Cilia, ciliary concanavalin A-binding proteins, and mating recognition in Tetrahymena thermophila

HAP2-Mediated Gamete Fusion: Lessons From the World of Unicellular Eukaryotes

Most, if not all the cellular requirements for fertilization and sexual reproduction arose early in evolution and are retained in extant lineages of single-celled organisms including a number of important model organism species. In recent years, work in two such species, the green alga, Chlamydomonas reinhardtii, and the free-living ciliate, Tetrahymena thermophila, have lent important new insights into the role of HAP2/GCS1 as a catalyst for gamete fusion in organisms ranging from protists to flowering plants and insects. Here we summarize the current state of knowledge around how mating types from these algal and ciliate systems recognize, adhere and fuse to one another, current gaps in our understanding of HAP2-mediated gamete fusion, and opportunities for applying what we know in practical terms, especially for the control of protozoan parasites.

Primary ciliary dyskinesia relative protein ZMYND10 is involved in regulating ciliary function and intraflagellar transport in Paramecium tetraurelia

Cilia are highly conserved in most eukaryotes and are regarded as an important organelle for motility and sensation in various species. Cilia are microscopic, hair-like cytoskeletal structures that protrude from the cell surface. The major focus in studies of cilia has been concentrated on the ciliary dysfunction in vertebrates that causes multisymptomatic diseases, which together are referred to as ciliopathies. To date, the understanding of ciliopathies has largely depended on the study of ciliary structure and function in different animal models. Zinc finger MYND-type containing 10 (ZMYND10) is a ciliary protein that was recently found to be mutated in patients with primary ciliary dyskinesia (PCD). In Paramecium tetraurelia, we identified two ZMYND10 genes, arising from a whole-genome duplication. Using RNAi, we found that the depletion of ZMYND10 in P. tetraurelia causes severe ciliary defects, thus provoking swimming dysfunction and lethality. Moreover, we found that the absence of ZMYND10 caused the abnormal localization of the intraflagellar transport (IFT) protein IFT43 along cilia. These results suggest that ZMYND10 is involved in the regulation of ciliary function and IFT, which may contribute to the study of PCD pathogenesis.

Primary cilia utilize glycoprotein-dependent adhesion mechanisms to stabilize long-lasting cilia-cilia contacts

Background

The central tenet of cilia function is sensing and transmitting information. The capacity to directly contact extracellular surfaces would empower primary cilia to probe the environment for information about the nature and location of nearby surfaces. It has been well established that flagella and other motile cilia perform diverse cellular functions through adhesion. We hypothesized that mammalian primary cilia also interact with the extracellular environment through direct physical contact.

Methods

We identified cilia in rod photoreceptors and cholangiocytes in fixed mouse tissues and examined the structures that these cilia contact in vivo. We then utilized an MDCK cell culture model to characterize the nature of the contacts we observed.

Results

In retina and liver tissue, we observed that cilia from nearby cells touch one another. Using MDCK cells, we found compelling evidence that these contacts are stable adhesions that form bridges between two cells, or networks between many cells. We examined the nature and duration of the cilia-cilia contacts and discovered primary cilia movements that facilitate cilia-cilia encounters. Stable adhesions form as the area of contact expands from a single point to a stretch of tightly bound, adjacent cilia membranes. The cilia-cilia contacts persisted for hours and were resistant to several harsh treatments such as proteases and DTT. Unlike many other cell adhesion mechanisms, calcium was not required for the formation or maintenance of cilia adhesion. However, swainsonine, which blocks maturation of N-linked glycoproteins, reduced contact formation. We propose that cellular control of adhesion maintenance is active because cilia adhesion did not prevent cell division; rather, contacts dissolved during mitosis as cilia were resorbed.

Conclusions

The demonstration that mammalian primary cilia formed prolonged, direct, physical contacts supports a novel paradigm: that mammalian primary cilia detect features of the extracellular space, not just as passive antennae, but also through direct physical contact. We present a model for the cycle of glycoprotein-dependent contact formation, maintenance, and termination, and discuss the implications for potential physiological functions of cilia-cilia contacts.

A cytogenetic study of development in mechanically disrupted pairs of Tetrahymena thermophila

We examined the nuclear behavior of mating Tetrahymena cells that had been mechanically disrupted at various times throughout conjugation. Disruption was achieved by agitating conjugating Tetrahymena in the presence of 0.1-3 mm glass beads. Two minutes of agitation with 1 mm beads yielded optimal pair disruption (70%) with high viability (92%). Disrupting pairs between 0-4.7 h after the initiation of mating produced mostly disrupted conjugants in which development was aborted. However, as many as 20% of these early disrupted conjugants completed development even without their mating partners. After 5 h the percentage of disrupted conjugants completing development increased dramatically, reaching 80% by 6.7 h. These results support a model suggesting that events associated with nuclear exchange and fusion 5 h into conjugation trigger a commitment to completion of the postzygotic developmental program. The early conjugants that completed development following disruption suggest that development can be sustained even in the absence of a mating partner. This represents a novel method of bringing the micronuclear genome into macronuclear expression with minimal cytoplasmic exchange between partners. We discuss these results in light of a model relating cortical and nuclear signaling events that reciprocally drive conjugal development.

Affinity-purification of concanavalin A-binding ciliary glycoconjugates of starved and feeding Tetrahymena thermophila

Development of mating competency in Tetrahymena thermophila requires starvation for at least 70 min in low ionic strength buffer. Pair formation between conjugating cells is blocked at early stages by the lectin Concanavalin A (Con A). To investigate the role of Con A-binding proteins in this induced cellular change and pairing, and to confirm and extend an earlier study from our laboratory, a method was developed for preparation of Con A-binding proteins from ciliary membrane-rich fractions of T. thermophila. Con A-binding ciliary proteins were prepared from non-starved and starved cells from two wild type strains and a mating mutant, RH179E1. Comparison of these proteins by SDS-PAGE revealed on overall reduction in number of wild-type bands after starvation. In particular, a major band at 28 kDa was present in non-starved cells and absent in starved cells. However, in the mating mutant, no change in banding profile was seen after starvation: the 28 kDa band was present in both non-starved and starved cells. This, Con A-binding ciliary membrane proteins undergo a major change during starvation-induced development of mating competency in wild-type T. thermophila. In contrast, the mutant differed from wild-type in overall composition of its ciliary Con A-binding glycoproteins and in the response of these proteins to starvation, suggesting that it may be deficient in its ability to be initiated by starvation. Our results are consistent with the hypothesis that a change affecting ciliary membrane Con A-binding proteins is essential for the cellular response to mating signals.

Differential staining of apoptotic nuclei in living cells: application to macronuclear elimination in Tetrahymena

Acridine orange (AO) has been used as a vital fluorescent stain to identify apoptotic cells in Drosophila, but little is known about what structures are stained. We explored the specificity of AO staining while studying nuclear apoptosis in Tetrahymena. Using AO alone or together with the vital nuclear stain Hoechst 33342 (HO), we find that lysosomes are generally clustered around the degenerating nucleus and that such nuclei are stained an orange-red color, like lysosomes. Significantly, the combined dyes, more so than with AO alone, distinguish between apoptotic and normal (or necrotic) nuclei by a clear color difference. Moreover, these dyes differentially stain apoptotic and normal nuclei in avian chondrocytes. The differential staining results are nullified in fixed cells or in cytoskeletal preparations treated with RNAse. Similarly, lysosomotrophic agents eliminate the differential staining. Our results are consistent with acidification of the apoptotic nucleus, possibly by fusion with lysosomes. However, even under basic conditions, the macronucleus condenses and is eliminated, suggesting that, if the nucleus is becoming acidified, acidification by itself is not essential for nuclear elimination. The differential staining procedure may provide a useful method for specifically identifying apoptotic cells and separating them for further analysis.

Temperature regulation of the Tetrahymena mimbres glycosylphosphatidylinositol-anchored protein lipid composition

By incubating Tetrahymena mimbres cells with [3H]myristic acid, [3H]ethanolamine, [3H]inositol, and [3H]mannose, proteins having apparent molecular masses of 23 and 63 kDa were identified as the cells' principal glycosylphosphatidylinositol (GPI)-anchored proteins. These proteins accounted for as much as 2-5% of the whole cell proteins, with the higher levels being recovered from non-growing cells. The two proteins, gpi 23 and gpi 63, were purified to near homogeneity through Triton X-114/water partitioning followed by preparative SDS/PAGE. The lipid components of the GPI anchors were determined by chemical and enzymic hydrolysis. Both proteins were anchored by ceramides, with the principal long-chain base being C18 sphinganine containing an O-methyl group at the 3 position. O-Methylation was shown not to be an artifact of hydrolysis. When T. mimbres was cultured at 15 degrees C, the ceramide fatty acid component of the GPI anchors was principally palmitic acid (75% in gpi 23 and 76% in gpi 63). GPI anchors from 28 degrees C-grown cells contained mainly stearic acid (79% in gpi 23 and 70% in gpi 63). Temperature change had little effect on the long-chain-base composition. The direction of temperature-induced lipid change in the protein-bound anchors was the same as found in the inositolphosphorylceramide putative precursors of the protein anchors described in the accompanying paper [Hung, Ko and Thompson (1995) Biochem. J. 307, 107-113], but the detailed fatty acid compositions of the precursors and the protein-bound lipids were quite different. The precise metabolic regulation of anchor lipid chain length supports the concept that composition of the lipid anchor is important in the function and/or metabolism of the anchored protein.