| Abstract | The neuregulins (NRGs) are cell-cell signaling proteins that are ligands for receptor tyrosine kinases of the ErbB family. The neuregulin family of genes has four members: NRG1, NRG2, NRG3, and NRG4. Relatively little is known about the biological functions of the NRG2, 3, and 4 proteins, and they are considered in this review only briefly. The NRG1 proteins play essential roles in the nervous system, heart, and breast. There is also evidence for involvement of NRG signaling in the development and function of several other organ systems, and in human disease, including the pathogenesis of schizophrenia and breast cancer. There are many NRG1 isoforms, raising the question "Why so many neuregulins?" Study of mice with targeted mutations ("knockout mice") has demonstrated that isoforms differing in their N-terminal region or in their epidermal growth factor (EGF)-like domain differ in their in vivo functions. These differences in function might arise because of differences in expression pattern or might reflect differences in intrinsic biological characteristics. While differences in expression pattern certainly contribute to the observed differences in in vivo functions, there are also marked differences in intrinsic characteristics that may tailor isoforms for specific signaling requirements, a theme that will be emphasized in this review. |
| Abstract | Neuregulins (NRGs) comprise a large family of EGF-like signaling molecules involved in cell-cell communication during development and disease. The neuregulin family of ligands has four members: NRG1, NRG2, NRG3, and NRG4. Relatively little is known about the biological functions of the NRG2, 3, and 4 proteins. In contrast, the NRG1 proteins have been demonstrated to play important roles during the development of the nervous system, heart, and mammary glands. For example, NRG1 has essential functions in the development of neural crest cells and some of their major derivatives, like Schwann cells and sympathetic neurons. NRG1 controls the trabeculation of the myocardial musculature and the ductal differentiation of the mammary epithelium. Moreover, there is emerging evidence for the involvement of NRG signals in the development and function of several other organ systems, and in human disease, including breast cancer and schizophrenia. Many different isoforms of the Neuregulin-1 gene are synthesized. Such isoforms differ in their tissue-specific expression patterns and their biological activities, thereby contributing to the great diversity of the in vivo functions of NRG1. Neuregulins transmit their signals to target cells by interacting with transmembrane tyrosine kinase receptors of the ErbB family. This family includes four members, the epidermal growth factor receptor (EGF-R, ErbB1, ErbB2, ErbB3, and ErbB4). Receptor-ligand interaction induces the heterodimerization of receptor monomers, which in turn results in the activation of intracellular signaling cascades and the induction of cellular responses including proliferation, migration, differentiation, and survival or apoptosis. In vivo, functional NRG1 receptors are heterodimers composed of ErbB2 with either an ErbB3, or ErbB4 molecule. The tissue-specific distribution of the different receptor types further contributes to the diversity and specificity of the biological functions of this signaling pathway. It is a typical feature of the Neuregulin-1/ErbB signaling pathway to control sequential steps during the development of a particular organ system. For example, this pathway functions in early precursor proliferation, maturation, as well as in the myelination of Schwann cells. The systematic analysis of genetic models that have been established by the help of conventional as well as conditional gene targeting strategies in mice was instrumental for the uncovering of the multitude of biological functions of this signaling system. In this review the basic biology of the Neuregulin-1/ErbB system and how it relates to the in vivo functions were discussed with special emphasis to transgenic techniques in mice. |