Interaction of Hm C1q with leech microglial cells: involvement of C1qBP-related molecule in the induction of cell chemotaxis

Background In invertebrates, the medicinal leech is considered to be an interesting and appropriate model to study neuroimmune mechanisms. Indeed, this non-vertebrate animal can restore normal function of its central nervous system (CNS) after injury. Microglia accumulation at the damage site has been shown to be required for axon sprouting and for efficient regeneration. We characterized Hm C1q as a novel chemotactic factor for leech microglial cell recruitment. In mammals, a C1q-binding protein (C1qBP alias gC1qR), which interacts with the globular head of C1q, has been reported to participate in C1q-mediated chemotaxis of blood immune cells. In this study, we evaluated the chemotactic activities of a recombinant form of Hm C1q and its interaction with a newly characterized leech C1qBP that acts as its potential ligand. Methods Recombinant Hm C1q (r Hm C1q) was produced in the yeast Pichia pastoris . Chemotaxis assays were performed to investigate r Hm C1q-dependent microglia migration. The involvement of a C1qBP-related molecule in this chemotaxis mechanism was assessed by flow cytometry and with affinity purification experiments. The cellular localization of C1qBP mRNA and protein in leech was investigated using immunohistochemistry and in situ hybridization techniques. Results r Hm C1q-stimulated microglia migrate in a dose-dependent manner. This r Hm C1q-induced chemotaxis was reduced when cells were preincubated with either anti- Hm C1q or anti-human C1qBP antibodies. A C1qBP-related molecule was characterized in leech microglia. Conclusions A previous study showed that recruitment of microglia is observed after Hm C1q release at the cut end of axons. Here, we demonstrate that r Hm C1q-dependent chemotaxis might be driven via a Hm C1q-binding protein located on the microglial cell surface. Taken together, these results highlight the importance of the interaction between C1q and C1qBP in microglial activation leading to nerve repair in the medicinal leech.