Ciliary dynein arms: Cytoplasmic preassembly, intraflagellar transport, and axonemal docking

Abstract

The microtubular scaffold of motile cilia-the axoneme, is decorated with dynein arms, which are large multiprotein complexes essential for ciliary motility. Dynein arms are arranged along the length of the axoneme in a precise repeating pattern, converting chemical energy from ATP hydrolysis into ciliary mechanical movement.How these complicated molecular machines are assembled coordinately and accurately, starting from mere polypeptide chains in the cytoplasm, remains a fascinating yet perplexing question. Rapidly emerging evidence, from multiple studies carried out with different model organisms and with various methodologies, has highlighted the existence of a dedicated assembly pathway. Here, we summarize recent progress made in clarifying the axonemal dynein arm assembly process, focusing on individual assembly steps, including cytoplasmic preassembly, intraflagellar transport, and axonemal docking. K E Y W O R D S dynein arm assembly, dynein arms, motile cilia 1 | INTRODUCTION Motile cilia are microtubule-based organelles that appear as hair-like protrusions on the cell surface and are present in diverse organisms across many evolutionary lineages. The microtubule scaffold of the cilium, the axoneme, is typically organized in a unique "9 + 2" pattern, in which a pair of central microtubules is surrounded by nine peripheral microtubule doublets (each doublet comprising a complete A and an incomplete B tubule) in a circular arrangement (Figure 1) (Ishikawa et al., 2021; Ishikawa, 2017). Importantly, a variety of biological processes, ranging from the free movement of individual cells such as protozoans like Paramecium and spermatozoa to the flow of mucus over the surface of the respiratory tract and circulation of cerebrospinal fluid within the brain ventricular cavities of mammals, all depend on the coordinated beating of motile cilia (F. Zhou & Roy, 2015).Ciliary motility is imparted by molecular motors known as outer and inner dynein arms (ODAs and IDAs, respectively), which are stably attached to the surface of the A tubule of the peripheral axonemal microtubules (Figure 1) (Ishikawa et al., 2021; Ishikawa, 2017). Dynein arms are very large, complex macromolecular assemblies that consist of multiple protein subunits (the estimated size can range from 1-to 2-MDa), overall contributing for ATP catalysis, microtubule binding activity, as well as regulatory input (King, 2016). The core catalytic subunits are comprised of dynein heavy chains (HCs), which have a length of around 4500 amino acid residues (around 500 KDa in size). Additionally, HCs associate and complex with multiple intermediate chains (ICs) and light chains (LCs), forming the dynein motors. Moreover, with the help of docking complexes (DC), dynein motors are able to stably associate with the axonemal microtubules, thereby enabling sliding motion of adjacent microtubule doublets and producing the overall oscillatory movement of the axoneme.Due to their elaborate structure, absence or mutation of even a single structural component of the dynein...

Ordered deployment of distinct ciliary beating machines in growing axonemes of vertebrate multiciliated cells

“…This series of results raises several interesting points: First, deployment of all IDA and ODA subunits examined here lagged behind Enkur, and thus behind the axoneme generally, suggesting that neither IDA nor ODA deployment is directly integrated with axoneme growth. Second, the kinetics for two ODA subunits were similar, as were those for two IDA(f) subunits, consistent with the fact that axonemal dyneins are pre-assembled in the cytoplasm and deployed to axonemes as intact motors (Qiu and Roy, 2022). Third, the divergent kinetics of ODA, IDA(f), and IDA(a,c,d) suggest those intact motors are deployed to the axoneme independently.…”

“…Finally, the distinct rates observed for IDA and ODA deployment add yet another piece to the complex puzzle surrounding the regulation of these motors (Desai et al, 2018; Qiu and Roy, 2022). Dynein arms generally are thought to be transported by IFT (Hou et al, 2007; Pazour et al, 1998; Qin et al, 2004; Viswanadha et al, 2014), and our data suggest not only that ODAs and IDAs display widely divergent rates of axonemal deployment, but also that even different IDA sub-types display subtly distinct rates.…”

Gene dosage of independent dynein arm motor preassembly factors influences cilia assembly in Chlamydomonas reinhardtii

“…Genetic analyses identified proteins required for the proper assembly of ODAs and IDAs known as dynein arm preassembly factors, or DNAAFs [32][33][34]. DNAAFs localize primarily to the cytoplasm, although there are a few exceptions [35][36][37][38].…”

AXDND1 is required to balance spermatogonial commitment and for sperm tail formation in mice and humans

“…Dynein complexes are large multi-unit assemblies that play critical roles in protein transport and cell motility (7). There are two types of dynein complexes, the cytoplasmic dyneins, which transport cellular cargoes along microtubules towards their minus end within the cell cytoplasm (7-10), and axonemal dyneins, which drive the ATPdependent beating of all motile cilia, including the sperm tail (11)(12)(13).…”