Fast axonal transport of tyrosine sulfate-containing proteins: preferential routing of sulfoproteins toward nerve terminals

Nonsulfated cholecystokinins in cerebral neurons

Cholecystokinin (CCK) is a widely expressed neuropeptide system originally discovered in the gut. Both cerebral and peripheral neurons as well as endocrine I-cells in the small intestine process proCCK to tyrosyl-O-sulfated and α-carboxyamidated peptides. Recently, we reported that gut endocrine I-cells also synthetize nonsulfated CCK in significant amounts. Accordingly, we have now examined whether porcine and rat cerebral tissues (four cortical regions, hypothalamus and cerebellum) also synthesize nonsulfated CCK. A new, specific radioimmunoassay showed that all brain samples from pigs (n=15) and rats (n=6) contained nonsulfated CCK. The highest concentrations were measured in the neocortex; 4.7±0.25pmol/g (7.4%) in the rat and 4.3±1.88pmol/g (2.3%) in the pig. Chromatography of porcine cortical extracts revealed that 96.4% of the CCK was O-sulfated CCK-8. A higher fraction of the larger peptides (CCK-58 and CCK-33) was nonsulfated in comparison with the shorter forms (CCK-22 and CCK-8), i.e., 8.1% and 4.3% versus 0.9% and 1.5%. Immunohistochemical analysis of the rat brain showed an overall similar distribution pattern in selected regions when comparing the antibody specific for nonsulfated CCK-8 with an antibody recognizing both sulfated and nonsulfated CCK. However, nonsulfated CCK immunoreactivity was stronger than that of sulfated CCK in cell bodies and weaker in nerve terminals. We conclude that only a small fraction of neuronal CCK is nonsulfated. The intracellular distribution of nonsulfated CCK in neurons suggests that they contribute only modestly to the CCK transmitter activity.

Reversal of rapidly transported protein and organelles at an axonal lesion

The time required for both rapid axonally transported organelles (vesicles and tubulo-vesicular structures) and proteins to undergo anterograde to retrograde reversal at a crush site was examined using sciatic nerve preparations obtained from Xenopus laevis. The transport and reversal of a pulse of newly synthesized 35S-labeled proteins was studied with a position-sensitive detector of ionizing radiation. Organelle transport and reversal were studied using video microscopy. Both protein and organelle reversal were assessed in two bathing media: a physiological saline and a medium that was compatible with the intracellular environment (internal medium). The time required for protein transport to reverse at a ligature was determined as a function of the time interval between the application of the ligature and the arrival of the pulse at the ligature (lesion time). In physiological saline, reversal times were greatest, about 3.5 h, when the lesion time was 1 h or less and decreased to approximately 1.5 h for lesion times of 4-12 h. When corrected for the approximately 2 mm length of degeneration caused by the saline, the results were similar to those obtained in internal medium and indicated a minimal reversal time for proteins of about 2 h. Organelle transport was examined close to narrow lesions in single myelinated axons. That the organelles moving away from the lesion represented organelles that had undergone reversed transport was suggested by observation of the reversal of individual organelles, and by a correlation between the flux of organelles towards and away from the lesion. Analysis of organelle flux within and adjacent to a segment of axon isolated by two lesions indicated that 70-80% of organelles moving away from a lesion represented reversed transport. Observations in internal medium were consistent with a reversal time of < 15 min, and in physiological saline < 30 min. The substantially smaller reversal time for organelle transport as compared to protein transport is consistent either with the existence of two types of organelles with different reversal times and hence different reversal mechanisms, or with the possibility that during reversal proteins are off-loaded from carrier organelles and subsequently up-loaded to different organelles.

Sulfation of proteins in the primate cerebellum and young rat brain

Protein tyrosine sulfotransferase activity in a 20,000 g sedimentable fraction of monkey cerebellum was demonstrated. Both endogenous proteins and the exogenous substrate poly (Glu, Ala, Tyr) random copolymer were sulfated. The copolymer in the low molecular mass range (approx 20 kDa) was preferentially sulfated. Addition of copolymer inhibited sulfation of endogenous proteins. Mg2+ and Mn2+ promoted sulfation. 35S-Labeled proteins from monkey cerebellum and young (10 days old) rat brain were subjected to lectin-Sepharose chromatography to identify the presence of sulfated glyco-proteins. Labeled proteins from both these sources could bind and get eluted from Concanavalin A-Sepharose and Ricinus Communis agglutinin-Sepharose column suggesting the presence of mannose or galactose containing glycosulfoproteins.

The physical biochemistry and molecular genetics of sulfate activation

This article is an overview of current research in the area of sulfate activation. Emphasis is placed on presenting unresolved issues in an appropriate context for critical evaluation by the reader. The energetics of sulfate activation is reevaluated in light of recent findings that demonstrate that the synthesis of activated sulfate is thermodynamically driven by GTP hydrolysis. The structural and functional bases of this GTPase activation are discussed in detail. The bonding and hydrolysis of the high-energy, phosphoric-sulfuric acid anhydride bond of activated sulfate are presented along with an analysis of the importance of the divalent cation and pyrophosphate protonation in the equilibria governing activated sulfate formation. The molecular genetics of sulfate assimilation in prokaryotes is reviewed with an emphasis on the regulation of the pathway. Recent discoveries connecting sulfate activation to plant/microbe symbiogenesis are presented, as are several examples of the importance of activated sulfate in human metabolism and disease.

Relationships between the rapid axonal transport of newly synthesized proteins and membranous organelles

Rapid axonal transport is generally viewed as being exactly analogous to the secretory process in nonneuronal cells. The cell biology of rapid axonal transport is reviewed, the central concern being to explore those aspects that do not fit into the general secretory model and which may thus represent specific neuronal adaptations. Particular attention is paid to the relationship between the transport of newly synthesized proteins and of the membranous organelles that act as carriers. Sites in the transport sequence at which the behavior of axonal transport may differ from the secretory model are at the initiation of axonal transport at the trans-side of the Golgi apparatus, within the axon where molecules are deposited from the moving phase to a stationary phase, and at nerve terminals or axonal lesions where transport reversal takes place.

Heat stress increases delivery of a unique sub-population of proteins conveyed by fast axonal transport

The effect of heat stress on protein synthesis and fast axonal transport was examined in vitro in bullfrog dorsal root ganglion (DRG) and associated spinal/sciatic nerve. Qualitative and quantitative changes of individual 35S-methionine-labelled proteins were determined following DRG labelling and fast transport in respective nerves via two-dimensional gel electrophoresis/autoradiography. Elevation of temperature from 18 degrees C to 33 degrees C for up to 6 hr resulted in a marked increase in synthesis of five individual DRG species of approximately 74,000 daltons that comigrate with heat shock proteins (HSPs). A quantitative comparison of species within this subset revealed two subgroups differentially affected by stress. The three most basic proteins were induced to approximately 1300% of unstressed controls after 6 hr of stress, while the two most acidic species demonstrated an increase to only 300% of controls over the same period. The relative abundance of 25 additional DRG proteins were uneffected by heat stress. Of 70 35S-labelled fast-transported proteins similarly analyzed, 15, comprising 5 families, were consistently transported at greater than 150% of controls following up to 6 hr of heat stress. Over this period all 15 proteins shared a similar profile of abundance relative to non-induced proteins. Transport was elevated to the greatest extent after 2 hr of stress, declined after 3 hr, and tended to rebound at later times. The remaining 55 fast-transported protein spots analyzed were unaffected. An increased delivery of this unique sub-population of 15 fast-transported proteins suggests a possible involvement in early cellular events that mediate heat stress in the nervous system.

Acrylamide-induced increases in deposition of axonally transported glycoproteins in rat sciatic nerve

The axonal transport of proteins, glycoproteins, and gangliosides in sensory neurons of the sciatic nerve was examined in adult rats exposed to acrylamide via intraperitoneal injection (40 mg/kg of body weight/day for nine consecutive days). The L5 dorsal root ganglion was injected with either [35S]methionine to label proteins or [3H]glucosamine to label, more specifically, glycoproteins and gangliosides. At times ranging from 2 to 6 h later, the sciatic nerve and injected ganglion were excised and radioactivity in consecutive 5-mm segments determined. In both control and acrylamide-treated animals, outflow profiles of [35S]methionine-labeled proteins showed a well defined crest which moved down the nerve at a rate of approximately 340 mm/day. Similar outflow profiles and transport rates were seen for [3H]glucosamine-labeled glycoproteins in control animals. However, in animals treated with acrylamide, the crest of transported labeled glycoprotein was severely attenuated as it moved down the nerve. This finding suggests that in acrylamide-treated animals, axonally transported glycoproteins were preferentially transferred (unloaded or exchanged against unlabeled molecules) from the transport vector to stationary axonal structures. We also examined the clearance of axonally transported glycoproteins distal to a ligature on the nerve. The observed impairment of clearance in acrylamide-treated animals relative to controls is supportive of the above hypothesis. Acrylamide may directly affect the mechanism by which axonally transported material is unloaded from the transport vector. Alternatively, the increased rate of unloading might reflect an acrylamide-induced increase in the demand for axonally transported material.

Identification of the plasma membrane proteolipid protein as a constituent of brain coated vesicles and synaptic plasma membrane

We have analyzed brain coated vesicles and synaptic plasma membrane for the presence of the plasma membrane proteolipid protein. Coated vesicles were isolated from calf brain gray matter with a final purification on Sephacryl S-1000 and reisolated twice by chromatography to ensure homogeneity. Fractions were analyzed by gel electrophoresis, immunoblotting for clathrin heavy chain, and by electron microscopy. Using an immunoblotting assay we were able to demonstrate the presence of the plasma membrane proteolipid protein in these coated vesicles at a significant level (i.e., approximately 1% of the bilayer protein of these vesicles). Reisolation of coated vesicles did not diminish the concentration of the protein in this fraction. Removal of the clathrin coat proteins or exposure of the coated vesicles to 0.1 M Na2CO3 showed that the plasma membrane proteolipid protein is not removed during uncoating and lysis but is intrinsic to the membrane bilayer of these vesicles. These studies demonstrate that plasma membrane proteolipid protein represents a significant amount of the bilayer protein of coated vesicles, suggesting that these vesicles may be a transport vehicle for the intracellular movement of the plasma membrane proteolipid protein. Isolation of synaptic plasma membranes proteolipid adult rat brain and estimation of the plasma membrane proteolipid protein content using the immunoblotting method confirmed earlier studies that show this protein is present in this membrane fraction at high levels as well (approximately 1-2%). The level of this protein in the synaptic plasma membrane suggests that the synaptic plasma membrane is one major site to which these vesicles may be targeted or from which the protein is being retrieved.

Lead-dependent deposits in diverse synaptic vesicles: suggestive evidence for the presence of anionic binding sites

We have observed electron dense deposits dependent on incubation of aldehyde-fixed tissues with lead ions within synaptic vesicles of several types of neurons that differ in the neurotransmitters utilized and in the secretory granules of the adrenal medulla. Evidently, vesicle components that can interact with lead ions are widespread. A plausible explanation for the occurrence of the deposits is the presence of anionic binding sites within the vesicles. This would agree well with other biochemical, cytochemical, and immunocytochemical evidence, such as that indicating the presence of sulfated macromolecules in certain synaptic vesicles. Anionic binding sites could play significant roles by participating in processes such as Ca2+ storage, stabilization of pH gradients, or the control of osmotic phenomena.

Complex compartmentation of tyrosine sulfate-containing proteins undergoing fast axonal transport

The compartmentation of fast-transported proteins that possess sulfated tyrosine residues--sulfoproteins--has been examined for further resolution of the possible significance of sulfated tyrosine in routing and delivery of fast-transported proteins. In vitro fast axonal transport of [35S]methionine- or 35SO4-labeled proteins was measured in dorsal root ganglion neurons for analysis of protein compartmentation en route and in synaptic regions. When membrane fractions were exposed to Na2CO3 for separation of "lumenal" and peripheral membrane proteins from integral components of the membrane, approximately 20% of the [35S]methionine incorporated into fast-transported proteins was present in a carbonate-releasable form in the axon, whereas 53% of the incorporated 35SO4 was released by carbonate. Eighty percent of the 35SO4 in this releasable fraction was acid labile, typical of sulfate ester-linked to tyrosine. Sulfoproteins were also detected in synaptosomes and were released into the extracellular medium in a calcium-dependent fashion, an observation suggesting that fast-transported sulfoproteins are secreted. Of the remaining 47% of the fast-transported 35SO4-labeled proteins resistant to carbonate treatment (the integral membrane protein fraction), nearly 60% of the 35SO4 was acid labile. Other membrane stripping agents, such as 0.1 M NaOH, 0.5 M NaCl, or mild trypsin treatment, failed to remove acid-labile 35SO4-labeled species from carbonate-treated membrane. Quantitative comparisons of several of the most abundant sulfoproteins resolved via two-dimensional gel electrophoresis confirmed that approximately 7% of each of the species remained associated with carbonate-treated membranes, presumably as integral membrane components.(ABSTRACT TRUNCATED AT 250 WORDS)

Destinations of some fast-transported proteins in sensory neurons of bullfrog sciatic nerve

Many characteristics of proteins that are fast axonally transported have been described, but the destinations of most within the neuron remain unknown. We have studied the destinations of some fast-transported proteins in sensory neurons of the bullfrog sciatic nerve, specifically to determine which may be deposited in axons and which may be destined for more distal, possibly terminal, areas. Dorsal root ganglia were pulse-labeled with [35S]methionine in vitro, following which they were separated from the sciatic nerve. After additional periods of transport, radioactive proteins from two areas of the nerve were separated by two-dimensional polyacrylamide gel electrophoresis and used to develop x-ray film. The first area contained the wavefront of transported radioactivity (wavefront region), whereas the second area was taken from nerve through which the wavefront had already passed (plateau region). The amount of radioactivity in certain fast-transported protein species from each area was determined by computer analysis of digitized video images of fluorographs. Certain proteins were preferentially left behind the wavefront and, therefore, may supply axon and possibly other nerve components, whereas other proteins were found almost exclusively in the wavefront and, hence, may supply more distal, possibly terminal, areas.

A double-isotope procedure for examining protein microheterogeneity: multiple forms of fast-transported glycoproteins and sulfoproteins possess a common polypeptide chain

Several fast-transported proteins that appear as single bands after sodium dodecyl sulfate-polyacrylamide gel electrophoresis resolve into multiple spots during isoelectric focusing. A method was devised for determining if such microheterogeneity in net charge indicates that individual polypeptides have been posttranslationally modified to differing extents. Dorsal root ganglia were pulse-labeled with [35S]methionine and either [3H]leucine or [3H]proline, proteins fast-transported into peripheral sensory axons were separated by two-dimensional gel electrophoresis, and isotope incorporation ratios of proteins associated with individual gel spots were determined. When four microheterogeneous glycoproteins were analyzed, each protein "family" showed markedly similar isotope ratios for its three to seven characteristic spots. Such ratios differed between families by almost twofold. In addition, a group of nonglycosylated, sulfate-containing proteins was identified as a family on the basis of the similar isotope incorporation ratios of its component spots. These results suggest that protein microheterogeneity can result from variable sulfation of tyrosine residues as well as from variation in sialic acid-containing oligosaccharide side-chains. More generally, the method can be utilized to test for protein microheterogeneity in cases where the amounts of protein are too low to permit peptide mapping analysis and where the nature of the charge-altering modification is unknown.