Fluorescence labeling of flagellar membranes

Ciliogenesis requires sphingolipid-dependent membrane and axoneme interaction

Cilium formation and regeneration requires new protein synthesis, but the underlying cytosolic translational reprogramming remains largely unknown. Using ribosome footprinting, we performed global translatome profiling during cilia regeneration in Chlamydomonas and uncovered that flagellar genes undergo an early transcriptional activation but late translational repression. This pattern guided our identification of sphingolipid metabolism enzymes, including serine palmitoyltransferase (SPT), as essential regulators for ciliogenesis. Cryo-electron tomography showed that ceramide loss abnormally increased the membrane-axoneme distance and generated bulged cilia. We found that ceramides interact with intraflagellar transport (IFT) particle proteins that IFT motors transport along axoneme microtubules (MTs), suggesting that ceramide-IFT particle-IFT motor-MT interactions connect the ciliary membrane with the axoneme to form rod-shaped cilia. SPT-deficient vertebrate cells were defective in ciliogenesis, and SPT mutations from patients with hereditary sensory neuropathy disrupted cilia, which could be restored by sphingolipid supplementation. These results reveal a conserved role of sphingolipid in cilium formation and link compromised sphingolipid production with ciliopathies.

Comparative Analysis of Ciliary Membranes and Ectosomes

Primary and motile cilia/flagella function as cellular antennae, receiving signals from the environment and subsequently activating signaling pathways that are critical for cellular homeostasis and differentiation [1-3]. Recent work with the green alga Chlamydomonas and the nematode C. elegans demonstrated that ectosomes can be released from the cilium and can mediate the intercellular communication [4-9]. To better understand the function of flagellar ectosomes, we have compared their protein composition to that of the flagellar membrane from which they are derived. Ectosomes released from flagella have a unique protein composition, being enriched in a subset of flagellar membrane proteins, proteases, proteins from the endosomal sorting complex required for transport (ESCRT) [10-12], small GTPases, and ubiquitinated proteins. Live imaging showed that an ESCRT-related protein (PDCD6) was enriched in ectosomes released from flagella during gamete activation. We devised a sensitive and rapid assay to monitor ectosome release using luciferase fused to PDCD6 and a mutated ubiquitin. Ectosome release increased when cells underwent flagellar resorption. Knockdown of two ESCRT-related proteins, PDCD6 and VPS4, attenuated ectosome release during flagellar shortening and shortening was slowed. These data suggest that the ESCRT proteins mediate ectosome release and thereby influence flagellar shortening in Chlamydomonas. In addition, the prevalence of receptors such as agglutinin and ubiquitinated proteins in ciliary ectosomes suggests that they are involved in cell signaling and turnover of ciliary proteins.

The biparental transmission of the mitochondrial genome in Chlamydomonas reinhardtii visualized in living cells

In the isogamous green alga Chlamydomonas reinhardtii, the chloroplast genome is transmitted from the mt+ parent, while the mitochondrial genes are believed to be inherited from the mt- parent. Chloroplast nucleoids have been visualized by DAPI (4,6-diamidino-2-phenylindole) staining, and the preferential digestion of the mt- chloroplast nucleoids has been observed in young zygotes. However, the mitochondrial nucleoids have never been visualized, and their behavior is only deduced from genetic and biochemical studies. We discovered that the mitochondrial and chloroplast genomes can be visualized simultaneously in living cells, using the fluorescent dye SYBR Green I. The ability to visualize the mitochondrial and chloroplast genome in vivo permits the direct observation of the number, distribution and behavior of the chloroplast and mitochondrial nucleoids in young zygotes. Using this method, the biparental transmission of the mitochondrial genome was revealed.