Spatial and temporal expression pattern of Runx3 (Aml2) and Runx1 (Aml1) indicates non-redundant functions during mouse embryogenesis

The Runx3 transcription factor regulates development and survival of TrkC dorsal root ganglia neurons

The RUNX transcription factors are important regulators of linage-specific gene expression in major developmental pathways. Recently, we demonstrated that Runx3 is highly expressed in developing cranial and dorsal root ganglia (DRGs). Here we report that within the DRGs, Runx3 is specifically expressed in a subset of neurons, the tyrosine kinase receptor C (TrkC) proprioceptive neurons. We show that Runx3-deficient mice develop severe limb ataxia due to disruption of monosynaptic connectivity between intra spinal afferents and motoneurons. We demonstrate that the underlying cause of the defect is a loss of DRG proprioceptive neurons, reflected by a decreased number of TrkC-, parvalbumin- and beta-galactosidase-positive cells. Thus, Runx3 is a neurogenic TrkC neuron-specific transcription factor. In its absence, TrkC neurons in the DRG do not survive long enough to extend their axons toward target cells, resulting in lack of connectivity and ataxia. The data provide new genetic insights into the neurogenesis of DRGs and may help elucidate the molecular mechanisms underlying somatosensory-related ataxia in humans.

Impaired vascular invasion of Cbfa1-deficient cartilage engrafted in the spleen

Chondrocyte maturation and vascular invasion of cartilage are essential in the process of endochondral ossification. Cbfal-deficient (Cbfa1-/-) mice displayed a complete absence of osteoblast and osteoclast maturation as well as severely inhibited chondrocyte maturation in most parts of the skeleton. Although chondrocyte maturation and mineralization were observed in restricted areas of Cbfa1-/- mouse skeleton, vascular invasion of calcified cartilage was never noted. To investigate the possibility of chondrocyte maturation and vascular invasion in Cbfal-/- cartilage and the role of the hematopoietic system in the process of vascular invasion, we transplanted embryonic day 18.5 (E18.5) Cbfa1-/- femurs, which are composed of immature chondrocytes, into spleens of normal mice. One week later, the transplanted femurs contained terminally differentiated chondrocytes expressing osteopontin, bone sialoprotein (BSP), and matrix metalloproteinase (MMP) 13. In the diaphyses of the transplants, the cartilage matrix was mineralized and the cartilage was invaded by vascular vessels and osteoclasts. However, chondrocyte maturation and vascular invasion were severely retarded in comparison with transplants of E14.5 wild-type femurs, in which the cartilage was rapidly replaced by bone, and neither mature osteoblasts nor bone formation were observed. In primary culture of Cbfa1-/- chondrocytes, transforming growth factor (TGF) beta1, platelet-derived growth factor (PDGF), interleukin (IL)-1beta, and thyroid hormone (T3) induced osteopontin and MMP-13 expression. These findings indicated that factors in the hematopoietic system are able to support vascular invasion of cartilage independent of Cbfal but are less effective without it, suggesting that Cbfal functions in cooperation with factors from bone marrow in the process of growth plate vascularization.

The RUNX3 gene--sequence, structure and regulated expression

The RUNX3 gene belongs to the runt domain family of transcription factors that act as master regulators of gene expression in major developmental pathways. In mammals the family includes three genes, RUNX1, RUNX2 and RUNX3. Here, we describe a comparative analysis of the human chromosome 1p36.1 encoded RUNX3 and mouse chromosome 4 encoded Runx3 genomic regions. The analysis revealed high similarities between the two genes in the overall size and organization and showed that RUNX3/Runx3 is the smallest in the family, but nevertheless exhibits all the structural elements characterizing the RUNX family. It also revealed that RUNX3/Runx3 bears a high content of the ancient mammalian repeat MIR. Together, these data delineate RUNX3/Runx3 as the evolutionary founder of the mammalian RUNX family. Detailed sequence analysis placed the two genes at a GC-rich H3 isochore with a sharp transition of GC content between the gene sequence and the downstream intergenic region. Two large conserved CpG islands were found within both genes, one around exon 2 and the other at the beginning of exon 6. RUNX1, RUNX2 and RUNX3 gene products bind to the same DNA motif, hence their temporal and spatial expression during development should be tightly regulated. Structure/function analysis showed that two promoter regions, designated P1 and P2, regulate RUNX3 expression in a cell type-specific manner. Transfection experiments demonstrated that both promoters were highly active in the GM1500 B-cell line, which endogenously expresses RUNX3, but were inactive in the K562 myeloid cell line, which does not express RUNX3.