Biology subjects

Bursac, N.

Publications and source records attributed to Bursac, N..

8 recordsLinked to original sources

bioRxiv2025

Organ-specific rewiring of mitochondrial integrity through COX7A dictates cellular ploidy control

To achieve proper cell and tissue size, cytoplasmic and nuclear growth must be coordinated. Disrupting this coordination causes birth defects and disease. In natures largest cells, nuclear growth occurs through polyploidization (whole-genome-duplication). How the massive nuclear growth of polyploid cells is coordinated with cytoplasmic growth processes such as mitochondrial biogenesis is relatively unclear. Here, focusing on one of natures most commonly polyploid organs-the heart-we uncover cross-talk between cytoplasmic mitochondrial biogenesis/integrity and nuclear growth/polyploidy. From a human-to-fly screen, we uncover novel regulators of cardiomyocyte ploidy, including mitochondrial integrity regulators. In comparing these cardiac hits with a parallel screen in another polyploid tissue, the salivary gland, we discovered two opposing roles for Cytochrome-c-oxidase-subunit-7A (COX7A). While salivary gland COX7A preserves mitochondrial integrity to promote polyploidy and optimal organ growth, cardiac COX7A instead suppresses mitochondrial biogenesis to repress polyploidy and prevent hypertrophic organ growth. Among all electron transport chain genes, only COX7A functions as a cardiac growth repressor. Fly hearts with compromised COX7A show abnormally high cardiac output. Human COX7A1, a mitochondrial-localized protein, similarly represses polyploidy of human iPSC-derived cardiomyocytes. In summary, our human-fly-human approach reveals conserved rewiring of mitochondrial integrity in heart tissue that switches COX7As role from ploidy promotion to repression. Our findings reveal fundamental cross-talk between mitochondrial biogenesis and genome duplication that are critical in growing metazoan tissues.

developmental biology↗

bioRxiv2025

Spatial and longitudinal tracking of enhancer-AAV vectors that target transgene expression to injured mouse myocardium

Tissue regeneration enhancer elements (TREEs) direct expression of target genes in injured and regenerating tissues. Additionally, TREEs of zebrafish origin were shown to direct the expression of transgenes in border zone regions after cardiac injury when packaged into recombinant AAV vectors and introduced into mice. Future implementation of TREEs into AAV-based vectors as research tools and potential gene therapy modalities will require a more detailed understanding of expression dynamics and potential off-target effects. Here, we applied in vivo bioluminescent imaging to mice systemically injected with AAV vectors containing different combinations of capsids, enhancers, and timing of delivery. Longitudinal tracking of TREE-based expression over time revealed distinct amplitudes and durations of reporter gene expression in the injured heart directed by different TREEs. The liver-de-targeted AAV capsid, AAV.cc84, was able to deliver TREEs either pre- or post-cardiac injury to negate off-target expression in the liver while maintaining transduction in the heart. By screening AAV9-based capsid libraries dosed systemically in mice post-cardiac injury, we discovered a new capsid variant, AAV.IR41, with enhanced transduction at cardiac border zone regions, and with elevated transduction of TREE driven transgenes versus conventional AAV9 vectors. In vivo bioluminescence imaging offers insights into how enhancers and engineered capsids can be implemented to modulate spatiotemporal transgene expression for future targeted therapies.

developmental biology↗

bioRxiv2025

Long-term editing of brain circuits in mice using an engineered electrical synapse

Electrical signaling across distinct populations of brain cells underpins cognitive and emotional function; however, approaches that selectively regulate electrical signaling between two cellular components of a mammalian neural circuit remain sparse. Here, we engineered an electrical synapse composed of two connexin proteins found in Morone americana (white perch fish) - connexin34.7 and connexin35 - to accomplish mammalian circuit modulation. By exploiting protein mutagenesis, devising a new in vitro system for assaying connexin hemichannel docking, and performing computational modeling of hemichannel interactions, we uncovered a structural motif that contributes to electrical synapse formation. Targeting these motifs, we designed connexin34.7 and connexin35 hemichannels that dock with each other to form an electrical synapse, but not with other major connexins expressed in the mammalian central nervous system. We validated this electrical synapse in vivo using C. elegans and mice, demonstrating that it can strengthen communication across neural circuits composed of pairs of distinct cell types and modify behavior accordingly. Thus, we establish Long-term integration of Circuits using connexins (LinCx) for precision circuit-editing in mammals.

neuroscience↗

bioRxiv2023

Conserved Chamber-Specific Polyploidy Maintains Heart Function in Drosophila

Developmentally programmed polyploidy (whole-genome-duplication) of cardiomyocytes is common across evolution. Functions of such polyploidy are essentially unknown. Here, we reveal roles for precise polyploidy levels in cardiac tissue. We highlight a conserved asymmetry in polyploidy level between cardiac chambers in Drosophila larvae and humans. In Drosophila, differential Insulin Receptor (InR) sensitivity leads the heart chamber to reach a higher ploidy/cell size relative to the aorta chamber. Cardiac ploidy-reduced animals exhibit reduced heart chamber size, stroke volume, cardiac output, and acceleration of circulating hemocytes. These Drosophila phenotypes mimic systemic human heart failure. Using human donor hearts, we reveal asymmetry in nuclear volume (ploidy) and insulin signaling between the left ventricle and atrium. Our results identify productive and likely conserved roles for polyploidy in cardiac chambers and suggest precise ploidy levels sculpt many developing tissues. These findings of productive cardiomyocyte polyploidy impact efforts to block developmental polyploidy to improve heart injury recovery.

developmental biology↗

bioRxiv2022

BRAF-V600E-Mediated Erk Activation Promotes Sustained Cell Cycling and Broad Transcriptional Changes in Neonatal Cardiomyocytes

Mitogens capable of promoting cardiomyocyte proliferation represent important targets for functional heart regeneration following myocardial infarction. We previously described an ERK-dependent pro-proliferative tissue phenotype following overexpression of constitutively-active (ca) human ERBB2 in both neonatal rat ventricular myocytes (NRVMs) and human iPSC-derived cardiomyocytes (hiPSC-CMs). Since ERBB2 canonically regulates multiple other pathways in addition to ERK, it is unclear whether ERK activation alone can drive CM proliferation. Here, we activated ERK in a targeted fashion by CM-specific lentiviral expression of a constitutively active mutant of BRAF, BRAF-V600E (caBRAF), in cultured NRVMs and examined the effects on engineered NRVM tissue proliferation, morphology, and function. caBRAF expression induced ERK activation, tissue growth, loss of contractile function, and increased tissue stiffness, all of which were sustained for at least 4 weeks in vitro. From bulk RNA-sequencing analysis of engineered tissues, we found that caBRAF had broad transcriptomic effects on CMs and induced a shift to glycolytic metabolism. Together, this work shows that direct ERK activation is sufficient to modulate CM cycling and functional maturation in a cell-autonomous fashion and could offer a potential target for cardiac regenerative therapies.

cell biology↗

bioRxiv2021

Long-term precision editing of neural circuits using engineered gap junction hemichannels

The coordination of activity between brain cells is a key determinant of neural circuit function; nevertheless, approaches that selectively regulate communication between two distinct cellular components of a mammalian circuit remain sparse. To address this gap, we developed a novel class of gap junctions by selectively engineering two connexin proteins found in Morone americana (white perch fish): connexin34.7 (Cx34.7) and connexin35 (Cx35). By iteratively exploiting protein mutagenesis, a novel in vitro assay of connexin docking, and computational modeling of connexin hemichannel interactions, we uncovered a pattern of structural motifs that contribute to hemichannel docking compatibility. Targeting these motifs, we designed Cx34.7 and Cx35 hemichannels that dock with each other, but not with themselves, nor with other major connexins expressed in the mammalian central nervous system. We validated these hemichannels in vivo using C. elegans and mice, demonstrating that they can facilitate communication across neural circuits composed of pairs of distinct cell types and modify behavior accordingly. Thus, we establish a potentially translational approach, Long-term integration of Circuits using connexins (LinCx), for context-precise circuit-editing with unprecedented spatiotemporal specificity in mammals.

neuroscience↗

bioRxiv2021

FETCH: A platform for high-throughput quantification of gap junction hemichannel docking

Gap junctions are membrane spanning channels that connect the cytoplasm of apposed cells, allowing for the passage of small molecules and ions. They are formed by the connexin (Cx) family of proteins which assemble into hexameric hemichannels on each cell and dock to create gap junctional channels between two cells. Despite importance of various Cx isoforms in human physiology and disease, available tools for screening and discriminating their interactions such as hemichannel compatibility, docking and permeability are limited. Here, we developed FETCH (flow enabled tracking of connexosomes in HEK cells), a method which utilizes the generation of annular gap junctions (connexosomes) as downstream indicators of hemichannel compatibility for intercellular docking. First, we show that fluorescent connexosomes create a cellular phenotype that is detectable by flow cytometry analysis. We then show that FETCH identifies homotypic and heterotypic docking of many single isoform connexin hemichannels. Finally, we demonstrate that FETCH captures the impact of disease-relevant connexin protein mutations on gap junction formation. Thus, we establish a new flow cytometry-based method that is amenable to the high-throughput classification of gap junction hemichannel docking.

cell biology↗

bioRxiv2020

Human Erbb2-induced Erk Activity Robustly Stimulates Cycling and Functional Remodeling of Rat and Human Cardiomyocytes

Multiple mitogenic pathways capable of promoting mammalian cardiomyocyte (CM) proliferation have been identified as potential candidates for functional heart repair following myocardial infarction. However, it is unclear whether the effects of these mitogens are species-specific and how they directly compare in the same cardiac setting. Here, we examined how CM-specific lentiviral expression of various candidate mitogens affects human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and neonatal rat ventricular myocytes (NRVMs) in vitro. In 2D-cultured CMs from both species, and in highly mature 3D-engineered cardiac tissues generated from NRVMs, a constitutively-active mutant form of the human gene Erbb2 (cahErbb2) was the most potent tested mitogen. Persistent expression of cahErbb2 induced CM proliferation, sarcomere loss, and remodeling of tissue structure and function, which were attenuated by small molecule inhibitors of Erk signaling. These results suggest transient activation of Erbb2/Erk axis in cardiomyocytes as a potential strategy for regenerative heart repair.

cell biology↗