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The function of Smad4 in skeletal development
Smad4 is the central intracellular mediator of transforming growth factor-β (TGF- β) and BMP signaling pathways. To study the role of Smad4 in skeletal development, we introduced a conditional mutation of the gene in chondrocytes using Cre-loxP system. A high level of Smad4 gene deletion was verified in Cre expressing mice. In vivo, deletion of Smad4 resulted in normal early limb development. That is, the mesenchyme condensed in the correct pattern. However, later in development, depletion of Smad4 resulted in defects in the development of the skeletal elements. The skull was smaller and the limbs were shorter than controls, the deltoid tuberosity was missing, and the joints of the phalanges were fused. The rib cage was smaller and flattened which could lead to respiratory distress and subsequent post-natal death. The abrogation of Smad4 in chondrocytes resulted in dwarfism with a severely disorganized growth plate characterized by expanded resting zone of chondrocytes, reduced chondrocyte proliferation. My data also indicated that loss of Smad4 has a huge impact on the activation of Smad2 and p38 and that phosphorylation of Smad2 and p38 are Smad4 dependent. However, Smad4 is not required for the activation of Smad1/5. Therefore, since loss of Smad4 caused a decreased pSmad2 and p-p38 level, the cells might respond to TGF-β or BMP signaling with a compensatory upregulation of pSmad1/5 level. All vii these data provided evidence demonstrating that Smad4-mediated TGF-β and BMP signals are required for maintaining the normal organization of chondrocytes in the growth plate, and that Smad4 is required for normal development of the skeleton.