Neuregulin 1 (NRG1) plays a critical role in myelination. However, little is known about regulatory mechanisms of NRG1 signaling. We show here that Erbin, a protein that contains leucine-rich repeats (LRR) and a PSD95-Dlg-Zol (PDZ) domain and that interacts specifically with ErbB2, is necessary for NRG1 signaling and myelination of peripheral nervous system (PNS). In Erbin null mice, myelinated axons were hypomyelinated with reduced expression of P0, a marker of mature myelinating Schwann cells (SCs), whereas unmyelinated axons were aberrantly ensheathed in Remak bundles, with increased numbers of axons in the bundles and in pockets. The morphological deficits were associated with decreased nerve conduction velocity and increased sensory threshold to mechanistic stimulation. These phenotypes were duplicated in erbin ⌬C/⌬C mice, in which Erbin lost the PDZ domain to interact with ErbB2. Moreover, ErbB2 was reduced at protein levels in both Erbin mutant sciatic nerves, and ErbB2 became unstable and NRG1 signaling compromised when Erbin expression was suppressed. These observations indicate a critical role of Erbin in myelination and identify a regulatory mechanism of NRG1 signaling. Our results suggest that Erbin, via the PDZ domain, binds to and stabilizes ErbB2, which is necessary for NRG1 signaling that has been implicated in tumorigenesis, heart development, and neural function.ErbB2 ͉ PDZ ͉ protein stability ͉ Schwann cells ͉ Akt M yelin sheath wraps axons to ensure the velocity and timing of action potential propagation and insulates neuronal activity. Impaired myelin formation and maintenance have been implicated in various neurological and psychiatric disorders including schizophrenia, multiple sclerosis, and Charcot-Marie-Tooth neuropathy disease (1, 2). Myelination is a tightly controlled, complex process. In the peripheral nervous system (PNS), neuregulin 1 (NRG1) has emerged as a key axon-derived factor that regulates myelination. Disruption of NRG1 signaling by ablating either the EGF domain that is contained in all isoforms or type III isoform leads to an almost complete loss of SCs and of the sensory and motor neurons that they support (3, 4). Recent studies have suggested that the level of NRG1 in the axon is critical for myelination (5, 6).NRG1 acts by stimulating a family of single transmembrane receptor tyrosine kinases called ErbB proteins (1). Although NRG1 was isolated as a ''ligand'' to activate ErbB2, it does not interact with the receptor (7). However, ErbB3 can bind NRG1 but its homodimers are catalytically inactive, indicating impaired kinase function (8). Therefore, ErbB2 and ErbB3, major ErbB proteins in SCs, need to form heterodimers with each other to be functional (9). This notion is supported by mouse genetic studies that mutation of NRG1, ErbB2, or ErbB3 genes causes severe deficits of peripheral neurons and SCs (3, 4, 10-13). Disruption of NRG1/ErbB signaling by a dominant negative approach led to deficits in myelinating and nonmyelinating SCs (14,15). Intracellularly, NRG1 stimu...