Dap12 expression in activated microglia from retinoschisin-deficient retina and its PU.1-dependent promoter regulation

Several alterations in the expression of immune-related transcripts were identified recently in the degenerating retina of the retinoschisin knockout (Rs1h(-/Y)) mouse, including the strong expression of the adaptor protein Dap12. As Dap12 is found in leukocytes, we hypothesized that its disease-related expression may be confined to activated retinal microglia cells. To test this hypothesis, we established a procedure for isolation and culture of retinal microglia cells and performed genome-wide expression profiling from Rs1h(-/Y) and control microglia. While retaining their activated state in culture, ex vivo microglia expressed high levels of Dap12 and the transcription factor PU. 1. The activation-dependent induction of Dap12 was also confirmed in the microglia cell line BV-2 following in vitro stimulation. To examine the transcriptional regulation of Dap12 further, macrophage cell lines were transfected with several Dap12 reporter constructs. Promoter deletion assays and site-directed mutagenesis experiments demonstrated an essential role of evolutionarily conserved PU. 1 consensus sites in the proximal -104/+118 Dap12 promoter. In vitro and in vivo binding of PU. 1 to this promoter region was demonstrated using EMSA and chromatin immunoprecipitation. Knockdown of PU. 1 by RNA interference caused a significant reduction of endogenous Dap12 expression and re-expression, and activation of PU. 1 in PU. 1(-/-) progenitor cells induced Dap12 transcription. Taken together, our results indicate that activated microglia from degenerating retinae express high levels of Dap12 and PU. 1, and PU. 1 controls the myeloid-specific regulation of Dap12 directly and may also play a general role in microglia gene expression during retinal degeneration.