Biology subjects

Long, A. F.

Publications and source records attributed to Long, A. F..

5 recordsLinked to original sources

bioRxiv2025

Dynamic remodeling of centrioles and the microtubule cytoskeleton in the lifecycle of chytrid fungi

Cells reorganize in space and time to move and divide - complex behaviors driven by their internal cytoskeleton. While we have substantial knowledge of the molecular parts and rules of cytoskeletal assembly, we know less about how structures are remodeled, for example to interconvert centrioles from the ciliary base to the centrosome. To study this in an evolutionary context we use the chytrid fungus, Rhizoclosmatium globosum, a member of the zoosporic fungi which have centrioles and cilia, lost in other fungal lineages. Chytrids undergo reorganization of their microtubule cytoskeleton as they cycle from zoospore to multinucleated coenocyte. We use comparative bioinformatics, RNA sequencing, and expansion microscopy to map the microtubule cytoskeleton over the chytrid lifecycle. We find that when zoospores encyst, cilia are retracted into the cytoplasm and degraded, and centrioles detach but are protected from degradation. A shortened proximal centriole then forms the mitotic centrosome and ultimately elongates to form cilia at the end of the mitotic cycles, driven by a conserved transcriptional program. Thus, structural remodeling of the chytrid centriole is coupled temporally to ciliated stages rather than mitotic cycles, which may serve as a mechanism to tune microtubule organization to meet the needs of different lifecycle stages.

cell biology↗

bioRxiv2021

ECM dimensionality tunes actin tension to modulate the endoplasmic reticulum and spheroid phenotype

Primary tissue organoids and cell spheroids recapitulate tissue physiology with remarkable fidelity. We investigated how engagement with a three dimensional laminin-rich extracellular matrix supports the polarized, stress resilient spheroid phenotype of mammary epithelial cells. Cells within a three dimensional laminin-rich extracellular matrix decreased and redistributed the actin crosslinker filamin to reduce their cortical actin tension. Cells with low cortical actin tension had increased plasma membrane protrusions that promoted negative plasma membrane curvature and fostered protein associations with the plasma membrane, consistent with efficient protein secretion. By contrast, cells engaging a laminin-rich extracellular matrix in two dimensions had high filamin-dependent cortical actin tension, exhibited compromised endoplasmic reticulum function including increased expression of PKR-like Endoplasmic Reticulum Kinase signaling effectors, and had compromised protein secretion. Cells with low filamin-mediated cortical actin tension and reduced endoplasmic reticulum stress response signaling secreted, and assembled, a polarized endogenous basement membrane and survived better, and their spheroids were more resistant to exogenous stress. The findings implicate filamin-dependent cortical actin tension in endoplasmic reticulum function and highlight a role for mechanics in organoid homeostasis.

cell biology↗

bioRxiv2019

Individual kinetochore-fibers locally dissipate force to maintain robust mammalian spindle structure

At cell division, the mammalian kinetochore binds many spindle microtubules that make up the kinetochore-fiber. To segregate chromosomes, the kinetochore-fiber must be dynamic and generate and respond to force. Yet, how it remodels under force remains poorly understood. Kinetochore-fibers cannot be reconstituted in vitro, and exerting controlled forces in vivo remains challenging. Here, we use microneedles to pull on mammalian kinetochore-fibers and probe how sustained force regulates their dynamics and structure. We show that force lengthens kinetochore-fibers by persistently favoring plus-end polymerization, not by increasing polymerization rate. We demonstrate that force suppresses depolymerization at both plus- and minus-ends, rather than sliding microtubules within the kinetochore-fiber. Finally, we observe that kinetochore-fibers break but do not detach from kinetochores or poles. Together, this work suggests an engineering principle for spindle structural homeostasis: different physical mechanisms of local force dissipation by the k-fiber limit force transmission to preserve robust spindle structure. These findings may inform how other dynamic, force-generating cellular machines achieve mechanical robustness.

cell biology↗

bioRxiv2019

Microneedle manipulation of the mammalian spindle reveals specialized, short-lived reinforcement near chromosomes

The spindle generates force to segregate chromosomes at cell division. In mammalian cells, kinetochore-fibers connect chromosomes to the spindle. The dynamic spindle anchors kinetochore-fibers in space and time to coordinate chromosome movement. Yet, how it does so remains poorly understood as we lack tools to directly challenge this anchorage. Here, we adapt microneedle manipulation to exert local forces on the spindle with spatiotemporal control. Pulling on kinetochore-fibers reveals that the spindle retains local architecture in its center on the seconds timescale. Upon pulling, sister, but not neighbor, kinetochore-fibers remain tightly coupled, restricting chromosome stretching. Further, pulled kinetochore-fibers freely pivot around poles but not around chromosomes, retaining their orientation within 3 {micro}m of chromosomes. This local reinforcement has a 20 s lifetime, and requires the microtubule crosslinker PRC1. Together, these observations indicate short-lived, specialized reinforcement of the kinetochore-fiber in the spindle center. This could help the spindle protect local structure near chromosomes from transient forces while allowing its remodeling over longer timescales, thereby supporting robust chromosome attachments and movements.

cell biology↗

bioRxiv2017

The mammalian kinetochore independently regulates its passive and active force-generating interfaces with microtubules

The kinetochore links chromosomes to dynamic spindle microtubules and drives both chromosome congression and segregation. To do so, the kinetochore must hold on to depolymerizing and polymerizing microtubules. At metaphase, one sister kinetochore couples to depolymerizing microtubules, pulling its sister along polymerizing microtubules [1,2]. Distinct kinetochore-microtubule interfaces mediate these behaviors: active interfaces transduce microtubule depolymerization into mechanical work, and passive interfaces generate friction as the kinetochore slides along microtubules [3,4]. We do not know the physical and molecular nature [5-7] of these interfaces, or how they are regulated to support diverse mitotic functions in mammalian cells. To address this question, we focus on the mechanical role of the essential load-bearing protein Hec1 [8-11]. Hec1s affinity for microtubules is regulated by Aurora B phosphorylation on its N-terminal tail [12-15], but its role at the passive and active interfaces remains unclear. Here, we use laser ablation to trigger cellular pulling on mutant kinetochores and decouple sisters in vivo, and thereby separately probe Hec1s role as it moves on polymerizing versus depolymerizing microtubules. We show that Hec1 phosphorylation tunes passive friction along polymerizing microtubules, modulating both the magnitude and timescale of responses to force. In contrast, we find that Hec1 phosphorylation does not affect the kinetochores ability to grip depolymerizing microtubules, or switch to this active force-generating state. Together, the data suggest that different kinetochore interfaces engage with growing and shrinking microtubules, and that passive friction can be regulated without disrupting active force generation. Through this mechanism, the kinetochore can modulate its grip on microtubules as its functional needs change during mitosis, and yet retain its ability to couple to microtubules powering chromosome movement.

cell biology↗