Biology subjects

Majumder, A.

Publications and source records attributed to Majumder, A..

At least 37 records · Page 2Linked to original sources

bioRxiv2024

Differential expression of oncogenic lncRNAs NEAT1 and MALAT1 in 2D monolayer vs. 3D tumoroid culture and its implications in cancer progression

Long non-coding RNAs (lncRNAs) have emerged as crucial regulators of cellular processes, overturning their previous classification as "junk" DNA. Their involvement in various cellular functions makes them potential therapeutic targets in a range of diseases, particularly in tumorigenesis and cancer. Among these lncRNAs, MALAT1 (Metastasis-Associated Lung Adenocarcinoma Transcript 1) and NEAT1 (Nuclear-Enriched Abundant Transcript 1) are prominent oncogenic lncRNAs with complex regulatory roles across multiple cancer types. Although the roles of these lncRNAs in cancer have been extensively studied, the majority of these investigations have been conducted in 2D culture systems. While 2D cell cultures are well-established and widely accepted models in cell biology, they lack the physiological complexity of 3D tumor architecture. In contrast, 3D cultures, where cells grow as three-dimensional clusters, better mimic in vivo conditions, making them essential for understanding tumor microenvironment (TME) dynamics. Despite the importance of 3D models, there is a lack of literature exploring lncRNA expression in 2D monolayers versus 3D cultures across different cancer types. This study examines the expression and function of the lncRNAs NEAT1 and MALAT1 in 3D tumoroids compared to 2D monolayer cultures, aiming to bridge the gap between in vitro models and the complex in vivo tumor microenvironment. We addressed this gap by quantifying the expression differences of NEAT1 and MALAT1 using qRT-PCR in breast cancer, liver cancer, cervical cancer, and glioblastoma (GBM) cells grown in both 2D and 3D cultures. Significant variations in NEAT1 and MALAT1 expression were observed between 2D and 3D cultures across these cancer types, signifying the need to study lncRNAs in 3D micro-environments. Furthermore, we established correlations between NEAT1 expression and cancer traits such as stemness, invasion, glucose transporter expression, and epithelial-mesenchymal transition (EMT) in GBM 3D tumoroids compared to 2D monolayers. Using siRNA to downregulate NEAT1 in GBM tumoroids, we demonstrated that reducing NEAT1 expression to the levels comparable to 2D cultures led to a decrease in the expression of mRNA markers associated with stemness, invasion, glucose transporters, and EMT. Additionally, siRNA-mediated downregulation of NEAT1 in 3D tumoroids directly impacted cancer properties, as validated by phenotypic assays, where reduced proliferation, migration, and invasion were observed in GBM (comparing 3D to 2D models). Therefore, our work provides new insights into the expression of two key oncogenic lncRNAs in 3D microenvironments of various cancers and lays the foundation for integrating lncRNAs as important molecular players within 3D culture systems, offering a better understanding of the in vivo complexity of the TME.

cancer biology↗

bioRxiv2024

Lipid Nanoparticle-Associated Inflammation is Triggered by Sensing of Endosomal Damage: Engineering Endosomal Escape without Side Effects

Lipid nanoparticles (LNPs) have emerged as the dominant platform for RNA delivery, based on their success in the COVID-19 vaccines and late-stage clinical studies in other indications. However, we and others have shown that LNPs induce severe inflammation, and massively aggravate pre-existing inflammation. Here, using structure-function screening of lipids and analyses of signaling pathways, we elucidate the mechanisms of LNP-associated inflammation and demonstrate solutions. We show that LNPs hallmark feature, endosomal escape, which is necessary for RNA expression, also directly triggers inflammation by causing endosomal membrane damage. Large, irreparable, endosomal holes are recognized by cytosolic proteins called galectins, which bind to sugars on the inner endosomal membrane and then regulate downstream inflammation. We find that inhibition of galectins abrogates LNP-associated inflammation, both in vitro and in vivo. We show that rapidly biodegradable ionizable lipids can preferentially create endosomal holes that are smaller in size and reparable by the endosomal sorting complex required for transport (ESCRT) pathway. Ionizable lipids producing such ESCRT-recruiting endosomal holes can produce high expression from cargo mRNA with minimal inflammation. Finally, we show that both routes to non-inflammatory LNPs, either galectin inhibition or ESCRT-recruiting ionizable lipids, are compatible with therapeutic mRNAs that ameliorate inflammation in disease models. LNPs without galectin inhibition or biodegradable ionizable lipids lead to severe exacerbation of inflammation in these models. In summary, endosomal escape induces endosomal membrane damage that can lead to inflammation. However, the inflammation can be controlled by inhibiting galectins (large hole detectors) or by using biodegradable lipids, which create smaller holes that are reparable by the ESCRT pathway. These strategies should lead to generally safer LNPs that can be used to treat inflammatory diseases.

bioengineering↗

bioRxiv2023

Synergistic computational and experimental studies of a phosphoglycosyltransferase membrane/ligand ensemble

Complex glycans serve important functions in all living systems. Many of these intricate and byzantine biomolecules are assembled employing biosynthetic pathways wherein the constituent enzymes are membrane associated. A signature feature of the stepwise assembly processes is the essentiality of unusual linear long-chain polyprenol phosphate-linked substrates, such as un-decaprenol phosphate in bacteria. In this study we focus on a small enzyme, PglC from Campylobacter, structurally characterized for the first time in 2018, as a detergent solubilized construct. PglC is a monotopic phosphoglycosyl transferase (PGT), that embodies the functional core structure of the entire enzyme superfamily and catalyzes the first membrane-committed step in a glycoprotein assembly pathway. The size of the enzyme is significant as it enables high level computation and relatively facile, for a membrane protein, experimental analysis. Our ensemble computational and experimental results reveal a specific interaction of undecaprenol phosphate with PGT cationic residues and suggest a role for critical conformational transitions and electrostatic steering in substrate recognition, overcoming significant energetic barriers to binding. The study highlights that computation, guided by fundamental chemical principles, can advance the study of biochemical processes at membrane bilayers and provide chemical insight at a molecular level that cannot be derived by experiment alone. Insert Table of Contents artwork here O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=169 SRC="FIGDIR/small/539694v1_ufig1.gif" ALT="Figure 1"> View larger version (100K): org.highwire.dtl.DTLVardef@19a38d9org.highwire.dtl.DTLVardef@a41d8org.highwire.dtl.DTLVardef@168e48borg.highwire.dtl.DTLVardef@1749bd8_HPS_FORMAT_FIGEXP M_FIG C_FIG

biochemistry↗

bioRxiv2023

Chromatin condensation delays human mesenchymal stem cells senescence by safeguarding nuclear damages during long-term in vitro expansion

Human mesenchymal stem cells (hMSCs) are multipotent cells that can differentiate into adipocytes, chondrocytes and osteoblasts. Due to their differentiation potential, hMSCs are among the most frequently used cells for therapeutic applications in tissue engineering and regenerative medicine. However, the number of cells obtained through isolation alone is insufficient for hMSC-based therapies and basic research, necessitating their in-vitro expansion. Conventionally, this is often carried out on rigid surfaces such as tissue culture petriplates (TCPs). However, during in-vitro expansion, hMSCs lose their proliferative ability and multilineage differentiation potential, making them unsuitable for clinical use. Although multiple approaches have been tried to maintain hMSC stemness over prolonged expansion, finding a suitable culture system to achieve this remains an unmet need. Recently, few research groups including ours have shown that hMSCs maintain their stemness over long passages when cultured on soft substrate. In addition, it has been shown that hMSCs cultured on soft substrates have more condensed chromatin and lower levels of histone acetylation compared to those cultured on stiff substrates. It has also been shown that condensing/decondensing chromatin by deacetylation/acetylation can delay/hasten replicative senescence in hMSCs during long-term expansion on TCPs. However, how chromatin condensation/decondensation influences nuclear morphology and DNA damage - which are strongly related to the onset of senescence and cancer - is still not known. To answer this question, here we cultured hMSCs for long duration (P4-P11) in presence of epigenetic modifiers histone acetyltransferase inhibitor (HATi) which promotes chromatin condensation by preventing histone acetylation and histone deacetylase inhibitor (HDACi) which promotes chromatin decondensation and investigated their effect on various nuclear markers related to senescence and cancer. We have found that consistent acetylation causes severe nuclear abnormalities whereas chromatin condensation by deacetylation helps in safeguarding nucleus from damages caused by in-vitro expansion.

cell biology↗

bioRxiv2023

A new competitive strategy to unveil the antibiotic-producing Actinobacteria

The bacterial phylum Actinobacteria encompasses microorganisms with incomparable metabolic versatility and deep resource of medicines. However, the recent decrease in the discovery rate of antibiotics warrants innovative strategies to harness actinobacterial resources for lead discovery. Indeed, microbial culturing efforts measuring the outcomes of specific genera lagged behind the detected microbial potential. Herein, we used a distinct competitive strategy that exploits competitive microbial interactions to accelerate the diversification of strain libraries producing antibiotics. This directed-evolution-based strategy shifted the diversity of Actinobacteria over the experimental time course (0-8 days) and led to the isolation of Actinobacterial strains with distinct antimicrobial spectrum against pathogens. To understand the competitive interactions over experimental time, the metagenomic community sequencing revealed that actinobacterial members from families Nocardiaceae and Cellulomonadaceae with relatively increased abundances towards end, are thus competitively advantageous. Whilst comparing the Actinobacteria retrieved in the competitive strategy to that of the routinely used isolation method, the Actinobacteria genera identified from competitive communities differed relatively in abundances as well as antimicrobial spectrum compared to actinobacterial strains retrieved in classical method. In sum, we present a strategy that influences microbial interactions to accelerate the likelihood of potential actinobacterial strains with antimicrobial potencies.

microbiology↗

bioRxiv2022

Lithography-less, frugal and long-term diffusion-based static gradient generating microfluidic device for high-throughput drug testing

Drug testing is a vital step in identification of the potential efficacy of any new/existing drug and/or combinations of drugs. The conventional methods of testing the efficacy of new drugs using multi-well plates are time consuming, prone to evaporation loss and manual error. Microfluidic devices with automated generation of concentration gradient provide a promising alternative. The implementation of such microfluidic devices is still limited owing to the additional expertise and facilities required to fabricate and run these devices. Conventional microfluidic devices also need pumps, tubings, valves, and other accessories, making them bulky and nonportable. To address these problems, we have developed a method for fabricating microfluidic structures using a nonconventional technique by exploiting the Saffman-Taylor instability in lifted Hele-Shaw cell. Multi-channel structure molds with varying dimensions were fabricated by shaping ceramic polymer slurry and retaining the shape. Further using the mold thus made, polydimethyl siloxane (PDMS) devices offering static, stable, diffusion-based gradient were casted using soft lithography. We have demonstrated with COMSOL simulation, as well as using Fluorescein isothiocyanate (FITC), a fluorescent dye, that the concentration gradient can be generated in this device, which remains stable for at least 5 days. Using this multichannel device, in vitro drug efficacy was validated with two drugs namely-Temozolomide (TMZ) and Curcumin, one FDA approved and one under research, on glioblastoma cells (U87MG). The resulting IC50 values were consistent with those reported in literature. We have also demonstrated the possibility of conducting molecular assays post-drug testing in the device by microtubule staining after curcumin treatment on cervical cancer cells (HeLa). In summary, we have demonstrated a i) user-friendly, ii) portable, static drug testing platform that iii) does not require further accessories and can create iv) a stable gradient for long duration. Such a device can reduce the time, manual errors, fabrication and running expenditure, and resources to a great extent in drug testing. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=142 SRC="FIGDIR/small/505813v1_ufig1.gif" ALT="Figure 1"> View larger version (79K): org.highwire.dtl.DTLVardef@e1f648org.highwire.dtl.DTLVardef@f029d7org.highwire.dtl.DTLVardef@14afd68org.highwire.dtl.DTLVardef@42b888_HPS_FORMAT_FIGEXP M_FIG C_FIG

cell biology↗

bioRxiv2022

Scalable large-area mesh-structured microfluidic gradient generator for drug testing applications

Microfluidic concentration gradient generators are useful in drug testing. drug screening, and other cellular applications to avoid manual errors, save time, and labor. However, expensive fabrication techniques make such devices prohibitively costly. Here, in the present work, we developed a microfluidic concentration gradient generator (CGG) using a recently proposed non-conventional photolithography-less method. In this method, ceramic suspension fluid was shaped into a square mesh by controlling Saffman Taylor instability in a Multiport Lifted Hele-Shaw Cell (MLHSC). Using the shaped ceramic structure as template, the microfluidic concentration gradient generator (CGG) was prepared by soft lithography. The concentration gradient was characterized and effect of the flow rates were studied usingCOMSOL simulations. The simulation result was further validated by creating fluorescein dye (Fluorescein isothiocanate, FITC) gradient in the fabricated CGG. To demonstrate the use of this device for drug testing, we created various concentrations of an anticancer drug - curcumin - using the device and determined its inhibitory concentration on cervical cancer cell-line HeLa. We found that the IC50 of curcumin for HeLa to be 28.6 {+/-} 6.1 M which matched well with the conventional muti-well drug testing method (34.9 {+/-} 1.7 M). This method of CGG fabrication has multiple advantages over conventional photolithography such as: i) the channel layout and inlet-outlet arrangements can be changed by simply wiping the ceramic fluid before it solidifies, (ii) it is cost effective, (iii) large area patterning is easily achievable, and (iv) the method is scalable. This technique can be utilised to achieve broad range of concentration gradient to be used for various biological and non-biological applications. Table of Content O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=99 SRC="FIGDIR/small/500002v2_ufig1.gif" ALT="Figure 1"> View larger version (28K): org.highwire.dtl.DTLVardef@d0bb65org.highwire.dtl.DTLVardef@1bef09dorg.highwire.dtl.DTLVardef@a6ebe8org.highwire.dtl.DTLVardef@c9dd17_HPS_FORMAT_FIGEXP M_FIG C_FIG

bioengineering↗

bioRxiv2022

Histone deacetylase inhibitor overrides the effect of soft hydrogel on the mechanoresponse of human mesenchymal stem cells

Human Mesenchymal stem cells (hMSCs) are promising in regenerative medicine for their multi-lineage differentiation capability. It has been demonstrated that lineage specification is governed by both chemical and mechanical cues. Among all the different mechanical cues known to control hMSCs fate, substrate stiffness is the most well-studied. It has been shown that the naive mesenchymal stem cells when cultured on soft gel, they commit towards adipogenic lineage while on stiff gel they become osteogenic. Soft substrates also cause less cell spreading, less traction, less focal adhesion assembly and stress fibre formation. Furthermore, chromatin condensation increases when cells are cultured on soft substrates. As the nucleus has been postulated to be mechanosensor and mechanotransducer, in this paper we asked the question how mechanosensing and mechanoresponse process will be influenced if we reduce the chromatin condensation in hMSCs cultured on soft substrates by using an external reagent. To address this question, we treated hMSCs cultured on soft polyacrylamide (PA) gels with a histone deacetylase inhibitor (HDACi) called Valproic Acid (VA) which decondense the chromatin by hyperacetylation of histone proteins. We found that the treatment with VA overrides the effect of soft substrates on hMSCs morphology, cellular traction, nuclear localization of mechnosensory protein YAP (yes-associated protein), and differentiation. VA treated cells behaved as if they are on stiff substrates in all aspects tested here. In contrast, when we increase chromatin condensation using histone acetyl transferase inhibitor (HATi), adipogenic differentiation on soft gel increases. Furthermore, we have shown that VA controls hMSCs differentiation via activation of ERK/MAPK pathway by increasing the p-ERK expression. Collectively, these findings demonstrate for the first time that inhibiting histone deacetylase can override the mechanoresponse of hMSCs. This work will help us to fundamentally understand the mechanosignalling process and to control the hMSCs differentiation in tissue engineering and regenerative medicine. SignificanceTo harness the clinical potential of Human mesenchymal stem cells (hMSCs), it is important to understand the working mechanism of various signals in deciding cell fate. It has been shown in the recent past that the substrate rigidity influence hMSCs differentiation. Though the mechanism is not fully understood, the mechanosignals are known to alter the cell fate via reorganizing the chromatin packing. In this work, we ask if altering chromatin packing can change the cell fate as dictated by the mechanical properties of the substrates. We have shown that almost all the known effector of mechanosignals get modified when we tweak the chromatin packing via histone deacetylase inhibitor (HDACi). This study is helpful for developing a better understanding of the mechano-signalling process. It gives us a tool to override the effect of the substrates, particularly when the mechanical properties of biomaterials are not conducive for the desired differentiation outcome. Additionally, as HDACi are being used in the therapy of cancer and different neurodegenerative diseases, our understanding of their crosstalk with tissue stiffness in controlling cell fate will help to understand any unknown and undesirable side effects of using HDACi as therapeutic agent.

cell biology↗

bioRxiv2021

Extracellular Matrix protein gelatin provides higher expansion, reduces size heterogeneity, and maintains cell stiffness in a long-term culture of mesenchymal stem cells.

Extracellular matrices (ECM) present in our tissues play a significant role in maintaining tissue homeostasis through various physical and chemical cues such as topology, stiffness, and secretion of biochemicals. They are known to influence the behaviour of resident stem cells. It is also known that ECM type and coating density on cell culture plates strongly influence in vitro cellular behaviour. However, the influence of ECM protein coating on long term mesenchymal stem cell expansion has not been studied yet. To address this gap, we cultured bone-marrow derived hMSCs for multiple passages on the tissue culture plastic plates coated with 25 g/ml of various ECM proteins. We found that cells on plates coated with ECM proteins had much higher proliferation compared to the regular tissue culture plates. Further, gelatin coated plates helped the cells to grow faster compared to collagen, fibronectin, and laminin coated plates. Additionally, the use of Gelatin showed less size heterogeneity among the cells when expanded from passages P3 to P9. Gelatin also helped in maintaining cellular stiffness which was not observed across other ECM proteins. In summary, in this research, we have shown that gelatin which is the least expensive compared to other ECM proteins provides a better platform for mesenchymal stem cell expansion.

cell biology↗

bioRxiv2021

Substrate stiffness controls the cell cycle of human mesenchymal stem cells via cellular traction

The microenvironment of human mesenchymal stem cells (hMSCs) regulates their self-renewal and differentiation properties. Previously it was shown that hMSCs remained quiescent on soft (0.25 kPa) polyacrylamide (PA) gels but re-entered into cell cycle on a stiff (7.5 kPa) gel. However, how cells behave on intermediate stiffness and what intracellular factors transmit mechanical changes to cell interior thereby regulating cell cycle remained unknown. In this work we demonstrated that PA gels between 1 and 5 kPa act as a mechanical switch in regulating cell cycle of hMSCs. By experiments on cell-cycle exit and re-entry, we found that hMSCs demonstrated a sharp transition from quiescence to proliferation between 1 and 5 kPa. Further studies with ROCK inhibitor Y-27632 revealed that contractile proteins, but not cell spread area, accounts for the sensitivity of hMSCs towards substrate stiffness and hence correlates with their changes in cell cycle. These observations therefore suggest that substrate stiffness regulates hMSC proliferation through contractile forces as generated by cellular contractile proteins in a unique pattern which is distinct from other cell types as studied. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=182 SRC="FIGDIR/small/469207v1_ufig1.gif" ALT="Figure 1"> View larger version (83K): org.highwire.dtl.DTLVardef@a4fff3org.highwire.dtl.DTLVardef@9fb415org.highwire.dtl.DTLVardef@e7d633org.highwire.dtl.DTLVardef@9fba04_HPS_FORMAT_FIGEXP M_FIG C_FIG

bioengineering↗

bioRxiv2021

Adaptation in outbred sexual yeast is repeatable, polygenic, and favors rare haplotypes

We describe the results of a 200 generation Evolve and Resequence (E&R) study initiated from an outbred dipliod recombined synthetic base population derived from 18 genetically diverse founders. Replicate populations were maintained at large effective population sizes (>105 individuals), exposed to several different chemical challenges over 12 weeks of evolution, and whole-genome resequenced. Weekly forced outcrossing implies a per gene per cell-division recombination rate higher than that achieved in Drosophila E&R studies. In 55 sexual populations we observe large fitness gains and highly repeatable patterns of genome-wide haplotype change within each chemical challenge. There was little evidence for pervasive pleiotropy, as evidenced by patterns of haplotype change between drug treatments. Within treatment adaptation appears highly polygenic with almost the entire genome showing significant consistent haplotype change. Finally, adaptation was almost always associated with only one of the 18 founder alleles, suggesting selection primarily acts on rare variants private to a founder or haplotype blocks harboring multiple mutations. This observation contradicts the notion that adaptation is often due to subtle frequency shifts at intermediate frequency variants.

evolutionary biology↗

bioRxiv2021

The utility of a closed breeding colony of Peromyscus leucopus for dissecting complex traits

Although Peromyscus leucopus (deermouse) is not considered a genetic model system, its genus is well suited for addressing several questions of biologist interest, including the genetic bases of longevity, behavior, physiology, adaptation, and its ability to serve as a disease vector. Here we explore a diversity outbred approach for dissecting complex traits in Peromyscus leucopus, a non-traditional genetic model system. We take advantage of a closed colony of deer-mice founded from 38 individuals between 1982 and 1985 and subsequently maintained for 35+ years ([~]40-60 generations). From 405 low-pass ([~]1X) short-read sequenced deermice we accurately imputed genotypes at 17,751,882 SNPs. Conditional on observed genotypes for a subset of 297 individuals, simulations were conducted in which a QTL contributes 5% to a complex trait under three different genetic models. The power of either a haplotype- or marker-based statistical test was estimated to be 15-25% to detect the hidden QTL. Although modest, this power estimate is consistent with that of DO/HS mice and rat experiments for an experiment with [~]300 individuals. This limitation in QTL detection is mostly associated with the stringent significance threshold required to hold the genome-wide false positive rate low, as in all cases we observe considerable linkage signal at the location of simulated QTL, suggesting a larger panel would exhibit greater power. For the subset of cases where a QTL was detected, localization ability appeared very desirable at [~]1-2Mb. We finally carried out a GWAS on a demonstration trait, bleeding time. No tests exceeded the threshold for genome-wide significance, but one of four suggestive regions co-localizes with Von Willebrand factor. Our work suggests that complex traits can be dissected in founders-unknown P. leucopus colony mice in much the same manner as founders-known DO/HS mice and rats, with genotypes obtained from low pass sequencing data. Our results further suggest that the DO/HS approach can be powerfully extended to any system in which a founders-unknown closed colony has been maintained for several dozen generations.

genetics↗

bioRxiv2020

Two synthetic 18-way outcrossed populations of diploid budding yeast with utility for complex trait dissection

Advanced generation multi-parent populations (MPPs) are a valuable tool for dissecting complex traits, having more power than GWAS to detect rare variants, and higher resolution than F2 linkage mapping. To extend the advantages of MPPs in budding yeast, we describe the creation and characterization of two outbred MPPs derived from eighteen genetically diverse founding strains. We carried out de novo assemblies of the genomes of the eighteen founder strains, such that virtually all variation segregating between these strains is known and represent those assemblies as Santa Cruz Genome Browser tracks. We discover complex patterns of structural variation segregating amongst the founders, including a large deletion within the vacuolar ATPase VMA1, several different deletions within the osmosensor MSB2, a series of deletions and insertions at PRM7 and the adjacent BSC1, as well as copy number variation at the dehydrogenase ALD2. Resequenced haploid recombinant clones from the two MPPs have a median unrecombined block size of 66kb, demonstrating the population are highly recombined. We pool sequenced the two MPPs to 3270X and 2226X coverage and demonstrate that we can accurately estimate local haplotype frequencies using pooled data. We further down-sampled the poolseq data to ~20-40X and show that local haplotype frequency estimates remain accurate, with median error rate 0.8% and 0.6% at 20X and 40X, respectively. Haplotypes frequencies are estimated much more accurately than SNP frequencies obtained directly from the same data. Deep sequencing of the two populations revealed that ten or more founders are present at a detectable frequency for over 98% of the genome, validating the utility of this resource for the exploration of the role of standing variation in the architecture of complex traits.

genetics↗

bioRxiv2019

\"Viscotaxis\" - Directed Migration of Mesenchymal Stem Cells in Response to Loss Modulus Gradient

Directed cell migration in response to chemical and mechanical gradients plays a crucial role in physiological and pathological conditions. One such mechanical cues that is known to influences cell migration is the gradient of substrate elastic modulus (E). However, the elastic modulus alone cannot fully define the material properties of the cellular microenvironment, which often has both elastic and viscous characteristics. In this study, we investigated the influence of the gradient of viscous nature, as defined by loss modulus, G\", on cell migration. We cultured human mesenchymal stem cells (hMSCs) on a collagen-coated polyacrylamide gel with constant elastic property, as defined by the storage modulus G, but with the gradient of loss modulus G\". We found hMSCs to migrate from high to low loss modulus. We have termed this, thus far unreported, directional cellular migration as \"Viscotaxis\". We have confirmed uniform collagen density and constant storage modulus of the gel by fluorescence microscopy and atomic force microscopy to eliminate the possibilities of haptotaxis and durotaxis. We hypothesize that material creep in the high loss modulus region hinders the building up of the cellular traction, leading to a force asymmetry that drives the observed viscotaxis. To verify our hypothesis, we estimated the cellular traction on gels with high and low loss moduli. We indeed found that cells apply higher traction force on more elastic materials i.e. materials with low loss modulus. On the disruption of actomyosin contractility with myosin inhibitor blebbistatin and ROCK inhibitor Y27632, directional migration was lost. Further, we showed that cells can maintain a stable morphology on a low loss modulus substrate but due to its inability to build up stable cellular traction on a substrate with high loss modulus, the cell spreading remains in a dynamic state. Our findings in this paper highlight the importance of considering the viscous modulus while preparing stiffness-based substrates for the field of tissue engineering.

biophysics↗

bioRxiv2019

Asamataxis: A cooperative relayed migration in response to subsurface inhomogeneity leads to long-range self-patterning of cells

Cells self-organize to give patterns that are essential for tissue functioning. While the effects of biochemical and mechanical cues are relatively well studied, the role of stiffness inhomogeneity on cellular patterning is unexplored. Using a rigid structure embedded in soft polyacrylamide (PAA) gel, we show that such mechanical inhomogeneity leads to long-range self-organized cellular patterns. Our results reveal that this patterning depends on cellular traction and cell morphology. Depending on a suitable combination of cellular morphology and traction, the information about the presence of embedded structure gets relayed outward. In response to this relay, the cells reorient their axis and migrate towards the embedded structure leading to the observed long-range (20-35 cell length) patterning. To predict the possibility of pattern formation, we present a dimensionless number f combining the governing parameters. We have also shown that the pattern can be tailor-made by pre-designing sub-surface structures, a potential tool for tissue engineering. This mechanism of directed migration driven long-range pattern formation in response to mechanical inhomogeneity may be involved during several pathophysiological conditions, a proposition that needs further investigation.\n\nOne Sentence SummarySubstrate inhomogeneity and cooperative cellular traction together lead to cellular migration and long-range pattern formation.

biophysics↗

bioRxiv2019

Slow and ad hoc: Unravelling the two features characterizing the development of bacterial resistance to membrane active peptides

Membrane disrupting drugs such as antimicrobial peptides are being considered as a solution to counter the problem of antibiotic resistance. Although it can be intuitively imagined that bacteria will eventually develop resistance to this class of drugs as well, the concern has largely been ignored. Drawing upon the experimental data from the resistance of Staphylococcus aureus to antimicrobial peptides, we theoretically model the membrane adaptation under drug pressure. Using our model, we simulate the serial passage experiments with and without the drug pressure, and use the comparisons with experiments to estimate the unknown kinetic parameters. While the development of resistance to enzyme or membrane targeting drugs are both driven by spontaneous mutations, an additional lysylation step required in the latter slows the development of resistance. By quantifying the tradeoff between the gain in fitness under drug pressure and a loss in growth due to membrane modification, our model shows a fast reversal of membrane composition in drug free conditions, re-sensitizing the bacterium to the drugs.

biophysics↗

bioRxiv2018

Computationally designing antimicrobial peptides for Acinectobacter Baumannii

Acinetobacter Baumannii, which is mostly contracted in hospital stays, has been developing resistance to all available antibiotics, including the last line of drugs, such as carbapenem. Because of its quick adaptation there is an immediate need to design new antibiotics, possibly antimicrobial peptides (AMPs) to which bacteria do not develop resistance easily. Our threefold goal was to curate the available activity of AMPs on the same strain of A. Baumannii, build a neural network model for predicting their activity and use it to rationally pre-screen for lead generation from the thousands of naturally occurring AMPs. By curating and analyzing the recent activity data from 81 AMPs on ATCC 19606 strain, we develop a quantitative AMP activity prediction model. We selected three other models with comparable performance against a test set with known activities. With the goal of inspiring further studies on AMP drug candidates and their rational shortlisting, we made activity predictions for the entire database of AMPs using all the models. To handle the uncertainty of training with a small data set, highlighted peptides which had consistent results from all models.

biochemistry↗

bioRxiv2018

Soft Substrate Maintains Proliferative and Multi-lineage Potential of Human Mesenchymal Stem Cells on Long Term Expansion

Human mesenchymal stem cells (hMSCs), when cultured on tissue culture plate (TCP) for in vitro expansion, they spontaneously lose their proliferative capacity and multi-lineage differentiation potential. They also lose their distinct spindle morphology and become large and flat. After a certain number of population doubling, they enter into permanent cell cycle arrest, called senescence. This is a major roadblock for clinical use of hMSCs which demands large number of cells. A cell culture system is needed which can maintain the stemness of hMSCs over long term passages yet simple to use. In this study, we explore the role of substrate rigidity in maintaining stemness. hMSCs were serially passaged on TCP and 5 kPa poly-acrylamide gel for 20 population doubling. It was found that while on TCP, cell growth reached a plateau at cumulative population doubling (CPD) = 12.5, on 5 kPa gel, they continue to proliferate linearly till we monitored (CPD = 20). We also found that while on TCP, late passage MSCs lost their adipogenic potential, the same was maintained on soft gel. Cell surface markers related to MSCs were also unaltered. We demonstrated that this maintenance of stemness was correlated with delay in onset of senescence, which was confirmed by {beta}-gal assay and by differential expression of vimentin, Lamin A and Lamin B. As preparation of poly-acrylamide gel is a simple, well established, and well standardized protocol, we believe that this system of cell expansion will be useful in therapeutic and research applications of hMSCs.\n\nOne Sentence SummaryhMSCs retain their stemness when expanded in vitro on soft polyacrylamide gel coated with collagen by delaying senescence.\n\nSignificance StatementFor clinical applications, mesenchymal stem cells (MSCs) are required in large numbers. As MSCs are available only in scarcity in vivo, to fulfill the need, extensive in vitro expansion is unavoidable. However, on expansion, they lose their replicative and multi-lineage differentiation potential and become senescent. A culture system that can maintain MSC stemness on long-term expansion, without compromising the stemness, is need of the hour. In this paper, we identified polyacrylamide (PAA) hydrogel of optimum stiffness that can be used to maintain stemness of MSCs during in vitro long term culture. Large quantity of MSCs thus grown can be used in regenerative medicine, cell therapy, and in treatment of inflammatory diseases.

bioengineering↗