An evaluation of Hsp90 as a mediator of cortical patterning in Tetrahymena.

This study asks two questions: 1) whether Hsp90 is involved in the regulation of cortical patterning in Tetrahymena, and 2) if it is, whether specific defects in this regulation can be attributed to functional insufficiency of the Hsp90 molecule. To address question 1, we compared the effects of a specific inhibitor of Hsp90, geldanamycin, on population growth and on development of the oral apparatus in two Tetrahymena species, T. pyriformis and T. thermophila. We observed that geldanamycin inhibits population growth in both species at very low concentrations, and that it has far more severe effects on oral patterning in T. pyriformis than in T. thermophila. These effects are parallel to those of high temperature in the same two species, and provide a tentative affirmative answer to the first question. To address question 2, we ascertained the base sequence of the genes that encode the Hsp90 molecules which are induced at high temperatures in both Tetrahymena species, as well as corresponding sequences in Paramecium tetraurelia. Extensive comparative analyses of the deduced amino acid sequences of the Hsp90 molecules of the two Tetrahymena species indicate that on the basis of what we currently know about Hsp90 both proteins are equally likely to be functional. Phylogenetic analyses of Hsp90 amino acid sequences indicate that the two Tetrahymena Hsp90 molecules have undergone a similar number of amino acid substitutions from their most recent common ancestor, with none of these corresponding to any known functionally critical region of the molecule. Thus there is no evidence that the Hsp90 molecule of T. pyriformis is functionally impaired; the flaw in the control of cortical patterning is more likely to be caused by defects in mechanism(s) that mediate the response to Hsp90, as would be expected from the "Hsp90 capacitor" model of Rutherford and Lindquist.

The effects of supraoptimal temperatures on population growth and cortical patterning in Tetrahymena pyriformis and Tetrahymena thermophila: a comparison.

In this investigation, we compare the multiplication rates and morphogenetic responses of the two most studied Tetrahymena species, T. pyriformis and T. thermophila, at supraoptimal temperatures. Although the upper temperature limits differ greatly in the two species, the pattern of growth responses to high temperature is for the most part similar, with some differences in detail. The transient recovery of cell division at the highest temperature that allows cell division, characteristic of T. pyriformis, is observed in a less distinct form in T. thermophila. Moreover, there is a remarkable difference in developmental response, with drastic abnormalities in patterning of oral structures during the transient recovery of cell division in T. pyriformis, and far more limited abnormalities under similar conditions in T. thermophila. The abnormalities result from spatial disorder in the alignment and orientation of basal body pairs within the early oral primordium, followed by failures in the realignment that normally occurs as oral structures (membranelles and undulating membrane) mature. Both the initial spatial disorder and the failures in realignment are far more severe in T. pyriformis than in T. thermophila.

HSP70 and HSP90 homologs are associated with tubulin in hetero-oligomeric complexes, cilia and the cortex of Tetrahymena.

We show in the present study that homologs of hsp90 and hsp70 are induced by heat shocks in Tetrahymena and appear to form a high molecular mass complex (approximately 700 kDa) with tubulin. Three members of the hsp70 family (hsp72, 73, and 78) and one member of the hsp90 family (hsp82) have been identified by immunological or by a combination of immunological and sequencing methods. The known components of the 700 kDa complex and the conditions under which it can be recovered suggest that it may be an induced protective assemblage rather than a normal processing intermediate. Immunoblotting and immunofluorescence studies suggest further that large amounts of hsp73 and lesser amounts of hsp82 are associated with mature microtubules in both cilia and the cortex in this cell type. Some site-specific localizations of the identified heat shock proteins were also noted in non-microtubular components of the cell cortex.

Monoclonal antibodies reveal complex structure in the membrane skeleton of Tetrahymena.

Twelve monoclonal antibodies were raised that are specific for the membrane skeleton of Tetrahymena. Five were directed against T. pyriformis and seven were directed against T. thermophila. Some cross-reactivity between species was found. Each monoclonal antibody recognized one of the three major components of epiplasm, i.e. the bands A, B, and C identified in electrophoretic separations of epiplasmic proteins. It was found, using these antibodies, that the epiplasmic proteins A, B and C have overlapping but independent distributions within the cell.

Cellular polarity in ciliates: persistence of global polarity in a disorganized mutant of Tetrahymena thermophila that disrupts cytoskeletal organization.

Much of the cell surface on the ciliate Tetrahymena thermophila is covered by a polarized lattice of cytoskeletal structures that are associated with basal bodies of the ciliary rows. Unique structural landmarks, including an oral apparatus and contractile vacuole pores, develop before cell division in localized domains located, respectively, posterior and anterior to the transverse fission zone. All of these structures can be visualized by specific monoclonal antibodies. A single-locus recessive mutation, disorganized-A (disA), primarily affects the striated rootlets of the ciliary-row basal bodies and brings about a severe disorganization in the positioning and orientation of these basal bodies and associated cytoskeletal elements. Nonetheless, the new oral apparatus, contractile vacuole pores, and other unique structures appeared at or near their normal sites along the anteroposterior axis of disA cells, indicating that the positioning of these localized structures is not dependent on the integrity of the ciliary rows. Abnormalities were present in the details of construction of some of the localized structures and in aspects of cell shape that may be influenced by these details. In the main, however, analysis of disA mutant cells indicates that intracellular domains near the cell poles develop independently of the vectorial polarity of the ciliary rows.

"Fenestrin" and conjugation in Tetrahymena thermophila.

Certain monoclonal antibodies interact with proteins of Tetrahymena thermophila found in the conjugation junction as well as around the gametic nuclei (pronuclei) of conjugating cells; they also react with the oral primordium and fission zone of vegetative cells and with the cytoproct and contractile vacuole pores of all cells. One of these (FXIX-3A7) was investigated in detail. Immunogold labelling suggests that the material labelled by the 3A7 monoclonal antibody, which we call "fenestrin," is located beneath the epiplasm (membrane skeleton). Immunoblots reveal that the major and perhaps sole antigen is a 64 kDa polypeptide, found in two isoelectric variants. Developmental studies implicate fenestrin in two processes involved in conjugation. The first is "tip transformation." During preliminary starvation ("initiation"), labelling of fenestrin first appeared as a spot at the anterior end of starved mature cells, then after mixing of different mating types ("costimulation") it extended posteriorly along the anterior suture. After pairing, this region spread to form a widened plate. The second process is pronuclear transfer. Fenestrations representing channels between the conjugating cells began to appear 0.5 to 1 h after the conjugants united, and eventually merged to form a small number of temporary large holes during exchange of the transfer pronuclei. A fenestrin envelope also enclosed both the transfer and resident pronuclei; a strand of fenestrin connected the two. Shortly after pronuclear transfer, both transfer and resident pronuclei were released from fenestrin caps and fused to produce a zygotic nucleus (synkaryon) not associated with fenestrin Fenestrin thus appears to be intimately involved in the process of pronuclear exchange.

Hypoangular: a gene potentially involved in specifying positional information in a ciliate, Tetrahymena thermophila.

In Tetrahymena, two unique cell-surface structures, the oral apparatus and the cytoproct, are formed at opposite ends of one ciliary row, the reference meridian, which is propagated longitudinally during clonal growth. A third set of unique structures, the contractile vacuole pore(s) (CVP), is located at a nearly constant proportion of the cell circumference to the cell's right of the reference meridian. Three allelic recessive temperature-sensitive mutations, collectively named hypoangular (hpo), alter both the geometry of propagation of the reference meridian and the location of the CVPs. In mutant cells, the reference meridian typically undergoes a steady rightward shift in successive cell generations ("cortical slippage"); concomitantly, CVP sets come to lie closer to the reference meridian. Although CVP location is still proportional to the cell circumference, the constant of proportionality (the "CVP angle") is reduced. Another effect is an alteration in the widths of morphogenetic domains within the cortex. As the temperature is raised (made more restrictive), these effects are accentuated and the CVP angle becomes reduced further. At the extreme, the CVP angle collapses to zero and less, i.e., there is a topological switch such that CVPs come to lie to the left of the reference meridian, and the direction of cortical slippage reverses from rightward to leftward. These observations are hard to reconcile with existing formal models of pattern specification in this system and suggest that the hpo locus might specify a key component of the intracellular positional system.

Oral assembly in left-handed Tetrahymena thermophila.

We have investigated oral development in a non-genetically derived left-handed (LH) form of Tetrahymena thermophila, in which the large-scale asymmetry of arrangement of cortical structures is reversed whereas the local asymmetry of ciliary architecture remains normal. Approximately 1/2 of the oral apparatuses (OAs) of LH cells develop in the form of superficial mirror-images of OAs of RH cells. In most of these OAs, membranelles are assembled from the cells' anterior to posterior. Nonetheless, the posterior ends of these membranelles undergo the basal body displacements that lead to a "sculptured" appearance, so that the membranelles of LH OAs become organized as rotational permutations of membranelles of normal RH OAs. Many of these membranelles re-orient to a normal orientation near the end of oral development. Membranelles and undulating membranes (UMs) may develop independently of each other, and formation of postciliary microtubules of UMs is separate from that of ribbed wall microtubules. In some cases, the entire OA develops and remains as a 180 degrees rotational permutation of the normal, resembling the inverted OAs of mirror-image doublets and LH cells of Glaucoma scintillans described by Suhama. We present a model for these complex developmental outcomes. These developmental patterns resemble those described previously and less completely for "secondary" OAs of cells with mirror-image global patterns, including janus cells. The present study demonstrates that such alterations in oral development are not a direct outcome of genotypic changes.

Maintenance and regulation of cellular handedness in Tetrahymena.

The left-handed phenotype of Tetrahymena thermophila (LH) is a global mirror image of its right-handed counterpart (RH). LH cells are 'wound' in the opposite direction from that of RH cells with respect to the placement of all structures that are asymmetrically disposed on the cell circumference. However, the local geometry of ciliary rows, including the asymmetrically placed microtubule bands and other accessory structures, is identical in RH and LH cells. Populations of LH cells grow more slowly than those of RH cells, probably because of nutritional problems due to faulty construction of the cell mouth. LH cells, like RH cells, conjugate in a homopolar configuration, while LH cells mate with RH cells in a heteropolar union which suffices to initiate the conjugal nuclear events but is insufficient to allow survival of progeny. Subclonal analyses indicate that reversion of the LH to the RH form is relatively rare. However, the frequency of reversion is greatly increased by conditions that promote the formation of doublets by fission arrest. An analysis of intermediate doublet forms in such cultures strongly suggests that reversion takes place through a specific pathway, with LH-LH doublets regulating to LH-RH forms that then may give rise to RH singlets. The origin and fate of the LH-RH intermediate forms can be explained by applying a modified polar coordinate model of positional information with the proviso that there is a preferred direction for the intercalation of new positional values.

Non-genic inheritance of cellular handedness.

Ciliates exhibit an asymmetry in arrangement of surface structures around the cell which could be termed handedness. If the usual order of placement of structures defines a 'right-handed' (RH) cell, then a cell with this order reversed would be 'left-handed' (LH). Such LH forms appear to be produced in Tetrahymena thermophila through aberrant reorganization of homopolar doublets back to the singlet condition. Four clones of LH forms were selected and subjected to genetic analysis to test whether this drastic phenotypic alteration resulted from a nuclear genetic change. The results of this analysis indicate that the change in handedness is not due to a genetic change in either the micronucleus or macronucleus. The LH form can, under certain circumstances, revert to the RH form, but typically it propagates itself across both vegetative and sexual generations with similar fidelity. While this analysis does not formally rule out certain possibilities of nuclear genic control involving regulatory elements transmitted through the cytoplasm, when the circumstances of origin and propagation of the LH condition are taken into account direct cortical perpetuation seems far more likely. Here we outline a conceptual framework centred on the idea of longitudinally propagated positional information; the positive evidence supporting this idea as well as further application of the idea itself are presented in the accompanying paper.

Positional reorganization in compound janus cells of Tetrahymena thermophila.

The janus mutations of Tetrahymena thermophila convert the large-scale organization of the dorsal surface of the cell into a mirror-image of the ventral surface, which is characterized by a second, abnormal, oral apparatus and by contractile vacuole pores to the left of the second oral area rather than the usual right. This conversion could be due either to a local change in the response to an unaltered positional system or to a more global reorganization of the system itself. janus homopolar doublets were used to distinguish between these two alternatives. Homopolar doublets can be made by fusing two similarly oriented cells in side-by-side parabiosis. Non-janus homopolar doublets typically possess two sets of normal oral structures with contractile vacuole pores to the right of each of them. In janus doublets, there are up to four sets of oral structures, with the abnormal oral structures located between the two sets of normal oral structures; contractile vacuole pores are situated to the right of the normal oral areas and to the left of the abnormal oral structures. Non-janus homopolar doublets are known to propagate their compound condition for a number of cell divisions, but also to regulate toward the singlet state through a progressive reduction in number of ciliary rows followed by loss of one of the two sets of major cell surface structures. janus homopolar doublets go through a corresponding regulation. As a consequence, the location of the abnormal oral structures relative to the normal ones is more variable in janus doublets than in janus singlets. Sometimes the abnormal oral structures shift to a position close to their normal counterparts and then the intervening CVP sets disappear. There is evidence for occasional fusion of an abnormal oral area with an adjacent normal oral apparatus, a condition that may be transitional to the singlet state. These observations are inconsistent with the idea of a fixed positional system and strongly suggest a global reorganization of the surface pattern in a manner consistent with predictions of an intercalation model that was first proposed to explain the regulation of non-janus doublets to singlets.

Intracellular pattern reversal in Tetrahymena thermophila. I. Evidence for reverse intercalation in unbalanced doublets.

Homopolar doublets of Tetrahymena thermophila possess two sets of similar cell surface structures, the most prominent of which are the complex and asymmetrical oral apparatuses. These initially are located on opposite surfaces of the duplex cell, but tend to shift so that they are no longer directly opposite each other. The two sets of oral structures are then separated by one wider and one narrower arc of cell surface. When one arc becomes sufficiently narrow, a new third oral apparatus with partially reversed internal asymmetry frequently becomes interposed between the two preexisting oral apparatuses, always within the narrower arc. After this happens, the reliability of development of new oral structures, particularly of the interposed ones, is reduced. Contractile vacuole pores, typically present within both arcs of homopolar doublets, tend to disappear from the narrower arcs. This anomalous partial triplet condition appears to be a transient intermediate stage in the reversion of homopolar doublets to normal singlets. We interpret the interposition of a transient third oral system in doublets that are regulating toward the singlet state as being a consequence of reverse intercalation of new positional values subsequent to excessive crowding of the preexisting positional values. This interpretation is an adaptation of the shortest-distance intercalation rule of the polar coordinate model applied in an intracellular and morphallactic context.

Intracellular pattern reversal in Tetrahymena thermophila. II. Transient expression of a janus phenocopy in balanced doublets.

Homopolar doublets of Tetrahymena thermophila which have two normal oral systems directly opposite one another may undergo a global transformation of cell surface geometry to create transient imitations of mirror-image configurations brought about by mutations at janus gene loci. The process by which a typical doublet transforms into a janus-like organization involves loss of capacity to form oral structures at one of the two normal oral meridians, followed by interpolation of reversed oral structures at a new location to the cell's right of the disappearing normal oral meridian. At the same time, the contractile vacuole pore (CVP) set on the side of the cell that is undergoing the transformation shifts to the left. The combination of these events creates a symmetrical large-scale organization in which both of the CVP sets are situated on one side of the cell, between the normal and the partially reversed oral apparatus. This unilateral positioning of CVP sets is commonly manifested even when reversed oral structures are absent. These configurations probably represent intermediate stages in the transformation of balanced typical doublets into singlets. We propose that this pathway of regulation from the doublet to the singlet state, like the more common one that starts from unbalanced typical doublets (described in the preceding paper), involves reverse intercalation. The remarkable resemblance between the transient configuration described here and the stable configuration of janus mutant cells leads us to suggest that the phenotype of the mutant is also a consequence of reverse-intercalation, in that case provoked by a loss of capacity to maintain positional values rather than by a geometrical instability in the system of positional values.

How the mirror-image pattern specified by a janus mutation of Tetrahymena thermophila comes to expression.

The initial changes of cell-surface organization that occurred as the recessive janA1 (janus) mutation of Tetrahymena thermophila first became expressed were elucidated in a special mating scheme in which old macronuclei homozygous for janA+ were synchronously replaced by new macronuclei homozygous for janA1. During this period of onset of expression, the number, regularity, and asymmetry of the ciliary rows remained unchanged. New normal (primary) oral apparatuses (OAs) continued to be formed posterior to old OAs, as in normal cells. At about four fissions after conjugation, abnormal (secondary) OAs with a partial reversal of asymmetry began to appear nearly opposite to the primary OAs, close to but not at the eventual circumferential position of janA1 secondary OAs. The array of contractile vacuole pores (CVPs), normally located adjacent to two ciliary rows centered near 22% of the cell circumference to the right of the primary oral meridian, underwent a two-step transformation: first, the number of adjacent ciliary rows bearing CVPs increased to 3, 4, and sometimes 5, then "skipped" rows appeared within this broadened CVP-arc to split the single set of CVPs into two separated subsets. The CVP transformations occurred gradually and progressively. They began prior to the expression of secondary OAs but accelerated as secondary OAs appeared. As the CVP arc became broader, its midpoint shifted somewhat to the right, away from the primary oral meridian, but ended up close to halfway between the primary and secondary oral meridians. The data provide a better fit to an intercalation model than to an alternative double-gradient model, suggesting that the janA1 mutation alters the large-scale organization of positional values by preventing the expression of a subset of these values and thus provoking reverse-intercalation of the remainder.

Mutational analysis of patterning of oral structures in Tetrahymena. I. Effects of increased size on organization.

The oral apparatus (OA) of the ciliated protozoan Tetrahymena thermophila consists of four ordered arrays of ciliary units. In wild-type cells, these arrays are constant in spatial organization and vary little in size except during extreme starvation. Recessive mutations at five gene loci are known to increase the size of the OA. They do this by increasing the length of the ciliary arrays, without affecting their width and often without increasing their number beyond the usual four. Comparison of the oral arrays over a large range of sizes has revealed: (1) that the lengths of the anterior two of three parallel arrays (membranelles) are rather tightly coordinated; (2) that the specific basal body configurations resulting from remodelling of the membranelles are only slightly affected by large changes in lengths of membranelles; and (3) that the third membranelle is restricted to a nearly constant length, except in the very largest OAs in which the structure is lengthened but interrupted by a gap in the middle. This gap may reveal the spatial extent of a putative zone of basal body regression. These phenomena are not specific to any of the genotypes utilized in this investigation; the effect of the mutations is to loosen quantitative restrictions and thus reveal underlying associations and constraints.

Mutational analysis of patterning of oral structures in Tetrahymena. II. A graded basis for the individuality of intracellular structural arrays.

The ciliary arrays of the oral apparatus of the ciliated protozoan Tetrahymena thermophila each have their own unique 'pattern signature', which varies little so long as the number of arrays remains the same. In this study, we analyse the consequence of increases in the number of these arrays (membranelles) brought about by certain mutations. In oral apparatuses of mutant cells, the addition of a membranelle is associated with specific alterations in at least one of the other membranelles. The features that are altered include the relative lengths of membranelles, the state of ciliation of basal bodies located at specific positions within these membranelles, and the spatial configurations resulting from displacement of ciliary units during late oral development. The final organization of each membranelle depends upon its relative position along the length of the oral apparatus. This indicates that the membranelles are not individually 'named' by the organism, and suggests that the unit of pattern organization is the membranelle field as a whole. In the Discussion, we consider means for testing whether the same underlying idea might also apply to multicellular systems, such as the vertebrate limb, in which spatially ordered differences appear to be superimposed upon a fundamental repeating pattern.

Regulation of the pattern of basal bodies within the oral apparatus of Tetrahymena thermophila.

The number and arrangement of basal bodies included in the four compound ciliary organelles making up the mature oral apparatus of Tetrahymena thermophila ordinarily vary only slightly. Severe starvation brings about formation of oral structures with a reduced number of basal bodies within these organelles, and sometimes with a complete loss of one of the component organelles. Such reductions are stringently specified in spatial terms, but they do not represent simple and proportional shrinkage of the organelle complex. Instead, certain spatial features remain essentially unaltered, while others undergo major quantitative reductions, resulting in large changes in the internal proportions of the structures. This selective regulation can be explained in terms of the different parallel and sequential processes taking place during the development of this organelle complex. There is also no strict proportionality between the size of the oral apparatus and that of the cell; instead, oral apparatuses become relatively larger as cells become smaller. This is due in part to the inherent temporal discontinuity of oral development, but there is probably also a real change in the oral/body size relation at the time of oral development. The 'French flag' rule fails when applied to the relative sizes and internal proportions of organelle systems in this and in other ciliates.

Discontinuities and overlaps in patterning within single cells.

Ciliates such as Tetrahymena manifest longitudinal vegetative growth and periodic equatorial subdivision. Evidence is presented suggesting that this subdivision involves the formation of discontinuities associated with the fission zone that closely resemble expressions of the segment border in multicellular organisms. Unlike latitudes, cellular longitudes can potentially maintain clonal continuity. Features of the system of longitudinal positioning of contractile vacuole pores (c.v.ps) in wild-type cells are suggestive of a circular positional system wrapped around the cell circumference, with a reference border coinciding with the axis of oral development. This border marks a discontinuity that, unlike the fission zone, can be clonally propagated. A recessive mutant, janus (jan), brings about alterations in c.v.p. positioning that suggest that a second longitudinal reference border is located about 45% of the cell circumference to the cell's right of the first. This second border, along which abnormal oral structures sporadically appear, seems to maintain a positional system oriented in a direction opposite to the primary system. When jan first comes to expression in cells previously jan+, the pattern of c.v.p. longitudes changes gradually from that characteristic of wild-type cells to the jan pattern; this change begins before abnormal oral structures first appear along the second reference border. We suggest that the two reference borders, and the positional systems that they control, might be present in wild-type as well as jan cells. The oppositely directed positional systems are likely to overlap. A simple model is proposed to illustrate how overlapping positional systems might cooperate to generate patterns such as those observed.