Stimulation-induced changes in filopodial dynamics determine the action radius of growth cones in the snail Helisoma trivolvis

A review of the actions of Nitric Oxide in development and neuronal function in major invertebrate model systems

Ever since the late-eighties when endothelium-derived relaxing factor was found to be the gas nitric oxide, endogenous nitric oxide production has been observed in virtually all animal groups tested and additionally in plants, diatoms, slime molds and bacteria. The fact that this new messenger was actually a gas and therefore didn't obey the established rules of neurotransmission made it even more intriguing. In just 30 years there is now too much information for useful comprehensive reviews even if limited to animals alone. Therefore this review attempts to survey the actions of nitric oxide on development and neuronal function in selected major invertebrate models only so allowing some detailed discussion but still covering most of the primary references. Invertebrate model systems have some very useful advantages over more expensive and demanding animal models such as large, easily identifiable neurons and simple circuits in tissues that are typically far easier to keep viable. A table summarizing this information along with the major relevant references has been included for convenience.

Nitric oxide synthase regulates morphogenesis of zebrafish spinal cord motoneurons

Nitric oxide (NO) is a signaling molecule that is synthesized in a range of tissues by the NO synthases (NOSs). In the immature nervous system, the neuronal isoform of NOS (NOS1) is often expressed during periods of axon outgrowth and elaboration. However, there is little direct molecular evidence to suggest that NOS1 influences these processes. Here we address the functional role of NOS1 during in vivo zebrafish locomotor circuit development. We show that NOS1 is expressed in a population of interneurons that lie close to nascent motoneurons of the spinal cord. To determine how this protein regulates spinal network assembly, we perturbed NOS1 expression in vivo with antisense morpholino oligonucleotides. This treatment dramatically increased the number of axon collaterals formed by motoneuron axons, an effect mimicked by pharmacological inhibition of the NO/cGMP signaling pathway. In contrast, exogenous elevation of NO/cGMP levels suppressed motor axon branching. These effects were not accompanied by a change in motoneuron number, suggesting that NOS1 does not regulate motoneuron differentiation. Finally we show that perturbation of NO signaling affects the ontogeny of locomotor performance. Our findings provide evidence that NOS1 is a key regulator of motor axon ontogeny in the developing vertebrate spinal cord.

Nitric oxide acts as a volume transmitter to modulate electrical properties of spontaneously firing neurons via apamin-sensitive potassium channels

Nitric oxide (NO) is a radical and a gas, properties that allow NO to diffuse through membranes and potentially enable it to function as a "volume messenger." This study had two goals: first, to investigate the mechanisms by which NO functions as a modulator of neuronal excitability, and second, to compare NO effects produced by NO release from chemical NO donors with those elicited by physiological NO release from single neurons. We demonstrate that NO depolarizes the membrane potential of B5 neurons of the mollusk Helisoma trivolvis, initially increasing their firing rate and later causing neuronal silencing. Both effects of NO were mediated by inhibition of Ca-activated iberiotoxin- and apamin-sensitive K channels, but only inhibition of apamin-sensitive K channels fully mimicked all effects of NO on firing activity, suggesting that the majority of electrical effects of NO are mediated via inhibition of apamin-sensitive K channels. We further show that single neurons release sufficient amounts of NO to affect the electrical activity of B5 neurons located nearby. These effects are similar to NO release from the chemical NO donor NOC-7 [3-(2-hydroxy-1-methyl-2-nitrosohydazino)-N-methyl-1-propyanamine], validating the use of NO donors in studies of neuronal excitability. Together with previous findings demonstrating a role for NO in neurite outgrowth and growth cone motility, the results suggest that NO has the potential to shape the development of the nervous system by modulating both electrical activity and neurite outgrowth in neurons located in the vicinity of NO-producing cells, supporting the notion of NO functioning as a volume messenger.

Nitric oxide release from a single cell affects filopodial motility on growth cones of neighboring neurons

Nitric oxide (NO), a gaseous messenger, has been reported to be involved in a variety of functions in the nervous system, ranging from neuronal pathfinding to learning and memory. We have shown previously that the application of NO via NO donors to growth cones of identified Helisoma buccal neurons B5 in vitro induces an increase in filopodial length, a decrease in filopodial number, and a slowing in neurite advance. It is unclear, however, whether NO released from a physiological source would affect growth cone dynamics. Here we used cell bodies of identified neurons known to express the NO synthesizing enzyme nitric oxide synthase (NOS) as a source of constitutive NO production and tested their effect on growth cones of other cells in a sender-receiver paradigm. We showed that B5 cell bodies induced a rapid increase in filopodial length in NO-responsive growth cones, and that this effect was blocked by the NOS inhibitor 7-NI, suggesting that the effect was mediated by NO. Inhibition of soluble guanylyl cyclase (sGC) with ODQ blocked filopodial elongation induced by B5 somata, confirming that NO acted via sGC. We also demonstrate that the effect of NO was reversible and that a cell releasing NO can affect growth cones over a distance of at least 100 microm. Our results suggest that NO released from a physiological source can affect the motility of nearby growth cones and thus should be considered a signaling molecule with the potential to affect the outcome of neuronal pathfinding in vivo.

Control of neurite outgrowth and growth cone motility by phosphatidylinositol-3-kinase

Phosphatidylinositol-3-kinase (PI-3K) has been reported to affect neurite outgrowth both in vivo and in vitro. Here we investigated the signaling pathways by which PI-3K affects neurite outgrowth and growth cone motility in identified snail neurons in vitro. Inhibition of PI-3K with wortmannin (2 microM) or LY 294002 (25 microM) resulted in a significant elongation of filopodia and in a slow-down of neurite outgrowth. Experiments using cytochalasin and blebbistatin, drugs that interfere with actin polymerization and myosin II activity, respectively, demonstrated that filopodial elongation resulting from PI-3K inhibition was dependent on actin polymerization. Inhibition of strategic kinases located downstream of PI-3K, such as Akt, ROCK, and MEK, also caused significant filopodial elongation and a slow-down in neurite outgrowth. Another growth cone parameter, filopodial number, was not affected by inhibition of PI-3K, Akt, ROCK, or MEK. A detailed study of growth cone behavior showed that the filopodial elongation induced by inhibiting PI-3K, Akt, ROCK, and MEK was achieved by increasing two motility parameters: the rate with which filopodia extend (extension rate) and the time that filopodia spend elongating. Whereas the inhibition of ROCK or Akt (both activated by the lipid kinase activity of PI-3K) and MEK (activated by the protein kinase activity of PI-3K) had additive effects, simultaneous inhibition of Akt and ROCK showed no additive effect. We further demonstrate that the effects on filopodial dynamics investigated were calcium-independent. Taken together, our results suggest that inhibition of PI-3K signaling results in filopodial elongation and a slow-down of neurite advance, reminiscent of growth cone searching behavior.

Local activation of the nitric oxide/cyclic guanosine monophosphate pathway in growth cones regulates filopodial length via protein kinase G, cyclic ADP ribose and intracellular Ca2+ release

Nitric oxide (NO) is a gaseous messenger that has been shown to affect growth cone motility and neurite outgrowth in several model systems, but how NO brings about its effects is not understood. We have previously demonstrated that global and long-term application of NO to Helisoma trivolvis B5 neurons results in a transient increase in filopodial length, decrease in filopodial number and decrease in neurite outgrowth, all of which are mediated via soluble guanylyl cyclase (sGC) and involve an increase in the intracellular Ca2+ concentration [S. Van Wagenen & V. Rehder (1999)Journal of Neurobiology, 39, 168-185; K.R. Trimm & V. Rehder (2004) European Journal of Neuroscience, 19, 809-818]. The goal of the current study was twofold: to investigate the effects of short-term NO exposure on individual growth cones and to further elucidate the downstream pathway through which NO exerts its effects. Local application of the NO donor NOC-7 for 10-20 ms via puffer micropipette resulted in a transient increase in filopodial length and a small decrease in filopodial number. We show evidence that these effects of NO are mediated via sGC, protein kinase G and cyclic ADP ribose, resulting in the release of Ca2+ from intracellular stores, probably of the ryanodine-sensitive type. These results suggest that growth cones expressing sGC are highly sensitive to local and short-term exposure to NO, which they may experience during pathfinding, and that the stereotyped response of transient filopodial elongation seen in B5 neurons in response to NO requires intracellular Ca2+ release.

Actin-Based Motility in the Net Slime MouldLabyrinthula: Evidence for the Role of Myosin in Gliding Movement

In contrast to crawling movement (e.g. in amoebae and tissue cells) the other major class of substratum-associated motility in eukaryotes, gliding, has received relatively little attention. The net slime mold Labyrinthula provides a useful laboratory model for studying this process since it exhibits a particular kind of gliding in its plasmodial stage. Here nucleated spindle cells glide along self-established cytoplasmic trackways in a predominantly unidirectional manner, at 1-2 microm/s. These trackways, upon which gliding is dependent, are held by filopodial tethers some distance off the well-developed reticulopodial mesh anchoring the plasmodium onto the substratum. Reflection interference microscopy resolves this matrix in live plasmodia. The axially disposed cytoskeletal elements of the trackways are revealed by rhodamine-labelled phalloidin to be rich in F-actin. A weft of peripheral, rapidly extending filopodia (50 microm/min) typifies the expanding regions of the plasmodium. Here spindle cells are recruited before emigrating into newly differentiated trackways. Immunoblotting whole plasmodia or a sucrose-soluble cytoplasmic extract reveals a single actin-positive band of Mr 48 kDa. Polyclonal antibodies to two distinct myosin peptide sequences identify a single myosin HC (Mr 96 kDa) in immunoblots. Gliding was reversibly blocked by 10 mM 2,3-butanedione-2-monoxime, a myosin ATPase inhibitor, but it was insensitive to the actin-binding drugs cytochalasin D and phalloidin. We suggest that the force (>50 pN) for gliding motility results from interaction of myosin molecules, associated with the spindle cells, with trackway F-actin via the bothrosomes.

Nitric oxide acts as a slow-down and search signal in developing neurites

Nitric oxide (NO) has been demonstrated to act as a signaling molecule during neuronal development, but its precise function is unclear. Here we investigate whether NO might function at the neuronal growth cone to affect growth cone motility. We have previously demonstrated that growth cones of identified neurons from the snail Helisoma trivolvis show a rapid and transient increase in filopodial length in response to NO, which was regulated by soluble guanylyl cyclase (sGC) [S. Van Wagenen and V. Rehder (1999) J. Neurobiol., 39, 168-185]. Because in vivo studies have demonstrated that growth cones have longer filopodia and advance more slowly in regions where pathfinding decisions are being made, this study aimed to establish whether NO could function as a combined 'slow-down and search signal' for growth cones by decreasing neurite outgrowth. In the presence of the NO donor NOC-7, neurites of B5 neurons showed a concentration-dependent effect on neurite outgrowth, ranging from slowing at low, stopping at intermediate and collapsing at high concentrations. The effects of the NO donor were mimicked by directly activating sGC with YC-1, or by increasing its product with 8-bromo-cGMP. In addition, blocking sGC in the presence of NO with NS2028 blocked the effect of NO, suggesting that NO affected outgrowth via sGC. Ca2+ imaging of growth cones with Fura-2 indicated that [Ca2+]i increased transiently in the presence of NOC-7. These results support the hypothesis that NO can function as a potent slow/stop signal for developing neurites. When coupled with transient filopodia elongation, this phenomenon emulates growth cone searching behavior.

Filopodial behavior is dependent on the phosphorylation state of neuronal growth cones

Several lines of evidence suggest that phosphorylation events play an important role in transducing neurite outgrowth signals. Here we tested if such phosphorylation events altered filopodial dynamics on neuronal growth cones and thereby might affect pathfinding decisions. The general protein kinase inhibitor K252a caused an increase in the overall length of filopodia, thereby increasing the action radius of a growth cone. Application of specific kinase inhibitors demonstrated that myosin light chain kinase, Ca/calmodulin-dependent kinase II, and protein kinase A were likely not involved in this filopodial response. Inhibition of protein kinase C (PKC) with calphostin C or cerebroside, however, induced filopodial elongation similar to that seen with K252a. Activation of PKC with the phorbol ester PMA produced the opposite effect, namely filopodial shortening. Consistent with this finding, the protein phosphatase activator C(2)-ceramide resulted in a significant increase in filopodial length, whereas application of the protein phosphatase inhibitor okadaic acid caused the opposite effect, filopodial shortening. Lastly, the tyrosine kinase inhibitor genistein also caused filopodial elongation, and this effect could be negated by the tyrosine phosphatase inhibitor sodium ortho-vanadate. Using the calcium indicator fura-2, we further showed that these drugs did not cause a measurable change in the free intracellular calcium concentration ([Ca(2+)](i)) in growth cones. Taken together, these results suggest that the action radius of a growth cone and its resulting pathfinding abilities could be rapidly altered by contact with extracellular cues, leading to changes in the activity of protein kinases and phosphatases.

Giant identified NO-releasing neurons and comparative histochemistry of putative nitrergic systems in gastropod molluscs

Gastropod molluscs provide attractive model systems for behavioral and cellular analyses of the action of nitric oxide (NO), specifically due to the presence of many relatively giant identified nitrergic neurons in their CNS. This paper reviews the data relating to the presence and distribution of NO as well as its synthetic enzyme NO synthase (NOS) in the CNS and peripheral tissues in ecologically and systematically different genera representing main groups of gastropod molluscs. Several species (Lymnaea, Pleurobranchaea, and Aplysia) have been analyzed in greater detail with respect to immunohistochemical, biochemical, biophysical, and physiological studies to further clarify the functional role of NO in these animals.