25
phenotype, binds basement membrane perlecan as well as other components, to
suppress vascular morphogenesis [87]. The roles of these individual components
need to be investigated in more detail in future studies.
1.3.15 Critical Functional Role for Fibronectin Matrix
Assembly During Vascular Development
A series of studies indicate that fibronectin gene knockouts result in an embryonic
lethal phenotype during vasculogenesis [42, 57]. Also, evidence has been presented
that fibronectin alternative splicing (IIIA and IIIB isoforms) is important during
these events [6]. Further work will be necessary to elucidate why these particular
fibronectin isoforms are playing a role during these processes. Vessel diameters
from these animals are extremely wide which, as discussed above, may be secondary to defects in proper EC-pericyte interactions causing abnormal basement membrane formation or reduced adhesiveness to these remodeled, but abnormal,
matrices. EC-specific knockout of the α5 integrin also shows phenotypes that are
manifested in a wider vessel phenotype, which appears to be further enhanced by
knockout of αv integrins, another class of fibronectin receptors [57].
Since we observed strong fibronectin upregulation in ECs, as well as deposition
selectively in EC-pericyte cocultures, we performed additional experiments to determine if fibronectin matrix assembly affected EC tube maturation events in this system. We incorporated a 70 kDa N-terminal fragment of fibronectin, which is known
to block fibronectin matrix assembly [106], to assess if it had any influence during
these events. Our work shows that disruption of fibronectin matrix assembly affects
EC tube width by significantly increasing it [94], suggesting that deposited fibronectin may play a role in restricting vascular tube diameter. Interestingly, this treatment
also markedly disrupted collagen type IV matrix deposition [94] while having a
lesser influence on laminin assembly. In support of these findings are experiments
showing that selective blockade of EC α5β1 integrin, a fibronectin receptor, also
significantly increases vascular tube width in the EC-pericyte cocultures, but not in
the EC-only cultures, where this receptor appears to play little role [94].
1.3.16 Important Functional Role for Collagen Type IV
in EC-Pericyte Tube Co-Assembly and Maturation
Events
In addition to the critical roles for fibronectin and nidogen-1, as bridging proteins
for ECM assembly, collagen type IV is another key basement membrane component
with affinity for both of these bridging molecules. Interestingly, pericyte-induced
fibronectin assembly around developing tubes appears to be involved in collagen
type IV assembly [94]. To assess which cell types contributed the collagen type IV
1 Molecular Control of Capillary Tube Morphogenesis and Maturation Through…
phenotype, binds basement membrane perlecan as well as other components, to
suppress vascular morphogenesis [87]. The roles of these individual components
need to be investigated in more detail in future studies.
1.3.15 Critical Functional Role for Fibronectin Matrix
Assembly During Vascular Development
A series of studies indicate that fibronectin gene knockouts result in an embryonic
lethal phenotype during vasculogenesis [42, 57]. Also, evidence has been presented
that fibronectin alternative splicing (IIIA and IIIB isoforms) is important during
these events [6]. Further work will be necessary to elucidate why these particular
fibronectin isoforms are playing a role during these processes. Vessel diameters
from these animals are extremely wide which, as discussed above, may be secondary to defects in proper EC-pericyte interactions causing abnormal basement membrane formation or reduced adhesiveness to these remodeled, but abnormal,
matrices. EC-specific knockout of the α5 integrin also shows phenotypes that are
manifested in a wider vessel phenotype, which appears to be further enhanced by
knockout of αv integrins, another class of fibronectin receptors [57].
Since we observed strong fibronectin upregulation in ECs, as well as deposition
selectively in EC-pericyte cocultures, we performed additional experiments to determine if fibronectin matrix assembly affected EC tube maturation events in this system. We incorporated a 70 kDa N-terminal fragment of fibronectin, which is known
to block fibronectin matrix assembly [106], to assess if it had any influence during
these events. Our work shows that disruption of fibronectin matrix assembly affects
EC tube width by significantly increasing it [94], suggesting that deposited fibronectin may play a role in restricting vascular tube diameter. Interestingly, this treatment
also markedly disrupted collagen type IV matrix deposition [94] while having a
lesser influence on laminin assembly. In support of these findings are experiments
showing that selective blockade of EC α5β1 integrin, a fibronectin receptor, also
significantly increases vascular tube width in the EC-pericyte cocultures, but not in
the EC-only cultures, where this receptor appears to play little role [94].
1.3.16 Important Functional Role for Collagen Type IV
in EC-Pericyte Tube Co-Assembly and Maturation
Events
In addition to the critical roles for fibronectin and nidogen-1, as bridging proteins
for ECM assembly, collagen type IV is another key basement membrane component
with affinity for both of these bridging molecules. Interestingly, pericyte-induced
fibronectin assembly around developing tubes appears to be involved in collagen
type IV assembly [94]. To assess which cell types contributed the collagen type IV
1 Molecular Control of Capillary Tube Morphogenesis and Maturation Through…