The fine structure of protonephridia in Gnathostomulida and their comparison within Bilateria

Summary The fine structure of the protonephridia of Haplognathia rosea (Filospermoidea) and Gnathostomula paradoxa (Bursovaginoidea) is described. Each protonephridium consists of three different cells: (1) a monociliated terminal cell which constitutes the filtration area, (2) a nonciliated canal cell showing a special protonephridial outlet system, and (3) an intraepidermal cell — the nephroporus cell — constituting the nephroporus. The protonephridia are arranged serially. There is no canal system connecting the protonephridial units.Protonephridial characters in other Bilateria are considered. The pattern of characters in the protonephridia in the last common gnathostomulid stem species and presumed apomorphies in the protonephridia of the Gnathostomulida investigated are discussed.Abbreviations used in figures ac acessory centriole - AC additional epidermal cell - bb basal body - bl basal lamina - bm bundle of microvilli - c cilium - cc cilium duct cell - cd cilium duct - cr ciliary rootlet - crs structures resembling ciliary rootlets - di diplosome - ds desmosome - dy dictyosome - f filtration area - g granules - m mitochondrium - mv microvillus - n nucleus - NC nephroporus cell - np nephroporus - oc outlet canal - TC terminal cell - tl tubules of lacunar system     

Feinstruktur des Riechepithels von Calamoichthys calabaricus J. A. Smith (Pisces,Brachiopterygii)

Zusammenfassung Die vorliegende Untersuchung befaßt sich mit der Feinstruktur der Riechfalten von Calamoichthys calabaricus unter besonderer Berücksichtigung des Riech- und Flimmerepithels. — Das Flimmerepithel ist aus 3 Zelltypen aufgebaut: Flimmerzellen, wenigen Stützzellen und Basalzellen. Die Flimmerzellen sind mitochondrienreich und tragen bis zu 160 Flimmerhärchen pro Zelle. Diese Kinocilien besitzen an ihren Basalkörpern Zilienwurzeln, von denen ein Teil ins Zellinnere bis in Kernnähe zieht, während der andere Teil parallel zur Oberfläche verläuft und benachbarte Basalapparate verbindet. — Auch das Riechepithel, das gegen das Flimmerepithel scharf abgesetzt ist, besteht aus 3 Zelltypen: Rezeptoren, Stützzellen und Basalzellen. Die Rezeptoren haben eine einheitliche Gestalt und Struktur. Sie sind schlank keulenförmig und überragen mit einer kleinen Vesicula olfactoria die Epitheloberfläche. Seitlich sitzen an der Vesicula — unter konstant 25–27° Ablenkung von der Senkrechten — in der Regel 12 sensorische Cilien, die alle auf gleicher Höhe entspringen. Basal setzt sich das Rezeptorperikaryon in ein Axon fort. Die Axone mehrerer Rezeptoren vereinigen sich noch innerhalb des Epithels zu Bündeln, die durch die Basalmembran ins Bindegewebe ziehen. Die stark osmiophilen Stützzellen des Riechepithels durchziehen das Riechepithel von der Basalmembran bis zur Epitheloberfläche und tragen einzelne Cilien. Der verbreiterte Apikalteil der Stützzellen enthält zahlreiche Schleimvesikel, die auf eine sezernierende Funktion dieser Zellen hinweisen. Die präparative Behandlung von Riechepithelien wird kritisch diskutiert.

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Ultrastructure of the olfactory epithelium of Calamoichthys calabaricus J. A. Smith (Pisces, Brachiopterygii)

Summary The ciliary epithelium of the olfactory folds in Calamoichthys calabaricus is composed of ciliary cells, supporting cells, and basal cells. All ciliary cells contain numerous mitochondria and bear up to 160 kinocilia. Some rootlets of the basal bodies of the kinocilia, project towards the nucleus, while others run parallel to the epithelial surface and connect with neighbouring basal bodies. Ciliary and olfactory epithelia are separated from each other. — The olfactory epithelium contains olfactory receptor cells, supporting cells, and basal cells. The club shaped olfactory receptor cells have a uniform ultrastructure. The terminal portions of the olfactory dendrites form small olfactory vesicles which are seen above the olfactory surface. 12 sensory cilia project constantly to the more basal portion of the olfactory vesicles, each cilium forming a 25–27° angle with the vertical cell axis. Basally, an axon originates from each olfactory receptor cell. Axons from a number of olfactory receptor cells may combine to form bundles within the epithelium. The supporting cells of the olfactory epithelia are strongly osmiophilic. Supporting cells occur in all parts of the olfactory epithelium and bear few cilia. Numerous mucous vesicles, located within the apical region of the supporting cells, probably have a secretory function.

Mit Unterstützung durch die Deutsche Forschungsgemeinschaft.     

Generation second messengers by prostanoids in the iris-sphincter and ciliary muscles of cows, cats and humans

We have examined the generation of second messengers after stimulation of feline, bovine, human iris-sphincter and ciliary muscles by selected prostaglandins (PGs). The tissues, labeled or unlabeled with ³H-myo-inositol, were stimulated by a range of concentrations of 16, 16-dimethyl PGE₂, 11-deoxy PGE₁, 17-phenyl trinor PGE₂ and PGF_2α. In both tissues of all three species, 16, 16-dimethyl PGE₂ and 11-deoxy PGE₁ stimulated the formation of cyclic AMP. Butaprost, an EP₂ receptor agonist, which was tested only in feline ciliary muscle, generated cyclic AMP. In the feline iris-sphincter and in bovine and feline ciliary muscles, 17-phenyl trinor PGE₂, an EP₁ receptor agonist, significantly increased inositol phosphate turnover. The FP receptor agonist, PGF_2α stimulated inositol phosphate turnover in the bovine, feline, and human iris-sphincter muscles and in human ciliary muscles. Feline and bovine ciliary muscles did not respond to PGF_2α.These results suggest that EP₁ receptors are present in feline iris-sphincter muscle and in bovine and feline ciliary muscles. The EP₂ receptors exist in both tissue. These results also suggest the presence FP receptors in bovine, feline, and human iris-sphincter and in human ciliary muscles. Bovine and feline ciliary muscles do not appear to express FP receptors.     

Effect of leukotriene C4 exposure on ciliated cells of the nasal mucosa

To clarify the effects of leukotriene C4 (LTC4) on human ciliated epithelium, ciliary activity of the ethmoid sinus mucosa was measured photoelectrically in tissue culture. At concentrations ranging from 10⁻⁶M to 10⁻⁹M, LTC4 showed minimal effects on the ciliated epithelium during the initial 30 minutes of exposure; thereafter, ciliary inhibition was observed in a concentration- and time-dependent manner. Irrigation of the mucosa with culture medium 15 minutes after exposure prevented the LTC4-induced ciliary inhibition. However, irrigation 60 minutes after exposure failed to inhibit 10⁻⁸M LTC4-induced ciliary dysfunction and mucosal damage. The LTC4-induced ciliary inhibition was blocked in the presence of FPL-55712 and/or Ly-171883, both leukotriene receptor antagonists. L-serine and sodium tetraborate complex (SBC), a γ-glutamyl transpeptidase (γ-GTP) inhibitor, also inhibited the LTC4-induced ciliary inhibition. These findings indicate that LTC4 is converted to LTD4 by γ-GTP during 60 minutes of exposure, and LTC4 itself has minimal direct effects on the ciliated cells.     

Recruitment of β-Arrestin into Neuronal Cilia Modulates Somatostatin Receptor Subtype 3 Ciliary Localization

Primary cilia are essential sensory and signaling organelles present on nearly every mammalian cell type. Defects in primary cilia underlie a class of human diseases collectively termed ciliopathies. Primary cilia are restricted subcellular compartments, and specialized mechanisms coordinate the localization of proteins to cilia. Moreover, trafficking of proteins into and out of cilia is required for proper ciliary function, and this process is disrupted in ciliopathies. The somatostatin receptor subtype 3 (Sstr3) is selectively targeted to primary cilia on neurons in the mammalian brain and is implicated in learning and memory. Here, we show that Sstr3 localization to cilia is dynamic and decreases in response to somatostatin treatment. We further show that somatostatin treatment stimulates β-arrestin recruitment into Sstr3-positive cilia and this recruitment can be blocked by mutations in Sstr3 that impact agonist binding or phosphorylation. Importantly, somatostatin treatment fails to decrease Sstr3 ciliary localization in neurons lacking β-arrestin 2. Together, our results implicate β-arrestin in the modulation of Sstr3 ciliary localization and further suggest a role for β-arrestin in the mediation of Sstr3 ciliary signaling.     

Ubiquitin chains earmark GPCRs for BBSome-mediated removal from cilia

Regulated trafficking of G protein–coupled receptors (GPCRs) controls cilium-based signaling pathways. β-Arrestin, a molecular sensor of activated GPCRs, and the BBSome, a complex of Bardet–Biedl syndrome (BBS) proteins, are required for the signal-dependent exit of ciliary GPCRs, but the functional interplay between β-arrestin and the BBSome remains elusive. Here we find that, upon activation, ciliary GPCRs become tagged with ubiquitin chains comprising K63 linkages (UbK63) in a β-arrestin–dependent manner before BBSome-mediated exit. Removal of ubiquitin acceptor residues from the somatostatin receptor 3 (SSTR3) and from the orphan GPCR GPR161 demonstrates that ubiquitination of ciliary GPCRs is required for their regulated exit from cilia. Furthermore, targeting a UbK63-specific deubiquitinase to cilia blocks the exit of GPR161, SSTR3, and Smoothened (SMO) from cilia. Finally, ubiquitinated proteins accumulate in cilia of mammalian photoreceptors and Chlamydomonas cells when BBSome function is compromised. We conclude that Ub chains mark GPCRs and other unwanted ciliary proteins for recognition by the ciliary exit machinery.     

Untersuchungen an der Regio olfactoria des Aals,Anguilla anguilla L.

Zusammenfassung Das Flimmerepithel von Anguilla anguilla besteht aus 4 Zellarten: Flimmerzellen, Stützzellen, Basalzellen und Schleimbecherzellen. Flimmerzellen enthalten im oberen Zelldrittel zahlreiche Mitochondrien und tragen an ihrer Oberfläche bis zu 140 Kinocilien. Die Basalkörper dieser Kinocilien haben lange Wurzelfilamente, von denen ein Teil ins Zellinnere zieht; der andere Teil verläuft parallel zur Oberfläche und verbindet benachbarte Basalapparate. — Ein Übergangsepithel verknüpft das Flimmerepithel mit dem Riechepithel. Im Riechepithel finden sich außer den Zellarten des Flimmerepithels die Rezeptoren. Bei einheitlichem Aufbau des Zellkörpers lassen sich aufgrund rein morphologischer Unterschiede der Vesiculae olfactoriae 3 Rezeptortypen unterscheiden: 1. Cilien-Rezeptor, 2. Mikrovilli-Rezeptor und 3. Pfriem-Rezeptor. — Der Cilien-Rezeptor trägt unterhalb der Vesicula olfactoria in einer Einschnürung 4–8 sensorische Cilien, die alle auf gleicher Höhe entspringen. Zwei gegenüberliegende sensorische Cilien schließen einen konstanten Winkel von 60° ein. — Der Mikrovilli-Rezeptor trägt auf seiner abgerundeten Vesicula olfactoria 30 bis 60 Mikrovilli von 0,1 m Dicke und bis zu 5 m Länge. Der Mikrovillus wird von einem zentralen, 160 Å weiten, Tubulus durchzogen. Unterhalb der Vesicula olfactoria liegen mehrere Centriolen. Die Rezeptornatur dieser Zellen wird durch ein Axon unterstrichen. — Der Pfriem-Rezeptor besitzt eine 0,8 m breite und bis zu 4 m lange Vesicula olfactoria ohne sensorische Cilien und ohne Mikrovilli. Im Lumen der Vesicula olfactoria befinden sich neben Neurotubuli auch Fibrillen von 40–50 Å Durchmesser, die gebündelt auftreten. An der Basis des Köpfchens liegen mehrere Centriolen.

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Studies of the regio olfactoria in the eel, Anguilla anguilla I. Fine structure of the olfactory epithelium

Summary The ciliary and olfactory epithelia of the olfactory folds in Anguilla anguilla were studied with the electron microscope. The ciliary epithelium is composed of ciliary cells, supporting cells, basal cells, and mucous cells. The ciliary cells contain numerous mitochondria in their apical portion and bear up to 140 cilia. The ciliary basal bodies have rootlets, some of which project towards the central part of the cell, and others parallel to the cell surface thereby connecting neighbouring basal bodies. A transitional epithelium is located between the ciliary and olfactory epithelia. The olfactory epithelium is composed of the same 4 cell types of the ciliary epithelium and besides contains three morphologically different receptor cell types: ciliary receptor cells, microvillous receptor cells, and receptors with a single rodshaped free appendage. The ciliary receptors have 4 to 8 sensory cilia which project from below the vesicula olfactoria, each forming a constant angle of about 30° with the vertical cell axis. The vesicula olfactoria of the microvillous receptors bears from 30 to 60 microvilli, each of 0.1 m diameter and up to 5 m length. Each microvillus of this receptor type contains a central tubulus of 160 Å diameter. Few centrioles are located closely to the vesicula olfactoria. The third receptor type, which has neither cilia nor microvilli, is characterised by a single rod-shaped appendage of 0.8 m diameter which projects up to 4 m above the epithelial surface. This appendage contains neurotubules and fibril bundles; some centrioles lie close to the base of the appendage.

     

Zwei Muskelrezeptoren an der Mandibel von Leuctra (Insecta,Plecoptera) — Beispiele für nicht-ciliäre Sinnesorgane mit Tubularkörper-ähnlichen Strukturen

Zusammenfassung Beide Muskelrezeptoren an der Mandibel von Leuctra ziehen vom vorderen Tentorium-Arm zur Mandibel-Basis. Der ventrale Rezeptor besteht aus zwei dünnen Muskelfasern (6 bis 7 m Durchmesser) und mindestens 10 multiterminalen Sinneszellen, deren Dendriten sich im Innern der Fasern verzweigen und an den Z-Scheiben enden. Der dorsale Rezeptor besitzt drei ähnlich dünne Muskelfasern, aber nur eine einzelne multiterminale Sinneszelle. Ihre Dendriten enden im Ansatzgebiet des Muskels, zwischen Muskelfasern und Epidermiszellen.Beide Rezeptoren haben im wesentlichen denselben Feinbau wie bei Coleopteren, jedoch treten — besonders beim dorsalen Rezeptor — in den Dendriten-Enden Strukturen auf, die den Tubularkörpern bei Mechanosensillen ähneln.

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Two muscle receptor organs at the mandible of Leuctra (Insecta, Plecoptera) — examples for non-ciliary sense organs with tubular-body-like structures

Summary The muscle receptor organs of the mandible of Leuctra extend between the anterior tentorial arm and the mandible base. The ventral receptor is composed of two thin muscle fibres (6–7 m in diameter) and at least ten multiterminal sensory cells, the dendrites of which branch in the interior of the fibres and end near the z-bands. The dorsal receptor organ consists of three muscle fibres of similar diameter and only one multiterminal sensory cell. The dendritic ends lie at the distal end of the muscle, where muscle fibres and epidermal cells make contact.Both receptor organs essentially show the same ultrastructural characteristics as in Coleoptera. However, the dorsal receptor organ in particular possesses organelles in its dendritic ends, which look like the tubular bodies in ciliary mechanoreceptors.

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Mit Unterstützung durch die Deutsche Forschungsgemeinschaft     

Smoothened determines β-arrestin–mediated removal of the G protein–coupled receptor Gpr161 from the primary cilium

Dynamic changes in membrane protein composition of the primary cilium are central to development and homeostasis, but we know little about mechanisms regulating membrane protein flux. Stimulation of the sonic hedgehog (Shh) pathway in vertebrates results in accumulation and activation of the effector Smoothened within cilia and concomitant disappearance of a negative regulator, the orphan G protein–coupled receptor (GPCR), Gpr161. Here, we describe a two-step process determining removal of Gpr161 from cilia. The first step involves β-arrestin recruitment by the signaling competent receptor, which is facilitated by the GPCR kinase Grk2. An essential factor here is the ciliary trafficking and activation of Smoothened, which by increasing Gpr161–β-arrestin binding promotes Gpr161 removal, both during resting conditions and upon Shh pathway activation. The second step involves clathrin-mediated endocytosis, which functions outside of the ciliary compartment in coordinating Gpr161 removal. Mechanisms determining dynamic compartmentalization of Gpr161 in cilia define a new paradigm for down-regulation of GPCRs during developmental signaling from a specialized subcellular compartment.     

Embryology of the Turbellaria and its phylogenetic significance

Developmental characters — including oocyte and yolk cell structure, patterns of cleavage, and modes of gastrulation — are presented and examined in relation to the phylogeny of the Turbellaria. Eggshell granules, which have been demonstrated to occur in the oocytes of entolecithal eggs and the yolk cells of ectolecithal eggs, are compared among species, and their potential value as a taxonomic character is discussed. The quartet 4d spiral cleavage of the entolecithal egg of polyclads is described as reminiscent of the primitive pattern of early development for the Turbellaria. This is compared to duet spiral cleavage of acoels, and possible phylogenetic schemes involving the two types of spiral cleavage are reviewed. The link between the precise spiral cleavage, which characterizes development of most archoophorans, and blastomere separation (Blastomeren-Anarchie), which occurs in several neoophoran orders, is established by the occurrence of quartet 4d spiral cleavage in one neoophoran order, and of both quartet spiral cleavage and Blastomeren-Anarchie in different species of a second neoophoran order. The epibolic gastrulation of polyclads is described as primitive for the Turbellaria because of its similarity to that of other members of the Spiralia. Although no identical process occurs in neoophoran development, the earlier event of formation of the hull membrane in some neoophorans, and the later event of formation of the definitive epidermis in all neoophorans studied are presented as processes of possible homology to the epibolic gastrulation of polyclads. The lack of correspondence between polyclads and neoophorans in the relationship of the definitive body axes to the egg axis is discussed, and an hypothesis is advanced to account for the differences. The phylogenetic relationships indicated by known developmental phenomena differ only slightly from the scheme presented by Karling in 1974.     

Ciliary transport regulates PDGF-AA/αα signaling via elevated mammalian target of rapamycin signaling and diminished PP2A activity

Primary cilia are built and maintained by intraflagellar transport (IFT), whereby the two IFT complexes, IFTA and IFTB, carry cargo via kinesin and dynein motors for anterograde and retrograde transport, respectively. Many signaling pathways, including platelet- derived growth factor (PDGF)-AA/αα, are linked to primary cilia. Active PDGF-AA/αα signaling results in phosphorylation of Akt at two residues: P-Akt^T308 and P-Akt^S473, and previous work showed decreased P-Akt^S473 in response to PDGF-AA upon anterograde transport disruption. In this study, we investigated PDGF-AA/αα signaling via P-Akt^T308 and P-Akt^S473 in distinct ciliary transport mutants. We found increased Akt phosphorylation in the absence of PDGF-AA stimulation, which we show is due to impaired dephosphorylation resulting from diminished PP2A activity toward P-Akt^T308. Anterograde transport mutants display low platelet-derived growth factor receptor (PDGFR)α levels, whereas retrograde mutants exhibit normal PDGFRα levels. Despite this, neither shows an increase in P-Akt^S473 or P-Akt^T308 upon PDGF-AA stimulation. Because mammalian target of rapamycin complex 1 (mTORC1) signaling is increased in ciliary transport mutant cells and mTOR signaling inhibits PDGFRα levels, we demonstrate that inhibition of mTORC1 rescues PDGFRα levels as well as PDGF-AA–dependent phosphorylation of Akt^S473 and Akt^T308 in ciliary transport mutant MEFs. Taken together, our data indicate that the regulation of mTORC1 signaling and PP2A activity by ciliary transport plays key roles in PDGF-AA/αα signaling.     

Nematode Chemosensilla: Form and Function

As an introduction to a symposium of nematode chemoreception, the anatomy of nematode chemosensilla, their distribution on plant parasitic nematodes, and their possible functional roles is briefly reviewed. Comparison of nematode chemosensilla with those of other animals shows their greater resemblance to olfactory primary sense cells of vertebrates. Although the sensory process is obviously derived from a cilium, the absence of many ciliary features is noted. Retention of the ciliary necklace may be important functionally. A simple model is proposed, wherein binding of stimulant molecules to receptors in the membrane of the cilium-derived process results in entry of Na⁺ and Ca⁺⁺ (the latter via the ciliary necklace) to produce a receptor potential that spreads along the dendrite to the cell body where action potentials continue along the short axon to synapses.     

Sense organs in the femur of the stick insect and their relevance to the control of position of the femur-tibia-joint

Summary The sensory innervation pattern is described for the femur of the middle and the hind legs ofCarausius morosus. — In one of the nerves (F121) extracellular recordings show a unit which mirrors the tension of the flexor tibiae muscle (tension receptor). The tension receptor increases the firing rate of the slow extensor tibiae motoneuron. It measures the tension of one or more muscle fibres of the anterior side near the distal end of the muscle. The anatomical basis of this receptor is uncertain. — Another receptor was found on the ventral side of the distal end of the apodeme of the extensor tibiae muscle (apodeme receptor). Recordings from this receptor could not be obtained inCarausius. But inExtatosoma tiaratum it responded to stretching of the nerve. In the natural position it shows a minimum of excitation in the 90°-position of the femur-tibia-joint and an increase in firing rate for both flexion and extension. — Tactile hairs react phasically and have no special sensitivity for one direction. Two receptors at the dorsal side of the femur-tibia-joint (RDAL and RDPL), which are situated in the same position as inSchistocerca hind legs, react phasically to extension movements and fire tonically in the most extended position of the joint. — The influence of these receptors on the position of the femur-tibia-joint is only weak.Supported by Deutsche Forschungsgemeinschaft     

The accessory outer segment of rods and cones in the retina of the guppy,Poecilia reticulata P. (teleostei)

Summary The ultrastructure of the accessory outer segment (AOS) — a ciliumlike structure emanating from the inner segment and running alongside the outer segment of photoreceptors — is described. The AOS occurs in both rods and cones of Poecilia reticulata. Its ultrastructure, including the arrangement of microtubules, which originate from the ciliary stalk, is the same in rods and cones. The cone-AOS is connected with the outer segment by a thin plasmabridge, whereas the rod-AOS lies embedded within the outer segment. The outer segment of the cone, in contrast to that of the rod, is separated from the pigment epithelium by a large extracellular space. An intimate contact, however, is secured by the AOS; its membrane is closely appositioned to the pigment epithelium membrane. The functional significance of the AOS and its possible occurrence in other vertebrate classes, are discussed.     

Fine structure of the ocellus of Sarsia tubulosa (Hydrozoa,Anthomedusae)

Summary The fine structure of the ocellus of Sarsia tubulosa is described. The ocellar cup is formed of pigment cells and receptor cells. The receptor cells outnumber the pigment cells in almost a 2:1 ratio. Lateral extensions of neighbouring pigment cells enclose a distal region of 2 to 10 receptor cells. The receptor cell body is 5–7 m in diameter with an apical extension (20–60 m long) that reaches the ocellar cavity. A cilium (9+2 microtubules) arises from the distal part of the receptor cell. The ciliary membrane forms lateral microvilli. The tips of a number of cilia are swollen into large vesicles forming a cornea. The central region of the ocellar cavity contains extracellular electron dense homogeneous material surrounded by swollen ciliary tips and small vesicles. The close apposition between the plasma membrane covering the distal part of adjacent receptor cells as well as the adjacent ciliary shafts suggests the presence of gap junctions. The basal part of each receptor cell forms an axon. The axons of receptor cells form 3 to 4 nerve bundles that join to form the optic nerve. Synapses occur between receptor cell bodies, between axons and receptor cell bodies and among axons.     

Mapping of the α4 subunit gene (GABRA4) to human chromosome 4 defines an α2—α4—β1—γ1 gene cluster: further evidence that modern GABAA receptor gene clusters are derived from an ancestral cluster

We demonstrated previously that an α₁—β₂—γ₂ gene cluster of the γ-aminobutyric acid (GABA_A) receptor is located on human chromosome 5q34–q35 and that an ancestral α—β—γ gene cluster probably spawned clusters on chromosomes 4, 5, and 15. Here, we report that the α₄ gene (GABRA4) maps to human chromosome 4p14–q12, defining a cluster comprising the α₂, α₄, β₁, and γ₁ genes. The existence of an α₂—α₄—β₁—γ₁ cluster on chromosome 4 and an α₁—α₆—β₂—γ₂ cluster on chromosome 5 provides further evidence that the number of ancestral GABA_A receptor subunit genes has been expanded by duplication within an ancestral gene cluster. Moreover, if duplication of the α gene occurred before duplication of the ancestral gene cluster, then a heretofore undiscovered subtype of α subunit should be located on human chromosome 15q11–q13 within an α₅—α_x—β₃—γ₃ gene cluster at the locus for Angelman and Prader—Willi syndromes.     

Basis of Chloride Transport in Ciliary Epithelium

The aqueous humor is formed by the bilayered ciliary epithelium. The pigmented ciliary epithelium (PE) faces the stroma and the nonpigmented ciliary epithelium (NPE) contacts the aqueous humor. Cl⁻ secretion likely limits the rate of aqueous humor formation. Many transport components underlying Cl⁻ secretion are known. Cl⁻ is taken up from the stroma into PE cells by electroneutral transporters, diffuses to the NPE cells through gap junctions and is released largely through Cl⁻ channels. Recent work suggests that significant Cl⁻ recycling occurs at both surfaces of the ciliary epithelium, providing the basis for modulation of net secretion. The PE-NPE cell couplet likely forms the fundamental unit of secretion; gap junctions within the PE and NPE cell layers are inadequate to maintain constancy of ionic composition throughout the epithelium under certain conditions. Although many hormones, drugs and signaling cascades are known to have effects, a persuasive model of the regulation of aqueous humor formation has not yet been developed. cAMP likely plays a central role, potentially both enhancing and reducing secretion by actions at both surfaces of the ciliary epithelium. Among other hormone receptors, A₃ adenosine receptors likely alter intraocular pressure by regulating NPE-cell Cl⁻ channel activity. Recently, functional evidence for the regional variation in ciliary epithelial secretion has been demonstrated; the physiologic and pathophysiologic implications of this regional variation remain to be addressed.This revised version was published online in June 2005 with a corrected cover date.     

Mechanisms of interaction between central neurons and abdominal stretch receptors in the crayfish

Mechanisms of interaction between central and receptor neurons of the crayfish (the principal inhibitory neuron — PIN — and the slow-adapting stretch receptor — SAR) when functioning under different conditions were investigated: during regular spontaneous activity of SAR, grouped discharges of PIN, and regular spontaneous activity of PIN. A close connection was found between the various parameters of the PIN and SAR responses. Adaptation of SAR to the action of adequate, regular repetitive stimulation takes place faster in the presence of stationary background activity of PIN. The appearance and disappearance of SAR spike activity are determined by the ratio between the firing rates of SAR and PIN: at the moment of changeover the neuron with the higher firing rate is predominant.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Institute of Zoology, Academy of Sciences of the Moldavian SSR, Kishinev. Translated from Neirofiziologiya, Vol. 4, No. 4, pp. 429–438, July–August, 1972.     

Cooperation of the IFT-A complex with the IFT-B complex is required for ciliary retrograde protein trafficking and GPCR import

Cilia sense and transduce extracellular signals via specific receptors. The intraflagellar transport (IFT) machinery mediates not only bidirectional protein trafficking within cilia but also the import/export of ciliary proteins across the ciliary gate. The IFT machinery is known to comprise two multisubunit complexes, namely, IFT-A and IFT-B; however, little is known about how the two complexes cooperate to mediate ciliary protein trafficking. We here show that IFT144–IFT122 from IFT-A and IFT88–IFT52 from IFT-B make major contributions to the interface between the two complexes. Exogenous expression of the IFT88(Δα) mutant, which has decreased binding to IFT-A, partially restores the ciliogenesis defect of IFT88-knockout (KO) cells. However, IFT88(Δα)-expressing IFT88-KO cells demonstrate a defect in IFT-A entry into cilia, aberrant accumulation of IFT-B proteins at the bulged ciliary tips, and impaired import of ciliary G protein–coupled receptors (GPCRs). Furthermore, overaccumulated IFT proteins at the bulged tips appeared to be released as extracellular vesicles. These phenotypes of IFT88(Δα)-expressing IFT88-KO cells resembled those of IFT144-KO cells. These observations together indicate that the IFT-A complex cooperates with the IFT-B complex to mediate the ciliary entry of GPCRs as well as retrograde trafficking of the IFT machinery from the ciliary tip.