Invited Discussion

The authors of this work have conducted a study to evaluate the usefulness of polylactic-co-glycolic acid (PLGA) microspheres as a carrier for nerve growth factor for slow release in the context of peripheral-nerve regeneration. The efficacy of the presented mode of delivery over time was tested both with a quantitative protein assay (ELISA) and a common cellular bioassay (PC12). The results indicate that protein delivery was maintained for 48 hr. However, biologic activity showed a marked decrease at 24 hr. The authors conclude that future release protocols must not only determine protein quantification, but also include appropriate bioassays.

I (and I suppose most readers of this journal) am interested in nerve regeneration. I am not a biomaterials scientist, nor am I particularly interested in release curves. But nerve regeneration is a complex biologic phenomenon, and the search for alternatives to a conventional nerve graft may also direct our interests into these areas of research.

Neural tissue engineering seeks to provide strategies to optimize and enhance regeneration through the use of three-dimensionl scaffolds, the delivery of growth-promoting molecules, and the use of neuronal support cells or genetically engineered cells. In the past few years, much research activity has been directed toward the development of a clinically useful bioengineered conduit capable of supporting peripheral-nerve regeneration over longer distances.

Regenerating axons require a structured environment that provides polymeric networks of several types of different macromolecules, which not only serve as physical cues for directive axonal growth, but are also needed for non neural cell populations for differentiation and physical integration. In recent years, the importance of different surface molecules to aid in nerve regeneration has been more appreciated. It has been understood that protruding growth cones of the regenerating axons need a permissive substratum to successfully regenerate across longer distances. In conventional nerve grafts, extracellular matrix molecules, such as laminin-1 and fibronectin, as well as more specialized cell surface molecules from either the integrin or the cadherin family, are ubiquitous. Furthermore, this matrix is evidently necessary for appropriate differentiation and induction of specific gene expression of a number of non-neural cells, such as Schwann cells, fibroblasts, or migrating macrophages. These, in turn, will provide the necessary humeral environment for regenerating nerve fibers to survive the transient targetless state. It thus follows that a tissue-engineered graft must exhibit these qualities.

Over the last decade, a number of models have been presented that have investigated the usefulness of different biologic or synthetic matrices in a nerve regeneration paradigm. Still, axonal regeneration did not generally occur, until recently, over longer (> 3 cm) distances. One problem of the presented paradigms may be that a growth-promoting environment includes not only physical cues, but also a rich spectrum of different growth factors provided by non-neuronal cells. Recent publications have clearly demonstrated the critical role of reactive Schwann cells during axonal regeneration. After nerve injury, these cells undergo a dramatic change, and will express an entirely different set of genes than before injury, providing an environment supportive of axonal regeneration. Since nerve regeneration is a lengthy process, these changes have to be maintained over longer periods of time. Therefore, if a tissue-engineered graft is to be successful, it must provide this environment over long periods of time. Furthermore, the spectrum, concentration, and sequence of tropic factors supplied by regenerative Schwann cells are highly complex, and can hardly be mimicked by a release dictated by different, however stable, release curves. Finally, as this current work has once more shown, released protein may still be recognizable by protein quantification techniques (ELISA), but nonetheless be biologically non-functional.

Biomaterial-based approaches to nerve repair show promise for certain clinical applications but, at this point, do not provide sufficient support to bridge large gaps or to stimulate CNS repair. Future biologic or synthetic polymers may, however, serve as scaffolds for cell adhesion/differentiation and provide surface cues to protruding growth cones. The complex humoral environment must be provided by non-neural cells continually interacting/responding to the needs of the moment.