Nir2, a novel regulator of cell morphogenesis

ER-organelle contacts: A signaling hub for neurological diseases

Neuronal health is closely linked to the homeostasis of intracellular organelles, and organelle dysfunction affects the pathological progression of neurological diseases. In contrast to isolated cellular compartments, a growing number of studies have found that organelles are largely interdependent structures capable of communicating through membrane contact sites (MCSs). MCSs have been identified as key pathways mediating inter-organelle communication crosstalk in neurons, and their alterations have been linked to neurological disease pathology. The endoplasmic reticulum (ER) is a membrane-bound organelle capable of forming an extensive network of pools and tubules with important physiological functions within neurons. There are multiple MCSs between the ER and other organelles and the plasma membrane (PM), which regulate a variety of cellular processes. In this review, we focus on ER-organelle MCSs and their role in a variety of neurological diseases. We compared the biological effects between different tethering proteins and the effects of their respective disease counterparts. We also discuss how altered ER-organelle contacts may affect disease pathogenesis. Therefore, understanding the molecular mechanisms of ER-organelle MCSs in neuronal homeostasis will lay the foundation for the development of new therapies targeting ER-organelle contacts.

YAP Signaling Regulates the Cellular Uptake and Therapeutic Effect of Nanoparticles

Interactions between living cells and nanoparticles are extensively studied to enhance the delivery of therapeutics. Nanoparticles size, shape, stiffness, and surface charge are regarded as the main features able to control the fate of cell-nanoparticle interactions. However, the clinical translation of nanotherapies has so far been limited, and there is a need to better understand the biology of cell-nanoparticle interactions. This study investigates the role of cellular mechanosensitive components in cell-nanoparticle interactions. It is demonstrated that the genetic and pharmacologic inhibition of yes-associated protein (YAP), a key component of cancer cell mechanosensing apparatus and Hippo pathway effector, improves nanoparticle internalization in triple-negative breast cancer cells regardless of nanoparticle properties or substrate characteristics. This process occurs through YAP-dependent regulation of endocytic pathways, cell mechanics, and membrane organization. Hence, the study proposes targeting YAP may sensitize triple-negative breast cancer cells to chemotherapy and increase the selectivity of nanotherapy.

In vivo identification of novel TGIF2LX target genes in colorectal adenocarcinoma using the cDNA-AFLP method

BACKGROUND AND STUDY AIMS

Homeobox-containing genes are composed of a group of regulatory genes encoding transcription factors involved in the control of developmental processes. The homeodomain proteins could activate or repress the expression of downstream target genes. This study was conducted to in vivo identify the potential target gene(s) of TGIF2LX in colorectal adenocarcinoma.

METHODS

A human colorectal adenocarcinoma cell line, SW48, was transfected with the recombinant pEGFPN1-TGIF2LX. The cells were injected subcutaneously into the flank of the three groups of 6-week-old female athymic C56BL/6 nude mice (n = 6 per group). The transcript profiles in the developed tumours were investigated using the cDNA amplified fragment length polymorphism (cDNA-AFLP) technique.

RESULTS

The real-time RT-PCR and DNA sequencing data for the identified genes indicated that the N-terminal domain-interacting receptor 1 (Nir1) gene was suppressed whereas Nir2 and fragile histidine triad (FHIT) genes were upregulated followed by the overexpression of TGIF2LX gene.

CONCLUSION

Downregulation of Nir1 and upregulation of Nir2 and FHIT genes due to the overexpression of TGIF2LX suggests that the gene plays an important role as a suppressor in colorectal adenocarcinoma.

The lipid-transfer protein Nir2 enhances epithelial-mesenchymal transition and facilitates breast cancer metastasis

The involvement of epithelial-mesenchymal transition (EMT) in breast cancer metastasis has been demonstrated in many studies. However, the intracellular proteins and signaling pathways that regulate EMT have not been fully identified. Here, we show that the lipid-transfer protein Nir2 (also known as PITPNM1) enhances EMT in mammary epithelial and breast cancer cells. Nir2 overexpression decreases the expression of epithelial markers and concomitantly increases the expression of mesenchymal markers, whereas silencing of Nir2 expression by small hairpin RNA (shRNA) has opposite effects. Additionally, Nir2 expression is increased during EMT and affects cell morphology, whereas Nir2 depletion attenuates growth factor-induced cell migration. These effects of Nir2 on EMT-associated processes are mainly mediated through the PI3K/AKT and the ERK1/2 pathways. Nir2 depletion also inhibits cell invasion in vitro and lung metastasis in animal models. Immunohistochemical analysis of breast cancer tissue samples reveals a correlation between high Nir2 expression and tumor grade, and Kaplan-Meier survival curves correlate Nir2 expression with poor disease outcome. These results suggest that Nir2 not only enhances EMT in vitro and breast cancer metastasis in animal models, but also contributes to breast cancer progression in human patients.

Pitpnm1 is expressed in hair cells during development but is not required for hearing

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We studied the expression of Pitpnm1 in the developing mouse inner ear.

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We covered several ages between E14.5 and P5, and also looked at adults.

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Pitpnm1 is expressed in the inner hair cells from before birth to adulthood.

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Pitpnm1 is expressed transiently in the outer hair cells at early postnatal stages.

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Mice lacking Pitpnm1 display no obvious auditory defects.

Deafness is a genetically complex disorder with many contributing genes still unknown. Here we describe the expression of Pitpnm1 in the inner ear. It is expressed in the inner hair cells of the organ of Corti from late embryonic stages until adulthood, and transiently in the outer hair cells during early postnatal stages. Despite this specific expression, Pitpnm1 null mice showed no hearing defects, possibly due to redundancy with the paralogous genes Pitpnm2 and Pitpnm3.

The Diverse Biological Functions of Phosphatidylinositol Transfer Proteins in Eukaryotes

Phosphatidylinositol/phosphatidylcholine transfer proteins (PITPs) remain largely functionally uncharacterized, despite the fact that they are highly conserved and are found in all eukaryotic cells thus far examined by biochemical or sequence analysis approaches. The available data indicate a role for PITPs in regulating specific interfaces between lipid-signaling and cellular function. In this regard, a role for PITPs in controlling specific membrane trafficking events is emerging as a common functional theme. However, the mechanisms by which PITPs regulate lipid-signaling and membrane-trafficking functions remain unresolved. Specific PITP dysfunctions are now linked to neurodegenerative and intestinal malabsorption diseases in mammals, to stress response and developmental regulation in higher plants, and to previously uncharacterized pathways for regulating membrane trafficking in yeast and higher eukaryotes, making it clear that PITPs are integral parts of a highly conserved signal transduction strategy in eukaryotes. Herein, we review recent progress in deciphering the biological functions of PITPs, and discuss some of the open questions that remain.

Regulation of phosphoinositide levels by the phospholipid transfer protein Sec14p controls Cdc42p/p21-activated kinase-mediated cell cycle progression at cytokinesis

Sec14p is an essential phosphatidylcholine/phosphatidylinositol transfer protein with a well-described role in the regulation of Golgi apparatus-derived vesicular transport in yeast. Inactivation of the CDP-choline pathway for phosphatidylcholine synthesis allows cells to survive in the absence of Sec14p function through restoration of Golgi vesicular transport capability. In this study, Saccharomyces cerevisiae cells containing a SEC14 temperature-sensitive allele along with an inactivated CDP-choline pathway were transformed with a high-copy-number yeast genomic library. Genes whose increased expression inhibited cell growth in the absence of Sec14p function were identified. Increasing levels of the Rho GTPase Cdc42p and its direct effector kinases Cla4p and Ste20p prevented the growth of cells lacking Sec14p and CDP-choline pathway function. Growth suppression was accompanied by an increase in large and multiply budded cells. This effect on polarized cell growth did not appear to be due to an inability to establish cell polarity, since both the actin cytoskeleton and localization of the septin Cdc12p were unaffected by increased expression of Cdc42p, Cla4p, or Ste20p. Nuclei were present in both the mother cell and the emerging bud, consistent with Sec14p regulation of the cell cycle subsequent to anaphase but prior to cytokinesis/septum breakdown. Increased expression of phosphatidylinositol 4-kinases and phosphatidylinositol 4-phosphate 5-kinase prevented growth arrest by CDC42, CLA4, or STE20 upon inactivation of Sec14p function. Sec14p regulation of phosphoinositide levels affects cytokinesis at the level of the Cdc42p/Cla4p/Ste20p signaling cascade.

RdgB proteins: Functions in lipid homeostasis and signal transduction

The RdgBs are a group of evolutionarily conserved molecules that contain a phosphatidylinositol transfer protein (PITP) domain. However in contrast to classical PITPs (PITPalpha) with whom they share the conserved PITP domain, these proteins also contain several additional sequence elements whose functional significance remains unknown. The founding member of the family DrdgB alpha (Drosophila rdgB) appears to be essential for sensory transduction and maintenance of ultra structure in photoreceptors (retinal sensory neurons). Although proposed to support the maintenance of phosphatidylinositol 4, 5 bisphosphate [PI (4, 5) P(2)] levels during G-protein coupled phospholipase C activity in these cells, the biochemical mechanism of DrdgB alpha function remains unresolved. More recently, a mammalian RdgB protein has been implicated in the maintenance of diacylglycerol (DAG) levels and secretory function at Golgi membranes. In this review we discuss existing work on the function of RdgB proteins and set out future challenges in understanding this group of lipid transfer proteins.

The wild-type Ras: road ahead

The cellular Ras is known to play an important role in cellular proliferation mediated by growth factor receptor. Evidence also points to its role in growth arrest. Substantiated proof for growth-suppressive activity of wild-type Ras comes from studies that showed 1) loss of wild-type ras allele in tumors, 2) suppression of growth in cells transformed by oncogenic ras upon overexpression of wild-type Ras, and 3) up-regulation of Ras expression during postnatal development and following growth arrest of untransformed cells in culture. To understand the mechanism by which the wild-type Ras brings about these diverse actions, we evaluated its well-known role in actively proliferating cells and its less understood role in growth arrest. This led to the proposal that wild-type Ras in either GDP or GTP-bound state can antagonize the function of oncogenic Ras.-Singh, A., Sowjanya, A. P., Ramakrishna, G. The wild-type Ras: road ahead.

Differential regulation of endoplasmic reticulum structure through VAP-Nir protein interaction

The endoplasmic reticulum (ER) exhibits a characteristic tubular structure that is dynamically rearranged in response to specific physiological demands. However, the mechanisms by which the ER maintains its characteristic structure are largely unknown. Here we show that the integral ER-membrane protein VAP-B causes a striking rearrangement of the ER through interaction with the Nir2 and Nir3 proteins. We provide evidence that Nir (Nir1, Nir2, and Nir3)-VAP-B interactions are mediated through the conserved FFAT (two phenylalanines (FF) in acidic tract) motif present in Nir proteins. However, each interaction affects the structural integrity of the ER differently. Whereas the Nir2-VAP-B interaction induces the formation of stacked ER membrane arrays, the Nir3-VAP-B interaction leads to a gross remodeling of the ER and the bundling of thick microtubules along the altered ER membranes. In contrast, the Nir1-VAP-B interaction has no apparent effect on ER structure. We also show that the Nir2-VAP-B interaction attenuates protein export from the ER. These results demonstrate new mechanisms for the regulation of ER structure, all of which are mediated through interaction with an identical integral ER-membrane protein.

Patellin1, a novel Sec14-like protein, localizes to the cell plate and binds phosphoinositides

Membrane trafficking is central to construction of the cell plate during plant cytokinesis. Consequently, a detailed understanding of the process depends on the characterization of molecules that function in the formation, transport, targeting, and fusion of membrane vesicles to the developing plate, as well as those that participate in its consolidation and maturation into a fully functional partition. Here we report the initial biochemical and functional characterization of patellin1 (PATL1), a novel cell-plate-associated protein that is related in sequence to proteins involved in membrane trafficking in other eukaryotes. Analysis of the Arabidopsis genome indicated that PATL1 is one of a small family of Arabidopsis proteins, characterized by a variable N-terminal domain followed by two domains found in other membrane-trafficking proteins (Sec14 and Golgi dynamics domains). Results from immunolocalization and biochemical fractionation studies suggested that PATL1 is recruited from the cytoplasm to the expanding and maturing cell plate. In vesicle-binding assays, PATL1 bound to specific phosphoinositides, important regulators of membrane trafficking, with a preference for phosphatidylinositol(5)P, phosphatidylinositol(4,5)P(2), and phosphatidylinositol(3)P. Taken together, these findings suggest a role for PATL1 in membrane-trafficking events associated with cell-plate expansion or maturation and point to the involvement of phosphoinositides in cell-plate biogenesis.

The role of the Nir/rdgB protein family in membrane trafficking and cytoskeleton remodeling

The Nir/rdgB family of proteins has been identified in a variety of eukaryotic organisms, ranging from worms to mammals. The Drosophila retinal degeneration B (rdgB), a protein that is required for photoreceptor cell viability and light response, was the first to be identified. It consists an amino-terminal phosphatidylinositol (PI)-transfer domain and was proposed to play an essential role in photoreceptor membrane renewal and biogenesis. The other Nir/rdgB family members are functionally and structurally related to the Drosophila homolog and are implicated in regulation of lipid trafficking, metabolism, and signaling. Recent advances have revealed that Nir/rdgB proteins are also involved in regulation of cytoskeletal elements. Thus, these family members exert a broad spectrum of cellular functions and are involved in multiple cellular processes. The physiological functions of these closely related proteins are described in this review.

Mitotic phosphorylation of the peripheral Golgi protein Nir2 by Cdk1 provides a docking mechanism for Plk1 and affects cytokinesis completion

The rearrangement of the Golgi apparatus during mitosis is regulated by several protein kinases, including Cdk1 and Plk1. Several peripheral Golgi proteins that dissociate from the Golgi during mitosis are implicated in regulation of cytokinesis or chromosome segregation, thereby coordinating mitotic and cytokinetic events to Golgi rearrangement. Here we show that, at the onset of mitosis, Cdk1 phosphorylates the peripheral Golgi protein Nir2 at multiple sites; of these, S382 is the most prominent. Phosphorylation of Nir2 by Cdk1 facilitates its dissociation from the Golgi apparatus, and phospho-Nir2(pS382) is localized in the cleavage furrow and midbody during cytokinesis. Mitotic phosphorylation of Nir2 is required for docking of the phospho-Ser/Thr binding module, the Polo box domain of Plk1, and overexpression of a Nir2 mutant, which fails to interact with Plk1, affects the completion of cytokinesis. These results demonstrate a mechanism for coordinating mitotic and cytokinetic events with Golgi rearrangement during cell division.

The formins: active scaffolds that remodel the cytoskeleton

Evolutionarily conserved in eukaryotes, formin homology (FH) proteins, or formins, exert their effects on the actin and microtubule (MT) networks during meiosis, mitosis, the maintenance of cell polarity, vesicular trafficking, signaling to the nucleus and embryonic development. Once thought to be only molecular scaffolds that indirectly affected cellular functions through the binding of other proteins, recent in vitro studies have illustrated that they can function as actin nucleators in the formation of new filaments. The connection between formins and MTs is less well understood. In yeast, the MT effects appear to be dependent on the ability of formins to generate polarized actin cables whereas, in mammalian cells, formin signals that cause MT stabilization and polarization might be more direct. A subclass of formins, the Diaphanous-related formins (Drfs), can act as effectors for Rho small GTPases, yet it is not clear what GTPase binding contributes to formin function.

The RhoA-binding protein, rhophilin-2, regulates actin cytoskeleton organization

The Rho GTPases regulate the actin cytoskeleton through interactions with various downstream effector molecules. Here we have identified a ubiquitously expressed human RhoA-binding protein, designated Rhophilin-2. Rhophilin-2 shows 40% amino acid similarity to human Rhophilin-1 and contains an N-terminal Rho-binding, a central Bro1-like, and a C-terminal PDZ domain. Glutathione S-transferase-capture experiments revealed that Rhophilin-1 and Rhophilin-2 interacted with both GDP- and GTP-bound RhoA in vitro. Despite the ability of Rhophilin-1 and Rhophilin-2 to interact with RhoA in a nucleotide-independent fashion, Rho-induced serum response element transcriptional activity was not altered by expression of either of these molecules. Although Rhophilin-2-expressing HeLa cells showed a loss of actin stress fibers, Rhophilin-1 expression had no noticeable effect on the actin cytoskeleton. Coexpression of Rhophilin-2 with a constitutively active Rho mutant reversed the disassembly phenotype, in which the coexpressing cells were more spread and less contracted than Rho alone-expressing cells. Expression of various Rhophilin-2 deletion and point mutants containing the N-terminal RhoA-binding domain but lacking other regions suggested that the disassembly of F-actin stress fibers was not simply caused by Rho sequestration. In addition, the Bro1 and PDZ domains of Rhophilin-2 were required for disassembly. RhoA activity assays also revealed that Rhophilin-2-expressing cells showed increased levels of RhoA-GTP suggesting that the Rhophilin-2-induced disassembly of stress fibers was not mediated by decreased RhoA activity. Based on the biochemical and biological activity, Rhophilin-2 may function normally in a Rho pathway to limit stress fiber formation and/or increase the turnover of F-actin structures in the absence of high levels of RhoA activity.

Targeting of Nir2 to lipid droplets is regulated by a specific threonine residue within its PI-transfer domain

Nir2, like its Drosophila homolog retinal degeneration B (RdgB), contains an N-terminal phosphatidylinositol-transfer protein (PI-TP)-like domain. Previous studies have suggested that RdgB plays an important role in the fly phototransduction cascade and that its PI-transfer domain is critical for this function. In this domain, a specific mutation, T59E, induces a dominant retinal degeneration phenotype. Here we show that a similar mutation, T59E in the human Nir2 protein, targets Nir2 to spherical cytosolic structures identified as lipid droplets by the lipophilic dye Nile red. A truncated Nir2T59E mutant consisting of only the PI-transfer domain was also targeted to lipid droplets, whereas neither the wild-type Nir2 nor the Nir2T59A mutant was associated with lipid droplets under regular growth conditions. However, oleic-acid treatment caused translocation of wild-type Nir2, but not translocation of the T59A mutant, to lipid droplets. This treatment also induced partial targeting of endogenous Nir2, which is mainly associated with the Golgi apparatus, to lipid droplets. Targeting of Nir2 to lipid droplets was attributed to its enhanced threonine phosphorylation. These results suggest that a specific threonine within the PI-transfer domain of Nir2 provides a regulatory site for targeting to lipid droplets. In conjunction with the role of PI-TPs in lipid transport, this targeting may affect intracellular lipid trafficking and distribution and may provide the molecular basis underlying the dominant effect of the RdgB-T59E mutant on retinal degeneration.

Nir2, a human homolog of Drosophila melanogaster retinal degeneration B protein, is essential for cytokinesis

Cytokinesis, the final stage of eukaryotic cell division, ensures the production of two daughter cells. It requires fine coordination between the plasma membrane and cytoskeletal networks, and it is known to be regulated by several intracellular proteins, including the small GTPase Rho and its effectors. In this study we provide evidence that the protein Nir2 is essential for cytokinesis. Microinjection of anti-Nir2 antibodies into interphase cells blocks cytokinesis, as it results in the production of multinucleate cells. Immunolocalization studies revealed that Nir2 is mainly localized in the Golgi apparatus in interphase cells, but it is recruited to the cleavage furrow and the midbody during cytokinesis. Nir2 colocalizes with the small GTPase RhoA in the cleavage furrow and the midbody, and it associates with RhoA in mitotic cells. Its N-terminal region, which contains a phosphatidylinositol transfer domain and a novel Rho-inhibitory domain (Rid), is required for normal cytokinesis, as overexpression of an N-terminal-truncated mutant blocks cytokinesis completion. Time-lapse videomicroscopy revealed that this mutant normally initiates cytokinesis but fails to complete it, due to cleavage furrow regression, while Rid markedly affects cytokinesis due to abnormal contractility. Rid-expressing cells exhibit aberrant ingression and ectopic cleavage sites; the cells fail to segregate into daughter cells and they form a long unseparated bridge-like cytoplasmic structure. These results provide new insight into the cellular functions of Nir2 and introduce it as a novel regulator of cytokinesis.