Microbial Challenge Promotes the Regenerative Process of the Injured Central Nervous System of the Medicinal Leech by Inducing the Synthesis of Antimicrobial Peptides in Neurons and Microglia

Abstract

Following trauma, the CNS of the medicinal leech, unlike the mammalian CNS, has a strong capacity to regenerate neurites and synaptic connections that restore normal function. In this study, we show that this regenerative process is enhanced by a controlled bacterial infection, suggesting that induction of regeneration of normal CNS function may depend critically upon the coinitiation of an immune response. We explore the interaction between the activation of a neuroimmune response and the process of regeneration by assaying the potential roles of two newly characterized antimicrobial peptides. Our data provide evidence that microbial components differentially induce the transcription, by microglial cells, of both antimicrobial peptide genes, the products of which accumulate rapidly at sites in the CNS undergoing regeneration following axotomy. Using a preparation of leech CNS depleted of microglial cells, we also demonstrate the production of antimicrobial peptides by neurons. Interestingly, in addition to exerting antibacterial properties, both peptides act as promoters of the regenerative process of axotomized leech CNS. These data are the first to report the neuronal synthesis of antimicrobial peptides and their participation in the immune response and the regeneration of the CNS. Thus, the leech CNS appears as an excellent model for studying the implication of immune molecules in neural repair.

Macin Family of Antimicrobial Proteins Combines Antimicrobial and Nerve Repair Activities

“…As the macins share high sequence identity including eight conserved cysteine residues, adoption of the same fold is likely. Nonetheless, both theromacin and neuromacin appear to act mainly against Gram-positive bacteria as no activity has been found against Gram-negative bacteria, albeit only a few strains have been tested so far (8,11). Whereas the antimicrobial activity of neuromacin appears to involve membrane permeabilization (8), the antimicrobial mechanism of theromacin remains to be determined.…”

“…Nonetheless, both theromacin and neuromacin appear to act mainly against Gram-positive bacteria as no activity has been found against Gram-negative bacteria, albeit only a few strains have been tested so far (8,11). Whereas the antimicrobial activity of neuromacin appears to involve membrane permeabilization (8), the antimicrobial mechanism of theromacin remains to be determined. Interestingly, neuromacin apparently is involved in the repair process of the central nervous system of the medicinal leech that is mediated by migration of microglia to the site of lesion and a probable cytoskeletondriven axon outgrowth (8).…”

“…All PCR products were subcloned into pGEM-T easy vector (Promega), and several different cDNA clones were sequenced (GenBank TM accession number EU149766). NM cDNA was characterized in a previous study (GenBank accession number EU156754) (8).…”

“…Whereas the antimicrobial activity of neuromacin appears to involve membrane permeabilization (8), the antimicrobial mechanism of theromacin remains to be determined. Interestingly, neuromacin apparently is involved in the repair process of the central nervous system of the medicinal leech that is mediated by migration of microglia to the site of lesion and a probable cytoskeletondriven axon outgrowth (8). However, the nerve repair mechanism of neuromacin remains elusive, and the capacity of theromacin and hydramacin-1 to promote the same is not known.…”

“…Besides being antimicrobially active, several AMPs possess further biological activities. Among others, they were found to be mediators of the adaptive immune system, participate in wound healing and angiogenesis, provide chemoattraction, modulate inflammation responses, and promote nerve repair (1,8). To date, 1923 AMPs are deposited in the antimicrobial database (9), but for almost 80%, the tertiary structures, which are essential to better understand the molecular mechanisms of AMPs, are unknown.…”