Biology subjects

Moitra, S.

Publications and source records attributed to Moitra, S..

4 recordsLinked to original sources

bioRxiv2023

How long is long? Word length effects in reading correspond to minimal graphemic units: An MEG study in Bangla

This paper presents a magnetoencephalography (MEG) study on reading in Bangla, an east Indo-Aryan language predominantly written in an abugida script. The study aims to uncover how visual stimuli are processed and mapped onto abstract linguistic representations in the brain. Specifically, we investigate the neural responses that correspond to word length in Bangla, a language with a unique orthography that introduces multiple ways to measure word length. Our results show that MEG signals localised in the anterior left fusiform gyrus, at around 130ms, are highly correlated with word length when measured in terms of the number of minimal graphemic units in the word rather than independent graphemic units (ak[s]ar) or phonemes. Our findings suggest that minimal graphemic units could serve as a suitable metric for measuring word length in non-alphabetic orthographies such as Bangla.

neuroscience↗

bioRxiv2022

Sphingolipids protect ergosterol in the Leishmania major membrane from sterol-binding toxins

Susceptibility of Leishmania to the first line treatment amphotericin B remains poorly understood. Amphotericin B targets ergosterol, so one approach to improving drug efficacy and reducing side effects could be improving access to ergosterol. While the surface exposure of ergosterol in Leishmania is unknown, sterols in mammalian cells can be sheltered from sterol-binding agents by membrane components, including sphingolipids. Here, we tested the ability of the Leishmania major sphingolipids inositol phosphorylceramide (IPC), and ceramide to shelter ergosterol by preventing binding and cytotoxicity of the sterol-specific toxins streptolysin O and perfringolysin O using flow cytometry. In contrast to mammalian systems, Leishmania sphingolipids did not preclude toxin binding to sterols in the membrane. However, IPC interfered with cytotoxicity. Ceramide reduced perfringolysin O, but not streptolysin O, cytotoxicity in cells. Ceramide sensing was controlled by the toxin L3 loop. Ceramide was sufficient to protect L. major promastigotes from amphotericin B. We propose a mechanism whereby pore-forming toxins engage additional lipids like ceramide to determine the optimal environment to sustain pore formation. Thus, L. major offers a genetically tractable model organism for understanding toxin-membrane interactions. Furthermore, our findings suggest targeting ceramide may enhance the efficacy of ergosterol-targeting anti-leishmanial drugs. Abstract ImportanceLeishmaniasis is a neglected tropical disease with [~]1.5-2 million new cases and [~]70,000 deaths annually. One first-line treatment for leishmaniasis is liposomal amphotericin B, which is expensive and damages the kidneys. Cost and side effects can be minimized by improving efficacy. To improve efficacy, we must learn how amphotericins target--ergosterol--is protected by other components of Leishmania. The human ergosterol equivalent is protected by components called sphingolipids. We tested the ability of sphingolipids to protect ergosterol using pore-forming toxins. Pore-forming toxins use ergosterol to bind and kill Leishmania. Unlike human cells, toxins bound to ergosterol--indicating that they had access--when sphingolipids were present. However, sphingolipids protected Leishmania from toxins and amphotericin. Thus, Leishmania organizes sterol-protective components differently from humans. Further, toxins and Leishmania serve as a system to understand fundamental rules governing sterol-protecting component membrane organization. We can use this information to help improve drugs targeting sterols.

microbiology↗

bioRxiv2020

De novo synthesis of phosphatidylcholine is essential for the promastigote but not amastigote stage in Leishmania major

Phosphatidylcholine (PC) is the most abundant type of phospholipids in eukaryotes constituting ~30% of total lipids in Leishmania. PC synthesis mainly occurs via the choline branch of the Kennedy pathway (choline {Rightarrow} choline-phosphate {Rightarrow} CDP-choline {Rightarrow} PC) and the N-methylation of phosphatidylethanolamine (PE). In addition, Leishmania parasites can acquire PC and other lipids from the host or culture medium. In this study, we assessed the function and essentiality of choline ethanolamine phosphotransferase (CEPT) in Leishmania major which is responsible for the final step of the de novo synthesis of PC and PE. Our data indicate that CEPT is localized in the endoplasmic reticulum and possesses the activity to generate PC from CDP-choline and diacylglycerol. Targeted deletion of CEPT is only possible in the presence of an episomal CEPT gene in the promastigote stage of L. major. These chromosomal null parasites require the episomal expression of CEPT to survive in culture, confirming its essentiality during the promastigote stage. In contrast, during in vivo infection of BALB/c mice, these chromosomal null parasites appeared to lose the episomal copy of CEPT while maintaining normal levels of virulence, replication and cellular PC. Therefore, while the de novo synthesis of PC/PE is indispensable for the proliferation of promastigotes, intracellular amastigotes appear to acquire most of their lipids through salvage and remodeling.

microbiology↗

bioRxiv2020

Sterol 14α-demethylase is vital for mitochondrial functions and stress tolerance in Leishmania major

Sterol 14--demethylase (C14DM) is a key enzyme in the biosynthesis of sterols and the primary target of azoles. In Leishmania major, genetic or chemical inactivation of C14DM leads to accumulation of 14-methylated sterol intermediates and profound plasma membrane abnormalities including increased fluidity and failure to maintain ordered membrane microdomains. These defects likely contribute to the hypersensitivity to heat and severely reduced virulence displayed by the C14DM-null mutants (c14dm-). In addition to plasma membrane, sterols are present in intracellular organelles. In this study, we investigated the impact of C14DM ablation on mitochondria. Our results demonstrate that c14dm- mutants have significantly higher mitochondrial membrane potential than wild type parasites. Such high potential leads to the buildup of reactive oxygen species in the mitochondria, especially under nutrient-limiting conditions. Consistent with these mitochondrial alterations, c14dm- mutants show impairment in respiration and are heavily dependent on glucose uptake and glycolysis to generate energy. Consequently, these mutants are extremely sensitive to glucose deprivation and such vulnerability can be rescued through the supplementation of glucose or glycerol. In addition, the accumulation of oxidants may also contribute to the heat sensitivity exhibited by c14dm-. Finally, genetic or chemical ablation of C14DM causes increased susceptibility to pentamidine, an antimicrobial agent with activity against trypanosomatids. In summary, our investigation reveals that alteration of sterol synthesis can negatively affect multiple cellular processes in Leishmania parasites and make them vulnerable to clinically relevant stress conditions. AUTHOR SUMMARYSterols are well recognized for their stabilizing effects on the plasma membrane, but their functions in intracellular organelles are under explored, which hampers the development of sterol synthesis inhibitors as drugs. Our previous studies have demonstrated significant plasma membrane instability in the sterol biosynthetic mutant c14dm- in Leishmania major, a pathogenic protozoan responsible for cutaneous leishmaniasis causing 1-1.5 million infections a year. While the plasma membrane defects have undoubtedly contributed to the reduced virulence exhibited by c14dm- mutants, it was not clear whether other cellular processes were also affected. In this study, we revealed profound mitochondrial dysfunctions and elevated level of reactive oxygen species in c14dm- mutants. These sterol mutants rely heavily on glycolysis to generate energy and are extremely sensitive to glucose restriction. In addition, the accumulation of oxidants appears to be responsible (at least in part) for the previously observed heat sensitivity in c14dm- mutants. Thus, genetic or chemical inactivation of C14DM can influence the functions of cellular organelles beyond the plasma membrane. These findings shed light on the mechanism of action for azole compounds and provide new insight into the roles of sterol biosynthesis in Leishmania parasites.

microbiology↗