Neural stem cell therapy and inflammation status of post-traumatic syringomyelia

Abstract

Post-traumatic syringomyelia (PTS) is a severe complication of chronic spinal cord injury (SCI) with intraspinal expanding cysts as the major pathology. PTS occurs in up to 30 % of SCI patients, and time of diagnosis is usually years after the primary injury. The sign of PTS is the deterioration of existing SCI symptoms, with further loss of motor, sensory and autonomic functions and increased neuropathic pain and spasticity. PTS used to be overlooked due to the limited availability of MRI, which is the critical diagnostic method.

The exact mechanism behind PTS is not clear yet, but disturbance of CSF flow by subarachnoid scar tissue with imbalance of in- and outflow of CSF in the spinal cord parenchyma is the most recognized factor. The surgical treatment, releasing the tethered spinal cord, with or without shunting to drain the fluid-filled cyst is not always effective. Therefore, it is essential to further investigate pathophysiological mechanisms and possible risk factors for the development of PTS, to identify therapeutic strategies in addition to surgery and improve the efficiency of treatment.

Neural stem/progenitor cell (NPC) with the ability to proliferate and differentiate into multiple lineages in CNS have been the focus of regenerative research in CNS disorders and insults such as SCI. NPCs can protect injured tissue, promote tissue repair and axon regeneration by releasing trophic factors, providing supportive tissue, modulating inflammation and differentiating into neurons, oligodendrocytes and astrocytes to replace the lost cells, and modulate the pathophysiology at the lesion site. NPCs can be transplanted to the injured recipient, or similar endogenous cells may be stimulated by various factors. In Paper I, we tested if NPC transplantation could improve tissue repair in PTS. We first developed a clinically relevant rat model of PTS mimicking the clinical situation by injecting blood into the subarachnoid space in combination with a mild traumatic SCI. The injection of blood was used to induce tethering of spinal cord without introduction of any foreign neurotoxic materials which could be toxic to the transplanted cells. Our rat model showed pathology similar to clinical PTS, including the appearance in MRI. With this model, we demonstrated the safety and efficacy of two types of human NPCs, embryonic/fetal NPCs and induced pluripotent stem cells (iPSC) derived NPCs-neuroepithelial stem cells (NESCs). We found that both of them can completely prevent the expansion of cysts in PTS and showed no sign of tumor formation after long-term survival. The therapeutic effect on cyst expansion did not correlate with the survival of transplanted cells and occurred in spite of limited differentiation of grafted cells.

To move further to clinical translation, in Paper II, we transplanted GMP-compliant iPSC-derived NESCs to the same rat model as in Paper I. We observed considerably better survival of transplanted cells than Paper I. Importantly, the transplanted cells not only prevented the cyst expansion but also modulated major pathophysiological SCI process, reducing astrocytic scar and inflammation, but also stimulating endogenous regeneration, enhancing proliferation of oligodendrocyte progenitor cells (OPCs) and supporting regeneration of serotonergic axon regrowth.

A better understanding of mechanism behind PTS will hopefully bring better therapeutic strategies for clinical treatments. In Paper III, we investigated and compared the pathophysiological processes focusing on inflammation and glial cell reaction to find additional mechanisms behind the development from SCI to PTS. We found signs of more pronounced inflammation at molecular and cellular level, less astrocyte-associated repair in PTS than SCI, and more severe tissue degenerative processes in PTS than SCI.

Taken together, with all the studies in this thesis, we have gained better understanding of the factors that could be related to the progression of PTS, and application of cell therapy which pave the way for improving the therapeutic strategies for PTS patients in a near future.