Review

Combinatorial strategies with Schwann cell transplantation to improve repair of the injured spinal cord

The spinal cord has a limited potential to regenerate after traumatic injury, due to extensive tissue destruction and a natural healing process, which creates physical, cellular, and molecular barriers in an attempt to preserve residual function. In this chapter, we focus on tissue-engineering approaches that use biomaterials as combinatorial therapies for spinal cord injury (SCI) repair. SCI is first described as the disruption of anatomic organization, along with its clinical and epidemiological consequences. Key tissue-engineering principles to facilitate spinal cord regeneration using biomaterial platforms in multimodal approaches are discussed. Bioengineering considerations for material fabrication in macro- and microarchitectures, and for integrated biocompatibility in animal models of SCI are addressed. The applications of natural and synthetic polymer scaffolds with cellular and molecular functionalization, and their outcomes in regenerating spinal cord tissue after injury, are presented in detail. The chapter concludes with perspectives on the clinical translation of these technologies to improve neurologic function in patients with SCI.

Schwann cell (SC) transplantation has been comprehensively studied as a strategy for spinal cord injury (SCI) repair. SCs are neuroprotective and promote axon regeneration and myelination. Nonetheless, substantial SC death occurs post-implantation, which limits therapeutic efficacy. The use of extracellular matrix (ECM)-derived matrices, such as Matrigel, supports transplanted SC survival and axon growth, resulting in improved motor function. Because appropriate matrices are needed for clinical translation, we test here the use of an acellular injectable peripheral nerve (iPN) matrix. Implantation of SCs in iPN into a contusion lesion did not alter immune cell infiltration compared to injury only controls. iPN implants were larger and contained twice as many SC-myelinated axons as Matrigel grafts. SC/iPN animals performed as well as the SC/Matrigel group in the BBB locomotor test, and made fewer errors on the grid walk at 4 weeks, equalizing at 8 weeks. The fact that this clinically relevant iPN matrix is immunologically tolerated and supports SC survival and axon growth within the graft offers a highly translational possibility for improving efficacy of SC treatment after SCI. To our knowledge, it is the first time that an injectable PN matrix is being evaluated to improve the efficacy of SC transplantation in SCI repair.

Melatonin research has been experiencing hyper growth in the last two decades; this relates to its numerous physiological functions including anti-inflammation, oncostasis, circadian and endocrine rhythm regulation, and its potent antioxidant activity. Recently, a large number of studies have focused on the role of melatonin in the regeneration of cells or tissues after their partial loss. In this review, we discuss the recent findings on the molecular involvement of melatonin in the regeneration of various tissues including the nervous system, liver, bone, kidney, bladder, skin, and muscle, among others.

Schwann cell-seeded nerve guidance channels are designed to assist post-traumatic nerve regeneration in the PNS. Chitosan is a natural polymer well suited for tissue engineering as it is biocompatible, non-immunogenic, and biodegradable. Electrospun chitosan nanofibers utilized in nerve guidance channels have the capacity for guiding axonal growth within the channel lumen yet are limited in their capacity to maintain structural integrity within physiological environments. To address this, we attempted genipin crosslinking of chitosan nanofibers. Compared to neat chitosan nanofibers, genipin-treated nanofibers exhibited increased stiffness, resistance to swelling and lysozymal degradation. Furthermore, alignment and proliferation of purified Schwann cell cultures upon genipin-treated substratum was enhanced. When dorsal root ganglion explants were utilized as an in vitro model of peripheral nerve regeneration, emigrating neurons and Schwann cells assumed the uniaxial pattern of aligned electrospun chitosan nanofibers. Neurite growth along the nanofibers led, reaching a frontier more than twice that of the pursuant Schwann cells. Critically, neurite growth rate upon genipin-treated nanofibers demonstrated a 100% increase. Altogether, genipin treatment improves upon the physical and biological properties of chitosan nanofibers towards their utility in nerve guidance channel design.

Among the potential therapies designed to repair the injured spinal cord is cell transplantation, notably the use of autologous adult human Schwann cells (SCs). Here, we detail some of the critical research accomplished over the last four decades to establish a foundation that enables these cells to be tested in clinical trials. New culture systems allowed novel information to be gained about SCs, including discovering ways to stimulate their proliferation to acquire adequately large numbers for transplantation into the injured human spinal cord. Transplantation of rat SCs into rat models of spinal cord injury has demonstrated that SCs promote repair of injured spinal cord. Additional work required to gain approval from the Food and Drug Administration for the first SC trial in the Miami Project is disclosed. This trial and a second one now underway are described.

This article reviews all historical literature in which rodent-derived myelinating cells have been engrafted into the contused adult rodent spinal cord. From 2500 initial PubMed citations identified, human cells grafts, bone mesenchymal stem cells, olfactory ensheathing cells, non-myelinating cell grafts, and rodent grafts into hemisection or transection models were excluded, resulting in the 67 studies encompassed in this review. Forty five of those involved central nervous system (CNS)-derived cells, including neural stem progenitor cells (NSPCs), neural restricted precursor cells (NRPs) or oligodendrocyte precursor cells (OPCs), and 22 studies involved Schwann cells (SC). Of the NSPC/NPC/OPC grafts, there was no consistency with respect to the types of cells grafted and/or the additional growth factors or cells co-grafted. Enhanced functional recovery was reported in 31/45 studies, but only 20 of those had appropriate controls making conclusive interpretation of the remaining studies impossible. Of those 20, 19 were properly powered and utilized appropriate statistical analyses. Ten of those 19 studies reported the presence of graft-derived myelin, 3 reported evidence of endogenous remyelination or myelin sparing, and 2 reported both. For the SC grafts, 16/21 reported functional improvement, with 11 having appropriate cellular controls and 9/11 using proper statistical analyses. Of those 9, increased myelin was reported in 6 studies. The lack of consistency and replication among these preclinical studies are discussed with respect to the progression of myelinating cell transplantation therapies into the clinic.