Biology subjects

Najafi, J.

Publications and source records attributed to Najafi, J..

5 recordsLinked to original sources

bioRxiv2026

The Endoplasmic Reticulum as a Cytoplasmic Anchor for Microtubule-Driven Aster Centration

The proper positioning of microtubule (MT) asters is essential to regulate division positioning and multi-cellular morphogenesis, yet the mechanisms regulating this process remain poorly understood. Using sea urchin zygotes as a model, we combined high-resolution imaging, perturbation assays, and 3D modeling to investigate the nature of forces driving sperm aster centration after fertilization. We show that centering asters are highly anisotropic, featuring long microtubules extending at the aster front that may engage with organelles in bulk cytoplasm to generate pulling forces. While vesicular compartments, like acidic organelles, mitochondria or yolk, contributed minimally, we found that the endoplasmic reticulum (ER) undergoes a massive compaction around the centering aster and drags cytosol to produce a reactive centering force on the aster. Perturbing ER organization by cytoplasmic stratification or Atlastin-mediated fragmentation disrupted aster centration. A two-fluid model suggests that the frictional interaction between the compacting ER and cytosol fluid is sufficient to recapitulate key features of aster centration. These findings identify the ER as an important cytoplasmic anchor for MT force exertion in bulk cytoplasm, to guide aster centering motion and influence early embryonic division geometries. More broadly, our results highlight the role of cytoplasmic architecture in shaping the mechanics of organelle positioning during development.

cell biology↗

bioRxiv2023

Cell shape anisotropy enhances cytoplasm viscoelastic resistance to stabilize mitotic spindle position during early embryo development

Cell geometry is a key parameter for the regulation of mitotic spindle positioning during early embryo development and tissue morphogenesis. To date, however, we still lack an understanding for how intracellular forces that position, orient or hold mitotic spindles depend on cell geometry. Here, we used in vivo magnetic tweezers to directly measure the forces that maintain the mitotic spindle in the center of sea urchin cells that adopt different shapes during early embryo development. We found that spindles are held by viscoelastic forces that progressively increase in amplitude as cells become more elongated during early development. By coupling direct cell shape manipulations and in vivo force measurements, we establish how spindle associated forces increase in dose dependence with cell shape anisotropy. Cytoplasm flow analysis and hydrodynamic simulations suggest that this geometry-dependent mechanical enhancement results from a stronger hydrodynamic coupling between the spindle and cell boundaries, which dampens cytoplasm flows and spindle mobility as cells become more elongated. These findings establish how cell shape affects spindle associated forces, and suggest a novel mechanism for shape-sensing and division positioning mediated by intracellular hydrodynamics with functional implications for early embryo morphogenesis.

cell biology↗

bioRxiv2022

PAMP-Induced secreted Peptide-Like 6 (PIPL6) functions as an amplifier of plant immune response through RLK7 and WRKY33 module

Plant peptide hormones are engaged in the regulation of plant developmental programs and immunity. PAMP-Induced Peptide (PIP) hormones are new class of signaling peptide with diverse functional roles in the regulation of plant development and stress responses. In this study, we have investigated the function of PAMP-Induced secreted Peptide-Like 6 (PIPL6) as an amplifier of plant immunity against necrotrophic fungal pathogens in Arabidopsis thaliana. We have applied an integrated omics approach to unveil the function and downstream signaling pathways initiated by PIPL6. PIPL6 is highly and transiently induced by treatment with different elicitors. Exogenous application of synthetic peptide designed from the C-terminal conserved domain of PIPL6 resulted in strong transcriptional induction of many genes involved in the regulation of plant immunity. Further gene expression analysis revealed that induction of marker genes by PIPL6 peptide requires the receptor-like kinase 7 (RLK7). Immunoblotting and gene expression analysis demonstrated that exogenous applications of PIPL6 peptide activates MAPK6, MAPK3, and WRKY33 module in an RLK7-dependent manner. The levels of salicylic acid, jasmonic acid, camalexin, and glucosinolates were differentially regulated in PIPL6 knock-down and overexpression lines challenged by necrotrophic pathogen Botrytis cinerea. Bioassays using the necrotrophic fungal pathogens Botrytis cinerea and Alternaria brassicae showed that pipl6 knock-down lines were more susceptible to these pathogens while PIPL6 overexpression lines exhibited enhanced resistance. Altogether, these results indicate that the PIPL6 peptide functions as a new damage-associated molecular pattern (DAMP) and acts as an amplifier of Arabidopsis immunity.

plant biology↗

bioRxiv2022

Size and position dependent cytoplasm viscoelasticity through hydrodynamic interactions with the cell surface

Many studies of cytoplasm rheology have focused on small components in the sub-micrometer scale. However, the cytoplasm also baths large organelles like nuclei, microtubule asters or spindles that often take significant portions of cells and move across the cytoplasm to regulate cell division or polarization. Here, we translated passive components of sizes ranging from few up to ~50 percent of the cell diameter, through the vast cytoplasm of live sea urchin eggs, with calibrated magnetic forces. Creep and relaxation responses indicate that for objects larger than the micron size, the cytoplasm behaves as a Jeffreys material, viscoelastic at short time-scales and fluidizing at longer times. However, as components size approached that of cells, cytoplasm viscoelastic resistance increased in a non-monotonic manner. Flow analysis and simulations suggest that this size-dependent viscoelasticity emerges from hydrodynamic interactions between the moving object and the static cell surface. This effect also yields to position-dependent viscoelasticity with objects initially closer to the cell surface being harder to displace. These findings suggest that the cytoplasm hydrodynamically couples large organelles to the cell surface to restrain their motion, with important implications for cell shape sensing and cellular organization. Significance StatementLarge-sized organelles like nuclei or mitotic spindles typically translocate through the cytoplasm to regulate cell division or polarity, but their frictional interaction with the cytoplasm and the cell surface remain poorly addressed. We used in vivo magnetic tweezers, to move passive components in a range of size in the cytoplasm of living cells. We found that the mobility of objects with sizes approaching that of cells, can be largely reduced as a result of hydrodynamic interactions that couple objects and the cell surface through the cytoplasm fluid.

biophysics↗

bioRxiv2021

Contribution of cytoplasm viscoelastic properties to mitotic spindle positioning

Cells are filled with macromolecules and polymer networks that set scale-dependent viscous and elastic properties to the cytoplasm. Although the role of these parameters in molecular diffusion, reaction kinetics and cellular biochemistry is being increasingly recognized, their contributions to the motion and positioning of larger organelles, such as mitotic spindles for cell division remain unknown. Here, using magnetic tweezers to displace and rotate mitotic spindles in living embryos, we uncovered that the cytoplasm can impart viscoelastic reactive forces that move spindles, or passive objects with similar size, back to their original position. These forces are independent of cytoskeletal force generators, yet reach hundreds of piconewtons and scale with cytoplasm crowding. Spindle motion shears and fluidizes the cytoplasm, dissipating elastic energy and limiting spindle recoils with functional implications for asymmetric and oriented divisions. These findings suggest that bulk cytoplasm material properties may constitute important control elements for the regulation of division positioning and cellular organization. Significance StatementThe regulation of mitotic spindle positioning is a key process for tissue architecture, embryo development and stem cells. To date, most models have assumed that spindles are positioned by forces exerted by polar cytoskeleton networks, like microtubule asters or acto-myosin bundles. Here, using in situ magnetic tweezers to apply calibrated forces and torques to mitotic spindles in live dividing sea urchin cells, we found that the viscoelastic properties of the cytoplasm medium in which spindles are embedded can hold spindles in place, and move them back if their original position is perturbed. These viscoelastic forces are large and may significantly participate in the force balance that position and orient mitotic spindles in many cell types.

cell biology↗