Caspase-dependent non-apoptotic processes in development

Abstract

Caspases are at the core of executing apoptosis by orchestrating cellular destruction with proteolytic cascades. Caspase-mediated proteolysis also controls diverse nonlethal cellular activities such as proliferation, differentiation, cell fate decision, and cytoskeletal reorganization. During the last decade or so, genetic studies of Drosophila have contributed to our understanding of the in vivo mechanism of the non-apoptotic cellular responses in developmental contexts. Furthermore, recent studies using C. elegans suggest that apoptotic signaling may play unexpected roles, which influence ageing and normal development at the organism level. In this review, we describe how the caspase activity is elaborately controlled during vital cellular processes at the level of subcellular localization, the duration and timing to avoid full apoptotic consequences, and also discuss the novel roles of non-apoptotic caspase signaling in adult homeostasis and physiology.

Figure 1

Conserved core caspase-signaling cascades. (a) In C. elegans, apoptotic signals transcriptionally upregulate EGL-1(BH3-only protein), of which binding to CED-9 (BCL2-family protein) releases CED-4 and consequently promotes the activation of caspase CED-3. (b) In Drosophila, different developmental signals become apoptotic stimuli and activate IAP antagonists Reaper, Hid, and Grim (RHG). RHG proteins localize to mitochondria and promote degradation of Drosophila inhibitor of apoptosis protein 1 (DIAP1). DIAP1 inhibits Dronc and effector caspases Drice and Dcp-1 so that the degradation of DIAP1 release caspases. Dronc and its activator Ark form the apoptosome complex, which activates downstream effector caspases. (c) In mammals, the interplay between pro-apoptotic and anti-apoptotic BCL-2 family proteins controls the release of mitochondria-localized proteins including Cyt-c and IAP antagonists such as Smac, HtrA2, and ARTS. Binding of Cyt-c to the Apaf-1 promotes apoptosome assembly, which recruits pro-caspase-9 and facilitates the proteolytic cascade. IAP antagonists release caspase-9 from the inhibition by XIAP, resulting in the activation of the apoptosome and the downstream effector caspases (caspase-3 and caspase-7). Functional homologous proteins across species are similarly represented by the color and shape

Figure 2

Localized caspase activation during Drosophila sperm individualization. (a) A schematic illustration of sperm individualization in male Drosophila. Four spermatids in a cyst are represented. Upon individualization, the individualization complex (IC, red triangle) is formed, ICs then move along the spermatid from the head to the tail direction. Along with IC movement, excess cytoplasm and organelles are removed, which accumulate in the cystic bulge (CB) and then are eventually deposited into the waste bag (WB). (b) A model of localized caspase activation in maturing spermatids. The active or inactive state of CRL3 complex is controlled by the testis-specific isoform of A-Sβ and the pseudosubstrate inhibitor Soti. The active CRL3 complex activates caspases through the ubiquitination and degradation of IAP-like protein dBruce that inhibit caspases. Soti expression forms a gradient in spermatids from the tail to the head direction, allowing the spatial gradient of dBruce distribution by inhibiting the CRL3 complex activity. The graded localization of dBruce leads to an inverse gradient of the caspase activity in spermatids from the head to the tail direction. The testis-specific A-Sβ localizes to mitochondria and competes with Soti, contributing to the CRL3 activation at the surface of the mitochondria. The CRL3 complex consists of Cullin-3, Klhl10 and Roc1b that recruits an ubiquitin (Ub)-conjugating enzyme (E2)

Figure 3

Non-apoptotic caspase signaling during SOP cell fate decision and cytoskeletal reorganization in Drosophila. Caspase signaling is involved in the non-apoptotic cellular processes in Drosophila, such as SOP cell fate decision and cytoskeletal reorganization during cellular shaping and border cell migration. Caspase activity is transiently and temporally controlled by the turnover of DIAP1. DIAP1 degradation is promoted by its DmIKKε-induced phosphorylation in normal development, which results in caspase activity at the level required for non-apoptotic functions. Shaggy (Sgg) protein was identified as a substrate for caspases during SOP cell specification and it becomes activated after processing by caspases (Sgg46 as inactive and Sgg10 as active form). DIAP1 positively regulates F-actin polymerization through the downregulating Dronc so that the proper amount of DIAP1 is required for border cell migration and arista morphogenesis. Dronc substrate specificity during non-apoptotic functions is conferred by the unconventional myosin CK, which physically interacts with Dronc and inactive Sgg46, and active Sgg10 negatively influences SOP cell fate determination and cytoskeletal remodeling

Figure 4

Apoptosis signaling affects longevity following mtROS in C. elegans. After treatment of the superoxide generator paraquat or mutations in structural components of mitochondrial respiratory chain (isp-1, nuo-6) lead to increased longevity in C. elegans, which is mediated by mtROS. The intrinsic apoptosis pathway (CED-9, CED-4, and CED-3) is sensitive to mtROS and is utilized as a protective mechanism for organism survival. Although the components of the core apoptosis pathway are utilized, the BH3-only protein CED-13 is required for the longevity phenotype in response to mtROS (left panel), instead of EGL-1, which is required for apoptosis (right panel)