Replacement of m-calpain by mu-calpain during maturation of megakaryocytes and possible involvement in platelet formation

Release of intracellular calcium stores facilitates coxsackievirus entry into polarized endothelial cells

Group B coxsackieviruses (CVB) are associated with viral-induced heart disease and are among the leading causes of aseptic meningitis worldwide. Here we show that CVB entry into polarized brain microvasculature and aortic endothelial cells triggers a depletion of intracellular calcium stores initiated through viral attachment to the apical attachment factor decay-accelerating factor. Calcium release was dependent upon a signaling cascade that required the activity of the Src family of tyrosine kinases, phospholipase C, and the inositol 1,4,5-trisphosphate receptor isoform 3. CVB-mediated calcium release was required for the activation of calpain-2, a calcium-dependent cysteine protease, which controlled the vesicular trafficking of internalized CVB particles. These data point to a specific role for calcium signaling in CVB entry into polarized endothelial monolayers and highlight the unique signaling mechanisms used by these viruses to cross endothelial barriers.

Enteroviruses are associated with a number of diverse syndromes such as myocarditis, febrile illness, and are the main causative agents of aseptic meningitis. No effective therapeutics exist to combat non-poliovirus enterovirus infections. A better understanding of the mechanisms by which these viruses infect host cells could lead to the design of effective therapeutic interventions. In this study, we found that intracellular calcium stores in polarized endothelial monolayers are depleted upon exposure to coxsackievirus B (CVB) and that this release is mediated by viral attachment to its receptor decay-accelerating factor. We also discovered that the calcium release requires the activation of signaling molecules involved in calcium signaling such as Src tyrosine kinases, phospholipase C, and the inositol 1,4,5-trisphosphate receptor isoform 3 on the ER membrane. Furthermore, we found that a calcium-activated cystein protease, calpain-2, was activated and necessary for proper viral trafficking inside the cell. Interestingly, we found that this signaling cascade was critical for CVB internalization into the endothelium, but was not involved in CVB entry into the epithelium. This is an important advance in our understanding of how enteroviruses hijack host endothelial cell signaling mechanisms in order to facilitate their entry and eventual spread.

In vivo platelet production from mature megakaryocytes: does platelet release occur via proplatelets?

Although platelets are universally accepted to be born from megakaryocytes (MKs), the mechanism by which platelets are formed and released from MKs in vivo remains controversial. One theory, known as the proplatelet theory, postulates that platelets are released from proplatelet processes protruding from MKs into sinusoids located in the bone marrow hematopoietic compartment. Proplatelet formation (PPF) has been observed in in vitro experiments involving detailed analyses of related molecular events. PPF has also been used as a marker of MK maturation. However, PPF is suggested to be a nonphysiological phenomenon. On the other hand, transmission electron microscopy (TEM) analyses have revealed platelet formation via explosive fragmentation of MK cytoplasm in bone marrow and lung capillaries prepared by immersion fixation. Moreover, TEM and scanning electron microscopy studies of liquid-cultured MKs kept in suspension show that platelet formation occurs without PPF. Rather, an explosive and global fragmentation of the MK cytoplasm composed of platelet territories has been reported as the mechanism of platelet formation. In addition, in vivo and ex vivo observations of platelet release from MKs with phase-contrast microscopy strongly support the explosive-fragmentation theory. With all observations taken into account, PPF may not be a prerequisite for platelet release from MKs under real-life conditions. In this review, a new "protoplatelet" concept is proposed to support the explosive-fragmentation theory. Additionally, the role of the lungs in platelet production is reviewed and discussed.

Unique in vivo modifications of coagulation factor V produce a physically and functionally distinct platelet-derived cofactor: characterization of purified platelet-derived factor V/Va

Platelet- and plasma-derived factor Va (FVa) serve essential cofactor roles in prothrombinase-catalyzed thrombin generation. Platelet-derived FV/Va, purified from Triton X-100 platelet lysates was composed of a mixture of polypeptides ranging from approximately 40 to 330 kDa, mimicking those visualized by Western blotting of platelet lysates and releasates with anti-FV antibodies. The purified, platelet-derived protein expressed significant cofactor activity such that thrombin activation led to only a 2-3-fold increase in cofactor activity yet expression of a specific activity identical to that of purified, plasma-derived FVa. Physical and functional differences between the two cofactors were identified. Purified, platelet-derived FVa was 2-3-fold more resistant to activated protein C-catalyzed inactivation than purified plasma-derived FVa on the thrombin-activated platelet surface. The heavy chain subunit of purified, platelet-derived FVa contained only a fraction ( approximately 10-15%) of the intrinsic phosphoserine present in the plasma-derived FVa heavy chain and was resistant to phosphorylation at Ser(692) catalyzed by either casein kinase II or thrombin-activated platelets. MALDI-TOF mass spectrometric analyses of tryptic digests of platelet-derived FV peptides detected an intact heavy chain uniquely modified on Thr(402) with an N-acetylglucosamine or N-acetylgalactosamine, whereas Ser(692) remained unmodified. N-terminal sequencing and MALDI-TOF analyses of platelet-derived FV/Va peptides identified the presence of a full-length heavy chain subunit, as well as a light chain subunit formed by cleavage at Tyr(1543) rather than Arg(1545) accounting for the intrinsic levels of cofactor activity exhibited by native platelet-derived FVa. These collective data are the first to demonstrate physical differences between the two FV cofactor pools and support the hypothesis that, subsequent to its endocytosis by megakaryocytes, FV is modified to yield a platelet-derived cofactor distinct from its plasma counterpart.

Suppression of oxidative stress-induced hepatocyte injury by calpain antisense

BACKGROUND

Calpain, a cytosolic Ca(2+)-dependent proteinase, plays a pivotal role in cell injury. In this study, we investigated the effect of calpain-mu antisense oligonucleotide on oxidative stress induced-hepatocyte injury.

MATERIALS AND METHODS

Hemagglutinating virus of Japan liposome complex with one of three types of antisense oligonucleotide (AS-1, AS-2, AS-3) or scramble oligonucleotide was added to the culture medium of HuH7 cells and incubated for 6 days. The expression of calpain-mu protein was examined by Western blotting. After the addition of tert-butyl hydroperoxide, bleb formation was examined by phase contrast microscopy, and cell viability was assessed by the release of lactate dehydrogenase.

RESULTS

Incubation of HuH7 cells with AS-2 resulted in a decrease in the amount of calpain on day 4 and a further decrease to almost undetectable levels on day 6, whereas scramble oligonucleotide had no effect. Bleb formation was observed 120 min after the addition of tert-butyl hydroperoxide in scramble oligonucleotide-treated cells as in untreated cells. In contrast, it was rarely observed in AS-2-treated cells. Lactate dehydrogenase release was significantly suppressed in AS-2-treated cells, compared with that in scramble oligonucleotide treated-cells.

CONCLUSIONS

Our findings suggest that calpain activation is involved in the pathogenesis of oxidative stress injury and that transfection of calpain antisense may potentially protect against ischemia/reperfusion liver injury.

Multiple Forms of the SH2-Containing Inositol Phosphatase, SHIP, Are Generated by C-Terminal Truncation

The SH2-containing inositol phosphatase, SHIP, often appears as multiple bands in anti-SHIP immunoblots. To characterize these bands, antisera were generated against the N-terminal (anti-N), mid-region (anti-M), and C-terminal (anti-C) portions of SHIP. Immunoprecipitation and immunoblotting studies showed that 145-, 135-, 125-, and 110-kD bands were detected in lysates from the murine hematopoietic cell line, DA-ER, with either anti-N or anti-M antisera, whereas only the 145- and 135-kD bands were recognized by the anti-C antiserum. This finding suggested that the smaller proteins might be C-terminal truncations of the full-length SHIP. To confirm this and determine if these proteins arose through alternate splicing or posttranslational cleavage, a 5′-hemagglutin (HA)-tagged full-length SHIP cDNA was expressed in these cells. We observed, via Western analysis with anti-HA antibodies, the same 4 bands with either anti-N or anti-M and only the 145- and 135-kD bands with anti-C immunoprecipitation. After interleukin-3 stimulation of HA-SHIP–expressing DA-ER cells, only the 145-kD form coprecipitated with Shc, raising the possibility that different forms of SHIP may have distinct intracellular sites. This was confirmed by subcellular fractionation, which showed that only the 110-kD form is present in the cytoskeleton of DA-ER cells. This 110-kD form possesses the same PIP3 5-ptase activity as the 145-kD form and can be generated from the latter in vitro by digestion with calpain. It is therefore possible that the different forms of SHIP are generated in vivo by calpain-mediated C-terminal truncations and perform distinct functions within hematopoietic cells.

© 1998 by The American Society of Hematology.

Multiple Forms of the SH2-Containing Inositol Phosphatase, SHIP, Are Generated by C-Terminal Truncation

The SH2-containing inositol phosphatase, SHIP, often appears as multiple bands in anti-SHIP immunoblots. To characterize these bands, antisera were generated against the N-terminal (anti-N), mid-region (anti-M), and C-terminal (anti-C) portions of SHIP. Immunoprecipitation and immunoblotting studies showed that 145-, 135-, 125-, and 110-kD bands were detected in lysates from the murine hematopoietic cell line, DA-ER, with either anti-N or anti-M antisera, whereas only the 145- and 135-kD bands were recognized by the anti-C antiserum. This finding suggested that the smaller proteins might be C-terminal truncations of the full-length SHIP. To confirm this and determine if these proteins arose through alternate splicing or posttranslational cleavage, a 5′-hemagglutin (HA)-tagged full-length SHIP cDNA was expressed in these cells. We observed, via Western analysis with anti-HA antibodies, the same 4 bands with either anti-N or anti-M and only the 145- and 135-kD bands with anti-C immunoprecipitation. After interleukin-3 stimulation of HA-SHIP–expressing DA-ER cells, only the 145-kD form coprecipitated with Shc, raising the possibility that different forms of SHIP may have distinct intracellular sites. This was confirmed by subcellular fractionation, which showed that only the 110-kD form is present in the cytoskeleton of DA-ER cells. This 110-kD form possesses the same PIP3 5-ptase activity as the 145-kD form and can be generated from the latter in vitro by digestion with calpain. It is therefore possible that the different forms of SHIP are generated in vivo by calpain-mediated C-terminal truncations and perform distinct functions within hematopoietic cells.

© 1998 by The American Society of Hematology.

The calpain-calpastatin system and cellular proliferation and differentiation in rodent osteoblastic cells

The calpain-calpastatin system, which consists of calpains I and II (two ubiquitously distributed calcium-activated papain-like cysteine proteases), as well as calpastatin (the endogenous calpain inhibitor), plays an important role in cell proliferation and differentiation in many tissues. However, its contribution to the regulation of osteoprogenitor or pluripotent stem cell proliferation and differentiation into osteoblasts remains poorly defined. In these studies, rat pluripotent mesodermal cells (ROB-C26) and mouse MC3T3-E1 preosteoblasts were induced to differentiate into osteoblasts by long-term culture or in response to bone morphogenetic protein (BMP). The occurrence and distribution of calpain-calpastatin system proteins were determined by immunofluorescent microscopy, measurement of calcium-dependent proteolytic activity, and Western blotting. Treatment of intact MC3T3-E1 cells with an irreversible, membrane-permeable cysteine protease inhibitor attenuated proliferation and alkaline phosphatase upregulation under differentiation-enhancing conditions. Calpain II activity increased during differentiation of MC3T3-E1 cells in postconfluent culture. When ROB-C26 cells were maintained in long-term culture, neutral protease, calpain I, and calpain II activities increased 2- to 3-fold in the absence of BMP. In the presence of partially purified native BMP, neutral protease and calpain I activities also increased similarly, but calpain II activity increased by 10-fold in 3 days. The maximal increase in alkaline phosphatase occurred 4 to 11 days after the calpain II activity had peaked. Induction of differentiation in long-term MC3T3-E1 cultures was associated with higher calpain II and 70- and 110-kDa calpastatin protein levels and lower 17-kDa calpastatin degradation product levels. In conclusion, cysteine protease activity is essential for preosteoblastic proliferation and differentiation. The calpain-calpastatin system is regulated during osteoprogenitor proliferation and differentiation, as it is in other cells, and bone morphogenetic protein is a specific regulator of calpain II.

A variety of calpain/calpastatin systems in mammalian erythrocytes

Calpain and its endogenous inhibitor, calpastatin, were isolated from erythrocytes of various mammals and their properties were compared. It has been widely believed that mammalian erythrocytes contain only mu-calpain. However, rat and human erythrocytes were found to contain two species of calpain, identified as mu-calpain and m-calpain from their elution positions on DEAE-cellulose column chromatography and their Ca(2+)-requirements. Thus, it is apparent that rat and human erythrocytes contain not only mu-calpain, but m-calpain as well. On the other hand, rabbit erythrocytes contain only mu-calpain. Western blot analysis showed that human and rabbit erythrocytes contain predominantly 70-kDa calpastatin (erythrocyte-type), but unnegligible amounts of 110-kDa calpastatin (tissue-type) are also present. Rat erythrocytes were shown to contain a calpastatin with a molecular mass of approx. 100 kDa almost exclusively; this molecular mass was in perfect coincidence with the mass of the calpastatin in rat lung. These results strongly suggest that rat erythrocytes contain a tissue-type calpastatin. No essential change in the calpain/calpastatin system during maturation of rabbit reticulocytes into mature erythrocytes was observed.