Biology subjects

Mitchell, J. A.

Publications and source records attributed to Mitchell, J. A..

26 records · Page 2Linked to original sources

bioRxiv2020

Resistance of endothelial cells to SARS-CoV-2 infection in vitro

RationaleThe secondary thrombotic/vascular clinical syndrome of COVID-19 suggests that SARS-CoV-2 infects not only respiratory epithelium but also the endothelium activating thrombotic pathways, disrupting barrier function and allowing access of the virus to other organs of the body. However, a direct test of susceptibility to SARS-CoV-2 of authentic endothelial cell lines has not been performed. ObjectiveTo determine infectibility of primary endothelial cell lines with live SARS-CoV-2 and pseudoviruses expressing SARS-CoV-2 spike protein. Methods and ResultsExpression of ACE2 and BSG pathways genes was determined in three types of endothelial cells; blood outgrowth, lung microvascular and aortic endothelial cells. For comparison nasal epithelial cells, Vero E6 cells (primate kidney fibroblast cell line) and HEK 293T cells (human embryonic kidney cells) transfected with either ACE2 or BSG were used as controls. Endothelial and Vero E6 cells were treated with live SARS-CoV-2 virus for 1 hour and imaged at 24 and 72 hours post infection. Pseudoviruses containing SARS-CoV-2, Ebola and Vesicular Stomatis Virus glycoproteins were generated and added to endothelial cells and HEK 239Ts for 2 hours and infection measured using luminescence at 48 hours post infection. Compared to nasal epithelial cells, endothelial cells expressed low or undetectable levels of ACE2 and TMPRSS2 but comparable levels of BSG, PPIA and PPIB. Endothelial cells showed no susceptibility to live SARS-CoV-2 or SARS-CoV-2 pseudovirus (but showed susceptibility to Ebola and Vesicular Stomatitis Virus). Overexpression of ACE2 but not BSG in HEK 239T cells conferred SARS-CoV-2 pseudovirus entry. Endothelial cells primed with IL-1{beta} remained resistant to SARS-CoV-2. ConclusionEndothelial cells are resistant to infection with SARS-CoV-2 virus, in line with relatively low levels of ACE2 and TMPRSS2, suggesting that the vascular dysfunction and thrombosis seen in severe COVID-19 is a result of factors released by adjacent infected cells (e.g. epithelial cells) and/or circulating, systemic inflammatory mediators.

immunology↗

bioRxiv2020

A computationally designed fluorescent biosensor for D-serine

Solute-binding proteins (SBPs) have evolved to balance the demands of ligand affinity, thermostability and conformational change to accomplish diverse functions in small molecule transport, sensing and chemotaxis. Although the ligand-induced conformational changes that occur in SBPs make them useful components in biosensors, they are challenging targets for protein engineering and design. Here we have engineered a D-alanine-specific SBP into a fluorescent biosensor with specificity for the signaling molecule D-serine (D-serFS). This was achieved through binding site and remote mutations that improved affinity (KD = 6.7 {+/-} 0.5 M), specificity (40-fold increase vs. glycine), thermostability (Tm = 79 {degrees}C) and dynamic range (~14%). This sensor allowed measurement of physiologically relevant changes in D-serine concentration using two-photon excitation fluorescence microscopy in rat brain hippocampal slices. This work illustrates the functional trade-offs between protein dynamics, ligand affinity and thermostability, and how these must be balanced to achieve desirable activities in the engineering of complex, dynamic proteins.

biochemistry↗

bioRxiv2020

Systemic analysis of putative SARS-CoV-2 entry and processing genes in cardiovascular tissues identifies a positive correlation of BSG with age in endothelial cells

COVID-19, caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has rapidly spread throughout the world with unprecedented global healthcare and socio-economic consequences. There is now an established secondary syndrome of COVID-19 characterised by thrombosis, vascular dysfunction and hypertension, seen in those most severely affected. Advancing age in adults is the single most significant risk factor for hospitalisation and death with COVID-19. In light of the cardiovascular/thrombotic sequalae associated with severe COVID-19 disease and the overwhelming risk that increased age carries, in this study, our aim was to obtain mechanistic insight by interrogating gene expression profiles in cardiovascular tissues and cells. Our focus was on the two putative receptors for SARS-CoV-2, ACE2 and BSG along with a selected range of genes thought to be involved in virus binding/processing. In this study we have made four important observations: (i)Cardiovascular tissues and/or endothelial cells express the required genes for SARS-CoV-2 infection, (ii) SASR-CoV-2 receptor pathways, ACE2/TMPRSS2 and BSG/PPIB(A) polarise to lung/epithelium and vessel/endothelium respectively, (iii) expression of SARS-CoV-2 host genes are, on the whole, relatively stable with age and (iv) notable exceptions were ACE2 which decreases with age in some tissues and BSG which increases with age in endothelial cells. Our data support the idea that that BSG is the dominate pathway utilised by SARS-CoV-2 in endothelial cells and are the first to demonstrate a positive correlation with age. We suggest BSG expression in the vasculature is a critical driver which explains the heightened risk of severe disease and death observed in those >40 years of age. Since BSG is utilised by other pathogens our findings have implications beyond the current pandemic. Finally, because BSG is functions in a range of cardiovascular diseases and fibrosis, our observations may have relevance to our understanding of the diseases associated with aging.

immunology↗

bioRxiv2020

A flexible repertoire of transcription factor binding sites and diversity threshold determines enhancer activity in embryonic stem cells

Transcriptional enhancers are critical for development, phenotype evolution and often mutated in disease contexts; however, even in well-studied cell types, the sequence code conferring enhancer activity remains unknown. We found genomic regions with conserved binding of multiple transcription factors in mouse and human embryonic stem cells (ESCs) contain on average 12.6 conserved transcription factor binding sites (TFBS). These TFBS are a diverse repertoire of 70 different sequences representing the binding sites of both known and novel ESC regulators. Remarkably, using a diverse set of TFBS from this repertoire was sufficient to construct short synthetic enhancers with activity comparable to native enhancers. Site directed mutagenesis of conserved TFBS in endogenous enhancers or TFBS deletion from synthetic sequences revealed a requirement for more than ten different TFBS. Furthermore, specific TFBS, including the OCT4:SOX2 co-motif, are dispensable, despite co-binding the OCT4, SOX2 and NANOG master regulators of pluripotency. These findings reveal a TFBS diversity threshold overrides the need for optimized regulatory grammar and individual TFBS that bind specific master regulators.

genomics↗

bioRxiv2020

The pregnant myometrium is epigenetically activated at contractility-driving gene loci prior to the onset of labor in mice

During gestation, uterine smooth muscle cells transition from a state of quiescence to one of contractility, but the molecular mechanisms underlying this transition at a genomic level are not well-known. To better understand these events, we evaluated the epigenetic landscape of the mouse myometrium during pregnancy, labor and post-partum. We established gestational timepoint-specific enrichment profiles involving histone H3K27 acetylation (H3K27ac), H3K4 tri-methylation (H3K4me3), and RNA polymerase II (RNAPII) occupancy by chromatin immunoprecipitation sequencing (ChIP-seq), as well as gene expression profiles by total RNA-sequencing (RNA-seq). Our findings reveal that 533 genes, including known contractility-driving genes (Gja1, Fos, Oxtr, Ptgs2), are upregulated during active labor due to an increase in transcription at gene bodies. Their promoters and putative intergenic enhancers, however, are epigenetically activated by H3K27ac as early as day 15, four days prior to the onset of active labor on day 19. In fact, we find that the majority of genome-wide H3K27ac or H3K4me3 peaks identified during active labor are present in the myometrium on day 15. Despite the early presence of H3K27ac at labor-associated genes, both an increase in non-coding enhancer RNA (eRNA) production, and in recruitment of RNAPII to corresponding genes occur during active labor, at labor upregulated gene loci. Our findings indicate that epigenetic activation of the myometrial genome precedes active labor by at least four days in the mouse model, suggesting the myometrium is poised for rapid activation of contraction-associated genes in order to exit the state of quiescence.

genomics↗

bioRxiv2019

Metal-organic framework (MOF) nanomedicine preparations of sildenafil designed for the future treatment of pulmonary arterial hypertension

Pulmonary Arterial Hypertension (PAH) is an aggressive disease with poor prognosis, no available cure, and low survival rates. Currently, there are several classes of vasodilator drugs that are widely used as treatment strategies for PAH. These include (i) endothelin-1 receptor antagonists, (ii) phosphodiesterase type 5 inhibitors, (iii) prostacyclin analogues, and (iv) soluble guanylate cyclase activators. Despite their clinical benefits, these therapies are hindered by their systemic side effects. This limitation could be overcome by controlled drug release, with future modifications for targeted drug delivery, using a nanomedicine approach. In the current study, we have evaluated one particular nanomedicine platform (the highly porous iron-based metal-organic framework (MOF) commonly referred to as MIL-89) as a carrier of the PAH drug sildenafil. We have previously shown that MIL-89 is relatively non-toxic in a range of human cell types and well tolerated in vivo. Here we prepared a nano-formulation of MIL-89 (nanoMIL-89) and then successfully charged it with a payload of sildenafil (sil@nanoMIL-891). sil@nanoMIL-89 was then shown to release sildenafil in a biphasic manner with an initial rapid release over 6 hours followed by a more sustained release over 72 hours. Both nanoMIL-89 and sil@nanoMIL-89 were relatively non-toxic when incubated with human endothelial cells for 24 hours. Finally, the vasodilator effect of sil@nanoMIL-89 was measured over 8 hours using isolated mouse aorta. Consistent with drug release kinetics, sil@nanoMIL-89 induced vasodilation after a lag phase of >4 hours. Thus, in sil@nanoMIL-89, we have produced a nano-formulation of sildenafil displaying delayed drug release corresponding to vasodilator activity. Further pharmacological assessment of sil@nanoMIL-89, including in PAH models, is now required and constitutes the subject of ongoing investigation.

pharmacology and toxicology↗

bioRxiv2019

KLF4 protein stability regulated by interaction with pluripotency transcription factors overrides transcriptional control

Embryonic stem (ES) cells are regulated by a network of transcription factors which maintain the pluripotent state. Differentiation relies on downregulation of pluripotency transcription factors disrupting this network. While investigating transcriptional regulation of the pluripotency transcription factor Klf4, we observed homozygous deletion of distal enhancers caused 17 fold decrease in Klf4 transcript but surprisingly decreased protein levels by less than 2 fold indicating post-transcriptional control of KLF4 protein overrides transcriptional control. The lack of sensitivity of KLF4 to transcription is due to high protein stability (half-life >24hr). This stability is context dependent and disrupted during differentiation, evidenced by a shift to a half-life of <2hr. KLF4 protein stability is maintained through interaction with other pluripotency transcription factors (NANOG, SOX2 and STAT3) that together facilitate association of KLF4 with RNA polymerase II. In addition, the KLF4 DNA binding and transactivation domains are required for optimal KLF4 protein stability. Post-translational modification of KLF4 destabilizes the protein as cells exit the pluripotent state and mutations that prevent this destabilization also prevent differentiation. These data indicate the core pluripotency transcription factors are integrated by post-translational mechanisms to maintain the pluripotent state, and identify mutations that increase KLF4 protein stability while maintaining transcription factor function.

developmental biology↗

bioRxiv2018

Variational Infinite Heterogeneous Mixture Model for Semi-supervised Clustering of Heart Enhancers

MotivationPMammalian genomes can contain thousands of enhancers but only a subset are actively driving gene expression in a given cellular context. Integrated genomic datasets can be harnessed to predict active enhancers. One challenge in integration of large genomic datasets is the increasing heterogeneity: continuous, binary and discrete features may all be relevant. Coupled with the typically small numbers of training examples, semi-supervised approaches for heterogeneous data are needed; however, current enhancer prediction methods are not designed to handle heterogeneous data in the semi-supervised paradigm.\n\nResultsWe implemented a Dirichlet Process Heterogeneous Mixture model that infers Gaussian, Bernoulli and Poisson distributions over features. We derived a novel variational inference algorithm to handle semi-supervised learning tasks where certain observations are forced to cluster together. We applied this model to enhancer candidates in mouse heart tissues based on heterogeneous features. We constrained a small number of known active enhancers to appear in the same cluster, and 47 additional regions clustered with them. Many of these are located near heart-specific genes. The model also predicted 1176 active promoters, suggesting that it can discover new enhancers and promoters.\n\nAvailabilityWe created the dphmix Python package: https://pypi.org/project/dphmix/\n\nContactalan.moses@utoronto.ca

bioinformatics↗