Finding Fixes for Injured Nerves
I first became aware of Matt’s research in early 2005. This was catalyzed when he kindly shared excellent IHC images his lab generated using our BDNF and NT-3 antibodies. I was impressed with him and his team’s data and related publications. I did not understand the context of his work and the potential future impact on people suffering peripheral nerve and spinal cord injury (SCI) and wanted to learn more.
He has generously taken the time to open up my view on spinal cord injury (SCI) and what are the challenges in finding therapies and cures. I had equated success soley with restoring mobility. I knew little of the bigger complexites and problems faced by sufferers of SCI.
More than 300,000 people in the United States and Canada suffer from SCI. The ecomonic cost is in the 10s of billions. One of the horrors of SCI is lost mobility.
People with SCI also suffer from a host of problems related to loss of senory and autonomic functions. Sensory and autonomic nerves in the periperal nervous systems (PNS) connect to the spinal cord dorsally. This is different and separate from those controlling movement and motor function. A little know fact is autonomic dysfunctions represent the primary causes of morbidity and mortality following SCI.
So what are the functions that are most important to SCI patients and how should they be prioritized for basic research and drug discovery? Here are two publications that provide insight:
Kim D. Anderson, Ph.D. Targeting Recovery: Priorities of the Spinal Cord-Injured Population. October 1, 2004, 21(10): 1371-1383. doi:10.1089/neu.2004.21.1371.
J A Inskip, L M Ramer, M S Ramer and A V Krassioukov. Autonomic assessment of animals with spinal cord injury: tools, techniques and translation. Spinal Cord advances online publication 10 June 2008; doi: 10.1038/sc.2008.61
The Funding Gap
If we look through the eyes of those suffering from SCI, we know that there are a mryiad of health issues that are outside the problem of lost mobility. I fear the public including those responsible for funding define cure as “the paralyzed can walk”. This is evidenced by a gap between motor vs. sensory/autonomic research and priorities. This gap needs to be closed. The work of Matt and his colleagues represents progress and needs to be supported with funding growth. This backstory highlights how research could feed the discovery of therapies that would answer the recovery priorities of SCI pateints. They, after all, know best.
The story starts in 2000 and highlights Matt’s research at King’s College London. This research was done in collaboration with Dr. Stephen McMahon and Dr. John Priestly.
They showed that regeneration in damaged rat sensory neurons was possible. Injured dorsal roots, treated with nerve growth factor (NGF), neurotrophin-3 (NT3) and glial-cell-line-derived neurotrophic factor (GDNF), but not brain-derived neurotrophic factor (BDNF), resulted in selective regrowth of damaged axons across the dorsal root entry zone and into the spinal cord. Dorsal horn neurons were found to be synaptically driven by peripheral nerve stimulation in rats treated with NGF, NT3 and GDNF, demonstrating functional reconnection. In behavioural studies, rats treated with NGF and GDNF recovered sensitivity to noxious heat and pressure:
Matt S. Ramer, John V. Priestley & Stephen B. McMahon. Functional regeneration of sensory axons into the adult spinal cord. Nature 403, 312-316 (20 January 2000) | doi:10.1038/35002084.
This is a tight rope act. While there is opportunity for regeneration, there are also inhibitors to nerve growth at work. Regeneration becomes more problematic as a function of time. Of the neurotrophins that promote regeneration, NT-3 appears to best at combating the competing inhibitory effects of proteins like NOGO-A. These inhibitory proteins are suspected to be secreted by astrocytes and microglia:
Matt S. Ramer, Ishwari Duraisingam, John V. Priestley, and Stephen B. McMahon. Two-Tiered Inhibition of Axon Regeneration at the Dorsal Root Entry Zone. The Journal of Neuroscience, April 15, 2001, 21(8):2651-2660.
Images: Axon growth 2 weeks after rhizotomy plus immediate NT-3 treatment. A, In intact animals, CTB-labeled terminals are present in lamina I and III, but absent from lamina II. B, Regenerating axons grow along the pial surface of the cord and in the superficial laminae of the gray matter, avoiding the degenerating cuneate fasciculus. C, Dark-field micrograph of B. Scale bar: B, 100 µm. D, Dark-field parasaggital section from a 2 week rhizotomized and NT-3-treated rat. E, Same section as in D, immunostained for CTB. CTB-labeled axons can be seen on the pial surface (arrowheads) and within the cord. Many axons have turned to grow in a rostrocaudal direction but appear to do so in the superficial laminae of the gray matter rather than the white matter. Some individual axons can be traced for up to 2 mm. F, In zones in which the density of regenerated axons is greatest, they form a longitudinal bundle in the gray matter, with few axons in the more superficial white matter (arrows). G, Many axons possess terminal swellings that may be growth cones or termination bulbs. Scale bar: E, 300 µm. The Journal of Neuroscience, April 15, 2001, 21(8):2651-2660.
Through the Looking Glass
Matt and his colleagues continue to gain understanding and refine methods for nerve regeneration. They are also studying plasticity and how these neurons connect to sensory and autonomic neurons in the PNS. This is analogous to re-wiring what was once severed. This would enable restoring of functions important to sufferers of SCI. The related good news is that even partial reconnection enable restoration of these lost functions.
Stepping through the looking glass involves understanding the specific role of these neurons. His recent works include:
Matt Ramer. Anatomical and functional characterization of neuropil in the gracile fasciculus. The Journal of Comparative Neurology. 10.1002/cne.21785.
- Neurokinin-1 (NK 1) Receptor-Detects a band at 80-90 kDa on Western blots of membranes prepared from cells transfected with the rat substance P receptor (Vigna et al., 1994); stainingin rat spinal cord was blocked by preabsorbing the antiserum with the immunizing peptide (Mantyh et al., 1995)-Dilution 1:2,000
- Substance P-The distribution of immunoreactivity in rat spinal cord is identical to that described previously (Hunt et al.,1981); in dual-labeling experiments, it labels the same structures as a polyclonal rabbit anti-SP (1:1,000; Peninsula/Bachem; T-4107; data not shown).
Here Matt and his team report on the morphology, inputs, projections, and functional properties of these neurons. Small fusiform and larger lentiform neurons are most abundant in the gracile fasciculus of the cervical and lumbar enlargements and are absent from the cuneate fasciculus and corticospinal tract. Many have dendrites that run along the dorsal pia, and, although in transverse sections these neurons appear isolated from the gray matter, they are also connected to area X by varicose and sometimes loosely fasciculated dendrites. These neurons receive neurochemically diverse, compartmentalized synaptic inputs (primary afferent, intrinsic and descending), half express the substance P receptor, and some project supraspinally. Unlike substantia gelatinosa neurons, they do not express protein kinase C gamma. Functionally, they have small receptive fields, which are somatotopically appropriate with respect to their anterior-posterior position along the neuraxis. They respond to innocuous and/or noxious mechanical stimulation of the distal extremities, and some are prone to central sensitization or windup. Morphologically, neurochemically, and functionally, therefore, these cells most closely resemble neurons in laminae III-VI in the dorsal horn.
There is hope for SCI patients. It is clear that related research and funding needs to expand dramatically beyond the current narrow focus on restored motor function and mobility. The priorities are documented and understood. The story continues. Real progress will be marked by answering these priorities with restored function. Sensing pain, pressure, temperature, etc. where today there is only nothingness. Controlling autonomic functions that pose such a risk to SCI sufferers. I will continue to report the progress of Dr. Matt Ramer and his colleagues. Godspeed to them.