Biomaterials for nerve regeneration

Primary authors: Josh Rastatter, Paul Dodd, Edwin Lynch, Ashley Stratman

I. Motivation

     Research in nerve regeneration is a highly motivated area of study. Potential benefactors range from car accident victims who have suffered spinal cord injuries, to people who genetically inherited Huntington's Disease. It is estimated that in the United States there are 5,000 cervical spinal cord injuries per year. For an average 30 year old tetrapeligic to get lifetime care, it costs an average of 1 million US dollars. Huntington's disease is a genetically inherited condition that occurs in about 3-7 out of 1,000,000 people according to John Hopkins University.  Research in nerve regeneration could one day cure some of these ailments.

A. Huntington's Disease

Huntington's disease is a degenerative disease that targets the nervous system. The lethal disease is caused by a dominant allele located on the tip of chromosome 4. Huntington's is inherited in the classic mendalionian fashion. The allele codes for protiens that destroy the nervous system. Once the the destruction of the nervous system begins, it is irreversible and fatal. Unfortunately symptoms of Huntington's disease don't show up until age 35-45. At that age many people have already had children, and each child has a 50% chance of inheriting the disease. In the United States 30,000 people love and cope with this lethal disease. The CBS special shown below helps explain the impact Huntington's causes and how important nerve research really is.

     1. Symptoms

          -Random jerky movements, also known as chorea

          -Lack of coordination

          -Slurring of speech

          -Trouble chewing or swallowing

          -Loss of memory, decreased critical thinking skills, loss of overall cognitive aptitude

          -Depression

     2. Managment

          -Physical therapy

          -Speech therapy

          -Antidepressants

          -Dense nutritional diet

     3. Prognosis & Conclusion

         The Prognosis for a Huntington's disease patient is bleak. The life expectancy is 15-20 years after the first onset of symptoms. Huntington's will slowly and  painfully eat away at the nervous system. However, Huntington's doesn't directly cause death. Other complications such as pneumonia, and heart faluire are what lead to mortality. If there was a way to heal the nervous system before deathly symptoms set in Huntington's patients could benefit greatly.

B. Spinal Injuries

Serious spinal cord injuries can happen to anyone. They are caused by car accidents, sports, or just falling the wrong way. Nerve regeneration could help all spinal cord injuries, but we will focus on the quadriplegiacs who would benefit most. Quadriplegia is a condition in which all four appendages are at least partially paralyzed. The obvious benefit of nerve regeneration is that the damaged nerve causing the paralysis could be repaired. Which would eliminate the paralysis.

     1. Symptoms

          -Paralysis of the limbs

          -Potential impairment of the bowels, bladder, and other internal functions

          -Sexual impotence

          -Pressure sores

          -Osteoporosis

The research in biomaterials to promote nerve regeneration is urgent.  Among the other treatments, biomaterials shows the most potential because it is one of the only therapies to help recover functionality, reduce Glial scarring, (in spinal cord and peripheral nerve injuries) as well as to give promise of beneficial effects with diseases like Huntington's and Parkinson's.  However, there are barriers to overcome.  Delivery systems and specific growth factors are being researched, which will allow for nerves in the Central Nervous System (CNS) to repair themselves and hopefully regain functionality.  These methods are trying to reduce the immunogenic responses using a combination of nerve transplants, degradable mesh-like polymers, gene therapy, and a combination of all in between.   

Background

The nervous system has complex structure and specialized cellular organization. A neuron has three major sections: the cell body,  where the organelles and nucleus are kept, the dendrites, which reach out from the cell body and search for the axons of other neurons, and the axons, which extend from the cell body to deliver messages from one neuron to another. The axons use junctions called synapses to chemically transmit their signals to other cells.  The axons are coated in a myelin sheath and Schwann cells. When damaged, the nerve cell sheds the myelin, a growth inhibitor, while the macrophages and the Schwann cells neutralize the myelin to promote axonal growth. 

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II. Previous work

     Understanding that peripheral nerves can secrete growth factors to promote the axonal growth and furthermore repair the nerve, David and Aguayo were the first to preform a successful peripheral nerve graft in the CNS in 1981.  Not only did it imply that there are specific growth factors absent from the CNS but also that there are axonal growth inhibitors.  In 1988, Caroni and Schwab showed that the the major inhibitor was CNS myelin.  The problem was becoming clearer. In the CNS important growth factors are missing and major inhibitors are present. This was important at the time because it proved that the spinal cord could regenerate in the proper environmental conditions, a contemporary notion in the scientific community. Therapuetic methods began to be developed.  The idea is to create an enviroment that is favorable for cell growth.  One way to do to this is with a tissue bridge ("Transplantation of Schwann Cells and Ensheathing Glia to Improve Regeneration in Adult Spinal Cord" Plant et al.).  A tissue bridge is a degradable scaffold or tube filled with extracellular matrix molecules, Schwann cells, glial, or fibroblasts genetically engineered to produce neurotrophins.

     Since the axons in the CNS cannot regenerate themeselves, they cannot cross large gaps that could be created by a large amount of tissue damage and surgical procedures; therefore the axons need a direct path (the scaffolds) to grow. Above are some pictures of such devices.  On the left, is an example of a polymer matrix. The right two are example of nerve conduits.  In Sweeden, researchers used multiple peripheral nerve transplant approach to treat a spine injury on a rat. They used eighteen pieces of a peripheral nerve was used to restore a 3-5mm gap in the rat's spine. The pieces were glued together with fibrin glue, which contained acidic fobroblast growth factor (FGF-1).  The research team reported functional recovery including limb locomotion (Cheng et al., 1996).  When Schwann cell were injected, they were able to induce "sprouting" of the corticospinal tract fibers.  The study showed that after 6 weeks the Schwann cells were still present and had myeliated the region of transplant. The cells could be integrated in the system, interacting directly with the host cells (Plant et al.).

     The biomaterials used in nerve regeneration must be compatable and incorporate the desired chemical or biological effect. This is one of the weaknesses of the current methods.  Large amounts of peripheral nerve can be required for a transplant.  However, this can also trigger an immune response by the host compromising the effectivness of the procedure and the health of the patient.  There are some cases where a transplant will not work due to incompatibility of the nerve endings on the spinal cord. However, this is one of the only treatments that has promise to recover functionality of the affected area.  Traditionally hydrogels are the most common biomaterial used for nerve regeneration. The hydrogels are very easy to work with. They are made out of nutrient rich, non toxic, water soluble polymers. The hydrogels mechanically behave like soft tissue, and can be delivered in a liquid state. A problem with hydrogels is that engineers find it difficult to control the pore size or other mechanical properties. Synthetic polymers are also used for nerve regeneration. The polymers usually consist of degradable material. Advantages to synthetic conduits include the ability for engineers to be highly specific and precise with the polymer properties. However, the mechanics of synthetic polymers can cause problems. They are very rigid and are not always very biocompatable. Sometimes the synthetic conduit is so sterile that cells do not grow and flourish on the polymer, which defeats the purpose. Such systems are used in both peripheral nerve transplants and Schwann cell transplantation.  While Schwann cells may be adventageous for axonal sprouting there are challenges as well.  The cells can only survive up to one week out side the body and when frozen lose their regenerative ability.

III. Future Strategies

Axon growth is highly random and often, if the gap between the two nerves is too large, the axons never rejoin and form their native organization, resulting in lost function of the nerves.  However, if these nerves can be aligned and reoriented the nerve function could potentially be partially recovered.  One possible way to realign the nerves is with magnetic microparticles.  These magnetic nanoparticles create mechanical tension which stimulates growth and elongation of the axons in the peripheral nervous system.  Currently, the only treatment offered is to take another part of the patient’s body to form a bridge between the two nerves.  The University of California, Berkeley research team is currently developing a polymer fiber to serve as the bridge between the nerves and align these nanofibers by attatching magnetic nanofibers.  These polymers are made by electrospinning, which involves the use of an electrical field to spin the thin fibers.  The benefits of artificial bridges is that the patient is not required to donate autografts from other parts of the body, usually resulting in loss of function.  Artificial bridges allow a patient to keep their autografts as well as hopefully regain use of the damaged nerves.  The picture below illustrates ideal axonal growth, allowing the neural function to rejoin with the skeletal muscle fibers:

Another major issue with nerve regeneration is the central nervous system’s lack of regeneration capabilities.  Injuries to the spinal cord and CNS often result in paralyzed patients because the nerves are not able to repair themselves.  The use of magnetic fields, however, creates a mechanical tension that promotes axon growth, potentially allowing the CNS axons to repair themselves.  This mechanical tension may be able to promote growth like Schwann cells promote growth in the peripheral nervous system.  This technology is still in the early steps of development, however, but the idea proves to be promising.

Ever since the human genome project, scientists have been able to narrow down exactly which diseases are caused by which genes.  Huntington’s disease has successfully been linked to a single gene.  Some research has been done on mice with turning single genes on and off, so as to potentially avoid getting a certain trait or disease.  Scientists have been able to turn off several genes in mice, however, at the cost of other unwanted effects.  Some mice lost their motor functions when certain genes were tampered with, which obviously would not be a desirable result.  With more ideas and research the idea of simply turning off the Huntington’s gene is a possiblility, although current technologies are more focused on treating and slowing the disease than actually fighting it.  Perhaps once Huntington’s is understood much more thoroughly in a few decades, more promising research will develop with which Huntington’s Disease can be completely avoided.

Citations

Axonal Regeneration in the Central Nervous System, edited by Nicholas A. Ingoglia, Marion Murray, 2001

Biology 8th Edition, Campbell, Reece, 2008

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