Plastic Adaptation: A Neuronal Imperative Capable of Confounding the Goals of Stem Cell Replacement Therapy for either Huntington’s or Parkinson’s Disease

Abstract

Although stem cell transplant therapy offers considerable promise for deteriorative diseases, the efficacy of its application may be mitigated by endogenous compensatory mechanisms in the host brain. Plastic compensation follows neurodegeneration, beginning at its very onset and minimizing early symptom expression. As researchers attempt to correlate symptom remission with the ability of transplanted cells to adopt specific cell phenotypes, they need to be vigilant of the possibility that competing, local compensatory effects may be altering the outcome. Clearly plastic compensatory mechanisms could confound desired transplant-derived improvements by supplanting the beneficial contributions of the transplants. As circuit-level adaptations occur, more explicit explorations of their relevance to neuronal transplantation success are needed. Conceptual models of undirected transplanted cells adopting preconceived appropriate roles require revision. The notion that newly transplanted neuronal precursors will incorporate themselves into host circuitry with mutual cooperation across both parties (i.e., transplant and host) without some symbiosis-promoting mechanism is naïve. Undirected local circuits could react to newly transplanted additions as intruders. We advocate that appropriate signaling from transplanted cells to the host environment is required to optimize the therapeutic relevance of transplantation. This review surveys critical signaling mechanisms that might promote symbiotic interdependence between the host and new transplants.