24
recruitment [94]. Although increased production of individual basement membrane
components was observed, this upregulation was not as marked as that observed in
the immunostaining experiments. We utilized our human EC and bovine pericyte
coculture system to determine which cell type produces particular ECM components over time, to regulate basement membrane matrix assembly (using speciesspecific RT-PCR primer sets) [94]. Major findings were that ECs increased the
production of fibronectin selectively in the presence of pericytes (and not in their
absence) and nidogen-1 was induced in pericytes that occurred selectively in the
presence of ECs [94]. We also observed induction of particular laminin isoforms as
well as perlecan at the mRNA level, which occurred through EC-pericyte interactions. Thus, EC-pericyte contacts during tube co-assembly events affected mRNA
and protein levels for key basement membrane matrix molecules [94]. Interestingly,
both fibronectin and nidogen-1 are known to bridge key molecules that compose the
basement membrane matrix [34, 76, 89]. Fibronectin shows affinity for collagen
type IV and perlecan, while nidogen-1 binds collagen type IV and laminin isoforms.
It is possible that these ECM components initiate a nidus which leads to the assembly of the insoluble matrix surrounding the EC-lined tubes that control basement
membrane deposition, as observed by electron microscopy. Most ECM proteins
have self-assembly functions, but they need to interact with each other to create the
complex meshwork that is characteristic of fully assembled basement membrane
matrices. It is also intriguing that collagen type IV, a fundamental basement membrane component greatly responsible for its structural integrity, shows affinity for
both fibronectin and nidogen-1, which are selectively affected by EC-pericyte interactions [94].
One question that is of great interest is how continuous basement membrane
assembly is accomplished along EC-lined tubes, despite the fact that pericytes are
only one-fifth to one-fourth of the total number of ECs. We believe that this occurs
due to the motility of pericytes along the abluminal EC tube surface which scans
along the tubes to stimulate the deposition of the basement membrane in a continuous manner [94]. Furthermore, the movement of both pericytes and ECs along each
other, within vascular guidance tunnels, will almost certainly exert mechanical
stress on the newly deposited ECM to facilitate basement membrane assembly.
Thus, it is intriguing that fibronectin, a mechanosensitive ECM component whose
assembly is facilitated by cell-exerted tensional forces, is a critical protein that only
strongly deposits around EC-lined tubes when pericytes are present along the tube
surface [94]. Another interesting possibility is that the presence of pericytes, along
the EC abluminal surface (and in a polarized fashion) [94], may stimulate the directional secretion and deposition of basement membrane components from both cell
types toward each other. Thus, both mechanical forces and vectorial secretion
mechanisms may play a fundamental role in how pericyte recruitment to EC-lined
tubes leads to vascular basement membrane matrix assembly, a major step toward
further tube maturation and stabilization. As discussed above, the deposition of laminin isoforms may represent stimuli for ECs to stop undergoing morphogenesis and
become a stable tube structure with a quiescent layer of ECs. Another molecule,
TIMP-3, whose deposition in the basement membrane would lead to a similar
G. E. Davis
recruitment [94]. Although increased production of individual basement membrane
components was observed, this upregulation was not as marked as that observed in
the immunostaining experiments. We utilized our human EC and bovine pericyte
coculture system to determine which cell type produces particular ECM components over time, to regulate basement membrane matrix assembly (using speciesspecific RT-PCR primer sets) [94]. Major findings were that ECs increased the
production of fibronectin selectively in the presence of pericytes (and not in their
absence) and nidogen-1 was induced in pericytes that occurred selectively in the
presence of ECs [94]. We also observed induction of particular laminin isoforms as
well as perlecan at the mRNA level, which occurred through EC-pericyte interactions. Thus, EC-pericyte contacts during tube co-assembly events affected mRNA
and protein levels for key basement membrane matrix molecules [94]. Interestingly,
both fibronectin and nidogen-1 are known to bridge key molecules that compose the
basement membrane matrix [34, 76, 89]. Fibronectin shows affinity for collagen
type IV and perlecan, while nidogen-1 binds collagen type IV and laminin isoforms.
It is possible that these ECM components initiate a nidus which leads to the assembly of the insoluble matrix surrounding the EC-lined tubes that control basement
membrane deposition, as observed by electron microscopy. Most ECM proteins
have self-assembly functions, but they need to interact with each other to create the
complex meshwork that is characteristic of fully assembled basement membrane
matrices. It is also intriguing that collagen type IV, a fundamental basement membrane component greatly responsible for its structural integrity, shows affinity for
both fibronectin and nidogen-1, which are selectively affected by EC-pericyte interactions [94].
One question that is of great interest is how continuous basement membrane
assembly is accomplished along EC-lined tubes, despite the fact that pericytes are
only one-fifth to one-fourth of the total number of ECs. We believe that this occurs
due to the motility of pericytes along the abluminal EC tube surface which scans
along the tubes to stimulate the deposition of the basement membrane in a continuous manner [94]. Furthermore, the movement of both pericytes and ECs along each
other, within vascular guidance tunnels, will almost certainly exert mechanical
stress on the newly deposited ECM to facilitate basement membrane assembly.
Thus, it is intriguing that fibronectin, a mechanosensitive ECM component whose
assembly is facilitated by cell-exerted tensional forces, is a critical protein that only
strongly deposits around EC-lined tubes when pericytes are present along the tube
surface [94]. Another interesting possibility is that the presence of pericytes, along
the EC abluminal surface (and in a polarized fashion) [94], may stimulate the directional secretion and deposition of basement membrane components from both cell
types toward each other. Thus, both mechanical forces and vectorial secretion
mechanisms may play a fundamental role in how pericyte recruitment to EC-lined
tubes leads to vascular basement membrane matrix assembly, a major step toward
further tube maturation and stabilization. As discussed above, the deposition of laminin isoforms may represent stimuli for ECs to stop undergoing morphogenesis and
become a stable tube structure with a quiescent layer of ECs. Another molecule,
TIMP-3, whose deposition in the basement membrane would lead to a similar
G. E. Davis