Morphology of a WGA-filled Purkinje cell, visualized by immunostaining. Courtesy of Allan Basbaum, University of California, San Francisco, California, USA.

To understand how neuronal circuits work, we also need to know about their anatomy. The discovery, around 20 years ago, that the plant lectin wheatgerm agglutinin (WGA) can be transported anterogradely along axons and across synapses, led to the development of transneuronal tracing techniques to visualize neuronal pathways and circuits. More recently, transgenic mice have been generated that express WGA under the control of tissue-specific gene promoters, allowing circuits to be traced from defined regions of the central nervous system (CNS). However, making a different transgenic line for each experiment is very laborious, and the range of good region-specific promoters is limited. To address these problems, Braz et al. have engineered a transgenic mouse that allows a more flexible approach.

The authors constructed a gene-expression vector in which the WGA gene was positioned downstream of a LacZ reporter gene. The expression of LacZ was driven by the chicken β-actin promoter, which is active in most tissues. The LacZ coding sequence was flanked with loxP sites, so that it could be excised in the presence of Cre recombinase, thereby bringing the WGA gene under the control of the β-actin promoter. The authors used this vector to create a transgenic mouse line, which they named ZW. By breeding these mice with lines in which different promoters controlled Cre recombinase expression, they could express WGA in a tissue-specific manner. As examples, they traced the neuronal pathways from the eye and the cerebellum by expressing Cre from retinal and Purkinje-cell-specific promoters, respectively.

Of course, the applications of this technique will be limited by the availability of mouse lines that express Cre in an appropriate pattern. However, the ZW mouse has another advantage; the WGA gene can also be activated at any time and in any region of the CNS, by injection of a Cre-expressing viral vector. To demonstrate the efficacy of this technique, the authors used it to trace neuronal circuits in the cerebellum and visual cortex of adult mice.

Although the β-actin promoter is generally thought to be ubiquitously active, Braz et al. found that it caused WGA to be expressed in a mosaic pattern in the ZW mice. However, they managed to turn this potential drawback to their advantage, by showing that it enabled individual neurons to be distinguished from their unlabelled neighbours. This provided a single-cell resolution that was comparable to Golgi staining, with the bonus of making it possible to study connectivity as well as morphology.

As transgenic mouse lines that express Cre recombinase from tissue-specific promoters become more widely available, it will be possible to use the ZW mouse to trace neuronal circuits in a wide variety of brain regions. The fact that the activation of the WGA gene can also be controlled in space and time using a Cre-expressing viral vector provides an additional level of flexibility, making this mouse an extremely valuable tool for visualizing both developing and adult neuronal circuits.