Caspase activity is involved in, but is dispensable for, early motoneuron death in the chick embryo cervical spinal cord

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Autophagy: A Key Regulator of Homeostasis and Disease: An Overview of Molecular Mechanisms and Modulators

Autophagy is a highly conserved lysosomal degradation pathway active at basal levels in all cells. However, under stress conditions, such as a lack of nutrients or trophic factors, it works as a survival mechanism that allows the generation of metabolic precursors for the proper functioning of the cells until the nutrients are available. Neurons, as post-mitotic cells, depend largely on autophagy to maintain cell homeostasis to get rid of damaged and/or old organelles and misfolded or aggregated proteins. Therefore, the dysfunction of this process contributes to the pathologies of many human diseases. Furthermore, autophagy is highly active during differentiation and development. In this review, we describe the current knowledge of the different pathways, molecular mechanisms, factors that induce it, and the regulation of mammalian autophagy. We also discuss its relevant role in development and disease. Finally, here we summarize several investigations demonstrating that autophagic abnormalities have been considered the underlying reasons for many human diseases, including liver disease, cardiovascular, cerebrovascular diseases, neurodegenerative diseases, neoplastic diseases, cancers, and, more recently, infectious diseases, such as SARS-CoV-2 caused COVID-19 disease.

In vivo detection of programmed cell death during mouse heart development

Despite the great progress on the cell biology of programmed cell death (PCD), its incidence and exact time course during embryonic and particular heart development are still unclear. This is also due to the lack of models enabling to directly identify and monitor PCD cells at different time points in vivo. Herein we report generation of transgenic murine embryonic stem cell and mouse models expressing secreted Annexin V-YFP under control of the CAG promoter. This enables to visualize and quantify PCD in vitro and in vivo during embryonic development. At early embryonic stages we found Annexin V-YFP⁺ fluorescent cells in known areas of PCD, such as the otic ring and at the site of neural tube closing, underscoring its specificity for detection of PCD. We have focused our detailed analysis primarily on PCD in the embryonic heart for a better understanding of its role during development. Our findings reveal that PCD peaks at early stages of cardiogenesis (E9.5-E13.5) and strongly decreases thereafter. Moreover, the PCD cells in the heart are predominantly cardiomyocytes, and an unexpected area of prominent cardiac PCD are the ventricular trabeculae (E9.5-E14.5). Thus, the sA5-YFP mouse line provides novel insight into the incidence and relevance of cardiac PCD during embryonic development ex- and in vivo.

Caspase-independent programmed cell death triggers Ca2PO4 deposition in an in vitro model of nephrocalcinosis

Nephrocalcinosis involves the deposition of microscopic crystals in the tubular lumen or interstitium. While the clinical, biochemical, and genetic aspects of the diseases causing nephrocalcinosis have been elucidated, little is known about the cellular events in this calcification process. We previously reported a phenomenon involving the spontaneous formation of Ca2PO4 nodules in primary papillary renal cells from a patient with medullary nephrocalcinosis harboring a rare glial cell-derived neurotrophic factor (GDNF) gene variant. We also demonstrated that cultivating GDNF-silenced human kidney-2 (HK-2) cells in osteogenic conditions for 15 days triggered Ca2PO4 deposits. Given the reportedly close relationship between cell death and pathological calcification, aim of the present study was to investigate whether apoptosis is involved in the calcification of GDNF-silenced HK-2 cells under osteogenic conditions. Silenced and control cells were cultured in standard and osteogenic medium for 1, 5, and 15 days, and any Ca2PO4 deposition was identified by means of von Kossa staining and environmental SEM (ESEM) analyses. Based on the results of annexin V and propidium iodide (PI) analysis, and terminal deoxynucleotidyl transferase dUTP-biotin nick end labeling (TUNEL) assay, the silenced cells in the osteogenic medium showed a significant increase in the percentage of cells in the late phase of apoptosis and an increased Ca2PO4 deposition at 15 days. The results of quantitative real-time PCR (qRT-PCR) of BAX and BCL2, and in-cell Western analysis of caspases indicated that the cell death process was independent of caspase-3, -6, -7, and -9 activation, however. Using this model, we provide evidence of caspase-independent cell death triggering the calcification process in GDNF-silenced HK-2 cells.

Anti-apoptotic BCL-2 family members in development

Almost 30 years ago it was first appreciated that anti-apoptotic B-cell lymphoma-2 (BCL-2) prevents the induction of apoptosis not only in malignant cells, but also in normal cellular lineages. This critical observation has rapidly evolved from merely identifying new BCL-2 family members to understanding how their biochemical interactions trigger the cell death process, and, more recently, to pharmacological inhibition of anti-apoptotic BCL-2 function in disease. Indeed, the proper regulation of apoptosis is important in many aspects of life including development, homeostasis, and disease biology. To better understand these processes, scientists have used many tools to assess the contribution of individual anti-apoptotic BCL-2 family members. This review will focus on the prominent roles for BCL-2 and other pro-survival family members in promoting the development of mammals during early embryogenesis, neurogenesis, and hematopoiesis.

Motor neurons with limb-innervating character in the cervical spinal cord are sculpted by apoptosis based on the Hox code in chick embryo

In the developing chick embryo, a certain population of motor neurons (MNs) in the non-limb-innervating cervical spinal cord undergoes apoptosis between embryonic days 4 and 5. However, the characteristics of these apoptotic MNs remain undefined. Here, by examining the spatiotemporal profiles of apoptosis and MN subtype marker expression in normal or apoptosis-inhibited chick embryos, we found that this apoptotic population is distinguishable by Foxp1 expression. When apoptosis was inhibited, the Foxp1⁺ MNs survived and showed characteristics of lateral motor column (LMC) neurons, which are of a limb-innervating subtype, suggesting that cervical Foxp1⁺ MNs are the rostral continuation of the LMC. Knockdown and misexpression of Foxp1 did not affect apoptosis progression, but revealed the role of Foxp1 in conferring LMC identity on the cervical MNs. Furthermore, ectopic expression of Hox genes that are normally expressed in the brachial region prevented apoptosis, and directed Foxp1⁺ MNs to LMC neurons at the cervical level. These results indicate that apoptosis in the cervical spinal cord plays a role in sculpting Foxp1⁺ MNs committed to LMC neurons, depending on the Hox expression pattern.

Non-apoptotic cell death in animal development

Programmed cell death (PCD) is an important process in the development of multicellular organisms. Apoptosis, a form of PCD characterized morphologically by chromatin condensation, membrane blebbing, and cytoplasm compaction, and molecularly by the activation of caspase proteases, has been extensively investigated. Studies in Caenorhabditis elegans, Drosophila, mice, and the developing chick have revealed, however, that developmental PCD also occurs through other mechanisms, morphologically and molecularly distinct from apoptosis. Some non-apoptotic PCD pathways, including those regulating germ cell death in Drosophila, still appear to employ caspases. However, another prominent cell death program, linker cell-type death (LCD), is morphologically conserved, and independent of the key genes that drive apoptosis, functioning, at least in part, through the ubiquitin proteasome system. These non-apoptotic processes may serve as backup programs when caspases are inactivated or unavailable, or, more likely, as freestanding cell culling programs. Non-apoptotic PCD has been documented extensively in the developing nervous system, and during the formation of germline and somatic gonadal structures, suggesting that preservation of these mechanisms is likely under strong selective pressure. Here, we discuss our current understanding of non-apoptotic PCD in animal development, and explore possible roles for LCD and other non-apoptotic developmental pathways in vertebrates. We raise the possibility that during vertebrate development, apoptosis may not be the major PCD mechanism.

Sensory pathways in the human embryonic spinal accessory nerve with special reference to the associated lower cranial nerve ganglia

BACKGROUND AND PURPOSE

Muscles supplied by the spinal accessory nerve are particularly prone to the development of trigger points characteristic of myofascial pain. This study aimed to confirm sensory pathways in the spinal accessory nerve and to describe sensory ganglion cell distributions along the lower cranial nerve roots.

METHODS

Using sagittal sections of ten human embryos at 6-7 weeks and horizontal sections of three 15- to 16-week-old embryos, we analyzed ganglion cell distributions along the lower cranial nerve roots, including the spinal accessory (XI) nerve.

RESULTS

In all ten 6- to 7-week-old embryos, the XI nerve root contained abundant ganglion cells, which were evenly distributed along the XI nerve root at levels between the jugular foramen and the dorsal root of the second cervical nerve. However, the hypoglossal (XII) nerve roots did not contain ganglion cells and did not communicate with nearby roots in the dural space. Thus, the so-called Froriep's occipital ganglion is unlikely to be associated with the XII nerve but rather with the XI nerve roots. According to observations of three larger fetuses (15-16 weeks), most of Froriep's ganglion cells seemed to have degenerated during early fetal life.

CONCLUSION

Nociceptive sensory pathways in the adult human XI nerve may be much more limited in number than would be expected based on previous animal studies. However, it is possible that sensory ganglion cells in the embryonic XI nerve root send axons toward the developing spinal accessory nerve fibers outside of the jugular foramen.

Elucidation of target muscle and detailed development of dorsal motor neurons in chick embryo spinal cord

The avian cervical spinal cord includes motoneurons (MNs) that send their axons through the dorsal roots. They have been called dorsal motoneurons (dMNs) and assumed to correspond to MNs of the accessory nerve that innervate the cucullaris muscle (SAN-MNs). However, their target muscles have not been elucidated to date. The present study sought to determine the targets and the specific combination of transcription factors expressed by dMNs and SAN-MNs and to describe the detailed development of dMNs. Experiments with tracing techniques confirmed that axons of dMNs innervated the cucullaris muscle. Retrogradely labeled dMNs were distributed in the ventral horn of C3 and more caudal segments. In most cases, some dMNs were also observed in the C2 segment. It was also demonstrated that SAN-MNs existed in the ventral horn of the C1-2 segments and the adjacent caudal hindbrain. Both SAN-MNs and dMNs expressed Isl1 but did not express Isl2, MNR2, or Lhx3. Rather, these MNs expressed Phox2b, a marker for branchial motoneurons (brMNs), although the intensity of expression was weaker. Dorsal MNs and SAN-MNs were derived from the Nkx2.2-positive precursor domain and migrated dorsally. Dorsal MNs remain in the ventral domain of the neural tube, unlike brMNs in the brainstem. These results indicate that dMNs and SAN-MNs belong to a common MN population innervating the cucullaris muscle and also suggest that they are similar to brMNs of the brainstem, although there are differences in Phox2b expression and in the final location of each population. J. Comp. Neurol. 521: 2987-3002, 2013. © 2013 Wiley Periodicals, Inc.

Maturation of the olfactory sensory neurons by Apaf-1/caspase-9-mediated caspase activity

Although the apoptotic role of caspases has been largely understood, accumulating evidence in Drosophila suggests that caspases also control other processes than apoptotic cell death. However, how caspases contribute to the development of the mammalian nervous system remains obscure. Here, we provide unique evidence that Apaf-1/caspase-9-mediated caspase signaling regulates the development of olfactory sensory neurons (OSNs), which includes axonal projection, synapse formation, and maturation of these neurons. This caspase signaling leads to a cleavage of Semaphorin 7A, a membrane-anchored semaphorin that is required for the proper axonal projection. Mutant mice deficient for apaf-1 or caspase-9 exhibit misrouted axons, impaired synaptic formation, and defects in the maturation of OSNs without affecting the number of these cells. Our findings suggest that Apaf-1/caspase-9-mediated nonapoptotic caspase signaling is required for the proper neural network formation during olfactory development.

Neuroprotective effects of caspase-3 inhibition on functional recovery and tissue sparing after acute spinal cord injury

STUDY DESIGN

We used gene microarrays and found that caspase-related death genes were upregulated. We tested caspase inhibition and evaluated its effect on the spinal cord after traumatic injury.

OBJECTIVE

The logical extension of previous studies was to determine whether downstream CASP genes might also be involved and whether inhibition might prevent injury-induced cell death.

SUMMARY OF BACKGROUND DATA

Apoptotic cell death occurs in all endogenous cellular compartments of the spinal cord, peaking at 3 days after injury in neurons, astrocytes, and microglia. The downstream effector caspase-3 cleaves several important cellular sites after being activated by upstream initiator caspases. Along with others, we have previously identified caspase signature cleavage of PARP, alpha-fodrin, and DFF45/ICAD in the injured rat spinal cord. We also showed rapid upregulation of caspase-3 gene expression along with localization of active caspase-3 in neurons and activated microglia after SCI. Others have reported that a more general active-site mimetic peptide ketone, benzylocarbonyl-Val-Ala-Asp-fluromethylketone (zVAD-fmk) was neuroprotective after rat spinal cord injury (SCI).

METHODS

In this study, we administered the caspase-3 subfamily tetrapeptide cell permeable inhibitor Z-Asp(O-Me)-Glu(O-Me)-Val-Asp(O-Me) fluoromethyl ketone (DEVD-fmk) intraperitoneally 1 hour after laminectomy and moderate (25 g cm force) SCI in rats. RESULTS.: We used the open field locomotor rating (LRS) over a 14-day course and found statistically significant improvement in DEVD-fmk-treated rats, LRS, 9.8 +/- 0.93 SEM, compared with vehicle, 6.6 +/- 0.4 (P < 0.05). Histologic analysis of percent spinal cord tissue volume spared was 50% greater for DEVD-fmk versus control (P < 0.5).

CONCLUSION

These results indicate neuroprotection at both the cellular level and with substantial functional recovery, suggesting caspase-3 inhibition may be a viable therapy in the early hours after experimental SCI.

Developing postmitotic mammalian neurons in vivo lacking Apaf-1 undergo programmed cell death by a caspase-independent, nonapoptotic pathway involving autophagy

Previous studies have shown that caspases and Apaf-1 are required for the normal programmed cell death (PCD) in vivo of immature postmitotic neurons and mitotically active neuronal precursor cells. In contrast, caspase activity is not necessary for the normal PCD of more mature postmitotic neurons that are establishing synaptic connections. Although normally these cells use caspases for PCD, in the absence of caspase activity these neurons undergo a distinct nonapoptotic type of degeneration. We examined the survival of these more mature postmitotic neuronal populations in mice in which Apaf-1 has been genetically deleted and find that they exhibit quantitatively normal PCD of developing postmitotic neurons. We next characterized the morphological mode of PCD in these mice and show that the neurons degenerate by a caspase-independent, nonapoptotic pathway that involves autophagy. However, autophagy does not appear to be involved in the normal PCD of postmitotic neurons in which caspases and Apaf-1 are present and functional because quantitatively normal neuronal PCD occurred in the absence of a key gene required for autophagy (ATG7). Finally, we examined the possible role of another caspase-independent type of neuronal PCD involving the apoptosis-inducing factor (AIF). Mice deficient in AIF also exhibit quantitatively normal PCD of postmitotic neurons after caspase inhibition. Together, these data indicate that, when key components of the type 1 apoptotic pathway (i.e., caspases and Apaf-1) are perturbed in vivo, developing postmitotic neurons nonetheless undergo quantitatively normal PCD by a caspase-independent pathway involving autophagy and not requiring AIF.

Survival and death of mature avian motoneurons in organotypic slice culture: trophic requirements for survival and different types of degeneration

We have developed an organotypic culture technique that uses slices of chick embryo spinal cord, in which trophic requirements for long-term survival of mature motoneurons (MNs) were studied. Slices were obtained from E16 chick embryos and maintained for up to 28 days in vitro (DIV) in a basal medium. Under these conditions, most MNs died. To promote MN survival, 14 different trophic factors were assayed. Among these 14, glial cell line-derived neurotrophic factor (GDNF) and vascular endothelial growth factor were the most effective. GDNF was able to promote MN survival for at least 28 DIV. K(+) depolarization or caspase inhibition prevented MN death but also induced degenerative-like changes in rescued MNs. Agents that elevate cAMP levels promoted the survival of a proportion of MNs for at least 7 DIV. Examination of dying MNs revealed that, in addition to cells exhibiting a caspase-3-dependent apoptotic pattern, some MNs died by a caspase-3-independent mechanism and displayed autophagic vacuoles, an extremely convoluted nucleus, and a close association with microglia. This organotypic spinal cord slice culture may provide a convenient model for testing conditions that promote survival of mature-like MNs that are affected in late-onset MN disease such as amyotrophic lateral sclerosis.

Lysosomal cathepsins in embryonic programmed cell death

During limb development, expression of cathepsin D and B genes prefigure the pattern of interdigital apoptosis including the differences between the chick and the webbed digits of the duck. Expression of cathepsin L is associated with advanced stages of degeneration. Analysis of Gremlin-/- and Dkk-/- mouse mutants and local treatments with BMP proteins reveal that the expression of cathepsin B and D genes is regulated by BMP signaling, a pathway responsible for triggering cell death. Further cathepsin D protein is upregulated in the preapoptotic mesenchyme before being released into the cytosol, and overexpression of cathepsin D induces cell death in embryonic tissues by a mechanism including mitochondrial permeabilization and nuclear translocation of AIF. Combined inhibition of cathepsin and caspases suggests a redundancy in the apoptotic molecular machinery, providing evidence for compensatory activation mechanisms in the cathepsin pathway when caspases are blocked. It is concluded that lysosomal enzymes are functionally implicated in embryonic programmed cell death.

Distinct susceptibility of developing neurons to death following Bax overexpression in the chicken embryo

Bax is a proapoptotic protein that is required for programmed cell death (PCD) of many neuronal populations. Here we show that, during an early period of retinal PCD and in naturally occurring sensory and motor neuron (MN) death in the spinal cord, Bax delivery results in enhanced death of these neural populations. In contrast, Bax overexpression fails to enhance an early phase of MN death that occurs in the cervical spinal cord, although overexpressed Bax appears to be activated in dying MNs. Bax overexpression does not also affect the survival of immature neurons prior to the PCD period. Taken together, these data provide the first in vivo evidence suggesting that Bax appears to act selectively as an executioner only in neurons undergoing PCD. Furthermore, although Bax appears to mediate the execution pathway for PCD, the effect of Bax overexpression on susceptibility to death differs between different neuronal populations.

Alix, making a link between apoptosis-linked gene-2, the endosomal sorting complexes required for transport, and neuronal death in vivo

Alix/apoptosis-linked gene-2 (ALG-2)-interacting protein X is an adaptor protein involved in the regulation of the endolysosomal system through binding to endophilins and to endosomal sorting complexes required for transport (ESCRT) proteins, TSG101 and CHMP4b. It was first characterized as an interactor of ALG-2, a calcium-binding protein necessary for cell death, and several observations suggest a role for Alix in controlling cell death. We used electroporation in the chick embryo to test whether overexpressed wild-type or mutated Alix proteins influence cell death in vivo. We show that Alix overexpression is sufficient to induce cell death of neuroepithelial cells. This effect is strictly dependent on its capacity to bind to ALG-2. On the other hand, expression of Alix mutants lacking the ALG-2 or the CHMP4b binding sites prevents early programmed cell death in cervical motoneurons at day 4.5 of chick embryo development. This protection afforded by Alix mutants was abolished after deletion of the TSG101, but not of the endophilin, binding sites. Our results suggest that the interaction of the ALG-2/Alix complex with ESCRT proteins is necessary for naturally occurring death of motoneurons. Therefore, Alix represents a molecular link between the endolysosomal system and the cell death machinery.

Redundant cell death mechanisms as relics and backups

Here we review recent observations indicating the existence of redundant cell death mechanisms. We speculate that this redundancy reflects a particular evolutionary history for cellular demise. Autophagic or apoptotic elements might have been added to a primordial death mechanism, initially improving cell dismantling and later acquiring the ability to act themselves as death effectors. The resulting redundancy of cell death mechanisms has pathophysiological implications.

Identification of midkine as a mediator for intercellular transfer of drug resistance

Resistance to cytotoxic agents is a major limitation for their clinical use to treat human cancers. Tumors become resistant to chemotherapy when a subset of cells undergoes molecular changes leading to overexpression of drug transport proteins, alterations in drug-target interactions or reduced ability to commit apoptosis. However, such changes may not be sufficient to explain why both resistant and nonresistant cells survive drug's action in tumors that ultimately become drug resistant. We hypothesized that, in such tumors, a cytoprotective relationship may exist between drug-resistant and neighboring drug-sensitive cells. The present study addresses the possibility that drug-resistant cells secrete in their culture medium factors able to protect sensitive cells from drug toxicity. A survival molecule, midkine, was identified by cDNA array to be expressed only in drug-resistant cells. Midkine-enriched fractions obtained by affinity chromatography exert a significant cytoprotective effect against doxorubicin in the wild-type drug-sensitive cells. Moreover, transfection of these cells with the midkine gene caused a decreased response to doxorubicin. The underlying mechanism of this cytoprotection appeared to imply activation of the Akt pathway and inhibition of drug-induced proliferation arrest as well as apoptotic cell death. These findings provide evidence for the existence of intercellular cytoprotective signals such as the one mediated by midkine, originating from cells with acquired drug resistance to protect neighboring drug-sensitive cells and thus contribute to development of resistance to chemotherapy.

Cell death independent of caspases: a review

Patterns of cell death have been divided into apoptosis, which is actively executed by specific proteases, the caspases, and accidental necrosis. However, there is now accumulating evidence indicating that cell death can occur in a programmed fashion but in complete absence and independent of caspase activation. Alternative models of programmed cell death (PCD) have therefore been proposed, including autophagy, paraptosis, mitotic catastrophe, and the descriptive model of apoptosis-like and necrosis-like PCD. Caspase-independent cell death pathways are important safeguard mechanisms to protect the organism against unwanted and potential harmful cells when caspase-mediated routes fail but can also be triggered in response to cytotoxic agents or other death stimuli. As in apoptosis, the mitochondrion can play a key role but also other organelles such as lysosomes and the endoplasmic reticulum have an important function in the release and activation of death factors such as cathepsins, calpains, and other proteases. Here we review the various models of PCD and their death pathways at molecular and organelle level and discuss the relevance of the growing knowledge of caspase-independent cell death pathways for cancer.

Multiple cell death programs: Charon's lifts to Hades

Cells use different pathways for active self-destruction as reflected by different morphology: while in apoptosis (or "type I") nuclear fragmentation associated with cytoplasmic condensation but preservation of organelles is predominant, autophagic degradation of cytoplasmic structures preceding nuclear collapse is a characteristic of a second type of programmed cell death (PCD). The diverse morphologies can be attributed--at least to some extent--to distinct biochemical and molecular events (e.g. caspase-dependent and -independent death programs; DAP-kinase activity, Ras-expression). However, apoptosis and autophagic PCD are not mutually exclusive phenomena. Rather, diverse PCD programs emerged during evolution, the conservation of which apparently allows cells a flexible response to environmental changes, either physiological or pathological.

Motoneuronal death during spinal cord development is mediated by oxidative stress

The involvement of reactive oxygen species (ROS) in neuronal death has been determined in culture, and in association with several neurodegenerative disorders. We examined whether ROS participate in the cell death observed during spinal cord development. We found that the general pattern of high ROS levels, gene expression for some antioxidant enzymes, and motoneuron death correlated positively along spinal cord development. ROS were reduced in spinal cords cultured in the presence of a synthetic superoxide dismutase and catalase mimetic, with a concomitant reduction in cell death and an increase in the number of motoneurons. The number of motoneurons was higher in spinal cords treated with the antioxidant than in those treated with caspase inhibitors. In general, the increase in motoneuron survival did not correlate with the reduction in cells undergoing DNA degradation in the motoneuronal region. These results suggest that ROS are signaling molecules controlling caspase-dependent and caspase-independent programmed motoneuron death, and support the hypothesis that this mechanism is abnormally turned on in some neurodegenerative disorders and aging.

Growth factor deprivation induces an alternative non-apoptotic death mechanism that is inhibited by Bcl2 in cells derived from neural precursor cells

Although apoptosis has been considered the typical mechanism for physiological cell death, presently alternative mechanisms need to be considered. We previously showed that fibroblast growth factor-2 (FGF2) could act as a survival factor for neural precursor cells. To study the death mechanism activated by the absence of this growth factor, we followed the changes in cell morphology and determined cell viability by staining with several dyes after FGF2 removal from mesencephalic neural-progenitor-cell cultures. The changes observed did not correspond to those associated with apoptosis. After 48 h in the absence of FGF2, cells began to develop vacuoles in their cytoplasm, a phenotype that became very obvious 3-5 days later. Double-membrane vacuoles containing cell debris were observed. Vacuolated cells did not stain with either ethidium bromide or trypan Blue, and did not show chromatin condensations. Nonetheless, during the course of culture, vacuolated cells formed aggregates with highly condensed chromatin and detached from the plate. Neural progenitor cells grown in the presence of FGF2 did not display any of those characteristics. The vacuolated phenotype could be reversed by the addition of FGF2. Typical autophagy inhibitors such as 3-MA and LY294002 inhibited vacuole development, whereas a broad-spectrum caspase inhibitor did not. Interestingly, Bcl-2 overexpression retarded vacuole development. In conclusion, we identified a death autophagy-like mechanism activated by the lack of a specific survival factor that can be inhibited by Bcl2. We propose that anti-apoptotic Bcl2 family members are key molecules controlling death activation independently of the cell degeneration mechanism used.

Rapid induction of the intrinsic apoptotic pathway byL-glutamine starvation

While the amino acid L-glutamine is known to play a role in the survival of several cell types, the underlying molecular mechanisms are still poorly defined. We show in this report that L-glutamine starvation rapidly triggered apoptosis in Sp2/0-Ag14 hybridoma cells. This process involved the activation of both caspases-9 and -3, suggesting that L-glutamine deprivation initiated an intrinsic apoptotic pathway in Sp2/0-Ag14 cells. Supporting this idea, the cytosolic release of the mitochondrial proteins SMAC/DIABLO and cytochrome c (Cyt c) was observed, with an initial limited leakage occurring during the first 30 min of L-glutamine deprivation, followed by a greater release after 60 min. The latter occurred simultaneously with the translocation of the pro-apoptotic protein Bax to the mitochondria. Finally, a decline in XIAP levels and the activation of caspases-3 and -9 were observed. Thus, L-glutamine deprivation of Sp2/0-Ag14 cells rapidly triggers intracellular events, which target the mitochondria, leading to the cytosolic release of apoptogenic factors, the activation of caspases-9 and -3, and the commitment to the death program. This work introduces the Sp2/0Ag14 hybridoma as a unique model for the study of the molecular events underlying the pro-survival function of L-glutamine.

Senescence-initiated Reversal of Drug Resistance

The present study was undertaken to verify whether induction of senescence could be sufficient to reverse drug resistance and, if so, to determine the underlying mechanism(s). Our findings indicated that cotreatment of drug-resistant neuroblastoma cells with doxorubicin, at sublethal concentrations, in combination with the pan-caspase inhibitor, Q-VD-OPH, elicited a strong reduction of cell viability that occurred in a caspase-independent manner. This was accompanied by the appearance of a senescence phenotype, as evidenced by increased p21/WAF1 expression and senescence-associated beta-galactosidase activity. Experiments using specific inhibitors of major cellular proteases other than caspases have shown that inhibition of cathepsin L, but not proteasome or cathepsin B, was responsible for the senescence-initiated reversal of drug resistance. This phenomenon appeared to be general because it was valid for other drugs and drug-resistant cell lines. A nonchemical approach, through cell transfection with cathepsin L small interfering RNA, also strongly reversed drug resistance. Further investigation of the underlying mechanism revealed that cathepsin L inhibition resulted in the alteration of intracellular drug distribution. In addition, in vitro experiments have demonstrated that p21/WAF1 is a substrate for cathepsin L, suggesting that inhibition of this enzyme may result in p21/WAF1 stabilization and its increased accumulation. All together, these findings suggest that cathepsin L inhibition in drug-resistant cells facilitates induction of senescence and reversal of drug resistance. This may represent the basis for a novel function of cathepsin L as a cell survival molecule responsible for initiation of resistance to chemotherapy.

GDNF-deprived sympathetic neurons die via a novel nonmitochondrial pathway

The mitochondrial death pathway is triggered in cultured sympathetic neurons by deprivation of nerve growth factor (NGF), but the death mechanisms activated by deprivation of other neurotrophic factors are poorly studied. We compared sympathetic neurons deprived of NGF to those deprived of glial cell line-derived neurotrophic factor (GDNF). In contrast to NGF-deprived neurons, GDNF-deprived neurons did not die via the mitochondrial pathway. Indeed, cytochrome c was not released to the cytosol; Bax and caspase-9 and -3 were not involved; overexpressed Bcl-xL did not block the death; and the mitochondrial ultrastructure was not changed. Similarly to NGF-deprived neurons, the death induced by GDNF removal is associated with increased autophagy and requires multiple lineage kinases, c-Jun and caspase-2 and -7. Serine 73 of c-Jun was phosphorylated in both NGF- and GDNF-deprived neurons, whereas serine 63 was phosphorylated only in NGF-deprived neurons. In many NGF-deprived neurons, the ultrastructure of the mitochondria was changed. Thus, a novel nonmitochondrial caspase-dependent death pathway is activated in GDNF-deprived sympathetic neurons.

Neuromuscular development after the prevention of naturally occurring neuronal death by Bax deletion

The removal of excess neurons by programmed cell death (PCD) is believed to be critical for the proper development and function of the nervous system. A major role of this neuronal loss is to attain quantitative matching of neurons with their targets and afferents. Because motoneurons (MNs) in Bax knock-out (Bax KO) mice fail to undergo PCD in the face of normal target muscle development, we asked whether the excess rescued neurons in Bax KO mice can develop normally. We observed many small atrophied MNs in postnatal Bax KO mice, and these failed to innervate limb muscle targets. When examined embryonically during the PCD period, however, these excess MNs had initiated target innervation. To examine whether a limitation in trophic factor availability is responsible for postnatal MN atrophy and loss of innervation, we applied glial cell line-derived neurotrophic factor (GDNF) to neonatal mice. GDNF injection for 7-14 d induced the regrowth and reinnervation of muscle targets by atrophic MNs in Bax KO mice and prevented the normal postnatal death of MNs in wild-type mice. These results indicate that, although initially all of the MNs, including those rescued by Bax deletion, are able to project to and innervate targets, because of limited target-derived signals required for maintaining innervation and growth, only a subpopulation can grow and retain target contacts postnatally. Although sensory neurons in the dorsal root ganglia are also rescued from PCD by Bax deletion, their subsequent development is less affected than that of MNs.

Inhibition of caspases promotes long-term survival and reinnervation by axotomized spinal motoneurons of denervated muscle in newborn rats

We examined whether (1) a pan-caspase inhibitor, Boc-D-FMK, exerts long-term neuroprotective effects on spinal motoneurons (MNs) after root avulsion in neonatal rats and (2) whether the rescued spinal MNs regenerate their axons into a peripheral nerve (PN) graft and reinnervate a previously denervated target muscle. Eight weeks after root avulsion, 67% of spinal MNs remained in the Boc-D-FMK-treated group, whereas all MNs died in the sham control group. By 12 weeks postinjury, however, all Boc-D-FMK treated MNs died. In the regeneration experiment, a PN graft was implanted at different times after injury. The animals were allowed to survive for 4 weeks following the operation. Without caspase inhibition, MNs did not regenerate at any time point. In animals treated with Ac-DEVD-CHO, a caspase-3-specific inhibitor, and Boc-D-FMK, 44 and 62% of MNs, respectively, were found to regenerate their axons into a PN graft implanted immediately after root avulsion. When the PN graft was implanted 2 weeks after injury, however, MNs failed to regenerate following Ac-DEVD-CHO treatment, whereas 53% of MNs regenerated their axons into the graft after treatment with Boc-D-FMK. No regeneration was observed when a PN graft was implanted later than 2 weeks after injury. In the reinnervation study, injured MNs and the target biceps muscle were reconnected by a PN bridge implanted 2 weeks after root avulsion with administration of Boc-D-FMK. Eight weeks following the operation, 39% of MNs reinnervated the biceps muscle. Morphologically normal synapses and motor endplates were reformed in the muscle fibers. Collectively, these data provide evidence that injured neonatal motoneurons can survive and reinnervate peripheral muscle targets following inhibition of caspases.

Caspase inhibition switches doxorubicin-induced apoptosis to senescence

The inhibition of apoptosis is generally believed to be a major determinant of resistance to chemotherapy. However, recent findings have shown that caspase inhibitors do not protect cancer cells from death by cytotoxic agents, but may switch drug-induced apoptosis to an alternative 'default death'. The primary goals of this study were to determine the major characteristics of the 'default death' and the mechanism by which this switch is activated. For this purpose, we first investigated putative cell death modes induced by doxorubicin. Molecular markers associated with these death modes were utilized to identify the default death resulting from the inhibition of apoptosis. Our findings demonstrated that doxorubicin induced at least three distinct types of cell death, senescence, apoptosis and a type of necrosis, which were concentration dependent. Specific molecular markers such as p21/WAF1, activated caspase-3 and activated Akt were associated with these death modes. The pan-caspase inhibitor (Q-VD-OPH) greatly reduced doxorubicin-induced caspase-3 activation but did not protect cells against drug toxicity. The combination of doxorubicin and Q-VD-OPH caused an increased expression of p21/WAF1 and senescence -associated -beta-galactosidase activity, but did not alter Akt activation. Collectively, these findings suggest that the inhibition of apoptosis may lead to an increased expression of cell cycle inhibitors and cellular senescence.

Steroid-triggered programmed cell death of a motoneuron is autophagic and involves structural changes in mitochondria

Neuronal death occurs during normal development and disease and can be regulated by steroid hormones. In the hawkmoth, Manduca sexta, individual accessory planta retractor (APR) motoneurons undergo a segment-specific pattern of programmed cell death (PCD) at pupation that is triggered directly and cell autonomously by the steroid hormone 20-hydroxyecdysone (20E). APRs from abdominal segment six [APR(6)s] die by 48 hours after pupal ecdysis (PE; entry into the pupal stage), whereas APR(4)s survive until adulthood. Cell culture experiments showed previously that 20E acts directly on APRs to trigger PCD, with intrinsic segmental identity determining which APRs die. The APR(6) death pathway includes caspase activation and loss of mitochondrial function. We used transmission electron microscopy to investigate the ultrastructure of APR somata before and during PCD. APR(4)s showed normal ultrastructure at all stages examined, as did APR(6)s until approximately stage PE. During APR(6) death, there was massive accumulation of autophagic bodies and vacuoles, mitochondria became ultracondensed and aggregated into compact clusters, and ribosomes aggregated in large blocks. Nuclear ultrastructure remained normal, without chromatin condensation, until the nuclear envelope fragmented late in the death process. Light microscopic immunocytochemistry showed that dying APR(6)s were TUNEL-positive, which is diagnostic of fragmented DNA. These observations indicate that the steroid-induced, caspase-dependent, cell-autonomous PCD of APR(6)s is autophagic, not apoptotic, and support an early role for mitochondrial alterations during PCD. This system permits the study of neuronal death in response to its bona fide developmental signal, the rise in a steroid hormone.

Caspase-independent cell deaths

A very common and the best understood of the mechanisms of physiological cell death is apoptosis, resulting from the activation, through either of two primary pathways, of site-specific proteases called caspases. There are, however, many other routes to cell death, prominently including autophagy and proteasomal degradation of critical constituents of cells. These routes are frequently seen in experimental situations in which initiator or effector caspases are inhibited or blocked through genetic means, but they are also encountered during normal physiological and pathological processes. Most frequently, autophagic or proteasomal degradation is used to eliminate massive cytoplasm of very large cells, especially post-mitotic cells, and these pathways are prominent even though caspase genes, messages, and pro-enzymes are found in the cells. These forms of cell death are fully physiological and not simply a default pathway for a defective cell; and they are distinct from necrosis. We do not yet understand the extent to which the pathways are linked, what mechanisms trigger the caspase-independent deaths, and how the choices are made.

Bcl-2 rescues motoneurons from early cell death in the cervical spinal cord of the chicken embryo

Motoneurons (MNs) in the cervical spinal cord of the chicken embryo undergo programmed cell death (PCD) between embryonic day (E) 4 and E5. The intracellular molecules regulating this early phase of PCD remain unknown. Here we show that introduction of Bcl-2 by a replication-competent avian retroviral vector prevented MN degeneration at E4.5, whereas the expression of the green fluorescent protein (GFP) was ineffective. Bcl-2 expression did not affect the number of Islet-1/2-positive MNs at the onset of cell death (E4). However, when examined at the end of the cell death period (E5.5), the number of Islet-1/2-positive MNs was clearly increased in Bcl-2-transfected embryos compared with control and GFP-transfected embryos. Activation of caspase-3, which is normally observed in this early MN death, was also prevented by Bcl-2. Thus, MNs in the cervical spinal cord appear to use intracellular pathway(s) for early PCD that is responsive to Bcl-2.

Proliferation and apoptosis in the developing human neocortex

The cell kinetics of the developing central nervous system (CNS) is determined by both proliferation and apoptosis. In the human neocortex at week 6 of gestation, proliferation is confined to the ventricular zone, where mitotic figures and nuclear immunoreactivity for proliferating cell nuclear antigen (PCNA) are detectable. Cell division is symmetric, with both daughter cells reentering mitosis. At week 7, the subventricular zone, a secondary proliferative zone, appears. It mainly gives rise to local circuit neurons and glial cells. Around week 12, the ventricular and subventricular zones are thickest, and the nuclear PCNA label is strongest, indicating that proliferation peaks at this stage. Thereafter, asymmetric division becomes the predominant mode of proliferation, with one daughter cell reentering mitosis and the other one migrating out. Towards late gestation, the ventricular and subventricular zones almost completely disappear and proliferation shifts towards the intermediate and subplate zones, where mainly glial cells are generated. A remnant of the subventricular zone with proliferative activity persists into adulthood. In general, proliferation follows a latero-medial gradient in the neocortex lasting longer in its lateral parts. Apoptotic nuclei have been detected around week 5, occurring in low numbers in the ventricular zone at this stage. Apoptotic cell death increases around midgestation and then spreads throughout all cortical layers, with most dying cells located in the ventricular and subventricular zones. This spatial distribution of apoptosis extends into late gestation. During the early postnatal period, most apoptotic cells are still located in the subcortical layers. During early embryonic development, proliferation and apoptosis are closely related, and are probably regulated by common regulators. In the late fetal and early postnatal periods, when proliferation has considerably declined in all cortical layers, apoptosis may occur in neurons whose sprouting axons do not find their targets.

Expression of apoptosis-inducing factor during early neural differentiation in the chick embryo

The distribution of apoptosis-inducing factor (AIF) immunoreactivity has been studied in the developing somites and nervous system of the chick embryo at embryonic day 4. AIF was found to be expressed primarily in the cytoplasm of cells of the ventral motor roots, at the points of their insertion into the neural tube. Co-localization of mitochondrial AIF immunoreactivity with the epitopes recognized by the monoclonal antibodies HNK-1 and 1E8 suggests that the AIF may be present in Schwann cell precursors as well as in nerve fibres. AIF immunoreactivity was not observed in either cell bodies in the neural tube, or in the somitic tissue surrounding the ventral roots. The results are consistent with the hypothesis that AIF may be involved in neuronal cell death during development, and that target-derived neuronal survival factors may act by controlling AIF activity.

Roles of caspases in the programmed cell death of motoneurons in vivo

Cysteine proteases comprising the caspase family have been considered one of the major executioners of programmed cell death. However, detailed analyses of the programmed cell death of developing motoneurons in mice following the genetic deletion of two key caspases, casp-3 and casp-9, and in the chick embryo following treatment with caspase inhibitors, indicate that normal amounts of cell loss occur although the death process is delayed. Motoneurons undergoing programmed cell death without caspase activities exhibit a nonapoptotic morphology in which nuclear changes such as chromatin condensation are absent or reduced and which exhibit extensive cytoplasmic vacuolization such as is rarely observed in degenerating control neurons. These results suggest that caspases are involved in, but are not indispensable for, the developmental death of motoneurons, and that one function of caspases may be to facilitate the removal of cells that are destined to die. Possible alternative caspase-independent pathways for the programmed death of motoneurons are discussed.

Long-lasting aberrant tubulovesicular membrane inclusions accumulate in developing motoneurons after a sublethal excitotoxic insult: a possible model for neuronal pathology in neurodegenerative disease

We have previously shown that chronic treatment of chick embryos [from embryonic day 5 (E5) to E9] with NMDA rescues spinal cord motoneurons (MNs) from programmed cell death. In this situation, MNs exhibit a reduced vulnerability to acute excitotoxic lesions and downregulate NMDA and AMPA-kainate receptors. Here, we report that this treatment results in long-lasting sublethal structural changes in MNs. In Nissl-stained sections from the spinal cord of NMDA-treated embryos, MNs display an area adjacent to an eccentrically positioned nucleus in which basophilia is excluded. Ultrastructurally, MNs accumulate tubulovesicular structures surrounded by Golgi stacks. Thiamine pyrophosphatase but not acid phosphatase was detected inside the tubulovesicular structures, which are resistant to disruption by brefeldin A or monensin. Immunocytochemistry reveals changes in the content and distribution of calcitonin gene-related peptide, the KDEL receptor, the early endosomal marker EEA1, and the recycling endosome marker Rab11, indicating that a dysfunction in membrane trafficking and protein sorting occurs in these MNs. FM1-43, a marker of the endocytic pathway, strongly accumulates in MNs from isolated spinal cords after chronic NMDA treatment. Changes in the distribution of cystatin C and presenilin-1 and an accumulation of amyloid precursor protein and beta-amyloid product were also observed in NMDA-treated MNs. None of these alterations involve an interruption of MN-target (muscle) connections, as detected by the retrograde tracing of MNs with cholera toxin B subunit. These results demonstrate that chronic NMDA treatment induces severe changes in the motoneuronal endomembrane system that may be related to some neuropathological alterations described in human MN disease.

Programmed cell death of developing mammalian neurons after genetic deletion of caspases

An analysis of programmed cell death of several populations of developing postmitotic neurons after genetic deletion of two key members of the caspase family of pro-apoptotic proteases, caspase-3 and caspase-9, indicates that normal neuronal loss occurs. Although the amount of cell death is not altered, the death process may be delayed, and the cells appear to use a nonapoptotic pathway of degeneration. The neuronal populations examined include spinal interneurons and motor, sensory, and autonomic neurons. When examined at both the light and electron microscopic levels, the caspase-deficient neurons exhibit a nonapoptotic morphology in which nuclear changes such as chromatin condensation are absent or reduced; in addition, this morphology is characterized by extensive cytoplasmic vacuolization that is rarely observed in degenerating control neurons. There is also reduced terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling in dying caspase-deficient neurons. Despite the altered morphology and apparent temporal delay in cell death, the number of neurons that are ultimately lost is indistinguishable from that seen in control animals. In contrast to the striking perturbations in the morphology of the forebrain of caspase-deficient embryos, the spinal cord and brainstem appear normal. These results are consistent with the growing idea that the involvement of specific caspases and the occurrence of caspase-independent programmed cell death may be dependent on brain region, cell type, age, and species or may be the result of specific perturbations or pathology.