NRP1 is a membrane-bound coreceptor to a tyrosine kinase receptor for both vascular endothelial growth factor (VEGF) and semaphorin family members. NRP1 plays versatile roles in angiogenesis, axon guidance, cell survival, migration, and invasion.
CHEMICAL STRUCTURE AND IMAGES
The two neuropilin genes, Nrp1 and Nrp2, share 45% homology and map to different chromosomes. In humans, NRP1 is located on chromosome 10, and NRP2 is located on chromosome 2. Both genes are composed of 17 exons, but NRP2 is expressed as several alternatively spliced transcripts (Neuropilin-1 is required for vascular development and is a mediator of VEGF-dependent angiogenesis in zebrafish. 2002). Both Nrps have similar overall structure but differ significantly in their expression patterns, regulation, and ligand binding specificities.
Figure 1. Neuropilin-1 domains and structure.
As depicted in Figure 1, the extracellular part of the Nrp proteins is composed of three unique domains referred to as a1/a2, b1/b2, and c. Among three vertebrate species, all the domains are highly conserved. A database search has shown that a number of molecules share protein motifs similar to the domains in the extracellular part of Nrp proteins. The a1/a2 domains have striking similarities to a motif found in the complement components C1r and C1s and bone morphogenetic protein-1. A motif similar to the b1/b2 domains of Nrp proteins has been found in coagulation factors V and VIII and the extracellular part of the receptor tyrosine kinases discoidin domain receptor. The central portion of the c domain coincides with a module designated as the MAM domain, which is contained in such functionally diverse proteins as the receptor protein tyrosine phosphatase μ and the metalloendopeptidases meprins (Immunoglobulin superfamily cell adhesion molecules: zippers and signals.2007). The short 40 aminoacid long intracellular domains of Nrp proteins are markedly conserved among three vertebrate species, indicating the importance of this part for neuropilin function, despite its having no homology with other proteins reported to date. The only information available raise from the analysis of the last three C-terminal aminoacids (S-E-A) of Nrp1 that constitute a typical class I consensus binding sequence (S/T-X-Φ) for PSD-95/DLG /ZO-1 (PDZ) domains. Indeed, the C-terminal SEA sequence of Nrp1 interacts with the PDZ domain of the endocytic adaptor protein GIPC1, whose knockdown during development results in an altered arterial branching (Selective regulation of arterial branching morphogenesis by synectin.2006).
AMINO ACID COMPOSITION:
The Protein Aminoacids Percentage gives useful information on the local environment and the metabolic status of the cell (starvation, lack of essential AA, hypoxia).
Nrp1 is synthesized by endothelial cells, which have direct contact with blood, therefore, in the environment, rich with oxygen.Nrp1 is also expressed in neurons and epithelial cells.
(The semaphorins: versatile regulators of tumour progression and tumour angiogenesis.2008)
Neuropilin1 has three isoforms:
neuropilin 1 isoform a: this variant represents the longest transcript and it encodes the longest protein;
neuropilin 1 isoform b: this variant uses a different splice site in the 3' end and contains a different segment for the 3' end of the coding region and the 3' UTR, compared to variant a. The resulting protein (isoform b) has a shorter C-terminus when compared to isoform a;
neuropilin 1 isoform c: this variant lacks an alternate in-frame coding region segment, uses a different splice site in the 3' end and contains a different segment for the 3' end of the coding region and the 3' UTR, compared to variant a. The resulting protein (isoform c) has a shorter C-terminus when compared to isoform a.
Plasmamembrane, enrichments at the level of focal adhesions, colocalized with vinculin.(Neuropilin-1/GIPC1 signaling regulates alpha5beta1 integrin traffic and function in endothelial cells.2009).
Biological functions :
axon guidance,axonal fasciculation, cell adhesion, cell differentiation, cell migration,cell-cell signaling, dendrite development, heart development,multicellular organismal development, negative regulation of axon extension involved in axon guidance, nervous system development, organ morphogenesis, patterning of blood vessels, positive regulation of cell proliferation, response to wounding, signal transduction, vascular endothelial growth factor receptor signaling pathway.
Although Nrps initially received their name from their neuronal localization, it is now known that Nrps are expressed in adult human organs by a variety of tissue types (Identification of a natural soluble neuropilin-1 that binds vascular endothelial growth factor: In vivo expression and antitumor activity.2000). Nrp1 and Nrp2 have specific and overlapping expression patterns in several tissues. For example, both sensory and sympathetic neurons express Nrp1, whereas Nrp2 is found only on sympathetic, but not on sensory neurons.
It has been shown that the b1/b2 domains of the Nrp1 protein are the sites of the heterophilic cell adhesion. More recently, it was shown that Nrp1 could act as a general regulator of EC adhesion to several ECM proteins, such as FN and laminin, through an unknown mechanism (Neuropilin-1 regulates attachment in human endothelial cells independently of vascular endothelial growth factor receptor-2.2005).
Nrp proteins as co-receptors of the chemorepulsive guidance factors SEMA3
The first Nrp ligands to be characterized were the proteins encoded by the SEMA3 gene subfamily.Nrp1 and Nrp2 (The neuropilins and their role in tumorigenesis and tumor progression.2006) are vertebrate transmembrane glycoproteins acting as key components of SEMA3 receptors (Figure 2). Nrp-1 and Nrp-2 form ligand independent homo- and heterodimers. SEMA3 elicit Nrp noncovalent oligomerization, which is absolutely required for biological response (Semaphorin III: role in neuronal development and structural plasticity.1998, Neuropilin-semaphorin III/D-mediated chemorepulsive signals play a crucial role in peripheral nerve projection in mice.1997). Nrp-1 homodimers confer responsiveness to SEMA3A and SEMA3E, Nrp-2 homodimers to SEMA3F, Nrp-1/Nrp-2 heterodimers to SEMA3B and SEMA3C. The a1-a2 and b1-b2 domains of Nrp bind respectively sema and Ig-basic domains of SEMA3, defining the specificity. The MAM/c domain is pivotal for Nrp oligomerization in response to SEMA3 and biological activity. The intracellular domain of the Nrps is short, and it was therefore assumed that it does not suffice to transduce biological signals. This view is supported by experiments that have shown that although Nrp1 is required for SEMA3A induced collapse of axonal growth cones, deletion of the cytoplasmic domain of Nrp1 does not inhibit SEMA3A activity, suggesting the existence of independent signal transducing moieties. These were later found to be the products of genes encoding type-A and type-D plexins (Figure 2), which are responsible for the biochemical signaling that results in the inhibition of integrin function upon SEMA3 stimulation of neurons and ECs.
Figure 2. Simplified model of signaling through Neuropilin receptor complexes.
(Left) Functional association of Nrp-1 and Nrp-2 receptors with Plexin-signaling receptors transduce Sema3 signals. Nrp-1 is known to bind Sema3A and Sema3C, while Nrp-2 binds Sema3C and Sema3F. (Right) In the vascular system, Nrps associate with VEGF tyrosine kinase receptors to transduce VEGF signals. Nrp-1 was shown to bind different VEGF family members, as well as the VEGF165 isoform (The interaction of Neuropilin-1 and Neuropilin-2 with tyrosine-kinase receptors for VEGF.2002). Sema, sema domain; PSI, Plexin Semaphorin Integrin domain; IPT, Integrin Plexin Transcription factor domain; SP, Sex Plexin domain; Ig, Immunoglobulin-like domain; TyrK, Tyrosine kinase domain.
Nrp protein as enhancer of the chemoattractive guidance factor VEGF-A
The angiogenic factor VEGF-A is considered to be a major angiogenic factor that plays an essential role in embryonic vasculogenesis and angiogenesis as well as in tumor angiogenesis. Multiple forms of human VEGF-A are produced as a result of alternative splicing (The splice variants of vascular endothelial growth factor (VEGF) and their receptors.2001). It was found that the VEGF-A 165 induced migration of cells expressing recombinant VEGFR-2 receptors was enhanced in the presence of Nrp1. In-contrast, VEGF-A 121 induced cell migration was unaffected by the presence of Nrp1 (Neuropilin-1 is expressed by endothelial and tumor cells as an isoform-specific receptor for vascular endothelial growth factor.1998). Thus, Nrp1 seems to function as an enhancer of VEGFR-2 activity in the presence of VEGF-A 165. This effect is probably the result of complex formation between VEGFR-2 and Nrp1.
Figure3. Functions of neuropilin domains.