Disruption in mice of key autophagy genes, such as Atg7 or Atg5, causes neurodegeneration and ubiquitin-rich inclusions ( Hara et al., 2006 and Komatsu et al., 2006). The neurodegeneration is associated with accumulation of aberrant organelles and stacks of cisternal membranes in the dystrophic axons of autophagy-deficient neurons ( Komatsu et al., 2007). Consistent with a protective function of autophagy, pharmacological AZD6244 nmr enhancement of autophagy
can rescue neurons from the toxicity associated with aggregated misfolded proteins or proteasome inhibition ( Pan et al., 2008, Pandey et al., 2007 and Tsvetkov et al., 2010). Neurodegenerative diseases—which involve death of neurons, degeneration of axons, loss of synapses, and impairment INCB018424 mw of synaptic plasticity—may be a pathological manifestation of cellular processes that are used normally in development, such as apoptosis, neurite pruning, and synapse elimination. In this context, it is interesting that molecular players in physiological
plasticity and pathological neurodegeneration are often shared, such as the involvement of proteolytic caspase-3 in LTD and neuronal cell death (Li et al., 2010). In this section, we will focus on how proteolytic pathways are dysregulated in AD and PD. AD is characterized by protein deposits composed of Aβ peptide (plaques) and hyperphosphorylated tau (tangles), both of which probably contribute to synaptic dysfunction and neuronal death (Ross and Poirier, 2004). In AD brains, ubiquitin immunoreactivity accumulates in intracellular aggregates suggesting UPS dysfunction (Chu et al., 2000). Reduced proteasome activity is reported in brain regions affected by AD, such as the hippocampus (Keck et al., 2003 and Keller et al., 2000). Similarly, primary neurons isolated from APP transgenic mice show decreased proteasome activity (Almeida et al., 2006). Interestingly, transduction of and UCH-L1, a DUB that promotes proteasomal degradation, reverses behavioral deficits in AD model mice (Gong et al., 2006 and Smith et al., 2009), consistent with an impairment of
UPS in AD. Overexpression of an anomalous form of ubiquitin found in some AD patients (UBB+1; generated by a non-DNA-encoded dinucleotide deletion in ubiquitin transcripts) impairs proteasomal degradation and induces neuronal death (Lam et al., 2000 and Tan et al., 2007). Defective proteasomal degradation of hyperphosphorylated tau may contribute to the buildup of tangles. Tau interacts with CHIP, an E3 ubiquitin ligase required for degradation of soluble phosphorylated tau (Dickey et al., 2006 and Shimura et al., 2004). In AD, the mechanism of stabilization and accumulation of hyperphosphorylated tau may involve inhibition of tau interaction with CHIP (Dickey et al., 2006). In addition to phosphorylation, tau is also acetylated; acetylation impairs the proteosomal degradation and enhances the accumulation of tau (Min et al., 2010). Impairment of autophagy is also implicated in AD.