Molecular links between autophagy and apoptosis

Sepsis-induced acute kidney injury

PURPOSE OF REVIEW

Sepsis is a common and frequently fatal condition in which mortality has been consistently linked to increasing organ dysfunction. For example, acute kidney injury (AKI) occurs in 40-50% of septic patients and increases mortality six to eight-fold. However, the mechanisms by which sepsis causes organ dysfunction are not well understood and hence current therapy remains reactive and nonspecific.

RECENT FINDINGS

Recent studies have challenged the previous notion that organ dysfunction is solely secondary to hypoperfusion, by showing, for example, that AKI occurs in the setting of normal or increased renal blood flow; and that it is characterized not by acute tubular necrosis or apoptosis, but rather by heterogeneous areas of colocalized sluggish peritubular blood flow and tubular epithelial cell oxidative stress. Evidence has also shown that microvascular dysfunction, inflammation, and the metabolic response to inflammatory injury are fundamental pathophysiologic mechanisms that may explain the development of sepsis-induced AKI.

SUMMARY

The implications of these findings are significant because in the context of decades of negative clinical trials in the field, the recognition that other mechanisms are at play opens the possibility to better understand the processes of injury and repair, and provides an invaluable opportunity to design mechanism-targeted therapeutic interventions.

Sensory Neurons from Tau Transgenic Mice and Their Utility in Drug Screening

Tau misfolding is a major cause of neurodegeneration, tauopathies being a growing group of diseases in which tau forms insoluble aggregates, best known in Alzheimer disease as neurofibrillary tangles (NFTs). Many transgenic mouse models of tauopathies have been generated, but it has been difficult to demonstrate disease in primary brain neurons from these mice because neurons need to be harvested within a few days of birth and tau fails to produce NFTs. Transgenic mice have been generated that express the 0N4R isoform of human tau mutated at amino acid 301 (P301S mice) under the Thy1.2 promoter. These mice, which model an inherited form of frontotemporal dementia, develop NFTs around 5 months of age. Taking advantage of the fact that Thy1.2 is expressed in the peripheral nervous system, we found that dorsal root ganglion (DRG) neurons express P301S tau and develop tau pathology along a similar time course to that found in central nervous system neurons in mice. Thus, NFTs are well-developed around 5 months of age. Because DRG neurons can be cultured from adult mice for months, they have proven to be an excellent model for studying how tau pathology develops and for screening compounds that may ameliorate tau pathology. Here we present a detailed protocol for the preparation of long-term DRG neuron cultures and describe how to study whether activation of autophagy ameliorates tau pathology.

A unified theory of sepsis-induced acute kidney injury: inflammation, microcirculatory dysfunction, bioenergetics, and the tubular cell adaptation to injury

Given that the leading clinical conditions associated with acute kidney injury (AKI), namely, sepsis, major surgery, heart failure, and hypovolemia, are all associated with shock, it is tempting to attribute all AKI to ischemia on the basis of macrohemodynamic changes. However, an increasing body of evidence has suggested that in many patients, AKI can occur in the absence of overt signs of global renal hypoperfusion. Indeed, sepsis-induced AKI can occur in the setting of normal or even increased renal blood flow. Accordingly, renal injury may not be entirely explained solely on the basis of the classic paradigm of hypoperfusion, and thus other mechanisms must come into play. Herein, we put forward a "unifying theory" to explain the interplay between inflammation and oxidative stress, microvascular dysfunction, and the adaptive response of the tubular epithelial cell to the septic insult. We propose that this response is mostly adaptive in origin, that it is driven by mitochondria, and that it ultimately results in and explains the clinical phenotype of sepsis-induced AKI.

Caspase inhibitors protect neurons by enabling selective necroptosis of inflamed microglia

Microglia are resident brain macrophages, which can cause neuronal loss when activated in infectious, ischemic, traumatic, and neurodegenerative diseases. Caspase-8 has both prodeath and prosurvival roles, mediating apoptosis and/or preventing RIPK1-mediated necroptosis depending on cell type and stimulus. We found that inflammatory stimuli (LPS, lipoteichoic acid, or TNF-α) caused an increase in caspase-8 IETDase activity in primary rat microglia without inducing apoptosis. Inhibition of caspase-8 with either Z-VAD-fmk or IETD-fmk resulted in necrosis of activated microglia. Inhibition of caspases with Z-VAD-fmk did not kill non-activated microglia, or astrocytes and neurons in any condition. Necrostatin-1, a specific inhibitor of RIPK1, prevented microglial caspase inhibition-induced death, indicating death was by necroptosis. In mixed cerebellar cultures of primary neurons, astrocytes, and microglia, LPS induced neuronal loss that was prevented by inhibition of caspase-8 (resulting in microglial necroptosis), and neuronal death was restored by rescue of microglia with necrostatin-1. We conclude that the activation of caspase-8 in inflamed microglia prevents their death by necroptosis, and thus, caspase-8 inhibitors may protect neurons in the inflamed brain by selectively killing activated microglia.

Endoplasmic reticulum stress triggers autophagy in malignant glioma cells undergoing cyclosporine a-induced cell death

Autophagy is a conserved, self-digestion process that is activated in response to nutrient limitation but acting also as an alternative death mechanism under certain conditions. It is accompanied by the progressive formation of vesicle structures from autophagosomes to autophagolysosomes orchestrated by autophagy effectors (Atg proteins) and modulators (that is, mTOR-mammalian target of rapamycin as a negative regulator). Malignant gliomas are highly resistant to current therapies that induce apoptosis, thus induction of the alternative cell death is an attractive strategy. We demonstrate that cyclosporine A (CsA, an immunophilin/calcineurin inhibitor) induces cell death with some apoptotic features but also accompanied by the appearance of numerous cytoplasmic vacuoles, immunostained for endoplasmic reticulum (ER) and autophagy markers. The induction of ER stress in glioma cells by CsA was evidenced by detection of unfolded protein response activation (phosphorylation of PERK, accumulation of IRE1α) and accumulation of ER stress-associated proteins (BIP and CHOP). Formation of the acidic vesicular organelles, increase of autophagic vacuoles, GFP-LC3 punctation (microtubule-associated protein light chain 3) and LC3-II accumulation upon CsA treatment confirmed activation of autophagy. Decrease of phosphorylation of 4E-BP1, p70S6K1 and its downstream target S6 ribosomal protein demonstrate inhibition of mTOR signaling by CsA. Salubrinal and silencing of PERK and IRE1α partially blocked CsA-induced accumulation of LC3-II. It suggests that ER stress precedes CsA-induced autophagy. Surprisingly, silencing of autophagy effectors ULK1, Atg5 or Atg7 increased the level of active caspases 3, 7 and PARP degradation in CsA-treated cells. Our results demonstrate that CsA induces both apoptosis and autophagy in malignant glioma cells via induction of ER stress and inhibition of mTOR/p70S6K1 pathway, however autophagy is cytoprotective in this context.

Role of inositol trisphosphate receptors in autophagy in DT40 cells

Previous studies have shown that small interfering RNA knockdown and pharmacological inhibition of inositol 1,4,5-trisphosphate receptors (IP(3)Rs) stimulate autophagy. We have investigated autophagy in chicken DT40 cell lines containing targeted deletions of all three IP(3)R isoforms (triple knock-out (TKO) cells). Using gel shifts of microtubule-associated protein 1 light chain 3 as a marker of autophagy, we find that TKO cells have enhanced basal autophagic flux even under nutrient-replete conditions. Stable DT40 cell lines derived from TKO cells containing the functionally inactive D2550A IP(3)R mutant did not suppress autophagy in the same manner as wild-type receptors. This suggests that the channel function of the receptor is important in its regulatory role in autophagy. There were no marked differences in the phosphorylation state of AMP-activated protein kinase, Akt, or mammalian target of rapamycin between wild-type and TKO cells. The amount of immunoprecipitated complexes of Bcl-2-Beclin-1 and Beclin-1-Vps34 were also not different between the two cell lines. The major difference noted was a substantially decreased mTORC1 kinase activity in TKO cells based on decreased phosphorylation of S6 kinase and 4E-BP1. The discharge of intracellular stores with thapsigargin stimulated mTORC1 activity (measured as S6 kinase phosphorylation) to a greater extent in wild-type than in TKO cells. We suggest that basal autophagic flux may be negatively regulated by IP(3)R-dependent Ca(2+) signals acting to maintain an elevated mTORC1 activity in wild-type cells and that Ca(2+) regulation of this enzyme is defective in TKO cells. The protective effect of a higher autophagic flux in cells lacking IP(3)Rs may play a role in the delayed apoptotic response observed in these cells.