LRRK2 and human disease: a complicated question or a question of complexes?

Expression analysis of Lrrk1, Lrrk2 and Lrrk2 splice variants in mice

Missense mutations in the leucine-rich repeat kinase 2 gene (LRRK2) are linked to autosomal dominant forms of Parkinson's disease (PD). In order to get insights into the physiological role of Lrrk2, we examined the distribution of Lrrk2 mRNA and different splice variants in the developing murine embryo and the adult brain of Mus musculus. To analyse if the Lrrk2-paralog, Lrrk1, may have redundant functions in PD-development, we also compared Lrrk1 and Lrrk2 expression in the same tissues. Using radioactive in situ hybridization, we found ubiquitous expression of both genes at low level from embryonic stage E9.5 onward, which progressively increased up until birth. The developing central nervous system (CNS) displayed no prominent Lrrk2 mRNA signals at these time-points. However, in the entire postnatal brain Lrrk2 became detectable, showing strongest level in the striatum and the cortex of adult mice; Lrrk1 was only detectable in the mitral cell layer of the olfactory bulb. Thus, due to the non-overlapping expression patterns, a redundant function of Lrrk2 and Lrrk1 in the pathogenesis of PD seems to be unlikely. Quantification of Lrrk2 mRNA and protein level in several brain regions by real-time PCR and Western blot verified the striatum and cortex as hotspots of postnatal Lrrk2 expression. Strong expression of Lrrk2 is mainly found in neurons, specifically in the dopamine receptor 1 (DRD1a) and 2 (DRD2)-positive subpopulations of the striatal medium spiny neurons. Finally, we identified 2 new splice-variants of Lrrk2 in RNA-samples from various adult brain regions and organs: a variant with a skipped exon 5 and a truncated variant terminating in an alternative exon 42a. In order to identify the origin of these two splice variants, we also analysed primary neural cultures independently and found cell-specific expression patterns for these variants in microglia and astrocytes.

Evolution of neurodegeneration

A number of neurodegenerative diseases principally affect humans as they age and are characterized by the loss of specific groups of neurons in different brain regions. Although these disorders are generally sporadic, it is now clear that many of them have a substantial genetic component. As genes are the raw material with which evolution works, we might benefit from understanding these genes in an evolutionary framework. Here, I will discuss how we can understand whether evolution has shaped genes involved in neurodegeneration and the implications for practical issues, such as our choice of model systems for studying these diseases, and more theoretical concerns, such as the level of selection against these phenotypes.

Current understanding of LRRK2 in Parkinson's disease: biochemical and structural features and inhibitor design

Since leucine-rich repeat kinase 2 (LRRK2) was linked to Parkinson's disease in 2004, kinase activity of LRRK2 has been believed to play a critical role in the pathogenesis of Parkinson's disease. As a result, identification of LRRK2 inhibitors has been a focus for drug discovery. However, most LRRK2 mutations do not simply increase kinase activity. In this review we summarize the potential mechanisms that regulate the kinase activity of LRRK2. We outline some currently available kinase inhibitors, including the identification of a DFG-out (type-II) inhibitor. Finally, we discuss the relationship of LRRK2 with tau and α-synuclein. The fact that all three proteins are autophapgy-related provides a future strategy for the identification of LRRK2 physiological substrate(s).

LRRK2: cause, risk, and mechanism

In 2004 it was first shown that mutations in LRRK2 can cause Parkinson's disease. This initial discovery was quickly followed by the observation that a single particular mutation is a relatively common cause of Parkinson's disease across varied populations. Further genetic investigation has revealed a variety of genetic ties to Parkinson's disease across this gene. These include common alleles with quite broad effects on risk, likely through both alterations at the protein sequence level, and in the context of expression. A great deal of functional characterization of LRRK2 and disease-causing mutations in this protein has occurred over the last 9 years, and considerable progress has been made. Particular attention has been paid to the kinase activity of LRRK2 as a therapeutic target, and while it is no means certain that this is viable target it is likely that this hypothesis will be tested in clinical trials sooner rather than later. We believe that the future goals for LRRK2 research are, while challenging, relatively clear and that the next 10 years of research promises to be perhaps more exciting than the last.

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

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

Discovery of highly potent, selective, and brain-penetrable leucine-rich repeat kinase 2 (LRRK2) small molecule inhibitors

There is a high demand for potent, selective, and brain-penetrant small molecule inhibitors of leucine-rich repeat kinase 2 (LRRK2) to test whether inhibition of LRRK2 kinase activity is a potentially viable treatment option for Parkinson's disease patients. Herein we disclose the use of property and structure-based drug design for the optimization of highly ligand efficient aminopyrimidine lead compounds. High throughput in vivo rodent cassette pharmacokinetic studies enabled rapid validation of in vitro-in vivo correlations. Guided by this data, optimal design parameters were established. Effective incorporation of these guidelines into our molecular design process resulted in the discovery of small molecule inhibitors such as GNE-7915 (18) and 19, which possess an ideal balance of LRRK2 cellular potency, broad kinase selectivity, metabolic stability, and brain penetration across multiple species. Advancement of GNE-7915 into rodent and higher species toxicity studies enabled risk assessment for early development.

High LRRK2 levels fail to induce or exacerbate neuronal alpha-synucleinopathy in mouse brain

The G2019S mutation in the multidomain protein leucine-rich repeat kinase 2 (LRRK2) is one of the most frequently identified genetic causes of Parkinson’s disease (PD). Clinically, LRRK2(G2019S) carriers with PD and idiopathic PD patients have a very similar disease with brainstem and cortical Lewy pathology (α-synucleinopathy) as histopathological hallmarks. Some patients have Tau pathology. Enhanced kinase function of the LRRK2(G2019S) mutant protein is a prime suspect mechanism for carriers to develop PD but observations in LRRK2 knock-out, G2019S knock-in and kinase-dead mutant mice suggest that LRRK2 steady-state abundance of the protein also plays a determining role. One critical question concerning the molecular pathogenesis in LRRK2(G2019S) PD patients is whether α-synuclein (aSN) has a contributory role. To this end we generated mice with high expression of either wildtype or G2019S mutant LRRK2 in brainstem and cortical neurons. High levels of these LRRK2 variants left endogenous aSN and Tau levels unaltered and did not exacerbate or otherwise modify α-synucleinopathy in mice that co-expressed high levels of LRRK2 and aSN in brain neurons. On the contrary, in some lines high LRRK2 levels improved motor skills in the presence and absence of aSN-transgene-induced disease. Therefore, in many neurons high LRRK2 levels are well tolerated and not sufficient to drive or exacerbate neuronal α-synucleinopathy.