The Arabidopsis proteasome RPT5 subunits are essential for gametophyte development and show accession-dependent redundancy

We investigated the role of the ubiquitin proteasome system (UPS), which allows proteins to be selectively degraded, during gametophyte development in Arabidopsis thaliana. Three mutant alleles altering the UPS were isolated in the Wassilewskija (Ws) accession: they affect the Regulatory Particle 5a (RPT5a) gene, which (along with RPT5b) encodes one of the six AAA-ATPases of the proteasome regulatory particle. In the heterozygous state, all three mutant alleles displayed 50% pollen lethality, suggesting that RPT5a is essential for male gametophyte development. However, a fourth mutant in the Columbia (Col) accession did not display such a phenotype because the RPT5b Col allele complements the rpt5a defect in the male gametophyte, whereas the RPT5b Ws allele does not. Double rpt5a rpt5b mutants showed a complete male and female gametophyte lethal phenotype in a Col background, indicating that RPT5 subunits are essential for both gametophytic phases. Mitotic divisions were affected in double mutant gametophytes correlating with an absence of the proteasome-dependent cyclinA3 degradation. Finally, we show that RPT5b expression is highly increased when proteasome functioning is defective, allowing complementation of the rpt5a mutation. In conclusion, RPT5 subunits are not only essential for both male and female gametophyte development but also display accession-dependent redundancy and are crucial in cell cycle progression.

Somatostatin binding sites in the white matter of the developing human brainstem: inverse relationship with the myelination process

The ontogeny of somatostatin binding sites was studied in eight fiber tracts of the human lower brainstem in 18 fetuses and infants aged from 21 weeks postconceptional to 6 months postnatal, and in two adults. The study was performed by means of quantitative autoradiography using [125I-Tyr0,DTrp8]somatostatin-14 as a radioligand. For all structures examined, the highest densities of binding sites were detected in the younger stages and the density of sites decreased during development. These results reveal the existence of a close inverse relationship between the density of somatostatin receptors and the myelination process in the fiber tracts of the human brainstem.

Increased density of somatostatin binding sites in respiratory nuclei of the brainstem in sudden infant death syndrome

Sudden infant death syndrome is the primary cause of mortality in children aged one to six months in industrialized countries. Although the etiology of this syndrome is still unknown, subtle abnormalities in the neuronal circuitry involved in the control of respiratory activity are suspected. Since stereotaxic administration of somatostatin in the brainstem of rat and cat produces fatal apnea, we have compared the densities of somatostatin binding sites in the respiratory centers of 11 cases of sudden infant death syndrome and six control infants without neuronal disease. The density of binding sites was measured in 17 structures of the pons and medulla oblongata by means of quantitative in vitro autoradiography using iodinated [Tyr0,D-Trp8]somatostatin-14 as a radioligand. The density of somatostatin binding sites was significantly higher in the medial and lateral parabrachial nuclei in the sudden infant death syndrome group than in the control group. In six other nuclei, the median of the receptor density was higher in the sudden infant death syndrome group than the maximum values measured in the control group. The presence of high concentrations of somatostatin binding sites in several respiratory nuclei of the brainstem in approximately half of the sudden infant death syndrome victims suggests that the decrease in receptor density that normally occurs during ontogeny was delayed in these infants. In particular, the high level of somatostatin binding sites in the medial and lateral parabrachial nuclei of sudden infant death syndrome suggests that the delayed maturation of these receptors may be associated with a deficit of the hyperventilatory response to hypoxia.

Ontogeny of somatostatin binding sites in respiratory nuclei of the human brainstem

The ontogeny of somatostatin binding sites was studied in 16 respiratory nuclei of the human brainstem, from 19 postconceptional weeks to 6 months postnatal, by quantitative autoradiography using [(125)I-Tyr0,DTrp8]S14 as a radioligand. In the early gestational stages (19-21 postconceptional weeks), moderate to high concentrations of [(125)I-Tyr0,DTrp8]S14 binding sites were found in all nuclei, the highest density being measured in the locus coeruleus. From 19 weeks of fetal life to 6 months postnatal, a decrease in the density of labeling was observed in all nuclei. The most dramatic reduction in site density (80-90%) was found in the ventral part of the nucleus medullae oblongata lateralis and in the nucleus paragigantocellularis lateralis. A 70-80% decrease was detected in the dorsal part of the nucleus tractus solitarius, the nucleus nervi hypoglossi, the ventral part of the nucleus medullae oblongatae centralis, the nucleus ambiguus, the nucleus paragigantocellularis dorsalis, and the nucleus gigantocellularis, and a 60-70% decrease in the nucleus parabrachialis medialis, the ventrolateral and ventromedial parts of the nucleus tractus solitarius, and the nucleus praepositus hypoglossi. A 50-60% decrease was observed in the caudal part of the nucleus tractus solitarius, the nucleus dorsalis motorius nervi vagi, and the nucleus parabrachialis lateralis, whereas in the nucleus locus coeruleus, the concentration of recognition sites decreased by only 30%. The profiles of the decrease in site density differed in the various structures. In the majority of the nuclei, a gradual diminution of binding density was observed either throughout the developmental period studied or mainly during fetal life. Conversely, in two nuclei, i.e., the nucleus parabrachialis lateralis and the locus coeruleus, an abrupt decrease occurred around birth. The differential decrease in the density of somatostatin binding sites observed in respiratory nuclei during development, together with the observation that microinjection of somatostatin in some of these nuclei causes ventilatory depression and apnea, strongly suggests that the somatostatinergic systems of the human brainstem are involved in the maturation of the respiratory control.

Distribution of somatostatin receptors in the adult human brainstem

The neuropeptide somatostatin is widely distributed in the central nervous system of rat and human. Somatostatin-containing neurons are particularly abundant in the hypothalamus, the cerebral cortex and the limbic system. Somatostatin is also present in a number of discrete structures in the brainstem and spinal cord. The localization of somatostatin receptors provides valuable information regarding the possible roles of the peptide in the brain. In the present study, we have investigated the precise distribution of somatostatin binding sites in the human lower brainstem by quantitative autoradiography, using [125I- Tyr0,DTrp8]S14 as a radioligand. The tissues were collected from two individuals, aged 50 and 67 years, who had no antecedent of neurological disorders. The binding of the radioligand was visualized in 73 distinct anatomical regions of the medulla and pons and quantified by computer-assisted image analysis. Somatostatin binding sites were present in sensory nuclei, the highest densities being observed in the trigeminal complex (spinalis oralis and interpolaris) and in the nucleus (N.) tractus solitarii. Moderate to low densities of binding sites were detected in the N. vestibularis medialis and spinalis, and in the N. nervus trigemini sensibilis principalis. Many relay nuclei of the ascending somatosensory pathways contained moderate to high densities of binding sites: the inferior olivary complex, the N. arcuatus and the N. praepositus hypoglossi. Binding sites were also present in several motor nuclei such as the N. nervi hypoglossi, the N. dorsalis motorius nervi vagi, the N. nervi facialis and the N. nervi abducentis. Moderate to low concentrations of binding sites were detected in nuclei related to the reticular formation including the N. raphae pallidus, the N. parabrachialis and the N. supratrochlearis. The N. locus coeruleus exhibited a very high concentration of somatostatin binding sites in both individuals. The present data, together with previous studies on the distribution of somatostatin-immunoreactive fibers in the human brainstem, suggest that somatostatin may be involved in (i) sensory processes including vestibular sensitivity, somatosensoriality and proprioception, (ii) sleep-waking cycle and arousal and (iii) control of various neurovegetative functions including regulation of cardiovascular and respiratory activities as well as gastric acid secretion.

Anatomical distribution of somatostatin receptors in the brainstem of the human fetus

The distribution of somatostatin binding sites was studied in the pons and medulla oblongata of three human fetuses (gestional ages 26, 28 and 30 weeks). The study was carried out by in vitro quantitative autoradiography using either [125I-Tyr0,D-Trp8]somatostatin-14 or [125I-Tyr11]somatostatin-14 as radioligands. Somatostatin binding sites were observed in a number of nuclei subserving sensory, motor or integrative functions within the pons and medulla. In addition, discrete tracts also contained significant amounts of binding sites. Among structures involved in sensory processes, a high density of binding sites (40-60 fmol/ mg wet tissue) was measured in the dorsal cochlear nucleus and in the nucleus tractus spinalis trigemini caudalis. Moderate to high levels of binding sites (30-40 fmol/mg wet tissue) were detected in the other sensory cranial nerve nuclei. A moderate density of sites (15-30 fmol/mg wet tissue) was measured in most motor nuclei, the highest concentrations being observed in the dorsal motor nucleus of the vagus nerve, the facial nucleus, the hypoglossal nucleus and the nucleus ambiguus. The griseum pontis and the nucleus corporis pontobulbaris contained very high (> 60 fmol/mg wet tissue) and high concentrations of somatostatin binding sites, respectively, while the other relay nuclei contained low to moderate levels of binding. In monoaminergic nuclei, very high and moderate to high concentrations of somatostatin binding sites were measured in the nucleus locus coeruleus and in its dorsal subnucleus, respectively. Moderate densities of sites were detected in the ventral subnucleus of the nucleus locus coeruleus and in the different parts of the raphe. In the white matter, low levels of binding were measured in the inferior cerebellar peduncle, the lateral and median lemnisci and the tractus solitarius. Conversely, moderate to high concentrations of somatostatin binding sites were measured in the median and superior cerebellar peduncles. The pyramis contained a very high density of recognition sites. A marked heterogeneity in the density of binding sites was observed within a few structures particularly in the medial accessory olivary of nucleus and the medial longitudinal fasciculus. Selective ligands were used to determine the pharmacological profile of the [Tyr11]somatostatin-14 binding sites in various brainstem regions. In the dorsal cochlear nucleus and the pyramis, all somatostatin binding sites belonged to the SSA subtype. Conversely, in the lateral paragigantocellular nucleus, all somatostatin binding sites belonged to the SSB subtype. The other regions studied contained various proportions of SSA and SSB subtypes. In conclusion, the present study shows that high concentrations of somatostatin receptors are present in many regions of the human fetus brainstem. These data support the concept that somatostatin could be involved in the maturation of brain structures.

ERGIC-53, a membrane protein of the endoplasmic reticulum-Golgi intermediate compartment, is identical to MR60, an intracellular mannose-specific lectin of myelomonocytic cells

A mannose-specific membrane lectin (MR60) isolated from human myelomonocytic HL60 cells by affinity chromatography is expressed in intracellular organelles of immature monocytes (Pimpaneau, V., Midoux, P., Monsigny, M., and Roche, A. C. (1991) Carbohydr. Res. 213, 95-108). It is not present at the cell surface and is immunochemically and structurally distinct from the M(r) 175,000 mannose receptor of mature macrophages. MR60 cDNA was isolated and characterized; on the basis of its sequence, MR60 is not related to any known mammalian lectins. Surprisingly, MR60 was found to be identical to ERGIC-53 (Schindler, R., Itin, C., Zerial, M., Lottspeich, F., and Hauri, H.P. (1993) Eur. J. Cell Biol. 61, 1-9), a type I integral membrane protein, defined as a marker of the intermediate compartment that recycles between the Golgi apparatus and endoplasmic reticulum; MR60/ERGIC-53 shares with VIP-36 significant homologies with leguminous plant lectins (Fiedler, K., and Simmons, K. (1994) Cell 77, 625-626). We extend these findings in evidencing a structural homology between MR60/ERGIC-53 and mammalian galectins (soluble beta galactose binding proteins). MR60/ERGIC-53 is the first lectin characterized as an endoplasmic reticulum-Golgi protein. Accordingly, this intracellular mannose binding protein could be involved in the traffic of glycoproteins between endoplasmic reticulum and the Golgi apparatus.

Characterization and cellular localization by monoclonal antibodies of the 60 kDa mannose specific lectin of human promyelocytic cells, HL60

Myelomonocytic lineage cells express an M(r) 60,000 mannose specific lectin, MR60 (Pimpaneau et al. (1991), Carbohydr Res 213: 95-108). Under non-reducing conditions, this protein migrates as a 120,000 protein. MR60 does not contain any N-glycan moiety cleavable by the action of N-glycanase. MR60 induces a sugar selective aggregation of beads coated with glycosylated albumin: beads bearing alpha-D-mannosyl residues are aggregated while beads bearing alpha-D-glucosyl residues are not. A monoclonal antibody Lec101B, specific for MR60, recognizes a single M(r) 60,000 protein by Western blotting. This monoclonal antibody does not label the cell surface of cells expressing MR60, but decorates intracellular vesicles upon permeabilization of these cells.

Sugar-dependent nuclear import of glycoconjugates from the cytosol

Large proteins (above 40 kDa) do not migrate from the cytosol to the nucleus except when they contain or are substituted with specific peptides called nuclear localization sequence. Accordingly fluorescein-labeled serum albumin, introduced into the cytosol upon electroporation, does not leave the cytosol. Conversely the same protein substituted with about 25 sugar residues migrate from the cytosol to the nucleus in a manner dependent on the nature of the sugar linked to serum albumin, on the temperature, and on the incubation time. Serum albumin substituted with glucose, fucose, or mannose residues enter the nucleus at 37 degrees C within 30 min while sugar-free serum albumin or serum albumin substituted with galactoside or 6-phosphomannoside residues do not. At 4 degrees C, all of those proteins stay in the cytosol. Thus, glycosylated proteins can enter the nucleus from the cytosol by making use of the sugars borne either by a new type of nuclear import or by making a complex with lectins acting as shuttle between cytosol and nucleus.

Endocytosis of alpha 1-acid glycoprotein variants by human monocytic lineage cells

Human alpha 1-acid glycoprotein (AGP or orosomucoid) is a major glycoprotein of plasma. AGP can be separated on immobilized concanavalin A into three variants bearing none (AGP A), one (AGP B) or two (AGP C) biantennary glycans. In this paper, we show, using flow cytometry and confocal microscopy, that AGP C which is eluted from concanavalin A with mannose, binds to human monocytes, monocyte-derived macrophages as well as human promonocytic cell lines such as THP1 or U937. Conversely HL60, a promyelocytic cell line, does not express the surface AGP C binding protein. AGP C is internalized and degraded with an efficiency depending on the state of differentiation of these cells. In contrast, AGP A which is not recognized by concanavalin A, does not bind to any of these cells.