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40
Pluripotent embryonic stem cells (ESCs) form embryoid bodies
in vitro that contain isolated areas of contractile SMCs induced
by endogenous TGF-β.
162
Because undifferentiated ESCs have
the potential to form teratocarcinomas, the ability to isolate pure
populations of differentiated cells is essential for use of ESCs in
tissue-engineering applications. An alternative method for tis-
CH
sue regeneration is to reprogram somatic cells to resemble ESCs.
3
Somatic cells can be induced to form pluripotent stem cells (iPS)
by addition of defined factors such as Sox2, Oct4, KLF4, and c-myc
(reviewed in
163
).
Multipotent adipose-derived mesenchymal stem cells (MSCs)
are candidates for a VSMC source for tissue-engineered blood
vessels because these MSCs can be easily obtained from human
lipoaspirate, readily expanded in culture, and differentiated into
contractile VSMC-like cells in culture media containing both TGF-β
and BMP-4.
164
Initial hypotheses for the origin of neointimal VSMCs proposed that injury-induced growth factors and ECM proteolysis
caused a VSMC phenotypic switch from a quiescent, contractile phenotype to a synthetic type, resulting in proliferation and
migration of a small number of clonal or oligoclonal VSMCs from
the underlying media into the intima where remodeling led to
plaque formation and lumen occlusion. Subsequent evidence
suggested that circulating bone marrow–derived SMC progenitor cells may contribute to normal vascular injury repair and
formation of the neointima in vascular lesions.
2,158
However, the
origin of intimal VSMCs from bone marrow–derived progenitor
cells in the blood in response to injury or disease has been dis-
165,166
puted.
In long-term studies of transplanted bone marrow
cells into lethally irradiated mice with wire injury, the bone marrow–derived cells, initially found in high numbers in the neointima, were not stable residents, and the few remaining after
16 weeks did not exhibit definitive VSMC marker proteins calponin and SM MHC. Additionally, the adventitial layer of the wall
serves as a niche for wall-derived MSCs and VSMC progenitor
161
cells,
including resident stem cell antigen-1 (Sca-1)-positive
cells, maintained in the adventitia by Shh signaling and myocardin transcriptional corepressors, which are capable of differentiating into VSMCs.
167
This population of Sca1+ progenitor cells in
the arterial adventitia could contribute to vessel wall remodeling in injury/disease. The prevailing hypothesis is that neointimal
VSMCs originate from the injured media and also from local resident progenitors in the adventitia.
166
The nature of VSMC phenotype plasticity, exemplified in distinct genetic expression patterns of marker genes and thus in
differential functions, complicates the definition and identification of VSMCs derived from bone marrow resident and circulating stem/progenitor cells.
168
The safe and effective use of
regenerative VSMCs in translational clinical therapy for cardiovascular disease awaits further methodologies for identifying, producing, and isolating cells that will differentiate into
VSMCs.
Conclusions
The protean nature of VSMCs is fundamental not only to their
contractile and synthetic functions within the normal vessel wall
during development and maturation, but also to vascular remodeling in response to injury and disease. As evidenced in this chapter,
recent advances in studies from animal models, the clinic, and
basic cell biology laboratories have enhanced our understanding of the factors, both intrinsic in the genetic code and extrinsic
in environmental cues, that regulate and control VSMC plasticity.
Future challenges include how to translate this understanding into
developing clinically effective pharmacological interventions for
treatment of cardiovascular disease and into producing functional
tissue-engineered vascular constructs for diseased/injured vessel
replacement.
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CHAPTER
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4 Connective Tissues
of the Subendothelium
Rajendra Raghow
Varieties of Blood Vessels and Their
Connective Tissue
The vascular system consists of a massive network of tubular
channels that circulate blood to transport nutrients and oxygen
to the tissues; blood vessels also serve as conduits for leukocytes
that carry on immunological surveillance and need to move
rapidly to sites of injury and inflammation. The vascular endothelium and its specialized extracellular matrix (ECM), owing
to their location between circulating blood and underlying tissues, have evolved with unique structural and functional properties that ensure optimal tissue homeostasis. The elastic fibers and
tensile forces–bearing networks of ECM that reside in the vessel
wall maintain their histological integrity in the face of enormous
mechanical load. Yet, the organization of the vessel walls allows
leukocytes to move through them without any obvious leakage.
The mechanical function of the vascular ECM has been recognized for a long time. In recent years, compelling data have accumulated to indicate that molecular components of ECM provide
informational cues to the endothelial cells (ECs) and vascular
smooth muscle cells (VSMCs) to regulate their proliferation, differentiation, and death. Additionally, ECM can sequester a number
of growth factors and cytokines, thereby modulating their spatial and temporal actions to regulate disparate physiological and
pathological responses of the vascular tissues.
The evolutionary transition from an open to a closed circula-
tory system is clearly reflected in the architecture of the blood ves-
1,2
sels.
The size and anatomical organization of individual vessels
vary with their specific locations and functions in the body. The
major vessels that carry blood directly from the heart are capable of storing and releasing large amounts of energy during the
cardiac cycle. As a result, the walls of large arteries are relatively
thick and more elastic to allow their expansion and contraction in
response to the systolic and diastolic cycles of the heart. Without
such elasticity, the intense surge in pressure as blood is ejected
from the heart would inhibit its emptying, and the pressure in the
vessels would fall too low for the heart to refill. The elasticity of
large arteries enables them to store a portion of the stroke volume
with each systole and discharge that volume with diastole. Thus,
the unique structure of large arteries allows the flow of blood from
the heart to be continuous, smooth, and efficient.
The smaller arteries are more rigid. Regulation of blood flow
in small arteries is facilitated by the contractile activity of their
smooth muscle cells (SMCs), which control the size of the vessel
lumen, depending on the rate of blood flow in a given location.
Capillaries contain only one layer of endothelial cells (ECs) with
an underlying basement membrane. This thin-walled structure of
capillaries permits rapid exchange of water, nutrients, and metabolic products between blood and interstitial fluids. Capillaries
deliver blood to the venous system at a much lower pressure.
Consequently, veins and venules have thinner walls, less ECM, and
a larger lumen than their arterial counterparts. They also have far
fewer SMCs and are equipped with valves to prevent reversal of
blood flow due to hydrostatic forces.
The walls of the large arteries contain three identifiable layers. The
luminal surface of arteries contains a single layer of polygonal ECs
connected by gap junctions. This cell layer rests on a basement membrane, which in turn is supported by a network of elastic fibers in a
fenestrated plate called the internal elastic lamina. This region of the
wall is called the tunica intima. The middle layer, called the tunica
media
, represents the bulk of the vessel wall, contains few elastic fibers
but has a large number of VSMCs, with their long axes perpendicular
to the lumen axis.
synthesize the major components of ECM that ultimately define the
mechanical properties of the vessel. The extracellular space contains
a variable mixture of collagen fibers in a continuous sheath adjacent
to the elastic fibers. The external elastic lamina separates the medial
and adventitial layers of the vessel wall. The outermost layer of the
vessel wall, the tunica adventitia, consists primarily of collagen-rich
ECM and the vasa vasorum, a network of vessels that supplies nutrients and O
anatomy and high collagen content of the tunica adventitia help
prevent arterial rupture at extremely high pressures, the adventitia is
highly susceptible to vascular inflammation.
The walls of smaller arteries are intermediate in size. The
tunica intima is relatively thin, as is the medial layer. The tunica
adventitia of small arteries usually contains more densely packed
collagen fibers arranged longitudinally along the vessel axis.
Arterioles have simpler walls; their EC layer is surrounded by
VSMCs, and the adventitia is smaller and more pliable compared
with those of larger arteries.
are surrounded by a few SMCs that control the amount of blood
passing through them. The walls of arterial and venous capillaries
are lined with flat ECs surrounded by a basement membrane; a
discontinuous sheath of pericytes and a fibrous reticulum, made
primarily of type III collagen, are attached to the basement membrane. The walls of venules also contain a reticular network of
collagen fibers derived from type III collagen, along with smaller
quantities of type I collagen fibers.
3
Smooth muscle cells residing in the tunica media
to the outer portion of arterial walls. Although the unique
2
1,3
Capillaries adjoining the arterioles
Vascular Morphogenesis
and Extracellular Matrix
Two distinct processes, vasculogenesis and angiogenesis,
are involved in the formation of blood vessels in vertebrates.
Vasculogenesis is de novo vessel formation that primarily
occurs in the developing embryo. Conversely, angiogenesis is
the process by which new vessels are sprouted from preexisting
blood vessels throughout life. During early embryogenesis, ECs
begin the process of vasculogenesis by forming a network of
capillaries in the absence of blood flow. Following the onset of
blood circulation, primitive capillary networks are transformed
into arteries and veins to form the fully functional closed circulatory system in the developing fetus. For obvious reasons, the
mechanisms of vasculogenesis and angiogenesis have received
intense scrutiny in recent years. Although both vasculogenesis
and angiogenesis are orchestrated by interactions among the
ECs, hematopoietic cells, and VSMCs, the detailed molecular
mechanisms involved in these processes are distinct.
The preceding overview underscores the striking structural and
phenotypic diversity of different branches of the vascular tree.
Therefore it is not surprising that the vascular ECM displays similar complexity depending on its location in the vasculature.
caveat notwithstanding, all vascular ECM is composed of fibrillar
and nonfibrillar components. The fibrillar component of the vascular connective tissue is mainly collagen, and a diversity of proteins and proteoglycans (PGs) make up the rest. What follows is an
overview of the structural and functional properties of the major
macromolecules that characterize the vascular ECM. For a more
detailed discussion of the individual classes of ECM macromolecules, astute readers will need to consult specialized reviews and
critical commentaries, a number of which are cited in the chapter.
2–5
This
43

44
Collagens
Twenty-eight genetically distinct types of collagen comprising 43
unique α chains have been identified in vertebrates (
The vast majority of these collagens exist in humans.
their domain organization and other structural features (
CH
collagens may be categorized as (1) fibril-forming collagens repre-
4
sented by types I, II, III, V, XI, XXIV, and XXVII; (2) fibril-associated collagens with interrupted triple helices (FACIT; e.g., IX, XII, XIV, XVI, XIX,
XX, XXI, XXII, and XXVI collagens); (3) collagens capable of forming
hexagonal network (e.g., VIII, X); (4) basement membrane collagens represented by IV collagen; (5) collagens that assemble into
beaded filaments (e.g., type VI); (6) anchoring fiber-forming collagens (e.g., VII); (7) plasma membrane-spanning types XIII, XVII,
XXIII, and XXV collagens; and (8) collagens with unique domain
organization, represented by types XV and XVIII.
We should note that the nomenclature of proteins as collagens and their classification into different types is somewhat
arbitrary, since collagen fibrils invariably consist of more than
one type of collagen. For instance, type I collagen fibrils contain small amounts of type III, V, and XII; similarly, type II collagen
fibrils contain significant amounts of collagen types IX and XI.
Even more strikingly, types V and IX collagen are known to form
hybrid fibrils. The discovery of collagens that have extensive nontriple-helical domains and several proteins that contain triplehelical domains, such as C1q, adiponectin, acetyl cholinesterase,
and ectodysplasin (see
what constitutes a “true collagen” and how it should be classified. Although several collagen types are found in the vasculature, collagen types I and III are the dominant constituents of the
blood vessel wall.
our discussion because they are not relevant to the ECM of the
vascular endothelium.
Fibrillar Collagens
The collagen molecule, the basic unit of collagen fibers, has an
asymmetrical, rodlike structure composed of three polypeptide
chains called a chains. Because of the Gly-X-Y repeating units
and their stereochemistry, each α chain forms a minor helix
(
Fig. 4-2). Three α chains wind around a common axis to form a
right-handed triple helix. In some collagens, all three α chains are
identical, while in others two or three unique α chains form the
triple-helical molecule. Type I and type III collagens are the most
abundant collagens in the blood vessel and together form the striated fibrils. With the exception of types XXV and XXVII, fibrillar
collagens form an uninterrupted triple-helical domain of approximately 300 nm. The type I collagen α chains contain 338 Gly-X-Y
repeats, and there are 341 such triplets in type III α chains. At
both the NH
of nonhelical sequences of approximately 15 to 20 amino acid residues, referred to as telopeptides.
Because of their similarities, type I and type III collagens are dis-
cussed together here. The type I collagen molecule is a heterotrimer of two identical α chains, α1(I), and a different α chain, α2(I),
and has the chain structure [α1(I)]
molecule is formed by three identical α chains and has the chain
structure [αI(III)]
repeating triplet sequence of [Gly-X-Y]
amino acid but are most frequently proline or hydroxyproline. The
amino acid residues in the Y position are nearly always hydroxylated (4-hydroxyproline). The configuration of the amino acids
forces the α chain to assume a left-handed helix, thus allowing
α chains to form a right-handed supercoil with a one-amino-acid
stagger between adjacent chains. The presence of glycine (without
a bulky side chain) as every third amino acid is critical because it
will occupy the center position within the triple helix. Substitution
of any other amino acid for glycine in the Gly-X-Y leads to disruption of the triple helix.
and COOH ends of each α chain are short segments
2
Fig. 4-1), further challenge the notion of
6,7,9
Collagen types II and X are excluded from
α2(I)]. The type III collagen
2
. The helical domain of the α chain contains a
3
, where X and Y may be any
n
Table 4-1) .
6–9
Based on
Fig. 4-1),
The collagen triple helix is further stabilized by interchain
hydrogen bonds contributed by hydroxyproline residues. Thus,
the collagen molecule is a long cylindrical rod with dimensions of 1.5 nm × 300 nm. Under physiological conditions of ionic
strength, pH, and temperature, collagen molecules spontaneously
aggregate into striated fibrils. Fibril formation occurs by lateral
aggregation of collagen molecules, in which each neighboring
row of molecules is displaced along its long axis by a distance of
68 nm. In addition, within the same row, there is a gap of approximately 40 nm between the end of one molecule and the beginning
of the next (see Figs. 4-1 and 4-2). The short nonhelical telopep-
tides at the NH
the gap or hole zone of the fibril and are therefore accessible to
and COOH ends of each α chain are located in
2
enzymes that regulate collagen cross-linking.
Network-Forming Collagens
As shown in Figure 4-1, collagen types IV (α1-α6 chains), VI
(α1-α5 chains), VIII (α1-α2 chains) and X are known to form
networks in the ECM of basement membranes. The supramolecular organization and function of type IV collagen has been
extensively characterized. Six different α polypeptide chains of
collagen IV are each encoded by an evolutionary conserved
gene. The amino and carboxyl propeptides of type IV collagen
remain as integral parts of the molecules when they are deposited in the basement membrane. As a result, rather than forming
a quarter-stagger, side-by-side alignment of individual molecules, as seen in types I, II, and III collagens, type IV collagen α
chains form chicken-wire structures by end-to-end associations
stabilized by lysine-derived cross-linking and interchain disulfide bonds (
are more closely related to each other than to α3(IV)1, α4(IV),
α5(IV), and α6(VI); the latter share a high degree of sequence
homology with each other. The amino terminal domains of
α1(IV) and α2(IV) collagen chains are 143 and 167 amino
acids, respectively; the NH
are much smaller (ranging in size from 13 to 19 amino acids).
Theoretically, all six α chains of type IV collagen may combine
randomly to generate 56 unique triple-helical permutations.
However, as shown in
branes the most common composition of triple-helical fibrils is
[α1(IV)1]
are also present in basement membrane.
Organization of the type IV collagen genes is unusual. The
COLA4A1 and COLA4A2 genes are paired head-to-head on the
same chromosome and are transcribed in opposite directions.
The pairs of COLA3A4 and COLA4A4 and COLA4A5 and COLA4A6
genes are similarly arranged, except each pair is located on a different
chromosome. Type IV collagen genes are very large, as exemplified
by COLA4A1 and COLA4A5 genes that exceed 100 kb in size.
Type VI collagen, another network-forming molecule, is represented by six distinct α chains in the mouse and five α chains in
humans; the gene encoding the putative α4(VI) collagen chain
is not functional in humans. Heterotrimers of different α chains,
encoded by unique genes, form the basic unit of type VI collagen.
Alternate splicing of messenger ribonucleic acids (mRNAs) generates additional variants of α2 (VI) and α3 (VI) chains.
X-Y domains of α chains of type VI collagen microfibrils are rather
short (about 330 amino acid residues) and are flanked by a number of von Willebrand factor (vWF) A domains.
Type VI collagen forms relatively unusual aggregates by a stepwise assembly into the triple-helical monomeric units that form
dimers in an antiparallel fashion. The dimers in turn form tetramers, held together by disulfide bonds, to create scissors-like structures. The supramolecular assemblies of type VI collagen, formed
by end-to-end associations of tetramers, appear as beads on a
string, as revealed by electron microscopy.
structures have been observed in vascular subendothelium and
skeletal muscle basement membranes. Type VI collagen microfibrils exhibit unique adhesive properties to other ECM components,
Fig. 4-3). The α1(IV) and α2(IV) collagen chains
-termini of the other four α chains
2
Figure 4-4, in vascular basement mem-
α2(IV). The [α3(IV)1]2 α4(IV) and [α5(IV)1]2 α6(IV)
2
9,10
7,9
The Gly-
9
These characteristic

TABLE 4-1 Collagen Types, Constituent α-Chains, and Their Genes*
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TYPE αCHAINS GENE # AMINO ACIDS TISSUE DISTRIBUTION
I α1(I)
II α1(II) COL2A1 1487 Cartilage, vitreous humor, cornea
III α1(III) COL3A1 1466 Tissues containing collagen I, except bone and tendon
IV α1(IV) COL4A1 1669 Basement membranes (BM)
V α1(V ) COL5A1 1838 Tissues containing collagen I
VI α1(VI) COL6A1 1028 Most connective tissues
VII α1(VII) COL7A1 2944 Anchoring fibrils
VIII α1(VIII) COL8A1 744 Many tissues
IX α1(IX) COL9A1 921 Tissues containing collagen II
X α1(X) COL10A1 680 Hypertrophic cartilage
XI α1(XI) COL11A1 1806 Tissues containing collagen II
XII α1(XII)
XIII α1(XIII) COL13A1 717 Many tissues
XIV α1(XIV) COL14A1 1796 Tissues containing collagen I
XV α1(XV) COL15A1 1388 Many tissues in the BM zone
XVI α1(XVI) COL16A1 1604 Many tissues
XVII α1(XVII) COL17A1 1497 Skin hemidesmosomes
XVIII α1(XVIII) COL18A1 1516 Many tissues in the BM zone
XIX α1(XIX) COL19A1 1142 Many tissues in the BM zone
XX
XXI α1(XXI) COL21A1 957 Fetal tissues and blood vessels
XXII α1(XXII) COL22A1 1626 BM of myotendinous junctions
XXIII α1(XXIII) COL23A1 540 Lung, kidney, brain, tumor cells
XXIV α1(XXIV) COL24A1 1714
XXV α1(XXV) COL25A1 654 Amyloid plaques
XXVI α1(XXVI) COL26A1 439 Testis and ovaries
XXVII α1(XXVII) COL27A1 1630 Early development of many tissues
XXVIII α1(XXVIII) COL28A1 1125 Dorsal root ganglia
*
Adapted from Myllyharju J, Kivirikko KI: Collagens, modifying enzymes and their mutations from humans, flies and worms. Trends Genet 20:33–43, 2004; and Gordon MK, Hahn RK: Collagens. Cell
Tissue Res 339:247–257, 2010.
α2(I)
α2(IV) COL4A2 1712
α3(IV) COL4A3 1670
α4(IV) COL4A4 1690
α5(IV) COL4A5 1685
α6(IV) COL4A6 1691
α2(V ) COL5A2 1499
α3(V ) COL5A3 1745
α2(VI) COL6A2 1019
α3(VI) COL6A3 3177
α4(VI) COL6A4 2611
α2(VIII) COL8A2 703
α2(IX) COL9A2 869
α3(IX) COL9A3
α2(XI) COL11A2 1736
α3(XI) COL11A3 1806
COL1AI
COL1A2
COL12A1 3063 Tissues containing collagen I
1464
1366
Most connective tissues, especially in bone, tendon, ligament
45
CH
4
ConnECTivE TissuEs of THE subEndoTHElium
such as other collagens, heparin, and vWF, and may be involved
in the adhesion of platelets and SMCs. In the medial layer, type VI
collagen facilitates interaction between SMCs and elastin by bridging the elastin fibers and cells.
11
As illustrated in Figure 4-5 (see discussion in “Metalloproteinases”), types VIII and X collagens comprise a unique subfamily
of collagens that form hexagonal networks. These relatively short
collagens, containing noncollagenous domains on their NH2 and
COOH termini, are collectively known as the multiplexin family of col-
lagens. Type VIII collagen is expressed in many tissues, especially in
the endothelium, while type X is exclusively associated with hypertrophic chondrocytes during cartilage and bone development.
The preponderance of evidence to date indicates that the two
α chains of collagen VIII, encoded by COL8A1 and COL8A2,

46
CH
Fibril-forming collagens, types I, II, III, V, XI, XXIV, and XXVII
Genes: COL1A1, COL1A2, COL2A1, COL3A1, COL5A1, COL5A2,
COL5A3, COL5A4, COL11A1, COL11A2, COL24A1, COL27A1
Triple helical region
N pro-
4
A
peptide
100 nm
C propeptide
300 nm
Collagens forming hexagonal networks, types VIII and X
Genes: COL8A1, COL8A2, COL10A1
VIII X
100 nm
C
100 nm
The family of type IV collagens
Genes: COL4A1, COL4A2, COL4A3, COL4A4, COL4A5, COL4A6
7S
100 nm
D
Type VII collagen forming anchoring fibrils
Gene: COL7A1
VII
100 nm
Dimer
200 nm
Dimer
Tetramer
7S
200 nm
Collagens with transmembrane domains,
types XIII, XVII, XXIII, and XXV
Genes: COL13A1, COL17A1, COL23A1,
COL25A1
XIII
XVII
XXIII
PM
XXV
100 nm
F
Basement
membrane
Anchoring fibril
Anchoring
plaque
G
FACIT and related collagens, types IX, XII, XIV, XVI,
XIX, XX, XXI, XXII, and XXVI
Genes: COL9A1, COL9A2, COL9A3, COL12A1,
COL14A1, COL16A1, COL19A1, COL20A1,
COL21A1, COL22A1, COL26A1
GAG
IX
GAG
Type II fibril
XII and XIV
Type I fibril
B
100 nm
100 nm
Type VI collagen forming beaded filaments
Genes: COL6A1, COL6A2, COL6A3
VI Dimer Tetramer Beaded filament
E
100 nm
100 nm
Type XV and XVIII collagens
Gene: COL15A1, COL18A1
XV
XVIII
100 nm
GAG
H
Restin
Endostatin
Proteins containing triple-helical collagenous domains
C1q Adiponectin Ficolin
20 nm
Collectin
(surfactant
protein)
Acetyl-
cholinesterase
Macrophage
receptor
Ectodysplasin
PM PM
I
FIGURE 41 Classification of superfamily of vertebrate collagens. Based on their primary structure, domain organization, and ability to form supramolecular
assemblies, all currently known collagens may be divided into nine families. These include (A) fibril-forming collagens, (B) fibril-associated collagens with interrupted
triple helices (FACIT collagens), located on the surface of collagen fibrils, and structurally related collagens, (C) collagens capable of forming hexagonal networks,
(D) the family of type IV collagens located in the basement membranes, (E) type VI collagen that forms beaded filaments, (F) collagen that forms anchoring filaments
of basement membranes, (G) collagens with transmembrane domains, and (H) the family of XV and XVIII collagens. The supramolecular organization of collagens
in (G) and (H) are not known. Polypeptide chains found in the 27 collagen types, each consisting of three chains, are encoded by 42 unique genes (written in blue).
A number of proteins possess collagenous domains (I) but are not considered to be bona fide collagens. The N- and C-terminal noncollagenous domains of these
proteins are shown in dark pink, and noncollagenous domains interrupting the collagen triple helix in light blue. For acetylcholinesterase, the catalytic domain (shown
in green) and the tail domain are encoded by separate exons. GAG, glycosaminoglycan. (From Myllyharju J, Kivirikko KI: Collagens, modifying enzymes and their mutations
in humans, flies and worms. Trends Genet 20:33–43, 2004.)
assemble into homotrimers of α1(VIII) and α2(VIII) (Fig. 4-6) .
Hexagonal aggregates of type VIII collagen have been observed
both
in vivo (e. g., Descemet's membrane of the cornea) and
in vitro with purified protein. It is believed that type VIII collagen is
capable of assuming other forms of macromolecular aggregates,
since hexagonal lattices have yet to be observed in the subendothelial ECM.
9
Fibril-Associated Collagens with Interrupted
Triple Helices
As the name suggests, the FACIT collagens (types IX, XII, XIV, XVI,
XIX, XX, XXI, XXII, and XXVI) do not form fibrils themselves but
associate with other fibril-forming collagens.
the prototype of this group, is cross-linked to the surface of type II
9
Type IX collagen,

47
Late transport vesicles and
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Endoplasmic reticulum
OH
OH
OH
OH
O-Gal
OH
Glc
Gal
O
OH
OH
OH
OH
OH
OH
O
Gal
OH
Glc
Gal
O
OH
OH
OH
OH
OH
O
Gal
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
O-Gal
SH
SH
OH
(Manin
GlcNAc
OH
OH
OH
OH
Glc
SH
SH
(Manin
GlcNAc
S
S
Polypeptide synthesis
Collagen prolyl 4-hydroxylase
OH
Lysyl hydroxylase
Prolyl 3-hydroxylase
Collagen gal-transferase and
glc-transferase
N glycosylated residue
Assembly of three
procollagen chains
S
Protein disulfide isomerase
S
Assembly of triple helix
Secretion of procollagen
in transport vesicles
extracellular matrix
N and C
proteinases
Cleavage of
propeptides
Assembly into
collagen fibrils
Lysyl oxidase
Formation of
covalent cross links
CH
4
ConnECTivE TissuEs of THE subEndoTHElium
FIGURE 42 An overview of the main steps involved in the synthesis of fibril-forming collagens. The α-polypeptide chains are synthesized on membrane-
bound ribosomes and secreted into the lumen of the endoplasmic reticulum (ER). The main steps in collagen biosynthesis are (i) cleavage of the signal peptide (not
shown), (ii) hydroxylation of specific proline and lysine residues, (iii) glycosylation of certain asparagine residues in the C-peptide, and (iv) formation of intramolecular
and intermolecular disulfide bonds. A nucleus for the assembly of the triple helix is formed in the C-terminal region after the C propeptides of three α-chains become
registered with each other and ~100 proline residues in each α-chain have been hydroxylated to 4-hydroxyproline. The triple helix formation proceeds toward
the N-terminus in a zipper-like fashion. Procollagen molecules are transported from the ER to Golgi, where they begin to associate laterally and exit the cell via
secretory vesicles. This is followed by cleavage of N and C propeptides, spontaneous self-assembly of the collagen molecules into fibrils, and formation of cross-links.
(From Myllyharju J, Kivirikko KI: Collagens, modifying enzymes and their mutations in humans, flies and worms. Trends Genet 20:33–43, 2004.)
collagen fibrils in cartilage (see Fig. 4-1); type XII and type XIV
collagens are found in both cartilage and noncartilaginous tissues,
where they are involved in controlling the diameter of collagen
fibrils (see
Fig. 4-1). The other FACIT-like collagens (e.g., types XVI,
XIX, and XXII) are localized in specialized basement membranes.
For instance, XVI collagen is associated with fibrillin 1 near the
epidermal basement membrane. Collagen types XXI and XXII are
closely related to each other in structure and are involved in formation of supramolecular aggregates in the basement membranes
of myotendinous junctions.
12,13
As a key constituent of cutaneous
basement membranes, anchoring fibrils of type VII collagen form
a structural continuum between the dermis and epidermis of normal human skin. The vWF A–like domain in collagen VII binds to
fibrils of type I collagen
in vitro.
14
to each other in their primary structure, but the patterns of their
expression appear to be unique. Type XXV collagen is enriched in
the senile plaques of Alzheimer's disease brains.
9,12,13,15
High expression of full-length collagen XXIII is found in the lungs, whereas its
shed form is enriched in brain, suggesting that shedding of XXIII
collagen occurs in a tissue-specific manner.
Collagen types XV and XVIII are highly pertinent to the EC
biology in several ways.
16,17
The full-length types XV and XVIII
collagen are basement membrane components; their triple-helical domains share a high degree of homology. Collagen types XV
and XVIII were initially identified as PG core proteins containing chondroitin sulfate and heparan sulfate (HS) side chains,
respectively. The COOH-terminal domains of XV and XVIII collagens can be cleaved to generate biologically active peptides,
endostatin and restin, respectively; these peptides inhibit migra-
Minor Collagen Types with Unique Structures
As illustrated in Figure 4-1, the transmembrane collagens (types
XIII, XVII, XXIII, and XXV) contain a cytoplasmic domain, a
membrane-spanning hydrophobic domain, and extracellular triplehelical domains interspersed with noncollagenous domains; these
collagens may also exist in a soluble form. Type XVII collagen is
a unique member of this group that is expressed on the basal surface of keratinocytes that bind to laminin found in the basement
membrane; compared with the other three members of this group,
type XVII has a rather large intracellular domain whose function
remains unknown. Collagen types XIII, XXIII, and XXV are similar
tion of ECs and thus potently block angiogenesis.
binant collagen XV binds to fibronectin (FN), laminin, and
vitronectin (VN) but not to fibrillar collagens, fibril-associated
collagens, or decorin.
18
Finally, collagens XXVI and XXVIII are newly discovered collagens that are unique both with regard to their structures and tissue-specific distributions. The triple-helical domain of type XXVI
is rather small, with only 146 Gly-X-Y repeats. Expression of type
XXVI collagen occurs predominantly in testis and ovary. The von
Willebrand factor–A domains flank the triple-helical structure of
type XXVIII collagen that is almost exclusively expressed in the
peripheral nerves.
19,20
In vitro, recom-

48
Human
col (IV)
Signal
peptide
N-terminal
domain (7S)
Central triple-helical
interrupted domain
C-terminal
domain (NC1)
CH
4
Chain
A
1-
2-
3-
4-
N-terminal
7S domain
1
2
3
4
5
6
Monomer
NC1 interactions
Trimer
Sheet structure
143 aa
157
13
26
15
29
Central
domain
1271 aa
1303
1398
1396
1416
1407
7S NC1
C-terminal
NC1 domain Interruptions
228 aa
227
231
230
228
227
B
FIGURE 43 A, Linear structures of human collagen IV α-chains. Six different
genes encode collagen IV α-chains. Each polypeptide is composed of three
distinct domains: a cysteine-rich N-terminal 7 S domain, a central triple-helical
domain with multiple small interruptions (boxes), and a globular C-terminal
noncollagenous NCl domain. The NCl and central triple-helical domains are of
an equivalent size, whereas 7 S domains are shorter in the cases of α3, α4, α5,
and α6 compared with α1 and α2. On the basis of sequence homology, type IV
collagen α-chains can be divided in two groups: the α1-like (α1, α2, α5) and the
α2-like (α2, α4, α6). B, Assembly of collagen IV α chains. Assembly of trimers is
dependent on the association of NCl domains, followed by formation of triplehelical structure and 7 S domains in a spider-shaped structure; the two trimers
interact head-to-head through their NCl domains, forming a sheet structure.
Several trimers can also lace together along their triple-helical domains,
thickening the structure. (Adapted from Company of Biologists Ltd., Ortega N,
Werb Z: New functional roles for noncollagenous domains of basement membrane
collagens. J Cell Sci 115:4201, 2002.)
Regulation of Collagen Biosynthesis
Collagen chains are synthesized as prepro-α chains from which
the hydrophobic leader sequence is removed prior to secretion, and the pro-α chains are secreted into the extracellular
space (see
peptide (N-peptide) and a COOH propeptide (C-peptide). The
N-peptide consists of a 139-residue sequence that precedes a
17-residue sequence of nonhelical telopeptide. This is followed
by a 1014 amino acids-long Gly-X-Y helical sequence attached
sequentially to a 26 residues-long COOH telopeptide and a
262-residues-long nonhelical C-peptide. The domain organization of pro-α2(I) and pro-α1(III) chains are similar except for
minor variations in the number of amino acid residues.
The genomic organization and chromosomal locations for
genes that encode collagens have been studied. In humans, the
genes encoding 43 distinct α chains are dispersed on at least 15
Fig. 4-2). The pro-α1(I) chain contains an NH2 pro-
21
26
26
25
23
25
6,7,21
FIGURE 44 Localization of the a1·a2 and a1·a2·a5·a6 networks of
type IV collagen in vascular basement membranes (BMs). Schematic
diagram of a large artery (aorta) depicts its multilayered structure (right).
Endothelial cells (En) rest on a subendothelial BM, which contains the α1∙α2(IV)
collagen network (right). Smooth muscle cells (SMCs) in the media are
surrounded by smooth muscle BM and are sandwiched between an internal and
external elastic lamina (IEL and EEL, respectively). The α1∙α2 and α1∙α2∙α5∙α6
networks of type IV collagen coexist in smooth muscle BM (right). (Adapted from
Borza DB, Bondar O, Ninomya Y, et al: The NCl domain of collagen IV encodes a novel
network composed of the alpha-1, alpha-2, alpha-5, and alpha-6 chains in smooth
muscle basement membranes. J Biol Chem 276:28532, 2001.)
chromosomes. Unlike majority of the collagen-encoding genes, the
six homologous α-chains of type IV collagen are encoded by genes
that are located in pairs with head-to-head orientation on chromosomes 13 (COL4A1 and COL4A2), 2 (COL4A3 and COL4A4),
and the X chromosome (COL4A5 and COL4A6). Interestingly, the
promoters of these pairs of type IV collagens overlap, suggesting
a coordinate regulation of the gene pairs. The precise molecular mechanisms of this regulation, however, remain incompletely
6,7,9,21,22
known.
The molecular events involved in procollagen biosynthesis, from
transcription and splicing of mRNA to its transport and translation
in the cytoplasm, are nearly identical to most other proteins synthesized by eukaryotic cells. Regulation at the level of transcription and mRNA turnover appears to be involved in the coordinated
synthesis of two pro-α1(I) chains for every one of pro-α2(I) chain.
Most cells that produce type I collagen also produce type III collagen in variable amounts, depending on the specific type of tissue,
its age, and the physiological and pathological situations.
The molecular mechanisms of regulation of biosynthesis of a
number of collagens have been studied to varying degrees, both
in physiological and pathological settings. Regulation of genes that
encode α chains of type I collagen has been studied extensively
and is briefly summarized. Transcriptional regulation of genes that
generate fibrillar (COL1A1, COL1A2, COL3A1) and basement membrane (e.g., COL4A1-6) collagens evidently involves both genomic
and epigenomic (deoxyribonucleic acid [DNA] methylation and
posttranslational modification of histones) mechanisms. Although
collagen genes are predominantly regulated at the level of transcription, a number of reports indicate that posttranscriptional regulation is also exerted under some conditions.
The cis-acting elements of COLA1 and COLA2 genes are modularly organized on either side of the transcription start point
(TSP). The regulatory elements are distributed over a distance
of 100 to 150 kb of genomic DNA, depending on the specific
gene and the assays used to study their transcriptional and posttranscriptional regulation. The tissue-specific and inducible
activation of collagen genes involves complex interactions
among the cis-acting modules of their promoters and enhancers. Promoters of COLA1 and COLA2 genes contain TATA boxes
located 25 to 35 bp upstream of the TSP. Existence of a number of enhancer and repressor cis-elements around the TSP
and in the first intron of COLA1 gene has been demonstrated.

Archetypal MMPs Furin-activatable MMPs
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49
2
Zn
C
2
Zn
C
2
Zn
C
Gelatinases
2
Zn
C
Matrilysins
2
Zn
C
Collagenases
MMP-1, (MMP 1a, MMP1b),
MMP-8, MMP-13
Stromelysins
MMP-3, MMP-10
Other MMPs
MMP-12, MMP-19
MMP-20, MMP-27
Gelatinases
MMP-2, MMP-9
Matrilysins
MMP-7, MMP-26
Signal
peptide
Propeptide
Furin
cleavage
Catalytic
2
Zn
C
2
Zn
C
2
Zn
C
2
Zn
C
Hemopexin
Cytoplasmic
tail
Immunoglobulin
Cysteine
array
Secreted
MMP-11, MMP-21, MMP-28
Type I transmembrane MT-MMPs
MT1-MMP, MT2-MMP
MT3-MMP, MT5-MMP
GPI anchored MMPs
MT4-MMP, MT6-MMP
Type II transmembrane MT-MMPs
MMP-23A, MMP-23B
Fibronectin
Transmembrane
type I
GPI anchor
Transmembrane
type II
2
Zn
C
Hinge
Linker
Zinc ion
Cysteine
FIGURE 45 Domain structure of matrix metalloproteinases (MMPs). The MMPs are multidomain enzymes that have a pro-domain, an enzymatic domain,
a zinc-binding domain, and a hemopexin/vitronectin (VN)-like domain (except in MMP-7 and MMP-26). Additionally, membrane-type MMPs contain membrane
anchor, with some membrane type (MT)-MMPs also possessing a cytoplasmic domain and a carboxyl terminus. Gelatinases contain a gelatin-binding domain with
three fibronectin (FN)-like repeats. In particular, MMP-9 also contains a serine- threonine- and O-glycosylated domain. N-glycosylated sites, one of which is conserved
in most MMPs, are denoted with a Y symbol. Part of the propeptide, which contains the chelating cysteine, and part of the zinc-binding domain with three histidines
are indicated with one letter code for amino acids. (Adapted from Hu J, et al: Matrix metalloproteinase inhibitors as therapy for inflammatory and vascular disease. Nat Rev
Drug Discov 6:480–498, 2007.)
CH
4
ConnECTivE TissuEs of THE subEndoTHElium
A key role for CAAT-binding factor, Sp1, Sp3, Ap1, nuclear factor
(NF)-κB, and SMADs has been reported for several collagen
genes; a number of orientation-dependent enhancer-like elements have also been documented.
23,24
Fibrillar and nonfibrillar collagens found in subendothelial
ECM are regulated by many cytokines and growth factors; collagen gene expression in response to cytokines (e.g., transforming
growth factor [TGF]-β, tumor necrosis factor [TNF]-α, interleukins
[ILs]), glucocorticoids, estrogen, androgen, and retinoids has been
reported. The signaling cascades initiated by intrinsic and exogenous regulators impinge on a distinct set of cis-acting elements
that bind to constitutive and inducible transcription factors. The
emerging theme from these studies is that various cis- and trans-
acting factors interact to recruit selective transcriptional coactivators and co-repressors in response to specific stimuli.
23,24
However,
the precise mechanisms that determine combinatorial interactions under physiological and inflammatory conditions remain to
be elucidated.
Following translation, pre-procollagen α chains are chaperoned
from the endoplasmic reticulum (ER) to the Golgi. It has been
reported that the heat shock protein-47 (Hsp47) functions as a
collagen-specific chaperone; thus, hsp47 is presumed to provide a
quality control mechanism needed for proper maturation of newly
synthesized procollagen chains. To demonstrate a role of hsp47
in vivo Nagai and coworkers25 inactivated Hsp47 gene by homol-
ogous recombination. The mutant embryos died in utero before
11.5 days of postcoitus development as a result of severely reduced
levels of mature type I collagen in their tissues.
As shown in
Figure 4-2, fibrillar and nonfibrillar collagens also
undergo a number of posttranslational modifications for proper
maturation; these include proteolysis of signal peptides, hydroxylation of key proline and lysine residues, glycosylation, and formation of interchain and intrachain disulfide bridges.
6,7,21
Thus,
optimal biosynthesis and assembly of collagens depends on a
number of key enzymes. These include three hydroxylases, two
collagen-specific glycosyl transferases, two unique proteinases
that cleave the NH
oxidase that is needed for cross-link formation. The posttrans-
- and COOH-termini, and a collagen-specific
2
lational processing of the procollagen molecules also needs a
peptidyl proline cis-trans isomerase and a protein disulfide isomerase (PDI).
Vitamin C–dependent 4-prolyl hydroxylase, an α
located in the ER, plays a central role in collagen synthesis
-tetramer
2β2
because 4-proline hydroxylation is obligatory for cross-link formation. In humans, there are three known isozymes of 4-prolyl hydroxylases, each with a distinct α subunit, but all contain PDI as their
β subunit. Hydroxylation of lysine is carried out by lysyl hydroxylase, which also uses the same cofactors as prolyl hydroxylase and
reacts only with a lysine residue in the Y position of the Gly-X-Y
triplets. There are three known isozymes of lysyl hydroxylase in
humans. The under-hydroxylation of procollagen leads to reduced
secretion and rapid degradation. Deficiency of lysine hydroxylase
is associated with skeletal deformities, tissue fragility, and vascular
malformations.
6,7,21
Several collagens undergo glycosylation; both galactose
and glucose residues are attached to some hydroxylysine residues during pre-procollagen biosynthesis. The enzyme UDP
galactose:hydroxylysine galactosyltransferase adds a galactose
residue to the hydroxyl group of hydroxylysine. The UDP glucose galactosyl:hydroxylysine glucosyltransferase then transfers
a glucose residue to the hydroxylysine-linked galactose. The two
enzymes act in sequence so that galactose is added first, with glucose added only to galactose. Glycosylation occurs during nascent
chain synthesis and before the formation of triple helices. Only two
of seven hydroxylysine residues of α1(I), α2(I), and α1(III) contain
the disaccharide; most of the hydroxylysine residues are glycosylated in other collagens. Glycosylation of some hydroxylysine residues imparts stability to the cross-link.
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