The R1441C-LRRK2 mutation induces myeloid immune cell exhaustion in an age- and sex-dependent manner

Considering age is the greatest risk factor for many neurodegenerative diseases, aging, in particular aging of the immune system, is the most underappreciated and understudied contributing factor in the neurodegeneration field. Genetic variation around the LRRK2 gene affects risk of both familial and sporadic Parkinson's disease (PD). The leucine-rich repeat kinase 2 (LRRK2) protein has been implicated in peripheral immune signaling, however, the effects of an aging immune system on LRRK2 function have been neglected to be considered. We demonstrate here that the R1441C mutation induces a hyper-responsive phenotype in macrophages from young female mice, characterized by increased effector functions, including stimulation-dependent antigen presentation, cytokine release, phagocytosis, and lysosomal function. This is followed by age-acquired immune cell exhaustion in a Lrrk2-kinase-dependent manner. Immune-exhausted macrophages exhibit suppressed antigen presentation and hypophagocytosis, which is also demonstrated in myeloid cells from R1441C and Y1699C-PD patients. Our novel findings that LRRK2 mutations confer immunological advantage at a young age but may predispose the carrier to age-acquired immune exhaustion have significant implications for LRRK2 biology and therapeutic development. Indeed, LRRK2 has become an appealing target in PD, but our findings suggest that more research is required to understand the cell-type specific consequences and optimal timing of LRRK2-targeting therapeutics.

APOE and immunity: Research highlights

INTRODUCTION

At the Alzheimer's Association's APOE and Immunity virtual conference, held in October 2021, leading neuroscience experts shared recent research advances on and inspiring insights into the various roles that both the apolipoprotein E gene (APOE) and facets of immunity play in neurodegenerative diseases, including Alzheimer's disease and other dementias.

METHODS

The meeting brought together more than 1200 registered attendees from 62 different countries, representing the realms of academia and industry.

RESULTS

During the 4-day meeting, presenters illuminated aspects of the cross-talk between APOE and immunity, with a focus on the roles of microglia, triggering receptor expressed on myeloid cells 2 (TREM2), and components of inflammation (e.g., tumor necrosis factor α [TNFα]).

DISCUSSION

This manuscript emphasizes the importance of diversity in current and future research and presents an integrated view of innate immune functions in Alzheimer's disease as well as related promising directions in drug development.

Disease mechanisms as subtypes: Lysosomal dysfunction in the endolysosomal Parkinson's disease subtype

Parkinson's disease (PD) remains one of the most prevalent neurodegenerative disorders. It has become increasingly recognized that PD is not one disease but a constellation of many, with distinct cellular mechanisms driving pathology and neuronal loss in each given subtype. Endolysosomal trafficking and lysosomal degradation are crucial to maintain neuronal homeostasis and vesicular trafficking. It is clear that deficits in endolysosomal signaling data support the existence of an endolysosomal PD subtype. This chapter describes how cellular pathways involved in endolysosomal vesicular trafficking and lysosomal degradation in neurons and immune cells can contribute to PD. Last, as inflammatory processes including phagocytosis and cytokine release are central in glia-neuron interactions, a spotlight on the role of neuroinflammation plays in the pathogenesis of this PD subtype is also explored.

Gene therapy restores dopamine transporter expression and ameliorates pathology in iPSC and mouse models of infantile parkinsonism

Most inherited neurodegenerative disorders are incurable, and often only palliative treatment is available. Precision medicine has great potential to address this unmet clinical need. We explored this paradigm in dopamine transporter deficiency syndrome (DTDS), caused by biallelic loss-of-function mutations in SLC6A3, encoding the dopamine transporter (DAT). Patients present with early infantile hyperkinesia, severe progressive childhood parkinsonism, and raised cerebrospinal fluid dopamine metabolites. The absence of effective treatments and relentless disease course frequently leads to death in childhood. Using patient-derived induced pluripotent stem cells (iPSCs), we generated a midbrain dopaminergic (mDA) neuron model of DTDS that exhibited marked impairment of DAT activity, apoptotic neurodegeneration associated with TNFα-mediated inflammation, and dopamine toxicity. Partial restoration of DAT activity by the pharmacochaperone pifithrin-μ was mutation-specific. In contrast, lentiviral gene transfer of wild-type human SLC6A3 complementary DNA restored DAT activity and prevented neurodegeneration in all patient-derived mDA lines. To progress toward clinical translation, we used the knockout mouse model of DTDS that recapitulates human disease, exhibiting parkinsonism features, including tremor, bradykinesia, and premature death. Neonatal intracerebroventricular injection of human SLC6A3 using an adeno-associated virus (AAV) vector provided neuronal expression of human DAT, which ameliorated motor phenotype, life span, and neuronal survival in the substantia nigra and striatum, although off-target neurotoxic effects were seen at higher dosage. These were avoided with stereotactic delivery of AAV2.SLC6A3 gene therapy targeted to the midbrain of adult knockout mice, which rescued both motor phenotype and neurodegeneration, suggesting that targeted AAV gene therapy might be effective for patients with DTDS.

LRRK2 interacts with the vacuolar-type H+-ATPase pump a1 subunit to regulate lysosomal function

Lysosomal dysfunction lies at the centre of the cellular mechanisms underlying Parkinson’s disease although the precise underlying mechanisms remain unknown. We investigated the role of leucine-rich repeat kinase 2 (LRRK2) on lysosome biology and the autophagy pathway in primary neurons expressing the human LRRK2-G2019S or LRKK2-R1441C mutant or the human wild-type (hWT-LRRK2) genomic locus. The expression of LRRK2-G2019S or hWT-LRRK2 inhibited autophagosome production, whereas LRRK2-R1441C induced a decrease in autophagosome/lysosome fusion and increased lysosomal pH. In vivo data from the cortex and substantia nigra pars compacta of aged LRRK2 transgenic animals revealed alterations in autophagosome puncta number reflecting those phenotypes seen in vitro. Using the two selective and potent LRRK2 kinase inhibitors, MLi-2 and PF-06447475, we demonstrated that the LRRK2-R1441C-mediated decrease in autolysosome maturation is not dependent on LRRK2 kinase activity. We showed that hWT-LRRK2 and LRRK2-G2019S bind to the a1 subunit of vATPase, which is abolished by the LRRK2-R1441C mutation, leading to a decrease in a1 protein and cellular mislocalization. Modulation of lysosomal zinc increased vATPase a1 protein levels and rescued the LRRK2-R1441C-mediated cellular phenotypes. Our work defines a novel interaction between the LRRK2 protein and the vATPase a1 subunit and demonstrates a mode of action by which drugs may rescue lysosomal dysfunction. These results demonstrate the importance of LRRK2 in lysosomal biology, as well as the critical role of the lysosome in PD.

LRRK2 BAC transgenic rats develop progressive, L-DOPA-responsive motor impairment, and deficits in dopamine circuit function

Mutations in leucine-rich repeat kinase 2 (LRRK2) lead to late-onset, autosomal dominant Parkinson's disease, characterized by the degeneration of dopamine neurons of the substantia nigra pars compacta, a deficit in dopamine neurotransmission and the development of motor and non-motor symptoms. The most prevalent Parkinson's disease LRRK2 mutations are located in the kinase (G2019S) and GTPase (R1441C) encoding domains of LRRK2. To better understand the sequence of events that lead to progressive neurophysiological deficits in vulnerable neurons and circuits in Parkinson's disease, we have generated LRRK2 bacterial artificial chromosome transgenic rats expressing either G2019S or R1441C mutant, or wild-type LRRK2, from the complete human LRRK2 genomic locus, including endogenous promoter and regulatory regions. Aged (18-21 months) G2019S and R1441C mutant transgenic rats exhibit L-DOPA-responsive motor dysfunction, impaired striatal dopamine release as determined by fast-scan cyclic voltammetry, and cognitive deficits. In addition, in vivo recordings of identified substantia nigra pars compacta dopamine neurons in R1441C LRRK2 transgenic rats reveal an age-dependent reduction in burst firing, which likely results in further reductions to striatal dopamine release. These alterations to dopamine circuit function occur in the absence of neurodegeneration or abnormal protein accumulation within the substantia nigra pars compacta, suggesting that nigrostriatal dopamine dysfunction precedes detectable protein aggregation and cell death in the development of Parkinson's disease. In conclusion, our longitudinal deep-phenotyping provides novel insights into how the genetic burden arising from human mutant LRRK2 manifests as early pathophysiological changes to dopamine circuit function and highlights a potential model for testing Parkinson's therapeutics.

Cellular processes associated with LRRK2 function and dysfunction

Mutations in the leucine-rich repeat kinase 2 (LRRK2)-encoding gene are the most common cause of monogenic Parkinson's disease. The identification of LRRK2 polymorphisms associated with increased risk for sporadic Parkinson's disease, as well as the observation that LRRK2-Parkinson's disease has a pathological phenotype that is almost indistinguishable from the sporadic form of disease, suggested LRRK2 as the culprit to provide understanding for both familial and sporadic Parkinson's disease cases. LRRK2 is a large protein with both GTPase and kinase functions. Mutations segregating with Parkinson's disease reside within the enzymatic core of LRRK2, suggesting that modification of its activity impacts greatly on disease onset and progression. Although progress has been made since its discovery in 2004, there is still much to be understood regarding LRRK2's physiological and neurotoxic properties. Unsurprisingly, given the presence of multiple enzymatic domains, LRRK2 has been associated with a diverse set of cellular functions and signalling pathways including mitochondrial function, vesicle trafficking together with endocytosis, retromer complex modulation and autophagy. This review discusses the state of current knowledge on the role of LRRK2 in health and disease with discussion of potential substrates of phosphorylation and functional partners with particular emphasis on signalling mechanisms. In addition, the use of immune cells in LRRK2 research and the role of oxidative stress as a regulator of LRRK2 activity and cellular function are also discussed.