An In-depth Guide to the Ultrastructural Expansion Microscopy (U-ExM) of Chlamydomonas reinhardtii

Expansion microscopy is an innovative method that enables super-resolution imaging of biological materials using a simple confocal microscope. The principle of this method relies on the physical isotropic expansion of a biological specimen cross-linked to a swellable polymer, stained with antibodies, and imaged. Since its first development, several improved versions of expansion microscopy and adaptations for different types of samples have been produced. Here, we show the application of ultrastructure expansion microscopy (U-ExM) to investigate the 3D organization of the green algae Chlamydomonas reinhardtii cellular ultrastructure, with a particular emphasis on the different types of sample fixation that can be used, as well as compatible staining procedures including membranes. Graphical overview.

CCDC66 regulates primary cilium length and signaling via interactions with transition zone and axonemal proteins

The primary cilium is a microtubule-based organelle that serves as a hub for many signaling pathways. It functions as part of the centrosome or cilium complex, which also contains the basal body and the centriolar satellites. Little is known about the mechanisms by which the microtubule-based ciliary axoneme is assembled with a proper length and structure, particularly in terms of the activity of microtubule-associated proteins (MAPs) and the crosstalk between the different compartments of the centrosome or cilium complex. Here, we analyzed CCDC66, a MAP implicated in cilium biogenesis and ciliopathies. Live-cell imaging revealed that CCDC66 compartmentalizes between centrosomes, centriolar satellites, and the ciliary axoneme and tip during cilium biogenesis. CCDC66 depletion in human cells causes defects in cilium assembly, length and morphology. Notably, CCDC66 interacts with the ciliopathy-linked MAPs CEP104 and CSPP1, and regulates axonemal length and Hedgehog pathway activation. Moreover, CCDC66 is required for the basal body recruitment of transition zone proteins and intraflagellar transport B (IFT-B) machinery. Overall, our results establish CCDC66 as a multifaceted regulator of the primary cilium and provide insight into how ciliary MAPs and subcompartments cooperate to ensure assembly of functional cilia.

The ciliopathy protein CCDC66 controls mitotic progression and cytokinesis by promoting microtubule nucleation and organization

Precise spatiotemporal control of microtubule nucleation and organization is critical for faithful segregation of cytoplasmic and genetic material during cell division and signaling via the primary cilium in quiescent cells. Microtubule-associated proteins (MAPs) govern assembly, maintenance, and remodeling of diverse microtubule arrays. While a set of conserved MAPs are only active during cell division, an emerging group of MAPs acts as dual regulators in dividing and nondividing cells. Here, we elucidated the nonciliary functions and molecular mechanism of action of the ciliopathy-linked protein CCDC66, which we previously characterized as a regulator of ciliogenesis in quiescent cells. We showed that CCDC66 dynamically localizes to the centrosomes, the bipolar spindle, the spindle midzone, the central spindle, and the midbody in dividing cells and interacts with the core machinery of centrosome maturation and MAPs involved in cell division. Loss-of-function experiments revealed its functions during mitotic progression and cytokinesis. Specifically, CCDC66 depletion resulted in defective spindle assembly and orientation, kinetochore fiber stability, chromosome alignment in metaphase as well as central spindle and midbody assembly and organization in anaphase and cytokinesis. Notably, CCDC66 regulates mitotic microtubule nucleation via noncentrosomal and centrosomal pathways via recruitment of gamma-tubulin to the centrosomes and the spindle. Additionally, CCDC66 bundles microtubules in vitro and in cells by its C-terminal microtubule-binding domain. Phenotypic rescue experiments showed that the microtubule and centrosome-associated pools of CCDC66 individually or cooperatively mediate its mitotic and cytokinetic functions. Collectively, our findings identify CCDC66 as a multifaceted regulator of the nucleation and organization of the diverse mitotic and cytokinetic microtubule arrays and provide new insight into nonciliary defects that underlie ciliopathies.

The ciliopathy-linked protein CCDC66 is only known for its ciliary functions. This study reveals that CCDC66 also has extensive non-ciliary functions, localizing to the spindle poles, spindle midzone, central spindle and midbody throughout cell division, where it regulates mitosis and cytokinesis by promoting microtubule nucleation and organization.

Unraveling the mysteries of centriolar satellites: time to rewrite the textbooks about the centrosome/cilium complex

Centriolar satellites are membraneless granules that localize and move around centrosomes and cilia. Once referred to as structures with no obvious function, research in the past decade has identified satellites as key regulators of a wide range of cellular and organismal processes. Importantly, these studies have revealed a substantial overlap between functions, proteomes, and disease links of satellites with centrosomes and cilia. Therefore, satellites are now accepted as the “third component” of the vertebrate centrosome/cilium complex, which profoundly changes the way we think about the assembly, maintenance, and remodeling of the complex at the cellular and organismal levels. In this perspective, we first provide an overview of the cellular and structural complexities of centriolar satellites. We then describe the progress in the identification of the satellite interactome, which have paved the way to a molecular understanding of their mechanism of action and assembly mechanisms. After exploring current insights into their functions as recently described by loss-of-function studies and comparative evolutionary approaches, we discuss major unanswered questions regarding their functional and compositional diversity and their functions outside centrosomes and cilia.