Randall L. Kincaid

Phosphorylation of Syk Activation Loop Tyrosines Is Essential for Syk Function AN IN VIVO STUDY USING A SPECIFIC ANTI-Syk ACTIVATION LOOP PHOSPHOTYROSINE ANTIBODY

Syk is an important protein-tyrosine kinase in immunoreceptor signaling. FcεRI aggregation in mast cells induces tyrosine phosphorylation and increased enzymatic activity of Syk. The two adjacent tyrosines in the Syk activation loop are thought to be important for the propagation of FcεRI signaling. To evaluate the phosphorylation of these tyrosines in vivo and further understand the relationship of Syk tyrosine phosphorylation with its function, an antibody was developed specific for phosphorylated tyrosines in the activation loop of Syk. FcεRI aggregation on mast cells induced the phosphorylation of both tyrosine residues of the activation loop. The kinase activity of Syk played the major role in phosphorylating its activation loop tyrosines both in vivoand in vitro. In FcεRI-stimulated mast cells, the total Syk tyrosine phosphorylation paralleled the phosphorylation of its activation loop tyrosines and downstream propagation of signals for histamine release. In contrast, the cell surface binding of anti-ganglioside monoclonal antibody AA4 induced only strong general tyrosine phosphorylation of Syk and minimal histamine release and weak phosphorylation of activation loop tyrosines. These results demonstrate that phosphorylation of the activation loop tyrosines is important for mediating receptor signaling and is a better marker of Syk function than is total Syk tyrosine phosphorylation.

[43] Assay of cyclic nucleotide phosphodiesterase using radiolabeled and fluorescent substrates

Publisher Summary This chapter describes the assay developed in laboratory that yields quantitative recovery of product with very low background. The reaction catalyzed by cyclic nucleotide phosphodiesterases involves the breaking of a high-energy phosphodiester bond in a metaldependent, hydrolytic process. For most enzymes, the preferred metal cofactors are divalent cations such as Mg 2+ or Mn 2+ , although other metals, such as Co 2+ , Ni 2+ , and Zn 2+ , can support lower activity; Ca 2+ , by itself, is virtually ineffective. The pH optimum for all cyclic nucleotide phosphodiesterases appears to be between 7.0 and 8.0. The release of a substantial amount of energy accompanies the reaction, but no direct link between this event and another energy-requiring reaction has been found. Of the many methods developed for assay of phosphodiesterase, most employ a radiolabeled cyclic nucleotide substrate. In addition, a phosphodiesterase assay using a fluorescent cAMP analog is described in the chapter, with specific emphasis on its value for measurement of activity at high enzyme concentrations.

Expression of the calmodulin-dependent protein phosphatase, calcineurin, in rat brain : developmental patterns and the role of nigrostriatal innervation

The distribution of neurons expressing the calmodulin-dependent protein phosphatase, calcineurin (CN) was characterized in developing and adult rat brain using a combination of immunocytochemical, immunoblot and in situ hybridization approaches. Immunoblot analysis revealed a strong increase postnatally in CN protein expression. Four differently-charged isoforms of CN were observed in adult brain with apparent regional differences in isoform expression. Immunocytochemistry showed highest levels of CN in hippocampus, striatum, substantia nigra, amygdala and septal nuclei with immunoreactivity first appearing in striatum and septal nuclei, followed by hippocampus, neocortex and limbic structures. In situ hybridization demonstrated that mRNA for the catalytic subunit of CN was seen as early as postnatal day (PND) 1 in striatum, cortex and hippocampus. Since immunoreactivity was not detectable until day 4, this suggests that mRNA expression may precede that of protein by several days in these regions. Lesioning of developing and adult nigrostriatal dopamine neurons either with 6-hydroxydopamine or by surgical hemitransection had little effect on expression of CN, suggesting that CN expression is not influenced transsynaptically by dopamine. Collectively, these findings demonstrate that CN protein and mRNA expression are subject to regional and temporal control during brain development suggesting that specific synaptic connections may influence CN gene expression. However, in striatum, dopaminergic innervation does not appear to affect CN levels.

Structure and expression of two isoforms of the murine calmodulin-dependent protein phosphatase regulatory subunit (calcineurin B).

Murine cDNAs representing distinct genes for the regulatory subunits of calmodulin-dependent protein phosphatase (CaM-PrP) were cloned from a testis library, using probes prepared by PCR amplification of brain and testis mRNA. The cDNA sequence of the brain-specific isoform (beta 1) encodes a 170 amino acid protein (M(r) approximately 19.3 kDa), whereas that for the testis isoform (beta 2) contains 179 residues (M(r) approximately 20.7 kDa); these two sequences show approximately 80% amino acid identity. An oligonucleotide probe for the brain isoform hybridized to a single mRNA of 3.6 kilobases (kb) in many tissues, whereas using the beta 2 probe, two mRNAs of 1.8 and 0.8 kb were detected only in testis. The mRNA for the testis-specific isoform increases markedly during development, its pattern being virtually identical to that of mRNA for a testicular form of the catalytic subunit (alpha 3). These data are consistent with the biological co-regulation of catalytic and regulatory subunits of a testis-specific isoenzyme during germ cell maturation.

Molecular cloning and chromosomal mapping of the human gene for the testis-specific catalytic subunit of calmodulin-dependent protein phosphatase (calcineurin A)

A cDNA for an alternatively spliced variant of the testis-specific catalytic subunit of calmodulin dependent protein phosphatase (CaM-PrP) was cloned from a human testis library. The nucleotide sequence of 2134 base pairs (bp) encodes a protein of 502 amino acids (Mr approximately 57,132) and pI 7.0. The cDNA sequence differs from the murine form of this gene by a 30 bp deletion in the coding region, the position of which matches those in the two other genes for the catalytic subunit. These data indicate that this alternative splicing event arose prior to the divergence of the three genes. The deduced sequence of the human protein is only 88% identical to the homologous murine form, in striking contrast to the other two CaM-PrP catalytic subunits which are highly conserved between mouse and human (approximately 99%); this indicates a more rapid rate of evolution for the testis-specific gene. Analysis of Southern blots containing DNA from human-hamster somatic cell hybrids show that the gene is on human chromosome 8.

Differential subcellular localization of neural isoforms of the catalytic subunit of calmodulin-dependent protein phosphatase (calcineurin) in central nervous system neurons: immunohistochemistry on formalin-fixed paraffin sections employing antigen retrieval by microwave irradiation.

We examined the immunohistochemical distribution of the two mammalian isoforms of calcineurin catalyic subunits, A alpha and A beta, in central nervous system (CNS) tissues of cows, rats, and humans. Cryostat sections and paraffin sections of parformaldehyde-fixed tissues were stained with antipeptide antibodies for each isoform. The same localization pattern was observed in both cryostat and paraffin sections. In the latter, the intensity of the staining was dramatically enhanced by microwave irradiation. Calcineurin isoforms were localized in a variety of nerve cells but not in neuroglial cells. Their differential expression as the A alpha isoform in the nucleus and the A beta isoform in the cytoplasm was present in a variety of CNS nerve cells, most distinctively in Purkinje cells of the cerebellum and pyramidal cells of the cerebrum, irrespective of species. These results suggest that each isform has distinct intracellular sites of action in CNS neurons and that the phenomenon has been conserved during mammalian evolution.

[56] Preparation of fluorescent, cross-linking, and biotinylated calmodulin derivatives and their use in studies of calmodulin-activated phosphodiesterase and protein phosphatase

Publisher Summary This chapter summarizes work that have carried out with three derivatives of calmodulin (CaM), each of which has unique advantages for the investigation of structure and function: 5-dimethylaminonaphthalene-1-sulfonyl(dansyl)-CaM, a fluorescent derivative; 3-(2-pyridyldithio)propionyl(PDP)-CaM, an activated sulfhydryl cross-linking derivative; and biotinyl-ɛ-aminocaproyl(Bio)-CaM, a biotin-containing derivative that forms stable complexes with avidin-enzyme conjugates. As the primary amino acid sequence of CaM is known, the site(s) of modification have, in some instances, been exactly defined using proteolytic and chemical cleavage of the modified protein. Thus, chemical modification can provide a valuable adjunct to classical physical methods for investigation of CaM structure and function. In addition to methods for their syntheses, procedures for characterization and use are outlined in the chapter. Properties and uses of Bio-CaM are also summarized in the chapter.

Molecular cloning of a full-length cDNA encoding the catalytic subunit of human calmodulin-dependent protein phosphatase (calcineurin Aα)

A complementary DNA for human calcineurin A alpha (protein phosphatase-2B), encoding a protein of 521 amino acids, was isolated from a hippocampus library. The deduced human sequence differs from that of mouse in only two amino acids, demonstrating that the structure of this catalytic subunit has been strictly conserved during mammalian evolution. Such high homology is in contrast to that seen for calcineurin A gamma, an isoform that shows only 88% identity between human and mouse (Muramatsu, T. and Kincaid, R.L. (1992) Biochem. Biophys. Res. Commun. 188, 265-271).

Calcineurin immunoreactivity in Alzheimer's disease.

Aberrant phosphorylation of tau is linked to formation of the paired helical filaments (PHF) seen in Alzheimers disease. Protein kinases such as mitogen-activated protein kinase, and calcium-regulated protein kinases may, in part, be responsible for addition of phosphate groups to serine residues of PHFtau; however, less is known concerning the phosphatases which regulate tau. In this report, we used several well-characterized antibodies to document calcineurin immunoreactivity in brain tissue from patients with Alzheimers disease. We now report that levels of immunoreactive calcineurin are not significantly altered in neocortex and cerebellum of Alzheimers patients relative to similar regions of age-matched controls. Immunocytochemical studies indicated that calcineurin immunoreactivity was present in dendrites and perikarya of many different neuronal populations in both control and Alzheimer brain. When specific antibodies against PHFtau were used in double-labeling experiments with anti-calcineurin antibodies, calcineurin immunoreactivity was seen in association with neurofibrillary tangles. However, calcineurin was not seen in all tangle bearing neurons. These data suggest that calcineurin levels per se are not significantly altered in Alzheimers disease, but that calcineurin is distributed around some neurofibrillary tangles and may play a role in regulation of tau phosphorylation.

Developmental expression of calmodulin-dependent cyclic nucleotide phosphodiesterase in rat brain

The patterns of expression of calmodulin-dependent cyclic nucleotide phosphodiesterase (CaM-PDE) have been studied in developing and adult rat brain using affinity-purified polyclonal antibodies against CaM-PDE. An immunocytochemical map of adult brain regions expressing CaM-PDE, constructed from serial coronal brain sections, illustrated that CaM-PDE was expressed in specific neuronal subpopulations throughout the adult rat brain. Immunoblot analysis coupled with subcellular fractionation indicated that CaM-PDE was primarily localized to cytoplasmic fractions, with a small amount associated with synaptosomal membranes. Immunoblots from developing brain indicated that CaM-PDE expression increased dramatically during postnatal days 7-20 (PND 7-20); parallel increases in CaM-PDE enzyme activity occurred during this same time. Immunocytochemical studies indicated that several distinct patterns of CaM-PDE expression occurred during development. Neocortex showed low levels of CaM-PDE immunoreactivity in neuronal somata of layers III, V and VI on PND 4 that increased by PND 11; the adult somatodendritic pattern of immunoreactivity was observed by PND 60. Similar patterns were observed in cerebellar Purkinje cells, with somatodendritic staining observed by PND 12. By contrast, caudate-putamen, the inferior olive and the hypoglossal nuclei expressed high levels of CaM-PDE on PND 4, with levels considerably lower in the adult animal. The different patterns of expression suggest that in neocortex and cerebellum, CaM-PDE increases during the period of neuronal differentiation and active synaptogenesis, while in the caudate-putamen, inferior olive and hypoglossal nucleus, high levels may be required early in development.