Leucine-rich repeat kinase 2 (LRRK2), a complex kinase/GTPase mutated in Parkinson's disease, has been shown to physically and functionally interact with cytoskeletal-related components in different brain cells. Neurons greatly rely on a functional cytoskeleton for many homeostatic processes such as local and long-distance vesicle transport, synaptic plasticity, and dendrites/axons growth and remodeling. Here, we will review the available data linking LRRK2 and the cytoskeleton, and discuss how this may be functionally relevant for the well-established roles of LRRK2 in intracellular trafficking pathways and outgrowth of neuronal processes in health and disease conditions.

Introduction

Intracellular vesicles, of any kind, need physical tracks to move from one cellular site to another, and these tracks are provided by cytoskeletal elements. Vesicle traffic is particularly important in neuronal cells, where the distance to cover between the somata and the processes (dendrites or axons) can be incredibly long if compared with the typical distances between cellular compartments of non-neuronal cells. The actin cytoskeleton is essential to shape the morphology of pre- and postsynaptic elements, to support neurite outgrowth and growth cone navigation and to control the traffic of neurotransmitter-containing presynaptic vesicles and postsynaptic receptor-containing endosomes [1]. On the other hand, microtubules (MTs) serve as railways for long-distance transport, but also provide mechanical support and control local signaling events (reviewed in ref. [2]).

Accumulating evidence indicates that the Parkinson's disease (PD) kinase LRRK2 (leucine-rich repeat kinase 2) is physically and functionally associated with cytoskeletal-related components in the neuronal cell [3]. Here, we will review the literature supporting a link between LRRK2 and the cytoskeleton and discuss how this may be functionally relevant for the well-established roles of LRRK2 in intracellular vesicle traffic and neurite outgrowth in health and PD conditions.

LRRK2: an overview

LRRK2 belongs to the family of ROCO proteins, characterized by the presence of the bidomain ROC (Ras of complex proteins) and COR (C-terminal of ROC) [4]. ROC functions as a GTPase and represents a signaling output, whereas COR operates as a dimerization device [57]. Another signaling output in LRRK2 is a serine-threonine kinase domain, which undergoes intramolecular autophosphorylation [5,8] and, more recently, it was shown to phosphorylate a subset of Rab GTPases [9]. LRRK2 also contains protein-to-protein interaction domains, indicating an additional scaffolding role in assembling cellular components — e.g. cytoskeleton and vesicles — and signal transduction complexes. Among all the genetic contributors of PD, gain-of-function mutations in the LRRK2 gene represent the single most common cause of disease [10], making LRRK2 kinase activity the most pursued target in quest of disease-modifying therapies. PD-segregating mutations sit in the catalytic core of the protein and can affect directly either the kinase (G2019S and I2020T) or the GTPase (R1441C/G/H and Y1699C) activities (Figure 1). Noteworthy, all PD-linked LRRK2 mutations confer enhanced kinase activity toward substrates in cells [9]. LRRK2 is expressed in multiple tissues and organs, with high expression in the brain, lungs, kidneys and circulating immune cells [11]. Within the brain, LRRK2 is found in neuronal and glial cells, with high expression in the striatum, which receives the dopaminergic projecting fibers from the substantia nigra pars compacta (SNpc). LRRK2 has been linked to multiple cellular processes, including autophagy, endolysosomal pathways, synaptic vesicles trafficking and cytoskeletal dynamics [3,12]. Phosphorylation of Ser910 and Ser935 by CK1alpha and IκB kinases (IKKs) is required for the binding with 14-3-3 proteins and LRRK2 subcellular relocalization [13,14] (Figure 1). Of note, the majority of LRRK2 pathogenic mutations display decreased Ser910/935 phosphorylation and 14-3-3 binding, a biochemical state that can be phenocopied by LRRK2 pharmacological inhibition [15]. Intriguingly, the cellular pool of dephosphorylated LRRK2 redistributes into MTs and vesicle-related structures [15,16].

LRRK2 domain organization and cytoskeletal-related interacting proteins.

Figure 1.
LRRK2 domain organization and cytoskeletal-related interacting proteins.

Schematic representation of LRRK2 domain organization with pathogenic mutations and their effect on enzymatic activities, major phosphorylation sites and binding of 14-3-3 proteins with regulation by phosphatases (mainly PP1A) and casein kinase 1 (CK1alpha) and IKKs.

MTs-related interactors/substrates are in green, while actin-related interactors/substrates are in red.

Figure 1.
LRRK2 domain organization and cytoskeletal-related interacting proteins.

Schematic representation of LRRK2 domain organization with pathogenic mutations and their effect on enzymatic activities, major phosphorylation sites and binding of 14-3-3 proteins with regulation by phosphatases (mainly PP1A) and casein kinase 1 (CK1alpha) and IKKs.

MTs-related interactors/substrates are in green, while actin-related interactors/substrates are in red.

LRRK2 and microtubules

A recent computational analysis of LRRK2 interactome reveals that 40% of LRRK2 interactors contributes toward the enrichment of terms associated with the cytoskeleton [17]. Accordingly, many studies established a connection between LRRK2 and both MTs and filamentous actin (F-actin), with multiple cytoskeletal-related proteins nominated as LRRK2 binders and/or substrates (Figure 1). MTs are polymers of α and β-tubulins, covering a critical role in neuronal homeostasis, including neurite outgrowth, bidirectional transport of proteins/organelles from the soma to the periphery and synaptic plasticity. Early studies showed that LRRK2 interacts with tubulins through its ROC domain [18] and colocalizes with βIII tubulin in mouse brains [19]. Moreover, recombinant human LRRK2 was reported to phosphorylate β-tubulin purified from mouse brain at Thr107, with phosphorylation significantly enhanced by the G2019S mutation [20]. Mechanistically, this phosphorylation was suggested to enhance MT stability in the presence of MT-associated proteins (MAPs) [20]. Of interest, Law et al. [21] showed increased β-tubulin acetylation in LRRK2 knockout (KO) models, suggesting that LRRK2 binding to MTs might interfere with tubulin acetylation. The same authors recently found that α-tubulin is also more acetylated in the kidneys of KO mice using an unbiased high-throughput proteomic screen [22]. In the same study, they further observed increased expression of the actin-binding protein coronin 1C and decreased expression of the MAP gephyrin in Lrrk2 KO kidneys [22], further supporting a link between LRRK2 and both actin and MT cytoskeleton. Of interest, Blanca Ramirez et al. [23] showed that all pathogenic LRRK2 mutants, with the exception of G2019S and I2010T, as well as pharmacologically inhibited LRRK2, show enhanced colocalization with acetylated and detyrosinated MTs, two post-translational modifications generally associated with stable MTs. Through a comprehensive analysis of LRRK2 cellular phosphorylation, autophosphorylation, 14-3-3 binding and GTP binding, the authors concluded that increased LRRK2 decoration of MTs correlates with enhanced GTP binding [23]. While increased colocalization of pathogenic [16] or inhibited LRRK2 [24] with MTs was previously reported, this study provides a mechanistic model of GTP-dependent LRRK2-MTs association, consistent with the previously described interaction between tubulins and the Roc/GTPase domain of LRRK2 [18]. In line with this study, Godena et al. [25] reported that R1441C and Y1699C LRRK2 mutants impair axonal transport in primary neurons and in Drosophila by increasing the binding with MTs. Although the authors found that mutant LRRK2 bind deacetylated rather than acetylated MTs in vitro [25], all these studies agree that LRRK2 can transfer to MTs and that pathogenic mutations enhance this association. LRRK2 also interacts with the MAP Tau and facilitates its phosphorylation via cyclin-dependent kinase 5 (Cdk5) [26]. Tau accumulation and hyperphosphorylation has been observed in multiple models expressing mutant LRRK2 [2729] and mutant G2019S LRRK2, but not wild type, markedly enhances neuron-to-neuron transmission of Tau in mice [30], supporting the pathological evidence that a sub-group of LRRK2 patients exhibit Tau pathology [31]. Interestingly, Tau as well as other MAPs bundle actin and cross-link the cellular cytoskeleton formed by MTs and actin filaments [32].

Therefore, pathogenic LRRK2 can affect both on microtubule and actin network exerting detrimental effect upstream of Tau.

LRRK2 and actin dynamics

In addition of interacting with MTs, LRRK2 also binds actin and promotes its polymerization [33]. LRRK2 was shown to phosphorylate the ERM protein moesin in vitro, which, together with ezrin and radixin, links the actin cytoskeleton to the plasma membrane [34]. ERMs are localized at the actin-rich sites in filopodia where they control neurite outgrowth by regulating filopodia architecture [35]. When neurons isolated from mutant LRRK2-G2019S-expressing mice are cultured in vitro, they exhibit reduced neurite outgrowth compared with non-transgenic neurons [3638], and the phosphorylation state of ERM and F-actin content is increased in LRRK2-G2019S neurons, which may, in part, explain the neurite outgrowth defects reported in these animals. Of note, while LRRK2 efficiently phosphorylates ERM proteins in vitro [34], cellular phosphorylation of ERM by LRRK2 has not been demonstrated, suggesting that the observed increased ERM phosphorylation in LRRK2-G2019S neurons is likely an indirect phenomenon.

Neurite shortening is a mutant LRRK2-associated phenotype that has been reported in dozens of studies (Table 1). In 2006, MacLeod et al. [38] were the first to observe decreased neurite length and complexity in mutant LRRK2 systems and several following studies replicated this finding across multiple experimental systems, e.g. primary mouse cultures, neuron-derived IPSCs and brain slices, with some evidence suggesting that LRRK2 KO exerts the opposite effect (Table 1). P21-activated kinase 6 (PAK6), a guanine nucleotide-dependent interactor of the ROC/GTPase domain of LRRK2 [36,39,40], is particularly interesting in the context of actin dynamics and neurite outgrowth. PAKs comprise a family of serine-threonine kinases playing a central role in signal transduction. In contrast with class I PAKs (PAK1–3) which are activated by Rho GTPase binding, class II PAKs (PAK4–6) are relocalized (not activated) by GTPases within specific signaling sites and locally activated by binding with SH3 domains which release pseudo-substrate inhibition [41]. One of the best-characterized functions of these kinases is their role in actin cytoskeleton reorganization via the LIM kinase–cofilin pathway. When activated, PAKs phosphorylate LIMK1, which phosphorylates cofilin causing inhibition of its actin-severing activity and stabilization of F-actin [40,42]. Of note, PAK6 is highly and almost exclusively expressed in the brain, with elevated expression in the dopaminergic fibers of the SNpc [43]. Pak6 KO mice are viable and fertile, whereas double Pak5/Pak6 KO exhibit cognitive and locomotor activity defects and neurite shortening [44]. We showed that the kinase activity of PAK6 stimulates neurite complexity in vivo in the striatum through the LIMK–cofilin pathway and in a LRRK2-dependent manner, a finding that functionally links the two kinases with downstream impact on actin cytoskeleton-dependent neurite remodeling [40] (Figure 2). More recently, we further demonstrated that PAK6 binds and phosphorylates 14-3-3γ at Ser59. Phosphorylation of this site results in loss of 14-3-3γ binding from phospho-Ser935 in LRRK2, with consequent LRRK2 cellular dephosphorylation [39]. Of relevance for PD, a constitutively active form of PAK6 rescues the neurite shortening phenotype exhibited by primary cortical neurons isolated from BAC-G2019S LRRK2 mice, through a mechanism dependent on 14-3-3γ phosphorylation at Ser59 [39]. Since both pharmacological inhibition and kinase hyperactive PD mutants result in LRRK2 dephosphorylation at Ser910/Ser935 [15,24] and given the observed protective role of PAK6 kinase activity toward G2019S-dependent neurite shortening [39], we can speculate that PAK6-mediated dephosphorylation reduces LRRK2 kinase activity similar to kinase inhibition (Figure 2).

Proposed LRRK2–PAK6–actin axis in remodeling neurite complexity and predicted impact of LRRK2 KO on the process (left-hand side).

Figure 2.
Proposed LRRK2–PAK6–actin axis in remodeling neurite complexity and predicted impact of LRRK2 KO on the process (left-hand side).

The rescue of the neurite shortening phenotype exhibited by G2019S LRRK2 neurons operated by PAK6 phosphorylation of 14-3-3γ is also depicted on the right-hand side.

Figure 2.
Proposed LRRK2–PAK6–actin axis in remodeling neurite complexity and predicted impact of LRRK2 KO on the process (left-hand side).

The rescue of the neurite shortening phenotype exhibited by G2019S LRRK2 neurons operated by PAK6 phosphorylation of 14-3-3γ is also depicted on the right-hand side.

Table 1
List of studies reporting the effects of LRRK2 mutations or gene KO on neurite outgrowth and morphology
LRRK2 genotype Effect on neurites Experimental model References 
Overexpressed WT, G2019S Shortening Differentiated SH-SY5Y cells [45
G2019S BAC mice Shortening Cortical neuron cultures [39
Overexpressed WT, G2019S, R1441C G2019S and R1441C: shortening Differentiated SH-SY5Y cells and mouse primary cortical neurons [46
ntg, G2019S Shortening hiPS cell lines differentiated into DAergic neurons [47
ntg, G2019S Shortening hiPS cell lines differentiated into DAergic neurons [48
G2019S Drosophila melanogaster and G2019S knock-in mice Shortening DAergic neurons in Drosophila larvae and mouse primary hippocampal and nigral neurons [49
WT, G2019S and KO BAC mice G2019S: shortening; KO: outgrowth Primary hippocampal cultures [50
AAV WT, G2019S injected rats Striatal axonal degeneration Striatal tissue slices [51
Overexpressed WT, G2019S Shortening Rat and human cortical neurons [52
siRNA-mediated knock-down Outgrowth SH-SY5Y cells and mice hippocampal neurons [53
DOX-induced expression of WT, G2019S Shortening MN9D cell lines [54
ntg, G2019S Shortening hiPS cell lines differentiated into DAergic neurons [55
KO, WT and G2019S BAC mice KO: outgrowth; G2019S: no effect Hippocampal neurons [37
WT, G2019S and R1441G overexpression; WT, G2019S BAC mice G2019S and R1441G: shortening Rat embryonic hippocampal neuron [8
WT, G2019S overexpression Shortening Rat primary cortical neurons [56
WT, G2019S, R1441C overexpression Shortening Mouse E15 cortical neurons [57
R1441G BAC mice Shortening Primary hippocampal neurons [58
I2020T transgenic mice Shortening Midbrain primary cultures [59
ntg, G2019S Shortening hiPS cell lines differentiated into DAergic neurons [60
WT, G2019S overexpression Shortening Rat primary cortical neurons [61
WT, G2019S BAC mice Shortening Primary midbrain cultures [62
WT, G2019S, R1441C overexpression Shortening Differentiated SH-SY5Y cells [63
WT, G2019S BAC mice Shortening Hippocampal tissue slices [64
G2019S, R1441C, G2385R transgenic D. melanogaster G2019S: degeneration; R1441C, G2385R: no effect DA neurons in A6 segment slices [65
shRNA-mediated knock-down Shortening NIH3T3 cells [33
KO, WT, G2019S and Y1699C BAC, knock-in mice G2019S and Y1699C BAC: shortening; KO: outgrowth; G2019S knock-in: no effect Primary hippocampal/midbrain neurons [66
WT, G2019S and R1441C overexpression; shRNA-mediated gene knock-down G2019S, R1441C: shortening; knock-down: outgrowth PC12 cells and rat primary hippocampal neuron cells [67
WT, G2019S BAC mice Shortening Primary hippocampal neurons [68
WT, G2019S overexpression Shortening Differentiated SH-SY5Y cells [69
WT, G2019S, I2020T, R1441G and Y1699C overexpression WT, G2019S, I2020T, R1441G: shortening; Y1699C: no effect Primary cortical cultures [38
LRRK2 genotype Effect on neurites Experimental model References 
Overexpressed WT, G2019S Shortening Differentiated SH-SY5Y cells [45
G2019S BAC mice Shortening Cortical neuron cultures [39
Overexpressed WT, G2019S, R1441C G2019S and R1441C: shortening Differentiated SH-SY5Y cells and mouse primary cortical neurons [46
ntg, G2019S Shortening hiPS cell lines differentiated into DAergic neurons [47
ntg, G2019S Shortening hiPS cell lines differentiated into DAergic neurons [48
G2019S Drosophila melanogaster and G2019S knock-in mice Shortening DAergic neurons in Drosophila larvae and mouse primary hippocampal and nigral neurons [49
WT, G2019S and KO BAC mice G2019S: shortening; KO: outgrowth Primary hippocampal cultures [50
AAV WT, G2019S injected rats Striatal axonal degeneration Striatal tissue slices [51
Overexpressed WT, G2019S Shortening Rat and human cortical neurons [52
siRNA-mediated knock-down Outgrowth SH-SY5Y cells and mice hippocampal neurons [53
DOX-induced expression of WT, G2019S Shortening MN9D cell lines [54
ntg, G2019S Shortening hiPS cell lines differentiated into DAergic neurons [55
KO, WT and G2019S BAC mice KO: outgrowth; G2019S: no effect Hippocampal neurons [37
WT, G2019S and R1441G overexpression; WT, G2019S BAC mice G2019S and R1441G: shortening Rat embryonic hippocampal neuron [8
WT, G2019S overexpression Shortening Rat primary cortical neurons [56
WT, G2019S, R1441C overexpression Shortening Mouse E15 cortical neurons [57
R1441G BAC mice Shortening Primary hippocampal neurons [58
I2020T transgenic mice Shortening Midbrain primary cultures [59
ntg, G2019S Shortening hiPS cell lines differentiated into DAergic neurons [60
WT, G2019S overexpression Shortening Rat primary cortical neurons [61
WT, G2019S BAC mice Shortening Primary midbrain cultures [62
WT, G2019S, R1441C overexpression Shortening Differentiated SH-SY5Y cells [63
WT, G2019S BAC mice Shortening Hippocampal tissue slices [64
G2019S, R1441C, G2385R transgenic D. melanogaster G2019S: degeneration; R1441C, G2385R: no effect DA neurons in A6 segment slices [65
shRNA-mediated knock-down Shortening NIH3T3 cells [33
KO, WT, G2019S and Y1699C BAC, knock-in mice G2019S and Y1699C BAC: shortening; KO: outgrowth; G2019S knock-in: no effect Primary hippocampal/midbrain neurons [66
WT, G2019S and R1441C overexpression; shRNA-mediated gene knock-down G2019S, R1441C: shortening; knock-down: outgrowth PC12 cells and rat primary hippocampal neuron cells [67
WT, G2019S BAC mice Shortening Primary hippocampal neurons [68
WT, G2019S overexpression Shortening Differentiated SH-SY5Y cells [69
WT, G2019S, I2020T, R1441G and Y1699C overexpression WT, G2019S, I2020T, R1441G: shortening; Y1699C: no effect Primary cortical cultures [38

Abbreviations: DAergic, dopaminergic; DOX, doxycycline; hiPS, human-induced pluripotent stem cells; ntg, non-transgenic.

Supporting these observations made with PAK6, a recent study showed that expression of a constitutively active form of the homologous kinase PAK4 protects from dopaminergic neuron loss in the 6-hydroxydopamine and alpha-synuclein rat models through the CRTC1–CREB pathway [70]. All these findings support the notion that PAK activity may represent a therapeutic solution that deserves further investigation.

Recently, LRRK2 has been shown to bind and phosphorylate the actin remodeling protein WAVE2 to regulate phagocytosis in myeloid cells [71]. Alteration of WAVE2 phosphorylation in the presence of Lrrk2-G2019S mutation induces dopaminergic cell loss in neurons co-cultures with microglia, supporting the notion that other brain cells, in addition to neurons, possibly mediate mutant LRRK2 toxicity. Along these lines, LRRK2 was found to bind and phosphorylate focal adhesion kinase (FAK), another kinase that signals upstream actin and MT cytoskeleton. Specifically, Lrrk2-G2019S transgenic microglia show reduced FAK phosphorylation at Y397 and retarded motility as well as response to brain injury [72,73].

Altogether, these findings indicate that LRRK2 regulates actin–cytoskeleton dynamics by interacting and/or phosphorylating multiple targets, which may play different roles in different cellular types but that ultimately affect neuronal homeostasis in the presence of mutant LRRK2.

LRRK2 at the crossroad between the cytoskeleton and vesicle dynamics

Although the role of LRRK2 in orchestrating vesicle dynamics is not new in the field, the recent findings showing a functional link between LRRK2 and a subset of Rab GTPases have provided a mechanistic model [71]. We will not review here the literature linking LRRK2 with organelle transport processes, for which we refer the reader to some excellent reviews [12,74,75]. Instead, we will discuss how LRRK2 may act as a bridge between vesicles and the cytoskeleton using synaptic vesicle trafficking and autophagy as paradigms.

Multiple studies showed that LRRK2 is physically and functionally associated with both synaptic vesicles and F-actin at the presynaptic site [76,77]. At the presynapse, F-actin is required for the correct development and organization of the presynaptic bouton, but it is also crucial for proper vesicle exo-endocytosis and clustering, via direct interactions with key presynaptic components such as synapsins (reviewed in ref. [78]). Thus, interaction with F-actin may grant LRRK2 access to selected presynaptic targets and influence their function through phosphorylation. Accordingly, many presynaptic proteins have been nominated as LRRK2 interactors or substrates. Among them, N-ethylmaleimide-sensitive fusion (NSF) protein, Endophilin A and auxilin are also substrates of LRRK2 kinase activity [77,79,80]. LRRK2 phosphorylation of NSF at T645 enhances NSF ATPase activity and SNARE complex disassembling rate [77], while phosphorylation of both Endophilin A and auxilin is involved in regulating the processes of endocytosis at the presynaptic bouton [30,80]. Thus, LRRK2 influences the exo–endocytic machinery by phosphorylating presynaptic components and mutant LRRK2 may impair these processes. Accumulating evidence indicates that LRRK2 also acts at the postsynaptic compartments, where it controls dopamine receptor traffic in striatal neurons [81]. One possibility is that the observed receptor trafficking defects depend on deregulated F-actin dynamics. In support of this notion, Cai and collaborators observed a substantial decrease of dendritic spines and increase of dendritic filopodia in developing spiny projecting neurons (SPNs) in Lrrk2 KO brains compared with wild type, as well as a higher proportion of thin and ‘less mushroom’ spines and increased phospho-cofilin in KO neurons within the first postnatal days [82]. Another study found that developing Lrrk2-G2019S SPNs possess larger spines and parallel larger postsynaptic activity [83]. However, the impact of LRRK2 on postsynaptic spine morphology in the adult brain is unknown at this time.

In addition of orchestrating organelle traffic at the synaptic compartments, the recent research around LRRK2 points to a clear involvement of the kinase in endolysosomal and autophagy pathways [12,17]. A specific role for LRRK2 as a negative regulator of autophagy has been proposed, since either its knock-down or the inhibition of its kinase activity promotes autophagosome formation. Of note, this phenotype is generally concomitant with an accumulation of enlarged lysosomes and is shared among LRRK2 pathological mutations (reviewed in ref. [84]), suggesting that LRRK2 likely affects the pathway at different steps, via mechanisms that are only partially resolved. To this regard, we cannot exclude that the specificity of the signaling is guaranteed through the involvement of different domains. In addition to the dual enzymatic activity, LRRK2 indeed possesses a WD40 domain that acquires particular relevance being a common motif among many autophagy proteins because of its ability to bind phosphoinositide [85]. An additional degree of complication adds up if we consider the relation of LRRK2 with the cytoskeleton. That microfilaments and MTs play fundamental roles in coordinating the delivery of membranes first and shaped vesicles later during autophagy is well acknowledged, even though the mechanics of the process have not been entirely elucidated. Despite the first indications coming from yeast of actin cytoskeleton only participating to selective forms of autophagy [86,87], studies in mammalian cells suggest that it is also required for the autophagic response to starvation [88]. Interestingly, the KO of a key autophagy gene, i.e. Atg7, in mouse causes severe defects in actin organization [89,90]. In addition, recent work in mammalian models attributes to the actin cytoskeleton the important tasks of (1) providing and supporting the membranes for the formation of the autophagosome, (2) loading the growing phagophore with the materials that need to be digested and (3) guiding, in cooperation with MTs, the shaped autophagosomes to the lysosomes throughout the cell (reviewed in refs [91,92]). In this scenario, LRRK2 might contribute to the regulation of autophagy not only through a direct action on the autophagic machinery (e.g. by phosphorylation of key autophagic components) [80], but also by remodeling the actin cytoskeleton. It is acknowledged that an actin scaffold helps shaping the autophagosome, and that the severing protein cofilin takes part in actin redistribution [93]. Thus, LRRK2 pathological mutations might lead to an aberrant activation of the PAK6/LIMK1/cofilin pathway, with defects in microfilament recruitment and/or mobilization and, in turn, consequences on autophagosome formation. In addition to this, PAK1 has been implicated in autophagy and identified as the initiator of the AKT/mTOR/ULK1 cascade [9496]. Even though PAK6 has yet to be associated to this process, we cannot exclude the involvement of the LRRK2/PAK6 axis in multiple, parallel pathways ultimately converging on the same cellular process.

Conclusions

Recent exciting evidence supports a clear link between the PD-associated kinase LRRK2 and cytoskeletal dynamics. LRRK2 may bridge vesicles and the cytoskeleton providing the scaffold for the assembly of local signaling and favor the mechanics of the movement, and/or travel from one site (vesicle) to another (cytoskeleton) in response to specific stimuli, locally influencing the activity of the machinery of organelle transport and membrane remodeling via phosphorylation of ad hoc substrates. Accordingly, the major cellular processes influenced by LRRK2 activity are critically linked to MT and actin dynamics (Figure 3). Alterations of the delicate cytoskeletal network due to LRRK2 mutations can profoundly affect neuronal homeostasis, from deregulation of neuronal process formation and growth, to impairment of intracellular vesicle dynamics, such as autophagy and synaptic vesicle trafficking. Future studies addressing the importance of these processes in the contest of brain cells and, in particular, of dopaminergic neurons will be of key importance to untangle the complexity of the molecular pathways leading to PD.

Major cellular processes related to LRRK2 function linked with cytoskeletal dynamics.

Figure 3.
Major cellular processes related to LRRK2 function linked with cytoskeletal dynamics.

LRRK2 has been associated with different intracellular processes that heavily rely on a functional cytoskeleton, including neurite outgrowth, axonal transport, trans-Golgi transport, endocytosis and phagocytosis, macroautophagy and synaptic vesicle trafficking.

Figure 3.
Major cellular processes related to LRRK2 function linked with cytoskeletal dynamics.

LRRK2 has been associated with different intracellular processes that heavily rely on a functional cytoskeleton, including neurite outgrowth, axonal transport, trans-Golgi transport, endocytosis and phagocytosis, macroautophagy and synaptic vesicle trafficking.

Abbreviations

     
  • COR

    C-terminal of ROC

  •  
  • ERM

    ezrin, radixin, moesin

  •  
  • F-actin

    filamentous actin

  •  
  • FAK

    focal adhesion kinase

  •  
  • IKKs

    IκB kinases

  •  
  • IPSC

    induced pluripotent stem cell

  •  
  • KO

    knockout

  •  
  • LIMK1

    LIM domain kinase 1

  •  
  • LRRK2

    leucine-rich repeat kinase 2

  •  
  • MAPs

    MT-associated proteins

  •  
  • MTs

    microtubules

  •  
  • NSF

    N-ethylmaleimide-sensitive fusion

  •  
  • PAK6

    P21-activated kinase 6

  •  
  • PD

    Parkinson's disease

  •  
  • ROC

    Ras of complex proteins

  •  
  • SNpc

    substantia nigra pars compacta

  •  
  • SPNs

    spiny projecting neurons

Funding

This work was supported by the Michael J Fox Foundation for Parkinson's Research and by the University of Padova [STARS Grants, LRRKing-Role of the Parkinson’s disease kinase LRRK2 in shaping neurites and synapses, funding: Euros 139848 2018 and PRID 2017 grants].

Competing Interests

The Authors declare that there are no competing interests associated with the manuscript.

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