Loss of Necdin in the Mouse Impairs the Migration of Neurons in the Developing Nervous System

Alysa A. Tennese, Jason R. Bush, and Rachel Wevrick

Department of Medical Genetics, University of Alberta, Edmonton, Alberta, Canada

Some clinical aspects of Prader-Willi syndrome (PWS) suggest dysfunction of the autonomic nervous system (ANS). The peripheral part of the ANS executes functions that are abnormal in PWS individuals including: feeding, drinking, thermoregulation, intestinal motility, reproduction, and reaction to stress and infection. Necdin is one of the genes inactivated in individuals with PWS and is located on chromosome 7C in the mouse in a region orthologous to 15q12. The expression of necdin in the mouse is highest in tissues relevant to PWS, including the central and peripheral nervous systems, including autonomic neurons, and muscle. Necdin has been implicated in diverse cellular processes in the nucleus by promoting the differentiation of specific neuronal subtypes, and in the cytoplasm by interacting with neurotrophin receptors and proteins involved in axonal outgrowth and cytoskeleton reorganization. Cell migration requires cytoskeletal reorganization to bring about extension of the leading edge in response to cues in the extracellular matrix, nuclear translocation into the leading edge, and retraction of the trailing process. Axonal outgrowth also requires cytoskeletal reorganization, both for elongation and for transport of signaling molecules bi-directionally along the nascent axons.

We previously determined that loss of necdin in mice causes axonal extension, bundling, and branching defects in cultured sympathetic chain ganglia neurons. Therefore, we examined the sympathetic nervous system during prenatal development in necdin-null mouse embryos using immunohistochemistry on sections and in whole tissues. We identified a defect in the size and location of the superior cervical ganglia (SCG), the most rostral ganglia in the sympathetic chain, in necdin-null embryos. The SCG appears normal at midgestation, embryonic days 11-12, but does not migrate towards the head at later stages in development as is normally observed in control embryos. The SCG innervates the salivary glands, pupillary muscle, pineal gland, and nasal mucosa. However, in necdin null embryos, a decrease in innervation of these target tissues and an increase in cell death are observed at embryonic day 18, just before birth. As the survival of neurons requires nerve growth factors produced by target tissues, the reduction in axonal outgrowth likely causes the increased apoptosis in the maturing SCG neurons.

Many cell types, including neurons, muscle cells, and chondrocytes, migrate from where they are first born to their final location where they perform essential functions. To address the molecular mechanisms by which necdin might promote proper migration, we cultured fibroblasts from necdin-null embryos and control littermates. Necdin-null fibroblasts show reduced migration in cell culture wound-healing assays, suggesting that loss of necdin may affect the ability of the cytoskeleton to reorganize itself in response to environmental cues. We propose a novel role for necdin in the migration of neurons and other cell types in which it is expressed. Understanding the function of necdin in cytoskeletal reorganization and cellular migration may identify a cause for many characteristics of the PWS phenotype.