29
assembly [18, 93]. These Factors, by acting on ECs, are also critical in facilitating
pericyte recruitment (through PDGF-BB and HB-EGF) which is required for capillary basement membrane matrix assembly, a fundamental step in vessel
maturation.
Finally, we have addressed a functional role for VEGF during these events, by
acting as an upstream primer of EC responses to the Factors [93]. VEGF pretreatment leads to upregulation of c-Kit, IL-3Rα, and CXCR4, which are the key receptors necessary to act in conjunction with FGF and insulin receptors [93]. The
addition of a combination of FGF and insulin together fails to support EC tubulogenesis. In contrast, addition of all five Factors leads to marked EC tubulogenesis.
Importantly, VEGF pretreatment of ECs for 8 h or more leads to significant increases
in their response to the downstream Factors [93]. Furthermore, VEGF priming also
enhances pericyte recruitment responses, during EC-pericyte tube co-assembly
[93]. This work strongly suggests that VEGF’s functional role needs to be reinterpreted, in that it fails to directly stimulate EC tubulogenesis but can clearly act as an
upstream EC primer/activator, which allows for enhanced responses to downstream
Factors. The unique signaling features of VEGF priming must be investigated and
distinguished from those supplied by the Factors, which separately leads to the
major EC tubulogenic process. Thus, these unique signaling events control distinct
steps in the process of EC tube morphogenesis which then leads to pericyte recruitment to establish capillary networks that mature over time.
1.4 Future Directions
It is clear that major advances have occurred over the past two decades in elucidating molecular mechanisms that underlie the ability of vessels to form, mature, and
regress. In our view, it is this type of mechanistic research that will most likely lead
to the generation of novel therapeutic strategies to manipulate blood vessels in the
context of disease. It is also critical that both in vitro and in vivo approaches be
continued and appreciated by individuals who focus on either side of these strategies. As the in vitro models and experimental strategies have evolved, it is more and
more evident that very rapid advances are occurring in this area. Particular assay
systems have repeatedly been shown to accurately reflect the biology of developing
and postnatal vessels in vivo, and thus, these systems represent a critical experimental approach to rapidly advance the field.
In terms of key future directions, it is clear that more cytokine and growth factor
research needs to be coupled with signal transduction studies to elucidate when and
where particular molecules act to control the development, maturation, and stability
of the vasculature. A key aspect of this question is how distinct growth factors or
combinations (and their unique downstream signals) are necessary to create and
maintain arteries, capillaries, veins, and the lymphatic vasculature. Our recent findings showing that five growth factors, SCF, IL-3, SDF-1α, FGF-2, and insulin, are
required in combination to support human capillary tube assembly is an example of
1 Molecular Control of Capillary Tube Morphogenesis and Maturation Through…
assembly [18, 93]. These Factors, by acting on ECs, are also critical in facilitating
pericyte recruitment (through PDGF-BB and HB-EGF) which is required for capillary basement membrane matrix assembly, a fundamental step in vessel
maturation.
Finally, we have addressed a functional role for VEGF during these events, by
acting as an upstream primer of EC responses to the Factors [93]. VEGF pretreatment leads to upregulation of c-Kit, IL-3Rα, and CXCR4, which are the key receptors necessary to act in conjunction with FGF and insulin receptors [93]. The
addition of a combination of FGF and insulin together fails to support EC tubulogenesis. In contrast, addition of all five Factors leads to marked EC tubulogenesis.
Importantly, VEGF pretreatment of ECs for 8 h or more leads to significant increases
in their response to the downstream Factors [93]. Furthermore, VEGF priming also
enhances pericyte recruitment responses, during EC-pericyte tube co-assembly
[93]. This work strongly suggests that VEGF’s functional role needs to be reinterpreted, in that it fails to directly stimulate EC tubulogenesis but can clearly act as an
upstream EC primer/activator, which allows for enhanced responses to downstream
Factors. The unique signaling features of VEGF priming must be investigated and
distinguished from those supplied by the Factors, which separately leads to the
major EC tubulogenic process. Thus, these unique signaling events control distinct
steps in the process of EC tube morphogenesis which then leads to pericyte recruitment to establish capillary networks that mature over time.
1.4 Future Directions
It is clear that major advances have occurred over the past two decades in elucidating molecular mechanisms that underlie the ability of vessels to form, mature, and
regress. In our view, it is this type of mechanistic research that will most likely lead
to the generation of novel therapeutic strategies to manipulate blood vessels in the
context of disease. It is also critical that both in vitro and in vivo approaches be
continued and appreciated by individuals who focus on either side of these strategies. As the in vitro models and experimental strategies have evolved, it is more and
more evident that very rapid advances are occurring in this area. Particular assay
systems have repeatedly been shown to accurately reflect the biology of developing
and postnatal vessels in vivo, and thus, these systems represent a critical experimental approach to rapidly advance the field.
In terms of key future directions, it is clear that more cytokine and growth factor
research needs to be coupled with signal transduction studies to elucidate when and
where particular molecules act to control the development, maturation, and stability
of the vasculature. A key aspect of this question is how distinct growth factors or
combinations (and their unique downstream signals) are necessary to create and
maintain arteries, capillaries, veins, and the lymphatic vasculature. Our recent findings showing that five growth factors, SCF, IL-3, SDF-1α, FGF-2, and insulin, are
required in combination to support human capillary tube assembly is an example of
1 Molecular Control of Capillary Tube Morphogenesis and Maturation Through…