Neurodegenerative disorders such as Parkinson and Alzheimer disease cause motor and cognitive dysfunction and belong to a heterogeneous group of common and disabling disorders. Although the complex molecular pathophysiology of neurodegeneration is largely unknown, major advances have been achieved by elucidating the genetic defects underlying mendelian forms of these diseases. This has led to the discovery of common pathophysiological pathways such as enhanced oxidative stress, protein misfolding and aggregation and dysfunction of the ubiquitin-proteasome system. Here, we describe loss-of-function mutations in a previously uncharacterized, predominantly neuronal P-type ATPase gene, ATP13A2, underlying an autosomal recessive form of early-onset parkinsonism with pyramidal degeneration and dementia (PARK9, Kufor-Rakeb syndrome). Whereas the wild-type protein was located in the lysosome of transiently transfected cells, the unstable truncated mutants were retained in the endoplasmic reticulum and degraded by the proteasome. Our findings link a class of proteins with unknown function and substrate specificity to the protein networks implicated in neurodegeneration and parkinsonism.

SummaryLoss of neuronal proteostasis, a common feature of the aging brain, is accelerated in neurodegenerative disorders, including different types of tauopathies. Aberrant turnover of tau, a microtubule‐stabilizing protein, contributes to its accumulation and subsequent toxicity in tauopathy patients’ brains. A direct toxic effect of pathogenic forms of tau on the proteolytic systems that normally contribute to their turnover has been proposed. In this study, we analyzed the contribution of three different types of autophagy, macroautophagy, chaperone‐mediated autophagy, and endosomal microautophagy to the degradation of tau protein variants and tau mutations associated with this age‐related disease. We have found that the pathogenic P301L mutation inhibits degradation of tau by any of the three autophagic pathways, whereas the risk‐associated tau mutation A152T reroutes tau for degradation through a different autophagy pathway. We also found defective autophagic degradation of tau when using mutations that mimic common posttranslational modifications in tau or known to promote its aggregation. Interestingly, although most mutations markedly reduced degradation of tau through autophagy, the step of this process preferentially affected varies depending on the type of tau mutation. Overall, our studies unveil a complex interplay between the multiple modifications of tau and selective forms of autophagy that may determine its physiological degradation and its faulty clearance in the disease context.

SUMMARY Together with GTP and the initiator methionyl-tRNA, the translation initiation factor eIF2 forms a ternary complex that binds the 40S ribosome and then scans an mRNA to select the AUG start codon for protein synthesis. Here, we show that a human X-chromosomal neurological disorder characterized by intellectual disability and microcephaly is caused by a missense mutation in eIF2γ (encoded by EIF2S3), the core subunit of the heterotrimeric eIF2 complex. Biochemical studies of human cells overexpressing the eIF2γ mutant and of yeast eIF2γ with the analogous mutation revealed a defect in binding the eIF2β subunit to eIF2γ. Consistent with this loss of eIF2 integrity, the mutation in yeast eIF2γ impaired translation start codon selection and eIF2 function in vivo in a manner that was suppressed by overexpression of eIF2β. These findings directly link intellectual disability to impaired translation initiation, and provide a mechanistic basis for the human disease due to partial loss of eIF2 function.

The Saccharomyces cerevisiae gene YPK9 encodes a putative integral membrane protein which is 58% similar and 38% identical in amino acid sequence to the human lysosomal P5B ATPase ATP13A2. Mutations in ATP13A2 have been found in patients with Kufor-Rakeb syndrome, a form of juvenile Parkinsonism. We report that Ypk9p localizes to the yeast vacuole and that deletion of YPK9 confers sensitivity for growth for cadmium, manganese, nickel or selenium. These results suggest that Ypk9p may play a role in sequestration of divalent heavy metal ions. Further studies on the function of Ypk9p/ATP13A2 may help to define the molecular basis of Kufor-Rakeb syndrome and provide a potential link to environmental factors such as heavy metals contributing to some forms of Parkinsonism.

Kufor-Rakeb syndrome (KRS) is a rare form of autosomal recessive juvenile or early-onset, levodopa responsive parkinsonism and has been associated with mutations in ATP13A2(also known as PARK9), a lysosomal type 5 P-type ATPase. Recently, we identified novel compound heterozygous mutations, c.3176T>G (p.L1059R) and c.3253delC (p.L1085WfsX1088) in ATP13A2 of two siblings affected with KRS. When overexpressed, wild-type ATP13A2 localized to Lysotracker-positive and LAMP2-positive lysosomes while both truncating and missense mutated ATP13A2 were retained in the endoplasmic reticulum (ER). Both mutant proteins were degraded by the proteasomal but not the lysosomal pathways. In addition, ATP13A2 mRNA with c.3253delC was degraded by nonsense-mediated mRNA decay (NMD), which was protected by cycloheximide treatment. To validate our findings in a biologically relevant setting, we used patient-derived human olfactory neurosphere cultures and fibroblasts and demonstrated persistent ER stress by detecting upregulation of unfolded protein response-related genes in the patient-derived cells. We also confirmed NMD degraded ATP13A2 c.3253delC mRNA in the cells. These findings indicate that these novel ATP13A2 mutations are indeed pathogenic and support the notion that mislocalization of the mutant ATP13A2, resultant ER stress, alterations in the proteasomal pathways and premature degradation of mutant ATP13A2 mRNA contribute to the aetiology of KRS.