Molecular and Cellular Biology, March 2001, p. 2213-2220, Vol. 21, No. 6
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.6.2213-2220.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Section of Hematology-Oncology Research, Departments of Medicine, Anatomy, and Cellular Biology, St. Elizabeth's Medical Center, Tufts University School of Medicine, Boston, Massachusetts 02135,1 and Division of Hematology-Oncology, New England Medical Center, Departments of Medicine and Biochemistry, Tufts University School of Medicine, Boston, Massachusetts 021112
Received 15 September 2000/Returned for modification 8 November 2000/Accepted 27 December 2000
Conventional calpains are ubiquitous calcium-regulated cysteine
proteases that have been implicated in cytoskeletal organization, cell
proliferation, apoptosis, cell motility, and hemostasis. There are two
forms of conventional calpains: the µ-calpain, or calpain I, which
requires micromolar calcium for half-maximal activation, and the
m-calpain, or calpain II, which functions at millimolar calcium
concentrations. We evaluated the functional role of the 80-kDa
catalytic subunit of µ-calpain by genetic inactivation using
homologous recombination in embryonic stem cells. The
µ-calpain-deficient mice are viable and fertile. The complete
deficiency of µ-calpain causes significant reduction in platelet
aggregation and clot retraction but surprisingly the mutant mice
display normal bleeding times. No detectable differences were observed
in the cleavage pattern and kinetics of calpain substrates such as the
3 subunit of
IIb
3 integrin, talin, and ABP-280 (filamin).
However, µ-calpain null platelets exhibit impaired tyrosine
phosphorylation of several proteins including the
3 subunit of
IIb
3 integrin, correlating with the agonist-induced reduction in
platelet aggregation. These results provide the first direct evidence
that µ-calpain is essential for normal platelet function, not by
affecting the cleavage of cytoskeletal proteins but by potentially
regulating the state of tyrosine phosphorylation of the platelet proteins.
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