Major influence of cell differentiation status on characteristics of proteoglycans synthesized by cultured rabbit renal proximal tubule cells: role of insulin and dexamethasone

Stoichiometric limitations in assembly of active recombinant retrovirus

Although recombinant retroviruses are widely used in gene therapy and as gene transfer vehicles for basic biological studies, their titers are very low as compared to other recombinant viral systems, e.g., adenovirus. We investigated the rate-limiting steps in production of LacZ-encoding ecotropic (CRE BAG 2) and amphotropic (Psi-CRIP) retrovirus. We found that ecotropic retrovirus producer cells produced a large number of inactive viral particles because they were severely limited by the amount of mRNA that was packaged into viral capsids. Introduction of the gene for green fluorescence protein (GFP) increased retroviral titers 40-fold, without affecting the viral matrix protein, p30, or the activity of reverse transcriptase. Surprisingly, while transfer of GFP gene increased retrovirus production, beta-gal activity and X-gal titer decreased significantly. Quantitative real-time polymerase chain reaction (PCR) showed that although producer cells synthesized similar amounts of both mRNAs, retroviral supernatants contained significantly lower amount of LacZ mRNA, possibly due to competition between LacZ and GFP mRNAs for encapsidation into virions. In contrast to ecotropic producers, introduction of GFP gene copies into amphotropic producers resulted in a moderate twofold increase in retrovirus production. However, delivery of genes encoding for the viral proteins gp70 and p30 increased virus production by fivefold, suggesting that amphotropic producers may also be limited by synthesis of structural viral proteins. Our data show that in addition to the amount of viral genome or proteins, assembly of viral components into active viral particles may limit production of high titer retroviral preparations.

Changes in expression and subcellular localization of annexin IV in rabbit kidney proximal tubule cells during primary culture

In the present study, we investigated the polarized expression of annexin IV at various stages in the growth of rabbit kidney proximal tubule cells (PTC) in primary cultures. The results of immunoblotting analysis and indirect immunofluorescence studies using a specific anti-annexin IV monoclonal antibody, indicated that annexin IV is expressed in proximal tubule cultured cells, although it was not detected in the proximal tubules present in frozen sections of kidney cortex and freshly isolated proximal tubule cells. In either non-confluent or confluent cells which remained attached to the collagen-coated support, annexin IV was mainly concentrated around the nucleus, whereas in PTC forming the monolayer of domes, it was restricted to the basolateral membrane domain. This basolateral localization was identical to that observed in other polarized epithelial cell types such as enterocytes. When the domes burst, the cells returned to the collagen-coated support and the annexin IV was again localized around the nuclei. The fact that the change of localization was very rapid suggested the existence of a considerable difference between the differentiation states of dome forming and adherent confluent cells. Moreover, a transient association of annexin IV with the basal body of apically located cilia also seemed to be correlated with a particular polarization state and/or differentiation states of adherent cultured cells, corresponding to the beginning of the polarized expression of aminopeptidase N, a hydrolase located in the apical brush border membrane, and to the falling of cells onto the support, subsequent to the bursting of the domes. In conclusion, these results provide evidence that annexin IV may constitute a new marker of the basolateral membrane domain of polarized epithelial renal cells in primary cultures.

Calcium oxalate crystallizing properties of polyanions elaborated by cultured renal proximal tubular cells

To study the influence of renal polyanions on crystallization of urinary calcium oxalate, we recovered polyanionic macromolecules from media conditioned by primary cultures of renal proximal tubular epithelial cells of rats in serum-free, hormonally defined medium. Cells cultured on microporous supports showed a higher degree of morphological and functional proximal differentiation into a polarized monolayer than those on plastic impervious substrata. Papainization of the polyanions yielded the glycosaminoglycans chondroitin/dermatan sulphate and heparan sulphate. These accounted respectively for 60% and 80% of the crystal nucleation-promoting activities of polyanions recovered from the apical and basal media conditioned by polarized cultures on microporous supports. Similar relative activities were observed among the urinary glycosaminoglycans and polyanions similarly tested. Primary cultures of polarized proximal tubular epithelial cells are useful then as an in vitro model to study the crystallizing activities of polyanionic macromolecules produced by renal cells.