Imaginal cell-specific accumulation of the multicatalytic proteinase complex (proteasome) during post-embryonic development in the tobacco hornworm, Manduca sexta

Differential control of cell death and gene expression during two distinct phases of hormonally-regulated muscle death in the tobacco hawkmoth Manduca sexta

In larvae of the tobacco hawkmoth Manduca sexta, the intersegmental muscles (ISMs) span eight abdominal segments and represent the major muscle group. Following pupation, the ISMs in the first two and last two segments undergo programmed cell death (PCD), while the remaining four segments persist until the time of adult eclosion, when they too undergo PCD. ISM death at adult eclosion is initiated by a decline in the circulating ecdysteroid titer and requires de novo gene expression. In this study we have investigated the hormonal regulation and the patterns of gene expression that accompany both early and late ISM death. We find that distinct endocrine cues regulate these two periods of muscle death. Even though the middle segments of ISMs are exposed to the same endocrine environment as the adjacent cells that die following pupation, they do not express death-associated transcripts until they are specifically signaled to die following adult eclosion. These data indicate that subsets of homologous muscles appear to make segment-specific decisions to couple their endogenous cell death programs to distinctly different developmental cues. Nevertheless, once cell death is initiated, they utilize many of the same molecular components.

Cell death in the heart

The role of apoptosis in cardiac disease remains controversial. Much of the apoptosis detected, by chemical or molecular means, reflects inflammatory reaction and responding blood cells rather than myocytes, though their apoptosis in situ may exacerbate a bad situation, and their direct action against myocytes has not been excluded definitely. Myocyte apoptosis may reflect end-stage cardiac failure rather than causing it. If this is the case, then preventing apoptosis so that the cells can undergo necrosis does not accomplish much. Apoptosis is a consistent and important finding in many forms of cardiovascular disease. As determined by ultra-structure, apoptosis is common in cardiomyocytes, fibroblasts, vascular endothelial cells, and smooth muscle cells in cardiovascular disease of many origins. (62) Even though smooth muscle cells in atheromatous plaques appear to be necrotic,l it is likely that this is an evolved situation of apoptotic cells that were not removed. Given the prevalence of apoptotic processes in diseased heart and the very limited capacity of this organ to repair itself, (56) it is appropriate and justified to continue to explore the significance of apoptosis in cardiac disease and, above all, to explore the use of antiapoptotic agents in acute situations. Researchers must pay explicit attention to how they document cell death and in what tissues or cells it occurs. Otherwise, clinicians risk being deluded by preservation of morphology in nonfunctional cells and by confusion of what happened and where death occurred in the sequence of causality. Cell death in the heart is a matter of substantial theoretical and practical concern. A major problem in analyzing it is that, although apoptosis may be demonstrated easily in myocytes, particularly embryonic myocytes, under conditions of culture, interpretation is much more complex in an intact organ. The first issue is one of timing. In situations of severe, acute loss of cells, such as in an infarct, apoptotic cells may not be cleared rapidly and may progress to a more oncotic or necrotic morphology. Second, in situations of inflammation, biochemical or molecular techniques may confound apoptosis of inflammatory cells with apoptosis of myocytes. Third, priorities in the sequence of apoptosis differ between large, generally nonmitotic cells with massive cytoplasm (as differentiated myocytes) and small mitotic cells in culture, which usually are studied. The appearance and many markers of physiological cell death may differ from the most widely recognized forms of apoptosis, including late collapse of the nucleus and primacy of lysosomal or other proteases as opposed to caspases. Investigators should always strive to establish multiple criteria for apoptosis, with good documentation of timing and cell type. When these factors are taken into consideration, it seems that aggressive action against apoptosis may be of value in acute situations, such as infarct, in which buying short increments of time may reduce damage. In more chronic situations, much of the apoptosis detected derives from invading lymphocytes, mast cells, or other cells relating to inflammation. The apoptosis of these cells may exacerbate an already difficult situation, and intervention may prove of value. Otherwise, apoptosis of myocytes is more typically an end-stage situation, and it is more fruitful to alleviate the problem before this stage is reached.

Cloning and analysis of small cytoplasmic leucine-rich repeat protein (SCLP), a novel, phylogenetically-conserved protein that is dramatically up-regulated during the programmed death of moth skeletal muscle

We used the abdominal intersegmental muscles (ISMs) of the moth Manduca sexta as a source of transcripts that are dramatically up-regulated during programmed cell death. One of these transcripts, Small Cytoplasmic Leucine-Rich Repeat Protein (SCLP), encodes a protein of approximately 24 kD that contains four perfect and two imperfect leucine-rich repeat (LRR) motifs. DNA sequence database analysis suggests that SCLP is a phylogenetically-conserved gene of unknown function. Both Northern and Western blots demonstrated that SCLP is expressed in the ISMs at all stages examined, but increases greater than 10-fold when the cells become committed to die. This increase in expression is regulated by the same change in the circulating ecdysteroid titer that controls death. Low levels of SCLP expression are also seen in flight muscle and fat body, but not in ovary, male sexual accessory gland, or Malpighian tubules. Immunohistochemical analysis demonstrates that SCLP is a cytoplasmic protein. Western blot analysis of proteins from the fly Drosophila suggests that an SCLP-related protein is expressed at the larval and pupal stages, but not in embryos or adults. Targeted expression of moth SCLP to a variety of different tissues in Drosophila using the Gal4/UAS P element system failed to generate an overt phenotype. These data are interpreted as suggesting that whereas SCLP presumably plays an important role in programmed cell death of muscle, perhaps by acting as an adaptor protein, its expression is insufficient to initiate death by itself.

Subunit compositions and catalytic properties of proteasomes from developmental temperature- sensitive mutants of Drosophila melanogaster

Two dominant temperature-sensitive (DTS) Drosophila mutants are missense mutations of proteasome genes encoding beta-type subunits beta6/C5 (DTS5) and beta2/Z (DTS7). At nonpermissive temperature (29 degrees C), heterozygotes (DTS5/+ and DTS7/+) develop normally until metamorphosis; pupae fail to mature and die before eclosion. Proteasomes were purified from wild-type (WT) and heterozygous adult flies raised at permissive temperature (25 degrees C). Two-dimensional gel electrophoresis separated at least 28 proteins, 13 of which were identified with monospecific antibodies to alpha6/C2 (five species), alpha2/C3 (three species), alpha7/C8 (three species), alpha5/zeta, and beta1/Y subunits. Both quantitative and qualitative differences were observed between WT and DTS/+ proteasomes, with DTS5/+ deviating more from WT than DTS7/+ proteasomes. In DTS5/+ there was a shift to more acidic species of C2 and C3 and a shift to less acidic species of 32-kDa subunits (#3-#7) recognized by an anti-alpha subunit monoclonal antibody (MCP222) and were losses of two 32-kDa subunits (#2 and #3), decreases in Y (25 kDa; 2-fold) and 31-kDa (#9; 2-fold) subunits, and increases in 52-kDa (#1; 1.9-fold) and 24-kDa (#13; 2.3-fold) subunits. In DTS7/+ there was a less pronounced shift to acidic species of C3 and no pI shift in C2 species and subunits #3-#7 and were decreases in #9 (2.5-fold) and #14 (3-fold) and a loss of #2. The three C8 species were similar between WT, DTS5/+, and DTS7/+ proteasomes. Qualitatively, the most dramatic difference was the appearance of a new 24-kDa subunit (#16) in DTS/+ preparations, with about a 14-fold greater amount of #16 in DTS7/+ than in DTS5/+ proteasomes. Catalytically, WT and DTS/+ proteasomes had similar peptidase activities, although the DTS/+ proteasomes were slightly more sensitive to SDS and elevated temperatures in vitro. The incorporation of DTS subunits apparently altered proteasome assembly and/or processing at permissive temperature with little effect on catalytic activities. These data suggest that at nonpermissive temperature, assembly/processing is more severely affected, producing DTS-containing complexes that lack functions essential for cellular proliferation and differentiation at metamorphosis.

Changes in proteasome expression and activity during differentiation of neuronal precursor NTera 2 clone D1 cells

The effect of differentiation of the human neuronal progenitor cell line NTera 2 clone D1 (NT2/D1) by retinoic acid on components of the proteasome system was studied. The chymotrypsin-like and peptidylglutamyl peptide bond hydrolyzing activities of the proteasome increased five weeks after retinoic acid, and following treatment with mitotic inhibitors returned to levels detected in non-differentiated cells. A selective induction of the MHC class II region encoded LMP7 and LMP2 proteasome subunits occurred during differentiation, whereas there were no changes in the expression of the constitutive LMP2 counterpart (delta-subunit) or the constitutive C2 subunit. Immunofluorescence revealed marked LMP7 accumulation in fully differentiated cells, with no changes in the labeling pattern of the constitutive proteasome antigens. The expression of the alpha-subunit of the PA28 proteasome activator was down-regulated in fully differentiated neurons, but was not correlated with changes in enzymatic activity. Changes in proteasome activity and composition may contribute to the processes leading to differentiation of human neurons in vitro and to the properties of fully differentiated neurons.

Intracellular proteinases of invertebrates: calcium-dependent and proteasome/ubiquitin-dependent systems

Cytosolic proteinases carry out a variety of regulatory functions by controlling protein levels and/or activities within cells. Calcium-dependent and ubiquitin/proteasome-dependent pathways are common to all eukaryotes. The former pathway consists of a diverse group of Ca(2+)-dependent cysteine proteinases (CDPs; calpains in vertebrate tissues). The latter pathway is highly conserved and consists of ubiquitin, ubiquitin-conjugating enzymes, deubiquitinases, and the proteasome. This review summarizes the biochemical properties and genetics of invertebrate CDPs and proteasomes and their roles in programmed cell death, stress responses (heat shock and anoxia), skeletal muscle atrophy, gametogenesis and fertilization, development and pattern formation, cell-cell recognition, signal transduction and learning, and photoreceptor light adaptation. These pathways carry out bulk protein degradation in the programmed death of the intersegmental and flight muscles of insects and of individuals in a colonial ascidian; molt-induced atrophy of crustacean claw muscle; and responses of brine shrimp, mussels, and insects to environmental stress. Selective proteolysis occurs in response to specific signals, such as in modulating protein kinase A activity in sea hare and fruit fly associated with learning; gametogenesis, differentiation, and development in sponge, echinoderms, nematode, ascidian, and insects; and in light adaptation of photoreceptors in the eyes of squid, insects, and crustaceans. Proteolytic activities and specificities are regulated through proteinase gene expression (CDP isozymes and proteasomal subunits), allosteric regulators, and posttranslational modifications, as well as through specific targeting of protein substrates by a diverse assemblage of ubiquitin-conjugases and deubiquitinases. Thus, the regulation of intracellular proteolysis approaches the complexity and versatility of transcriptional and translational mechanisms.

UFD1L, a developmentally expressed ubiquitination gene, is deleted in CATCH 22 syndrome

The CATCH 22 acronym outlines the main clinical features of 22q11.2 deletions (cardiac defects, abnormal facies, thymic hypoplasia, cleft palate and hypocalcemia), usually found in DiGeorge (DGS) and velo-cardio-facial (VCFS) syndromes. Hemizygosity of this region may also be the cause of over 100 different clinical signs. The CATCH 22 locus maps within a 1.5 Mb region, which encompasses several genes. However, no single defect in 22q11.2 hemizygous patients can be ascribed to any gene so far isolated from the critical region of deletion. We have identified a gene in the CATCH 22 critical region, whose functional features and tissue-specific expression suggest a distinct role in embryogenesis. This gene, UFD1L, encodes the human homolog of the yeast ubiquitin fusion degradation 1 protein (UFD1p), involved in the degradation of ubiquitin fusion proteins. Cloning and characterization of the murine homolog (Ufd1l) showed it to be expressed during embryogenesis in the eyes and in the linear ear primordia. These data suggest that the proteolytic pathway that recognizes ubiquitin fusion proteins for degradation is conserved in vertebrates and that the UFD1L gene hemizygosity is the cause of some of the CATCH 22-associated developmental defects.