This protocol describes a method for converting established porcine 3D organoids into single cells, which are then seeded onto transwell membranes to form an intact monolayer. This configuration grants access to the apical side of the cells, facilitating the use of Ussing chambers to monitor absorptive and secretory processes.
The initial and crucial step in this multi-step process is the precise disintegration of the 3D organoids. Achieving uniform seeding of the single cells is essential for the formation of a functional monolayer27. Therefore, careful monitoring of the disintegration process is necessary. Incubation with trypsin/EDTA solution should be adjusted accordingly - prolonged if cell clumps are observed or shortened if excessive cell death is noted during cell counting. It is important to note that variations in the size of the organoids, which may result from differences in user handling, can impact the disintegration process and should be taken into consideration.
The increasing TEER values, which reach a plateau phase at day 9 for the colon organoids or day 18 in the case of the jejunum organoids, indicate an intact monolayer28 as it has already been shown in well-established human cell culture systems of the intestinal tract29,30 and also in porcine organoids15,17. However, these publications show TEER values of more than 750 Ω*cm2 after 1-4 days15 or 500-2,500 Ω*cm2 after 3 days of cultivation17 which exceed values obtained in this study. These differences are presumably derived from different media compositions or seeding densities of the cells. Furthermore, these conditions may also influence cell proliferation and differentiation. While monolayers in previous studies developed monolayers within a few days15,16,17, our protocol results in a slower monolayer formation, followed by a subsequent differentiation phase. This extended timeline is likely due to specific cultivation conditions. The increasing TEER itself is facilitated by the increasing expression of expression of tight junction proteins such as claudins (e.g., claudin 2 and claudin 3) or occludin, leading to a higher interconnection of the individual cells31,32.
The experimental approach utilizes the Ussing chamber technique to monitor active transport processes across the epithelial layer. Notably, the basal Isc values differed between jejunal and colonic monolayers, with colonic monolayers displaying higher Isc values than their jejunal counterparts. This observation suggests that the variations between the two epithelia are likely to result from differing levels of electrogenic transport processes. These findings align with those observed in native porcine epithelium, highlighting the high degree of comparability of this model to the in vivo situation23. In contrast to the differences in Isc values, the Rt values, which are the reciprocal of tissue conductance, are comparable between both organoid types, although they remain higher than those recorded in native tissues19,23,33.
In summary, colon and jejunum organoids exhibit distinct TEER values, as well as differences in basal Isc and basal Rt, which determine the Ussing chamber setup. These differences align with findings from previous studies on native pigs small and large intestinal tissue23,34,35. The occurrence of similar differences in our in vitro system, as observed in vivo, further strengthens the comparability between the two conditions.
In addition to basal parameters, functional changes driven by active transport processes across the epithelium are crucial for establishing a functional intestinal model. In this study, we examined the effects of forskolin, which activates adenylate cyclase36, leading to elevated cyclic adenosine monophosphate (cAMP) levels. This, in turn, activates the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, resulting in increased chloride secretion. Consequently, a significant rise in the Isc was observed in both jejunal and colonic monolayers. These results closely resemble the data obtained from native porcine tissue in these intestinal segments, underscoring the validity of the model20,22,23,37. Besides cAMP-dependent chloride secretion is present in both intestinal segments, segment-specific transport processes may be investigated in the Ussing chamber in future studies. These could include the absorption of monosaccharides such as glucose38 by the sodium-dependent glucose transporter 139 in the small intestine or the influence of the sodium transport by the amiloride-sensitive sodium channel in the large intestine40.
Overall, depending on the species from which the organoids are derived, and the specific method used for generating the 3D organoids, protocol modifications may be required. Furthermore, the growth of organoids may be influenced by whether they are generated from fresh tissue, as in this study, or from frozen crypts17. These adjustments could involve extending or shortening incubation and cultivation times at some stages of the protocol. The present study reveals differences in both cultivation parameters and experimental outcomes among the organoid types investigated. These findings indicate that organoids derived from different segments of the intestinal tract may require further adjustments and the protocol needs validation during the cultivation process depending on the abovementioned reasons.
Further limitations of this protocol include the need for more structural investigations, as it focuses solely on electro- and transport physiological properties. Additionally, careful handling of the organoids is crucial during both cultivation and Ussing chamber experiments due to their high vulnerability. For instance, excessive aeration in the Ussing chamber can cause cells to detach from the transwells and disrupt the monolayer due to shear stress, and this must be avoided.
In conclusion, the presented method enables the generation of porcine 2D monolayers characterized by increasing TEER values, tissue integrity, and functional capabilities, as demonstrated by the chloride secretion through the CFTR channels. By combining the relatively new organoid system - which is still undergoing refinement, particularly in the case of farm animal organoids - with the well-established Ussing chamber technique, we provide a crucial foundation for further validation and comparison with data obtained from native tissue. Additionally, this approach offers a valuable platform for addressing pathophysiological and pharmacological research questions.