Early lysosome defects precede neurodegeneration with Aβ and tau aggregation

Background Loss‐of‐function mutations in the X‐linked endosomal Na + /H + Exchanger 6 (NHE6) cause Christianson syndrome (CS) in males. CS involves endosome dysfunction leading to early cerebellar degeneration, as well as later‐onset cortical and subcortical neurodegeneration, potentially including tau deposition as reported in postmortem studies. In addition, there is reported evidence of modulation of amyloid beta (Aβ) levels in experimental models wherein NHE6 expression was targeted. For in vivo studies, rat models may have an advantage over mouse models for the study of neurodegeneration, as rat brain can demonstrate robust deposition of endogenously‐expressed Aβ and tau in certain pathological states. Mouse models generally do not show the accumulation of insoluble, endogenously‐expressed (non‐transgenic) tau or Aβ. Method We generated an NHE6‐null rat model of CS using CRISPR‐Cas9 genome‐editing technologies to study neurodegeneration in CS and the possibility of Aβ and tau pathology. Result Using this novel rat model, we were able to follow the sequence of pathogenic events in CS. NHE6‐null rats demonstrate an early and rapid loss of Purkinje cells in the cerebellum, as well as a more protracted neurodegenerative course in the cerebrum. In both the cerebellum and cerebrum, lysosome deficiency is an early pathogenic event, preceding autophagic dysfunction. Microglial and astrocyte activation also occur early. In the hippocampus and cortex, lysosome defects precede loss of pyramidal cells. Importantly, we subsequently observe biochemical and in situ evidence of both Aβ and tau aggregation in the aged NHE6‐null hippocampus and cortex (but not in the cerebellum). Tau deposition is widely distributed, including cortical and subcortical distributions. Interestingly, we observe tau deposition in both neurons and glia, as has been reported in CS postmortem studies previously. Conclusion This new model is among very few examples of a genetically modified animal that exhibits neurodegeneration with deposition of endogenously‐expressed Aβ and tau. This NHE6‐null rat will serve as a new robust model for CS. Furthermore, these studies provide evidence for linkages between endo‐lysosome dysfunction and neurodegeneration involving protein aggregations, including Aβ and tau. Therefore these studies may provide insight into mechanisms of more common neurodegenerative disorders, including Alzheimer’s Disease and related dementias.