Altered apoptosis regulation in Kufor-Rakeb syndrome patients with mutations in the ATP13A2 gene

Genetic Evidence for Endolysosomal Dysfunction in Parkinson’s Disease: A Critical Overview

Parkinson’s disease (PD) is the second most common neurodegenerative disorder in the aging population, and no disease-modifying therapy has been approved to date. The pathogenesis of PD has been related to many dysfunctional cellular mechanisms, however, most of its monogenic forms are caused by pathogenic variants in genes involved in endolysosomal function (LRRK2, VPS35, VPS13C, and ATP13A2) and synaptic vesicle trafficking (SNCA, RAB39B, SYNJ1, and DNAJC6). Moreover, an extensive search for PD risk variants revealed strong risk variants in several lysosomal genes (e.g., GBA1, SMPD1, TMEM175, and SCARB2) highlighting the key role of lysosomal dysfunction in PD pathogenesis. Furthermore, large genetic studies revealed that PD status is associated with the overall “lysosomal genetic burden”, namely the cumulative effect of strong and weak risk variants affecting lysosomal genes. In this context, understanding the complex mechanisms of impaired vesicular trafficking and dysfunctional endolysosomes in dopaminergic neurons of PD patients is a fundamental step to identifying precise therapeutic targets and developing effective drugs to modify the neurodegenerative process in PD.

Pallidal degenerations and related disorders: an update

Neurodegenerative disorders involving preferentially the globus pallidus, its efferet and afferent circuits and/or related neuronal systems are rare. They include a variety of both familial and sporadic progressive movement disorders, clinically manifesting as choreoathetosis, dystonia, Parkinsonism, akinesia or myoclonus, often associated with seizures, mental impairment and motor or cerebellar symptoms. Based on the involved neuronal systems, this heterogenous group has been classified into several subgroups: "pure" pallidal atrophy (PPA) and extended forms, pallidonigral and pallidonigrospinal degeneration (PND, PNSD), pallidopyramidal syndrome (PPS), a highly debatable group, pallidopontonigral (PPND), nigrostriatal-pallidal-pyramidal degeneration (NSPPD) (Kufor-Rakeb syndrome /KRS), pallidoluysian degeneration (PLD), pallidoluysionigral degeneration (PLND), pallidoluysiodentate atrophy (PLDA), the more frequent dentatorubral-pallidoluysian atrophy (DRPLA), and other hereditary multisystem disorders affecting these systems, e.g., neuroferritinopathy (NF). Some of these syndromes are sporadic, others show autosomal recessive or dominant heredity, and for some specific gene mutations have been detected, e.g., ATP13A2/PARK9 (KRS), FTL1 or ATP13A2 (neuroferritinopathy), CAG triple expansions in gene ATN1 (DRPLA) or pA152T variant in MAPT gene (PNLD). One of the latter, and both PPND and DRPLA are particular subcortical 4-R tauopathies, related to progressive supranuclear palsy (PSP), corticobasal degeneration (CBD) and frontotemporal lobe degeneration-17 (FTLD-17), while others show additional 3-R and 4-R tauopathies or TDP-43 pathologies. The differential diagnosis includes a large variety of neurodegenerations ranging from Huntington and Joseph-Machado disease, tauopathies (PSP), torsion dystonia, multiple system atrophy, neurodegeneration with brain iron accumulation (NBIA), and other extrapyramidal disorders. Neuroimaging data and biological markers have been published for only few syndromes. In the presence of positive family histories, an early genetic counseling may be effective. The etiology of most phenotypes is unknown, and only for some pathogenic mechanisms, like polyglutamine-induced oxidative stress and autophagy in DRPLA, mitochondrial dysfunction induced by oxidative stress in KRS or ferrostasis/toxicity and protein aggregation in NF, have been discussed. Currently no disease-modifying therapy is available, and symptomatic treatment of hypo-, hyperkinetic, spastic or other symptoms may be helpful.

ATP13A2 facilitates HDAC6 recruitment to lysosome to promote autophagosome-lysosome fusion

Mutations in ATP13A2 cause Kufor-Rakeb syndrome, an autosomal recessive form of juvenile-onset atypical Parkinson's disease (PD). Recent work tied ATP13A2 to autophagy and other cellular features of neurodegeneration, but how ATP13A2 governs numerous cellular functions in PD pathogenesis is not understood. In this study, the ATP13A2-deficient mouse developed into aging-dependent phenotypes resembling those of autophagy impairment. ATP13A2 deficiency impaired autophagosome-lysosome fusion in cultured cells and in in vitro reconstitution assays. In ATP13A2-deficient cells or Drosophila melanogaster or mouse tissues, lysosomal localization and activity of HDAC6 were reduced, with increased acetylation of tubulin and cortactin. Wild-type HDAC6, but not a deacetylase-inactive mutant, restored autophagosome-lysosome fusion, antagonized cortactin hyperacetylation, and promoted lysosomal localization of cortactin in ATP13A2-deficient cells. Mechanistically, ATP13A2 facilitated recruitment of HDAC6 and cortactin to lysosomes. Cortactin overexpression in cultured cells reversed ATP13A2 deficiency-associated impairment of autophagosome-lysosome fusion. PD-causing ATP13A2 mutants failed to rescue autophagosome-lysosome fusion or to promote degradation of protein aggregates and damaged mitochondria. These results suggest that ATP13A2 recruits HDAC6 to lysosomes to deacetylate cortactin and promotes autophagosome-lysosome fusion and autophagy. This study identifies ATP13A2 as an essential molecular component for normal autophagy flux in vivo and implies potential treatments targeting HDAC6-mediated autophagy for PD.

Orchestrated increase of dopamine and PARK mRNAs but not miR-133b in dopamine neurons in Parkinson's disease

Progressive loss of substantia nigra dopamine neurons (SN DA) is a hallmark of aging and of Parkinson's disease (PD). Mutations in PARK genes cause familial PD forms. Increased expression of alpha-synuclein (PARK4) is a disease-triggering event in familial PD and also observed in SN DA neurons in sporadic PD but related transcriptional changes are unknown. With optimized single-cell quantitative real-time polymerase chain reaction analysis, we compared messenger RNA and microRNA levels in SN DA neurons from sporadic PD patients and controls. Non-optimally matched donor ages and RNA integrities are common problems when analyzing human samples. We dissected the influence of distinct ages and RNA integrities of our samples by applying a specifically-optimized, linear-mixed-effects model to quantitative real-time polymerase chain reaction-data. We identified that elevated alpha-synuclein messenger RNA levels in SN DA neurons of human PD brains were positively correlated with corresponding elevated levels of mRNAs for functional compensation of progressive SN DA loss and for enhanced proteasomal (PARK5/UCHL1) and lysosomal (PARK9/ATPase13A2) function, possibly counteracting alpha-synuclein toxicity. In contrast, microRNA miR-133b levels, previously implicated in transcriptional dysregulation in PD, were not altered in SN DA neurons in PD.