Developmentally determined intersectional genetic strategies to dissect adult somatosensory circuit function

Improvements in the speed and cost of expression profiling of neuronal tissues offer an unprecedented opportunity to define ever finer subgroups of neurons for functional studies. In the spinal cord, single cell RNA sequencing studies[1][1],[2][2] support decades of work on spinal cord lineage studies[3][3]–[5][4] offering a unique opportunity to probe adult function based on developmental lineage. While Cre/Flp recombinase intersectional strategies remain a powerful tool to manipulate spinal cord neurons[6][5]–[8][6], the field lacks genetic tools and strategies to restrict manipulations to the adult mouse spinal cord at the speed at which new tools develop. This study establishes a new workflow for intersectional mouse-viral strategies to dissect adult spinal cord circuit function based on developmental genetic lineage. To restrict manipulations to the spinal cord, we generate a brain-sparing Hoxb8FlpO mouse line restricting Flp-recombinase expression to caudal tissue. Recapitulating endogenous Hoxb8 gene expression[9][7], Flp-dependent reporter expression is present in the caudal embryo starting day 10.5. This expression restricts FlpO activity in the adult to the caudal brainstem and below. Hoxb8FlpO heterozygous and homozygous mice do not develop any of the sensory or locomotor phenotypes evident in Hoxb8 heterozygous or mutant animals[10][8],[11][9], suggesting normal developmental function of the Hoxb8 gene and protein in Hoxb8FlpO mice. Comparing the variability of brain recombination to available caudal Cre and Flp lines[12][10],[13][11] we show that Hoxb8FlpO activity is not present in the brain above the caudal brainstem, independent of mouse genetic background. Lastly, we combine the Hoxb8FlpO mouse line with a dorsal horn developmental lineage Cre mouse line to express GFP in developmentally determined dorsal horn populations. Using GFP dependent Cre recombinase viruses[14][12], we target this lineage in the adult to show how this strategy can incorporate viral tools in a modular fashion. In summary, this new mouse line and viral approach provides a blueprint to dissect adult somatosensory circuit function using Cre/Flp genetic tools to target spinal cord interneurons based on genetic lineage. In brief We describe the generation of a Hoxb8FlpO mouse line that targets Flp-recombinase expression to the spinal cord, dorsal root ganglia, and caudal viscera. This line can be used in intersectional Cre/Flp strategies to restrict manipulations to the caudal nervous system. Additionally, we describe an intersectional genetics+viral strategy to convert developmental GFP expression into Cre expression, allowing for modular incorporation of viral tools into intersectional genetics. This approach allows for manipulation of a developmentally determined lineage in the adult. This strategy is also more accessible than traditional intersectional genetics, and can adapt to the constantly evolving available viral repertoire. Highlights ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-1 [2]: #ref-2 [3]: #ref-3 [4]: #ref-5 [5]: #ref-6 [6]: #ref-8 [7]: #ref-9 [8]: #ref-10 [9]: #ref-11 [10]: #ref-12 [11]: #ref-13 [12]: #ref-14