Primary cilia have a length-dependent persistence length

Microstructure-based modeling of primary cilia mechanics

A primary cilium, made of nine microtubule doublets enclosed in a cilium membrane, is a mechanosensing organelle that bends under an external mechanical load and sends an intracellular signal through transmembrane proteins activated by cilium bending. The nine microtubule doublets are the main load-bearing structural component, while the transmembrane proteins on the cilium membrane are the main sensing component. No distinction was made between these two components in all existing models, where the stress calculated from the structural component (nine microtubule doublets) was used to explain the sensing location, which may be totally misleading. For the first time, we developed a microstructure-based primary cilium model by considering these two components separately. First, we refined the analytical solution of bending an orthotropic cylindrical shell for individual microtubule, and obtained excellent agreement between finite element simulations and the theoretical predictions of a microtubule bending as a validation of the structural component in the model. Second, by integrating the cilium membrane with nine microtubule doublets and simulating the tip-anchored optical tweezer experiment on our computational model, we found that the microtubule doublets may twist significantly as the whole cilium bends. Third, besides being cilium-length-dependent, we found the mechanical properties of the cilium are also highly deformation-dependent. More important, we found that the cilium membrane near the base is not under pure in-plane tension or compression as previously thought, but has significant local bending stress. This challenges the traditional model of cilium mechanosensing, indicating that transmembrane proteins may be activated more by membrane curvature than membrane stretching. Finally, we incorporated imaging data of primary cilia into our microstructure-based cilium model, and found that comparing to the ideal model with uniform microtubule length, the imaging-informed model shows the nine microtubule doublets interact more evenly with the cilium membrane, and their contact locations can cause even higher bending curvature in the cilium membrane than near the base.

Microstructure-Based Modeling of Primary Cilia Mechanics

A primary cilium, made of nine microtubule doublets enclosed in a cilium membrane, is a mechanosensing organelle that bends under an external mechanical load and sends an intracellular signal through transmembrane proteins activated by cilium bending. The nine microtubule doublets are the main load-bearing structural component, while the transmembrane proteins on the cilium membrane are the main sensing component. No distinction was made between these two components in all existing models, where the stress calculated from the structural component (nine microtubule doublets) was used to explain the sensing location, which may be totally misleading. For the first time, we developed a microstructure-based primary cilium model by considering these two components separately. First, we refined the analytical solution of bending an orthotropic cylindrical shell for individual microtubule, and obtained excellent agreement between finite element simulations and the theoretical predictions of a microtubule bending as a validation of the structural component in the model. Second, by integrating the cilium membrane with nine microtubule doublets, we found that the microtubule doublets may twist significantly as the whole cilium bends. Third, besides being cilium-length-dependent, we found the mechanical properties of the cilium are also highly deformation-dependent. More important, we found that the cilium membrane near the base is not under pure in-plane tension or compression as previously thought, but has significant local bending stress. This challenges the traditional model of cilium mechanosensing, indicating that transmembrane proteins may be activated more by membrane curvature than membrane stretching. Finally, we incorporated imaging data of primary cilia into our microstructure-based cilium model, and found that comparing to the ideal model with uniform microtubule length, the imaging-informed model shows the nine microtubule doublets interact more evenly with the cilium membrane, and their contact locations can cause even higher bending curvature in the cilium membrane than near the base.

SIGNIFICANCE

Factors regulating the mechanical response of a primary cilium to fluid flow remain unclear. Modeling the microtubule doublet as a composite of two orthotropic shells and the ciliary axoneme as an elastic shell enclosing nine such microtubule doublets, we found that the length distribution of microtubule doublets (inferred from cryogenic electron tomography images) is the primary determining factor in the bending stiffness of primary cilia, rather than just the ciliary length. This implies ciliary-associated transmembrane proteins may be activated by membrane curvature changes rather than just membrane stretching. These insights challenge the traditional view of ciliary mechanosensation and expands our understanding of the different ways in which cells perceive and respond to mechanical stimuli.

Quantum noise may limit the mechanosensory sensitivity of cilia in the left-right organizer of the vertebrate bodyplan

Could nature be harnessing quantum mechanics in cilia to optimize the sensitivity of the mechanism of left-right symmetry breaking during development in vertebrates? I evaluate whether mechanosensing - i.e., the detection of a left-right asymmetric signal through mechanical stimulation of sensory cilia, as opposed to biochemical signalling - might be functioning in the embryonic left-right organizer of the vertebrate bodyplan through quantum mechanics. I conclude that there is a possible role for quantum biology in mechanosensing in cilia. The system may not be limited by classical thermal noise, but instead by quantum noise, with an amplification process providing active cooling.

Fluid-structure interaction modelling of neighboring tubes with primary cilium analysis

We have developed a numerical model of two osculating cylindrical elastic renal tubules to investigate the impact of neighboring tubules on the stress applied to a primary cilium. We hypothesize that the stress at the base of the primary cilium will depend on the mechanical coupling of the tubules due to local constrained motion of the tubule wall. The objective of this work was to determine the in-plane stresses of a primary cilium attached to the inner wall of one renal tubule subject to the applied pulsatile flow, with a neighboring renal tube filled with stagnant fluid in close proximity to the primary tubule. We used the commercial software COMSOL^Ⓡ to model the fluid-structure interaction of the applied flow and tubule wall, and we applied a boundary load to the face of the primary cilium during this simulation to produces a stress at its base. We confirm our hypothesis by observing that on average the in-plane stresses are greater at the base of the cilium when there is a neighboring renal tube versus if there is no neighboring tube at all. In combination with the hypothesized function of a cilium as a biological fluid flow sensor, these results indicate that flow signaling may also depend on how the tubule wall is constrained by neighboring tubules. Our results may be limited in their interpretation due to the simplified nature of our model geometry, and further improvements to the model may potentially lead to the design of future experiments.

Crosstalk between cilia and autophagy: implication for human diseases

Macroautophagy/autophagy is a self-degradative process necessary for cells to maintain their energy balance during development and in response to nutrient deprivation. Autophagic processes are tightly regulated and have been found to be dysfunctional in several pathologies. Increasing experimental evidence points to the existence of an interplay between autophagy and cilia. Cilia are microtubule-based organelles protruding from the cell surface of mammalian cells that perform a variety of motile and sensory functions and, when dysfunctional, result in disorders known as ciliopathies. Indeed, selective autophagic degradation of ciliary proteins has been shown to control ciliogenesis and, conversely, cilia have been reported to control autophagy. Moreover, a growing number of players such as lysosomal and mitochondrial proteins are emerging as actors of the cilia-autophagy interplay. However, some of the published data on the cilia-autophagy axis are contradictory and indicate that we are just starting to understand the underlying molecular mechanisms. In this review, the current knowledge about this axis and challenges are discussed, as well as the implication for ciliopathies and autophagy-associated disorders.

Microscopic artificial cilia - a review

Cilia are microscopic hair-like external cell organelles that are ubiquitously present in nature, also within the human body. They fulfill crucial biological functions: motile cilia provide transportation of fluids and cells, and immotile cilia sense shear stress and concentrations of chemical species. Inspired by nature, scientists have developed artificial cilia mimicking the functions of biological cilia, aiming at application in microfluidic devices like lab-on-chip or organ-on-chip. By actuating the artificial cilia, for example by a magnetic field, an electric field, or pneumatics, microfluidic flow can be generated and particles can be transported. Other functions that have been explored are anti-biofouling and flow sensing. We provide a critical review of the progress in artificial cilia research and development as well as an evaluation of its future potential. We cover all aspects from fabrication approaches, actuation principles, artificial cilia functions - flow generation, particle transport and flow sensing - to applications. In addition to in-depth analyses of the current state of knowledge, we provide classifications of the different approaches and quantitative comparisons of the results obtained. We conclude that artificial cilia research is very much alive, with some concepts close to industrial implementation, and other developments just starting to open novel scientific opportunities.

Signal transduction in primary cilia - analyzing and manipulating GPCR and second messenger signaling

The primary cilium projects from the surface of most vertebrate cells, where it senses extracellular signals to regulate diverse cellular processes during tissue development and homeostasis. Dysfunction of primary cilia underlies the pathogenesis of severe diseases, commonly referred to as ciliopathies. Primary cilia contain a unique protein repertoire that is distinct from the cell body and the plasma membrane, enabling the spatially controlled transduction of extracellular cues. G-protein coupled receptors (GPCRs) are key in sensing environmental stimuli that are transmitted via second messenger signaling into a cellular response. Here, we will give an overview of the role of GPCR signaling in primary cilia, and how ciliary GPCR signaling can be targeted by pharmacology, chemogenetics, and optogenetics.

Primary cilium: a paradigm for integrating mathematical modeling with experiments and numerical simulations in mechanobiology

Primary cilia are non-motile, solitary (one per cell) microtubule-based organelles that emerge from the mother centriole after cells have exited the mitotic cycle. Identified as a mechanosensing organelle that responds to both mechanical and chemical stimuli, the primary cilium provides a fertile ground for integrative investigations of mathematical modeling, numerical simulations, and experiments. Recent experimental findings revealed considerable complexity to the underlying mechanosensory mechanisms that transmit extracellular stimuli to intracellular signaling many of which include primary cilia. In this invited review, we provide a brief survey of experimental findings on primary cilia and how these results lead to various mathematical models of the mechanics of the primary cilium bent under an external forcing such as a fluid flow or a trap. Mathematical modeling of the primary cilium as a fluid-structure interaction problem highlights the importance of basal anchorage and the anisotropic moduli of the microtubules. As theoretical modeling and numerical simulations progress, along with improved state-of-the-art experiments on primary cilia, we hope that details of ciliary regulated mechano-chemical signaling dynamics in cellular physiology will be understood in the near future.