Components of the plasma membrane of growing axons. I. Size and distribution of intramembrane particles

The plasmalemma of mature and growing olfactory axons of the bullfrog has been studied by freeze-fracture. Intramembrane particles (IMPs) of mature olfactory axons are found to be uniformly distributed along the shaft. However, during growth, a decreasing gradient of IMP density is evident along the somatofugal axis. The size histograms of axolemmal IMPs from different segments of growing nerve reveal regional differences in the particle composition. The distribution of each individual size class of particles along the growing nerve forms a decreasing gradient in the somatofugal direction; the slope of these gradients varies directly with particle diameter. These size-dependent density gradients are consistent with a process of lateral diffusion of membrane components that are inserted proximally into the plasma membrane. The membrane composition of the growth cone, however, appears to be independent of these diffusion gradients; it displays a mosaic pattern of discrete domains of high and low particle densities. The relative IMP profiles of these growth cone regions are similar to one another but contain higher densities of large IMPs than the neighboring axonal shaft. The shifting distributions of intramembrane particles that characterize the sprouting neuron give new insights into cellular processes that may underlie the establishment of the functional polarity of the neuron and into the dynamics of axolemmal maturation.     

Isolation of membranes enriched in "tetrodotoxin-insensitive" saxitoxin-binding sites from mammalian ventricle. Receptor solubilization

Purification and characterization of Na+ channel protein from mammalian ventricular myocytes has heretofore been complicated by the low concentration of Na+ channels and by the finding that mammalian ventricles contain both tetrodotoxin (TTX)-sensitive channels (TSC), with high affinity for saxitoxin (STX), and TTX-insensitive channels (TIC), with low affinity for STX. Most (perhaps all) of the sodium current for myocardial cell action potentials is carried by TIC. Most, if not all, of the cardiac TSC reside in nerves innervating the heart. To isolate TIC in sufficient quantity for further study, we prepared t-tubular sarcolemmal vesicles from large (sheep) hearts with techniques designated to minimize contamination from nerve plasmalemma. Discontinuous sucrose density gradient centrifugation of these membranes produced membrane vesicles, some of which contained no detectable TSC (range 94-100% TIC, or 0-6% TSC), at a concentration of 200-1500 fmol total sites/mg protein, with yields of 4.0-25.0 mg protein/100 g starting material (ventricle). TTX-insensitive STX-binding sites were solubilized from the membranes by 1% digitonin (and with less stability by Triton X-100). The equilibrium binding constant and dissociation rate coefficient for STX binding to the digitonin-solubilized sites were similar to those of the binding sites for the unsolubilized membranes. Unlabeled TTX competed with [3H]STX for the site with 14 times less affinity than did unlabeled STX. Digitonin-solubilized sites had a half-life for STX binding of about 24 h. Binding could be further stabilized by addition of Mg2+ or Ca2+ and exogenous phospholipid.     

Molecular structure of the axolemma of developing axons following altered gliogenesis in rat optic nerve

Axonal and axolemmal development of fibers from rat optic nerves in which gliogenesis was severely delayed by systemic injection of 5-azacytidine (5-AZ) was examined by freeze-fracture electron microscopy. In neonatal (0-2 days) rat optic nerves, all fibers lack myelin, whereas in the adult, virtually all axons are myelinated. The axolemma of neonatal premyelinated fibers is relatively undifferentiated. The P-fracture face (P-face) displays a moderate (approximately 550/micron 2) density of intramembranous particles (IMPs), whereas the E-fracture face (E-face) has few IMPs (approximately 125/micron 2) present. By 14 days of age, approximately 25% of the axons within control optic nerves are ensheathed or myelinated, with the remaining axons premyelinated. The ensheathed and myelinated fibers display increased axonal diameter compared to premyelinated axons, and these larger caliber fibers exhibit marked axonal membrane differentiation. Notably, the P-face IMP density of ensheathed and myelinated fibers is substantially increased compared to premyelinated axolemma, and, at nodes of Ranvier, the density of E-face particles is moderately high (approximately 1300/micron 2), in comparison to internodal or premyelinated E-face axolemma. In optic nerves from 14-day-old 5-AZ-treated rats, few oligodendrocytes are present, and the percentage of myelinated fibers is markedly reduced. Despite delayed gliogenesis, some unensheathed axons within 5-AZ-treated optic nerves display an increased axonal diameter compared to premyelinated fibers. Most of these large caliber fibers also exhibit a substantial increase in P-face IMP density. Small (less than 0.4 micron) diameter unensheathed axons within treated optic nerves maintain a P-face IMP density similar to that of control premyelinated fibers. Regions of increased E-face particle density were not observed. The results demonstrate that some aspects of axolemma differentiation continue despite delayed gliogenesis and the absence of glial ensheathment, and suggest that axolemmal ultrastructure is, at least in part, independent of glial cell association.     

Identification of an Axolemma-Enriched Fraction from Peripheral Nerve

Abstract: A method has been devised for the fractiona-tion of whole peripheral nerve. The procedure utilizes differential centrifugation and separation on a linear sucrose gradient (10–40%, wt/wt). A membrane fraction localized between 26% and 29% sucrose was not only enriched for the plasma membrane markers, 5'-nucleotidase and acetylcholinesterase (AChE), but also possessed the highest binding of [³H]saxitoxin, a specific marker for sodium channels. Neurons in the lumbar dorsal roots and ventral horns of rats were injected with [³H]fucose to label glycoproteins associated with the axolemma from sciatic nerve. Fractionation of the labeled nerves demonstrated a coincidence in the distribution of [³H]fucose-labeled material and AChE activity in the sucrose density gradient. The increase in the specific activity of marker enzymes for plasma membrane, sodium channels, and labeled membrane, previously demonstrated to be of axolemmal origin, identified the 26–29% region of the sucrose gradient as enriched for axolemma derived from peripheral nerve.     

Development of saxitoxin-conjugated affinity gels

Saxitoxin (STX) and its analogues accumulated in bivalves cause food poisoning through the blockade of sodium channels in the nervous system. In the current studies, STX-conjugated agarose gels as affinity chromatography reagents were prepared for investigation of the fate of the toxins in natural environments and in the human body. A carboxyl moiety was introduced through positions C11 and C13 to leave the most characteristic part of the molecule intact. Two types of synthesized derivatives, 11-(2-carboxyethylthio)saxitoxin and 13-O-hemisuccinyldecarbamoylsaxitoxin, were successfully conjugated to Sepharose 4B in high yield. Affinity gels containing 500 nmol of STX or decarbamoylsaxitoxin per milliliter of gel were accomplished by masking the residual amino groups by acetylation. Finally, the STX-conjugated affinity gel was effective for concentrating STX-binding proteins from pufferfish and bullfrog plasma.     

Development of the axon membrane during differentiation of myelinated fibres in spinal nerve roots

Previous studies by a number of workers have shown that the axon membrane in normal mature myelinated fibres is highly differentiated, with the nodal axolemma exhibiting characteristics different to those of the internodal axolemma. However, the development of this axolemmal heterogeneity has not been previously explored. In the present study we used cytochemical methods to examine the development of nodal axolemma during the differentiation of myelinated fibres in rat spinal roots. The staining properties characteristic of normal nodal membrane appear in the axon, at gaps between Schwann cells, before the development of mature compact myelin or well defined paranodal axon--Schwann cell specializations close to the region of nodal axolemmal differentiation. These results are consistent with the hypothesis that the axon membrane differentiates into nodal and internodal regions before, or early in the process of, myelination, and suggest that the differentiation of the axon membrane may provide a signal demarcating the region to be covered by the myelin-forming cell.     

Ionic channels and nerve membrane constituents. Tetrodotoxin-like interaction of saxitoxin with cholesterol monolayers

Saxitoxin (STX) and tetrodotoxin (TTX) have the same striking property of blocking the Na⁺ channels in the axolemma. Experiments with nerve plasma membrane components of the squid Dosidicus gigas have shown that TTX interacts with cholesterol monolayers. Similar experiments were carried out with STX. The effect of STX on the surface pressure-area diagrams of lipid monolayers and on the fluorescence emission spectra of sonicated nerve membranes was studied. The results indicate a TTX-like interaction of STX with cholesterol monolayers. The expansion of the monolayers caused by 10^-6 M STX was 2.2 A²/cholesterol molecule at 25°C. From surface pressure measurements at constant cholesterol area (39 A²/molecule) in media with various STX concentrations, it was calculated that the STX/cholesterol surface concentration ratio is 0.54. The apparent dissociation constant of the STX-cholesterol monolayer complex is 4.0 x 10^-7 M. The STX/cholesterol ratio and the apparent dissociation constant are similar to those determined for TTX. The presence of other lipids in the monolayers affects the STX-cholesterol association. The interactions of STX and TTX with cholesterol monolayers suggest (a) that cholesterol molecules may be part of the nerve membrane Na⁺ channels, or (b) that the toxin receptor at the nerve membrane shares similar chemical features with the cholesterol monolayers.     

Saxitoxin binding in nerves from walking legs of the lobster Homarus americanus. Two classes of receptors

The binding of exchange-labeled saxitoxin (STX) to sodium channels has been investigated in the nonmyelinated fibers of the walking leg nerves of the lobster. The properties of the STX binding site differed systematically among the nerves from different walking legs. The equilibrium dissociation constant for STX binding (KSTX) to the front legs is approximately twice that for the binding to the rear legs; the average ratio of KSTX (front): KSTX (rear) from five separate experiments was 1.80 +/- 0.21 (mean +/- SE). The actual KSTX values ranged from 124.0 to 22.7 nM for the front leg nerves and from 8.6 to 12.7 nM for the rear leg nerves. KSTX values for the middle two walking leg nerves fell between those for the front and rear legs. The inhibitory dissociation constant for tetrodotoxin (KTTX), calculated from tetrodotoxin's inhibition of labeled STX binding, was 3.02 +/- 0.27 nM for the front legs and 2.20 +/- 0.33 nM for the rear legs. The ratio KSTX: KTTX was different in the front and rear leg nerves, being 5.5 and 4.2, respectively. The apparent P pKa of the STX receptor also differed between the two legs, being 4.6 +/- 0.3 for the front legs and 5.1 +/- 0.1 for the rear legs. These results demonstrate that one tissue type in one organism can contain different toxin binding sites. The difference in the receptors can be qualitatively accounted for by the location of an additional negative charge near the receptor site of the rear walking leg.     

Sodium channels in axons and glial cells of the optic nerve of Necturus maculosa

Experiments investigating both the binding of radioactively labelled saxitoxin (STX) and the electrophysiological response to drugs that increase the sodium permeability of excitable membranes were conducted in an effort to detect sodium channels in glial cells of the optic nerve of Necturus maculosa, the mudpuppy. Glial cells in nerves from chronically enucleated animals, which lack optic nerve axons, show no saturable uptake of STX whereas a saturable uptake is clearly present in normal optic nerves. The normal nerve is depolarized by aconitine, batrachotoxin, and veratridine (10(-6)-10(-5) M), whereas the all-glial preparation is only depolarized by veratridine and at concentrations greater than 10(-3) M. Unlike the depolarization caused by veratridine in normal nerves, the response in the all-glial tissue is not blocked by tetrodotoxin nor enhanced by scorpion venom (Leiurus quinquestriatus). In glial cells of the normal nerve, where axons are also present, the addition of 10(-5) M veratridine does lead to a transient depolarization; however, it is much briefer than the axonal response to veratridine in this same tissue. This glial response to veratridine could be caused by the efflux of K+ from the drug-depolarized axons, and is similar to the glial response to extracellular K+ accumulation resulting from action potentials in the axon.     

Action of the sterol-binding form of filipin on the lobster axon membrane

The present work demonstrates by electron microscopy and spectrofluorimetry the interaction of the sterol-binding form of filipin with the axolemma of lobster nerves. The results also indicate that the axolemma cholesterol accessible to filipin seems to play no specific role in the functioning of the tetrodotoxin receptors and sodium channels.     

Microtubule stability along mammalian peripheral nerves

Microtubule (MT) number, axonal area, and MT density were examined in unmyelinated axons of rat cervical vagus nerve. Study of nerve regions proximal (1-5 mm) and distal (35-40 mm) to the nodosum ganglion in controls (incubation at 37 degrees C for 1 h) showed that the number of MT per axon is significantly less in distal than in proximal nerve regions. Cooling (incubation at 0 degree C for 1 h) caused a significant reduction in the number of MT per axon in both nerve regions. The unmyelinated axons from both nerve regions showed a comparable reduction in MT number by cooling, indicating that axonal MT stability to cold was not significantly different between these two nerve regions. In these nerves no detectable changes were found in cross-axonal area of unmyelinated axons between distal and proximal nerve regions. In another experimental series, in distal nerve regions (35-40 mm from the nodosum ganglion) the number of MT was not further reduced in nerves incubated at 0 degree C by increasing the incubation time. Similar results were obtained from colchicine treated nerves (incubation at 37 degrees C, with 10 mM colchicine for 1 and 2 h). Distal nerve regions (35-40 mm from the nodosum ganglion) showed a similar reduction in the number of MT per axon when nerves were incubated at 0 degree C or with colchicine, suggesting that this drug, as well as cold, may be affecting a similar population of axonal MT, i.e., MT susceptible to anti-MT agents. These results indicate that approximately one-half of the axonal MT are stable to cold as well as to colchicine in rat unmyelinated axons.     

Axonal growth during regeneration: a quantitative autoradiographic study

The intraaxonal distribution of labeled glycoproteins in the regenerating hypoglossal nerve of the rabbit was studied by use of quantitative electron microscope autoradiography. 9 d after nerve crush, glycoproteins were labeled by the administration of [3H]fucose to the medulla. The distribution of transported 3H-labeled glycoproteins was determined 18 h later in segments of the regenerating nerve and in the contralateral, intact nerve. At the regenerating tip, the distribution was determined both in growth cones and in non-growth cone axons, 6 and 18 h after labeling. The distribution within the non-growth cone axons of the tips was quite different at 6 and 18 h. At 6 h, the axolemma region contained < 10% of the radioactivity; at 18 h, it contained virtually all the radioactivity. In contrast, the distribution within the growth cones was similar at both time intervals, with 30% of the radioactivity over the axolemmal region. Additional segments of the regenerating nerve also showed a preferential labeling of the axolemmal region. In the intact nerve, 3H-labeled glycoproteins were uniformly distributed. These results suggest that: (a) in this system the labeled glycoproteins reaching the tip of the regenerating axons are inserted into the axolemma between 6 and 18 h after leaving the neuronal perikaryon; (b) at the times studied, there is a fairly constant ratio between glycoproteins reaching the growth cone through axoplasmic transport and glycoproteins inserted into the growth cone axolemma; (c) the axolemma elongates by continuous insertion of membrane precursors at the growth cone; the growth cone then advances, leaving behind an immature axon with a newly formed axolemma; and (d) glycoproteins are preferentially inserted into the axolemma along the entire regenerating axon.     

Ionic channel distribution in regenerating mouse motor endings

One of the nerves supplying the Triangularis sterni muscle of the adult mouse was crushed and focal extracellular recordings were made from the regenerating motor nerve terminals. Stimulation of the nerve elicited 4 types of signals depending upon the position of the electrode along the newly formed terminal branches. Specific ionic channel blockers were applied in the bath or iontophoretically to identify the nature of the different signal components. Sodium and potassium channels appeared to be located along most of the length of the newly formed terminals. This overlapping distribution differs considerably from their segregated localization previously described for mature endings. During maturation of regenerating endings, potassium channels disappear from the heminodal area and the density of sodium channels is greatly reduced along the remainder of the branches. Presynaptic evidence for polyneuronal reinnervation of single endplates is presented.     

Reconstitution of the voltage-sensitive sodium channel of rat brain from solubilized components

The voltage-sensitive sodium channel of rat brain synaptosomes was solubilized with sodium cholate. The solubilized sodium channel migrated on a sucrose density gradient with an apparent S20,w of approximately 12 S, retained [3H]saxitoxin ([3H]STX) binding activity that was labile at 36 degrees C but no longer bound 125I-labeled scorpion toxin (125I-ScTX). Following reconstitution into phosphatidylcholine vesicles, the channel regained 125I-ScTX binding and thermal stability of [3H]STX binding. Approximately 50% of the [3H]STX binding activity and 58% of 125I-ScTX binding activity were recovered after reconstitution. The reconstituted sodium channel bound STX and ScTX with KD values of 5 and 10 nM, respectively. Under depolarized conditions, veratridine enhanced the binding of 125I-ScTX with a K0.5 of 20 microM. These KD and K0.5 values are similar to those of the native synaptosome sodium channel. 125I-ScTX binding to the reconstituted sodium channel, as with the native channel, was voltage dependent. The KD for 125I-ScTX increased with depolarization. This voltage dependence was used to demonstrate that the reconstituted channel transports Na+. Activation of sodium channels by veratridine under conditions expected to cause hyperpolarization of the reconstituted vesicles increased 125I-ScTX binding 3-fold. This increased binding was blocked by STX with K0.5 = 5 nM. These data indicate that reconstituted sodium channels can transport Na+ and hyperpolarize the reconstituted vesicles. Thus, incorporation of solubilized synaptosomal sodium channels into phosphatidylcholine vesicles results in recovery of toxin binding and action at each of the three neurotoxin receptor sites and restoration of Na+ transport by the reconstituted channels.     

The binding of saxitoxin to axolemma of mammalian brain. Cooperative competition between saxitoxin and sodium ion.

Saturable, high affinity binding of tritium-labeled saxitoxin ([3H]STX) to axolemma-enriched membranes from white matter of bovine brain was identified. The apparent [3H]STX equilibrium dissociation constant (Kd*) was strongly affected by the cationic environment:choline ion had little effect; cesium ion increased the mammalian axolemma Kd* in a simple competitive manner. In contrast, sodium ion more dramatically increased the Kd*--this effect was highly cooperative between 75 and 200 mM sodium (Hill coefficient of 2.85). The cooperativity is most pronounced at the normally expected [sodium] external to the axon in the mammalian central nervous system. This sodium-specific cooperative modification of the STX binding site (the hypothetical "ion selectivity filter" of the axonal Na+ gate) may be indicative of some as yet undefined regulatory mechanism of the Na+ gate in mammalian myelinated axons.     

Detection of sodium channel distribution in rat sciatic nerve following lysophosphatidylcholine-induced demyelination

Summary In vivo application of lysophosphatidylcholine (LPC) to rat sciatic nerve induces impaired hind leg movement within 2 days which is recovered by 6 days. Segmental demyelination was seen at 2 days after LPC application, and remyelination had barely started in a few axons by 6 days. Using sodium channel-specific monoclonal antibodies and immunofluorescence microscopy, we observed altered distribution of sodium channels in demyelinated axons. Bright fluorescent labeling was found along the segmentally demyelinated axolemma at 6 days in contrast to the dim staining of the demyelinated nerve found at 2 days. In addition, radioimmunoassays detected an elevated number of antibody binding sites on sciatic nerve trunk from the sixth day. Our data provide the immunocytochemical evidence for the assumption that recruitment of sodium channels into demyelinated axolemma contributes to the recovery of function following axon demyelination by LPC.     

Nerve growth factor and fibroblast growth factor influence post-fusion expression of Na-channels in cultured rat skeletal muscle

We have examined effects of nerve growth factor (NGF) and fibroblast growth factor (FGF) on the density of tetrodotoxin (TTX)-sensitive Na-channels in cultured rat skeletal muscle. Measurements were made of specific binding of [3H]saxitoxin (STX) and the frequency and rate of rise of spontaneously occurring action potentials, the physiological expression of Na-channel density. Cells were transferred to various growth conditions at 6 days in vitro, and measurements were made beginning 24 hr later. Both growth factors (GF) caused dose-related increases in Na-channels compared with myotubes maintained in normal, serum-supplemented growth medium. Maximum effects occurred with a concentration of NGF of 50 ng/ml and FGF of 15 ng/ml. Scatchard analysis of specific STX binding showed an increase in Bmax with no significant change in Kd. Similar increases occurred on rate of rise and frequency spontaneous action potential. Treatment of cultures with cycloheximide or actinomycin D, inhibitors of protein and RNA synthesis, completely prevented the increase in STX-binding induced by GF treatment. The results indicate that NGF and FGF have important effects on regulation of excitable cell gene products after differentiation.     

Sodium and potassium channels in regenerating and developing mammalian myelinated nerves

A study has been made of how the normal complementary distribution of sodium and potassium channels in mammalian myelinated nerve fibres (all the sodium channels being in the node with all the potassium channels in the internode) is altered in regenerating and in developing rabbit sciatic nerves. In regenerating nerve fibres, where a marked increase in the number of nodes per unit length occurs, there is a corresponding increase in the sodium channel content (determined from the maximum saturable binding of labelled saxitoxin), consistent with the idea that the number of sodium channels per node remains roughly constant. The use of 4-aminopyridine, which by blocking potassium channels prolongs the action potential, has shown that both in regenerating nerve fibres and in developing nerve fibres potassium currents contribute to the mammalian action potential. In both cases, with the passage of time, the sensitivity to 4-aminopyridine progressively decreases.     

Adaptive evolution of tetrodotoxin resistance in animals

Tetrodotoxin (TTX), first isolated from pufferfish (tetraodontids), is a highly potent neurotoxin that selectively binds to voltage-gated sodium channels (Na(v)) in muscle and nerve tissues causing paralysis and death. Saxitoxin (STX) is a TTX-related neurotoxin produced by dinoflagellates. Recent investigations have implicated diverse substitutions in the P-loop regions of skeletal muscle and neuronal Na(v) channels in the convergent evolution of neurotoxin resistance in pufferfish, garter snakes and softshell clams, which has enabled them to feed on TTX- and STX-bearing organisms.     

Incorporation of axonally transported glycoproteins into axolemma during nerve regeneration

The insertion of axonally transported fucosyl glycoproteins into the axolemma of regenerating nerve sprouts was examined in rat sciatic motor axons at intervals after nerve crush. [(3)H]Fucose was injected into the lumbar ventral horns and the nerves were removed at intervals between 1 and 14 d after labeling. To follow the fate of the “pulse- labeled” glycoproteins, we examined the nerves by correlative radiometric and EM radioautographic approaches. The results showed, first, that rapidly transported [(3)H]fucosyl glycoproteins were inserted into the axolemma of regenerating sprouts as well as parent axons. At 1 d after delivery, in addition to the substantial mobile fraction of radioactivity still undergoing bidirectional transport within the axon, a fraction of label was already associated with the axolemma. Insertion of labeled glycoproteins into the sprout axolemma appeared to occur all along the length of the regenerating sprouts, not just in sprout terminals. Once inserted, labeled glycoproteins did not undergo extensive redistribution, nor did they appear in sprout regions that formed (as a result of continued outgrowth) after their insertion. The amount of radioactivity in the regenerating nerves decreased with time, in part as a result of removal of transported label by retrograde transport. By 7-14 d after labeling, radioautography showed that almost all the remaining radioactivity was associated with axolemma. The regenerating sprouts retained increased amounts of labeled glycoproteins; 7 or 14 d after labeling, the regenerating sprouts had over twice as much of radioactivity as comparable lengths of control nerves or parent axons. One role of fast axonal transport in nerve regeneration is the contribution to the regenerating sprout of glycoproteins inserted into the axolemma; these membrane elements are added both during longitudinal outgrowth and during lateral growth and maturation of the sprout.