The signals that determine whether axons are ensheathed or myelinated by Schwann cells have long been elusive. We now report that threshold levels of neuregulin-1 (NRG1) type III on axons determine their ensheathment fate. Ensheathed axons express low levels whereas myelinated fibers express high levels of NRG1 type III. Sensory neurons from NRG1 type III deficient mice are poorly ensheathed and fail to myelinate; lentiviral-mediated expression of NRG1 type III rescues these defects. Expression also converts the normally unmyelinated axons of sympathetic neurons to myelination. Nerve fibers of mice haploinsufficient for NRG1 type III are disproportionately unmyelinated, aberrantly ensheathed, and hypomyelinated, with reduced conduction velocities. Type III is the sole NRG1 isoform retained at the axon surface and activates PI 3-kinase, which is required for Schwann cell myelination. These results indicate that levels of NRG1 type III, independent of axon diameter, provide a key instructive signal that determines the ensheathment fate of axons.

No abstract

During peripheral nervous system development, Schwann cells are precisely matched to the axons that they support. This is mediated by axonal neuregulins that are essential for Schwann cell survival and differentiation. Here, we show that sensory and motor axons rapidly release heparin-binding forms of neuregulin in response to Schwann cell-derived neurotrophic factors in a dose-dependent manner. Neuregulin release occurs within minutes, is saturable, and occurs from axons that were isolated using a newly designed chamber slide apparatus. Although NGF and glial cell line-derived neurotrophic factor (GDNF) were the most potent neurotrophic factors to release neuregulin from sensory neurons, GDNF and BDNF were most potent for motor neurons and were the predominant neuregulin-releasing neurotrophic factors produced by cultured Schwann cells. Comparable levels of neuregulin could be released at a similar rate from neurons after protein kinase C activation with the phorbol ester, phorbol 12-myristate 13-acetate, which has also been shown to promote the cleavage and release of neuregulin from its transmembrane precursor. The rapid release of neuregulin from axons in response to Schwann cell-derived neurotrophic factors may be part of a spatially restricted system of communication at the axoglial interface important for proper peripheral nerve development, function, and repair.

Multiple mechanisms are likely to account for the link between obesity and increased risk of post-menopausal breast cancer. Two adipokines, leptin and adiponectin, are of particular interest due to their opposing biological functions and associations with breast cancer risk. In the current study, we investigated the effects of leptin and adiponectin on normal breast epithelial stem cells. Levels of leptin in human adipose explant-derived conditioned media positively correlated with the size of the normal breast stem cell pool. By contrast, an inverse relationship was found for adiponectin. Moreover, a strong linear relationship was observed between the leptin/adiponectin ratio in adipose conditioned media and breast stem cell self-renewal. Consistent with these findings, exogenous leptin stimulated whereas adiponectin suppressed breast stem cell self-renewal. In addition to local in-breast effects, circulating factors including leptin and adiponectin may contribute to the link between obesity and breast cancer. Increased levels of leptin and reduced amounts of adiponectin were found in serum from obese compared to age-matched lean post-menopausal women. Interestingly, serum from obese women increased stem cell self-renewal by 30% compared to only 7% for lean control serum. Taken together, these data suggest a plausible explanation for the obesity-driven increase in post-menopausal breast cancer risk. Leptin and adiponectin may function as both endocrine and paracrine/juxtacrine factors to modulate the size of the normal stem cell pool. Interventions that disrupt this axis and thereby normalize breast stem cell self-renewal could reduce the risk of breast cancer.

Proper, graded communication between different cell types is essential for normal development and function. In the nervous system, heart, and for some cancer cells, part of this communication requires signaling by soluble and membrane-bound factors produced by the NRG1 gene. We have previously shown that glial-derived neurotrophic factors activate a rapid, localized release of soluble neuregulin from neuronal axons that can, in turn promote proper axoglial development (Esper, R. M., and Loeb, J. A. (2004) J. Neurosci. 24, 6218 -6227). Here we elucidate the mechanism of this localized, regulated release by implicating the delta isoform of protein kinase C (PKC). Blocking the PKC delta isoform with either rottlerin, a selective antagonist, or small interference RNA blocks the regulated release of neuregulin from both transfected cells and primary neuronal cultures. PKC activation also leads to the rapid phosphorylation of the pro-NRG1 cytoplasmic tail on serine residues adjacent to the membrane-spanning segment, that, when mutated markedly reduce the rate of NRG1 activity release. These findings implicate this specific PKC isoform as an important factor for the cleavage and neurotrophin-regulated release of soluble NRG1 forms that have important effects in nervous system development and disease.The neuregulins (NRGs) 2 are a family of growth and differentiation factors with a broad range of functions during development and in the adult. NRGs are necessary for glial and cardiac development and participate in a wide range of biologic processes ranging from proper formation of peripheral nerves and the neuromuscular junction to tumor growth (2-9). The NRGs have also been implicated as both potential mediators and therapeutic targets for a number of human diseases including cancer, schizophrenia, and multiple sclerosis (10 -12). NRGs function as mediators of cell-to-cell communication through a multitude of alternatively spliced isoforms arising from at least four distinct genes that bind to and activate members of the epidermal growth factor receptor family HER-2/3/4 (ErbB-2/3/4) (13)(14)(15)(16)(17)(18)(19).Although all known isoforms of the NRG1 gene have an epidermal growth factor-like domain sufficient to bind to and activate its receptors (20), products of this gene are divided into three classes based on structurally and functionally different N-terminal regions (21) The type I and II forms have a unique N-terminal, heparin-binding Ig-like domain (22)(23)(24)(25)(26). This Iglike domain potentiates the biological activities of soluble NRG1 forms and leads to their highly selective tissue distributions through its affinity for specific cell-surface heparan sulfates (12,20,27,28). These forms are first expressed as transmembrane precursors (pro-NRG1) that undergo proteolytic cleavage to release their soluble ectodomains. The type III NRG1 forms, on the other hand, are not typically released from cells, because their N-terminal domain consists of a cysteinerich domain that can serve as a membrane tether making t...