An In-Depth Analysis of Neuropilin 1
Nicole Dano and Jessica Hrnciar
Date Published: December, 14 2020
Major Functions (14)
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COVID-19 Mediator
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Axon Guidance
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HTLV-1 Infection
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Angiogenesis
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Cell Adhesion
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Growth Factor Binding
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Heparin Binding
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Metal Ion Binding
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Protein Kinase Binding
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Coreceptor Activity
Common Functions
COVID-19 Mediator
COVID recognizes NRP-1
Spike protein binds to NRP-1
Cell uptakes COVID-19 virus
After its outbreak in December of 2019, COVID-19 has wreaked havoc worldwide. Figuring out how this virus entered cells began with the recognition of the Ace2 receptor (24). Ace2 allows the virus to enter and infect the cell. However, along with Ace2, NRP-1 was just recently discovered to also aid in the virus's entry into a cell. The spike protein along the outer portion of the COVID virus attaches itself to the binding pocket on the NRP-1 receptor, which is located on the surface of a healthy human cell (25, 26). After it is attached, the virus enters the cell and replicates. In a recent study, it was discovered that in the presence of NRP1, the virus is enhanced, leading to more severe infection and a larger spread of the virus.
Angiogenesis
Within the process of angiogenesis, endothelial cells are grown from pre-existing vasculature to then form brand new capillary networks. Tip cells, a subsection of endothelial cells, later lead to endothelial sprouts. A lack of NRP1 leads to a smaller number of these tip cells. However, an abundance of NRP1 was recently shown to improve the number of endothelial tips during angiogenesis (27). NRP1 is known to regulate VEGFR2 response to VEGF triggers. Impairing NRP1 leads to many vascular defects. One of the defects being in the reduction of intersomitic vessels, which is a process that greatly depends on angiogenesis (27). Not only does NRP1 regulate angiogenesis with the help of VEGFR2, but it was recently discovered that NRP1 can regulate angiogenesis on its own through cell adhesion.
Biochemical Pathways
Axon Guidance
Source: "Reference Pathway" (28)
The axon guidance pathway is involved in the formation of neuronal circuits by allowing axons to travel through existing tissue to form a synapse at their designated target. This is done by repulsive and attractive cues that act over long and short distances to help a growing axon navigate its way to its target (29). NRP1 is capable of binding semaphorin 3A, which is a molecule involved in axon repulsion. However, NRP1 will often associate with Plexin a to form a receptor with a higher affinity for semaphorin 3A (30). Once semaphorin 3A binds this complex several different events can occur. A lot of the other downstream molecules involved are from the Ras superfamily (31). The binding of semaphorin 3A to the NRP1-Plexin A complex can activate Rac, which will then activate PAK. PAK activation leads to LIMK activation, which will inhibit Cofflin. When Cofflin is inhibited, it will lead to axon repulsion. The seMaphorin 3A bound to NRP1-Plexin A can also activate RhoD which will inhibit a few other molecules and also lead to axon repulsion. This pathway also shows L1CAM interacting with NRP1. L1CAM can also form a complex with NRP1 to allow the bound semaphorin 3A molecule to lead to proper signal transduction, and when L1CAM is mutated it may lead to abnormal axon guidance (32).
Human T-cell leukemia virus 1 (HTLV-1) infection
Source: "Reference Pathway" (33)
This pathway shows how the human T lymphotropic virus 1 enters and infects the cell. When NRP1 is expressed on CD4 T cells it can directly interact with molecules trying to enter the cell. One of NRP1’s main functions is to act as a receptor and depending on where it is expressed it is a receptor for many different things. NRP1 in CD4 T cells interacts with glucose transporter GLUT1 to allow HTLV1 to enter the cell (34). This entry occurs via the interaction of heparan sulfate proteoglycans (HSPG) with the NRP1 and GLUT1 complex (35). The virus has glycoproteins 21 and 46 on its surface which directly interact with the HSPG, NRP1, and GLUT1 to allow viral entry.
Tissue Expression of NRP1 Protein and All Isoforms
Developmental Stages
Fetal Development (2):
​Week 10- adrenal, heart, intestines, kidney, lung, stomach​​​​
Week 11- heart, intestine
Week 15- intestine
Week 16-adrenal, kidney, stomach
Week 17-heart, intestine, lung
Week 18- adrenal, heart, stomach
Week 20- adrenal, heart, intestine, kidney, lung, stomach
All stages after birth (37) (newborn-elderly) have a wide range of tissue expression for NRP1 ranging from the calcaneal tendon at adolescence through 79yo to subcutaneous fat from early adulthood to late adulthood.
Sub-Cellular Localization of NRP1
Source: "Sub-Cellular Localization" (14)
Different proteins have different areas of the cell where they are expressed and located. The image provided from UniProt shows regions of the cell where nRP1 is found in yellow. This means that NRP1 is commonly found to be in the extracellular region or secreted, mitochondria, and plasma membrane. Genecards lists several other cellular compartments where NRP1 can be found, but it lists the plasma membrane or extracellular membrane with the highest confidence (38). This means that most of the NRP1 protein in the body is found in the plasma membrane or in the extracellular surface, which makes sense since NRP1 is commonly used as a receptor and found on the cell surface. NRP1 is very rarely found in the nucleus or a lysosome (38).
In regards to NRP1 specific isoforms, isoform a is most likely found in the endoplasmic reticulum (44%) or Golgi apparatus (33%), and less likely in the plasma membrane (22%) (8). This finding was interesting because isoform a often acts as a receptor, so we thought its localization in the plasma membrane would be higher. Isoform i is most likely an excreted protein found in the extracellular space (55%), but it can also be found in the endoplasmic reticulum (22%) or the Golgi apparatus (11%) (8). Interestingly, both isoforms are possibly found in the endoplasmic reticulum or Golgi apparatus, even though NRP1 overall had low confidence for localization in both of those organelles.
The localization of a protein is directly tied to its function because a protein cannot properly do its job if it is not in the right location. NRP1 was found most commonly to localize in the plasma membrane (38). Most of the major functions of NRP1 involve it acting as a receptor in an abundance of pathways. In order for NRP1 to act as a receptor, it must be localized in the plasma membrane so that it can interact with both the outside and inside of the cell. On the outside, NRP1 can bind to molecules floating around, and on the inside of the cell, it can facilitate signal transduction to make something happen. NRP1 is also commonly excreted. This allows it to act as a signaling molecule, ensuring it can perform the function of cell signaling. NRP1 has a variety of functions, but its role in cell signaling is carried out due to its localization in the plasma membrane or extracellular space.
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Five Protein Interactions
KDR
NRP1
AGER
FLNA
VEGFA
ITGA5
The NRP1 protein interacts KDR, FLNA, AGER, VEGFA, ITGA5 along with many other proteins (38).
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KDR: (kinase insert domain receptor, VEGFR) is the major growth factor for endothelial cells and plays a major role in angiogenesis. Coexpression between NRP1 and KDR leads to an increase in binding between KDR and VEGF165, which is very important in angiogenesis (38).
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FLNA: (Filamin A) is an actin-binding protein. Its function is to link actin filaments to glycoproteins found on the cell membrane. Filamin-A binds to NRP1 and acts as a focal adhesion regulator during vascular development (38).
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VEGFA: (Vascular Endothelial Growth Factor A) functions to promote endothelial cell proliferation, inhibits apoptosis, and also aids in blood vessel permeabilization. When NRP1 and VEGFA have bound together a signal pathway for axon guidance is initiated (38).
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ITGA5: Is made up of two subunits, one alpha, and one beta. These two subunits' main function is involved in cell surface adhesion and signaling. NRP1 is known to interact with TGA5 at its adhesion sites. The interaction between NRP1 and TGA5 plays a role in causing defects during angiogenesis (38).
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AGER: (advanced glycosylation end-product specific receptor) is a multiligand receptor and functions with other particles that are involved in homeostasis and development. Both NRP1 and AGER are receptors involved in the use of oncolytic viruses and CAR T-cells (38).