26
that was deposited extracellularly during these events, we performed siRNA suppression experiments revealing that ECs were the predominant source of collagen
type IV [94]. Knockdown of collagen type IV in ECs strongly decreased collagen
type IV assembly around tubes and resulting again in increased vascular tube width,
an indicator of dysfunctional interactions between ECs and pericytes [94].
Knockdown of collagen type IV in pericytes had lesser, but nonetheless significant,
inhibitory effects on both collagen type IV deposition and vessel tube width [94].
This suggests that both ECs and pericytes contribute collagen type IV during basement membrane assembly.
1.3.17 Pericyte TIMP-3 Contributes to Vascular Basement
Membrane Matrix Assembly by Increasing Collagen
Type IV Deposition or Stability
Another contributing role of pericytes during this process is the delivery of TIMP-3,
a basement membrane and ECM-binding protein. As discussed earlier, TIMP-3
plays a critical role in pericyte-induced tube stabilization by blocking MMP-1,
MMP-10, and ADAM-15, which promote vascular regression events as well as
inhibiting further morphogenic events by blocking MT1-MMP [87]. In this work,
we show that TIMP-3 plays yet another role, by facilitating collagen type IV assembly in EC-pericyte cocultures. siRNA suppression of pericyte TIMP-3 results in
markedly decreased collagen type IV assembly [94], which may be due to less
deposition or increased turnover due to lack of inhibition of MT1-MMP (which
degrades type IV collagen). With decreased collagen type IV assembly around EC
tubes, there was a significant increase in vessel diameter [94]. Thus, collagen type
IV assembly may be a primary determinant of vascular tube diameter. It is particularly intriguing to consider that EC-only tubes, which are not surrounded by basement membranes, become very wide during morphogenic events. This suggests a
lack of inhibitory signals. EC-pericyte co-assembled tubes are much narrower, suggesting that the inhibitory signals are delivered to ECs through interactions with the
assembled basement membrane to suppress further morphogenesis and promote
maturation. The marked differences in vessel diameter in these two situations demonstrate functional evidence for both the production and deposition of basement
membrane matrices but also reveal that ECs recognize the proteins and respond by
restricting tube diameter. Decreased vessel diameter and tube network areas are
measurements that reflect the ability of pericytes to negatively regulate vascular
tube morphogenesis while at the same time preventing pro-regressive stimuli from
acting. Thus, EC tube diameter is also an important indicator of dysfunctional
EC-pericyte interactions that lead to a variety of vessel abnormalities (which frequently show increased vessel diameter). In addition, it is well known that basement
membranes can facilitate cell polarity functions, by enhancing cell-cell contacts
mediated through junctional contacts, such as through adherens and tight junctional
G. E. Davis
that was deposited extracellularly during these events, we performed siRNA suppression experiments revealing that ECs were the predominant source of collagen
type IV [94]. Knockdown of collagen type IV in ECs strongly decreased collagen
type IV assembly around tubes and resulting again in increased vascular tube width,
an indicator of dysfunctional interactions between ECs and pericytes [94].
Knockdown of collagen type IV in pericytes had lesser, but nonetheless significant,
inhibitory effects on both collagen type IV deposition and vessel tube width [94].
This suggests that both ECs and pericytes contribute collagen type IV during basement membrane assembly.
1.3.17 Pericyte TIMP-3 Contributes to Vascular Basement
Membrane Matrix Assembly by Increasing Collagen
Type IV Deposition or Stability
Another contributing role of pericytes during this process is the delivery of TIMP-3,
a basement membrane and ECM-binding protein. As discussed earlier, TIMP-3
plays a critical role in pericyte-induced tube stabilization by blocking MMP-1,
MMP-10, and ADAM-15, which promote vascular regression events as well as
inhibiting further morphogenic events by blocking MT1-MMP [87]. In this work,
we show that TIMP-3 plays yet another role, by facilitating collagen type IV assembly in EC-pericyte cocultures. siRNA suppression of pericyte TIMP-3 results in
markedly decreased collagen type IV assembly [94], which may be due to less
deposition or increased turnover due to lack of inhibition of MT1-MMP (which
degrades type IV collagen). With decreased collagen type IV assembly around EC
tubes, there was a significant increase in vessel diameter [94]. Thus, collagen type
IV assembly may be a primary determinant of vascular tube diameter. It is particularly intriguing to consider that EC-only tubes, which are not surrounded by basement membranes, become very wide during morphogenic events. This suggests a
lack of inhibitory signals. EC-pericyte co-assembled tubes are much narrower, suggesting that the inhibitory signals are delivered to ECs through interactions with the
assembled basement membrane to suppress further morphogenesis and promote
maturation. The marked differences in vessel diameter in these two situations demonstrate functional evidence for both the production and deposition of basement
membrane matrices but also reveal that ECs recognize the proteins and respond by
restricting tube diameter. Decreased vessel diameter and tube network areas are
measurements that reflect the ability of pericytes to negatively regulate vascular
tube morphogenesis while at the same time preventing pro-regressive stimuli from
acting. Thus, EC tube diameter is also an important indicator of dysfunctional
EC-pericyte interactions that lead to a variety of vessel abnormalities (which frequently show increased vessel diameter). In addition, it is well known that basement
membranes can facilitate cell polarity functions, by enhancing cell-cell contacts
mediated through junctional contacts, such as through adherens and tight junctional
G. E. Davis