The claustrum is a subcortical structure located between the insula and the putamen which is present in all amniotes. Its exact function is unclear. Due to its extensive connectivity with all neocortical regions, it has been proposed to play a role in integrating sensory information into conscious experience. Further research has then demonstrated that the claustrum could be implied in attention and salience processing, the generation of slow wave oscillation and attentional set shifting.
The study of the claustrum requires a proper delineation of the neuronal population that composes it, and accurate tools to target it. So far, the function of the claustrum was mainly investigated in mice using stereotaxic targeting in combination or not with different genetic markers of the structure. However, the claustrum’s deep location, extensive connectivity, and elongated shape on the one hand, and the lack of specificity of the markers on the other hand have severely limited its investigation. Therefore, in search for a more accurate claustrum marker, the Rodriguez and Carleton’s laboratories identified a gene whose expression is mainly restricted to claustrum excitatory neurons. We termed this gene Ghost, and generated a series of mouse lines expressing the Cre recombinase under the control of its promoter. These Ghost-Cre lines consist of a knock-in (Ghost-IRES-Cre, referred to as KI) and two BAC transgenic lines (Tg61 and Tg62).
In this work, I aim to determine the expression pattern of the different Ghost-Cre lines in the developing and adult mouse brain using a Cre-dependant tdTomato-expressing reporter allele to map onsets of transgene expression. In KI animals, I found that the Cre is mainly expressed in cell populations which sustain Ghost transcription throughout the animal's life, namely, the claustrum, the endopiriform nucleus and the thalamic reticular nucleus. In the BAC transgenic Tg62 animals, I found that the transgene mainly recapitulates the endogenous spatiotemporal pattern of Ghost, only with slightly later onsets (in days) during post-natal development. In the BAC transgenic Tg61 animals, I found that Cre expression is mainly restricted to claustrum neurons, making it a promising tool for studying this structure.
