Distribution and Activation of cAMP- and cGMP-Dependent Protein Kinases in Highly Purified Human Platelet Plasma and Intracellular Membranes

Previously cAMP- and cGMP-dependent protein kinases (cAMP-PK, cGMP-PK) have been found predominantly associated with the particulate fraction in human platelets. We now report the distribution and activation of cAMP-PK and cGMP-PK in highly purified fractions of human platelet plasma (PM) and intracellular membranes (IM) prepared using high voltage free flow electrophoresis. Two non-hydrolys-able analogues of cAMP and cGMP namely Sp-5,6-DCl-cBiMPS and 8-p-CPT-cGMP have been used to activate cAMP-PK and cGMP-PK respectively. Addition of either agonist with [γ32P]ATP stimulated the endogenous activity of cAMP-PK or cGMP-PK in PM but not in IM. With PM Sp-5,6-DCl-cBiMPS stimulated the phosphorylation of protein substrates of Mr 16,22,24,46-50,66,90,160 and 250 kDa. A specific peptide inhibitor of cAMP-PK inhibited the phosphorylation of all of the substrates by Sp-5,6-DCl-cBiMPS. 8-pCPT-cGMP also induced the phosphorylation of a number of substrates particularly 16,22, 46-50, 90 and 250 kDa proteins. Inclusion of the cAMP-PK inhibitor peptide totally blocked the phosphorylation of the 16 and 22 kDa proteins, partially inhibited phosphorylation of 46-50 and 90 kDa proteins and had no effect on the 250 kDa protein indicating the 46-50, 90 and 250 kDa proteins were also cGMP-PK substrates. Western blotting with antibodies to cGMP-PK and the catalytic subunit of cAMP-PK revealed the presence of the kinases to be exclusively associated with PM with no detection in IM.

The presence of cAMP-PK substrates in IM was investigated by exogenous addition of catalytic subunit of cAMP-PK. Phosphoproteins of Mr 16, 22, 27, 30,45, 75,116 and 250 kDa were detected. A range of antibodies to cAMP-PK substrates were used to identify and localise the substrates. These antibodies revealed GPIb and VASP to be exclusively associated with PM fractions. Rap IB was also predominantly associated with PM with a small level detected in IM. Antibodies to the IP3 receptor (18A10 and 4C11) revealed the protein to be predominantly associated with IM. Additionally the antibody 4C11 recognised a 230 kDa protein band in PM that was not seen in IM. From the known specificity of these antibodies the results confirm the presence of a type IIP3 receptor in IM and a distinct (possible type III) IP3 receptor with the PM. the 16, 22, 27, 30, 75 and 116 kDa proteins in IM represent nwly detected substartes for camp-pk of presently unknown identity.

Distinct localization and function of (1,4,5)IP(3) receptor subtypes and the (1,3,4,5)IP(4) receptor GAP1(IP4BP) in highly purified human platelet membranes

Platelet activation is associated with an increase of cytosolic Ca(++) levels. The (1,4,5)IP(3) receptors [(1,4,5)IP(3)R] are known to mediate Ca(++) release from intracellular stores of many cell types. Currently there are at least 3 distinct subtypes of (1,4, 5)IP(3)R-type I, type II, and type III-with suggestions of distinct roles in Ca(++) elevation. Specific receptors for (1,3,4,5)IP(4) belonging to the GAP1 family have also been described though their involvement with Ca(++) regulation is controversial. In this study we report that platelets contain all 3 subtypes of (1,4,5)IP(3)R but in different amounts. Type I and type II receptors are predominant. In studies using highly purified platelet plasma (PM) and intracellular membranes (IM) we report a distinct localization of these receptors. The PM fractions were found to contain the type III (1,4,5)IP(3)R and GAP1(IP4BP) in contrast to IM, which contained type I (1,4,5)IP(3)R. The type II receptor exhibited a dual distribution. In studies examining the labeling of surface proteins with biotin in intact platelets only the type III (1,4,5)IP(3)R was significantly labeled. Immunogold studies of ultracryosections of human platelets showed significantly more labeling of the PM with the type III receptor antibodies than with type I receptor antibodies. Ca(++) flux studies were carried out with the PM to demonstrate in vitro function of inositol phosphate receptors. Ca(++) release activities were present with both (1,4,5)IP(3) and (1, 3,4,5)IP(4) (EC(50) = 1.3 and 0.8 micromol/L, respectively). Discrimination of the Ca(++)-releasing activities was demonstrated with cyclic adenosine monophosphate (cAMP)-dependent protein kinase (cAMP-PK) specifically inhibiting (1,4,5)IP(3) but not (1,3,4, 5)IP(4)-induced Ca(++) flux. In experiments with both PM and intact platelets, the (1,4,5)IP(3)Rs but not GAP1(IP4BP) were found to be substrates of cAMP-PK and cGMP-PK. Thus the Ca(++) flux property of (1,3,4,5)IP(4) is insensitive to cAMP-PK. These studies suggest distinct roles for the (1,4,5)IP(3)R subtypes in Ca(++) movements, with the type III receptor and GAP1(IP4BP) associated with cation entry in human platelets and the type I receptor involved with Ca(++) release from intracellular stores.

Expression of human TRPC genes in the megakaryocytic cell lines MEG01, DAMI and HEL

Store-regulated Ca2+ entry represents a major mechanism for Ca2+ influx in non-excitable cells although many details remain to be evaluated including the identification of cation entry channels. Recently human homologues of the Drosophila proteins TRP and TRPL, have been described (TRPC1, TRPC1A, HTRP1) and suggested as candidate cation channels. In this study we sought to examine if the producers of blood platelets, megakaryocytic cells (using the cell lines MEG01, DAMI, HEL), expressed these genes. RNA was prepared from the cell lines and platelets and converted to cDNA. The cDNA was then subjected to 30-35 cycles of PCR using gene specific primers for TRPC1-3. PCR products of the expected sizes were observed for all three TRPC genes in the three cell lines. Direct sequencing confirmed their identity. Additionally for TRPC1, a larger species, and for TRPC2, a smaller species was detected in all three cell lines with sequencing revealing the fragments to contain TRPC sequence, suggesting that they were either products of alternative splicing events or from closely related genes. These results suggest that TRPC genes are expressed in megakaryocytic cell lines and that the TRPC proteins may play a role in mediating cation influx in both megakaryocytes and platelets.