The kakapo Mutation Affects Terminal Arborization and Central Dendritic Sprouting of Drosophila Motorneurons

The lethal mutation l(2)CA4 causes specific defects in local growth of neuronal processes. We uncovered four alleles of l(2)CA4 and mapped it to bands 50A-C on the polytene chromosomes and found it to be allelic to kakapo (Prout et al. 1997. Genetics. 146:275– 285). In embryos carrying our kakapo mutant alleles, motorneurons form correct nerve branches, showing that long distance growth of neuronal processes is unaffected. However, neuromuscular junctions (NMJs) fail to form normal local arbors on their target muscles and are significantly reduced in size. In agreement with this finding, antibodies against kakapo (Gregory and Brown. 1998. J. Cell Biol. 143:1271–1282) detect a specific epitope at all or most Drosophila NMJs. Within the central nervous system of kakapo mutant embryos, neuronal dendrites of the RP3 motorneuron form at correct positions, but are significantly reduced in size. At the subcellular level we demonstrate two phenotypes potentially responsible for the defects in neuronal branching: first, transmembrane proteins, which can play important roles in neuronal growth regulation, are incorrectly localized along neuronal processes. Second, microtubules play an important role in neuronal growth, and kakapo appears to be required for their organization in certain ectodermal cells: On the one hand, kakapo mutant embryos exhibit impaired microtubule organization within epidermal cells leading to detachment of muscles from the cuticle. On the other, a specific type of sensory neuron (scolopidial neurons) shows defects in microtubule organization and detaches from its support cells.     

Fine structure of sensory tubes on the antennule of Conchoecia spinirostris (Ostracoda,Crustacea)

Summary In addition to setae, the first antennae of Conchoecia spinirostris also bear soft sensory tubes (4 tubes + 1 seta; 2 tubes + 3 setae). These tubes were examined electron microscopically. Each tube is divided into 4 regions: the stem, the bulbous region, the main region, and the tip. A tube contains 40–60 multiciliated dendrites, some hypodermal cells, and nonneuronal cells, and it has a specialized cuticle. Each dendrite develops within the tube, on the terminal 5–8 m of its inner dendritic segment, approx. 25 cilia in a 9 × 2 + 0 pattern, whose rootlets are absent or only poorly developed. Each cilium splits up into 9 ramifications which extend into the tip. These ramifications partly take a spirallike course and form a ring in the distal main part beneath the cuticle. Their membranes often dilate into spindleshaped swellings. In the center of the middle and distal parts of the main region approx. 7 dendrites without cilia are located, one of them reaches into the tip. The poreless cuticle is extremely delicate and electron lucid. In contrast to the cuticle of the setae it is elastic and soft. Special substructures are described. The tubes are completely covered by a filamentous surface coat. Because of the structure and the thin walled nature of the cuticle, permeability for dissolved substances is assumed. The ciliary ramifications are likely to represent the receptive apparatus. The sensory tubes are interpreted as chemoreceptors. They can best be compared with the chemoreceptors of certain crustaceans, but differ strongly from the types of sensilla found in insects.Supported by project 3540 of the Fonds zur Förderung der wissenschaftlichen Forschung in Österreich. The author is deeply indebted to Prof. Gamulin (Dubrovnik) for his support and to Prof. R. Riedl and Dr. W. Klepal for helpful discussions     

Comparison of active transport in neuronal axons and dendrites

This paper presents a theoretical study, based on modified Smith-Simmons equations, that compares transport of intracellular organelles in two different neurite outgrowths, dendrites and axons. It is demonstrated that the difference in microtubule polarity orientations in dendrites and axons has significant implications on motor-assisted transport in these neurite outgrowths. The developed approach presents a qualitative theoretical basis for understanding important questions such as why axons exhibit almost an unlimited grows potential in vitro while dendrites remain relatively short. It is shown that the difference in a microtubule polarity arrangement between axons and dendrites may be a regulatory mechanism for limiting dendritic growth. Other biological implications of the developed theory as well as other possible reasons for the difference in microtubule structure between axons and dendrites are discussed.     

Method of modelling intracellular transport in branching neurites: application to axons and dendrites of Drosophila sensory neurons

This paper develops a method of calculating the transport of intracellular organelles in neurons with branching neurites which is based on the Smith–Simmons equations of motor-assisted transport. The method is aimed at understanding the effects of microtubule (MT) polarity orientation in branching neurites on transport of organelles at the fundamental level. The method is applied to calculating the organelle transport in axons and dendrites of Drosophila neurons, using the map of MT orientation in such neurons developed by Stone et al. (Mol Biol Cell 19:4122–4129, 2008). The proximal dendrite is assumed to branch and form two distal dendrites. Two different MT polarity arrangements in a proximal dendrite are considered, and implications of these MT arrangements on organelle transport are analysed. It is demonstrated that the MT arrangement found in Drosophila dendrites (MTs have their minus ends out in a proximal dendrite) results in much more efficient motor-driven transport than the structure with a mixed MT orientation in proximal dendrites.     

Modelling active transport in Drosophila unipolar motor neurons

This paper develops a model for simulating organelle transport in Drosophila unipolar motor neurons. The paper is motivated by a recent experimental investigation by Stone et al. (Microtubules have opposite orientation in axons and dendrites of Drosophila neurons. Mol Biol Cell.19:4122-4129) who proposed a map of microtubule (MT) orientation in Drosophila neurons, and explained why dynein mutations selectively impede dendritic growth without having much effect on axonal growth. Two different approaches to modelling the effect of dynein mutations are utilised: one through assuming a reduced average velocity of a dynein mutant motor and the other through assuming its decreased processivity (an increased detachment rate from MTs). Modified Smith–Simmons equations are used for developing a continuum model of the process. Distributions of organelle concentrations as well as distributions of diffusion, motor-driven and total organelle fluxes are simulated.     

Statins decrease dendritic arborization in rat sympathetic neurons by blocking RhoA activation

Clinical and experimental evidence suggest that statins decrease sympathetic activity, but whether peripheral mechanisms involving direct actions on post-ganglionic sympathetic neurons contribute to this effect is not known. Because tonic activity of these neurons is directly correlated with the size of their dendritic arbor, we tested the hypothesis that statins decrease dendritic arborization in sympathetic neurons. Oral administration of atorvastatin (20 mg/kg/day for 7 days) significantly reduced dendritic arborization in vivo in sympathetic ganglia of adult male rats. In cultured sympathetic neurons, statins caused dendrite retraction and reversibly blocked bone morphogenetic protein-induced dendritic growth without altering cell survival or axonal growth. Supplementation with mevalonate or isoprenoids, but not cholesterol, attenuated the inhibitory effects of statins on dendritic growth, whereas specific inhibition of isoprenoid synthesis mimicked these statin effects. Statins blocked RhoA translocation to the membrane, an event that requires isoprenylation, and constitutively active RhoA reversed statin effects on dendrites. These observations that statins decrease dendritic arborization in sympathetic neurons by blocking RhoA activation suggest a novel mechanism by which statins decrease sympathetic activity and protect against cardiovascular and cerebrovascular disease.     

Flat-floored Air-lifted Platform: A New Method for Combining Behavior with Microscopy or Electrophysiology on Awake Freely Moving Rodents

It is widely acknowledged that the use of general anesthetics can undermine the relevance of electrophysiological or microscopical data obtained from a living animal’s brain. Moreover, the lengthy recovery from anesthesia limits the frequency of repeated recording/imaging episodes in longitudinal studies. Hence, new methods that would allow stable recordings from non-anesthetized behaving mice are expected to advance the fields of cellular and cognitive neurosciences. Existing solutions range from mere physical restraint to more sophisticated approaches, such as linear and spherical treadmills used in combination with computer-generated virtual reality. Here, a novel method is described where a head-fixed mouse can move around an air-lifted mobile homecage and explore its environment under stress-free conditions. This method allows researchers to perform behavioral tests (e.g., learning, habituation or novel object recognition) simultaneously with two-photon microscopic imaging and/or patch-clamp recordings, all combined in a single experiment. This video-article describes the use of the awake animal head fixation device (mobile homecage), demonstrates the procedures of animal habituation, and exemplifies a number of possible applications of the method.