The Actin and Myosin Filaments of Human and Bovine Blood Platelets

Abstract

A B S T R A C T The contractility of platelets has been attributed to an actomyosin-like protein which has been well defined on a physicochemical basis. Moreover, platelets contain ±80 A filaments which resemble actin filaments in smooth muscle. Studies were undertaken on human and bovine platelets to better define the morphologic structures which may subserve this contractile function. In order to identify actin, the ability of the filaments to react with heavy meromyosin (HMM) was tested. Accordingly, platelets were glycerinated and treated with HMM. In addition, platelet actin was extracted, reacted with HMM, and examined by negative staining. In both instances typical arrowhead structures with clearly defined polarity and a periodicity of ±360 A formed. As is the case with purified muscle actin, the complexes were dissociable with Mg-ATP. The formation of myosin-like filaments was observed when osmotically shocked platelets were incubated with MgCla and excess ATP. These "thick" filaments measured 250-300 A in width, tapered at both ends and often occurred in clumps. They resembled aggregates of thick filaments described in contracted smooth muscle. Extraction of platelets by methods suitable for the demonstration of myosin showed filaments with an average length of 0.3 /, a smooth shaft, and frayed or bulbous ends. These appeared identical to those seen in synthetically prepared myosin of striated muscle. It is suggested that the filaments described here represent the actin and myosin of platelets.

Instability of the Amyloidogenic Cystatin C Variant of Hereditary Cerebral Hemorrhage with Amyloidosis, Icelandic Type

“…In order to understand the consequences of the amino acid substitution, we studied cell lines stably overexpressing the normal or the variant genes. While a cell of origin for amyloid fibrils is not apparent, it has been proposed that cystatin C in serum and CSF is synthesized and secreted by monocytes and glial cells, respectively (31,32). Furthermore, monocytes/macrophages and glial cells have been implicated in the development of many kinds of amyloidosis (31)(32)(33)(34)(35).…”

“…While a cell of origin for amyloid fibrils is not apparent, it has been proposed that cystatin C in serum and CSF is synthesized and secreted by monocytes and glial cells, respectively (31,32). Furthermore, monocytes/macrophages and glial cells have been implicated in the development of many kinds of amyloidosis (31)(32)(33)(34)(35). Therefore, human astrocytic glioma cell line U251 was used for permanent overexpression of the wild type and the mutated cystatin C. Human kidney 293 cell line was used to test whether the results obtained are cell type-specific.…”

The Tumor-Promoter Phorbol Ester (12-0-Tetradecanoyl-Phorbol-13-Acetate), a Potent Aggregating Agent for Blood Platelets

“…At least some of this protein is localized at the cell surface (40,43,44), and is presumably responsible for the ouabaininsensitive ecto-ATPase activity of platelets (12) . Because actomyosin has been implicated in platelet aggregation, Puszkin and Zucker (45) tested the effect of phorbol ester on actomyosin isolated from platelets and skeletal muscle .…”

Platelet interaction with modified articular cartilage. Its possible relevance to joint repair.

“…To improve contrast of the platelet surface coat and the inter-platelet bridges, ruthenium red at a concentration of 1 mg/ml was added to the fixatives essentially as described by Luft (20). Postfixation was achieved in 2% osmium tetroxide for 2 h. Dehydration and embedding in Epon (Shell Chemical Co., New York) was carried out as in previous reports (21). Thin sections were cut with an LKB ultrotome equipped with a diamond knife (LKB Instruments, Inc., Rockville, Md.…”

“…In general, these studies Received for publication 21 October 1976 and in revised form 7 December 1976. have been aimed at the elucidation of hemostasis or the pathogenesis of atherosclerosis (for review see 4). The possibility that platelets may also serve a crucial function in connective tissue repair has sometimes been raised but has never been pursued in depth.…”