Dendritic Cilia Research Articles

The β2-adrenergic receptor as a surrogate odorant receptor in mouse olfactory sensory neurons.

In the mouse, mature olfactory sensory neurons (OSNs) express one allele of one of the ~1200 odorant receptor (OR) genes, which encode G-protein coupled receptors (GPCRs). Axons of OSNs that express the same OR coalesce into homogeneous glomeruli at conserved positions in the olfactory bulb. ORs are involved in OR gene choice and OSN axonal wiring, but the mechanisms remain poorly understood. One approach is to substitute an OR genetically with another GPCR, and to determine in which aspects this GPCR can serve as a surrogate OR under experimental conditions. Here, we characterize a novel gene-targeted mouse strain in which the mouse β2-adrenergic receptor (β2AR) is coexpressed with tauGFP in OSNs that choose the OR locus M71 for expression (β2AR→M71-GFP). By crossing these mice with β2AR→M71-lacZ gene-targeted mice, we find that differentially tagged β2AR→M71 alleles are expressed monoallelically. The OR coding sequence is thus not required for monoallelic expression - the expression of one of the two alleles of a given OR gene in an OSN. We detect strong β2AR immunoreactivity in dendritic cilia of β2AR→M71-GFP OSNs. These OSNs respond to the β2AR agonist isoproterenol in a dose-dependent manner. Axons of β2AR→M71-GFP OSNs coalesce into homogeneous glomeruli, and β2AR immunoreactivity is detectable within these glomeruli. We do not find evidence for expression of endogenous β2AR in OSNs of wild-type mice, also not in M71-expressing OSNs, and we do not observe overt differences in the olfactory system of β2AR and β1AR knockout mice. Our findings corroborate the experimental value of the β2AR as a surrogate OR, including for the study of the mechanisms of monoallelic expression.

Intraflagellar transport/Hedgehog-related signaling components couple sensory cilium morphology and serotonin biosynthesis in Caenorhabditis elegans.

Intraflagellar transport in cilia has been proposed as a crucial mediator of Hedgehog signal transduction during embryonic pattern formation in both vertebrates and invertebrates. Here, we show that the Hh receptor Patched-related factor DAF-6 and intraflagellar transport modulate serotonin production in Caenorhabditis elegans animals, by remodeling the architecture of dendritic cilia of a pair of ADF serotonergic chemosensory neurons. Wild-type animals under aversive environment drastically reduce DAF-6 expression in glia-like cells surrounding the cilia of chemosensory neurons, resulting in cilium structural remodeling and upregulation of the serotonin-biosynthesis enzyme tryptophan hydroxylase tph-1 in the ADF neurons. These cellular and molecular modifications are reversed when the environment improves. Mutants of daf-6 or intraflagellar transport constitutively upregulate tph-1 expression. Epistasis analyses indicate that DAF-6/intraflagellar transport and the OCR-2/OSM-9 TRPV channel act in concert, regulating two layers of activation of tph-1 in the ADF neurons. The TRPV signaling turns on tph-1 expression under favorable and aversive conditions, whereas inactivation of DAF-6 by stress results in further upregulation of tph-1 independently of OCR-2/OSM-9 activity. Behavioral analyses suggest that serotonin facilitates larval animals resuming development when the environment improves. Our study revealed the cilium structure of serotonergic neurons as a trigger of regulated serotonin production, and demonstrated that a Hedgehog-related signaling component is dynamically regulated by environment and underscores neuroplasticity of serotonergic neurons in C. elegans under stress and stress recovery.

Presynaptic Cyclic Nucleotide-Gated Ion Channels Modulate Neurotransmission in the Mammalian Olfactory Bulb

Cyclic nucleotide-gated channels (CNGCs) on the dendritic cilia of olfactory receptor neurons (ORNs) are critical for sensory transduction in the olfactory system. Do CNGCs also play a role in the axons and/or nerve terminals of ORNs? We find that the cyclic nucleotides cAMP and cGMP can both facilitate and depress synaptic transmission between olfactory nerve fibers and their targets in olfactory bulb glomeruli. Cyclic nucleotides increase intracellular Ca 2+ in ORN terminals and enhance spontaneous transmitter release; at higher concentrations, cyclic nucleotides depress evoked transmission by altering olfactory nerve excitability. Cyclic nucleotides have no effect on transmission or nerve excitability, however, in mice lacking olfactory CNGCs. Taken together, our results identify a novel role for presynaptic CNGCs in modulating neurotransmission.

Guanylyl cyclases as a family of putative odorant receptors.

Mammals can discriminate among a large number (> 10,000) of unique odorants. The most highly supported explanation for this ability is that olfactory neurons express a large number of seven transmembrane receptors that are not spatially organized at the level of the olfactory epithelium, but whose axonal projections form a distinct pattern within the olfactory bulb. The odor-induced signaling pathway in olfactory neurons includes a Gs-like protein (G(olf)) that activates a specific adenylyl cyclase (type III) isoform, resulting in elevations of cyclic AMP and subsequent activation of a cyclic nucleotide-gated channel. The channel also can be regulated by cyclic GMP. Recently, an olfactory neuron-specific guanylyl cyclase was discovered in rodents, and subsequently a large family of sensory neuronal guanylyl cyclases was identified in nematodes. These guanylyl cyclases are concentrated in the plasma membrane of the dendritic cilia and contain extracellular domains that retain many of the primary sequence characteristics of guanylyl cyclases known to be receptors for various peptides. Thus, the guanylyl cyclases appear to represent a second family of odorant/pheromone receptors.

Ultrastructural studies of microtubules and microtubule organizing centers of the vertebrate olfactory neuron.

The olfactory neuron is specialized along its length into highly determined morphological regions. These regions include the dendritic cilia, dendritic vesicle, dendritic shaft proper, perikaryon, axon, zone of transition where the axon widens as it approaches its termination, and the axon terminal. Except for the zone of transition and the terminal, characteristic populations of microtubules occur in these compartments. In the olfactory vesicle, three discrete microtubule organizing centers (MTOCs) nucleate microtubules: the basal body, the lateral foot associated with the body, and dense masses of nearby material. Little is known about MTOCs elsewhere in the neuron, although the polarity of the axonal microtubules indicate that they originate at or near the perikaryon. An attempt is made to summarize what is known of the origin, structure, distribution, and function of microtubules in vertebrate olfactory neurons, which are useful model systems in which to study microtubules. Information about olfactory neuron microtubules may be applicable to neurons in general (e.g., the discovery that axons contain microtubules of uniform polarity was first made in the olfactory neuron) or to microtubules in other eukaryotic cells.

THE ULTRASTRUCTURE OF AN INTEGUMENTAL MICROTRICH SENSILLUM IN GAMMARUS SETOSUS (AMPHIPODA)

The dorsolateral body surface in Gammarus setosus is equipped with integumental microtrich sensilla whose sockets are oval depressions in the cuticle. Each socket is based on 4 epidermal cells. Cell 1 forms a smooth dome on one side of the rim over the base of the microtrich, and lines the bottom of the depression with tubercles. Cells 2 and 3 flank cell 1 and meet opposite the dome where cell 4 completes the depression. The microtrich has an apical pore under a tapered hood and its surface is decorated by a scaliculus. The 8 microtrich receptor cells are at the base of the epidermis within the basal lamina. Each group is at a distance of 100 um or more from the socket it innervates. The axons associate in bundles and on each side join the tegumentary nerve behind the eyes, pass through the olfactory lobe cell layer into the tritocerebrum. Each microtrich receptor unit receives 8 unbranched dendrites. The neurons and their processes are sheathed by 4 serially arranged auxiliary cells: the thecogen, trichogen, and 2 tormogen, which are surrounded by accessory cells with dense nuclei and cytoplasm. The receptorlymph cavity delimited by the thecogen cell does not expand the receptor unit at the level of the stepwise ciliary connections. The basal bodies of the 9 + 0 cilia are unusual in having 9 doublet microtubules embedded in dense material and 9 peripheral filaments. The cuticle of the microtrich is derived from the processes of the trichogen and 2 thecogen cells which can be identified during ecdysis. In the intermolt the processes are withdrawn from the receptor canal into recesses within the endocuticle below the sensillum. Since all 8 dendritic cilia are structurally similar and receive stimuli via the apical pore, and none is associated with a scolopale or ends in a tubular body, the microtrich sensilla are presumed to be chemoreceptive.

Ultrastructure of the sensory epithelia of oral tube, fungiform sensory bodies, and terminal knobs of tentacles of Ovatella myosotis Draparnaud (Archaeopulmonata, Gastropoda).

Sensory epithelia of the oral tube, a fungiform body anterior to the tentacles and of the terminal knob of tentacles, were studied in Ovatella myosotis by electron microscopy. All three epithelia consist of columnar support cells, sensory cells, and, except in the oral tube, numerous goblet cells. The epithelia differ significantly in their apical differentiations. In the oral tube an outer layer is formed by irregularly bent villi of support cells completely embedded in a surface coat. Cilia and cytofila of the dendrites of sensory cells intertwine throughout the entire depth of the villous layer. In the fungiform sensory body some of the villi of support cells are singly branched. Their basal region is free of a surface coat. In this region cytofila and cilia of dendrites form a spongy layer, some cytofila extending into the surface coat. In the tentacular terminal knob the villi of the support cells branch dichotomously once or twice, a single villus thus ending with 2-4 tips. Only these terminal twigs are invested with the surface coat. The cytofila and dendritic cilia are confined to a broad spongy layer underneath. Three types of dendrites are present. They differ in their number of cilia, structure of basal bodies and occurrence in the three epithelia. Dendritic cytofila are most abundant in the tentacular terminal knob and least numerous in the oral tube. The observations are discussed with respect to corresponding epithelia in other pulmonates, the homology of the fungiform body, and possible functional correlates of structural features.

Unusual fine structure of sensory hair triad of the millipede, Polyxenus.

Three sensory hairs are inserted at right angles to teach other at the temporal region of the head of Polyxenus. They display structural features characteristic of trichobothria and of scolopidia as well. They also manifest several unusual details: 1) The dendritic cilia are enclosed within a capsule formed by enveloping cells. 2) The dendritic cilia are interconnected by desmosome-like junctions. 3) The 9 X 2+0 organization of the dendritic ciliary microtubules is maintained over the entire length of the cilia. 4) Neither a tubular body nor even so much as an inflation of the dendritic cilia develop. 5) Pores and pore tubules occur in the lower halves of the hairs. This uncommon combination of structural details renders the modality of the adequate stimulus uncertain. Though most features suggest a mechanoreceptor, the hairs could function as chemo-, hygro- and/or thermoreceptors as well.

Antennal Sensory Hairs in Talitrid Amphipods (Crustacea)

AbstractSensory hairs from antennulae and antennae of the two species Orchestia platensis Krøyer and Talorchestia deshayesii Audouin were studied by means of light microscopy and scanning and transmission electron microscopy. The complicated cuticular structures of the hair wall are described and figured. The central hair cavity opens in a rather wide terminal pore. Dendritic cilia from sensory neurons with perikarya inside the antennae run the whole length of the hair and terminate in the pore opening. The distal parts of the cilia contain osmiophilic material. It is concluded on morphological grounds that the hairs are chemosensory organs.