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

Abstract

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.

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BibTeX RIS

BibTeX

@misc{salle2026endoplasmic,
  title = {{The Endoplasmic Reticulum as a Cytoplasmic Anchor for Microtubule-Driven Aster Centration}},
  author = {J. Salle and D. Wörthmüller and J. Najafi and C. Municio-Diaz and H. Palenzuela and A. Remond and I. Mengoni and A. Guillou and R. Le Borgne and C. Pioche-Durieu and J.-M. Verbavatz and N. Joly and J.-F. Joanny and S. Dmitrieff and N. MINC},
  howpublished = {bioRxiv},
  year = {2026},
  month = oct,
  doi = {10.64898/2026.10.22.760944},
  url = {https://www.biorxiv.org/content/10.64898/2026.10.22.760944v1},
  note = {CC BY-NC-ND},
}

RIS

TY  - UNPB
TI  - The Endoplasmic Reticulum as a Cytoplasmic Anchor for Microtubule-Driven Aster Centration
AU  - J. Salle
AU  - D. Wörthmüller
AU  - J. Najafi
AU  - C. Municio-Diaz
AU  - H. Palenzuela
AU  - A. Remond
AU  - I. Mengoni
AU  - A. Guillou
AU  - R. Le Borgne
AU  - C. Pioche-Durieu
AU  - J.-M. Verbavatz
AU  - N. Joly
AU  - J.-F. Joanny
AU  - S. Dmitrieff
AU  - N. MINC
PY  - 2026
DA  - 2026/10/22
PB  - bioRxiv
DO  - 10.64898/2026.10.22.760944
UR  - https://www.biorxiv.org/content/10.64898/2026.10.22.760944v1
AB  - 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.
ER  -

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