Neuropilins were discovered as receptors for axon guidance factors
belonging to class-3 semaphorin subfamily. Neuropilins were subsequently found
to function as receptors for vascular endothelial growth factor. We have cloned
two chick isoforms of neuropilin-2 cDNA, and found that while neuropilin-1 is
expressed preferentially in arteries, neuropilin-2 is preferentially expressed
in veins. Using double in-situ hybridization experiments, we found that six
somite embryos contain blood island subpopulations that express neuropilin-1 or
neuropilin-2 as well as a subpopulation that co-expresses both neuropilins. In
13 somite embryos, the expression of neuropilin-1 and neuropilin-2 is
segregated between the arterial and venous parts of the plexus despite the
absence of blood flow. In 26 somite embryos, posterior blood islands expressing
exclusively neuropilin-2 appear to undergo fusion to form the posterior sinus
vein and its tributaries, suggesting that the venous identity of these veins
may be established prior to their formation. Potential sites of semaphorin activity
are expected to occur where neuropilins and plexins are co-expressed. Such
sites were found in veins and in intra-embryonic arteries of 26 somite embryos,
and in blood islands subpopulations of six somite embryos. Using a whole mount
binding assay we found that semaphorin-3f bound exclusively to neuropilin-2 expressing
cells regardless of the presence of plexin-A1 or plexin-A2. We have also found
that semaphorin-3f inhibits sprouting from venous rings. Semaphorin-3f also
inhibited sprouting from arterial rings, although less efficiently than from
venous rings.
