J. Deschamps

Rnf2 (Ring1b) deficiency causes gastrulation arrest and cell cycle inhibition

The highly homologous Rnf2 (Ring1b) and Ring1 (Ring1a) proteins were identified as in vivo interactors of the Polycomb Group (PcG) protein Bmi1. Functional ablation of Rnf2 results in gastrulation arrest, in contrast to relatively mild phenotypes in most other PcG gene null mutants belonging to the same functional group, among which is Ring1. Developmental defects occur in both embryonic and extraembryonic tissues during gastrulation. The early lethal phenotype is reminiscent of that of the PcG-gene knockouts Eed and Ezh2, which belong to a separate functional PcG group and PcG protein complex. This finding indicates that these biochemically distinct PcG complexes are both required during early mouse development. In contrast to the strong skeletal transformation in Ring1 hemizygous mice, hemizygocity for Rnf2 does not affect vertebral identity. However, it does aggravate the cerebellar phenotype in a Bmi1 null-mutant background. Together, these results suggest that Rnf2 or Ring1-containing PcG complexes have minimal functional redundancy in specific tissues, despite overlap in expression patterns. We show that the early developmental arrest in Rnf2-null embryos is partially bypassed by genetic inactivation of the Cdkn2a (Ink4a/ARF) locus. Importantly, this finding implicates Polycomb-mediated repression of the Cdkn2a locus in early murine development.

Loss of Hoxb8 alters spinal dorsal laminae and sensory responses in mice

Although Hox gene expression has been linked to motoneuron identity, a role of these genes in development of the spinal sensory system remained undocumented. Hoxb genes are expressed at high levels in the dorsal horn of the spinal cord. Hoxb8 null mutants manifest a striking phenotype of excessive grooming and hairless lesions on the lower back. Applying local anesthesia underneath the hairless skin suppressed excessive grooming, indicating that this behavior depends on peripheral nerve activity. Functional ablation of mouse Hoxb8 also leads to attenuated response to nociceptive and thermal stimuli. Although spinal ganglia were normal, a lower postmitotic neural count was found in the dorsalmost laminae at lumbar levels around birth, leading to a smaller dorsal horn and a correspondingly narrowed projection field of nociceptive and thermoceptive afferents. The distribution of the dorsal neuronal cell types that we assayed, including neurons expressing the itch-specific gastrin-releasing peptide receptor, was disorganized in the lumbar region of the mutant. BrdU labeling experiments and gene-expression studies at stages around the birth of these neurons suggest that loss of Hoxb8 starts impairing development of the upper laminae of the lumbar spinal cord at approximately embryonic day (E)15.5. Because none of the neuronal markers used was unexpressed in the adult dorsal horn, absence of Hoxb8 does not impair neuronal differentiation. The data therefore suggest that a lower number of neurons in the upper spinal laminae and neuronal disorganization in the dorsal horn underlie the sensory defects including the excessive grooming of the Hoxb8 mutant.

Hoxb8-Cre Mice: a Tool for Brain-Sparing Conditional Gene Deletion

The spinal cord is the first site of temporal and spatial integration of nociceptive signals in the pain pathway. Neuroplastic changes occurring at this site contribute critically to various chronic pain syndromes. Gene targeting in mice has generated important insights into these processes. However, the analysis of constitutive (global) gene‐deficient mice is often hampered by confounding effects arising from supraspinal sites. Here, we describe a novel Cre mouse line that expresses the Cre recombinase under the transcriptional control of the Hoxb8 gene. Within the neural axis of these mice, Hoxb8‐Cre expression is found in spinal cord neurons and glial cells, and in virtually all neurons of the dorsal root ganglia, but spares the brain apart from a few cells in the spinal trigeminal nucleus. The Hoxb8‐Cre mouse line should be a valuable new tool for the in vivo analysis of peripheral and spinal gene functions in pain pathways. genesis 48:596–602, 2010.