Thomas A. Keil

Decoding Cilia Function: Defining Specialized Genes Required for Compartmentalized Cilia Biogenesis

The evolution of the ancestral eukaryotic flagellum is an example of a cellular organelle that became dispensable in some modern eukaryotes while remaining an essential motile and sensory apparatus in others. To help define the repertoire of specialized proteins needed for the formation and function of cilia, we used comparative genomics to analyze the genomes of organisms with prototypical cilia, modified cilia, or no cilia and identified approximately 200 genes that are absent in the genomes of nonciliated eukaryotes but are conserved in ciliated organisms. Importantly, over 80% of the known ancestral proteins involved in cilia function are included in this small collection. Using Drosophila as a model system, we then characterized a novel family of proteins (OSEGs: outer segment) essential for ciliogenesis. We show that osegs encode components of a specialized transport pathway unique to the cilia compartment and are related to prototypical intracellular transport proteins.

FUNCTIONAL MORPHOLOGY OF INSECT MECHANORECEPTORS

This paper reviews the structure and function of insect mechanoreceptors with respect to their cellular, subcellular, and cuticular organization. Four types are described and their function is discussed: 1, the bristles; 2, the trichobothria; 3, the campaniform sensilla; and 4, the scolopidia. Usually, bristles respond to touch, trichobothria to air currents and sound, campaniform sensilla to deformation of the cuticle, and scolopidia to stretch. Mechanoreceptors are composed of four cells: a bipolar sensory neuron, which is enveloped by the thecogen, the trichogen, and the tormogen cells. Apically, the neuron gives off a ciliary dendrite which is attached to the stimulus‐transmitting cuticular structures. In types 1–3, the tip of the dendrite contains a highly organized cytoskeletal complex of microtubules, the “tubular body,” which is connected to the dendritic membrane via short rods, the “membrane‐integrated cones” (MICs). The dendritic membrane is attached to the cuticle via fine attachment fibers. The hair‐type sensilla (types 1, 2) are constructed as first‐order levers, which transmit deflection of the hair directly to the dendrite tip. In campaniform sensilla (type 3), there is a cuticular dome instead of a hair and the dendrite is stimulated by deformation of the cuticle. In these three types, a slight lateral compression of the dendrite tip is most probably the effective stimulus. In scolopidia, the dendritic membrane is most probably stimulated by stretch. On the subcellular level, connectors between the cytoskeleton, the dendritic membrane, and extracellular (cuticular) structures are present in all four types and are thought to be engaged in membrane depolarization. Microsc. Res. Tech. 39:506–531, 1997.

Mechanosensitive and Olfactory Sensilla of Insects

Insects are the most successful land dwellers among the invertebrates. They owe this achievement largely to their cuticle, which provides mechanical support and protects against water loss at the same time. This exoskeleton forms a barrier between the environment and the interior milieu, so that sense organs need unique adaptations to remain accessible to external stimuli.

Morphology and Development of the Peripheral Olfactory Organs

The olfactory organ of an insect is formed by a pair of head appendages, the antennae which carry arrays of innervated hair structures, the sensilla. The antennae are the most important multimodal sensory organs for the insects and their relatives, bearing not only the sensilla of olfaction, but also those of taste, mechano-, hygro-, and thermoreception, and sometimes sensors for CO2. For many insects, the olfactory sense, and therefore the antenna, is of utmost importance not only in their search for food for themselves or their offspring, but for intraspecific communication as well, for example in ants or moths.

Formation of cristae and crista junctions in mitochondria depends on antagonism between Fcj1 and Su e/g

Crista junctions (CJs) are important for mitochondrial organization and function, but the molecular basis of their formation and architecture is obscure. We have identified and characterized a mitochondrial membrane protein in yeast, Fcj1 (formation of CJ protein 1), which is specifically enriched in CJs. Cells lacking Fcj1 lack CJs, exhibit concentric stacks of inner membrane in the mitochondrial matrix, and show increased levels of F1FO–ATP synthase (F1FO) supercomplexes. Overexpression of Fcj1 leads to increased CJ formation, branching of cristae, enlargement of CJ diameter, and reduced levels of F1FO supercomplexes. Impairment of F1FO oligomer formation by deletion of its subunits e/g (Su e/g) causes CJ diameter enlargement and reduction of cristae tip numbers and promotes cristae branching. Fcj1 and Su e/g genetically interact. We propose a model in which the antagonism between Fcj1 and Su e/g locally modulates the F1FO oligomeric state, thereby controlling membrane curvature of cristae to generate CJs and cristae tips.

A notch-independent activity of suppressor of hairless is required for normal mechanoreceptor physiology.

Suppressor of Hairless [Su(H)]/Lag-1/RBP-Jkappa/CBF1 is the only known transducing transcription factor for Notch receptor signaling. Here, we show that Su(H) has three distinct functions in the development of external mechanosensory organs in Drosophila: Notch-dependent transcriptional activation and a novel auto-repression function, both of which direct cell fate decisions, and a novel auto-activation function required for normal socket cell differentiation. This third phase of activity, the first known Notch-independent activation function for Su(H) in development, depends on a cell type-specific autoregulatory enhancer that is active throughout adult life and is required for proper mechanoreception. These results establish a direct link between a broadly deployed cell signaling pathway and an essential physiological function of the nervous system.

Comparative morphogenesis of sensilla: A review

Abstract The “typical” insect sensillum is formed by a fixed number of cells: one or several bipolar sensory neurons are enveloped by the glia-like thecogen, the trichogen, and the tormogen cells. These cells arise via differential mitoses from an epidermal sensillum mother cell, which is “singled out” in the epidermis by the action of proneural and neurogenic genes, and then inhibits its neighbours from becoming sensillum mother cells themselves (“lateral inhibition”). Morphogenesis begins with the formation of a primary ciliary dendrite (9 × 2 + 0) by the neuron that grows above the epidermal surface. The trichogen cell then develops an apical sprout backed by a microtubular cytoskeleton, which will secrete the cuticle of the sensory hair, whereas the tormogen cell forms the hair socket. After finishing cuticle formation, both cells retract and form the subcuticular sensillum lymph cavity. In hemimetabolous insects preparing for molting, the dendrite leaves the new sensillum via an apical or a lateral pore, remaining connected with the old sensillum. During adult development of holometabolous insects, the primary dendrites also project from the newly forming hairs, being lost when cuticle secretion starts. The definite sensory dendrites grow into the hair shaft, whereas the trichogen cell retracts from the latter in most species.

Contacts of pore tubules and sensory dendrites in antennal chemosensilla of a silkmoth: Demonstration of a possible pathway for olfactory molecules

Antennal olfactory hairs of Antheraea polyphemus were investigated by means of transmission electron microscopy. Adequate preservation of dendrites and extracellular pore tubules is obtained by mechanical opening of the hair lumen and subsequent chemical fixation. The dendritic membrane has a cell coat. The dendrites contain microfilamentous structures in addition to their cytoplasmatic microtubules. The extracellular pore tubules traverse the hair cuticle and reach into the hair lumen for maximally 350 nm. Their diameter varies between 20 and 40 nm, depending on the preparation method. They consist of an electron-dense wall and an electron-lucent core. The wall has a helical substructure and is covered with a fuzzy coat. Contacts of pore tubules and dendritic membranes occur wherever dendrites are near the inner surface of the hair cuticle. Some of the pore tubules terminate approximately at right angles on the dendritic membrane, others lie against the membrane. The contact seems to be made via the surface coats of the tubules and the membrane. The structure of pore tubules which had been negatively stained with uranyl acetate is similar to the conventionally thin-sectioned material. The observations provide support for earlier assumptions that pore tubules are the pathways by which odor molecules reach the dendritic membrane.

Surface coats of pore tubules and olfactory sensory dendrites of a silkmoth revealed by cationic markers

Negatively charged surface coats have been demonstrated on the pore tubules and dendritic membranes of olfactory hairs of male Antheraea polyphemus silkmoths by application of the cationic markers lanthanum (La3+), ruthenium red (RR), and cationized ferritin (CF). Lanthanum and RR diffused readily into the apically opened hairs, whereas CF penetrated only for a relatively short distance. Deposits of the markers are distributed as follows: the inner surfaces of the hair walls are stained by RR and to a small degree by CF; the surfaces of the pore tubules and the dendritic membranes are stained by all three markers. The pore tubules have the strongest affinity for CF. The number of pore tubule-membrane contacts seems to be increased by the cationic dyes. The dendrites are often penetrated by RR, which forms deposits on the inner membrane leaflets, the cytoplasmic microtubules, and microfilaments, and by La3+, but never by CF. The observations provide support for the assumption that, first, the pore tubule-membrane contacts are formed via surface coats of both structures, possibly influenced by cations and, second, that the dendrites remain intact after pinching off the hair tips.

Diffusion barriers in silkmoth sensory epithelia: Application of lanthanum tracer to olfactory sensilla of Antheraea polyphemus and Bombyx mori

The distribution of diffusion barriers in silkmoth olfactory sensilla has been investigated with ionic lanthanum. The tracer was applied from the apical side of the sensory epithelium by first pinching off the hair tips and then dipping the antennal branches into the La(NO(3))(3) solution. The tracer neither passed the apical septate junctions between the dendrite and the thecogen cell nor those between thecogen, trichogen, and tormogen cells, nor the tight contact between the apical membrane of the tormogen cell and the cuticle. After perfusing the hemolymph space with La(NO(3))(3) solution, the tracer was found in the clefts between the thecogen, trichogen, tormogen, and epidermis cells, but not in those between the receptor cells and the thecogen cell, or between the axon and the glial envelope. Lanthanum neither entered the receptor-lymph space nor the subcuticular space. Therefore, (i) receptor-lymph space, subcuticular space, and hemolymph space are isolated from each other, and (ii) the cleft between thecogen and sensory cell is separated from the hemolymph as well as from the receptor-lymph spaces. Furthermore, the results indicate that pleated septate junctions form the diffusion barriers in silkmoth olfactory sensilla.