Mohanish Deshmukh

Caspase inhibitor affords neuroprotection with delayed administration in a rat model of neonatal hypoxic-ischemic brain injury.

Programmed cell death (apoptosis) is a normal process in the developing nervous system. Recent data suggest that certain features seen in the process of programmed cell death may be favored in the developing versus the adult brain in response to different brain injuries. In a well characterized model of neonatal hypoxia-ischemia, we demonstrate marked but delayed cell death in which there is prominent DNA laddering, TUNEL-labeling, and nuclei with condensed chromatin. Caspase activation, which is required in many cases of apoptotic cell death, also followed a delayed time course after hypoxia-ischemia. Administration of boc-aspartyl(OMe)-fluoromethylketone, a pan-caspase inhibitor, was significantly neuroprotective when given by intracerebroventricular injection 3 h after cerebral hypoxia-ischemia. In addition, systemic injections of boc-aspartyl(OMe)-fluoromethylketone also given in a delayed fashion, resulted in significant neuroprotection. These findings suggest that caspase inhibitors may be able to provide benefit over a prolonged therapeutic window after hypoxic-ischemic events in the developing brain, a major contributor to static encephalopathy and cerebral palsy.

Guidelines for the use and interpretation of assays for monitoring cell death in higher eukaryotes

Cell death is essential for a plethora of physiological processes, and its deregulation characterizes numerous human diseases. Thus, the in-depth investigation of cell death and its mechanisms constitutes a formidable challenge for fundamental and applied biomedical research, and has tremendous implications for the development of novel therapeutic strategies. It is, therefore, of utmost importance to standardize the experimental procedures that identify dying and dead cells in cell cultures and/or in tissues, from model organisms and/or humans, in healthy and/or pathological scenarios. Thus far, dozens of methods have been proposed to quantify cell death-related parameters. However, no guidelines exist regarding their use and interpretation, and nobody has thoroughly annotated the experimental settings for which each of these techniques is most appropriate. Here, we provide a nonexhaustive comparison of methods to detect cell death with apoptotic or nonapoptotic morphologies, their advantages and pitfalls. These guidelines are intended for investigators who study cell death, as well as for reviewers who need to constructively critique scientific reports that deal with cellular demise. Given the difficulties in determining the exact number of cells that have passed the point-of-no-return of the signaling cascades leading to cell death, we emphasize the importance of performing multiple, methodologically unrelated assays to quantify dying and dead cells.

Evidence of a Novel Event during Neuronal Death: Development of Competence-to-Die in Response to Cytoplasmic Cytochrome c

Sympathetic neurons undergoing programmed cell death after nerve growth factor (NGF) deprivation are shown to exhibit a protein synthesis-dependent, BAX-dependent loss of cytochrome c from the mitochondria. However, cytoplasmic microinjection of cytochrome c was insufficient to induce cell death in NGF-maintained sympathetic neurons. In contrast, microinjection of cytochrome c rapidly induced a caspase-dependent death in NGF-deprived, Bax-deficient or NGF-deprived, cycloheximide-treated neurons. Cells needed to be deprived of NGF for 15-20 hr before they acquired competence to die with injection of cytochrome c. These data suggest that NGF deprivation induced the translocation of cytochrome c and another event, which we term as competence-to-die, that was independent of macromolecular synthesis and BAX function. Both these processes were required for neurons to undergo apoptosis.

Glucose metabolism inhibits apoptosis in neurons and cancer cells by redox inactivation of cytochrome c

Neurons and cancer cells use glucose extensively, yet the precise advantage of this adaptation remains unclear. These two seemingly disparate cell types also show an increased regulation of the apoptotic pathway, which allows for their long-term survival. Here we show that both neurons and cancer cells strictly inhibit cytochrome c-mediated apoptosis by a mechanism dependent on glucose metabolism. We report that the pro-apoptotic activity of cytochrome c is influenced by its redox state and that increases in reactive oxygen species (ROS) following an apoptotic insult lead to the oxidation and activation of cytochrome c. In healthy neurons and cancer cells, however, cytochrome c is reduced and held inactive by intracellular glutathione (GSH), generated as a result of glucose metabolism by the pentose phosphate pathway. These results uncover a striking similarity in apoptosis regulation between neurons and cancer cells and provide insight into an adaptive advantage offered by the Warburg effect for cancer cell evasion of apoptosis and for long-term neuronal survival.

miR-29b is activated during neuronal maturation and targets BH3-only genes to restrict apoptosis

The execution of apoptosis is critical for proper development of the nervous system. However, it is equally important that neurons strictly inhibit apoptosis after development to ensure their survival throughout the lifetime of the organism. Here we show that a microRNA, miR-29b, is markedly induced with neuronal maturation and functions as a novel inhibitor of neuronal apoptosis. The prosurvival function of miR-29b is mediated by targeting genes in the proapoptotic BH3-only family. Our results identify a unique strategy evolved by maturing neurons that uses a single microRNA to inhibit the multiple, redundant BH3-only proteins that are key initiators of apoptosis.

Critical function of endogenous XIAP in regulating caspase activation during sympathetic neuronal apoptosis

In sympathetic neurons, unlike most nonneuronal cells, growth factor withdrawal–induced apoptosis requires the development of competence in addition to cytochrome c release to activate caspases. Thus, although most nonneuronal cells die rapidly with cytosolic cytochrome c alone, sympathetic neurons are remarkably resistant unless they develop competence. We have identified endogenous X-linked inhibitor of apoptosis protein (XIAP) as the essential postcytochrome c regulator of caspase activation in these neurons. In contrast to wild-type neurons that are resistant to injection of cytochrome c, XIAP-deficient neurons died rapidly with cytosolic cytochrome c alone. Surprisingly, the release of endogenous Smac was not sufficient to overcome the XIAP resistance in sympathetic neurons. In contrast, the neuronal competence pathway permitted cytochrome c to activate caspases by inducing a marked reduction in XIAP levels in these neurons. Thus, the removal of XIAP inhibition appears both necessary and sufficient for cytochrome c to activate caspases in sympathetic neurons. These data identify a critical function of endogenous XIAP in regulating apoptosis in mammalian cells.

Yeast ribosomal protein L1 is required for the stability of newly synthesized 5S rRNA and the assembly of 60S ribosomal subunits.

Ribosomal protein L1 from Saccharomyces cerevisiae binds 5S rRNA and can be released from intact 60S ribosomal subunits as an L1-5S ribonucleoprotein (RNP) particle. To understand the nature of the interaction between L1 and 5S rRNA and to assess the role of L1 in ribosome assembly and function, we cloned the RPL1 gene encoding L1. We have shown that RPL1 is an essential single-copy gene. A conditional null mutant in which the only copy of RPL1 is under control of the repressible GAL1 promoter was constructed. Depletion of L1 causes instability of newly synthesized 5S rRNA in vivo. Cells depleted of L1 no longer assemble 60S ribosomal subunits, indicating that L1 is required for assembly of stable 60S ribosomal subunits but not 40S ribosomal subunits. An L1-5S RNP particle not associated with ribosomal particles was detected by coimmunoprecipitation of L1 and 5S rRNA. This pool of L1-5S RNP remained stable even upon cessation of 60S ribosomal subunit assembly by depletion of another ribosomal protein, L16. Preliminary results suggest that transcription of RPL1 is not autogenously regulated by L1.

Reduced Apaf-1 levels in cardiomyocytes engage strict regulation of apoptosis by endogenous XIAP

Overexpression studies have identified X-linked inhibitor of apoptosis protein (XIAP) as a potent inhibitor of caspases. However, the exact function of endogenous XIAP in regulating mammalian apoptosis is less clear. Endogenous XIAP strictly regulates cytochrome c–dependent caspase activation in sympathetic neurons but not in many mitotic cells. We report that postmitotic cardiomyocytes, unlike fibroblasts, are remarkably resistant to cytosolic microinjection of cytochrome c. The cardiomyocyte resistance to cytochrome c is mediated by endogenous XIAP, as XIAP-deficient cardiomyocytes die rapidly with cytosolic cytochrome c alone. Importantly, we found that cardiomyocytes, like neurons, have markedly reduced Apaf-1 levels and that this decrease in Apaf-1 is directly linked to the tight regulation of caspase activation by XIAP. These data identify an important function of XIAP in cardiomyocytes and point to a striking similarity in the regulation of apoptosis in postmitotic cells.

Decreased apoptosome activity with neuronal differentiation sets the threshold for strict IAP regulation of apoptosis.

Despite the potential of the inhibitor of apoptosis proteins (IAPs) to block cytochrome c–dependent caspase activation, the critical function of IAPs in regulating mammalian apoptosis remains unclear. We report that the ability of endogenous IAPs to effectively regulate caspase activation depends on the differentiation state of the cell. Despite being expressed at equivalent levels, endogenous IAPs afforded no protection against cytochrome c–induced apoptosis in naïve pheochromocytoma (PC12) cells, but were remarkably effective in doing so in neuronally differentiated cells. Neuronal differentiation was also accompanied with a marked reduction in Apaf-1, resulting in a significant decrease in apoptosome activity. Importantly, this decrease in Apaf-1 protein was directly linked to the increased ability of IAPs to stringently regulate apoptosis in neuronally differentiated PC12 and primary cells. These data illustrate specifically how the apoptotic pathway acquires increased regulation with cellular differentiation, and are the first to show that IAP function and apoptosome activity are coupled in cells.

Distinct pathways mediate axon degeneration during apoptosis and axon-specific pruning.

Neurons can activate pathways that destroy the whole cell via apoptosis or selectively degenerate only the axon (pruning). Both apoptosis and axon degeneration require Bax and caspases. Here we demonstrate that despite this overlap, the pathways mediating axon degeneration during apoptosis versus axon pruning are distinct. While caspase-6 is activated in axons following nerve growth factor (NGF) deprivation, microfluidic chamber experiments reveal that caspase-6 deficiency only protects axons during axon-specific but not whole-cell (apoptotic) NGF deprivation. Strikingly, axon-selective degeneration requires the apoptotic proteins Caspase-9 and Caspase-3 but, in contrast to apoptosis, not Apaf-1. Additionally, cell bodies of degenerating axons are protected from caspase activation by protea some activity and XIAP. Also, mature neurons restrict apoptosis but remain permissive for axon degeneration, further demonstrating the independent regulation of these two pathways. These results reveal insight into how neurons allow for precise control over apoptosis and axon-selective degeneration pathways, thereby permitting long-term plasticity without risking neurodegeneration.