Caspase recruitment domain of procaspase-2 could be a target for SUMO-1 modification through Ubc9

Stepwise phosphorylation and SUMOylation of PIDD1 drive PIDDosome assembly in response to DNA repair failure

SUMOylation regulates numerous cellular stress responses, yet targets in the apoptotic machinery remain elusive. We show that a single, DNA damage-induced monoSUMOylation event controls PIDDosome (PIDD1/RAIDD/caspase-2) formation and apoptotic death in response to unresolved DNA interstrand crosslinks (ICLs). SUMO-1 conjugation occurs on conserved K879 in the PIDD1 death domain (DD); is catalyzed by PIAS1 and countered by SENP3; and is triggered by ATR phosphorylation of neighboring T788 in the PIDD1 DD, which enables PIAS1 docking. Phospho/SUMO-PIDD1 proteins are captured by nucleolar RAIDD monomers via a SUMO-interacting motif (SIM) in the RAIDD DD, thus compartmentalizing nascent PIDDosomes for caspase-2 recruitment. Denying SUMOylation or the SUMO-SIM interaction spares the onset of PIDDosome assembly but blocks its completion, thus eliminating the apoptotic response to ICL repair failure. Conversely, removal of SENP3 forces apoptosis, even in cells with tolerable ICL levels. SUMO-mediated PIDDosome control is also seen in response to DNA breaks but not supernumerary centrosomes. These results illuminate PIDDosome formation in space and time and identify a direct role for SUMOylation in the assembly of a major pro-apoptotic device.

Procaspase-1 patrolled to the nucleus of proatherogenic lipid LPC-activated human aortic endothelial cells induces ROS promoter CYP1B1 and strong inflammation

To determine the roles of nuclear localization of pro-caspase-1 in human aortic endothelial cells (HAECs) activated by proatherogenic lipid lysophosphatidylcholine (LPC), we examined cytosolic and nuclear localization of pro-caspase-1, identified nuclear export signal (NES) in pro-caspase-1 and sequenced RNAs. We made the following findings: 1) LPC increases nuclear localization of procaspase-1 in HAECs. 2) Nuclear pro-caspase-1 exports back to the cytosol, which is facilitated by a leptomycin B-inhibited mechanism. 3) Increased nuclear localization of pro-caspase-1 by a new NES peptide inhibitor upregulates inflammatory genes in oxidative stress and Th17 pathways; and SUMO activator N106 enhances nuclear localization of pro-caspase-1 and caspase-1 activation (p20) in the nucleus. 4) LPC plus caspase-1 enzymatic inhibitor upregulates inflammatory genes with hypercytokinemia/hyperchemokinemia and interferon pathways, suggesting a novel capsase-1 enzyme-independent inflammatory mechanism. 5) LPC in combination with NES inhibitor and caspase-1 inhibitor upregulate inflammatory gene expression that regulate Th17 activation, endotheli-1 signaling, p38-, and ERK- MAPK pathways. To examine two hallmarks of endothelial activation such as secretomes and membrane protein signaling, LPC plus NES inhibitor upregulate 57 canonical secretomic genes and 76 exosome secretomic genes, respectively, promoting four pathways including Th17, IL-17 promoted cytokines, interferon signaling and cholesterol biosynthesis. LPC with NES inhibitor also promote inflammation via upregulating ROS promoter CYP1B1 and 11 clusters of differentiation (CD) membrane protein pathways. Mechanistically, all the LPC plus NES inhibitor-induced genes are significantly downregulated in CYP1B1-deficient microarray, suggesting that nuclear caspase-1-induced CYP1B1 promotes strong inflammation. These transcriptomic results provide novel insights on the roles of nuclear caspase-1 in sensing DAMPs, inducing ROS promoter CYP1B1 and in regulating a large number of genes that mediate HAEC activation and inflammation. These findings will lead to future development of novel therapeutics for cardiovascular diseases (CVD), inflammations, infections, transplantation, autoimmune disease and cancers. (total words: 284).

Alterations in the nucleocytoplasmic transport in apoptosis: Caspases lead the way

Apoptosis is a mode of regulated cell death that is indispensable for the morphogenesis, development and homeostasis of multicellular organisms. Caspases are cysteine-dependent aspartate-specific proteases, which function as initiators and executors of apoptosis. Caspases are cytosolic proteins that can cleave substrates located in different intracellular compartments during apoptosis. Many years ago, the involvement of caspases in the regulation of nuclear changes, a hallmark of apoptosis, was documented. Accumulated data suggest that apoptosis-associated alterations in nucleocytoplasmic transport are also linked to caspase activity. Here, we aim to discuss the current state of knowledge regarding this process. Particular attention will be focused on caspase nuclear entry and their functions in the demolition of the nucleus upon apoptotic stimuli.

Post-translational Modification of Caspases: The Other Side of Apoptosis Regulation

Apoptosis is a crucial program of cell death that controls development and homeostasis of multicellular organisms. The main initiators and executors of this process are the Cysteine-dependent ASPartate proteASES - caspases. A number of regulatory circuits tightly control caspase processing and activity. One of the most important, yet, at the same time still poorly understood control mechanisms of activation of caspases involves their post-translational modifications. The addition and/or removal of chemical groups drastically alters the catalytic activity of caspases or stimulates their nonapoptotic functions. In this review, we will describe and discuss the roles of key caspase modifications such as phosphorylation, ubiquitination, nitrosylation, glutathionylation, SUMOylation, and acetylation in the regulation of apoptotic cell death and cell survival.

Caspase-2 short isoform interacts with membrane-associated cytoskeleton proteins to inhibit apoptosis

Caspase-2 (casp-2) is the most conserved caspase across species, and is one of the initiator caspases activated by various stimuli. The casp-2 gene produces several alternative splicing isoforms. It is believed that the long isoform, casp-2L, promotes apoptosis, whereas the short isoform, casp-2S, inhibits apoptosis. The actual effect of casp-2S on apoptosis is still controversial, however, and the underlying mechanism for casp-2S-mediated apoptosis inhibition is unclear. Here, we analyzed the effects of casp-2S on DNA damage induced apoptosis through "gain-of-function" and "loss-of-function" strategies in ovarian cancer cell lines. We clearly demonstrated that the over-expression of casp-2S inhibited, and the knockdown of casp-2S promoted, the cisplatin-induced apoptosis of ovarian cancer cells. To explore the mechanism by which casp-2S mediates apoptosis inhibition, we analyzed the proteins which interact with casp-2S in cells by using immunoprecipitation (IP) and mass spectrometry. We have identified two cytoskeleton proteins, Fodrin and α-Actinin 4, which interact with FLAG-tagged casp-2S in HeLa cells and confirmed this interaction through reciprocal IP. We further demonstrated that casp-2S (i) is responsible for inhibiting DNA damage-induced cytoplasmic Fodrin cleavage independent of cellular p53 status, and (ii) prevents cisplatin-induced membrane blebbing. Taken together, our data suggests that casp-2S affects cellular apoptosis through its interaction with membrane-associated cytoskeletal Fodrin protein.

Regulation of Neuronal Protein Trafficking and Translocation by SUMOylation

Post-translational modifications of proteins are essential for cell function. Covalent modification by SUMO (small ubiquitin-like modifier) plays a role in multiple cell processes, including transcriptional regulation, DNA damage repair, protein localization and trafficking. Factors affecting protein localization and trafficking are particularly crucial in neurons because of their polarization, morphological complexity and functional specialization. SUMOylation has emerged as a major mediator of intranuclear and nucleo-cytoplasmic translocations of proteins involved in critical pathways such as circadian rhythm, apoptosis and protein degradation. In addition, SUMO-regulated re-localization of extranuclear proteins is required to sustain neuronal excitability and synaptic transmission. Thus, SUMOylation is a key arbiter of neuronal viability and function. Here, we provide an overview of recent advances in our understanding of regulation of neuronal protein localization and translocation by SUMO and highlight exciting areas of ongoing research.

SUMOylation pathway in Trypanosoma cruzi: functional characterization and proteomic analysis of target proteins

SUMOylation is a relevant protein post-translational modification in eukaryotes. The C terminus of proteolytically activated small ubiquitin-like modifier (SUMO) is covalently linked to a lysine residue of the target protein by an isopeptide bond, through a mechanism that includes an E1-activating enzyme, an E2-conjugating enzyme, and transfer to the target, sometimes with the assistance of a ligase. The modification is reversed by a protease, also responsible for SUMO maturation. A number of proteins have been identified as SUMO targets, participating in the regulation of cell cycle progression, transcription, translation, ubiquitination, and DNA repair. In this study, we report that orthologous genes corresponding to the SUMOylation pathway are present in the etiological agent of Chagas disease, Trypanosoma cruzi. Furthermore, the SUMOylation system is functionally active in this protozoan parasite, having the requirements for SUMO maturation and conjugation. Immunofluorescence analysis showed that T. cruzi SUMO (TcSUMO) is predominantly found in the nucleus. To identify SUMOylation targets and get an insight into their physiological roles we generated transfectant T. cruzi epimastigote lines expressing a double-tagged T. cruzi SUMO, and SUMOylated proteins were enriched by tandem affinity chromatography. By two-dimensional liquid chromatography-mass spectrometry a total of 236 proteins with diverse biological functions were identified as potential T. cruzi SUMO targets. Of these, metacaspase-3 was biochemically validated as a bona fide SUMOylation substrate. Proteomic studies in other organisms have reported that orthologs of putative T. cruzi SUMOylated proteins are similarly modified, indicating conserved functions for protein SUMOylation in this early divergent eukaryote.

Caspase-2: controversial killer or checkpoint controller?

The caspases are an evolutionarily conserved family of cysteine proteases, with essential roles in apoptosis or inflammation. Caspase-2 was the second caspase to be cloned and it resembles the prototypical nematode caspase CED-3 more closely than any other mammalian protein. An absence of caspase-2-specific reagents and the subtle phenotype of caspase-2-deficient mice have hampered definition of the physiological role of caspase-2 and identification of factors regulating its activity. Although some data implicate caspase-2 in apoptotic pathways, a link with apoptosis has been less firmly established for caspase-2 than for some other caspases. Emerging evidence suggests that caspase-2 regulates the cell cycle and may act as a tumour suppressor. This article critically reviews the current state of knowledge regarding the biochemistry and biology of this controversial caspase.

The enigma of caspase-2: the laymen's view

Proteolysis of cellular substrates by caspases (cysteine-dependent aspartate-specific proteases) is one of the hallmarks of apoptotic cell death. Although the activation of apoptotic caspases is considered a 'late-stage' event in apoptosis signaling, past the commitment stage, one caspase family member, caspase-2, splits the cell death community into half - those searching for evidence of an apical initiator function of this molecule and those considering it as an amplifier of the apoptotic caspase cascade, at best, if relevant for apoptosis at all. This review screens past and present biochemical as well as genetic evidence for caspase-2 function in cell death signaling and beyond.

C-terminal cleavage of the amyloid-beta protein precursor at Asp664: a switch associated with Alzheimer's disease

In addition to the proteolytic cleavages that give rise to amyloid-beta (Abeta), the amyloid-beta protein precursor (AbetaPP) is cleaved at Asp664 intracytoplasmically. This cleavage releases a cytotoxic peptide, APP-C31, removes AbetaPP-interaction motifs required for signaling and internalization, and is required for the generation of AD-like deficits in a mouse model of the disease. Although we and others had previously shown that Asp664 cleavage of AbetaPP is increased in AD brains, the distribution of the Asp664-cleaved forms of AbetaPP in non-diseased and AD brains at different ages had not been determined. Confirming previous reports, we found that Asp664-cleaved forms of AbetaPP were increased in neuronal cytoplasm and nuclei in early-stage AD brains but were absent in age-matched, non-diseased control brains and in late-stage AD brains. Remarkably, however, Asp664-cleaved AbetaPP was prominent in neuronal somata and in processes in entorhinal cortex and hippocampus of non-diseased human brains at ages <45 years. Our observations suggest that Asp664 cleavage of AbetaPP may be part of the normal proteolytic processing of AbetaPP in young (<45 years) human brain and that this cleavage is down-regulated with normal aging, but is aberrantly increased and altered in location in early AD.

GMEB1, a novel endogenous caspase inhibitor, prevents hypoxia- and oxidative stress-induced neuronal apoptosis

The interaction of glucocorticoid modulatory element-binding protein 1 (GMEB1) with procaspase-2, -8, or -9 prevents caspase oligomerization and maturation. In the present study, we examined the effect of GMEB1 on neuronal apoptosis induced by hypoxia and oxidative stress. GMEB1 effectively attenuated caspase activation and apoptosis caused by these stresses in human neuroblastoma SK-N-MC cells, indicating that it functions as a potent inhibitor of caspase activation and apoptosis in response to oxidative stress. We propose that GMEB1 blocks pro-apoptosis signals induced by a variety of stresses.

SUMO on the road to neurodegeneration

Sumoylation is a post-translational modification by which small ubiquitin-like modifiers (SUMO) are covalently conjugated to target proteins. This reversible pathway provides a rapid and efficient way to modulate the subcellular localization, activity and stability of a wide variety of substrates. Similar to its well-known cousin ubiquitin, SUMO co-localize with the neuronal inclusions associated with several neurodegenerative diseases, including multiple system atrophy, Huntington's disease and other related polyglutamine disorders. The identification of huntingtin, ataxin-1, tau and alpha-synuclein as SUMO substrates further supports the involvement of sumoylation in the pathogenesis of this family of neurological diseases. In addition to direct targeting of these constituent proteins, sumoylation also impacts other disease pathways such as oxidative stress, protein aggregation and proteasome-mediated degradation. This review highlights the recent advances in understanding the contributions of SUMO to neurodegeneration and the underlying pathogenic mechanisms of these diseases.

Relationship between SUMO-1 modification of caspase-7 and its nuclear localization in human neuronal cells

The aim of this study was to elucidate the role of SUMO-1 modification in caspase-7 in neuronal cells. We have previously demonstrated that procaspase-2 could be a possible target for SUMO-1 modification. In the present study, we attempted to investigate whether other caspases also interact with Ubc9/SUMO-1. The specific binding of SUMO-1 with caspase-7 was observed in mammalian cells. Deletion mutant analysis revealed that a SUMO-1 modification site may be located in at least N-terminal p20 subunit of caspase-7. Furthermore, SUMO-1-modified caspase-7 appeared as a dot-like structure in nuclear localization. These findings suggest that SUMO-1 modification in caspase-7 may be linked to specific its localization in the nucleus and may therefore contribute to the cleavage of nuclear substrates during neuronal apoptosis.