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)     

Fast-Transported Glycoproteins and Nonglycosylated Proteins Contain Sulfate

35SO4-labeled fast-transported proteins of bullfrog dorsal root ganglion neurons were separated by two-dimensional gel electrophoresis, and their mobilities were compared to similar species labeled with [3H]mannose or [3H]fucose. Fluorography revealed regions of poorly resolved, high molecular weight material, likely to represent sulfated proteoglycans, as well as many well resolved spots that corresponded in mobility to individual [35S]methionine-labeled fast-transported proteins. The majority of these well resolved spots appeared as "families," previously identified as glycoproteins based on their labeling with sugars. Thus, sulfate can be a contributor to the carbohydrate side-chain charge that underlies microheterogeneity. The most heavily 35SO4-labeled species, however, corresponded to fast-transported proteins that were not labeled by either sugar. The relative acid labilities of 35SO4 associated with individual species cut from the gel confirmed the assignments of these spots as glycoproteins or nonglycoproteins. A group of spots intermediate in their acid lability was also detected, suggesting that some proteins may contain sulfate linked to carbohydrate as well as to amino acid residues.     

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.     

The decreased synthesis of chondroitin sulfate-containing extracellular proteoglycans by SV40 transformed Balb/c 3T3 cells.

Balb/c 3T3 cells synthesize 5--10 times more 35SO2/4- -labeled extracellular proteoglycan per cell than do Balb/c 3T3 cells transformed by SV40 (SV3T3). The extracellular 35SO2/4- -labeled proteoglycans of the Balb/c 3T3 and SV3T3 cells differ markedly in their acid mucopolysaccharide composition. Extracellular Balb/c 3T3 proteoglycans contain about 70--80% chondroitin sulfate, most of which is chondroitin 4-sulfate, and small amounts of heparan sulfate and/or heparin. On the other hand, extracellular SV3T3 proteoglycans contain 65-75% heparan sulfate and/or heparin and less than 15% chondroitin sulfate. Analysis of extracellular 35SO2/4- -labeled proteoglycan by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals that Balb/c 3T3 alone synthesizes a class of proteoglycans capable of migrating in a 10% separating gel. This class of proteoglycans, designated as fraction C, accounts for up to 45% of the total extracellular Balb/c 3T3 35 SO2/4- -labeled proteoglycans and contains chondroitin sulfate extracellular SV3T3 proteoglycans. The absence of this and other classes of chondroitin sulfate-containing proteoglycans can account for the 5-10-fold decreased synthesis of 35SO2/4- -labeled proteoglycans by SV3T3 cells when compared to Balb/c 3T3 cells.     

Complex-type N-linked oligosaccharides of gp120 from human immunodeficiency virus type 1 contain sulfated N-acetylglucosamine.

The major envelope glycoproteins gp120 and gp41 of human immunodeficiency virus type 1, the causative agent for human AIDS, contain numerous N-linked oligosaccharides. We report here our discovery that N-acetylglucosamine residues within the complex-type N-linked oligosaccharides of both gp120 and its precursor, gp160, are sulfated. When human Molt-3 cells persistently infected with human T-cell leukemia virus IIIB were metabolically radiolabeled with 35SO4, gp160, gp120, and to some extent gp41 were radiolabeled. The 35SO4-labeled oligosaccharides were quantitatively released by N-glycanase treatment and were bound by immobilized Ricinus communis agglutinin I, a lectin that binds to terminal beta-galactosyl residues. The kinetics of release of sulfate upon acid hydrolysis from 35SO4-labeled gp120 indicate that sulfation occurs in a primary sulfate ester linkage. Methylation analysis of total glycopeptides from Molt-3 cells metabolically radiolabeled with [3H]glucosamine demonstrates that sulfation occurs at the C-6 position of N-acetylglucosamine. Fragmentation of the gp120-derived 35SO4-labeled glycopeptides by treatment with hydrazine and nitrous acid and subsequent reduction generated galactosyl-anhydromannitol-6-35SO4, which is the expected reaction product from GlcNAc-6-sulfate within a sulfated lactosamine moiety. Charge analysis of the [3H]galactose- and [3H]glucosamine-labeled glycopeptides from gp120 and gp160 indicates that approximately 14% of the complex-type N-linked oligosaccharides are sulfated.     

Secretion of 35SO4-labeled proteins from isolated rat hepatocytes

Sulfation is a Golgi-specific modification of secretory proteins. We have characterized the proteins that are labeled with 35SO4 in cultures of rat hepatocytes and studied their transport to the medium. Analysis by polyacrylamide gel electrophoresis showed that of the five most heavily labeled proteins, four had well-defined mobilities--apparent molecular masses of 188, 142, 125, and 82 kDa--whereas one was electrophoretically heterogeneous--apparent molecular mass of 35-45 kDa. Judging by their relatively high resistance to acid treatment, the sulfate residues in the 125- and 35-45-kDa proteins were linked to carbohydrate. Some of the secreted proteins were sialylated. In samples of pulse-labeled cells, there appeared to be no unsialylated forms, indicating that sulfation occurred after sialylation, presumably in the trans Golgi. Kinetic experiments showed that the cellular half-life was the same for all the sulfated proteins--about 8 min--consistent with the idea that transport from the Golgi complex to the cell surface occurs by liquid bulk flow.     

Sulfate transport-deficient mutants of Chinese hamster ovary cells. Sulfation of glycosaminoglycans dependent on cysteine

We isolated 59 Chinese hamster ovary cell mutants defective in 35SO4 incorporation into glycosaminoglycans. Thirty-five mutants incorporated [6-3H]glucosamine into glycosaminoglycans normally, suggesting that they were specifically impaired in sulfate incorporation. Cell hybridization studies revealed that the 35 mutants defined a unique complementation group. Pulse-labeling one of the mutants with 35SO4 showed that it possessed a defect in a saturable, 4-acetamido-4-isothiocyanostilbene-2,2'-disulfonic acid-sensitive transport system required for sulfate uptake. Despite the dramatic reduction in 35SO4 incorporation, the mutant synthesized sulfated heparan and chondroitin chains. Incubation of the mutant with [35S]cysteine resulted in the formation of 35SO4, which was subsequently incorporated into the glycosaminoglycans. Similar results were obtained when wild-type cells were incubated in sulfate-free growth medium containing [35S]cysteine, and isotope dilution analysis indicated that about 15 microM of sulfate was derived from cysteine catabolism. We also found that the sulfate transport deficiency rendered the mutant resistant to 5 microM sodium chromate, whereas wild-type cells did not divide under these conditions. However, the mutant also did not proliferate in medium containing 5 microM chromate when grown in the presence of wild-type cells, suggesting that chromate was transported through cell-cell contacts. Since co-cultivating sulfate transport-deficient mutants with mutants defective in xylosyltransferase or galactosyltransferase I partially restored 35SO4 incorporation into glycosaminoglycans, intercellular sulfate transport occurred as well. Therefore, the availability of sulfate for glycosaminoglycan synthesis depends on sulfate uptake, turnover of sulfur-containing amino acids, and sulfate transport between cells.     

Glycosylation as a Criterion for Defining Subpopulations of Fast-Transported Proteins

The role carbohydrate residues may play in the sorting of newly synthesized fast-transported proteins during the initiation of fast axonal transport has been examined by identifying individual fast-transported glycoproteins that contain either or both fucose and galactose. [3H]Fucose or [3H]galactose was incorporated together with [35S]methionine in vitro in bullfrog dorsal root ganglia. Fast-transported proteins that accumulated proximal to a ligature on the spinal nerve were separated via two-dimensional gel electrophoresis, and 92 gel spots were analyzed quantitatively for the presence of 35S and 3H. Of these spots, 56 (61%) contained either or both fucose and galactose. Glycomoieties were generally associated with families of charged spots whose isoelectric points could be altered with neuraminidase treatment. Single spots tended to be unglycosylated and were unaffected by neuraminidase. The prevalence of glycoproteins was considerably greater in the higher-molecular weight range. Of the 55 spots analyzed with molecular weight greater than approximately 35,000 daltons, 89% were glycosylated, whereas only 19% of the 37 spots with lower molecular weight contained sugar moieties. When considered in light of previous studies in which similar subpopulations have been described, the current findings suggest that the presence or absence of glycomoieties may represent another criterion by which proteins are sorted during the initiation of fast axonal transport.     

Evidence that all newly synthesized proteins destined for fast axonal transport pass through the Golgi apparatus

Effects of the sodium ionophore, monensin, were examined on the passage from neuronal cell body to axon of materials undergoing fast intracellular transport. In vitro exposure of bullfrog dorsal root ganglia to concentrations of drug less than 1.0 micron led to a dose-dependent depression in the amount of fast-transported [3H]leucine- or [3H]glycerol-labeled material appearing in the nerve trunk. Incorporation of either precursor was unaffected. Exposure of a desheathed nerve trunk to similar concentrations of monensin, while ganglia were incubated in drug-free medium, had no effect on transport. With [3H]fucose as precursor, fast transport of labeled glycoproteins was depressed to the same extent as with [3H]leucine; synthesis, again, was unaffected. By contrast, with [3H]galactose as precursor, an apparent reduction in transport of labeled glycoproteins was accounted for by a marked depression in incorporation. The inference from these findings, that monensin acts to block fast transport at the level of the Golgi apparatus, was supported by ultrastructural examination of the drug-treated neurons. An extensive and selective disruption of Golgi saccules was observed, accompanied by an accumulation of clumped smooth membranous cisternae. Quantitative analyses of 48 individual fast-transported protein species, after separation by two-dimensional gel electrophoresis, revealed that monensin depresses all proteins to a similar extent. These results indicate that passage through the Golgi apparatus is an obligatory step in the intracellular routing of materials destined for fast axonal transport.     

Transport of heparan sulfate into the nuclei of hepatocytes

Monolayer cultures of a rat hepatocyte cell line shown previously to accumulate a nuclear pool of free heparan sulfate chains that are enriched in sulfated glucuronic acid (GlcA) residues (Fedarko, N.S., and Conrad, H.E., (1986) J. Cell Biol. 587-599) were incubated with 35SO4(2-), and the rate of appearance of heparan [35S]sulfate in the nuclei was measured. Heparan [35S]sulfate began to accumulate in the nuclei 2 h after the administration of 35SO4(2-) to the cells and reached a steady state level after 20 h. Heparan [35S]sulfate was lost from the nuclei of prelabeled cells with a t1/2 of 8 h. Chloroquine did not inhibit the transport of heparan sulfate into the nucleus, but increased the t1/2 for the exit of heparan sulfate from the nucleus to 20 h and led to a doubling of the steady state level of nuclear heparan sulfate. Heparan [35S]sulfate which was obtained from the medium or from the cell matrix of a labeled culture and which contained only low levels of GlcA-2-SO4 residues was incubated with cultures of unlabeled cells, and the uptake of the exogenous heparan [35S]sulfate was studied. At 37 degrees C the cells took up proteoheparan [35S]sulfate and transported about 10% of the internalized heparan [35S]sulfate into the nucleus, where it appeared as free chains. The heparan [35S]sulfate isolated from the nucleus was enriched in GlcA-2-SO4 residues, whereas the heparan [35S]sulfate remaining in the rest of the intracellular pool showed a corresponding depletion in GlcA-2-SO4 residues. At 16 degrees C, where endocytosed materials do not enter the lysosomes, the cells also transported exogenous proteoheparan [35S]sulfate to the nucleus with similar processing. Thus, the metabolism of exogenous heparan sulfate by hepatocytes follows the same pathway observed in continuously labeled cells and does not involve lysosomal processing of the internalized heparan sulfate.     

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.     

Secretion of sulfated and nonsulfated forms of parathyroid chromogranin A (secretory protein-I)

Chromogranin A (secretory protein-I) is an acidic, sulfated glycoprotein found in secretory granules of most endocrine cells but not in exocrine or epithelial cells. Parathyroid chromogranin A is sulfated on tyrosine residues, whereas adrenal chromogranin A appears to be sulfated mainly on oligosaccharide residues. Chromogranin B, on the other hand, is tyrosine-sulfated in the bovine adrenal whereas this protein is absent from the parathyroid. The role of this tissue- or species-specific sulfation of chromogranin is not known. Tyrosine sulfation is a common post-translational modification of proteins in the exocytotic pathway and has been suggested to play a role in the sorting or intracellular transport of secretory proteins. To test this, porcine parathyroid tissue slices were metabolically labeled with 35SO4 and [3H]Lys, and the tissue and incubation medium analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and immunoprecipitation with chromogranin A-specific antiserum or by radioimmunoassay for parathormone. Secretion of total and 3H-labeled chromogranin A was about 3- and 7-fold higher, respectively, at 0.5 mM than at 3.0 mM Ca2+, and secretion of 35SO4-labeled chromogranin A was 67-fold higher. This indicates that either sulfated chromogranin A is directed primarily to the Ca2+-regulated pathway or that sulfation occurs following sorting to this pathway. Sodium chlorate (1-10 mM) inhibited sulfation in a dose-dependent manner by up to 95% but it had no effect on the onset or rate of chromogranin A secretion. These data indicate that regulated secretion of parathyroid chromogranin A does not require sulfation of tyrosine residues.     

Characterization of a heparan sulfate proteoglycan that copurifies with the neural cell adhesion molecule

We have demonstrated previously that the neural cell adhesion molecule (NCAM) interacts with a neuronal heparan sulfate proteoglycan. The binding of this proteoglycan(s) by NCAM appears to be required for NCAM-mediated cell adhesion, although the mechanism is unclear. In the present study we show that a heparan sulfate proteoglycan copurifies with NCAM, and provide an initial biochemical characterization of the proteoglycan. The copurification of a heparan sulfate proteoglycan with NCAM was demonstrated following immunopurification of NCAM from a detergent extract of cell membranes derived from Na2(35)SO4-labeled neural retinal cells. A large-molecular-weight, 35SO4-labeled molecule copurified with NCAM isolated from these neural cell cultures, and was resistant to chondroitinase ABC treatment, but degraded completely by nitrous acid treatment. These results indicate that the molecule is a heparan sulfate proteoglycan. Although this proteoglycan copurifies with NCAM, it is not detected when the neuron-glia cell adhesion molecule (NgCAM) is immunopurified using the 8D9 monoclonal antibody. The heparan sulfate proteoglycan may also be a membrane-associated proteoglycan since it interacts with phenyl-Sepharose. Molecular weight characterization of the proteoglycan by gel filtration chromatography indicates a molecular weight of 400-520 kDa. The heparan sulfate glycosaminoglycan chains were shown to have an average molecular weight of approximately 40 kDa, and the polypeptide backbone was estimated to be 120 kDa by polyacrylamide gel electrophoresis. These data therefore demonstrate that a neuronal heparan sulfate proteoglycan copurifies with NCAM.     

Molecular cloning of squid N-acetylgalactosamine 4-sulfate 6-O-sulfotransferase and synthesis of a unique chondroitin sulfate containing E-D hybrid tetrasaccharide structure by the recombinant enzyme

N-Acetylgalactosamine 4-sulfate 6-O-sulfotransferase (GalNAc4S-6ST) transfers sulfate to position 6 of GalNAc(4SO(4)) residues in chondroitin sulfate (CS). We previously purified squid GalNAc4S-6ST and cloned a cDNA encoding the partial sequence of squid GalNAc4S-6ST. In this paper, we cloned squid GalNAc4S-6ST cDNA containing a full open reading frame and characterized the recombinant squid GalNAc4S-6ST. The cDNA predicts a Type II transmembrane protein composed of 425 amino acid residues. The recombinant squid GalNAc4S-6ST transferred sulfate preferentially to the internal GalNAc(4SO(4)) residues of chondroitin sulfate A (CS-A); nevertheless, the nonreducing terminal GalNAc(4SO(4)) could be sulfated efficiently when the GalNAc(4SO(4)) residue was included in the unique nonreducing terminal structure, GalNAc(4SO(4))-GlcA(2SO(4))-GalNAc(6SO(4)), which was previously found in CS-A. Shark cartilage chondroitin sulfate C (CS-C) and chondroitin sulfate D (CS-D), poor acceptors for human GalNAc4S-6ST, served as the good acceptors for the recombinant squid GalNAc4S-6ST. Analysis of the sulfated products formed from CS-C and CS-D revealed that GalNAc(4SO(4)) residues included in a tetrasaccharide sequence, GlcA-GalNAc(4SO(4))-GlcA(2SO(4))-GalNAc(6SO(4)), were sulfated efficiently by squid GalNAc4S-6ST, and the E-D hybrid tetrasaccharide sequence, GlcA-GalNAc(4,6-SO(4))-GlcA(2SO(4))-GalNAc(6SO(4)) was generated in the resulting sulfated glycosaminoglycans. These observations indicate that the recombinant squid GalNAc4S-6ST is a useful enzyme for preparing a unique chondroitin sulfate containing the E-D hybrid tetrasaccharide structure.     

Differential effects of cobalt on the initiation of fast axonal transport

Effects of Co²⁺ on the fast axonal transport of individual proteins were examined in vitro in bullfrog spinal/sciatic nerves.³⁵S-methionine-labeled proteins, fast-transported in control and Co²⁺-treated preparations were separated via two-dimensional gel electrophoresis. While the overall amount of protein transported was reduced, no qualitative differences could be seen when gel fluorographic patterns were compared. Quantitative analyses of the 48 most abundantly transported species revealed two significantly different populations (p < 0.01) differentially sensitive to Co²⁺ and distinguishable to a large extent by molecular weight. Those proteins less sensitive to Co²⁺ ranged from ~20,000 to 35,000 daltons while those more sensitive to Co²⁺ were >~35,000 daltons. The finding that all proteins are affected by Co²⁺ supports the proposal that fast-transported proteins are subject to a common Co²⁺-sensitive, Ca²⁺-requiring step. The observed differential effects are consistent with more than one Ca²⁺-dependent step occurring during the initiation phase of fast transport.This research was supported by a Muscular Dystrophy Association postdoctoral fellowship to G.C.S., and by research grants from NSF (BNS 79-24125) and the National Multiple Sclerosis Society (RG 1296-A-1) to R.H.     

Heparan sulfate-chondroitin sulfate hybrid proteoglycan of the cell surface and basement membrane of mouse mammary epithelial cells

Chondroitin sulfate represents approximately 15% of the 35SO4-labeled glycosaminoglycans carried by the proteoglycans of the cell surface and of the basolateral secretions of normal mouse mammary epithelial cells in culture. Evidence is provided that these chondroitin sulfate-carrying proteoglycans are hybrid proteoglycans, carrying both chondroitin sulfate and heparan sulfate chains. Complete N-desulfation but limited O-desulfation, by treatment with dimethyl sulfoxide, of the proteoglycans decreased the anionic charge of the chondroitin sulfate-carrying proteoglycans to a greater extent than it decreased the charge of their constituent chondroitin sulfate chains. Partial depolymerization of the heparan sulfate residues of the proteoglycans with nitrous acid or with heparin lyase also reduced the effective molecular radius of the chondroitin sulfate-carrying proteoglycans. The effect of heparin lyase on the chondroitin sulfate-carrying proteoglycans was prevented by treating the proteoglycan fractions with dimethyl sulfoxide, while the effect of nitrous acid on the dimethyl sulfoxide-treated proteoglycans was prevented by acetylation. This occurrence of heparan sulfate-chondroitin sulfate hybrid proteoglycans suggests that the substitution of core proteins by heparan sulfate or chondroitin sulfate chains may not solely be determined by the specific routing of these proteins through distinct chondroitin sulfate and heparan sulfate synthesizing mechanisms. Moreover, regional and temporal changes in pericellular glycosaminoglycan compositions might be due to variable postsynthetic modification of a single gene product.     

A unique nonreducing terminal modification of chondroitin sulfate by N-acetylgalactosamine 4-sulfate 6-o-sulfotransferase

N-Acetylgalactosamine 4-sulfate 6-O-sulfotransferase (GalNAc4S-6ST) transfers sulfate from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to position 6 of N-acetylgalactosamine 4-sulfate (GalNAc(4SO4)). We previously identified human GalNAc4S-6ST cDNA and showed that the recombinant GalNAc4S-6ST could transfer sulfate efficiently to the nonreducing terminal GalNAc(4SO4) residues. We here present evidence that GalNAc4S-6ST should be involved in a unique nonreducing terminal modification of chondroitin sulfate A (CSA). From the nonreducing terminal of CS-A, a GlcA-containing oligosaccharide (Oligo I) that could serve as an acceptor for GalNAc4S-6ST was obtained after chondroitinase ACII digestion. Oligo I was found to be GalNAc(4SO4)-GlcA(2SO4)-GalNAc(6SO4) because GalNAc(4SO4) and deltaHexA(2SO4)-GalNAc(6SO4) were formed after chondroitinase ABC digestion. When Oligo I was used as the acceptor for GalNAc4S-6ST, sulfate was transferred to position 6 of GalNAc(4SO4) located at the nonreducing end of Oligo I. Oligo I was much better acceptor for GalNAc4S-6ST than GalNAc(4SO4)-GlcAGalNAc(6SO4). An oligosaccharide (Oligo II) whose structure is identical to that of the sulfated Oligo I was obtained from CS-A after chondroitinase ACII digestion, indicating that the terminal modification occurs under the physiological conditions. When CS-A was incubated with [35S]PAPS and GalNAc4S-6ST and the 35S-labeled product was digested with chondroitinase ACII, a 35S-labeled trisaccharide (Oligo III) containing [35S]GalNAc(4,6-SO4) residue at the nonreducing end was obtained. Oligo III behaved identically with the sulfated Oligos I and II. These results suggest that GalNAc4S-6ST may be involved in the terminal modification of CS-A, through which a highly sulfated nonreducing terminal sequence is generated.     

The decreased synthesis of chondroitin sulfate-containing extracellular proteoglycans by SV40 transformed Balb/c 3T3 cells

Balb/c 3T3 cells synthesize 5–10 times more ³⁵SO₄^2?-labeled extracellular proteoglycan per cell than do Balb/c 3T3 cells transformed by SV40 (SV3T3). The extracellular ³⁵SO₄^2?-labeled proteoglycans of the Balb/c 3T3 and SV3T3 cells differ markedly in their acid mucopolysaccharide composition. Extracellular Balb/c 3T3 proteoglycans contain about 70–80% chondroitin sulfate, most of which is chondroitin 4-sulfate, and small amounts of heparan sulfate and/or heparin. On the other hand, extracellular SV3T3 proteoglycans contain 65–75% heparan sulfate and/or heparin and less than 15% chondroitin sulfate. Analysis of extracellular ³⁵SO₄^2?-labeled proteoglycan by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals that Balb/c 3T3 alone synthesizes a class of proteoglycans capable of migrating in a 10% separating gel. This class of proteoglycans, designated as fraction C, accounts for up to 45% of the total extracellular Balb/c 3T3 ³⁵SO₄^2?-labeled proteoglycans and contains chondroitin sulfate exclusively. It is altogether absent in the extracellular SV3T3 proteoglycans. The absence of this and other classes of chondroitin sulfate-containing proteoglycans can account for the 5–10-fold decreased synthesis of ³⁵SO₄^2?-labeled proteoglycans by SV3T3 cells when compared to Balb/c 3T3 cells.     

Subsets of Axonally Transported and Periaxonal Polypeptides Are Released from Regenerating Nerve

Using two-dimensional polyacrylamide gel electrophoresis to analyze proteins, we have found subsets of periaxonal and fast-transported axoplasmic proteins that are released in vitro from regenerating sciatic nerve into a surrounding bath. Of the fast-transported proteins that are released from nerve, there is a subset of at least five polypeptides that appears in greater relative abundance in the bath than in the nerve. Some of these released, fast-transported proteins are glycosylated. Several periaxonally synthesized polypeptides are released in significantly greater amounts from regenerating nerve, and of these polypeptides, two are released in greater amounts from nerve only at regions of regeneration or distal to regeneration. These released polypeptides do not represent the most abundant of the locally synthesized proteins. The released, fast-transported and periaxonal proteins may play a role in intercellular signaling or in modulation of the extracellular environment during nerve regeneration.     

The in vitro cell culture age and cell density of articular chondrocytes alter sulfated-proteoglycan biosynthesis

The effect of cell culture age and concomitant changes in cell density on the biosynthesis of sulfated-proteoglycan by rabbit articular chondrocytes in secondary monolayer culture was studied. Low density (LD, 2 d), middle density (MD, 5-7 d), and high density (HD, 12-15 d) cultures demonstrated changes in cellular morphology and rates of DNA synthesis. DNA synthesis was highest at LD to MD densities, but HD cultures continued to incorporate [3H]-thymidine. LD cultures incorporated 35SO4 into sulfated-proteoglycans at a higher rate than MD or LD cultures. The qualitative nature of the sulfated-proteoglycans synthesized at the different culture ages were analyzed by assessing the distribution of incorporated 35SO4 in associative and dissociative CsCl density gradients and by elution profiles on Sepharose CL-2B. Chondrocytes deposited into the extracellular matrix (cell-associated fraction) 35SO4-labeled proteoglycan aggregate. More aggregated proteoglycan was found in the MD and HD cultures than at LD. A 35SO4-labeled aggregated proteoglycan of smaller hydrodynamic size than that found in the cell-associated fraction was secreted into the culture medium at each culture age. The proteoglycan monomer (A1D1) of young and older cultures had similar hydrodynamic sizes at all cell culture ages and cell densities. The glycosaminoglycan chains of A1D1 were hydrodynamically larger in the younger LD cultures than in the older HD cultures and consisted of only chondroitin 6 and 4 sulfate chains. A small amount of chondroitin 4,6 sulfate was detected, but no keratan sulfate was measured. The A1D2 fractions of young LD cultures contained measurable amounts of dermatan sulfate; no dermatan sulfate was found in older MD or HD cultures. These studies indicated that chondrocytes at LD synthesized a proteoglycan monomer with many of the characteristics of young immature articular cartilage of rabbits. These results also indicated that rapidly dividing chondrocytes were capable of synthesizing proteoglycans which form aggregates with hyaluronic acid. Culture age and cell density appears primarily to modulate the synthesis of glycosaminoglycan types and chain length. Whether or not these glycosaminoglycans are found on the same or different core proteins remains to be determined.