Molecular weight distribution of proteins synthesized in single, identified neurons of Aplysia

A first partial Aplysia californica proteome

Aplysia proteins have not been studied systematically and it was therefore the aim of the study to carry out protein profiling in ganglia from Aplysia californica (AC). AC ganglia were extirpated, proteins extracted and run on 2DE with subsequent in-gel digestion, followed by identification of proteins by nano-LC-ESI-MS/MS on an ion trap. Proteins were identified based upon a public Aplysia EST database. Out of 408 picked spots, 276 spots were identified corresponding to 172 ESTs and 118 individual proteins. The range of sequence coverage was between 14 and 80% and the average amount of peptides used for the identification of proteins was 9 (from 3 to 24). Mean score for protein identification was 516. Comparison of protein levels between cerebral, pleural, pedal and abdominal ganglia revealed a series of significant differences including: signaling, metabolism, cytoskeleton and structural, redox, chaperone, replication/transcription and electron/proton transport proteins. The generation of a protein map complements transcriptional studies carried out in AC ganglia. The findings provide the basis for investigation into post-translational modifications, splice variants and assist in the generation of antibodies against AC proteins. Moreover, differences in protein expression between ganglia may be valuable for the design of future studies in neurobiology of AC.

Chemical cytometry: fluorescence-based single-cell analysis

Cytometry deals with the analysis of the composition of single cells. Flow and image cytometry employ antibody-based stains to characterize a handful of components in single cells. Chemical cytometry, in contrast, employs a suite of powerful analytical tools to characterize a large number of components. Tools have been developed to characterize nucleic acids, proteins, and metabolites in single cells. Whereas nucleic acid analysis employs powerful polymerase chain reaction-based amplification techniques, protein and metabolite analysis tends to employ capillary electrophoresis separation and ultrasensitive laser-induced fluorescence detection. It is now possible to detect yoctomole amounts of many analytes in single cells.

Calcium-binding proteins in Aplysia neurons

1. Calcium (Ca)-binding proteins of neuronal ganglia and of single, identified neurons of the marine mollusk, Aplysia californica, were investigated. Using transblot/45Ca overlays two proteins, at Mr 45,000 and Mr 23,000, with a high Ca-binding ability were found. 2. Western blot analysis revealed that the protein at Mr 45,000 could be separated by 2D-PAGE into proteins with Mr 40,000 and Mr 43,000. The protein at Mr 40,000 immunocross-reacted with antisera directed against parvalbumin and rat calbindin D-28K, indicating a novel Ca-binding protein sharing common antigenic determinants for both proteins. 3. The protein at Mr 23,000 could be separated into a group of proteins with Mr 13,000-20,000 which showed a high degree of similarity to sarcoplasmatic calcium-binding proteins (SCP). 4. We further investigated the protein pattern of single, identified neurons of different electrical activity (bursting, beating, and silent) by 2D-PAGE. Major differences were found in the range of low Mr and low pI, where Ca-binding proteins are generally located. A protein at high concentrations characteristic for silent cells migrated at a position similar to crayfish SCP. 5. The results show that various Ca-binding proteins are characteristic for neurons in the Aplysia nervous system and support the idea that they may effect the electrical behavior of nerve cells.

Influence of a stationary magnetic field on bioelectric properties of snail neurons

Identified cells of Helix lucorum L. received 20 min exposures to 23, 120, or 200 mT stationary magnetic field (MFs). Resting potentials and input resistances were measured. Controls were instituted for temperature changes and for mechanical and other sources of artifact. Resting potentials did not change with MF exposure. Input resistances decreased significantly in normally silent cells during MF exposure, but increased significantly in spontaneously active cells. The magnitudes of changes were monotonically related to strength of the MF. Changes in excitatory postsynaptic potentials were observed during MF exposure. Elimination of perineuronal glia by proteolytic enzymes eliminated the MF effects.

Aplysia californica neurons R3-R14: primary structure of the myoactive histidine-rich basic peptide and peptide I

The R3-R14 neurons of the marine mollusc Aplysia are neuroendocrine cells that express a gene encoding peptides I, II and histidine-rich basic peptide (HRBP), a myoactive peptide that excites Aplysia heart and enhances gut motility in vitro. Peptide II has been chemically characterized (35), but the complete primary structures of peptide I and HRBP have not been established by amino acid sequence analysis. HRBP, peptide I, and the prohormone (proHRBP) were therefore purified from acid extracts of Aplysia californica neural tissue using sequential gel filtration and reverse-phase high-performance liquid chromatography and chemically characterized. Amino acid sequence analysis demonstrated that HRBP was a 43-residue peptide whose sequence was: less than Glu-Val-Ala-Gln-Met-His-Val-Trp-Arg-Ala-Val-Asn-His-Asp-Arg-Asn-His-Gly- Thr-Gly - Ser-Gly-Arg-His-Gly-Arg-Phe-Leu-Ile-Arg-Asn-Arg-Tyr-Arg-Tyr-Gly-Gly-Gly- His-Leu - Ser-Asp-Ala-COOH. Compositional and sequence analyses of peptide I and proHRBP demonstrated that peptide I was a 26-residue peptide with the following sequence: NH2-Glu-Glu-Val-Phe-Asp-Asp-Thr-Asp-Val-Gly-Asp-Glu-Leu-Thr-Asn-Ala- Leu-Glu-Ser-Val-Leu-Thr-Asp-Phe-Lys-Asp-COOH. These results demonstrated that the pro-HRBP sequence predicted by nucleotide sequence analysis of a cDNA clone (24) was in fact synthesized in R3-R14 neurons. Hydrophilicity and hydrophobicity profiles of preproHRBP, combined with charge distribution profiles and predictive secondary structural analysis, showed that cleavage at dibasic sequences was strongly associated with peaks of hydrophilicity in alpha-helical regions of the preprohormone.

Purification and sequencing of neuropeptides contained in neuron R15 of Aplysia californica

R15 is a large identified neuron present in the abdominal ganglion of the mollusc Aplysia. Previous studies have indicated that this neuron may play a role in water balance and possibly renovascular functions. A peptidic factor contained in the neuron R15 has been shown to increase the water content of Aplysia. To determine the structure of the peptides contained in R15, we purified the extracts of 820 R15 cells by means of two steps of reverse-phase HPLC. The purification yielded a number of peptides, only one of which, R15 alpha 1, resulted in water uptake when injected into animals. Determination of the amino acid content and sequence analysis of the R15 alpha 1 peptide demonstrated that this peptide contains 38 residues, including two cysteines. The peptide failed to react with iodoacetate, indicating that the two cysteines are connected by a disulfide bridge. To confirm the assigned structure, the peptide was synthesized with a disulfide bridge. The chromatographic properties and bioactivity of the synthetic material were identical to those of the native peptide. Several other R15 peptides were inactive in the bioassay for water uptake. The sequence of one of these peptides (R15 beta) was determined, and it was established that the peptide contains 28 residues. Amino acid analysis of three other peaks was performed. One of these peaks contained a peptide (R15 beta f) whose amino acid composition suggests that it is a fragment of the R15 beta peptide. The other two peaks contained peptides with identical amino acid compositions, suggesting that they are variants of a single peptide (R15 gamma). The amino acid sequences of all the peptides identified in neuron R15 correspond to stretches of a polyprotein encoded by a recently sequenced R15 cDNA.

Localization of Aplysia neurosecretory peptides to multiple populations of dense core vesicles

Many neurons in the mollusc Aplysia are identifiable and provide a useful model system for investigating the cellular mechanisms used by the neuroendocrine system to mediate simple behaviors. In this study we determined the subcellular localization of eight Aplysia neuropeptides using immunogold labeling techniques, and analyzed the size distribution of dense core and granular vesicles in peptidergic neurons. Recent observations demonstrate that many neurons use multiple chemical messengers. Thus, an understanding of the functional significance of cotransmitters requires an analysis of their relative subcellular distributions. The peptides are expressed in a subset of neurons, or the exocrine atrial gland, and are primarily localized to dense core vesicles. Multiple regions of precursors which are cleaved into several components are co-localized. Each neuron has a distinct size distribution of peptide-containing dense core vesicles ranging in size from 65 to 600 nm. The atrial gland contains very large (up to 2 micron) peptide-containing granules. Single neurons have multiple populations of granules whose quantal sizes agree with predictions based on physical constraints. Some cells contain very large peptide-containing granules which are found in the cell soma and not in processes. Thus, the genetic determination of neuronal cell type includes not only transmitter choices but also multiple modes of packaging the intercellular messengers.

Identification and primary structural analysis of peptide II, an end-product of precursor processing in cells R3-R14 of Aplysia

Peptide II, which is encoded on a gene for a precursor protein in abdominal ganglion neurons R3-R14, was purified from extracts of abdominal ganglia of Aplysia californica. Native peptide II comigrates with synthetic standards on HPLC under isocratic conditions. Amino acid sequence and composition analyses indicate that the sequence of peptide II is Glu-Ala-Glu-Glu-Pro-Ser-Phe-Met-Thr-Arg-Leu, as predicted from the precursor. The molluscan cardioexcitatory peptide Phe-Met-Arg-Phe-amide was also identified in abdominal ganglion extracts by similar means. The large amount of peptide II recovered (100 ng/ganglion), and its location on the precursor between two pairs of basic residues, strongly suggest that the precursor is processed into peptide II and at least two other peptides. Although cells R3-R11 have been postulated to play a role in cardiovascular control, peptide II was without effect at less than or equal to 10(-4) M concentrations on identified abdominal ganglion neurons, the gastroesophageal artery or the heart. The physiological role of peptide II therefore remains to be elucidated.

Peripheral axons of the parabolic burster neuron R15

In a combined electrophysiological and anatomical study, the parabolic burster neuron R15 was found to project axons through the genito-pericardial nerve onto the pericardial wall and digestive gland sheath and, more variably, into the heart and pericardial coelom. Projection into these tissues is consistent with the hypothesis that R15 is neurosecretory and may play a role in circulation and/or ion-water regulation in Aplysia.

Low molecular weight proteins of Aplysia neurosecretory cells

The proteins of identified cells from the Aplysia californica central nervous system were labeled with radioactive amino acids and fractionated on SDS acrylamide gels containing 6 M urea. Most of the large cells contain prominent, cell-specific protein products in the molecular weight range between 3 and 30 KD. The molecular weights of the largest specific prevalent protein products are in good agreement with the predicted molecular weights of precursors as determined from an analysis of cDNA clones homologous to mRNA's specifically expressed in several of these neurons. Biologically active peptides have been found in many of these cells. These data, and other indirect evidence suggests that the synthesis of a large amount of a particular protein in this molecular weight range is indicative of the synthesis of a neurosecretory product. We conclude that most, if not all, large neurons in the Aplysia central nervous system are peptidergic.

Neuropeptides: mediators of behavior in Aplysia

The Aplysia neuroendocrine system is a particularly advantageous model for cellular and molecular studies because of the relatively small number and large size of its component neurons. Recombinant DNA techniques have been used to isolate the genes that encode the precursors of peptides expressed in identified neurons of known function. The organization and developmental expression of these genes have been examined in detail. Several of the genes encode precursors of multiple biologically active peptides that are expressed in cells which also contain classical transmitters. These studies, as well as immunohistochemical studies and the use of intracellular recording and voltage clamp techniques are the first steps toward revealing the mechanisms by which neuropeptides govern simple behaviors.

A cDNA clone encoding neuropeptides isolated from Aplysia neuron L11

Single nerve cells can use more than one substance as extracellular chemical messengers. Classical transmitters have been shown to coexist in the same neuron and possibly even in the same vesicle as neuroactive peptides. Furthermore, multiple neuroactive peptides, which are thought to be coreleased, are often encoded in the same precursor assuring stoichiometric synthesis. The precise organization of multiple message systems and the physiological significance of the coexistence is poorly understood. The abdominal ganglion of the gastropod mollusc Aplysia contains a number of identified neurons that are cotransmitter candidates. One such cell, L11, is cholinergic and probably also uses biologically active peptides. Differential screening with labeled cDNA was used to isolate cDNA clones expressed specifically in the bag cells and abdominal ganglion neurons L11 or R15. Analysis of an L11-specific clone suggests that it encodes a 14.7-kDa protein that is the precursor for the secreted peptides. The poly(A)+ RNA transcript is approximately equal to 1.2 kilobases and there are 1-3 copies of this gene in the Aplysia haploid genome.

The pharmacology of molluscan neurons

It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)

Gene isolation with cDNA probes from identified Aplysia neurons: neuropeptide modulators of cardiovascular physiology

The Aplysia abdominal ganglion neurons, R3-R14, modulate cardiovascular activity. In vitro translations of poly(A)+ RNA from these cells suggest that they contain a prevalent mRNA encoding a 14 kd protein. Utilizing differential screening techniques with 32P-labeled cDNA synthesized from the poly(A)+ RNA of identified neurons, we isolated the corresponding gene. The Aplysia haploid genome contains a single copy of this sequence, which is interrupted by two large introns and spans approximately 7 kb of genomic DNA. The R3-R14 neurons specifically express this gene, resulting in the synthesis of a 1.25 kb mRNA not found in other abdominal ganglion cells or in the head ganglia. The gene was shown to encode a 13.5 kd precursor, which is proteolytically cleaved into at least three peptides with molecular weights of 5.0, 3.3, and 1.3 kd. These peptides and glycine are thought to act as chemical messengers in the central nervous system and peripherally.

Distribution of soluble proteins within spinal motoneurons: a quantitative two-dimensional electrophoretic analysis

Soluble protein fractions obtained from bovine lumbar spinal motoneuron cell bodies, ventral gray matter, and ventral and dorsal roots were analyzed by two-dimensional gel electrophoresis. Each extract was separated into Coomassie blue-stained patterns of up to 350 polypeptides ranging in isoelectric point from pH 4 to 8 and in molecular weight from 10,000 to 200,000. Visual inspection of the protein pattern of the isolated cell bodies showed it to be substantially different from those of ventral gray matter and the spinal roots, while the patterns obtained from ventral and dorsal roots were indistinguishable. Computer-assisted densitometry of the major soluble proteins from spinal roots showed no quantitative difference between the predominant proteins in ventral and dorsal root extracts. Differences of 10-fold or more were common when the major proteins of the isolated perikarya were compared with those of the other fractions. Since most of the soluble proteins extracted from ventral and dorsal roots were probably derived from the axoplasm of motor and sensory nerves, respectively, these results are interpreted to mean that large differences exist in the distribution of individual soluble proteins between the cell body and axon of spinal motoneurons, while the major soluble proteins of spinal motor and sensory axons are highly similar.

Changes in hormone activity of single neurosecretory cell bodies during a physiological secretion cycle

Single neurosecretory cell bodies were dissected from the ventral ganglionic mass of Rhodnius prolixus, lysed in distilled water, and bioassayed for diuretic hormone (DH) activity on isolated malpighian tubules. DH was found in a least 10 somata within the ganglion; electron micrographs of isolated cells show a large population of elementary neurosecretory granules. Quantitative measures of hormone activity were made by bioassaying somata from unfed 5th instar larvae, and at the following times after feeding; 1 h, 4 h (near the end of DH-mediated diuresis), 1 day, 5 days, 10 days, 17 days and 21 days (just after the moult to adult). DH activity in cell bodies drops significantly within 1 h after feeding, and remains low long after hormone secretion ceases. Restocking of the soma with active hormone occurs during the period 10-21 days after feeding.

Biosynthesis and processing of presumed neurosecretory proteins in single identified neurons of Aplysia californica

The biosynthesis and processing of low molecular weight protein (presumed neurosecretory protein) in cells R15, R14 and L11 of Aplysia californica was studied at high resolution by polyacrylamide slab gel electrophoresis in sodium dodecylsulfate. The number of low molecular weight proteins detected in each cell ranges from 3 in R14 and L11 to 5 to 6 in R15. In each of the cells studied, the low molecular weight protein consists of a primary precursor of ca. 12,000 daltons, and its proteolytic processing products. In each cell, the smallest protein, or in the case of R14, one of the two smallest proteins, accumulates to a significant extent, suggesting that it might correspond to a final processed neurohormone. In cell R15, the biosynthesis of the primary precursor and its subsequent processing to smaller peptides is largely unaffected by removal of extracellular calcium, by replacement of calcium with cobalt or by inhibition of spontaneous bursting via stimulation of the brachial nerve.

Specific, water-soluble polypeptides in identified neurons of Aplysia californica

Application of an ethylene glycol lysis technique to extract water-soluble, low molecular weight polypeptides in Aplysia neurons, was used in conjunction with microgradient gel electrophoresis and micro-isoelectric focusing, to identify unique polypeptides in specific, identified neurons. The polypeptides found in neurons R15, R3-13, R14, and the bag cells were particularly abundant, consistent with the previously suggested neurosecretory role for these cells. Water extraction of the strongly basic polypeptides (pI 10.7) in R3-13 and R14 required an acidic lysis medium.

Further identification of neurons in the abdominal ganglion of Aplysia using behavioral criteria

This review is an updating of the paper by Frazier et at. 13 in which 30 individual cells and 8 cell clusters were identified in the abdominal ganglion of of Aplysia californica on the basis of morphological, physiological and pharmacological criteria. Since that time, a number of neurobiological studies have utilized the neurons of the abdominal ganglion for behavioral studies. Based on these new investigations, 32 additional cells and 2 additional cell clusters have been identified, bringing the total of identified cells in the ganglion to 62 individual cells and 10 cell clusters. Additional features of the previously identified cells have also emerged. Much of the new information concerns the role of identified cells in behavior. We have summarized the current list of identified cells in the abdominal ganglion emphasizing these behavioral features. We also review the known synaptic connections made by identified interneurons in the ganglion. Included are descriptions of 6 new interneurons that connect to other cells in the ganglion. A major conclusion from this survey is that behavioral criteria permit resolution between cells in identified cell clusters that could previously not be distinguished using other criteria.

A comparison of the 12,000 dalton proteins synthesized by Aplysia neurons L11 and R15

The 12,000 dalton proteins of neurons L11 and R15 of the Aplysia abdominal ganglion were labeled by incubation of the ganglion in [3H]leucine and compared in terms of their subcellular localization, solubility in various media, and molecular charge. Both proteins are cytoplasmic constituents. Their solubility behavior is identical: both are insoluble in aqueous media of low and high ionic strength as well as chloroform-methanol, and both are solubilized by Triton X-100 + urea and by LIS. They are essentially identical in molecular weight as determined by SDS gel electrophoresis but differ by a single charge per molecule at low pH. The broad similarity between these proteins suggests that they could serve similar functions, while the observed charge difference might be important in terms of previously discovered differences in their processing.

Alteration of protein metabolism in individual, identified neurons from Aplysia

A search for control mechanisms governing protein metabolism in neurons from Aplysia californica has uncovered two examples of altered patterns of newly synthesized proteins: (1) The pattern of newly synthesized proteins in the R2 neuron is altered when protein synthesis occurs at elevated temperatures (22-30 degrees C as compared with 13-15 degrees C). (2) The processing of newly synthesized 12,000 dalton (12k) material to 6-9,000 dalton (6-9k) size in the R15 neuron (Strumwasser, F. and Wilson, D.F. [1976], J. Gen. Physiol., in press) can be blocked by certain ion replacements. If acetate replaces chloride in the incubation medium during the synthesis of 12k material, an early step in the processing, prior to the actual breakdown of 12k material, is blocked. Experiments with RNA-synthesis inhibitors indicate that none of the mRNAs which code for abundantly synthesized protein species in the R2 or R15 neurons have short (less than 4 hr) half-lives. This result has implications for an earlier report of regulation of protein synthesis in the R15 neuron.

Patterns of proteins synthesized in the R15 neuron of Aplysia. Temporal studies and evidence for processing

The time-course of changes in the pattern of newly synthesized proteins in the R15 neuron of the parietovisceral ganglion of Aplysia californica has been studied at 14 degrees C. 5% polyacrylamide gels containing sodium dodecyl sulfate (SDS) have been used to separate newly synthesized (leucine-labeled) proteins from the neuron. We have demonstrated that the pattern of newly synthesized proteins from the R15 neuron does not change significantly if 5-h pulses of labeled leucine are given during the first 72 h of in vitro incubation of the excised ganglion. However, the level of leucine incorporation begins to decline somewhere between 17 and 43 h after the ganglion is isolated; at 43 and 69 h the levels of incorporation fell to 29 and 10% of the initial level, respectively. A number of conclusions have been drawn from the use of a sequential, double-label type of experiment in the same cell. There is processing of SDS-soluble, 12,000-dalton (12k) material to 6,000-9,000-dalton (6-9k) material. These materials are the two major peaks on gels after long labeling periods and together account for about 35% of all newly synthesized proteins. After synthesis of 12k material, there is a gradual disappearance of 12k (half-life about 8 h) and simultaneous appearance of 6-9k material on the gels, as the postsynthesis "chase" period of ganglia incubation is increased. The processing of 12k to 6-9k material occurs even in the presence of anisomycin, a protein syntehsis inhibitor, during the chase period. While the rate of 12k to 6-9k conversion can vary from cell to cell, it appears to remain consistent within, and is characteristic of, any individual R15. We detect no circadian rhythm in either the rate of 12k synthesis or the rate of 12k to 6-9k processing with 5-h label periods. These results are discussed in relation to the roles of 12k and 6-9k material in the R15 neuron.

Low molecular weight specific proteins in identified molluscan neurons. I. Synthesis and storage

Low molecular weight specific proteins in phenotypically distinct, identified neurons of Aplysia californica, have been detected using a high resolution acid-urea polyacrylamide gel system, and the molecular weight of these proteins labeled by in vitro incubation in [3H]leucine were determined by two different methods. The relative mobilities of these proteins on the acid-urea gel differed significantly, even though they appeared to co-electrophorese on SDS gelss. These identified neurons, and the specific proteins that they synthesize and store represent excellent model systems for the study of specific protein regulation in neurons.

Functional correlates of low molecular weight peptide synthesis in Aplysia neurons

An attempt was made to determine whether the synthesis of low molecular weight proteins by certain neurons of Aplysia could be correlated with either pacemaker activity or neurosecretion. Electrophysiological tests indicated that the spontaneous generation of action potentials in cells L8 and L9 is due to an endogenous pacemaker mechanism. Gel electrophoresis of proteins synthesized by these neurons showed no evidence of low molecular weight material. The non-neurosecretory interneuron L10 synthesizes a 12,000 dalton protein, whereas the silent neurosecretory cell L5 synthesizes a lower molecular weight peptide. Taken with previous findings, these results indicate that there is no correlation between pacemaker activity and the synthesis of these peptides. There is, however, a strict correlation between neurosecretory activity and the synthesis of proteins of molecular weights lower than 12,000 daltons.

Sodium dodecyl sulfate in protein chemistry

This review summarizes in a brief manner the main aspects of the application of sodium dodecyl sulfate (SDS) to protein chemistry. The principal problems of SDS-polyacrylamide gel electrophoresis are described, as well as the anomalous behavior of protein-SDS complexes and the inactivation of enzymes due to variable binding of SDS to the polypeptides studied. The particular value of SDS in elucidating the protein composition of biological membranes and in membrane-reconstitution experiments is discussed.

Studies on bursting pacemaker potential activity in molluscan neurons. I. Membrane properties and ionic contributions

Bursting pacemaker potential (BPP) activity of identified molluscan neurons has been studied using cells from Aplysia and Otala. The results presented in this paper indicate that (1) a potassium conductance mediates the hyperpolarizing phase of the BPP; (2) the BPP amplitude is directly dependent on [Na+]0; (3) BPP activity requires the presence of divalent cations and is prevented by C02+ and La3+, but not D-600; (4) the apparent increase in membrane resistance during the depolarizing phase of the Bd can be accounted for by the movement of the membrane potential along the non-linear portion of the I-V curve; and (5) non-linear I-V relations and a minimal effective membrane resistance are pre-requisite to BPP generation. Coupled with recent observations on the presence of an inward current in these cells, the results suggest that the mechanisms underlying the BPP are similar to those proposed to describe the myocardial pacemaker potential: the hyperpolarizing phase is due to activation of a potassium conductance which slowly inactivates, resulting in a gradula deplorization until a voltage-dependent inward current is activated which then leads to an increasingly rapid deplorization and initiation of the burst of spikes. It would appear that Na+ may play the major role in carrying the inward current, although a secondary role for divalent cations cannot be discounted.

Synthesis of glycoproteins in a single identified neuron of Aplysia californica

Incorporation of L-[(3)H]fucose into glycoproteins was studied in R2, the giant neuron in the abdominal ganglion of Aplysia. [(3)H]fucose injected directly into the cell body of R2 was readily incorporated into glycoproteins which, as shown by autoradiography, were confined almost entirely to the injected neuron. Within 4 h after injection, 67% of the radioactivity in R2 had been incorporated into glycoproteins; at least 95% of these could be sedimented by centrifugation at 105,000 g, suggesting that they are associated with membranes. Extraction of the particulate fraction with sodium dodecyl sulfate (SDS), followed by gel filtration on Sephadex G-200 and polyacrylamide gel electrophoresis in SDS revealed the presence of only five major radioactive glycoprotein components which ranged in apparent molecular weight from 100,000 to 200,000 daltons. Similar results were obtained after intrasomatic injection of [(3)H]N-acetylgalactosamine. Mild acid hydrolysis of particulate fractions released all of the radioactivity in the form of fucose. When ganglia were incubated in the presence of [(3)H]fucose, radioactivity was preferentially incorporated into glial cells and connective tissue. In contrast to the relatively simple electrophoretic patterns obtained from cells injected with [(3)H]fucose, gel profiles of particulate fractions labeled with [(14)C]valine were much more complex.

Axonal transport of newly synthesized glycoproteins in a single identified neuron of Aplysia californica

Increasing amounts of glycoprotein synthesized from L-[(3)H]fucose injected into the cell body of R2, an identified Aplysia neuron, were found in the right pleuro-abdominal connective. Autoradiography revealed that the glycoproteins were localized in the axon of R2. Glycoproteins appearing in the axon presumably were synthesized in the cell body, since no significant incorporation was observed when [(3)H]fucose was injected directly into the axon. [(3)H]glycoproteins were detected in the connective after a delay of 1 h after intrasomatic injection. Thereafter, transport from the cell body was rapid, and by 10 h after injection, 45% of the total neuronal [(3)H]glycoprotein had appeared in the axon. By analysing the radioactivity in cell body and connective 4, 10, and 15 h after injection, we found that [(3)H]glycoproteins were transported selectively compared to nonmacromolecular material. Sequential sectioning of the connective revealed that [(3)H]glycoproteins were transported in discrete waves. The population of membrane-associated [(3)H]glycoproteins in the axon differed from that in the cell body. Two of the five somatic components appeared to be transported preferentially. In addition a new component appeared in the axon 10 h after injection.