The Drosophila anterior-posterior axis is polarized by asymmetric myosin activation
Hélène Doerflinger1, Vitaly Zimyanin1, Daniel St Johnston1
1The Gurdon Institute and the Department of Genetics, University of Cambridge, Tennis Court Road, Cambridge CB2 1QN, UK.
Abstract:
The Drosophila anterior-posterior axis is specified at mid-oogenesis when the Par-1 kinase is recruited to the posterior cortex of the oocyte, where it polarizes the microtubule cytoskeleton to define where the axis determinants, bicoid and oskar mRNAs, localize. This polarity is established in response to an unknown signal from the follicle cells, but how this occurs is unclear. Here we show that the myosin chaperone Unc-45 and non-muscle myosin II (MyoII) are required upstream of Par-1 in polarity establishment. Furthermore, the myosin regulatory light chain (MRLC) is di-phosphorylated at the oocyte posterior in response to the follicle cell signal, inducing longer pulses of myosin contractility at the posterior that may increase cortical tension. Overexpression of MRLC-T21A that cannot be di-phosphorylated or treatment with the myosin light-chain kinase inhibitor ML-7 abolishes Par-1 localization, indicating that the posterior of MRLC di-phosphorylation is essential for both polarity establishment and maintenance. Thus, asymmetric myosin activation polarizes the anterior-posterior axis by recruiting and maintaining Par-1 at the posterior cortex. This raises an intriguing parallel with anterior-posterior axis formation in C. elegans, where MyoII also acts upstream of the PAR proteins to establish polarity, but to localize the anterior PAR proteins rather than Par-1.
Insights
Myosin II activation is crucial for establishing the anterior-posterior axis in Drosophila oocytes. Di-phosphorylation of myosin regulatory light chain at the posterior cortex is essential for recruiting Par-1 kinase and maintaining polarity.
Area of Science:
- Developmental biology
- Cell biology
- Molecular biology
Background:
- The anterior-posterior (A-P) axis in Drosophila oocytes is critical for development.
- Par-1 kinase recruitment to the posterior cortex initiates axis specification.
- The follicle cell signal initiating this polarity remains poorly understood.
Purpose of the Study:
- To investigate the upstream mechanisms regulating Par-1 localization and A-P axis establishment.
- To elucidate the role of myosin in polarity establishment in response to follicle cell signals.
Main Methods:
- Genetic manipulation of myosin components (Unc-45, MyoII, MRLC).
- Phosphorylation site mutagenesis of MRLC (MRLC-T21A).
- Pharmacological inhibition of myosin light-chain kinase (ML-7).
- Assessment of Par-1 localization and cortical tension.
Main Results:
- Unc-45 and non-muscle myosin II (MyoII) are required upstream of Par-1.
- Myosin regulatory light chain (MRLC) undergoes di-phosphorylation at the oocyte posterior.
- Posterior MRLC di-phosphorylation induces myosin contractility pulses, potentially increasing cortical tension.
- Inhibition of MRLC di-phosphorylation or myosin activity disrupts Par-1 localization.
Conclusions:
- Asymmetric myosin activation, driven by MRLC di-phosphorylation, is essential for recruiting and maintaining Par-1 at the posterior cortex.
- Myosin II acts upstream of Par-1 to polarize the A-P axis in Drosophila oocytes.
- This mechanism parallels MyoII's role in polarity establishment in C. elegans.
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