“Patching” the Primate Spinal Cord With Human Neural Stem Cells

An interdisciplinary team made of neuroscientists, neurologists, and neurosurgeons, led by Dr. Mark Tuzynski at the University of California – San Diego (UCSD, San Diego, CA), reported this past February in Nature Medicine the successful grafting of human spinal cord-derived neural progenitor cells (NPCs) into the spinal cord of rhesus monkeys (Macaca mulatta) with high level (i.e., cervical) injuries. The grafts not only survived for at least 9 months post-injury, and but also developed large numbers of axons that traveled over long distances within the injured spinal cord and were also penetrated by host axons.

Over the course of the last three decades of spinal cord injury (SCI), it has become clear that regeneration of injured axons is a difficult and exhausting task. However, restoration of this long-distance fibers might harbour the answer to improve functional outcomes below the level of injury. After a SCI, numerous hostile extrinsic changes such as an inhibitory cellular matrix, high expression of inhibitory myelin-associated proteins, and low concentrations of neurotrophic factors, as well as neuronal-intrinsic factors, actively hinder axonal regeneration. During development, neurons from the spinal cord have active robust growth mechanisms that do not respond to inhibitory cue found in the adult central nervous system.

With this in mind, this group of researchers used their grafting expertise in rodents to test whether grafts (with 20 million cells) derived from human neural progenitor and neural stem cells (NSCs) could (1) survive and differentiate into neurons, (2) support growth of host (injured) axons, (3) extend axons into the host spinal cord, and (4) form functional synapses with host neurons below the lesion, this in order to provide a relay circuit to the injured spinal cord, and improve functional outcomes. To assess this, 9 adult rhesus monkeys were subjected to right-side C7 hemisections 14-days before grafting. Matching the period required for stabilization of patient with SCI before becoming candidates for NCS therapy.

Critical measures were taken to optimize the delivery of the grafts, such as the use of the reverse Trendelenburg position (operative table tilted in a 30 degrees angle, head upwards) to keep cerebrospinal fluid away from the lesion during the grafting procedure. This action, coupled to increased concentrations of fibrin-thrombin, and an immunosuppression protocol with three drugs (tacrolimus, mycophenolate mofetil, and prednisone), resulted in increased survival of the grafts, which reached at least 9  months. By combining intracortical injections with biotinylated dextran amine (BDA), and immunofluorescence for neuron-specific and glial-specific cytoskeletal markers (class III β-tubulin,  NF70, NF200, SMI32, and GFAP) and the postsynaptic marker HOMER, the researchers identified that transplanted grafts were able to develop long-distance axons within the host spinal cord (mostly of immature nature as evidenced by lack of NF200 immunoreactivity), promote host axonal regeneration, and demonstrated for the first time the regeneration primate corticospinal axons into human NPC grafts. Ultimately, surviving grafts led to significant improvements in object manipulation tasks, locomotor recovery, and peak performance.

The present study represents a massive leap in SCI research, moving from rodents to primates, offering significant translational value and understanding of interspecies differences in stem cell biology and regenerative mechanisms. Future studies will look to verify these exciting results and optimize the grafting of human NPCs by improving delivery, immunosuppressive therapy, and selection of more fitting NPCs and NCs.