Axonal Sprouting and Growth after Neural Injury
ASNR Foundation thanks the Craig H. Neilsen Foundation for their generous support of this Symposium
Organized by S. Thomas Carmichael, MD, PhD
 Larry Benowitz, PhD |
Inosine enhances the rewiring of injured CNS pathways The inability of neurons to regenerate axons after CNS injury, combined with the weak capacity of uninjured neurons to form compensatory connections, severely limits recovery after stroke, spinal cord injury, or other types of CNS damage. Inosine is a metabolite of adenosine that crosses the cell membrane of neurons and activates Mst3b, a component of the cell-signaling pathway that controls axon outgrowth. Following a unilateral stroke in the rat motor cortex, inosine enhances the ability of motor neurons in the undamaged hemisphere to extend axon collaterals across the spinal cord midline into areas that have lost their normal innervation and improves skilled use of the impaired forelimb. This effect is further enhanced by combining inosine with environmental enrichment or with an agent that counteracts myelin-associated growth inhibitors. In a spinal cord injury (SCI) model, inosine increases the sprouting of serotonergic axons that regulate intrinsic spinal cord activity, along with the ability of damaged corticospinal axons to extend collateral branches rostral to the injury site. These collateral branches can form synaptic connections with interneurons that, in the case of incomplete SCI, retain projections from rostral to distal segments of the spinal cord. The formation of these “detour circuits” helps restore volitional control to the hindlimbs. In the optic nerve, Mst3b is activated by the myeloid cell-derived growth factor oncomodulin (Ocm), and combining Ocm with a cAMP analog and pten gene deletion enables retinal ganglion cells to regenerate injured axons from the eye to the brain. Thus, inosine and certain growth factors activate Mst3b and can help improve recovery after stroke, spinal cord injury, and optic nerve damage. Support: NINDS, NEI, Dept. of Defense, Dr. Miriam and Sheldon Adelson Medical Research Foundation. |
| Molecules that Enhance Axonal Sprouting and Recovery after Stroke Stroke causes death, destruction and functional impairment. Stroke also induces a limited amount of repair and recovery. Understanding the normal mechanisms of tissue repair after stroke may allow development of novel therapies to promote more complete recovery in this diseases. Of the mechanisms of tissue repair in stroke, the formation of new connections (axonal sprouting) in brain adjacent to the stroke site occurs in rodents, primates and (with a strong correlative marker) in humans. In pre-clinical models of stroke, axonal sprouting in peri-infarct brain is causally related to recovery of motor function. We have identified several molecules that promote axonal sprouting and recovery. These molecules fall into several categories of brain repair. One category consists of molecules that are induced by stroke and control changes in gene expression that induce a growth program in neurons, such as the molecule ATRX. A second category includes molecules that are induced by stroke and normally block axonal sprouting. These “growth inhibitors” can themselves be blocked to produce an increase new connections and enhanced recovery. These include Ephrin-A5 and the Nogo receptor 1. A third class of molecules is emerging from our most recent studies and includes the actual triggers for axonal sprouting and recovery—the molecules induced by stroke that turn on this overall growth program. The major molecule in this class is GDF10, a TGFb family member. This molecule is induced by stroke in adjacent brain tissue, promotes the formation of new connections and is responsible for a large degree of normal neurological recovery in pre-clinical stroke models. Overall, these molecular systems provide insights into normal stroke recovery and possible candidates for stroke neural repair. Support: NINDS, Richard Merkin Foundation for Neural Repair at UCLA, Dr. Miriam and Sheldon Adelson Medical Research Foundation. |
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 Shelly Sakiyama-Elbert, PhD |
Temporally Regulated GDNF Delivery Prevent Axon Trapping in Long Peripheral Nerve Injuries The use of growth factors, such as glial cell line-derived neurotrophic factor (GDNF), for the treatment of peripheral nerve injury has been useful in promoting axon survival and regeneration. Unfortunately, finding a method that delivers the appropriate spatial and temporal release profile to promote functional recovery has proven difficult. Some release methods result in burst release profiles too short to remain effective over the regeneration period, however prolonged exposure to GDNF can result in axonal entrapment at the site of release. Thus, GDNF was delivered in both a spatially and temporally-controlled manner using a two-phase system comprised of an affinity-based release system and conditional lentiviral GDNF over-expression from SCs. Briefly, SCs were transduced with a tetracycline-inducible (Tet-On) GDNF over-expressing lentivirus prior to transplantation. Three-centimeter acellular nerve allografts (ANAs) were modified by injection of a GDNF-releasing fibrin scaffold under the epineurium, and then used to bridge a 3 cm sciatic nerve defect. To encourage growth past the ANA, GDNF-SCs were transplanted into the distal nerve and doxycycline was administered for varying time periods to determine the optimal duration of GDNF expression in the distal nerve. Live imaging and histomorphometric analysis determined that 6 weeks of doxycycline treatment resulted in enhanced regeneration compared to shorter or longer delivery periods. This enhanced regeneration resulted in increased gastrocnemius and tibialis anterior muscle mass for animals receiving doxycycline for 6 weeks. The results of this study demonstrate that strategies providing spatial and temporal control of delivery can improve axonal regeneration and functional muscle reinnervation. |
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Nociceptive Afferent Sprouting, Hypereflexia and Anatomical and physiological plasticity following neural injury is usually considered necessary for recovery of lost functions such as movement. The same plasticity, however, may also underlie the emergence of so-called “positive phenomena” such as spasticity, neuropathic pain, and autonomic dysfunction. In this presentation, we will discuss the plasticity of nociceptive cutaneous afferents in the spinal cord and relate those to changes in a nociceptive reflex and in autonomic function following spinal cord injury. We will demonstrate afferent sprouting in both expected and unexpected sites and relate that to nociceptive hypereflexia. We will also demonstrate the relationships between pain afferent subtypes and distinct cardiovascular functions, namely blood pressure and heart rate, both before and after spinal cord injury. Finally, we will demonstrate the treatment effects of a two week course of several pain medications on the development of dysautonomia following spinal cord injury. |
 Keith Tansey, MD, PhD |
