The phagocytic integrins and complement receptors M 2/CR3 and X 2/CR4 are classically
associated with the phagocytosis of iC3b-opsonized particles. The activation of this receptor is
dependent on signals derived from other receptors (inside-out signaling) with the crucial involvement
of the Rap1-RIAM-Talin-1 pathway. Here, we analyze the implication of RIAM and its binding
partner VASP in the signaling events occurring downstream of 2 integrins (outside-in) during
complement-mediated phagocytosis. To this end, we used HL-60 promyelocytic cell lines deficient in
RIAM or VASP or overexpressing EGFP-tagged VASP to determine VASP dynamics at phagocytic
cups. Our results indicate that RIAM-deficient HL-60 cells presented impaired particle internalization
and altered integrin downstream signaling during complement-dependent phagocytosis. Similarly,
VASP deficiency completely blocked phagocytosis, while VASP overexpression increased the random
movement of phagocytic particles at the cell surface, with reduced internalization. Moreover,
the recruitment of VASP to particle contact sites, amount of pSer157-VASP and formation of actin-rich
phagocytic cups were dependent on RIAM expression. Our results suggested that RIAM worked as a
relay for integrin complement receptors in outside-in signaling, coordinating integrin activation and
cytoskeletal rearrangements via its interaction with VASPThis work has been supported by Ministerio Español de Economía y Competitividad (MINECO)
grants: SAF2016-77096-R (ELD and CC), BIO2014:54164-R (PAR), BIO2017-86500-R (MY-M), EI/MICIU EXPLORA
BIO2017-91272-EXP & FEDER (PI17/02303) (S.R.-P). AT-G is supported by a predoctoral fellowship from MINECO,
JLS-T is supported by a predoctoral fellowship from Universidad Complutense de Madrid, RT-R is supported by a
postdoctoral fellowship from Asociación Española Contra el Cáncer (AECC
