Finding a key protein for nerve sprouting

Posted by Sam Maddox in Research News on April 26, 2016

After nerve damage, axons that escaped injury often shoot out new branches. This process, which happens in both in the peripheral nervous system and in the central nervous system (brain and spinal cord), is called collateral sprouting. This new nerve growth can be useful – sprouts can reroute nerve signals around broken pathways, thus restoring function. Alas, sprouts can also have a flip side – post-injury axon growth can contribute to pain, or to autonomic dysreflexia and related blood pressure problems.

Jeff Petruska and his research group at the Kentucky Spinal Cord Injury Research Center at the University of Louisville have been working over the past few years to better understand sprouting and how the nervous system responds after injury. He and his team want to know, what are the molecules that regulate sprouting, and might it be possible to tune the axon response so sprouting is always beneficial?

They started by using a microarray analysis, using powerful computers to sort through thousands of gene candidate proteins that are switched on, or upregulated, in sensory axons that sprout after nearby nerves are cut. A protein that showed up repeatedly was CD2-associated protein (CD2AP), previously known for its role in kidney function.

Petruska and his team, in their recent publication, “The Adaptor Protein CD2AP Is a Coordinator of Neurotrophin Signaling-Mediated Axon Arbor Plasticity” in the Journal of Neuroscience, have now spelled out the role of CD2AP in the nervous system, and in the process of collateral sprouting.

Before we get to the paper, a brief word about Petruska. He’s well-known around the Reeve Foundation research department. Before he got his own lab at U of L, he was with the Lorne Mendell lab at NYU Stony Brook. Mendell’s lab is one of seven that form the Reeve International Research Consortium on Spinal Cord Injury, a collaborative lab alliance to accelerate development of treatments.

Each Consortium PI, or principal investigator, keeps two or three post-doctoral students, known as Associates, to manage the day-to-day tasks of Consortium projects. Petruska was Mendell’s Associate. Mendell is a coauthor on the new paper, as are former Consortium PIs Mary Bunge (Miami Project) and Fred Gage (Salk Institute). Lawrence Moon and Caitlin Hill are also listed as coauthors; both were Bunge lab Associates in Miami.

From an interview I did with Petruska in 2013:

The Consortium is an amazing entity; it trains its Associates in how science is really being done now. You have to be multidisciplinary. You have to collaborate. The Consortium is about working across disciplines, across many labs.

I learned more from the Consortium than I would have by going on to a second post-doc. I set up a number of collaborations that have been invaluable as I moved on and started my own lab. I was attracted to the Kentucky SCI program because it reminded me of the Consortium model. I interviewed with a bunch of places that all had really great stuff going on. But this place felt the closest to that collaborative model.

Petruska’s current research began at the Mendell lab. “The question I asked is: How do adult neurons that are not injured extend new branches of their axons? Of those that are injured, some extend axon branches using regeneration. But some axons that are not injured grow and change their connections too – collateral sprouting. So I put this idea in front of the Consortium, all of the principal investigators, the advisory panel – they said ‘do it.’ That was a powerful expression of faith and confidence and very meaningful to me as a postdoctoral fellow.

Once set up in Louisville, Petruska put a team together. “I’m not a molecular biologist. So that’s where I went to Lawrence Moon. He had just completed his own micro-array experiment, which was outstanding. He compared the genes in the central nervous system neurons that could regenerate after injury with those that could not regenerate.”

Petruska also recruited his own post-doc, molecular and cellular biologist Ben Harrison, who had been working in the cancer field. The university had a computational/bioinformatics expert, Eric Rouchka, to develop tools to analyze huge datasets. “Together we’ve gone from a broad data set to one that identifies, for the first time, the specific genes necessary for collateral sprouting.” And now, a specific protein.

CD2AP is what is termed an adaptor protein – it assembles other proteins that control sprouting from a nerve cell so it can connect to other nerve cells, skin, and organs.

Here’s Harrison, first author on the new paper, from a UK press release:

CD2AP brings in all the correct players, forms a multiprotein complex, and coordinates that multiprotein complex to achieve growth of the neurons. There are a whole bunch of proteins that it could bring together, but it only brings together the correct proteins to create the correct response. In this case, it changes the structure of the axons through sprouting and elongation.

Through targeting this molecule, we could help the body's natural healing process to coordinate the appropriate growth.

The study with CD2AP shows how the nervous system remodels itself after injury, recruiting growth molecules such as NGF. NGF bumps up CD2AP, which in turn activates growth mechanisms of nerve cells. Said Harrison, "People have been studying nerve growth factor and the responses it induces for a while, but this protein (CD2AP) forms a nice link between NGF and the response in the cell." Meanwhile, other research has linked CD2AP mutations and poor NGF signaling to Alzheimer’s disease, so the studies by the Petruska lab may be relevant beyond spinal cord injury.

Petruska’s work is closely linked to other research being conducted at the U of L Kentucky Spinal Cord Injury Research Center. He thinks understanding the molecular processes of sprouting could perhaps hasten development of other therapies, such as locomotor training now being done at the U of L Frazier Rehab Center [Susan Harkema lab and home of epidural stimulation and the Reeve Big Idea].

Petruska, from the U of L press release:

We are starting to discover that there are different modes of nerve growth and different sets of genes that control different kinds of growth. This is particularly important as it relates to locomotor training. When you train, you enhance the growth factor environment of the injured spinal cord, and those growth factors are involved in the axon plasticity. This mode that we study is dependent on the growth factors.

From Harrison:

My dream is to one day do a clinical trial with a drug that targets this protein [CD2AP] and can enhance the ability of the patients to respond to the activity-based rehabilitation (locomotor training) that they are doing at Frazier Rehab.