Jcb_201612148 1..15

Amyloid plaques, a defining feature of Alzheimer’s disease (AD) brain pathology, have long been recognized to contain an extracellular aggregate of the β-amyloid peptide that is surrounded by microglia and an abundance of swollen axons (Terry et al., 1964; Itagaki et al., 1989; Tsai et al., 2004; Fiala, 2007; Condello et al., 2011). These axons contain a massive accumulation of organelles that resemble lysosomes and/or hybrid organelles arising from the fusion of lysosomes with late endosomes and autophagosomes (subsequently referred to as lysosomes for simplicity; Terry et al., 1964; Su et al., 1993; Cataldo et al., 1994; Nixon et al., 2005; Condello et al., 2011; Gowrishankar et al., 2015). Despite their long-known and robust occurrence, the disease relevance of these lysosome-filled axonal swellings has not been established. Our current understanding of AD has been greatly influenced by the identification of mutations in the amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) genes as causes of early onset, familial forms of the disease (Selkoe and Hardy, 2016). These advances in the understanding of human AD genetics have supported the development of transgenic mice that develop amyloid plaque pathology due to the expression of mutant forms of APP and presenilin. Even in these mouse models, the amyloid plaques are surrounded by swollen, lysosome-filled axons (Yu et al., 2005; Dikranian et al., 2012; Kandalepas et al., 2013; Gowrishankar et al., 2015). The possible disease relevance of these lysosome-filled axonal swellings is supported by human and mouse studies that revealed striking enrichment for both APP and BACE1 within the swollen axons at amyloid plaques (Cras et al., 1991; Cummings et al., 1992; Kandalepas et al., 2013; Yoon et al., 2013; Gowrishankar et al., 2015; Yuan et al., 2016). Furthermore, a recent investigation of the subcellular localization of PSENs revealed lysosomal localization of AD-related PSEN1 mutants and the constitutive localization of WT PSEN2 to lysosomes (Sannerud et al., 2016). The presence of PSEN1 within lysosome-like organelles at plaques is further supported by observations in the human AD brain (Yu et al., 2005). Although such observations strongly suggest that the lysosome-filled axonal swellings at amyloid plaques could act as important sites of Aβ peptide production, this model is largely based on descriptive studies and lacks direct evidence of causality. The high abundance of lysosomes within axonal swellings at amyloid plaques and their potential role as sites of APP processing also raise questions concerning the fundamental mechanisms that govern axonal lysosome abundance. Multiple studies have identified late endosomes and autophagosomes within distal regions of axons that likely play key housekeeping functions by sequestering old or damaged proteins and organelles (Hollenbeck, 1993; Maday et al., 2012; Maday and Holzbaur, 2014; Cheng et al., 2015). However, to degrade and recycle their contents, these organelles must acquire lysosomal properties such as hydrolytic enzymes and a highly acidic lumen. To this end, these organelles are thought to undergo a maturation process within axons that is coupled with their retrograde axonal transport toward the neuronal cell body (Hollenbeck, 1993; Overly Lysosomes robustly accumulate within axonal swellings at Alzheimer’s disease (AD) amyloid plaques. However, the underlying mechanisms and disease relevance of such lysosome accumulations are not well understood. Motivated by these problems, we identified JNK-interacting protein 3 (JIP3) as an important regulator of axonal lysosome transport and maturation. JIP3 knockout mouse neuron primary cultures accumulate lysosomes within focal axonal swellings that resemble the dystrophic axons at amyloid plaques. These swellings contain high levels of amyloid precursor protein processing enzymes (BACE1 and presenilin 2) and are accompanied by elevated Aβ peptide levels. The in vivo importance of the JIP3-dependent regulation of axonal lysosomes was revealed by the worsening of the amyloid plaque pathology arising from JIP3 haploinsufficiency in a mouse model of AD. These results establish the critical role of JIP3-dependent axonal lysosome transport in regulating amyloidogenic amyloid precursor protein processing and support a model wherein Aβ production is amplified by plaque-induced axonal lysosome transport defects. Impaired JIP3-dependent axonal lysosome transport promotes amyloid plaque pathology