Coordinated Movement of Bile Canalicular Networks Reconstructed by Rat Small Hepatocytes

A modular analysis of bile canalicular function and its implications for cholestasis

Hepatocytes produce bile components and secrete them into a lumen, known as a bile canaliculus, that is formed by the apical membranes of adjoining hepatocytes. Bile canaliculi merge to form tubular structures that subsequently connect to the canal of Hering and larger intra- and extrahepatic bile ducts formed by cholangiocytes, which modify bile and enable flow through the small intestine. The major functional requirements for bile canaliculi are the maintenance of canalicular shape to preserve the blood-bile barrier and regulation of bile flow. These functional requirements are mediated by functional modules, primarily transporters, the cytoskeleton, cell-cell junctions, and mechanosensing proteins. I propose here that bile canaliculi behave as robust machines whereby the functional modules act in a coordinated manner to perform the multistep task of maintaining canalicular shape and bile flow. Cholestasis, the general term for aberrant bile flow, stems from drug/toxin-induced or genetic dysregulation of one or more of the protein components in the functional modules. Here, I discuss the interactions between components of the various functional modules in bile canaliculi and describe how these functional modules regulate canalicular morphology and function. I use this framework to provide a perspective on recent studies of bile canalicular dynamics.

Bile canaliculi contract autonomously by releasing calcium into hepatocytes via mechanosensitive calcium channel

Drug-induced hepatocellular cholestasis leads to altered bile flow. Bile is propelled along the bile canaliculi (BC) by actomyosin contractility, triggered by increased intracellular calcium (Ca²⁺). However, the source of increased intracellular Ca²⁺ and its relationship to transporter activity remains elusive. We identify the source of the intracellular Ca²⁺ involved in triggering BC contractions, and we elucidate how biliary pressure regulates Ca²⁺ homeostasis and associated BC contractions. Primary rat hepatocytes were cultured in collagen sandwich. Intra-canalicular Ca²⁺ was measured with fluo-8; and intra-cellular Ca²⁺ was measured with GCaMP. Pharmacological modulators of canonical Ca²⁺-channels were used to study the Ca²⁺-mediated regulation of BC contraction. BC contraction correlates with cyclic transfer of Ca²⁺ from BC to adjacent hepatocytes, and not with endoplasmic reticulum Ca²⁺. A mechanosensitive Ca²⁺ channel (MCC), Piezo-1, is preferentially localized at BC membranes. The Piezo-1 inhibitor GsMTx-4 blocks the Ca²⁺ transfer, resulting in cholestatic generation of BC-derived vesicles whereas Piezo-1 hyper-activation by Yoda1 increases the frequency of Ca²⁺ transfer and BC contraction cycles. Yoda1 can recover normal BC contractility in drug-induced hepatocellular cholestasis, supporting that Piezo-1 regulates BC contraction cycles. Finally, we show that hyper-activating Piezo-1 can be exploited to normalize bile flow in drug-induced hepatocellular cholestasis.

Actomyosin contractility drives bile regurgitation as an early response during obstructive cholestasis

BACKGROUND & AIMS

A wide range of liver diseases manifest as biliary obstruction, or cholestasis. However, the sequence of molecular events triggered as part of the early hepatocellular homeostatic response in obstructive cholestasis is poorly elucidated. Pericanalicular actin is known to accumulate during obstructive cholestasis. Therefore, we hypothesized that the pericanalicular actin cortex undergoes significant remodeling as a regulatory response to obstructive cholestasis.

METHODS

In vivo investigations were performed in a bile duct-ligated mouse model. Actomyosin contractility was assessed using sandwich-cultured rat hepatocytes transfected with various fluorescently labeled proteins and pharmacological inhibitors of actomyosin contractility.

RESULTS

Actomyosin contractility induces transient deformations along the canalicular membrane, a process we have termed inward blebbing. We show that these membrane intrusions are initiated by local ruptures in the pericanalicular actin cortex; and they typically retract following repair by actin polymerization and actomyosin contraction. However, above a certain osmotic pressure threshold, these inward blebs pinch away from the canalicular membrane into the hepatocyte cytoplasm as large vesicles (2-8μm). Importantly, we show that these vesicles aid in the regurgitation of bile from the bile canaliculi.

CONCLUSION

Actomyosin contractility induces the formation of bile-regurgitative vesicles, thus serving as an early homeostatic mechanism against increased biliary pressure during cholestasis.

LAY SUMMARY

Bile canaliculi expand and contract in response to the amount of secreted bile, and resistance from the surrounding actin bundles. Further expansion due to bile duct blockade leads to the formation of inward blebs, which carry away excess bile to prevent bile build up in the canaliculi.

Effect of HSP27 and Cofilin in the injury of hypoxia/reoxygenation on hepatocyte membrane F-actin microfilaments

Hypoxia-reoxygenation (H/R) injury hepatocyte models were established to simulate the ischemia/reperfusion injury of transplanted organ. Through the study of the molecular mechanism of H/R on the F-actin damage of the liver cytomembrane, the mechanism of F-actin damage induced by ischemia and reperfusion was studied from the level of cell and molecule.The hypoxic environment of cells in vitro was simulated by chemical hypoxia agent CoCl2. Liver cells were detected by MTT, H/R group was subdivided into 3 subgroups: H/R 2, 4, and 6 h. Changes of cell shape and the growth state, apoptosis, ultrastructural changes, and the changes in F-actin microfilament content were observed. Heat shock protein 27 (HSP27), Cofilin, and F-actin gene and protein levels were determined by real-time polymerase chain reaction and western blot assay, respectively.Cells showed circular adherence growth under normal circumstances, while the spindle cells and shedding cells were significantly increased in H/R groups. Apoptosis cells in H/R group were increased significantly with the extension of hypoxia time. The number of endoplasmic reticulum was decreased significantly in the H/R group, the mitochondrion hydropic was degenerated and the glycogen was disappeared. The F-actin fibers in the H/R group were disordered, the morphology of the fibers was obviously decreased, and the fluorescence staining decreased obviously (P < .05). The transcription and expression levels of HSP27, Cofilin, and F-actin were significantly lower than those in the control group (P < .05).These results demonstrate that H/R can affect the correct assembly of F-actin microfilaments and weakens the normal cycle of F-actin microfilaments through inhibiting the protein expression and gene transcription of HSP27 and Cofilin in hepatocytes, thereby changing the skeleton of F-actin microfilaments.

Hepatocellular carcinoma-targeted drug discovery through image-based phenotypic screening in co-cultures of HCC cells with hepatocytes

Background

Hepatocellular carcinoma (HCC) is one of the most common malignant cancers worldwide and is associated with substantial mortality. Because HCCs have strong resistance to conventional chemotherapeutic agents, novel therapeutic strategies are needed to improve survival in HCC patients.

Methods

Here, we developed a fluorescence image-based phenotypic screening system in vitro to identify HCC-specific drugs in co-cultures of HCC cells with hepatocytes. To this end, we identified two distinctive markers of HCC, CHALV1 and AFP, which are highly expressed in HCC cell lines and liver cancer patient-derived materials. We applied these markers to an HCC-specific drug screening system.

Results

Through pilot screening, we identified three anti-folate compounds that had HCC-specific cytotoxicity. Among them, pyrimethamine exhibited the greatest HCC-specific cytotoxicity. Interestingly, pyrimethamine significantly increased the size and number of lysosomes and subsequently induced the release of cathepsin B from the lysosome to the cytosol, which triggered caspase-3-dependent apoptosis in Huh7 (HCC) but not Fa2N-4 cells (immortalized hepatocytes). Importantly, Fa2N-4 cells had strong resistance to pyrimethamine relative to Huh7 cells in 2D and 3D culture systems.

Conclusion

These results demonstrate that this in vitro image-based phenotypic screening platform has the potential to be widely adopted in drug discovery research, since we promptly estimated anticancer activity and hepatotoxicity and elucidated functional roles of pyrimethamine during the apoptosis process in HCC.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2816-x) contains supplementary material, which is available to authorized users.

Engineering of implantable liver tissues

In this chapter, from the engineering point of view, we introduce the results from our group and related research on three typical configurations of engineered liver tissues; cell sheet-based tissues, sheet-like macroporous scaffold-based tissues, and tissues based on special scaffolds that comprise a flow channel network. The former two do not necessitate in vitro prevascularization and are thus promising in actual human clinical trials for liver diseases that can be recovered by relatively smaller tissue mass. The third approach can implant a much larger mass but is still not yet feasible. In all cases, oxygen supply is the key engineering factor. For the first configuration, direct oxygen supply using an oxygen-permeable polydimethylsiloxane membrane enables various liver cells to exhibit distinct behaviors, complete double layers of mature hepatocytes and fibroblasts, spontaneous thick tissue formation of hepatocarcinoma cells and fetal hepatocytes. Actual oxygen concentration at the cell level can be strictly controlled in this culture system. Using this property, we found that initially low then subsequently high oxygen concentrations were favorable to growth and maturation of fetal cells. For the second configuration, combination of poly-L: -lactic acid 3D scaffolds and appropriate growth factor cocktails provides a suitable microenvironment for the maturation of cells in vitro but the cell growth is limited to a certain distance from the inner surfaces of the macropores. However, implantation to the mesentery leaves of animals allows the cells again to proliferate and pack the remaining spaces of the macroporous structure, suggesting the high feasibility of 3D culture of hepatocyte progenitors for liver tissue-based therapies. For the third configuration, we proposed a design criterion concerning the dimensions of flow channels based on oxygen diffusion and consumption around the channel. Due to the current limitation in the resolution of 3D microfabrication processes, final cell densities were less than one-tenth of those of in vivo liver tissues; cells preferentially grew along the surfaces of the channels and this fact suggested the necessity of improved 3D fabrication technologies with higher resolution. In any case, suitable oxygen supply, meeting the cellular demand at physiological concentrations, was the most important factor that should be considered in engineering liver tissues. This enables cells to utilize aerobic respiration that produces almost 20 times more ATP from the same glucose consumption than anaerobic respiration (glycolysis). This also allows the cells to exhibit their maximum reorganization capability that cannot be observed in conventional anaerobic conditions.

Ductular network formation by rat biliary epithelial cells in the dynamical culture with collagen gel and dimethylsulfoxide stimulation

Formation of bile ducts in culture is important for reconstructing hepatic organoids with bile drainage systems. However, morphogenic factors of biliary epithelial cells (BECs) have been poorly understood because of the lack of experimental models. Here, we demonstrated that rat BECs formed bile ductular networks in dynamic culture, when culture conditions were sequentially controlled. BEC morphogenesis was achieved through two-dimensional culture on collagen gel, collagen gel sandwich configuration, and 1% dimethylsulfoxide stimulation. In this culture system, BECs developed into large bile duct structures (LBDs) that formed interconnected networks of continuous lumens. LBD luminal surfaces possessed well developed microvilli, consisted of 7 to 10 BECs, and their inner diameters measured 20 to 50 microm. Quantitative PCR analysis revealed that the cells in LBDs expressed apical and basal domain markers of BECs. Immunofluorescent staining identified apical domain markers such as Cl(-)/HCO(3)(-) anion exchanger 2 and cystic fibrosis transmembrane regulator on the luminal surface of LBDs, responding to secretin stimulation as well as laminin protein surrounding LBDs. Furthermore, the cells in LBDs transported metabolized fluorescein from the basal side to the luminal space, further demonstrating that the reconstructed LBDs were functionally and morphologically similar to the bile ducts in vivo. The culture model described here will be useful in reconstructing hepatic tissues as well as in understanding the mechanism of bile duct development and its disruption in disease.

Reconstruction of 3D stacked-up structures by rat small hepatocytes on microporous membranes

The three-dimensional (3D) culture of hepatocytes is essential for the reconstruction of functional hepatic tissues in vitro. In the present experiment, we developed a 3D-culture method in order to reconstruct hepatic cordlike structures by stacking up two-dimensional (2D) tissues composed of rat small hepatocytes (SHs), which are hepatic progenitor cells. Pairs of membranes were prepared and the cells were separately cultured on each membrane. After the SH colonies had developed, one membrane was inverted on top of the other to form an SH bilayer. Thereafter, we investigated whether the stacked cells were organized into differentiated tissues. In the 3D stacked-up structures, bile canaliculi (BC) started to form and gradually developed into anastomosing networks. Transmission electron microscopy revealed that the SHs of the upper and lower layers adhered to one another, and that BC formed between them. Bile canalicular proteins localized on the lumina of the tubular structures. Furthermore, the cells within the structures exhibited mRNA transcription of the hepatic-differentiation markers and maintained a relatively high level of albumin secretion. We conclude that highly differentiated 3D tissues, including functional BC, can be reconstructed by stacking up layers of SHs. This 3D stacked-up culture is useful for the reconstruction of tissue-engineered livers.