Using reporter genes to label selected neuronal populations in transgenic mice for gene promoter, anatomical, and physiological studies

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Abstract

This review summarizes recent work on the use of reporter genes to label selected neuronal populations in transgenic mice, with particular emphasis on gonadotropin-releasing hormone (GnRH) neurons. Reporter genes discussed are the lacZ, green fluorescent protein (GFP), luc, and bla genes, which encode the reporter proteins β-galactosidase, GFP, luciferase, and β-lactamase, respectively. Targeted transgenic expression of these reporter proteins is obtained by fusing the corresponding reporter gene, with or without a subcellular localization signal, to a cell type- or brain region-specific gene promoter. Mice carrying GnRH promoter-driven reporter genes have proven useful for revealing the promoter elements required for cell type-specific expression of GnRH, the full anatomical profile of the GnRH neuronal network, and its electrophysiological activity, suggesting that similar approaches will assist in elucidating the properties of other neuronal populations as well.

Introduction

Certain types of neurons in the brain are difficult to study because they cannot be identified by location or morphological criteria alone. One approach to identify such neurons is to tag them with a reporter protein. Recent work on the gonadotropin-releasing hormone (GnRH) neuronal network has demonstrated that such an approach can be quite powerful, yielding important new information about the anatomical distribution and electrophysiological activity of a neuronal population, as well as the promoter elements required for its specific expression of genes. This review discusses several commonly used reporter proteins and their past, present, and future application, with particular emphasis on the GnRH neuronal network.

Section snippets

Reporter proteins

A reporter protein is a protein whose expression is linked to the expression of a gene of interest and that can be visualized as a result of its bioluminescence, fluorescence, or, if it is an enzyme, due to the bioluminescence, fluorescence, or color of the product of the reaction it catalyzes. Compared to non-enzymatic reporter proteins, enzymatic reporter proteins are more sensitive detectors of gene expression because each molecule of an enzymatic reporter protein can catalyze the reaction

GnRH neurons and GnRH-reporter mice

GnRH–LacZ and GnRH–GFP mice, in which the GnRH gene promoter drives the expression of the lacZ gene or a humanized, red-shifted variant of the GFP gene, are striking examples of tagging a neuronal population with a reporter gene for gene promoter, anatomical, and physiological studies. GnRH–LacZ and GnRH–GFP mice, along with GnRH–luciferase and GnRH–Bla mice, are discussed below following a brief description of the GnRH neuronal network.

Simultaneous use of two or more reporter genes

Tagging a selected neuronal population with two or more reporter genes can result in both high sensitivity of detection in fixed tissue and identification of reporter-tagged neurons in live tissue. Such tagging can be obtained by breeding mice with transgenes containing different reporters, or by using a transgene with a bi-directional cytomegalovirus promoter that can be transactivated by a cell type- or brain region-specific transactivator to drive the expression of two different reporters.

Future developments

Likely future developments include the generation of even brighter GFP's and the enhancement of red fluorescent proteins from species other than jellyfish (Matz et al., 1999, Tsien, 1999). In combination with GFP, the red fluorescent proteins should provide a convenient and powerful approach to double-label neurons that contain two peptides or proteins of interest for physiological studies.

Conclusion

The results from the GnRH–LacZ and GnRH–GFP mice described in this review clearly demonstrate that using reporter genes to tag selected neuronal populations is an important approach that enables one to perform gene promoter, anatomical, and physiological investigations that were previously difficult or impossible. Such an approach, in its various present and future forms, some of which have been described in this review, will undoubtedly assist greatly in deepening our understanding of neuronal

Acknowledgements

D.J.S. was supported by an Alexander von Humboldt Foundation Research Fellowship, by SFB Grants 317 and 488 of the Deutsche Forschungsgemeinschaft, and by an NIH Individual National Research Service Award (F32-NS10085). Funded in part by grants from the Volkswagen Foundation and the German Chemical Society to P.H.S.

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