Physicochemical controls on the initiation of phytoplankton bloom during the winter monsoon in the Arabian Sea.

Occurrence of phytoplankton bloom in the northern Arabian Sea (NAS) during the winter monsoon is perplexing. The convective mixing leads to a deeper and well-oxygenated (> 95% saturation) mixed layer. We encountered low chlorophyll conditions though the nutrient conditions were favorable for a bloom. The mean ratio of silicate (Si) to DIN (Dissolved Inorganic Nitrogen: nitrate + nitrite + ammonium) in the euphotic zone was 0.52 indicating a "silicate-stressed" condition for the proliferation of diatoms. Also, the euphotic depth was much shallower (~ 49 m) than the mixed layer (~ 110 m) suggesting the Sverdrup critical depth limitation in the NAS. We show that the bloom in this region initiates only when the mixed layer shoals towards the euphotic zone. Our observations further suggest that two primary factors, the stoichiometric ratio of nutrients, especially the Si/DIN ratio, in the mixed layer and re-stratification of the upper water column, govern the phytoplankton blooming in NAS during the later winter monsoon. The important finding of the present study is that the Sverdrup's critical depth limitation gives rise to the observed low chl-a concentration in the NAS, despite having enough nutrients.

FtsA-FtsZ interaction in Vibrio cholerae causes conformational change of FtsA resulting in inhibition of ATP hydrolysis and polymerization.

Proper interaction between the divisome proteins FtsA and FtsZ is important for the bacterial cell division which is not well characterized till date. In this study, the objective was to understand the mechanism of FtsA-FtsZ interaction using full-length recombinant proteins. We cloned, over-expressed, purified and subsequently characterized FtsA of Vibrio cholerae (VcFtsA). We found that VcFtsA polymerization assembly was dependent on Ca2+ ions, which is unique among FtsA proteins reported until now. VcFtsA also showed ATPase activity and its assembly was ATP dependent. Binding parameters of the interaction between the two full-length proteins, VcFtsA, and VcFtsZ determined by fluorescence spectrophotometry yielded a Kd value of around 38 µM. The Kd value of the interaction was 3 µM when VcFtsA was in ATP bound state. We found that VcFtsZ after interacting with VcFtsA causes a change of secondary structure in the later one leading to loss of its ability to hydrolyze ATP, subsequently halting the VcFtsA polymerization. On the other hand, a double-mutant of VcFtsA (VcFtsA-D242E,R300E), that does not bind to VcFtsZ, polymerized in the presence of VcFtsZ. Though FtsA proteins among different organisms show 70-80% homology in their sequences, assembly of VcFtsA showed a difference in its regulatory processes.

Role of Andaman and Nicobar Islands in eddy formation along western boundary of the Bay of Bengal.

Eddies along western boundary of the Bay of Bengal (WBoB) play an important role in regulating regional climate and marine productivity of the north Indian Ocean. In this paper, role of Andaman and Nicobar islands (ANIs) in the formation of eddies along the WBoB is studied using an ocean general circulation model. Our analysis shows that, in the absence of ANIs, there is a significant reduction in the total number of mesoscale eddies in this region. The impact is particularly evident for the cyclonic eddies as a reduction of ~50% can be noticed in the absence of the islands. In contrast, influence of ANIs on anticyclonic eddies is not homogeneous in the WBoB; while absence of ANIs significantly increases anticyclonic eddies in the central part of the WBoB, a decrease can be noticed in the southern part. We further show that the reduction in number of cyclonic eddies along the WBoB is primarily driven by reduced baroclinic and barotropic instabilities. This process is more conspicuous during winter (October-January) season compared to summer (June-September) and spring (February-May) seasons.

Author Correction: Annihilation of the Somali upwelling system during summer monsoon.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Annihilation of the Somali upwelling system during summer monsoon.

Somali upwelling system during northern summer is believed to be the largest upwelling region in the Indian Ocean and has motivated some of the early studies on the Indian Ocean. Here we present results from observations and ocean model to show that the upwelling along the Somali coast is limited to the early phase of the summer monsoon and later primarily limited to the eddy dominated flows in the northern and some extent in the southern part of the coast. Major part of the Somali coast (~60% of the entire coastal length) shows prominent downwelling features driven by offshore negative windstress curl and subsurface entrainment mixing. Further, we show that the surface cooling of coastal waters are dominantly driven by subsurface entrainment and surface heat fluxes. These findings not only augment the existing knowledge of the Somali upwelling system, but also have serious implications on the regional climate. Most importantly, our analysis underscores the use of alongshore winds only to project future (climate driven) changes in the upwelling intensity along this coast.

MiR-16 targets Bcl-2 in paclitaxel-resistant lung cancer cells and overexpression of miR-16 along with miR-17 causes unprecedented sensitivity by simultaneously modulating autophagy and apoptosis.

Non-small cell lung cancer is one of the most aggressive cancers as per as the mortality and occurrence is concerned. Paclitaxel based chemotherapeutic regimes are now used as an important option for the treatment of lung cancer. However, resistance of lung cancer cells to paclitaxel continues to be a major clinical problem nowadays. Despite impressive initial clinical response, most of the patients eventually develop some degree of paclitaxel resistance in the course of treatment. Previously, utilizing miRNA arrays we reported that downregulation of miR-17 is at least partly involved in the development of paclitaxel resistance in lung cancer cells by modulating Beclin-1 expression [1]. In this study, we showed that miR-16 was also significantly downregulated in paclitaxel resistant lung cancer cells. We demonstrated that anti-apoptotic protein Bcl-2 was directly targeted miR-16 in paclitaxel resistant lung cancer cells. Moreover, in this report we showed that the combined overexpression of miR-16 and miR-17 and subsequent paclitaxel treatment greatly sensitized paclitaxel resistant lung cancer cells to paclitaxel by inducing apoptosis via caspase-3 mediated pathway. Combined overexpression of miR-16 and miR-17 greatly reduced Beclin-1 and Bcl-2 expressions respectively. Our results indicated that though miR-17 and miR-16 had no common target, both miR-16 and miR-17 jointly played roles in the development of paclitaxel resistance in lung cancer. miR-17 overexpression reduced cytoprotective autophagy by targeting Beclin-1, whereas overexpression of miR-16 potentiated paclitaxel induced apoptotic cell death by inhibiting anti-apoptotic protein Bcl-2.

Dimethyl sulphoxide and Ca2+ stimulate assembly of Vibrio cholerae FtsZ.

We cloned, overexpressed and purified Vibrio cholerae FtsZ protein for the first time. We used several complementary techniques to probe and compare the comparative assembly properties of recombinant Vibrio cholerae FtsZ (VcFtsZ) and Escherichia coli FtsZ (EcFtsZ). We observed that VcFtsZ polymerized at a slower rate than EcFtsZ and interestingly its polymerization was highly dependent on the presence of Ca(2+) ion. Furthermore, DMSO specifically modulated the polymerization of VcFtsZ, promoted polymer bundling and increased the stability of the VcFtsZ protofilaments. Whereas DMSO showed no significant stimulatory effect on the assembly and bundling of EcFtsZ. Transmission electron microscopy experiments demonstrated that in presence of 8% DMSO the average thickness of the VcFtsZ polymers were increased significantly. DMSO specifically stabilized the VcFtsZ polymers against dilution induced disassembly and it reduced the GTPase activity of VcFtsZ. These results collectively suggested that despite lot of sequence homology, the assembly of VcFtsZ and EcFtsZ are differently regulated processes. We expect to use this knowledge of assembly properties of VcFtsZ for screening of small molecules against VcFtsZ for development of anti-cholera agent.

miR-17-5p downregulation contributes to paclitaxel resistance of lung cancer cells through altering beclin1 expression.

Non-small-cell lung cancer (NSCLC) is one of the most leading causes of cancer-related deaths worldwide. Paclitaxel based combination therapies have long been used as a standard treatment in aggressive NSCLCs. But paclitaxel resistance has emerged as a major clinical problem in combating non-small-cell lung cancer and autophagy is one of the important mechanisms involved in this phenomenon. In this study, we used microRNA (miRNA) arrays to screen differentially expressed miRNAs between paclitaxel sensitive lung cancer cells A549 and its paclitaxel-resistant cell variant (A549-T24). We identified miR-17-5p was one of most significantly downregulated miRNAs in paclitaxel-resistant lung cancer cells compared to paclitaxel sensitive parental cells. We found that overexpression of miR-17-5p sensitized paclitaxel resistant lung cancer cells to paclitaxel induced apoptotic cell death. Moreover, in this report we demonstrated that miR-17-5p directly binds to the 3'-UTR of beclin 1 gene, one of the most important autophagy modulator. Overexpression of miR-17-5p into paclitaxel resistant lung cancer cells reduced beclin1 expression and a concordant decease in cellular autophagy. We also observed similar results in another paclitaxel resistant lung adenosquamous carcinoma cells (H596-TxR). Our results indicated that paclitaxel resistance of lung cancer is associated with downregulation of miR-17-5p expression which might cause upregulation of BECN1 expression.

Thymoquinone inhibits microtubule polymerization by tubulin binding and causes mitotic arrest following apoptosis in A549 cells.

Microtubule-Targeting agents (MTA) are indispensable for cancer therapeutics. We here report thymoquinone (TQ) as a new MTA that already has been appreciated for its anticancer effects. TQ induced G2/M cell cycle arrest in human non-small lung epithelial cells (A549) and majority of arrested cells were in mitosis. TQ depolymerized the microtubule (MT) network and disrupted mitotic spindle organization of A549 cells. MT depolymerization by TQ was followed by apoptosis and subsequent loss in cell viability (IC50 value of ∼10 µM). Interestingly, TQ didn't affect the MT network of normal HUVEC cells at and below the IC50 concentration for A549 cells. TQ also inhibited tubulin polymerization in cell-free system with an IC50 of 27 µM and bound to tubulin heterodimers at a single site with a dissociation constant of 1.19 µM at 25 °C. Binding of TQ to tubulin quenched the tryptophan fluorescence of protein in a time-dependent manner. The TQ-tubulin binding kinetics was biphasic in nature and equilibrated in 30 min. TQ competed with colchicine for tubulin binding with a Ki of 1.9 µM as determined by modified Dixon plot analysis, this suggests TQ may bind tubulin at or near the colchicine binding site and in silico modeling study supported that. Our results establish a novel antimitotic mechanism of TQ by its direct binding to tubulin-MT network in A549 cells.

Blood compatible N-maleyl chitosan-graft-PAMAM copolymer for enhanced gene transfection.

To improve transfection efficiency, we prepared N-maleyl chitosan-graft-polyamidoamine (NMCTS-graft-PAMAM) copolymer. Self-assembled NMCTS-graft-PAMAM/pDNA complexes were prepared by complex coacervation method at different N/P (nitrogen to phosphate ratio) ratios. The copolymer effectively formed complexes with pDNA at lower N/P ratio (N/P ratio 1.0) than that of unmodified chitosan (N/P ratio 2.0) and the complexes were spherical with particle size of 100-150 nm. The copolymer showed significant protection of DNA from nuclease attack with lower toxicity against HeLa cell. The copolymer also showed no noticeable hemolytic effects up to 10mg/mL indicating no detectable disturbance of the red blood cell membranes. The transfection efficiency of the copolymer was increased significantly compared to that of chitosan and reached up to 36±2% at N/P ratio 7.0 which was higher than that of PEI (30±3% at N/P ratio 10). Therefore, the copolymer may be a strong alternative candidate as effective nonviral vector.