Patricia Gómez-Suaga

Leucine-rich repeat kinase 2 regulates autophagy through a calcium-dependent pathway involving NAADP

Mutations in the leucine-rich repeat kinase-2 (LRRK2) gene cause late-onset Parkinson’s disease, but its physiological function has remained largely unknown. Here we report that LRRK2 activates a calcium-dependent protein kinase kinase-β (CaMKK-β)/adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway which is followed by a persistent increase in autophagosome formation. Simultaneously, LRKR2 overexpression increases the levels of the autophagy receptor p62 in a protein synthesis-dependent manner, and decreases the number of acidic lysosomes. The LRRK2-mediated effects result in increased sensitivity of cells to stressors associated with abnormal protein degradation. These effects can be mimicked by the lysosomal Ca2+-mobilizing messenger nicotinic acid adenine dinucleotide phosphate (NAADP) and can be reverted by an NAADP receptor antagonist or expression of dominant-negative receptor constructs. Collectively, our data indicate a molecular mechanism for LRRK2 deregulation of autophagy and reveal previously unidentified therapeutic targets.

LRRK2 delays degradative receptor trafficking by impeding late endosomal budding through decreasing Rab7 activity

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene cause late-onset autosomal dominant Parkinsons disease (PD), and sequence variations at the LRRK2 locus are associated with increased risk for sporadic PD. LRRK2 contains both GTPase and kinase domains flanked by protein interaction motifs, and mutations associated with familial PD have been described for both catalytic domains. LRRK2 has been implicated in diverse cellular processes, and recent evidence pinpoints to an important role for LRRK2 in modulating a variety of intracellular membrane trafficking pathways. However, the underlying mechanisms are poorly understood. Here, by studying the classical, well-understood, degradative trafficking pathway of the epidermal growth factor receptor (EGFR), we show that LRRK2 regulates endocytic membrane trafficking in an Rab7-dependent manner. Mutant LRRK2 expression causes a slight delay in early-to-late endosomal trafficking, and a pronounced delay in trafficking out of late endosomes, which become aberrantly elongated into tubules. This is accompanied by a delay in EGFR degradation. The LRRK2-mediated deficits in EGFR trafficking and degradation can be reverted upon coexpression of active Rab7 and of a series of proteins involved in bridging the EGFR to Rab7 on late endosomes. Effector pulldown assays indicate that pathogenic LRRK2 decreases Rab7 activity both in cells overexpressing LRRK2, as well as in fibroblasts from pathogenic mutant LRRK2 PD patients when compared with healthy controls. Together, these findings provide novel insights into a previously unknown regulation of Rab7 activity by mutant LRRK2 which impairs membrane trafficking at very late stages of the endocytic pathway.

A link between LRRK2, autophagy and NAADP-mediated endolysosomal calcium signalling.

Mutations in LRRK2 (leucine-rich repeat kinase 2) represent a significant component of both sporadic and familial PD (Parkinsons disease). Pathogenic mutations cluster in the enzymatic domains of LRRK2, and kinase activity seems to correlate with cytotoxicity, suggesting the possibility of kinase-based therapeutic strategies for LRRK2-associated PD. Apart from cytotoxicity, changes in autophagy have consistently been observed upon overexpression of mutant, or knockdown of endogenous, LRRK2. However, delineating the precise mechanism(s) by which LRRK2 regulates autophagy has been difficult. Recent data suggest a mechanism involving late steps in autophagic-lysosomal clearance in a manner dependent on NAADP (nicotinic acid-adenine dinucleotide phosphate)-sensitive lysosomal Ca2+ channels. In the present paper, we review our current knowledge of the link between LRRK2 and autophagic-lysosomal clearance, including regulation of Ca2+-dependent events involving NAADP.

Novel insights into the neurobiology underlying LRRK2-linked Parkinson's disease.

Mutations in LRRK2 (leucine-rich repeat kinase 2) are found associated with both sporadic and familial Parkinsons disease (PD). Pathogenic mutations are localized to the catalytic domains of LRRK2, including kinase and GTPase domains. Altered catalytic activity correlates with neurotoxicity, indicating that targeting those activities may provide clues as to novel therapeutic strategies for LRRK2-linked PD. However, the cellular readout of such altered catalytic activities remains largely unknown. Recent cell biological studies have started to highlight possible early cellular events which are altered in the presence of pathogenic LRRK2 and may ultimately lead to neuronal demise, and these studies link altered LRRK2 function to various abnormal endolysosomal vesicular trafficking events. This review examines our current knowledge of LRRK2 neurobiology and how pathogenic mutations may lead to neurodegeneration in PD.

A Link between Autophagy and the Pathophysiology of LRRK2 in Parkinson's Disease.

Parkinsons disease is a debilitating neurodegenerative disorder, and its molecular etiopathogenesis remains poorly understood. The discovery of monogenic forms has significantly advanced our understanding of the molecular mechanisms underlying PD, as it allows generation of cellular and animal models carrying the mutant gene to define pathological pathways. Mutations in leucine-rich repeat kinase 2 (LRRK2) cause dominantly inherited PD, and variations increase risk, indicating that LRRK2 is an important player in both genetic and sporadic forms of the disease. G2019S, the most prominent pathogenic mutation, maps to the kinase domain and enhances enzymatic activity of LRRK2, which in turn seems to correlate with cytotoxicity. Since kinases are druggable targets, this has raised great hopes that disease-modifying therapies may be developed around modifying LRRK2 enzymatic activity. Apart from cytotoxicity, changes in autophagy have been consistently reported in the context of G2019S mutant LRRK2. Here, we will discuss current knowledge about mechanism(s) by which mutant LRRK2 may regulate autophagy, which highlights additional putative therapeutic targets.

Alterations in late endocytic trafficking related to the pathobiology of LRRK2-linked Parkinson's disease

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene comprise the most common cause of familial Parkinsons disease (PD), and variants increase the risk for sporadic PD. LRRK2 displays kinase and GTPase activity, and altered catalytic activity correlates with neurotoxicity, making LRRK2 a promising therapeutic target. Despite the importance of LRRK2 for disease pathogenesis, its normal cellular function, and the mechanism(s) by which pathogenic mutations cause neurodegeneration remain unclear. LRRK2 seems to regulate a variety of intracellular vesicular trafficking events to and from the late endosome in a manner dependent on various Rab proteins. At least some of those events are further regulated by LRRK2 in a manner dependent on two-pore channels (TPCs). TPCs are ionic channels localized to distinct endosomal structures and can cause localized calcium release from those acidic stores, with downstream effects on vesicular trafficking. Here, we review current knowledge about the link between LRRK2, TPC- and Rab-mediated vesicular trafficking to and from the late endosome, highlighting a possible cross-talk between endolysosomal calcium stores and Rab proteins underlying pathomechanism(s) in LRRK2-related PD.

Iron overload causes endolysosomal deficits modulated by NAADP-regulated 2-pore channels and RAB7A.

ABSTRACT Various neurodegenerative disorders are associated with increased brain iron content. Iron is known to cause oxidative stress, which concomitantly promotes cell death. Whereas endolysosomes are known to serve as intracellular iron storage organelles, the consequences of increased iron on endolysosomal functioning, and effects on cell viability upon modulation of endolysosomal iron release remain largely unknown. Here, we show that increasing intracellular iron causes endolysosomal alterations associated with impaired autophagic clearance of intracellular protein aggregates, increased cytosolic oxidative stress and increased cell death. These effects are subject to regulation by NAADP, a potent second messenger reported to target endolysosomal TPCNs (2-pore channels). Consistent with endolysosomal iron storage, cytosolic iron levels are modulated by NAADP, and increased cytosolic iron is detected when overexpressing active, but not inactive TPCNs, indicating that these channels can modulate endolysosomal iron release. Cell death triggered by altered intralysosomal iron handling is abrogated in the presence of an NAADP antagonist or when inhibiting RAB7A activity. Taken together, our results suggest that increased endolysosomal iron causes cell death associated with increased cytosolic oxidative stress as well as autophagic impairments, and these effects are subject to modulation by endolysosomal ion channel activity in a RAB7A-dependent manner. These data highlight alternative therapeutic strategies for neurodegenerative disorders associated with increased intracellular iron load.

Parkinson disease-associated mutations in LRRK2 cause centrosomal defects via Rab8a phosphorylation

BackgroundMutations in LRRK2 are a common genetic cause of Parkinson’s disease (PD). LRRK2 interacts with and phosphorylates a subset of Rab proteins including Rab8a, a protein which has been implicated in various centrosome-related events. However, the cellular consequences of such phosphorylation remain elusive.MethodsHuman neuroblastoma SH-SY5Y cells stably expressing wildtype or pathogenic LRRK2 were used to test for polarity defects in the context of centrosomal positioning. Centrosomal cohesion deficits were analyzed from transiently transfected HEK293T cells, as well as from two distinct peripheral cell types derived from LRRK2-PD patients. Kinase assays, coimmunoprecipitation and GTP binding/retention assays were used to address Rab8a phosphorylation by LRRK2 and its effects in vitro. Transient transfections and siRNA experiments were performed to probe for the implication of Rab8a and its phosphorylated form in the centrosomal deficits caused by pathogenic LRRK2.ResultsHere, we show that pathogenic LRRK2 causes deficits in centrosomal positioning with effects on neurite outgrowth, cell polarization and directed migration. Pathogenic LRRK2 also causes deficits in centrosome cohesion which can be detected in peripheral cells derived from LRRK2-PD patients as compared to healthy controls, and which are reversed upon LRRK2 kinase inhibition. The centrosomal cohesion and polarity deficits can be mimicked when co-expressing wildtype LRRK2 with wildtype but not phospho-deficient Rab8a. The centrosomal defects induced by pathogenic LRRK2 are associated with a kinase activity-dependent increase in the centrosomal localization of phosphorylated Rab8a, and are prominently reduced upon RNAi of Rab8a.ConclusionsOur findings reveal a new function of LRRK2 mediated by Rab8a phosphorylation and related to various centrosomal defects.

Autophagic Pathology and Calcium Deregulation in Neurodegeneration

Over the last decades, substantial efforts have been made towards understanding the key players underlying neurodegeneration. However, despite extensive research efforts, the exact molecular mechanism(s) remain unclear, and much less is certain about possible common target(s) amongst distinct age-dependent neurodegenerative disorders. Whilst the precise mechanism(s) underlying neurodegeneration amongst the different diseases remain to be determined, a number of cellular processes have been suggested to be involved in all of them, including protein accumulation and aggregation, oxidative stress, mitochondrial deficits, Ca2 + dyshomeostasis and impairments in lysosomal degradation pathways including macroautophagy. The various possible pathogenic factors are not mutually exclusive, and the aim of much current research is to elucidate the correlation between them to establish successful strategies in limiting the disease process. Here, we summarize recent data that pinpoint Ca2 + dyshomeostasis as a key player underlying neurodegeneration in the context of macroautophagy deregulation. We will provide a brief overview of recent work towards addressing how macroautophagy and Ca2 + deregulation may cause cellular dysfunction linked to the pathogenesis of several neurodegenerative disorders, with emphasis on Parkinson’s disease (PD).