Laura Kallay

Scribble associates with two polarity proteins, Lgl2 and Vangl2, via distinct molecular domains

Scribble (Scrib) is a large multi‐domain cytoplasmic protein that was first identified through its requirement for the establishment of epithelial polarity. We tested the hypotheses that Scrib asssociates with the basolateral membrane via multiple domains, binds specific protein partners, and is part of a multimeric complex. We generated a series of EGFP‐tagged Scrib fusion proteins and examined their membrane localizations in two types of polarized mammalian epithelial cells using biochemical and morphological approaches. We found that Scribs Leucine‐rich‐repeat (LRR) and PDS‐95/Discs Large/ZO‐1 (PDZ) domains independently associate with the plasma membrane in both cell types. We identified multiple large Scrib complexes, demonstrated that Scrib and the cytoplasmic protein Lethal giant larvae2 (Lgl2) co‐IP and that this association occurs via Scribs LRR domain. Further, this report demonstrates that the membrane protein Vangl2 binds selectively to specific PDZ domains in Scrib. Our identification of Scribs associations highlights its function in multiple biologic pathways and sets the stage for future identification of more proteins that must interact with Scribs remaining domains. J. Cell. Biochem. 99: 647–664, 2006.

Genome-Wide Analysis Reveals the Vacuolar pH-Stat of Saccharomyces cerevisiae

Protons, the smallest and most ubiquitous of ions, are central to physiological processes. Transmembrane proton gradients drive ATP synthesis, metabolite transport, receptor recycling and vesicle trafficking, while compartmental pH controls enzyme function. Despite this fundamental importance, the mechanisms underlying pH homeostasis are not entirely accounted for in any organelle or organism. We undertook a genome-wide survey of vacuole pH (pHv) in 4,606 single-gene deletion mutants of Saccharomyces cerevisiae under control, acid and alkali stress conditions to reveal the vacuolar pH-stat. Median pHv (5.27±0.13) was resistant to acid stress (5.28±0.14) but shifted significantly in response to alkali stress (5.83±0.13). Of 107 mutants that displayed aberrant pHv under more than one external pH condition, functional categories of transporters, membrane biogenesis and trafficking machinery were significantly enriched. Phospholipid flippases, encoded by the family of P4-type ATPases, emerged as pH regulators, as did the yeast ortholog of Niemann Pick Type C protein, implicated in sterol trafficking. An independent genetic screen revealed that correction of pHv dysregulation in a neo1ts mutant restored viability whereas cholesterol accumulation in human NPC1−/− fibroblasts diminished upon treatment with a proton ionophore. Furthermore, while it is established that lumenal pH affects trafficking, this study revealed a reciprocal link with many mutants defective in anterograde pathways being hyperacidic and retrograde pathway mutants with alkaline vacuoles. In these and other examples, pH perturbations emerge as a hitherto unrecognized phenotype that may contribute to the cellular basis of disease and offer potential therapeutic intervention through pH modulation.

Vestibular Hair Bundles Control pH with (Na+, K+)/H+ Exchangers NHE6 and NHE9

In hair cells of the inner ear, robust Ca2+/H+ exchange mediated by plasma-membrane Ca2+-ATPase would rapidly acidify mechanically sensitive hair bundles without efficient removal of H+. We found that, whereas the basolateral membrane of vestibular hair cells from the frog saccule extrudes H+ via an Na+-dependent mechanism, bundles rapidly remove H+ in the absence of Na+ and HCO3−, even when the soma is acidified. K+ was fully effective and sufficient for H+ removal; in contrast, Rb+ failed to support pH recovery. Na+/H+-exchanger isoform 1 (NHE1) was present on hair-cell soma membranes and was likely responsible for Na+-dependent H+ extrusion. NHE6 and NHE9 are organellar isoforms that can appear transiently on plasma membranes and have been proposed to mediate K+/H+ exchange. We identified NHE6 in a subset of hair bundles; NHE9 was present in all bundles. Heterologous expression of these isoforms in yeast strains lacking endogenous exchangers conferred pH-dependent tolerance to high levels of KCl and NaCl. NHE9 preferred cations in the order K+, Na+ ≫ Rb+, consistent with the relative efficacies of these ions in promoting pH recovery in hair bundles. Electroneutral K+/H+ exchange, which we propose is performed by NHE9 in hair bundles, exploits the high-K+ endolymph, responds only to pH imbalance across the bundle membrane, is unaffected by the +80 mV endocochlear potential, and uses mechanisms already present in the ear for K+ recycling. This mechanism allows the hair cell to remove H+ generated by Ca2+ pumping without ATP hydrolysis in the cell.

Mutational analysis of the intramembranous H10 loop of yeast Nhx1 reveals a critical role in ion homoeostasis and vesicle trafficking

Yeast Nhx1 [Na+(K+)/H+ exchanger 1] is an intracellular Na+(K+)/H+ exchanger, localizing to the late endosome where it is important for ion homoeostasis and vesicle trafficking. Phylogenetic analysis of NHE (Na+/H+ exchanger) sequences has identified orthologous proteins, including HsNHE6 (human NHE6), HsNHE7 and HsNHE9 of unknown physiological role. These appear distinct from well-studied mammalian plasma membrane isoforms (NHE1-NHE5). To explore the differences between plasma membrane and intracellular NHEs and understand the link between ion homoeostasis and vesicle trafficking, we examined the consequence of replacing residues in the intramembranous H10 loop of Nhx1 between transmembrane segments 9 and 10. The critical role for the carboxy group of Glu355 in ion transport is consistent with the invariance of this residue in all NHEs. Surprisingly, residues specifically conserved in the intracellular isoforms (such as Phe357 and Tyr361) could not be replaced with closely similar residues (leucine and phenylalanine) found in the plasma membrane isoforms without loss of function, revealing unexpected side chain specificity. The trafficking phenotypes of all Nhx1 mutants, including hygromycin-sensitivity and missorting of carboxypeptidase Y, were found to directly correlate with pH homoeostasis defects and could be proportionately corrected by titration with weak base. The present study demonstrates the importance of the H10 loop of the NHE family, highlights the differences between plasma membrane and intracellular isoforms and shows that trafficking defects are tightly coupled with pH homoeostasis.

Transcript profile of cellular senescence-related genes in Fuchs endothelial corneal dystrophy

Fuchs endothelial corneal dystrophy (FECD) is a genetically heterogeneous disease. Hypothesizing that cellular senescence may be relevant in FECD pathogenesis, genetically undifferentiated late-onset FECD endothelial samples were analyzed to identify common changes of specific senescence-related transcripts. Total RNA was extracted from 21 FECD endothelial samples retrieved from patients undergoing lamellar keratoplasty due to clinically diagnosed end-stage FECD and from 12 endothelial samples retrieved from normal autopsy eyes. Taqman low density array (TLDA) cards were used to analyze differential expression of 89 cellular senescence-related transcripts. Result validation was performed using individual real-time PCR assays. TLDA-analysis demonstrated differential expression of 31 transcripts (fold-change >1.5; p < 0.05). Thereof, 27 showed significant up-regulation and 4 significant down-regulation. Markedly elevated mRNA-levels of the constitutively active and reactive oxygen species-generating enzyme NOX4 were found in all evaluable FECD samples. In addition, increased expression of CDKN2A and its transcriptional activators ETS1 and ARHGAP18 (SENEX) along with decreased expression of CDKN2A inhibitor ID1 were detected in FECD samples. Consistent over-expression of NOX4 in FECD endothelial samples suggests a role as pathogenic factor and as a potential new treatment target in FECD. Transcriptional up-regulation of the CDKN2A-pathway provides further evidence for increased cellular senescence in FECD endothelium.

Unconventional Chemiosmotic Coupling of NHA2, a Mammalian Na+/H+ Antiporter, to a Plasma Membrane H+ Gradient

Background: Inwardly directed Na+ gradients have failed to reveal transport activity of NHA2 in mammalian cells, although it functionally complements salt tolerance in yeast. Results: NHA2 mediates sodium-lithium countertransport, and cation efflux using an inwardly directed H+ gradient. Conclusion: NHA2 is chemiosmotically coupled to the V-ATPase. Significance: H+-driven salt and pH homeostasis in the kidney may be important in hypertension. Human NHA2, a newly discovered cation proton antiporter, is implicated in essential hypertension by gene linkage analysis. We show that NHA2 mediates phloretin-sensitive Na+-Li+ counter-transport (SLC) activity, an established marker for hypertension. In contrast to bacteria and fungi where H+ gradients drive uptake of metabolites, secondary transport at the plasma membrane of mammalian cells is coupled to the Na+ electrochemical gradient. Our findings challenge this paradigm by showing coupling of NHA2 and V-type H+-ATPase at the plasma membrane of kidney-derived MDCK cells, resulting in a virtual Na+ efflux pump. Thus, NHA2 functionally recapitulates an ancient shared evolutionary origin with bacterial NhaA. Although plasma membrane H+ gradients have been observed in some specialized mammalian cells, the ubiquitous tissue distribution of NHA2 suggests that H+-coupled transport is more widespread. The coexistence of Na+ and H+-driven chemiosmotic circuits has implications for salt and pH regulation in the kidney.

Endosomal Na+ (K+)/H+ exchanger Nhx1/Vps44 functions independently and downstream of multivesicular body formation.

Background: Nhx1/Vps44 is proposed to be a Class E gene involved in formation of the multivesicular body (MVB). However, this hypothesis has not been tested. Results: Nhx1 is not required for cargo sorting or MVB formation and shows synthetic phenotypes with select ESCRT mutants. Conclusion: Nhx1 functions independently of the ESCRT pathway. Significance: Nhx1 may have a post-ESCRT role in endosomal membrane fusion. The multivesicular body (MVB) is an endosomal intermediate containing intralumenal vesicles destined for membrane protein degradation in the lysosome. In Saccharomyces cerevisiae, the MVB pathway is composed of 17 evolutionarily conserved ESCRT (endosomal sorting complex required for transport) genes grouped by their vacuole protein sorting Class E mutant phenotypes. Only one integral membrane protein, the endosomal Na+ (K+)/H+ exchanger Nhx1/Vps44, has been assigned to this class, but its role in the MVB pathway has not been directly tested. Herein, we first evaluated the link between Nhx1 and the ESCRT proteins and then used an unbiased phenomics approach to probe the cellular role of Nhx1. Select ESCRT mutants (vps36Δ, vps20Δ, snf7Δ, and bro1Δ) with defects in cargo packaging and intralumenal vesicle formation shared multiple growth phenotypes with nhx1Δ. However, analysis of cellular trafficking and ultrastructural examination by electron microscopy revealed that nhx1Δ cells retain the ability to sort cargo into intralumenal vesicles. In addition, we excluded a role for Nhx1 in Snf7/Bro1-mediated cargo deubiquitylation and Rim101 response to pH stress. Genetic epistasis experiments provided evidence that NHX1 and ESCRT genes function in parallel. A genome-wide screen for single gene deletion mutants that phenocopy nhx1Δ yielded a limited gene set enriched for endosome fusion function, including Rab signaling and actin cytoskeleton reorganization. In light of these findings and the absence of the so-called Class E compartment in nhx1Δ, we eliminated a requirement for Nhx1 in MVB formation and suggest an alternative post-ESCRT role in endosomal membrane fusion.

Microrna-29b Overexpression Decreases Extracellular Matrix mrna and Protein Production in Human Corneal Endothelial Cells

Purpose: MicroRNAs are small noncoding RNAs that regulate gene expression at the posttranscriptional level. We reported that levels of microRNA (miR)-29 family are decreased in corneas of patients with Fuchs endothelial corneal dystrophy (FECD). The miR-29 family regulates the production of extracellular matrix (ECM) proteins. Accumulation of ECM proteins in Descemet membrane is an important pathologic change in FECD. In this study, we transfected miR-29b into human corneal endothelial cells and tissues and evaluated ECM protein expression levels. Methods: An immortalized Fuchs human corneal endothelial cell line (iFECD) was established by infection of corneal endothelial cells from patients with FECD with hTERT lentivirus. MiR-29b was transfected into iFECD, and the expression levels of ECMs collagen type 1 alpha 1 (COL1A1), collagen type 4 alpha 1 (COL4A1), and laminin gamma 1 (LAMC1) were evaluated with quantitative reverse transcriptase–polymerase chain reaction (qRT-PCR) and Western blot. Expression level of LAMC1 protein in miR-29b−transfected donor corneal endothelium was also evaluated by Western blot. Results: Compared with control, miR-29b expression level after transfection of iFECD was increased to 335.6% (±91.0%), and ECM expression levels were significantly decreased. Compared with control, qRT-PCR demonstrated reduction of ECM to the following levels: COL1A1: 1.9% (±0.4%); COL4A1: 7.1% (±1.7%); and LAMC1: 21.5% (±2.7%). Western blot showed reduced protein expression: COL1A1: 4.8% (±3.2%); COL4A1: 42.5% (±25.0%); and LAMC1: 44.8% (±3.1%). In miR-29b−transfected corneal tissue, LAMC1 protein expression level was decreased to 14.4% (±20.5%). Conclusions: Overexpression of miR-29b decreased ECM protein production in human corneal endothelial cells. Thus, miR-29 replacement therapy might be a new treatment strategy for FECD aimed at reducing pathologic production of ECM proteins in Descemet membrane.

Use of human serum for human corneal endothelial cell culture

Background/aims To study human corneal endothelial cells (HCECs) cultured in vitro with human serum (HS) supplemented media (HS-SM) compared with HCEC cultured in fetal bovine serum (FBS) supplemented media (FBS-SM). Methods One cornea from a donor aged 21 years and a pair of corneas from a 16 year-old donor were obtained from the eye bank and used to create two different cell populations. At the first passage, the cell populations were equally divided and seeded in two different wells containing FBS-SM or HS-SM. In subsequent passages, HS-SM was compared with FBS-SM by morphology, growth curves, immunohistochemistry and real-time reverse-transcriptase PCR for endothelial cell markers. Results No difference in morphology could be seen in P2, P5 or in any other passages for cells grown in the two media. By growth curves, cell counts were similar in FBS-SM and HS-SM from days 1 to 5, with a trend towards higher cell counts in HS-SM at day 7. Cells grown in FBS-SM and HS-SM media showed similar expression of endothelial cell markers when assessed by immunohistochemistry and real-time reverse-transcriptase PCR. Conclusions HS-SM was similar to FBS-SM for HCEC culture when assessed by cell morphology, proliferation and protein/gene expression.

Epidemiology and Genetic Basis of Fuchs Endothelial Corneal Dystrophy

Fuchs endothelial corneal dystrophy (FECD) is a progressive disease of the corneal endothelium characterized by the thickening of Descemet membrane, formation of guttae, and loss of endothelial cells, resulting in corneal edema and a significant decrease in visual acuity. First described over one century ago by Austrian ophthalmologist Ernst Fuchs, significant strides have been made in our understanding of this complex disease. In this chapter, we review the epidemiology and genetic basis of FECD and discuss potential medical therapies.