Phosphorylation-dependent regulation of ryanodine receptors: a novel role for leucine/isoleucine zippers.

Ryanodine receptors (RyRs), intracellular calcium release channels required for cardiac and skeletal muscle contraction, are macromolecular complexes that include kinases and phosphatases. Phosphorylation/dephosphorylation plays a key role in regulating the function of many ion channels, including RyRs. However, the mechanism by which kinases and phosphatases are targeted to ion channels is not well understood. We have identified a novel mechanism involved in the formation of ion channel macromolecular complexes: kinase and phosphatase targeting proteins binding to ion channels via leucine/isoleucine zipper (LZ) motifs. Activation of kinases and phosphatases bound to RyR2 via LZs regulates phosphorylation of the channel, and disruption of kinase binding via LZ motifs prevents phosphorylation of RyR2. Elucidation of this new role for LZs in ion channel macromolecular complexes now permits: (a) rapid mapping of kinase and phosphatase targeting protein binding sites on ion channels; (b) predicting which kinases and phosphatases are likely to regulate a given ion channel; (c) rapid identification of novel kinase and phosphatase targeting proteins; and (d) tools for dissecting the role of kinases and phosphatases as modulators of ion channel function.

Lutropin-choriogonadotropin receptor: an unusual member of the G protein-coupled receptor family.

A complementary DNA (cDNA) for the rat luteal lutropin-choriogonadotropin receptor (LH-CG-R) was isolated with the use of a DNA probe generated in a polymerase chain reaction with oligonucleotide primers based on peptide sequences of purified receptor protein. As would be predicted from the cDNA sequence, the LH-CG-R consists of a 26-residue signal peptide, a 341-residue extracellular domain displaying an internal repeat structure characteristic of members of the leucine-rich glycoprotein (LRG) family, and a 333-residue region containing seven transmembrane segments. This membrane-spanning region displays sequence similarity with all members of the G protein-coupled receptor family. Hence, the LH-CG-R gene may have evolved by recombination of LRG and G protein-coupled receptor genes. Cells engineered to express LH-CG-R cDNA bind human choriogonadotropin with high affinity and show an increase in cyclic adenosine monophosphate when exposed to hormone. As revealed by RNA blot analysis and in situ hybridization, the 4.4-kilobase cognate messenger RNA is prominently localized in the rat ovary.

Characterization of an antiserum to the rat luteal luteinizing hormone/chorionic gonadotropin receptor.

The LH/CG receptor was purified from detergent extracts of rat luteal tissue using hCG affinity and wheat germ agglutinin chromatography. Analysis of the purified material by silver staining of sodium dodecyl sulfate-polyacrylamide gels (with or without reducing agents) revealed a prominent broad band corresponding to a 93K protein and several minor contaminants. That the 93K band represents the LH/CG receptor is supported by the following. 1) This band is absent in material purified from rat luteal tissue in which the LH/CG receptor had been down-regulated. 2) [125I]iodohCG binding to Western blots of both the initial detergent extract and the purified material resulted in binding to a 93K protein. The 93K protein representing the LH/CG receptor was excised after sodium dodecyl sulfate-gel electrophoresis of the purified material and was electroeluted. Half of the electroeluted receptor was incubated with reducing agents. After mixing with nonreduced receptor, this mixture was used to immunize one rabbit. Immune, but not preimmune, serum (or immunoglobulin G purified thereof) recognized a 93K protein on Western blots of partially purified (approximately 10% pure) rat luteal receptor. No other bands were seen. Using this approach it was determined that the antiserum recognized reduced, nonreduced, denatured, and native forms of the receptor. A polyclonal antibody to the LH/CG receptor will be useful for studies on the structure and regulation of this receptor.

Regulators for the rat clusterin gene: DNA methylation and cis-acting regulatory elements.

Clusterin, also known as sulphated glycoprotein-2 or testosterone-repressed prostate message-2, is a ubiquitous protein found in a variety of tissues and species. In the reproductive tract of the male rat, clusterin is regulated in a complex age-dependent and cell-specific manner. It is expressed at high levels in the epididymis and testis and at very low levels in the prostate under basal conditions. The expression of this gene in the prostate and seminal vesicles is associated with androgen withdrawal, while in the testis clusterin mRNA is repressed by cyclic AMP (cAMP). To understand the mechanisms that control the expression of the clusterin gene better, we isolated and characterized the gene encoding rat clusterin, and analysed its cytosine methylation pattern in various tissues. Several putative regulatory DNA elements were identified, including a consensus AP-1 site in the 5' flanking region. Two AP-1 sites and two transforming growth factor-beta inhibitory elements, one AP-2 site and eight half-sites for glucocorticoid/androgen response elements were found within the first intron, and one cAMP response element was found in the first exon. The cytosine methylation pattern indicated that testicular or epididymal DNA in the rat is hypomethylated in the region between positions -534 and -99 of the clusterin gene, when compared with tissues with lower levels of expression such as prostate as well as liver, lung, kidney and spleen.

Analysis of the rat clusterin gene promoter and cyclic AMP-regulated mRNA stability in testicular cells.

Clusterin, also known as SGP-2 or TRPM-2, is expressed in the male reproductive tissues at different levels. The genomic structure of the rat clusterin gene was recently reported by our laboratory and others. In this study, we have determined the promoter responsible for the basal expression of the rat clusterin gene in testicular cells by analyzing the transient expression of the bacterial chloramphenicol acetyl transferase (CAT) reporter gene in MA-10 cells driven by different segments of the 5'-flanking region and the first intron of the clusterin gene. The region required for maximal basal expression was identified at -266 to +54. Addition of DNA fragments of the rat clusterin gene from -1298 to -266 bp, or from +54 to +1153 to (-266/+54)CAT resulted in a 87% decrease in CAT activity, suggesting the presence of inhibitory DNA elements in both the 5'-flanking region and the first intron. When DNA fragment in the first intron, +1153 to +2874, was included, CAT activity in the (-266/+2874)CAT construct increased to 70% of the clusterin promoter (-266/+54)CAT, indicating that stimulatory DNA elements may be present in this region of the first intron. Treatment of MA-10 cells with cyclic AMP (cAMP) neither decreased CAT activity driven by any of the clusterin/CAT chimeric plasmids examined in transient transfection studies, nor reduced the synthesis of nuclear clusterin RNA in nuclear run-on assays, indicating that the reduction of clusterin mRNA levels by cAMP previously reported in our laboratory is not exerted at the transcriptional level. Furthermore, addition of transcriptional or translational inhibitors (actinomycin D and cycloheximide respectively) abolished the cAMP effect observed in MA-10 cells. In summary, we have demonstrated that the basal transcription of the rat clusterin gene in testicular cells is under the control of both positive and negative regulatory sequences at the 5'-flanking region as well as in the first intron. The reduction of clusterin mRNA after exposure of MA-10 cells to cAMP is not due to a decrease in its transcriptional activity, but rather to an increase in the degradation of this mRNA through synthesis of a destabilizing protein(s) and its mRNA.

Transcriptional features of fractionated human breast tumor chromatin.

By means of controlled mechanical shearing it was possible to fractionate human breast tumor chromatin in "active" and "inactive" species, according to their in vitro template activity, using Escherichia coli RNA polymerase. The "active" molecules can be resolved in three well defined regions in an exponential sucrose gradient, showing approximately 300 times the efficiency for synthesizing ribonucleic acid, relative to the chromatin which migrated fastest. This technique could provide the initial tool to isolate eu-and heterochromatin from native human chromatin.

PKA phosphorylation dissociates FKBP12.6 from the calcium release channel (ryanodine receptor): defective regulation in failing hearts.

The ryanodine receptor (RyR)/calcium release channel on the sarcoplasmic reticulum (SR) is the major source of calcium (Ca2+) required for cardiac muscle excitation-contraction (EC) coupling. The channel is a tetramer comprised of four type 2 RyR polypeptides (RyR2) and four FK506 binding proteins (FKBP12.6). We show that protein kinase A (PKA) phosphorylation of RyR2 dissociates FKBP12.6 and regulates the channel open probability (Po). Using cosedimentation and coimmunoprecipitation we have defined a macromolecular complex comprised of RyR2, FKBP12.6, PKA, the protein phosphatases PP1 and PP2A, and an anchoring protein, mAKAP. In failing human hearts, RyR2 is PKA hyperphosphorylated, resulting in defective channel function due to increased sensitivity to Ca2+-induced activation.

Intracellular calcium release channel expression during embryogenesis.

The release of intracellular calcium (Ca2+) via either inositol 1,4, 5-trisphosphate receptors (IP3R) or ryanodine receptors (RyR) activates a wide variety of signaling pathways in virtually every type of cell. In the present study we demonstrate that at early stages of development IP3R mRNA and functional IP3-gated Ca2+ release channels are widely expressed in virtually all tissues in murine embryos. As organogenesis proceeds, more specialized RyR channels are expressed in many cell types and the triggering mechanisms for intracellular Ca2+ release become more diverse to include IP3-dependent and voltage-dependent and Ca2+-induced Ca2+ release. As development proceeds virtually all cell types continue to express IP3R channels but in excitable cells including skeletal and cardiac muscles the major Ca2+ release channels are RyRs. This developmental switch from predominantly IP3-mediated to both IP3-mediated and IP3-independent pathways for intracellular Ca2+ release is consistent with data showing that IP3R plays an important regulatory role in cellular proliferation and apoptosis, whereas RyR is required for other cellular functions including muscle contraction.

FKBP12 binding modulates ryanodine receptor channel gating.

The ryanodine receptor (RyR1)/calcium release channel on the sarcoplasmic reticulum of skeletal muscle is comprised of four 565,000-dalton RyR1s, each of which binds one FK506 binding protein (FKBP12). RyR1 is required for excitation-contraction coupling in skeletal muscle. FKBP12, a cis-trans peptidyl-prolyl isomerase, is required for the normal gating of the RyR1 channel. In the absence of FKBP12, RyR1 channels exhibit increased gating frequency, suggesting that FKBP12 "stabilizes" the channel in the open and closed states. We now show that substitution of a Gly, Glu, or Ile for Val2461 in RyR1 prevents FKBP12 binding to RyR1, resulting in channels with increased gating frequency. In the case of the V2461I mutant RyR1, normal channel function can be restored by adding FKBP12.6, an isoform of FKBP12. These data identify Val2461 as a critical residue required for FKBP12 binding to RyR1 and demonstrate the functional role for FKBP12 in the RyR1 channel complex.

Transcriptional features of fractionated human breast tumor chromatin.

By means of controlled mechanical shearing it was possible to fractionate human breast tumor chromatin in [quot ]active[quot ] and [quot ]inactive[quot ] species, according to their in vitro template activity, using Escherichia coli RNA polymerase. The [quot ]active[quot ] molecules can be resolved in three well defined regions in an exponential sucrose gradient, showing approximately 300 times the efficiency for synthesizing ribonucleic acid, relative to the chromatin which migrated fastest. This technique could provide the initial tool to isolate eu-and heterochromatin from native human chromatin.

Transcriptional features of fractionated human breast tumor chromatin

By means of controlled mechanical shearing it was possible to fractionate human breast tumor chromatin in [quot ]active[quot ] and [quot ]inactive[quot ] species, according to their in vitro template activity, using Escherichia coli RNA polymerase. The [quot ]active[quot ] molecules can be resolved in three well defined regions in an exponential sucrose gradient, showing approximately 300 times the efficiency for synthesizing ribonucleic acid, relative to the chromatin which migrated fastest. This technique could provide the initial tool to isolate eu-and heterochromatin from native human chromatin.