Early lysosome defects precede neurodegeneration with amyloid-β and tau aggregation in NHE6-null rat brain

Abstract

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. We have recently shown that loss of NHE6 causes defects in endosome maturation and trafficking underlying lysosome deficiency in primary mouse neurons in vitro. 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. Therefore, to study neurodegeneration in CS and the possibility of Aβ and tau pathology, we generated an NHE6-null rat model of CS using CRISPR-Cas9 genome-editing. Here, we present the sequence of pathogenic events in neurodegenerating NHE6-null male rat brains across the lifespan. 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. In summary, this experimental 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.

Gait as a quantitative translational outcome measure in Angelman syndrome

“…DigiGait avoids many of these pitfalls and appears to be a versatile tool, sensitive to various treatments, and has an established record of use as a strong longitudinal/ repeat testing outcome measure (Hansen & Pulst, 2013). Compared to rotarod, DigiGait is more sensitive to severe deficits and can be used to collect motor data in animals unable to stand on a rod (Lee et al, 2021). Treadmill gait analysis, using ventral imaging, such as DigiGait and the Noldus CatWalk, allows for 1) unhindered access to an ambulating mouse, 2) walking that is unaffected by the experimenter's grasping or "scruffing," and 3) detection of nuanced and fine-grained phenotypes (Lei et al, 2014).…”

The Less Well-Known Little Brothers: The SLC9B/NHA Sodium Proton Exchanger Subfamily—Structure, Function, Regulation and Potential Drug-Target Approaches

“…Given the known association of other endosomal NHEs (NHE6 and NHE9) with neurological diseases (e.g. NHE6 for Christianson syndrome and NHE9 for autism and ADHD) ( Kondapalli et al, 2013 ; Ouyang et al, 2013 ; Pescosolido et al, 2014 ; Schwede et al, 2014 ; Prasad et al, 2017 ; Gao et al, 2019 ; Lee et al, 2021 ; Pescosolido et al, 2021 ) and potential functional redundancy of endosomal NHEs, it is possible that NHA2 also has an important, not yet understood, function in the brain or other organs that have not yet been studied in detail.…”

Mutations in α-synuclein, TDP-43 and tau prolong protein half-life through diminished degradation by lysosomal proteases

“…As the engines of autophagy, lysosomes have been increasingly recognized as key players in neurodegeneration, with mutations in lysosome-resident proteins including GBA, PGRN and CTSD implicated in PD, FTD and AD, respectively 54 . De cient lysosomal function has also been identi ed as an early pathogenic event in animal models of neurodegeneration 55 . Our results further highlight lysosomal proteases as signi cant contributors to PD, FTD, ALS and AD pathobiology that could be differentially targeted for disease-speci c therapeutic purposes.…”