Synthesis and Turnover of Embryonic Sea Urchin Ciliary Proteins during Selective Inhibition of Tubulin Synthesis and Assembly

When ciliogenesis first occurs in sea urchin embryos, the major building block proteins, tubulin and dynein, exist in substantial pools, but most 9+2 architectural proteins must be synthesized de novo. Pulse-chase labeling with [³H]leucine demonstrates that these proteins are coordinately up-regulated in response to deciliation so that regeneration ensues and the tubulin and dynein pools are replenished. Protein labeling and incorporation into already-assembled cilia is high, indicating constitutive ciliary gene expression and steady-state turnover. To determine whether either the synthesis of tubulin or the size of its available pool is coupled to the synthesis or turnover of the other 9+2 proteins in some feedback manner, fully-ciliated mid- or late-gastrula stage Strongylocentrotus droebachiensis embryos were pulse labeled in the presence of colchicine or taxol at concentrations that block ciliary growth. As a consequence of tubulin autoregulation mediated by increased free tubulin, no labeling of ciliary tubulin occurred in colchicine-treated embryos. However, most other proteins were labeled and incorporated into steady-state cilia at near-control levels in the presence of colchicine or taxol. With taxol, tubulin was labeled as well. An axoneme-associated 78 kDa cognate of the molecular chaperone HSP70 correlated with length during regeneration; neither colchicine nor taxol influenced the association of this protein in steady-state cilia. These data indicate that 1) ciliary protein synthesis and turnover is independent of tubulin synthesis or tubulin pool size; 2) steady-state incorporation of labeled proteins cannot be due to formation or elongation of cilia; 3) substantial tubulin exchange takes place in fully-motile cilia; and 4) chaperone presence and association in steady-state cilia is independent of background ciliogenesis, tubulin synthesis, and tubulin assembly state.     

The intraflagellar transport protein IFT20 is associated with the Golgi complex and is required for cilia assembly

Eukaryotic cilia are assembled via intraflagellar transport (IFT) in which large protein particles are motored along ciliary microtubules. The IFT particles are composed of at least 17 polypeptides that are thought to contain binding sites for various cargos that need to be transported from their site of synthesis in the cell body to the site of assembly in the cilium. We show here that the IFT20 subunit of the particle is localized to the Golgi complex in addition to the basal body and cilia where all previous IFT particle proteins had been found. In living cells, fluorescently tagged IFT20 is highly dynamic and moves between the Golgi complex and the cilium as well as along ciliary microtubules. Strong knock down of IFT20 in mammalian cells blocks ciliary assembly but does not affect Golgi structure. Moderate knockdown does not block cilia assembly but reduces the amount of polycystin-2 that is localized to the cilia. This work suggests that IFT20 functions in the delivery of ciliary membrane proteins from the Golgi complex to the cilium.     

An essential quality control mechanism at the eukaryotic basal body prior to intraflagellar transport

Constructing a eukaryotic cilium/flagellum is a demanding task requiring the transport of proteins from their cytoplasmic synthesis site into a spatially and environmentally distinct cellular compartment. The clear potential hazard is that import of aberrant proteins could seriously disable cilia/flagella assembly or turnover processes. Here, we reveal that tubulin protein destined for incorporation into axonemal microtubules interacts with a tubulin cofactor C (TBCC) domain-containing protein that is specifically located at the mature basal body transitional fibres. RNA interference-mediated ablation of this protein results in axonemal microtubule defects but no effect on other microtubule populations within the cell. Bioinformatics analysis indicates that this protein belongs to a clade of flagellum-specific TBCC-like proteins that includes the human protein, XRP2, mutations which lead to certain forms of the hereditary eye disease retinitis pigmentosa. Taken with other observations regarding the role of transitional fibres in cilium/flagellum assembly, we suggest that a localized protein processing capacity embedded at transitional fibres ensures the 'quality' of tubulin imported into the cilium/flagellum, and further, that loss of a ciliary/flagellar quality control capability may underpin a number of human genetic disorders.     

The cytoplasmic tail of fibrocystin contains a ciliary targeting sequence

Sensory functions of primary cilia rely on ciliary-localized membrane proteins, but little is known about how these receptors are targeted to the cilium. To further our understanding of this process, we dissected the ciliary targeting sequence (CTS) of fibrocystin, the human autosomal recessive polycystic kidney disease gene product. We show that the fibrocystin CTS is an 18-residue motif localized in the cytoplasmic tail. This motif is sufficient to target green fluorescent protein (GFP) to cilia of ciliated cells and targets GFP to lipid rafts if the cells are not ciliated. Rab8, but not several other Rabs implicated in ciliary assembly, binds to the CTS in a coimmunoprecipitation assay. Dominant-negative Rab8 interacts more strongly than wild-type or constitutively active Rab8, and coexpression of this dominant-negative mutant Rab8 blocks trafficking to the cilium. This suggests that the CTS functions by binding regulatory proteins like Rab8 to control trafficking through the endomembrane system and on to the cilium.     

Molecular chaperones in cilia and flagella: implications for protein turnover

The mechanisms of protein incorporation and turnover in 9+2 ciliary axonemes are not known. Previous reports of an HSP70-related protein, first in Chlamydomonas flagella and then in sea urchin embryonic cilia, suggested a potential role in protein transport or incorporation. The present study further explores this and other chaperones in axonemes from a representative range of organisms. Two-dimensional gel electrophoresis proved identity between the sea urchin ciliary 78 kDa HSP and a constitutive cytoplasmic HSP70 cognate (pI = 5.71). When isolated flagella from mature sea urchin sperm were analyzed, the same total amount and distribution of 78 kDa protein as in cilia were found. Antigens of similar size were detected in ctenophore comb plate, molluscan gill, and rabbit tracheal cilia. Absent from sea urchin sperm flagella, TCP-1alpha was detected in sea urchin embryonic and rabbit tracheal cilia; the latter also contained HSP90, detected by two distinct antibodies. Tracheal cilia were shown to undergo axonemal protein turnover while tracheal cells mainly synthesized ciliary proteins. TCP-1alpha progressively appeared in regenerating embryonic cilia only as their growth slowed, suggesting a regulatory role in incorporation or turnover. These results demonstrate that chaperones are widely distributed ciliary and flagellar components, potentially related to axonemal protein dynamics.     

Ciliary protein turnover continues in the presence of inhibitors of golgi function: evidence for membrane protein pools and unconventional intracellular membrane dynamics

The intimate association of the Golgi apparatus with cilia suggests a functional alliance. To explore the relationship between the synthesis and processing of membrane constituents and the turnover or regeneration of cilia, parallel cultures of gastrula-stage sea urchin embryos were pulse-chase labeled with (3)H-leucine in the presence of monensin, brefeldin A, or colchicine. Steady-state labeled cilia were isolated, and the embryos were allowed to regenerate cilia, which were then isolated after the equivalent of two normal regeneration times. Regeneration was absent in colchicine, minimal in monensin, and inhibited about 40% by brefeldin A. Both monensin and brefeldin A effectively inhibited the post-translational processing of prominent phosphatidylinositoylated and palmitoylated membrane proteins and the axoneme-associated transmembrane Spec3 protein, yet most other membrane plus matrix and 9+2 axonemal proteins were labeled to levels indistinguishable from untreated controls. However, total protein analysis of the membrane plus matrix fractions showed a substantial increase in glycoproteins and the calsequestrin-like protein ECaSt/PDI after treatment at steady-state with all three inhibitors and after regeneration in brefeldin A. Other constituents of this compartment, such as membrane-associated tubulin, calmodulin, and a 53-kDa calcium-binding protein, were unchanged. Therefore, inhibition of Golgi function via three different mechanisms left 9+2 protein turnover undiminished but resulted in an accumulation, in the cilium, of already-processed membrane pool constituents and a normally ER-resident protein. A disproportionate elevation of HSP70 suggests that a novel stress response may be involved in inhibiting ciliary regeneration or promoting glycoprotein augmentation.     

Architecture and function of IFT complex proteins in ciliogenesis

Cilia and flagella (interchangeable terms) are evolutionarily conserved organelles found on many different types of eukaryotic cells where they fulfill important functions in motility, sensory reception and signaling. The process of Intraflagellar Transport (IFT) is of central importance for both the assembly and maintenance of cilia, as it delivers building blocks from their site of synthesis in the cell body to the ciliary assembly site at the tip of the cilium. A key player in this process is the multi-subunit IFT-complex, which acts as an adapter between the motor proteins required for movement and the ciliary cargo proteins. Since the discovery of IFT more than 15 years ago, considerable effort has gone into the purification and characterization of the IFT complex proteins. Even though this has led to very interesting findings and has greatly improved our knowledge of the IFT process, we still know very little about the overall architecture of the IFT complex and the specific functions of the various subunits. In this review we will give an update on the knowledge of the structure and function of individual IFT proteins, and the way these proteins interact to form the complex that facilitates IFT.     

Tubulin transport by IFT is upregulated during ciliary growth by a cilium-autonomous mechanism

The assembly of the axoneme, the structural scaffold of cilia and flagella, requires translocation of a vast quantity of tubulin into the growing cilium, but the mechanisms that regulate the targeting, quantity, and timing of tubulin transport are largely unknown. In Chlamydomonas, GFP-tagged α-tubulin enters cilia as an intraflagellar transport (IFT) cargo and by diffusion. IFT-based transport of GFP-tubulin is elevated in growing cilia and IFT trains carry more tubulin. Cells possessing both nongrowing and growing cilia selectively target GFP-tubulin into the latter. The preferential delivery of tubulin boosts the concentration of soluble tubulin in the matrix of growing versus steady-state cilia. Cilia length mutants show abnormal kinetics of tubulin transport. We propose that cells regulate the extent of occupancy of IFT trains by tubulin cargoes. During ciliary growth, IFT concentrates soluble tubulin in cilia and thereby promotes elongation of the axonemal microtubules.     

Differential protein synthesis and utilization during cilia formation in sea urchin embryos.

Pulse labeling with [¹⁴C]leucine, hypertonic deciliation, fractionation of axonemes by differential solubilization, and autoradiographic analysis of electrophoretically resolved components reveal that the onset of ciliogenesis is marked by the de novo synthesis of numerous architectural proteins of the “9 + 2” axoneme. The synthesis of most of these components continues, some at reduced rates, after full growth of cilia at hatching. Deciliation results in enhanced synthesis of these minor components, dynein, and tubulin. The A- and B-tubulin dimers, derived from the respective subfibers, have essentially identical specific activities after regeneration in the presence of isotope. Subsequent regeneration in cold leucine demonstrates substantial pools of most of the architectural proteins, but at least two such proteins (nexin and ribbon component-20) are made quantally and in limiting amounts in response to each regeneration. Such second regeneration cilia (whose pools were labeled during the first regeneration) have a decreased specific activity of B-tubulin (10–15%) and an increased specific activity of A-tubulin (30–35%), indicating a limited pool of the former but an apparent retarded synthesis, delayed activation, or initial compartmentalization of the latter. This 45% difference in specific activity of the two tubulin dimer pools offers independent evidence that chemically unique tubulin dimers form the structurally unique subfibers. During natural ciliary augmentation or after stimulation by repeated deciliation, the bulk of the initial incorporation occurs in the quantal, minor components, while newly synthesized dynein and tubulin are not maximally utilized until the succeeding generation. The limited, quantal synthesis of microtubule-associated proteins may be a control mechanism for ciliary assembly or elongation, while a delayed utilization of the major proteins of the axoneme may reflect a replenishment of pools and a requisite activation or post-translational modification of stored components.     

Regulation of Microtubule Synthesis and Utilization during Early Embryonic Development of the Sea Urchin

Microtubules deployed during early development of the sea urchinembryo are derived both from a preexisting pool of subunitspresent in the egg and from microtubule protein subunits synthesizedin the embryo. Several aspects of microtubule protein synthesisand utilization are reviewed. Microtubule protein synthesisin early development utilizes oogenetic messenger RNA species.Translation of this mRNA is under regulation. Microtubule proteinsynthesis rises concomitantly with overall protein synthesisat fertilization, but rises at a relatively higher rate laterin cleavage stages. Microtubule protein labeled with [³H]-leucinein early development is incorporated into cilia, indicatingthat newly synthesized protein enters the pool of subunits usedin organelle assembly. The microtubule protein pool comprisesabout 1%of the soluble protein of the egg, and remains constantin size at least until the blastula stage. Direct pool sizeestimates are consistent with results of experiments on recruitmentof microtubule protein subunits into the mitotic apparatus andinto regenerating cilia. Soluble and particulate colchicinebinding fractions, which have been reported from several systems,appear to be present in sea urchin embryos. The possible roleof such fractions are discussed, as are aspects of the regulationof ciliary assembly.     

A synthetic derivative of plant allylpolyalkoxybenzenes induces selective loss of motile cilia in sea urchin embryos.

Polyalkoxybenzenes are plant components displaying a wide range of biological activities. In these studies, we synthesized apiol and dillapiol isoxazoline analogues of combretastatins and evaluated their effect on sea urchin embryos. We have shown that p-methoxyphenyl isoxazoline caused sea urchin embryo immobilization due to the selective excision of motile cilia, whereas long immotile sensory cilia of apical tuft remained intact. This effect was completely reversed by washing the embryos. The compound did not alter cell division, blastulae hatching, and larval morphogenesis. In our hands, the molecule would serve as a convenient tool for in vivo studying morphogenetic processes in the sea urchin embryo. We anticipate that both the assay and the described derivative could be used for studies in ciliary function in embryogenesis.     

Intraflagellar transport proteins in ciliogenesis of photoreceptor cells

Background information. The assembly and maintenance of cilia depend on IFT (intraflagellar transport) mediated by molecular motors and their interplay with IFT proteins. Here, we have analysed the involvement of IFT proteins in the ciliogenesis of mammalian photoreceptor cilia. Results. Electron microscopy revealed that ciliogenesis in mouse photoreceptor cells follows an intracellular ciliogenesis pathway, divided into six distinct stages. The first stages are characterized by electron‐dense centriolar satellites and a ciliary vesicle, whereas the formations of the ciliary shaft and the light‐sensitive outer segment discs are features of the later stages. IFT proteins were associated with ciliary apparatus during all stages of photoreceptor cell development. Conclusions. Our data conclusively provide evidence for the participation of IFT proteins in photoreceptor cell ciliogenesis, including the formation of the ciliary vesicle and the elongation of the primary cilium. In advanced stages of ciliogenesis the ciliary localization of IFT proteins indicates a role in IFT as is seen in mature cilia. A prominent accumulation of IFT proteins in the periciliary cytoplasm at the base of the cilia in these stages most probably resembles a reserve pool of IFT molecules for further delivery into the growing ciliary shaft and their subsequent function in IFT. Nevertheless, the cytoplasmic localization of IFT proteins in the absence of a ciliary shaft in early stages of ciliogenesis may indicate roles of IFT proteins beyond their well‐established function for IFT in mature cilia and flagella.     

Heterotrimeric Kinesin-II Is Required for the Assembly of Motile 9+2 Ciliary Axonemes on Sea Urchin Embryos

Heterotrimeric kinesin-II is a plus end– directed microtubule (MT) motor protein consisting of distinct heterodimerized motor subunits associated with an accessory subunit. To probe the intracellular transport functions of kinesin-II, we microinjected fertilized sea urchin eggs with an anti–kinesin-II monoclonal antibody, and we observed a dramatic inhibition of ciliogenesis at the blastula stage characterized by the assembly of short, paralyzed, 9+0 ciliary axonemes that lack central pair MTs. Control embryos show no such defect and form swimming blastulae with normal, motile, 9+2 cilia that contain kinesin-II as detected by Western blotting. Injection of anti–kinesin-II into one blastomere of a two-cell embryo leads to the development of chimeric blastulae covered on one side with short, paralyzed cilia, and on the other with normal, beating cilia. We observed a unimodal length distribution of short cilia on anti–kinesin-II–injected embryos corresponding to the first mode of the trimodal distribution of ciliary lengths observed for control embryos. This short mode may represent a default ciliary assembly intermediate. We hypothesize that kinesin-II functions during ciliogenesis to deliver ciliary components that are required for elongation of the assembly intermediate and for formation of stable central pair MTs. Thus, kinesin-II plays a critical role in embryonic development by supporting the maturation of nascent cilia to generate long motile organelles capable of producing the propulsive forces required for swimming and feeding.     

Development of the Nervous System of Sea Urchin Embryos: Formation of Ciliary Bands and the Appearance of Two Types of Ectoneural Cells in the Pluteus

An ultrastructural study of the larval integument of the sea urchin, Hemicentrotus pulcherrimus , was conducted with special emphasis on the development of the nervous system in relation to the formation of ciliary bands. In the integument of 4-armed pluteus larvae, cells associated with the ciliary band, which have 200 nm-thick projections at their apices, and cells in the squamous epithelium, which have a cilium and long, fine radiating processes in the apical region, were observed. Both cell types have axons at their basal ends that form nerve bundles beneath the ciliary bands, where the axons make contact with ectodermal effector cells with motile cilia. The cilia and other apical projections of these ectoneural cells run parallel to the surface of the cells, and are under the hyaline layer. The axoneme of the cilium has a typical "9 + 2" microtubular arrangement, but generally has no dynein arms. These ectoneural cells are more frequent on the oral surface than on the antioral surface.     

Preferential incorporation of tubulin into the junctional region of ciliary outer doublet microtubules: a model for treadmilling by lattice dislocation

Even in the presence of colchicine or Taxol(R), sea urchin embryonic cilia undergo substantial steady-state turnover, with a rate of tubulin incorporation approaching half that seen in full regeneration [Stephens: Mol Biol Cell 8:2187-2198, 1997]. Preliminary experiments suggest that tubulin incorporates differentially into the most stable portion of the outer doublet, the junctional protofilaments [Stephens: Cell Struct Funct 24:413-418, 1999]. To explore this possibility further, embryos of the sea urchin Tripneustes gratilla, a ciliary length inducible system [Stephens: J Exp Zool 269:106-115, 1994a], were pulse labeled with (3)H leucine during steady-state turnover or induced elongation, followed by regeneration in the presence of unlabeled leucine. Cilia were isolated by hypertonic shock and fractionated into detergent-soluble membrane plus matrix, thermally-solubilized microtubule walls, and insoluble 9-fold symmetric remnants of A-B junctional protofilaments plus associated architectural elements. The fractions were resolved by SDS-PAGE and the specific activity of alpha-tubulin was determined. In cilia undergoing turnover or elongation during an isotope pulse, the specific activity of tubulin in the junctional region approximated that of precursor membrane plus matrix tubulin but surpassed that of the tubule wall by a factor of approximately 1.5. In cilia regenerated during an isotope chase, the specific activity of junctional tubulin exceeded that of both the membrane plus matrix and the tubule wall by a similar factor. These data indicate that tubulin is preferentially incorporated into junctional protofilaments during steady-state turnover, induced elongation and regeneration. A model for directional incorporation based on surface lattice discontinuities in the outer doublet is proposed.     

Calreticulin reveals a critical Ca(2+) checkpoint in cardiac myofibrillogenesis

Approximately 10% of the photoreceptor outer segment (OS) is turned over each day, requiring large amounts of lipid and protein to be moved from the inner segment to the OS. Defects in intraphotoreceptor transport can lead to retinal degeneration and blindness. The transport mechanisms are unknown, but because the OS is a modified cilium, intraflagellar transport (IFT) is a candidate mechanism. IFT involves movement of large protein complexes along ciliary microtubules and is required for assembly and maintenance of cilia. We show that IFT particle proteins are localized to photoreceptor connecting cilia. We further find that mice with a mutation in the IFT particle protein gene, Tg737/IFT88, have abnormal OS development and retinal degeneration. Thus, IFT is important for assembly and maintenance of the vertebrate OS.     

Arf4 Is Required for Mammalian Development but Dispensable for Ciliary Assembly

The primary cilium is a sensory organelle, defects in which cause a wide range of human diseases including retinal degeneration, polycystic kidney disease and birth defects. The sensory functions of cilia require specific receptors to be targeted to the ciliary subdomain of the plasma membrane. Arf4 has been proposed to sort cargo destined for the cilium at the Golgi complex and deemed a key regulator of ciliary protein trafficking. In this work, we show that Arf4 binds to the ciliary targeting sequence (CTS) of fibrocystin. Knockdown of Arf4 indicates that it is not absolutely required for trafficking of the fibrocystin CTS to cilia as steady-state CTS levels are unaffected. However, we did observe a delay in delivery of newly synthesized CTS from the Golgi complex to the cilium when Arf4 was reduced. Arf4 mutant mice are embryonic lethal and die at mid-gestation shortly after node formation. Nodal cilia appeared normal and functioned properly to break left-right symmetry in Arf4 mutant embryos. At this stage of development Arf4 expression is highest in the visceral endoderm but we did not detect cilia on these cells. In the visceral endoderm, the lack of Arf4 caused defects in cell structure and apical protein localization. This work suggests that while Arf4 is not required for ciliary assembly, it is important for the efficient transport of fibrocystin to cilia, and also plays critical roles in non-ciliary processes.     

Redistribution of the kinesin-II subunit KAP from cilia to nuclei during the mitotic and ciliogenic cycles in sea urchin embryos

KAP is the non-motor subunit of the heteromeric plus-end directed microtubule (MT) motor protein kinesin-II essential for normal cilia formation. Studies in Chlamydomonas have demonstrated that kinesin-II drives the anterograde intraflagellar transport (IFT) of protein complexes along ciliary axonemes. We used a green fluorescent protein (GFP) chimera of KAP, KAP-GFP, to monitor movements of this kinesin-II subunit in cells of sea urchin blastulae where cilia are retracted and rebuilt with each mitosis. As expected if involved in IFT, KAP-GFP localized to apical cytoplasm, basal bodies, and cilia and became concentrated on basal bodies of newly forming cilia. Surprisingly, after ciliary retraction early in mitosis, KAP-GFP moved into nuclei before nuclear envelope breakdown, was again present in nuclei after nuclear envelope reformation, and only decreased in nuclei as ciliogenesis reinitiated. Nuclear transport of KAP-GFP could be due to a putative nuclear localization signal and nuclear export signals identified in the sea urchin KAP primary sequence. Our observation of a protein involved in IFT being imported into the nucleus after ciliary retraction and again after nuclear envelope reformation suggests KAP115 may serve as a signal to the nucleus to reinitiate cilia formation during sea urchin development.