Biology subjects

Weinberger, L. S.

Publications and source records attributed to Weinberger, L. S..

11 recordsLinked to original sources

bioRxiv2024

Quantitative comparison of single-cell RNA sequencing versus single-molecule RNA imaging for quantifying transcriptional noise

Stochastic fluctuations (noise) in transcription generate substantial cell-to-cell variability. However, how best to quantify genome-wide noise, remains unclear. Here we utilize a small-molecule perturbation (IdU) to amplify noise and assess noise quantification from numerous scRNA-seq algorithms on human and mouse datasets, and then compare to noise quantification from single-molecule RNA FISH (smFISH) for a panel of representative genes. We find that various scRNA-seq analyses report amplified noise, without altered mean-expression levels, for [~]90% of genes and that smFISH analysis verifies noise amplification for the vast majority of genes tested. Collectively, the analyses suggest that most scRNA-seq algorithms are appropriate for quantifying noise including a simple normalization approach, although all of these systematically underestimate noise compared to smFISH. From a practical standpoint, this analysis argues that IdU is a globally penetrant noise-enhancer molecule--amplifying noise without altering mean-expression levels--which could enable investigations of the physiological impacts of transcriptional noise. MOTIVATIONCell-to-cell variability in isogenic populations is predominantly attributed to the stochastic fluctuations (i.e., noise) in transcription. However, the quantification of this noise, particularly on a genome-wide scale, remains an open question. To address this general question, here we utilize a small-molecule perturbation reported to amplify transcriptional noise. Previous single-cell RNA-sequencing (scRNA-seq) analysis indicated that the pyrimidine nucleobase 5'-iodo-2'-deoxyuridine (IdU) amplifies noise but technical drawbacks of scRNA-seq may have obscured the penetrance and degree of noise amplification. Consequently, here we assess numerous scRNA-seq algorithms, on two different scRNA-seq datasets for a human and mouse cell type, for their ability to quantify noise amplification and then compare the results to noise quantification from single-molecule RNA FISH (smFISH) imaging for a panel of representative genes. The specific questions addressed are whether IdU amplifies noise in a globally or partially penetrant manner, and what scRNA-seq algorithm is most appropriate for quantifying transcriptional noise. O_FIG O_LINKSMALLFIG WIDTH=184 HEIGHT=200 SRC="FIGDIR/small/607289v1_ufig1.gif" ALT="Figure 1"> View larger version (26K): org.highwire.dtl.DTLVardef@1f9b17aorg.highwire.dtl.DTLVardef@1f2d410org.highwire.dtl.DTLVardef@1407cfaorg.highwire.dtl.DTLVardef@cd2bc9_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOGraphical AbstractC_FLOATNO C_FIG

systems biology↗

bioRxiv2020

Phenotypic distribution of the stem-like melanoma initiating state depends on CDKN2A status

Human cancers can exhibit phenotype switching, resulting in cells that are more metastatic or that are more tolerant to treatment. However, the relationship between these aggressive states is not well understood. We investigated the dynamics of phenotypic switching in human melanoma cells by monitoring the fluorescent activity from a transgenic reporter of BRN2 promoter activation. Melanoma cells exhibit heterogeneous BRN2 expression patterns that are reestablished upon isolation and clonal outgrowth. Specifically, stable BRN2 expression was generally inherited over multiple generations while undergoing occasional bidirectional interconversion. We found that BRN2 low cells were required for tumor initiation and metastasis in animal engraftment assays but were more sensitive to targeted BRAF inhibition. In contrast, the BRN2 high state was not tumorigenic, but entry into this state was uniform and persistent among cells tolerant to targeted BRAF therapy. Single-cell RNA sequencing analyses revealed core programs exclusive to either the BRN2 high or low cells, each of which is present in ex vivo tumors, demonstrating the physiological relevance of these states. Our findings emphasize that one challenge of effectively targeting phenotype switching in melanoma as a therapeutic strategy could be balancing distinct aggressive phenotypes so that sensitizing tumors to BRAF inhibition does not inadvertently lead to further dissemination. TeaserUnraveling melanomas shape-shifting behavior: insights into how cancer cells swap between metastasis and drug evasion.

cancer biology↗

bioRxiv2020

Discovery of a Cellular Mechanism Regulating Transcriptional Noise

Stochastic fluctuations in gene expression (‘noise’) are often considered detrimental but, in other fields, fluctuations are harnessed for benefit (e.g., ‘dither’ or amplification of thermal fluctuations to accelerate chemical reactions). Here, we find that DNA base-excision repair amplifies transcriptional noise, generating increased cellular plasticity and facilitating reprogramming. The DNA-repair protein Apex1 recognizes modified nucleoside substrates to amplify expression noise—while homeostatically maintaining mean levels of expression— for virtually all genes across the transcriptome. This noise amplification occurs for both naturally occurring base modifications and unnatural base analogs. Single-molecule imaging shows amplified noise originates from shorter, but more intense, transcriptional bursts that occur via increased DNA supercoiling which first impedes and then accelerates transcription, thereby maintaining mean levels. Strikingly, homeostatic noise amplification potentiates fate-conversion signals during cellular reprogramming. These data suggest a functional role for the observed occurrence of modified bases within DNA in embryonic development and disease.Competing Interest StatementThe authors have declared no competing interest.View Full Text

systems biology↗

bioRxiv2020

Chromatin remodeler Brahma safeguards canalization in cardiac mesoderm differentiation

Differentiation proceeds along a continuum of increasingly fate-restricted intermediates, referred to as canalization1-4. Canalization is essential for stabilizing cell fate, but the mechanisms underlying robust canalization are unclear. Here we show that deletion of the BRG1/BRM-associated factor (BAF) chromatin remodeling complex ATPase gene Brm (encoding Brahma) results in a radical identity switch during directed cardiogenesis of mouse embryonic stem cells (ESCs). Despite establishment of well-differentiated precardiac mesoderm, Brm-null cells subsequently shifted identities, predominantly becoming neural precursors, violating germ layer assignment. Trajectory inference showed sudden acquisition of non-mesodermal identity in Brm-null cells, consistent with a new transition state inducing a fate switch referred to as a saddle-node bifurcation3,4. Mechanistically, loss of Brm prevented de novo accessibility of cardiac enhancers while increasing expression of the neurogenic factor POU3F1 and preventing expression of the neural suppressor REST. Brm mutant identity switch was overcome by increasing BMP4 levels during mesoderm induction, repressing Pou3f1 and re-establishing a cardiogenic chromatin landscape. Our results reveal BRM as a compensable safeguard for fidelity of mesoderm chromatin states, and support a model in which developmental canalization is not a rigid irreversible path, but a highly plastic trajectory that must be safeguarded, with implications in development and disease.

developmental biology↗

bioRxiv2020

A Molecular Mechanism for Probabilistic Bet-hedging and its Role in Viral Latency

Probabilistic bet hedging, a strategy to maximize fitness in unpredictable environments by matching phenotypic variability to environmental variability, is theorized to account for the evolution of various fate-specification decisions, including viral latency. However, the molecular mechanisms underlying bet-hedging remain unclear. Here, we report that large variability in protein abundance within individual herpesvirus virion particles enables probabilistic bet hedging between viral replication and latency. Super-resolution imaging of individual virions of the human herpesvirus cytomegalovirus (CMV) showed that virion-to-virion levels of pp71 tegument protein--the major viral transactivator protein--exhibit extreme variability. This super-Poissonian tegument variability promoted alternate replicative strategies: high virion pp71 levels enhance viral replicative fitness but, strikingly, impede silencing, whereas low virion pp71 levels reduce fitness but promote silencing. Overall, the results indicate that stochastic tegument packaging provides a mechanism enabling probabilistic bet hedging between viral replication and latency. SIGNIFICANCEProbabilistic bet hedging is a generalized diversification strategy to maximize fitness in unpredictable environments, and has been proposed as an evolutionary basis for herpesvirus latency. However, the molecular mechanisms enabling probabilistic bet hedging have remained elusive. Here, we find that the human herpesvirus cytomegalovirus--a major cause of birth defects and transplant failures--utilizes stochastic variability in the abundance of a protein packaged into individual viral particles to enable probabilistic bet hedging between alternate viral states.

molecular biology↗

bioRxiv2019

Discovery and Engineering of a Therapeutic Interfering Particle (TIP): a combination self-renewing antiviral

Population-level control of HIV-1 faces recognized challenges, including the evolution of viral resistance and adherence issues in resource-limited settings. It has long been proposed that viral deletion mutants that conditionally self-renew at the expense of the wild-type virus (i.e., Defective Interfering Particles, DIPs1) could constitute a long-term intervention that circumvents adherence challenges and has a high genetic barrier to resistance, echoing recent approaches2. Theories predict3, 4 that DIPs could be engineered into a therapy for HIV-1 (i.e., Therapeutic interfering particles or TIPs) provided they stably persist in patients (R0>1) by spreading to new cells during active infection (hence, a self-replenishing antiviral). To date, DIPs amenable to such engineering have remained elusive for HIV-1. Here we report the discovery of an HIV-1 DIP and its subsequent engineering into a TIP. The TIP interferes with HIV-1 replication at multiple stages of the viral lifecycle, including genome packaging, virion maturation, and reverse transcription, essentially acting as a combination antiviral. In humanized mice, the TIP suppressed HIV-1 replication by ten-fold and significantly protected CD4+ T cells from HIV-1 mediated depletion. These data provide proof-of-concept for a class of biologic with the potential to circumvent significant barriers to HIV-1 control.

synthetic biology↗

bioRxiv2018

Feedback-mediated signal conversion promotes viral fitness

A fundamental signal-processing problem is how biological systems maintain phenotypic states (i.e., canalization) long after degradation of initial catalyst signals. For example, to efficiently replicate, herpesviruses (e.g., human cytomegalovirus, HCMV) rapidly counteract cell-mediated silencing using trans-activators packaged in the tegument of the infecting virion particle. But, the activity of these tegument trans-activators is inherently transient--they undergo immediate proteolysis but delayed synthesis--and how transient activation sustains lytic viral gene expression despite cell-mediated silencing is unclear. By constructing a two-color, conditional-feedback HCMV mutant, we find that positive feedback in HCMVs Immediate Early 1 (IE1) protein is of sufficient strength to sustain HCMV lytic expression. Single-cell time-lapse imaging and mathematical modeling show that IE1 positive feedback converts transient transactivation signals from tegument pp71 proteins into sustained lytic expression, which is obligate for efficient viral replication, whereas attenuating feedback decreases fitness by promoting a reversible silenced state. Together, these results identify a regulatory mechanism enabling herpesviruses to sustain expression despite transient activation signals--akin to early electronic transistors--and expose a potential target for therapeutic intervention.

systems biology↗

bioRxiv2017

Cytoplasmic import and processing of mRNA amplify transcriptional bursts accounting for the majority of cellular noise

Transcription is an episodic process characterized by probabilistic bursts; but how these bursts are modulated by cellular physiology remains unclear and has implications for how selection may act on these fluctuations. Using simulations and single-molecule RNA counting, we examined how cellular processes influence cell-to-cell variability (noise). The results show that RNA noise is amplified in the cytoplasm compared to the nucleus in ~85% of genes across diverse promoters, genomic loci, and cell types (human and mouse). Surprisingly, measurements show further amplification of RNA noise in the cytoplasm, fitting a model of biphasic mRNA conversion between translation- and degradation-competent states. The multi-state translation-degradation of mRNA also causes substantial noise amplification in protein levels, ultimately accounting for ~74% of intrinsic protein variability in cell populations. Overall, the results demonstrate how transcriptional bursts are intrinsically amplified by mRNA processing and indicate mechanisms through which noise could act as a substrate for evolutionary selection.

cell biology↗

bioRxiv2017

Non-latching positive feedback enables robust bimodality by de-coupling expression noise from the mean

Fundamental to biological decision-making is the ability to generate bimodal expression patterns where two alternate expression states simultaneously exist. Here, we use a combination of single-cell analysis and mathematical modeling to examine the sources of bimodality in the transcriptional program controlling HIVs fate decision between active replication and viral latency. We find that the HIV Tat protein manipulates the intrinsic toggling of HIVs promoter, the LTR, to generate bimodal ON-OFF expression, and that transcriptional positive feedback from Tat shifts and expands the regime of LTR bimodality. This result holds for both minimal synthetic viral circuits and full-length virus. Strikingly, computational analysis indicates that the Tat circuits non-cooperative non-latching feedback architecture is optimized to slow the promoters toggling and generate bimodality by stochastic extinction of Tat. In contrast to the standard Poisson model, theory and experiment show that non-latching positive feedback substantially dampens the inverse noise-mean relationship to maintain stochastic bimodality despite increasing mean-expression levels. Given the rapid evolution of HIV, the presence of a circuit optimized to robustly generate bimodal expression appears consistent with the hypothesis that HIVs decision between active replication and latency provides a viral fitness advantage. More broadly, the results suggest that positive-feedback circuits may have evolved not only for signal amplification but also for robustly generating bimodality by decoupling expression fluctuations (noise) from mean expression levels.

systems biology↗

bioRxiv2017

HIVs Feedback Circuit Breaks The Fundamental Limit On Noise Suppression To Stabilize Fate

Diverse biological systems utilize gene-expression fluctuations ( noise) to drive lineage-commitment decisions1-5. However, once a commitment is made, noise becomes detrimental to reliable function6,7 and the mechanisms enabling post-commitment noise suppression are unclear. We used time-lapse imaging and mathematical modeling, and found that, after a noise-driven event, human immunodeficiency virus (HIV) strongly attenuated expression noise through a non-transcriptional negative-feedback circuit. Feedback is established by serial generation of RNAs from post-transcriptional splicing, creating a precursor-product relationship where proteins generated from spliced mRNAs auto-deplete their own precursor un-spliced mRNAs. Strikingly, precursor auto-depletion overcomes the theoretical limits on conventional noise suppression--minimizing noise far better than transcriptional auto-repression--and dramatically stabilizes commitment to the active-replication state. This auto-depletion feedback motif may efficiently suppress noise in other systems ranging from detained introns to non-sense mediated decay.

cell biology↗

bioRxiv2017

Transient Thresholding: A Mechanism Enabling Non-Cooperative Transcriptional Circuitry To Form A Switch

Threshold generation in fate-selection circuits is often achieved through deterministic bistability, which requires cooperativity (i.e., nonlinear activation) and associated hysteresis. However, the Tat positive-feedback loop that controls HIVs fate decision between replication and proviral latency lacks self-cooperativity and deterministic bistability. Absent cooperativity, it is unclear how HIV can temporarily remain in an off state long enough for the kinetically slower epigenetic silencing mechanisms to act-- expression fluctuations should rapidly trigger active positive feedback and replication, precluding establishment of latency. Here, using flow cytometry and single-cell imaging, we find that the Tat circuit exhibits a transient activation threshold. This threshold largely disappears after [~]40 hours--accounting for the lack of deterministic bistability--and promoter activation shortens the lifetime of this transient threshold. Continuous differential equation models do not recapitulate this phenomenon. However, chemical reaction (master equation) models where the transcriptional transactivator and promoter toggle between inactive and active states can recapitulate the phenomenon since they intrinsically create a single-molecule threshold transiently requiring excess molecules in the inactive state to achieve at least one molecule (rather than a continuous fractional value) in the active state. Given the widespread nature of promoter toggling and transcription factor modifications, transient thresholds may be a general feature of inducible promoters.

biophysics↗