Murine neural stem cells engineered to express the neural adhesion molecule L1 under the control of the human GFAP promoter promote functional recovery after transplantation in a mouse spinal cord injury model in Mus musculus (Linneaus, 1758)

Spinal cord injury is a devastating neurological disorder. Thus, more research efforts on combinational approaches are needed to facilitate regenerative effects. The transplantation of neural stem cells (NSC) can promote survival or replacement of injured neurons, axonal growth, and reconnection with appropriate targets without tumor formation. However, grafted neural stem cells differentiate mainly into astrocytes with only a minor fraction differentiating into neurons or oligodendrocytes thus hampering their application. Therefore, one aim of stem cell research is to improve neuronal differentiation of neural stem cells. Moreover, the predominant astrocytic differentiation could be taken advantage of in order to deliver therapeutic genes beneficial for regeneration. An ideal candidate for a therapeutic target gene is the gene encoding the neural adhesion molecule L1. L1 plays an important role in the development and regeneration of the central and peripheral nervous system and improves neuronal differentiation, cell survival and neurite outgrowth. To realize these aims, transgenic mice which ectopically express L1 under the control of the human GFAP promoter were used. In these mice neural cell adhesion molecule L1 is ectopically expressed in radial glia and mature astrocytes possibly promoting survival or replacement of injured neurons, axonal growth, and reconnection of axons with appropriate targets. Consequently, neural stem cells overexpressing L1 under the influence of the human GFAP promoter (hGFAP-L1 NSCs) were derived from these mice, analyzed in vitro, and transplanted into a mouse model of spinal cord injury to evaluate their regenerative potential. It could be shown that L1 was expressed in radial glial cells and astrocytes differentiated from these hGFAP-L1 NSCs. Furthermore, L1 expressing-radial glia cells (L1-imm cells) were immunoisolated from cultures of hGFAP-L1 NSCs applying L1 antibodies in vitro. Both hGFAP-L1 NSCs and L1-imm cells showed increased neuronal differentiation, enhanced migration, and decreased astrocytic differentiation when compared to wild type (WT) NSCs in vitro. Moreover, ectopic expression of L1 on the cell surface of astrocytes enhanced neuronal differentiation, decreased astrocytic differentiation, and enhanced neurite outgrowth of cocultured wild type neural stem cells in vitro. Mice transplanted with hGFAP-L1 NSCs and L1-imm cells into the compression-lesioned spinal cord showed better locomotor functional recovery as compared to mice that had been injected with WT NSCs or sham-injected with PBS. A novel electrophysiological H/M reflex recording method could confirm these findings at low frequency stimulation. Morphological analysis revealed increased neuronal differentiation and migration of transplanted cells as well as increased numbers of endogenous catecholaminergic (tyrosine hydroxylase [TH]+) axons caudal to the lesion site in the hGFAP-L1 NSC and L1-imm cell group when compared to the WT NSC and PBS groups. Furthermore, soma size and synaptic coverage of host motoneurons increased caudal to the lesion site in the hGFAP-L1 NSC and L1-imm cell group in comparison to the other experimental groups. The host tissue showed a decreased microgial reaction after syngeneic transplantation of NSCs into the lesioned spinal cord but no difference in the glial reaction was observed. These findings indicate that hGFAP-L1 NSCs and L1-imm cells are capable of improving functional recovery in a syngeneic transplantation paradigm by modulating regenerative processes in the injured spinal cord thus emphasizing the beneficial potential of the neural cell adhesion molecule L1.