Biology subjects

Sarikas, S.

Publications and source records attributed to Sarikas, S..

4 recordsLinked to original sources

bioRxiv2025

Presenilin-1 controls glucose metabolism and identity of pancreatic beta cells

AbstractPresenilin 1 is an endoplasmic reticulum protein, most known for its role in pathogenesis of familial Alzheimers Disease (AD). Presenilin 1 has been attributed roles in intracellular calcium homeostasis in the brain, as well as in the pancreatic beta cells, where it has been shown to be fundamental for glucose-induced insulin secretion. Functional similarity of presenilin 1 in regulation of intracellular calcium homeostasis in the brain and pancreas prompted us to investigate a prevalent assumption that associates AD and diabetes mellitus. By examining pancreatic islets from AD model mice, we have found deficits in initial phase of glucose-induced calcium signaling and insulin secretion. Furthermore, these transgenic mice showed a tendency towards reduced expression of mature beta cell markers, which was even more pronounced in islets and beta cell lines with a transient knock down of presenilin 1. We demonstrate here that presenilin 1 controls beta cell glycolysis by regulating sub-cellular calcium homeostasis and, in doing so, contributes to preservation of beta cell identity.

cell biology↗

bioRxiv2022

Real-time dynamics of individual chemoreceptor mRNA molecules reveals translation hotspots at the inner membrane of Escherichia coli

Since bacteria lack a nucleus, the location of mRNA molecules is determined by the different characteristics of the encoded proteins, and the transcriptome is spatially arranged into cytosolic and membrane-associated mRNA. While translation of membrane protein-encoding mRNA has been studied in great mechanistic detail using biochemical methods, the spatiotemporal dynamics of this process remains poorly understood at the subcellular level. Here, we investigate the dynamics of individual fluorescently labelled mRNA molecules encoding the transmembrane serine chemoreceptor Tsr, to probe the mechanism of membrane protein translation. Analysis of tsr mRNA diffusion in the proximity of the plasma membrane revealed distinct diffusive modes that reflect the state of the mRNA molecule and its involvement in the process of active translation into the Sec secretion system. We find that the composition, and hence the fluidity of the membrane affects diffusion of membrane targeted mRNAs. Moreover, Tsr translation occurs in localized membrane regions, similar to eukaryotic hotspots. The hotspot localization coincides with the physical location of the transcribed gene, which itself is displaced towards the inner membrane. These findings suggest that inner membrane protein translation is a spatially defined process that occurs in hotspots, indicative of long-lived transertion sites. Our results show an additional layer of spatio-temporal structuring within the bacterial cell, thus revealing a qualitatively different understanding of the basic process of transcription and translation in bacteria. Significance StatementA large fraction of the bacterial proteome is directly synthesized into the inner membrane, and this process shapes the overall distribution of mRNA transcripts within the cell. Although highly dynamic in their nature, bacterial transcriptomes have mostly been studied in fixed cells. Here, we track individual mRNA molecules encoding the serine chemoreceptor in living bacterial cells and find that translation occurs in membrane hotspots that were previously exclusive to eukaryotes. Our results indicate an additional layer of spatio-temporal structuring within the bacterial cell that impacts our understanding of transcription and translation in bacteria.

microbiology↗

bioRxiv2021

Intracellular Ca2+ channels initiate physiological glucose signaling in beta cells examined in situ

The release of peptide hormones is predominantly regulated by a transient increase in cytosolic Ca2+ concentration ([Ca2+]c). To trigger exocytosis, Ca2+ ions enter the cytosol from intracellular Ca2+ stores or from the extracellular space. The molecular events of late stages of exocytosis, and their dependence on [Ca2+]c, were extensively described in isolated single cells from various endocrine glands. Notably less work has been done on endocrine cells in situ to address the heterogeneity of [Ca2+]c events contributing to a collective functional response of a gland. For this beta cell collectives in a pancreatic islet are particularly well suited as they are the smallest, experimentally manageable functional unit, where [Ca2+]c dynamics can be simultaneously assessed on both cellular and collective level. Here we measured [Ca2+]c transients across all relevant timescales, from a sub-second to a minute time range, using high-resolution imaging with low-affinity Ca2+ sensor. We quantified the recordings with a novel computational framework for semi-automatic image segmentation and [Ca2+]c event identification. Our results demonstrate that under physiological conditions the duration of [Ca2+]c events is variable, and segregated into 3 reproducible modes, sub-second, second and tens of seconds time range, and are a result of a progressive temporal summation of the shortest events. Using pharmacological tools we show that activation of intracellular Ca2+ receptors is both sufficient and necessary for glucose-dependent [Ca2+]c oscillations in beta cell collectives, and that a subset of [Ca2+]c events could be triggered even in the absence of Ca2+ influx across the plasma membrane. In aggregate, our experimental and analytical platform was able to readily address the involvement of intracellular Ca2+ receptors in shaping the heterogeneity of [Ca2+]c responses in collectives of endocrine cells in situ.

physiology↗

bioRxiv2020

Structure and Evolution of Constitutive Bacterial Promoters

Predicting gene expression levels from any DNA sequence is a major challenge in biology. Using libraries with >25,000 random mutants, we developed a biophysical model that accounts for major features of {sigma}70-binding bacterial promoters to accurately predict constitutive gene expression levels of any sequence. We experimentally and theoretically estimated that 10-20% of random sequences lead to expression and 82% of non-expressing sequences are one point mutation away from a functional promoter. Generating expression from random sequences is pervasive, such that selection acts against {sigma}70-RNA polymerase binding sites even within inter-genic, promoter-containing regions. The pervasiveness of {sigma}70- binding sites, which arises from the structural features of promoters captured by our biophysical model, implies that their emergence is unlikely the limiting step in gene regulatory evolution.

evolutionary biology↗