Vitamins, Harmones and Hemoglobin
Which of the following is NOT a component of inter photoreceptor matrix? (AIPG 2008)
a. Mimecan / osteoglycin ,isolated from bone,was originally called osteoinductive factor and later renamed mimecan/osteoglycin). It belongs to a family of small leucine-rich proteoglycans (SLRPs)that are secreted into the extracellular matrix.
b. SLRPs, abundant in the bone matrix, cartilage cells, and connective tissues, are important for collagen fibrillogenesis, cellular growth, differentiation, and migration.
c. The physiological functions of mimecan have not yet been fully understood.
d. Initial studies have shown that mimecan can induce ectopic bone formation; subcutaneous implantation of mimecan plus TGF-→ type 1 or 2 into rats induces bone formation at the implantation site.
e. In atherosclerotic lesions, mimecan mRNA was down-regulated in the media and up-regulated in the activated endothelium and thick neointima, whereas the protein accumulated in the front edge of migrating smooth muscle cells.
f. More recently, both pro-and mature mimecan have been shown to regulate type I collagen fibrillogenesis, and conversion of mimecan from the precursor to mature form, through cleavage by BMP-1, potentiates the ability of mimecan to modulate the formation of collagen fibrils.
g. Osteonectin, also known as SPARC and BM-40 ,is a 43-kDa Ca 2+ binding glycoprotein with complex biological functions.
h. Osteonectin/SPARC is believed to regulate cell growth through interactions with the extracellular matrix. Increased secretion of osteonectin/SPARC is associated with endothelial cell injury in vitro and the inhibition of cell spreading on collagen.
i. It can also induce cell rounding in cultured endothelial cells and fibroblasts. Osteonectin/SPARC will associate directly with platelet derived growth factor (PDGF)and modulate its activity.
j. Osteonectin/SPARC is performing an important function in the retina. Before more specific functions can be explored, some basic properties of the protein need to be determined, such as its turnover in the macula versus peripheral retina, possible feedback mechanisms that affect regulation of transcription, its calcium binding activity, and which retinal cell type(s)may be synthesizing it.
k. This preferential expression and secretion by the macula is rather unique and may be of particular interest to macular degenerations .
l. Photoreceptor degeneration is followed by remodeling of both neuronal and glial cells as well as the vasculature. Genetic evidence for tissue and vessel remodeling in the retina is based on the overexpression of aquaporin 4,matrix metalloproteinase tissue inhibitor of metalloproteinase 3,and fibroblast growth factor-1 (cluster 1).
m. Matrix metalloproteinase and their tissue inhibitors have been shown to be involved in extracellular matrix remodeling in a variety of brain pathologies including optic nerve inflammation.