Centrin is a conserved protein that forms diverse associations with centrioles and MTOCs in Naegleria and other organisms

Novel cytoskeletal traits in the intestinal parasites (Squirmida, Platyproteum vivax) of Pacific peanut worms (Sipuncula, Phascolosoma agassizii)

The cytoskeletal organization of a squirmid, namely Platyproteum vivax, was investigated with confocal laser scanning microscopy (CLSM) to refine inferences about convergent evolution among intestinal parasites of marine invertebrates. Platyproteum inhabits Pacific peanut worms (Phascolosoma agassizii) and has traits that are similar to other lineages of myzozoan parasites, namely gregarine apicomplexans within Selenidium, such as conspicuous feeding stages, called "trophozoites," capable of dynamic undulations. SEM and CLSM of P. vivax revealed an inconspicuous flagellar apparatus and a uniform array of longitudinal microtubules organized in bundles (LMBs). Extreme flattening of the trophozoites and a consistently oblique morphology of the anterior end provided a reliable way to distinguish dorsal and ventral surfaces. CLSM revealed a novel system of microtubules oriented in the flattened dorsoventral plane. Most of these dorsoventral microtubule bundles (DVMBs) had a punctate distribution and were evenly spaced along a curved line spanning the longitudinal axis of the trophozoites. This configuration of microtubules is inferred to function in maintaining the flattened shape of the trophozoites and facilitate dynamic undulations. The novel traits in Platyproteum are consistent with phylogenomic data showing that this lineage is only distantly related to Selenidium and other marine gregarine apicomplexans with dynamic intestinal trophozoites.

The Impact of Centrosome Pathologies on Ovarian Cancer Development and Progression with a Focus on Centrosomes as Therapeutic Target

The impact of centrosome abnormalities on cancer cell proliferation has been recognized as early as 1914 (Boveri, Zur Frage der Entstehung maligner Tumoren. Jena: G. Fisher, 1914), but vigorous research on molecular levels has only recently started when it became fully apparent that centrosomes can be targeted for new cancer therapies. While best known for their microtubule-organizing capabilities as MTOC (microtubule organizing center) in interphase and mitosis, centrosomes are now further well known for a variety of different functions, some of which are related to microtubule organization and consequential activities such as cell division, migration, maintenance of cell shape, and vesicle transport powered by motor proteins, while other functions include essential roles in cell cycle regulation, metabolic activities, signal transduction, proteolytic activity, and several others that are now heavily being investigated for their role in diseases and disorders (reviewed in Schatten and Sun, Histochem Cell Biol 150:303-325, 2018; Schatten, Adv Anat Embryol Cell Biol 235:43-50, 2022a; Schatten, Adv Anat Embryol Cell Biol 235:17-35, 2022b).Cancer cell centrosomes differ from centrosomes in noncancer cells in displaying specific abnormalities that include phosphorylation abnormalities, overexpression of specific centrosomal proteins, abnormalities in centriole and centrosome duplication, formation of multipolar spindles that play a role in aneuploidy and genomic instability, and several others that are highlighted in the present review on ovarian cancer. Ovarian cancer cell centrosomes, like those in other cancers, display complex abnormalities that in part are based on the heterogeneity of cells in the cancer tissues resulting from different etiologies of individual cancer cells that will be discussed in more detail in this chapter.Because of the critical role of centrosomes in cancer cell proliferation, several lines of research are being pursued to target centrosomes for therapeutic intervention to inhibit abnormal cancer cell proliferation and control tumor progression. Specific centrosome abnormalities observed in ovarian cancer will be addressed in this chapter with a focus on targeting such aberrations for ovarian cancer-specific therapies.

Analysis of Predicted Amino Acid Sequences of Diatom Microtubule Center Components

Diatoms synthesize species-specific exoskeletons inside cells under the control of the cytoskeleton and microtubule center. Previous studies have been conducted with the visualization of the microtubule center; however, its composition has not been studied and reliably established. In the present study, several components of MTOC in diatoms, GCP (gamma complex proteins), Aurora A, and centrins have been identified. Analysis of the predicted amino acid sequences of these proteins revealed structural features typical for diatoms. We analyzed the conserved amino acids and the motives necessary for the functioning of proteins. Phylogenetic analysis of GCP showed that all major groups of diatoms are distributed over phylogenetic trees according to their systematic position. This work is a theoretical study; however, it allows drawing some conclusions about the functioning of the studied components and possible ways to regulate them.

Pericentriolar matrix (PCM) integrity relies on cenexin and polo-like kinase (PLK)1

Polo-like-kinase (PLK) 1 activity is associated with maintaining the functional and physical properties of the centrosome's pericentriolar matrix (PCM). In this study, we use a multimodal approach of human cells (HeLa), zebrafish embryos, and phylogenic analysis to test the role of a PLK1 binding protein, cenexin, in regulating the PCM. Our studies identify that cenexin is required for tempering microtubule nucleation by maintaining PCM cohesion in a PLK1-dependent manner. PCM architecture in cenexin-depleted zebrafish embryos was rescued with wild-type human cenexin, but not with a C-terminal cenexin mutant (S796A) deficient in PLK1 binding. We propose a model where cenexin's C terminus acts in a conserved manner in eukaryotes, excluding nematodes and arthropods, to sequester PLK1 that limits PCM substrate phosphorylation events required for PCM cohesion.

Moonlighting at the Poles: Non-Canonical Functions of Centrosomes

Centrosomes are best known as the microtubule organizing centers (MTOCs) of eukaryotic cells. In addition to their classic role in chromosome segregation, centrosomes play diverse roles unrelated to their MTOC activity during cell proliferation and quiescence. Metazoan centrosomes and their functional doppelgängers from lower eukaryotes, the spindle pole bodies (SPBs), act as important structural platforms that orchestrate signaling events essential for cell cycle progression, cellular responses to DNA damage, sensory reception and cell homeostasis. Here, we provide a critical overview of the unconventional and often overlooked roles of centrosomes/SPBs in the life cycle of eukaryotic cells.

Using Paramecium as a Model for Ciliopathies

Paramecium has served as a model organism for the studies of many aspects of genetics and cell biology: non-Mendelian inheritance, genome duplication, genome rearrangements, and exocytosis, to name a few. However, the large number and patterning of cilia that cover its surface have inspired extraordinary ultrastructural work. Its swimming patterns inspired exquisite electrophysiological studies that led to a description of the bioelectric control of ciliary motion. A genetic dissection of swimming behavior moved the field toward the genes and gene products underlying ciliary function. With the advent of molecular technologies, it became clear that there was not only great conservation of ciliary structure but also of the genes coding for ciliary structure and function. It is this conservation and the legacy of past research that allow us to use Paramecium as a model for cilia and ciliary diseases called ciliopathies. However, there would be no compelling reason to study Paramecium as this model if there were no new insights into cilia and ciliopathies to be gained. In this review, we present studies that we believe will do this. For example, while the literature continues to state that immotile cilia are sensory and motile cilia are not, we will provide evidence that Paramecium cilia are clearly sensory. Other examples show that while a Paramecium protein is highly conserved it takes a different interacting partner or conducts a different ion than expected. Perhaps these exceptions will provoke new ideas about mammalian systems.

Archaeal Origins of Eukaryotic Cell and Nucleus

Symbiosis is a major evolutionary force, especially at the cellular level. Here we discuss several older and new discoveries suggesting that besides mitochondria and plastids, eukaryotic nuclei also have symbiotic origins. We propose an archaea-archaea scenario for the evolutionary origin of the eukaryotic cells. We suggest that two ancient archaea-like cells, one based on the actin cytoskeleton and another one based on the tubulin-centrin cytoskeleton, merged together to form the first nucleated eukaryotic cell. This archaeal endosymbiotic origin of eukaryotic cells and their nuclei explains several features of eukaryotic cells which are incompatible with the currently preferred autogenous scenarios of eukaryogenesis.

Study on the interaction of Zea mays L. centrin and melittin

Zea mays L. centrin (Zmcen) is a 20 kDa calcium binding protein also known as caltractin.

Proteomic analysis of gametophytic sex expression in the fern Ceratopteris thalictroides

Ceratopteris thalictroides, a model fern, has two kinds of gametophytes with different sex expression: male and hermaphrodite. Hermaphroditic gametophytes have one or several archegonia beneath the growing point and a few antheridia at the base or margin. Male gametophytes show a spoon-like shape with much longer than the width and produce many antheridia at the margin and surface. The results of chlorophyll fluorescence detection showed that the photochemical efficiency of hermaphrodites was higher than that of males. By using two-dimensional electrophoresis and mass spectrometry, the differentially abundant proteins in hermaphroditic and male gametophytes were identified. A total of 1136 ± 55 protein spots were detected in Coomassie-stained gels of proteins from hermaphroditic gametophytes, and 1130 ± 65 spots were detected in gels of proteins from male gametophytes. After annotation, 33 spots representing differentially abundant proteins were identified. Among these, proteins involved in photosynthesis and chaperone proteins were over-represented in hermaphrodites, whereas several proteins involved in metabolism were increased in male gametophytes in order to maintain their development under relatively nutritionally deficient conditions. Furthermore, the differentially abundant cytoskeletal proteins detected in this study, such as centrin and actin, may be involved in the formation of sexual organs and are directly related to sex expression. These differentially abundant proteins are important for maintaining the development of gametophytes of different sexes in C. thalictroides.

Functions and dysfunctions of the mammalian centrosome in health, disorders, disease, and aging

Since its discovery well over 100 years ago (Flemming, in Sitzungsber Akad Wissensch Wien 71:81-147, 1875; Van Beneden, in Bull Acad R Belg 42:35-97, 1876) the centrosome is increasingly being recognized as a most impactful organelle for its role not only as primary microtubule organizing center (MTOC) but also as a major communication center for signal transduction pathways and as a center for proteolytic activities. Its significance for cell cycle regulation has been well studied and we now also know that centrosome dysfunctions are implicated in numerous diseases and disorders including cancer, Alstrom syndrome, Bardet-Biedl syndrome, Huntington's disease, reproductive disorders, and several other diseases and disorders. The present review is meant to build on information presented in the previous review (Schatten, in Histochem Cell Biol 129:667-686, 2008) and to highlight functions of the mammalian centrosome in health, and dysfunctions in disorders, disease, and aging with six sections focused on (1) centrosome structure and functions, and new insights into the role of centrosomes in cell cycle progression; (2) the role of centrosomes in tumor initiation and progression; (3) primary cilia, centrosome-primary cilia interactions, and consequences for cell cycle functions in health and disease; (4) transitions from centrosome to non-centrosome functions during cellular polarization; (5) other centrosome dysfunctions associated with the pathogenesis of human disease; and (6) centrosome functions in oocyte germ cells and dysfunctions in reproductive disorders and reproductive aging.

Evolutionary Cell Biology of Proteins from Protists to Humans and Plants

During evolution, the cell as a fine-tuned machine had to undergo permanent adjustments to match changes in its environment, while "closed for repair work" was not possible. Evolution from protists (protozoa and unicellular algae) to multicellular organisms may have occurred in basically two lineages, Unikonta and Bikonta, culminating in mammals and angiosperms (flowering plants), respectively. Unicellular models for unikont evolution are myxamoebae (Dictyostelium) and increasingly also choanoflagellates, whereas for bikonts, ciliates are preferred models. Information accumulating from combined molecular database search and experimental verification allows new insights into evolutionary diversification and maintenance of genes/proteins from protozoa on, eventually with orthologs in bacteria. However, proteins have rarely been followed up systematically for maintenance or change of function or intracellular localization, acquirement of new domains, partial deletion (e.g. of subunits), and refunctionalization, etc. These aspects are discussed in this review, envisaging "evolutionary cell biology." Protozoan heritage is found for most important cellular structures and functions up to humans and flowering plants. Examples discussed include refunctionalization of voltage-dependent Ca²⁺ channels in cilia and replacement by other types during evolution. Altogether components serving Ca²⁺ signaling are very flexible throughout evolution, calmodulin being a most conservative example, in contrast to calcineurin whose catalytic subunit is lost in plants, whereas both subunits are maintained up to mammals for complex functions (immune defense and learning). Domain structure of R-type SNAREs differs in mono- and bikonta, as do Ca²⁺ -dependent protein kinases. Unprecedented selective expansion of the subunit a which connects multimeric base piece and head parts (V0, V1) of H⁺ -ATPase/pump may well reflect the intriguing vesicle trafficking system in ciliates, specifically in Paramecium. One of the most flexible proteins is centrin when its intracellular localization and function throughout evolution is traced. There are many more examples documenting evolutionary flexibility of translation products depending on requirements and potential for implantation within the actual cellular context at different levels of evolution. From estimates of gene and protein numbers per organism, it appears that much of the basic inventory of protozoan precursors could be transmitted to highest eukaryotic levels, with some losses and also with important additional "inventions."

Biology and pathogenesis of Naegleria fowleri

Naegleria fowleri is a protist pathogen that can cause lethal brain infection. Despite decades of research, the mortality rate related with primary amoebic meningoencephalitis owing to N. fowleri remains more than 90%. The amoebae pass through the nose to enter the central nervous system killing the host within days, making it one of the deadliest opportunistic parasites. Accordingly, we present an up to date review of the biology and pathogenesis of N. fowleri and discuss needs for future research against this fatal infection.

The sporulation of the green alga Ulva prolifera is controlled by changes in photosynthetic electron transport chain

Sporulation and spore release are essential phases of the life cycle in algae and land plants. Ulva prolifera, which is an ideal organism for studying sporulation and spore release, was used as the experimental material in the present study. The determination of photosynthetic parameters, combined with microscopic observation, treatment with photosynthetic inhibitors, limitation of carbon acquisition, and protein mass spectrometry, was employed in this experiment. Cycle electron transport (CEF) was found enhanced at the onset of sporangia formation. The inhibition effect of dibromothymoquinone (DBMIB) towards sporulation was always strong during the sporulation process whereas the inhibition effect of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) was continuously declined accompanied with the progress of sporulation. The changes of photosynthesis resulted from the limitation of CO₂ acquisition could stimulate sporulation onset. Quantitative protein analysis showed that enzymes involved in carbon fixation, including RUBISCO and pyruvate orthophosphate dikinase, declined during sporogenesis, while proteins involved in sporulation, including tubulin and centrin, increased. These results suggest that enhanced cyclic electron flow (CEF) and oxidation of the plastoquinone pool are essential for sporangia formation onset, and changes in photosynthetic electron transport chain have significant impacts on sporulation of the green algae.

Naegleria: a classic model for de novo basal body assembly

The amoeboflagellate Naegleria was one of the first organisms in which de novo basal body/centriole assembly was documented. When in its flagellate form, this single-celled protist has two flagella that are templated by two basal bodies. Each of these basal bodies is structurally well conserved, with triplet microtubules and well-defined proximal cartwheel structures, similar to most other eukaryotic centrioles. The basal bodies are anchored to the nucleus by a single, long striated rootlet. The Naegleria genome encodes many conserved basal body genes whose expression is induced prior to basal body assembly. Because of the rapid and synchronous differentiation from centriole-less amoebae to temporary flagellates with basal bodies, Naegleria offers one of the most promising systems to study de novo basal body assembly, as well as the mechanisms regulating the number of centrioles assembled per cell.

Rapid centriole assembly in Naegleria reveals conserved roles for both de novo and mentored assembly

Centrioles are eukaryotic organelles whose number and position are critical for cilia formation and mitosis. Many cell types assemble new centrioles next to existing ones ("templated" or mentored assembly). Under certain conditions, centrioles also form without pre-existing centrioles (de novo). The synchronous differentiation of Naegleria amoebae to flagellates represents a unique opportunity to study centriole assembly, as nearly 100% of the population transitions from having no centrioles to having two within minutes. Here, we find that Naegleria forms its first centriole de novo, immediately followed by mentored assembly of the second. We also find both de novo and mentored assembly distributed among all major eukaryote lineages. We therefore propose that both modes are ancestral and have been conserved because they serve complementary roles, with de novo assembly as the default when no pre-existing centriole is available, and mentored assembly allowing precise regulation of number, timing, and location of centriole assembly.

The centrosome: a prospective entrant in cancer therapy

INTRODUCTION

The centrosome plays an essential role in the cell cycle. The centrosome and its associated proteins assist in nucleating and organizing microtubules. A structural or a functional aberration in the centrosome is known to cause abnormal cell proliferation leading to tumors. Therefore, the centrosome is considered as a promising anti-cancer target.

AREAS COVERED

This review begins with a brief introduction to the centrosome and its role in the cell cycle. We elaborate on the centrosome-associated proteins that regulate microtubule dynamics. In addition, we discuss the centrosomal protein kinase targets such as cyclin-dependent, polo-like and aurora kinases. Inhibitors targeting these kinases are undergoing clinical trials for cancer chemotherapy. Further, we shed light on new approaches to target the centrosomal proteins for cancer therapy.

EXPERT OPINION

Insights into the functioning of the centrosomal proteins will be extremely beneficial in validating the centrosome as a target in cancer therapy. New strategies either as a single entity or in combination with current chemotherapeutic agents should be researched or exploited to reveal the promises that the centrosome holds for future cancer therapy.

Posttranscriptional control over rapid development and ciliogenesis in Marsilea

Marsilea vestita is a semiaquatic fern that produces its spores (meiotic products) as it undergoes a process of natural desiccation. During the period of desiccation, the spores mature, and produce large quantities of pre-mRNA, which is partially processed and stored in nuclear speckles and can remain stable during a period of extended quiescence in the dry spore. Rehydration of the spores initiates a highly coordinated developmental program, featuring nine successive mitotic division cycles that occur at precise times and in precise planes within the spore wall to produce 39 cells, 32 of which are spermatids. The spermatids then undergo de novo basal body formation, the assembly of a massive cytoskeleton, nuclear and cell elongation, and finally ciliogenesis, before being released from the spore wall. The entire developmental program requires only 11 h to reach completion, and is synchronous in a population of spores rehydrated at the same time. Rapid development in this endosporic gametophyte is controlled posttranscriptionally, where stored pre-mRNAs, many of which are intron-retaining transcripts, are unmasked, processed, and translated under tight spatial and temporal control. Here, we describe posttranscriptional mechanisms that exert temporal and spatial control over this developmental program, which culminates in the production of ∼140 ciliary axonemes in each spermatozoid.

Such small hands: the roles of centrins/caltractins in the centriole and in genome maintenance

Centrins are small, highly conserved members of the EF-hand superfamily of calcium-binding proteins that are found throughout eukaryotes. They play a major role in ensuring the duplication and appropriate functioning of the ciliary basal bodies in ciliated cells. They have also been localised to the centrosome, which is the major microtubule organising centre in animal somatic cells. We describe the identification, cloning and characterisation of centrins in multiple eukaryotic species. Although centrins have been implicated in centriole biogenesis, recent results have indicated that centrosome duplication can, in fact, occur in the absence of centrins. We discuss these data and the non-centrosomal functions that are emerging for the centrins. In particular, we discuss the involvement of centrins in nucleotide excision repair, a process that repairs the DNA lesions that are induced primarily by ultraviolet irradiation. We discuss how centrin may be involved in these diverse processes and contribute to nuclear and cytoplasmic events.

Centrins in unicellular organisms: functional diversity and specialization

Centrins (also known as caltractins) are conserved, EF hand-containing proteins ubiquitously found in eukaryotes. Similar to calmodulins, the calcium-binding EF hands in centrins fold into two structurally similar domains separated by an alpha-helical linker region, shaping like a dumbbell. The small size (15-22 kDa) and domain organization of centrins and their functional diversity/specialization make them an ideal system to study protein structure-function relationship. Here, we review the work on centrins with a focus on their structures and functions characterized in unicellular organisms.

The Sperm Centrosome: Its Role and Significance in Nature and Human Assisted Reproduction

In humans and other non-rodent mammalian species, the sperm's centriole-centrosome complex is an essential component for successful fertilization and serves as template for all centrioles during subsequent cell divisions, embryo development, divisions of most adult somatic cells, as well as in primary cilia formation and functions. Dysfunctions of this complex can be causes for infertility, developmental disorders, and play a role in various adulthood diseases. While assisted reproductive technology (ART) has been able to overcome sperm motility dysfunctions by employing intracytoplasmic sperm injection (ICSI), we currently do not yet have therapies to overcome dysfunctions of the centriole-centrosome complex although several lines of investigations have addressed the causes for centriole-centrosome dysfunctions and implications for sperm aster formation and union of the parental genomes. The present review highlights the importance of the centriole-centrosome complex and its significance for fertilization and embryo development.

Extremes in rapid cellular morphogenesis: post-transcriptional regulation of spermatogenesis in Marsilea vestita

The endosporic male gametophyte of the water fern, Marsilea vestita, provides a unique opportunity to study the mechanisms that control cell fate determination during a burst of rapid development. In this review, we show how the spatial and temporal control of development in this simple gametophyte involves several distinct modes of RNA processing that allow the translation of specific mRNAs at distinct stages during gametogenesis. During the early part of development, nine successive cell division cycles occur in precise planes within a closed volume to produce seven sterile cells and 32 spermatids. There is no cell movement in the gametophyte; so, cell position and size within the spore wall define cell fate. After the division cycles have been completed, the spermatids become sites for the de novo formation of basal bodies, for the assembly of a complex cytoskeleton, for nuclear and cell elongation, and for ciliogenesis. In contrast, the adjacent sterile cells exhibit none of these changes. The spermatids differentiate into multiciliated, corkscrew-shaped gametes that resemble no other cells in the entire plant. Development is controlled post-transcriptionally. The transcripts stored in the microspore are released (unmasked) in the gametophyte at different times during development. At the start of these studies, we identified several key mRNAs that undergo translation at specific stages of gametophyte development. We developed RNA silencing protocols that enabled us to block the translation of these proteins and thereby establish their necessity and sufficiency for the completion of specific stages of gametogenesis. In addition, RNAi enabled us to identify additional proteins that are essential for other phases of development. Since the distributions of mRNAs and the proteins they encode are not identical in the gametophyte, transcript processing is apparently important in allowing translation to occur under strict temporal and spatial control. Transcript polyadenylation occurs in the spermatogenous cells in ways that match the translation of specific mRNAs. We have found that the exon junction complex plays key roles in transcript regulation and modifications that underlie cell specification in the gametophyte. We have recently become interested in the mechanisms that control the unmasking of the stored transcripts and have linked the synthesis and redistribution of spermidine in the gametophyte to the control of mRNA release from storage during early development and later to basal body formation, cytoskeletal assembly, and nuclear and cell elongation in the differentiating spermatids.

Naegleria gruberi de novo basal body assembly occurs via stepwise incorporation of conserved proteins

Centrioles and basal bodies are discrete structures composed of a cylinder of nine microtubule triplets and associated proteins. Metazoan centrioles can be found at mitotic spindle poles and are called basal bodies when used to organize microtubules to form the core structure of flagella. Naegleria gruberi, a unicellular eukaryote, grows as an amoeba that lacks a cytoplasmic microtubule cytoskeleton. When stressed, Naegleria rapidly (and synchronously) differentiates into a flagellate, forming a complete cytoplasmic cytoskeleton de novo, including two basal bodies and flagella. Here, we show that Naegleria has genes encoding conserved centriole proteins. Using novel antibodies, we describe the localization of three centrosomal protein homologs (SAS-6, gamma-tubulin, and centrin-1) during the assembly of the flagellate microtubule cytoskeleton. We also used these antibodies to show that Naegleria expresses the proteins in the same order as their incorporation into basal bodies, with SAS-6 localizing first, followed by centrin and finally gamma-tubulin. The similarities between basal body assembly in Naegleria and centriole assembly in animals indicate that mechanisms of assembly, as well as structure, have been conserved throughout eukaryotic evolution.

Transcriptome analysis of functional differentiation between haploid and diploid cells of Emiliania huxleyi, a globally significant photosynthetic calcifying cell

BACKGROUND

Eukaryotes are classified as either haplontic, diplontic, or haplo-diplontic, depending on which ploidy levels undergo mitotic cell division in the life cycle. Emiliania huxleyi is one of the most abundant phytoplankton species in the ocean, playing an important role in global carbon fluxes, and represents haptophytes, an enigmatic group of unicellular organisms that diverged early in eukaryotic evolution. This species is haplo-diplontic. Little is known about the haploid cells, but they have been hypothesized to allow persistence of the species between the yearly blooms of diploid cells. We sequenced over 38,000 expressed sequence tags from haploid and diploid E. huxleyi normalized cDNA libraries to identify genes involved in important processes specific to each life phase (2N calcification or 1N motility), and to better understand the haploid phase of this prominent haplo-diplontic organism.

RESULTS

The haploid and diploid transcriptomes showed a dramatic differentiation, with approximately 20% greater transcriptome richness in diploid cells than in haploid cells and only

CONCLUSIONS

This study permitted the identification of genes likely involved in diploid-specific biomineralization, haploid-specific motility, and transcriptional control. Greater transcriptome richness in diploid cells suggests they may be more versatile for exploiting a diversity of rich environments whereas haploid cells are intrinsically more streamlined.

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MESH Headings

• Cell Cycle/genetics
• Cluster Analysis
• Diploidy
• Expressed Sequence Tags
• Flagella/genetics
• Flow Cytometry
• Gene Expression Profiling
• Gene Expression Regulation
• Gene Library
• Haploidy
• Histones/genetics
• Histones/metabolism
• Membrane Transport Proteins/genetics
• Membrane Transport Proteins/metabolism
• Photosynthesis/genetics
• Phytoplankton/cytology
• Phytoplankton/genetics
• Phytoplankton/growth & development
• Reproducibility of Results
• Reverse Transcriptase Polymerase Chain Reaction
• Sequence Homology, Nucleic Acid
• Time Factors

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Affiliation(s)

Peter von Dassow Evolution du Plancton et PaleOceans, Station Biologique de Roscoff, CNRS UPMC UMR7144, 29682 Roscoff, France
Hiroyuki Ogata Information Génomique et Structurale, CNRS - UPR2589, Institut de Microbiologie de la Méditerranée, Parc Scientifique de Luminy - 163 Avenue de Luminy - Case 934, FR- 13288, Marseille cedex 09, France
Ian Probert Evolution du Plancton et PaleOceans, Station Biologique de Roscoff, CNRS UPMC UMR7144, 29682 Roscoff, France
Patrick Wincker Genoscope, 2 Rue Gaston Crémieux, 91057 Evry, France
Corinne Da Silva Genoscope, 2 Rue Gaston Crémieux, 91057 Evry, France
Stéphane Audic Information Génomique et Structurale, CNRS - UPR2589, Institut de Microbiologie de la Méditerranée, Parc Scientifique de Luminy - 163 Avenue de Luminy - Case 934, FR- 13288, Marseille cedex 09, France
Jean-Michel Claverie Information Génomique et Structurale, CNRS - UPR2589, Institut de Microbiologie de la Méditerranée, Parc Scientifique de Luminy - 163 Avenue de Luminy - Case 934, FR- 13288, Marseille cedex 09, France
Colomban de Vargas Evolution du Plancton et PaleOceans, Station Biologique de Roscoff, CNRS UPMC UMR7144, 29682 Roscoff, France

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The role of centrosomes in mammalian fertilization and its significance for ICSI

Centrosome integrity is critically important for successful fertilization and embryo development. In humans, the sperm contributes the dominant centrosomal material containing centrioles and centrosomal components onto which oocyte centrosomal proteins assemble after sperm incorporation to form the sperm aster that is essential for uniting sperm and oocyte pronuclei. Increasingly, dysfunctional sperm centrosomes have been identified as a factor for sperm-derived infertility and heterologous Intracytoplasmic Sperm Injection (ICSI) has been used to assess centrosome and sperm aster formation and clearly established a relationship between infertility and sperm centrosomal dysfunction. ICSI has been used successfully to provide novel treatment to overcome male factor infertility and it may open up new possibilities to correct specific sperm-related centrosome dysfunctions at molecular levels. New data indicate that it is now possible to replace dysfunctional centrosomes with functional donor sperm centrosomes which may provide new treatment for couples in which infertility is a result of centrosome-related sperm dysfunctions.

hPOC5 is a centrin-binding protein required for assembly of full-length centrioles

Centrin has been shown to be involved in centrosome biogenesis in a variety of eukaryotes. In this study, we characterize hPOC5, a conserved centrin-binding protein that contains Sfi1p-like repeats. hPOC5 is localized, like centrin, in the distal portion of human centrioles. hPOC5 recruitment to procentrioles occurs during G2/M, a process that continues up to the full maturation of the centriole during the next cell cycle and is correlated with hyperphosphorylation of the protein. In the absence of hPOC5, RPE1 cells arrest in G1 phase, whereas HeLa cells show an extended S phase followed by cell death. We show that hPOC5 is not required for the initiation of procentriole assembly but is essential for building the distal half of centrioles. Interestingly, the hPOC5 family reveals an evolutionary divergence between vertebrates and organisms like Drosophila melanogaster or Caenorhabditis elegans, in which the loss of hPOC5 may correlate with the conspicuous differences in centriolar structure.

Dictyostelium discoideum CenB is a bona fide centrin essential for nuclear architecture and centrosome stability

Centrins are a family of proteins within the calcium-binding EF-hand superfamily. In addition to their archetypical role at the microtubule organizing center (MTOC), centrins have acquired multiple functionalities throughout the course of evolution. For example, centrins have been linked to different nuclear activities, including mRNA export and DNA repair. Dictyostelium discoideum centrin B is a divergent member of the centrin family. At the amino acid level, DdCenB shows 51% identity with its closest relative and only paralog, DdCenA. Phylogenetic analysis revealed that DdCenB and DdCenA form a well-supported monophyletic and divergent group within the centrin family of proteins. Interestingly, fluorescently tagged versions of DdCenB were not found at the centrosome (in whole cells or in isolated centrosomes). Instead, DdCenB localized to the nuclei of interphase cells. This localization disappeared as the cells entered mitosis, although Dictyostelium cells undergo a closed mitosis in which the nuclear envelope (NE) does not break down. DdCenB knockout cells exhibited aberrant nuclear architecture, characterized by enlarged and deformed nuclei and loss of proper centrosome-nucleus anchoring (observed as NE protrusions). At the centrosome, loss of DdCenB resulted in defects in the organization and morphology of the MTOC and supernumerary centrosomes and centrosome-related bodies. The multiple defects that the loss of DdCenB generated at the centrosome can be explained by its atypical division cycle, transitioning into the NE as it divides at mitosis. On the basis of these findings, we propose that DdCenB is required at interphase to maintain proper nuclear architecture, and before delocalizing from the nucleus, DdCenB is part of the centrosome duplication machinery.

The mammalian centrosome and its functional significance

Primarily known for its role as major microtubule organizing center, the centrosome is increasingly being recognized for its functional significance in key cell cycle regulating events. We are now at the beginning of understanding the centrosome’s functional complexities and its major impact on directing complex interactions and signal transduction cascades important for cell cycle regulation. The centrosome orchestrates entry into mitosis, anaphase onset, cytokinesis, G1/S transition, and monitors DNA damage. Recently, the centrosome has also been recognized as major docking station where regulatory complexes accumulate including kinases and phosphatases as well as numerous other cell cycle regulators that utilize the centrosome as platform to coordinate multiple cell cycle-specific functions. Vesicles that are translocated along microtubules to and away from centrosomes may also carry enzymes or substrates that use centrosomes as main docking station. The centrosome’s role in various diseases has been recognized and a wealth of data has been accumulated linking dysfunctional centrosomes to cancer, Alstrom syndrome, various neurological disorders, and others. Centrosome abnormalities and dysfunctions have been associated with several types of infertility. The present review highlights the centrosome’s significant roles in cell cycle events in somatic and reproductive cells and discusses centrosome abnormalities and implications in disease.

Temperature-shock induction of multiple flagella induces additional synthesis of flagellum specific mRNAs and tubulin

Four mRNAs (alpha- and beta-tubulin, flagellar calmodulin and Class-I), specifically expressed when Naegleria amebae differentiate into flagellates, were followed at 5-10 min intervals during the temperature-shock induction of multiple flagella in order to better understand how basal body and flagellum number are regulated. Surprisingly, tubulin synthesis continued during the 37 min temperature shock. An initial rapid decline in alpha- and beta-tubulin and flagellar calmodulin mRNAs was followed by a rapid re-accumulation of mRNAs before the temperature was lowered. mRNA levels continued to increase until they exceeded control levels by 4-21%. Temperature shock delayed flagella formation 37 min, produced twice as much tubulin protein synthesis and three fold more flagella. Labeling with an antibody against Naegleria centrin suggested that basal body formation was also delayed 30-40 min. An extended temperature shock demonstrated that lowering the temperature was not required for return of mRNAs to near control levels suggesting that induction of multiple flagella and the formation of flagella per se are affected in different ways. We suggest that temperature-shock induction of multiple flagella reflects increased mRNA accumulation combined with interference with the regulation of the recently reported microtubule-nucleating complex needed for basal body formation.

Centrins in retinal photoreceptor cells: regulators in the connecting cilium

Changes in the intracellular Ca2+ concentration regulate the visual signal transduction cascade directly or more often indirectly through Ca2+-binding proteins. Here we focus on centrins, which are members of a highly conserved subgroup of the EF-hand superfamily of Ca2+-binding proteins in photoreceptor cells of the vertebrate retina. Centrins are commonly associated with centrosome-related structures. In mammalian retinal photoreceptor cells, four centrin isoforms are expressed as prominent components in the connecting cilium linking the light-sensitive outer segment compartment with the metabolically active inner segment compartment. Our data indicate that Ca2+-activated centrin isoforms assemble into protein complexes with the visual heterotrimeric G-protein transducin. This interaction of centrins with transducin is mediated by binding to the betagamma-dimer of the heterotrimeric G-protein. More recent findings show that these interactions of centrins with transducin are reciprocally regulated via site-specific phosphorylations mediated by the protein kinase CK2. The assembly of centrin/G-protein complexes is a novel aspect of translocation regulation of signalling proteins in sensory cells, and represents a potential link between molecular trafficking and signal transduction in general.

Functional diversification of centrins and cell morphological complexity

In addition to their key role in the duplication of microtubule organising centres (MTOCs), centrins are major constituents of diverse MTOC-associated contractile arrays. A centrin partner, Sfi1p, has been characterised in yeast as a large protein carrying multiple centrin-binding sites, suggesting a model for centrin-mediated Ca2+-induced contractility and for the duplication of MTOCs. In vivo validation of this model has been obtained in Paramecium, which possesses an extended contractile array - the infraciliary lattice (ICL) - essentially composed of centrins and a huge Sfi1p-like protein, PtCenBP1p, which is essential for ICL assembly and contractility. The high molecular diversity revealed here by the proteomic analysis of the ICL, including ten subfamilies of centrins and two subfamilies of Sf1p-like proteins, led us to address the question of the functional redundancy, either between the centrin-binding proteins or between the centrin subfamilies. We show that all are essential for ICL biogenesis. The two centrin-binding protein subfamilies and nine of the centrin subfamilies are ICL specific and play a role in its molecular and supramolecular architecture. The tenth and most conserved centrin subfamily is present at three cortical locations (ICL, basal bodies and contractile vacuole pores) and might play a role in coordinating duplication and positioning of cortical organelles.

Molecular characterization of centriole assembly in ciliated epithelial cells

Ciliated epithelial cells have the unique ability to generate hundreds of centrioles during differentiation. We used centrosomal proteins as molecular markers in cultured mouse tracheal epithelial cells to understand this process. Most centrosomal proteins were up-regulated early in ciliogenesis, initially appearing in cytoplasmic foci and then incorporated into centrioles. Three candidate proteins were further characterized. The centrosomal component SAS-6 localized to basal bodies and the proximal region of the ciliary axoneme, and depletion of SAS-6 prevented centriole assembly. The intraflagellar transport component polaris localized to nascent centrioles before incorporation into cilia, and depletion of polaris blocked axoneme formation. The centriolar satellite component PCM-1 colocalized with centrosomal components in cytoplasmic granules surrounding nascent centrioles. Interfering with PCM-1 reduced the amount of centrosomal proteins at basal bodies but did not prevent centriole assembly. This system will help determine the mechanism of centriole formation in mammalian cells and how the limitation on centriole duplication is overcome in ciliated epithelial cells.

Reorganization of centrosomal marker proteins coincides with epithelial cell differentiation in the vertebrate lens

The differentiation of epithelial cells in the vertebrate lens involves a series of changes that includes the degradation of all intracellular organelles and a dramatic elongation of the cells. The latter is accompanied by a substantial remodelling of the cytoskeleton and changes in the distribution of the actin, microtubule and intermediate filament cytoskeletons during lens cell differentiation have been well documented. There have, however, been no studies of microtubule organizing centres (MTOCs) and specifically centrosomes during lens cell differentiation. We have investigated the fate of the centrosomal MTOCs during cellular differentiation in the bovine lens using gamma-tubulin, ninein, centrin 2 and centrin 3 as markers. Our studies show that these markers oscillate between a clear centrosome-based association in epithelial cells and a defocused cluster in lens fibre cells. Our data further reveal a transient loss of signal for the typical centrosomal marker gamma-tubulin as the lens epithelial cells begin to differentiate into lens fibre cells. This marker apparently disappears in the most distal epithelial cells at the lens equator, only to reappear in early lens fibre cells. The changes in gamma-tubulin distribution are mirrored by the other centrosomal markers, centrins 2 and 3 and ninein that also show a similar transient loss of their signals and subsequent clustering at the apical ends of differentiating fibre cells. The transient loss of staining for these centrosomal markers in the most posterior epithelial cells is a distinctive feature that precedes lens cell elongation. The dramatic reorganization of MTOC markers coincides with gap junction reorganization as seen by the loss of connexin 43 (alpha1-connexin) in these lens epithelial cells suggesting that these events mark a significant change preceding subsequent cell elongation and differentiation into fibre cells.

High sensitivity of human centrin 2 toward radiolytical oxidation: C-terminal tyrosinyl residue as the main target

Centrins are calcium-binding proteins that play a significant role in the maintenance of the centrosomal organization, mainly in the continuity between centrosome and microtubular network. Recent data showed that centrosome duplication abnormalities, like overduplication for example, could be due to hydrogen peroxide, suggesting an important impact of oxidative stress. To challenge this hypothesis, we performed one-electron oxidation experiments with human centrin 2, starting from azide radicals. Our results first revealed several intermolecular cross-links generating dimers, tetramers, hexamers, and higher molecular mass species. Dimers result from covalent bond linking the C-terminal tyrosines of each monomer. Second, the methionyl residue at position 19 was oxidized on the monomeric centrin. Further, electron microscopy experiments on centrin 2 showed a preexisting hexameric organization that was stabilized by covalent bonds as a result of irradiation. Overall, these results show that centrin 2 is highly sensitive to ionizing radiation, which could have important consequences on its biological functions.

Centrosomal protein centrin is not detectable during early pre-implantation development but reappears during late blastocyst stage in porcine embryos

Centrin is an evolutionarily conserved 20 kDa, Ca+2-binding, calmodulin-related protein associated with centrioles and basal bodies of phylogenetically diverse eukaryotic cells. Earlier studies have shown that residual centrosomes of non-rodent mammalian spermatozoa retain centrin and, in theory, could contribute this protein for the reconstruction of the zygotic centrosome after fertilization. The present work shows that CEN2 and CEN3 mRNA were detected in germinal vesicle-stage (GV) oocytes, MII oocytes, and pre-implantation embryos from the two-cell through the blastocyst stage, but not in spermatozoa. Boar ejaculated spermatozoa possess centrin as revealed by immunofluorescence microscopy and western blotting. Immature, GV oocytes possess speckles of centrin particles in the perinuclear area, visualized by immunofluorescence microscopy and exhibit a 19 kDa band revealed by western blotting. Mature MII stage oocytes lacked centrin that could be detected by immunofluorescence or western blotting. The sperm centrin was lost in zygotes after in vitro fertilization. It was not detectable in embryos by immunofluorescence microscopy until the late blastocyst stage. Embryonic centrin first appeared as fine speckles in the perinuclear area of some interphase blastocyst cells and as putative centrosomes of the spindle poles of dividing cells. The cells of the hatched blastocysts developed centrin spots comparable with those of the cultured cells. Some blastomeres displayed undefined curved plate-like centrin-labeled structures. Anti-centrin antibody labeled interphase centrosomes of cultured pig embryonic fibroblast cells as distinct spots in the juxtanuclear area. Enucleated pig oocytes reconstructed by electrofusion with pig fibroblasts displayed centrin of the donor cell during the early stages of nuclear decondensation but became undetectable in the late pronuclear or cleavage stages. These observations suggest that porcine zygotes and pre-blastocyst embryonic cells lack centrin and do not retain exogenously incorporated centrin. The early embryonic centrosomes function without centrin. Centrin in the blastocyst stage embryos is likely a result of de novo synthesis at the onset of differentiation of the pluripotent blastomeres.

Three-dimensional analysis of mitosis and cytokinesis in the binucleate parasite Giardia intestinalis

In the binucleate parasite Giardia intestinalis, two diploid nuclei and essential cytoskeletal structures including eight flagella are duplicated and partitioned into two daughter cells during cell division. The mechanisms of mitosis and cytokinesis in the binucleate parasite Giardia are poorly resolved, yet have important implications for the maintenance of genetic heterozygosity. To articulate the mechanism of mitosis and the plane of cell division, we used three-dimensional deconvolution microscopy of each stage of mitosis to monitor the spatial relationships of conserved cytological markers to the mitotic spindles, the centromeres and the spindle poles. Using both light- and transmission electron microscopy, we determined that Giardia has a semi-open mitosis with two extranuclear spindles that access chromatin through polar openings in the nuclear membranes. In prophase, the nuclei migrate to the cell midline, followed by lateral chromosome segregation in anaphase. Taxol treatment results in lagging chromosomes and half-spindles. Our analysis supports a nuclear migration model of mitosis with lateral chromosome segregation in the left-right axis and cytokinesis along the longitudinal plane (perpendicular to the spindles), ensuring that each daughter inherits one copy of each parental nucleus with mirror image symmetry. Fluorescence in situ hybridization (FISH) to an episomal plasmid confirms that the nuclei remain separate and are inherited with mirror image symmetry.

NgUNC-119, Naegleria homologue of UNC-119, localizes to the flagellar rootlet

The UNC-119 family of proteins is ubiquitous in animals. The expression of UNC-119 is prominent in neural tissues including photoreceptor cells. Homologues of UNC-119 are also found in ciliated (or flagellated) single-celled organisms; however, the cellular distribution of this protein in protists is unknown. We cloned and characterized a homologue of unc-119 from the ameboflagellate Naegleria gruberi (Ngunc-119) and identified the cellular distribution of the protein. The Ngunc-119 open reading frame contained 570 nucleotides encoding a protein of 189 amino acids with a predicted molecular weight of 22.1 kDa, which is similar to that of Paramecium UNC-119 and Trypanosoma UNC-119. These three proteins are 46-48% identical in their amino acid sequences. The smaller NgUNC-119 corresponds to the conserved C-terminal 3/4 of the UNC-119 from multi-cellular organisms. The amino acid sequence of NgUNC-119 is 43-50% identical to that of the conserved C-terminal regions. NgUNC-119 was not found in growing amoebae but accumulated rapidly after the initiation of differentiation into flagellates. Indirect immunofluorescence staining of differentiating N. gruberi showed that NgUNC-119 begins to concentrate at a spot near the nucleus of differentiating cells and then elongates into a filamentous structure. Purification and indirect immunofluorescence staining of the Naegleria flagellar rootlet suggested that NgUNC-119 is a component of the flagellar rootlet.

Insights into functional aspects of centrins from the structure of N-terminally extended mouse centrin 1

Centrins are members of the family of Ca(2+)-binding EF-hand proteins. In photoreceptor cells, centrin isoform 1 is specifically localized in the non-motile cilium. This connecting cilium links the light-sensitive outer segment with the biosynthetic active inner segment of the photoreceptor cell. All intracellular exchanges between these compartments have to occur through this cilium. Three-dimensional structures of centrins from diverse organisms are known, showing that the EF-hand motifs of the N-terminal domains adopt closed conformations, while the C-terminal EF-hand motifs have open conformations. The crystal structure of an N-terminally extended mouse centrin 1 (MmCen1-L) resembles the overall structure of troponin C in its two Ca(2+) bound form. Within the N-terminal extension in MmCen1-L, residues W24 and R25 bind to the C-terminal domain of centrin 1 in a target-protein-like geometry. Here, we discuss this binding mode in connection with putative interaction sites of the target-protein transducin and the self-assembly of centrins.

Structural role of Sfi1p-centrin filaments in budding yeast spindle pole body duplication

Centrins are calmodulin-like proteins present in centrosomes and yeast spindle pole bodies (SPBs) and have essential functions in their duplication. The Saccharomyces cerevisiae centrin, Cdc31p, binds Sfi1p on multiple conserved repeats; both proteins localize to the SPB half-bridge, where the new SPB is assembled. The crystal structures of Sfi1p–centrin complexes containing several repeats show Sfi1p as an α helix with centrins wrapped around each repeat and similar centrin–centrin contacts between each repeat. Electron microscopy (EM) shadowing of an Sfi1p–centrin complex with 15 Sfi1 repeats and 15 centrins bound showed filaments 60 nm long, compatible with all the Sfi1 repeats as a continuous α helix. Immuno-EM localization of the Sfi1p N and C termini showed Sfi1p–centrin filaments spanning the length of the half-bridge with the Sfi1p N terminus at the SPB. This suggests a model for SPB duplication where the half-bridge doubles in length by association of the Sfi1p C termini, thereby providing a new Sfi1p N terminus to initiate SPB assembly.

Centrin scaffold in Chlamydomonas reinhardtii revealed by immunoelectron microscopy

In the flagellate green alga Chlamydomonas reinhardtii the Ca(2+)-binding EF-hand protein centrin is encoded by a single-copy gene. Previous studies have localized the protein to four distinct structures in the flagellar apparatus: the nucleus-basal body connector, the distal connecting fiber, the flagellar transitional region, and the axoneme. To explain the disjunctive distribution of centrin, the interaction of centrin with as yet unknown specific centrin-binding proteins has been implied. Here, we demonstrate using serial section postembedding immunoelectron microscopy of isolated cytoskeletons that centrin is located in additional structures (transitional fibers and basal body lumen) and that the centrin-containing structures of the basal apparatus are likely part of a continuous filamentous scaffold that extends from the nucleus to the flagellar bases. In addition, we show that centrin is located in the distal lumen of the basal body in a rotationally asymmetric structure, the V-shaped filament system. This novel centrin-containing structure has also been detected near the distal end of the probasal bodies. Taken together, these results suggest a role for a rotationally asymmetric centrin "seed" in the growth and development of the centrin scaffold following replication of the basal apparatus.

A trial to restore defective human sperm centrosomal function

BACKGROUND

In human fertilization, sperm centrosome function is essential for male and female pronuclear movement and fusion. In this study, we investigated the possibility of restoring human sperm centrosomal function in sperm exhibiting abnormalities in microtubule organization.

METHODS

Semen was obtained from both a fertile donor and a patient with dysplasia of the fibrous sheath (DFS). Following heterologous ICSI using human sperm, we examined microtubules and chromatin configuration in bovine oocytes. Sperm were treated with dithiothreitol (DTT) prior to ICSI, while the oocytes were treated with the cytoskeletal stabilizer paclitaxel after ICSI.

RESULTS

The combination of DTT and paclitaxel treatment induced microtubule organization in dead sperm from the fertile donor following heterologous ICSI. This treatment, however, was not effective for DFS sperm. In addition, expression of centrin, a protein functioning within the sperm centrosome, was reduced in DFS sperm from that of the normal levels observed in fertile donor sperm.

CONCLUSION

These results indicate that sperm centrosomal function could be induced by the treatment of sperm with DTT before ICSI and of oocytes with paclitaxel after ICSI. DFS sperm are likely to exhibit such severe dysfunction of sperm centrosome that cannot be compensated for by this treatment; therefore, this method may be a practical way to discern the degree of sperm centrosomal dysfunction.

Localization of centrins in the hypotrich ciliate Paraurostyla weissei

Centrins are ubiquitous cytoskeletal proteins that are generally associated with the centrosome and form large cytoskeletal networks in protists. To obtain more data on the respective role of different centrin proteins, we studied their distribution and behavior in one ciliate species, Paraurostyla weissei, using specific antibodies. In this species, only two major proteins of 21 and 24 kDa corresponding to centrins, were identified by 1D and 2D electrophoresis. Immunofluorescence analysis showed that these two proteins displayed non-overlapping localization in the interphase cell and during morphogenesis. Both centrin proteins localize on the fibrous network linking the oral basal bodies in the interphase cell and in the form of marginal dots, which correspond to the proximal ends of the striated rootlets; the 21 kDa centrin was also detected within the basal bodies, whereas the 24 kDa centrin allowed identifying new structures, the frontal dashes. During morphogenesis, the 21 kDa centrin locates at the basal bodies, while the 24 kDa centrin is detected along the striated rootlets and in close association with the basal bodies pairs. These data are discussed in terms of the potential roles of the two centrins in different cellular functions.

Differential expression and interaction with the visual G-protein transducin of centrin isoforms in mammalian photoreceptor cells

Photoisomerization of rhodopsin activates a heterotrimeric G-protein cascade leading to closure of cGMP-gated channels and hyperpolarization of photoreceptor cells. Massive translocation of the visual G-protein transducin, Gt, between subcellular compartments contributes to long term adaptation of photoreceptor cells. Ca(2+)-triggered assembly of a centrin-transducin complex in the connecting cilium of photoreceptor cells may regulate these transducin translocations. Here we demonstrate expression of all four known, closely related centrin isoforms in the mammalian retina. Interaction assays revealed binding potential of the four centrin isoforms to Gtbetagamma heterodimers. High affinity binding to Gtbetagamma and subcellular localization of the centrin isoforms Cen1 and Cen2 in the connecting cilium indicated that these isoforms contribute to the centrin-transducin complex and potentially participate in the regulation of transducin translocation through the photoreceptor cilium. Binding of Cen2 and Cen4 to Gbetagamma of non-visual G-proteins may additionally regulate G-proteins involved in centrosome and basal body functions.

Cell Biology of Normal and Abnormal Ciliogenesis in the Ciliated Epithelium

Ciliogenesis is divided into four stages: (1) generation of centrioles, (2) migration of duplicated centrioles, (3) formation of the basal body-associated structures, and (4) elongation of cilia. The ultrastructural profile of ciliogenesis is fundamentally the same among various kinds of animal species. In acentriolar centriologenesis, centrioles are generated around deuterosomes by the use of fibrous granules. Components of the centriolar precursor structures, and genes that regulate the differentiation of ciliated cells, have been revealed. Ciliary abnormalities are classified into two categories: specific congenital defects of ciliary structure and acquired nonspecific anomalies of the ciliary apparatus. When ciliogenesis is disturbed, various nonspecific ciliary abnormalities develop in the cell. Inhibition of centriole migration results in the development of intracytoplasmic axonemes, cilia within periciliary sheaths, and intracellular ciliated vacuoles. Swollen cilia and the bulging type of compound cilia are formed during ciliary budding and elongation. Primary cilia can also develop from one of a pair of centrioles. They lack dynein arms and are immobile, but work as a mechanosensor and play a role during morphogenesis of the kidney. Abnormal function or structure of primary cilia results in the development of polycystic kidney disease. The axonemes of primary cilia or monocilia in the embryonic node cells are associated with dynein arms and move vortically. They have a role in determining the left-right (L-R) asymmetry of the fetus. This review also discusses the ciliogenesis of a primary cilium in the cell.