Neuregulin isoforms in dorsal root ganglion neurons: Effects of the cytoplasmic domain on localization and membrane shedding of Nrg‐1 type I

Abstract

Embryonic sensory neurons express membrane‐anchored growth factors that stimulate proliferation and differentiation of Schwann cells. The most important of these are members of the neuregulin‐1 (Nrg‐1) family that activate the erbB2/erbB3 receptor kinase on Schwann cells. Nrg‐1 growth factors display a complex pattern of alternative mRNA splicing. We investigated the expression of the Nrg‐1 type I in rat embryo dorsal root ganglion (DRG) neurons. Nrg‐1 type I mRNA was abundantly expressed in DRG neurons; molecular cloning identified three distinct isoforms. The most prominent structural difference produced by alternative splicing was truncation of the C‐terminal cytoplasmic domain. In sensory neurons and other cells, Nrg‐1 type I proteins with the full‐length 374‐amino‐acid cytoplasmic domain were expressed on the cell surface. In contrast, an isoform with a partially truncated cytoplasmic domain was retained in an intracellular compartment. Deletion studies demonstrated the presence of a cryptic intracellular retention signal that was exposed in the truncated cytoplasmic domain. Cell surface Nrg‐1 type I molecules were subject to protease‐dependent release of the biologically active ectodomain. As a consequence of their intracellular localization, the Nrg‐1 type I isoform with a truncated cytoplasmic domain was not subject to membrane shedding. Nrg‐1 type I ectodomain release was accelerated by factors present in Schwann cell‐conditioned medium. In cells with active Nrg‐1 type I ectodomain, shedding products corresponding to the cytoplasmic domain were not detected, because of rapid γ‐secretase‐ and proteasome‐dependent degradation. These results demonstrate that sensory neurons express alternatively spliced neuregulin polypeptides with distinct subcellular localizations and processing.