by New York University

Nerve growth factor receptor TrkA (in green) and co-receptor NRP1 (in purple) are coexpressed in the same nerve cells in mice. Credit: Bunnett Lab

Researchers at the NYU Pain Research Center have found a new receptor for nerve growth factor that plays an important role in pain signaling, even though it does not signal on its own, according to a study published in the Journal of Clinical Investigation. The findings hold promise for finding new treatments for arthritis and other forms of inflammatory and cancer pain, without the side effects that led recent therapies to fail in clinical trials.

“Nerve growth factor is unusual because it’s one of the few patient-validated targets for pain,” said Nigel Bunnett, professor and chair of the Department of Molecular Pathobiology at NYU College of Dentistry and the study’s senior author. “We wanted to think of a way of circumventing side effects in an effort to find safer, non-opioid therapies for arthritis and other forms of chronic pain.”

Nerve growth factor is a protein that stimulates the development of neurons. It is also a powerful driver of pain in animals and humans, and is released by cells from injured or diseased tissue. To transmit pain signals, nerve growth factor binds to a receptor called tropomyosin receptor kinase A, or TrkA.

Monoclonal antibodies—lab-made proteins that mimic our natural antibodies and bind to specific proteins to treat diseases—have emerged as a promising treatment for chronic pain by targeting nerve growth factor and sequestering it. In large clinical trials, monoclonal antibodies relieved osteoarthritis pain better than placebo or other drugs, but because some patients experienced worsening joint damage, the treatments were not approved.

How a non-signaling receptor regulates pain signals

In a series of studies using mouse and human neurons, the researchers discovered a new receptor for nerve growth factor: neuropilin-1 (NRP1), a protein expressed in neurons and other cell types.

To determine this, they observed that nerve growth factor has a stretch of amino acids that is known to allow other proteins to bind to NPR1. NRP1 was also expressed in the same cells at the nerve growth factor receptor TrkA.

Examining pain-sensing neurons, the researchers found that NRP1 could bind to nerve growth factor with high affinity, and when NRP1 was blocked in neurons from both mice and humans, it inhibited nerve growth factor from signaling pain. The researchers concluded that NRP1 is a co-receptor for nerve growth factor, as—unlike TrkA—NRP1 is not known to signal on its own.

“Our findings suggest that neuropilin-1 is required for nerve growth factor to signal pain, even if it is indirectly regulating it,” said Bunnett.

In further cellular studies, the researchers discovered two mechanisms that explain the NRP1’s role in pain. First, when binding to nerve growth factor, NRP1 increases the local concentration of nerve growth factor that is presented to TrkA, the signaling receptor.

In addition, NRP1 was found to be a molecular chaperone, or a protein that aids in the trafficking of other proteins in the cell—in this case, TrkA. NRP1 interacts with TrkA and brings it from the interior of the cell to the plasma membrane on the surface. This increases the amount of TrkA on the cell’s surface to recognize nerve growth factor and signal pain.

The researchers then used molecular modeling to better understand the interactions between nerve growth factor, TrkA, and NRP1 at the surface of cells. Their modeling suggests that two molecules of nerve growth factor, two molecules of TrkA, and two molecules of NRP1 together form a pain signaling complex.

Finally, the researchers identified a protein, G Alpha Interacting Protein C-terminus 1 (GIPC1), that appears to play a critical role in connecting TrkA and NRP1 and signaling pain. GIPC1 links TrkA and NRP1 to a specific molecule that transports the pain signaling complex into the cell’s interior, which may lead to sustained or chronic pain.

A ‘springboard’ for pain treatments

Given the newfound role of NRP1 in nerve growth factor pain signaling, the researchers envision several ways that this knowledge can be used to redeploy existing therapies to treat pain and create new ones.

Blocking NRP1 with established compounds is one option, as NRP1 inhibitors—including monoclonal antibodies—have already been developed to treat cancer.

“We could test these monoclonal antibodies that target NRP1 in models of pain,” said Bunnett. “Because these treatments would target receptors at the surface of pain-sensing neurons, this specificity might avoid the side effects seen with other monoclonal antibodies that sequester all nerve growth factor in the body.”

The researchers are also harnessing their new understanding of the pain signaling complex, determining the sites at which nerve growth factor, TrkA, and NRP1 interact, and generating peptides that disrupt them. In their study, the researchers created one such peptide that blocked the ability of nerve growth factor to interact with NRP1, which stopped pain in cellular studies.

“We can use this information as a springboard to develop new peptide-based analgesics that prevent this signaling complex from forming,” said Bunnett.