Ken Ichi Wada

Hippo pathway regulation by cell morphology and stress fibers

The Hippo signaling pathway plays an important role in regulation of cell proliferation. Cell density regulates the Hippo pathway in cultured cells; however, the mechanism by which cells detect density remains unclear. In this study, we demonstrated that changes in cell morphology are a key factor. Morphological manipulation of single cells without cell-cell contact resulted in flat spread or round compact cells with nuclear or cytoplasmic Yap, respectively. Stress fibers increased in response to expanded cell areas, and F-actin regulated Yap downstream of cell morphology. Cell morphology- and F-actin-regulated phosphorylation of Yap, and the effects of F-actin were suppressed by modulation of Lats. Our results suggest that cell morphology is an important factor in the regulation of the Hippo pathway, which is mediated by stress fibers consisting of F-actin acting upstream of, or on Lats, and that cells can detect density through their resulting morphology. This cell morphology (stress-fiber)-mediated mechanism probably cooperates with a cell-cell contact (adhesion)-mediated mechanism involving the Hippo pathway to achieve density-dependent control of cell proliferation.

Modulating F-actin organization induces organ growth by affecting the Hippo pathway.

The Hippo tumour suppressor pathway is a conserved signalling pathway that controls organ size. The core of the Hpo pathway is a kinase cascade, which in Drosophila involves the Hpo and Warts kinases that negatively regulate the activity of the transcriptional coactivator Yorkie. Although several additional components of the Hippo pathway have been discovered, the inputs that regulate Hippo signalling are not fully understood. Here, we report that induction of extra F‐actin formation, by loss of Capping proteins A or B, or caused by overexpression of an activated version of the formin Diaphanous, induced strong overgrowth in Drosophila imaginal discs through modulating the activity of the Hippo pathway. Importantly, loss of Capping proteins and Diaphanous overexpression did not significantly affect cell polarity and other signalling pathways, including Hedgehog and Decapentaplegic signalling. The interaction between F‐actin and Hpo signalling is evolutionarily conserved, as the activity of the mammalian Yorkie‐orthologue Yap is modulated by changes in F‐actin. Thus, regulators of F‐actin, and in particular Capping proteins, are essential for proper growth control by affecting Hippo signalling.

DNA damage sensible engineered promoter for cellular biosensing of cytotoxicity.

We have established a cytotoxic sensor cell line by transfecting HepG2 cells with a luciferase protein plasmid derived from the heat shock protein 70B′ (HSP70B′) promoter, which is induced by cytotoxic reagents. HSP70B genes are up‐regulated by a wide‐range of cytotoxic stimulators, in particular, those that denature proteins. However, the HSP70B genes do not respond to DNA damage. We used a PCR array to detect marker genes of DNA damage‐related cytotoxic stimulation and found the BTG2 gene to be one such gene. Analysis of the BTG2 gene functional promoter region by transfection of various deletion constructs into HepG2 cells indicated that the p53 and NFY biding sites on BTG2 are important for the response to DNA damage. We then constructed HepG2 sensor cells using the functional BTG2 promoter, and found that these sensor cells can specifically detect the cytotoxicity accompanied by DNA strand breaks with high sensitivity. Biotechnol. Bioeng. 2009;102: 1460–1465.

Reproducible fashion of the HSP70B' promoter‐induced cytotoxic response on a live cell‐based biosensor by cell cycle synchronization

Live cell‐based sensors potentially provide functional information about the cytotoxic effect of reagents on various signaling cascades. Cells transfected with a reporter vector derived from a cytotoxic response promoter can be used as intelligent cytotoxicity sensors (i.e., sensor cells). We have combined sensor cells and a microfluidic cell culture system that can achieve several laminar flows, resulting in a reliable high‐throughput cytotoxicity detection system. These sensor cells can also be applied to single cell arrays. However, it is difficult to detect a cellular response in a single cell array, due to the heterogeneous response of sensor cells. The objective of this study was cell homogenization with cell cycle synchronization to enhance the response of cell‐based biosensors. Our previously established stable sensor cells were brought into cell cycle synchronization under serum‐starved conditions and we then investigated the cadmium chloride‐induced cytotoxic response at the single cell level. The GFP positive rate of synchronized cells was approximately twice as high as that of the control cells, suggesting that cell homogenization is an important step when using cell‐based biosensors with microdevices, such as a single cell array. Biotechnol. Bioeng. 2010;107: 561–565.

Cryopreservation of adhered mammalian cells on a microfluidic device: Toward ready-to-use cell-based experimental platforms

In this paper, we describe cryopreservation of mammalian cells in the adhered state on a microfluidic device (microdevice) for the first time. HeLa, NIH3T3, MCF-7, and PC12 cells were cultured on a microdevice in which a commercial polystyrene dish surface was used as the cell adhesion surface. Without cell-detaching treatment, the microdevice was stored in a freezer at -80°C. After thawing, we observed a greater number of live cells on the microdevice than those on a control culture dish. Although the effectiveness of the microdevice varied depending on the cell type and surface coating, the trend was consistent. We confirmed that the phenotype of the PC12 cells to differentiate into neuron-like cells was kept after the on-chip cryopreservation, and that the results of cytotoxicity test of cisplatin against the HeLa cells were essentially unchanged by the on-chip cryopreservation. These findings will open up a new possibility of ready-to-use cell-based experimental platforms.

Cell fusion through a microslit between adhered cells and observation of their nuclear behavior.

This paper describes a novel cell fusion method which induces cell fusion between adhered cells through a microslit for preventing nuclear mixing. For this purpose, a microfluidic device which had ∼ 100 cell pairing structures (CPSs) making cell pairs through microslits with 2.1 ± 0.3 µm width was fabricated. After trapping NIH3T3 cells with hydrodynamic forces at the CPSs, the cells were fused through the microslit by the Sendai virus envelope method. With following timelapse observation, we discovered that the spread cells were much less susceptible to nuclear migration passing through the microslit compared with round cells, and that cytoplasmic fraction containing mitochondria was transferred through the microslit without nuclear mixing. These findings will provide an effective method for cell fusion without nuclear mixing, and will lead to an efficient method for reprograming and transdifferentiation of target cells toward regenerative medicine.

Effects of ROCK inhibitor Y‐27632 on cell fusion through a microslit

We previously reported a direct cytoplasmic transfer method using a microfluidic device, in which cell fusion was induced through a microslit (slit‐through‐fusion) by the Sendai virus envelope (HVJ‐E) to prevent nuclear mixing. However, the method was impractical due to low efficiency of slit‐through‐fusion formation and insufficient prevention of nuclear mixing. The purpose of this study was to establish an efficient method for inducing slit‐through‐fusion without nuclear mixing. We hypothesized that modulation of cytoskeletal component can decrease nuclear migration through the microslit considering its functions. Here we report that supplementation with Y‐27632, a specific ROCK inhibitor, significantly enhances cell fusion induction and prevention of nuclear mixing. Supplementation with Y‐27632 increased the formation of slit‐through‐fusion efficiency by more than twofold. Disruption of F‐actin by Y‐27632 prevented nuclear migration between fused cells through the microslit. These two effects of Y‐27632 led to promotion of the slit‐through‐fusion without nuclear mixing with a 16.5‐fold higher frequency compared to our previous method (i.e., cell fusion induction by HVJ‐E without supplementation with Y‐27632). We also confirmed that mitochondria were successfully transferred to the fusion partner under conditions of Y‐27632 supplementation. These findings demonstrate the practicality of our cell fusion system in producing direct cytoplasmic transfer between live cells. Biotechnol. Bioeng. 2015;112: 2334–2342.

Microfluidic perfusion cell culture system confined in 35 mm culture dish for standard biological laboratories

An extremely simple, self-standing microfluidic cell culture system is reported. The whole system is confined in a 35 mm culture dish, and requires only a standard CO2 incubator. The culture medium is perfused by gravity. We successfully cultured NIH3T3-derived cells up to 10 days with a viability of ∼90%.