Temporary Sequestration of Potassium by Mitochondria in Astrocytes

Increases in extracellular potassium concentration ([K⁺]_o), which can occur during neuronal activity and under pathological conditions such as ischemia, lead to a variety of potentially detrimental effects on neuronal function. Although astrocytes are known to contribute to the clearance of excess K⁺_o, the mechanisms are not fully understood. We examined the potential role of mitochondria in sequestering K⁺ in astrocytes. Astrocytes were loaded with the fluorescent K⁺ indicator PBFI and release of K⁺ from mitochondria into the cytoplasm was examined after uncoupling the mitochondrial membrane potential with carbonyl cyanide m-chlorophenylhydrazone (CCCP). Under the experimental conditions employed, transient applications of elevated [K⁺]_o led to increases in K⁺ within mitochondria, as assessed by increases in the magnitudes of cytoplasmic [K⁺] ([K⁺]_i) transients evoked by brief exposures to CCCP. When mitochondrial K⁺ sequestration was impaired by prolonged application of CCCP, there was a robust increase in [K⁺]_i upon exposure to elevated [K⁺]_o. Blockade of plasmalemmal K⁺ uptake routes by ouabain, Ba²⁺, or a mixture of voltage-activated K⁺ channel inhibitors reduced K⁺ uptake into mitochondria. Also, reductions in mitochondrial K⁺ uptake occurred in the presence of mito-K_ATP channel inhibitors. Rises in [K⁺]_i evoked by brief applications of CCCP following exposure to high [K⁺]_o were also reduced by gap junction blockers and in astrocytes isolated from connexin43-null mice, suggesting that connexins also play a role in K⁺ uptake into astrocyte mitochondria. We conclude that mitochondria play a key role in K⁺_o handling by astrocytes.     

The chromatin-remodeling factor CHD4 coordinates signaling and repair after DNA damage

In response to ionizing radiation (IR), cells delay cell cycle progression and activate DNA repair. Both processes are vital for genome integrity, but the mechanisms involved in their coordination are not fully understood. In a mass spectrometry screen, we identified the adenosine triphosphate–dependent chromatin-remodeling protein CHD4 (chromodomain helicase DNA-binding protein 4) as a factor that becomes transiently immobilized on chromatin after IR. Knockdown of CHD4 triggers enhanced Cdc25A degradation and p21^Cip1 accumulation, which lead to more pronounced cyclin-dependent kinase inhibition and extended cell cycle delay. At DNA double-strand breaks, depletion of CHD4 disrupts the chromatin response at the level of the RNF168 ubiquitin ligase, which in turn impairs local ubiquitylation and BRCA1 assembly. These cell cycle and chromatin defects are accompanied by elevated spontaneous and IR-induced DNA breakage, reduced efficiency of DNA repair, and decreased clonogenic survival. Thus, CHD4 emerges as a novel genome caretaker and a factor that facilitates both checkpoint signaling and repair events after DNA damage.     

Repression of sigK intervening (skin) element gene expression by the CI-like protein SknR and effect of SknR depletion on growth of Bacillus subtilis cells

The Bacillus subtilis phage DNA-like sigK intervening (skin) element (48 kb) is excised from the chromosome by DNA rearrangement, and a composite gene, sigK (spoIIIC and spoIVCB), is created on the chromosome during sporulation. In this study, we first focused on the role of sknR (skin repressor), which has homology with the gene encoding the Xre repressor of defective phage PBSX. The depletion of SknR caused overexpression of the region between yqaF and yqaN (the yqaF-yqaN operon) and a growth defect in B. subtilis. Point mutation analysis and an electrophoretic mobility shift assay (EMSA) suggested that SknR functions as a negative regulator of gene expression in the yqaF-yqaN operon of the skin element through direct interaction with operators of 2-fold symmetry located in the intergenic region between sknR and yqaF. Deletion analysis revealed that the lethal effect of depletion of SknR was related to overexpression of yqaH and yqaM, whose products were previously reported to associate with DnaA and DnaC, respectively. Furthermore, overexpression of either yqaH or yqaM caused cell filamentation and abnormal chromosome segregation, which suggested that overproduction of these proteins inhibits DNA replication. Moreover, overexpression of yqaM inhibited the initiation of replication. Taken together, these data demonstrate that the B. subtilis skin element carries lethal genes, which are induced by the depletion of sknR.     

Untangling a Decline in Tropical Forest Resilience: Constraints on the Sustainability of Shifting Cultivation Across the Globe

Shifting cultivators depend on forest biomass inputs to nourish their crops. For them, forest resilience has an immediate impact: it affects crop productivity. A decline in the rate of recovery following shifting cultivation would ultimately affect local, regional and global carbon budgets, with feedbacks to climate. Yet the long-term impacts of shifting cultivation have been quantified in only six locations. In this study, we reanalyze data from these locations to determine whether the rate of biomass recovery is the same from cycle to cycle. Further, using case studies in Southern Yucatan, Mexico and West Kalimantan, Indonesia, we investigate the ecological and socioeconomic factors that affect forest resilience and thus determine whether or not shifting cultivation is sustainable. The reanalysis links aboveground biomass recovery following shifting cultivation to site productivity, forest age, fallow length, history of cultivation, and soil texture. Across locations, biomass accumulation rate declines by 9.3 percent with each cycle of shifting cultivation. Per cycle change in biomass accumulation rate is significantly more negative in younger forests and forests that experience a shorter fallow period. However, more detailed analyses for two case studies suggest that a purely ecological framework is of limited effectiveness in explaining variability in the effect of repeated shifting cultivation. Rather, socioeconomic factors such as migration, subsidies, roads, and settlement history can alter the outcome of shifting cultivation by limiting the accumulation and use of local knowledge.     

Liprin-α1 affects the distribution of low-affinity β1 integrins and stabilizes their permanence at the cell surface

Integrins mediate the interaction between cells and extracellular matrix by assembling adhesive structures that need to be dynamically modulated to allow cell motility. We have recently identified liprin-α1 as an essential regulator of integrin dynamics required for efficient cell motility. Here we investigated the effects of liprin-α1 expression on β1 integrin receptors. We found that increased levels of liprin-α1 affected the localization of inactive, low-affinity integrins, while increasing the average size of β1 integrin-positive focal adhesions. Although a direct interaction between β1 integrins and liprin-α1 could not be revealed biochemically, a striking colocalization between redistributed inactive β1 integrins and liprin-α1 was observed. The tight association of overexpressed and endogenous liprin-α1 to the cytoplasmic side of the ventral plasma membrane suggested a possible role of liprin in stabilizing integrin receptors at the cell surface. In support of this hypothesis, we demonstrated an inhibitory effect of liprin overexpression on antibody-induced β1 integrin internalization. On the other hand, depletion of endogenous liprin-α by small interfering RNA increased the rate of integrin internalization. Overall, these results support the hypothesis that liprin-α1 exerts its action on focal adhesion turnover by influencing the localization and stability of integrin receptors at the cell surface.     

Cellular analysis of host cell infection by different developmental stages of Trypanosoma cruzi

Trypanosoma cruzi is an obligate intracellular parasite that infects phagocytic and non-phagocytic mammalian cells by a complex process that appears to involve several discrete steps. Even though the infection process was described many years ago, the molecular mechanisms involved remain poorly understood. As fluorescent proteins have proven to be excellent tools for live-cell imaging, we used EGFP- and DsRed1-1-transfected trypomastigotes, amastigotes and epimastigotes to study the infection process in living cells. Contrary to what has been reported, our results showed that epimastigotes are as infective as trypomastigotes and amastigotes. Besides, differences in replication, differentiation and parasite release times were observed among the stages. Our results suggest that the different developmental stages use distinct attachment and invasion mechanisms. We propose that fluorescent-based plasmid expression systems are good models for studying the infection process of intracellular microorganisms and could offers insights about the molecular mechanisms involved.