Spontaneous Ca2+ Influx in Drosophila Pupal Neurons Is Modulated by IP3-Receptor Function and Influences Maturation of the Flight Circuit

Transcriptome analysis of Apis mellifera antennae reveals molecular divergence underlying the division of labour in worker bees

The olfactory system plays a fundamental role in mediating insect behaviour. Worker bees exhibit an age-dependent division of labour, performing discrete sets of behaviours throughout their lifespan. The behavioural states of bees rely on their sense of the environment and chemical communication via their olfactory system, the antennae. However, the olfactory adaptation mechanism of worker bees during their behavioural development remains unclear. In this study, we conducted a comprehensive and quantitative analysis of antennal gene expression in the Apis mellifera of newly emerged workers, nurses, foragers and defenders using RNA-seq. We found that the antenna tissues of honey bees continued developing after transformation from newly emerged workers to adults. Additionally, we identified differentially expressed genes associated with bee development and division of labour. We validated that major royal jelly protein genes are highly and specifically expressed in nurse honey bee workers. Furthermore, we identified and validated significant alternative splicing events correlated with the development and division of labour. These findings provide a comprehensive transcriptome profile and a new perspective on the molecular mechanisms that may underlie the worker honey bee division of labour.

Orai-mediated calcium entry determines activity of central dopaminergic neurons by regulation of gene expression

Maturation and fine-tuning of neural circuits frequently require neuromodulatory signals that set the excitability threshold, neuronal connectivity, and synaptic strength. Here, we present a mechanistic study of how neuromodulator-stimulated intracellular Ca2+ signals, through the store-operated Ca2+ channel Orai, regulate intrinsic neuronal properties by control of developmental gene expression in flight-promoting central dopaminergic neurons (fpDANs). The fpDANs receive cholinergic inputs for release of dopamine at a central brain tripartite synapse that sustains flight (Sharma and Hasan, 2020). Cholinergic inputs act on the muscarinic acetylcholine receptor to stimulate intracellular Ca2+ release through the endoplasmic reticulum (ER) localised inositol 1,4,5-trisphosphate receptor followed by ER-store depletion and Orai-mediated store-operated Ca2+ entry (SOCE). Analysis of gene expression in fpDANs followed by genetic, cellular, and molecular studies identified Orai-mediated Ca2+ entry as a key regulator of excitability in fpDANs during circuit maturation. SOCE activates the transcription factor trithorax-like (Trl), which in turn drives expression of a set of genes, including Set2, that encodes a histone 3 lysine 36 methyltransferase (H3K36me3). Set2 function establishes a positive feedback loop, essential for receiving neuromodulatory cholinergic inputs and sustaining SOCE. Chromatin-modifying activity of Set2 changes the epigenetic status of fpDANs and drives expression of key ion channel and signalling genes that determine fpDAN activity. Loss of activity reduces the axonal arborisation of fpDANs within the MB lobe and prevents dopamine release required for the maintenance of long flight.

A Comparative Perspective on Functionally-Related, Intracellular Calcium Channels: The Insect Ryanodine and Inositol 1,4,5-Trisphosphate Receptors

Calcium (Ca²⁺) homeostasis is vital for insect development and metabolism, and the endoplasmic reticulum (ER) is a major intracellular reservoir for Ca²⁺. The inositol 1,4,5- triphosphate receptor (IP₃R) and ryanodine receptor (RyR) are large homotetrameric channels associated with the ER and serve as two major actors in ER-derived Ca²⁺ supply. Most of the knowledge on these receptors derives from mammalian systems that possess three genes for each receptor. These studies have inspired work on synonymous receptors in insects, which encode a single IP₃R and RyR. In the current review, we focus on a fundamental, common question: “why do insect cells possess two Ca²⁺ channel receptors in the ER?”. Through a comparative approach, this review covers the discovery of RyRs and IP₃Rs, examines their structures/functions, the pathways that they interact with, and their potential as target sites in pest control. Although insects RyRs and IP₃Rs share structural similarities, they are phylogenetically distinct, have their own structural organization, regulatory mechanisms, and expression patterns, which explains their functional distinction. Nevertheless, both have great potential as target sites in pest control, with RyRs currently being targeted by commercial insecticide, the diamides.

Target Molecules of STIM Proteins in the Central Nervous System

Stromal interaction molecules (STIMs), including STIM1 and STIM2, are single-pass transmembrane proteins that are located predominantly in the endoplasmic reticulum (ER). They serve as calcium ion (Ca²⁺) sensors within the ER. In the central nervous system (CNS), they are involved mainly in Orai-mediated store-operated Ca²⁺ entry (SOCE). The key molecular components of the SOCE pathway are well-characterized, but the molecular mechanisms that underlie the regulation of this pathway need further investigation. Numerous intracellular target proteins that are located in the plasma membrane, ER, cytoskeleton, and cytoplasm have been reported to play essential roles in concert with STIMs, such as conformational changes in STIMs, their translocation, the stabilization of their interactions with Orai, and the activation of other channels. The present review focuses on numerous regulators, such as Homer, SOCE-associated regulatory factor (SARAF), septin, synaptopodin, golli proteins, partner of STIM1 (POST), and transcription factors and proteasome inhibitors that regulate STIM-Orai interactions in the CNS. Further we describe novel roles of STIMs in mediating Ca²⁺ influx via other than Orai pathways, including TRPC channels, VGCCs, AMPA and NMDA receptors, and group I metabotropic glutamate receptors. This review also summarizes recent findings on additional molecular targets of STIM proteins including SERCA, IP₃Rs, end-binding proteins (EB), presenilin, and CaMKII. Dysregulation of the SOCE-associated toolkit, including STIMs, contributes to the development of neurodegenerative disorders (e.g., Alzheimer's disease, Parkinson's disease, and Huntington's disease), traumatic brain injury, epilepsy, and stroke. Emerging evidence points to the role of STIM proteins and several of their molecular effectors and regulators in neuronal and glial physiology and pathology, suggesting their potential application for future therapeutic strategies.

Type 3 Inositol 1,4,5-Trisphosphate Receptor is a Crucial Regulator of Calcium Dynamics Mediated by Endoplasmic Reticulum in HEK Cells

Being the largest the Ca²⁺ store in mammalian cells, endoplasmic reticulum (ER)-mediated Ca²⁺ signalling often involves both Ca²⁺ release via inositol 1, 4, 5-trisphosphate receptors (IP₃R) and store operated Ca²⁺ entries (SOCE) through Ca²⁺ release activated Ca²⁺ (CRAC) channels on plasma membrane (PM). IP₃Rs are functionally coupled with CRAC channels and other Ca²⁺ handling proteins. However, it still remains less well defined as to whether IP₃Rs could regulate ER-mediated Ca²⁺ signals independent of their Ca²⁺ releasing ability. To address this, we generated IP₃Rs triple and double knockout human embryonic kidney (HEK) cell lines (IP₃Rs-TKO, IP₃Rs-DKO), and systemically examined ER Ca²⁺ dynamics and CRAC channel activity in these cells. The results showed that the rate of ER Ca²⁺ leakage and refilling, as well as SOCE were all significantly reduced in IP₃Rs-TKO cells. And these TKO effects could be rescued by over-expression of IP₃R3. Further, results showed that the diminished SOCE was caused by NEDD4L-mediated ubiquitination of Orai1 protein. Together, our findings indicate that IP₃R3 is one crucial player in coordinating ER-mediated Ca²⁺ signalling.

Synergistically regulated spontaneous calcium signaling is attributed to cartilaginous extracellular matrix metabolism

Ca²⁺ has been recognized as a key molecule for chondrocytes, however, the role and mechanism of spontaneous [Ca ²⁺ ] _i signaling in cartilaginous extracellular matrix (ECM) metabolism regulation are unclear. Here we found that spontaneous Ca ²⁺ signal of in-situ porcine chondrocytes was [Ca ²⁺ ] _o dependent, and mediated by [Ca ²⁺ ] _i store release. T-type voltage-dependent calcium channel (T-VDCC) mediated [Ca ²⁺ ] _o influx was associated with decreased cell viability and expression levels of ECM deposition genes. Further analysis revealed that chondrocytes expressed both inositol 1,4,5-trisphosphate receptor (InsP3R) and Orai isoforms. Inhibition of endoplasmic reticulum (ER) Ca ²⁺ release and store-operated calcium entry significantly abolished spontaneous [Ca ²⁺ ] _i signaling of in-situ chondrocytes. Moreover, blocking ER Ca ²⁺ release with InsP3R inhibitors significantly upregulated ECM degradation enzymes production, and was accompanied by decreased proteoglycan and collagen type II intensity. Taken together, our data provided evidence that spontaneous [Ca ²⁺ ] _i signaling of in-situ porcine chondrocytes was tightly regulated by [Ca ²⁺ ] _o influx, InsP3Rs mediated [Ca ²⁺ ] _i store release, and Orais mediated calcium release-activated calcium channels activation. Both T-VDCC mediated [Ca ²⁺ ] _o influx and InsP3Rs mediated ER Ca ²⁺ release were found crucial to cartilaginous ECM metabolism through distinct regulatory mechanisms.

Stable STIM1 Knockdown in Self-Renewing Human Neural Precursors Promotes Premature Neural Differentiation

Ca²⁺ signaling plays a significant role in the development of the vertebrate nervous system where it regulates neurite growth as well as synapse and neurotransmitter specification. Elucidating the role of Ca²⁺ signaling in mammalian neuronal development has been largely restricted to either small animal models or primary cultures. Here we derived human neural precursor cells (NPCs) from human embryonic stem cells to understand the functional significance of a less understood arm of calcium signaling, Store-operated Ca²⁺ entry or SOCE, in neuronal development. Human NPCs exhibited robust SOCE, which was significantly attenuated by expression of a stable shRNA-miR targeted toward the SOCE molecule, STIM1. Along with the plasma membrane channel Orai, STIM is an essential component of SOCE in many cell types, where it regulates gene expression. Therefore, we measured global gene expression in human NPCs with and without STIM1 knockdown. Interestingly, pathways down-regulated through STIM1 knockdown were related to cell proliferation and DNA replication processes, whereas post-synaptic signaling was identified as an up-regulated process. To understand the functional significance of these gene expression changes we measured the self-renewal capacity of NPCs with STIM1 knockdown. The STIM1 knockdown NPCs demonstrated significantly reduced neurosphere size and number as well as precocious spontaneous differentiation toward the neuronal lineage, as compared to control cells. These findings demonstrate that STIM1 mediated SOCE in human NPCs regulates gene expression changes, that in vivo are likely to physiologically modulate the self-renewal and differentiation of NPCs.

Store-Operated Ca2+ Entry in Drosophila Primary Neuronal Cultures

Intracellular calcium signals in neurons frequently derive from the stimulation of G protein-coupled receptors (GPCR) by neurotransmitters, neuropeptides, and neurohormones. GPCR stimulation in neurons leads to generation of inositol 1,4,5-trisphosphate (IP₃), which in turn activates endoplasmic reticulum (ER)-localized IP₃ receptors. The IP₃ receptor (IP₃R) is a ligand-gated Ca²⁺ channel, which releases Ca²⁺ from ER stores. In Drosophila neurons it has been shown that depletion of ER Ca²⁺ store is followed by store-operated Ca²⁺ entry (SOCE) through STIM and Orai, the ER Ca²⁺ sensor and the plasma membrane Ca²⁺ channel respectively. The elucidation of this Ca²⁺ signaling pathway in neurons has in part been possible due to the ease of genetic manipulation in Drosophila, which has allowed neuron-specific knockdown of various proteins of interest. This has been followed by standardization of conditions for culturing neurons from dissected brains of the relevant genotypes, such that they could be used for robust Ca²⁺ measurements by imaging with standard Ca²⁺ indicator dyes. Protocols for measurement of IP₃-mediated Ca²⁺ release, passive depletion of ER Ca²⁺ store, and SOCE in primary cultures of Drosophila neurons are described here.