Next generation SARM1 knockout and epitope tagged CRISPR-Cas9-generated isogenic mice reveal that SARM1 does not participate in regulating nuclear transcription, despite confirmation of protein expression in macrophages

Abstract

SARM1 is an ancient and highly conserved TIR-domain containing protein, with a diverse range of proposed roles in both innate immunity and neuronal death and degeneration. Murine SARM1 has been reported to regulate the transcription of specific chemokines in both neurons and macrophages, however the extent and mechanism by which SARM1 contributes to transcription regulation remains to be fully understood. Here, using RNA sequencing we identify differential gene expression in bone marrow-derived macrophages (BMDM) from C57BL/6 congenic 129 ES cell-derived Sarm1-/- mice compared to wild type (WT). However, we show that passenger genes which are derived from the 129 donor strain of mice flank the Sarm1 locus, confounding interpretation of results, since many of the identified differentially regulated genes come from the region containing passenger genes. To re-examine the transcriptional role of SARM1 in the absence of such passenger genes, we generated three different Sarm1-/- mice using CRISPR/Cas9 technology. Vincristine treatment of ex vivo cultured post-natal neurons from these mice confirmed SARM1's previously identified key function as an executor of axon degeneration. However, using these mice, we show that the absence of SARM1 has no impact on transcription of genes previously shown to be altered in macrophages or in the brainstem. To gain further insight into SARM1 function, we generated and characterized a mouse expressing epitope-tagged SARM1, as it has been difficult to date to confirm which cells and tissues express SARM1 protein. In these mice we see high SARM1 protein expression in the brain and brainstem, and lower but detectable levels in macrophages. Overall, the generation of these next generation SARM1 knockout and epitope-tagged mice has clarified that SARM1 is expressed in mouse macrophages but has no general role in transcriptional regulation in these cells, and has provided important new animal models to further explore SARM1 function.