Resistivity of axoplasm. II. Internal resistivity of giant axons of squid and Myxicola

The specific resistivity of the axoplasm of giant axons of squid and Myxicola was measured utilizing a single metal microelectrode subjected to alternating current in a circuit in which the voltage output varies with the conductivity of the thin layer of fluid at the exposed electrode tip. The average specific resistivity of stellar axons of Loligo pealei was 31 omegacm (1.55 times seawater [X SW]) while for Loligo opalescens it was 32 omegacm (1.30 X SW). Smaller giant axons had a higher average resistivity. Myxicola giant axons had a resistivity of 68 omegacm (2.7 X SW) in normal seawater, and 53 omegacm (2.1 X SW) in a hypertonic high-Mg++ seawater. The temperature dependence of squid axon resistivity does not differ from that of an equally conductive dilution of seawater.     

Identification of the subunit proteins of 10-nm neurofilaments isolated from axoplasm of squid and Myxicola giant axons

Neurofilaments were isolated from the axoplasm of the giant axons of Myxicola infundibulum and squid. The axoplasm was fractionated by discontinuous sucrose gradient centrifugation and gel filtration on Sepharose 4B. The fractions were monitored for neurofilaments by electron microscopy. When isolated in the presence of chelating agents, the neurofilaments of Myxicola are composed almost entirely of protein subunits with mol wt of 150,000 and 160,000. Squid neurofilaments contain two major proteins with mol wt of 200,000 and 60,000. These proteins are compared with other intermediate filament proteins which have been reported in the literature.     

Nucleotide specificity for the bidirectional transport of membrane-bounded organelles in isolated axoplasm.

Video microscopy of isolated axoplasm from the squid giant axon permits correlated quantitative analyses of membrane-bounded organelle transport both in the intact axoplasm and along individual microtubules. As a result, the effects of experimental manipulations on both anterograde and retrograde movements of membrane-bounded organelles can be evaluated under nearly physiological conditions. Since anterograde and retrograde fast axonal transport are similar but distinct cellular processes, a systematic biochemical analysis is important for a further understanding of the molecular mechanisms for each. In this series of experiments, we employed isolated axoplasm of the squid to define the nucleoside triphosphate specificity for bidirectional organelle motility in the axon. Perfusion of axoplasm with 2-20 mM ATP preserved optimal vesicle velocities in both the anterograde and retrograde directions. Organelle velocities decreased to less than 50% of optimal values when the axoplasm was perfused with 10-20 mM UTP, GTP, ITP, or CTP with simultaneous depletion of endogenous ATP with hexokinase. Under the same conditions, TTP and ATP-gamma-S were unable to support significant levels of transport. None of the NTPs tested had a differential effect on anterograde vs. retrograde movement of vesicles. Surprisingly, several inconsistencies were revealed when a comparison was made between these results and nucleoside triphosphate specificities that have been reported for putative organelle motors by using in vitro assays. These data may be used in conjunction with data from well-defined in vitro assays to develop models for the molecular mechanisms of axonal transport.     

The early changes in the axoplasm during wallerian degeneration

Axoplasmic changes were studied in the saphenous nerve of the albino rat during the early stages of Wallerian degeneration. The axons were examined at 0, 24, and 48 hours after the surgical transection of the nerve. The material was fixed in 2 per cent OsO(4) in phosphate buffer (pH 7.2-7.5) with sucrose (added to a final osmolar concentration of approximately 0.37 M). The earliest changes were seen in the endoplasmic reticulum which became fragmented into rows of small vesicles. Then, between 24 and 48 hours, the neurofilaments underwent complete disintegration and the axoplasm became filled with finely granular material which later formed irregular clumps surrounded by a structureless matrix, probably fluid in vivo. The fragmentation of the neurofilaments was accompanied by pronounced swelling of the mitochondria.     

Uptake and release of 45Ca by Myxicola axoplasm

The binding and release of 45Ca by axoplasm isolated from Myxicola giant axons were examined. Two distinct components of binding were observed, one requiring ATP and one not requiring ATP. The ATP- dependent binding was largely prevented by the addition of mitochondrial inhibitors, whereas the ATP-independent component was unaffected by these inhibitors. The ATP-independent binding accounted for roughly two-thirds of the total 45Ca uptake in solutions containing an ionized [Ca2+] = 0.54 microM and was the major focus of this investigation. This fraction of bound 45Ca was released from the axoplasm at a rate that increased with increasing concentrations of Ca2+ in the incubation fluid. The ions Cd2+ and Mn2+ were also able to increase 45Ca efflux from the sample, but Co2+, Ni2+, Mg2+, and Ba2+ had no effect. The concentration-response curves relating the 45Ca efflux rate coefficients to the concentration of Ca2+, Cd2+, and Mn2+ in the bathing solution were S-shaped. The maximum rate of efflux elicited by one of these divalent ions could not be exceeded by adding a saturating concentration of a second ion. Increasing EGTA concentration in the bath medium from 100 to 200 microM did not increase 45Ca efflux; yet increasing the concentration of the EGTA buffer in the uptake medium from 100 to 200 microM and keeping ionized Ca2+ constant caused more 45Ca to be bound by the axoplasm. These results suggest the existence of high-affinity, ATP-independent binding sites for 45Ca in Myxicola axoplasm that compete favorably with 100 microM EGTA. The 45Ca efflux results are interpreted in terms of endogenous sites that interact with Ca2+, Cd2+, or Mn2+.     

The flow properties of axoplasm in a defined chemical environment: influence of anions and calcium.

The flow properties of axoplasm have been studied in a defined chemical environment. Axoplasm extruded from squid giant axons was introduced into porous cellulose acetate tubes of diameter roughly equal to that of the original axon. Passage of axoplasm along the tube rapidly coated the tube walls with a layer of protein. By measuring the rate of low back and forth along the tube, the rheological properties of the axoplasm plug were investigated at a range of pressures and in a variety of media. Axoplasm behaves as a classical Bingham body the motion of which can be characterized by a yield stress (theta) and a plastic viscosity (eta p). In a potassium methanesulphonate medium containing 65 nM free Ca2+, theta averaged 109 +/- 46 dyn/cm2 and eta p1 146 +/- 83 P. These values were little affected by ATP, COLCHICINE, CYTOCHOLASIN B or by replacing K by Na but were sensitive to the anion composition of the medium. The effectiveness of different anions at reducing theta and eta p1 was in the order SCN greater than I greater then Br greater than Cl greater than methanesulphonate. Theta and eta p1 were also drastically reduced by increasing the ionized Ca. This effect required millimolar amounts of Ca, was unaffected by the presence of ATP and was irreversible. It could be blocked by the protease inhibitor TLCK. E.p.r. measurements showed that within the matrix of the axoplasm gel there is a watery space that is largely unaffected by anions or calcium.     

A comparison of measurements of intracellular Ca by Ca electrode and optical indicators

Squid giant axons were injected with aequorin or arsenazo III and impaled with a Ca-sensing electrode. The light output of aequorin or the spectrophotometer output when measuring arsenazo was compared with the voltage output of the electrode when the squid axon was depolarized with high-K solutions, when the seawater was made Na-free, or when the axon was tetanized for several minutes. The results from these treatments were that the optical response rose (as much as 50-fold) with all treatments known to increase Ca entry, while the electrode remained unaffected by these treatments. If axons previously subjected to Ca load are treated with electron-transport poisons such as CN, it is known that [Ca]i rises after a time necessary to deplete ATP stores. In such axons one expects a rise of [Ca]i in axoplasm which does not necessarily have to be uniform although the source of such Ca is the mitochondria and these are uniformly distributed in axoplasm. Under conditions of CN application, the optical signals from aequorin or arsenazo and Ca electrode output do rise together when [Ca]i is high, but there is a region of [Ca]i concentration where aequorin light output or arsenazo absorbance rises while electrode output does not. Axons not loaded with Ca but injected with apyrase and vanadate have mitochondria that still retain some Ca and this can be released by CN in a truly uniform manner. The results show that such a release (which is small) can be readily measured with aequorin, but again the Ca electrode is insensitive to such [Ca]i change.     

In situ accumulation of calcium by organelles of squid axoplasm

Existing morphological and physiological evidence indicates that axoplasm of squid axons sequesters calcium by both mitochondrial and non-mitochondrial buffers. The present work demonstrates that essentially all of the non-mitochondrial component is located in organelles. Extruded axoplasm was loaded with varying amounts of calcium by mixing with small volumes of solutions containing pH buffered ⁴⁵Ca. Ethyleneglycol-bis(β-amino-ethyl ether)N,N′-tetraacetic acid (EGTA) or diethylenetriamine pentaacetic acid (DTPA) was used to stabilize the free calcium. The axoplasm was then sucked up in a polyethylene tube and centrifuged at 100,000 g for 2–3 hours to produce a loose pellet comprising 10–20% of the axoplasm volume. After centrifugation, the tube was frozen, sliced into segments, and counted by liquid scintillation. No significant pellet accumulation of exogenous calcium occurred at physiological concentrations of free calcium (ca. 50 nM); however, a threshold for accumulation existed at 150–200 nM. Essentially complete pellet sequestration of the exogenous load occurred at a free calcium concentration above 1 μM. About half of the pellet buffering capacity was sensitive to carbonyl cyanide, p-trifluoromethoxy phenylhydrazone (FCCP). Variation of exogenous load between 0.1 – 3 mmole/kg axoplasm did not affect the buffering capacity of either the FCCP sensitive or insensitive components when the free calcium concentration was above threshold.     

The presence of transfer RNA in the axoplasm of the squid giant axon

Previous work has revealed that 4S RNA is the primary species of RNA in the axoplasm from the giant axons of the squid and Myxicola. This study shows that axoplasmic 4S RNA from the squid giant axon has the functional properties of tRNA. Axoplasmic RNA was charged with amino acids by aminoacyl-tRNA synthetases prepared from squid brain. The aminoacylation was prevented by incubating the RNA with RNase prior to running the reaction. The amino acid-RNA complex was labile at pH 9, which is characteristic of the acyl linkage between an amino acid and its tRNA. Aminoacyl-tRNA synthetase activity was also present in the axoplasm, primarily in the soluble fraction.     

Phospholipid synthesis in the squid giant axon: incorporation of lipid precursors

The squid giant axon and extruded axoplasm from the giant axon were used to study the capacity of axoplasm for phospholipid synthesis. Extruded axoplasm, suspended in chemically defined media, catalyzed the synthesis of phospholipids from all of the precursors tested. 32P-Labeled inorganic phosphate and gamma-labeled ATP were actively incorporated into phosphatidylinositol phosphate, while [2-3H]myo-inositol and L-[3H(G)]serine were actively incorporated into phosphatidylinositol and phosphatidylserine, respectively. Though less well utilized. [2-3H]glycerol was incorporated into phosphatidic acid, phosphatidylinositol, and triglyceride, and methyl-3H]choline and [1-3H]ethanolamine were incorporated into phosphatidylcholine and phosphatidylethanolamine, respectively. Isolated squid giant axons were incubated in artificial seawater containing the above precursors. The axoplasm was extruded following the incubations. Although most of the product lipids were recovered in the sheath (composed of cortical axoplasm, axolemma, and surrounding satellite cells), significant amounts (4-20%) were present in the extruded axoplasm. With tritiated choline and myo-inositol, the major labeled phospholipids found in both the extruded axoplasm and the sheath were phosphatidylcholine and phosphatidylinositol, respectively. With both glycerol and phosphate, phosphatidylethanolamine was a major labeled lipid in both axoplasm and sheath. These findings demonstrate that all classes of phospholipids are formed by endogenous synthetic enzymes in axoplasm. In addition, we feel that the different patterns of incorporation by intact axons and extruded axoplasm indicate that surrounding sheath cells contribute lipids to axoplasm. A comprehensive picture of axonal lipid metabolism should include axoplasmic synthesis and glial-axon transfer as pathways complementing the axonal transport of perikaryally formed lipids.     

Calcium measurement in the periphery of an axon

Aequorin was microinjected into squid giant axons, the axons were stimulated, and the change in light emission was followed. This response was compared with that found when the axon, in addition to being microinjected with aequorin, is also injected with the dye phenol red. Large concentrations of phenol red injected into axons result in a high probability that photons emitted by aequorin, when it reacts with Ca in the core of the axoplasm, will be absorbed before they escape from the axon; photons produced by the aequorin reaction at the periphery of the axoplasm are much less likely to be absorbed. This technique thus favors observing changes in Cai taking place in the periphery of the axon. Stimulation in 50 mM Ca seawater of an aequorin-phenol red-injected axon at 180 s-1 for 1 min produces a scarcely detectable change in Cai; the addition of 2 mM cyanide (CN) to the seawater produces an easily measureable increase in Cai, suggesting that mitochondrial buffering in the periphery is substantial. Making the pH of the axoplasm of a normal axon alkaline with 30 mM NH4+ -50 mM Ca seawater, reduces the resting glow of the axon but results in an even more rapid increase in Cai with stimulation. In a phenol red-injected axon, this treatment results in a measureable response to stimulation in the absence of CN.     

ATP-dependent calcium accumulation by non-mitochondrial organelles of axoplasm isolated from Myxicola giant axons

Axoplasm from freshly isolated Myxicola giant axons was mixed with small volumes of 'artificial axoplasm' containing 45Ca and either CaEGTA/EGTA or CaDTPA/DTPA buffers giving various nominal values of [Ca2+]. The axoplasm samples were centrifuged at 100 000 X g for 30 min to form a pellet and the percentage of 45Ca bound to the pellet was determined. The fraction of bound calcium rose with increasing values of [Ca2+] along an S-shaped curve. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) was used to reveal the presence of mitochondrial Ca uptake. At physiological values of [Ca2+], around 100 nM, Ca uptake was insensitive to FCCP. As [Ca2+] was elevated, increasing sensitivity to FCCP was noted above [Ca2+] = 0.5 microM. At low values of [Ca2+], including the physiological range, Ca binding was significantly reduced by vanadate and quercetin, agents known to inhibit Ca uptake mediated by Ca2+-activated ATPase reactions. Inhibition of Ca binding by these agents was approximately 50% at physiological values of [Ca2+]. ATP depletion decreased the percentage of Ca binding by the pellet at physiological [Ca2+]. The results suggest that about 50% of the Ca buffering by particulate matter in axoplasm is via organelles requiring intact Ca2+-ATPase reaction at physiological values of [Ca2+].     

A comparison of measurements of intracellular Ca by Ca electrode and optical indicators

Squid giant axons were injected with aequorin or arsenazo III and impaled with a Ca-sensing electrode. The light output of aequorin or the spectrophotometer output when measuring arsenazo was compared with the voltage output of the electrode when the squid axon was depolarized with high-K solutions, when the seawater was made Na-free, or when the axon was tetanized for several minutes. The results from these treatments were that the optical response rose (as much as 50-fold) with all treatments known to increase Ca entry, while the electrode remained unaffected by these treatments. If axons previously subjected to Ca load are treated with electron-transport poisons such as CN, it is known that [Ca]_i rises after a time necessary to deplete ATP stores. In such axons one expects a rise of [Ca]_i in axoplasm which does not necessarily have to be uniform although the source of such Ca is the mitochondria and these are uniformly distributed in axoplasm. Under conditions of CN application, the optical signals from aequorin or arsenazo and Ca electrode output do rise together when [Ca]_i is high, but there is a region of [Ca]_i concentration where aequorin light output or arsenazo absorbance rises while electrode output does not. Axons not loaded with Ca but injected with apyrase and vanadate have mitochondria that still retain some Ca and this can be released by CN in a truly uniform manner. The results show that such a release (which is small) can be readily measured with aequorin, but again the Ca electrode is insensitive to such [Ca]_i change.     

Bidirectional transport of fluorescently labeled vesicles introduced into extruded axoplasm of squid Loligo pealei

A reconstituted model was devised to study the mechanisms of fast axonal transport in the squid Loligo pealei. Axonal vesicles were isolated from axoplasm of the giant axon and labeled with rhodamine-conjugated octadecanol, a membrane-specific fluorescent probe. The labeled vesicles were then injected into a fresh preparation of extruded axoplasm in which endogenous vesicle transport was occurring normally. The movement of the fluorescent, exogenous vesicles was observed by epifluorescence microscopy for as long as 5 min without significant photobleaching, and the transport of endogenous, nonfluorescent vesicles was monitored by video-enhanced differential interference-contrast microscopy. The transport of fluorescent, exogenous vesicles was shown to be bidirectional and ATP-dependent and occurred at a mean rate of 6.98 +/- 4.11 micron/s (mean +/- standard deviation, n = 41). In comparison, the mean rate of transport of nonfluorescent, endogenous vesicles in control axoplasm treated with vesicle buffer alone was 4.76 +/- 1.60 micron/s (n = 64). These rates are slightly higher than the mean rate of endogenous vesicle movement in extruded axoplasm (3.56 +/- 1.05 micron/s, n = 40) not subject to vesicles or vesicle buffer. Not all vesicles and organelles, exogenous or endogenous, were observed to move. In experiments in which proteins of the surface of the fluorescent vesicles were digested with trypsin before injection, no movement of the fluorescent vesicles was observed, although the transport of endogenous vesicles and organelles appeared to proceed normally. The results summarized above indicate that isolated vesicles, incorporated into axoplasm, move with the characteristics of fast axonal transport. Because the vesicles are fluorescent, they can be readily distinguished from nonfluorescent, endogenous vesicles. Moreover, this system permits vesicle characteristics to be experimentally manipulated, and therefore may prove valuable for the elucidation of the mechanisms of fast axonal transport.     

Calcium content and net fluxes in squid giant axons

Axons freshly dissected from living specimens of the tropical squid Dorytheutis plei have a calcium content of 68 mumol/kg of axoplasm. Fibers stimulated at 100 impulses/s in 100 mM Ca seawater increase their Ca content by 150 mumol/kg.min; axons placed in 3 Ca (choline) seawater increase their Ca content by 12 mumol/kg.min. Axons loaded with 0.2--1.5 mmol Ca/kg of axoplasm extruded Ca with a half time of 15--30 min when allowed to recover in 3 Ca (Na) seawater. The half time for recovery of loaded axons poisoned with carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and iodoacetic acid (IAA) is about the same as control axons. Axons placed in 40 mM Na choline seawater (to reduce chemical gradient for Na) or in 40 mM Na, 410 mM K seawater to reduce the electrochemical gradient for Na to near zero either fail to lose previously loaded Ca or gain further Ca.     

Intracellular calcium buffering capacity in isolated squid axons

Changes in ionized calcium were studied in axons isolated from living squid by measuring absorbance of the Ca binding dye Arsenazo III using multiwavelength differential absorption spectroscopy. Absorption changes measured in situ were calibrated in vitro with media of ionic composition similar to axoplasm containing CaEGTA buffers. Calcium loads of 50-2,500 μmol/kg axoplasm were induced by microinjection, by stimulation in 112 mM Ca seawater, or by soaking in choline saline with 1-10 mM Ca. Over this range of calcium loading of intact axoplasm, the ionized calcium in the axoplasm rose about 0.6 nM/μM load. Similar loading in axons preteated with carbonyl cyanide 4- trifluoromethoxyphenylhydrazone (FCCP) to inhibit the mitochondrial proton gradient increased ionized calcium by 5-7 percent of the imposed load, i.e. 93-95 percent of the calcium load was buffered by a process insensitive to FCCP. This FCCP- insensitive buffer system was not saturated by the largest calcium loads imposed, indicating a capacity of at least several millimolar. Treatment of previously loaded axons with FCCP or apyrase plus cyanide produced rises in ionized calcium which could be correlated with the extent of the load. Analysis of results indicated that, whereas only 6 percent of the endogenous calcium in fresh axons is stored in the FCCP-sensitive (presumably mitochondrial) buffer system, about 30 percent of an imposed exogenous load in the range of 50-2,500 μM is taken up by this system.     

Dynamic instability and motile events of native microtubules from squid axoplasm

Native microtubules from extruded axoplasm of squid giant axons were used as a paradigm to characterize the motion of organelles along free microtubules and to study the dynamics of microtubule length changes. The motion of large round organelles was visualized by AVEC-DIC microscopy and analyzed at a temporal resolution of 10 frames per second. The movements were smooth and showed no major changes in velocity or direction. During translocation, the organelles paused very rarely. Superimposed on the rather constant mean velocity was a velocity fluctuation, which indicated that the organelles are subject to considerable thermal motion during translocation. Evidence for a regular low-frequency oscillation was not found. The thermal motion was anisotropic such that axial motion was less restricted than lateral motion. We conclude that the crossbridge connecting the moving organelle to the microtubule has a flexible region that behaves like a hinge, which permits preferential movement in the direction parallel to the microtubule. The dynamic changes in length of native microtubules were studied at a temporal resolution of 1 Hz. About 98% of the native microtubules maintained their length ("stable" microtubules), while 2% showed phases of growing and/or shrinking typical for dynamic instability ("dynamic" microtubules). Gliding and organelle motion were not influenced by dynamic length changes. Transitions between growing and shrinking phases were low-frequency events (1-10 minutes per cycle). However, a new type of microtubule length fluctuation, which occurred at a high frequency (a few seconds per cycle), was detected. The length changes were in the 1-3 micron range. The latter events were very prominent at the (+) ends. It appears that the native axonal microtubules are much more stable than the purified microtubules and the microtubules of cultured cells that have been studied thus far. Potential mechanisms accounting for the three states of microtubule stability are discussed. These studies show that the native microtubules from squid giant axons are a very useful paradigm for studying microtubule-related motility events and microtubule dynamics.     

K+-selective microelectrode study of internally dialyzed squid giant axons.

Intracellular potassium activity, (aK)i, and axoplasmic K+ concentration, [K+]i, were measured by means of K+-selective microelectrodes and atomic absorption spectroscopy, respectively, in squid giant axons dialyzed with K+-free dialysis solution and bathed in K+-free artificial sea water. (aK)i measurements indicated that axoplasmic free K+ could be depleted by dialysis, whereas [K+]i measurements on axoplasm extruded from these axons suggest substantial retention of K+ (15.5 +/- 1.7 mmol/kg axoplasm K+; n = 9). In comparison, [K+]i in axoplasm extruded from freshly dissected axons was 330 +/- 16 mmol/kg axoplasm (n = 6). These data suggest that approximately 5% of the axoplasmic K+ ions are not easily removed by dialysis and that these ions are either bound to macromolecular sites or sequestered into membrane-enclosed organelles.     

Internally perfused Myxicola giant axons showing long-term survival.

An improved method for internally perfusing the Myxicola giant axon based on removing the axoplasm by dispersing it in KCl-KF salt solutions is described. Proteolytic enzymes are not introduced. With this improved method perfused preparations show long-term stability of their electrical properties and the ability to generate action potentials for many hours. Mean initial values for resting membrane potential, action potential amplitude, and peak inward current were -68 mV, 118 mV, and 3.62 mA/cm2, respectively. Mean resting membrane resistance was 75% of that in intact axons. In one series of voltage clamp experiments, perfused preparations remained excitable for a mean period of 5 1/2 h, but this period could exceed 10 h. 4 min are needed for exchange of internal solutions. At least 50 mM KF is required both in the axoplasm liquefying solution and in the standard perfusate to obtain stable preparations.     

Magnesium influx in dialyzed squid axons

Summary The influx of magnesium from seawater into squid giant axons has been measured under conditions where internal solute control in the axon was maintained by dialysis. Mg influx is smallest (1 pmol/cm² sec) when both Na and ATP have been removed from the axoplasm by dialysis. The addition of 3mm ATP to the dialysis fluid gives a Mg influx of 2.5 pmol/cm² sec while the addition of [Na] _i and [ATP] _i gives 3 pmol/cm² sec as a value for Mg influx; this corresponds well with fluxes measured in intact squid giant axons.The Mg content of squid axons is 6 mmol/kg axoplasm; this is unaffected by soaking axons in Li or Na seawater for periods of up to 100 min.