PD has a complex etiology, underscored by multifaceted interactions between genetics and environment on the background of aging. Among the more than a dozen genes linked to Mendelian inherited forms of PD, mutations in the gene encoding the protein leucine-rich repeat kinase 2 (LRRK2) are associated with autosomal dominant risk for PD2. Moreover, RAB29 is one of five genes identified within the PARK16 locus that has been linked to human PD2,3.
Often, a lack of ideal research tools and models are limiting factors in testing new scientific concepts. When scientists must make their own research tools, it inefficiently expends time and resources and constrains replicability, quality control, and wide access to the tools. Through its research tools program, The Michael J. Fox Foundation for Parkinson's Research (MJFF) addresses this by partnering with field-wide experts to identify what tools are missing from the toolbox and working with qualified vendors to produce and distribute the tools without restrictions to access.
In its continued efforts to facilitate access to improved preclinical models for parkinsonism, MJFF partnered with Taconic Biosciences to generate a new genetically engineered mouse model for PD, culminating in the recent launch of the Rab29 Overexpression Mouse model (Taconic Model #16552).
Taconic Launches a Rab29 Overexpression Mouse Model for Parkinson's DiseaseThe ideal preclinical model for Parkinson's disease (PD) would meet the following criteria:
- Displays insidious multi-domain symptomology emergence (both within the central nervous system and peripheral compartments) exacerbated by aging
- Presents a progressive pathophysiology
- Has the ability to recapitulate the stereotyped motor deficits of the human condition, in a manner that is reversible by exogenous dopamine
- Involves pathology within the dopaminergic neuron cell bodies and neural terminals of the nigrostriatal system as well as within non-nigral neural nuclei
LRRK2 in familial and idiopathic PDAccounting for approximately 5% of familial PD cases, LRRK2 represents one of the most commonly mutated genes linked to heritable PD and is also observed to be mutated in approximately 1% of sporadic PD patients4. Due to its linkage with genetic cases of PD as well as its association with sporadic PD and combined with its ubiquitous expression and putative kinase function, LRRK2 has emerged as an attractive candidate for disease-modifying therapeutic development for PD5.
Structure and Function of LRRK2LRRK2 is a large, multi-domain protein kinase consisting of an armadillo repeat domain, an akyrin domain, leucine-rich repeats, a ROC-type GTPase domain that closely resembles a Rab GTPase and is associated with a COR domain, a serine/threonine protein kinase domain, and a WD40 repeat-containing domain6. The most common pathogenic mutation in LRRK2 — G2019S — lies within the catalytic domain and increases LRRK2 kinase activity7-9.
Taconic offers two MJFF-sponsored LRRK2 G2019S models: a LRRK2 G2019S KI Mouse model with targeted insertion of the mouse LRRK2 gene harboring the G2019S mutation and a human LRRK2 G2019S Rat model, with random insertion of the human LRRK2 gene into the Sprague Dawley® rat.
Functionally, LRRK2 has been implicated in numerous pathways in PD, including inflammation and immune system dysfunction10,11, aberrant autophagosome, endosome, and lysosome trafficking/function12, and synapse development, maturation, and function13, among others.
Identification of Rab Proteins as LRRK2 SubstratesWhile the toxic gain of function by increased LRRK2 kinase activity downstream of the G2019S mutation was appreciated for over a decade, bona fide, endogenous substrates of LRRK2's kinase activity long-eluded the PD research community. In the last couple of years however, phosphoproteomic analyses revealed a subset of Rab GTPases regulated by LRRK214. Ten Rab proteins were further shown to be endogenous substrates of LRRK2 and/or regulators of LRRK2 signaling, including Rab8a, Rab10, Rab12, Rab29, and Rab3514,15. Notably, Rab29 is also known as Rab7L1. More information on Rab29 nomenclature can be found at Rab29 GeneCard, UniProt, and Gene - NCBI.
Physiological and Pathophysiological Roles of Rab29Rab GTPases are master regulators of membrane trafficking. In the majority of cases reported to date, Rab GTPase phosphorylation is predicted to impede Rab protein function; however, the discovery of LRRK2 as a protein exhibiting preferential binding to phosphorylated Rabs and subsequent increased LRRK2 activity suggests that LRRK2-mediated pathology may be attributed to more complex Rab protein interactions than previously appreciated16.
Since its discovery and validation as a substrate of LRRK2 kinase activity, Rab29 has emerged as a master regulator of LRRK2 signaling and has been implicated in the following cellular functions/pathways:
Implications for Rab29-related LRRK2 Signaling in PDMutations that prevent LRRK2 from interacting with either Rab29 or GTP robustly inhibit LRRK2 phosphorylation at a number of important sites (including Ser910, Ser935, Ser955, and Ser97315), many of which are markers of toxic LRRK2 function25,26 or are associated with pathogenic LRRK2 mutations27.
Moreover, by nature of its regulation of LRRK2 activity, Rab29 is potentially implicated by association with aberrant LRRK2-related functions in PD, including defective ciliogenesis and centromere function, axonal/neurite morphology, Golgi and lysosomal homeostasis, etc., but the specific role of Rab29 in some of these LRRK2-related pathways may not yet be established in the published literature. As the discovery of the Rab29-LRRK2 axis was only recently described, many gaps in Rab29-mediated homeostasis and Parkinson's disease-associated pathogenic signaling remain to be explored. Research tools like the Rab29 Overexpression Mouse model will be instrumental in such studies.
The Rab29 Toolbox for PD ResearchersTaconic's launch of the PD-relevant Rab29 Overexpression Mouse is part of a growing collection of preclinical research tools in the Rab29 space. In addition to supporting the development of this Rab29 Overexpression Mouse now available at Taconic, MJFF — in partnership with Abcam Inc., and in collaboration with the Alessi Lab at the University of Dundee — also sponsored the generation and commercial distribution of custom-made rabbit monoclonal antibodies to detect total and phosphorylated versions of Rab proteins, including Rab29. These antibodies are available at Abcam and have been tested and validated in multiple applications (e.g., immunoprecipitation, immunoblotting, and immunocytochemistry) for detecting Rab protein expression in diverse cells and tissues28. Moreover, anti-Rab10 antibodies have been successfully deployed to detect total and phosphorylated Rab10 in human neutrophils29, tangibly demonstrating the conserved expression and function of LRRK2-relevant Rab proteins across mouse and human phylogeny and the tantalizing potential for development of relevant LRRK2-Rab biomarker assays. However, the assessment of pRab29 in human leukocytes has not yet been reported in the published literature.
Prospective Rab29 Research — Role in PD Novel Diagnostics and TherapiesNotably, LRRK2 kinase activity is aberrantly increased in dopamine neurons in postmortem human brain tissue from idiopathic PD cases30. This further supports the notion that LRRK2 kinase inhibitors and potentially inhibitors of Rab29 (due to its role in regulation of LRRK2 activity) may be useful therapeutics for idiopathic PD patients as well as PD patients harboring pathogenic LRRK2 mutations.
Collectively, LRRK2-related Rab signaling is an exciting and emerging new direction in Parkinson's disease research and therapeutic development. Early stage clinical trials testing LRRK2 kinase inhibitors in PD patients are already underway. Further understanding of Rab29-regulated LRRK2 kinase activation in clinically relevant paradigms will likely be instrumental in accelerating drug and biomarker discovery for PD; in addition to providing insight into LRRK2-mediated pathology in PD, Rab29-mediated LRRK2 signaling may also be amenable for leverage as a diagnostic biomarker or for therapeutic engagement in LRRK2 mutation-driven or idiopathic versions of PD.
Rab29 is a very new player in the PD landscape and much remains to be studied in this nacent sub-field. Taconic's Rab29 Overexpression Mouse model is a new and important tool in the toolbox for this emerging area of Parkinson's disease research.