A role for SHARPIN in platelet linear protein ubiquitination and function

SHARPIN (Src homology 3 and multiple ankyrin repeat domains protein (SHANK)- associated RH domain-interacting protein) as part of the linear ubiquitin chain assembly complex (LUBAC) catalyses the addition of linear (Met1-linked) ubiquitin chains to substrates. As part of this complex SHARPIN acts as a multi-functional modulator of immune/inflammatory responses through regulation of NfkB activation. In addition, SHARPIN can act as a negative regulator of integrin function. Despite platelets being anucleate cells several studies have determined potential roles for both ubiquitination and NfkB in regulating platelet function. However, little is known about either linear ubiquitination and/or SHARPIN in mouse platelets. In this study, we evaluated platelet function in mice with impaired SHARPIN expression. We confirmed that SHARPIN was expressed in platelets from wild-type mice but not in mice homozygous for SHARPINcpdm allele (cpdm/cpdm) and that this correlated with a reduction in linear ubiquitination. Platelet function in response to thrombin was unaffected. In contrast, CRP-XL-and U46619-mediated platelet responses and thrombus formation under flow on a collagen-coated surface were significantly reduced in the cpdm/cpdm mice. This was associated with impaired U46619-mediated intracellular signalling as well as a reduction in CRP-mediated ERK phosphorylation. Despite the reported role for NfκB in regulating platelet function, inhibiting IκBα phosphorylation did not recapitulate the cpdm/cpdm phenotype. Together, these data indicate that the lack of SHARPIN and linear ubiquitination results in impaired thrombosis and platelet functional responses to CRP and U46619. This phenotype is independent of NfκB pathway inhibition but may involve alternative signalling pathways regulated by linear ubiquitination.