Jennie R. Chin

Extracellular Matrix Remodeling during Morphogenesisa

The role of proteinases in extracellular matrix remodeling during the developmental program of bone, cartilage, muscle, and epithelial differentiation in the mandibular arch during embryogenesis was investigated. ECM changes accompany morphogenesis during development. The most dramatic changes occur during development of bone and cartilage. The expression of matrix metalloproteinases (MMPs) was altered by regulating expression of MMPs by growth factors, by inhibiting MMP activity, and by genetic ablation of MMPs. The data point to critical roles for MMPs in cartilage development and endochondral bone formation. MMPs appear to regulate not only ECM degradation but also programmed cell death, cell migration, and invasion during these morphogenic processes. The data suggest that matrix metalloproteinases play a pivotal role in the morphogenesis of structures derived from epithelium (oral sulcus), somitic mesoderm (tongue), and cranial neural crest (Meckels cartilage).

Epidermal Growth Factor Receptor-deficient Mice Have Delayed Primary Endochondral Ossification Because of Defective Osteoclast Recruitment

The epidermal growth factor receptor (EGFR) and its ligands function in diverse cellular functions including cell proliferation, differentiation, motility, and survival. EGFR signaling is important for the development of many tissues, including skin, lungs, intestines, and the craniofacial skeleton. We have now determined the role of EGFR signaling in endochondral ossification. We analyzed long bone development in EGFR-deficient mice. EGFR deficiency caused delayed primary ossification of the cartilage anlage and delayed osteoclast and osteoblast recruitment. Ossification of the growth plates was also abnormal resulting in an expanded area of growth plate hypertrophic cartilage and few bony trabeculae. The delayed osteoclast recruitment was not because of inadequate expression of matrix metalloproteinases, including matrix metalloproteinase-9, which have previously been shown to be important for osteoclast recruitment. EGFR was expressed by osteoclasts, suggesting that EGFR ligands may act directly to affect the formation and/or function of these cells. EGFR signaling regulated osteoclast formation. Inhibition of EGFR tyrosine kinase activity decreased the generation of osteoclasts from cultured bone marrow cells.

BIOSYNTHETIC RADIOLABELING OF CELLULAR AND SECRETED PROTEINS OF MONONUCLEAR PHAGOCYTES

Publisher Summary This chapter discusses the biosynthetic radio-labeling of cellular and secreted proteins of mononuclear phagocytes. A complete description of the phenotype of mononuclear phagocytes involves defining all the biochemical and functional properties of the cells. One approach is to examine the pattern of total transcribable mRNA present in a macrophage at any given point in its history. This can be achieved either by translating isolated mRNA in cell-free systems, a process unsuitable for the examination of many samples, or by studying the translation of mRNA into proteins using the machinery of a live cell. If radio-labeled amino acids are present during translation in live cells, the resulting biosynthesized proteins can be analyzed by polyacrylamide gel electrophoresis to give detailed fingerprints of specific macrophage phenotypes. In conjunction with other methods, such as specific immunoprecipitation of labeled proteins, it is possible to examine detail changes in the properties of macrophages that may not be seen with a single assay such as receptor binding or quantification of a secreted proteolytic enzyme.

Control of apolipoprotein E secretion by macrophages

Mononuclear phagocytes are active secretory cells (1). Starting at the promonocyte stage, they secrete into their extracellular milieu at least 50 different proteins, peptides, and low molecular weight metabolites (Table I). These products are not secreted coordinately by all mononuclear phagocytes, but vary with the functional states of the cells. Because the secretion products of macrophages are particularly sensitive to alteration in response to inflammation and other changes in the macrophage environment (Table II), the secretory profile of a macrophage is particularly useful in defining its phenotype (2).