Development of FKBP5 ASOs to mitigate tau pathology

Background: Alzheimer’s disease (AD) is a progressive neurodegenerative disorder characterized by the accumulation of abnormal protein aggregates, including tau tangles, in the brain. FKBP51 (encoded by FKBP5 gene) is a chaperone protein that regulates tau and has been implicated in AD pathology. FKBP51 overexpression in the brains of tau transgenic mice correlates with increased tau oligomers and neuronal toxicity, whereas siRNA mediated knockdown of FKBP5 decreased tau levels in cell culture models. In this study, we developed antisense oligonucleotides (ASOs) to target FKBP5, in an effort to lower tau levels. We hypothesize that ASOs targeting FKBP5 will reduce FKBP51, which will decrease tau levels and improve tau-associated deficits. Method: We designed and synthesized vivo-morpholino ASOs that specifically bind to and target FKBP5 mRNA degradation. We tested three ASOs in vitro using human neuroglioma and mouse hippocampal neuronal cultures but encountered tissue toxicity when using direct injections into the brains of wild-type mice. To overcome this, we designed and synthesized locked-nucleic acid (LNA) GapmeR ASOs targeting regions conserved between human and mouse FKBP5. These have been tested in human neuroglioma and mouse hippocampal neuroblastoma cells lines for their ability to decrease dexamethasone induced FKBP5 upregulation and assessed for toxicity. Result: Initial screening of vivo morpholino ASOs resulted in 50% - 80% reduction in dexamethasone induced FKBP51 levels at 10 µM. This was further corroborated in mouse primary cortico-hippocampal neurons where FKBP51 levels were reduced by 50% - 75%, which correlated with significant reduction in total tau levels. However, in vivo pilot studies showed tissue toxicity for our control construct. LNA GapmeR ASOs reduced FKBP51 levels at much lower concentrations (<10 nM) with no apparent toxicity. Conclusion: Fkbp5 ASOs have shown promising results at decreasing FKBP51 and total tau levels. Future studies will include further testing in vivo for efficacy and safety, followed by studies in tau transgenic mice.