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The ins and outs of extracellular matrix assembly
Author(s) -
Kadler Karl E.
Publication year - 2004
Publication title -
international journal of experimental pathology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.671
H-Index - 72
eISSN - 1365-2613
pISSN - 0959-9673
DOI - 10.1111/j.0959-9673.2004.0369c.x
Subject(s) - extracellular matrix , procollagen peptidase , microbiology and biotechnology , tendon , fibrillogenesis , collagen, type i, alpha 1 , chemistry , connective tissue , fibril , secretion , golgi apparatus , biology , anatomy , biochemistry , endoplasmic reticulum , genetics
To a rough approximation, the adult human comprises 2–5 kg of cells. The remaining mass originates from the extracellular matrix (ECM). The ECM is highly organized, which is surprising considering the paucity of cells and the fact that the ECM comprises some of the largest and most insoluble macromolecules encoded by the genome. The high level of supramolecular order of the ECM is particularly evident in tendon in which millimetre‐long collagen fibrils lie parallel to each other and to the tendon‐long axis (Canty & Kadler 2002). The work in our laboratory is focused on understanding the molecular and cellular basis of collagen fibrillogenesis. The work is directly relevant to understanding the secretion and assembly of large proteins, tissue homeostasis and embryonic development, as well as understanding the aetiology of heritable and acquired diseases of connective tissue including fibrosis. Materials and methods A multidisciplinary approach is being used including protein biochemistry, recombinant protein expression, immunofluorescence, immunoEM and serial section reconstruction from electron micrographs. Results Using pulse‐chase in organ cultures of embryonic chick tendon we show that approximately 50% of newly synthesized type‐I procollagen is secreted to the ECM where it is cleaved to type‐I collagen, and approximately 50% of the procollagen is cleaved to collagen inside the cells. ImmunoEM shows collagen in specialized secretory vesicles, which bud from the trans‐face of the Golgi apparatus. Serial section reconstruction of E13 chick metatarsal tendon and E15.5 mouse tail tendon shows cross‐striated nascent collagen fibrils, which are approximately 1 µm in length and identical to collagen early collagen fibrils (Graham et al . 2000), enclosed within tube‐like secretory vesicles. The nascent fibrils are secreted via nozzles (plasma membrane protrusions) into tunnel‐shaped spaces between the cells. Bone morphogenetic protein‐1 (ProBMP‐1) is cleaved to active BMP‐1 in the TGN by dibasic convertases (Graham et al . 2000) (e.g. furin) and thereby initiates the assembly of collagen fibrils. Further studies have identified the minimal structure of procollagen C‐proteinase activity of BMP‐1 (Leighton & Kadler 2003). Discussion In vitro studies in the 1980s established that collagen fibrillogenesis resembles a crystallization process, in that it proceeds by distinct nucleation and propagation phases. However, there was no obvious mechanism for how the two phases occurred separately in vivo . Our recent studies show that the nucleation phase occurs in specialized secretory vesicles. The early fibrils are deposited in the ECM via nozzles which are long protrusions of the plasma membranes. The propagation phase for fibril assembly occurs in the ECM. Our studies have direct relevance to the assembly of other matrix polymers, e.g. fibrillin and type‐VI collagen, as well as understanding the molecular basis of ECM assembly in diseases such as fibrosis and wound healing.