Draper-dependent glial phagocytic activity is mediated by Src and Syk family kinase signalling

Abstract

The cellular machinery promoting phagocytosis of corpses of apoptotic cells is well conserved from worms to mammals. An important component is the Caenorhabditis elegans engulfment receptor CED-1 (ref. 1) and its Drosophila orthologue, Draper 2 . The CED-1/Draper signalling pathway is also essential for the phagocytosis of other types of 'modified self' including necrotic cells 3 , developmentally pruned axons 4,5 and dendrites 6 , and axons undergoing Wallerian degeneration 7 . Here we show that Drosophila Shark, a non-receptor tyrosine kinase similar to mammalian Syk and Zap-70, binds Draper through an immunoreceptor tyrosine-based activation motif (ITAM) in the Draper intracellular domain. We show that Shark activity is essential for Draper-mediated signalling events in vivo, including the recruitment of glial membranes to severed axons and the phagocytosis of axonal debris and neuronal cell corpses by glia. We also show that the Src family kinase (SFK) Src42A can markedly increase Draper phosphorylation and is essential for glial phagocytic activity. We propose that ligand-dependent Draper receptor activation initiates the Src42A-dependent tyrosine phosphorylation of Draper, the association of Shark and the activation of the Draper pathway. These Draper-Src42A-Shark interactions are strikingly similar to mammalian immunoreceptor-SFK-Syk signalling events in mammalian myeloid and lymphoid cells 8,9 . Thus, Draper seems to be an ancient immunoreceptor with an extracellular domain tuned to modified self, and an intracellular domain promoting phagocytosis through an ITAM-domain-SFK-Syk-mediated signalling cascade.Developing tissues produce excessive numbers of cells and selectively destroy a subpopulation through programmed cell death to regulate growth. Rapid clearance of cell corpses is essential for maintaining tissue homeostasis and preventing the release of potentially cytotoxic or antigenic molecules from dying cells, and defects in cell corpse clearance are closely associated with autoimmune and inflammatory diseases 10-13 . In C. elegans the CED-1 receptor is expressed in engulfing cells, where it acts to recognize cell corpses and drive their phagocytosis 1 . CED-1 promotes engulfment through an intracellular NPXY motif, a binding site for proteins containing a phosphotyrosine-binding (PTB) domain, and a YXXL motif, a potential interaction site for proteins containing SH2 domains 1 . The PTB domain adaptor protein CED-6 can bind the NPXY motif of , is required for cell corpse Glia are the primary phagocytic cell type in the developing and mature brain. Glia rapidly engulf neuronal cell corpses produced during development, as well as neuronal debris generated during axon pruning 19,20 or during Wallerian degeneration in the adult brain 21 . In Drosophila, glial phagocytosis of these engulfment targets requires Draper, the fly orthologue of CED-1 (refs 2 , 4-6 ). Draper, like CED-1, contains 15 extracellular atypical epidermal growth factor (EGF) repeats, a single transmembrane domain, and NPXY an...

Retinal Axon Guidance Requires Integration of Eya and the Jak/Stat Pathway into Phosphotyrosine-Based Signaling Circuitries in Drosophila

“…The tyrosine kinases from our short list of candidates also have potential links to immunity. shark and Src42A are important for Draper-mediated glial phagocytosis of axonal debris (Ziegenfuss et al 2008) which, if left uncleared, stimulates an immune response, and Src64B overexpression has been shown to be sufficient for inducing an immune response in larva (Williams 2009). Even if Eya's threonine phosphatase activity is not involved, extending exploration of the interactions we have uncovered in the context of axon guidance to other biological contexts in which the candidate interacting partner is known to function may provide new insight into the cytoplasmic signaling networks in which Eya participates during development and disease.…”

Alleviation of Neurotoxicity by Microglial Human Siglec-11

“…However, we also showed that Siglec-11-expressing microglia have impaired capacity to phagocytose apoptotic neuronal material. Thus Siglec-11 signaling via ITIM might antagonize the phagocytosis-associated ITAM-Syk signaling pathways (Ziegenfuss et al, 2008). Consequently, higher neurite and neuronal cell body density in the cocultures with Siglec-11-expressing microglia might result either from less phagocytosis or reduced production of microglial proinflammatory mediators.…”

The c-Jun kinase signaling cascade promotes glial engulfment activity through activation of draper and phagocytic function

“…Src42a phosphorylates an intracellular immunoreceptor tyrosinebased activation motif (ITAM) domain on Draper, the nonreceptor tyrosine kinase Shark then binds to the Draper ITAM, and engulfment is activated. 16 The phosphotyrosine-binding domain-containing protein dCed-6 also appears to signal downstream of the Draper receptor, 17 but additional signaling molecules that promote engulfment activity in glia remain poorly defined. Here we show that the Drosophila c-Jun amino-terminal kinase (dJNK) plays a critical role in glial responses to degenerating axonal debris.…”