Building a bridge towards guided neuronal regeneration in spinal cord repair
Approach: Research motivations post neuronal injury
1. Nerve Repair Pathways
There are two major nerve repair pathways post neuronal injury, 1) nerve protection and 2) axonal regeneration. Nerve protection refers to the mechanisms within the nervous system that are involved in prevention of further nerve cell degradation post neuronal injury. Specifically, in the region involved in SCI, the central nervous system (brain and spinal cord), the glycoprotein erythropoietin (Epo) is involved in neuroprotection. Epo has both direct and indirect effects in limiting the number of neurons undergoing apoptosis, including protecting nerve cells against hypoxia-induced glutamate toxicity, enhancing expression of anti-oxidant enzymes to protect against free radical degradation, and affecting neurotransmitter release to increase blood flow to the affected region. [1]
Axonal/neuronal regeneration refers to the restoration of the connected neural pathway via new axon growth. Axonal regeneration has been demonstrated in the peripheral nervous system (PNS) and follows the basic process of Wallerian degeneration, axonal regeneration, and nerve innervation. [2] As shown below, axonal regeneration begins via Wallerian degeneration, the preliminary process of clearing via Schwann cells and macrophages to remove damaged axon and myelin from the injured nerve site.
The proximal stump is the end of the neuron still attached to the healthy cell body. The axon regenerates from the proximal stump to the distal stump, the injured end of the axon that must be cleared to connect to the growing axon. Schwann cells and fibroblasts are important in axonal regeneration. The basic process is as follows: 1) Schwann cells fills the interval between the stumps, 2) The neurilemma (Schwann sheath) forms a protective tube guiding proximal to distal axonal growth, 3) Axonal sprouting and growth. The regeneration is driven by a growth cone, which is the cone-shaped, actin-motivated concentration of the axon moving towards the distal stump, as in the video shown below. Neurite-promoting factors, neurotrophic factors, and electrical stimulation can all contribute towards nerve reinnervation. [3]
http://www.youtube.com/watch?v=62HPVy9myK8&feature=related [4]
Axonal regeneration in the central nervous system (CNS) is much more unsuccessful, hence the major significance of finding treatment to such conditions as SCI. Axonal regeneration in the CNS shares similar reconstitution properties to the regeneration pathways in the PNS such as sprouting of a growth cone; however, the absence of Schwann cells and the neural tube to guide axonal growth, as well as the inability for the growth cone to cross the glial scar (i.e. the scarring of the injured area) limit a similar successful process in the CNS. [5]
2. An engineering approach
While there are a number of biologically active molecules which would trigger nerve cell growth, the integration of an engineering approach to guiding the regeneration process could logically yield a highly successful result. First, there are a number of developed biomaterials, with controllable porosity, strength, and functional properties, that can be optimized towards creating the right scaffold. The wealth of information regarding cell behavior, in migration, proliferation, differentiation, and communication and that too in interaction with various biomaterials could be directed towards developing a cellular bridge that could integrate the mechanical and biological processes of axonal regeneration. Surely, while a combinatorial therapeutic approach would logically be most effective, building a bridge could be the most important step in therapy.
A number of developed strategies aimed at promoting this successful guided regeneration strategy have had their shortcomings. The promotion of axonal regeneration via glial cells (in a similar manner to promotion in the PNS system) both in isolation and via construction of a guided channel such as an artificial neural tube or bridge was met with limited success. One successful example is shown below.[6, 7]
In the guided approach, axons could cross the on-ramp, grow across the bridge, but became stuck at the off-ramp. [8] There were two major problems: 1) the Schwann cells (PNS glial cells) could motivate growth in the PNS-like environment created in the bridge, but once the axonal growth reached the CNS environment at the distal stump, growth was arrested, and 2) the Schwann cells resisted integration with the astrocytes (CNS glial cells) creating a cellular boundary blocking completion of nerve regeneration.
Several developed grafts were formulated from olfactory sheathing glia (as seen in the figure below) [9] or genetically engineered fibroblasts [9], and various other strategies with cells attached to tubes have had success in promoting regeneration, but have lacked a physical guide such as the PNS neural tube that can catalyze the overall nerve connection and reinnervation. The engineering of biocompatible cell-free bridges with limited interference in the inflammatory reactions would be a successful research strategy to provide the guidance for axonal regeneration.
