Comparison of the properties of two kinds of preparations of human blood platelet actin with sarcomeric actin

Mammalian Actins: Isoform-Specific Functions and Diseases

Actin is the central building block of the actin cytoskeleton, a highly regulated filamentous network enabling dynamic processes of cells and simultaneously providing structure. Mammals have six actin isoforms that are very conserved and thus share common functions. Tissue-specific expression in part underlies their differential roles, but actin isoforms also coexist in various cell types and tissues, suggesting specific functions and preferential interaction partners. Gene deletion models, antibody-based staining patterns, gene silencing effects, and the occurrence of isoform-specific mutations in certain diseases have provided clues for specificity on the subcellular level and its consequences on the organism level. Yet, the differential actin isoform functions are still far from understood in detail. Biochemical studies on the different isoforms in pure form are just emerging, and investigations in cells have to deal with a complex and regulated system, including compensatory actin isoform expression.

The pathogenesis of adolescent idiopathic scoliosis: review of the literature

STUDY DESIGN

Review of the literature on the pathogenesis of adolescent idiopathic scoliosis (AIS).

OBJECTIVE

To discuss the different theories that have appeared on this subject.

SUMMARY OF BACKGROUND DATA

The pathogenesis of AIS, a condition exclusive to humans, has been the subject of many studies. Over the years, practically every structure of the body has been mentioned in the pathogenesis of AIS; however, the cause of this spinal deformity remains little understood. The pathogenesis of this condition is termed multifactorial.

METHODS

PubMed and Google Scholar electronic databases were searched focused on parameters concerning the pathogenesis of adolescent idiopathic scoliosis. The search was limited to the English language.

RESULTS

No single causative factor for the development of idiopathic scoliosis has been identified, it is thus termed multifactorial. AIS is a complex genetic disorder. The fully erect posture, which is unique to humans, seems to be a prerequisite for the development of AIS.

CONCLUSION

Although any or all of the mentioned factors in this review may play a certain role in the initiation and progression of AIS at a certain stage, the presented material suggests that in the observed deformation, genetics, and the unique mechanics of the fully upright human spine play a decisive role.

Functional specificity of actin isoforms

Actin, one of the main proteins of muscle and cytoskeleton, exists as a variety of highly conserved isoforms whose distribution in vertebrates is tissue-specific. Synthesis of specific actin isoforms is accompanied by their subcellular compartmentalization, with both processes being regulated by factors of cell proliferation and differentiation. Actin isoforms cannot substitute for each other, and the high-level synthesis of exogenous actins leads to alterations in cell organization and morphology. This indicates that the highly conserved actins are functionally specialized for the tissues in which they predominate. The first goal of this review is to analyze the data on the polymerizability of actin isoforms to show that cytoskeleton isoactins form less stable polymers than skeletal muscle actin. This difference correlates with the dynamics of actin microfilaments versus the stability of myofibrillar systems. The three-dimensional actin structure as well as progress in the analysis of conformational changes in both the actin monomer and the filament allows us to view the data on the structure and polymerization of isoactins in terms of structure-function relationships within the actin molecule. Most of the amino acid substitutions that distinguish actin isoforms are located apart from actin-actin contact sites in the polymer. We suggest that these substitutions can modulate the ability of actin monomers to form more or less stable polymers by long-range (allosteric) regulation of the contact sites.

Immune response of rabbits to native G-actins

Actins are highly conserved proteins and are therefore claimed to be not very immunogenic without prior denaturation or chemical modification. We have obtained in rabbits high-titered antibodies to "native" G-actins from chicken and man, and assayed their cross-reaction using an enzyme immunoassay, Western blotting and immunohistochemistry. The antigens differ in their ability to induce antibody formation (chicken gizzard actin [(beta), gamma] greater than chicken skeletal actin [alpha] = human platelet actin [beta, (gamma)]). Antibodies to skeletal actin [alpha] are muscle-specific and mainly directed against the homologous region comprising the N-terminus (residues 1-226). Antibodies to gizzard actin [(beta), gamma] cross-react, to a lesser extent, with the alpha and beta, (gamma) isoforms. They show no regional specificity within the homologous antigen. Antibodies to the tryptic core fragment (residues 69-374) of skeletal actin react with fragments comprising the C-terminal part of muscular actins. Antibodies to platelet actin [beta, (gamma)] cross-react with muscular actins, recognizing not the native, but slightly degraded molecules. Platelet actin induces the formation of high-titered albumin antibodies for hitherto unknown reasons.

Properties of two isoforms of human blood platelet alpha-actinin

The structural and functional properties of the aa (2 X 97 kDa) and cc (2 X 94 kDa) isoforms of platelet alpha-actinin have been compared. Structural differences between aa and cc are revealed by their peptide maps, obtained from limited proteolysis, and by their immunological cross-reactivity. Both isoforms stimulate the Mg ATPase activity of actomyosin, bind to F-actin (high-speed sedimentation) and cross-link or gel actin filaments (low-speed sedimentation and viscometry), in a calcium-dependent manner. The study of the interaction with F-actin indicates that the binding of 1 molecule of aa or cc alpha-actinin/9-11 actin monomers is sufficient to produce maximal gelation in the presence of EGTA. CaCl2 at 0.1 mM strongly inhibits the binding of aa to F-actin and weakly that of cc, while it inhibits similarly the cross-linking of either aa or cc. The cross-linking efficiency of cc is 9, 7, 1.7 and 1.3 times higher than that of aa at 4, 20, 30 and 37 degrees C, respectively. The bb form (2 X 96 kDa), which is a proteolytic product of aa [Y. Gache et al. (1984) Biochem. Biophys. Res. Commun. 124, 877-881], behaves roughly as aa, but the calcium sensitivity of its binding to F-actin is intermediate between that of aa and cc. These results suggest that the effect of Ca2+ concentration on the binding of platelet alpha-actinin to F-actin may be partly dissociated from the effect on the cross-linking.

Microvascular pericytes contain muscle and nonmuscle actins

We have affinity-fractionated rabbit antiactin immunoglobulins (IgG) into classes that bind preferentially to either muscle or nonmuscle actins. The pools of muscle- and nonmuscle-specific actin antibodies were used in conjunction with fluorescence microscopy to characterize the actin in vascular pericytes, endothelial cells (EC), and smooth muscle cells (SMC) in vitro and in situ. Nonmuscle-specific antiactin IgG stained the stress fibers of cultured EC and pericytes but did not stain the stress fibers of cultured SMC, although the cortical cytoplasm associated with the plasma membrane of SMC did react with nonmuscle-specific antiactin. Whereas the muscle-specific antiactin IgG failed to stain EC stress fibers and only faintly stained their cortical cytoplasm, these antibodies reacted strongly with the fiber bundles of cultured SMC and pericytes. Similar results were obtained in situ. The muscle-specific antiactin reacted strongly with the vascular SMC of arteries and arterioles as well as with the perivascular cells (pericytes) associated with capillaries and post-capillary venules. The non-muscle-specific antiactin stained the endothelium and the pericytes but did not react with SMC. These findings indicate that pericytes in culture and in situ possess both muscle and nonmuscle isoactins and support the hypothesis that the pericyte may represent the capillary and venular correlate of the SMC.

Isolation and characterization of human blood platelet gelsolin

A 90-kDa protein-actin stable complex was purified from blood platelets by a short and efficient procedure giving at the same time actin used in polymerization assays. 90-kDa protein free of actin was prepared from the complex by 8 M ureau treatment and renaturation. By its molecular mass, immunological cross-reactivity with macrophage gelsolin and its effect on G- and F-actin the 90-kDa protein appears as the platelet gelsolin.