P62/SQSTM1 is a novel leucine-rich repeat kinase 2 (LRRK2) substrate that enhances neuronal toxicity

Dual-specificity kinase DYRK3 phosphorylates p62 at the Thr-269 residue and promotes melanoma progression

Melanoma is a type of skin cancer that originates in melanin-producing melanocytes. It is considered a multifactorial disease caused by both genetic and environmental factors, such as UV radiation. Dual-specificity tyrosine-phosphorylation-regulated kinase (DYRK) phosphorylates many substrates involved in signaling pathways, cell survival, cell cycle control, differentiation, and neuronal development. However, little is known about the cellular function of DYRK3, one of the five members of the DYRK family. Interestingly, it was observed that the expression of DYRK3, as well as p62 (a multifunctional signaling protein), is highly enhanced in most melanoma cell lines. This study aimed to investigate whether DYRK3 interacts with p62, and how this affects melanoma progression, particularly in melanoma cell lines. We found that DYRK3 directly phosphorylates p62 at the Ser-207 and Thr-269 residue. Phosphorylation at Thr-269 of p62 by DYRK3 increased the interaction of p62 with tumor necrosis factor receptor-associated factor 6 (TRAF6), an already known activator of mammalian target of rapamycin complex 1 (mTORC1) in the mTOR-involved signaling pathways. Moreover, the phosphorylation of p62 at Thr-269 promoted the activation of mTORC1. We also found that DYRK3-mediated phosphorylation of p62 at Thr-269 enhanced the growth of melanoma cell lines and melanoma progression. Conversely, DYRK3 knockdown or blockade of p62-T269 phosphorylation inhibited melanoma growth, colony formation, and cell migration. In conclusion, we demonstrated that DYRK3 phosphorylates p62, positively modulating the p62-TRAF6-mTORC1 pathway in melanoma cells. This finding suggests that DYRK3 suppression may be a novel therapy for preventing melanoma progression by regulating the mTORC1 pathway.

The PB1 and the ZZ domain of the autophagy receptor p62/SQSTM1 regulate the interaction of p62/SQSTM1 with the autophagosome protein LC3B

Autophagy is a highly conserved cellular process that allows degradation of large macromolecules. p62/SQSTM1 is a key adaptor protein that interacts both with material to be degraded and with LC3 at the autophagosome, enabling degradation of cargos such as protein aggregates, lipid droplets and damaged organelles by selective autophagy. Dysregulation of autophagy contributes to the pathogenesis of many diseases. In this study, we investigated if the interaction of p62/SQSTM1 with LC3B could be regulated. We purified full-length p62/SQSTM1 and established an in vitro assay that measures the interaction with LC3B. We used the assay to determine the role of the different domains of p62/SQSTM1 in the interaction with LC3B. We identified a mechanism of regulation of p62/SQSTM1 where the ZZ and the PB1 domains regulate the exposure of the LIR-sequence to enable or inhibit the interaction with LC3B. A mutation to mimic the phosphorylation of a site on the ZZ domain leads to increased interaction with LC3B. Also, a small compound that binds to the ZZ domain enhances interaction with LC3B. Dysregulation of these mechanisms in p62/SQSTM1 could have implications for diseases where autophagy is affected. In conclusion, our study highlights the regulated nature of p62/SQSTM1 and its ability to modulate the interaction with LC3B through a LIR-sequence Accessibility Mechanism (LAM). Furthermore, our findings suggest the potential for pharmacological modulation of the exposure of LIR, paving the way for future therapeutic strategies.

Proximity labeling reveals dynamic changes in the SQSTM1 protein network

Sequestosome1 (SQSTM1) is an autophagy receptor that mediates degradation of intracellular cargo, including protein aggregates, through multiple protein interactions. These interactions form the SQSTM1 protein network that are mediated by SQSTM1 functional interaction domains, which include LIR, PB1, UBA and KIR. Despite various attempts to unravel the complexity of the SQSTM1 protein network, our understanding of the relationship of various components in cellular physiology and disease states continues to evolve. To investigate the SQSTM1 protein interaction network, we performed proximity profile labeling by fusing TurboID with the human protein SQSTM1 (TurboID::SQSTM1). This chimeric protein displayed well-established SQSTM1 features including: production of SQSTM1 intracellular bodies, binding to known SQSTM1 interacting partners via defined functional SQSTM1 interacting domains and capture of novel SQSTM1 interactors. Strikingly, aggregated tau protein altered the protein interaction network of SQSTM1 to include many stress-associated proteins. Overall, our work reveals the dynamic landscape of the SQSTM1 protein network and offers a resource to study SQSTM1 function in cellular physiology and disease state.

Leucine-rich repeat kinase 2 at a glance

Leucine-rich repeat kinase 2 (LRRK2) is a multidomain scaffolding protein with dual guanosine triphosphatase (GTPase) and kinase enzymatic activities, providing this protein with the capacity to regulate a multitude of signalling pathways and act as a key mediator of diverse cellular processes. Much of the interest in LRRK2 derives from mutations in the LRRK2 gene being the most common genetic cause of Parkinson's disease, and from the association of the LRRK2 locus with a number of other human diseases, including inflammatory bowel disease. Therefore, the LRRK2 research field has focused on the link between LRRK2 and pathology, with the aim of uncovering the underlying mechanisms and, ultimately, finding novel therapies and treatments to combat them. From the biochemical and cellular functions of LRRK2, to its relevance to distinct disease mechanisms, this Cell Science at a Glance article and the accompanying poster deliver a snapshot of our current understanding of LRRK2 function, dysfunction and links to disease.

Overview of the Impact of Pathogenic LRRK2 Mutations in Parkinson's Disease

Leucine-rich repeat kinase 2 (LRRK2) is a large protein kinase that physiologically phosphorylates and regulates the function of several Rab proteins. LRRK2 is genetically implicated in the pathogenesis of both familial and sporadic Parkinson's disease (PD), although the underlying mechanism is not well understood. Several pathogenic mutations in the LRRK2 gene have been identified, and in most cases the clinical symptoms that PD patients with LRRK2 mutations develop are indistinguishable from those of typical PD. However, it has been shown that the pathological manifestations in the brains of PD patients with LRRK2 mutations are remarkably variable when compared to sporadic PD, ranging from typical PD pathology with Lewy bodies to nigral degeneration with deposition of other amyloidogenic proteins. The pathogenic mutations in LRRK2 are also known to affect the functions and structure of LRRK2, the differences in which may be partly attributable to the variations observed in patient pathology. In this review, in order to help researchers unfamiliar with the field to understand the mechanism of pathogenesis of LRRK2-associated PD, we summarize the clinical and pathological manifestations caused by pathogenic mutations in LRRK2, their impact on the molecular function and structure of LRRK2, and their historical background.

The APP intracellular domain promotes LRRK2 expression to enable feed-forward neurodegenerative mechanisms in Parkinson's disease

Gain-of-function mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are common in familial forms of Parkinson's disease (PD), which is characterized by progressive neurodegeneration that impairs motor and cognitive function. We previously demonstrated that LRRK2-mediated phosphorylation of β-amyloid precursor protein (APP) triggers the production and nuclear translocation of the APP intracellular domain (AICD). Here, we connected LRRK2 to AICD in a feed-forward cycle that enhanced LRRK2-mediated neurotoxicity. In cooperation with the transcription factor FOXO3a, AICD promoted LRRK2 expression, thus increasing the abundance of LRRK2 that promotes AICD activation. APP deficiency in LRRK2^G2019S mice suppressed LRRK2 expression, LRRK2-mediated mitochondrial dysfunction, α-synuclein accumulation, and tyrosine hydroxylase (TH) loss in the brain, phenotypes associated with toxicity and loss of dopaminergic neurons in PD. Conversely, AICD overexpression increased LRRK2 expression and LRRK2-mediated neurotoxicity in LRRK2^G2019S mice. In LRRK2^G2019S mice or cultured dopaminergic neurons from LRRK2^G2019S patients, treatment with itanapraced reduced LRRK2 expression and was neuroprotective. Itanapraced showed similar effects in a neurotoxin-induced PD mouse model, suggesting that inhibiting the AICD may also have therapeutic benefits in idiopathic PD. Our findings reveal a therapeutically targetable, feed-forward mechanism through which AICD promotes LRRK2-mediated neurotoxicity in PD.

Acetylation of p62 regulates base excision repair through interaction with APE1

p62, a well-known adaptor of autophagy, plays multiple functions in response to various stresses. Here, we report a function for p62 in base excision repair that is distinct from its known functions. Loss of p62 impairs base excision repair capacity and increases the sensitivity of cancer cells to alkylating and oxidizing agents. In response to alkylative and oxidative damage, p62 is accumulated in the nucleus,acetylated by hMOF,and deacetylated by SIRT7, and acetylated p62 is recruited to chromatin. The chromatin-enriched p62 directly interacts with APE1, a key enzyme of the BER pathway, and promotes its endonuclease activity, which facilitates BER and cell survival. Collectively, our findings demonstrate that p62 is a regulator of BER and provide further rationale for targeting p62 as a cancer therapeutic strategy.

p62 works as a hub modulation in the ageing process

p62 (also known as SQSTM1) is widely used as a predictor of autophagic flux, a process that allows the degradation of harmful and unnecessary components through lysosomes to maintain protein homeostasis in cells. p62 is also a stress-induced scaffold protein that resists oxidative stress. The multiple domains in its structure allow it to be connected with a variety of vital signalling pathways, autophagy and the ubiquitin proteasome system (UPS), allowing p62 to play important roles in cell proliferation, apoptosis and survival. Recent studies have shown that p62 is also directly or indirectly involved in the ageing process. In this review, we summarize in detail the process by which p62 regulates ageing from multiple ageing-related signs with the aim of providing new insight for the study of p62 in ageing.

Lack of p62 Impairs Glycogen Aggregation and Exacerbates Pathology in a Mouse Model of Myoclonic Epilepsy of Lafora

Lafora disease (LD) is a fatal childhood-onset dementia characterized by the extensive accumulation of glycogen aggregates—the so-called Lafora Bodies (LBs)—in several organs. The accumulation of LBs in the brain underlies the neurological phenotype of the disease. LBs are composed of abnormal glycogen and various associated proteins, including p62, an autophagy adaptor that participates in the aggregation and clearance of misfolded proteins. To study the role of p62 in the formation of LBs and its participation in the pathology of LD, we generated a mouse model of the disease (malin^KO) lacking p62. Deletion of p62 prevented LB accumulation in skeletal muscle and cardiac tissue. In the brain, the absence of p62 altered LB morphology and increased susceptibility to epilepsy. These results demonstrate that p62 participates in the formation of LBs and suggest that the sequestration of abnormal glycogen into LBs is a protective mechanism through which it reduces the deleterious consequences of its accumulation in the brain.

Protein phosphatase 2A holoenzymes regulate leucine-rich repeat kinase 2 phosphorylation and accumulation

LRRK2 is a highly phosphorylated multidomain protein and mutations in the gene encoding LRRK2 are a major genetic determinant of Parkinson's disease (PD). Dephosphorylation at LRRK2's S910/S935/S955/S973 phosphosite cluster is observed in several conditions including in sporadic PD brain, in several disease mutant forms of LRRK2 and after pharmacological LRRK2 kinase inhibition. However, the mechanism of LRRK2 dephosphorylation is poorly understood. We performed a phosphatome-wide reverse genetics screen to identify phosphatases involved in the dephosphorylation of the LRRK2 phosphosite S935. Candidate phosphatases selected from the primary screen were tested in mammalian cells, Xenopus oocytes and in vitro. Effects of PP2A on endogenous LRRK2 phosphorylation were examined via expression modulation with CRISPR/dCas9. Our screening revealed LRRK2 phosphorylation regulators linked to the PP1 and PP2A holoenzyme complexes as well as CDC25 phosphatases. We showed that dephosphorylation induced by different kinase inhibitor triggered relocalisation of phosphatases PP1 and PP2A in LRRK2 subcellular compartments in HEK-293 T cells. We also demonstrated that LRRK2 is an authentic substrate of PP2A both in vitro and in Xenopus oocytes. We singled out the PP2A holoenzyme PPP2CA:PPP2R2 as a powerful phosphoregulator of pS935-LRRK2. Furthermore, we demonstrated that this specific PP2A holoenzyme induces LRRK2 relocalization and triggers LRRK2 ubiquitination, suggesting its involvement in LRRK2 clearance. The identification of the PPP2CA:PPP2R2 complex regulating LRRK2 S910/S935/S955/S973 phosphorylation paves the way for studies refining PD therapeutic strategies that impact LRRK2 phosphorylation.

The Cross-Links of Endoplasmic Reticulum Stress, Autophagy, and Neurodegeneration in Parkinson's Disease

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder, and it is characterized by the selective loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc), as well as the presence of intracellular inclusions with α-synuclein as the main component in surviving DA neurons. Emerging evidence suggests that the imbalance of proteostasis is a key pathogenic factor for PD. Endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR) and autophagy, two major pathways for maintaining proteostasis, play important roles in PD pathology and are considered as attractive therapeutic targets for PD treatment. However, although ER stress/UPR and autophagy appear to be independent cellular processes, they are closely related to each other. In this review, we focused on the roles and molecular cross-links between ER stress/UPR and autophagy in PD pathology. We systematically reviewed and summarized the most recent advances in regulation of ER stress/UPR and autophagy, and their cross-linking mechanisms. We also reviewed and discussed the mechanisms of the coexisting ER stress/UPR activation and dysregulated autophagy in the lesion regions of PD patients, and the underlying roles and molecular crosslinks between ER stress/UPR activation and the dysregulated autophagy in DA neurodegeneration induced by PD-associated genetic factors and PD-related neurotoxins. Finally, we indicate that the combined regulation of ER stress/UPR and autophagy would be a more effective treatment for PD rather than regulating one of these conditions alone.

Parkinson's disease and mitophagy: an emerging role for LRRK2

Parkinson's disease (PD) is a progressive neurodegenerative disorder that affects around 2% of individuals over 60 years old. It is characterised by the loss of dopaminergic neurons in the substantia nigra pars compacta of the midbrain, which is thought to account for the major clinical symptoms such as tremor, slowness of movement and muscle stiffness. Its aetiology is poorly understood as the physiological and molecular mechanisms leading to this neuronal loss are currently unclear. However, mitochondrial and lysosomal dysfunction seem to play a central role in this disease. In recent years, defective mitochondrial elimination through autophagy, termed mitophagy, has emerged as a potential contributing factor to disease pathology. PINK1 and Parkin, two proteins mutated in familial PD, were found to eliminate mitochondria under distinct mitochondrial depolarisation-induced stress. However, PINK1 and Parkin are not essential for all types of mitophagy and such pathways occur in most cell types and tissues in vivo, even in the absence of overt mitochondrial stress - so-called basal mitophagy. The most common mutation in PD, that of glycine at position 2019 to serine in the protein kinase LRRK2, results in increased activity and this was recently shown to disrupt basal mitophagy in vivo. Thus, different modalities of mitophagy are affected by distinct proteins implicated in PD, suggesting impaired mitophagy may be a common denominator for the disease. In this short review, we discuss the current knowledge about the link between PD pathogenic mutations and mitophagy, with a particular focus on LRRK2.

LRRK2 at the Crossroad of Aging and Parkinson's Disease

Parkinson's disease (PD) is a heterogeneous neurodegenerative disease characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta and the widespread occurrence of proteinaceous inclusions known as Lewy bodies and Lewy neurites. The etiology of PD is still far from clear, but aging has been considered as the highest risk factor influencing the clinical presentations and the progression of PD. Accumulating evidence suggests that aging and PD induce common changes in multiple cellular functions, including redox imbalance, mitochondria dysfunction, and impaired proteostasis. Age-dependent deteriorations in cellular dysfunction may predispose individuals to PD, and cellular damages caused by genetic and/or environmental risk factors of PD may be exaggerated by aging. Mutations in the LRRK2 gene cause late-onset, autosomal dominant PD and comprise the most common genetic causes of both familial and sporadic PD. LRRK2-linked PD patients show clinical and pathological features indistinguishable from idiopathic PD patients. Here, we review cellular dysfunctions shared by aging and PD-associated LRRK2 mutations and discuss how the interplay between the two might play a role in PD pathologies.

LRRK2 at the pre-synaptic site: A 16-years perspective

Parkinson's disease is a common neurodegenerative disorder and is clinically characterized by bradykinesia, rigidity, and resting tremor. Missense mutations in the leucine-rich repeat protein kinase-2 gene (LRRK2) are a recognized cause of inherited Parkinson's disease. The physiological and pathological impact of LRRK2 is still obscure, but accumulating evidence indicates that LRRK2 orchestrates diverse aspects of membrane trafficking, such as membrane fusion and vesicle formation and transport along actin and tubulin tracks. In the present review, we focus on the special relation between LRRK2 and synaptic vesicles. LRRK2 binds and phosphorylates key actors within the synaptic vesicle cycle. Accordingly, alterations in dopamine and glutamate transmission have been described upon LRRK2 manipulations. However, the different modeling strategies and phenotypes observed require a critical approach to decipher the outcome of LRRK2 at the pre-synaptic site.

Synergistic Effects of Milk-Derived Exosomes and Galactose on α-Synuclein Pathology in Parkinson's Disease and Type 2 Diabetes Mellitus

Epidemiological studies associate milk consumption with an increased risk of Parkinson's disease (PD) and type 2 diabetes mellitus (T2D). PD is an α-synucleinopathy associated with mitochondrial dysfunction, oxidative stress, deficient lysosomal clearance of α-synuclein (α-syn) and aggregation of misfolded α-syn. In T2D, α-syn promotes co-aggregation with islet amyloid polypeptide in pancreatic β-cells. Prion-like vagal nerve-mediated propagation of exosomal α-syn from the gut to the brain and pancreatic islets apparently link both pathologies. Exosomes are critical transmitters of α-syn from cell to cell especially under conditions of compromised autophagy. This review provides translational evidence that milk exosomes (MEX) disturb α-syn homeostasis. MEX are taken up by intestinal epithelial cells and accumulate in the brain after oral administration to mice. The potential uptake of MEX miRNA-148a and miRNA-21 by enteroendocrine cells in the gut, dopaminergic neurons in substantia nigra and pancreatic β-cells may enhance miRNA-148a/DNMT1-dependent overexpression of α-syn and impair miRNA-148a/PPARGC1A- and miRNA-21/LAMP2A-dependent autophagy driving both diseases. MiRNA-148a- and galactose-induced mitochondrial oxidative stress activate c-Abl-mediated aggregation of α-syn which is exported by exosome release. Via the vagal nerve and/or systemic exosomes, toxic α-syn may spread to dopaminergic neurons and pancreatic β-cells linking the pathogenesis of PD and T2D.

LncRNA XIST sponges miR-199a-3p to modulate the Sp1/LRRK2 signal pathway to accelerate Parkinson's disease progression

In vitro and in vivo models of Parkinson’s disease were established to investigate the effects of the lncRNA XIST/miR-199a-3p/Sp1/LRRK2 axis. The binding between XIST and miR-199a-3p as well as miR-199a-3p and Sp1 were examined by luciferase reporter assay and confirmed by RNA immunoprecipitation analysis. Following the Parkinson’s disease animal behavioural assessment by suspension and swim tests, the brain tissue injuries were evaluated by hematoxylin and eosin, TdT-mediated dUTP-biotin nick end labelling, and tyrosine hydroxylase stainings. The results indicated that miR-199a-3p expression was downregulated, whereas that of XIST, Sp1 and LRRK2 were upregulated in Parkinson’s disease. Moreover, miR-199a-3p overexpression or XIST knockdown inhibited the cell apoptosis induced by MPP⁺ treatment and promoted cell proliferation. The neurodegenerative defects were significantly recovered by treating the cells with shXIST or shSp1, whereas miR-199a-3p inhibition or Sp1 and LRRK2 overexpression abrogated these beneficial effects. Furthermore, the results of our in vivo experiments confirmed the neuroprotective effects of shXIST and miR-199a-3p against MPTP-induced brain injuries, and the Parkinson’s disease behavioural symptoms were effectively alleviated upon shXIST or miR-199a-3p treatment. In summary, the results of the present study showed that lncRNA XIST sponges miR-199a-3p to modulate Sp1 expression and further accelerates Parkinson’s disease progression by targeting LRRK2.

Structure and function of p62/SQSTM1 in the emerging framework of phase separation

p62/SQSTM1 is a multiprotein interaction hub forming cellular punctate structures known as p62 bodies. p62 is centrally involved in the degradation of ubiquitinated cargo through autophagy, as well as in a wide range of signaling activities as part of the cellular response to nutrient sensing, oxidative stress, infection, immunity, and inflammation. Structural work has shown that p62 forms flexible filamentous assemblies composed of an N-terminal PB1-domain scaffold and a C-terminal binding platform, including folded recognition domains and structurally disordered binding motifs. In the cell, these filaments are part of cellular p62 bodies that display properties of liquid-liquid-phase separation. Here, we review the accumulated structural and functional work of p62 and integrate them with the emerging framework of filamentous biomolecular condensates.

Pathological Functions of LRRK2 in Parkinson's Disease

Mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) are common genetic risk factors for both familial and sporadic Parkinson’s disease (PD). Pathogenic mutations in LRRK2 have been shown to induce changes in its activity, and abnormal increase in LRRK2 kinase activity is thought to contribute to PD pathology. The precise molecular mechanisms underlying LRRK2-associated PD pathology are far from clear, however the identification of LRRK2 substrates and the elucidation of cellular pathways involved suggest a role of LRRK2 in microtubule dynamics, vesicular trafficking, and synaptic transmission. Moreover, LRRK2 is associated with pathologies of α-synuclein, a major component of Lewy bodies (LBs). Evidence from various cellular and animal models supports a role of LRRK2 in the regulation of aggregation and propagation of α-synuclein. Here, we summarize our current understanding of how pathogenic mutations dysregulate LRRK2 and discuss the possible mechanisms leading to neurodegeneration.

Parkinson's: A Disease of Aberrant Vesicle Trafficking

Parkinson's disease (PD) is a leading cause of neurodegeneration that is defined by the selective loss of dopaminergic neurons and the accumulation of protein aggregates called Lewy bodies (LBs). The unequivocal identification of Mendelian inherited mutations in 13 genes in PD has provided transforming insights into the pathogenesis of this disease. The mechanistic analysis of several PD genes, including α-synuclein (α-syn), leucine-rich repeat kinase 2 (LRRK2), PTEN-induced kinase 1 (PINK1), and Parkin, has revealed central roles for protein aggregation, mitochondrial damage, and defects in endolysosomal trafficking in PD neurodegeneration. In this review, we outline recent advances in our understanding of these gene pathways with a focus on the emergent role of Rab (Ras analog in brain) GTPases and vesicular trafficking as a common mechanism that underpins how mutations in PD genes lead to neuronal loss. These advances have led to previously distinct genes such as vacuolar protein-sorting-associated protein 35 (VPS35) and LRRK2 being implicated in a common signaling pathway. A greater understanding of these common nodes of vesicular trafficking will be crucial for linking other PD genes and improving patient stratification in clinical trials underway against α-syn and LRRK2 targets.

LRRK2 Phosphorylation, More Than an Epiphenomenon

Mutations in the Leucine Rich Repeat Kinase 2 (LRRK2) gene are linked to autosomal dominant Parkinson's disease (PD), and genetic variations at the LRRK2 locus are associated with an increased risk for sporadic PD. This gene encodes a kinase that is physiologically multiphosphorylated, including clusters of both heterologous phosphorylation and autophosphorylation sites. Several pieces of evidence indicate that LRRK2's phosphorylation is important for its pathological and physiological functioning. These include a reduced LRRK2 heterologous phosphorylation in PD brains or after pharmacological inhibition of LRRK2 kinase activity as well as the appearance of subcellular LRRK2 accumulations when this protein is dephosphorylated at heterologous phosphosites. Nevertheless, the regulatory mechanisms governing LRRK2 phosphorylation levels and the cellular consequences of changes in LRRK2 phosphorylation remain incompletely understood. In this review, we present current knowledge on LRRK2 phosphorylation, LRRK2 phosphoregulation, and how LRRK2 phosphorylation changes affect cellular processes that may ultimately be linked to PD mechanisms.

"LRRK2: Autophagy and Lysosomal Activity"

It has been 15 years since the Leucine-rich repeat kinase 2 (LRRK2) gene was identified as the most common genetic cause for Parkinson's disease (PD). The two most common mutations are the LRRK2-G2019S, located in the kinase domain, and the LRRK2-R1441C, located in the ROC-COR domain. While the LRRK2-G2019S mutation is associated with increased kinase activity, the LRRK2-R1441C exhibits a decreased GTPase activity and altered kinase activity. Multiple lines of evidence have linked the LRRK2 protein with a role in the autophagy pathway and with lysosomal activity in neurons. Neurons rely heavily on autophagy to recycle proteins and process cellular waste due to their post-mitotic state. Additionally, lysosomal activity decreases with age which can potentiate the accumulation of α-synuclein, the pathological hallmark of PD, and subsequently lead to the build-up of Lewy bodies (LBs) observed in this disorder. This review provides an up to date summary of the LRRK2 field to understand its physiological role in the autophagy pathway in neurons and related cells. Careful assessment of how LRRK2 participates in the regulation of phagophore and autophagosome formation, autophagosome and lysosome fusion, lysosomal maturation, maintenance of lysosomal pH and calcium levels, and lysosomal protein degradation are addressed. The autophagy pathway is a complex cellular process and due to the variety of LRRK2 models studied in the field, associated phenotypes have been reported to be seemingly conflicting. This review provides an in-depth discussion of different models to assess the normal and disease-associated role of the LRRK2 protein on autophagic function. Given the importance of the autophagy pathway in Parkinson's pathogenesis it is particularly relevant to focus on the role of LRRK2 to discover novel therapeutic approaches that restore lysosomal protein degradation homeostasis.

Guilt-by-Association - Functional Insights Gained From Studying the LRRK2 Interactome

The Parkinson's disease-associated Leucine-rich repeat kinase 2 (LRRK2) is a complex multi-domain protein belonging to the Roco protein family, a unique group of G-proteins. Variants of this gene are associated with an increased risk of Parkinson's disease. Besides its well-characterized enzymatic activities, conferred by its GTPase and kinase domains, and a central dimerization domain, it contains four predicted repeat domains, which are, based on their structure, commonly involved in protein-protein interactions (PPIs). In the past decades, tremendous progress has been made in determining comprehensive interactome maps for the human proteome. Knowledge of PPIs has been instrumental in assigning functions to proteins involved in human disease and helped to understand the connectivity between different disease pathways and also significantly contributed to the functional understanding of LRRK2. In addition to an increased kinase activity observed for proteins containing PD-associated variants, various studies helped to establish LRRK2 as a large scaffold protein in the interface between cytoskeletal dynamics and the vesicular transport. This review first discusses a number of specific LRRK2-associated PPIs for which a functional consequence can at least be speculated upon, and then considers the representation of LRRK2 protein interactions in public repositories, providing an outlook on open research questions and challenges in this field.

Advances in elucidating the function of leucine-rich repeat protein kinase-2 in normal cells and Parkinson's disease

Autosomal dominant missense mutations that hyperactivate the leucine-rich repeat protein kinase-2 (LRRK2) are a common cause of inherited Parkinson's disease and therapeutic efficacy of LRRK2 inhibitors is being tested in clinical trials. In this review, we discuss the nuts and bolts of our current understanding of how the LRRK2 is misregulated by mutations and how pathway activity is affected by LRRK2 binding to membrane, microtubule filaments, and 14-3-3, as well as by upstream components such as Rab29 and VPS35. We discuss recent work that points toward a subset of Rab proteins comprising key physiological substrates that bind new sets of effectors, such as RILPL1/2, JIP3 and JIP4 after phosphorylation by LRRK2. We explore what is known about how LRRK2 regulates ciliogenesis, the endosomal-lysosomal system, immune responses and interplay with alpha-synuclein and tau and how this might be linked to Parkinson's' disease.

'High vault-age': non-coding RNA control of autophagy

RNA-binding proteins typically change the fate of RNA, such as stability, translation or processing. Conversely, we recently uncovered that the small non-coding vault RNA 1-1 (vtRNA1-1) directly binds to the autophagic receptor p62/SQSTM1 and changes the protein's function. We refer to this process as 'riboregulation'. Here, we discuss this newly uncovered vault RNA function against the background of three decades of vault RNA research. We highlight the vtRNA1-1-p62 interaction as an example of riboregulation of a key cellular process.

DNA damage and repair in Parkinson's disease: Recent advances and new opportunities

Parkinson's disease (PD) is the most common movement neurodegenerative disorder. Although our understanding of the underlying mechanisms of pathogenesis in PD has greatly expanded, this knowledge thus far has failed to translate into disease-modifying therapies. Therefore, it is of the utmost urgency to interrogate further the multifactorial etiology of PD. DNA repair defects cause many neurodegenerative diseases. An exciting new PD research avenue is the role that DNA damage and repair may play in neuronal death. The goal of this mini-review was to discuss the evidence for the types of DNA damage that accumulates in PD, which has provided clues for which DNA repair pathways, such as DNA double-strand break repair, are dysfunctional. We further highlight compelling data for activation of the DNA damage response in familial and idiopathic PD. The significance of DNA damage and repair is emerging in the PD field and linking these insights to PD pathogenesis may provide new insights into PD pathophysiology and consequently lead to new therapies.

Pathways of protein synthesis and degradation in PD pathogenesis

Since the discovery of protein aggregates in the brains of individuals with Parkinson's disease (PD) in the early 20th century, the scientific community has been interested in the role of dysfunctional protein metabolism in PD etiology. Recent advances in the field have implicated defective protein handling underlying PD through genetic, in vitro, and in vivo studies incorporating many disease models alongside neuropathological evidence. Here, we discuss the existing body of research focused on understanding cellular pathways of protein synthesis and degradation, and how aberrations in either system could engender PD pathology with special attention to α-synuclein-related consequences. We consider transcription, translation, and post-translational modification to constitute protein synthesis, and protein degradation to encompass proteasome-, lysosome- and endoplasmic reticulum-dependent mechanisms. Novel findings connecting each of these steps in protein metabolism to development of PD indicate that deregulation of protein production and turnover remains an exciting area in PD research.

Leucine-rich repeat kinase 2 and lysosomal dyshomeostasis in Parkinson disease

Over the last two decades, a number of studies have underlined the importance of lysosomal-based degradative pathways in maintaining the homeostasis of post-mitotic cells, and revealed the remarkable contribution of a functional autophagic machinery in the promotion of longevity. In contrast, defects in the clearance of organelles and aberrant protein aggregates have been linked to accelerated neuronal loss and neurological dysfunction. Several neurodegenerative disorders, among which Alzheimer disease (AD), Frontotemporal dementia, and Amyotrophic Lateral Sclerosis to name a few, are associated with alterations of the autophagy and endo-lysosomal pathways. In Parkinson disease (PD), the most prevalent genetic determinant, Leucine-rich repeat kinase 2 (LRRK2), is believed to be involved in the regulation of intracellular vesicle traffic, autophagy and lysosomal function. Here, we review the current understanding of the mechanisms by which LRRK2 regulates lysosomal-based degradative pathways in neuronal and non-neuronal cells and discuss the impact of pathogenic PD mutations in contributing to lysosomal dyshomeostasis.

Neuron-autonomous susceptibility to induced synuclein aggregation is exacerbated by endogenous Lrrk2 mutations and ameliorated by Lrrk2 genetic knock-out

Neuronal aggregates containing α-synuclein are a pathological hallmark of several degenerative diseases; including Parkinson’s disease, Parkinson’s disease with dementia and dementia with Lewy bodies. Understanding the process of α-synuclein aggregation, and discovering means of preventing it, may help guide therapeutic strategy and drug design. Recent advances provide tools to induce α-synuclein aggregation in neuronal cultures. Application of exogenous pre-formed fibrillar α-synuclein induces pathological phosphorylation and accumulation of endogenous α-synuclein, typical of that seen in disease. Genomic variability and mutations in α-synuclein and leucine-rich repeat kinase 2 proteins are the major genetic risk factors for Parkinson’s disease. Reports demonstrate fibril-induced α-synuclein aggregation is increased in cells from leucine-rich repeat kinase 2 pathogenic mutant (G2019S) overexpressing mice, and variously decreased by leucine-rich repeat kinase 2 inhibitors. Elsewhere in vivo antisense knock-down of leucine-rich repeat kinase 2 protein has been shown to protect mice from fibril-induced α-synuclein aggregation, whereas kinase inhibition did not. To help bring clarity to this issue, we took a purely genetic approach in a standardized neuron-enriched culture, lacking glia. We compared fibril treatment of leucine-rich repeat kinase 2 germ-line knock-out, and G2019S germ-line knock-in, mouse cortical neuron cultures with those from littermates. We found leucine-rich repeat kinase 2 knock-out neurons are resistant to α-synuclein aggregation, which predominantly forms within axons, and may cause axonal fragmentation. Conversely, leucine-rich repeat kinase 2 knock-in neurons are more vulnerable to fibril-induced α-synuclein accumulation. Protection and resistance correlated with basal increases in a lysosome marker in knock-out, and an autophagy marker in knock-in cultures. The data add to a growing number of studies that argue leucine-rich repeat kinase 2 silencing, and potentially kinase inhibition, may be a useful therapeutic strategy against synucleinopathy.

LRRK2 inhibition prevents endolysosomal deficits seen in human Parkinson's disease

LRRK2 has been implicated in endolysosomal function and likely plays a central role in idiopathic Parkinson’s disease (iPD). In iPD, dopaminergic neurons within the substantia nigra are characterized by increased LRRK2 kinase activity, endolysosomal deficits, and accumulation of autophagic vesicles with incompletely degraded substrates, including α-synuclein. Although LRRK2 has been implicated in endolysosomal and autophagic function, it remains unclear whether inhibition of LRRK2 kinase activity can prevent endolysosomal deficits or reduce dopaminergic neurodegeneration. In this study, we characterized the endolysosomal and autophagic defects in surviving dopaminergic neurons of iPD patient brain tissue. We next showed that these defects could be reproduced reliably in vivo using the rotenone model of iPD. Results suggested that there was impaired endosomal maturation, resulting in lysosomal dysfunction and deficits in protein degradation. A highly selective, brain-penetrant LRRK2 kinase inhibitor not only improved apparent endosomal maturation and lysosomal function, but also prevented rotenone-induced neurodegeneration in vivo. The fact that a LRRK2 kinase inhibitor was capable of preventing the neuropathological and endolysosomal abnormalities observed in human iPD suggests that LRRK2 inhibitors may have broad therapeutic utility in iPD, not only in those who carry a LRRK2 mutation.

The dynamic switch mechanism that leads to activation of LRRK2 is embedded in the DFGψ motif in the kinase domain

Little is known about the regulation of Leucine-rich repeat kinase 2 (LRRK2) associated with familial Parkinson’s disease (PD). To test whether the kinase domain drives LRRK2 activation, we applied the spine concept that describes the core architecture of every protein kinase. We discovered that mutation of Y2018, a regulatory spine residue, to Phe in the DFGψ motif created a hyperactive kinase similar to the PD-associated mutation G2019S. The hydroxyl moiety of Y2018 thus serves as a “brake,” stabilizing the inactive conformation; simply removing it destroys a key inhibitory hydrogen-bonding node. These data reveal an LRRK2-specific regulatory mechanism, confirming that the kinase domain functions as a classical kinase that controls overall conformational dynamics in full-length LRRK2 and drives therapeutic strategies.

Leucine-rich repeat kinase 2 (LRRK2) is a large multidomain protein, and LRRK2 mutants are recognized risk factors for Parkinson’s disease (PD). Although the precise mechanisms that control LRRK2 regulation and function are unclear, the importance of the kinase domain is strongly implicated, since 2 of the 5 most common familial LRRK2 mutations (G2019S and I2020T) are localized to the conserved DFGψ motif in the kinase core, and kinase inhibitors are under development. Combining the concept of regulatory (R) and catalytic (C) spines with kinetic and cell-based assays, we discovered a major regulatory mechanism embedded within the kinase domain and show that the DFG motif serves as a conformational switch that drives LRRK2 activation. LRRK2 is quite unusual in that the highly conserved Phe in the DFGψ motif, which is 1 of the 4 R-spine residues, is replaced with tyrosine (DY²⁰¹⁸GI). A Y2018F mutation creates a hyperactive phenotype similar to the familial mutation G2019S. The hydroxyl moiety of Y2018 thus serves as a “brake” that stabilizes an inactive conformation; simply removing it destroys a key hydrogen-bonding node. Y2018F, like the pathogenic mutant I2020T, spontaneously forms LRRK2-decorated microtubules in cells, while the wild type and G2019S require kinase inhibitors to form filaments. We also explored 3 different mechanisms that create kinase-dead pseudokinases, including D2017A, which further emphasizes the highly synergistic role of key hydrophobic and hydrophilic/charged residues in the assembly of active LRRK2. We thus hypothesize that LRRK2 harbors a classical protein kinase switch mechanism that drives the dynamic activation of full-length LRRK2.

SQSTM1/p62: A Potential Target for Neurodegenerative Disease

Neurodegenerative diseases, characterized by a progressive loss of brain function, affect the lives of millions of individuals worldwide. The complexity of the brain poses a challenge for scientists trying to map the biochemical and physiological pathways to identify areas of pathological errors. Brain samples of patients with neurodegenerative diseases have been shown to contain large amounts of misfolded and abnormally aggregated proteins, resulting in dysfunction in certain brain centers. Removal of these abnormal molecules is essential in maintaining protein homeostasis and overall neuronal health. Macroautophagy is a major route by which cells achieve this. Administration of certain autophagy-enhancing compounds has been shown to provide therapeutic effects for individuals with neurodegenerative conditions. SQSTM1/p62 is a scaffold protein closely involved in the macroautophagy process. p62 functions to anchor the ubiquitinated proteins to the autophagosome membrane, promoting degradation of unwanted molecules. Modulators targeting p62 to induce autophagy and promote its protective pathways for aggregate protein clearance have high potential in the treatment of these conditions. Additionally, causal relationships have been found between errors in regulation of SQSTM1/p62 and the development of a variety of neurodegenerative disorders, including Alzheimer's, Parkinson's, Huntington's, amyotrophic lateral sclerosis, and frontotemporal lobar degeneration. Furthermore, SQSTM1/p62 also serves as a signaling hub for multiple pathways associated with neurodegeneration, providing a potential therapeutic target in the treatment of neurodegenerative diseases. However, rational design of a p62-oriented autophagy modulator that can balance the negative and positive functions of multiple domains in p62 requires further efforts in the exploration of the protein structure and pathological basis.

Rab GTPases as Physiological Substrates of LRRK2 Kinase

LRRK2 (Leucine-Rich Repeat Kinase 2) is a gene whose specific mutations cause Parkinson's disease (PD), the most common neurodegenerative movement disorder. LRRK2 harbors GTPase and kinase activities, two enzyme activities that play critical roles in the regulation of cellular signal transduction. Among the several LRRK2 pathogenic mutations, the most prevalent G2019S mutation increases its kinase activity when compared with the wild-type (WT), suggesting that LRRK2 kinase substrates are potential culprits of PD pathogenesis. Although there were several studies to identify LRRK2 kinase substrates, most of them mainly employed in vitro kinase assays. Therefore, it remains uncertain whether the identified substrates were real physiological substrates. However, efforts to determine physiological LRRK2 kinase substrates have recently identified several members of the Rab GTPase family as physiological LRRK2 kinase substrates. A conserved threonine or serine in the switch II domain of certain Rab GTPase family members (Rab3A/B/C/D, Rab5A/B, Rab8A/B, Rab10, Rab12, Rab29, Rab35 and Rab43) has been pinpointed to be phosphorylated by LRRK2 in cells using sophisticated phosphoproteomics technology in combination with LRRK2-specific kinase inhibitors. The Rab GTPases regulate vesicle trafficking, suggesting that LRRK2 may be a regulator of such vesicle trafficking, confirming previously suggested LRRK2 functions. However, how the consequence of the LRRK2-mediated Rab phosphorylation is related to PD pathogenesis is not clear. This review briefly summarizes the recent results about LRRK2-mediated Rab phosphorylation studies.

Kinase activity of mutant LRRK2 manifests differently in hetero-dimeric vs. homo-dimeric complexes

The Parkinson's disease (PD) protein leucine-rich repeat kinase 2 (LRRK2) exists as a mixture of monomeric and dimeric species, with its kinase activity highly concentrated in the dimeric conformation of the enzyme. We have adapted the proximity biotinylation approach to study the formation and activity of LRRK2 dimers isolated from cultured cells. We find that the R1441C and I2020T mutations both enhance the rate of dimer formation, whereas, the G2019S kinase domain mutant is similar to WT, and the G2385R risk factor variant de-stabilizes dimers. Interestingly, we find a marked departure in the kinase activity between G2019S-LRRK2 homo-dimers and wild-type-G2019S hetero-dimers. While the homo-dimeric G2019S-LRRK2 exhibits the typical robust enhancement of kinase activity, hetero-dimers comprised of wild-type (WT) and G2019S-LRRK2 exhibit kinase activity similar to WT. Dimeric complexes of specific mutant forms of LRRK2 show reduced stability following an in vitro kinase reaction, in LRRK2 mutants for which the kinase activity is similar to WT. Phosphorylation of the small GTPase Rab10 follows a similar pattern in which hetero-dimers of WT and mutant LRRK2 show similar levels of phosphorylation of Rab10 to WT homo-dimers; while the levels of pRab10 are significantly increased in cells expressing mutant homo-dimers. Interestingly, while the risk variant G2385R leads to a de-stabilization of LRRK2 dimers, those dimers possess significantly elevated kinase activity. The vast majority of familial LRRK2-dependent PD cases are heterozygous; thus, these findings raise the possibility that a crucial factor in disease pathogenesis may be the accumulation of homo-dimeric mutant LRRK2.

The Roles of Ubiquitin-Binding Protein Shuttles in the Degradative Fate of Ubiquitinated Proteins in the Ubiquitin-Proteasome System and Autophagy

The ubiquitin-proteasome system (UPS) and autophagy are the two major intracellular protein quality control (PQC) pathways that are responsible for cellular proteostasis (homeostasis of the proteome) by ensuring the timely degradation of misfolded, damaged, and unwanted proteins. Ubiquitination serves as the degradation signal in both these systems, but substrates are precisely targeted to one or the other pathway. Determining how and when cells target specific proteins to these two alternative PQC pathways and control the crosstalk between them are topics of considerable interest. The ubiquitin (Ub) recognition code based on the type of Ub-linked chains on substrate proteins was believed to play a pivotal role in this process, but an increasing body of evidence indicates that the PQC pathway choice is also made based on other criteria. These include the oligomeric state of the Ub-binding protein shuttles, their conformation, protein modifications, and the presence of motifs that interact with ATG8/LC3/GABARAP (autophagy-related protein 8/microtubule-associated protein 1A/1B-light chain 3/GABA type A receptor-associated protein) protein family members. In this review, we summarize the current knowledge regarding the Ub recognition code that is bound by Ub-binding proteasomal and autophagic receptors. We also discuss how cells can modify substrate fate by modulating the structure, conformation, and physical properties of these receptors to affect their shuttling between both degradation pathways.

Wild-type and mutant (G2019S) leucine-rich repeat kinase 2 (LRRK2) associate with subunits of the translocase of outer mitochondrial membrane (TOM) complex

Leucine-rich repeat kinase 2 (LRRK2) is important in various cellular processes including mitochondrial homeostasis and mutations in this gene lead to Parkinson's disease (PD). However, the full spectrum of LRRK2's functions remain to be elucidated. The translocase of outer mitochondrial membrane (TOM) complex is essential for the import of almost all nuclear-encoded mitochondrial proteins and is fundamental for cellular survival. Using co-immunoprecipitation, super-resolution structured illumination microscopy (SR-SIM), and 3D virtual reality (VR) assisted co-localization analysis techniques we show that wild-type and mutant (G2019S) LRRK2 associate and co-localize with subunits of the TOM complex, either under basal (dimethyl sulfoxide, DMSO) or stress-induced (carbonyl cyanide m-chlorophenyl hydrazine, CCCP) conditions. Interestingly, LRRK2 interacted with TOM40 under both DMSO and CCCP conditions, and when the PD causing mutation, G2019S was introduced, the association was not altered. Moreover, overexpression of G2019S LRRK2 resulted in the formation of large, perinuclear aggregates that co-localized with the TOM complex. Taken together, this is the first study to show that both WT and mutant LRRK2 associate with the TOM complex subunits. These findings provide additional evidence for LRRK2's role in mitochondrial function which has important implications for its role in PD pathogenesis.

The emerging interrelation between ROCO and related kinases, intracellular Ca2+ signaling, and autophagy

ROCO kinases form a family of proteins characterized by kinase activity in addition to the presence of the so-called ROC (Ras of complex proteins)/COR (C-terminal of ROC) domains having a role in their GTPase activity. These are the death-associated protein kinase (DAPK) 1 and the leucine-rich repeat kinases (LRRK) 1 and 2. These kinases all play roles in cellular life and death decisions and in autophagy in particular. Related to the ROCO kinases is DAPK 2 that however cannot be classified as a ROCO protein due to the absence of the ROC/COR domains. This review aims to bring together what is known about the relation between these proteins and intracellular Ca²⁺ signals in the induction and regulation of autophagy. Interestingly, DAPK 1 and 2 and LRRK2 are all linked to Ca²⁺ signaling in their effects on autophagy, though in various ways. Present evidence supports an upstream role for LRRK2 that via lysosomal and endoplasmic reticulum Ca²⁺ release can trigger autophagy induction. In contrast herewith, DAPK1 and 2 react on existing Ca²⁺ signals to stimulate the autophagic pathway. Further research will be needed for obtaining a full understanding of the role of these various kinases in autophagy and to assess their exact relation with intracellular Ca²⁺ signaling as this would be helpful in the development of novel therapeutic strategies against neurodegenerative disorders, cancer and auto-immune diseases. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Roco Proteins and the Parkinson's Disease-Associated LRRK2

Small G-proteins are structurally-conserved modules that function as molecular on-off switches. They function in many different cellular processes with differential specificity determined by the unique effector-binding surfaces, which undergo conformational changes during the switching action. These switches are typically standalone monomeric modules that form transient heterodimers with specific effector proteins in the 'on' state, and cycle to back to the monomeric conformation in the 'off' state. A new class of small G-proteins called "Roco" was discovered about a decade ago; this class is distinct from the typical G-proteins in several intriguing ways. Their switch module resides within a polypeptide chain of a large multi-domain protein, always adjacent to a unique domain called COR, and its effector kinase often resides within the same polypeptide. As such, the mechanisms of action of the Roco G-proteins are likely to differ from those of the typical G-proteins. Understanding these mechanisms is important because aberrant activity in the human Roco protein LRRK2 is associated with the pathogenesis of Parkinson's disease. This review provides an update on the current state of our understanding of the Roco G-proteins and the prospects of targeting them for therapeutic purposes.

Analysis of macroautophagy related proteins in G2019S LRRK2 Parkinson's disease brains with Lewy body pathology

LRRK2, the gene encoding the multidomain kinase Leucine-Rich Repeat Kinase 2 (LRRK2), has been linked to familial and sporadic forms of Parkinson's disease (PD), as well as cancer, leprosy and Crohn's disease, establishing it as a target for discovery therapeutics. LRRK2 has been associated with a range of cellular processes, however its physiological and pathological functions remain unclear. The most prevalent LRRK2 mutations in PD have been shown to affect macroautophagy in various cellular models while a role in autophagy signalling has been recapitulated in vivo. Dysregulation of autophagy has been implicated in PD pathology, and this raises the possibility that differential autophagic activity is relevant to disease progression in PD patients carrying LRRK2 mutations. To examine the relevance of LRRK2 to the regulation of macroautophagy in a disease setting we examined the levels of autophagic markers in the basal ganglia of G2019S LRRK2 PD post-mortem tissue, in comparison to pathology-matched idiopathic PD (iPD), using immunoblotting (IB). Significantly lower levels of p62 and LAMP1 were observed in G2019S LRRK2 PD compared to iPD cases. Similarly, an increase in ULK1 was observed in iPD but was not reflected in G2019S LRRK2 PD cases. Furthermore, examination of p62 by immunohistochemistry (IH) recapitulated a distinct signature for G2019S PD. IH of LAMP1, LC3 and ULK1 broadly correlated with the IB results. Our data from a small but pathologically well-characterized cases highlights a divergence of G2019S PD carriers in terms of autophagic response in alpha-synuclein pathology affected brain regions compared to iPD.

Physiological and pathological functions of LRRK2: implications from substrate proteins

Leucine-rich repeat kinase 2 (LRRK2) encodes a 2527-amino acid (aa) protein composed of multiple functional domains, including a Ras of complex proteins (ROC)-type GTP-binding domain, a carboxyl terminal of ROC (COR) domain, a serine/threonine protein kinase domain, and several repeat domains. LRRK2 is genetically involved in the pathogenesis of both sporadic and familial Parkinson's disease (FPD). Parkinson's disease (PD) is the second most common neurodegenerative disorder, manifesting progressive motor dysfunction. PD is pathologically characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta, and the presence of intracellular inclusion bodies called Lewy bodies (LB) in the remaining neurons. As the most frequent PD-causing mutation in LRRK2, G2019S, increases the kinase activity of LRRK2, an abnormal increase in LRRK2 kinase activity is believed to contribute to PD pathology; however, the precise biological functions of LRRK2 involved in PD pathogenesis remain unknown. Although biochemical studies have discovered several substrate proteins of LRRK2 including Rab GTPases and tau, little is known about whether excess phosphorylation of these substrates is the cause of the neurodegeneration in PD. In this review, we summarize latest findings regarding the physiological and pathological functions of LRRK2, and discuss the possible molecular mechanisms of neurodegeneration caused by LRRK2 and its substrates.

p62/SQSTM1 - steering the cell through health and disease

SQSTM1 (also known as p62) is a multifunctional stress-inducible scaffold protein involved in diverse cellular processes. Its functions are tightly regulated through an extensive pattern of post-translational modifications, and include the isolation of cargos degraded by autophagy, induction of the antioxidant response by the Keap1-Nrf2 system, as well as the regulation of endosomal trafficking, apoptosis and inflammation. Accordingly, malfunction of SQSTM1 is associated with a wide range of diseases, including bone and muscle disorders, neurodegenerative and metabolic diseases, and multiple forms of cancer. In this Review, we summarize current knowledge regarding regulation, post-translational modifications and functions of SQSTM1, as well as how they are dysregulated in various pathogenic contexts.