Aged Tmem106b knockout mice display gait deficits in coincidence with Purkinje cell loss and only limited signs of non‐motor dysfunction

Abstract

Genetic variants in TMEM106B are a major risk factor for several neurodegenerative diseases including frontotemporal degeneration, limbic-predominant age-related TDP-43 encephalopathy, Parkinson's disease, late-onset-Alzheimer's disease and constitute a genetic determinant of differential aging. TMEM106B encodes an integral lysosomal membrane protein but its precise physiological function in the central nervous system remains enigmatic. Presently, we aimed to increase understanding of TMEM106B contribution to general brain function and aging. We analyzed an aged cohort of Tmem106b knockout-, heterozygote and wild-type mice in a behavioral test battery including assessments of motor function as well as, social, emotional and cognitive function. Aged Tmem106b knockout (KO) mice displayed diverse behavioral deficits including motor impairment, gait defects and reduced startle reactivity. In contrast, no prominent deficits were observed in social, emotional or cognitive behaviors. Histologically, we observed late-onset loss of Purkinje cells followed by reactive gliosis in the cerebellum, which likely contributed to progressive decline in motor function and gait defects in particular. Reactive gliosis was not restricted to the cerebellum but observed in different areas of the brain including the brain stem and parts of the cerebral cortex. Surviving Purkinje cells showed vacuolated lysosomes in the axon initial segment, implicating TMEM106B-dependent lysosomal trafficking defects as the underlying cause of axonal and more general neuronal dysfunction contributing to behavioral impairments. Our experiments help to elucidate how TMEM106B affects spatial neuronal homeostasis and exemplifies a critical role of TMEM106B in neuronal cells for survival.

Loss of Tmem106b leads to cerebellum Purkinje cell death and motor deficits

“…This analysis showed that the loss of Tmem106b results in an approximately 2‐fold increase of immunoreactivity of Gfap ( p < 0.01) and Iba‐1 ( p < 0.05) in the cerebellum as compared to WT mice (Figure 1F–H). Similar to the observation from Stroobants et al (1), gliosis was pronounced in the molecular layer and white matter tract. We also observed that there were significantly more lysosomal enlarged Purkinje neurons and microglia cells in Tmem106b−/− as compared to WT mice as evidenced by Lamp1 immunofluorescence staining (Figure S1).…”

“…We also demonstrated that Tmem106b deficiency results in lysosomal enlargement in both Purkinje cells and microglia, and increased neuro-inflammation including complement system activation. Together with the independent findings from Stroobants et al (1), these data suggest that, in addition to myelination (8), Tmem106b also plays important role in maintaining the health and survival of cerebellar Purkinje cells during aging.…”

“…The enlarged lysosomes in Purkinje cells were mainly located in the axon hillock (Figure S1A). Instead of single enlarged lysosomes as observed in Stroobants et al (1), we often observed multiple clustered enlarged lysosomes. It is possible that, with time, these clustered enlarged lysosomes might fuse to form single enlarged lysosomes.…”

“…Importantly, regardless of the age of the animals, Tmem106b+/− mice showed no signs of Purkinje cell loss ( Tmem106b+/− vs WT, p > 0.05 at all ages) (Figure 1B–E). Notably, in the study from Stroobants et al (1), prominent loss of Purkinje cells was demonstrated at 18 months of age whereas 9‐months was described as an age preceding prominent neuronal loss. While it is difficult to compare the results from independent studies, the timing of the age‐dependent Purkinje cell loss observed in our Tmem106b−/− mice appears potentially similar to this model.…”

“…With great interest, we read the work of Stroobants et al, published online October 5th 2020 in Brain Pathology , in which the authors reported late‐onset cerebellar Purkinje cell loss and progressive decline in motor function and gait deficits in their CRISPR Tmem106b−/− mouse model (1). Here, we report similar age‐dependent Purkinje cell death and motor deficits in a conventional Tmem106b−/− mouse model.…”