Brain-derived neurotrophic factor (BDNF) is a neurotrophin that, through the activation of its full length receptor, TrkB-FL, plays a pivotal role in neuroprotection, namely against neuronal toxicity mediated by amyloid-β peptide (Aβ). In astrocytes, the increase of calcium (Ca2+) signaling due the increase of metabotropic glutamate receptor type 5 (mGluR5) levels, induced by Aβ, has been considered deleterious for astrocytic function. In adition BDNF also increases intracellular calcium concentration ([Ca²⁺]ᵢ), in astrocytes, via activation of the truncated TrkB receptor isoform, TrkB-Tc. While the role of BDNF, in neurons, is well established, in terms of neuroprotection, its role in astrocytes, particularly in Aβ-induced toxicity conditions, remains less clear. Thus, this study aimed to evaluate the interplay between BDNF and Aβ in the modulation of [Ca2+]i signaling in primary cultures of cortical astrocytes. Ca2+ transients were induced by the activation of mGluR5 through the application of its agonist DHPG. In astrocytes pre-exposed to Aβ25-35 (10 µM, for 48-72 h), the Ca2+ transient amplitude was significantly increased compared to the control. A similar increase was observed in astrocytes incubated for 48 h with BDNF (20 ng/mL), or when astrocytes were simultaneously exposed to BDNF and Aβ. The effect of BDNF was mediated by TrkB-Tc since it was prevented by a cocktail of the three siRNAs against TrkB-Tc expression. mGluR5 levels were significantly increased in astrocytes pre-exposed to Aβ, while exposure to BDNF did not affect mGluR5 levels. Importantly, while the presence of Aβ did affect TrkB-Tc receptor levels in astrocytes, the presence of BDNF prevented the increase in mGluR5 levels caused by Aβ thus precluding a further exacerbation of Ca2+ transients caused by Aβ.

Defective insulin secretion is a hallmark of diabetes mellitus. Glucose-induced Ca2+ oscillations are critical for the stimulation of insulin secretion, though the mechanisms through which these propagate across the islet are poorly understood. Here, we use beta cell-targeted GCaMP6f to explore the role of endoplasmic reticulum (ER) Ca2+ mobilization in response to submaximal (11 mM) or hyperglycemic (25 mM) glucose, mimicking diabetes. Inhibition of inositol 1,4,5-trisphosphate (IP3) receptors, and other ion channels, with 2-aminoethoxydiphenyl borate (2-APB), had minimal effects on the initial peak or intercellular connectivity provoked by 11 mM glucose. However, 2-APB lowered subsequent glucose-induced cytosolic Ca2+increases and connectivity at both 11 and 25 mM glucose. Unexpectedly, the activation of IP3 receptors with the muscarinic acetylcholine receptor agonist carbachol had minimal impact on the initial peak elicited by 11 mM glucose, but Ca2+ waves at 11 and 25 mM glucose were more poorly coordinated. To determine whether ER calcium mobilization was sufficient to initiate Ca2+ waves we next blocked sarco(endo)plasmic Ca2+ ATPase (SERCA) pumps with thapsigargin, whilst preventing plasma membrane depolarization with the KATP-channel opener, diazoxide. Under these conditions, an initial cytosolic Ca2+increase was followed by secondary Ca2+ waves that subsided slowly. The application of carbachol alongside diazoxide still enhanced Ca2+dynamics, though activity was uncoordinated. After genetic deletion of SERCA2 in beta cells, Ca2+wave frequency, but not connectivity, were lowered. Our results show that ER Ca2+ mobilization plays a relatively minor role in the initiation and propagation of Ca2+ waves in response to glucose but is needed for sustained Ca2+waves.

Reticulocalbin1 (RCN1), a calcium-binding protein localized in the endoplasmic reticulum (ER), is implicated in cancer progression, but its role in the immune system remains poorly understood. To clarify the function of RCN1, we generated a RCN1-deficient (Rcn1-/-) mice using CRISPR-Cas9 system. Immunological characterization by flow cytometry revealed that while T cell populations in the spleen were unaffected, the proportion of CD8 single-positive (SP) thymocytes was significantly reduced in Rcn1-/- mice. In contrast, in vitro stimulation of splenic CD8+ T cells revealed no significant differences in the expression of cell surface activation markers or cytokines between Rcn1+/+ and Rcn1-/- mice. Functional analysis showed that at baseline, cytosolic calcium levels were significantly higher in Rcn1+/+ CD8+ T cells. In contrast, stimulus-induced calcium responses, including ER calcium release, extracellular calcium entry, and TCR-dependent calcium flux, were preserved in both genotypes. Collectively, our results suggest that RCN1 contributes to the fine-tuning of ER-associated calcium handling in naïve (non-activated) T cells, thereby modulating basal cytosolic calcium levels and supporting efficient thymic CD8⁺ T cell development.

Cardiac remodeling, including hypertrophy, is associated with alterations in cytosolic Ca2+ homeostasis of cardiac myocytes that spill over into the nucleoplasm. To test whether nuclear Ca2+ signaling acts causally on the development of cardiac hypertrophy, we expressed parvalbumin to buffer nuclear Ca2+ and we blocked nuclear Ca2+-calmodulin signaling by Adeno-associated virus (AAV)-mediated expression of the calmodulin (CaM) binding-peptide nlsCaMBP4, respectively, in the nuclei of ES cell-derived (Cor.At) and neonatal rat ventricular cardiac myocytes (NRVCM). Expression of nlsCaMBP4, but not parvalbumin, leads to a significant reduction of hypertrophic growth induced by phenylephrine (PE). Expression of nlsCaMBP4 did not alter the amplitude of electrically-evoked intracellular Ca2+ transients in NRVCMs in the absence or presence of PE, and did not affect the PE-evoked increase in store-operated Ca2+ entry. Transcriptome analysis on NRVCMs expressing nlsCaMBP4 revealed that induction of classical hypertrophy markers such as ANF and BNP or MEF2 target genes (such as Srpk3, Xirp1 and Xirp2) were not reduced by nlsCaMBP4 expression. Further analysis of the nuclear Ca2+-calmodulin-regulated gene pool revealed differential expression of genes involved in mRNA translation, including the translation initiation factor subunits Eif2s1, Eif3d and Eif5, whose upregulation was absent in nlsCaMBP4-treated myocytes. Puromycin assays showed that inhibition of Ca2+-calmodulin signaling prevented catecholamine-evoked protein translation, suggesting that Ca2+-calmodulin signaling in the nucleus of cardiac myocytes regulates translation via transcriptional control mechanisms. However, future studies are needed to identify the exact molecular components and machinery that integrate Ca2+-calmodulin-dependent regulation of transcription, protein translation, and development of cardiac myocyte hypertrophy.

The actin cytoskeleton is a dynamic network present in all eukaryotic cells. It plays a central role in various cellular processes, including cell shape maintenance, motility, intracellular transport, and cell division. The actin cytoskeleton consists of actin filaments and a diverse array of associated actin-binding proteins (ABPs), which regulate the assembly, organization, and functions of actin filaments. S100 proteins, a family of low-molecular-weight Ca²⁺-binding proteins, have emerged as important regulators of actin filaments. They exert their regulatory functions either directly, through interactions with actin and actin-binding proteins (ABPs), or indirectly, by modulating Ca2+ release and thereby influencing actin-dependent contractility. This review article provides a comprehensive overview of current literature on the S100-dependent regulation of actin cytoskeleton dynamics in diverse cellular contexts. Specifically, it highlights the role of S100 proteins in modulating striated muscle contractility, actin-myosin interactions in smooth muscle, mechanotransduction, stress fiber assembly, lamellipodia formation, actin cortex organization, and structural organization of the actin cytoskeleton within synapses.