SMaRT human neural stem cells found to degrade scar and optimise regeneration after traumatic cervical spinal cord injury

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Christopher Ahuja

Researchers at the University of Toronto (Toronto, Canada) have released “exciting” proof-of-concept data that genetically-engineered SMaRT cells can degrade CSPGs in vitro and that human neural stem cell (NSC) grafts can form long axonal processes in the chronic cervical spinal cord injury (SCI) niche. The data were presented at the 46th annual meeting of the Cervical Spine Research Society (CSRS; 6–8 December 2018, Scottsdale, USA) by Christopher S Ahuja (University of Toronto, Toronto, Canada).

According to the researchers, scar-modifying enzymes can enhance NSC-mediated recovery, however, non-specific administration via an intrathecal catheter increases the risk of off-target CNS effects. Ahuja and colleagues in Michael Fehlings’ lab therefore aimed to generate a novel, genetically-engineered line of hiPS-NSCs, termed Spinal Microenvironment Modifying and Regenerative Therapeutic (SMaRT) cells, capable of locally expressing a scar-degrading enzyme to enhance functional recovery without the risk of nonspecific administration.

The researchers found that the scar-degrading ENZYME and fluorescent reporter are expressed by the transgenic SMaRT cells. “Importantly,” Ahuja remarked, “SMaRT cells retain their human NSC characteristics.” The expressed enzyme appropriately degrades human CSPGs and allows neurons to extend into CSPG-rich regions in vitro. They also found that conditioned SMaRT cell media degrades in situ rodent CSPGs in ex vivo injured cord cryosections.

While blinded behavioural analyses are ongoing, Ahuja noted, an interim histologic analysis of several animals shows that grafted human cells are extending remarkable long (medulla to mid-thoracic) axons through rodent white matter at eight weeks’ post-transplant. The investigators found that the graft further evolves by 32 weeks’ post-transplant demonstrating more numerous, thinner, and longer processes with positive staining for mature neuron markers such as NF200.

Ahuja described how using non-viral techniques, a scar degrading enzyme was genetically integrated into hiPS-NSCs under a human promoter and a monoclonal line was generated by fluorescence activated cell sorting. Enzyme expression and activity was extensively characterised in vitro by biomechanical assays, slot blot, and cell culture assay.

Further to this, T-cell deficient RNU rats with chronic (8 week) C6–7 clip-contusion injuries were randomised to receive (1) NSCs, (2) SMaRT enzyme-expressing NSCs, (3) vehicle control, or (4) sham surgery (laminectomy alone). Behavioural assessments are completed to different timepoints post-injury representing stages of chronic human injury with analyses ongoing.

On the significance of the research, Ahuja told Spinal News International that “currently, no effective regenerative therapy exists for millions of individuals living with chronic spinal cord injuries. This work shows that human neural stem cells can be bioengineered to express an enzyme capable of breaking down a component of chronic scar. It also provides evidence that long-distance neurite outgrowth is possible in the typically challenging chronic spinal cord injury microenvironment.”

Ahuja mentioned some limitations of the study. “While the neurite outgrowth by grated cells is impressive,” he commented, “the bigger question is how to leverage these findings to produce meaningful functional recovery.” He added that “hopefully, the ongoing behavioural analyses will provide insight into the cells’ effects on sensory and motor improvement in these animals.”

Looking forward, Ahuja remarked that “the next steps for SMaRT cells are to assess their safety, biodistribution, and efficacy in a larger animal model. In parallel, we are developing protocols to generate these unique bioengineered cells in a clinical-grade, or Good Manufacturing Practice (GMP), facility.”

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