Nerve regeneration. From Santiago Ramón y Cajal’s “Estudios sobre la degeneración y regeneración del sistema nervioso” (1913-14). Via Scholarpedia.
Researchers have tried for a century to get injured nerves in the brain and spinal cord to regenerate. Various combinations of growth-promoting and growth-inhibiting molecules have been found helpful, but results have often been hard to replicate. There have been some notable glimmers of hope in recent years, but the goal of regenerating a nerve fiber enough to wire up properly in the brain and actually function again has been largely elusive.
“The majority of axons still cannot regenerate,” says Zhigang He, PhD, a member of the F.M. Kirby Neurobiology Center at Boston Children’s Hospital. “This suggests we need to find additional molecules, additional mechanisms.”
Microarray analyses – which show what genes are transcribed (turned on) in injured nerves – have helped to some extent, but the plentiful leads they turn up are hard to analyze and often don’t pan out. The problem, says Judith Steen, PhD, who runs a proteomics lab at the Kirby Center, is that even when the genes are transcribed, the cell may not actually build the proteins they encode.
That’s where proteomics comes in. “By measuring proteins, you get a more direct, downstream readout of the system,” Steen says.
In a paper last week in Neuron, Steen and He describe using proteomics techniques to study mice with optic nerve injury – a classic, easy-to-study type of central nervous system injury. Together with fellows Stephane Belin, PhD, and Homaira Nawabi, PhD, they used quantitative mass spectrometry to identify and quantify proteins produced by the injured retinal ganglion cells (RGCs), which run through the optic nerve from the retina to the brain. Using bioinformatics analyses, they compared their readouts with those from uninjured RGCs to look for differences. They also looked at system-wide patterns indicating proteins that act together in concert.
“This approach gave us a nice 30,000-foot view of what pathways are changed in the system as a whole in response to injury,” says Steen. “It showed us the main pathways we should perturb to get regeneration.”
The pathways matched many that were previously identified, but also included some new players, such as TGF-b, NFkb, Huntingtin and, most notably, the oncogene/cell growth promoter c-myc. When the team induced the mice to make more c-myc near the injury site, nerve regeneration was promoted, even when some time had elapsed after the injury.
When the researchers combined add-back of c-myc with deletion of two other known inhibitors of regeneration (PTEN and SOCS3), the effect was synergistic. Survival of the injured RGCs was dramatically improved, and the cells’ axons grew all the way to and beyond the optic chiasm, the part of the brain where the optic nerves cross – an impressive degree of regeneration.
Fibers from the injured nerve crossing the optic chiasm (courtesy Zhigang He)
The researchers caution that there is risk in stimulating c-myc (a tumor promoter) and deleting PTEN (a tumor suppressor), as both strategies could also promote cancer – the likely reason that these pathways are shut down once our nervous system has developed. However, there may be ways to mimic these pathways, or ways of stimulating them in a targeted, temporary fashion with small molecules, the researchers suggest.
Steen and He are now starting to test other proteins and pathways identified through their analysis and are finding some regenerative promise. “You may have to invoke several pathways simultaneously,” notes Steen. “You can’t say there’s one magic bullet.”
They are also using proteomics to look for new pathways to target in degenerative neurologic disorders such as Alzheimer’s disease, frontotemporal dementia, spinal muscular atrophy and amyotrophic lateral sclerosis, as well as sensory nerve injury.