Dynamic regulation of hepatic lipid metabolism by torsinA and its activators.

Depletion of torsinA from hepatocytes leads to reduced liver triglyceride secretion and marked hepatic steatosis. TorsinA is an atypical ATPase that lacks intrinsic activity unless it is bound to its activator, lamina-associated polypeptide 1 (LAP1) or luminal domain-like LAP1 (LULL1). We previously demonstrated that depletion of LAP1 from hepatocytes has more modest effects on liver triglyceride secretion and steatosis development than depletion of torsinA. We now show that depletion of LULL1 alone does not significantly decrease triglyceride secretion or cause steatosis. However, simultaneous depletion of both LAP1 and LULL1 leads to defective triglyceride secretion and marked steatosis similar to that observed with depletion of torsinA. Depletion of both LAP1 and torsinA from hepatocytes generated phenotypes similar to those observed with only torsinA depletion, implying that the 2 proteins act in the same pathway in liver lipid metabolism. Our results demonstrate that torsinA and its activators dynamically regulate hepatic lipid metabolism.

Engraftment and injury repair in regionally conditioned rat lung in vivo by lung progenitors derived from human pluripotent stem cells.

Although lung disease is a major cause of mortality, the mechanisms involved in human lung regeneration are unclear because of the lack of experimental models. Here we report a novel model where human pluripotent stem cell-derived expandable cell lines sharing features of airway secretory and basal cells engraft in the distal rat lung after conditioning by locoregional de-epithelialization followed by irradiation and immunosuppression. The engrafting cells, which we named distal lung epithelial progenitors (DLEPs), contributed to alveolar epithelial cells and generated 'KRT5-pods', structures involved in distal lung repair after severe injury, but only rarely to distal airways. Most strikingly, however, injury induced by the conditioning regimen was largely prevented by the engrafting DLEPs. The approach described here provides a model to study mechanisms involved in human lung regeneration, and potentially lays the foundation for the preclinical development of cell therapy to treat lung injury and disease.

Functional interaction of torsinA and its activators in liver lipid metabolism.

TorsinA is an atypical ATPase that lacks intrinsic activity unless it is bound to its activators lamina-associated polypeptide 1 (LAP1) in the perinuclear space or luminal domain-like LAP1 (LULL1) throughout the endoplasmic reticulum. However, the interaction of torsinA with LAP1 and LULL1 has not yet been shown to modulate a defined physiological process in mammals in vivo . We previously demonstrated that depletion of torsinA from mouse hepatocytes leads to reduced liver triglyceride secretion and marked steatosis, whereas depletion of LAP1 had more modest similar effects. We now show that depletion of LULL1 alone does not significantly decrease liver triglyceride secretion or cause steatosis. However, simultaneous depletion of both LAP1 and LULL1 from hepatocytes leads to defective triglyceride secretion and marked steatosis similar to that observed with depletion of torsinA. Our results demonstrate that torsinA and its activators dynamically regulate a physiological process in mammals in vivo .

Interaction of Human OATP1B1 with PDZK1 Is Required for Its Trafficking to the Hepatocyte Plasma Membrane.

Uptake of xenobiotics by hepatocytes is mediated by specific proteins, including organic anion transporting polypeptides (OATPs), residing on the basolateral (sinusoidal) plasma membrane. Many of the OATPs have PDZ consensus binding sites, determined by their C-terminal 4 amino acids, while others do not. Mouse and rat OATP1A1 are associated with PDZK1, which is necessary for their trafficking to the plasma membrane. humanOATP1B1 (hOATP1B1) is a major drug transporter in human liver. Although localized to the plasma membrane, it was thought to lack a PDZ consensus motif, suggesting that the trafficking paradigm for murine OATPs is not applicable to human liver. The aim of the present study was to determine whether hOATP1B1 is a ligand for hPDZK1. hOATP1B1 immunoprecipitates with hPDZK1 following co-expression in 293T cells as well as in normal human liver. Co-expression with each of the 4 PDZ domains revealed interaction with domain 1 only. A truncated version of hOATP1B1 that lacks its terminal 4 amino acid PDZ binding motif as well as hOATP1B3, which does not contain a PDZ binding consensus motif, failed to interact with hPDZK1. Immunofluorescence microscopy of hOATP1B1 in stably transfected HeLa cells that endogenously express hPDZK1 showed that it distributes predominantly along the plasma membrane whereas hOATP1B1 lacking its terminal 4 amino acids distributes primarily intracellularly with little plasma membrane localization. Similar to findings in rats and mice, human OATP1B1 is a ligand for PDZK1 and requires interaction with PDZK1 for optimal trafficking to the hepatocyte plasma membrane. SIGNIFICANCE: Previous studies suggested that OATP1B1, a major xenobiotic transporter in human liver, does not have a PDZ binding consensus motif and does not follow the paradigm for subcellular trafficking and function that was established for OATP1A1 in murine liver. We now demonstrated that OATP1B1 but not OATP1B3 has a PDZ binding consensus motif that mediates binding to PDZK1 and is required for its trafficking to the plasma membrane. Such interaction could be an important previously unrecognized modulator of transport function.

DNA damage primes hematopoietic stem cells for direct megakaryopoiesis.

Hematopoietic stem cells (HSCs) reside in the bone marrow (BM), can self-renew, and generate all cells of the hematopoietic system. 1 Most hematopoietic lineages arise through successive, increasingly lineage-committed progenitors. In contrast, megakaryocytes (MKs), hyperploid cells that generate platelets essential to hemostasis, can derive rapidly and directly from HSCs. 2 The underlying mechanism is unknown however. Here we show that DNA damage and subsequent arrest in the G2 phase of the cell cycle rapidly induce MK commitment specifically in HSCs, but not in progenitors, through an initially predominantly post-transcriptional mechanism. Cycling HSCs show extensive replication-induced DNA damage associated with uracil misincorporation in vivo and in vitro . Consistent with this notion, thymidine attenuated DNA damage, rescued HSC maintenance and reduced the generation of CD41 + MK-committed HSCs in vitro . Similarly, overexpression of the dUTP-scavenging enzyme, dUTPase, enhanced in vitro maintenance of HSCs. We conclude that a DNA damage response drives direct megakaryopoiesis and that replication stress-induced direct megakaryopoiesis, at least in part caused by uracil misincorporation, is a barrier to HSC maintenance in vitro . DNA damage-induced direct megakaryopoiesis may allow rapid generation of a lineage essential to immediate organismal survival, while simultaneously removing damaged HSCs and potentially avoiding malignant transformation of self-renewing stem cells.

"You have to believe the patient": What do people with fibromyalgia find helpful (and hindering) when accessing health care?

Background: Fibromyalgia (FM) is a complex, still poorly understood, and difficult-to-treat chronic pain condition for which many people struggle to find adequate care. Aims: This study investigated the research question, "What do people accessing health care services for fibromyalgia perceive as helpful, hindering, and absent but desired?" with the aim of identifying clear, implementable changes for clinical practice. Materials and methods: This study used the enhanced critical incident technique (ECIT), a qualitative research method that focuses on helping, hindering, and desired factors, to explore the health care experiences of 14 individuals (12 women and 2 men) diagnosed with FM. Results: Using qualitative data analysis, results identified three categories of health care experiences: (1) systemic navigation, including financial and economic security; accessibility, flexibility, and continuity of care; and diversity of treatment options; (2) clinician-patient alliance, including invalidation and prejudice; therapeutic bond; and clinician-patient alignment on treatment plan; and (3) patient self-management strategies, including information-seeking and education, self-advocacy, social supports, symptom management strategies, and other coping strategies. Participants tended to conceptualize their health care concerns as a multilayered systemic problem. Conclusions: Participants described a medical system they perceived as poorly equipped to support their needs and tended to invalidate their health concerns. Helping experiences tended to be the result of unique efforts on the part of individual clinicians. Findings emphasize the importance of recognizing the complexities and psychological impact of pain, trusting clinician-patient relationships, multidisciplinary/interdisciplinary care within a biopsychosocial framework, and improved education and awareness around psychosocial aspects of FM and effective management of chronic pain.

Contexte: La fibromyalgie est une maladie chronique complexe, encore mal comprise et difficile à traiter, pour laquelle beaucoup de gens ont du mal à trouver des soins adéquats.Objectifs: Cette étude s'est penchée sur la question de recherche : « Qu'est-ce que les personnes qui ont accès aux soins de santé pour la fibromyalgie perçoivent comme utile, gênant, et absent mais désiré? ¼ en vue de définir des changements clairs et réalisables pour la pratique clinique.Matériaux et méthodes: Cette étude a utilisé la technique des incidents critiques améliorée, une méthode de recherche qualitative qui met l'accent sur les facteurs aidants, entravants et souhaités, afin d'étudier les expériences de soins de santé de 14 personnes (12 femmes et 2 hommes) ayant reçu un diagnostic de fibromyalgie.Résultats: À l'aide d'une analyse qualitative des données, les résultats ont permis de définir trois catégories d'expériences de soins de santé : (1) la navigation systémique, y compris la sécurité financière et économique; l'accessibilité, la flexibilité et la continuité des soins; et la diversité des options de traitement; (2) l'alliance clinicien-patient, y compris l'invalidation et les préjugés; le lien thérapeutique; et l'alignement clinicien-patient sur le plan de traitement; et (3) les stratégies d'auto-prise en charge des patients, y compris la recherche d'information et l'éducation, l'autonomie sociale, les soutiens sociaux, les stratégies de prise en charge des symptômes et d'autres stratégies d'adaptation. Les participants avaient tendance à conceptualiser leurs préoccupations en matière de soins de santé comme un problème systémique à plusieurs niveaux.Conclusions: Les participants ont décrit un système médical qu'ils percevaient comme mal outillé pour répondre à leurs besoins et qui avait tendance à invalider leurs préoccupations en matière de santé. Les expériences d'aide tendaient à être le résultat d'efforts isolés de la part de certains cliniciens. Les résultats soulignent l'importance de reconnaître la complexité et les répercussions psychologiques de la douleur, des relations de confiance entre cliniciens et patients, des soins multidisciplinaires ou interdisciplinaires dans un cadre biopsychosocial. Ils soulignent aussi l'importance d'améliorer l'éducation et la sensibilisation aux aspects psychosociaux de la fibromyalgie et de la prise en charge efficace de la douleur chronique.

Human pluripotent stem cell-derived respiratory airway progenitors generate alveolar epithelial cells and recapitulate features of idiopathic pulmonary fibrosis.

Human lungs contain unique cell populations in distal respiratory airways (RAs). These populations accumulate in patients with lung injury, chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF). Their lineage potentials and roles are unknown, however. As they are absent in rodents, deeper understanding of these cells requires a human in vitro model. Here we report the generation from human pluripotent stem cells (hPSCs) of expandable spheres (induced respiratory airway progenitors (iRAPs)) consisting of all RA-associated cell types. iRAPs could differentiate into type 1 (AT1) and type 2 alveolar (AT2) epithelial cells in defined conditions, showing that alveolar cells can be derived from RAs. iRAPs with deletion of HPS1, which causes pulmonary fibrosis in humans, display defects that are hallmarks of IPF, indicating involvement of intrinsic dysfunction of RA-associated cells in IPF. iRAPs thus provide a model to gain insight into human lung regeneration and into pathogenesis of IPF.

Deletion of the Pseudorabies Virus gE/gI-US9p complex disrupts kinesin KIF1A and KIF5C recruitment during egress, and alters the properties of microtubule-dependent transport in vitro.

During infection of neurons by alphaherpesviruses including Pseudorabies virus (PRV) and Herpes simplex virus type 1 (HSV-1) viral nucleocapsids assemble in the cell nucleus, become enveloped in the cell body then traffic into and down axons to nerve termini for spread to adjacent epithelia. The viral membrane protein US9p and the membrane glycoprotein heterodimer gE/gI play critical roles in anterograde spread of both HSV-1 and PRV, and several models exist to explain their function. Biochemical studies suggest that PRV US9p associates with the kinesin-3 motor KIF1A in a gE/gI-stimulated manner, and the gE/gI-US9p complex has been proposed to recruit KIF1A to PRV for microtubule-mediated anterograde trafficking into or along the axon. However, as loss of gE/gI-US9p essentially abolishes delivery of alphaherpesviruses to the axon it is difficult to determine the microtubule-dependent trafficking properties and motor-composition of Δ(gE/gI-US9p) particles. Alternatively, studies in HSV-1 have suggested that gE/gI and US9p are required for the appearance of virions in the axon because they act upstream, to help assemble enveloped virions in the cell body. We prepared Δ(gE/gI-US9p) mutant, and control parental PRV particles from differentiated cultured neuronal or porcine kidney epithelial cells and quantitated the efficiency of virion assembly, the properties of microtubule-dependent transport and the ability of viral particles to recruit kinesin motors. We find that loss of gE/gI-US9p has no significant effect upon PRV particle assembly but leads to greatly diminished plus end-directed traffic, and enhanced minus end-directed and bidirectional movement along microtubules. PRV particles prepared from infected differentiated mouse CAD neurons were found to be associated with either kinesin KIF1A or kinesin KIF5C, but not both. Loss of gE/gI-US9p resulted in failure to recruit KIF1A and KF5C, but did not affect dynein binding. Unexpectedly, while KIF5C was expressed in undifferentiated and differentiated CAD neurons it was only found associated with PRV particles prepared from differentiated cells.

Rat Organic Anion Transport Protein 1A1 Interacts Directly With Organic Anion Transport Protein 1A4 Facilitating Its Maturation and Trafficking to the Hepatocyte Plasma Membrane.

Organic anion transport proteins (OATPs) on the basolateral surface of hepatocytes mediate uptake of a number of drugs and endogenous compounds. Previous studies showed that rat OATP1A1 (rOATP1A1) has a postsynaptic density protein, drosophila disc large tumor suppressor, zonula occludens-1 protein (PDZ) consensus binding motif at its C-terminus and binds to PDZ domain containing 1 (PDZK1), which is required for its cell-surface localization. PDZK1 associates with rOATP1A1-containing endocytic vesicles within cells, mediating recruitment of motor proteins required for microtubule-based trafficking to the plasma membrane. rOATP1A4 also traffics to the plasma membrane, although it lacks a PDZ binding consensus sequence. The current study was designed to test the hypothesis that trafficking of rOATP1A4 to the plasma membrane requires its direct interaction with rOATP1A1 resulting in a complex that traffics through the cell in common subcellular vesicles in which the cytosolic tail of rOATP1A1 is bound to PDZK1. We found that 74% of rOATP1A4-containing rat liver endocytic vesicles (n = 12,044) also contained rOATP1A1. Studies in transfected HEK293 cells showed surface localization of rOATP1A1 only when coexpressed with PDZK1 whereas rOATP1A4 required coexpression with rOATP1A1 and PDZK1. Studies in stably transfected HeLa cells that constitutively expressed PDZK1 showed that coexpression of rOATP1A4 with rOATP1A1 resulted in more rapid appearance of rOATP1A4 on the plasma membrane and faster maturation to its fully glycosylated form. Similar results were observed on immunofluorescence analysis of single cells. Immunoprecipitation of rat liver or transfected HeLa cell lysates with rOATP1A1 antibody specifically co-immunoprecipitated rOATP1A4 as determined by western blotting. Conclusion: These studies indicate that optimal rOATP1A4 trafficking to the cell surface is dependent upon coexpression and interaction with rOATP1A1. As rOATP1A1 binds to the chaperone protein, PDZK1, rOATP1A4 functionally hitchhikes through the cell with this complex.

Genes and Pathways Promoting Long-Term Liver Repopulation by Ex Vivo hYAP-ERT2 Transduced Hepatocytes and Treatment of Jaundice in Gunn Rats.

Hepatocyte transplantation is an attractive alternative to liver transplantation. Thus far, however, extensive liver repopulation by adult hepatocytes has required ongoing genetic, physical, or chemical injury to host liver. We hypothesized that providing a regulated proliferative and/or survival advantage to transplanted hepatocytes should enable repopulation in a normal liver microenvironment. Here, we repopulated livers of DPPIV- (dipeptidyl peptidase-4) rats and Ugt1a1 (uridinediphosphoglucuronate glucuronosyltransferase 1a1)-deficient Gunn rats (model of Crigler-Najjar syndrome type 1), both models without underlying liver injury, for up to 1 year by transplanting lenti-hYAP-ERT2 (mutated estrogen receptor ligand-binding domain 2)-transduced hepatocytes (YAP-Hc). Yap (yes-associated protein) nuclear translocation/function in YAP-Hc was regulated by tamoxifen. Repopulating YAP-Hc and host hepatocytes were fluorescence-activated cell sorting-purified and their transcriptomic profiles compared by RNAseq. After 1 year of liver repopulation, YAP-Hc clusters exhibited normal morphology, integration into hepatic plates and hepatocyte-specific gene expression, without dysplasia, dedifferentiation, or tumorigenesis. RNAseq analysis showed up-regulation of 145 genes promoting cell proliferation and 305 genes suppressing apoptosis, including hepatocyte growth factor and connective tissue growth factor among the top 30 in each category and provided insight into the mechanism of cell competition that enabled replacement of host hepatocytes by YAP-Hc. In Gunn rats transplanted with YAP-Hc+tamoxifen, there was a 65%-81% decline in serum bilirubin over 6 months versus 8%-20% with control-Hc, representing a 3-4-fold increase in therapeutic response. This correlated with liver repopulation as demonstrated by the presence of Ugt1a1-positive hepatocyte clusters in livers and western blot analysis of tissue homogenates. Conclusion: Tamoxifen-regulated nuclear translocation/function of hYAP-ERT2 enabled long-term repopulation of DPPIV-/- and Gunn rat livers by hYAP-ERT2-transduced hepatocytes without tumorigenesis. This cell transplantation strategy may offer a potential therapy for most of the inherited monogenic liver diseases that do not exhibit liver injury.

Defective recruitment of motor proteins to autophagic compartments contributes to autophagic failure in aging.

Inability to preserve proteostasis with age contributes to the gradual loss of function that characterizes old organisms. Defective autophagy, a component of the proteostasis network for delivery and degradation of intracellular materials in lysosomes, has been described in multiple old organisms, while a robust autophagy response has been linked to longevity. The molecular mechanisms responsible for defective autophagic function with age remain, for the most part, poorly characterized. In this work, we have identified differences between young and old cells in the intracellular trafficking of the vesicular compartments that participate in autophagy. Failure to reposition autophagosomes and lysosomes toward the perinuclear region with age reduces the efficiency of their fusion and the subsequent degradation of the sequestered cargo. Hepatocytes from old mice display lower association of two microtubule-based minus-end-directed motor proteins, the well-characterized dynein, and the less-studied KIFC3, with autophagosomes and lysosomes, respectively. Using genetic approaches to mimic the lower levels of KIFC3 observed in old cells, we confirmed that reduced content of this motor protein in fibroblasts leads to failed lysosomal repositioning and diminished autophagic flux. Our study connects defects in intracellular trafficking with insufficient autophagy in old organisms and identifies motor proteins as a novel target for future interventions aiming at correcting autophagic activity with anti-aging purposes.

Reduction of organelle motility by removal of potassium and other solutes.

There are surprisingly few studies that describe how the composition of cell culture medium may affect the trafficking of organelles. Here we utilize time lapse multi-channel fluorescent imaging to show that short term exposure of Huh-7 cells to medium lacking potassium, sodium, or chloride strongly reduces but does not eliminate the characteristic back and forth and cell-traversing movement of fluorescent EGF (FL-EGF) containing organelles. We focused on potassium because of its relatively low abundance in media and serum and its energy requiring accumulation into cells. Upon exposure to potassium free medium, organelle motility declined steadily through 90 min and then persisted at a low level. Reduced motility was confirmed in 5 independent cell lines and for organelles of the endocytic pathway (FL-EGF and Lysotracker), autophagosomes (LC3-GFP), and mitochondria (TMRE). As has been previously established, potassium free medium also inhibited endocytosis. We expected that diminished cellular metabolism would precede loss of organelle motility. However, extracellular flux analysis showed near normal mitochondrial oxygen consumption and only a small decrease in extracellular acidification, the latter suggesting decreased glycolysis or proton efflux. Other energy dependent activities such as the accumulation of Lysotracker, TMRE, DiBAC4(3), and the exclusion of propidium iodide remained intact, as did the microtubule cytoskeleton. We took advantage of cell free in vitro motility assays and found that removal of potassium or sodium from the reconstituted cytosolic medium decreased the movement of endosomes on purified microtubules. The results indicate that although changes in proton homeostasis and cell energetics under solute depletion are not fully understood, potassium as well as sodium appear to be directly required by the motile machinery of organelles for optimal trafficking.

Experimental Model for Successful Liver Cell Therapy by Lenti TTR-YapERT2 Transduced Hepatocytes with Tamoxifen Control of Yap Subcellular Location.

Liver repopulation by transplanted hepatocytes has not been achieved previously in a normal liver microenvironment. Here we report that adult rat hepatocytes transduced ex vivo with a lentivirus expressing a human YapERT2 fusion protein (hYapERT2) under control of the hepatocyte-specific transthyretin (TTR) promoter repopulate normal rat liver in a tamoxifen-dependent manner. Transplanted hepatocytes expand very slowly but progressively to produce 10% repopulation at 6 months, showing clusters of mature hepatocytes that are fully integrated into hepatic parenchyma, with no evidence for dedifferentiation, dysplasia or malignant transformation. Thus, we have developed the first vector designed to regulate the growth control properties of Yap that renders it capable of producing effective cell therapy. The level of liver repopulation achieved has significant translational implications, as it is 2-3x the level required to cure many monogenic disorders of liver function that have no underlying hepatic pathology and is potentially applicable to diseases of other tissues and organs.

The Na(+)-Taurocholate Cotransporting Polypeptide Traffics with the Epidermal Growth Factor Receptor.

Na(+)-taurocholate cotransporting polypeptide (ntcp) mediates bile acid transport, also serving as the hepatitis B virus receptor. It traffics in vesicles along microtubules, requiring activity of protein kinase C (PKC)ζ for motility. We have now found that the epidermal growth factor receptor (EGFR) is the target of PKCζ activity and that EGFR and ntcp colocalize in vesicles. ntcp-containing vesicles that are not associated with EGFR have reduced microtubule-based motility, consistent with intracellular accumulation and reduced surface expression of ntcp in cells following EGFR knockdown.

Hepatocytes maintain greater fluorescent bile acid accumulation and greater sensitivity to drug-induced cell death in three-dimensional matrix culture.

Primary hepatocytes undergo phenotypic dedifferentiation upon isolation from liver that typically includes down regulation of uptake transporters and up regulation of efflux transporters. Culturing cells between layers of collagen in a three-dimensional (3D) "sandwich" is reported to restore hepatic phenotype. This report examines how 3D culturing affects accumulation of fluorophores, the cytotoxic response to bile acids and drugs, and whether cell to cell differences in fluorescent anion accumulation correlate with differences in cytotoxicity. Hepatocytes were found to accumulate fluorescent bile acid (FBA) at significantly higher levels than the related fluorophores, carboxyfluorescein diacetate, (4.4-fold), carboxyfluorescein succinimidyl ester (4.8-fold), and fluorescein (30-fold). In 2D culture, FBA accumulation decreased to background levels by 32 h, Hoechst nuclear accumulation strongly decreased, and nuclear diameter increased, indicative of an efflux phenotype. In 3D culture, FBA accumulation was maintained through 168 h but at 1/3 the original intensity. Cell to cell differences in accumulated FBA did not correlate with levels of liver zonal markers L-FBAP (zone 1) or glutamine synthetase (zone 3). Cytotoxic response to hydrophobic bile acids, acetaminophen, and phalloidin was maintained in 3D culture, and cells with higher FBA accumulation showed 12-18% higher toxicity than the total population toward hydrophobic bile acids (P < 0.05). Long-term imaging showed oscillations in the accumulation of FBA over periods of hours. Overall, the studies suggest that high accumulation of FBA can indicate the sensitivity of cultured hepatocytes to hydrophobic bile acids and other toxins.

Oatp1a1 requires PDZK1 to traffic to the plasma membrane by selective recruitment of microtubule-based motor proteins.

Previous studies identified a family of organic anion transport proteins (OATPs), many of which have C-terminal PDZ binding consensus sequences. In particular, the C-terminal four amino acids of Oatp1a1, a transporter on rat and mouse hepatocytes, comprise a consensus binding site for PDZK1. In PDZK1 knockout mice and in transfected cells where PDZK1 expression was knocked down, Oatp1a1 accumulates in intracellular vesicles. The present study tests the hypothesis that Oatp1a1 traffics to and from the cell surface in vesicles along microtubules, and that PDZK1 guides recruitment of specific motors to these vesicles. Oatp1a1-containing vesicles were prepared from wild-type and PDZK1 knockout mice. As seen by immunofluorescence, kinesin-1, a microtubule plus-end directed motor, was largely associated with vesicles from wild-type mouse liver, whereas dynein, a minus-end directed motor, was largely associated with vesicles from PDZK1 knockout mouse liver. Quantification of motility on directionally marked microtubules following addition of 50 µM ATP showed that wild-type vesicles moved equally toward the plus and minus ends whereas PDZK1 knockout vesicles moved predominantly toward the minus end, consistent with net movement toward the cell interior. These studies provide a novel mechanism by which PDZK1 regulates intracellular trafficking of Oatp1a1 by recruiting specific motors to Oatp1a1-containing vesicles. In the absence of PDZK1, Oatp1a1-containing vesicles cannot recruit kinesin-1 and associate with dynein as a predominant minus-end directed motor. Whether this is a result of direct interaction of the Oatp1a1 cytoplasmic domain with dynein or with a dynein-containing protein complex remains to be established.

In vitro motility of liver connexin vesicles along microtubules utilizes kinesin motors.

Trafficking of the proteins that form gap junctions (connexins) from the site of synthesis to the junctional domain appears to require cytoskeletal delivery mechanisms. Although many cell types exhibit specific delivery of connexins to polarized cell sites, such as connexin32 (Cx32) gap junctions specifically localized to basolateral membrane domains of hepatocytes, the precise roles of actin- and tubulin-based systems remain unclear. We have observed fluorescently tagged Cx32 trafficking linearly at speeds averaging 0.25 µm/s in a polarized hepatocyte cell line (WIF-B9), which is abolished by 50 µM of the microtubule-disrupting agent nocodazole. To explore the involvement of cytoskeletal components in the delivery of connexins, we have used a preparation of isolated Cx32-containing vesicles from rat hepatocytes and assayed their ATP-driven motility along stabilized rhodamine-labeled microtubules in vitro. These assays revealed the presence of Cx32 and kinesin motor proteins in the same vesicles. The addition of 50 µM ATP stimulated vesicle motility along linear microtubule tracks with velocities of 0.4-0.5 µm/s, which was inhibited with 1 mM of the kinesin inhibitor AMP-PNP (adenylyl-imidodiphosphate) and by anti-kinesin antibody but only minimally affected by 5 µM vanadate, a dynein inhibitor, or by anti-dynein antibody. These studies provide evidence that Cx32 can be transported intracellularly along microtubules and presumably to junctional domains in cells and highlight an important role of kinesin motor proteins in microtubule-dependent motility of Cx32.

Heterogeneous accumulation of fluorescent bile acids in primary rat hepatocytes does not correlate with their homogenous expression of ntcp.

Sodium taurocholate-cotransporting polypeptide (ntcp) is considered to be a major determinant of bile acid uptake into hepatocytes. However, the regulation of ntcp and the degree that it participates in the accumulation of specific substrates are not well understood. We utilized fluorescent bile acid derivatives and direct quantitation of fluorescent microscopy images to examine the regulation of ntcp and its role in the cell-to-cell variability of fluorescent bile acid accumulation. Primary-cultured rat hepatocytes rapidly accumulated the fluorescent bile acids, chenodeoxycholylglycylamidofluorescein (CDCGamF), 7-ß- nitrobenzoxadiazole 3-α hydroxy 5-ß cholan-24-oic acid (NBD-CA), and cholyl-glycylamido-fluorescein (CGamF). However, in stably transfected HeLa cells, ntcp preferred CDCGamF, whereas the organic anion transporter, organic anion transporting polypeptide 1 (oatp1a1), preferred NBD-CA, and neither ntcp nor oatp1a1 showed strong accumulation of CGamF by these methods. Ntcp-mediated transport of CDCGamF was inhibited by taurocholate, cyclosporin, actin depolymerization, and an inhibitor of atypical PKC-ζ. The latter two agents altered the cellular distribution of ntcp as visualized in ntcp-green fluorescent protein-transfected cells. Although fluorescent bile acid accumulation was reproducible by the imaging assays, individual cells showed variable accumulation that was not attributable to changes in membrane permeability or cell viability. In HeLa cells, this was accounted for by variable levels of ntcp, whereas, in hepatocytes, ntcp expression was uniform, and low accumulation was seen in a large portion of cells despite the presence of ntcp. These studies indicate that single-cell imaging can provide insight into previously unrecognized details of anion transport in the complex environment of polarized hepatocytes.

Proteomic analysis of endocytic vesicles: Rab1a regulates motility of early endocytic vesicles.

Texas-Red-asialoorosomucoid (ASOR) fluorescence-sorted early and late endocytic vesicles from rat liver were subjected to proteomic analysis with the aim of identifying functionally important proteins. Several Rab GTPases, including Rab1a, were found. The present study immunolocalized Rab1a to early and late endocytic vesicles and examined its potential role in endocytosis. Huh7 cells with stable knockdown of Rab1a exhibited reduced endocytic processing of ASOR. This correlated with the finding that Rab1a antibody reduced microtubule-based motility of rat-liver-derived early but not late endocytic vesicles in vitro. The inhibitory effect of Rab1a antibody was observed to be specifically towards minus-end-directed motility. Total and minus-end-directed motility was also reduced in early endocytic vesicles prepared from Rab1a-knockdown cells. These results corresponded with virtual absence of the minus-end-directed kinesin Kifc1 from early endocytic vesicles in Rab1a knockdown cells and imply that Rab1a regulates minus-end-directed motility largely by recruiting Kifc1 to early endocytic vesicles.

PDZK1 binding and serine phosphorylation regulate subcellular trafficking of organic anion transport protein 1a1.

Although perturbation of organic anion transport protein (oatp) cell surface expression can result in drug toxicity, little is known regarding mechanisms regulating its subcellular distribution. Many members of the oatp family, including oatp1a1, have a COOH-terminal PDZ consensus binding motif that interacts with PDZK1, while serines upstream of this site (S634 and S635) can be phosphorylated. Using oatp1a1 as a prototypical member of the oatp family, we prepared plasmids in which these serines were mutated to glutamic acid [E634E635 (oatp1a1(EE)), phosphomimetic] or alanine [A634A635 (oatp1a1(AA)), nonphosphorylatable]. Distribution of oatp1a1(AA) and oatp1a1(EE) was largely intracellular in transfected human embryonic kidney (HEK) 293T cells. Cotransfection with a plasmid encoding PDZK1 revealed that oatp1a1(AA) was now expressed largely on the cell surface, while oatp1a1(EE) remained intracellular. To quantify these changes, studies were performed in HuH7 cells stably transfected with these oatp1a1 plasmids. These cells endogenously express PDZK1. Surface biotinylation at 4°C followed by shift to 37°C showed that oatp1a1(EE) internalizes quickly compared with oatp1a1(AA). To examine a physiological role for phosphorylation in oatp1a1 subcellular distribution, studies were performed in rat hepatocytes exposed to extracellular ATP, a condition that stimulates serine phosphorylation of oatp1a1 via activity of a purinergic receptor. Internalization of oatp1a1 under these conditions was rapid. Thus, although PDZK1 binding is required for optimal cell surface expression of oatp1a1, phosphorylation provides a mechanism for fast regulation of the distribution of oatp1a1 between the cell surface and intracellular vesicular pools. Identification of the proteins and motor molecules that mediate these trafficking events represents an important area for future study.