Testing the ion-current model for flagellar length sensing and IFT regulation

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Abstract

Eukaryotic cilia and flagella are microtubule-based organelles whose relatively simple shape makes them ideal for investigating the fundamental question of organelle size regulation. Most of the flagellar materials are transported from the cell body via an active transport process called intraflagellar transport (IFT). The rate of IFT entry into flagella, known as IFT injection, has been shown to negatively correlate with flagellar length. However, it remains unknown how the cell measures the length of its flagella and controls IFT injection. One of the most-discussed theoretical models for length sensing to control IFT is the ion-current model, which posits that there is a uniform distribution of Ca²⁺ channels along the flagellum and that the Ca²⁺ current from the flagellum into the cell body increases linearly with flagellar length. In this model, the cell uses the Ca²⁺ current to negatively regulate IFT injection. The recent discovery that IFT entry into flagella is regulated by the phosphorylation of kinesin through a calcium-dependent protein kinase has provided further impetus for the ion-current model. To test this model, we measured and manipulated the levels of Ca²⁺ inside of Chlamydomonas flagella and quantified IFT injection. Although the concentration of Ca²⁺ inside of flagella was weakly correlated with the length of flagella, we found that IFT injection was reduced in calcium-deficient flagella, rather than increased as the model predicted, and that variation in IFT injection was uncorrelated with the occurrence of flagellar Ca²⁺spikes. Thus, Ca²⁺ does not appear to function as a negative regulator of IFT injection, hence it cannot form the basis of a stable length control system.

Data availability

Modelling code has been uploaded to Github under repository https://github.com/ishikawaUCSF/temp and a link to the repository is provided in Materials and Methods

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Hiroaki Ishikawa

Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, United States

Competing interests
The authors declare that no competing interests exist.

"This ORCID iD identifies the author of this article:" 0000-0003-3984-3657
Jeremy Moore

Department of Biology, Kenyon College, Gambier, United States

Competing interests
The authors declare that no competing interests exist.
Dennis R Diener

Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany

Competing interests
The authors declare that no competing interests exist.
Markus Delling

Department of Physiology, University of California, San Francisco, San Francisco, United States

Competing interests
The authors declare that no competing interests exist.

"This ORCID iD identifies the author of this article:" 0000-0001-9556-2097
Wallace F Marshall

Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, United States

For correspondence
wallace.marshall@ucsf.edu

Competing interests
The authors declare that no competing interests exist.

"This ORCID iD identifies the author of this article:" 0000-0002-8467-5763

Funding

National Institutes of Health (R35GM130327)

Wallace F Marshall

National Institutes of Health (R01GM130908)

Markus Delling

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

Copyright

This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.