The complex relationships between microglia, alpha-synuclein, and LRRK2 in Parkinson’s disease

Abstract

The proteins alpha-synuclein (αSyn) and LRRK2 are both key players in the pathogenesis of the neurodegenerative disorder Parkinson’s disease (PD), but establishing a functional link between the two proteins has proven elusive. Research studies for these two proteins have traditionally and justifiably focused in neuronal cells, but recent studies indicate that each protein could play a greater pathological role elsewhere. αSyn is expressed at high levels within neurons, but they also secrete the protein into the extracellular milieu, where it can have broad ranging effects in the nervous system and relevance to disease etiology. Similarly, low neuronal LRRK2 expression and activity suggests that LRRK2-related functions could be more relevant in cells with higher expression, such as brain-resident microglia. Microglia are monocytic immune cells that protect neurons from noxious stimuli, including pathological αSyn species, and microglial activation is believed to contribute to neuroinflammation and neuronal death in PD. Interestingly, both αSyn and LRRK2 can be linked to microglial function. Secreted αSyn can directly activate microglia, and can be taken up by microglia for clearance, while LRRK2 has been implicated in the intrinsic regulation of microglial activation and of lysosomal degradation processes. Based on these observations, the present review will focus on how PD-associated mutations in LRRK2 could potentially alter microglial biology with respect to neuronally-secreted αSyn, resulting in cell dysfunction and neurodegeneration.

Compromised MAPK signaling in human diseases: an update

“…The symptoms of this disease include resting tremors, postural abnormalities, and akinesia due to the loss of dopaminergic neurons (Gandhi and Wood 2005). Many genes, including those for α-synuclein, LRRK2, DJ-1, PINK1, Parkin, and HtrA2 (Omi), have been associated with PD in recent years (Gautier et al 2014;Schapansky et al 2014) (Fig. 2e).…”

Role of the JAK/STAT signaling pathway in regulation of innate immunity in neuroinflammatory diseases

“…Leucine-rich repeat kinase 2 (LRRK2) is the most commonly mutated gene in idiopathic and familial PD [124,125]. LRRK2 expression is enriched in immune cells, especially in B cells, monocytes and DCs [126].…”

Ligands of Receptor for Advanced Glycation End-Products Produced by Activated Microglia are Critical in Neurodegenerative Diseases

“…Interestingly, in PD, secreted α-synuclein can activate microglia directly and LRRK2 has been related with the regulation of microglia activation in PD [62].…”