SEMA4D (sema domain, immunoglobulin domain (Ig), transmembrane domain (TM) and short cytoplasmic domain, (semaphorin) 4D)

Note	Sema4D is 862 amino acids with a predicted mass of 96.15 kd. Experimentally, Sema4D runs at about 150 kd on a Western blot.
	Fig. 1: Sema4D is composed of a Sema domain, a Cystine Rich domain, an Immunoglobulin-like domain, a transmembrane segment and a short cytoplasmic tail.
Description	The semaphorins have been shown to exert control over the proliferation and activation of lymphocytes (Hall et al., 1996; Kumanogoh et al., 2001; Wang et al., 2001) (for review: Bismuth et al., 2002), promote tumor growth and metastasis (Christensen et al., 1998) (for review: Kreuter et al., 2002) and regulate development of the lungs (Ito et al., 2000) and the heart and vasculature (Behar et al., 1996; Brown et al., 2001; Feiner et al., 2001; Torres-Vazquez et al., 2004). There are more than 20 known semaphorins grouped into eight classes: classes 1 and 2 are invertebrate semaphorins, classes 3 to 7 are found in vertebrates, and an eighth class, class V, has been identified in some non-neurotropic DNA viruses (for review: Semaphorin Nomenclature Committee, 1999). Sema4D is composed of a Sema domain, a Cystine Rich domain (also called the Plexin Repeat Domain or the Met Related Sequence), an Immunoglobulin-like domain, and a short cytoplasmic tail (Fig. 1). The Sema domain, a seven-bladed beta-propeller similar in topology to integrins (Love et al., 2003), occurs in the semaphorins and their receptors, the plexins, as well as in the hepatocyte growth factor (HGF) receptor family members Met and RON (for review: Gherardi et al., 2004). The Cystine Rich domain has an unknown function but is found in several different receptors. Three copies of this repeat are found in Plexin-B1, the receptor for Sema4D (Tamagnone et al., 1999), while the Met receptor contains one copy. Immunoglobulin domain family members include components of immunoglobulins and cell surface glycoproteins such as the T-cell receptors CD2, CD4, and CD8. The function of the Sema4D intracellular domain is not known, but it has been associated with a serine kinase activity, suggesting that bi-directional signaling may take place (Elhabazi et al., 1997).
Expression	Sema4D is expressed in many tissues including skeletal muscle, blood and bone marrow, lymphoid tissues such as the spleen and thymus, the testes, kidney, small intestine, prostate, heart, placenta, lung, pancreas and the peripheral and central nervous system, as well as in many carcinomas (Basile et al., 2006) and sarcomas (Ch'ng et al., 2007).
Localisation	Sema4D is a transmembrane protein bound to the cell surface, though it is sometimes found in a smaller, secreted form (Elhabazi et al., 2001; Basile et al., 2007b; Zhu et al., 2007).
Function	Sema4D is expressed on the surface of T, B and dendritic cells and modulates their function through either Plexin-B1 or CD72, a lower affinity receptor for Sema4D found in lymphoid tissues. (Kumanogoh et al., 2000) (for review: Moretti et al., 2006). There is evidence that the HIV-1 Tat protein upregulates the expression of Sema4D in immature dendritic cells, an effect that likely facilitates the expansion of HIV-1 infection (Izmailova et al., 2003). Sema4D also induces collapse of axonal growth cones during neural development and remodeling by binding and activating Plexin-B1 (Oinuma et al., 2004), which is why when many of the semaphorins were first characterized they were referred to as 'collapsins'. Sema4D is processed into a slightly smaller form that is shed by some cell types. Elhabazi et al. (2001) observed the release of soluble Sema4D from T lymphocytes upon the cleavage of the membrane bound protein at a cysteine residue located immediately adjacent to the transmembrane domain. Zhu, et al. (2007) have demonstrated that platelets express Sema4D, Plexin-B1, and CD72, and that Sema4D is gradually shed from the surface following platelet activation by ADAM17 (also called tumor-necrosis factor-alpha (TNF-a) converting enzyme, or TACE) in a process that promotes formation of a thrombus. Head and neck squamous cell carcinoma cells secrete a soluble form of Sema4D that promotes tumor-induced angiogenesis, in this case cleaved by the membrane type 1-matrix metalloproteinase (MT1-MMP, also called MMP14) (Basile et al, 2006). Upregulation of the MMPs occurs in cancer cells and has, in fact, been linked to the acquisition of an aggressive, more vascular and more invasive phenotype. Ligation of plexins by semaphorins initiates a signaling cascade that involves the G-protein-mediated pathways. For example, Plexin-A1 and Plexin-B1 are known to act as R-Ras GAPs (GTPase-activating proteins) when bound by their respective semaphorins (Oinuma et al., 2004). There is also data to suggest that Plexin-B1 may compete for Rac binding with PAK (p21-activated kinase) (Vikis et al., 2002). Therefore, in addition to inhibiting Ras signaling through its Ras GAP activity, Plexin-B1 may sequester Rac and inhibit PAK activation. The Rho specific GEFs (guanine nucleotide-exchange factors) LARG (leukemia-associated RhoGEF) and PRG (PDZ-RhoGEF) bind to the PDZ-binding motif at the C-terminus of Plexin-B1 and mediate activation of the small GTPase RhoA, and subsequently its downstream effector Rho Kinase (ROK), in response to Sema4D ligation (Driessens et al., 2001; Aurandt et al., 2002; Hirotani et al., 2002; Perrot et al., 2002; Swiercz et al., 2002; Basile et al., 2004; Basile et al., 2007a). Indeed, Sema4D-Plexin-B1 binding contributes to coordination of epithelial-mesenchymal interactions during organogenesis via modulation of RhoA signaling (Korostylev et al., 2008). Plexin-B1-mediated signaling begins with phosphorylation of a tyrosine residue in the intracellular Sex-Plex domain upon Sema4D binding (for review: Castellani et al., 2002). However, it was not known how Plexin-B1 or its downstream target proteins are phosphorylated, since Plexin-B1 is devoid of intrinsic tyrosine kinase activity. A search for the kinase associated with Plexin-B1 revealed that in MLP29 liver progenitor cells, Plexin-B1 interacted with the extracellular domain of the scatter factor receptor tyrosine kinase c-Met (Giordano et al., 2002). In fact, this Plexin-B1/ c-Met interaction may be responsible for a pro-migratory, angiogenic response observed in Sema4D treated endothelial cells (Conrotto et al., 2005) (Fig. 2A). Sema4D-mediated activation of Plexin-B1 also may promote cell migration by stimulating an intracellular kinase cascade that begins with the recruitment of PDZ RhoGEF and LARG to the C-terminal PDZ binding motif of Plexin-B1. This induces activation of RhoA and ROK and the subsequent phosphorylation and activation of the cytoplasmic tyrosine kinase PYK2, which then phosphorylates Plexin-B1 in the intracellular Sex-Plex domain in a step necessary for a cellular response (Basile et al., 2005) (Fig. 2B). In this model, signaling proceeds through Src, Akt and ERK and results in reorganization of the cytoskeleton (Basile et al., 2005; Aurandt et al., 2006; Basile et al., 2007a)(Fig. 2B). Interestingly, a recent study has shown that inhibition of migration may be elicited by Sema4D under certain conditions where Plexin-B1 preferentially associates with the receptor tyrosine kinase ErbB-2 instead of Met (Swiercz et al., 2008) (Fig. 2C).
	Fig. 2: Binding of Sema4D to Plexin-B1 via their Sema domains stimulates the tyrosine kinase activity of Met (A) or ErbB-2 (C), resulting in tyrosine phosphorylation of Plexin-B1 in the Sex-Plex domain and initiation of a pro- or anti- migratory response, respectively. Sema4D may also activate an intracellular tyrosine kinase cascade via PDZ RhoGEF or LARG, culminating in a RhoA and ROK-dependent activation of the non-receptor tyrosine kinase PYK2 (B). In turn, PYK2 tyrosine-phosphorylates Plexin-B1 and activates Src, Akt and ERK to elicit a pro-migratory response.
Homology	Sema4D exhibits homology with the semaphorins and c-Met and the Met-like protein tyrosine kinase RON, receptors collectively known as the scatter factor receptors (for review: Comoglio et al., 1996). The scatter factor receptors participate in branching morphogenesis, axonal guidance in neuronal tissues, and normal and aberrant proliferation and enhanced cell motility in many different cell types (for review: Vande Woude et al., 1997; Maina et al., 1998).

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