Morphological and biochemical characterization of basal and starvation-induced autophagy in isolated adult rat cardiomyocytes

Autophagy is simultaneously a mode of programmed cell death and an important physiological process for cell survival, but its pathophysiological significance in cardiac myocytes remains largely unknown. We induced autophagy in isolated adult rat ventricular cardiomyocytes (ARVCs) by incubating them in glucose-free, mannitol-supplemented medium for up to 4 days. Ultrastructurally, intracellular vacuoles containing degenerated subcellular organelles (e.g., mitochondria) were markedly apparent in the glucose-starved cells. Microtubule-associated protein-1 light chain 3 was significantly upregulated among the glucose-starved ARVCs than among the controls. After 4 days, glucose-starved ARVCs showed a significantly worse survival rate (19+/-5.2%) than the controls (55+/-8.3%, P<0.005). Most dead ARVCs in both groups showed features of necrosis, and the rate of apoptosis did not differ between the groups. Two inhibitors of autophagy, 3-methyladenine (3-MA) and leupeptin, significantly and dose-dependently reduced the viability of both control and glucose-starved ARVCs and caused specific morphological alterations; 3-MA reduced autophagic findings, whereas leupeptin greatly increased the numbers and the sizes of vacuoles that contained incompletely digested organelles. The knockdown of the autophagy-related genes with small interfering RNA also reduced the glucose-starved ARVCs viability, but rapamycin, an autophagy enhancer, improved it. Reductions in the ATP content of ARVCs caused by glucose depletion were exacerbated by the inhibitors while attenuated by rapamycin, suggesting that autophagy inhibition might accelerate energy depletion, leading to necrosis. Taken together, our findings suggest that autophagy in cardiomyocytes reflects a prosurvival, compensatory response to stress and that autophagic cardiomyocyte death represents an unsuccessful outcome due to necrosis.     

Fate of isolated adult cardiomyocytes undergoing starvation-induced autophagic degeneration

We induced autophagy in isolated adult rat ventricular cardiomyocytes by incubating them in glucose-free medium supplemented with mannitol for up to 4 days. The up-regulation of LC3, and vacuoles containing partially degraded subcellular organelles were readily apparent in glucose-starved cells. Most dead cells in both groups showed features of necrosis, although the survival rate was significantly lower among glucose-starved cells than among the controls. In contrast, the rate of apoptosis was about the same in both groups. Two inhibitors of autophagy, 3-methyladenine (3-MA) and leupeptin, significantly reduced the viability of both control and glucose-starved cells in a dose-dependent manner and caused specific morphological alterations: 3-MA reduced the number of autophagic vacuoles, whereas leupeptin greatly increased their number and size. Conversely, rapamycin, an enhancer of autophagy, improved the survival of glucose-starved cells. The reduction in cellular ATP caused by glucose depletion were exacerbated by the inhibitors but mitigated by rapamycin, suggesting that inhibition of autophagy may accelerate energy depletion, leading to necrosis. Our findings suggest that in cardiomyocytes autophagy is a compensatory, prosurvival response to stress and that autophagic death is an unsuccessful outcome brought on by necrosis.

Addendum to: Maruyama R, Goto K, Takemura G, Ono K, Nagao K, Horie T, Tsujimoto A, Kanamori H, Miyata S, Ushikoshi H, Nagashima K, Minatoguchi S, Fujiwara T, Fujiwara H. Morphological and biochemical characterization of basal and starvation-induced autophagy in isolated adult rat cardiomyocytes. Am J Phsyiol Heart Circ Physiol 2008; 295:H1599-607; PMID: 18708438; DOI:10.1152/ajpheart.91449.2007.     

Nerve growth factor (NGF) down-regulates the Bcl-2 homology 3 (BH3) domain-only protein Bim and suppresses its proapoptotic activity by phosphorylation

Bim is a proapoptotic, BH3-domain-only member of the Bcl-2 family that plays a role in death of trophic factor-deprived sympathetic neurons as well as in other paradigms of apoptotic death. We report here that nerve growth factor (NGF) leads to both a slow down-regulation of Bim expression in neuronal PC12 cells and rapid Bim phosphorylation. Both effects appear to be mediated by the MEK/MAPK pathway. An assay for Bim-mediated death revealed that NGF-promoted phosphorylation suppresses the proapoptotic activity of Bim. The phosphorylation sites responsible for this effect in the extra long form of rBim were identified as Ser-109 and Thr-110. Thus, NGF protects neurons from the proapoptotic effects of Bim both by acute phosphorylation and the longer term repression of expression.     

The trk proto-oncogene rescues NGF responsiveness in mutant NGF-nonresponsive PC12 cell lines.

The trk tyrosine kinase proto-oncogene product gp140prototrk binds nerve growth factor (NGF) and is rapidly and selectively activated by this neurotrophic factor. To determine whether gp140prototrk is involved in transducing a functional NGF signal, PC12 cell mutants (PC12nnr) deficient in high affinity NGF binding and unresponsive to NGF were used. Northern analysis revealed that these mutant cells have greatly reduced levels of trk expression. PC12nnr cultures were transiently transfected with expression vectors encoding the full-length rat trk cDNA and assessed for responsiveness to NGF. Expression of exogenous trk rescued the capacity for NGF-promoted neurite outgrowth, cellular hypertrophy, and serum-free survival by these cells. These results indicate that gp140prototrk is necessary for functional NGF signal transduction.     

Pivotal Role of Mitochondrial Calcium Uptake in Neural Cell Apoptosis and Necrosis

Abstract : Perturbed cellular calcium homeostasis has been implicated in both apoptosis and necrosis, but the role of altered mitochondrial calcium handling in the cell death process is unclear. The temporal ordering of changes in cytoplasmic ([Ca²⁺]C) and intramitochondrial ([Ca²⁺]M) calcium levels in relation to mitochondrial reactive oxygen species (ROS) accumulation and membrane depolarization (MD) was examined in cultured neural cells exposed to either an apoptotic (staurosporine ; STS) or a necrotic (the toxic aldehyde 4-hydroxynonenal ; HNE) insult. STS and HNE each induced an early increase of [Ca²⁺]C followed by delayed increase of [Ca²⁺]M. Overexpression of Bcl-2 blocked the elevation of [Ca²⁺]M and the MD in cells exposed to STS but not in cells exposed to HNE. The cytoplasmic calcium chelator BAPTA-AM and the inhibitor of mitochondrial calcium uptake ruthenium red prevented both apoptosis and necrosis. STS and HNE each induced mitochondrial ROS accumulation and MD, which followed the increase of [Ca²⁺]M. Cyclosporin A prevented both apoptosis and necrosis, indicating critical roles for MD in both forms of cell death. Caspase activation occurred only in cells undergoing apoptosis and preceded increased [Ca²⁺]M. Collectively, these findings suggest that mitochondrial calcium overload is a critical event in both apoptotic and necrotic cell death.     

Ectopic trkA expression mediates a NGF survival response in NGF- independent sensory neurons but not in parasympathetic neurons

We have investigated the role of trkA, the tyrosine kinase NGF receptor, in mediating the survival response of embryonic neurons to NGF. Embryonic trigeminal mesencephalic (TMN) neurons, which normally survive in the presence of brain-derived neurotrophic factor (BDNF) but not NGF, become NGF-responsive when microinjected with an expression vector containing trkA cDNA. In contrast, microinjection of ciliary neurotrophic factor (CNTF)-dependent embryonic ciliary neurons with the same construct does not result in the acquisition of NGF responsiveness by these neurons despite de novo expression of trkA mRNA and protein. The failure of trkA to result in an NGF-promoted survival response in ciliary neurons is not due to absence of the low-affinity NGF receptor, p75, in these neurons. Quantitative RT/PCR and immunocytochemistry showed that TMN and ciliary neurons both express p75 mRNA and protein. These findings not only provide the first direct experimental demonstration of trkA mediating a physiological response in an appropriate cell type, namely NGF-promoted survival of embryonic neurons, but indicate that not all neurons are able to respond to a trkA-mediated signal transduction event.     

Relationship of Intracellular Calcium to Dependence on Nerve Growth Factor in Dorsal Root Ganglion Neurons in Cell Culture

During development, neural crest-derived sensory neurons require nerve growth factor (NGF) for survival, but lose this dependency postnatally. Similarly, dissociated embryonic sensory neurons lose their NGF dependence during the first 3 weeks in cell culture. It has been hypothesized that, in sympathetic neurons, intracellular levels of calcium are related to trophic factor dependence. In vitro during the period in which embryonic-day-15 sensory neurons become independent of NGF, intracellular calcium concentrations progressively increased in parallel to the decline in NGF dependence. This elevation of intracellular calcium was directly related to the absolute age of the neurons, not to the length of time in culture. Without NGF, immature sensory, i.e., dependent, neurons survived in the presence of high extracellular potassium, a condition that produces elevated intracellular calcium. In another paradigm, measurements of intracellular calcium were determined in NGF-dependent neurons "committed to die" after NGF withdrawal. These measurements were determined prior to the time that extensive morphological changes, consistent with cell death, were noted by phase-contrast microscopy. No elevation in intracellular calcium was found in these dying neurons, but rather, a small decrease was observed prior to the disintegration of the neurons. These findings support the hypothesis that trophic factor dependence of neurons may be inversely related to levels of intracellular calcium.     

Iron promotes the survival and neurite extension of serum-starved PC12 cells in the presence of NGF by enhancing cell attachment

Delayed death of serum-starved PC12 cells on a poly-L-lysine (PLL) matrix was observed, even in the presence of NGF. NGF blocked the apoptotic death of attached but not detached cells, which suggests that delayed death may be related to cell detachment from the PLL matrix. Iron selectively blocked this anoikis-like death by increasing cell attachment. Interestingly, the addition of > 10 microM FeCl2 to the culture medium generated gelatinous iron precipitates, and the removal of the precipitates abolished the iron effect. Attachment experiments using poly-HEMA supported the role of iron precipitates on cell-to-matrix adhesion. The expression of integrin beta1, neither N-cadherin nor alpha/beta-catenin, was also significantly increased by iron. In addition to its effect on cell viability, iron promoted the outgrowth of neurites. Our results collectively indicate that iron functions as a necessary co-element for NGF by enhancing cell attachment, survival, and neurite extension.     

Dual effect of nerve growth factor on cell death of PC12 cells Induced by serum deprivation

The effect of nerve growth factor (NGF) on the cell death of PC12 cells that is induced by serum deprivation was examined in the floating and attached cells to the extracellular matrix. NGF suppressed cell death occurred in the floating cells. The onset of cell death in the attached cells was much slower than in the floating cells. Moreover, the cell death in the attached cells was either accelerated in a high-density culture (over approximately 50% confluent), or inhibited in a low-density culture by NGF. While nucleosomal DNA fragmentation and poly (ADP-ribose) polymerase degradation was observed in both the floating and attached cells, the incidence of nuclear fragmentation and chromatin condensation was much lower in the attached cells than in the floating cells. The delayed onset of cell death in the attached cells was due to the signals that are generated from the extracellular matrix that is formed by PC12 cells, together with cell-to-cell interaction. The acceleration of cell death in the NGF-treated cells was anoikis, caused by the loss of the anchorage of the cell via the action of increased activities of matrix metalloproteinases (MMP2, MMP9). These results suggest that NGF has a different role in the cell death of PC12 cells that is induced by serum deprivation, depending on the cell-matrix, as well as the cell-cell interaction.     

Proliferative inhibition by dominant-negative Ras rescues naive and neuronally differentiated PC12 cells from apoptotic death.

We have used the nerve growth factor (NGF)-responsive PC12 cell line as a model to examine the role of cell cycle progression in apoptotic neuronal cell death triggered by withdrawal of trophic support. Because p21 Ras plays a key role in mitogenic signaling, we tested whether interference with the activity of this protein would affect cell cycle progression and thereby apoptotic death after trophic factor deprivation. For this purpose, we exploited PC12 cells transfected with an inducible form of dominant-inhibitory Ras. In contrast to non-transfected and uninduced cells, which continue to synthesize DNA when deprived of trophic support, PC12 cells induced to express dominant-inhibitory Ras showed little thymidine incorporation. When non-transfected and uninduced cells were deprived of trophic support, these underwent rapid apoptotic death that could be prevented by NGF. However, cells in which dominant-inhibitory Ras was induced and which were consequently quiescent did not die upon withdrawal of trophic support and showed long-term survival in the absence of NGF or other trophic factors. Moreover, induction of dominant-inhibitory Ras also rescued non-dividing, neuronally differentiated PC12 cells from death caused by NGF withdrawal. These findings suggest a relationship between proliferative capacity and neuronal apoptosis and raise the hypothesis that following withdrawal of trophic support, neurons undergo an unsuccessful and fatal attempt to re-enter the cell cycle.     

Cyclosporin A inhibits caspase-independent death of NGF-deprived sympathetic neurons: a potential role for mitochondrial permeability transition

Opening of the permeability transition pore (PTP) has been implicated as an important mitochondrial event that occurs during apoptosis. We examined the role of the PTP in the well-characterized cell death of rat sympathetic neurons deprived of nerve growth factor (NGF) in vitro. Removal of NGF causes these neurons to undergo either a classic apoptotic cell death or, when treated with a broad-spectrum caspase inhibitor such as boc-aspartyl(OMe)-fluoromethylketone (BAF), a delayed, nonapoptotic cell death. The PTP inhibitor, cyclosporin A (CsA), blocked commitment-to-die in the presence of BAF, as defined by the ability of NGF readdition to rescue cells, but had little effect on commitment-to-die in the absence of BAF. CsA did not have trophic effects on BAF-saved cells, but did block the decrease in mitochondrial membrane potential. These data suggest that PTP opening is a critical event in caspase-independent, nonapoptotic (but not caspase-dependent, apoptotic) death of NGF-deprived rat sympathetic neurons.     

The role of calcium in cell death

Cell death is frequently encountered in human disease. Ischemia, chemicals, viruses, radiation and toxins are among its varied causes. The resulting pathology, however, is very uniform. The common pattern of altered morphology, coagulative necrosis, implies that, at some point, the diverse causes share common mechanisms. Recent evidence suggests that coagulative necrosis may ultimately reflect an alteration in the control of intracellular calcium homeostasis. Studies in intact animals and in cell culture suggest than an ultimate influx of calcium ions across injured plasma membranes and along a steep concentrations gradient converts potentially reversible alterations into the irreversible injury of cell death. The structural alterations in cellular constituents that characterize coagulative necrosis are themselves very likely the direct result of the action of an elevated calcium concentration on cellular macromolecules. The continuing need to identify the membrane alterations induced by the various causes of cell death in order to assess their potential reversibility in the absence of irreversible calcium accumulation is emphasized.     

Study of the mechanisms of hydrogen peroxide and hydroxyl free radical-induced cellular injury and calcium overload in cardiac myocytes.

There is evidence that myocardial injury, as would occur on post-ischemic reperfusion, may be caused by the generation of oxygen radicals, as well as by the induction of intracellular calcium overload; however, the relationship between these two mechanisms of injury is not known. To test the hypothesis that oxidants and oxygen radicals can cause cardiac myocyte injury and intracellular calcium overload, isolated adult rat ventricular myocytes were exposed to H2O2 (1-10 mM) and Fe3(+)-nitrilotriacetate. EPR measurements confirmed the production of the highly reactive .OH radical by this system. The oxygen radical generating system initially caused a transient augmentation of twitch amplitude in single field stimulated myocytes. This was followed by contractile oscillations occurring during the twitch prior to full cell relaxation, and spontaneous mechanical oscillations occurring between electrically stimulated contractions. Eventually, cells became inexcitable and abruptly underwent contracture. In the presence of lower bathing calcium concentrations, these oxidant-induced alterations were prevented or delayed. However, cells exposed to the radical generating system in the absence of extracellular calcium still eventually underwent contracture but stimulated contractions or mechanical oscillations were not seen. Measurements in single myocytes loaded with the fluorescent probe of intracellular calcium, Indo-1, demonstrated a rise in both systolic and diastolic fluorescence ratio, as well as oscillations and widening of the fluorescence transient, suggestive of cellular calcium loading, following exposure to the radical generating system. Injured myocytes did not take up trypan blue dye. Contractile dysfunction and calcium channel blocker, nitrendipine. NMR measurements of cellular [ATP] demonstrated that these alterations in cellular calcium preceded the depletion of ATP. Subsequent depletion of ATP was accompanied by the appearance of increased concentrations of sugar phosphates indicative of a block in glycolysis and ATP depletion correlated with cellular rigor. Thus, oxygen free radicals can cause cardiac myocyte injury with contractile abnormalities which occur due to myocyte calcium loading. The mechanism of oxidant-induced calcium loading is not due to nonspecific membrane damage, or energy depletion, but rather due to increased calcium influx through voltage gated calcium channels. This early calcium overload state as well as oxidant induced block of glycolysis result in cellular energy depletion and cell death with the induction of contracture.     

The Ability of Diphenylpiperazines to Prevent Neuronal Death in Dorsal Root Ganglion Neurons In Vitro After Nerve Growth Factor Deprivation and In Vivo After Axotomy

Abstract: The mechanism of neuroprotection by the calcium channel antagonist flunarizine against neuronal death is unknown. We investigated the ability of other calcium channel antagonists (cinnarizine, nimodipine, nicardipine, diltiazem, and verapamil), calmodulin antagonists, and calpain inhibitors to prevent neuronal death in rat dorsal root ganglion neurons in vitro after nerve growth factor (NGF) deprivation and the ability of cinnarizine and diltiazem to protect in vivo after axotomy. In vitro, only neurons treated with cinnarizine or flunarizine were protected from death after withdrawal. In vivo, cinnarizine, but not diltiazem, protected dorsal root ganglion neurons in rats after unilateral sciatic nerve crush. Intracellular calcium concentration ([Ca²⁺],) was evaluated with fura 2 after NGF deprivation In vitro. Neurons "committed to die" 24 h after NGF deprivation displayed a decline in [Ca^a+], before visible morphological deterioration consistent with cell death. The influx of extracellular calcium was not necessary to produce neuronal death. Neurons deprived of NGF gradually lost the ability to respond to elevated external potassium with an increase in [Ca²⁺], during the first 24 h after trophic factor deprivation. After 24 h, neurons deprived of NGF could not be rescued by readministration of NGF. Neurons protected from cell death with diphenylpiperazines maintained their response to high external potassium, suggesting continued membrane integrity. We speculate that diphenylpiperazines may protect sensory neurons via an unknown mechanism that stabilizes cell membranes.     

Nerve Growth Factor is a Potential Treated Target in Tg(SOD1*G93A)1Gur Mice

Nerve growth factor (NGF) is a protective factor of neural cells; the possible relationship between the NGF and the pathogenesis of amyotrophic lateral sclerosis (ALS) hasn’t been completely known. In this study, we observed and analyzed the expression and distribution of NGF, as well as the possible relationship between the NGF expression and distribution and the neural cell death in both SOD1 wild-type (WT) and Tg(SOD1*G93A)1Gur (TG) mice applying the fluorescence immunohistochemistry method. The results showed that the expression and distribution of NGF in the anterior horn (AH), the lateral horn (LH), and the surrounding central canal (CC) significantly increased at the supper early stage of ALS (Pre-onset stage) and the early stage (Onset stage), but the NGF expression and distribution in the AH, the LH, and the surrounding CC significantly reduced at the progression stage. The astrocyte, neuron, and oligodendrocyte produced the NGF and the neural precursor cells (NPCs) produced the NGF. The neural cell death gradually increased accompanying with the reduction of NGF expression and distribution. Our data suggested that the NGF was a protective factor of neural cells, because the neural cells in the AH, the LH, and the surrounding CC produced more NGF at the supper early and early stage of ALS; moreover, the NPCs produced the NGF. It implied that the NGF exerted the protective effect of neural cells, prevented from the neural cell death and aroused the potential of self-repair in the development of ALS.