Identification and characterization of metal uptake ABC transporters in Mycobacterium tuberculosis unveil their ligand specificity

Mycobacterium tuberculosis, one of the major threats to mankind, requires micronutrients like metal ions for their survival and pathogenicity inside the host system. Intracellular pathogens such as M. tuberculosis have co-evolved to combat the nutritional immunity developed by the host. It has developed eminent mechanisms to sequester essential metal ions from the host system. One such prominent mechanism to scavenge metal ions to thrive in the host cell involves ATP-binding cassette (ABC) transporters, which transport metal ions (in free and/or complex forms) across the cell membrane. This study employs a high-throughput data mining analysis to identify open reading frames (ORFs) encoding metal uptake ABC transporters in M. tuberculosis H37Rv. In total, 19 ORFs resulting in seven ABC transport systems and two P-type ATPases were identified, which are potentially involved in the uptake of different metal ions. The results also suggest the existence of a subunit sharing mechanism in M. tuberculosis where the transmembrane and nucleotide binding domains are shared among different ABC transport systems indicating the import of multiple substrates via a single ABC transporter. Thus, this study reflects an overview of the repertoire of metal-specific ABC transport systems in M. tuberculosis H37Rv, providing potential therapeutic targets for the future.

Mammalian copper homeostasis requires retromer-dependent recycling of the high-affinity copper transporter 1

The concentration of essential micronutrients, such as copper (used here to describe both Cu⁺ and Cu²⁺), within the cell is tightly regulated to avoid their adverse deficiency and toxicity effects. Retromer-mediated sorting and recycling of nutrient transporters within the endo-lysosomal network is an essential process in regulating nutrient balance. Cellular copper homeostasis is regulated primarily by two transporters: the copper influx transporter copper transporter 1 (CTR1; also known as SLC31A1), which controls the uptake of copper, and the copper-extruding ATPase ATP7A, a recognised retromer cargo. Here, we show that in response to fluctuating extracellular copper, retromer controls the delivery of CTR1 to the cell surface. Following copper exposure, CTR1 is endocytosed to prevent excessive copper uptake. We reveal that internalised CTR1 localises on retromer-positive endosomes and, in response to decreased extracellular copper, retromer controls the recycling of CTR1 back to the cell surface to maintain copper homeostasis. In addition to copper, CTR1 plays a central role in the trafficking of platinum. The efficacy of platinum-based cancer drugs has been correlated with CTR1 expression. Consistent with this, we demonstrate that retromer-deficient cells show reduced sensitivity to the platinum-based drug cisplatin.

Summary: CTR1 (SLC31A1) is the only known mammalian importer of copper. We show that CTR1 is a retromer complex cargo protein, and that retromer is required for cellular sensitivity to extracellular copper.

Reversal of behavioral decline and neuropathology by a complex vitamin supplement involves modulation of key neurochemical stressors

Metal ions are crucial for normal neurochemical signaling and perturbations in their homeostasis have been associated with neurodegenerative processes. Hypothesizing that in vivo modulation of key neurochemical processes including metal ion regulation (by transferrin receptor-1: TfR-1) in cells can improve disease outcome, we investigated the efficacy of a complex vitamin supplement (CVS) containing B-vitamins and ascorbic acid in preventing/reversing behavioral decline and neuropathology in rats. Wistar rats (eight weeks-old) were assigned into five groups (n = 8), including controls and those administered CVS (400 mg/kg/day) for two weeks before or after AlCl₃ (100 mg/kg)-induced neurotoxicity. Following behavioral assessments, prefrontal cortex (PFC) and hippocampus were prepared for biochemical analyses, histology and histochemistry. CVS significantly reversed reduction of exploratory/working memory, frontal-dependent motor deficits, cognitive decline, memory dysfunction and anxiety. These correlated with CVS-dependent modulation of TfP-1 expression that were accompanied by significant reversal of neural oxidative stress in expressed superoxide dismutase, nitric oxide, catalase, glutathione peroxidase and malondialdehyde. Furthermore, CVS inhibited neural bioenergetics dysfunction, with increased labelling of glucokinase within PFC and hippocampus correlating with increased glucose-6-phosphate dehydrogenase and decreased lactate dehydrogenase expressions. These relates to inhibition of over-expressed acetylcholinesterase and increased total protein synthesis. Histological and Nissl staining of thin sections corroborated roles of CVS in reversing AlCl₃-induced neuropathology. Summarily, we showed the role of CVS in normalizing important neurochemical molecules linking concurrent progression of oxidative stress, bioenergetics deficits, synaptic dysfunction and cellular hypertrophy during neurodegeneration.

Synergy and antagonism between iron chelators and antifungal drugs in Cryptococcus

Fungal infections remain very difficult to treat, and developing new antifungal drugs is difficult and expensive. Recent approaches therefore seek to augment existing antifungals with synergistic agents that can lower the therapeutic dose, increase efficacy and prevent resistance from developing. Iron limitation can inhibit microbial growth, and iron chelators have been employed to treat fungal infections. In this study, chequerboard testing was used to explore combinations of iron chelators with antifungal agents against pathogenic Cryptococcus spp. with the aim of determining how disruption to iron homeostasis affects antifungal susceptibility. The iron chelators ethylenediaminetetraacetic acid (EDTA), deferoxamine (DFO), deferiprone (DFP), deferasirox (DSX), ciclopirox olamine and lactoferrin (LF) were paired with the antifungal agents amphotericin B (AmB), fluconazole, itraconazole, voriconazole and caspofungin. All chelators except for DFO increased the efficacy of AmB, and significant synergy was seen between AmB and LF for all Cryptococcus strains. Addition of exogenous iron rescued cells from the antifungal effect of LF alone but could not prevent inhibition by AmB + LF, indicating that synergy was not due primarily to iron chelation but to other properties of LF that were potentiated in the presence of AmB. Significant synergy was not seen consistently for other antifungal-chelator combinations, and EDTA, DSX and DFP antagonised the activity of azole drugs in strains of Cryptococcus neoformans var. grubii. This study highlights the range of interactions that can be induced by chelators and indicates that most antifungal drugs are not enhanced by iron limitation in Cryptococcus.

Dynamic internalization and recycling of a metal ion transporter: Cu homeostasis and CTR1, the human Cu⁺ uptake system

Cu ion (Cu) entry into human cells is mediated by CTR1 (also known as SLC31A1), the high-affinity Cu transporter. When extracellular Cu is raised, the cell is protected against excess accumulation by rapid internalization of the transporter. When Cu is lowered, the transporter returns to the membrane. We show in HEK293 cells overexpressing CTR1 that expression of either the C-terminal domain of AP180 (also known as SNAP91), a clathrin-coat assembly protein that sequesters clathrin, or a dominant-negative mutant of dynamin, decreases Cu-induced endocytosis of CTR1, as does a dynamin inhibitor and clathrin knockdown using siRNA. Utilizing imaging, siRNA techniques and a new high-throughput assay for endocytosis employing CLIP-tag methodology, we show that internalized CTR1 accumulates in early sorting endosomes and recycling compartments (containing Rab5 and EEA1), but not in late endosomes or lysosomal pathways. Using live cell fluorescence, we find that upon extracellular Cu removal CTR1 recycles to the cell surface through the slower-recycling Rab11-mediated pathway. These processes enable cells to dynamically alter transporter levels at the plasma membrane and acutely modulate entry as a safeguard against excess cellular Cu.