Methods Used to Investigate the Plasmodium falciparum Digestive Vacuole

Plasmodium falciparum malaria remains a global health problem as parasites continue to develop resistance to all antimalarials in use. Infection causes clinical symptoms during the intra-erythrocytic stage of the lifecycle where the parasite infects and replicates within red blood cells (RBC). During this stage, P. falciparum digests the main constituent of the RBC, hemoglobin, in a specialized acidic compartment termed the digestive vacuole (DV), a process essential for survival. Many therapeutics in use target one or multiple aspects of the DV, with chloroquine and its derivatives, as well as artemisinin, having mechanisms of action within this organelle. In order to better understand how current therapeutics and those under development target DV processes, techniques used to investigate the DV are paramount. This review outlines the involvement of the DV in therapeutics currently in use and focuses on the range of techniques that are currently utilized to study this organelle including microscopy, biochemical analysis, genetic approaches and metabolomic studies. Importantly, continued development and application of these techniques will aid in our understanding of the DV and in the development of new therapeutics or therapeutic partners for the future.

Lamp1 mediates lipid transport, but is dispensable for autophagy in Drosophila

The endolysosomal system not only is an integral part of the cellular catabolic machinery that processes and recycles nutrients for synthesis of biomaterials, but also acts as signaling hub to sense and coordinate the energy state of cells with growth and differentiation. Lysosomal dysfunction adversely influences vesicular transport-dependent macromolecular degradation and thus causes serious problems for human health. In mammalian cells, loss of the lysosome associated membrane proteins LAMP1/2 strongly impacts autophagy and cholesterol trafficking. Here we show that the previously uncharacterized Drosophila Lamp1 is a bona fide homolog of vertebrate LAMP1/2. Surprisingly and in contrast to Lamp1/2 double mutant mice, Drosophila Lamp1 is not required for viability or autophagy, suggesting that autophagy defects in Lamp1/2 mutants may have indirect causes. However, Lamp1 deficiency results in an expansion of the acidic compartment in flies. Furthermore, we find that Lamp1 mutant larvae have defects in lipid metabolism as they show elevated levels of sterols and diacylglycerols (DAGs). Since DAGs are the main lipid species used for transport though the hemolymph (blood) in insects, our results indicate broader functions of Lamp1 in lipid transport. Our findings make Drosophila an ideal model to study the role of LAMP proteins in lipid assimilation without the confounding effects of their storage and without interfering with autophagic processes.

Acidic Compartment Size, Positioning, and Function during Myogenesis and Their Modulation by the Wnt/Beta-Catenin Pathway

Lysosomes and acidic compartments are involved in breaking down of macromolecules, membrane recycling, and regulation of signaling pathways. Here, we analyzed the role of acidic compartments during muscle differentiation and the involvement of the Wnt/beta-catenin pathway in lysosomal function during myogenesis. Acridine orange was used to localize and quantify acidic cellular compartments in primary cultures of embryonic muscle cells from Gallus gallus. Our results show an increase in acidic compartment size and area, as well as changes in their positioning during the initial steps of myogenesis. The inhibition of lysosomal function by either the chloroquine Lys05 or the downregulation of LAMP-2 with siRNA impaired chick myogenesis, by inhibiting myoblast fusion. Two activators of the Wnt/beta-catenin pathway, BIO and Wnt3a, were able to rescue the inhibitory effects of Lys05 in myogenesis. These results suggest a new role for the Wnt/beta-catenin pathway in the regulation of acidic compartment size, positioning, and function in muscle cells.

Extracellular Vesicles Hijack the Autophagic Pathway to Induce Tau Accumulation in Endolysosomes

Clinical progression of tauopathies is reflected by the transcellular propagation of pathogenic Tau seeds with the possible involvement of extracellular vesicles as transport vectors. However, the mechanism regulating extracellular vesicle cargo delivery to recipient cells is poorly understood. We established a cell model for investigating extracellular vesicle-delivery of membranes and proteins. In this model, extracellular vesicles are readily internalized and accumulate in endolysosomes. For the first time, we show that in this acidic compartment of recipient cells, extracellular vesicle-delivered Tau seeds cause the accumulation and abnormal folding of normal Tau by a process that requires the participation of autophagy. Endolysomes represent thus a cross-road where Tau seeds released from extracellular vesicles propagate on cellular Tau on its route for autophagy-mediated degradation, ultimately driving its accumulation, endolysosomal stress and cytotoxicity. Whilst, autophagy stimulation is considered as a viable solution to protect neurons from harmful cytosolic protein inclusions, our data suggest that this approach may favour the aberrant accumulation of neurodegeneration-associated proteins induced by exogenous pathogenic protein forms, with possible implications in the spreading of the disease.

Marginal protein stability drives subcellular proteome isoelectric point

There exists a positive correlation between the pH of subcellular compartments and the median isoelectric point (pI) for the associated proteomes. Proteins in the human lysosome—a highly acidic compartment in the cell—have a median pI of ∼6.5, whereas proteins in the more basic mitochondria have a median pI of ∼8.0. Proposed mechanisms reflect potential adaptations to pH. For example, enzyme active site general acid/base residue pKs are likely evolved to match environmental pH. However, such effects would be limited to a few residues on specific proteins, and might not affect the proteome at large. A protein model that considers residue burial upon folding recapitulates the correlation between proteome pI and environmental pH. This correlation can be fully described by a neutral evolution process; no functional selection is included in the model. Proteins in acidic environments incur a lower energetic penalty for burying acidic residues than basic residues, resulting in a net accumulation of acidic residues in the protein core. The inverse is true under alkaline conditions. The pI distributions of subcellular proteomes are likely not a direct result of functional adaptations to pH, but a molecular spandrel stemming from marginal stability.

SIDT2 is involved in the NAADP-mediated release of calcium from insulin secretory granules

The Sidt2 global knockout mouse (Sidt2−/−) has impaired insulin secretion. The aim of this study was to assess the role of SIDT2 protein in glucose-induced insulin secretion in primary cultured mouse β-cells. The major metabolic and electrophysiological steps of glucose-induced insulin secretion of primary cultured β-cells from Sidt2−/− mice were investigated. The β-cells from Sidt2−/− mice had normal NAD(P)H responses and KATP and KV currents. However, they exhibited a lower [Ca2+]i peak height when stimulated with 20mM glucose compared with those from WT mice. Furthermore, it took a longer time for the [Ca2+]i of β-cell from Sidt2−/− mice to reach the peak. Pretreatment with ryanodine or 2-aminoethoxydiphenyl borate (2-APB) did not change [Ca2+]i the response pattern to glucose in Sidt2−/− cells. Extraordinarily, pretreatment with bafilomycin A1(Baf-A1) led to a comparable [Ca2+]i increase pattern between these two groups, suggesting that calcium traffic from the intracellular acidic compartment is defective in Sidt2−/− β-cells. Bath-mediated application of 50nM nicotinic acid adenine dinucleotide phosphate (NAADP) normalized the [Ca2+]i response of Sidt2−/− β-cells. Finally, glucose-induced CD38 expression increased to a comparable level between Sidt2−/− and WT islets, suggesting that Sidt2−/− islets generated NAADP normally. We conclude that Sidt2 is involved in NAADP-mediated release of calcium from insulin secretory granules and thus regulates insulin secretion.

Exopolysaccharide Production in Response to Medium Acidification Is Correlated With an Increase in Competition for Nodule Occupancy

Sinorhizobium meliloti strains unable to utilize galactose as a sole carbon source, due to mutations in the De-Ley Doudoroff pathway (dgoK), were previously shown to be more competitive for nodule occupancy. In this work, we show that strains carrying this mutation have galactose-dependent exopolysaccharide (EPS) phenotypes that were manifested as aberrant Calcofluor staining as well as decreased mucoidy when in an expR+ genetic background. The aberrant Calcofluor staining was correlated with changes in the pH of the growth medium. Strains carrying dgoK mutations were subsequently demonstrated to show earlier acidification of their growth medium that was correlated with an increase expression of genes associated with succinoglycan biosynthesis as well as increased accumulation of high and low molecular weight EPS in the medium. In addition, it was shown that the acidification of the medium was dependent on the inability of S. meliloti strains to initiate the catabolism of galactose. To more fully understand why strains carrying the dgoK allele were more competitive for nodule occupancy, early nodulation phenotypes were investigated. It was found that strains carrying the dgoK allele had a faster rate of nodulation. In addition, nodule competition experiments using genetic backgrounds unable to synthesize either succinoglycan or EPSII were consistent with the hypothesis that the increased competition phenotype was dependent upon the synthesis of succinoglycan. Fluorescent microscopy experiments on infected root-hair cells, using the acidotropic dye Lysotracker Red DND-99, provide evidence that the colonized curled root hair is an acidic compartment.