Alistair N. Garratt

Conditional mutation of the ErbB2 (HER2) receptor in cardiomyocytes leads to dilated cardiomyopathy

The ErbB2 (Her2) proto-oncogene encodes a receptor tyrosine kinase, which is frequently amplified and overexpressed in human tumors. ErbB2 provides the target for a novel and effective antibody-based therapy (Trastuzumab/Herceptin) used for the treatment of mammary carcinomas. However, cardiomyopathies develop in a proportion of patients treated with Trastuzumab, and the incidence of such complications is increased by combination with standard chemotherapy. Gene ablation studies have previously demonstrated that the ErbB2 receptor, together with its coreceptor ErbB4 and the ligand Neuregulin-1, are essential for normal development of the heart ventricle. We use here Cre-loxP technology to mutate ErbB2 specifically in ventricular cardiomyocytes. Conditional mutant mice develop a severe dilated cardiomyopathy, with signs of cardiac dysfunction generally appearing by the second postnatal month. We infer that signaling from the ErbB2 receptor, which is enriched in T-tubules in cardiomyocytes, is crucial for adult heart function. Conditional ErbB2 mutant mice provide a model of dilated cardiomyopathy. In particular, they will allow a rigorous assessment of the role of ErbB2 in the heart and provide insight into the molecular mechanisms that underlie the adverse effects of anti-ErbB2 antibodies.

Short- and Long-Range Attraction of Cortical GABAergic Interneurons by Neuregulin-1

Most cortical interneurons arise from the subcortical telencephalon, but the molecules that control their migration remain largely unidentified. Here, we show that different isoforms of Neuregulin-1 are expressed in the developing cortex and in the route that migrating interneurons follow toward the cortex, whereas a population of the migrating interneurons express ErbB4, a receptor for Neuregulin-1. The different isoforms of Neuregulin-1 act as short- and long-range attractants for migrating interneurons, and perturbing ErbB4 function in vitro decreases the number of interneurons that tangentially migrate to the cortex. In vivo, loss of Neuregulin-1/ErbB4 signaling causes an alteration in the tangential migration of cortical interneurons and a reduction in the number of GABAergic interneurons in the postnatal cortex. These observations provide evidence that Neuregulin-1 and its ErbB4 receptor directly control neuronal migration in the nervous system.

Wnt signaling in stem and cancer stem cells

The functional versatility of Wnt/β-catenin signaling can be seen by its ability to act in stem cells of the embryo and of the adult as well as in cancer stem cells. During embryogenesis, stem cells demonstrate a requirement for β-catenin in mediating the response to Wnt signaling for their maintenance and transition from a pluripotent state. In adult stem cells, Wnt signaling functions at various hierarchical levels to contribute to specification of different tissues. This has raised the possibility that the tightly regulated self-renewal mediated by Wnt signaling in stem and progenitor cells is subverted in cancer cells to allow malignant progression. Intensive work is currently being performed to resolve how intrinsic and extrinsic factors that regulate Wnt/β-catenin signaling coordinate the stem and cancer stem cell states.

Tangential neuronal migration controls axon guidance: a role for neuregulin-1 in thalamocortical axon navigation.

Neuronal migration and axon guidance constitute fundamental processes in brain development that are generally studied independently. Although both share common mechanisms of cell biology and biochemistry, little is known about their coordinated integration in the formation of neural circuits. Here we show that the development of the thalamocortical projection, one of the most prominent tracts in the mammalian brain, depends on the early tangential migration of a population of neurons derived from the ventral telencephalon. This tangential migration contributes to the establishment of a permissive corridor that is essential for thalamocortical axon pathfinding. Our results also demonstrate that in this process two different products of the Neuregulin-1 gene, CRD-NRG1 and Ig-NRG1, mediate the guidance of thalamocortical axons. These results show that neuronal tangential migration constitutes a novel mechanism to control the timely arrangement of guidance cues required for axonal tract formation in the mammalian brain.

Neuregulin, a factor with many functions in the life of a schwann cell.

The signalling system comprising the ligand Neuregulin‐1, and its receptors, ErbB2 and ErbB3, plays multiple and important roles in glial development. These include functions in early development of neural crest cells, in expansion of the Schwann cell precursor pool and in myelination. Neuregulin is one of the crucial axon‐derived signals that influence development of Schwann cells. These are specialized cells that ensheath peripheral axons and provide electrical insulation. Schwann cells have also long been implicated in providing more than a simple ensheathing function. Compelling evidence for this has emerged from the analysis of mice lacking these cells, resulting from a non‐functional or compromised Neuregulin signalling system. They serve as a model to study glia–nerve interactions in vivo and indicate that Schwann cells provide important neurotrophic signals, and also cues that regulate perineurium development and nerve fasciculation. BioEssays 22:987–996, 2000.

Neuregulin-1/ErbB signaling serves distinct functions in myelination of the peripheral and central nervous system.

Understanding the control of myelin formation by oligodendrocytes is essential for treating demyelinating diseases. Neuregulin-1 (NRG1) type III, an EGF-like growth factor, is essential for myelination in the PNS. It is thus thought that NRG1/ErbB signaling also regulates CNS myelination, a view suggested by in vitro studies and the overexpression of dominant-negative ErbB receptors. To directly test this hypothesis, we generated a series of conditional null mutants that completely lack NRG1 beginning at different stages of neural development. Unexpectedly, these mice assemble normal amounts of myelin. In addition, double mutants lacking oligodendroglial ErbB3 and ErbB4 become myelinated in the absence of any stimulation by neuregulins. In contrast, a significant hypermyelination is achieved by transgenic overexpression of NRG1 type I or NRG1 type III. Thus, NRG1/ErbB signaling is markedly different between Schwann cells and oligodendrocytes that have evolved an NRG/ErbB-independent mechanism of myelination control.

A role of the cryptic gene in the correct establishment of the left–right axis

During vertebrate embryogenesis, a left-right axis is established. The heart, associated vessels and inner organs adopt asymmetric spatial arrangements and morphologies. Secreted growth factors of the TGF-beta family, including nodal, lefty-1 and lefty-2, play crucial roles in establishing left-right asymmetries [1] [2] [3]. In zebrafish, nodal signalling requires the presence of one-eyed pinhead (oep), a member of the EGF-CFC family of membrane-associated proteins [4]. We have generated a mutant allele of cryptic, a mouse EGF-CFC gene [5]. Homozygous cryptic mutants developed to birth, but the majority died during the first week of life because of complex cardiac malformations such as malpositioning of the great arteries, and atrial-ventricular septal defects. Moreover, laterality defects, including right isomerism of the lungs, right or left positioning of the stomach and splenic hypoplasia were observed. Nodal gene expression in the node was initiated in cryptic mutant mice, but neither nodal, lefty-2 nor Pitx2 were expressed in the left lateral plate mesoderm. The laterality defects observed in cryptic(-/-) mice resemble those of mice lacking the type IIB activin receptor or the homeobox-containing factor Pitx2 [6] [7] [8] [9], and are reminiscent of the human asplenic syndrome [10]. Our results provide genetic evidence for a role of cryptic in the signalling cascade that determines left-right asymmetry.

ErbB2 pathways in heart and neural diseases.

The proto-oncogene ErbB2 (also known as c-neu or HER2 in humans) encodes a receptor tyrosine kinase that is frequently overexpressed in human tumors. It is the target of a novel and effective antibody-based therapy for malignant mammary tumors (trastuzumab/Herceptin). Biochemical and genetic experiments have shown that ErbB2 acts as a coreceptor for other members of the ErbB family of receptor tyrosine kinases. In particular, signals are transduced by ErbB2/ErbB4, ErbB2/ErbB3, and ErbB2/EGF receptor heteromers. ErbB2/4 and ErbB2/ErbB3 heteromers transmit neuregulin-1 signals in the developing and adult heart, and in the peripheral nervous system, respectively. Of particular medical relevance are recent findings that relied on tissue-specific mutation of ErbB2 in cardiomyocytes, which revealed an essential function of ErbB2 in normal heart physiology and demonstrated that loss of cardiac ErbB2 can cause dilated cardiomyopathy in adult mice. Thus, ErbB2 is important not only in development, but also for the correct functioning of the differentiated myocardium. The conditional ErbB2 mutant mice provide a model for the principal side effects--cardiomyopathy and heart failure--that can be observed in patients undergoing chemotherapy with Trastuzumab.

Axonally derived neuregulin-1 is required for remyelination and regeneration after nerve injury in adulthood.

Neuregulin-1 (NRG1) plays a crucial role in axoglial signaling during the development of the peripheral nervous system, but its importance in adulthood after peripheral nerve injury remains unclear. We used single-neuron labeling with inducible Cre-mediated knock-out animals, which enabled visualization of a subset of adult myelinated sensory and motoneurons neurons in which Nrg1 was inducibly mutated by tamoxifen treatment. In uninjured mice, NRG1-deficient axons and the associated myelin sheath were normal, and the neuromuscular junction demonstrated normal apposition of presynaptic and postsynaptic components. After sciatic nerve crush, NRG1 ablation resulted in severe defects in remyelination: axons were either hypomyelinated or had no myelin sheath. NRG1-deficient axons were also found to regenerate at a slower rate. After nerve injury, the neuromuscular junction was reinnervated, but excess terminal sprouting was observed. Juxtacrine Neuregulin-1 signaling is therefore dispensable for maintenance of the myelin sheath in adult animals but has a key role in reparative processes after nerve injury.

ErbB2 Signaling in Schwann Cells Is Mostly Dispensable for Maintenance of Myelinated Peripheral Nerves and Proliferation of Adult Schwann Cells after Injury

Neuregulin/erbB signaling is critically required for survival and proliferation of Schwann cells as well as for establishing correct myelin thickness of peripheral nerves during development. In this study, we investigated whether erbB2 signaling in Schwann cells is also essential for the maintenance of myelinated peripheral nerves and for Schwann cell proliferation and survival after nerve injury. To this end, we used inducible Cre-loxP technology using a PLP-CreERT2 allele to ablate erbB2 in adult Schwann cells. ErbB2 expression was markedly reduced after induction of erbB2 gene disruption with no apparent effect on the maintenance of already established myelinated peripheral nerves. In contrast to development, Schwann cell proliferation and survival were not impaired in mutant animals after nerve injury, despite reduced levels of MAPK-P (phosphorylated mitogen-activated protein kinase) and cyclin D1. ErbB1 and erbB4 do not compensate for the loss of erbB2. We conclude that adult Schwann cells do not require major neuregulin signaling through erbB2 for proliferation and survival after nerve injury, in contrast to development and in cell culture.