ANGPTL7 and Its Role in IOP and Glaucoma

Purpose

Loss-of-function variants in the ANGPTL7 gene are associated with protection from glaucoma and reduced intraocular pressure (IOP). We investigated the role of ANGPTL7 in IOP homeostasis and its potential as a target for glaucoma therapeutics.

Methods

IOP, outflow facility, and outflow tissue morphology of Angptl7 knockout (KO) mice were assessed with and without dexamethasone (Dex). ANGPTL7 was quantified in conditioned media from human trabecular meshwork cells in response to Dex, in effluent from perfused human donor eyes, and in aqueous humor from human patients treated with steroids. Antibodies to ANGPTL7 were generated and tested in three-dimensional (3D) culture of outflow cells and perfused human donor eyes. Rabbits were injected intravitreally with a neutralizing antibody targeting ANGPTL7, and IOP was measured.

Results

IOP was significantly elevated, but outflow facility and outflow tissue morphology were not different between Angptl7 KO mice and littermates. When challenged with Dex, IOP increased in wild-type but not Angptl7 KO mice. In human samples, increased ANGPTL7 was seen in the aqueous humor of patients treated with steroids, regardless of glaucoma status. Using 3D culture, recombinant ANGPTL7 decreased, and ANGPTL7-blocking antibodies increased hydraulic conductivity. Significantly, outflow facility increased in human eyes treated ex vivo with ANGPTL7-blocking antibodies, and IOP decreased for 21 days in rabbits after a single injection of blocking antibodies.

Conclusions

Using multiple models, we have demonstrated that excess ANGPTL7 increases outflow resistance and IOP and that neutralizing ANGPTL7 has beneficial effects in both naïve and steroid-induced hypertensive eyes, thus motivating the development of ANGPTL7-targeting therapeutics for the treatment of glaucoma.

Testing the role of sensory systems in the migratory heading of a songbird

The identification of the sensory cues and mechanisms by which migratory birds are able to reach the same breeding and wintering grounds year after year has eluded biologists despite more than 50 years of intensive study. While a number of environmental cues have been proposed to play a role in the navigation of birds, arguments still persist about which cues are essential for the experience based navigation shown by adult migrants. To date, few studies have tested the sensory basis of navigational cues used during actual migration in the wild: mainly laboratory based studies or homing during the non-migratory season have been used to investigate this behaviour. Here we tested the role of olfactory and magnetic cues in the migration of the catbird (Dumetella carolinensis) by radio tracking the migration of birds with sensory manipulations during their actual migratory flights. Our data suggest that adult birds treated with zinc sulphate to produce anosmia were unable to show the same orientation as control adults, and instead reverted to a direction similar to that shown by juveniles making their first migration. The magnetic manipulation had no effect on the orientation of either adults or juveniles. These results allow us to propose that the olfactory sense may play a role in experience based migration in adult catbirds. While the olfactory sense has been shown to play a role in the homing of pigeons and other birds, this is the first time it has been implicated in migratory orientation.

Turnover and degradation of mitochondria and their proteins

Very little is known about intracellular protein degradation. We have concentrated on mitochondrial proteins for simplicity. Even this is a complex problem and several mechanisms may be involved. The half-lives of mitochondrial proteins vary widely. Using immunological, autoradiographic (ferritin and iodine) and biochemical techniques we are delineating the extent of intra-mitochondrial protein degradation, lysosomal participation in that degradation, and signals controlling the overall process. As a high yielding dividend of our autoradiographic and immunological experiments it seems possible to clarify such important questions as whether there is a selective inactivation and turnover of mitochondria and their proteins. Calculations of autophagy and vacuoles, the concentration of mitochondrial proteins within lysosomes, as well as key aspects of the initiation and regulation of protein turnover and its elasto-plastic control are presented.

[Identification and characterization of the first Spanish familial ligand-defective apolipoprotein B homozygote]

BACKGROUND

Familial ligand-defective apolipoprotein B 100 (FDB) is an autosomal inherited disease due to mutations on apo B 100, clinically indistinguishable from familial hypercholesterolemia (FH). We described the first Spanish homozygote for FDB.

METHODS

We have screened R3500Q mutation of apo B gene (PCR-SSCP analysis) in a large family with FDB and have identified the first Spanish homozygote for FDB.

RESULTS

The homozygote is a 58 year-old man with coronary heart disease, no presence of xanthomata and with total cholesterol and LDL cholesterol plasma levels of 415 and 352 mg/dl. The response to statins and resins was up to 42% for total cholesterol and 51% for LDLc plasma values. The LDL receptor activity was normal in the FDB homozygote.

CONCLUSIONS

We have identified and characterised the first Spanish homozygote for FDB (R3500Q mutation). Our data indicate a moderate lipoprotein phenotype in FDB homozygote, different as expected comparing to homozygous FH.

Import of a cytosolic protein into lysosomes by chaperone-mediated autophagy depends on its folding state

We have analyzed the folding state of cytosolic proteins imported in vitro into lysosomes, using an approach originally developed by Eilers and Schatz, (Eilers, M., and Schatz, G. (1986) Nature 322, 228-232) to investigate protein import into mitochondria. The susceptibility toward proteases of mouse dihydrofolate reductase (DHFR), synthesized in a coupled transcription-translation system with rabbit reticulocytes, decreased in the presence of its substrate analogue, methotrexate. This analogue complexes with high affinity with the in vitro synthesized DHFR and locks it into a protease-resistant folded conformation. DHFR was taken up by freshly isolated rat liver lysosomes and methotrexate reduced this uptake by about 80%. A chimeric DHFR protein, which carries the N-terminal presequence of subunit 9 of ATP synthase preprotein from Neurospora crassa fused to its N terminus, was taken up by lysosomes more efficiently. Again, methotrexate abolished the lysosomal uptake of the fusion protein, which was partially restored by washing of methotrexate from DHFR or by adding together methotrexate and dihydrofolate, the natural substrate of DHFR. Immunoblot analysis with anti-DHFR of liver lysosomes and of other fractions, isolated from rats starved for 88 h and treated with lysosomal inhibitors, suggests that DHFR is degraded by chaperone-mediated autophagy. Competition with ribonuclease A and stimulation by ATP/Mg(2+) and the heat shock cognate protein of 73 kDa show that the lysosomal uptake of the fusion protein also occurs by this pathway. It is concluded that the lysosomal uptake of cytosolic proteins by chaperone-mediated autophagy mainly occurs by passage of the unfolded proteins through the lysosomal membrane. Therefore, this mechanism is different from protein transport into peroxisomes, but similar to the import of proteins into the endoplasmic reticulum and mitochondria.

Electrothermal atomic absorption spectrometric diagnosis of familial hypercholesterolemia

We have developed a new nonradioactive assay to identify human low-density lipoprotein receptor defects. It is based on the incubation of cultured cells with colloidal gold-LDL conjugates and quantitation of the gold associated with the cells by electrothermal atomic absorption spectrometry. After an oxidative treatment with nitric and hydrochloric acids, the biological matrix interferes neither with the gold recovery nor with the gold measurements, which are linear, at least from 0.15 to 3 ng of gold. When cells expressing a functional LDL receptor are incubated with increasing amounts of colloidal-gold LDL conjugates, the obtained saturation curve parallels that described when [125I]LDL is used as ligand. Moreover, this new assay allows us to clearly distinguish among fibroblasts from normal subjects or from heterozygous or homozygous patients of familial hypercholesterolemia, a very common autosomal disease. The assay is easy to perform, is sensitive, and avoids the use of radioactive compounds. Therefore, it could be successfully employed in the clinical diagnosis of this disease. Furthermore, since the methodology developed here can be applied to quantify the association of other gold-conjugated ligands to cells, it could have a widespread use in a variety of clinical and basic research studies.

Subcellular localization of proteasomes and their regulatory complexes in mammalian cells

Proteasomes can exist in several different molecular forms in mammalian cells. The core 20S proteasome, containing the proteolytic sites, binds regulatory complexes at the ends of its cylindrical structure. Together with two 19S ATPase regulatory complexes it forms the 26S proteasome, which is involved in ubiquitin-dependent proteolysis. The 20S proteasome can also bind 11S regulatory complexes (REG, PA28) which play a role in antigen processing, as do the three variable gamma-interferon-inducible catalytic beta-subunits (e.g. LMP7). In the present study, we have investigated the subcellular distribution of the different forms of proteasomes using subunit specific antibodies. Both 20S proteasomes and their 19S regulatory complexes are found in nuclear, cytosolic and microsomal preparations isolated from rat liver. LMP7 was enriched approximately two-fold compared with core alpha-type proteasome subunits in the microsomal preparations. 20S proteasomes were more abundant than 26S proteasomes, both in liver and cultured cell lines. Interestingly, some significant differences were observed in the distribution of different subunits of the 19S regulatory complexes. S12, and to a lesser extent p45, were found to be relatively enriched in nuclear fractions from rat liver, and immunofluorescent labelling of cultured cells with anti-p45 antibodies showed stronger labelling in the nucleus than in the cytoplasm. The REG was found to be localized predominantly in the cytoplasm. Three- to six-fold increases in the level of REG were observed following gamma-interferon treatment of cultured cells but gamma-interferon had no obvious effect on its subcellular distribution. These results demonstrate that different regulatory complexes and subpopulations of proteasomes have different distributions within mammalian cells and, therefore, that the distribution is more complex than has been reported for yeast proteasomes.

Subcellular localization of proteasomes and their regulatory complexes in mammalian cells

Proteasomes can exist in several different molecular forms in mammalian cells. The core 20S proteasome, containing the proteolytic sites, binds regulatory complexes at the ends of its cylindrical structure. Together with two 19S ATPase regulatory complexes it forms the 26S proteasome, which is involved in ubiquitin-dependent proteolysis. The 20S proteasome can also bind 11S regulatory complexes (REG, PA28) which play a role in antigen processing, as do the three variable gamma-interferon-inducible catalytic beta-subunits (e.g. LMP7). In the present study, we have investigated the subcellular distribution of the different forms of proteasomes using subunit specific antibodies. Both 20S proteasomes and their 19S regulatory complexes are found in nuclear, cytosolic and microsomal preparations isolated from rat liver. LMP7 was enriched approximately two-fold compared with core alpha-type proteasome subunits in the microsomal preparations. 20S proteasomes were more abundant than 26S proteasomes, both in liver and cultured cell lines. Interestingly, some significant differences were observed in the distribution of different subunits of the 19S regulatory complexes. S12, and to a lesser extent p45, were found to be relatively enriched in nuclear fractions from rat liver, and immunofluorescent labelling of cultured cells with anti-p45 antibodies showed stronger labelling in the nucleus than in the cytoplasm. The REG was found to be localized predominantly in the cytoplasm. Three- to six-fold increases in the level of REG were observed following gamma-interferon treatment of cultured cells but gamma-interferon had no obvious effect on its subcellular distribution. These results demonstrate that different regulatory complexes and subpopulations of proteasomes have different distributions within mammalian cells and, therefore, that the distribution is more complex than has been reported for yeast proteasomes.

The Escherichia coli trmE (mnmE) gene, involved in tRNA modification, codes for an evolutionarily conserved GTPase with unusual biochemical properties

The evolutionarily conserved 50K protein of Escherichia coli, encoded by o454, contains a consensus GTP-binding motif. Here we show that 50K is a GTPase that differs extensively from regulatory GTPases such as p21. Thus, 50K exhibits a very high intrinsic GTPase hydrolysis rate, rather low affinity for GTP, and extremely low affinity for GDP. Moreover, it can form self-assemblies. Strikingly, the 17 kDa GTPase domain of 50K conserves the guanine nucleotide-binding and GTPase activities of the intact 50K molecule. Therefore, the structural requirements for GTP binding and GTP hydrolysis by 50K are without precedent and justify a separate classification in the GTPase superfamily. Immunoelectron microscopy reveals that 50K is a cytoplasmic protein partially associated with the inner membrane. We prove that o454 is allelic with trmE, a gene involved in the biosynthesis of the hypermodified nucleoside 5-methylaminomethyl-2-thiouridine, which is found in the wobble position of some tRNAs. Our results demonstrate that 50K is essential for viability depending on the genetic background. We propose that combination of mutations affecting the decoding process, which separately do not reveal an obvious defect in growth, can give rise to lethal phenotypes, most likely due to synergism.

A rapid procedure suitable to assess quantitatively the endocytosis of colloidal gold and its conjugates in cultured cells

We measured the endocytic uptake of low-density lipoproteins (LDLs) conjugated to colloidal gold in cultured cells, either by counting gold particles on electron micrographs or by inductively coupled plasma (ICP) mass spectrometry (MS). Both procedures are comparable but the latter requires a considerably shorter time and allows analysis of a much larger sample. In addition, ICP MS, compared to alternative radioactive or fluorescent procedures, offers the major advantage of using the same probe to quantify the endocytic uptake and to follow it by electron microscopy. Therefore, ICP MS analysis provides an easy, rapid, and sensitive quantification of endocytosis that complements the electron microscopic studies.

Acidic cytosolic proteins are preferentially imported into rat liver lysosomes

Previous studies have reported that lysosomes isolated from human diploid fibroblasts and from rat liver can selectively import and degrade specific proteins. We have now reinvestigated this selectivity using an in vitro assay with rat liver lysosomes and an extract of cytosolic proteins prepared from cultured cells labeled to equilibriums with [35S-]methionine. Analysis by two-dimensional gel electrophoresis and autoradiography of the cytosolic proteins bound to the lysosomal membrane and imported into the lysosomes shows that when all cytosolic proteins are simultaneously present in the in vitro assay the lysosomal uptake also occurs in a specific manner. These findings suggest that isolated lysosomes are able to discriminate among different proteins, selecting those with certain features for lysosomal degradation. Additional characterization of the cytosolic proteins which are selectively imported by lysosomes shows that a common structural feature of most, but not all, of these proteins is an acidic isoelectric point (pI <6.0) and a small or intermediate size. This observation is in agreement with earlier studies which established a relationship between the in vivo half-lives of cytosolic proteins in rat liver and their net charge, with acidic proteins, in general, being degraded more rapidly than neutral or basic proteins. The reasons for this preference are still uncertain, although a possible explanation is presented.

A population of rat liver lysosomes responsible for the selective uptake and degradation of cytosolic proteins

Two populations of rat liver lysosomes can be distinguished on the basis of their density. A major difference between these populations is that one contains the heat shock cognate protein of 73 kDa (hsc73) within the lysosomal lumen. The lysosomal fraction containing hsc73 exhibits much higher efficiencies in the in vitro uptake and degradation of glyceraldehyde-3-phosphate dehydrogenase and ribonuclease A, two well established substrates of the selective lysosomal pathway of intracellular protein degradation. Preloading of the lysosomal population that is devoid of lumenal hsc73 with hsc73 isolated from cytosol activated the selective transport of substrate proteins into these lysosomes. Furthermore, treatment of animals with leupeptin, an inhibitor of lysosomal cathepsins, or 88 h of starvation also increased the amount of hsc73 within their lysosomal lumen, and these in vivo treatments also activated the selective transport of substrate proteins in vitro. Thus, the hsc73 located within lysosomes appears to be required for efficient uptake of cytosolic proteins by these organelles. The difference in hsc73 content between the lysosomal populations appears to be due to differences in their ability to take up hsc73 combined with differences in the intralysosomal degradation rates of hsc73. The increased stability of hsc73 in one population of lysosomes is primarily a consequence of this lysosomal population's more acidic pH.

Use of inductively coupled plasma-mass spectrometry for the quantitation of the binding and uptake of colloidal gold-low-density lipoprotein conjugates by cultured cells

The binding and endocytic uptake of low-density lipoprotein (LDL) particles by cells, transiently or permanently transfected with the human LDL receptor cDNA, was investigated, under different situations, using colloidal gold-LDL conjugates. The amount of gold associated with the various cells, which bind and internalize LDL to different extents, was estimated by inductively coupled plasma-MS. In all cases, the existing differences in LDL binding and uptake were clearly detectable with this procedure. We conclude, therefore, that inductively coupled plasma-MS provides an appropriate assay system for the rapid quantitation of these processes. This procedure also recognizes differences in LDL receptor expression in human lymphocytes and, therefore, it could be of value for the differential diagnosis of LDL receptor defects in familial hypercholesterolemia in various cell types. In addition, this easily performed methodology can also be applied to a variety of other problems requiring quantitation of colloidal gold associated with cells.

Selective uptake and degradation of c-Fos and v-Fos by rat liver lysosomes

The transcription factor c-Fos is a short-lived protein and calpains and ubiquitin-dependent systems have been proposed to be involved in its degradation. In this report, we consider a lysosomal degradation pathway for c-Fos. Using a cell-free assay, we have found that freshly isolated lysosomes can take up and degrade c-Fos with high efficiency. v-Fos, the oncogenic counterpart of c-Fos, can also be taken up by lysosomes, yet the amount of incorporated protein is much lower. c-Fos uptake is independent of its phosphorylation state but it appears to be regulated by dimerization with differentially phosphorylated forms of c-Jun, while v-Fos escapes this regulation. Moreover, we show that c-Fos is immunologically detected in lysosomes isolated from the liver of rats treated with the protease inhibitor leupeptin. Altogether, these results suggest that lysosomes can also participate in the selective degradation of c-Fos in rat liver.

Subpopulations of proteasomes in rat liver nuclei, microsomes and cytosol

Mammalian proteasomes are composed of 14-17 different types of subunits, some of which, including major-histocompatibility-complex-encoded subunits LMP2 and LMP7, are non-essential and present in variable amounts. We have investigated the distribution of total proteasomes and some individual subunits in rat liver by quantitative immunoblot analysis of purified subcellular fractions (nuclei, mitochondria, microsomes and cytosol). Proteasomes were mainly found in the cytosol but were also present in the purified nuclear and microsomal fractions. In the nuclei, proteasomes were soluble or loosely attached to the chromatin, since they could be easily extracted by treatment with nucleases or high concentrations of salt. In the microsomes, proteasomes were on the outside of the membranes. Further subfractionation of the microsomes showed that the proteasomes in this fraction were associated with the smooth endoplasmic reticulum and with the cis-Golgi but were practically absent from the rough endoplasmic reticulum. Using monospecific antibodies for some proteasomal subunits (C8, C9, LMP2 and Z), the composition of proteasomes in nuclei, microsomes and cytosol was investigated. Although there appear not to be differences in proteasome composition in the alpha subunits (C8 and C9) in the different locations, the relative amounts of some beta subunits varied. Subunit Z was enriched in nuclear proteasomes but low in microsome-associated proteasomes, whereas LMP2, which was relatively low in nuclei, showed a small enrichment in the microsomes. These differences in subunit composition of proteasomes probably reflect differences in the function of proteasomes in distinct cell compartments.

Activation of a selective pathway of lysosomal proteolysis in rat liver by prolonged starvation

Lysosomal uptake and degradation of polypeptides such as glyceraldehyde-3-phosphate dehydrogenase (GAPDH), ribonuclease A (RNase A), and RNase S-peptide (residues 1-20 of RNase A) are progressively activated in rat liver by starvation before isolation of lysosomes. This pathway of proteolysis is selective, since it is stimulated by the heat shock cognate protein of 73 kDa (HSC73) and ATP-MgCl2, and lysosomal uptake of RNase A could be competed by GAPDH but not by ovalbumin. A portion of intracellular HSC73 is associated with certain lysosomes, and the amount of lysosomal HSC73 increases by 5- to 10-fold during prolonged starvation. The lysosome-associated HSC73 is primarily within the lysosomal lumen. Double immunogold labeling of lysosomes incubated in vitro with RNase A detects this protein substrate as well as HSC73 within lysosomes. More than two-thirds of the labeled lysosomes contain both RNase A and HSC73. The possible physiological significance of the activation of this selective pathway of lysosomal proteolysis in long-term starvation is discussed.

Degradation of proteasomes by lysosomes in rat liver

Proteasomes are high-molecular-mass multisubunit complexes which are believed, either by themselves or as a part of the 26S proteinase complex, to play a central role in extralysosomal pathways of intracellular protein breakdown. We have addressed the degradation of proteasomes in rat liver, investigating the possible role of lysosomes. Affinity-purified antibodies against rat liver proteasomes were used for immunoblot analysis of isolated lysosomes. Although proteasomes are not found in lysosomes from normally fed rats, they were found to accumulate in lysosomes of rats treated with leupeptin (an inhibitor of lysosomal proteases) and could also be detected in lysosomes isolated from livers of starved (24 h) rats. Proteinase-K treatment of these fractions, as well as immunogold procedures, show that a proportion of the proteasomes are inside lysosomes. Comparison of the amount of proteasomes found in lysosomes by immunoblotting with their experimentally determined half life (8.3 days) is consistent with an important role of these organelles in the degradation of rat liver proteasomes. Nevertheless, these data do not exclude the possibility that some nonlysosomal degradation of proteasome components also occurs. Since proteasomes were localized in autophagic vacuoles, it is likely that they are taken up mainly by nonselective autophagy. However, using an in vitro system, it was found that, under conditions of starvation, proteasomes may also be taken up into lysosomes and degraded via the heat-shock cognate protein of 73 kDa (hsc73)-mediated transport.

Selective binding and uptake of ribonuclease A and glyceraldehyde-3-phosphate dehydrogenase by isolated rat liver lysosomes

Ribonuclease A (RNase A) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) are selectively taken up and degraded by isolated rat liver lysosomes by very similar processes. The uptake and degradation of both of these proteins are stimulated by the heat shock cognate protein of 73 kDa and ATP/Mg2+. Both binding and uptake of RNase A and GAPDH by lysosomes are saturable, and uptake of RNase A and GAPDH requires a protease-sensitive component within the lysosomal membrane. GAPDH competes for binding and uptake of RNase A by lysosomes and vice versa while another protein, ovalbumin, does not compete. RNase S-peptide (amino acids 1-20 of RNase A) also competes for RNase A binding and uptake by lysosomes, while RNase S-protein (amino acids 21-124 of RNase A) does not compete. The uptake of RNase A by lysosomes appears to involve an intermediate step in which approximately 2 kDa of the polypeptide's COOH terminus remains outside lysosomes while the remainder is inside the lysosomal lumen.

Changes in proteasome localization during the cell cycle

We have investigated proteasome localization in synchronized cells using polyclonal anti-proteasome antibodies. Proteasomes were localized in the nucleus and cytoplasm at all phases of the cycle, but changes in localization were observed which explain the different immunofluorescence patterns found in asynchronous cells. In the nucleus, the intensity of staining in early S phase was low and showed a punctate distribution which changed to a more diffuse and intense labeling during S to G1. In the cytoplasm, proteasomes were concentrated in the perinuclear region at G1 and at the start of S phase and gradually moved towards the periphery of the cell as the cell cycle progressed to G2. No cell cycle-dependent changes were detected in the rate of synthesis or level of proteasomes. An apparent colocalization of proteasomes with elements of the cytoskeleton mainly observed in G2 was investigated further in PtK2 cells. The overall distribution of proteasomes and cell cycle-dependent changes in PtK2 cells were similar to those in L-132 cells. Double-label immunofluorescence studies using anti-proteasome and anti-cytokeratin (TROMA-1) antibodies showed that proteasomes do colocalize with intermediate filaments of the cytokeratin type, mainly during G2. In mitosis, proteasomes were found by immunogold electron microscopy to be localized around the chromosomes in both PtK2 and L-132 cells. Cell cycle-dependent changes in the localization of proteasomes suggest that they may have a regulatory function related to the cell cycle, for example, in the degradation of proteins which control its progression.

Uptake and degradation of glyceraldehyde-3-phosphate dehydrogenase by rat liver lysosomes

The molecular mechanisms involved in the degradation of individual cellular proteins are probably unique and characteristic. We have investigated in rat liver the degradation of glyceraldehyde-3-phosphate dehydrogenase, an abundant cytosolic enzyme of the glycolytic pathway. Immunoblot analysis of isolated liver lysosomes from rats treated with lysosomal inhibitors show that this protein is degraded, at least in part, by a lysosomal pathway. This pathway was further investigated by incubating the enzyme with lysosomes in a cell-free system, followed by proteolysis measurements, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of lysosomes, and electron microscopic immunocytochemistry. We postulate that the degradative mechanism of glyceraldehyde-3-phosphate dehydrogenase includes a temperature-dependent lysosomal pathway, different from classical nonspecific macroautophagy. The postulated pathway involves: binding of the enzyme to the lysosomal membrane, entry into the lysosomal matrix, and degradation. This cell-free system, which can also incorporate in vitro synthesized proteins, should allow further advances toward clarifying the complex signals that regulate protein degradation as well as its close interrelationship with protein synthesis.

Electron microscopic localization of the multicatalytic proteinase complex in rat liver and in cultured cells

The multicatalytic proteinase (MCP) prosome or proteasome is a large multifunctional complex which is believed to play a major role in non-lysosomal pathways of intracellular protein degradation and has recently been implicated in antigen processing. In this study, affinity-purified antibodies against rat liver MCP were used to investigate the localization of the proteinase both in rat liver and in growing human L-132 cells in culture, using electron microscopic immunogold techniques. Quantitation of the MCP in different subcellular localizations by morphometric analysis of electron micrographs showed the proportion in the nucleus to be 17% for hepatocytes and 51% for L-132 cells, demonstrating differences in the distribution of MCP in different cell types. In hepatocytes, 14% of the total MCP was found associated with the endoplasmic reticulum. The remainder was localized in the cytoplasmic matrix. Immunofluorescence studies with L-132 cells also showed a reaction in nuclei and cytoplasm. The localization of MCP is consistent with its proposed multiple functions in protein turnover, in the production of peptides for antigen presentation, and in RNA processing.

Cooperation of lysosomes and inner mitochondrial membrane in the degradation of carbamoyl phosphate synthetase and other proteins

Carbamoyl phosphate synthetase (CPS) from rat liver is proteolitically inactivated at acid pH by broken lysosomes. Inactivation increases when lysosomes are previously incubated with inner mitochondrial membrane, although this mitochondrial fraction does not inactivate CPS 'per se'. The increased degradation is due to membrane factor(s), most probably mitochondrial proteinase(s), solubilized by lysosomal matrix proteinases, after incubation of the inner mitochondrial membrane fraction with broken lysosomes. This (these ) factor(s) degrade(s) CPS and other proteins in the absence of lysosomal proteinases or when these are inhibited by leupeptin, chymostatin and pepstatin. We have also tested the possible regulation of this degradation and found that ATP and, particularly, acetyl glutamate accelerate the degradation of CPS by the factor(s) liberated from the inner mitochondrial membrane.

Levels of carbamoyl phosphate synthetase I in livers of young and old rats assessed by activity and immunoassays and by electron microscopic immunogold procedures

Carbamoyl phosphate synthetase I, the most abundant protein of rat liver mitochondria, plays a key role in synthesis of urea. Because aging affects some liver functions, and because there is no information on the levels of carbamoyl phosphate synthetase I during aging, we assayed the activity of this enzyme and determined immunologically the level of carbamoyl phosphate synthetase I in liver homogenates from young (4 months) and old (18 or 26 months) rats. In addition, we used electron microscopic immunogold procedures to locate and measure the amount of the enzyme in the mitochondrial matrix. There is no significant change in enzyme activity or enzyme protein content with age, although there is a higher concentration of the enzyme in the mitochondria (c. 1.5 times greater) from old rats, which is compensated by a decrease in the fractional volume of the mitochondrial compartment during aging.

Endocytosis of liposomes containing lysosomal proteins increases intracellular protein degradation in growing L-132 cells

We have used a new approach to test the possible participation of lysosomes in the degradation of long-lived proteins. Rat liver lysosomal proteins were introduced, via multilamellar liposomes, into L-132 cells. Viability and protein synthesis were not impaired by this treatment. The liposomal content was released into the lysosomes of the cultured cells, as revealed by ferritin uptake and electron microscopy. Degradation rates of long-lived proteins increased with the uptake of lysosomal proteases. However, the increased protein degradation of chloroquine and leupeptin, in contrast to the inhibition by these reagents of the increased protein degradation of cells 'starved' of serum (step-down conditions). This approach opens a new way of investigating the degradation of intracellular proteins in cultured cells.

Effects of centrifugation on the degradation of short-lived proteins in exponentially growing cultured cells

The degradation mechanisms of short-lived proteins in cultured cells are unknown, probably due to the lack of procedures which specifically affect the degradation of these proteins. We found that centrifugation of cultured cells, growing either in monolayer or in suspension, between 5000 and 25,000g for 30 min, inhibits (more than 50%) the degradation of short-lived proteins but not of long-lived proteins. Protein synthesis or cell viability is not affected. Centrifugation also disorganizes the Golgi apparatus, as checked by routine electron microscopy, and inhibits the degradation of endocytosed proteins (a lysosomal process which is controlled by the Golgi apparatus). Using different centrifugation speeds, a good correlation was found between alteration of the Golgi apparatus and inhibition of protein degradation.

Vanadate inhibits degradation of short-lived, but not of long-lived, proteins in L-132 human cells

Vanadate, at concentrations higher than 0.04 mM, inhibits the intracellular degradation of short-lived proteins in exponentially growing L-132 human cells. The inhibition is not due to a decrease in viability or in the ATP contents of the cells. Since vanadate decreases proteolysis in cell extracts, the inhibition appears to affect the proteinases which degrade these proteins. Under optimal nutritional conditions, the degradation of long-lived proteins is accelerated by vanadate, thus providing additional evidence that in exponentially growing cultured cells degradation of short- and long-lived proteins occurs by different processes. Vanadate also efficiently inhibits the lysosomal degradation of endocytosed proteins and of long-lived proteins under step-down conditions. However, this effect seems to be unrelated to the observed inhibition of degradation of short-lived proteins, because chloroquine and leupeptin, which inhibit degradation of proteins by lysosomes, do not modify the degradation of these proteins. Our results provide for the first time a probe which, owing to its opposite effects on the degradation of short- and long-lived proteins, could be useful to clarify the mechanisms involved in protein degradation in cultured cells.

Autophagy of mitochondria in rat liver assessed by immunogold procedures

Glutamate dehydrogenase and carbamoyl phosphate synthase-I were localized in rat liver by immunogold procedures, using monoclonal and polyclonal antibodies. As expected, there was extensive labeling in mitochondria. Label was also found in lysosomal autophagic vacuoles. When autophagy was stimulated by in vivo administration of the anti-microtubular agent vinblastine we found that: (a) carbamoyl phosphate synthase-I and glutamate dehydrogenase could be found in mitochondria within autophagic vacuoles; (b) the carbamoyl phosphate synthase-I and glutamate dehydrogenase content of the mitochondria sequestered into autophagic vacuoles is the same as that of the nearby "free" mitochondria; and (c) in the whole liver, autophagic vacuoles contain c. 1.5 times more glutamate dehydrogenase than carbamoyl phosphate synthase-I, in contrast to mitochondria which have c. three times more carbamoyl phosphate synthase-I than glutamate dehydrogenase. The latter finding could explain, at least partially, the difference in half-lives of these enzymes.

The mitochondrial probe rhodamine 123 inhibits in isolated hepatocytes the degradation of short-lived proteins

The fluorescent dye rhodamine 123 (R123) decreases the intracellular ATP levels and also inhibits the degradation of short-lived proteins in isolated hepatocytes. This inhibition affects lysosomal and, to some extent, non-lysosomal mechanisms. The degradation of short-lived proteins decreases more when ATP levels are less than 40% of those in control cells, in contrast to the reported linear correlation between ATP levels and degradation of long-lived proteins. R123 provides a powerful probe for clarifying the proteolytic mechanisms involved in degradation of short-lived proteins and the ATP requirements in protein degradation. Indeed, as illustrated, the results suggest different mechanisms for the degradation of short- and long-lived proteins. Moreover, they provide a warning for the clinical use of this reagent.

Immunohistochemical localization of glutamate dehydrogenase in rat liver: plasticity of distribution during development and with hormone treatment

In adult rat liver, glutamate dehydrogenase is present in high concentrations around the terminal portal (zone 1) and hepatic (zone 3) veins, whereas its concentration is low in the intermediate zone. Although the size and staining intensity of the periportal glutamate dehydrogenase-positive compartment are less than those of the pericentral compartment, it can expand under appropriate endocrine conditions, leading to a homogeneous distribution. At birth, glutamate dehydrogenase is also homogeneously distributed. Glutamate dehydrogenase disappears from the periportal compartment during the first postnatal week and reappears in that compartment after weaning. These observations indicate an independent regulation of glutamate dehydrogenase levels in the periportal and pericentral zone. The size of the periportal glutamate dehydrogenase-containing zone is appreciably smaller than that of carbamoylphosphate synthetase, whereas the pericentral glutamate dehydrogenase-containing zone is appreciably larger than that of glutamine synthetase. The heterogeneous distribution of glutamate dehydrogenase suggests the possibility that, under normal conditions, deamination of glutamate prevails in the periportal compartment and amination of glutamate in the pericentral compartment.

Differences in the half-lives of some mitochondrial rat liver enzymes may derive partially from hepatocyte heterogeneity

The different turnover rates of rat liver mitochondrial enzymes make autophagy unlikely to be the main mechanism for degradation of mitochondria. Although alternatives have been presented, hepatocyte heterogeneity has not been considered. Lighter hepatocytes isolated in a discontinuous Percoll gradient contain more glutamate dehydrogenase (GDH) (half-life 1 day) and a more active autophagic system than heavier hepatocytes. The latter contain more carbamoyl phosphate synthase (CPS) and ornithine carbamoyl transferase (OTC) (half-lives 8 days) but less lysosomal activity. As expected, isolated autophagic vacuoles contain, relative to the mitochondrial content, 3-times less OTC and CPS than GDH, probably reflecting a faster lysosomal engulfment of mitochondria in the light hepatocytes (which contain more GDH). These data may explain some of the half-life differences of the enzymes studied.

Analysis by flow cytometry of rat hepatocytes from different acinar zones

Many functional, morphological and biochemical differences among hepatocytes from different acinar zones have been described. Therefore, it will facilitate studies on liver metabolism rapid, non-destructive procedures to isolate hepatocytes from these zones. Flow cytometry is a new powerful tool which, however, has not been used thus far to accomplish the separation of hepatocytes from different acinar zones. We describe here various cytometric parameters which characterize hepatocyte populations, separated by isopycnic centrifugation in Percoll gradients. The intraacinar origin of the different hepatocytes was assessed by enzymatic and morphological measurements.

2,3-Bisphosphoglycerate inhibits ATP-stimulated proteolysis

Intracellular protein breakdown could be regulated at the substrate level by changes in the environment. Under in vitro conditions, ATP increases the proteolytic susceptibility of several mitochondrial and cytosolic proteins, while 2,3-bisphosphoglycerate not only has the opposite effect but also prevents the ATP-stimulated proteolysis. ATP and 2,3-bisphosphoglycerate, present at relatively high levels in many tissues, provide a good model of environmental components that may influence intracellular proteolysis.

Monoclonal antibodies used in immunocytochemical localization by electron microscopy of carbamoyl phosphate synthetase I in liver from rats fed high-protein diets

Carbamoyl phosphate synthetase I (CPS-I) is the most abundant protein of rat liver mitochondria. Biochemical measurements in liver homogenates have shown that the liver from rats fed a high-protein diet contains more CPS-I per gram tissue protein than controls. However, there is no information on changes in the intact tissue at the cellular and mitochondrial level. Therefore, monoclonal antibodies to beef liver CPS-I were produced by the hybridoma technique. Four clones, C-241/1A, B, C, and D secreted immunogammaglobulin (IgG) IgG1. Using C-241/C, we measured by electron microscopy immunogold procedures the labeling of CPS-I in mitochondria from liver of rats fed high protein (casein, 50 and 80% of total food intake) diets. CPS-I (expressed as gold particles/micron2 of mitochondrial cross-sectional area) was greater than in mitochondria from control rats (20% casein diet), whether the rats were fed for 1, 6, or 14 months on the high-protein diets. The immunocytochemical measurements shown here demonstrate that the increase in the level of CPS-I in high-protein diets is a reflection of both the larger number of CPS-I molecules per mitochondrial area and the larger proportion of the total hepatocyte volume occupied by mitochondria. Similar measurements were carried out with glutamate dehydrogenase (GDH) using previously characterized monoclonal antibodies. No differences in GDH labeling were found with high-protein diets. Interestingly, when mitochondria from hepatocytes of rats fed a high-protein diet were divided into two subpopulations on the basis of mitochondrial cross-sectional size (i.e., greater or less than 0.7 micron2), the large mitochondria had 1.2 times more CPS-I and 0.8 times less GDH than the small mitochondria nearby.

2,3-Bisphosphoglycerate protects mitochondrial and cytosolic proteins from proteolytic inactivation

2,3-bisphosphoglycerate at physiological concentration similar to that found in many tissues protects effectively ornithine transcarbamoylase (OTC) from proteolytic inactivation by broken lysosomes. 2,3-bisphosphoglycerate protects also many other mitochondrial and cytosolic proteins, such as glutamate dehydrogenase (GDH) an glyceraldehyde-3-phosphate dehydrogenase (GAPDH), from proteolysis by broken lysosomes and other proteases. It is, thus, suggested that 2,3-bisphosphoglycerate may play an important role in the control of the degradative rates of some proteins, which may explain its high concentration in certain cells.

Turnover of rat liver ornithine transcarbamylase

The relative half-life of ornithine transcarbamylase from rat liver has been determined using the double isotope technique and affinity chromatography. The calculated half-life (6-9 days) is similar to that of mitochondria and of the other mitochondrial enzyme of the urea cycle, carbamoyl-phosphate synthase. Therefore, both mitochondrial urea cycle enzymes are most probably degraded mainly via the lysosomal (autophagic) pathway of mitochondrial protein degradation.

The reduction-oxidation status may influence the degradation of glyceraldehyde-3-phosphate dehydrogenase

NADH and NADPH accelerate the 'in vitro' rate of proteolysis of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) by elastase and other proteases, including lysosomal proteases. NAD+ and NADP+ have the opposite effect. Since there is a good correlation between proteolytic susceptibility of proteins and their 'in vivo' degradation rates, a possible role of the reduction-oxidation status in controlling the intracellular degradation of GAPDH is advanced.

Electron microscopic localization of glutamate dehydrogenase in rat liver mitochondria by an immunogold procedure and monoclonal and polyclonal antibodies

Glutamate dehydrogenase (GDH) was localized in rat liver by indirect electron microscopic immunogold, using different sizes of gold particles and monoclonal and polyclonal antibodies. Using the protein A-gold technique in double immunocytochemical experiments, both antibodies, at their optimal dilutions, gave similar results. A novel assessment of the distribution of GDH was made by measurements of the number of gold particles per square micrometer of cross-sectional images of individual mitochondria. The data indicate intracellular homogeneity among mitochondria in individual parenchymal cells. The enzyme is almost absent in non-parenchymal cells. Finally, GDH was found mainly in association with the mitochondrial inner membrane.

Regulation of lysosomal autophagy in transformed and non-transformed mouse fibroblasts under several growth conditions

The role of the lysosomal system in accelerated protein degradation was investigated in 3T3 mouse fibroblasts and in the SV40 virus-transformed derivative, SV3T3. Rates of protein degradation and quantitative electron microscopic alterations in the lysosomal system were compared under four different growing conditions: exponential growth, confluent phase, serum deprivation, and confluent phase together with serum deprivation. We found a significant correlation between increases in rates of proteolysis of long-lived proteins and fractional volume of lysosomes, suggesting a causal relationship between the two, as well as a morphological explanation for the differences in rates of protein degradation in transformed and non-transformed cultured cells. The increase in lysosomal fractional volume resulted from an increase in dense bodies only (in serum-deprived exponential or confluent cultures) or from an increase in autophagic vacuoles and dense bodies (in serum-supplemented confluent cultures).

Homogeneity among mitochondria revealed by a constant proportion of their enzymes

The homogeneity or heterogeneity at the enzyme level of mitochondria has not been directly demonstrated and is important for many studies. To clarify this point, carbamoyl phosphate synthase (ammonia), glutamate dehydrogenase and mitochondrial adenosine triphosphatase (F1) were located in rat liver by immunolabeling using protein A-gold. Measurements of the number of gold particles per square micron of cross sectional images of mitochondria permit to assess the relative molecular concentration of the three enzymes and, most interestingly, it presents the first evidence that different mitochondria in rat liver cells have the same relative proportion of the three enzymes. Since they have vastly different half-lives, bulk or unregulated autophagy as the main mechanism regulating the turnover of these enzymes seems unlikely.

The precursor of rat liver mitochondrial glutamate dehydrogenase has enzymatic activity

The cytosolic precursor for the mitochondrial glutamate dehydrogenase of rat liver was synthesized in a cell-free reticulocyte lysate using messenger RNA from rat liver. To check whether this precursor had enzymatic activity, a highly sensitive fluorimetric method, which can measure picogram quantities of enzyme, was used together with competitive dissociation of the precursor from an immunoprecipitate with inactive glutamate dehydrogenase. Glutamate dehydrogenase activity, corresponding to that estimated from incorporation of [35S]-methionine, was detected in the precursor. The significance of this finding is discussed.

Exit of proteins and fragments thereof from mitochondria is accelerated by the import of cytosolic synthesized proteins

Most mitochondrial proteins are synthesized on cytosolic ribosomes and imported into mitochondria. Incubation of 35S-methionine labeled mitochondria from rat hepatocytes with proteins synthesized in a cell-free system, using messenger RNA from rat liver, dramatically increased the release of mitochondrial proteins and fragments thereof into the medium. Since the synthesized proteins include cytosolic precursors of mitochondrial proteins, our results strongly suggest that import of proteins from the cytosol into mitochondria influences the half-life of proteins in these organelles. The use of this simple approach--i.e. combining the study of protein import and exit with mitochondria--to further clarify intracellular protein turnover and its regulation is suggested.

Degradation of short-lived proteins is decreased by centrifugation

We have examined the effects of enucleation and of inhibitors of mRNA synthesis (actinomycin D and cordycepin) on protein turnover of HeLa cells. Enucleation markedly inhibited the rate of protein degradation for short-lived proteins. However, cells centrifuged in the absence of cytochalasin B at the speed required to obtain cytoplasts showed protein degradation rates identical to those of cytoplasts, while inhibitors of mRNA synthesis did not affect the process. Although enucleation may affect degradation of specific proteins, these results suggest that centrifugation is largely responsible for the inhibition of protein degradation in cytoplasts.

A comparative study of complex mitochondrial DNA in human lymphocytes transformed by Epstein-Barr virus and PHA

Mitochondrial DNA (mt DNA) molecules, isolated from normal human lymphocytes, lymphocytes stimulated with phytohemagglutinin (72 h) and a cultured Burkitt's lymphoma cell line, were examined by electron microscopy. Only monomeric forms of mt DNA were found in normal human lymphocytes, whereas in the other cell types, catenated forms were also observed. Unicircular complex forms were apparently absent in all the cell types studied, suggesting that these forms are not a common malignant transformation in human cells. No clear correlation between mitochondrial modifications--as detected by stereological analysis--and the presence of catenated forms could be established. It is suggested that other mechanisms rather than replication of mt DNA would appear to be the mechanism responsible for the formation of these forms.