Depolarization-induced changes in neurite elongation and intracellular Ca2+ in isolated Helisoma neurons

Canadian Association of Neuroscience Review: Axonal Regeneration in the Peripheral and Central Nervous Systems – Current Issues and Advances

Injured nerves regenerate their axons in the peripheral (PNS) but not the central nervous system (CNS). The contrasting capacities have been attributed to the growth permissive Schwann cells in the PNS and the growth inhibitory environment of the oligodendrocytes in the CNS. In the current review, we first contrast the robust regenerative response of injured PNS neurons with the weak response of the CNS neurons, and the capacity of Schwann cells and not the oligodendrocytes to support axonal regeneration. We then consider the factors that limit axonal regeneration in both the PNS and CNS. Limiting factors in the PNS include slow regeneration of axons across the injury site, progressive decline in the regenerative capacity of axotomized neurons (chronic axotomy) and progressive failure of denervated Schwann cells to support axonal regeneration (chronic denervation). In the CNS on the other hand, it is the poor regenerative response of neurons, the inhibitory proteins that are expressed by oligodendrocytes and act via a common receptor on CNS neurons, and the formation of the glial scar that prevent axonal regeneration in the CNS. Strategies to overcome these limitations in the PNS are considered in detail and contrasted with strategies in the CNS.

Acetylcholine elongates neuronal growth cone filopodia via activation of nicotinic acetylcholine receptors

In addition to acting as a classical neurotransmitter in synaptic transmission, acetylcholine (ACh) has been shown to play a role in axonal growth and growth cone guidance. What is not well understood is how ACh acts on growth cones to affect growth cone filopodia, structures known to be important for neuronal pathfinding. We addressed this question using an identified neuron (B5) from the buccal ganglion of the pond snail Helisoma trivolvis in cell culture. ACh treatment caused pronounced filopodial elongation within minutes, an effect that required calcium influx and resulted in the elevation of the intracellular calcium concentration ([Ca]i ). Whole-cell patch clamp recordings showed that ACh caused a reduction in input resistance, a depolarization of the membrane potential, and an increase in firing frequency in B5 neurons. These effects were mediated via the activation of nicotinic acetylcholine receptors (nAChRs), as the nAChR agonist dimethylphenylpiperazinium (DMPP) mimicked the effects of ACh on filopodial elongation, [Ca]i elevation, and changes in electrical activity. Moreover, the nAChR antagonist tubucurarine blocked all DMPP-induced effects. Lastly, ACh acted locally at the growth cone, because growth cones that were physically isolated from their parent neuron responded to ACh by filopodial elongation with a similar time course as growth cones that remained connected to their parent neuron. Our data revealed a critical role for ACh as a modulator of growth cone filopodial dynamics. ACh signaling was mediated via nAChRs and resulted in Ca influx, which, in turn, caused filopodial elongation.

Calcium and voltage dependent inactivation of sodium and calcium currents limits calcium influx in Helisoma neurons

The control of free intracellular calcium concentration ([Ca2+]i) is necessary for cell survival because of the ubiquitous and essential role this second messenger plays in regulating numerous intracellular processes. Calcium regulation in neurons is especially vigorous because of the large calcium influx that occurs through voltage-gated channels during membrane depolarization. In this study we examined changes in ionic currents that can limit calcium influx into neurons during electrical activity. We found that the [Ca2+]i in electrically stimulated Helisoma B4 neurons initially increased to a peak and then relaxed to lower concentrations in tandem with a decline in the action potential peak voltage. The decline in [Ca2+]i and the peak action potential voltage in this sodium and calcium driven neuron was found to be a dual manifestation of I(Na) and I(Ca) inactivation. I(Na) and I(Ca) both displayed voltage dependent inactivation. Additionally, I(Na) and I(Ca) progressively inactivated at [Ca2+]i above 200 nM, concentrations readily attained in electrically stimulated B4 neurons. Calcium and voltage dependent I(Na) and I(Ca) inactivation were found to reduce calcium influx during continuous electrical stimulation by decreasing both the magnitude of I(Ca) that could be activated and the percent of the available I(Ca) that would be activated due to the diminished peak action potential voltage. Calculations based on data herein suggest that the voltage and calcium dependent I(Na) and I(Ca) inactivation that occurs during continuous electrical stimulation dramatically reduces calcium influx in this sodium and calcium driven neuron and thus limits the increase in [Ca2+]i.

Toosendanin induces outgrowth of neuronal processes and apoptosis in PC12 cells

In the present study, the effects of toosendanin on cell differentiation and apoptosis were investigated in PC12 cells. The results showed that after 24-48 h of culture in a medium containing toosendanin (approximately 1-10x10(-7) M), cell differentiation and outgrowth of neuronal processes were promoted. Combined treatment with toosendanin and a calcium channel blocker, nifedipine or omega-conotoxin GVIA, resulted in a significant inhibition of the toosendanin-induced effects. Pretreatment of PC12 cells with BAPTA-AM also inhibited the toosendanin-induced effects; however, these effects were not inhibited by pertussis toxin and H-7 in the medium. Toosendanin also induced cell apoptosis. Based on the DNA content determined by flow cytometric analysis, the number of apoptotic cells significantly increased when the incubation time in the toosendanin-containing medium was lasted up to 72 h. Toosendanin at a higher concentration (> or =1 x 10(-6) M) caused cell death while it had no effect on cell division at concentrations lower than 1 x 10(-7) M.

Factors controlling axonal and dendritic arbors

The sculpting and maintenance of axonal and dendritic arbors is largely under the control of molecules external to the cell. These factors include both substratum-associated and soluble factors that can enhance or inhibit the outgrowth of axons and dendrites. A large number of factors that modulate axonal outgrowth have been identified, and the first stages of the intracellular signaling pathways by which they modify process outgrowth have been characterized. Relatively fewer factors and pathways that affect dendritic outgrowth have been described. The factors that affect axonal arbors form an incompletely overlapping set with those that affect dendritic arbors, allowing selective control of the development and maintenance of these critical aspects of neuronal morphology.

Actin dynamics and organization during growth cone morphogenesis in Helisoma neurons

Growth cone formation at the terminal region of severed axons is a fundamental step in neuronal regeneration. To understand the cytoskeletal events underlying this process, we have followed actin organization and dynamics as the severed, axonal stumps of Helisoma neurons transformed into mature growth cones. We identified three stages in growth cone morphogenesis: (1) formation, (2) expansion, and (3) maturation. The formation stage involved cytochalasin B-insensitive terminal swelling formation, followed by cytochalasin B-inhibited filopodial and lamellipodial formation. Time-lapse images of neurons injected with labeled actin showed actin ribs in nascent growth cones formed both by incorporation of filopodial actin bundles and de novo assembly at the leading edge. Phallacidin-stained growth cones revealed F-actin to be organized into bundles (ribs) and a meshwork throughout morphogenesis. Actin ribs represented the dominant F-actin population during the expansion stage and the early phase of maturation, whereas a meshwork organization dominated the late phase of maturation. During the expansion stage, growth cones exhibited a rapid retrograde flow (4.8 microns/min), as assessed with flow-coupled latex beads, and comparatively slow lamellipodial protrusion (0.3 micron/min). During the maturation stage, no net lamellipodial advancement occurred; however, the rate of retrograde flow was significantly faster in the early phase (5.0 microns/min) than the late phase (2.3 microns/min). This decrease in retrograde flow corresponded with a change in actin organization. Lateral movements of actin ribs (2.1 microns/min) also occurred throughout growth cone morphogenesis, but were most prominent during the expansion stage. These experiments provide evidence for de novo actin assembly during growth cone formation and demonstrate that temporal changes in actin organization and dynamics accompany growth cone morphogenesis.

Long-term depolarization changes morphological parameters of PC12 cells

It is well known that neuronal differentiation is strongly dependent on the intracellular level of free calcium ions ([Ca2+]i). In the present study the morphological and intracellular free calcium concentration changes were compared on PC12 pheochromocytoma cells cultured in control conditions and in a medium with high KCl level. Culturing PC12 cells in a medium with 20-30 mM KCl deprived of nerve growth factor supported cell proliferation and rapid growth of small neurite-like processes. However, their lengths did not increase with prolongation of the time of culturing. During culturing with 40 mM KCl the growth of these processes became blocked; the cells stopped proliferating and showed signs of degeneration. Measurements of [Ca2+]i level during the first days of PC12 cells culturing in a hyperpotassium medium indicate that such changes in this level could be an important factor in the induction of the observed morphological alterations; however, other effects induced by membrane depolarization may also be responsible for them.

Calcium influx alters actin bundle dynamics and retrograde flow in Helisoma growth cones

The ability of calcium (Ca(2+)) to effect changes in growth cone motility requires remodeling of the actin cytoskeleton. To understand the mechanisms involved, we evaluated the effect of elevated intracellular calcium ([Ca(2+)](i)) on actin bundle dynamics, organization, and retrograde flow in the large growth cones of identified Helisoma neurons. Depolarization with 15 mM KCl (high K(+)) for 30 min caused a rapid and sustained increase in [Ca(2+)](i) and resulted in longer filopodia, shorter actin ribs, and a decrease in lamellipodia width. Time-lapse microscopy revealed that increasing [Ca(2+)](i) affected actin bundle dynamics differently at the proximal and distal ends. Filopodial lengthening resulted from assembly-driven elongation of actin bundles whereas actin rib shortening resulted from a distal shift in the location of breakage. Buckling of ribs occurred before breakage, suggesting nonuniform forces were applied to ribs before shortening. Calcium (Ca(2+)) influx also resulted in a decrease in density of F-actin in bundles, as determined by contrast changes in ribs imaged by differential interference contrast microscopy and fluorescent intensity changes in rhodamine-labeled ribs. The velocity of retrograde flow decreased by 50% after elevation of [Ca(2+)](i). However, no significant change in retrograde flow occurred when the majority of changes in actin bundles were blocked by phalloidin. This suggests that inhibition of retrograde flow resulted from Ca(2+)-induced changes in the actin cytoskeleton. These results implicate Ca(2+) as a regulator of actin dynamics and, as such, provide a mechanism by which Ca(2+) can influence growth cone motility and behavior.

Extracellular potassium rapidly inhibits axonal transport of particles in cultured mouse dorsal root ganglion neurites

Changes in extracellular potassium concentration ([K+]o) modulate a variety of neuronal functions. However, whether axonal transport, which conveys materials to the appropriate destination for morphogenesis and other neuronal functions, depends on the extracellular K+ environment remains unclear. We therefore examined the effects of changes in [K+]o on axonal transport of particles visualized by video-enhanced microscopy in cultured mouse dorsal root gan-glion neurites. Increases in [K+]o (delta[K+]o > or = 2.5 mM) from control concentration (5 mM) inhibited both anterograde and retrograde axonal transport within a few minutes in a concentration-dependent manner. Conversely, removal of extracellular K+ induced the rapid facilitation of transport in both directions. These inhibitory and facilitatory responses were completely blocked by the K+ channel blocker tetraethylammonium (TEA), suggesting that the effect of changes in [K+]o involves the TEA-sensitive K+ channels. Increases in [K+]o provoked membrane depolarization in the absence and presence of TEA. Another depolarizing agent, veratridine, did not produce an effect on axonal transport. These results suggest that the extracellular K+-mediated inhibition of axonal transport does not depend on membrane depolarization. The inhibitory effect of increasing [K+]o on axonal transport was retained in calcium (Ca2+)-free extracellular medium, indicating that the inhibitory effect of extracellular K+ does not result from Ca2+ influx through voltage-dependent Ca2+ channels. In chloride (CI-)-free medium, increasing [K+]o failed to inhibit axonal transport, implying that the extracellular K+-mediated inhibition of axonal transport may be due to an increase in intracellular Cl- concentration associated with increases in the net inward movement of K+ and CI- across the membrane. Our results suggest that the extracellular K+ environment is involved in the rapid modulation of axonal transport of particles in dorsal root ganglion neurites.

Delayed retraction of filopodia in gelsolin null mice

Growth cones extend dynamic protrusions called filopodia and lamellipodia as exploratory probes that signal the direction of neurite growth. Gelsolin, as an actin filament-severing protein, may serve an important role in the rapid shape changes associated with growth cone structures. In wild-type (wt) hippocampal neurons, antibodies against gelsolin labeled the neurite shaft and growth cone. The behavior of filopodia in cultured hippocampal neurons from embryonic day 17 wt and gelsolin null (Gsn-) mice (Witke, W., A.H. Sharpe, J.H. Hartwig, T. Azuma, T.P. Stossel, and D.J. Kwiatkowski. 1995. Cell. 81:41-51.) was recorded with time-lapse video microscopy. The number of filopodia along the neurites was significantly greater in Gsn- mice and gave the neurites a studded appearance. Dynamic studies suggested that most of these filopodia were formed from the region of the growth cone and remained as protrusions from the newly consolidated shaft after the growth cone advanced. Histories of individual filopodia in Gsn- mice revealed elongation rates that did not differ from controls but an impaired retraction phase that probably accounted for the increased number of filopodia long the neutrite shaft. Gelsolin appears to function in the initiation of filopodial retraction and in its smooth progression.

Spatial organization of calcium dynamics in growth cones of sensory neurones

The concentration of calcium ions in the cytosol ([Ca2+]i) has a dominant influence on neuronal development. A [Ca2+]i rise can, depending on the amplitude and location, promote outgrowth or dramatically inhibit it. We have used the fluorescent calcium indicators Fura-2 and Fura-2 dextran to measure [Ca2+]i dynamics in sensory neurones from the adult rat. [Ca2+]i was low and uniform in advancing growth cones, even during specific behaviours such as protrusion, filling and consolidation. A brief train of action potentials caused [Ca2+]i to rise at the extreme leading edge of the growth cone. [Ca2+]i changes in more proximal regions of the growth cone were much smaller. This spatially organized [Ca2+]i change, which may result from a concentration of calcium channels at the growth cone leading edge, is likely to function in spontaneously active regenerating axons in vivo to specifically activate calcium-dependent processes at the growth cone tip.

Continuous exposure of cultured rat cerebellar macroneurons to ethanol-depressed NMDA and KCl-stimulated elevations of intracellular calcium

This series of experiments measured ethanol-induced changes in levels of free intracellular calcium. Cerebellar macroneurons, harvested from rat embryos on embryonic day 17, were cultured in the presence of 75 mM ethanol for 24, 48, or 96 hr. Intracellular calcium concentrations in control and ethanol-exposed neurons did not differ after 24 hr, but they were significantly elevated in the neurons exposed to ethanol for 48 or 96 hr. Similarly, increases in intracellular calcium elicited by stimulation with 50 microM NMDA were not significantly different in control and ethanol-exposed neurons after 24 hr. After 48 and 96 hr, however, NMDA-stimulated increases in intracellular calcium levels in control neurons were significantly greater than in the ethanol-exposed neurons. These results showed that, when calcium levels were elevated by prolonged exposure to ethanol, the neurons were significantly less responsive to NMDA stimulation. Increases in intracellular calcium elicited by stimulation with 30 mM KCI were not significantly different in the control and treated neurons after 24 and 48 hr of ethanol exposure. After 96 hr of exposure to ethanol, however, there was a significant increase in intracellular calcium levels in control neurons following KCI stimulation, but not in the ethanol-exposed neurons. The fact that neuronal responses to KCI stimulation were depressed only following 96 hr of exposure to ethanol makes it unlikely that voltage-regulated channels were the primary mediators of the ethanol-induced elevations in intracellular calcium in chronically exposed neurons.

Calcium transients in growth cones and axons of cultured Helisoma neurons in response to conditioning factors

Accumulating evidence indicates that cytosolic calcium levels regulate growth cone motility and neurite extension. The purpose of this study was to determine if intracellular calcium levels also influence the initiation of neurite extension induced by growth-promoting factors. An in vitro preparation of axotomized neurons that can be maintained in the absence of growth-promoting factors was utilized. The distal axons of cultured Helisoma neurons plated into defined medium do not extend neurites until they are exposed to Helisoma brain-conditioned medium. This provided the opportunity to study the intracellular changes associated with neurite extension. Cytosolic calcium levels were monitored with the calcium-sensitive dye fura 2 at the distal axon. In control medium calcium levels in the distal axon were constant. However, transient elevations in cytosolic calcium in the axonal growth cone occurred after addition of conditioned medium and coincident with the initiation of neurite extension. Application of calcium channel blockers showed that the transients resulted from calcium influx across the neuronal membrane. The transients, however, were not required for neurite extension, although they did influence the rate and extent of neurite outgrowth. Simultaneous extracellular patch recordings demonstrated that the calcium transients were correlated temporally with an increase in rhythmic spontaneous electrical activity of cells, suggesting that conditioned medium influences ionic membrane properties of these neurons.

The role of conditioning factors in the formation of growth cones and neurites from the axon stump after axotomy

Knowledge of the cellular events that underlie initiation of outgrowth is crucial to understanding regulation of development and regeneration of the nervous system. This study utilized a culture preparation in which growth cone formation could be studied independent of cellular responses to the presence of conditioning factors. Identified neurons were removed from the buccal ganglion of the mollusc, Helisoma trivolvis, and plated into defined culture medium. A large growth cone formed at the end of the attached axon stump. Although this axonal growth cone exhibited filopodial and lamellipodial activity, it did not advance across the substrate, suggesting that growth cone formation and motility were independent of the presence of conditioning factors. Axonal growth cones of identified neurons B19 and B5 exhibited differences in their morphological and behavioral properties. In response to the addition of conditioning factors, several new neurites extended from the periphery of the axonal growth cone. Extension of outgrowth from the axonal growth cone was accompanied by a redistribution of cytoskeletal elements in the axonal growth cone. Cytoskeletal staining revealed a loss of the peripheral actin filament network and microtubules were found to extend into the peripheral lamellipodium of the axonal growth cone, an area normally devoid of microtubule staining. Thus, these experiments indicate that growth cone formation is an intrinsic property of the distal axon stump and that neurite extension from this structure involves reorganization of the neuronal cytoskeleton.

Action potentials, calcium transients and the control of differentiation of excitable cells

Calcium influx via action potentials in differentiating nerve and muscle is regulated principally by the expression of potassium currents. Transient elevations of intracellular calcium in spontaneously active cells are necessary for normal neuronal development. The mechanisms that connect calcium elevations to long term developmental change are likely to be utilized in the mature nervous system.