Faculty Bibliography

Disruptions in the use of skeletal muscle lead to muscle atrophy. After short periods of disuse, muscle atrophy is reversible, and even after prolonged periods of inactivity, myofiber degeneration is uncommon. The pathways that regulate atrophy, initiated either by peripheral nerve damage, immobilization, aging, catabolic steroids, or cancer cachexia, however, are poorly understood. Previously, we found that Runx1 (AML1), a DNA-binding protein that is homologous to Drosophila Runt and has critical roles in hematopoiesis and leukemogenesis, is poorly expressed in innervated muscle, but strongly induced in muscle shortly after denervation. To determine the function of Runx1 in skeletal muscle, we generated mice in which Runx1 was selectively inactivated in muscle. Here, we show that Runx1 is required to sustain muscle by preventing denervated myofibers from undergoing myofibrillar disorganization and autophagy, structural defects found in a variety of congenital myopathies. We find that only 29 genes, encoding ion channels, signaling molecules, and muscle structural proteins, depend upon Runx1 expression, suggesting that their misregulation causes the dramatic muscle wasting. These findings demonstrate an unexpected role for electrical activity in regulating muscle wasting, and indicate that muscle disuse induces compensatory mechanisms that limit myofiber atrophy. Moreover, these results suggest that reduced muscle activity could cause or contribute to congenital myopathies if Runx1 or its target genes were compromised

How CD4(+)CD8(+) thymocytes commit to CD4 helper versus CD8 cytotoxic lineages is a central unresolved question in developmental immunology. In this issue, show that engineering CD4 for shutoff immediately after positive selection misdirects cells to the cytotoxic lineage. The result highlights the distinction between positive selection and lineage commitment and provides new impetus for reexamining lineage models

Parenchymal microglia are the principal immune cells of the brain. Time-lapse two-photon imaging of GFP-labeled microglia demonstrates that the fine termini of microglial processes are highly dynamic in the intact mouse cortex. Upon traumatic brain injury, microglial processes rapidly and autonomously converge on the site of injury without cell body movement, establishing a potential barrier between the healthy and injured tissue. This rapid chemotactic response can be mimicked by local injection of ATP and can be inhibited by the ATP-hydrolyzing enzyme apyrase or by blockers of G protein-coupled purinergic receptors and connexin channels, which are highly expressed in astrocytes. The baseline motility of microglial processes is also reduced significantly in the presence of apyrase and connexin channel inhibitors. Thus, extracellular ATP regulates microglial branch dynamics in the intact brain, and its release from the damaged tissue and surrounding astrocytes mediates a rapid microglial response towards injury

We examined the in vivo behavior of liver natural killer T cells (NKT cells) by intravital fluorescence microscopic imaging of mice in which a green fluorescent protein cDNA was used to replace the gene encoding the chemokine receptor CXCR6. NKT cells, which account for most CXCR6(+) cells in liver, were found to crawl within hepatic sinusoids at 10-20 microm/min and to stop upon T cell antigen receptor activation. CXCR6-deficient mice exhibited a selective and severe reduction of CD1d-reactive NKT cells in the liver and decreased susceptibility to T-cell-dependent hepatitis. CXCL16, the cell surface ligand for CXCR6, is expressed on sinusoidal endothelial cells, and CXCR6 deficiency resulted in reduced survival, but not in altered speed or pattern of patrolling of NKT cells. Thus, NKT cells patrol liver sinusoids to provide intravascular immune surveillance, and CXCR6 contributes to liver-based immune responses by regulating their abundance

RNA silencing has a known role in the antiviral responses of plants and insects. Recent evidence, including the finding that the Tat protein of human immunodeficiency virus (HIV) can suppress the host's RNA-silencing pathway and may thus counteract host antiviral RNAs, suggests that RNA-silencing pathways could also have key roles in mammalian virus-host interactions

The peripheral lymphocyte pool size is governed by homeostatic mechanisms. Thus, grafted T cells expand and replenish T cell compartments in lymphopenic hosts. Lymphopenia-driven proliferation of naive CD8+ T cells depends on self-peptide/MHC class I complexes and the cytokine IL-7. Lymphopenia-driven proliferation and maintenance of memory CD8+ T cells are MHC independent, but are believed to require IL-7 and contact with a bone marrow-derived cell that presents the cytokine IL-15 by virtue of its high affinity receptor (IL-15Ralpha). In this study we show that optimal spontaneous proliferation of grafted naive and memory CD8+ T cells in mice rendered lymphopenic through gene ablation or irradiation requires the presence of CD11chigh dendritic cells. Our results suggest a dual role of CD11chigh dendritic cells as unique APC and cytokine-presenting cells

Mice deficient for the chemokine receptor CXCR4 show premature translocation of granule cell neuroblasts from their germinal zone into the nascent cerebellum [Y.-R. Zuo et al. (1998) Nature, 393, 595-599]. Here, we used CXCR4-null mice to analyse the early development of cerebellar cortical inhibitory interneurons and pontine neurons which, in the adult, are synaptically integrated with granule cells. Cortical inhibitory interneuronal precursors normally invade the cerebellar anlage of CXCR4-deficient mice, but their dispersal is impeded by dislocated foci of proliferating granule cells, from which they are excluded. This is reminiscent of the strict exclusion of inhibitory interneuronal precursors from the superficial external granule cell layer. As inhibitory interneuronal precursors readily mingle with post-mitotic granule cells both in wild-type and CXCR4-null mice, these findings indicate that the developmentally regulated interactions between granule and inhibitory interneuronal precursors are independent of SDF-1/CXCR4 signalling. In contrast, the transit of pontine neurons from the rhombic lip through the anterior extramural stream to the basilar pons is disrupted in CXCR4-deficient animals. Migrating pontine neurons express CXCR4, and in CXCR4-null animals these cells are found displaced deep into the brainstem. Consequently, nascent pontine nuclei in CXCR4-deficient animals are hypoplastic. Moreover, they fail to express plexin D1, suggesting that SDF-1/CXCR4 signalling may also impinge on axon guidance critical to the orderly formation of granule cell mossy fibre afferents

The chemokine CXCL12/SDF-1 and its receptor CXCR4 regulate the development and the function of the hematopoietic system and control morphogenesis of distinct brain areas. Here, we demonstrate that inactivation of CXCR4 results in a massive loss of spinal cord motoneurons and dorsal root ganglion neurons and, subsequently, in a reduced innervation of the developing mouse fore- and hindlimbs. However, only the death of sensory neurons seems to be a direct consequence of receptor inactivation as suggested by the observations that DRG neurons, but not motoneurons, of wild-type animals express CXCR4 and respond to CXCL12 with an increase in cell survival. In contrast, the increased death of motoneurons in CXCR4-deficient animals seems to result from impaired limb myogenesis and a subsequent loss of muscle-derived neurotrophic support. In summary, our findings unravel a previously unrecognized complex role of CXCL12/CXCR4 in the control of limb neuromuscular development