Interactions between the matricellular protein BM-40 and fibrillar collagens type I, II and III

Introduction 
 
 
 
 
 2  
 
 
The Extracellular Matrix 
 
The evolution of multicellular organisms was mostly associated with the necessity of 
a controlled environment where cells could survive and interact, and where specialization 
could occur. This fundamental requirement was met by the formation of a network of 
connecting elements, the extracellular matrix (ECM), which not only permitted the inflow 
and outflow of materials essential for life, but also provided protection from external 
physical stress on the organism. 
The ECM can be defined as a complex network of macromolecules, composed of 
insoluble fibres, microfibrils and a wide range of soluble proteins, produced and secreted in 
the extracellular space by the cells of the connective tissues. The primary role of the ECM 
is to endow tissues with specific mechanical and physiochemical properties and to provide 
a scaffold for cell attachment and migration. Although the ECM is a quite stable structure 
that surrounds the cells, it has not to be considered as an inert scaffold that stabilizes the 
physical structure of tissues: the matrix plays a far more active and complex role in 
regulating the behaviour of the cells, influencing their development, migration, 
proliferation, shape and function. Cells are in fact continuously interacting with the matrix 
produced by themselves, thus maintaining an active and important exchange of information 
through the binding of specific cell surface receptors and growth factors with matrix 
components. 
We can distinguish at least three main classes of molecules composing the ECM and 
contributing to its functions: 
• insoluble proteins, like collagens and elastin, responsible for the resistance and the 
elastic properties of the connective tissues; 
• soluble macromolecules and various glycoproteins, such as proteoglycans, that 
contribute to the elasticity and the cohesion of the tissues by resisting the compression 
forces and allowing the diffusion of small molecules; 
• another class of molecules, termed “matricellular” proteins, like BM-40, 
thrombospondins and tenascins, that function as adaptors and modulators of cell-matrix 
interactions without having strictly or exclusively structural roles. 
Introduction 
 
 
 
 
 3  
 
 
The composition of the ECM changes according to the tissue: variations in the 
relative amounts of the different types of matrix macromolecules and the way they are 
organized in the extracellular matrix give rise to a surprisingly high diversity of forms, 
each adapted to the functional requirements of the particular tissue (Adams J.C. et al., 
1993; Lin C.Q. et al., 1993; Birk D.E. et al., 1991; Kreis T. et al., 1993). 
 
The Collagen Family 
 
Collagens represent one of the most important families of proteins of the matrix, both 
from a quantitative point of view and because of their contribution to the biomechanical 
properties of the matrix. The term “collagen” is often used as a generic term to indicate a 
wide range of highly characteristic fibrous proteins found in all multicellular organisms. 
They are secreted by connective tissue cells and share the basic structural motif of three 
polypeptide chains, called α chains, assembled into a characteristic triple helical 
conformation and, at a higher level, by the ability of forming characteristic supramolecular 
aggregates. 
So far, more than 40 vertebrate collagen genes have been described, the translation 
products of which combine to form 27 distinct homo- or heterotrimeric molecules (Table 
1). Although the collagen proteins differ considerably in size, structure, tissue distribution 
and function, the basic feature remains constant for all of them. This is the presence of 
either continuous or interrupted triple helical domains made up of multiple repeats of Gly-
X-Y sequences. 
In addition to the collagens there exists a number of secreted proteins which contain 
collagenous amino acid sequences and short triple helical regions. However, these proteins 
are not classified as collagens because they have no known structural role in the ECM. 
These include, for instance, the complement component C1q, the collectins and the 
macrophage scavenger receptor, which play an important role in immunological defence. 
Other examples are the enzyme acetylcholine esterase, the lung surfactant proteins, 
conglutinin and the serum mannan-binding protein (Brodsky B. et al., 1995). 
 
Introduction 
 
 
 
 
 4  
 
 
Table 1: Genetically distinct collagen types. 
 
Type Chain(s) Molecular forms Main Distribution Key Features 
 
I 
 
α1(I)  
α2(I) 
 
[α1(I)]
2
 α2(I) 
 
 
 
Very widespread: dermis, 
bone, ligament, tendon, etc 
 
Most abundant collagen type. Main 
constituent of major fibre bundles that gives 
strength to connective tissues 
  
 
[α1(I)]
3
 
Dermis, dentin Apparently a minor form 
 
II 
 
α1(II) 
 
 
[α1(II)]
3
 
 
Cartilage, intervertebral disc 
 
Main collagen of cartilage. Forms the major 
fibrils of this tissue 
 
III 
 
α1(III) 
 
 
[α1(III)]
3
 
 
Blood vessels, dermis, 
intestine, etc. 
 
Fibrillar, frequently associated with type I in 
extensible tissues. More abundant in foetal 
tissues 
 
IV 
 
α1(IV) 
α2(IV) 
 
[α1(IV)]
2 
α2(IV) 
[α3(IV)]
2 
α4(IV) (?) 
 
Basement membranes 
 
A non-fibril forming collagen. Forms 
extended  network 
 
α3(IV) [α5(IV)]
2
 α6(IV) (?) 
 
α4(IV) (?)  
 α5(IV) 
 
 
 α6(IV) 
 
 
V 
 
α1(V) 
α2(V) 
α3(V) 
 
[α1(V)]
3 
[α1(V)]
2
 α2(V)  
α1(V) α2(V) α3(V) 
 
Widespread in low quantity; 
appears associated with 
collagen I fibrils 
 
Forms fibrils; may form fibril core with 
collagen I 
 
VI 
 
α1(VI) 
α2(VI) 
 
α1(VI) α2(VI) α3(VI) 
 
Widespread 
 
Forms beaded filaments. Alternative spliced 
forms 
 
α3(VI) 
 
VII 
 
α1(VII) 
 
 
[α1(VII)]
3
 
 
 
Skin, oral mucosa, cervix 
 
Forms anchoring structure linking epithelial 
basement membrane to underlying tissue 
 
VIII 
 
α1(VIII)  
α2(VIII) 
 
[α1(VIII)]
3
 
[α2(VIII)]
3 
[α1(VIII)]
2
 α2(VIII) 
 
Associated with endothelial 
cell layers, e.g. Descemet’s 
membrane 
 
May form a hexagonal lattice in some 
tissues. Short chain length 
 
IX 
 
α1(IX)  
α2(IX)  
α3(IX) 
 
α1(IX) α2(IX) α3(IX) 
 
Cartilage, vitreous body 
 
A collagen II fibril-associated collagen with 
an interrupted triple helix (FACIT); can 
contain a glycosaminoglycan chain 
 
X 
 
α1(X) 
 
[α1(X)]
3
 
 
 
Hypertrophic mineralising 
cartilage 
 
A short-chain collagen; has similar structure 
to collagen VIII 
 
XI 
 
α1(XI)  
α2(XI) 
 
α1(XI) α2(XI) α3(XI) 
 
Cartilage, intervertebral disc 
 
Forms fibrils which are associated with 
collagen II 
 α3(XI) 
 
XII 
 
α1(XII) 
 
[α1(XII)]
3
 
 
Ligament, tendon 
 
A collagen I fibril-associated collagen with 
an interrupted triple helix (FACIT) 
 
XIII 
 
α1(XIII) 
 
(?) 
 
Widespread in low quantity 
 
Transmembrane collagen with an 
hydrophobic domain 
 
XIV 
 
α1(XIV) 
 
[α1(XIV)]
3
 
 
Skin, tendon 
 
A fibril-associated collagen with an 
interrupted triple helix (FACIT) 
 
XV 
 
α1(XV) 
 
(?) 
 
Expressed in fibroblasts, 
smooth muscle cells 
 
Triple helix is interrupted in several places 
 
XVI 
 
α1(XVI) 
 
(?) 
 
Expressed in fibroblasts, 
keratinocytes 
 
A fibril-associated collagen with an 
interrupted triple helix (FACIT) 
Introduction 
 
 
 
 
 5  
 
 
Table 1: Genetically distinct collagen types (continued from page 3). 
 
Type Chain(s) Molecular forms Main Distribution Key Features 
 
XVII 
 
α1(XVII) 
 
(?) 
 
Bullous pemphigoid antigen, 
expressed at dermal-epidermal 
junction
 
Triple helix is interrupted in several places. 
Transmembrane collagen with an 
hydrophobic domain 
 
XVIII 
 
α1(XVIII) 
 
(?) 
 
Expressed in highly vascularised 
tissues
 
Triple helix is interrupted in several places 
 
XIX 
 
α1(XIX) 
 
(?) 
 
Expressed in very small amounts by 
cultured skin fibroblasts and tumour 
cells
 
A fibril-associated collagen with an 
interrupted triple helix (FACIT) with five 
triple helical subdomains 
 
XX 
 
α1(XX) 
 
(?) 
 
Minor component of several 
connective tissues, such as sternal 
cartilage, cornea and tendon and 
embryonic tissue as corneal 
epithelium 
 
A fibril-associated collagen with an 
interrupted triple helix (FACIT) 
 
XXI 
 
α1(XXI) 
 
(?) 
 
Expressed in tissues with a muscle 
phenotype (heart, skeletal muscle, 
smooth muscle and placenta) 
 
A fibril-associated collagen with an 
interrupted triple helix (FACIT) 
 
XXII 
 
α1(XXII) 
 
 
 
Hair follicle 
 
Unspecified 
 
XXIII 
 
α1(XXIII) 
 
[α1(XXIII)]
3
 (?) 
 
Cornea, overexpressed in 
adenocarcinoma cells; cellular 
localization
 
Transmembrane collagen with an 
hydrophobic domain 
 
XXIV 
 
α1(XXIV) 
 
(?) 
 
Cartilage, retina, cornea, skin 
 
Fibrillar collagen close to collagens type V 
and XI
 
XXV 
 
α1(XXV) 
 
(?) 
 
Overexpressed in neurons 
 
Transmembrane collagen with an 
hydrophobic domain 
 
XXVI 
 
α1(XXVI) 
 
(?) 
 
Specifically expressed in testis and 
ovary
 
Unspecified, maybe related to collagen type 
XIII and XXV 
 
XXVII 
 
α1(XXVII) 
 
(?) 
 
Cartilage, eye, ear, lung and colon 
 
Fibrillar collagen 
 
 
Collagen structure 
 
The basic conformation of the triple helix has been deduced at the end of the sixties 
from X-ray diffraction studies on collagen in tendon. The comparison between the 
chemical and the crystallographic data has shown that each of the three polypeptide chains 
in the molecule forms an extended left-handed polyproline II helix, which is stabilized by 
the high imino acid content. The three chains are then supercoiled around a common axis 
in a right-handed manner to form the triple helix which is stabilized by hydrogen bonds 
between the chains (Fig. 1) (Rich A. et al., 1961; Ramachandran G.N., 1967; Fraser R.B.D. 
et al., 1973).