A novel role for the non-catalytic intracellular domain of Neprilysins in muscle physiology.

Neprilysins (Neps) are membrane-bound M13 endopeptidases responsible for the activation and/or inactivation of peptide signalling events on cell surfaces. By hydrolysing their respective substrates, mammalian Neps are crucial to the metabolism of numerous bioactive peptides, especially in the nervous, immune, cardiovascular and inflammatory systems. On the basis of their involvement in essential physiological processes, proteins of the Nep family constitute putative therapeutic agents as well as targets in different diseases, including Alzheimer's disease. We here demonstrate that overexpression of Neprilysin 4 (Nep4) in Drosophila melanogaster leads to a severe muscle degeneration phenotype. This phenotype is observed for overexpression of full-length Nep4 in somatic muscles and is accompanied by severely impaired movement of larvae and lethality in late larval development. On the contrary, down-regulation of expression caused only the latter two effects. By expressing several mutated and truncated forms of Nep4 in transgenic animals, we show that the intracellular domain is responsible for the observed phenotypes while catalytic activity of the enzyme was apparently dispensable. A yeast two-hybrid screen identified a yet uncharacterised carbohydrate kinase as a first interaction partner of the intracellular domain of Nep4. These data demonstrate that the physiological significance of Nep4 is not limited to its function as an active peptidase but that the enzyme's intracellular N-terminus is affecting muscle integrity, independent of the protein's enzymatic activity. To our knowledge, this is the first report of an intracellular Nep domain being involved in muscle integrity.