Biology subjects

Saurav, S.

Publications and source records attributed to Saurav, S..

4 recordsLinked to original sources

bioRxiv2026

SERCA2b loss of function drives pigmentation by inducing adaptive ER stress and enhancing mitochondrial calcium uptake: significance in pathological hyperpigmentation associated with Darier’s Disease

Pigmentation is a critical protective mechanism that safeguards the skin against UV-induced damage, whereas dysregulated pigmentation predisposes to pigmentary disorders and skin malignancies. Although calcium signaling has emerged as an important regulator of melanogenesis, the identity of the calcium-handling proteins and the molecular mechanisms linking calcium dynamics to pigmentation remain poorly understood. Here, we identify the ER calcium pump SERCA2b as a negative regulator of pigmentation through modulation of ER stress and mitochondrial calcium uptake. We demonstrate that SERCA2b expression inversely correlates with pigmentation levels, and gain- and loss-of-function studies establish SERCA2b as a suppressor of melanogenesis. Mechanistically, SERCA2b depletion induces adaptive ER stress, enhances ER-mitochondrial proximity, and promotes mitochondrial calcium uptake. Notably, mutations in SERCA2b are associated with Darier disease, a condition characterized by hyperpigmented skin lesions, although the underlying mechanism remains unknown. To address this, we generated SERCA2b mutants corresponding to variants identified in Indian Dariers disease patients and examined their effects on pigmentation, ER stress, and mitochondrial calcium dynamics. The mutant phenotypes closely recapitulated SERCA2b loss-of-function effects, demonstrating that adaptive ER stress and enhanced mitochondrial calcium signaling underlie hyperpigmentation associated with Dariers disease. Importantly, treatment with 4-phenylbutyrate (4-PBA), an FDA-approved ER stress alleviator, rescued mutant-induced hyperpigmentation, reduced ER stress, and normalized mitochondrial calcium uptake. Collectively, our findings uncover a previously unrecognized role of SERCA2b in skin pigmentation, establish a mechanistic link between SERCA2b mutations and hyperpigmentation, and identify adaptive ER stress pathways as potential therapeutic target for pigmentary disorders.

Cell Biology↗

bioRxiv2025

IP3R2 mediated inter-organelle Ca2+ signaling orchestrates melanophagy

Organelle dynamics and crosstalk play a critical role in cellular functions thereby regulating physiological processes and pathological conditions. A variety of cellular processes are outcome of a balance between organelle biogenesis and degradation. Pigmentation is one such homeostatic state that is a result of melanosome biogenesis and melanosome degradation. Although melanosome biogenesis is partially understood, the melanosome degradation i.e. melanophagy remains largely unappreciated. Here, we reveal that Inositol 1,4,5-trisphosphate receptor 2 (IP3R2) is a negative regulator of melanophagy. In this study, we developed two de novo ratio metric imaging probes to study melanophagy in live-cells. Using these probes, biochemical assays, ultrastructural studies, confocal microscopy, molecular analyses and calcium imaging; we demonstrate that IP3R2, but not IP3R1 or IP3R3, keeps melanophagy in check. In vivo studies in zebrafish model system further substantiate IP3R2s functional relevance in pigmentation. Mechanistically, IP3R2 silencing decreases mitochondrial Ca2+ uptake, augments ADP/ATP ratio and thereby activates melanophagy. Simultaneously, IP3R2 knockdown increases ER-lysosome proximity, enhances lysosomal Ca2+ levels and decreases lysosomal pH. This in turn activates lysosomal TRPML1 channel and stimulates nuclear translocation of TFEB transcription factor, which facilitates transcription of key autophagy and two known melanophagy drivers. Taken together, we uncover that IP3R2-mediated Ca2+ signaling across organelles is a critical determinant of melanophagy and thereby skin pigmentation. Hence, this signaling cascade offers potential therapeutic prospects for the management of pigmentary disorders and skin malignancies. Highlights[tpltrtarr] IP3R2, but not IP3R1 and IP3R3, is a critical positive regulator of melanogenesis in vitro and in vivo. [tpltrtarr]Generation and validation of two de novo ratio-metric live cells imaging probes reveal crucial role of IP3R2 in melanophagy. [tpltrtarr]IP3R2 knockdown decreases mitochondrial Ca2+ uptake, augments ADP/ATP ratio and thereby activates melanophagy via AMPK-ULK1 pathway. [tpltrtarr]IP3R2 silencing enhances ER-lysosomal proximity, elevates lysosomal Ca2+ levels and reduces lysosomal pH. [tpltrtarr]IP3R2 knockdown stimulates lysosomal TRPML1 channel activity thereby facilitating nuclear translocation of TFEB transcription factor. [tpltrtarr]TFEB transcriptionally upregulates genes involved in the melanophagy process, leading to enhanced degradation of melanosomes and decreased pigmentation.

cell biology↗

bioRxiv2024

Remodeling Ca2+ dynamics by targeting a promising E-box containing G-quadruplex at ORAI1 promoter in triple-negative breast cancer.

ORAI1 is an intrinsic component of store-operated calcium entry (SOCE) that strictly regulates Ca2+ influx in most non-excitable cells. ORAI1 has been extensively studied to have been overexpressed in various cancer phenotypes, and its signal transduction has been associated with oncotherapy resistance. There is extensive proteomic interaction of ORAI1 with other channels and effectors, resulting in various altered phenotypes. However, the transcription regulation of this gene is not well understood. We have found a putative G-quadruplex (G4) motif, ORAI1-Pu, in the upstream promoter region of the gene, having regulatory functions. High-resolution 3-D NMR structure elucidation suggests that ORAI1-Pu is a stable parallel-stranded G4, having an unusual 8-nt loop imparting dynamics without affecting the structural stability. The protruded loop further houses an E-box motif that provides a docking site for transcription factors like Zeb1. The G4 structure was also endogenously observed using Chromatin Immunoprecipitation (ChIP) with anti-G4 antibody (BG4) in the MDA-MB-231 cell line overexpressing ORAI1. Ligand-mediated stabilization suggested that the stabilized G4 represses transcription in cancer cell line MDA-MB-231. Downregulation of transcription further cascaded down to a decrease in Ca2+ entry by the SOCE pathway, as observed by Fura-2 confocal Ca2+ imaging.

biochemistry↗

bioRxiv2023

Telomeres control regulation of the human Telomerase (hTERT) gene through non-telomeric TRF2 and independent of Telomere looping

The function of the human telomerase reverse transcriptase (hTERT) in the synthesis and maintenance of chromosome ends, or telomeres, is widely understood. Whether and how telomeres, on the other hand, influence hTERT regulation is relatively less studied. We found hTERT was transcriptionally altered depending on telomere length (TL). This resulted from TL-dependent binding of TRF2 between telomeres and the hTERT promoter. hTERT promoter-bound TRF2 was non-telomeric and did not involve the looping of telomeres to the hTERT promoter. Cell lines from different tissue types (fibrosarcoma (HT1080), colon cancer (HCT116), and breast cancer (MDA-MB-231), engineered for either telomere elongation/shortening gave increase/decrease in hTERT, respectively. Mechanistically, we show hTERT promoter-bound non-telomeric TRF2 recruits the canonical PRC2-complex inducing repressor histone H3K27-trimethylation in a TL-dependent fashion. This was further supported by TL-dependent promoter activity from an exogenously inserted hTERT reporter. Increase in TL over days followed by gradual decline, resulted in activation followed by repression of hTERT in a concerted manner, further implicating TL as a key factor for hTERT regulation. Notably on reprogramming primary fibroblasts to induced pluripotent stem cells (iPSCs), TRF2 loss from the hTERT promoter was evident along with telomere elongation and hTERT upregulation. Conversely, on telomere shortening in iPSCs, hTERT promoter-bound TRF2 was restored with marked reduction in hTERT further supporting the causal role of TL in hTERT transcription. Mechanisms of tight control of hTERT by TL shown here are likely to have major implications in telomere-related physiologies, particularly, cancer, ageing and pluripotency. TeaserTelomere length controls hTERT expression by modulating TRF2 distribution and PRC2-mediated repression, highlighting a self-regulatory mechanism in cancer.

cancer biology↗