Autophagy alterations in Alzheimer’s disease

By Diana-Madalina Stan, PhD student, University of Salford

1. Alzheimer’s disease (AD), the most common cause of dementia, is a neurodegenerative disorder that mainly affects the elderly. AD International reported 46.8 million people suffering from AD in 2015 and the number is presumed to be 50 million at present. AD pathology includes abnormal protein deposits including neurofibrillary tangles and neuritic plaques. Aggregated tau protein undergoes abnormal hyperphosphorylation which changes the microtubule stability, the primary function of this protein (Lace, 2009). As a consequence, the cellular integrity and the cytoskeletal maintenance is altered (Mietelska-Porowska, 2014). Formation of neurofibrillary tangles, which are aggregates of abnormal tau, contributes to extensive loss of cortical neurons altering the brain’s essential cognitive and memory functions (Serrano-Pozo, 2011). Abnormal accumulation of beta amyloid have been shown to damage cells via neurotoxic events such as free radicals release, inflammatory responses in the neurons within the brain, disruption of Ca2+ channels (Carrillo-Mora, 2014), all leading to irreversible neuronal damage and clinical dementia symptoms which are triggered by high production of beta amyloid (Rajasekhar, 2015).

Preventing the build-up of these damaging proteins is a popular area of research but studies have had limited success. One of the most recent studies involves Professor Claude Wischik’s work at the University of Aberdeen, which tested a new drug called LMTM (leuco-methylthioninium bis hydromethanesulfonate) that acts as an inhibitor for tau protein aggregation. The Phase III Trial drug aimed to test its efficacy on the progression of mild to moderate AD and failed to slow cognitive or functional decline in patients already taking AD related medication. Only 15% of the subjects, who were treated with LMTM alone showed a significant effect with slower progression and brain shrinkage (Gauthier, 2016).

Autophagy is a dynamic intracellular pathway which degrades proteins and organelles into cellular contents that can be further recycled ensuring a permanent renewal of the proteome components by lysosomal degradation (Cuervo, 2014).

Besides removal of altered or dysfunctional cellular components, autophagy is also stimulated by nutrient starvation in order to re-use cellular constituents for energy such as amino-acids after protein degradation. Therefore, autophagy failure to remove damaged proteins and organelles result in accumulation of dysfunctional toxic material that numerous studies showed to contribute to neurodegenerative diseases, lysosomal diseases, cancer, aging, inflammation and development abnormalities as cellular integrity is modified (Levine, 2014).

Figure illustrating autophagy machineries within the cell

The mechanism of autophagy differentiates two main types of protein turnover (Mizushima, 2008). Macroautophagy (MA) is characterized by sequestration of large structures within a double-membrane vesicle called autophagosome that fuses with the lysosome. Here, the acidic lysosomal hydrolases have the function to degrade the contents and release them into the cytosol (Feng, 2014). Chaperone-mediated autophagy (CMA), a highly selective machinery, selects soluble cytosolic proteins containing a motif biochemically related to the pentapeptide KFERQ ( Lys-Phe-Glu-Arg-Gln) and carries them to the lysosome where proteins pass through the organelle membrane (Kaushik, 2011).

Studies show that failure of autophagy can lead to increased pathological proteins such as tau and beta-amyloid. A study on tau inducible cell culture model suggests that disruption of lysosomal systems of autophagy by chloroquine treatment on the cells can lead to an increase in tau aggregation and toxicity with delayed tau clearance (Friedman, 2014). Other studies on transgenic APP mice suggested that impaired degradation systems result in increased levels of beta-amyloid (Salminen, 2013). Beclin-1 deficiency, an important marker of MA induction, seems to lead to deposits of beta-amyloid in cultured neurons while overexpression of the MA biomarker reduces the accumulation of the plaques (Wang, 2009).

Autophagosomes and autophagic vacuoles accumulate in dystrophic dendrites due to impaired transport to the lysosomes and fail to complete the final steps of the pathway. Surprisingly, immature vacuoles were discovered to be a considerable Aβ-containing site while APP molecules (either complete or cleaved), were found abundantly in autophagic vacuoles of MA machinery (Yu et al., 2005). Tau, a long lived protein, was shown to be degraded by MA and CMA, too (Wang, 2012).

Since 2005, when Nixon’s group was the first to describe autophagy impairment due to incomplete development of autophagic vacuoles, numerous papers have presented experiments that strengthen the link between defective lysosomal mechanisms and neurodegeneration (Nixon et al., 2005). Previous studies suggest that macroautophagy and CMA are involved in the clearance of Aβ and tau, highlighting their importance in neurodegeneration. Their stimulation has been shown to enhance clearance of aggregates in AD, providing protection in neurons of cellular and animal models (Wang, 2009; Jiang, 2014).

Autophagy related studies have previously described impairment in AD brains but there is a lack of understanding the nature of these deficits. Also, there is not much information to describe whether different autophagy pathways work together to compensate the cellular imbalance. There is one review paper which suggests that CMA is characterized by a very organized and coordinated manner, working independently from the other lysosomal pathway (Kon, 2010), however this has not been demonstrated on post-mortem human brain tissue of patients with AD. It is unclear if autophagy impairment in AD brains manifest uniformly across different brain regions or whether the impairment varies across different stages of the disease. Previous studies on ALS transgenic mouse models (SOD1G93A) have suggested that the autophagy stimulating drug Rapamycin, which can rescue autophagy, leads to neuropathology and the symptoms can worsen. As a result, switching on the whole autophagy machinery can have irreversible damaging effects and the most suitable therapeutic intervention would be a highly specific drug that could target the defected step within the pathway (Zhang, 2011).

The aim of my research was to investigate alterations in macroautophagy and chaperone-mediated autophagy using post-mortem human brain tissue. Three different brain regions were used such as the hippocampus/temporal cortex, due to its involvement in memory formation, primarily affected in AD (Salminen, 2013), as well as regions that become involved with advancing disease progression: frontal and occipital lobes. The study aimed to explore the variation of defective autophagy pathways from early to late stages. Also, we sought to determine possible deficit locations in both lysosomal degradation pathways and how the two machineries interact when one becomes impaired. Understanding the nature and the locations of these deficits within the pathway can offer potential targets for novel therapeutic strategies to restore the protein degradation systems in the future. 

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