g., Roth and Balch, 2011). It seems likely that for each misfolding-prone protein certain types of neurons are more affected by how that protein disrupts cellular protein networks, and this may contribute to their selective vulnerability to a particular NDD (Figure 1). Indeed, consistent with dominant interference in subsets of neurons, genetic studies in C. elegans have provided evidence that disease-related human proteins such as α-synuclein preferentially form aggregates in certain worm neurons, where they

enhance the vulnerability of the same neurons to misfolding-prone protein species such as constructs with subthreshold polyglutamine stretches ( Brignull et al., 2006 and Lim et al., 2008). Furthermore, mutant misfolding Adriamycin concentration proteins associated with familial forms of NDDs can, at least to some extent, model the same diseases when expressed ubiquitously in evolutionarily distant model organisms such as zebrafish or Drosophila Crizotinib chemical structure (e.g., Lessing and Bonini, 2009, Sheng et al., 2010 and Xia, 2010). These studies are consistent with the notion that disease-associated misfolding proteins each interfere with cellular signaling and proteostasis networks in their own specific manners, thereby affecting preferentially

particular subtypes of neurons whose properties are evolutionarily conserved. Notably, the accumulation of misfolding proteins is often not sufficient to cause disease, and studies of human populations suggest how additional

factors have to combine with the age-related accumulation of misfolding proteins for disease to develop. Thus, the same types of characteristic macroscopic deposits can accumulate in the same neurons or the same brain regions in some but not all aging brains in the absence of major disease manifestations (Jellinger, 2004, Brignull et al., 2006 and Kern and Behl, 2009; but see Sperling et al., 2009 and Hedden et al., 2009). Recent studies have provided intriguing insights into how deposit accumulation may relate to dysfunction in the absence or presence of disease. The studies combined amyloid and functional brain imaging and revealed that aged persons with deposits, but without second noticeable AD, exhibit cognitive deficits involving cortical “default networks,” i.e., cortical areas that are active even when the brain is not engaged and which may be involved in off-line processing. Comparable impairments were detected in patients with mild cognitive deficits, which frequently progress to develop full-blown AD, suggesting that the amyloid deposits may be associated with very early stages of AD (Sperling et al., 2009 and Hedden et al., 2009). Such early stages may not necessarily progress to AD, and the mechanisms underlying disease conversion remain to be determined.