Alteration in the regulation of plasma membrane glycoproteins of the hepatocyte during ontogeny

Translational PBPK Modeling of the Protein Therapeutic and CD95L Inhibitor Asunercept to Develop Dose Recommendations for Its First Use in Pediatric Glioblastoma Patients

The protein therapeutic and CD95L inhibitor asunercept is currently under clinical investigation for the treatment of glioblastoma and myelodysplastic syndrome. The purpose of this study was to predict the asunercept pharmacokinetics in children and to give dose recommendations for its first use in pediatric glioblastoma patients. A physiologically-based pharmacokinetic (PBPK) model of asunercept in healthy and diseased adults was successfully developed using the available clinical Phase I and Phase II study data. This model was then extrapolated to different pediatric populations, to predict the asunercept exposure in children and to find equivalent starting doses. Simulation of the asunercept serum concentration-time curves in children between 1–18 years of age shows that a dosing regimen based on body weight results in a similar asunercept steady-state exposure in all patients (pediatric or adult) above 12 years of age. For children between 1–12 years, higher doses per kg body weight are recommended, with the highest dose for the very young patients. Translational PBPK modeling is strongly encouraged by regulatory agencies to help with the initial dose selection for pediatric trials. To our knowledge, this is the first report of pediatric PBPK to support the dose selection of a therapeutic protein before its administration to children.

Small hepatocytes in culture develop polarized transporter expression and differentiation

Rat small hepatocytes have been shown to proliferate in culture and to form organoids with differentiated hepatocytes in vitro. To evaluate the degree of polarized transporter differentiation of rat small hepatocytes during 9 weeks of culturing, we studied the time-dependent expression and subcellular localization of the major bile salt and organic anion transport systems of hepatocytes [i.e. the basolateral sodium-taurocholate co-transporting protein (Ntcp), organic-anion-transporting polypeptide 1b2 (Oatp1b2), the canalicular bile-salt export pump (Bsep) and multidrug-resistance-associated protein 2 (Mrp2)]. Small hepatocytes proliferated and differentiated in culture and formed sharply demarcated colonies as assessed by morphology, α-fetoprotein, albumin and Mrp1 expression. Polarized surface transporter expression was evident after 5 weeks of culturing for Ntcp, Oatp1b2 and Mrp2, and after 7 weeks for Bsep. After 9 weeks in culture, the vast majority of matured hepatocytes expressed Ntcp/Oatp1b2 at the basolateral and Bsep/Mrp2 at the canalicular plasma-membrane domains. This polarized transporter expression was accompanied by canalicular secretion of fluorescein-diacetate and cholylglycyl-fluorescein. Furthermore, an anastomizing three-dimensional network of bile canaliculi developed within piling-up colonies. These data demonstrate that cultured rat small hepatocytes acquire a fully differentiated transporter expression phenotype during their development into hepatic `organoid-like' clusters of mature hepatocytes. Thereby, the time-dependent sequence of transporter expression mirrored the ontogenesis of transporter expression in developing rat liver, supporting the concept that small hepatocytes correspond to the hepatocyte lineage derived from embryonic hepatoblasts and/or from a different pool of `committed hepatocyte progenitor cells'.

Differences between neonates and adults in carbohydrate sequences and reaction kinetics of plasmin and alpha(2)-antiplasmin

This study investigates reaction kinetics by slow-binding kinetics methods of both adult and fetal plasmin (Types 1 and 2) with adult and fetal alpha(2)-antiplasmin. In addition, carbohydrate sequences of Fetal and Adult Plasminogen Types 1 and 2, as well as fetal and adult alpha(2)-antiplasmin, were determined by mass spectrometric analysis. All curves of plasmin-alpha(2)-antiplasmin interaction followed the same pattern, indicating reversible slow-binding inhibition with an initial loose complex and a following tight complex. Differences between fetal and adult plasmin reactions with alpha(2)-antiplasmin were predominantly due to the initial loose complex. Values for K(i initial) in the reaction with adult alpha(2)-antiplasmin were 1.5 and 1.6 nM for Fetal Plasmin Types 1 and 2, respectively; compared to 0.3 and 0.7 nM for the corresponding adult types. Increasing concentrations of tranexamic acid resulted in a continuous increase of K(i initial) until a plateau was reached which was similar for all plasmin types. Almost identical values could be obtained when fetal alpha(2)-antiplasmin was used instead of adult alpha(2)-antiplasmin. Mass spectrometric analyses of the glycans present on plasminogen revealed a higher level of truncated N-glycans on the fetal material compared to the adult. The O-glycans of fetal and adult plasminogen were closely similar and only minor differences were observed between N-glycans of fetal and adult alpha(2)-antiplasmin. In conclusion, both fetal plasmin isoforms are less inhibited by alpha(2)-antiplasmin compared to the adult plasmin variants. These findings are important for the understanding of the physiology of the fibrinolytic system in neonates and provide further evidence that differences in glycosylation could be associated with marked effects on protein function.

Thyroglobulin is selected as luminal protein cargo for apical transport via detergent-resistant membranes in epithelial cells

Thyroid hormone synthesis by thyrocytes depends upon apical secretion of thyroglobulin (Tg), the glycoprotein prohormone. In stably transfected MDCK cells, recombinant Tg is also secreted apically. All secreted Tg has undergone Golgi carbohydrate modification, whereas most intracellular Tg (which is slow to exit the endoplasmic reticulum) is sensitive to digestion with endoglycosidase H. However, in MDCK cells and PC Cl3 thyrocytes, a subpopulation of newly synthesized recombinant and endogenous Tg, respectively, is recovered in a Triton X-100 insoluble, glycosphingolipid/cholesterol-enriched (GEM/raft) fraction, and this small subpopulation is overwhelmingly endoglycosidase H resistant. Upon apical secretion, Tg solubility is restored. Apical secretion of Tg is inhibited by cellular cholesterol depletion. In FRT cells, recombinant Tg becomes Triton X-100 insoluble within 60 min after synthesis and a portion is actually endoglycosidase H-sensitive, suggesting early Tg entry into GEMs/rafts. Interestingly in FRT cells, Tg remains associated with the apical plasma membrane upon exocytosis, and all surface Tg is GEM/raft-associated. Thus, Tg is the first secretory protein demonstrated to enter Triton X-100 insoluble membranes en route to the apical surface of epithelial cells. The data imply that Tg utilizes a cargo-selective mechanism for apical sorting.

Reversible induction of rat hepatoma cell polarity with bile acids

A dynamic model for inducing and isolating polarized cell colonies from differentiated rat hepatoma was established with chenodeoxycholic acid (CDCA). Cells were treated with 75 microM CDCA in a 1% solvent mix (DMSO/ethanol: 0.5%/0.5%) for 11 days and positive Fao-BA1 and C2rev7-BA1 clones were isolated, respectively, from Fao and C2rev7. Cell polarization in these two clones was demonstrated by (i) the detection of (gamma)-glutamyl transpeptidase activity (gamma)-GT) and the presence of specific proteins, namely aminopeptidase N (APN), bile acid export pump (Bsep), multidrug resistance-associated protein 2 (Mrp2) at the canalicular pole, (ii) the expression of tight junction (ZO-1) and basolateral (1–18) marker proteins, (iii) the presence of regular microvilli in the cavities sealed by tight junctions, and (iv) functional bile canaliculi-like structures with the capacity to metabolise and secrete carboxyfluorescein diacetate dye. The polarized phenotype was maintained for more than 200 cell generations in the presence of CDCA and could be modulated by cell density or omitting the inducing agent. Hence this cellular model is well suited for studies on hepatic differentiation, polarization and bile salt trafficking with therapeutic implications.

Apical endocytosis in rat hepatocytes In situ involves clathrin, traverses a subapical compartment, and leads to lysosomes

BACKGROUND & AIMS

This study demonstrates and characterizes apical (canalicular) endocytic pathways in hepatocytes in situ.

METHODS

Endocytic markers were administered by retrograde infusion through the common bile duct. Colocalization with proteins that are specific for various endocytic compartments was performed on stacks of deconvoluted confocal immunofluorescence images. The subcellular distribution of marker proteins was assessed by electron microscopy (EM).

RESULTS

Bulk-phase, as well as membrane-associated markers, were internalized readily at the apical cell pole. At the EM level, marker was found initially in 60-100-nm tubulovesicular structures and 150-200-nm cup-shaped vesicles, whereas multivesicular bodies and lysosomes became labeled after longer time intervals. Apical endocytosis involved clathrin and delivered marker to late endosomes (rab7(+), cathepsin D(+)), as well as lysosomes (rab7(-), cathepsin D(+)). Simultaneous labeling of the basolateral endocytic route resulted in overlap of both pathways in the late endosomal and lysosomal compartments. In addition, apical endocytosis involved a subapical compartment (endolyn-78(+), rab11(+), polymeric IgA receptor [pIgA-R(+)]) that is passed by the transcytotic route, thus constituting a crossroads. pIgA-R immunoreactivity, probably reflecting the cleaved receptor fragment, was associated with apical endocytic marker and colocalized with clathrin and later with cathepsin D.

CONCLUSIONS

Apical endocytosis involves coated pits/vesicles, leads to a subapical compartment, and plays a role in the retrieval of canalicular plasma membrane components for lysosomal degradation.

Immunohistochemical analysis of development of desmin-positive hepatic stellate cells in mouse liver

Development of desmin-positive hepatic stellate cells was studied in mice using double immunofluorescent techniques and in vitro cultures with special attention given to their cell lineages. Several studies recently reported on the presence of cells that are immunologically reactive with both antidesmin and anticytokeratin antibodies in young fetal rat livers, and suggested the possibility that these cells give rise to hepatocytes and hepatic stellate cells. At early stages of mouse liver development, stellate cells with desmin-positive filaments were scattered in the liver parenchyma. However, the stellate cells definitely differed from hepatoblasts and hepatocytes in terms of their morphology and expression of desmin and hepatoblast and hepatocyte-specific E-cadherin in the liver. Fetal hepatoblasts and hepatocytes did not react with antidesmin antibodies, nor did desmin-positive stellate cells express E-cadherin in vivo and in vitro. Thus it is likely that desmin-positive stellate cells and hepatoblasts belong to different cell lineages. In the fetal liver, the desmin-positive stellate cells surrounded blood vessels, and extended their processes to haematopoietic cells and megakaryocytes. Many, but not all, hepatoblasts and hepatocytes were observed to be associated with the stellate cells. At fetal stages, cellular processes positive for desmin in the stellate cells were also thick compared with those in the adult liver, in which desmin-positive stellate cells lay in Disse's space and were closely associated with all hepatocytes. These developmental changes in the geography of desmin-positive cells in the liver parenchyma and their morphology may be associated with their maturation and interactions with other cell types.

Proliferation and differentiation of fetal liver epithelial progenitor cells after transplantation into adult rat liver

To identify cells that have the ability to proliferate and differentiate into all epithelial components of the liver lobule, we isolated fetal liver epithelial cells (FLEC) from ED 14 Fischer (F) 344 rats and transplanted these cells in conjunction with two-thirds partial hepatectomy into the liver of normal and retrorsine (Rs) treated syngeneic dipeptidyl peptidase IV mutant (DPPIV(-)) F344 rats. Using dual label immunohistochemistry/in situ hybridization, three subpopulations of FLEC were identified: cells expressing both alpha-fetoprotein (AFP) and albumin, but not CK-19; cells expressing CK-19, but not AFP or albumin, and cells expressing AFP, albumin, and cytokeratins-19 (CK-19). Proliferation, differentiation, and expansion of transplanted FLEC differed significantly in the two models. In normal liver, 1 to 2 weeks after transplantation, mainly cells with a single phenotype, hepatocytic (expressing AFP and albumin) or bile ductular (expressing only CK-19), had proliferated. In Rs-treated rats, in which the proliferative capacity of endogenous hepatocytes is impaired, transplanted cells showed mainly a dual phenotype (expressing both AFP/albumin and CK-19). One month after transplantation, DPPIV(+) FLEC engrafted into the parenchyma exhibited an hepatocytic phenotype and generated new hepatic cord structures. FLEC, localized in the vicinity of bile ducts, exhibited a biliary epithelial phenotype and formed new bile duct structures or were incorporated into pre-existing bile ducts. In the absence of a proliferative stimulus, ED 14 FLEC did not proliferate or differentiate. Our results demonstrate that 14-day fetal liver contains lineage committed (unipotential) and uncommitted (bipotential) progenitor cells exerting different repopulating capacities, which are affected by the proliferative status of the recipient liver and the host site within the liver where the transplanted cells become engrafted. These findings have important implications in future studies directed toward liver repopulation and ex vivo gene therapy.

Quantitative analysis of cell allocation during liver development, using the spf(ash)-heterozygous female mouse

Mosaicism of ornithine transcarbamylase (OTC) expression in hepatocytes was quantitatively analyzed during liver development of the spf(ash)-heterozygous female mouse. Because the mosaic patterns depend on cell migration and cell mixing, such analysis could give insights on the growth pattern or allocation pattern of hepatocytes during liver development. Complex mosaic patterns of OTC-positive and -negative hepatocytes were observed in sections of fetal and postnatal livers. Sizes of patches, which were aggregates of OTC-positive or -negative hepatocytes, increased during development. Patches were slender and comparatively simple in 15.5- and 17.5-day fetal and neonatal livers. Quantitative analysis of patch shapes demonstrated that undulation of patches was maximal at 7 postnatal days. Patches with nodular shapes also started to increase in number at this stage. Isolated patches in sections of fetal livers and postnatal livers three-dimensionally connected with one another. However, especially in fetal livers, in which OTC-positive patches were minor, due to the presence of abundant hemopoietic cells, isolated three-dimensional patches consisting of approximately 5 to 70 cells were often found. They were shaped like slender branching or zigzag-shaped cords, but no definite orientation such as portal-central was observed in them at any stage. These results suggest that hepatocytes contiguously allocate their daughter cells as zigzag-shaped or branching cords at younger stages. Some hepatocytes grow with nodular formation after 7 postnatal days. Migration and mixing of hepatocytes appear to be more extensive at fetal stages than in the adult liver. Immunohistochemical analysis of intercellular junction proteins (E-cadherin, connexins 26 and 32, occludin, and ZO-1) also revealed that their expression and distribution changed in hepatocytes during development, which may be correlated with the OTC mosaic patterns.

Protein-restricted diet alters concentration of plasma membrane glycoproteins in rat liver

Malnutrition is known to have adverse effects on the physiology and morphology of the liver. The aim of this investigation was to examine the effect of protein restriction on the content of plasma membrane proteins residing in the sinusoidal and bile canalicular domains of rat liver. Post-weanling rats maintained on low protein isocaloric diets showed marked growth retardation concomitant with reduced liver protein concentration compared to control animals. The content of leucine aminopeptidase, a bile canalicular enzyme, and asialoglycoprotein receptor, a sinusoidal receptor, in livers of protein-restricted rats was 66% and 50%, respectively, of control livers. In contrast, the relative concentrations of dipeptidyl peptidase IV and a cell adhesion molecule (GP 110), both canalicular proteins, were 160% and 121%, respectively, in rat livers upon protein restriction. After a 4-week rehabilitation period, the concentrations of all canalicular membrane proteins were similar to those in control livers, while the sinusoidal receptor was only 68% of control values. Protein restriction was found to adversely affect the concentrations of protein constituents, but not their localization in the hepatocyte plasma membrane. In general, altered concentrations of hepatocyte membrane proteins were reversed on the administration of a normal protein diet.

Culture matrix configuration and composition in the maintenance of hepatocyte polarity and function

Several extracellular matrix (ECM) configurations involving type I collagen and Matrigel were examined for their ability to support differentiated function and polarity of cultured adult rat hepatocytes. Collagen sandwich- and Matrigel-based cultures yielded superior and comparable albumin secretion for at least 2 weeks. In collagen sandwich, hepatocytes were polygonal, and formed multicellular arrays. Collagen sandwich was also found to promote in vivo-like polarization of F-actin, cell adhesion molecules (E-cadherin), and lateral (Na+, K(+)-ATPase, glucose transporter) and apical (dipeptidyl peptidase, aminopeptidase) membrane polarity markers, but not the expression of the gap junction protein connexin 32 and the epidermal growth factor (EGF) receptor. In contrast, hepatocytes cultured in or on Matrigel were more rounded and formed aggregates. Matrigel-based cultures also elicited detectable levels of connexin and EGF receptor and an altered distribution of F-actin, E-cadherin, and apical and lateral membrane proteins. Composite sandwich configurations containing collagen I and Matrigel restored markers lacking in the collagen sandwich, and showed a variable morphology and membrane polarity. Hepatocyte polarity could thus be manipulated by the overall ECM composition. Furthermore, in composite sandwich cultures, these manipulations can be effected largely independent of changes in hepatocyte morphology and albumin secretion.

Changes in keratin expression during fetal and postnatal development of intestinal epithelial cells

We have investigated keratin expression in fetal, newborn and adult rat intestines by immunofluorescence staining, immunoblotting of two-dimensional gels and Northern blot analysis of total cellular RNAs. Keratin-type intermediate filaments, composed predominantly of keratin no. 19, were observed already in the undifferentiated stratified epithelium present at 15-16 days of gestation. The marked maturation and differentiation of the epithelium taking place at 18-19 days of gestation was characterized by the appearance of the differentiation-specific keratin no. 21 and by a significant increase in the relative amount of keratin no. 8. The keratin pattern typical of adult villus cells became established at the time of birth, and was marked by a considerable increase in the complexity of the keratin-related polypeptides detected on two-dimensional gels, indicative of extensive post-translational modification of all keratins. Starting at 20 days of gestation there was a major increase in the relative abundance of mRNAs coding for keratin nos. 8, 19 and 21; in contrast, the relative amount of keratin no. 18 mRNA reached a peak shortly after birth and declined to very low levels in adult intestine. These results demonstrated marked changes in keratin expression and post-translational processing taking place at key stages of intestinal development. The appearance of keratin no. 21 in coincidence with the formation of an adult-type brush border and terminal web would be consistent with it having an important role in the organization of the intermediate filament network in the apical cytoplasm of the differentiated intestinal cells.

Clonal analysis of sucrase-isomaltase expression in the human colon adenocarcinoma Caco-2 cells

To investigate the biosynthetic basis for the mosaic expression of brush border enzymes in confluent Caco-2 cells, a human colon carcinoma cell line exhibiting characteristics of adult small intestinal enterocytes, we have obtained a series of clones differing markedly in their growth rates, amounts of transforming growth factor-alpha/epidermal growth factor-like activity released into the culture medium, and sucrase-isomaltase (SI) activity. Other intestinal markers (aminopeptidase N, dipeptidylpeptidase IV, lactase, alkaline phosphatase and 'crypt cell antigen') displayed a much more limited variability in expression, suggesting that the Caco-2 cell clones we have obtained did not differ in their overall ability to differentiate. Immunofluorescence staining, metabolic labelling with radioactive methionine and hybridization analysis of SI mRNA abundance were used to investigate SI synthesis and its regulation in clones endowed with low, intermediate or high sucrase activity. The results obtained have demonstrated heterogeneous SI expression, even in clonal cell lines, and a negative correlation between SI expression and growth factor concentrations in the culture medium, suggesting an autocrine regulation of cell proliferation and differentiation in confluent Caco-2 cells. Pulse-chase experiments using the two clones endowed with the lowest and highest levels of SI activity, followed by immunoprecipitation of labelled SI with epitope-specific antibodies and SDS/PAGE analysis, suggested that both transcriptional and post-translational mechanisms play a role in the regulation of SI expression in intestinal cells.

Modulation of extracellular matrix proteins in rat liver during development

The expression and localization of extracellular matrix proteins in rat liver was investigated as a function of liver development. Levels of extracellular matrix proteins were measured by dot-blot or immunoblot protocols using monospecific antibodies prepared against collagen types I, III and IV; laminin; fibronectin; and fibronectin receptor. Proline and hydroxyproline levels from extracted liver collagen were quantitated by Pico Tag analysis. It was observed that the content of type IV collagen and fibronectin in the rat liver increased two to four times during the perinatal period. In contrast, levels of laminin and collagen types I and III decreased up to twofold in developing rat livers. The content of fibronectin receptor during ontogeny was decreased four times in an inverse relationship to fibronectin molecules. Fibronectin receptor and extracellular matrix proteins displayed no difference in apparent molecular weight as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis immunoblots. Indirect immunofluorescence staining of frozen thin liver sections revealed that the pattern of localization of extracellular matrix proteins in the nonvascular regions of fetal liver was punctate rather than restricted to a specific region such as the perisinusoidal area of adult livers. Similarly, fibronectin receptor was also present, mainly in the sinusoidal area of adult livers, whereas fetal sections were diffusely stained. Our findings suggest that the differential modulation of extracellular matrix proteins and their localization in the developing rat livers undergo a dramatic alteration in the composition and structural organization of matrix material, which may act to modulate proliferation and to promote the differentiation of liver cells during development.