Fucci System Research Articles

Unfolding the role of cell state on stress granule heterogeneity and function in pancreatic cancer.

e16357 Background: A major impediment to cancer therapy is the reality that cancer is a heterogeneous disease. A single tumor can consist of many genotypic or phenotypic profiles, which are marked by degrees of sensitivity to different therapeutics. Identifying and targeting origins of cancer cell heterogeneity is critical for predicting patient outcome and enhancing treatment efficacy. We observe a novel source of cancer cell heterogeneity that involves one of the largest stress-sensing platforms described to date: the stress granule (SG). Stress granules are non-membranous, cytoplasmic organelles that assemble in response to stress, and promote cell survival. Deregulation of SGs has been linked to several pathological conditions, including cancer, neurodegeneration, ischemia, and viral infections. SGs have been shown to promote tumor growth and metastasis, as well as contribute to mechanisms of chemo-resistance. Thus, these signaling hubs are emerging as a potential target in cancer. Methods: To induce SG formation, human pancreatic ductal adenocarcinoma (PDAC) cell lines were treated with sodium arsenite, thapsigargin, or exposed to hypoxic conditions. Immunoflourescent staining of core SG proteins was then conducted. To investigate cell cycle-driven mechanisms of SG heterogeneity, cell cycle stage was identified by expression of a Fluorescent ubiquitination-based cell cycle indicator (Fucci) lentiviral system. To investigate cPLA2’s role in SG formation, the chemical inhibitor Pyrrophenone, an shRNA targeting cPLA2, and an exogenous full-length or cleaved form of cPLA2, were used. In vivo, ES149 cells expressing Luciferase and the Fucci system were orthotopically implanted into C57BL/6 mice. Established tumors were treated every 3 days with Vehicle, Oxaliplatin, Pyrrophenone, or a combination of the latter. Tumor size was tracked via IVIS Spectrum. Fucci flourescence was used to identify cell cycle phase. SGs and cell death were assessed by immunoflourescent staining of tumors. Results: Cancer cells show a high degree of heterogeneity in their production of SGs. Cell cycle phase dictates SG heterogeneity. G2 phase cells have significantly higher levels of SG formation as compared cells in G1/S. This process is regulated by cleavage of the cPLA2 protein. Oxaliplatin administration increases G2 phase content in tumors. Co-treatment with Oxaliplatin and Pyrrophenone significantly decreases SG formation, increases cell death, and leads to tumor regressions in vivo. Conclusions: G2 phase cancer cells develop higher levels of SGs when exposed to stressful stimuli, as compared to G1/S phase counterparts. The cPLA2 protein is cleaved into a catalytically dead form that is present in G1/S phases, but absent in G2 phase. Oxaliplatin treatment in vivo leads to an increase in G2 phase tumor cells. When paired with cPLA2 inhibition, this results in increased cancer cell death and decreased tumor growth.

Prediction of cell cycle distribution after drug exposure by high content imaging analysis using low-toxic DNA staining dye.

Interference in cell cycle progression has been noted as one of the important properties of anticancer drugs. In this study, we developed the cell cycle prediction model using high-content imaging data of recipient cells after drug exposure and DNA-staining with a low-toxic DNA dye, SiR-DNA. For this purpose, we exploited HeLa and MCF7 cells introduced with a fluorescent ubiquitination-based cell cycle indicator (Fucci). Fucci-expressing cancer cells were subjected to high-content imaging analysis using OperettaCLS after 36-h exposure to anticancer drugs; the nuclei were segmented, and the morphological and intensity properties of each nucleus characterized by SiR-DNA staining were calculated using imaging analysis software, Harmony. For the use of training, we classified cells into each phase of the cell cycle using the Fucci system. Training data (n = 7500) and validation data (n = 2500) were randomly sampled and the binary classification prediction models for G1, early S, and S/G2/M phases of the cell cycle were developed using four supervised machine learning algorithms. We selected random forest as the model with the best performance through 10-fold cross-validation; the accuracy rate was approximately 75%-87%. Regarding feature importance, variables expected to be biologically related to the cell cycle, for example, signal intensity and nuclear size, were highly ranked, suggesting the validity of the model. These results showed that the cell cycle can be predicted in cancer cells by simply exploiting the current prediction model using fluorescent images of DNA-staining dye, and the model could be applied for the use of future exvivo drug sensitivity diagnosis.

Blue-to-Red TagFT, mTagFT, mTsFT, and Green-to-FarRed mNeptusFT2 Proteins, Genetically Encoded True and Tandem Fluorescent Timers.

True genetically encoded monomeric fluorescent timers (tFTs) change their fluorescent color as a result of the complete transition of the blue form into the red form over time. Tandem FTs (tdFTs) change their color as a consequence of the fast and slow independent maturation of two forms with different colors. However, tFTs are limited to derivatives of the mCherry and mRuby red fluorescent proteins and have low brightness and photostability. The number of tdFTs is also limited, and there are no blue-to-red or green-to-far-red tdFTs. tFTs and tdFTs have not previously been directly compared. Here, we engineered novel blue-to-red tFTs, called TagFT and mTagFT, which were derived from the TagRFP protein. The main spectral and timing characteristics of the TagFT and mTagFT timers were determined in vitro. The brightnesses and photoconversions of the TagFT and mTagFT tFTs were characterized in live mammalian cells. The engineered split version of the TagFT timer matured in mammalian cells at 37 °C and allowed the detection of interactions between two proteins. The TagFT timer under the control of the minimal arc promoter, successfully visualized immediate-early gene induction in neuronal cultures. We also developed and optimized green-to-far-red and blue-to-red tdFTs, named mNeptusFT and mTsFT, which were based on mNeptune-sfGFP and mTagBFP2-mScarlet fusion proteins, respectively. We developed the FucciFT2 system based on the TagFT-hCdt1-100/mNeptusFT2-hGeminin combination, which could visualize the transitions between the G1 and S/G2/M phases of the cell cycle with better resolution than the conventional Fucci system because of the fluorescent color changes of the timers over time in different phases of the cell cycle. Finally, we determined the X-ray crystal structure of the mTagFT timer and analyzed it using directed mutagenesis.

The Ubiquitin Ligase RNF138 Cooperates with CtIP to Stimulate Resection of Complex DNA Double-Strand Breaks in Human G1-Phase Cells.

DNA double-strand breaks (DSBs) represent the molecular origin of ionizing-radiation inflicted biological effects. An increase in the ionization density causes more complex, clustered DSBs that can be processed by resection also in G1 phase, where repair of resected DSBs is considered erroneous and may contribute to the increased biological effectiveness of heavy ions in radiotherapy. To investigate the resection regulation of complex DSBs, we exposed G1 cells depleted for different candidate factors to heavy ions or α-particle radiation. Immunofluorescence microscopy was used to monitor the resection marker RPA, the DSB marker γH2AX and the cell-cycle markers CENP-F and geminin. The Fucci system allowed to select G1 cells, cell survival was measured by clonogenic assay. We show that in G1 phase the ubiquitin ligase RNF138 functions in resection regulation. RNF138 ubiquitinates the resection factor CtIP in a radiation-dependent manner to allow its DSB recruitment in G1 cells. At complex DSBs, RNF138′s participation becomes more relevant, consistent with the observation that also resection is more frequent at these DSBs. Furthermore, deficiency of RNF138 affects both DSB repair and cell survival upon induction of complex DSBs. We conclude that RNF138 is a regulator of resection that is influenced by DSB complexity and can affect the quality of DSB repair in G1 cells.

Tracking the fate and migration of cells in live animals with cell-cycle indicators and photoconvertible proteins

Cell migration and cell proliferation are the basic principles that make up a living organism, and both biologically and medically. In order to understand living organism and biological phenomena, it is essential to track the migration, proliferation, and fate of cells in living cells and animals and to clarify the properties and molecular expression of cells. Recent developments in novel fluorescent proteins have made it possible to observe cell migration and proliferation as the cell cycle at the single-cell level in living individuals and tissues. Here, we introduce cell cycle visualization of living cells and animals by Fucci (Fluorescent Ubiquitination-based Cell Cycle Indicator) system and in situ cell labeling of cells and tracking cell migration by photoactivatable and photoconvertible proteins. In addition, we will present our established methods as an example of combines above tools with single-cell molecular expression analysis to reveal the fate of migrating cells at single cell level.

Abstract PS16-36: Paracrine signalling with stromal fibroblasts drives recovery of cancer cells after chemotherapy treatment

Abstract Introduction: The main cause of death for cancer patients is the development of metastasis. These arise mainly due to irresponsiveness of cancer cells to the administered therapy, which then fails to eliminate all cancer cells present in the patient. To overcome this problem, it is essential to understand which mechanisms are involved in the lack of treatment response. We are investigating how the tumour microenvironment (TME) affects the response of cancer cells to chemotherapy (CTX) and how it can be modulated to improve the outcome of patients to therapy. Materials and Methods: Co-culture of chemotherapy-treated breast cancer cell lines with primary fibroblasts isolated from breast cancer patients was performed to investigate if fibroblasts affect the response of tumour cells to commonly used agents, such as epirubicin and paclitaxel. Recovery of cells was assessed using colony formation assays (CFA) and cell cycle profiling by EdU and the FUCCI system. To further explore the complex crosstalk between cancer cells and fibroblasts in the context of CTX, gene expression analysis of both cell types was done using next generation sequencing. Validation and evaluation of the biological impact of the identified pathways was done using RT-qPCR, western-blot and perturbation experiments. Lastly, publicly available datasets for breast cancer were used to investigate the clinical relevance of our findings. Results and Discussion: We show that cancer cells utilize paracrine signalling with stromal fibroblasts to drive their recovery after treatment withdrawal. Cell cycle analysis and RNA-sequencing revealed an increase in cell cycle re-entry of CTX-treated cancer cells in co-culture with fibroblasts. In addition, we have successfully shown that treated cancer cells upregulate an important secreted factor that modulates fibroblasts into a pro-tumorigenic state. Moreover, analysis of human breast carcinomas supported the proposed role of the identified factor since its expression is inversely correlated with recurrence free survival (RFS). Moreover, expression of the gene signature identified in stromal fibroblasts in co-culture with CTX-treated cancer cells was equally associated with higher recurrence rates and a worse outcome in breast cancer patients. Conclusion: CTX-induced secretory profile of cancer cells orchestrates the reprogramming of stromal fibroblasts into a pro-tumorigenic state, which drives the expansion of cancer cells. Our study unravels a novel paracrine communication between cancer cells and stromal fibroblasts that ultimately results in the escape of malignant cells to treatment, highlighting the importance of the TME in drug response. Targeting of this axis could potentially improve the outcome of breast cancer patients to CTX treatment. Citation Format: Ana Maia, Zuguang Gu, André Koch, Mireia Berdiel-Acer, Rainer Will, Matthias Schlesner, Stefan Wiemann. Paracrine signalling with stromal fibroblasts drives recovery of cancer cells after chemotherapy treatment [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS16-36.

Visualization of Radiation-Induced Cell Cycle Kinetics with a Fluorescent Ubiquitination-Based Cell Cycle Indicator (Fucci).

Among the methods for detecting cell cycle kinetics in tumor cells, fluorescent ubiquitination-based cell cycle indicator (Fucci) is innovative because it allows observation in live cells without losing spatiotemporal information. We succeeded in using the Fucci system to visualize radiation-induced G2 arrest in tumor cells with deficient p53 function. Here we describe protocols for establishing Fucci-expressing cell lines and analyzing radiation-induced G2 arrest kinetics in three different models: monolayer cell cultures, spheroids, and xenografted solid tumors in mice.

Fluctuation in radioresponse of HeLa cells during the cell cycle evaluated based on micronucleus frequency

In this study, we examined the fluctuation in radioresponse of HeLa cells during the cell cycle. For this purpose, we used HeLa cells expressing two types of fluorescent ubiquitination-based cell cycle indicators (Fucci), HeLa-Fucci (CA)2 and HeLa-Fucci (SA), and combined this approach with the micronucleus (MN) assay to assess radioresponse. The Fucci system distinguishes cell cycle phases based on the colour of fluorescence and cell morphology under live conditions. Time-lapse imaging allowed us to further identify sub-positions within the G1 and S phases at the time of irradiation by two independent means, and to quantitate the number of MNs by following each cell through M phase until the next G1 phase. Notably, we found that radioresponse was low in late G1 phase, but rapidly increased in early S phase. It then decreased until late S phase and increased in G2 phase. For the first time, we demonstrated the unique fluctuation of radioresponse by the MN assay during the cell cycle in HeLa cells. We discuss the difference between previous clonogenic experiments using M phase-synchronised cell populations and ours, as well as the clinical implications of the present findings.

Abstract PO-040: Oral squamous carcinoma cells under TGF-β-induced cell cycle arrest represent highly motile and invasive population

Abstract Cancer cells forming primary tumors are heterogeneous and respond in various ways to the stimuli from tumor microenvironment. Transforming growth factor-β (TGF-β) has been implicated in progression of various types of epithelial cancer. TGF-β is known to induce epithelial-mesenchymal transition (EMT) leading to increased invasiveness and thus metastasis. In addition, it has been reported that TGF-β arrests the epithelial cell in early G1 phase. While TGF-β elicits multiple effects on epithelial cancer cells, heterogeneity of tumor mass results in a different response depending on the cell state, thus it remains to be elucidated whether the cells exhibiting TGF-β-induced EMT and high motility are also under the cell cycle arrest. To examine the correlation between TGF-β-induced motility and cell cycle arrest at a single cell level, we have utilized oral squamous carcinoma cell lines, carrying fluorescent ubiquitination-based cell cycle indicator (Fucci) system. Using quantitative RT-PCR, immunoblotting and FACS analysis, we determined the expression of cell cycle-related genes and characterized the population of cells residing in certain cell cycle phases upon TGF-β treatment. Cells treated with TGF-β were also tested for their migratory ability to compare motility of cells residing in different cycle phases. Surprisingly, cells residing in G1 phase were characterized by higher motility than cells residing in S/G2/M phases suggesting a correlation between TGF-β-dependent cell cycle progression and migration. Moreover, increased motility of cells residing in G1 phase did not resulted from the progression of EMT. These results were also confirmed by our in vivo studies. In orthotropic xenograft model, cells under G1 arrest represented the majority of cells forming metastatic lymph nodules, in contrast to primary tumor mass comprising almost of equal number of cells residing in S/G2/M and G1 phases. We also performed cDNA microarray analyses of G1 and S/G2/M phase SAS-Fucci cells and identified gene whose expression was associated with both TGF-β-induced migration and cell cycle arrest. In conclusion our data suggested that cells under G1 cell cycle arrest are motile, but this increased motility is not a result of TGF-β-dependent EMT. Our data also strengthen the importance of targeting both, proliferating cancer cells and cell cycle-arrested cancer cells for the effective anti-cancer therapy. Citation Format: Katarzyna A. Inoue, Kazuki Takahashi, Maki Saito, Atsushi Kaida, Akinari Sugauchi, Toshihiro Uchihashi, Yasuhiro Yoshimatsu, Susumu Tanaka, Masahiko Miura, Mikihiko Kogo, Tetsuro Watabe. Oral squamous carcinoma cells under TGF-β-induced cell cycle arrest represent highly motile and invasive population [abstract]. In: Proceedings of the AACR Virtual Special Conference on Tumor Heterogeneity: From Single Cells to Clinical Impact; 2020 Sep 17-18. Philadelphia (PA): AACR; Cancer Res 2020;80(21 Suppl):Abstract nr PO-040.

In vivo analysis of cardiomyocyte proliferation during trabeculation

Cardiomyocyte proliferation is crucial for cardiac growth, patterning and regeneration; however, few studies have investigated the behavior of dividing cardiomyocytes in vivo Here, we use time-lapse imaging of beating hearts in combination with the FUCCI system to monitor the behavior of proliferating cardiomyocytes in developing zebrafish. Confirming in vitro observations, sarcomere disassembly, as well as changes in cell shape and volume, precede cardiomyocyte cytokinesis. Notably, cardiomyocytes in zebrafish embryos and young larvae mostly divide parallel to the myocardial wall in both the compact and trabecular layers, and cardiomyocyte proliferation is more frequent in the trabecular layer. While analyzing known regulators of cardiomyocyte proliferation, we observed that the Nrg/ErbB2 and TGFβ signaling pathways differentially affect compact and trabecular layer cardiomyocytes, indicating that distinct mechanisms drive proliferation in these two layers. In summary, our data indicate that, in zebrafish, cardiomyocyte proliferation is essential for trabecular growth, but not initiation, and set the stage to further investigate the cellular and molecular mechanisms driving cardiomyocyte proliferation in vivo.

Tracking neural crest cell cycle progression in vivo.

Analysis of cell cycle entry/exit and progression can provide fundamental insights into stem cell propagation, maintenance, and differentiation. The neural crest is a unique stem cell population in vertebrate embryos that undergoes long-distance collective migration and differentiation into a wide variety of derivatives. Using traditional techniques such as immunohistochemistry to track cell cycle changes in such a dynamic population is challenging, as static time points provide an incomplete spatiotemporal picture. In contrast, the fluorescent, ubiquitination-based cell cycle indicator (Fucci) system provides in vivo readouts of cell cycle progression and has been previously adapted for use in zebrafish. The most commonly used Fucci systems are ubiquitously expressed, making tracking of a specific cell population challenging. Therefore, we generated a transgenic zebrafish line, Tg(-4.9sox10:mAG-gmnn(1/100)-2A-mCherry-cdt1(1/190)), in which the Fucci system is specifically expressed in delaminating and migrating neural crest cells. Here, we demonstrate validation of this new tool and its use in live high-resolution tracking of cell cycle progression in the neural crest and derivative populations.

Genetically Encoded Tools for Optical Dissection of the Mammalian Cell Cycle

Eukaryotic cells spend most of their life in interphase of the cell cycle. Understanding the rich diversity of metabolic and genomic regulation that occurs in interphase requires the demarcation of precise phase boundaries in situ. Here, we report the properties of two genetically encoded fluorescence sensors, Fucci(CA) and Fucci(SCA), which enable real-time monitoring of interphase and cell-cycle biology. We re-engineered the Cdt1-based sensor from theoriginal Fucci system to respond to S phase-specific CUL4Ddb1-mediated ubiquitylation alone or in combination with SCFSkp2-mediated ubiquitylation. In cultured cells, Fucci(CA) produced a sharp triple color-distinct separation of G1, S, and G2, while Fucci(SCA) permitted a two-color readout of G1 and S/G2. Fucci(CA) applications included tracking the transient G1 phase of rapidly dividing mouse embryonic stem cells and identifying a window for UV-irradiation damage in S phase. These results show that Fucci(CA) is an essential tool for quantitative studies of interphase cell-cycle regulation.

Radiosensitivity of quiescent and proliferating cells grown as multicellular tumor spheroids.

The multicellular spheroid model partly mimics tumor microenvironments in vivo and has been reported in plenty of studies regarding radiosensitivity. However, clear isolation of quiescent and proliferating cells in live conditions has been quite difficult owing to technical limitations; therefore, comprehensive characterization could not be done thus far. In this study, we succeeded in separately isolating different cell types using a fluorescent ubiquitination‐based cell cycle indicator (Fucci) and determining their radiosensitivities. Unexpectedly, proliferating cells were more radioresistant than quiescent cells due to the contact effect when spheroids were disaggregated immediately after irradiation. However, the radiosensitivity of quiescent cells was not influenced by mild hypoxia (hypoxia‐inducible factor‐1α‐positive but pimonidazole‐negative), but their radioresistance became similar to that of proliferating cells due to potentially lethal damage repair when disaggregated 24 h after irradiation. The Fucci system further allowed long‐term observation of cell kinetics inside of the spheroid following irradiation using real‐time confocal fluorescence scanning. Repeated cycles of recruitment from the quiescent to the proliferating phase resulted in cell loss from the outside of the spheroid toward the inside, causing gradual shrinkage. Interestingly, the central region of the spheroid entered a dormant stage approximately 40 days after irradiation and survived for more than 2 months. Using the Fucci system, we were able to comprehensively characterize the radiosensitivity of spheroids for the first time, which highlights the importance of cell cycle kinetics after irradiation in determining the radiosensitivity under tumor microenvironments.

4P IGF-IR, but not EGFR, regulates DNA damage response in HeLa cells following irradiation

Legal entity responsible for the study The roles of receptor tyrosine kinases in DNA damage response (DDR) are still largely unknown. In this study, we examined the possible involvement of insulin-like growth factor I receptor (IGF-IR) and epidermal growth factor receptor (EGFR) in the DNA damage response (DDR) following irradiation. Methods HeLa cells expressing the fluorescence ubiquitination-based cell cycle indicator (Fucci) probes (HeLa-Fucci cells) were used in this study. Kinetics of the Fucci fluorescence were detected by FACS and time-lapse imaging. NVP-AEW541, Tyrphostin AG1478, LY294002, PD98059, NU7026 were used as specific inhibitors for IGF-IR, EGFR, PI3-K, MEK, and DNA-PKcs, respectively. Phosphorylation of ERK1/2 and Akt was detected by western blotting. Double strand breaks (DSBs) were detected by immunofluorescence staining for 53BP1. Cells were irradiated using an RX-650 Cabinet X-radiator system (Faxitron). Results To investigate the possible involvement of EGFR and IGF-IR in G2 arrest, FACS analysis and time-lapse imaging of Fucci fluorescence were performed using specific inhibitors. Results showed that inhibition of IGF-IR, but not that of EGFR, prolonged G2 arrest, which was irrespective of cell cycle phases at irradiation, i.e., red (G1 phase) or green (S/G2 phases) phase in the Fucci system. Similarly, only inhibition of IGF-IR decreased the DSB repair activity. Hereafter, further analysis was focused on IGF-IR-associated events. We next attempted to identify the responsible IGF-IR- downstream signaling pathways. Irradiation phosphorylated both Akt and ERK; however, inhibition of IGF-IR abrogated activation of only Akt. Moreover, inhibition of PI3-K/Akt, but not that of MEK/ERK, prolonged G2 arrest, which mimicked that of IGF-IR. Inhibition of DNA-PKcs, a major factor of non-homologous end joining (NHEJ), also prolonged G2 arrest. Conclusions We conclude that irradiation is likely to preferentially activate the IGF- IR/PI3-K/Akt signaling pathway in HeLa-Fucci cells, which may enhance DSB repair, eventually contributing to reduction of G2 arrest in the DDR following irradiation. Legal entity responsible for the study Tokyo Medical and Dental University Funding Tokyo Medical and Dental University, MEXT Japan Disclosure All authors have declared no conflicts of interest.

Fluorescent indicators for simultaneous reporting of all four cell cycle phases.

A robust method for simultaneous visualization of all four cell cycle phases in living cells is highly desirable. We developed an intensiometric reporter of the transition from S to G2 phase and engineered a far-red fluorescent protein, mMaroon1, to visualize chromatin condensation in mitosis. We combined these new reporters with the previously described Fucci system to create Fucci4, a set of four orthogonal fluorescent indicators that together resolve all cell cycle phases.

KPU-300, a Novel Benzophenone-Diketopiperazine-Type Anti-Microtubule Agent with a 2-Pyridyl Structure, Is a Potent Radiosensitizer That Synchronizes the Cell Cycle in Early M Phase.

KPU-300 is a novel colchicine-type anti-microtubule agent derived from plinabulin (NPI-2358). We characterized the effects of KPU-300 on cell cycle kinetics and radiosensitization using HeLa cells expressing the fluorescent ubiquitination-based cell cycle indicator (Fucci). Cells treated with 30 nM KPU-300 for 24 h were efficiently synchronized in M phase and contained clearly detectable abnormal Fucci fluorescence. Two-dimensional flow-cytometric analysis revealed a fraction of cells distinct from the normal Fucci fluorescence pattern. Most of these cells were positive for an M phase marker, the phosphorylated form of histone H3. Cells growing in spheroids responded similarly to the drug, and the inner quiescent fraction also responded after recruitment to the growth fraction. When such drug-treated cells were irradiated in monolayer, a remarkable radiosensitization was observed. To determine whether this radiosensitization was truly due to the synchronization in M phase, we compared the radiosensitivity of cells synchronized by KPU-300 treatment and cells in early M phase isolated by a combined method that took advantage of shake-off and the properties of the Fucci system. Following normalization against the surviving fraction of cells treated with KPU-300 alone, the surviving fractions of cells irradiated in early M phase coincided. Taken together with potential vascular disrupting function in vivo, we propose a novel radiosensitizing strategy using KPU-300.

Analysis of cardiomyocyte movement in the developing murine heart

The precise assemblage of several types of cardiac precursors controls heart organogenesis. The cardiac precursors show dynamic movement during early development and then form the complicated heart structure. However, cardiomyocyte movements inside the newly organized mammalian heart remain unclear. We previously established the method of ex vivo time-lapse imaging of the murine heart to study cardiomyocyte behavior by using the Fucci (fluorescent ubiquitination-based cell cycle indicator) system, which can effectively label individual G1, S/G2/M, and G1/S-transition phase nuclei in living cardiomyocytes as red, green, and yellow, respectively. Global analysis of gene expression in Fucci green positive ventricular cardiomyocytes confirmed that cell cycle regulatory genes expressed in G1/S, S, G2/M, and M phase transitions were upregulated. Interestingly, pathway analysis revealed that many genes related to the cell cycle were significantly upregulated in the Fucci green positive ventricular cardiomyocytes, while only a small number of genes related to cell motility were upregulated. Time-lapse imaging showed that murine proliferating cardiomyocytes did not exhibit dynamic movement inside the heart, but stayed on site after entering the cell cycle.

Expanding the Utility of FUCCI Reporters Using FACS-Based Omics Analysis.

The FUCCI indicator system is a powerful tool for spatio-temporal analysis of the cell cycle, but its utility has been restricted so far to a limited range of applications. Here, we describe how to establish and validate the FUCCI system in murine pluripotent stem cells (PSCs) and describe the utility of transgenic FUCCI mice. We then describe how the FUCCI system can be used to generate material for a wide-range of omics-based applications in conjunction with FACS isolation of cells. This significantly broadens the potential applications of FUCCI reporters for studying the molecular basis of development and disease.

Fucci2a: A bicistronic cell cycle reporter that allows Cre mediated tissue specific expression in mice

Markers of cell cycle stage allow estimation of cell cycle dynamics in cell culture and during embryonic development. The Fucci system incorporates genetically encoded probes that highlight G1 and S/G2/M phases of the cell cycle allowing live imaging. However the available mouse models that incorporate Fucci are beset by problems with transgene inactivation, varying expression level, lack of conditional potential and/or the need to maintain separate transgenes—there is no transgenic mouse model that solves all these problems. To address these shortfalls we re-engineered the Fucci system to create 2 bicistronic Fucci variants incorporating both probes fused using the Thosea asigna virus 2A (T2A) self cleaving peptide. We characterize these variants in stable 3T3 cell lines. One of the variants (termed Fucci2a) faithfully recapitulated the nuclear localization and cell cycle stage specific florescence of the original Fucci system. We go on to develop a conditional mouse allele (R26Fucci2aR) carefully designed for high, inducible, ubiquitous expression allowing investigation of cell cycle status in single cell lineages within the developing embryo. We demonstrate the utility of R26Fucci2aR for live imaging by using high resolution confocal microscopy of ex vivo lung, kidney and neural crest development. Using our 3T3 system we describe and validate a method to estimate cell cycle times from relatively short time-lapse sequences that we then apply to our neural crest data. The Fucci2a system and the R26Fucci2aR mouse model are compelling new tools for the investigation of cell cycle dynamics in cell culture and during mouse embryonic development.

Derivation of a novel G2 reporter system.

Progression through G2 phase of the cell cycle is a technically difficult area of cell biology to study due to the lack of physical markers specific to this phase. The FUCCI system uses the biology of the cell cycle to drive fluorescence in select phases of the cell cycle. Similarly, a commercially available system has used a fluorescent analog of the Cyclin B1 protein to visualize cells from late S phase to the metaphase-anaphase transition. We have modified these systems to use the promoter and destruction box elements of Cyclin B1 to drive a cyan fluorescent protein. We demonstrate here that this is a useful tool for measuring the length of G2 phase without perturbing any aspect of cell cycle progression.