Melanin as an Endogenous MRI Contrast Agent for Stem Cell Delivery in Stroke

Abstract

In vivo measures of survival, growth, and migration of stem cell transplants for stroke therapy remain inadequate. Since such biomarkers in the central nervous system do not exist, we sought to investigate the potential of melanin as an endogenous MR contrast medium for monitoring cell lineage transplants. Expression of a human gene, tyrosinase, induces melanogenesis, but unlike particle-based tracking agents, tyrosinase incorporated into the host genome will be replicated during mitosis.Tyrosinase and the gene for a co-enzyme, tyrosinase-related protein 1, were placed under the CMV promoter. The construct was inserted into 293 HEK (human embryonal kidney) and iPS NPC (induced pluripotent neural progenitor) cells via plasmid transfection and viral transduction, respectively. Stroked mice were injected with melanin-producing cells. Control mice were injected with native cell lines. In vitro expression was measured by fluorescent microscopy, immunocytochemistry, spectroscopy, PCR, and MRI.Robust in vitro melanin production was achieved in both cell lineages demonstrated by significant T1 shortening on in vivo MRI. Pathologic correlation demonstrated colocalization of pigmented regions in the injection sites with human antinuclear antibody staining. Through induction of melanogenesis that is reproducible across multiple cell divisions, MR-based imaging of clinically relevant cell lineage transplants is possible.