The role of heterozygous lysosomal storage disorder alleles as risk factors for dementia

Abstract

Alzheimer’s disease, the most common form of dementia, is characterised by extracellular amyloid beta plaques and intraneuronal tau tangles. While it is not fully understood why these pathological hallmarks develop, several biological systems are believed to be involved. Of these, lysosomal network dysfunction is an increasingly recognised pathogenic factor. Lysosomal network disruptions, including upregulated endocytosis, aberrant trafficking and storage of undegraded substrates, commence from the earliest stages of Alzheimer’s disease. The importance of the lysosomal network for neuronal health is underscored by the existence of more than 50 lysosomal storage disorders, which arise from the deficiency of an enzyme or other protein required for lysosomal function. Many lysosomal storage disorders have a severe neurodegenerative phenotype, most are recessively inherited, and until recently, heterozygotes were considered asymptomatic carriers. However, there is increasing evidence of some pathophysiology in heterozygotes, especially during ageing. This study aimed to investigate the impact of a heterozygous lysosomal storage disorder allele (Hexb+/-) in an Alzheimer’s mouse model (AppNL-G-F/NL-G-F). We hypothesised Hexb heterozygosity would hasten and/or exacerbate pathology and disease-related signs in AppNL-G-F/NL-G-F mice. The AppNL-G-F/NL-GF knock-in mouse was novel and not extensively characterised at the beginning of this PhD study. We therefore performed a behavioural test battery, and examined the lysosomal network, in six-monthold AppNL-G-F/NL-G-F mice. AppNL-G-F/NL-G-F mice did not have memory impairments detectable in a Y-maze, novel object recognition test or Morris water maze at six-months. They did, however, exhibit reduced open field activity and lysosomal network disruptions. Lysosome-associated membrane protein 1 and cathepsins B, L and D accumulated at amyloid beta plaques in AppNL-G-F/NL-G-F mice, presenting at the earliest and smallest plaques observed. AppNL-G-F/NL-G-F mice also exhibited elevated activity of β- hexosaminidase and cathepsins D/E and elevated levels of cathepsin D and LC3-II in the cerebral cortex, as determined by Western blot. AppNL-G-F/NL-G-F mice were intercrossed with Hexb+/- mice to determine the effect of heterozygosity of Hexb in the Alzheimer’s mouse. No substantial memory impairments or increases in key neuropathological markers of Alzheimer’s disease were found. However, AppNL-G-F/NL-G-F; Hexb+/- mice demonstrated subtle impairments in behavioural flexibility during the reversal phase of the Morris water maze. AppNL-G-F/NL-G-F; Hexb+/+ mice had reduced activity in an open field and on the Y-maze. This phenotype was not exacerbated in AppNL-G-F/NL-G-F; Hexb+/- mice, but was reproduced by Hexb heterozygosity alone, in Appwt/wt; Hexb+/- mice. Heterozygosity of Hexb in AppNL-G-F/NL-G-F mice did not increase glial fibrillary acidic protein or ionised calcium binding adaptor molecule 1, markers of astrocytes and microglia, respectively. Nor did it lead to increased GM2 ganglioside (the substrate degraded by the enzyme encoded by Hexb), or related gangliosides, GM1 or GM3. Surprisingly, heterozygosity of Hexb resulted in less amyloid beta plaques in the orbital cortex and hippocampus of 46-week-old AppNL-G-F/NL-G-F mice and less Aβ42 in the hippocampus. In summary, Hexb heterozygosity in AppNL-G-F/NL-G-F mice did not induce substantial memory impairments or increase key neuropathological markers of Alzheimer’s disease, but led to subtle phenotypic alterations, suggesting that Hexb haploinsufficiency is not a dominant factor in the progression of Alzheimer’s disease.