Biology subjects

Jagannathan, M.

Publications and source records attributed to Jagannathan, M..

13 recordsLinked to original sources

bioRxiv2026

Protein-driven colloid-osmotic pressure controls nuclear size, organization, and function

The size of the cell nucleus is tightly controlled and changes in nuclear size correlate with altered nuclear function during development, cell differentiation and senescence. How nuclear size is regulated and whether changes thereof are functionally relevant remains unclear. Here, we demonstrate that nuclear size is determined by the osmotic pressure exerted by proteins in yeast, human cells and frog egg extracts. The biophysical model we establish solves the long-standing question of how the nuclear-to-cytoplasmic ratio is regulated and maintained. Furthermore, altering protein-driven osmotic balance modulates the physical properties of the nucleus, with a direct effect on chromatin organization and gene expression: Nuclear enlargement causes the dissolution of heterochromatic structures and derepression of subtelomeres and transposons, while simultaneously down-regulating highly expressed housekeeping genes. Importantly, these global transcriptional patterns closely mimic the gene expression changes that occur as yeast, human, and drosophila cells enlarge. Importantly, artificially forcing nuclear compression is sufficient to reverse these size-associated expression changes. Together, our findings provide a quantitative, mechanistic explanation for the coupling between the size of the nucleus and the cell, and they establish nuclear size as a modulator of chromatin organization and gene expression.

cell biology↗

bioRxiv2026

Nuclear stress bodies enable a germline-specific transcriptional stress response in Drosophila

The germline ensures the continuity of genetic information across generations, but how this immortal lineage functions under stress conditions remains incompletely understood. Here, we identify that heat shock factor (Hsf), a conserved master regulator of the stress response, drives the expression of transposable elements (TEs), in addition to molecular chaperones upon heat shock in Drosophila gonads. In germ cells, this potential intra-genomic conflict is countered by the formation of nuclear stress bodies (nSBs) at non-coding satellite DNA repeats. Using chemical and genetic perturbations, we demonstrate that nSBs are both necessary and sufficient to delay Hsf-dependent transcription. Notably, this nSB-mediated delay, in tandem with the piRNA pathway, allows germ cells to selectively express molecular chaperones, but not transposable elements, upon heat shock. Overall, we propose that this unique transcriptional stress response preserves germline function and evolutionary fitness, especially in natural populations routinely exposed to environmental stress.

cell biology↗

bioRxiv2026

Meiotic pairing through barcode-like satellite DNA repeats

During meiosis, chromosomes must find, pair, and synapse with their homologous partners in the crowded milieu of the nucleus1. Although homology detection is generally attributed to recombination, pairing and synapsis can occur in its absence2-10, suggesting alternate mechanisms that discriminate between homologous and non-homologous chromosomes. In many eukaryotes, tandem repeats known as satellite DNA facilitate inter-chromosomal associations11. Notably, their non-uniform distribution across chromosomes gives rise to homologue-specific satellite DNA barcodes12-14, which have been speculated to enable meiotic pairing15-18. However, satellite DNA function remains actively debated since these repeats cannot be manipulated in most model organisms. Here, we use satellite DNA deletion, duplication, and translocation strains that are unique to Drosophila to demonstrate that repeat mismatches perturb meiotic pairing, particularly at centromeres and pericentromeres. In the absence of satellite DNA homology, pairing is antagonized by the HORMAD protein, Mad2, while a Pachytene checkpoint 2 (Pch2)-dependent meiotic delay restores pairing. Remarkably, pairing defects are also observed in the progeny of D. melanogaster natural populations that have diverged in their satellite DNA content. Finally, compromised meiotic pairing is strongly correlated with mid-oogenesis cell death, a quality control mechanism that likely culls defective oocytes to prevent chromosome mis-segregation and aneuploidy. These findings resolve a long-standing debate on satellite DNA functionality by demonstrating a barcode-like role in homology detection. We propose that this repeat-based pairing mechanism exerts an underappreciated selective pressure, constraining the divergence of rapidly evolving satellite DNA within interbreeding natural populations.

genetics↗

bioRxiv2025

Dystonia-associated Torsins sustain CLCC1 function to promote membrane fusion of the nuclear envelope for NPC biogenesis

DYT1 early-onset dystonia is a severe, incurable disorder of the central nervous system caused by mutations in the gene encoding Torsin1A (Tor1A, DYT1). Torsins are ER-resident AAA+-ATPases implicated in lipid metabolism, nuclear pore complex (NPC) biogenesis, and lipoprotein secretion, yet their molecular function that underlies the disease pathology has remained incompletely understood. Here, we have utilized Drosophila melanogaster and human somatic cells as experimental models to shed light on their mode-of-action. Fly germ cells lacking dTorsin are arrested in development and display defects in the final steps of NPC biogenesis due to a failure in fusion of the inner and outer nuclear membranes. Using proximity labelling of Torsin1A in human cells, we identify the conserved membrane protein chloride channel CLIC-like protein 1 (CLCC1) as a novel Torsin binding partner. Absence of human CLCC1 or its Drosophila homolog dClcc1 phenocopied the membrane fusion defects at NPC assembly sites observed upon Torsin deletion. Furthermore, CLCC1 is enriched at arrested fusion sites, suggesting it to be a candidate for the elusive NE membrane fusogen. Importantly, CLCC1/dClcc1 overexpression is sufficient to rescue NPC biogenesis and developmental defects associated with Torsin-loss-of-function. Taken together, our data suggest that Torsin-regulated CLCC1 activity drives membrane fusion during NPC biogenesis and reveal that modulating CLCC1 expression is a promising therapeutic prospect for DYT1 dystonia.

cell biology↗

bioRxiv2025

Nuclear mechanostability emerges from satellite DNA condensation into chromocenters

As the largest organelle, the nucleus endures significant mechanical stresses over the cellular lifespan, and mechanostability, i.e. the ability to resist deformation, is critical for genome integrity and function. Here, we reveal that nuclear mechanostability is an emergent property arising from the clustering of satellite DNA repeats into nuclear condensates known as chromocenters. Targeted chromocenter disruption in Drosophila testes subjected to natural and artificial mechanical stress compromises nuclear mechanostability, leading to deformed nuclei, DNA damage, and chromosome breaks. Conversely, enhancing chromocenter coalescence through genetic means improves mechanostability. Molecular dynamics simulations suggest that chromocenters enable physically linked chromosomes to respond collectively, rather than individually, to mechanical challenge, and dissipate external forces over a larger nuclear surface. We propose that the satellite DNA-dependent mechanostability framework described here likely extends to other cells and tissues facing mechanical stress, and offers an explanation for the evolutionary success of these non-coding repeats across eukaryotes.

cell biology↗

bioRxiv2025

A conserved mechanism of membrane fusion in nuclear pore complex assembly

The nuclear pore complex (NPC) forms a large channel that spans the double lipid bilayer of the nuclear envelope and is the central gateway for macromolecular transport between the nucleus and cytoplasm in eukaryotes. NPC biogenesis requires the coordinated assembly of over 500 proteins culminating in the fusion of the inner and outer nuclear membranes. The molecular mechanism of this membrane fusion step that occurs in all eukaryotes is unknown. Here, we elucidate the mechanism by which two paralogous transmembrane proteins, Brl1 and Brr6, mediate membrane fusion in S. cerevisiae. Both proteins form multimeric, ring-shaped complexes with membrane remodeling activity. Brl1 is enriched at NPC assembly sites via a nuclear export sequence and then interacts with Brr6 across the nuclear envelope lumen through conserved hydrophobic loops. Disrupting this interaction blocks fusion and halts NPC assembly. Molecular dynamics simulations suggest that the Brl1-Brr6 complex drives membrane fusion by forming a channel across bilayers that enables lipid exchange. Phylogenetic analyses reveal that Brl1/Brr6 homologues are broadly distributed across eukaryotes, and functional experiments in human cells and D. melanogaster establish CLCC1 as an NPC fusogen in metazoans. Together, our results uncover a novel, conserved mechanism for membrane fusion in eukaryotes.

cell biology↗

bioRxiv2024

Patterns of crossover distribution in Drosophila mauritiana necessitate a re-thinking of the centromere effect on crossing over

We present a SNP-based crossover map for Drosophila mauritiana. Using females derived by crossing two different strains of D. mauritiana, we analyzed crossing over on all five major chromosome arms. Analysis of 105 male progeny allowed us to identify 327 crossover chromatids bearing single, double, or triple crossover events, representing 398 separate crossover events. We mapped these crossover sites along these five chromosome arms using a genome sequence map that includes the euchromatin-heterochromatin boundary. Confirming previous studies, we show that the overall crossover frequency in D. mauritiana is higher than is seen in D. melanogaster. Much of the increase in exchange frequency in D. mauritiana is due to a greatly diminished centromere effect. Using larval neuroblast metaphases from D. mauritiana -D. melanogaster hybrids we show that the lengths of the pericentromeric heterochromatin do not differ substantially between the two species, and thus cannot explain the observed differences in crossover distribution. Using a new and robust maximum likelihood estimation tool for obtaining Weinstein tetrad distributions, we observed an increase in bivalents with two or more crossovers when compared to D. melanogaster. This increase in crossing over along the arms of D. mauritiana likely reflects an expansion of the crossover-available euchromatin caused by the reduction in the centromere effect. The pattern of crossing over in D. mauritiana conflicts with the commonly accepted view of centromeres as polar suppressors of exchange (whose intensity is buffered by sequence non-specific heterochromatin) and demonstrates the importance of expanding such studies into other species of Drosophila. Article SummaryIn meiosis, homolog segregation is usually ensured by crossovers. The number and distribution of crossovers is in part regulated by cis-acting factors such as the cis- acting centromere effect, a polar suppression of exchange emanating from the vicinity of the centromere. We use SNP-based crossover mapping to show that in Drosophila mauritiana, the centromere effect is greatly reduced on four of the five major chromosome arms. We conclude that the centromere effect differs between Drosophila mauritiana and Drosophila melanogaster and that the ability to attenuate the centromere effect is not a general property of heterochromatin.

genetics↗

bioRxiv2023

A nuclear architecture screen in Drosophila identifies Stonewall as a link between chromatin position at the nuclear periphery and germline stem cell fate

The association of genomic loci to the nuclear periphery is proposed to facilitate cell-type specific gene repression and influence cell fate decisions. However, the interplay between gene position and expression remains incompletely understood, in part because the proteins that position genomic loci at the nuclear periphery remain unidentified. Here, we used an Oligopaint-based HiDRO screen targeting [~]1000 genes to discover novel regulators of nuclear architecture in Drosophila cells. We identified the heterochromatin-associated protein, Stonewall (Stwl), as a factor promoting perinuclear chromatin positioning. In female germline stem cells (GSCs), Stwl binds and positions chromatin loci, including GSC differentiation genes, at the nuclear periphery. Strikingly, Stwl-dependent perinuclear positioning is associated with transcriptional repression, highlighting a likely mechanism for Stwls known role in GSC maintenance and ovary homeostasis. Thus, our study identifies perinuclear anchors in Drosophila and demonstrates the importance of gene repression at the nuclear periphery for cell fate.

cell biology↗

bioRxiv2023

Genome organization regulates nuclear pore complex formation and promotes differentiation during Drosophila oogenesis

Genome organization can regulate gene expression and promote cell fate transitions. The differentiation of germline stem cells (GSCs) to oocytes in Drosophila involves changes in genome organization mediated by heterochromatin and the nuclear pore complex (NPC). Heterochromatin represses germ-cell genes during differentiation and NPCs anchor these silenced genes to the nuclear periphery, maintaining silencing to allow for oocyte development. Surprisingly, we find that genome organization also contributes to NPC formation, mediated by the transcription factor Stonewall (Stwl). As GSCs differentiate, Stwl accumulates at boundaries between silenced and active gene compartments. Stwl at these boundaries plays a pivotal role in transitioning germ-cell genes into a silenced state and activating a group of oocyte genes and Nucleoporins (Nups). The upregulation of these Nups during differentiation is crucial for NPC formation and further genome organization. Thus, crosstalk between genome architecture and NPCs is essential for successful cell fate transitions.

developmental biology↗

bioRxiv2023

Multi-tissue proteomics identifies a link between satellite DNA organization and transgenerational transposon repression in Drosophila

Non-coding satellite DNA repeats are abundant at the pericentromeric heterochromatin of eukaryotic chromosomes. During interphase, sequence-specific DNA-binding proteins cluster these repeats from multiple chromosomes into nuclear foci known as chromocenters. Despite the pivotal role of chromocenters in cellular processes like genome encapsulation and gene repression, the associated proteins remain incompletely characterized. Here, we use two satellite DNA-binding proteins, D1 and Prod, as baits to characterize the chromocenter-associated proteome in Drosophila embryos, ovaries, and testes through quantitative mass spectrometry. We identify D1- and Prod-associated proteins, including known heterochromatin proteins as well as proteins previously unlinked to satellite DNA or chromocenters, thereby laying the foundation for a comprehensive understanding of cellular functions enabled by satellite DNA repeats and their associated proteins. Interestingly, we find that multiple components of the transposon-silencing piRNA pathway are associated with D1 and Prod in embryos. Using genetics, transcriptomics, and small RNA profiling, we show that flies lacking D1 during embryogenesis exhibit transposon expression and gonadal atrophy as adults. We further demonstrate that this gonadal atrophy can be rescued by mutating the checkpoint kinase, Chk2, which mediates germ cell arrest in response to transposon mobilization. Thus, we reveal that a satellite DNA-binding protein functions during embryogenesis to silence transposons, in a manner that is heritable across later stages of development.

cell biology↗

bioRxiv2021

Defective satellite DNA clustering into chromocenters underlies hybrid incompatibility in Drosophila

Although rapid evolution of pericentromeric satellite DNA repeats is theorized to promote hybrid incompatibility (HI)(1-4), how divergent repeats affect hybrid cells remains poorly understood. Recently, we demonstrated that sequence-specific DNA-binding proteins cluster satellite DNA from multiple chromosomes into chromocenters, thereby bundling chromosomes to maintain the entire genome in a single nucleus(5, 6). Here we show that ineffective clustering of divergent satellite DNA in the cells of Drosophila hybrids results in chromocenter disruption, associated micronuclei formation and tissue atrophy. We further demonstrate that previously identified HI factors trigger chromocenter disruption and micronuclei in hybrids, linking their function to a conserved cellular process. Together, we propose a unifying framework that explains how the widely observed satellite DNA divergence between closely related species can cause reproductive isolation.

cell biology↗

bioRxiv2018

The modular mechanism of chromocenter formation in Drosophila

A central principle underlying the ubiquity and abundance of pericentromeric satellite DNA repeats in eukaryotes has remained poorly understood. In our previous study (Jagannathan et al., 2018), we proposed that the interchromosomal clustering of satellite DNAs into nuclear structures known as chromocenters ensures encapsulation of all chromosomes into a single nucleus. Chromocenter disruption led to micronuclei formation, resulting in cell death. Here we show that chromocenter formation is mediated by a modular network, where interactions between two sequence-specific satellite DNA-binding proteins, D1 and Prod, bound to their cognate satellite DNAs, bring the full complement of chromosomes into the chromocenter. D1 prod double mutants die during embryogenesis, exhibiting enhanced phenotypes associated with chromocenter disruption, revealing the universal importance of satellite DNAs and chromocenters. Taken together, we propose that interactions between chromocenter modules, consisting of satellite DNA binding proteins and their cognate satellite DNA, package the Drosophila genome within a single nucleus.

molecular biology↗

bioRxiv2018

A conserved function for pericentromeric satellite DNA

A universal and unquestioned characteristic of eukaryotic cells is that the genome is divided into multiple chromosomes and encapsulated in a single nucleus. However, the underlying mechanism to ensure such a configuration is unknown. Here we provide evidence that pericentromeric satellite DNA, which is often regarded as junk, is a critical constituent of the chromosome, allowing the packaging of all chromosomes into a single nucleus. We show that the multi AT-hook satellite DNA binding proteins, D. melanogaster D1 and mouse HMGA1, play an evolutionarily conserved role in bundling pericentromeric satellite DNA from heterologous chromosomes into chromocenters, a cytological association of pericentromeric heterochromatin. Defective chromocenter formation leads to micronuclei formation due to budding off of interphase nucleus and cell death. We propose that chromocenter and satellite DNA serves a fundamental role to achieve the universal characteristic of eukaryotic cells: full complement of the genome within a single nucleus.

cell biology↗