Strategies for endogenous spinal cord repair: HPMA hydrogel to recruit migrating endogenous stem cells.

Injury to the spinal cord disrupts ascending and descending axonal pathways and causes tissue damage with a subsequent limited cellular regeneration. Successful treatment would encompass the restoration of the cytoarchitecture, homeostasis and function all in dear need. Transplantation-based treatments using exogenous cells are the most favoured approach. Yet, with the advent of the stem cell concept and continuous progress in the field it became clear that the endogenous potential for repair is greater than previously thought. As an alternative to neural grafting, we and other researchers have aimed at understanding what are the elements needed for a successful repair with self progenitors that would give rise to the cell types needed to restore function of the central nervous system. Some studies involve both scaffolds and cell grafts. Here we describe studies on spinal cord repair using what we call "endogenous tissue engineering for regenerative medicine". The approach involves a hydrogel that mimics the natural milieu where endogenous pre-existing and newly formed cells populate the gel progressively allowing for the integration of CNS self populations leading to a successful recovery of function. Highlight aspects learned from this type of studies are that: Endogenous reconstruction of the injured spinal cord is possible by using the adequate support. The contribution of nestin-expressing progenitors to spinal cord regeneration is continuous and substantial both, in the reconstructed segment as well as, along the distal and caudal segments of the reconstructed spinal cord. Most of these cells appear to have been in a quiescent state until the injury occurred and only a small fraction of these neural progenitors was produced via cell proliferation. The hydrogel combined with exercise was necessary and sufficient to restore locomotor function in cats that underwent spinal transaction followed by reconstructive surgery. This recovery of function was first seen 28 days after surgery and continued to improve for at least 21 months. Therefore, endogenous pre-existing and newly formed cells populated the gel scaffold established contact with the non injured tissue and lead to recovery of function.

Development of a sialic acid-containing hydrogel of poly[N-(2-hydroxypropyl) methacrylamide]: characterization and implantation study.

This study describes the preparation and the characterization of poly[ N-(2-hydroxypropyl methacrylamide)] hydrogel with bulk-modified saccharidic portion of ganglioside GM 3 (Neu5Ac-alpha2,3-Gal-beta1,4-Glc). The 3'-sialyllactose is a bioactive epitope recognized by many cell surface receptors on viruses, bacteria, and human cells such as growth factor receptors. Acrylated 3'-sialyllactose was synthesized and incorporated into the macromolecular network of hydrogels by free radical cross-linking copolymerization. Fluorescence techniques coupled to confocal laser scanning microscopy was employed to characterize the binding and accessibility of the sialyl group in the polymer network by using a monoclonal antibody against GM 3 and the lectin wheat germ agglutinin. The morphology of the network was examined by scanning electron microscopy and confocal microscopy to image the gel morphology. The water content of sialyllactosyl-HPMA hydrogel compared to unmodified gel was characterized by swelling measurements and thermogravimetry. A preliminary implantation study in rat brain was performed to examine the biofunctionality of the sialyllactosyl hydrogel using an experimental model of Parkinson's disease.

Expression of heat shock protein (HSP)-25 and HSP-32 in the rat spinal cord reconstructed with Neurogel.

We recently showed a successful reconstruction of the cat spinal cord using NeuroGel a polymer hydrogel bridge between the two spinal stumps. The polymer graft supports axonal elongation, myelination and angiogenesis up to 21 months, Wallerian degeneration was diminished and gliotic scarring was prevented. In the present study, we report the expression patterns of two stress proteins, (HSPs) HSP-25 and HSP-32 after spinal cord hemisection with and without reparative surgery with NeuroGel. Double immunofluorescence using cell specific markers for neurons, astrocytes and oligodendrocytes (OL), in combination with antibodies for HSP-25 and 32 showed that mainly neurons express both proteins. Both HSPs displayed different temporal expression patterns in the reconstructed spinal cords with a concomitant reduction of secondary damage. In conclusion, Neurogel reconstruction of the spine during the acute phase considerably reduces secondary damage resulting in a rapid and stable regenerative response.

Prevention of gliotic scar formation by NeuroGel allows partial endogenous repair of transected cat spinal cord.

Spinal cords of adult cats were transected and subsequently reconnected with the biocompatible porous poly (N-[2-hydroxypropyl] methacrylamide) hydrogel, NeuroGel. Tissue repair was examined at various time points from 6-21 months post reconstructive surgery. We examined two typical phenomena, astrogliosis and scar formation, in spines reconstructed with the gel and compared them to those from transected non-reconstructed spines. Confocal examination with double immunostaining for glial fibrillary acidic protein (GFAP) and myelin basic protein (MBP) showed that the interface formed between the hydrogel and the spine stumps did prevent scar formation and only a moderate gliosis was observed. The gel implant provided an adequate environment for growth of myelinated fibers and we saw angiogenesis within the gel. Electron microscopy showed that regenerating axons were myelinated by Schwann cells rather than oligodendrocytes. Moreover, the presence of the gel implant lead to a considerable reduction in damage to distal caudal portions of the spine as assessed by the presence of more intact myelinated fibers and a reduction of myelin degradation. Neurologic assessments of hindlimb movement at various times confirmed that spinal cord reconstruction was not only structural but also functional. We conclude that NeuroGel lead to functional recovery by providing a favorable substrate for regeneration of transected spinal cord, reducing glial scar formation and allowing angiogenesis.

Homotopic septal grafts combined with a hydrogel bridge promote functional recovery in rats with fimbria-fornix lesions: A unit recording study.

Fimbria-fornix lesions abolish the hippocampal electrophysiological activity time-locked to the theta rhythm and alter some functional characteristics of place cells. The present experiment investigated whether homotopic grafts of fetal septal cells can alleviate some of these alter-ations when combined with a polymeric hydrogel bridging a fimbria-fornix lesion-cavity. Eleven months after grafting surgery, unit recordings were obtained from hippocampal neurons of seven rats [two sham-operated (S), two lesion-only (L) and three grafted (G)] while they explored a radial maze. The lesions induced dramatic loss of hippocampal acetylcholinesterase(AChE)-positive reaction products. Surviving grafts were found in the three grafted rats and several AChE-positive processes could be observed in the polymeric hydrogel, as well as in the most dorsal portion of the hippocampal parenchyma. Of 168 recorded units, 132 were hippocampal interneurons (i.e., fired rapidly everywhere in the maze), and 36 were pyramidal place cells (i.e., fired only when the rat was in a specific location in the maze, the place field). The overall firing characteristics of either cell type were similar in S, L and G rats. However, while none of the interneurons recorded from L rats was found to fire rhythmically, a significant proportion of interneurons recorded from S and G rats had an activity pattern time-locked to the theta rhythm [S: 16/19 (84 %); G : 22/70 (31 %)]. In addition, the increase in firing activity observed in interneurons recorded from S rats when they were moving was disrupted in cells from L rats, but partially restored in cells from G rats. Concerning place cells, most (93 %) place fields in S rats were stable relative to extra-maze cues when the radial maze was rotated, while they followed the maze rotation in both L and G rats. Because of the low number of rats used, the present results should be considered with caution. Nevertheless, they indicate graft-induced recovery of some properties of hippocampal function following fimbria-fornix damage, and suggest that homotopic transplants of projection neurons may foster some func-tional recovery when provided with a biomaterial allowing the host or grafted neurons to cross the lesion cavity.