Avalanche-like behavior in ciliary import

Area of Science:

• Cell Biology
• Biophysics
• Molecular Transport

Background:

• Cilia and flagella are essential microtubule-based organelles involved in motility and sensing.
• Intraflagellar transport (IFT) is crucial for assembling and maintaining these structures.
• The regulation of IFT protein delivery to the flagellar compartment is key to ciliogenesis, but the precise mechanisms controlling injection rates remain unclear.

Purpose of the Study:

• To investigate the regulatory mechanisms governing the rate of IFT protein injections into flagella.
• To determine if IFT injection dynamics exhibit self-organizing or avalanche-like behaviors.
• To explore potential physical mechanisms, such as the RanGTP gradient, that could sense and regulate flagellar length via IFT.

Main Methods:

• Utilized total internal reflection fluorescence microscopy (TIRFm) to observe IFT protein dynamics in live cells.
• Quantified the size, frequency, and temporal patterns of IFT injections at the flagellar base.
• Developed computational models based on avalanching principles to simulate and validate observed IFT dynamics.

Main Results:

• IFT injections occur as avalanche-like releases of accumulated IFT material.
• Injection dynamics display power-law distributions, negative correlations between size and frequency, quasiperiodicity, bursting, and long-memory effects.
• The recruitment of IFT material to the flagellar base is identified as a key regulated feature for controlling ciliary length.
• The RanGTP gradient is proposed as a theoretical flagellar length sensor regulating IFT accumulation.

Conclusions:

• IFT injection dynamics are governed by a self-organizing, avalanche-like physical mechanism.
• Flagellar length control is achieved through the regulated recruitment of IFT material, potentially sensed by the RanGTP gradient.
• This study reveals a physical basis for regulating complex intracellular transport pathways like IFT.