Calcium requirement for axoplasmic transport in mammalian nerve

Mitochondrial dysfunction in distal axons contributes to human immunodeficiency virus sensory neuropathy

OBJECTIVE

Accumulation of mitochondrial DNA (mtDNA) damage has been associated with aging and abnormal oxidative metabolism. We hypothesized that in human immunodeficiency virus-associated sensory neuropathy (HIV-SN), damaged mtDNA accumulates in distal nerve segments, and that a spatial pattern of mitochondrial dysfunction contributes to the distal degeneration of sensory nerve fibers.

METHODS

We measured levels of common deletion mutations in mtDNA and expression levels of mitochondrial respiratory chain complexes of matched proximal and distal nerve specimens from patients with and without HIV-SN. In mitochondria isolated from peripheral nerves of simian immunodeficiency virus (SIV)-infected macaques, a model of HIV-SN, we measured mitochondrial function and generation of reactive oxygen species.

RESULTS

We identified increased levels of mtDNA common deletion mutation in postmortem sural nerves of patients with HIV-SN as compared to uninfected patients or HIV patients without sensory neuropathy. Furthermore, we found that common deletion mutation in mtDNA was more prevalent in distal sural nerves compared to dorsal root ganglia. In a primate model of HIV-SN, freshly isolated mitochondria from sural nerves of macaques infected with a neurovirulent strain of SIV showed impaired mitochondrial function compared to mitochondria from proximal nerve segments.

INTERPRETATION

Our findings suggest that mtDNA damage accumulates in distal mitochondria of long axons, especially in patients with HIV-SN, and that this may lead to reduced mitochondrial function in distal nerves relative to proximal segments. Although our findings are based on HIV-SN, if confirmed in other neuropathies, these observations could explain the length-dependent nature of most axonal peripheral neuropathies.

Axonal mitochondrial transport and potential are correlated

Disruption of axonal transport leads to a disorganized distribution of mitochondria and other organelles and is thought to be responsible for some types of neuronal disease. The reason for bidirectional transport of mitochondria is unknown. We have developed and applied a set of statistical methods and found that axonal mitochondria are uniformly distributed. Analysis of fast axonal transport showed that the uniform distribution arose from the clustering of the stopping events of fast axonal transport in the middle of the gaps between stationary mitochondria. To test whether transport was correlated with ATP production, we added metabolic inhibitors locally by micropipette. Whereas applying CCCP (a mitochondrial uncoupler) blocked mitochondrial transport, as has been previously reported, treatment with antimycin (an inhibitor of electron transport at complex III) caused increases in retrograde mitochondrial transport. Application of 2-deoxyglucose did not decrease transport compared with the mannitol control. To determine whether mitochondrial transport was correlated with mitochondrial potential, we stained the neurons with the mitochondrial potential-sensing dye JC-1. We found that approximately 90% of mitochondria with high potential were transported towards the growth cone and approximately 80% of mitochondria with low potential were transported towards the cell body. These experiments show for the first time that a uniform mitochondrial distribution is generated by local regulation of the stopping events of fast mitochondrial transport, and that the direction of mitochondrial transport is correlated with mitochondrial potential. These results have implications for axonal clogging, autophagy, apoptosis and Alzheimer's disease.

Calcium-binding proteins in primate basal ganglia

This paper describes the distribution of the calcium-binding proteins calbindin-D28k. Parvalbumin and calretinin in primate basal ganglia. The data derive from immunocytochemical studies undertaken in squirrel monkeys (Saimiri sciureus) and in normal human individuals. In the striatum, calbindin labels medium-sized spiny projection neurons whereas parvalbumin and calretinin mark two separate classes of aspiny interneurons. The striatal matrix compartment is markedly enriched with calbindin while striatal patches (striosomes) display a calretinin-rich neuropil. In the pallidum, virtually all neurons contain parvalbumin but none express calbindin. Calretinin occurs only in a small subpopulation of both large and small pallidal neurons. In the subthalamic nucleus, there exists a multitude of parvalbumun-positive cells and fibers but the number of calretinin and calbindin-positive neuronal elements is small. In the substantia nigra/ventral tegmental area complex, calbindin and calretinin occur principally in dopaminergic neurons of the dorsal tier of the pars compacta and in those of the ventral tegmental area. Parvalbumin is strictly confined to the GABAergic neurons of the pars reticulata and lateralis. Calbindin-rich fibers abound in the pars reticulata and lateralis, while calretinin-positive axons are confined to the pars compacta. These results indicate that calbindin and parvalbumin are distributed according to a strikingly complementary pattern in primate basal ganglia. Calretinin is less ubiquitous but occurs in all basal ganglia components where it labels distinct subsets of neurons. Such highly specific patterns of distribution indicate that calbindin, parvalbumin and calretinin may work in synergy within primate basal ganglia.

Postischemic reperfusion causes a massive calcium overload in the myelinated spinal cord fibers

The visualization of Ca binding in the myelinated axons of lumbosacral segments of rabbit was done at the electron microscopic level using the spinal cord ischemia model. To assess the calcium accumulation, the binding agent pyroantimonate was used. Nonsignificant Ca2+ binding was found in the myelinated axons after 40 min of ischemia followed immediately by perfusion fixation. A high concentration of calcium pyroantimonate deposits, seen as electron dense particles, was detected in the myelin interlamellar clefts and axoplasm. The paranodal region was the most affected site.

Distribution of the inositol 1,4,5-trisphosphate receptor, P400, in adult rat brain

The distribution of the inositol 1,4,5-trisphosphate receptor protein, P400, was investigated in adult rat brain by immunocytochemistry with the monoclonal antibody 4C11 raised against mouse cerebellar inositol 1,4,5-trisphosphate receptor protein. Immunoreactive neuronal cell bodies were detected in the cerebral cortex, the claustrum, the endopiriform nucleus, the corpus callosum, the anterior olfactory nuclei, the olfactory tubercle, the nucleus accumbens, the lateral septum, the bed nucleus of the stria terminalis, the hippocampal formation, the dentate gyrus, the caudate-putamen, the fundus striatum, the amygdaloid complex, the thalamus, the caudolateral part of the hypothalamus, the supramammillary nuclei, the substantia nigra, the pedunculopontine tegmental nucleus, the ventrotegmental area, the Purkinje cells in the cerebellum, the dorsal cochlear nucleus, the subnucleus oralis and caudalis of trigeminal nerve, and the dorsal horn of the spinal cord. Immunoreactive fibres were found in the medial forebrain bundle, the globus pallidus, the stria terminalis, the pyramidal tract, the spinal tract of trigeminal nerve, and the ventral horn of spinal cord. Nerve fibres forming a dense plexus ending in terminal-like boutons were detected in relation to nonimmunoreactive neurons of the dentate, interpositus, and fastigial nuclei of the cerebellum and around neurons of the vestibular nuclei. This receptor protein binds a specific second messenger, inositol 1,4,5-trisphosphate, which produces a mobilization of intracellular Ca2+ and a modulation of transmitter release.

Fast axonal transport is modulated by altering trans-axolemmal calcium flux

Factors involved in fast axonal transport (motor proteins, microtubules, organelles, etc.) have been identified but the molecular mechanism controlling transport is unknown. We used video enhanced microscopy to directly evaluate the effect of calcium on fast axonal transport (FAxT). FAxT alterations included rapid speed decreases (within minutes) in Ca2+ free buffer and rapid speed increases (within seconds) when axons were treated with parathyroid hormone, BAY K 8644, or K+ depolarization. The speed increases were blocked by dihydropyridine Ca2+ channel antagonists. Ryanodine (20 microM), known to block calcium release from subcellular stores, caused a decrease in the rate of retrograde FAxT. Calcium ionophore A23187 (at 1 and 20 micrograms/ml) caused increases in FAxT, an effect also noted only in retrograde moving organelle traffic. Hyper- or hypo-tonic solutions produced no alterations making axoplasmic viscosity changes an unlikely explanation for the speed changes. Reproducible alteration of FAxT by manipulation of Ca2+ levels provides evidence that Ca2+ modulates fast axonal transport. Retrograde transport appears more sensitive to changes in Ca2+ and differential effects on antero- and retro-FAxT mechanisms suggest directional specificity for some of these signals which may be based upon the organelle size. Endogenous substances (e.g. PTH) that trigger axonal Ca2+ changes may rapidly modulate the rate of material delivery in axons. The results are discussed within the context of a Ca2+/calmodulin-dependent modification of the cytoskeletal matrix.

Calcium transfer at the blood-nerve barrier of the frog sciatic nerve

Calcium transfer across the blood-nerve barrier of the frog sciatic nerve was studied using an in situ perfusion technique and an in vivo i.v. bolus injection technique. The permeability-surface area product of 45Ca at the blood-nerve barrier, (PA)BNB, calculated from radioactivity in the desheathed nerve segment after 5 min of circulation of tracer, and corrected for the residual radioactivity in the blood space, equaled 4.4 +/- 0.4 (S.E.M.) X 10(-5) ml.s-1.g-1 wet wt. The (PA)BNB of 45Ca was independent of [Ca2+] in the perfusion medium between 0.18 and 18 mM. The permeability-surface area products of 45Ca across the perineurium [(PA)per] also was measured by an in situ incubation technique, and equaled 1.45 +/- 0.41 X 10(-5) ml.s-1.g-1 wet wt. (n = 8). The half time (t 1/2) for nerve calcium to equilibrate with plasma calcium was calculated to be 60 min. The low, passive permeability to calcium of the blood-nerve barrier probably limits marked calcium concentration changes in nerve endoneurium following transient changes of plasma calcium, but should not alter steady-state responses.

Cytochemical localization of Ca2+-ATPase activity in peripheral nerve

We used an electron microscopic cytochemical method to determine the localization of Ca2+-ATPase in rat peripheral nerve. We found that reaction product occurred along most cytoplasmic membranes in the dorsal root ganglia (DRG). Unmyelinated axons demonstrated reaction product on the axolemma diffusely along their length. Myelinated fibers, in contrast, had reaction product limited to the axolemma in the paranodal region. Internodal axolemma never showed reaction product and nodal axolemma was only occasionally stained, usually in sections reacted for the maximum times. Schwann cell plasma membranes uniformly showed reaction product. The restricted localization of Ca2+-ATPase to the paranodal region of myelinated fibers suggests that calcium efflux may occur principally at those sites.

Calcium regulation of pigment transport in vitro

Calcium has been implicated in the regulation of many cellular motility events. In this study we have examined the role of different Ca2+ concentrations on the in vitro transport of pigment within cultured chromatophores. Cells treated with Brij detergent for 1-2 min were stripped of their plasma membranes, leaving their cytoskeleton and associated pigment granules exposed to the external milieu. We found that retrograde pigment transport (aggregation) is induced upon addition of 1 mM MgATP2- with 10(-7) M free Ca2+, while an orthograde transport (redispersal) of pigment results from lowering the concentration of free Ca2+ to 10(-8) M while maintaining 1 mM MgATP2-. These Ca2+-regulated movements are ATP dependent but are apparently independent of cAMP and insensitive to calmodulin inhibitors. The observations reported here provide novel evidence that the concentration of free Ca2+ acts to regulate the direction of intracellular organelle transport.

Calcium and the aging nervous system

Many aspects of calcium homeostasis change with aging. Numerous calcium compartments complicate studies of altered calcium regulation. However, age-related decreases in calcium permeation across membranes and mobilization from organelles may be a common fundamental change. Deficits in ion movements appear to lead to altered coupling of calcium-dependent biochemical and neurophysiological processes and may lead to pathological and behavioral changes. The calcium-associated changes during aging probably do not occur with equal intensity in all cell types or in different parts of the same cell. Thus, cells or compartments with a high proportion of calcium activated processes would be more sensitive to diminished calcium availability. These age-related changes may predispose the brain to the development of age-related neurological disorders. The effects of decreased ion movement may be further aggravated by an age-related decline in other calcium-dependent processes. Depression of some of these calcium-dependent functions appears physiologically significant, since increasing calcium availability ameliorates age-related deficits in neurotransmission and behavior. A better understanding of the interactions between calcium homeostasis and calcium-dependent processes during aging will likely help in the design of more effective therapeutic strategies.

The use of the regenerating frog sciatic nerve for pharmacological studies of orthograde and retrograde axonal transport

The outgrowth region of the regenerating frog sciatic nerve shows an increased permeability for various drugs, which has been utilized for pharmacological studies of axonal transport. Six days after a bilateral crush lesion, the nerves, including the spinal ganglia, were incubated in a compartmented chamber. Orthograde transport was assessed from the proximodistal distribution and the accumulation of labelled proteins in the nerve growth region. Retrograde transport was examined by allowing orthogradely transported materials to reverse at the regenerating region and then to accumulate at a ligature during a second incubation period. The distribution of radioactivity along the nerve was assayed by fluorography of whole-mount nerve preparations or by scintillation counting. Fluorography made it possible to increase the spatial resolution and to demonstrate effects in the elongating part of the regenerating nerve. Colchicine at low concentrations (10-100 microM) only inhibited axonal transport in the outgrowth region (6 mm long at 6 days after crush) and along some mm of the nerve proximal to the crush. Compound 48/80 (50 micrograms/ml), the most specific calmodulin inhibitor so far described, inhibited the transport along the same part of the nerve. Cytochalasin B (10 micrograms/ml) inhibited transport by effects limited to the outgrowth region. Both orthograde and retrograde transport showed sensitivity to these and some other drugs. The regenerating frog sciatic nerve seems to have significant advantages for pharmacological studies of axonal transport.

Ultrastructural distribution of Ca++ within neurons. An oxalate pyroantimonate study

We used the oxalate-pyroantimonate technique to determine the ultrastructural distribution of Ca++ in neurons of the rat sciatic nerve. The content of the precipitate was confirmed by X-ray microanalysis and appropriate controls. In the cell bodies of the dorsal root ganglia, Ca++ precipitate was found in the Golgi, mitochondria, multivesicular bodies and large vesicles of the cytoplasm but not in lysosomes, and was prominently absent from regions of rough endoplasmic reticulum and ribosomes. It was seen in the nucleus but not in the nuclear bodies or nucleolus. Within the axon itself, Ca++ precipitate was also found sequestered in mitochondria and smooth endoplasmic reticulum. In addition Ca++ precipitate found diffusely throughout the axoplasm exhibited a discrete and heterogeneous distribution. In myelinated fibers the amount of precipitate decreased predictably in the axoplasm beneath the Schmidt-Lanterman cleft and in the paranodal regions at the nodes of Ranvier. This correlated with the presence of dense precipitate in the Schmidt-Lanterman cleft themselves and in the paranodal loops of myelin. Intracytoplasmic ionic Ca++ is maintained at 10(-7) M by balanced processes of influx, sequestration and extrusion. The irregular distribution of Ca++ precipitate in the axoplasm of myelinated fibers suggests that there may be specific regions of preferential efflux across the axolemma.

Inhibition of fast axonal transport in bullfrog nerves by dibenzazepine and dibenzocycloheptadiene calmodulin inhibitors

The effects of the calmodulin inhibitors amitriptyline, desipramine, imipramine, and clomipramine on fast axonal transport, oxidative metabolism, and density of axonal microtubules were measured in bullfrog spinal nerves in vitro. The four drugs tested inhibited the fast orthograde transport of [3H]leucine-labelled proteins and the fast retrograde transport of acetylcholinesterase at a concentration of 0.2 mM. Amitriptyline, desipramine, and imipramine were equipotent inhibitors of transport, and clomipramine was a more potent inhibitor than imipramine. The adenosine triphosphate content of the nerves was reduced by at most 19% by the compounds under study; such a reduction cannot account for the inhibition of fast axonal transport. Desipramine and imipramine had no significant effect on the density of microtubules in unmyelinated axons, whereas amitriptyline only reduced it by 18%; the inhibition of axonal transport by these three drugs can therefore not be explained by microtubule disruption. Clomipramine reduced microtubular density by 40%, and this effect may have contributed to the inhibition of fast axonal transport. The inhibition of fast axonal transport by desipramine, imipramine, and amitriptyline may be related to the inhibition of calmodulin function by these drugs. The similar potency of these three drugs as inhibitors of fast axonal transport goes in parallel with their known similar potency as calmodulin antagonists.

Inhibition of the retrograde axonal transport of dopamine-beta-hydroxylase antibodies by the calcium ionophore A23187

High levels of calcium, as well as calcium ionophores, have been reported to inhibit the anterograde transport of proteins. The effect of the calcium ionophore, A23187, on the retrograde axonal transport of proteins was therefore investigated. The uptake of antibodies to dopamine-beta-hydroxylase (anti-D beta H) by sympathetic nerve terminals in the iris and their subsequent accumulation in the superior cervical ganglion was inhibited by up to 65% by A23187 (6 nmol, i.o.). At this dose, catecholamine fluorescence in the iris was reduced, indicating a high rate of exocytosis, but tyrosine hydroxylase levels and the capacity of the treated irides to take up noradrenaline were unaffected. Higher amounts of A23187 (28 nmol, i.o.) did not cause a greater degree of inhibition of retrograde transport. However, this dose was toxic to the neurons, as shown by a 68% decrease in the ability of the nerve terminals in the iris to take up [3H]noradrenaline. This loss of function occurred gradually over a 12-h period. On the other hand, tyrosine hydroxylase levels were unaffected by 28 nmol A23187. The toxicity of A23187 may be a consequence of a build up in intracellular calcium, but such toxicity did not lead to any apparent loss of nerve terminals within a 3-day period.

Reduced axoplasmic somatostatin transport in hypothyroid rats

The effect of hypothyroidism on neuronal function was studied by measuring axoplasmic transport of immunoreactive somatostatin in rat sciatic nerve by the ligation technique. Accumulation of immunoreactive somatostatin proximal to a ligature was linear up to 8 h in normal, in thyroidectomized, and in parathyroidectomized rats. The transport rate was decreased by 38% in thyroidectomized rats as compared to normal rats and was unchanged in parathyroidectomized rats. Sciatic nerve content of somatostatin in hypothyroid rats did not differ from control. Reduced accumulation of immunoreactive somatostatin in hypothyroid rats may be due to a decrease in somatostatin synthesis or in axoplasmic transport, or to an increase in the degradation rate of the peptide.

Inhibition of the retrograde axonal transport of acetylcholinesterase by the anti-calmodulin agents amitriptyline and desipramine

The possible involvement of calmodulin in mediating the calcium requirement for retrograde axonal transport of acetylcholinesterase was studied in vitro in bullfrog spinal nerves, with the use of the calmodulin inhibitors amitriptyline and desipramine. When nerves were preincubated with 0.2 mM amitriptyline or desipramine for 5 h, and were then ligated and incubated for an additional 17-18 h in drug-containing medium, the accumulation of acetylcholinesterase distal to the ligature was significantly reduced as compared to contralateral control nerves maintained in drug-free medium. The identical degree of transport inhibition observed for both drugs is consistent with their similar anti-calmodulin activity.

Localization of calcium in nerve fibers

Using the desheathed nerve preparation, a pyroantimonate precipitation method was used to examine the distribution of electron-dense particles seen in various organelles of the nerve fibers following exposure of nerve to various levels of Ca2+ in vitro. The presence of Ca2+ in the electron-dense particles was indicated by their extraction with EGTA and by the use of energy-dispersive X-ray microanalysis. In normal Ringer or in a Ca2+ -free medium, electron-dense particles were seen associated with the outer membrane of the mitochondria, with the smooth endoplasmic reticulum (SER), along the axolemma and yet others scattered throughout the axoplasm. When nerves were incubated in media containing higher than normal concentrations of 20-60 mM Ca2+, an increase in the number of such electron-dense particles was seen in the axoplasm and within the mitochondrial matrix. Nerves loaded with a high concentration of 60mM Ca2+ could be depleted of these particles after transfer to a Ca2+ -free or low Ca2+ Ringer medium. The sequestration of Ca2+ in axonal organelles is discussed with respect to Ca2+-regulatory mechanisms in the axon needed to maintain a low level of Ca2+ which is optimal for the support of axoplasmic transport.

A paradigm for axonal transport

Axonal transport has been extensively studied for a period of 20-30 years, but there is still no general consensus concerning the mechanism by which this transport process operates. An important development in this regard is the recent studies in the physical biochemistry group in the Department of Biochemistry at Monash University where it has been demonstrated that ordered flows may be generated spontaneously in polymer systems under non-equilibrium conditions. The new phenomenon exhibits many novel features, particularly with respect to polymer transport, which bear marked similarity to the behaviour of components in axonal transport. This article sets out to essentially bring to the attention of those in the neurosciences some of the properties of ordered structured flows in polymer solutions. These properties may generate a different view in the understanding of the mechanism of axonal transport.

Ionic requirements for in vitro retrograde axonal transport of acetylcholinesterase

The ionic requirements for retrograde axonal transport of acetylcholinesterase were studied in vitro in desheathed spinal nerves of the bullfrog; the accumulation of enzyme activity distal to a ligature served to evaluate the status of retrograde transport. After an 18 h incubation in control medium, the 2 mm segment of nerve immediately distal to the ligature contained approximately twice as much acetylcholinesterase activity as other more distal segment. Accumulation of acetylcholinesterase distal to the ligature was reduced during incubation in Ca2+-free medium plus 1 mM EGTA; retrograde transport was not diminished by the substitution of sucrose for the NaCl of the medium. In comparison with ionic requirements for orthograde fast axonal transport of protein, retrograde transport of acetylcholinesterase thus appears to share a Ca2+ requirement, but not a requirement for NaCl.

Dependence of batrachotoxin block of axoplasmic transport on sodium

Batrachotoxin (BTX) in the low concentration range of 19-190 nM blocks axoplasmic transport in the desheathed cat peroneal nerve in vitro. When the level of Na+ in the incubation medium was reduced to 10 mM, the blocking effect of BTX was much diminished, and in an Na+-free medium BTX had no effect on transport at all. The blocking action of BTX with Na+ present was inhibited by increasing the concentration of Ca2+ in the experimental medium. Relatively small increases were effective with a maximum protection seen when the Ca2+ concentrations were 7-10 mM. The results support the view that an increase in axonal Na+ is inhibitory to the transport mechanism. The results are discussed on the basis of the recently developed transport filament model of axoplasmic transport which takes into account an obligatory role for Ca2+ in transport and its axonal regulation. The possible relation of intraaxonal Na+ concentration to the Ca2+ level is also discussed.

Calcium requirement for fast axonal transport in frog motoneurons

Calcium is required to sustain fast axonal transport in sensory neurons of frog and cat. We studied the Ca2+ dependence of fast axonal transport in the motoneurons of the lower spinal cord from frog. The accumulation of acetylcholinesterase at a crush on the ventral roots was used to follow axonal transport. Two types of experiments were performed: modification of the medium bathing the ventral roots, alone, and modification of the medium bathing the spinal cord and ventral roots. Incubation (17--18 h) of the ventral roots in Ca2+-free medium markedly inhibited acetylcholinesterase transport, a finding that demonstrates a Ca2+ requirement for fast axonal transport in motoneurons; when 4 mM MgCl2 was added to the Ca2+-free medium, transport was also greatly reduced. During incubation of the ventral roots in normal medium supplemented with 0.18 mM CoCl2 transport proceeded normally; but when the Co2+ concentration was raised to 1.8 mM, transport was diminished as drastically as in the Ca2+-free medium. Incubation of the spinal cord and ventral roots in medium containing 0.18 mM CoCl2 did not reduce the accumulation of acetylcholinesterase at the crush. Similarly, accumulation of acetylcholinesterase at a crush on the dorsal root was not significantly reduced by exposure of the dorsal root ganglion and root to 0.18 mM Co2+. Exposure of sensory cell bodies to 0.18 mM Co2+ thus produces differential effects on transport of acetylcholinesterase and on transport of newly synthesized radiolabeled protein.

Microscopic observations on endogenous fluorochromes within a nerve fibre excited by a 325 nm He-Cd laser

A microscopic procedure to observe endogenous fluorochromes in superfused tissues is described. In this procedure, a 325 nm beam from a He-Cd laser is directed through the objective lens of a microscope onto the surface of a brain slice. Fluorescent structures within the tissue can be observed, photographed and measured. A nerve fibre running from the corporis callosi to the dorsal cerebral cortex was studied with this approach. The 325 nm laser beam excited material with an emission peak at 420 nm within a fibre on the cut surface of a rat brain slice superfused with Krebs-Ringer. A similar fibre was observed in vivo in the exposed mouse brain. Packets of the fluorescent material appear to move within the fibre in vitro. The fluorescence of the fibre was rapidly diminished by cyanide, colchicine, rotenone and the withdrawal of Ca2+. Elevation of [K+] in the superfusate caused a "haloing' of the fibre's granularity without decreasing the total fluorescence. This phenomenon is possibly related to the depolarization by [K+] of neurones. Although the structure of the fluorochrome was not established, it is suggested that it may be a neurotransmitter-related substance undergoing axonal movement.

Divalent cations and fast axonal transport in chemically desheathed (Triton X-treated) frog sciatic nerve

We have studied the ability of divalent cations to restore to normal axonal transport (AXT) which was inhibited by deprivation of Ca2+ and/or Mg2+ ions. The epi- and perineurium of the frog sciatic nerve were damaged by a 30-s wash in Triton X-100 containing frog Ringer's. This treatment did not affect either AXT or nerve levels of Ca2+ and Mg2+, but made the ions more easily extractable with a Ca2+- and Mg2+-free Ringer's solution (CMFR). Inhibition of AXT was achieved by incubating Triton X-100-treated nerves in CMFR + EGTA for 5 h, followed by an additional incubation for 12 h in CMFR or Ringer's devoid of only Ca2+ (CFR). These treatments reduced Ca2+ and Mg2+ contents by 77% and 38% respectively. Addition of Ca2+ (1.1 mM) during the 12-h period stimulated AXT, measured as accumulation of 3H-labelled components in front of a ligature, several fold. Mg2+ could not substitute for Ca2+ but potentiated the stimulating effect of Ca2+. Addition of other divalent cations did not affect AXT (Sr2+ and Ba2+) or potentiated the inhibition caused by Ca2+-deprived medium (Mn2+ and Co2+). ATP and creatine phosphate contents were similar in nerves incubated in Ca2+-deprived medium and in Ca2+-containing Ringer's. Thus, inhibition of AXT in the former situation was not due to a decreased availability of high energy phosphates. Two calcium antagonists, D-600 and nifedipin, which are potent smooth muscle relaxants, effectively blocked AXT. The present results suggest that Ca2+ is specifically required to maintain AXT and that an analogy exists between Ca2+ regulation during smooth muscle contraction and AXT.

Subcellular localization of calcium in the coronet cells and tanycytes of the saccus vasculosus of the rainbow trout, Salmo gairdneri Richardson

The intracellular localization of calcium in the saccus vasculosus of the rainbow trout, Salmo gairdneri Richardson, was studied by means of ultracytochemical and X-ray microanalytical techniques. Using a variant of the glutaraldehyde/potassium pyroantimonate-osmium tetroxide method, Ca was detected in mitochondria, smooth endoplasmic reticulum and primary vesicles of coronet cells, and in mitochondria and smooth endoplasmic reticulum of tanycytes. Mitochondria and smooth endoplasmic reticulum in both cell types are considered as general Ca-stores. The primary vesicles in the ciliary globules of coronet cells are viewed as additional Ca-reservoirs. Possible roles of these Ca-stores in the regulation of transport activities of coronet cells in the homeostasis of the CSF are discussed.

A study of the motion of organelles which undergo retrograde and anterograde rapid axonal transport in Xenopus

1. Axonally transported organelles were detected optically in myelinated axons from Xenopus laevis at room temperature (21-23 degrees C). Details of the motion of organelles which were transported in the retrograde and anterograde directions were studied using filmed records.2. A group of 133 organelles with a mean retrograde velocity of 0.91 mum/sec was compared with a group of thirty-nine organelles with a mean anterograde velocity of 0.93 mum/sec.3. Averaged power spectra of the positional deviations about the mean positional change through time were constructed for organelles which travelled in the retrograde and anterograde directions. Most of the power in the two spectra was at frequencies below 0.2 Hz and each contained a single peak at 0.02-0.04 Hz. The power spectrum for retrograde organelle motion had a magnitude about twice that for anterograde organelle motion.4. Estimates of the instantaneous velocity of organelles which travelled in either direction varied smoothly with time. Instantaneous velocity was not a smooth function of organelle position, (i.e. was ;saltatory').5. Histograms of the estimates for the groups of organelles whose major motion was retrograde or anterograde were broad, covering a range of about 3 mum/sec, were unimodal, and passed through zero to include a small group of values which indicated motion in the opposite (minor) direction.6. Organelles spent, on average, more time moving in the minor direction the lower their mean velocity.7. The variation in instantaneous velocity was greater for organelles which travelled in the retrograde direction than for those which travelled in the anterograde direction. No correlation was found between the variation of instantaneous velocity and the mean velocity of the organelles.8. Images of organelles occasionally appeared to rotate while the organelle continued to move in the major direction of travel.9. Evidence is presented that spatially related properties of the axon influence organelle velocity and that this influence is common to organelles which travel in the two major directions.10. A hypothesis is presented to account for the findings. This supposes that each organelle travels through a stationary axoplasm and is propelled by the resultant of two opposing driving forces whose relative magnitude fluctuates with time. Spatially dependent properties of the axoplasm modify the postulated time-related cycle of motion.

Ca2+- or Mg2+-stimulated ATPase activity in bullfrog spinal nerve: relation to Ca2+ requirements for fast axonal transport

Adenosine triphosphatase (ATPase) activity stimulated by Ca2+ or Mg2+ was characterized in spinal nerve and spinal sensory ganglion of bullfrog. Enzyme activity of homogenates from both sources reached a maximum at a 1-2 mM concentration of either cation, although the level of maximal activity in nerve trunks was approximately twice that in ganglia. Enzyme activation was not observed with 2 mM-Sr2+ or Ba2+. Co2+ or Mn2+, at 2 mM, depressed Ca2+ activation of the enzyme by 50-60% in nerve but had no inhibitory effect on ganglia activity. In intact spinal ganglion/spinal nerve preparations, incubated for 20 h in medium containing 0.2 mM-Co2+, no effect was detected on Ca2+/Mg2+ ATPase activity in ganglia or nerve trunks whereas fast axonal transport was inhibited by 80%. Incubation in medium containing 0.02 mM-Hg2+ depressed enzyme activity in ganglia by 64% and in nerve trunks by 44%, whereas fast transport was again inhibited by 80%. When only nerve trunks were exposed to these ions, Hg2+ but not Co2+ was observed to slow the rate of fast axonal transport. The divalent cation specificity of the Ca2+/Mg2+ ATPase activity is distinct from the ion specificities, determined in previous work, of the Ca2+ requirement during initiation of fast axonal transport in the soma, and of the Ca2+ requirement during translocation in the axon. Thus, previous observations of Ca2+-dependent events in fast axonal transport cannot be taken per se to suggest the involvement of Ca2+/Mg+ ATPase in the transport process.

Inhibition of rapid axonal transport in vitro by the ionophores X-537 A and A 23187

The effects of the ionophores X-537 A and A 23187 on axonal transport (AXT)' high energy phosphates, levels of Ca2+ and ultrastructure were investigated in frog sciatic nerves in vitro. X-537 A at 0.09 muM blocked AXT of [3H]labelled proteins by 50% as judged by ligature experiments while the levels of ATP and CrP remained unchanged. Pulse-label experiments showed that the amount of transported material decreased whereas the rate of AXT was only slightly retarded. Elevated extracellular levels of Ca2+ (5mM) potentiated the inhibitory effect of the ionophore. In contrast, ruthenium red counteracted the ionophore induced inhibition of AXT. A 23187 at 10 muM but not at 2 muM inhibition AXT but also reduced the ATP and CrP levels. In pulse-label experiments A 23187 displayed similar but less pronounced effects than X-537 A. Both ionophores increased the total calcium content of the nerve and appeared to decrease the amount of axonal mictotubles (MT). As X-537 is a very potent inhibitor of AXT it may prove a valuable tool in exploring the relations between AXT, Ca2+ and MT.

Microtrabecular structure of the axoplasmic matrix: visualization of cross-linking structures and their distribution

Axoplasmic transport is a dramatic example of cytoplasmic motility. Constituents of axoplasm migrate as far as 400 mm/d or at approximately 5 micron/s. Thin-section studies have identified the major morphological elements within the axoplasm as being microtubules, neurofilaments (100-A filaments), an interconnected and elongated varicose component of smooth endoplasmic reticulum (SER), more dilated and vesicular organelles resembling portions of SER, multivesicular bodies, mitochondria, and, finally, a matrix of ground substance in which the tubules, filaments, and vesicles are suspended. In the ordinary thin-section image, the ground substance is comprised of wispy fragments which, in not being noticeably tied together, do not give the impression of representing more than a condensation of what might be a homogeneous solution of proteins. With the high-voltage microscope on thick (0.5-micron) sections, we have noticed, however, that the so-called wispy fragments are part of a three-dimensional lattice. We contend that this lattice is not an artifact of aldehyde fixation, and our contention is supported by its visability after rapid-freezing and freeze-substitution. This lattice or microtrabecular matrix of axoplasm was found to consist of an organized system of cross-bridges between microtubules, neurofilaments, cisternae of the SER, and the plasma membrane. We propose that formation and deformation of this system are involved in rapid axonal transport. To facilitate electron microscope visualization of the trabecular connections between elements of axoplasm, the following three techniques were used: first, the addition of tannic acid to the primary fixative, OsO4 postfixation, then en bloc staining in uranyl acetate for conventional transmission electron microscope (TEM); second, embedding tissue in polyethylene glycol for thin sectioning, dissolving out the embedding medium from the sections and blocks, critical-point-drying (J. J. Wolosewick, 1980, J. Cell Biol., 86:675-681.), and then observing the matrix-free sections with TEM or the blocks with a scanning electron microscope; and third, rapid freezing of fixed tissue followed by freeze-etching and rotary-shadowing with replicas observed by TEM. All of these procedures yielded images of cross-linking elements between neurofilaments and organelles of the axoplasm. These improvements in visualization should enable us to examine the distribution of trabecular links on motile axonal organelles.

The effects of the calcium ionophore, A23187, on the axoplasmic transport of dopamine beta-hydroxylase

1 The effects of the ionophore, A23187, on the intra-axonal transport of dopamine beta-hydroxylase (DBH) were investigated in the cat hypogastric nerve-inferior mesenteric ganglion preparation by monitoring, in vitro, the enzyme accumulation above a ligature, 2 to 2.5 cm distal to the ganglion. 2 DBH accumulation in the proximal segment immediately above the ligature (P1) increased linearly up to 6 h, during incubation in normal Krebs solution at 37 degrees C. The ionophore, A23187, interfered with the enzyme accumulation, but did not modify the previously accumulated DBH activity present in P1. 3 The blocking effects of A23187 on DBH transport were greatly impaired in the absence of extracellular calcium ions; an excess of calcium in the bathing solution (7.5 mM) itself blocked the enzyme transport by 50%. 4 A23187 did not significantly modify the levels of adenosine triphosphate (ATP) in the segments P1 and P2 of the nerve proximal to the ligature. 5 Nerves incubated in an A23187-containing medium showed many mitochondria of normal shape and fine structure; however, typical microtubules or filaments were not seen in these preparations. 6 The results suggest that the ionophore A23187, by considerably raising the axoplasmic ionized calcium levels, interferes with the assembling of microtubules. In this manner, the ionophore would inhibit the transport of adrenergic vesicles and therefore of DBH along the axon. The results also provide additional evidence in favour of the view that for the transport system to work adequately, it is necessary to maintain the intra-axoplasmic ionized calcium concentration between certain critical levels.

Fast axonal transport in the presence of high Ca2+: evidence that microtubules are not required

Microtubules have long been associated with the mechanism of fast axoplasmic transport, although experimental evidence to support an involvement has been equivocal. Electron microscopic studies demonstrated that incubation of the axons of excised rat sciatic nerves in media containing 75 mM Ca2+ caused complete loss of microtubules within 6 hr. To evaluate the role of microtubules in fast anterograde transport, studies of transport in nerves exposed to these conditions were undertaken. Prior to measurement of axoplasmic transport, nerves ligated distal to the dorsal root ganglia were preincubated in vitro in 75 mM Ca2+ for 0-6 hr. Fast axonal transport was subsequently monitored by measuring the amount of trichloroacetic acid-insoluble radioactivity that accumulated at the ligature after incubation for 12-18 hr with L-[3H]proline. Nerves in which microtubules had been depolymerized by preincubation in high Ca2+ maintained control levels of transport. We conclude that intact microtubules are not required for fast anterograde axoplasmic transport.

Turnaround of axoplasmic transport of selected particle-specific enzymes at an injury in control and diisopropylphosphorofluoridate-treated rats

Reversal of direction (turnaround) of axonal transport of particle-specific enzyme activities was studied at a ligature placed on rat sciatic nerve. In the principal experiment, the ligature remained on the nerve in vivo several hours, allowing enzyme activities (acetylcholinesterase, acid phosphatase, and monoamine oxidase) to accumulate immediately proximal to the tie. The nerve was then tied a second time, proximal to the first tie, and incubated in vitro for several more hours. Accumulation of enzyme activities just distal to the second tie was measured. This second accumulation, of activities traveling in the retrograde direction, was shown to be the result of turnaround in several ways. (1) The increase in activity distal to the second tie was equal to the decrease in activity proximal to the first. (2) The increase in enzyme activities distal to the second tie was greatly reduced when the accumulation proximal to the first tie was trapped by placing a third tie between the first and second ties. (3) It was shown that the activity that accumulated distal to the second tie could not have been in retrograde motion at the time of the first tie. (4) Accumulation distal to the second tie was not a function of the length of nerve segment included between the two ties. In contrast to the consistent occurrence of turnaround of orthograde flow, turnaround of retrograde flow could not be demonstrated. Turnaround transport was blocked by incubation in the cold and in the presence of NaCN or vinblastine. The turnaround process operated on all three enzymes studied, suggesting that it operates on lysosomes and mitochondria, as well as on the endoplasmic reticulum-like material bearing acetylcholinesterase. Evidence for the participation of the transport process in the renewal of AChE in the distal portions of the axon was obtained in experiments using diisopropylphosphorofluoridate and cycloheximide.

Axonal transport of the Ca2+-dependent protein modulator of 3':5'-cyclic-AMP phosphodiesterase in the rabbit visual system

Water-soluble proteins were extracted from individual retinas, optic nerves, combined optic tracts and lateral geniculate bodies, and superior colliculi of rabbits at 1, 3, and 18 days after injection of [3H]leucine into the right eye. The Ca2+-dependent protein modulator of 3':5'-cyclic-AMP phosphodiesterase (calmodulin) was isolated from these samples by a two-step polyacrylamide gel electrophoresis procedure. An analysis of the radioactivity incorporated into the total soluble proteins and the calmodulin revealed that most of the calmodulin was axonally transported at a slow rate (2--4 mm/day) and represented about 0.45% of the total transported soluble protein.

The synthesis and possible transport of specific proteins by cells associated with brain capillaries

Vinblastine causes alterations in the subcellular distribution of certain proteins synthesized by telencephalon slices. Proteins in various subcellular fractions were separated according to their molecular weight by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and radioactive proteins were determined by autofluorography. A microvascular fraction contained very high amounts of radioactivity in proteins with a molecular weight of 71,000. At least one of these proteins accumulated in the microvascular fraction when the telencephalon slices were incubated in vinblastine. At the same time these proteins became depleted in a myelinated axon fraction, microsomal fraction, and soluble/cytosol fraction. Vinblastine also affected the subcellular distribution of some proteins with a molecular weight below 27,00, but unlike the proteins of mol. wt. 71,000 none of these were synthesized at very high rates. Vinblastine did not effect the synthesis of protein in telencephalon slices, nor did it alter the subcellular fractionation of particles and organelles from slices. It is suggested that a non-neuronal vinblastine-sensitive protein translocation system is functioning within the cells of the microvascular network in telencephalon slices, and that at least one protein of 71,000 molecular weight and one protein with a molecular weight below 27,000 are transported on this system.