Formin proteins in megakaryocytes and platelets: regulation of actin and microtubule dynamics

Abstract

The platelet and megakaryocyte cytoskeletons are essential for formation and function of these cells. A dynamic, properly organised tubulin and actin cytoskeleton is critical for the development of the megakaryocyte and the extension of proplatelets. Tubulin in particular plays a pivotal role in the extension of these proplatelets and the release of platelets from them. Tubulin is further required for the maintenance of platelet size, and actin is the driving force for shape change, spreading and platelet contraction during platelet activation. Whilst several key proteins which regulate these cytoskeletons have been described in detail, the formin family of proteins has received less attention. Formins are intriguing as, although they were initially believed to simply be a nucleator of actin polymerisation, increasing evidence shows they are important regulators of the crosstalk between the actin and microtubule cytoskeletons. In this review, we will introduce the formin proteins and consider the recent evidence that they play an important role in platelets and megakaryocytes in mediating both the actin and tubulin cytoskeletons.

Keywords: Actin; Formin; macrothrombocytopenia; microtubules cytoskeleton; proplatelet formation.

Figure 1.

Generalised schematic of formin domains and structures. a) Typical domains present in members of the formin family and binding sites for proteins which interact with formins. GBD = GTPase binding domain, DID = diaphanous inhibitory domain, DD = dimerisation domain, FH1 = formin homology 1, FH2 = formin homology 2, FH3 = formin homology 3, DAD = diaphanous auto-regulatory domain. b) The homodimer is formed through interactions of the DD and FH2 domains on each half of the dimer with auto-inhibition being achieved by interaction of the DAD with DID domains (blue dashed line). c) The active conformation is achieved following binding of GTP-bound Rho GTPases to the GBD, via binding to dishevelled (Dvl) (DAAM1) or Rho kinase mediated phosphorylation at Thr¹¹⁴¹ (FHOD1). d) Formins nucleate linear actin filaments by binding of dimerised FH2 domains to actin dimers. Elongation of the filament occurs via processive capping; the binding of profilin:ATP G-actin to the FH1 domain allows the formin to facilitate addition of actin monomers to the growing actin filament whilst protecting it from capping proteins and depolymerisation. Figure adapted from (11,68).

Figure 2.

Mechanisms of actin and microtubule regulation and coordination by formins. Formins are proposed to have direct and indirect actions on actin filaments and microtubules. 1. Actin nucleation and elongation by formins occurs by processive capping of linear actin filaments. 2. Formins also bundle actin filaments together and 3 align them with microtubules. 4, 5. Formins can bind to and stabilise microtubules directly or via interaction with capping proteins including EB1. 6. Like for actin filaments, microtubules can also be brought in close proximity to each other by bundling regulated by formins. Figure adapted from (69,70).

Figure 3.

Expression of mammalian formin proteins during megakaryocyte development. a) Relative expression levels of the 15 mammalian formin proteins during the development of human megakaryocytes. CD34+ cells from umbilical cord blood were cultured in vitro in the presence of thrombopoietin and interleukin 1β and gene expression quantified by RNA-seq. Data are taken from the Blueprint epigenome project (http://www.blueprint-epigenome.eu/) and (35). * Note – no data was available for the expression of GRID2IP. b) Relative expression levels of the 15 mammalian formin proteins during the development of mouse megakaryocytes. Bone marrow cells from C57BL/6 mice were sorted using FACS and gene expression quantified by microarrays. Data are taken from the haemosphere database (http://haemosphere.org/) and (36). For both panels, Log2 of normalised gene expression is represented by a heat map where red equals increased gene expression. Gene names in this figure and throughout the review use the conventions of the HUGO Gene Nomenclature Committee (www.genenames.org) and the Mouse Genome Informatics (www.informatics.jax.org) for human and mouse genes, respectively.