Inositol (1,4,5)-trisphosphate receptor: characterization of neuron-specific alternative splicing in rat brain and peripheral tissues

Nuclear ALG-2 protein interacts with Ca2+ homeostasis endoplasmic reticulum protein (CHERP) Ca2+-dependently and participates in regulation of alternative splicing of inositol trisphosphate receptor type 1 (IP3R1) pre-mRNA

The intracellular Ca(2+) signaling pathway is important for the control of broad cellular processes from fertilization to cell death. ALG-2 is a Ca(2+)-binding protein that contains five serially repeated EF-hand motifs and interacts with various proteins in a Ca(2+)-dependent manner. Although ALG-2 is present both in the cytoplasm and in the nucleus, little is known about its nuclear function. Ca(2+) homeostasis endoplasmic reticulum protein (CHERP) was first identified as an endoplasmic reticulum protein that regulates intracellular Ca(2+) mobilization in human cells, but recent proteomics data suggest an association between CHERP and spliceosomes. Here, we report that CHERP, containing a Pro-rich region and a phosphorylated Ser/Arg-rich RS-like domain, is a novel Ca(2+)-dependent ALG-2-interactive target in the nucleus. Immunofluorescence microscopic analysis revealed localization of CHERP to the nucleoplasm with prominent accumulation at nuclear speckles, which are the sites of storage and modification for pre-mRNA splicing factors. Live cell time-lapse imaging showed that nuclear ALG-2 was recruited to the CHERP-localizing speckles upon Ca(2+) mobilization. Results of co-immunoprecipitation assays revealed binding of CHERP to a phosphorylated form of RNA polymerase II. Knockdown of CHERP or ALG-2 in HT1080 cells resulted in generation of alternatively spliced isoforms of the inositol 1,4,5-trisphosphate receptor 1 (IP3R1) pre-mRNA that included exons 41 and 42 in addition to the major isoform lacking exons 40-42. Furthermore, binding between CHERP and IP3R1 RNA was detected by an RNA immunoprecipitation assay using a polyclonal antibody against CHERP. These results indicate that CHERP and ALG-2 participate in regulation of alternative splicing of IP3R1 pre-mRNA and provide new insights into post-transcriptional regulation of splicing variants in Ca(2+) signaling pathways.

Differential neuronal localizations and dynamics of phosphorylated and unphosphorylated type 1 inositol 1,4,5-trisphosphate receptors

Type 1 inositol 1,4,5-trisphosphate receptors are phosphorylated by cyclic-AMP-dependent protein kinase A at serines 1589 and 1755, with serine 1755 phosphorylation greatly predominating in the brain. Inositol 1,4,5-trisphosphate receptor protein kinase A phosphorylation augments Ca(2+) release. To assess type 1 protein kinase A phosphorylation dynamics in the intact organism, we developed antibodies selective for either serine 1755 phosphorylated or unphosphorylated species. Immunohistochemical studies reveal marked variation in localization. For example, in the hippocampus the phosphorylated type 1 inositol 1,4,5-trisphosphate receptor is restricted to CA1, while the unphosphorylated receptor occurs ubiquitously in CA1-CA3 and dentate gyrus granule cells. Throughout the brain the phosphorylated type 1 inositol 1,4,5-trisphosphate receptor is selectively enriched in dendrites, while the unphosphorylated receptor predominates in cell bodies. Focal cerebral ischemia in rats and humans is associated with dephosphorylation of type 1 inositol 1,4,5-trisphosphate receptors, and glutamatergic excitation of cerebellar Purkinje cells mediated by ibogaine elicits dephosphorylation of type 1 inositol 1,4,5-trisphosphate receptors that precedes evidence of excitotoxic neuronal degeneration. We have demonstrated striking variations in regional and subcellular distribution of inositol 1,4,5-trisphosphate receptor phosphorylation that may influence normal physiological intracellular Ca(2+) signaling in rat and human brain. We have further shown that the subcellular distribution of inositol 1,4,5-trisphosphate receptor phosphorylation in neurons is regulated by excitatory neurotransmission, as well as excitotoxic insult and neuronal ischemia-reperfusion. Phosphorylation dynamics of type 1 inositol 1,4,5-trisphosphate receptors may modulate intracellular Ca(2+) release and influence the cellular response to neurotoxic insults.

Olfactory receptor neurons exist as distinct subclasses of immature and mature cells in primary culture

The processes of neuronal differentiation and survival are key questions in neurobiology. The olfactory system possesses unique regenerative capacity, as its neurons are continually replaced throughout adulthood from a maintained population of precursor cells. Primary cultures of olfactory epithelium enriched in olfactory neurons would provide a useful model to study the processes of neurogenesis, differentiation and senescence. To determine whether immature olfactory neurons could be isolated in primary culture and to investigate the mechanisms underlying these processes, culture conditions which selectively favored the presence of immature olfactory neurons were optimized. Using low plating densities, a population of cells was identified which, by reverse transcription-polymerase chain reaction, demonstrated messages for olfactory neuronal markers, including Golf, olfactory cyclic nucleotide-gated channel and olfactory marker protein, as well as the p75 low-affinity nerve growth factor receptor. Immunocytochemical analysis showed that these putative immature olfactory neurons possessed immunoreactivity to G(olf), neuron-specific tubulin, neural cell adhesion molecule, synaptophysin and neurofilament. These neurons were defined as olfactory receptor neuron-1 cells. Under these conditions, a separate class of rarely occurring cells with different morphology demonstrated immunoreactivity to mature markers, such as adenylyl cyclase III and olfactory marker protein. Electrophysiologically, these cells displayed properties consistent with those of acutely dissociated olfactory receptor neurons. Another class of rarer cells which represented less than 2% of cells in culture demonstrated immunoreactivity to glial fibrillary acidic protein. These cultures can serve as a model for in vitro analysis of olfactory receptor neuronal development and maintenance, and provide a potential substrate for the development of cell lines.

Muscle-specific mRNA isoform encodes a protein composed mainly of the N-terminal 175 residues of type 2 Ins(1,4,5)P3 receptor

We have found a novel isoform of the mouse type 2 Ins(1,4,5)P3 receptor [Ins(1,4,5)P3R] mRNA by reverse transcriptase-mediated PCR analysis. The novel isoform, which was expressed specifically in skeletal muscle and heart, was generated by the inclusion of a novel exon. As this exon contains a stop codon, the isoform encodes a putative protein (designated TIPR) consisting of 175 acid residues of the type 2 Ins(1,4,5)P3R and the following six residues derived from this exon. We transfected the cDNA of this isoform into COS-7 cells; these cells expressed a 24 kDa protein that was recognized by an antibody against TIPR produced in Escherichia coli. The isoform encoding TIPR was also found in human skeletal muscle and heart. The N-terminal region of Ins(1,4,5)P3R is suggested to have a role in ligand binding and to interact with the C-terminal channel domain of Ins(1,4,5)P3R itself. TIPR might regulate the Ins(1,4,5)P3 signal pathway in both muscles.

Isoform-specific regulation of the Na+/Ca2+ exchanger in rat astrocytes and neurons by PKA

The Na+/Ca2+ exchanger is a major transporter of Ca2+ in neurons and glial cells. The Na+/Ca2+ exchanger gene NCX1 expresses tissue-specific isoforms of the Na+/Ca2+ exchanger, and the isoforms have been examined here quantitatively using primary cultures of astrocytes and neurons. We present a PCR-based quantitative method, quantitative end-labeled reverse transcription-PCR (QERT-PCR), to determine the relative amounts of the NCX1 isoforms present in these cells. Six exons (A, B, C, D, E, and F) are alternatively spliced to produce the known NCX1 isoforms. Three exon B-containing isoforms (BDEF, BDF, and BD) are the predominant transcripts in primary rat cortical astrocytes and in C6 glioma cells. In contrast, exon A-containing isoforms (ADF and AD) are the predominant transcripts in primary rat hippocampal neurons. Functional differences between full-length constructs of NCX1 containing either the astrocyte isoform BD or the neuron isoform AD were examined in a Xenopus oocyte expression system. Although both isoforms function normally, the activity of the AD isoform can be increased 39% by activation of protein kinase A (PKA), whereas that of the BD isoform is not affected. We conclude that specific NCX1 isoforms are expressed in distinct patterns in astrocytes and neurons. Furthermore, the activity of a neuronal (but not glial) isoform of the Na+/Ca2+ exchanger can be altered by the activation of the PKA pathway.

Tissue-specific alternative splicing of mouse brain type ryanodine receptor/calcium release channel mRNA

We detected alternative splicing of the mouse brain type ryanodine receptor (RyR3) mRNA. The splicing variant was located in the transmembrane segment. The non-splicing type (RyR3-II) included a stretch of 341 bp, and that of the 13th codon was stop codon TAA. Reverse transcription-polymerase chain reaction (RT-PCR) analysis shows that RyR3-II mRNA was expressed in various peripheral tissues and brain at all developmental stages. However, interestingly, the splicing type (RyR3-I) mRNA was detected only in the cerebrum. These findings suggest that the splicing variants RyR3-I and RyR3-II may generate functional differences of RyR3 in a tissue-specific manner.

cDNA cloning reveals a tissue specific expression of alternatively spliced transcripts of the ryanodine receptor type 3 (RyR3) calcium release channel

Ryanodine receptors (RyRs) are a family of intracellular calcium release channels. Three cDNAs encoding different isoforms of RyR have been identified and cloned. We report the complete sequence of the mink RyR3 cDNA and the characterization of three alternative spliced regions. The first two splicing sites are represented by insertions of five and six amino acids, respectively. The third site is represented by a mutually exclusive splicing. The tissue distribution of the alternatively spliced transcripts revealed a ubiquitous expression of splicing site I and a differential distribution of sites II and III, indicating that a further level of complexity in RyR3 expression may result from alternative splicings in this gene.

Inositol 1,4,5-trisphosphate receptors in endocrine cells: localization and association in hetero- and homotetramers

The inositol 1,4,5-trisphosphate receptor (IP3R) is an intracellular calcium channel involved in coupling cell membrane receptors to calcium signal transduction pathways within cells including endocrine cells. Several isoforms (I, II, and III) of IP3Rs have been identified, which are encoded by separate genes, and are expressed in many tissues with differing patterns of cellular expression. We have generated specific affinity-purified polyclonal anti-peptide antibodies to each of the three isoforms. Western blot analysis of RINm5F and ATt20 cells shows high levels of endogenously expressed type I and type III IP3R, but undetectable levels of type II. Immunofluorescence studies revealed an endoplasmic reticulum-like pattern similar to BiP, an ER marker. In contrast with previous claims, both type I and type III IP3Rs were absent from the secretory granules of ATt20 cells. Western blots of sucrose gradients and gel filtration probed with antibodies to either type I or type III showed a molecular weight of greater than 1,000 kDa consistent with a tetrameric structure. Co-immunoprecipitation experiments indicated that most of the receptors were present as heterotetramers. Homotetramers were identified for the type III IP3R; however, type I homotetramers were undetectable. These data suggest that molecular association of IP3Rs into heterotetrameric forms can contribute to the complexity of the regulation of Ca2+ release from ER by IP3Rs within cells.

Isolation and characterization of vascular smooth muscle inositol 1,4,5-trisphosphate receptor

myo-Inositol 1,4,5-trisphosphate (InsP3) receptor of porcine aorta was purified to near homogeneity and its biochemical properties were compared with those of cerebellar InsP3 receptor of the same animal species. The aortic InsP3 receptor consisted of equal amounts of two polypeptides with slightly differing molecular masses of around 240 kDa and was found to possess a single population of InsP3-binding site (Kd of 1.2 nM). The InsP3 receptor purified from porcine cerebellum was also comprised of two polypeptides. However, the molecular mass was slightly but definitely larger, being 250 kDa, and the amounts of the two polypeptides were not equal. The aortic InsP3 receptor cross-reacted with polyclonal antibody specific to type 1 InsP3 receptor as did the cerebellar InsP3 receptor. The aortic InsP3 receptor bound to calmodulin-Sepharose in a Ca(2+)-dependent manner, while the cerebellar InsP3 receptor did not. Reverse transcriptase-PCR analysis revealed two splicing variants of the type 1 InsP3 receptor in porcine aortic smooth muscle distinct from those of the type 1 InsP3 receptor of porcine cerebellum. The possible relevance of this difference to difference in calmodulin-binding property was discussed.

Tissue-specific and developmentally regulated alternative splicing in mouse skeletal muscle ryanodine receptor mRNA

The ryanodine receptor is a channel for Ca2+ release from intracellular stores. By PCR analysis, we identified two alternatively spliced regions in mRNA of the mouse skeletal muscle ryanodine receptor (sRyR). The splice variants were characterized by the presence or absence of 15 bp (ASI) and 18 bp (ASII) exons. The exclusion of these exons results in the absence of the regions corresponding to Ala3481-Gln3485 and Val3865-Asn3870, respectively, of rabbit sRyR; these amino acid sequences exist in the modulatory region, where sites for phosphorylation and binding of Ca2+, calmodulin and ATP are postulated to be. We also detected sRyR in brain and heart as well as in skeletal muscle, and the splicing patterns were found to be tissue-specific. Only the ASII-lacking isoform was detected in heart, whereas in other tissues the ASII-containing isoform was predominant. The splicing patterns were also found to change during development. In skeletal muscle, the ASI-containing isoform increased gradually from embryo to adult. The ASII-lacking isoform abruptly increased upon birth, but the ASII-containing isoform increased steadily afterwards. In cerebrum, the ratio of the ASII-containing isoform to the ASII-lacking one increased abruptly during embryonic days 14 and 18. These findings suggest that the alternative splicing of ASI and ASII, by affecting the modulatory region, generates functionally different sRyR isoforms in a tissue-specific and developmentally regulated manner.

Partial calcium release in response to submaximal inositol 1,4,5-trisphosphate receptor activation

Even a prolonged application of a submaximal dose of Ins(1,4,5)P3 is unable to release the same amount of Ca2+ from the Ins(1,4,5)P3-sensitive store as a higher dose of Ins(1,4,5)P3. Low doses of Ins(1,4,5)P3 therefore only induce a partial release of the stored Ca2+. In this review, we will focus on the mechanisms that may contribute to this behaviour. Molecular heterogeneity of the Ins(1,4,5)P3 receptor can contribute to such behaviour if all the gene products and alternatively spliced isoforms would have different functional properties and be located in different store units. We will show that the control of the Ins(1,4,5)P3 receptor by by luminal Ca2+ also contributes to the partial release behaviour; it can set the sensitivity of the Ins(1,4,5)P3 receptor and the decreasing luminal Ca2+ concentration may inhibit further release while some Ca2+ is still left in the store. It is finally possible that the Ins(1,4,5)P3 receptor may adapt to a maintained stimulus.