How Does Intracellular Ca2+ Oscillate: By Chance or by the Clock?

Ca²⁺ oscillations have been considered to obey deterministic dynamics for almost two decades. We show for four cell types that Ca²⁺ oscillations are instead a sequence of random spikes. The standard deviation of the interspike intervals (ISIs) of individual spike trains is similar to the average ISI; it increases approximately linearly with the average ISI; and consecutive ISIs are uncorrelated. Decreasing the effective diffusion coefficient of free Ca²⁺ using Ca²⁺ buffers increases the average ISI and the standard deviation in agreement with the idea that individual spikes are caused by random wave nucleation. Array-enhanced coherence resonance leads to regular Ca²⁺ oscillations with small standard deviation of ISIs.     

How Selective is Autophagy?

No abstract available.     

How a RING Finger Protein and a Steroid Hormone Control Autophagy?

Previous work in our laboratory has indicated that the steroid hormone ecdysone triggers programmed autophagy in the fat body of Drosophila larvae by downregulating the class I phosphoinositide 3-kinase (PI3K) pathway. We recently found evidence that Deep orange (Dor), a Drosophila RING finger protein implicated in late-endosomal trafficking, controls ecdysone signaling as well as autolysosome fusion, thus exerting a dual regulation of autophagy. We found that dor mutants are defective in programmed autophagy. The mutant larvae showed impaired upregulation of ecdysone signaling during development, accompanied by a failure to downregulate the PI3K pathway. Downregulation of the PI3K pathway could be restored by feeding the dor mutants with ecdysone. Even though ecdysone signaling and autophagy were impaired in the dor mutants, we detected an accumulation of autophagosomes in dor mutant fat bodies. This could probably be attributed to the failure of autophagosomes to fuse with lysosomes. In this Addendum we review these findings and provide some speculations about how Dor may control both ecdysone signalling and autolysosomal fusion.

Addendum to:

A Dual Function for Deep Orange in Programmed Autophagy in the Drosophila melanogaster Fat Body

K. Lindmo, A. Simonsen, A. Brech, K. Finley, T.E. Rusten and H. Stenmark

Exp Cell Res 2006; Epub ahead of print     

Janus-Faced Autophagy: A Dual Role of Cellular Self-Eating in Neurodegeneration?

Autophagy is a highly regulated cellular pathway used by eukaryotic cells to consume parts of their constituents during development or starvation. It is associated with extensive rearrangements of intracellular membranes, and involves the cooperation of many gene products in the regulation and execution phase by largely unknown mechanisms. Recent results strongly indicate the role of autophagy in the degradation of damaged macromolecules, in particular misfolded, aberrant proteins, and in organelle turnover; in mutant mice with reduced autophagy, accumulation of abnormal cytosolic proteins as inclusion bodies and massive cell loss occur similarly to human neurodegenerative disorders. Thus, autophagy seems to prevent neurons from undergoing protein aggregation-induced degeneration. In contrast, we have shown that inactivation of genes involved in autophagosome formation suppresses neuronal demise induced by various hyperactivating ion channel mutations or by neurotoxins in the nematode Caenorhabditis elegans. These results raise the possibility that autophagy may also contribute to excitotoxic necrotic-like cell death. This way, autophagic degradation of cytoplasmic materials might have a dual role in the survival of neurons. Depending on the actual cellular milieu and insulting factor, it can act both as a protector and contributor to neuronal damage.

Addendum to:

Influence of Autophagy Genes on Ion Channel-Dependent Neuronal Degeneration in Caenorhabditis elegans

M.L. Tóth, P. Simon, A.L. Kovács,and T. Vellai

J Cell Sci 2007; 120:1134-41     

Does Autophagy Contribute To Cell Death?

Autophagy (specifically macroautophagy) is an evolutionarily conserved catabolic process where the cytoplasmic contents of a cell are sequestered within double membrane vacuoles, called autophagosomes, and subsequently delivered to the lysosome for degradation. Autophagy can function as a survival mechanism in starving cells. At the same time, extensive autophagy is commonly observed in dying cells, leading to its classification as an alternative form of programmed cell death. The functional contribution of autophagy to cell death has been a subject of great controversy. However, several recent loss-of-function studies of autophagy (Atg) genes have begun to address the roles of autophagy in both cell death and survival. Here, we review the emerging evidence in favor of and against autophagic cell death, discuss the possible roles that autophagic degradation might play in dying cells, and identify salient issues for future investigation.     

Myosin VI: How Do Charged Tails Exert Control?

Molecular dynamics simulations and single molecule experiments are used to suggest that charged helices in the medial tail domain participate in myosin VI dimerization (Kim et al., 2010), which reinforces the mechanism that unfolding of the three helix bundle in the proximal tail serves as a lever arm extension.     

The “Janus-Faced Role” of Autophagy in Neuronal Sickness: Focus on Neurodegeneration

The mature brain is a highly dynamic organ that constantly changes its organization by destroying and forming new connections. Collectively, these changes are referred to as brain plasticity and are associated with functional changes, such as memory, addiction, and recovery of function after brain damage. Neuronal plasticity is sustained by the fine regulation of protein synthesis and organelle biogenesis and their degradation to ensure efficient turnover. Thus, autophagy, as quality control mechanism of proteins and organelles in neurons, is essential to their physiology and pathology. Here, we review recent several findings proving that defects in autophagy affect neuronal function and impair functional recovery after brain insults, contributing to neurodegeneration, in chronic and acute neurological disorders. Thus, an understanding of the molecular mechanisms by which the autophagy machinery is finely regulated might accelerate the development of therapeutic interventions in many neurological disorders for which no cure is available.     

Lead (Pb) is Now a Non-Threshold Substance: How Does this Affect Soil Quality Guidelines?

Risk-based concentrations (RBCs) for lead (Pb) in soil were estimated using equations for development of Canadian soil quality guidelines. Based on the latest toxicological assessments by various health agencies, risk specific doses for Pb were defined for children (for impacts on intelligence quotient [IQ]) and adults (for impacts on systolic blood pressure [SBP] as well as protection of fetal effects in the case of pregnant women). The analysis suggests that a RBC in soil of 180 μg/g (dry weight) for residential and other areas where children routinely play is protective of a 1 IQ point decrement on a population basis and may actually be associated with decrements of less than 0.2 to 0.35 IQ points when the weight of evidence is considered. For soils that children do not contact on a frequent basis, RBCs as great as 8800 μg/g are considered to be protective of a 1 mmHg SBP increase in adults (as well as IQ effects to the fetus). It is stressed that non-soil sources of Pb may be even more important than soil. The approach may also be useful in jurisdictions outside of Canada as the importance of considering IQ decrements on a population basis rather than an individual basis and uncertainties in soil ingestion rates are considered.     

Intracellular quality control by autophagy: how does autophagy prevent neurodegeneration?

Autophagy is an intracellular bulk degradation process, through which a portion of cytoplasm is delivered to lysosomes to be degraded. In many organisms, the primary role of autophagy is adaptation to starvation. However, we have found that autophagy is also important for intracellular protein quality control. Atg5(-/-) mice die shortly after birth due, at least in part, to nutrient deficiency. These mice also exhibit an intracellular accumulation of protein aggregates in neurons and hepatocytes. We now report the generation of neural cell-specific Atg5-deficient mice. Atg5( flox/flox);Nestin-Cre mice show progressive deficits in motor function and degeneration of some neural cells. In autophagy-deficient cells, diffuse accumulation of abnormal proteins occurs, followed by the generation of aggregates and inclusions. This study emphasizes the point that basal autophagy is important even in individuals who do not express neurodegenerative disease-associated mutant proteins. Furthermore, the primary targets of autophagy are diffuse cytosolic proteins, not protein aggregates themselves.     

Postural Control in Dual-Task Situations: Does Whole-Body Fatigue Matter?

Postural control is important to cope with demands of everyday life. It has been shown that both attentional demand (i.e., cognitive processing) and fatigue affect postural control in young adults. However, their combined effect is still unresolved. Therefore, we investigated the effects of fatigue on single- (ST) and dual-task (DT) postural control. Twenty young subjects (age: 23.7 ± 2.7) performed an all-out incremental treadmill protocol. After each completed stage, one-legged-stance performance on a force platform under ST (i.e., one-legged-stance only) and DT conditions (i.e., one-legged-stance while subtracting serial 3s) was registered. On a second test day, subjects conducted the same balance tasks for the control condition (i.e., non-fatigued). Results showed that heart rate, lactate, and ventilation increased following fatigue (all p < 0.001; d = 4.2–21). Postural sway and sway velocity increased during DT compared to ST (all p < 0.001; d = 1.9–2.0) and fatigued compared to non-fatigued condition (all p < 0.001; d = 3.3–4.2). In addition, postural control deteriorated with each completed stage during the treadmill protocol (all p < 0.01; d = 1.9–3.3). The addition of an attention-demanding interference task did not further impede one-legged-stance performance. Although both additional attentional demand and physical fatigue affected postural control in healthy young adults, there was no evidence for an overadditive effect (i.e., fatigue-related performance decrements in postural control were similar under ST and DT conditions). Thus, attentional resources were sufficient to cope with the DT situations in the fatigue condition of this experiment.     

Does High-Dose Antimicrobial Chemotherapy Prevent the Evolution of Resistance?

High-dose chemotherapy has long been advocated as a means of controlling drug resistance in infectious diseases but recent empirical studies have begun to challenge this view. We develop a very general framework for modeling and understanding resistance emergence based on principles from evolutionary biology. We use this framework to show how high-dose chemotherapy engenders opposing evolutionary processes involving the mutational input of resistant strains and their release from ecological competition. Whether such therapy provides the best approach for controlling resistance therefore depends on the relative strengths of these processes. These opposing processes typically lead to a unimodal relationship between drug pressure and resistance emergence. As a result, the optimal drug dose lies at either end of the therapeutic window of clinically acceptable concentrations. We illustrate our findings with a simple model that shows how a seemingly minor change in parameter values can alter the outcome from one where high-dose chemotherapy is optimal to one where using the smallest clinically effective dose is best. A review of the available empirical evidence provides broad support for these general conclusions. Our analysis opens up treatment options not currently considered as resistance management strategies, and it also simplifies the experiments required to determine the drug doses which best retard resistance emergence in patients.