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Hypoxia Inducible Factor 1 alpha (HIF-1α) counteracts the acute death of cells transplanted into the injured spinal cord
Cellular transplantation is in clinical testing for a number of central nervous system disorders, including spinal cord injury (SCI). One challenge is acute transplanted cell death. To prevent this death, there is a need to both establish when the death occurs and develop approaches to mitigate its effects. Here, using luciferase (luc) and green fluorescent protein (GFP) expressing Schwann cell (SC) transplants in the contused thoracic rat spinal cord 7 days post-injury, we establish via in vivo bioluminescent (IVIS) imaging and stereology that cell death occurs prior to 2-3 days post-implantation. We then test an alternative approach to the current paradigm of enhancing transplant survival by including multiple factors along with the cells. To stimulate multiple cellular adaptive pathways concurrently, we activate the hypoxia inducible factor 1 alpha (HIF-1α) transcriptional pathway. Retroviral expression of VP16-HIF-1α in SCs increased HIF-α by 5.9-fold and its target genes implicated in oxygen transport and delivery (VEGF, 2.2-fold) and cellular metabolism (enolase, 1.7-fold). In cell death assays in vitro, HIF-1α protected cells from H2O2-induced oxidative damage. It also provided some protection against camptothecin-induced DNA damage, but not thapsigargin-induced endoplasmic reticulum stress or tunicamycin-induced unfolded protein response. Following transplantation, VP16-HIF-1α increased SC survival by 34.3%. The increase in cell survival was detectable by stereology, but not by in vivo luciferase or ex vivo GFP IVIS imaging. The results support the hypothesis that activating adaptive cellular pathways enhances transplant survival and identifies an alternative pro-survival approach that, with optimization, could be amenable to clinical translation.
To maximize the benefits of cellular transplants for human therapeutic use, there is a critical need to develop strategies that effectively promote transplant survival and permit rapid assessment of transplant survival. The current study: 1) identifies the narrow time window in which transplanted cells die within the injured rat spinal cord, thus establishing the time window in which cytoprotection should be targeted to counteract transplanted cell death; 2) tests the effects of elevating HIF-1α on spinal cord transplant survival, thus demonstrating that activating adaptive transcriptional pathways is protective in SCI, and; 3) demonstrates, by comparing three approaches to quantifying transplant survival, that until faster and more sensitive methods can be developed, stereology remains the most reliable method.