Musings of a Compulsive Thinker

Injury repair in the central nervous system

This is such a promising development. Also, you know, it's really cool science.

Injuries to the central nervous system (CNS) are notoriously difficult to repair, though in many cases the surrounding CNS tissues adapt to work around the resulting scar. This "glial scar" is very inhibitory to the regeneration of damaged axons, which carry outgoing signals from neurons (signal-conducting nerve cells). In order to understand the inhibitory nature of the glial scar and consequent failure of axon regeneration after CNS injury, we should first understand the cells and processes involved in the formation of the glial scar.

When CNS tissue is damaged it undergoes an injury response called reactive gliosis, or glial scarring. This consists of a series of cellular and molecular events that occur and change over a period of several days; the glial scar structure evolves over time as various cells arrive and participate at different times. The main cell types involved are the neurons themselves, as well as the surrounding glial tissue, which consists of astrocytes (general support cells, providing structural stability and helping to regulate the extracellular environment), microglia (immune cells, the "garbagemen" of the CNS), and oligodendrocytes (provide insulation for axons in the form of myelin sheaths).

Immediately following injury, myelin debris (from damaged oligodendrocytes) will be released into the neural environment as oligodendrocytes and other cells in the injured area are damaged and die. In the first few days following injury, the primary entering cells are microglia. The lesion (damaged area) also expands during this time. The mature glial scar consists mainly of a tightly-woven network of astrocyte processes.

Astrocytes around the lesion exhibit abnormal growth and some undergo cell division; the end result of this activity is the dense, predominantly astrocytic composition of the glial scar. It has been found that this tissue can be both inhibitory to and supportive of axonal regeneration, depending on changes in the CNS environment and/or the population of glial scars by an as-yet undetermined sub-type of astrocyte which is inhibitory.

Oligodendrocytes are directly damaged by traumatic injury to the CNS, causing the release of myelin debris and some oligodendrocyte death. Glial scars, therefore, usually contain some oligodendrocytes and myelin debris. It has been shown that mature oligodendrocytes and myelinated areas of the CNS are inhibitory to axon growth. Also, it has been seen that oligodendrocyte precursor cells are recruited en masse to CNS lesion sites; these cells express several proteoglycans (dense molecular complexes of proteins and polysaccharides) that are inhibitory to axon regeneration.

Microglia exhibit activation and division following injury, and migrate to the injury site, becoming more macrophage-like over time ("macrophage" essentially means "great eater," so you can guess what a macrophage does). Collective evidence suggests that microglia may actually support regeneration, as long as nothing happens to make them overtly toxic.

Interestingly, it has been shown that the eradication of all CNS glia from the lesioned area results in an environment in which robust axonal regeneration can occur for a period of about 4 days, until glia re-invade the area.

With this in mind, we turn to the potential use of a self-assembling nanofiber peptide scaffold. This novel scaffold is a hydrogel (content is over 99 percent water, with 1 to 10 milligrams of peptides per milliliter of water) that forms when a self-assembling peptide (SAP) solution is exposed to salt solution similar to that found in the human body. The figure at right (click to enlarge) shows (a) a molecular model of the SAP, (b) a microscopic image of the SAP nanofibers, (c) a microscopic image of the scaffold, and (d) a photograph of the hydrogel. The components of the scaffold are amphiphilic oligopeptides that have repeated alternating ionic hydrophilic & hydrophobic amino acids. These form beta-pleated sheets with distinct polar & non-polar surfaces. Structurally, macroscopic scaffolds have been formed in various shapes and sizes, depending on SAP concentration, total amount of SAP solution, salt concentration, and the geometry of the processing apparatus. These structures consist of individual interwoven fibers of about 10 to 20 nanometers (one-trillionth of a meter) in diameter, with the density of fibers correlating with the concentration of the SAP solution.

Here's the "really cool science" part. In one study, Holmes et al. seeded neural cells on SAP scaffolds. These cells underwent extensive outgrowth along the contours of the scaffolds; further evidence suggested that the scaffolds also supported the formation of functional neuron synapses. In another study conducted by Ellis-Behnke et al., a tissue gap was created in the hamster midbrain (by deep transection of the optic tract). When treated with SAP solution, this gap was seen to be reduced or completely eliminated by 72 hours post-surgery and in all subsequent examinations, as compared to the saline-treated controls, which remained visible in macroscopic examination at all times post-surgery. The figure at right shows typical examples from 30 days post-surgery of (a) the saline control case and (b) the SAP-treated case. The SAP-treated animals showed axon regeneration through the injury site; the control animals showed no axon regeneration. Here the SAP treatment was generally found to support axon regeneration with a correlating return of functional vision.

Both of the above studies included tests of the body's toleration of the SAP solution; after an injection of the SAP solution into muscle tissue, examinations found no detectable toxic effects and no observable signs of structural abnormalities, muscle necrosis, inflammation, or motor impairment. Where the SAP solution was injected into brain tissue, no apparent inflammatory response was found; in addition, it was discovered that the amino acid degradation products of the scaffold are mostly eliminated from the body within 3-4 weeks post-injection.

Also, one important feature of the SAP scaffold is its ability to fill irregular voids (because it is a hydrogel) such as injury sites in damaged tissue. This allows close contact between the scaffold nanofibers and surrounding tissues.

References:
> J.W. Fawcett et al.: "The glial scar and central nervous system repair" [PDF, 183 KB], Brain Research Bulletin, 1999.
> T.C. Holmes et al.: "Extensive neurite outgrowth and active synapse formation on self-assembling peptide scaffolds" [PDF, 522 KB], Proceedings of the National Academy of Sciences, 2000.
> R.G. Ellis-Behnke et al.: "Nano neuro knitting: Peptide nanofiber scaffold for brain repair and axon regeneration with functional return of vision" [PDF, 1.7 MB], Proceedings of the National Academy of Sciences, 2003.

--

Labels: engineering, science

posted by the alygator @ 11:00 AM