Long term culture of epithelia in a continuous fluid gradient for biomaterial testing and tissue engineering

Bridging the gap between traditional cell cultures and bioreactors applied in regenerative medicine: practical experiences with the MINUSHEET perfusion culture system

To meet specific requirements of developing tissues urgently needed in tissue engineering, biomaterial research and drug toxicity testing, a versatile perfusion culture system was developed. First an individual biomaterial is selected and then mounted in a MINUSHEET(®) tissue carrier. After sterilization the assembly is transferred by fine forceps to a 24 well culture plate for seeding cells or mounting tissue on it. To support spatial (3D) development a carrier can be placed in various types of perfusion culture containers. In the basic version a constant flow of culture medium provides contained tissue with always fresh nutrition and respiratory gas. For example, epithelia can be transferred to a gradient container, where they are exposed to different fluids at the luminal and basal side. To observe development of tissue under the microscope, in a different type of container a transparent lid and base are integrated. Finally, stem/progenitor cells are incubated in a container filled by an artificial interstitium to support spatial development. In the past years the described system was applied in numerous own and external investigations. To present an actual overview of resulting experimental data, the present paper was written.

Dietary salt intake modulates differential splicing of the Na-K-2Cl cotransporter NKCC2

Both sodium reabsorption in the thick ascending limb of the loop of Henle (TAL) and macula densa salt sensing crucially depend on the function of the Na/K/2Cl cotransporter NKCC2. The NKCC2 gene gives rise to at least three different full-length NKCC2 isoforms derived from differential splicing. In the present study, we addressed the influence of dietary salt intake on the differential splicing of NKCC2. Mice were subjected to diets with low-salt, standard salt, and high-salt content for 7 days, and NKCC2 isoform mRNA abundance was determined. With decreasing salt intake, we found a reduced abundance of the low-affinity isoform NKCC2A and an increase in the high-affinity isoform NKCC2B in the renal cortex and the outer stripe of the outer medulla. This shift from NKCC2A to NKCC2B during a low-salt diet could be mimicked by furosemide in vivo and in cultured kidney slices. Furthermore, the changes in NKCC2 isoform abundance during a salt-restricted diet were partly mediated by the actions of angiotensin II on AT1 receptors, as determined using chronic angiotensin II infusion. In contrast to changes in oral salt intake, water restriction (48 h) and water loading (8% sucrose solution) increased and suppressed the expression of all NKCC2 isoforms, without changing the distribution pattern of the single isoforms. In summary, the differential splicing of NKCC2 pre-mRNA is modulated by dietary salt intake, which may be mediated by changes in intracellular ion composition. Differential splicing of NKCC2 appears to contribute to the adaptive capacity of the kidney to cope with changes in reabsorptive needs.

Supportive development of functional tissues for biomedical research using the MINUSHEET® perfusion system

Functional tissues generated under in vitro conditions are urgently needed in biomedical research. However, the engineering of tissues is rather difficult, since their development is influenced by numerous parameters. In consequence, a versatile culture system was developed to respond the unmet needs.

Optimal adhesion for cells in this system is reached by the selection of individual biomaterials. To protect cells during handling and culture, the biomaterial is mounted onto a MINUSHEET® tissue carrier. While adherence of cells takes place in the static environment of a 24 well culture plate, generation of tissues is accomplished in one of several available perfusion culture containers. In the basic version a continuous flow of always fresh culture medium is provided to the developing tissue. In a gradient perfusion culture container epithelia are exposed to different fluids at the luminal and basal sides. Another special container with a transparent lid and base enables microscopic visualization of ongoing tissue development. A further container exhibits a flexible silicone lid to apply force onto the developing tissue thereby mimicking mechanical load that is required for developing connective and muscular tissue. Finally, stem/progenitor cells are kept at the interface of an artificial polyester interstitium within a perfusion culture container offering for example an optimal environment for the spatial development of renal tubules.

The system presented here was evaluated by various research groups. As a result a variety of publications including most interesting applications were published. In the present paper these data were reviewed and analyzed. All of the results point out that the cell biological profile of engineered tissues can be strongly improved, when the introduced perfusion culture technique is applied in combination with specific biomaterials supporting primary adhesion of cells.

Optimization of culture conditions for osteogenically-induced mesenchymal stem cells in β-tricalcium phosphate ceramics with large interconnected channels

The aim of this study was to optimize culture conditions for human mesenchymal stem cells (hMSCs) in β-tricalcium phosphate ceramics with large interconnected channels. Fully interconnected macrochannels comprising pore diameters of 750 µm and 1400 µm were inserted into microporous β-tricalcium phosphate (β-TCP) scaffolds by milling. Human bone marrow-derived MSCs were seeded into the scaffolds and cultivated for up to 3 weeks in both static and perfusion culture in the presence of osteogenic supplements (dexamethasone, β-glycerophosphate, ascorbate). It was confirmed by scanning electron microscopic investigations and histological staining that the perfusion culture resulted in uniform distribution of cells inside the whole channel network, whereas the statically cultivated cells were primarily found at the surface of the ceramic samples. It was also determined that perfusion with standard medium containing 10% fetal calf serum (FCS) led to a strong increase (seven-fold) of cell numbers compared with static cultivation observed after 3 weeks. Perfusion with low-serum medium (2% FCS) resulted in moderate proliferation rates which were comparable to those achieved in static culture, although the specific alkaline phosphatase (ALP) activity increased by a factor of more than 3 compared to static cultivation. Gene expression analysis of the ALP gene also revealed higher levels of ALP mRNA in low-serum perfused samples compared to statically cultivated constructs. In contrast, gene expression of the late osteogenic marker bone sialoprotein II (BSPII) was decreased for perfused samples compared to statically cultivated samples.

Achievements and challenges in bioartificial kidney development

Bioartificial kidneys (BAKs) combine a conventional hemofilter in series with a bioreactor unit containing renal epithelial cells. The epithelial cells derived from the renal tubule should provide transport, metabolic, endocrinologic and immunomodulatory functions. Currently, primary human renal proximal tubule cells are most relevant for clinical applications. However, the use of human primary cells is associated with many obstacles, and the development of alternatives and an unlimited cell source is one of the most urgent challenges. BAKs have been applied in Phase I/II and Phase II clinical trials for the treatment of critically ill patients with acute renal failure. Significant effects on cytokine concentrations and long-term survival were observed. A subsequent Phase IIb clinical trial was discontinued after an interim analysis, and these results showed that further intense research on BAK-based therapies for acute renal failure was required. Development of BAK-based therapies for the treatment of patients suffering from end-stage renal disease is even more challenging, and related problems and research approaches are discussed herein, along with the development of mobile, portable, wearable and implantable devices.

Tissue engineering of composite grafts: Cocultivation of human oral keratinocytes and human osteoblast-like cells on laminin-coated polycarbonate membranes and equine collagen membranes under different culture conditions

In complex craniomaxillofacial defects, the simultaneous reconstruction of hard and soft tissue is often necessary. Until now, oral keratinocytes and osteoblast-like cells have not been cocultivated on the same carrier. For the first time, the cocultivation of human oral keratinocytes and human osteoblast-like cells has been investigated in this study. Different carriers (laminin-coated polycarbonate and equine collagen membranes) and various culture conditions were examined. Human oral keratinocytes and human osteoblast-like cells from five patients were isolated from tissue samples, seeded on the opposite sides of the carriers and cultivated for 1 and 2 weeks under static conditions in an incubator and in a perfusion chamber. Proliferation and morphology of the cells were analyzed by EZ4U-tests, light microscopy, and scanning electron microscopy. Cocultivation of both cell-types seeded on one carrier was possible. Quantitative and qualitative growth was significantly better on collagen membranes when compared with laminin-coated polycarbonate membranes independent of the culture conditions. Using perfusion culture in comparison to static culture, the increase of cell proliferation after 2 weeks of cultivation when compared with the proliferation after 1 week was significantly lower, independent of the carriers used. In conclusion, the contemporaneous cultivation of human oral keratinocytes and human osteoblast-like cells on the same carrier is possible, a prerequisite for planned in vivo studies. As carrier collagen is superior to laminin-coated polycarbonate membranes. Regarding the development over time, the increase of proliferation rate is lower in perfusion culture. Examinations of cellular differentiation over time under various culture conditions will be subject of further investigations.

Stable ciliary activity in human nasal epithelial cells grown in a perfusion system

PURPOSE

Explore the usefulness of a perfusion system in order to establish human nasal epithelial cell cultures suitable for long-term in vitro ciliary beat frequency (CBF) and cilio-toxicity studies.

METHODS

The cells were obtained by protease digestion of nasal biopsy material. The cells were plated at a density of 0.8-1 x 10(6)/cm2 on Vitrogen-coated polyethylene terephthalate membranes, and cultured under submerged conditions in a CO2 incubator or in a perfusion system (initiated on days 8-9 after plating). The CBF was determined at 24.1 +/- 0.8 degrees C by a computerized microscope photometry system. The morphology of the cultured cells was characterized by transmission electron microscopy (TEM).

RESULTS

Under CO2 incubator culture conditions, stable ciliary activity was expressed and maintained from day 2 to day 24. Under perfusion system culture conditions, the CBF (mean+/-S.D., n = 4) amounted to 8.4 +/- 0.9 and 8.8 +/- 0.4 Hz on days 7 and 14, respectively. These values were lower as compared to the corresponding CBF obtained in the CO2 incubator cultures (9.5 +/- 0.6 and 9.9 +/- 1.0 Hz, respectively). Reference cilio-stimulatory (glycocholate) and cilio-inhibitory (chlorocresol) compounds were used to assess CBF reactivity. In the CO2 incubator and 7- and 14-days perfusion system cultures, glycocholate (0.5%) showed a reversible cilio-stimulatory effect of 23, 26 and 21%, respectively, while chlorocresol (0.005%) exerted a reversible cilio-inhibitory effect of 36, 40 and 36%, respectively. TEM revealed polarized cuboidal to columnar epithelial morphology, with well-differentiated ciliated cells under CO2 and perfusion system conditions (up to day 23).

CONCLUSION

Culturing human nasal epithelial cells on Vitrogen-coated polyethylene terephthalate membranes in submerged conditions in a CO2 incubator and in a perfusion system offers the possibility for long-term preservation (up to 22-24 days) of stable and reactive CBF in vitro.