Selected Current Research Projects
Understanding the competition between proliferation and differentiation that controls tissue size
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We have developed a live-cell imaging approach to measure both the cell-cycle and the precise timepoint of cell differentiation in thousands of live cells. We identified a critical threshold level of PPARG after which progenitor cells irreversibly differentiate into adipocytes (fat cells). We found that proliferation is a critical part of the terminal differentiation process, regulating how many times cells divide before they terminally differentiate, thereby controlling how many differentiated cells are produced to regulate tissue size. We make now use of the unique fluorescent differentiation and proliferation cell model we developed to understand the dynamic signaling mechanisms controlling terminal differentiation: (i) How progenitor cells first start and then stop proliferation, (ii) how they undergo irreversible differentiation, and (iii) how proliferation and differentiation are linked.
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Open projects make use of automated fluorescence microscopy, light sheet microscopy, spinning disk confocal microscopy, multiplexed fixed immunofluorescence and RNA FISH, and/or RNA-Seq and CHIP-seq analysis. We are using mammalian cultured cell and organoids, as well as mouse models.
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Early enforcement of cell identity by a functional component of the terminally differentiated state. PLoS Biol 2022
Flattening of circadian glucocorticoid oscillations drives acute hyperinsulinemia and adipocyte hypertrophy. Cell Rep 2022
Molecular Competition in G1 Controls When Cells Simultaneously Commit to Terminally Differentiate and Exit the Cell Cycle. Cell Rep 2020
Controlling low rates of cell differentiation through noise and ultrahigh feedback. Science 2014
Consecutive positive feedback loops create a bistable switch that controls preadipocyte-to-adipocyte conversion. Cell Rep 2012
How do cells integrate dynamically changing signals to decide between quiescence, proliferation, and senescence?
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We make use of unique fluorescence microscopy tools that we developed. We investigate how mitogen and stress stimuli control the decision of normal and cancer cells whether to proliferate, exit to quiescence, re-enter the cell cycle, or enter senescence.
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We developed live-cell fluorescent reporters to monitor the four signaling activities that control the proliferation decision, the activities of the protein kinases Cdk4/6 and Cdk2, the transcription factor E2F, and the E3 ubiquitin ligase APC/C. We combine these methods with small molecule signaling inhibitors, multiplexed fixed immunofluorescence, and single-cell RNA FISH and RNA-Seq analysis. We link new findings in cultured cell and organoid models to in vivo studies of normal and cancer cells in mice.
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Cdt1 inhibits CMG helicase in early S phase to separate origin licensing from DNA synthesis. Mol Cell 2023
CDC7-independent G1/S transition revealed by targeted protein degradation. Nature 2022
Palbociclib induces selective and immediate dissociation of p21 from cyclin D-CDK4 to inhibit CDK2. Nature Comm 2021
Altered G1 signaling order and commitment point in cells proliferating without CDK4/6 activity. Nature Comm 2021
Transient Hysteresis in CDK4/6 Activity Underlies Passage of the Restriction Point in G1. Mol Cell 2019
EMI1 switches from being a substrate to an inhibitor of APC/CCDH1 to start the cell cycle. Nature 2018
An intrinsic S/G2 checkpoint enforced by ATR. Science 2018
Competing memories of mitogen and p53 signalling control cell-cycle entry. Nature 2017
Irreversible APC(Cdh1) Inactivation Underlies the Point of No Return for Cell-Cycle Entry. Cell 2016
The Proliferation-Quiescence Decision Is Controlled by a Bifurcation in CDK2 Activity at Mitotic Exit. Cell. 2013.
How does the timing of circadian rhythms control tissue size?
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We discovered that a main driver of adipogenesis, glucocorticoid hormones, cause minimal adipogenesis in mice when they increase in a normal daily pattern during the wake period. However, when we mimicked stress signals that flatten glucocorticoid levels using glucocorticoid pellet implantation (which keeps glucocorticoid elevated during the rest period), fat tissues rapidly grew to twice the size in only three weeks. We were able to recapitulate the increase in adipogenesis in vitro using pulsed versus continuous adipogenic stimulation of adipocyte progenitor cells. We also developed the first cell model to monitor the circadian rhythm and terminal differentiation live in the same single cell and showed that progenitor adipocytes can only pass the threshold to terminally differentiate when their cell intrinsic circadian clock is in the rest period.
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We are pursuing two lines of research: (1) understanding the molecular mechanisms by which disruption of normal circadian hormone oscillations cause obesity. In particular, we want to understand the lean-to-fat mass shift and hyperinsulinemia that is caused by flattening of glucocorticoid circadian rhythms such as from chronic stress, jet-lag, irregular sleep and eating schedules. (2) understanding how the master clock in our brain and the peripheral clocks in the tissues and cells in our bodies interact to control tissue size and health. Our experiments make use of in vitro, organoid and mouse models and include studies how the dysregulation of glucocorticoid rhythms alters insulin secretion from beta-cell and preferential glucose uptake by adipocytes over muscle cells.
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The circadian clock mediates daily bursts of cell differentiation by periodically restricting cell differentiation commitment. PNAS 2022
Flattening of circadian glucocorticoid oscillations drives acute hyperinsulinemia and adipocyte hypertrophy. Cell Rep 2022
A Transcriptional Circuit Filters Oscillating Circadian Hormonal Inputs to Regulate Fat Cell Differentiation. Cell Metab 2018
Controlling low rates of cell differentiation through noise and ultrahigh feedback. Science 2014
How do spatial and temporal signals control cell polarization, movement, and proliferation?
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In recent work, we identified critical roles of the actin cortex and ER-PM contact sites in polarizing and sensitizing receptor signaling, which in turn controls whether and how cells migrate, proliferate, and differentiate.
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We are developing photolithography methods and new classes of fluorescent reporters to study the signaling processes controlling cell polarization and migration by focusing on the spatial and temporal control of receptor signaling at the plasma membrane. Open projects focus on developing approaches to spatially map differences in local receptor and signaling activities, understanding the consequence of rapid small molecule perturbations, mechanisms of drug action, performing genetic screens, and confocal and super-resolution fluorescence microscopy. The overall goal is to understand how the actin cortex and ER-PM contact sites regulate receptor signaling to control cell migration and proliferation.
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Structural mechanism for bidirectional actin cross-linking by T-plastin. PNAS 2022
Enhanced substrate stress relaxation promotes filopodia-mediated cell migration. Nat Materials 2022
Membrane proximal F-actin restricts local membrane protrusions and directs cell migration. Science 2020
T-Plastin reinforces membrane protrusions to bridge matrix gaps during cell migration. Nat Commun 2020
Efficient Front-Rear Coupling in Neutrophil Chemotaxis by Dynamic Myosin II Localization. Dev Cell 2020
PLEKHG3 enhances polarized cell migration by activating actin filaments at the cell front. PNAS 2016
Engulfed cadherin fingers are polarized junctional structures between collectively migrating endothelial cells. Nat Cell Biol 2016
Waves of actin and microtubule polymerization drive microtubule-based transport and neurite growth before single axon formation. Elife 2016
Locally excitable Cdc42 signals steer cells during chemotaxis. Nat Cell Biol 2016
A polarized Ca2+, diacylglycerol and STIM1 signalling system regulates directed cell migration. Nat Cell Biol 2014
A localized Wnt signal orients asymmetric stem cell division in vitro. Science 2013
Cooperative activation of PI3K by Ras and Rho family small GTPases. Mol Cell 2012