Author ORCID Identifier

https://orcid.org/0000-0002-4178-699X

Date of Award

Spring 5-6-2025

Document Type

Thesis (Ph.D.)

Department or Program

Biological Sciences

First Advisor

Erik E. Griffin

Second Advisor

James B. Moseley

Third Advisor

Amanda A. Amodeo

Abstract

Asymmetric cell divisions are essential processes across diverse organisms that give rise to two daughter cells with different cell fates, allowing to the generation of cell diversity. During these divisions, the establishment and maintenance of cell polarity is required to control the segregation of cell fate determinants along a polarity axis. Moreover, cell polarity must be coordinated with the cell cycle to ensure the precise spatiotemporal distribution of cell determinants.

In the C. elegans one-cell embryo, PAR proteins establish anterior-posterior polarity that directs the asymmetric segregation of cytoplasmic cell fate regulators. Following symmetry breaking, anterior PAR proteins (PAR-3, PAR-6, PKC-3) and posterior PAR proteins (PAR-1, PAR-2) define distinct cortical domains. These cortical PAR domains guide the polarization of downstream cytoplasmic factors such as MEX-5, MEX-1, PIE-1, and POS-1 by locally regulating their mobility. As a result, these factors become enriched in either the anterior or posterior cytoplasm, establishing cellular asymmetries required for the cell fate specification following the first division.

Han et al. (2018) demonstrated that during the asymmetric division of the C. elegans zygote, PLK-1, in complex with MEX-5/6, phosphorylates POS-1 to inhibit its retention in the anterior cytoplasm, thereby promoting its segregation to the posterior. This PLK-1 phosphorylation-dependent mechanism is crucial for the proper asymmetric segregation of POS-1. In this dissertation, I use quantitative live-cell imaging techniques, biochemical approaches and CRISPR-Cas9 based genome editing to understand the role of PLK-1 in regulating segregation of other cytoplasmic cell fate regulators during asymmetric divisions of the early C. elegans embryo. In Chapter 2, I show that PLK-1 phosphorylates MEX-1 to inhibit its retention in the anterior cytoplasm. While PLK-1 phosphorylation is essential for POS-1 function but contributes to MEX-1 function only at an elevated temperature. I also show that the disordered proteins MEG-1/2 are specifically involved in regulating the segregation of MEX-1.

In Chapter 3, I show that PIE-1 can also serve as a substrate of PLK-1. Preliminary data suggest that PLK-1-mediated phosphorylation regulates both cytoplasmic and centrosomal PIE-1 asymmetry during the early divisions of the C. elegans embryo. Loss of PIE-1 asymmetry on the centrosomes results in abnormal enrichment of nuclear PIE-1 in a somatic blastomere and leads to increased sterility. Taken together, my dissertation work provides molecular insights into the role of PLK-1 phosphorylation in regulating the polarization of cell fate determinants during the asymmetric division of the zygote.

Original Citation

Kim AJ, Griffin EE. PLK-1 Regulation of Asymmetric Cell Division in the Early C. elegans Embryo. Front Cell Dev Biol. 2021 Jan 21;8:632253. doi: 10.3389/fcell.2020.632253. PMID: 33553173; PMCID: PMC7859328.

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