Developmental changes in the oligosaccharide composition of synaptic junctional glycoproteins

Human-specific features and developmental dynamics of the brain N-glycome

Comparative "omics" studies have revealed unique aspects of human neurobiology, yet an evolutionary perspective of the brain N-glycome is lacking. We performed multiregional characterization of rat, macaque, chimpanzee, and human brain N-glycomes using chromatography and mass spectrometry and then integrated these data with complementary glycotranscriptomic data. We found that, in primates, the brain N-glycome has diverged more rapidly than the underlying transcriptomic framework, providing a means for rapidly generating additional interspecies diversity. Our data suggest that brain N-glycome evolution in hominids has been characterized by an overall increase in complexity coupled with a shift toward increased usage of α(2-6)-linked N-acetylneuraminic acid. Moreover, interspecies differences in the cell type expression pattern of key glycogenes were identified, including some human-specific differences, which may underpin this evolutionary divergence. Last, by comparing the prenatal and adult human brain N-glycomes, we uncovered region-specific neurodevelopmental pathways that lead to distinct spatial N-glycosylation profiles in the mature brain.

Human-specific features and developmental dynamics of the brain N-glycome

Comparative "omics" studies have revealed unique aspects of human neurobiology, yet an evolutionary perspective of the brain N-glycome is lacking. Here, we performed multi-regional characterization of rat, macaque, chimpanzee, and human brain N-glycomes using chromatography and mass spectrometry, then integrated these data with complementary glycotranscriptomic data. We found that in primates the brain N-glycome has evolved more rapidly than the underlying transcriptomic framework, providing a mechanism for generating additional diversity. We show that brain N-glycome evolution in hominids has been characterized by an increase in complexity and α(2-6)-linked N-acetylneuraminic acid along with human-specific cell-type expression of key glycogenes. Finally, by comparing the prenatal and adult human brain N-glycome, we identify region-specific neurodevelopmental pathways that lead to distinct spatial N-glycosylation profiles in the mature brain.

One-Sentence Summary

Evolution of the human brain N-glycome has been marked by an increase in complexity and a shift in sialic acid linkage.

The dynamic brain N-glycome

The attachment of carbohydrates to other macromolecules, such as proteins or lipids, is an important regulatory mechanism termed glycosylation. One subtype of protein glycosylation is asparagine-linked glycosylation (N-glycosylation) which plays a key role in the development and normal functioning of the vertebrate brain. To better understand the role of N-glycans in neurobiology, it's imperative we analyse not only the functional roles of individual structures, but also the collective impact of large-scale changes in the brain N-glycome. The systematic study of the brain N-glycome is still in its infancy and data are relatively scarce. Nevertheless, the prevailing view has been that the neuroglycome is inherently restricted with limited capacity for variation. The development of improved methods for N-glycomics analysis of brain tissue has facilitated comprehensive characterisation of the complete brain N-glycome under various experimental conditions on a larger scale. Consequently, accumulating data suggest that it's more dynamic than previously recognised and that, within a general framework, it has a given capacity to change in response to both intrinsic and extrinsic stimuli. Here, we provide an overview of the many factors that can alter the brain N-glycome, including neurodevelopment, ageing, diet, stress, neuroinflammation, injury, and disease. Given this emerging evidence, we propose that the neuroglycome has a hitherto underappreciated plasticity and we discuss the therapeutic implications of this regarding the possible reversal of pathological changes via interventions. We also briefly review the merits and limitations of N-glycomics as an analytical method before reflecting on some of the outstanding questions in the field.

Neural glycomics: the sweet side of nervous system functions

The success of investigations on the structure and function of the genome (genomics) has been paralleled by an equally awesome progress in the analysis of protein structure and function (proteomics). We propose that the investigation of carbohydrate structures that go beyond a cell's metabolism is a rapidly developing frontier in our expanding knowledge on the structure and function of carbohydrates (glycomics). No other functional system appears to be suited as well as the nervous system to study the functions of glycans, which had been originally characterized outside the nervous system. In this review, we describe the multiple studies on the functions of LewisX, the human natural killer cell antigen-1 (HNK-1), as well as oligomannosidic and sialic (neuraminic) acids. We attempt to show the sophistication of these structures in ontogenetic development, synaptic function and plasticity, and recovery from trauma, with a view on neurodegeneration and possibilities to ameliorate deterioration. In view of clinical applications, we emphasize the need for glycomimetic small organic compounds which surpass the usefulness of natural glycans in that they are metabolically more stable, more parsimonious to synthesize or isolate, and more advantageous for therapy, since many of them pass the blood brain barrier and are drug-approved for treatments other than those in the nervous system, thus allowing a more ready access for application in neurological diseases. We describe the isolation of such mimetic compounds using not only Western NIH, but also traditional Chinese medical libraries. With this review, we hope to deepen the interests in this exciting field.

Post-natal developmental changes in the composition of the rat neocortical N-glycome

Asparagine-linked glycosylation (N-glycosylation) plays a key role in many neurodevelopmental processes, including neural cell adhesion, neurite outgrowth and axon targeting. However, little is known about the dynamics of N-glycosylation during brain development and, in particular, how the N-glycome of the developing neocortex differs from that of the adult. The aim of this study, therefore, was to perform a thorough characterization of N-glycosylation in both the adult and neonatal rat neocortex in order to gain insights into the types of changes occurring in the N-glycome during neurodevelopment. To this end, we used hydrophilic interaction ultraperformance liquid chromatography coupled to electrospray ionization quadrupole time-of-flight mass spectrometry to compare the adult neocortical N-glycome with that of 24- and 48-h neonates. We report that the abundance of complex N-glycans is significantly lower in adults compared with neonates. Furthermore, the proportion of charged complex N-glycans is also greatly reduced. This decrease in the abundance of complex N-glycans is offset by a corresponding increase in the proportion of truncated and, to a lesser extent, hybrid N-glycans. Lastly, we report that although the proportion of oligomannose N-glycans remains constant at around 24%, the distribution of high-mannose subtypes shifts from predominantly large subtypes in neonates to smaller subtypes in the adult. In summary, our findings indicate that N-glycan synthesis in the rat neocortex is fundamentally different in neonates compared with adults with a general shift occurring from large, sialylated N-glycans towards smaller, neutral structures as neonates develop into adults, coupled with a parallel shift towards smaller oligomannose structures.

Synapsin I is an oligomannose-carrying glycoprotein, acts as an oligomannose-binding lectin, and promotes neurite outgrowth and neuronal survival when released via glia-derived exosomes

Oligomannosidic glycans play important roles in nervous system development and function. By performing a phage display screening with oligomannose-specific antibodies, we identified an oligomannose-mimicking peptide that was functionally active in modulating neurite outgrowth and neuron-astrocyte adhesion. Using the oligomannose-mimicking peptide in crosslinking experiments, synapsin I was identified as a novel oligomannose-binding protein in mouse brain. Further analyses not only verified that synapsin I is an oligomannose-binding lectin, but also indicated that it is a glycoprotein carrying oligomannose and Lewis(x). We also found that synapsin I is expressed in glia-enriched cultures and is released from glial cells via exosomes. Incubation of glial-derived exosomes in the presence of high KCl concentrations or subjecting glial cell cultures to either oxygen/glucose deprivation or hydrogen peroxide resulted in release of synapsin I from exosomes. Application of synapsin I promoted neurite outgrowth from hippocampal neurons and increased survival of cortical neurons upon hydrogen peroxide treatment or oxygen/glucose deprivation. Coculture experiments using wild-type hippocampal neurons and wild-type or synapsin-deficient glial cells showed enhanced neurite outgrowth when synapsin was expressed by glial cells. Synapsin-induced neurite outgrowth was dependent on oligomannose on synapsin I and the neural cell adhesion molecule NCAM at the neuronal cell surface. The data indicate that, under conditions of high neuronal activity and/or oxidative stress, synapsin can be released from glial-derived exosomes and promotes neurite outgrowth and neuronal survival by modulating the interactions between glia and neurons.

Change of oligosaccharides of rat brain microsomes depending on dietary fatty acids and learning task

We have analyzed oligosaccharide chains in brain microsomes of rats fed an n-3 polyunsaturated fatty acid-deficient (safflower oil group; S group) or -rich (perilla oil group; P group) diet before and after brightness-discrimination learning tasks. The amount of concanavalin A-binding sites (mainly mannoside) of the brain microsomes was found to be significantly less in the S group than the P group before the learning task. Detailed analysis of glycoprotein glycans demonstrated that high mannose type oligosaccharides were dominant in brain microsomes before the learning task in both dietary groups, whereas multiantennary complex-type oligosaccharides became dominant after the learning task and especially a tetra-antennary glycan, that had a core structure of the glycan of neural cell adhesion molecule, was more increased in the S-group than the P group. When polysialylated glycans were analyzed on serotonin-conjugated HPLC column, the glycans in the S-group microsomes before the learning task contained larger amount of higher affinity-polysialylated glycans to serotonin column than those in the P-group, and also contained larger amount of phosphoglycans that showed also high affinity to serotonin column than the P-group. Removal of mannoside from microsomes by alpha-mannosidase-treatment changed the membrane surface physical property, especially permittivity, as revealed by analysis of the interaction with 1-anilinonaphthalene-8-sulfonate. These results suggest that high mannose content and several multiantennary glycans including polysialylated and phospho-glycans were changed by dietary n-3 fatty acid deficiency and learning task in rat brain microsomal glycoproteins and that these changes may affect membrane functions through changes of membrane surface physical properties and reactivity against serotonin.

Developmental changes in Asn-linked neutral oligosaccharides in murine cerebrum

The changes in Asn-linked oligosaccharide composition in the murine cerebrum during development have been examined by high-performance liquid chromatography (HPLC) and electrospray ionization mass spectrometry (ESI-MS). The oligosaccharides, obtained from murine cerebrum in several developmental stages, were separated by HPLC on anion-exchange and reverse-phase columns. We found that two Asn-linked oligosaccharides, designated oligosaccharide I and oligosaccharide II, had their expression changed during postnatal development. Whereas oligosaccharide I was reduced during brain development, oligosaccharide II was increased. The structures of oligosaccharides I and II were analyzed by ESI-MS and sequential exoglycosidase digestions. Judging from the molecular and fragment ions in each oligosaccharide, the oligosaccharide I was composed of 5Hex+2HexNAc+ABOE (MW 1467.2) and the oligosaccharide II was 3Hex+4HexNAc+DoHex+ABOE (MW 1695.2). The results of sequential exoglycosidase digestion indicated that the oligosaccharide I was an oligomannose type saccharide and the oligosaccharide II was a biantennary complex type saccharide including fucose. The proposed structures are shown below. These results offer an important clue to the role of Asn-linked oligosaccharides associated with development of the central nervous system.

Characterisation of a novel glycoprotein (AvGp50) in the avian nervous system, with a monoclonal antibody

A size fractionated lentil lectin-positive fraction derived from a deoxycholate extract of 1-day-old chick forebrain membranes was used to generate a series of monoclonal antibodies (Mabs) against neural antigens. One of these, MabSA1.7 recognises a glycoprotein which is enriched in synaptic plasma membranes, designated AvGp50. Polyacrylamide gel electrophoresis and Western blots show that AvGp50 is comprised of at least two glycoforms, with M(r)s of 53 kDa and 49 kDa respectively. AvGp50 is nervous system specific and most abundantly expressed in the forebrain, tecta and cerebellum where its pattern of expression is developmentally regulated. Immunohistochemical data localises AvGp50 to regions characterised by highly concentrated synapses, in particular, the molecular and granule cell layers of the cerebellum and in the inner and outer plexiform layers in the retina. Solubilization of the protein with the detergent Triton X-100 shows that AvGp50 is predominantly a cytoskeletally associated glycoprotein. However, when a synaptic plasma membrane fraction was treated with Triton X-114, AvGp50 partitioned into the detergent phase. Digestion of the protein with N-glycanase cleaved five N-linked carbohydrate side chains and reduced the molecular weight to approximately 34 and 31 kDa. Removal of the carbohydrate side chains led to an almost complete loss of recognition of the 34 kDa glycoform by the MabSA1.7, suggesting that the monoclonal antibody recognises a carbohydrate rather than peptide epitope.

Molecular cloning of calmodulin mRNA species which are preferentially expressed in neurons in the rat brain

A cDNA clone designated NGB, which was isolated from a rat brain expression library, detected two mRNA species of 1.8 and 4.0 kb which are highly enriched in brain tissue. cDNAs NGB1 and NGB2 corresponding to these two mRNAs have been isolated and characterized. Sequence data showed that both mRNA species contain the same open reading frames but differ in their 3' untranslated regions. The open reading frame encodes a calmodulin protein of 148 amino acids. Both mRNA species are derived from the rat CaMI gene by utilization of different polyadenylation addition sites. Analysis of the 3' untranslated sequence which is unique to the larger mRNA species revealed a putative AU-rich 'destabilizer' sequence which is thought to be involved in mechanisms of selective mRNA breakdown. In situ hybridization studies revealed that the two calmodulin mRNAs are expressed strongly in neuronal cells in the adult rat brain. Levels of the two mRNA species increased during early postnatal development.

Phosphorylation of proteins of the postsynaptic density: effect of development on protein tyrosine kinase and phosphorylation of the postsynaptic density glycoprotein, PSD-GP180

The effect of development on the tyrosine kinase activity of postsynaptic densities (PSDs) has been determined. PSDs were prepared from the forebrains of rats ranging in postnatal age from 13 to 90 days and the phosphorylation of both exogenous and endogenous substrates by tyrosine kinase measured. PSDs exhibited tyrosine kinase activity at all ages examined. Phosphorylation of the exogenous substrates polyglutamyltyrosine (4:1) and [val5] angiotensin II increased twofold between days 17 and 22 and then decreased between days 30 and 90 to levels slightly lower than those present at 13 days. The phosphorylation of endogenous PSD proteins on tyrosine residues, assessed by alkali digestion of polyacrylamide gels of 32P-labelled PSD proteins and by measuring the formation of [32P] phosphotyrosine by PSDs incubated in the presence of [gamma-32P] ATP, closely paralleled the changes in total tyrosine kinase activity. Tyrosine phosphorylation of the PSD-specific glycoprotein, PSD-GP180, also showed a transient increase between days 22 and 30, although its concentration within the PSD continued to increase slowly up to 90 days. The results indicate that the tyrosine kinase activity of PSDs is developmentally regulated and that tyrosine phosphorylation of PSD proteins is limited by enzyme rather than substrate availability.

Molecular cloning of SC1: a putative brain extracellular matrix glycoprotein showing partial similarity to osteonectin/BM40/SPARC

We describe the cloning of SC1, a novel cDNA that was selected from a rat brain expression library using a mixed polyclonal antibody directed against synaptic junction glycoproteins. SC1 detects a 3.2 kb mRNA expressed throughout postnatal development of the brain and present at high levels in the adult. In situ hybridization reveals that the SC1 mRNA is expressed widely in the brain and is present in many types of neurons. DNA sequence data suggest that the SC1 product is a secreted, calcium binding glycoprotein. Strikingly, the carboxy-terminal region of the SC1 protein shows substantial similarity to the extracellular matrix glycoprotein osteonectin/BM40/SPARC. These data are consistent with the hypothesis that SC1 is an extracellular matrix glycoprotein in the brain.

Dolichol-linked glycoprotein synthesis in developing mammalian brain: maturational changes of the N-acetylglucosaminylphosphotransferase

The enzyme UDP-N-acetylglucosamine:dolichyl phosphate, N-acetylglucosamine-1-phosphate transferase (GlcNAc-1-P transferase), the first committed step in the dolichol-linked oligosaccharide pathway for glycoprotein biosynthesis, has been studied in developing rat brain. The enzyme was shown to be localized in microsomes, particularly heavy microsomes, and to be activated by Mg2+ and inhibited by tunicamycin. Study of the enzyme with brain development demonstrated two prominent findings. First, the accentuation of enzymatic activity caused by addition of a saturating concentration of dolichyl phosphate was greater in brain of older (3-4 weeks of age and subsequently) animals (25-fold) than in brain of younger (less than two weeks of age) animals (10-fold). This difference suggests that dolichyl phosphate may be limiting for GlcNAc-1-P transferase activity in endoplasmic reticulum of the older animals. Second, a marked (3.5-fold) increase in activity occurred over a discrete time period (3-4 weeks of postnatal life) during brain development. That this increase reflected an increase in enzyme amount rather than in catalytic efficiency was suggested by kinetic studies. Coupled with our previous demonstrations of increases in brain dolichol, dolichol kinase activity, and dolichyl phosphate levels during approximately the same developmental period (Sakakihara, Y. and Volpe, J.J., Dev. Brain Res., 14 (1984) 225-262; Volpe, J.J. et al., Dev. Brain Res., in press), the data suggest a temporally discrete period of activation of the dolichol-linked pathway to glycoproteins. Whether the pathway is regulated coordinately or sequentially is a fertile topic for future study.

Characterization of fucosyl oligosaccharides associated with synaptic membrane and synaptic junctional glycoproteins

Rats were administered [3H]fucose by intracranial injection and synaptic membranes (SMs) isolated 18 h later. Oligosaccharides associated with SM glycoproteins were prepared by hydrazinolysis and analyzed by a combination of affinity chromatography on concanavalin A (Con A)-agarose, ion-exchange chromatography on DEAE-cellulose, and gel permeation chromatography. Most (94%) of the [3H]fucose-labelled oligosaccharides were present in the fraction that did not bind to Con A. Of these 41% did not bind to DEAE-cellulose, indicating the absence of negatively charged groups and the remainder were resolved into four fractions of increasing acidity. Gel permeation chromatography of the fractions from the DEAE-cellulose column suggested that the major oligosaccharides corresponded to fucosylated triantennary structures containing varying amounts of sialic acid although more highly branched structures containing peripheral branches lacking one or more sugars may also have been present. Comparison of fucosyl oligosaccharides associated with SMs prepared from 10- and 28-day-old animals indicated that although the general oligosaccharide content was similar at both ages, membranes from younger animals were characterized by an increase in the proportion of highly acidic structures. Fucosylated glycans derived from synaptic junctional (SJ) glycoproteins were also characterized by a greater percentage of highly acidic structures than SMs. The results indicate that SMs and SJs are characterized by specific complements of fucosylated glycoprotein oligosaccharides.

Characterization of gp 50, a major glycoprotein present in rat brain synaptic membranes, with a monoclonal antibody

Several cell lines secreting monoclonal antibodies (Mabs) against a major forebrain synaptic membrane (SM) glycoprotein, gp 50, have been raised. Western blots show that the Mabs react with a polypeptide doublet of Mrs 49 and 45 kDa. These polypeptides exist solely in a concanavalin A (Con A) binding form. Removal of the Con A receptors by digestion with endo-beta-N-acetylglucosaminidase H (endo H) lowers the Mrs of the glycoprotein doublet to 36.5 and 34 kDa. Western blots of 2D polyacrylamide gels indicate that gp 50 exists in several isoforms. Solid phase radioimmunoassay (RIA) and Western blots of brain subcellular fractions show the antigenic material to be concentrated in the SM fraction, but to be present in much lower amounts in synaptic junctions and postsynaptic densities. Gp 50 appears to be brain specific. Regional distribution studies show that it is present in all brain regions but is two-fold concentrated in cerebellum, brainstem and midbrain compared to forebrain. Immunocytochemical studies of several brain regions show that gp 50-like immunoreactivity is neuron specific and is concentrated in selected neuronal species, particularly granule cells. In both cerebellar and hippocampal granule cells gp 50-like immunoreactivity is localized in the perikarya and primary dendrites. Though immunocytochemistry did not show staining of synaptic regions this may be due to masking of the reactive epitope. The results are discussed in terms of the molecular properties of gp 50 and its subcellular localization in brain tissue.

Dolichol kinase and the regulation of dolichyl phosphate levels in developing brain

The developmental changes of dolichol kinase activity and dolichyl phosphate levels have been studied in rat brain. Because both dolichol kinase activity and dolichyl phosphate were enriched in microsomes, detailed study of this subcellular fraction was carried out. Dolichol kinase specific activity in brain microsomes increased postnatally 3-fold to a maximum at ca. 30 days of age. This increase was observed whether exogenous dolichol was present or not and whether Zn2+ or Ca2+ was utilized as the cation for the enzyme. Zn2+ was the most effective cation in developing brain, as we have shown previously for adult brain (Sakakihara, Y. and Volpe, J.J., J. Biol. Chem., 260 (1985) 15413-15419). Although the Vmax for the enzyme increased by three-fold with development, the Km for dolichol and for CTP did not change, indicating that the developmental increase was not related to an alteration in catalytic efficiency of the enzyme. A striking and parallel increase in dolichyl phosphate levels in brain microsomes was defined with development. Levels were lowest in one-day-old animals and then increased ca. 13-fold to a maximum at 30 days of postnatal age. The parallel increase in dolichol kinase activity and dolichyl phosphate levels in microsomes of developing brain suggests that dolichol kinase is the principal determinant of cellular levels of dolichyl phosphate, the critical intermediate in the dolichol-linked pathway to glycoproteins.

Lectins demarcate the barrel subfield in the somatosensory cortex of the early postnatal mouse

Plant lectins were used to examine the disposition of glycosylated molecules in vibratome sections through the barrel subfield of mouse somatosensory cortex at selected times during postnatal development. The peroxidase conjugates of peanut agglutinin (PNA, specific for N-acetylgalactosamine), concanavalin A (specific for mannose), and wheat germ agglutinin (specific for N-acetylglucosamine and N-acetylneuraminic acid) were used to study lectin binding in aldehyde-fixed tissue sections of cortex. Following peroxidase cytochemistry and light microscopy, it was found that all three lectins bound in the region of the barrel subfield as early as postnatal day 3 (day of birth = postnatal day 1). The lectins bound to the prospective sides and/or septae of individual barrels in preference to the prospective hollows. This lectin demarcation of the barrel field occurred prior to the detection of this region with cresyl violet staining and was still demonstrable on postnatal day 6, when the individual barrels became discernible with cresyl violet. This suggests that the lectin binding material is present before the barrel field becomes a fully formed and organized region. A decrease in lectin affinity for binding sites in these tissue sections occurs during postnatal development (Cooper and Steindler: Soc. Neurosci. (Abstr.) 10: 43a, '84) and this study demonstrates that lectins do not delineate the barrel field of more mature animals (2-3 months old), whereas barrels can be detected with cresyl violet at this time. A preliminary electron microscope analysis of the postnatal day 6 somatosensory cortex demonstrates that the lectin PNA binds to elements of the forming neuropil and also to Golgi apparatus intermediate saccules in neuronal cells. The prospective barrel field can be detected with lectins during a critical period in development in which alterations can occur in the barrel field in response to peripheral deprivation (Jeanmonod et al: Neuroscience 6:1503-35, '81) and therefore we suggest that the glycans visualized with lectin-peroxidase conjugates denote possible candidates for molecules involved in shaping barrel structure.

Isolation of postsynaptic densities from day-old chicken brain

Synaptic plasma membranes from chicken brain were used to isolate a postsynaptic density (PSD) fraction using an aqueous two-phase polymer system and the detergent n-octyl glucoside. The protein and glycoprotein composition and the morphology of the day-old chicken brain PSD fraction were compared with a PSD fraction isolated from 12-week-old chicken brain. The PSD fraction from day-old chicken brain contained predominantly PSDs although, like the fraction from 12-week-old chicken, there was some membrane contamination. The major polypeptides in the day-old chicken fraction resolved by polyacrylamide gel electrophoresis comigrated with alpha- and beta-tubulin (Mr 57,000 and 55,000) and actin (Mr 45,000). The major PSD polypeptide (mPSDp) of 12-week-old chicken forebrain, which has a molecular weight of 52,000 was not a major component in day-old chicken. A polypeptide of molecular weight 63,000 was also far more prominent in the 12-week-old chicken PSD fraction whereas the reverse was true for a polypeptide of 31,000. Day-old chicken brain PSDs contained at least 14 concanavalin A-binding glycoproteins of high (greater than 85,000) molecular weight, the two most prominent having molecular weights of 170,000 and 180,000. In contrast to the polypeptide composition, the glycoprotein pattern of day-old chicken PSDs was very similar to that of the 12-week-old bird. Intraperitoneally injected [3H]fucose was incorporated into the glycoproteins of synaptic plasma membranes and PSDs from day-old chickens.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of the postsynaptic density glycoprotein gp180 by Ca2+/calmodulin-dependent protein kinase

Postsynaptic densities (PSDs) were prepared by the aqueous two-phase extraction of synaptic membranes in the presence of n-octyl glucoside. Incubation of postsynaptic densities with [gamma-32P]ATP resulted in the incorporation of 32P into a range of proteins. Isolation of glycoproteins from 32P-labelled PSDs by affinity chromatography on concanavalin A-agarose identified the postsynaptic glycoprotein of apparent Mr 180,000 (gp180) as a substrate for endogenous protein kinase(s). When the phosphorylation reaction was performed in the presence of Ca2+ and calmodulin, there was an overall 13-fold increase in the phosphorylation of PSD proteins. The largest effects of calmodulin were associated with two proteins of molecular weights 51,000 and 60,000, which showed average calmodulin-dependent increases in phosphorylation of 68-fold. The phosphorylation of gp180 was increased 7.5-fold in the presence of calmodulin. Fifty percent of maximum phosphorylation of proteins and glycoproteins occurred with a free Ca2+ concentration of 0.3 X 10(-6) M. The amounts 12.6 micrograms/ml and 9.1 micrograms/ml of calmodulin were required for 50% of maximum phosphorylation of proteins and glycoproteins, respectively. Peptide mapping experiments identified three major phosphorylation sites in gp180. The phosphorylation of all three sites was increased in the presence of calmodulin. Phosphoamino acid analysis of gp180 revealed that [32P]phosphoserine and [32P]phosphothreonine were both produced during the phosphorylation reaction, with phosphoserine being the predominant product. The phosphorylation of both amino acids was increased in the presence of calmodulin. [32P]phosphotyrosine was also identified as a product of the phosphorylation of gp180.

In vivo phosphorylation of the postsynaptic density glycoprotein gp180

Rats received intraventricular injections of [32P]PO4 and were killed after 30 min for the preparation of postsynaptic densities (PSDs). Gel electrophoretic analysis identified a number of PSD proteins that incorporated 32P under these conditions. Major proteins that were labelled with 32P had Mr of 185,000, 165,000, 140,000, 92,000, and 51,000. Of these p185, p165, and p140 were also labelled when PSDs were incubated with [gamma-32P]ATP in vitro. In contrast p92 and p51 were relatively poorly labelled under in vitro conditions. Analysis of glycoproteins isolated by chromatography on concanavalin A (Con A)-agarose demonstrated that greater than 70-80% of the 32P present in the glycoproteins eluted from Con A-agarose with alpha-methyl-D-mannopyranoside (Con A+ glycoproteins) was associated with the PSD specific glycoprotein gp180 following both in vivo and in vitro labelling. Phosphopeptide maps and phosphoamino acid analysis of gp180 indicated that similar sites were labelled in vitro and in vivo. Analysis of the subcellular distribution of glycoproteins that incorporated 32P during in vivo labelling demonstrated that gp180 was highly concentrated in PSDs, in accord with the previously suggested exclusive association of this glycoprotein with postsynaptic structures.

Phosphorylation of the postsynaptic density glycoprotein gp180 by endogenous tyrosine kinase

Incubation of postsynaptic densities (PSDs) with [gamma-32P]adenosine triphosphate (ATP) results in the phosphorylation of a number of proteins. Of these, phosphoproteins with apparent molecular weights (Mr) of 51,000, 180,000, 300,000, 320,000 and 370,000 contain 32P which is resistant to digestion with hot KOH suggesting the presence of [32P]phosphotyrosine residues. Phosphoamino acid analysis of total 32P-labelled PSDs identified [32P]phosphotyrosine as well as phosphoserine and phosphothreonine as products of the phosphorylation reaction. The PSD-specific glycoprotein gp180 was isolated from 32P-labelled PSDs and shown to contain [32P]phosphotyrosine. The results identify tyrosine kinase as a component of purified PSDs and gp180 as an endogenous substrate for this enzyme.

Dolichol in human brain: regional and developmental aspects

Distinct regional differences in dolichol content were defined in human brain from 15 to 76 years of age. Concerning the regional distribution of dolichol, levels were: higher in cortical gray matter than in subcortical white matter, highest among cortical regions in temporal gray matter, highest among all brain regions in thalamus, and lowest among all brain regions in lower brain stem and spinal cord. The developmental changes in the contents of dolichol were found to be different among brain regions. For example, among regions with the highest levels of dolichol, in thalamus there was a six to sevenfold increase, but in parietal gray matter, only a 2.5-fold increase. Regional and developmental changes in the proportions of the individual molecular species (isoprenologues) of dolichol were also observed. The findings indicate that the metabolism of dolichol is not uniform among regions of developing and aging human brain and may have implications for the role of dolichol in normal and diseased human brain.