIA. In addition, downregulation of myosin IIA in polarized MDCK cells was found to mimic the effects of stable expression of the N-terminal part of Ins(1,4,5)P(3)R on cell shape and to alter localization of endogenous Ins(1,4,5)P(3)Rs. Taken together, these results support a model in which the recruitment of Ins(1,4,5)P(3)Rs at the apex of the lateral membrane in polarized MDCK cells, involves myosin IIA and might be important for the regulation of cortical actin dynamics.

IP3 receptors: some lessons from DT40 cells

Inositol-1,4,5-trisphosphate receptors (IP3Rs) are intracellular Ca2+ channels that are regulated by IP3 and Ca2+ and are modulated by many additional signals. These properties allow them to initiate and, via Ca2+-induced Ca2+ release, regeneratively propagate Ca2+ signals evoked by receptors that stimulate formation of IP3. The ubiquitous expression of IP3R highlights their importance, but it also presents problems when attempting to resolve the behavior of defined IP3R. DT40 cells are a pre-B-lymphocyte cell line in which high rates of homologous recombination afford unrivalled opportunities to disrupt endogenous genes. DT40-knockout cells with both alleles of each of the three IP3R genes disrupted provide the only null-background for analysis of homogenous recombinant IP3R. We review the properties of DT40 cells and consider three areas where they have contributed to understanding IP3R behavior. Patch-clamp recording from the nuclear envelope and Ca2+ release from intracellular stores loaded with a low-affinity Ca2+ indicator address the mechanisms leading to activation of IP(3)R. We show that IP3 causes intracellular IP3R to cluster and re-tune their responses to IP3 and Ca2+, better equipping them to mediate regenerative Ca2+ signals. Finally, we show that DT40 cells reliably count very few IP3R into the plasma membrane, where they mediate about half the Ca2+ entry evoked by the B-cell antigen receptor.

Localization and socialization: experimental insights into the functional architecture of IP3 receptors

Inositol 1,4,5-trisphosphate (IP(3))-evoked Ca(2+) signals display great spatiotemporal malleability. This malleability depends on diversity in both the cellular organization and in situ functionality of IP(3) receptors (IP(3)Rs) that regulate Ca(2+) release from the endoplasmic reticulum (ER). Recent experimental data imply that these considerations are not independent, such that-as with other ion channels-the local organization of IP(3)Rs impacts their functionality, and reciprocally IP(3)R activity impacts their organization within native ER membranes. Here, we (i) review experimental data that lead to our understanding of the "functional architecture" of IP(3)Rs within the ER, (ii) propose an updated terminology to span the organizational hierarchy of IP(3)Rs observed in intact cells, and (iii) speculate on the physiological significance of IP(3)R socialization in Ca(2+) dynamics, and consequently the emerging need for modeling studies to move beyond gridded, planar, and static simulations of IP(3)R clustering even over short experimental timescales.

Targeting and clustering of IP3 receptors: key determinants of spatially organized Ca2+ signals

Inositol 1,4,5-trisphosphate receptors (IP3R) are intracellular Ca2+ channels that are almost ubiquitously expressed in animal cells. The spatiotemporal complexity of the Ca2+ signals evoked by IP3R underlies their versatility in cellular signaling. Here we review the mechanisms that contribute to the subcellular targeting of IP3R and the dynamic interplay between IP3R that underpin their ability to generate complex intracellular Ca2+ signals.

G-protein-coupled receptor kinase-interacting proteins inhibit apoptosis by inositol 1,4,5-triphosphate receptor-mediated Ca2+ signal regulation

The inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) is an intracellular IP(3)-gated calcium (Ca(2+)) release channel and plays important roles in regulation of numerous Ca(2+)-dependent cellular responses. Many intracellular modulators and IP(3)R-binding proteins regulate the IP(3)R channel function. Here we identified G-protein-coupled receptor kinase-interacting proteins (GIT), GIT1 and GIT2, as novel IP(3)R-binding proteins. We found that both GIT1 and GIT2 directly bind to all three subtypes of IP(3)R. The interaction was favored by the cytosolic Ca(2+) concentration and it functionally inhibited IP(3)R activity. Knockdown of GIT induced and accelerated caspase-dependent apoptosis in both unstimulated and staurosporine-treated cells, which was attenuated by wild-type GIT1 overexpression or pharmacological inhibitors of IP(3)R, but not by a mutant form of GIT1 that abrogates the interaction. Thus, we conclude that GIT inhibits apoptosis by modulating the IP(3)R-mediated Ca(2+) signal through a direct interaction with IP(3)R in a cytosolic Ca(2+)-dependent manner.

Insulin promotes the association of heat shock protein 90 with the inositol 1,4,5-trisphosphate receptor to dampen its Ca2+ release activity

The inositol 1,4,5-trisphosphate receptor (IP(3)R) is a Ca(2+) release channel that plays a pivotal role in regulating intracellular Ca(2+) levels in resting cells. Three isoforms of IP(3)Rs have been identified, and they all possess a large regulatory domain that covers about 60% of the protein. This regulation is accomplished by interaction with small molecules, posttranslational modifications, and mostly protein-protein interactions. In our search for new binding partners of the IP(3)R, we found that 90-kDa heat-shock protein (Hsp90) binds to the IP(3)R. This interaction increased on stimulation of HEK293T6.11 cells with insulin but not with G(q) protein-coupled receptor (G(q)PCR) agonists. Moreover, the Hsp90 inhibitor geldanamycin (GA) disrupted the interaction between Hsp90 and the IP(3)R. Pretreatment of HEK293T6.11 cells with GA greatly increased the intracellular Ca(2+) release induced by a G(q)PCR agonist. Insulin alone did not induce any intracellular Ca(2+) release. However, insulin diminished the intracellular Ca(2+) release induced by a G(q)PCR agonist. Interestingly, GA abolished the inhibitory effect of insulin on G(q)PCR-induced intracellular Ca(2+) release. Furthermore, in our search for a mechanistic explanation to this phenomenon, we found that inhibition of kinases activated downstream of the insulin receptor greatly increased the interaction between Hsp90 and the IP(3)R. Of greater interest, we found that the simultaneous inhibition of mammalian target of rapamycin and the Src kinase almost completely disrupted the interaction between Hsp90 and the IP(3)R. These results demonstrate that insulin promotes the interaction of Hsp90 with the IP(3)R to dampen its Ca(2+) release activity by a complex mechanism involving mammalian target of rapamycin and the Src kinase.

Cell density-dependent changes in intracellular Ca2+ mobilization via the P2Y2 receptor in rat bone marrow stromal cells

Bone marrow stromal cells (BMSCs) are an interesting subject of research because they have characteristics of mesenchymal stem cells. We investigated intracellular Ca(2+) signaling in rat BMSCs. Agonists for purinergic receptors increased intracellular Ca(2+) levels ([Ca(2+)](i)). The order of potency followed ATP = UTP > ADP = UDP. ATP-induced rise in [Ca(2+)](i) was suppressed by U73122 and suramin, but not by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), suggesting the functional expression of G protein-coupled P2Y(2) receptors. RT-PCR and immunohistochemical studies also showed the expression of P2Y(2) receptors. [Ca(2+)](i) response to UTP changed with cell density. The UTP-induced rise in [Ca(2+)](i) was greatest at high density. V(max) (maximum Ca(2+) response) and EC(50) (agonist concentration that evokes 50% of V(max)) suggest that the amount and property of P2Y(2) receptors were changed by cell density. Note that UTP induced Ca(2+) oscillation at only medium cell density. Pharmacological studies indicated that UTP-induced Ca(2+) oscillation required Ca(2+) influx by store-operated Ca(2+) entry. Carbenoxolone, a gap junction blocker, enhanced Ca(2+) oscillation. Immunohistochemical and quantitative real-time PCR studies revealed that proliferating cell nuclear antigen (PCNA)-positive cells declined but the mRNA expression level of the P2Y(2) receptor increased as cell density increased. Co-application of fetal calf serum with UTP induced Ca(2+) oscillation at high cell density. These results suggest that the different patterns observed for [Ca(2+)](i) mobilization with respect to cell density may be associated with cell cycle progression.

Assembly and dynamics of proteins of the longitudinal and junctional sarcoplasmic reticulum in skeletal muscle cells

The sarcoplasmic reticulum (SR) of skeletal muscle cells is a complex network of tubules and cisternae that share a common lumen delimited by a single continuous membrane. The SR contains longitudinal and junctional domains characterized by distinctive patterns of protein localization, but how SR proteins reach and/or are retained at these sites is not known. Here, we report that the organization of longitudinal SR proteins is a slow process characterized by temporally distinct patterns of protein localization. In contrast, junctional SR proteins rapidly and synchronously assembled into clusters which, however, merged into mature triadic junctions only after completion of longitudinal SR protein organization. Fluorescence recovery after photobleaching experiments indicated that SR organization was accompanied by significant changes in the dynamic properties of longitudinal and junctional proteins. The decrease in mobility that accompanied organization of the longitudinal SR proteins ank1.5-GFP and GFP-InsP3R1 was abrogated by deletion of specific binding sites for myofibrillar or cytoskeletal proteins, respectively. Assembly of junctional SR domains was accompanied by a strong decrease in mobility of junctional proteins that in triadin appeared to be mediated by its intraluminal region. Together, the data suggest that the organization of specific SR domains results from a process of membrane reorganization accompanied by the establishment of multiple protein-protein interactions with intrinsic and extrinsic cues.

Alternatively spliced exon 5 of the FERM domain of protein 4.1R encodes a novel binding site for erythrocyte p55 and is critical for membrane targeting in epithelial cells

Direct physical linkage of MAGUKs to the actin cytoskeleton was first established by the interaction of erythrocyte p55 with the FERM domain of protein 4.1R. Subsequently, it was reported that p55 binds to a 51-amino acid peptide, encoded by exon 10, located within the FERM domain of protein 4.1R. In this study, we investigated the nature of the p55-FERM domain binding interface and show that p55 binds to a second 35-amino acid peptide, encoded by an alternatively spliced exon 5, located within the FERM domain of protein 4.1R. Competition and Surface Plasmon Resonance-binding measurements suggest that the peptides encoded by exons 5 and 10 bind to independent sites within the D5 domain of p55. Interestingly, the full length 135 kDa isoform of protein 4.1R containing both exons 5 and 10 was targeted exclusively to the plasma membrane of epithelial cells whereas the same isoform without exon 5 completely lost its membrane localization capacity. Together, these results indicate that p55 binds to two distinct sites within the FERM domain, and the alternatively spliced exon 5 is necessary for the membrane targeting of protein 4.1R in epithelial cells. Since sequences similar to the exon 5-peptide of protein 4.1R and D5 domain of p55 are conserved in many proteins, our findings suggest that a similar mechanism may govern the membrane targeting of other FERM domain containing proteins.

Regulation of inositol 1,4,5-trisphosphate receptor-mediated calcium release by the Na/K-ATPase in cultured renal epithelial cells

It is known that the Na/K-ATPase alpha1 subunit interacts directly with inositol 1,4,5-triphosphate (IP(3)) receptors. In this study we tested whether this interaction is required for extracellular stimuli to efficiently regulate endoplasmic reticulum (ER) Ca(2+) release. Using cultured pig kidney LLC-PK1 cells as a model, we demonstrated that graded knockdown of the cellular Na/K-ATPase alpha1 subunit resulted in a parallel attenuation of ATP-induced ER Ca(2+) release. When the knockdown cells were rescued by knocking in a rat alpha1, the expression of rat alpha1 restored not only the cellular Na/K-ATPase but also ATP-induced ER Ca(2+) release. Mechanistically, this defect in ATP-induced ER Ca(2+) release was neither due to the changes in the amount or the function of cellular IP(3) and P2Y receptors nor the ER Ca(2+) content. However, the alpha1 knockdown did redistribute cellular IP(3) receptors. The pool of IP(3) receptors that resided close to the plasma membrane was abolished. Because changes in the plasma membrane proximity could reduce the efficiency of signal transmission from P2Y receptors to the ER, we further determined the dose-dependent effects of ATP on protein kinase Cepsilon activation and ER Ca(2+) release. The data showed that the alpha1 knockdown de-sensitized the ATP-induced ER Ca(2+) release but not PKCepsilon activation. Moreover, expression of the N terminus of Na/K-ATPase alpha1 subunit not only disrupted the formation of the Na/K-ATPase-IP(3) receptor complex but also abolished the ATP-induced Ca(2+) release. Finally, we observed that the alpha1 knockdown was also effective in attenuating ER Ca(2+) release provoked by angiotensin II and epidermal growth factor.

IP3 receptor/Ca2+ channel: from discovery to new signaling concepts

Inositol 1,4,5-trisphosphate (IP(3)) is a second messenger that induces the release of Ca(2+) from the endoplasmic reticulum (ER). The IP(3) receptor (IP(3)R) was discovered as a developmentally regulated glyco-phosphoprotein, P400, that was missing in strains of mutant mice. IP(3)R can allosterically and dynamically change its form in a reversible manner. The crystal structures of the IP(3)-binding core and N-terminal suppressor sequence of IP(3)R have been identified. An IP(3) indicator (known as IP(3)R-based IP(3) sensor) was developed from the IP(3)-binding core. The IP(3)-binding core's affinity to IP(3) is very similar among the three isoforms of IP(3)R; instead, the N-terminal IP(3) binding suppressor region is responsible for isoform-specific IP(3)-binding affinity tuning. Various pathways for the trafficking of IP(3)R have been identified; for example, the ER forms a meshwork upon which IP(3)R moves by lateral diffusion, and vesicular ER subcompartments containing IP(3)R move rapidly along microtubles using a kinesin motor. Furthermore, IP(3)R mRNA within mRNA granules also moves along microtubules. IP(3)Rs are involved in exocrine secretion. ERp44 works as a redox sensor in the ER and regulates IP(3)R1 activity. IP(3) has been found to release Ca(2+), but it also releases IRBIT (IP(3)R-binding protein released with IP(3)). IRBIT is a pseudo-ligand for IP(3) that regulates the frequency and amplitude of Ca(2+) oscillations through IP(3)R. IRBIT binds to pancreas-type Na, bicarbonate co-transporter 1, which is important for acid-base balance. The presence of many kinds of binding partners, like homer, protein 4.1N, huntingtin-associated protein-1A, protein phosphatases (PPI and PP2A), RACK1, ankyrin, chromogranin, carbonic anhydrase-related protein, IRBIT, Na,K-ATPase, and ERp44, suggest that IP(3)Rs form a macro signal complex and function as a center for signaling cascades. The structure of IP(3)R1, as revealed by cryoelectron microscopy, fits closely with these molecules.

Lipid rafts establish calcium waves in hepatocytes

BACKGROUND & AIMS

Polarity is critical for hepatocyte function. Ca(2+) waves are polarized in hepatocytes because the inositol 1,4,5-trisphosphate receptor (InsP3R) is concentrated in the pericanalicular region, but the basis for this localization is unknown. We examined whether pericanalicular localization of the InsP3R and its action to trigger Ca(2+) waves depends on lipid rafts.

METHODS

Experiments were performed using isolated rat hepatocyte couplets and pancreatic acini, plus SkHep1 cells as nonpolarized controls. The cholesterol depleting agent methyl-beta-cyclodextrin (mbetaCD) was used to disrupt lipid rafts. InsP3R isoforms were examined by immunoblot and immunofluorescence. Ca(2+) waves were examined by confocal microscopy.

RESULTS

Type II InsP3Rs initially were localized to only some endoplasmic reticulum fractions in hepatocytes, but redistributed into all fractions in mbetaCD-treated cells. This InsP3R isoform was concentrated in the pericanalicular region, but redistributed throughout the cell after mbetaCD treatment. Vasopressin-induced Ca(2+) signals began as apical-to-basal Ca(2+) waves, and mbetaCD slowed the wave speed and prolonged the rise time. MbetaCD had a similar effect on Ca(2+) waves in acinar cells but did not affect Ca(2+) signals in SkHep1 cells, suggesting that cholesterol depletion has similar effects among polarized epithelia, but this is not a nonspecific effect of mbetaCD.

CONCLUSIONS

Lipid rafts are responsible for the pericanalicular accumulation of InsP3R in hepatocytes, and for the polarized Ca(2+) waves that result. Signaling microdomains exist not only in the plasma membrane, but also in the nearby endoplasmic reticulum, which in turn, helps establish and maintain structural and functional polarity.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.

[Regulation of G protein-coupled receptor function by its binding proteins]

G protein-coupled receptors (GPCRs) are seven transmembrane receptors with an N-terminus in the extracellular region and C-terminus in the intracellular region. When an agonist binds to a GPCR, a signal is transduced into a cell through the activation of trimeric G proteins. Recently, it has been shown that the activities of GPCRs are regulated by multiple mechanisms. One of the mechanisms is regulation through the binding proteins to the carboxy (C)-terminus of GPCRs. In the present study, the binding partners for the C-terminus of the parathyroid hormone receptor (PTHR) and thromboxane A(2) receptor (TP) were searched for using yeast two-hybrid screening, and the functions of these proteins were investigated. We identified t-complex testis expressed-1 (Tctex-1) and 4.1G as associated proteins for the PTHR. Tctex-1 is one of the light chains of cytoplasmic dynein, which is a motor protein across microtubles. We found that Tctex-1 was involved in agonist-induced internalization of the PTHR. 4.1G, a cytoskeletal protein, facilitated the cell surface localization of the PTHR and augmented PHTR-mediated signal transduction. TPs consists of two splicing variants, TPalpha and TPbeta. As a result of yeast two-hybrid screening, two proteasomal proteins, proteasome activator PA28gamma and proteasome subunit alpha7, were identified as direct interacting proteins for TPbeta. TPbeta has a tendency to be retained in the intracellular compartment, probably due to its binding to proteasomes. We also demonstrated that TPalpha and TPbeta formed heterodimers, and the signal transduction through TPalpha was reduced by the formation of heterodimers. In conclusion, the proteins bound to GPCRs may regulate the intracellular traffic of GPCRs.

The inositol 1,4,5-trisphosphate receptor (IP3R) and its regulators: sometimes good and sometimes bad teamwork

In both nonexcitable and excitable cells, the inositol 1,4,5-trisphosphate receptor (IP(3)R) is the primary cytosolic target responsible for the initiation of intracellular calcium (Ca(2+)) signaling. To fulfill this function, the IP(3)R depends on interaction with accessory subunits and regulatory proteins. These include proteins that reside in the lumen of the endoplasmic reticulum (ER), such as chromogranin A and B and ERp44, and cytosolic proteins, such as neuronal Ca(2+) sensor 1, huntingtin, cytochrome c, IP(3)R-binding protein released with inositol 1,4,5-trisphosphate, Homer, and 4.1N. Specific interactions between these modulatory proteins and the IP(3)R have been described, making it clear that the controlled modulation of the IP(3)R by its binding partners is necessary for physiological cell regulation. The functional coupling of these modulators with the IP(3)R can control apoptosis, intracellular pH, the initiation and regulation of neuronal Ca(2+) signaling, exocytosis, and gene expression. The pathophysiological relevance of IP(3)R modulation is apparent when the functional interaction of these proteins is enhanced or abolished by mutation or overexpression. The subsequent deregulation of the IP(3)R leads to pathological changes in Ca(2+) signaling, signal initiation, the amplitude and frequency of Ca(2+) signals, and the duration of the Ca(2+) elevation. Consequences of this deregulation include abnormal growth and apoptosis. Complex regulation of Ca(2+) signaling is required for the cell to live and function, and this difficult task can only be managed when the IP(3)R teams up and acts properly with its numerous binding partners.

Expression of protein 4.1G in Schwann cells of the peripheral nervous system

The membrane-associated cytoskeletal proteins, including protein 4.1 family, play important roles in membrane integrity, protein targeting, and signal transduction. Although protein 4.1G (4.1G) is expressed ubiquitously in mammalian tissues, it can have very discrete distributions within cells. The present study investigated the expression and distributions of 4.1G in rodent sciatic nerve. Northern and Western blot analysis detected abundant 4.1G mRNA and protein in rat sciatic nerve extracts. Immunohistochemical staining with a 4.1G-specific antibody and double immunolabeling with E-cadherin, betaIV spectrin, and connexin 32 detected 4.1G in paranodal loops, Schmidt-Lanterman incisures, and periaxonal, mesaxonal, and abaxonal membranes of rodent sciatic nerve. Immunoelectron microscopy confirmed the immunodistribution of 4.1G in Schwann cells. In developing mouse sciatic nerves, 4.1G was diffusely distributed in immature Schwann cells and gradually localized at paranodes, incisures, and periaxonal and mesaxonal membranes during their maturation. These data support the concept that 4.1G plays an important role in the membrane expansion and specialization that occurs during formation and maintenance of myelin internodes in the peripheral nervous system.

Distinct Role of the N-terminal Tail of the Na,K-ATPase Catalytic Subunit as a Signal Transducer

Mounting evidence suggests that the ion pump, Na,K-ATPase, can, in the presence of ouabain, act as a signal transducer. A prominent binding motif linking the Na,K-ATPase to intracellular signaling effectors has, however, not yet been identified. Here we report that the N-terminal tail of the Na,K-ATPase catalytic alpha-subunit (alphaNT-t) binds directly to the N terminus of the inositol 1,4,5-trisphosphate receptor. Three amino acid residues, LKK, conserved in most species and most alpha-isoforms, are essential for the binding to occur. In wild-type cells, low concentrations of ouabain trigger low frequency calcium oscillations that activate NF-kappaB and protect from apoptosis. All of these effects are suppressed in cells overexpressing a peptide corresponding to alphaNT-t but not in cells overexpressing a peptide corresponding to alphaNT-t deltaLKK. Thus we have identified a well conserved Na,K-ATPase motif that binds to the inositol 1,4,5-trisphosphate receptor and can trigger an anti-apoptotic calcium signal.

Inositol 1,4,5-trisphosphate (IP3) receptors and their role in neuronal cell function

Inositol 1,4,5-trisphosphate (IP(3)) receptor is a Ca(2+) release channel localized on the endoplasmic reticulum (ER) and plays an important role in neuronal function. IP(3) receptor was discovered as a developmentally regulated protein missing in the cerebellar mutant mice. Recent studies indicate that IP(3)Rs are involved in early development and neuronal plasticity. IP(3) works to release IRBIT from the IP(3) binding core in addition to release Ca(2+). IRBIT binds to and activates Na, Bicarbonate cotransporter. Electron microscopic study show the IP(3) receptor has allosteric property to change its form from square to windmill in the presence of Ca(2+). IP(3)R associates with ERp44, a redox sensor, Homer, other proteins and is transported as vesicular ER on microtubules. All these data suggests IP(3) receptor/CA(2+) channel works as a signaling center inside cells.

4.1N binding regions of inositol 1,4,5-trisphosphate receptor type 1

Zhang et al. and Maximov et al. [S. Zhang, A. Mizutani, C. Hisatsune, T. Higo, H. Bannai, T. Nakayama, M. Hattori, and K. Mikoshiba, Protein 4.1N is required for translocation of inositol 1,4,5-trisphosphate receptor type 1 to the basolateral membrane domain in polarized Madin-Darby canine kidney cells, J. Biol. Chem. 278 (2003) 4048-4056; A. Maximov, T. S. Tang, and I. Bezprozvanny, Association of the type 1 inositol (1,4,5)-trisphosphate receptor with 4.1N protein in neurons, Mol. Cell. Neurosci. 22 (2003) 271-283.] reported that 4.1N is a binding partner of inositol 1,4,5-trisphosphate receptor type 1 (IP(3)R1), however the binding site of IP(3)R1 differed: the former determined the C-terminal 14 amino acids of the cytoplasmic tail (CTT14aa) as the binding site, while the latter assigned another segment, cytoplasmic tail middle 1 (CTM1). To solve this discrepancy, we performed immunoprecipitation and found that both the segments had binding activity to 4.1N. Both segments also interfered the 4.1N-regulated IP(3)R1 diffusion in neuronal dendrites. However, IP(3)R1 lacking the CTT14aa (IP(3)R1-DeltaCTT14aa) does not bind to 4.1N [S. Zhang, A. Mizutani, C. Hisatsune, T. Higo, H. Bannai, T. Nakayama, M. Hattori, and K. Mikoshiba, Protein 4.1N is required for translocation of inositol 1,4,5-trisphosphate receptor type 1 to the basolateral membrane domain in polarized Madin-Darby canine kidney cells, J. Biol. Chem. 278 (2003) 4048-4056.] and its diffusion constant is larger than that of IP(3)R1 full-length in neuronal dendrites [K. Fukatsu, H. Bannai, S. Zhang, H. Nakamura, T. Inoue, and K. Mikoshiba, Lateral diffusion of inositol 1,4,5-trisphosphate receptor type 1 is regulated by actin filaments and 4.1N in neuronal dendrites, J. Biol. Chem. 279 (2004) 48976-48982.]. We conclude that both the CTT14aa and CTM1 sequences can bind to 4.1N in peptide fragment forms. However, we propose that the responsible binding site for 4.1N binding in full-length tetramer form of IP(3)R1 is CTT14aa.

Increase in cell-surface localization of parathyroid hormone receptor by cytoskeletal protein 4.1G

The cell-surface localization of GPCRs (G-protein-coupled receptors) has emerged as one of critical factors of the GPCR-mediated signal transduction. It has been reported that the C-termini of GPCRs contain the sequences for sorting the receptors to cell surface. In the present study, we have searched for proteins that interact with the C-terminus of PTH (parathyroid hormone)/PTH-related protein receptor (PTHR) by using the yeast two-hybrid system, and identified a cytoskeletal protein 4.1G (generaltype 4.1 protein) as an interactant with the C-terminus. Immunohistochemical study revealed that both PTHR and 4.1G were co-localized on plasma membranes, when they were transiently expressed in COS-7 cells. When 4.1G or the C-terminal domain of 4.1G (4.1G-CTD), a dominant-negative form of 4.1G, was co-expressed with PTHR in COS-7 cells, 4.1G, but not 4.1G-CTD, facilitated the cell-surface localization of PTHR, determined by cell-surface biotinylation assay. PTH-(1-34) caused phosphorylation of ERK (extracellular-signal-regulated kinase) 1/2 in PTHR-expressed cells mainly mediated through EGF (epidermal growth factor) receptor. The phosphorylation was enhanced by the expression of 4.1G, but not 4.1G-CTD. PTH-(1-34) elevated [Ca2+]i (intracellular Ca2+ concentration) independent of EGF receptor activation, and the elevation was enhanced by the expression of 4.1G, but not 4.1G-CTD. These data indicate that 4.1G facilitates the cell-surface localization of PTHR through its interaction with the C-terminus of the receptor, resulting in the potentiation of PTHR-mediated signal transduction.

Visualization of inositol 1,4,5-trisphosphate receptor by atomic force microscopy

Inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) acts as a ligand-gated channel that mediates neuronal signals by releasing Ca(2+) from the endoplasmic reticulum. The three-dimensional (3D) structure of tetrameric IP(3)R has been demonstrated by using electron microscopy (EM) with static specimens; however, the dynamic aspects of the IP(3)R structure have never been visualized in a native environment. Here we attempt to measure the surface topography of IP(3)R in solution using atomic force microscopy (AFM). AFM revealed large protrusions extending approximately 4.3 nm above a flat membrane prepared from Spodoptera frugiperda (Sf9) cells overexpressing mouse type 1 IP(3)R (Sf9-IP(3)R1). The average diameter of the large protrusions was approximately 32 nm. A specific antibody against a cytosolic epitope close to the IP(3)-binding site enabled us to gold-label the Sf9-IP(3)R1 membrane as confirmed by EM. AFM images of the gold-labeled membrane revealed 7.7-nm high protrusions with a diameter of approximately 30 nm, which should be IP(3)R1-antibody complexes. Authentic IP(3)R1 immuno-purified from mouse cerebella had approximately the same dimensions as those of the IP(3)R-like protrusions on the membrane. Altogether, these results suggest that the large protrusions on the Sf9-IP(3)R1 membrane correspond to the cytosolic domain of IP(3)R1. Our study provides the first 3D representation of individual IP(3)R1 particles in an aqueous solution.

Overexpression of the small transmembrane and glycosylated protein SMAGP supports metastasis formation of a rat pancreatic adenocarcinoma line

Small cell transmembrane and glycosylated protein (SMAGP) was recently identified in the metastasizing rat pancreatic adenocarcinoma line BSp73ASML. SMAGP, an evolutionary conserved transmembrane protein, is expressed on lateral epithelial cell membranes. SMAGP expression was restricted to or was upregulated in several metastasizing as compared to nonmetastasizing human and rat tumor lines. In contrast to nontransformed tissue, SMAGP was mainly expressed in the cytoplasm, as has already been described for high-grade human colorectal cancer. This raised the question on the impact of SMAGP on tumor progression. To answer the question, metastasis formation was evaluated in the nonmetastasizing rat pancreatic adenocarcinoma subline BSp73AS (AS), which was stably transfected with SMAGP cDNA (AS-SMAGP). Cytoplasmic SMAGP expression promoted cell agglomeration, but inhibited tumor cell proliferation, adhesion to and migration toward vitronectin and matrigel invasion, which was accompanied by a failure of actin reorganization. AS-SMAGP clones strongly promoted metastasis formation by dislodgment of normal tissue; 82% of rats developed lymph node metastasis as compared to 22% of rats receiving AS or mock-cDNA-transfected AS cells. The incidence of lung metastasis was increased from 6% in AS to 98% in AS-SMAGP tumor-bearing rats. Thus, SMAGP strongly promotes tumor progression. This likely is due to redistribution from the plasma membrane into the cytoplasm. SMAGP redistribution does not only facilitate tumor cell detachment from neighboring cells and the extracellular matrix, but obviously contributes actively by a not yet defined mechanism to tumor cell agglomeration and capillary plugging.

Protein 4.1N does not interact with the inositol 1,4,5-trisphosphate receptor in an epithelial cell line

Cytosolic Ca2+ regulates a variety of cell functions, and the spatial patterns of Ca2+ signals are responsible in part for the versatility of this second messenger. The subcellular distribution of the inositol 1,4,5-trisphosphate receptor (IP3R) is thought to regulate Ca2+-signaling patterns but little is known about how the distribution of the IP3R itself is regulated. Here we examined the relationship between the IP3R and the cytoskeletal linker protein 4.1N in the polarized WIF-B cell line because protein 4.1N regulates targeting of the type I IP3R in neurons, but WIF-B cells do not express this cytoskeletal protein. WIF-B cells expressed all three isoforms of the IP3R, and each isoform was distributed throughout the cell. These cells did not express the ryanodine receptor. Photorelease of microinjected, caged IP3 induced a rapid rise in cytosolic Ca2+, but the increase began uniformly throughout the cell rather than at a specific initiation site. Expression of protein 4.1N was not associated with redistribution of the IP3R or changes in Ca2+-signaling patterns. These findings are consistent with the hypothesis that the subcellular distribution of IP3R isoforms regulates the formation of Ca2+ waves, and the finding that interactions between protein 4.1N and the IP3R vary among cell types may provide an additional, tissue-specific mechanism to shape the pattern of Ca2+ waves.

Dynamics of the Ins(1,4,5)P3 receptor during polarization of MDCK cells

BACKGROUND INFORMATION

The uneven distribution of the Ins(1,4,5)P3R [Ins(1,4,5)P3 receptor] within the ER (endoplasmic reticulum) membrane generates spatially complex Ca2+ signals. The ER is a dynamic network, which allows the rapid diffusion of membrane proteins from one part of the cell to another. However, little is known about the localization and the dynamics of the Ins(1,4,5)P3R in the ER of living cells. We have used a MDCK (Madin-Darby canine kidney) clone stably expressing the Ins(1,4,5)P3R1-GFP (where GFP stands for green fluorescent protein) to investigate the effect of cell polarity on the lateral mobility of the Ins(1,4,5)P3R.

RESULTS

In non-confluent MDCK cells, the chimaera is homogeneously distributed throughout the ER and the nuclear envelope. FRAP (fluorescence recovery after photobleaching) experiments showed that the receptor can move freely in the ER with a diffusion constant (D=0.01 microm2/s) approx. ten times lower than other ER membrane proteins. In confluent polarized cells, two populations of receptor can be defined: one population is distributed in the cytoplasm and is mobile but with a slower diffusion constant (D=0.004 microm2/s) compared with non-confluent cells, whereas the other population is concentrated at the periphery of the cells and is apparently immobile.

CONCLUSIONS

The observed differences in the mobility of the Ins(1,4,5)P3R are most probably due to its interactions with stable protein complexes that form at the periphery of the polarized cells.

Topographical significance of membrane skeletal component protein 4.1 B in mammalian organs

The polarized architecture of epithelial cells is a fundamental determinant of cell structures and functions. Both formation and orientation of proper epithelial polarity are needed for cell-cell or cell-matrix adhesion, signal transduction and cytoskeletal interactions of multimolecular complexes at apical, lateral and basal cell membranes. These cell membrane domains are usually segregated by some junctional complexes. Recent molecular genetic studies on the anchor structure between myelin sheaths and axons have indicated the specific molecular organization for polarization of axolemma and the myelin sheaths at paranodes, termed 'septate-like junctions'. It was also speculated that other mammalian organs may use a similar junctional system. The protein 4.1 B was originally found to be localized in paranodes and juxtaparanodes of myelinated nerve fibers. Our recent immunohistochemical studies on protein 4.1B have indicated its significance for the cell-cell and/or cell-matrix adhesion in various rodent organs. The protein 4.1 family of proteins have been supposed to possess variable molecular domains relating to cell adhesion, ion balance, receptor responses and signal transduction. Therefore, more precise studies on the molecular structure and the functional domains of protein 4.1B, as well as on its changes under physiological and pathological conditions, may provide a clue for organogenesis in various mammalian organs.

The inositol 1,4,5-trisphosphate receptors

The inositol (1,4,5)-trisphosphate receptors (InsP3R) are the intracellular calcium (Ca2+) release channels that play a key role in Ca2+ signaling in cells. Three InsP3R isoforms-InsP3R type 1 (InsP3R1), InsP3R type 2 (InsP3R2), and InsP3R type 3 (InsP3R3) are expressed in mammals. A single InsP3R isoform is expressed in Drosophila melanogaster (DmInsP3R) and Caenorhabditis elegans (CeInsP3R). The progress made during last decade towards understanding the function and the properties of the InsP3R is briefly reviewed in this chapter. The main emphasis is on studies that revealed structural determinants responsible for the ligand recognition by the InsP3R, ion permeability of the InsP3R, modulation of the InsP3R by cytosolic Ca2+, ATP and PKA phosphorylation and on the recently identified InsP3R-binding partners. The main focus is on the InsP3R1, but the recent information about properties of other InsP3R isoforms is also discussed.

Immunolocalization of protein 4.1B in the rat digestive system

Protein 4.1 family proteins are thought to interact with membrane proteins and also membrane skeletons. In this study, immunohistochemical studies by light and electron microscopy were performed with a specific antibody against protein 4.1B. Specific protein 4.1B immunolabeling was observed in simple columnar epithelium in the adult rat large intestine, small intestine and stomach. Protein 4.1B immunolabeling was localized along the membranes facing the adjacent cells (lateral portion) and also facing the extracellular matrix (basal portion). Moreover, a spatial protein 4.1B expression gradient was observed along the crypt-villus axis of the rat small and large intestinal epithelium: strong protein 4.1B expression was present within the villus, with the crypt showing barely any detectable protein 4.1B. The expression of protein 4.1B was not detected in the stratified squamous epithelium in the forestomach or the esophagus. By immunoelectron microscopy, the immunolabeling of the cells was observed to be restricted to the cytoplasmic side just beneath the plasma membrane, including the membranes adjacent to the next cells, except for the tight junctions. We conclude that the protein 4.1B expression pattern is related to the maturation of simple columnar epithelium in the rat digestive system, probably by the effect of adhesion.