The inositol 1,4,5-trisphosphate receptor (Itpr) gene family in Xenopus: identification of type 2 and type 3 inositol 1,4,5-trisphosphate receptor subtypes

Endoplasmic reticulum in oocytes: spatiotemporal distribution and function

ENDOPLASMIC RETICULUM IN OOCYTES

The storage and release of calcium ions (Ca2 +) in oocyte maturation and fertilization are particularly noteworthy features of the endoplasmic reticulum (ER). The ER is the largest organelle in the cell composed of rough ER, smooth ER, and nuclear envelope, and is the main site of protein synthesis, transport and folding, and lipid and steroid synthesis. An appropriate calcium signaling response can initiate oocyte development and embryogenesis, and the ER is the central link that initiates calcium signaling. The transition from immature oocytes to zygotes also requires many coordinated organelle reorganizations and changes. Therefore, the purpose of this review is to generalize information on the function, structure, interaction with other organelles, and spatiotemporal localization of the ER in mammalian oocytes. Mechanisms related to maintaining ER homeostasis have been extensively studied in recent years. Resolving ER stress through the unfolded protein response (UPR) is one of them. We combined the clinical problems caused by the ER in in vitro maturation (IVM), and the mechanisms of ER have been identified by single-cell RNA-seq. This article systematically reviews the functions of ER and provides a reference for assisted reproductive technology (ART) research.

Tracing the Evolutionary History of Inositol, 1, 4, 5-Trisphosphate Receptor: Insights from Analyses of Capsaspora owczarzaki Ca2+ Release Channel Orthologs

Cellular Ca(2+) homeostasis is tightly regulated and is pivotal to life. Inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) are the major ion channels that regulate Ca(2+) release from intracellular stores. Although these channels have been extensively investigated in multicellular organisms, an appreciation of their evolution and the biology of orthologs in unicellular organisms is largely lacking. Extensive phylogenetic analyses reveal that the IP3R gene superfamily is ancient and diverged into two subfamilies, IP3R-A and IP3R-B/RyR, at the dawn of Opisthokonta. IP3R-B/RyR further diversified into IP3R-B and RyR at the stem of Filozoa. Subsequent evolution and speciation of Holozoa is associated with duplication of IP3R-A and RyR genes, and loss of IP3R-B in the vertebrate lineages. To gain insight into the properties of IP3R important for the challenges of multicellularity, the IP3R-A and IP3R-B family orthologs were cloned from Capsaspora owczarzaki, a close unicellular relative to Metazoa (designated as CO.IP3R-A and CO.IP3R-B). Both proteins were targeted to the endoplasmic reticulum. However, CO.IP3R-A, but strikingly not CO.IP3R-B, bound IP3, exhibited robust Ca(2+) release activity and associated with mammalian IP3Rs. These data indicate strongly that CO.IP3R-A as an exemplar of ancestral IP3R-A orthologs forms bona fide IP3-gated channels. Notably, however, CO.IP3R-A appears not to be regulated by Ca(2+), ATP or Protein kinase A-phosphorylation. Collectively, our findings explore the origin, conservation, and diversification of IP3R gene families and provide insight into the functionality of ancestral IP3Rs and the added specialization of these proteins in Metazoa.

Characterization of a flatworm inositol (1,4,5) trisphosphate receptor (IP₃R) reveals a role in reproductive physiology

Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are intracellular Ca²⁺ channels that elevate cytoplasmic Ca²⁺ in response to the second messenger IP3. Here, we describe the identification and in vivo functional characterization of the planarian IP₃R, the first intracellular Ca²⁺ channel to be defined in flatworms. A single IP₃R gene in Dugesia japonica encoded a 2666 amino acid protein (Dj.IP₃R) that shared well conserved structural features with vertebrate IP₃R counterparts. Expression of an NH₂-terminal Dj.IP₃R region (amino acid residues 223-585) recovered high affinity ³H-IP₃ binding (0.9±0.1 nM) which was abolished by a single point mutation of an arginine residue (R495L) important for IP₃ coordination. In situ hybridization revealed that Dj.IP₃R mRNA was most strongly expressed in the pharynx and optical nerve system as well as the reproductive system in sexualized planarians. Consistent with this observed tissue distribution, in vivo RNAi of Dj.IP₃R resulted in a decreased egg-laying behavior suggesting Dj.IP₃R plays an upstream role in planarian reproductive physiology.

The Xenopus oocyte: a single-cell model for studying Ca2+ signaling

In the four decades since the Xenopus oocyte was first demonstrated to have the capacity to translate exogenous mRNAs, this system has been exploited for many different experimental purposes. Typically, the oocyte is used either as a "biological test tube" for heterologous expression of proteins without any particular cell biological insight or, alternatively, it is used for applications where cell biology is paramount, such as investigations of the cellular adaptations that power early development. In this article, we discuss the utility of the Xenopus oocyte for studying Ca(2+) signaling in both these contexts.

Differential regulation of the InsP₃ receptor type-1 and -2 single channel properties by InsP₃, Ca²⁺ and ATP

An elevation of intracellular Ca2+ levels as a result of InsP3 receptor (InsP3R) activity represents a ubiquitous signalling pathway controlling a wide variety of cellular events. InsP3R activity is tightly controlled by the levels of the primary ligands, InsP3, Ca2+ and ATP. Importantly, InsP3Rs are regulated by Ca2+ i in a biphasic manner. Ca2+ release through all InsP3R family members is also modulated dramatically by ATP, albeit with sub-type-specific properties. To ascertain if a common mechanism can account for ATP and Ca2+ regulation of these InsP3R family members, we examined the effects of [ATP] on the Ca2+ dependency of rat InsP3R-1 (rInsP3R-1) and mouse InsP3R-2 (mInsP3R-2) activity expressed in DT40-3KO cells. We used the on-nucleus patch clamp recording technique with various [ATP], [InsP3] and [Ca2+] in the patch pipette and measured single InsP3R channel activity in stably transfected DT40 cells. Under identical conditions, at saturating [InsP3] and [ATP], the activity of rInsP3R-1 and mInsP3R-2 was essentially identical in terms of single channel conductance, maximal achievable open probability (Po) and the [Ca2+] required for activation and inhibition of activity. However, in contrast to rInsP3R-1 at saturating [InsP3], the activity of mInsP3R-2 was unaffected by [ATP]. At lower [InsP3], ATP had dramatic effects on mInsP3R-2 Po, but unlike the rInsP3R-1, this did not occur by altering the relative Ca2+ dependency, but by simply increasing the maximally achievable Po at a particular [InsP3] and [Ca2+]. [InsP3] did not alter the biphasic regulation of activity by Ca2+ in either rInsP3R-1 or mInsP3R-2. Analysis of the single channel kinetics indicated that Ca2+ and ATP modulate the Po predominately by facilitating extended bursting activity of the channel but the underlying biophysical mechanism appears to be distinct for each receptor. Subtype-specific regulation of InsP3R channel activity probably contributes to the fidelity of Ca2+ signalling in cells expressing these receptor subtypes.

Evolution and functional diversity of the Calcium Binding Proteins (CaBPs)

The mammalian central nervous system (CNS) exhibits a remarkable ability to process, store, and transfer information. Key to these activities is the use of highly regulated and unique patterns of calcium signals encoded by calcium channels and decoded by families of specific calcium-sensing proteins. The largest family of eukaryotic calcium sensors is those related to the small EF-hand containing protein calmodulin (CaM). In order to maximize the usefulness of calcium as a signaling species and to permit the evolution and fine tuning of the mammalian CNS, families of related proteins have arisen that exhibit characteristic calcium binding properties and tissue-, cellular-, and sub-cellular distribution profiles. The Calcium Binding Proteins (CaBPs) represent one such family of vertebrate specific CaM like proteins that have emerged in recent years as important regulators of essential neuronal target proteins. Bioinformatic analyses indicate that the CaBPs consist of two subfamilies and that the ancestral members of these are CaBP1 and CaBP8. The CaBPs have distinct intracellular localizations based on different targeting mechanisms including a novel type-II transmembrane domain in CaBPs 7 and 8 (otherwise known as calneuron II and calneuron I, respectively). Recent work has led to the identification of new target interactions and possible functions for the CaBPs suggesting that they have multiple physiological roles with relevance for the normal functioning of the CNS.

Inositol (1,4,5)-trisphosphate receptor microarchitecture shapes Ca2+ puff kinetics

Inositol (1,4,5)-trisphosphate receptors (IP(3)Rs) release intracellular Ca(2+) as localized Ca(2+) signals (Ca(2+) puffs) that represent the activity of small numbers of clustered IP(3)Rs spaced throughout the endoplasmic reticulum. Although much emphasis has been placed on estimating the number of active Ca(2+) release channels supporting Ca(2+) puffs, less attention has been placed on understanding the role of cluster microarchitecture. This is important as recent data underscores the dynamic nature of IP(3)R transitions between heterogeneous cellular architectures and the differential behavior of IP(3)Rs socialized into clusters. Here, we applied a high-resolution model incorporating stochastically gating IP(3)Rs within a three-dimensional cytoplasmic space to demonstrate: 1), Ca(2+) puffs are supported by a broad range of clustered IP(3)R microarchitectures; 2), cluster ultrastructure shapes Ca(2+) puff characteristics; and 3), loosely corralled IP(3)R clusters (>200 nm interchannel separation) fail to coordinate Ca(2+) puffs, owing to inefficient triggering and impaired coupling due to reduced Ca(2+)-induced Ca(2+) release microwave velocity (<10 nm/s) throughout the channel array. Dynamic microarchitectural considerations may therefore influence Ca(2+) puff occurrence/properties in intact cells, contrasting with a more minimal role for channel number over the same simulated conditions in shaping local Ca(2+) dynamics.

Granulosa cells express three inositol 1,4,5-trisphosphate receptor isoforms: cytoplasmic and nuclear Ca2+ mobilization

BACKGROUND

Granulosa cells play an important endocrine role in folliculogenesis. They mobilize Ca2+ from intracellular stores by a coordinated action between 1,4,5 inositol trisphosphate and ryanodine receptors (IP3R and RyR). The aim of this study was to explore the isoforms of IP3Rs expressed in mouse C57BL/6 NHsd granulosa cells, characterizing their intranuclear localization and the relation with other Ca2+-handling proteins.

METHODS

Ovarian tissue and granulosa cells were analyzed by multiphotonic and confocal microscopy to determine the intracellular presence of IP3R types 1, 2 and 3, RyR, thapsigargin-sensitive Ca2+-ATPase, and endomembranes. Cellular fractionation and Western blot assays were also used to further confirm the nuclear occurrence of the three IP3R isoforms. Free nuclear and cytosolic Ca2+ concentrations were measured using Fluo-4 AM by confocal microscopy.

RESULTS

By using antibodies and specific fluorophores, was shown that granulosa cells endomembranes contain three isoforms of IP3R, the RyR, and the thapsigargin-sensitive Ca2+-ATPase (SERCA). Interestingly, all these proteins were also detected in the nuclear envelope and in well-defined intranuclear structures. Microsomal membranes depicted characteristic bands of the 3 types of IP3R, but also variants of lower molecular weight. Analysis of nuclear membranes and nucleoplasmic fraction confirmed the nuclear localization of the IP3R types 1, 2 and 3. We demonstrated ATP-induced Ca2+ transients in the nuclear and cytoplasmic compartments. Remarkably, the inhibitory effect on ATP-induced Ca2+ mobilization of brefeldin A was more accentuated in the cytoplasm than in the nucleus.

CONCLUSION

These findings provide evidence that granulosa cells, including nuclei, express the Ca2+-handling proteins that allow Ca2+ mobilization. All three IP3R were also detected in ovarian slices, including the nuclei of granulosa cells, suggesting that these cells use the three IP3R in situ to achieve their physiological responses.