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The Wiskott-Aldrich Syndrome: the Actin Cytoskeleton and Immune Cell Function

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The Wiskott-Aldrich Syndrome: the Actin Cytoskeleton and Immune Cell Function Disease Markers 29 (2010) 157–175 157 DOI 10.3233/DMA-2010-0735 IOS Press The Wiskott-Aldrich syndrome: The actin cytoskeleton and immune cell function Michael P. Blundella, Austen Wortha,b, Gerben Boumaa and Adrian J. Thrashera,b,∗ aMolecular Immunology Unit, UCL Institute of Child Health, London, UK bDepartment of Immunology, Great Ormond Street Hospital NHS Trust, Great Ormond Street, London, UK Abstract. Wiskott-Aldrich syndrome (WAS) is a rare X-linked recessive primary immunodeficiency characterised by immune dysregulation, microthrombocytopaenia, eczema and lymphoid malignancies. Mutations in the WAS gene can lead to distinct syndrome variations which largely, although not exclusively, depend upon the mutation. Premature termination and deletions abrogate Wiskott-Aldrich syndrome protein (WASp) expression and lead to severe disease (WAS). Missense mutations usually result in reduced protein expression and the phenotypically milder X-linked thrombocytopenia (XLT) or attenuated WAS [1–3]. More recently however novel activating mutations have been described that give rise to X-linked neutropenia (XLN), a third syndrome defined by neutropenia with variable myelodysplasia [4–6]. WASP is key in transducing signals from the cell surface to the actin cytoskeleton, and a lack of WASp results in cytoskeletal defects that compromise multiple aspects of normal cellular activity including proliferation, phagocytosis, immune synapse formation, adhesion and directed migration. Keywords: Wiskott-Aldrich syndrome, actin polymerization, lymphocytes, dendritic cells, migration, cell activation, immune cell function Abbreviations SNX9: Sorting nexin 9 TCR: T cell receptor Toca-1: Transducer of Cdc42-dependent actin assembly-1 Treg: Regulatory T cell AGM: Aorto-gonad-mesonephros VCA: Verprolin homology, central and acidic regions ARPC: aActin related protein complex component WAS: Wiskott-Aldrich syndrome CRIB: Cdc42/Rac interactive binding WAS KO: WAS knockout DC: Dendritic cells WASH: WASp and SCAR homolog EVH1: Ena-VASP homology domain 1 WASp: WAS protein FRET: Fluorescence Resonance Energy Transfer WAVE: WASp family verprolin-homologous protein GBD: GTPase binding domain WHAMM: WASP homolog associated with actin, membranes, GEF: Guanine exchange factor and microtubules IS: Immune synapse WIP: WASp interacting protein MZB: Marginal zone B cells XLN: X-linked neutropenia N-WASp: Neuronal WASp XLT: X- linked thrombocytopenia PH: Pleckstrin homology PIP2: Phosphatidylinositol (4,5)-bisphosphate PLCγ: Phospholipase C gamma 1. Introduction PSTPIP: Proline, serine threonine-rich phosphatase interacting protein Wiskott-Aldrich syndrome (WAS) is a rare X- linked recessive primary immunodeficiency charac- terised by immune dysregulation, microthrombocy- ∗ topaenia, eczema and lymphoid malignancies. Mu- Corresponding author: Adrian J. Thrasher, UCL Institute of tations in the WAS gene can lead to distinct syn- Child Health, Molecular Immunology Unit, 30 Guilford Street, Lon- don, WC1N 1EH, UK. Tel.: +44 (0)20 7905 2289; Fax: +44 (0)20 drome variations which largely, although not exclu- 7905 2810; E-mail: [email protected]. sively, depend upon the mutation. Premature termina- ISSN 0278-0240/10/$27.50 2010 – IOS Press and the authors. All rights reserved 158 M.P. Blundell et al. / The Wiskott-Aldrich syndrome: The actin cytoskeleton and immune cell function tion and deletions abrogate Wiskott-Aldrich syndrome protein (WASp) expression and lead to severe disease (WAS). Missense mutations usually result in reduced protein expression and the phenotypically milder X- linked thrombocytopenia(XLT) or attenuated WAS [1– 3]. More recently however novel activating mutations havebeen describedthat giverise to X-linkedneutrope- nia (XLN), a third syndrome defined by neutropenia Fig. 1. Domain structure of WASp family protein members. with variable myelodysplasia [4–6]. WASP is key in Schematic representation of the protein domains of individual mem- transducing signals from the cell surface to the actin bers of the WASp family. The C terminus, which is critical for regulation of actin polymerization, is highly conserved between the cytoskeleton, and a lack of WASp results in cytoskele- members, while the N terminus is divergent. EVH1, Ena Vasp ho- tal defects that compromise multiple aspects of normal mology domain; B, basic domain; GBD, GTPase binding domain; cellular activity including proliferation, phagocytosis, PPP, polyproline domain; V, verprolin homology domain; C, cen- tral domain; A, acidic domain; SH, Scar homology domain; N, immune synapse formation, adhesion and directed mi- N-terminal domain with insignificant homology to known protein; gration. CC, coiled-coils domain; WHD1/2, WASH homology domain 1 and 2. 3. EVH1 and WIP 2. Molecular structure The EVH1 domain is a proline-rich region that has been implicated in actin remodelling. This domain is WASp is a multidomain protein that lacks intrin- found in actin scaffold proteins, such as Mena, VASP sic catalytic activity. It acts as an adaptor to bring and Evl. The best characterised EVH1 domain binding together downstream mediators that facilitate Arp2/3- partners are members of the verprolin family of pro- mediated actin polymerization. WASp consists of an teins. These include WASp-interacting protein (WIP), N-terminal Ena-VASP homology domain 1 (EVH1), CR16 and WICH (WIP and CR16 homologousprotein, a basic domain, a GTPase binding domain (GBD), also referred to as WIRE) [12]. Of these, WIP is prob- polyproline domain and the C-terminal domain com- ably the best characterized. It is a 503-amino acid long prising of a cluster of verprolin homology (V), cen- proline-rich protein, which is expressed in many tissues tral (C) and acidic regions (A) (the VCA domain) although most abundantly in haematopoietic cells [13]. (Fig. 1). WASp was the first member of a family of While CR16 is known to be able to bind the WASp proteins that shares the ability to regulate Arp2/3 activ- EVH1 domain, it is not expressed in haematopoietic ity through their VCA domains. The other mammalian cells; therefore, CR16/WASp interactions are not likely members (Fig. 1) that share expression of the VCA to be physiologically relevant [14]. WICH shows some domain include neural WASp (N-WASp), WASp fami- ability to bind WASp, but this binding is much weaker ly verprolin-homologous protein (WAVE) 1–3 and the compared to N-WASp binding. WICH is expressed at more recently discovered Wiskott-Aldrich syndrome much lower levels in haematopoietic tissues compared protein and SCAR homolog (WASH) and WASP ho- to brain and gastrointestinal tissues, suggesting that N- WASp is the preferred binding partner to WICH [15]. molog associated with actin, membranes, and micro- Amino acids 416–488 of WIP were initially iden- tubules (WHAMM) [7–9]. Many studies have investi- tified as being required for WASp binding [16]. The gated isolated protein domains to gain insight into the crystal structure of WIP bound to the EVH1 domain of function of WASp family members and have extrap- N-WASp shows that a 25 amino acid fragment(aa 461– olated findings of one member to predict function of 485) is minimally required for high affinity N-WASp another. In particular, studies of N-WASp have been binding [17]. Later the same group showed that for used to predict function of WASp. The overall pro- optimal binding, a larger sequence of WIP (aa 451– tein sequence identity between WASp and N-WASp is 485)wraps aroundthe EVH1 domainof N-WASp; they around 50%, although sequence of the functional do- also identified three WIP epitopes that are involved in main is much higher. Importantly, however, expression WIP/N-WASp binding [18]. Many studies have used of WASp is restricted to haematopoietic cells while N-WASp to study the interaction of WASp with WIP. N-WASp is ubiquitously expressed [10,11]. It is likely that these interactions are similar, but that M.P. Blundell et al. / The Wiskott-Aldrich syndrome: The actin cytoskeleton and immune cell function 159 the WIP/WASp interaction is more relevant in vivo giv- bility of vaccinia virus [37]. These observations sug- en the similar tissue expression profiles. WIP inhibits gest that by binding to WASp, WIP prevents degrada- N-WASp-mediated activation of Arp2/3 by direct in- tion of WASp, keeps it in an inactive state and helps to teraction with N-WASp [19], and WIP maintains the achieve correct subcellular localization, thus providing autoinhibitory conformation of WASP in vivo [20]. A an important spatial and temporal regulation of WASp similar inhibitory mechanism exists for N-WASp, in activity. which the WIP/N-WASp complex cannot initiate actin polymerization whereas free N-WASp can induce actin polymerization [21]. Toca-1 (transducer of Cdc42- 4. Basic domain and PIP2 dependent actin assembly) could release WIP binding, enabling Cdc42 to activate the protein and initiate actin The basic domain is found in all members of the polymerization [21]. WASp family, although the specific sequence is poor- Compelling evidence that WIP binding is important ly conserved. In WASp, the basic domain consists of for WASp expression was provided by several groups 10 amino acids, of which six are lysine residues. The who have shown that when WIP is absent, no or very basic domain of N-WASp binds phosphatidylinositol low levels of WASp expression can be detected [22– (4,5)-bisphosphate (PIP2) in vitro [38,39]. WASp can 24]. Furthermore, reduced levels of WASp expression also bind PIP2, but the specific binding site remains in lymphocytes
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