Microtubule-dependent formation of podosomal adhesion structures in primary human macrophages

Podosomes are unique actin-rich adhesion structures of monocyte-derived cel ls such as macrophages and osteoclasts, They clearly differ from other subs tratum-contacting organelles like focal adhesions in morphological and func tional regards. Formation of podosomes has been shown to be dependent on th e small GTPase CDC42Hs and its effector Wiskott-Aldrich syndrome protein (W ASp), In this study, we investigated the functional relation between podoso mes and the microtubule system in primary human macrophages. We demonstrate that, in contrast to focal adhesions, assembly of podosomes in macrophages and their monocytic precursors is dependent on an intact microtubule syste m. In contrast, experiments using Wiskott-Aldrich syndrome (WAS) macrophage s indicate that the microtubule system is not reciprocally dependent on pod osomes, A potential linker between podosomes and microtubules may be WASp i tself, considering that microinjection of the WASp polyproline domain preve nts podosome reassembly. This polyproline domain is thought to link WASp to microtubules via CDC42 interacting protein 4 (CIP4). Consistently, macroph ages microinjected with CIP4 constructs deficient in either the microtubule - or the WASp-binding domain also fail to reassemble podosomes. In sum, our findings show that microtubules are essential for podosome formation in pr imary human macrophages and that WASp and CIP4 may be involved in this phen omenon.