Differential expression of fibromodulin mRNA associated with tendon fibril growth: isolation and characterization of a chicken fibromodulin cDNA

Abstract

A 450 bp cDNA fragment similar to that encoding bovine fibromodulin was isolated using a screening procedure to isolate genes differentially expressed between the pre- and post-growth phases of fibril growth in the developing chicken embryo metatarsal tendon. Using this fragment, a 2.4 kb cDNA clone for chicken fibromodulin was isolated from a λZAP library, and the 5´ rapid amplification of cDNA ends technique was employed to clone the 5´ end of the fibromodulin cDNA. The full-length cDNA contained an open reading frame coding for a 380-amino-acid protein. There was ∼80% similarity with human, rat and bovine fibromodulins, which confirmed its identity as fibromodulin. Structural features of the deduced sequence include an 18-amino-acid signal peptide, cysteine residues in conserved positions in the N- and C-terminal regions, and a central leucine-rich domain containing eleven repeats of the sequence LXXLXLXXNXL/I. Features unique to chicken fibromodulin include an additional glycosylation site as well as a decreased number of tyrosine residues that could be sulphated, and therefore potential changes in the charge of the molecule. In addition, there was little similarity among the untranslated regions. When compared with chicken decorin and lumican, fibromodulin showed greater similarity to the other keratan sulphate-containing proteoglycan, lumican. Northern blot analysis revealed a 6–8-fold increase in the fibromodulin mRNA level from day 14 to day 19 of development. In the chicken tendon, collagen fibril growth is a process characterized by a precipitous increase in length during a short developmental period. The necessary changes would require the expression of different genes regulating fibril formation and growth, and interactions between fibromodulin and collagen fibrils may participate in the regulation of collagen fibril growth and matrix assembly.

Homologous Sequence in Lumican and Fibromodulin Leucine-rich Repeat 5-7 Competes for Collagen Binding

“…During tendon development, both lumican and fibromodulin have a role in the initial assembly of intermediates, whereas fibromodulin facilitates growth steps leading to mature fibrils (18). It has also been demonstrated that the fibromodulin mRNA level is increased 6 -8-fold in chicken tendons between day 14 and day 19 of development, which coincide with a dramatic increase in collagen fibril length (28). Increased expression of fibromodulin with a higher affinity for collagen may displace lumican from collagen and appears to have a role in tendon developmental transitions.…”

Super-motifs and evolution of tandem leucine-rich repeats within the small proteoglycans?biglycan, decorin, lumican, fibromodulin, PRELP, keratocan, osteoadherin, epiphycan, and osteoglycin

“…Nine to twelve LRRs were assigned within class I and II: biglycan, [25][26][27][28][29] decorin, 30 -37 lumican, 38 -41 fibromodulin, [42][43][44] PRELP, 45,46 keratocan, [47][48][49] and osteoadherin. 24 Six to eight LRRs were assigned within class III: epiphycan, 50,51 and osteoglycin [21][22][23] ; however, the researchers who determined the osteoglycin sequence did not discuss repeat number.…”

Collagen fibrillogenesis in tendon development: Current models and regulation of fibril assembly

“…Adult tendons are comprised mainly of collagen type I molecules that are hierarchically organized into structural units: molecules aggregate to form fibrils; bundles of fibrils form fibers; fibers group into primary fiber bundles or fascicles; and fascicles group together to form tertiary fiber bundles ( Fig. 1) (Kastelic et al, 1978;Fleischmajer et al, 1988;Nurminskaya and Birk, 1996;Silver et al, 2003;Kuo et al, 2008). Endotendinous connective tissue (endotenon) surrounds the fiber bundles and contains blood vessels, lymphatics, and nerves.…”