PerspectiveNEURODEGENERATION

LRRK2 kinase in Parkinson's disease

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Science  06 Apr 2018:
Vol. 360, Issue 6384, pp. 36-37
DOI: 10.1126/science.aar5683

Despite intensive research, attempts to pause or even just slow the progression of Parkinson's disease (PD) have thus far failed. Although most cases of PD are idiopathic and with largely unknown aetiology, mutations in ∼20 genes, including LRRK2 (leucine-rich repeat kinase 2), cause rare genetic Parkinsonism. All pathogenic mutations in LRRK2 result in hyperactivation of the LRRK2 kinase, offering the prospect of elaborating disease-modifying treatments. Indeed, LRRK2 inhibitors have entered phase 1 clinical trials. Data are also emerging for LRRK2 involvement in idiopathic PD, suggesting that inhibitors may benefit patients beyond those carrying LRRK2 mutations. Recent advances point toward a role for LRRK2 in regulating autophagy, an intracellular process that delivers cytoplasmic constituents to the lysosome for degradation and recycling. LRRK2 phosphorylates a subgroup of RAB proteins and regulates their ability to bind cognate effector proteins. Additionally, LRRK2 is highly expressed in immune cells. Intriguing research indicates that, in early life, increased LRRK2 activity may protect against opportunistic pathogenic infection but then later increases the risk of developing PD, a concept called antagonistic pleiotropy.

LRRK2 signaling pathway

Mutations that activate LRRK2 kinase activity are associated with Parkinson's disease (PD). LRRK2 phosphorylates a subgroup of RAB GTPases that regulate various cellular processes, including vesicular trafficking and immune responses. LRRK2 is a possible therapeutic target for PD.

GRAPHIC: K. SUTLIFF/SCIENCE

Autosomal dominant missense mutations within the LRRK2 gene account for 1 to 2% of all cases of PD, and a much higher proportion in some populations (Ashkenazi Jews and North African Berbers) (1). LRRK2-associated PD closely resembles idiopathic disease in terms of late age of onset, signs, and symptoms (1). The penetrance of LRRK2 mutations is incomplete and dependent on age and specific mutation. For example, most carriers of the common Gly2019→Ser (G2019S) mutation may never develop disease (penetrance as low as 24%), whereas the Arg1441→Gly (R1441G) mutation appears to be more penetrant (up to 95% in later life) (2, 3). Variations at the LRRK2 locus also mildly increase the risk for idiopathic PD (1, 3). LRRK2 is a large protein (2527 residues) that, in addition to its kinase domain, possesses a second enzymatic guanosine triphosphatase (GTPase) domain (ROC-COR domain) as well as other motifs (see the figure). The pathogenic mutations cluster within the GTPase (for example, R1441G) as well as kinase (for example, G2019S) domains and stimulate protein kinase activity. The G2019S mutation results in a moderate increase in kinase activity (about twofold). Pathogenic mutations in the GTPase domain enhance GTP binding and stimulate LRRK2 activity to a greater extent than those in the kinase domain (around fourfold) through interactions with the RAB29 protein and the Golgi apparatus (4, 5).

Much research, based on studying localization and verifying consequences of manipulating the expression of wild-type and mutant forms of LRRK2, reveals that it plays a major role in vesicular membranes, as well as autophagy and lysosome function (6). The role of LRRK2 in vesicular membranes could be mediated by the ability of LRRK2 to phosphorylate a subgroup of RAB GTPases, including RAB8A (Thr72) and RAB10 (Thr73), at highly conserved sites located in the center of the effector binding motif (7, 8). RAB proteins are master regulators of membrane trafficking, orchestrating vesicle formation and vesicle movement along actin and tubulin networks, as well as membrane docking and fusion; all important aspects of autophagy and lysosome biology. Consistent with RAB proteins comprising relevant substrates for PD, all the pathogenic LRRK2 mutants tested enhance RAB phosphorylation in vivo (7, 8).

LRRK2 phosphorylation of RAB8A and RAB10 does not affect GTPase activity but instead prevents binding to well-characterized effectors, including the guanosine diphosphate-dissociation inhibitor and the guanine nucleotide exchange factor rabin-8 (7, 8). This is likely to affect localization and the GTP-loaded state of RAB proteins. Recent work revealed that the poorly studied proteins RAB-interacting lysosomal-like protein 1 (RILPL1) and RILPL2, previously implicated in regulating ciliary membrane content, specifically bind to RAB8A and RAB10 once they have been phosphorylated by LRRK2 (8). Interestingly, pathogenic LRRK2 mutations inhibit primary cilia formation induced by serum starvation (8), but it remains to be established whether this is mediated through LRRK2-phosphorylated RAB8A-RAB10 interaction with RILPL1 and RILPL2.

There is mounting evidence that disruption of RAB biology and membrane trafficking is involved in PD. Loss-of-function mutations in RAB39B and complex genetic changes within the Parkinson disease 16 (PARK16) locus encompassing RAB29, as well as mutations in the genes encoding at least three other components of the membrane trafficking machinery [vacuolar protein sorting-associated protein 35 (VPS35), VPS13C, and DnaJ homolog subfamily C member 6] are causally linked to PD (3, 4). Loss-of-function mutations in PTEN-induced putative kinase 1 (PINK1) are associated with young-onset recessive PD. PINK1 indirectly controls the phosphorylation of certain RAB GTPases, including RAB8A, at a site (Ser111) distinct from that of LRRK2-mediated phosphorylation (9). Furthermore, various RAB proteins, including LRRK2 substrates RAB3A and RAB8A, promote vesicular trafficking from the endoplasmic reticulum to the Golgi apparatus and thereby attenuate cytotoxicity linked to aggregated α-synuclein, a factor that is frequently associated with PD (10).

Inflammation plays an important role in the development of PD. LRRK2 is highly expressed in macrophages, monocytes, and neutrophils, suggesting that it functions in the defense against intracellular pathogens. In humans, single-nucleotide polymorphisms within or close to the LRRK2 gene have been linked to inflammatory conditions, including ulcerative colitis and Crohn's disease, and also increased susceptibility to leprosy infection (11). In mice, LRRK2 is required for mucosal immunity against the opportunistic pathogen Listeria monocytogenes and colocalizes with intracellular Salmonella typhimurium during bacterial infection in macrophages (11). The LRRK2 kinase is most closely related to the receptor-interacting protein (RIP) kinases, which are key regulators of inflammasomes. Inflammasomes are multiprotein signaling complexes that assemble upon stimulation of the innate immune system by pathogens or toxins and play a crucial role in the inflammatory response and host defense (12). LRRK2 may participate in modulating inflammasome assembly. Mice lacking LRRK2 or treated with an LRRK2 inhibitor exhibit an impaired ability to clear S. typhimurium infection (13). By contrast, mice expressing the kinase-activating G2019S LRRK2 mutant are protected from infection (13). Moreover, in response to S. typhimurium infection, LRRK2 interacts with and reportedly phosphorylates the inflammasome protein NLRC4 (NLR family CARD domain-containing protein 4) at a critical residue (Ser533) required for its assembly into inflammasomes (13). This leads to the activation of caspase 1, which processes and produces a variety of proinflammatory agents such as interleukin-1β, that if sustained over a long time could lead to neuroinflammation and increased PD susceptibility. Therefore, current understanding suggests that activation of LRRK2 by mutation promotes proinflammatory responses offering protection against infection and survival benefit earlier in life, before PD strikes.

Highly potent, selective, and brain penetrant LRRK2 inhibitors have been reported. Such drugs could benefit not only individuals bearing LRRK2 mutations but also other patients in whom LRRK2 activity is driving the disease. Much research is taking place to develop tests to interrogate LRRK2 activity and function in patients. This includes studying LRRK2 autophosphorylation and RAB protein phosphorylation in neutrophils, monocytes, and cerebrospinal fluid. Several pharmaceutical companies are in late stages of preclinical evaluation of LRRK2 inhibitors, and one has recently completed a phase 1 clinical trial in healthy volunteers. Studies in LRRK2-deficient rodents and nonhuman primates treated with LRRK2 inhibitors highlighted potential toxicity concerns in lung and kidney, resulting from loss of LRRK2 kinase activity that might be linked with altered autophagy and lysosome biology (14, 15). Therefore, lung and kidney function will need to be closely monitored in LRRK2 inhibitor clinical trials. Given the challenges of translating findings in immunity from mice to humans, LRRK2 inhibitor clinical trials present an opportunity for refining our understanding of LRRK2 in human immune function. Above all, it will be important to establish whether LRRK2 inhibition increases the risk of opportunistic infections.

LRRK2 lies at the nexus of an emerging signaling network of high relevance for understanding and developing treatments for PD. It will be essential to define in clearer detail the upstream and downstream components of this pathway. Understanding the interplay between LRRK2-mediated immune defense mechanisms and PD will undoubtedly reveal fascinating biology. Reports of monozygotic LRRK2 mutation-carrying twins discordant for PD and the incomplete penetrance of LRRK2 mutations highlight the importance of environment (for example, exposure to toxins, herbicides, pesticides, and fungicides), lifestyle (for example, smoking, exercise, diet), gut microbiome, and infection in the development of LRRK2-dependent PD. It is possible that these factors synergize with genetic mutations. In the future, aligning mechanistic insight into disease pathways with genetics and clinical phenotyping could define the roles that LRRK2 signaling plays in idiopathic PD. This will help justify the use of LRRK2 inhibitors in more patients. However, for now, the most exciting question will be whether LRRK2 inhibitors have disease-modifying effects in PD patients with LRRK2 mutations.

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