A Versatile Strategy for Genetic Manipulation of Cajal-Retzius Cells in the Adult Mouse Hippocampus.

Cajal-Retzius (CR) cells are transient neurons with long-lasting effects on the architecture and circuitry of the neocortex and hippocampus. Contrary to the prevailing assumption that CR cells completely disappear in rodents shortly after birth, a substantial portion of these cells persist in the hippocampus throughout adulthood. The role of these surviving CR cells in the adult hippocampus is largely unknown, partly because of the paucity of suitable tools to dissect their functions in the adult versus the embryonic brain. Here, we show that genetic crosses of the mouse line, widely used to target CR cells, to reporter mice induce reporter expression not only in CR cells but also progressively in postnatal dentate gyrus granule neurons. Such a lack of specificity may confound studies of CR cell function in the adult hippocampus. To overcome this, we devise a method that not only leverages the temporary CR cell-targeting specificity of the mice before the first postnatal week, but also capitalizes on the simplicity and effectiveness of freehand neonatal intracerebroventricular injection of adeno-associated virus. We achieve robust Cre-mediated recombination that remains largely restricted to hippocampal CR cells from early postnatal age to adulthood. We further demonstrate the utility of this method to manipulate neuronal activity of CR cells in the adult hippocampus. This versatile and scalable strategy will facilitate experiments of CR cell-specific gene knockdown and/or overexpression, lineage tracing, and neural activity modulation in the postnatal and adult brain.High-throughput and specific tools for genetic manipulation of neuronal subtypes are desirable for scalable experiments and accurate data interpretation. However, limitations in available tools present a challenge for certain cell types, such as Cajal-Retzius cells, a class of transient neurons of which a portion persists in the adult brain. Highlighting the limitation of Cre-driver mouse lines because of loss of specificity in adulthood, we demonstrate the use of neonatal intracerebroventricular delivery of adeno-associated viral vectors to specifically manipulate Cajal-Retzius cells in the adult hippocampus. Our strategy offers a framework to address similar issues with experiment throughput and specificity of other neuronal subtypes.