Biology subjects

Bahrami-Nejad, Z.

Publications and source records attributed to Bahrami-Nejad, Z..

5 recordsLinked to original sources

bioRxiv2022

The circadian clock mediates daily bursts of cell differentiation by periodically restricting cell differentiation commitment

Most mammalian cells have an intrinsic circadian clock that coordinates their metabolic activity with the daily rest and wake cycle. In addition, the circadian clock is known to regulate cell differentiation, but how continuous daily oscillations of the internal clock control a much longer, multi-day differentiation process is not known. Here we simultaneously monitor the circadian clock and progression of adipocyte differentiation live in single cells. Strikingly, we find a bursting behavior in the cell population whereby individual preadipocytes commit to differentiate primarily during a 12-hour window each day corresponding to the time of rest. Daily gating of differentiation occurs because cells can irreversibly commit to differentiate within a few hours, which is faster than the rest phase and much faster than the overall multi-day differentiation process. We show that the daily bursts in differentiation are driven by a variable and slow increase in expression of PPARG, the master regulator of adipogenesis, combined with rapid, clock-driven expression of CEBPA, which is in a fast positive feedback relationship with PPARG. During each rest cycle, the increase in CEBPA causes a brief step increase in PPARG so that some cells can reach the threshold to irreversibly commit to differentiate, causing the consecutive daily bursts in cell differentiation at the population level. Our findings are broadly relevant given that most differentiating somatic cells are regulated by the circadian clock. Having a restricted time each day when differentiation occurs may open therapeutic strategies to use timed treatment relative to the clock to promote tissue regeneration. Significance StatementCells rely on a circadian clock that coordinates cellular activities with the day-night cycle. Defects in circadian clock genes dysregulate cell differentiation processes in immune, muscle, skin and fat cells. However, how a perpetual daily clock can regulate a multi-day long cell differentiation process was not understood. Here we show that the circadian clock controls a fast upregulation of the transcription factor CEBPA during each daily rest phase which in turn controls a fast irreversible step during the overall slow multi-day differentiation of fat cells, causing daily bursts of cell differentiation. Our finding opens potential therapeutic strategies to enhance tissue regeneration by timing when during the day drugs are administered.

cell biology↗

bioRxiv2020

The highly expressed lipid buffer FABP4 enforces adipocyte cell identity by driving the initial cell differentiation process

How progenitor cells can attain a distinct differentiated cell identity is a challenging problem given that critical transcription factors are often not unique to a differentiation process and the fluctuating signaling environment in which cells exist. Here we test the hypothesis that a unique differentiated cell identity can result from a core component of the differentiated state doubling up as a signaling protein that also drives differentiation. Using live single-cell imaging in the adipocyte differentiation system, we show that progenitor fat cells (preadipocytes) can only commit to terminally differentiate after upregulating FABP4, a lipid buffer that is highly enriched in mature adipocytes. Upon induction of adipogenesis, we show that after a long delay, cells first abruptly start to engage a positive feedback between CEBPA and PPARG before then engaging, after a second delay, a positive feedback between FABP4 and PPARG. These sequential positive feedbacks both need to engage in order to drive PPARG levels past the threshold for irreversible differentiation. In the last step before commitment, PPARG transcriptionally increases FABP4 expression while fatty-acid loaded FABP4 binds to and increases PPARG activity. Together, our study suggests a control principle for robust cell identity whereby a core component of the differentiated state also promotes differentiation from its own progenitor state. HIGHLIGHTSO_LIFatty-acid loaded FABP4 binds to and increases PPARG expression, thereby turning on PPARG positive feedback loops that further increase PPARG expression. C_LIO_LIFABP4 critically controls the second phase of adipogenesis between activation of the feedback loops and reaching the threshold to differentiate. C_LIO_LIOnly a small fraction ([~]10%) of the FABP4 levels typically attained in mature fat cells is needed to commit cells to the differentiated state, thus providing an explanation for why maintenance of the mature adipocyte state is so robust. C_LI

cell biology↗

bioRxiv2019

Stochastic and coincident control of terminal cell differentiation and cell cycle exit

Terminal differentiation is essential for the development and maintenance of tissues in all multi-cellular organisms and is associated with a permanent exit from the cell cycle. Failure to permanently exit the cell cycle can result in cancer and disease. However, the molecular mechanisms and timing that coordinates differentiation commitment and cell cycle exit are not yet understood. Here using adipogenesis as a model system to track differentiation commitment in live cells, we show that a rapid switch mechanism engages exclusively in G1 to trigger a simultaneous commitment to differentiate and permanently exit from the cell cycle. We identify a signal integration mechanism whereby the strengths of both mitogen and differentiation stimuli control a molecular competition between cyclin D1 and PPARG-induced expression of the CDK inhibitor p21 which in turn regulates if and when the differentiation switch is triggered and when the proliferative window closes. In this way, the differentiation control system is able to couple mitogen and differentiation stimuli to sustain a long-term balance between terminally differentiating cells and maintaining the progenitor cell pool, a parameter of critical importance for enabling proper development of tissue domains and organs. HIGHLIGHTSO_LIProgenitor cells both commit to terminally differentiate and permanently exit the cell cycle at a precise time in G1 as a result of a competition process that can last over multiple cell cycles. C_LIO_LIPositive-feedback driven expression of PPARG and the parallel induction of p21 triggers a rapid commitment to terminally differentiate and then maintains a postmitotic adipocyte state. C_LIO_LIOpposing mitogen and adipogenic signals are funneled into a molecular competition in G1 phase that controls if and when cells commit to differentiate, which in turn regulates the number of differentiated cells produced while allowing for the maintenance of sufficient progenitor cells. C_LI

cell biology↗

bioRxiv2019

Loss of adipocyte identity through synergistic repression of PPARγ by TGF-β and mechanical stress

Adipocytes convert into myofibroblasts in a TGF-{beta}-dependent mouse model of fibrosis. The molecular steps and timing underlying this conversion are poorly understood, hindering development of antifibrotic therapies. Here we used two single-cell approaches, lineage tracing and live-cell imaging of an adipocyte marker PPAR{gamma}, to track the fate of adipocytes induced to convert by TGF-{beta}. We found that TGF-{beta} alone was not sufficient to activate the TGF-{beta} pathway and to induce myofibroblast conversion in cells with high PPAR{gamma} expression. However, robust conversion was observed when an additional PPAR{gamma}-inhibiting stimulus, mechanical stress applied by increasing adhesion area on a stiff matrix, was applied simultaneously with TGF-{beta}. We show that the PPAR{gamma} downregulation in response to increased adhesion area required both fibronectin and a sufficiently stiff extracellular matrix (ECM) and was partially mediated by Rho. Our results show for the first time the order of the molecular processes driving fat tissue fibrosis and the requirement for signal convergence for the loss of adipocyte identity.

cell biology↗

bioRxiv2018

Transcription factor dynamics reveals a circadian code for fat cell differentiation

Glucocorticoid and other adipogenic hormones are secreted in mammals in circadian oscillations. Loss of this circadian oscillation pattern during stress and disease correlates with increased fat mass and obesity in humans, raising the intriguing question of how hormone secretion dynamics affect the process of adipocyte differentiation. By using live, single-cell imaging of the key adipogenic transcription factors CEBPB and PPARG, endogenously tagged with fluorescent proteins, we show that pulsatile circadian hormone stimuli are rejected by the adipocyte differentiation control system, leading to very low adipocyte differentiation rates. In striking contrast, equally strong persistent signals trigger maximal differentiation. We identify the mechanism of how hormone oscillations are filtered as a combination of slow and fast positive feedback centered on PPARG. Furthermore, we confirm in mice that flattening of daily glucocorticoid oscillations significantly increases the mass of subcutaneous and visceral fat pads. Together, our study provides a molecular mechanism for why stress, Cushings disease, and other conditions for which glucocorticoid secretion loses its pulsatility can lead to obesity. Given the ubiquitous nature of oscillating hormone secretion in mammals, the filtering mechanism we uncovered may represent a general temporal control principle for differentiation.\n\nHIGHLIGHTO_LIWe found that the fraction of differentiated cells is controlled by rhythmic and pulsatile hormone stimulus patterns.\nC_LIO_LITwelve hours is the cutoff point for daily hormone pulse durations below which cells fail to differentiate, arguing for a circadian code for hormone-induced cell differentiation.\nC_LIO_LIIn addition to fast positive feedback such as between PPARG and CEBPA, the adipogenic transcriptional architecture requires added parallel slow positive feedback to mediate temporal filtering of circadian oscillatory inputs\nC_LI

cell biology↗