Engineering membrane proteins for nuclear medicine: applications for gene therapy and cell tracking

Nuclear imaging techniques such as PET and SPECT imaging are expected to play major roles in evaluating the efficacy of in vivo gene therapy. In particular, the quantification of vector delivery and imaging the efficacy of gene expression are of key interests in testing new treatment paradigms and in designing novel vectors. In this review article we illustrate how nuclear imaging can be used to image novel cell-surface expressed fusion proteins and how this strategy can be used to probe for phenotypic changes in genetically manipulated cells. Since the described approach uses new fusion proteins, typically not present on eukaryotic cells, such "artificial receptors" can be designed to bind radioisotopes currently in clinical use. The described fusion proteins consist of 1) a binding domain such as a peptide based chelator that binds 99mTc oxotechnetate and 2) a membrane anchoring domain. A variety of fusion proteins have been tested so far and the most promising one to date consists of a metallothionein (MT)-derived C-terminal peptide fused to a type II membrane protein markers containing the N-terminal membrane anchoring domain of neutral endopeptidase (PEP). Cell-surface expression of MT in transfected cells has been demonstrated using monoclonal antibodies in vitro. Both in vitro and in vivo transchelation experiments have confirmed expression of 99mTc-binding sites in eukaryotic cells. We expect the described approach to evolve into a useful strategy to "tag" transfected cells with 99mTc and thus assessing efficiency of gene delivery and expression.