Figure 1. LRRK2 signaling pathway.
Overview of LRRK2
Parkinson's disease (PD) is a common neurodegenerative disease and it is currently believed that the combination of environmental and genetic factors leads to the onset of PD with age. Up to now, 18PD genetic susceptibility loci have been mapped, and 13 PD pathogenic genes have been cloned. The leucine-rich repeat kinase 2 (LRRK2) gene located at the PARK8 locus is the most common causative gene of the dominant chromosomal dominant PD. How LRRK2 is involved in the pathogenesis of PD remains unclear, and it is currently believed that changes in LRRK2 kinase activity may be associated with the pathogenesis of PD. Lin et al. found that the increased expression level of LRRK2 may participate in the development of PD by affecting the function of microtubule system, Golgi and ubiquitin proteasome system. Therefore, the function and regulation mechanism of pathogenic genes are studied to elucidate the pathogenesis of PD.
LRRK2 protein is composed of seven functional domains: ANK, LRR, ROC, CDR, MAPKKK~H, and WD-40. Among them, ANK, LRR~H WD-40 are involved in protein interactions, ANKs are found in both prokaryotes and eukaryotes, and LRR can be involved in mediating GTPase and protein kinase activities. The gene mutation of the LR domain is closely related to the occurrence of Parkinson’s disease. The distribution of the LRRK2 gene in normal brain tissue is as follows: LRRK2 is widely expressed in mouse brain tissue by techniques such as in situ hybridization but shows some differences in different regions or nucleus expression. The expression levels are the highest in the cerebral cortex, hippocampus, caudate putamen and amygdala, and are expressed in the substantia nigral, thalamus and hypothalamic nuclei, and cereb Ellar Purkinje cell layer. The cerebral cortex, caudate putamen, and the substantia nigra Pukeken cells belong to the motor nervous system, and the high expression of the LRRK2 protein is found in these regions, which is consistent with the performance of the patient’s motor system dysfunction, suggesting that LRRK2 may be involved in the regulation of exercise. It has been shown that LRRK2 is a cytoplasmic protein, mainly in the cytoplasm, especially in the golgi apparatus. It can also be distributed in nerve cell dendrites, axons, and other parts. Recent studies have also shown that LRRK2 relates to lipid rafts, indicating that LRRK2 may be involved in the biosynthesis, degradation, transport and other processes of membrane phase structure. LRRK2 regulates synaptic vesicle circulation, neurite outgrowth and maintains the normal function of Golgi, enzymes because mitochondrial dysfunction may impair the survival of dopamine neurons. The LRRK2 protein is localized to the mitochondrial outer membrane, which may be involved in the regulation of mitochondrial function.
LRRK2 signaling pathway
LRRK2 signaling pathway cascade
The greatest function of the cascade of the LRRK2 signaling pathway is to affect the autophagy of the cells. The role of LLRK2 in autophagy plays a significant role in regulating the initiation process macroautophagy by acting on multiple autophagy initiation related proteins. Zhu et al. found that there is an interaction between LRRK2 and the classical autophagy initiation key molecule ULK1, and LRRK2 binds to ULK1 through its WD40 domain. LRRK2, which overexpresses WD40 domain deletion in rat renal tubular epithelial NRK cells, is unable to induce autophagy, whereas knockdown of ULK1 expression is also effective in inhibiting LRRK2-induced autophagy, indicating that an interaction between LRRK2 and ULK1 is crucial for the occurrence of autophagy. However, the specific mechanism of action between LRRK2 and ULK1 remains unclear, and whether LRRK2 functions by phosphorylating ULK1 or direct protein interactions remains to be further studied. Gomez-Suaga et al. found AMPK activity was increased by the overexpression of the wild-type LRRK2 kinase domain G2019S mutant kinase domain in HEK293 cells, and LRRK2-induced inhibition was inhibited by AMPK inhibitor compound C or overexpression of AMPK dominant negative mutation in cells. AMPK is activated by calcium-dependent protein kinase kinase β (CaMKKβ), whereas the CaMKKβ inhibitor STO-609 also significantly inhibits LRRK2-mediated autophagy. In addition, they also found that LRRK2 interacts with the nicotinic acid adenine dinucleotide phosphate receptor double-pore channel 2 (TPC2). The above results suggest that LRRK2 may act on TPC2 to lead to lysosomal Ca2+ outflow to the cytosol, cytosolic Ca2+ concentration increases, thereby activating Ca2+/CaMKKβ/AMPK, and LRRK2 can promote autophagy by activating the Ca2+/CaMKKβ/AMPK signaling pathway. Extracellular signal-regulated kinase (ERK) pathway can also induce autophagy, and inhibition of ERK activity by U0126 can effectively inhibit tumor necrosis factor-α-induced autophagy. LRRK2 regulates the development of autophagy by regulating the ERK signaling pathway. The study found that ERK1/2 Thr202/Tyr204 residues are elevated in fibroblasts from patients with G2019S mutant PD. U0126 is effective in reducing LC3 II levels in fibroblasts from G2019S mutant PD patients and inhibiting G2019S-mediated autophagy. During autophagosome formation, p62 can bind to LC3 and polyubiquitinated proteins and transport ubiquitinated proteins to autophagosomes to get involved in autophagosome formation. Park et al. found that LRRK2 interacts with p62, and knockdown of LRRK2 in primary cortical neurons results in a significant increase in phosphorylation levels of p62 Ser351 and Ser403 residues. Phosphorylation of p62 increases the binding capacity of p62 to the ubiquitin chain, which in turn may affect the binding of p62 to autophagy substrates. Therefore, it can be speculated that LRRK2 may regulate its phosphorylation by acting on p62 to regulate the formation of autophagosomes. Soukup et al. found that Endophilin A inactivation led to a significant decrease in Atg8-positive spots in synaptic cells of Drosophila motor neurons, suggesting that autophagy in synapses is dependent on Endophilin A. As a ubiquitin-like conjugated enzyme, ATG3 plays an important role in the lipidation of LC3 protein and participates in the formation of autophagosomes. LRRK2 phosphorylates the Endophilin a S75 residue, and knockout of LRRK in drosophila motoneurons significantly inhibits autophagy. LRRK may phosphorylate Endophilin A and recruit Atg3 into synaptic vesicles. Gomez Suaga et al also found that the number of intracellular alkalized lysosomes expressing the LRRK2 kinase domain was significantly increased. Further studies revealed that the nicotinic acid adenine dinucleotide phosphate antagonist NED19 inhibited LRRK2-induced lysosomal alkalization and autophagy, whichindicates that LRRK2 can also act on TPC2 leading to Ca2+ outflow in lysosomes, decreased Ca2+ concentration in lysosomes, and alkalization of lysosomes, leading to dysfunction of autophagy degradation. LRRK2 regulates autophagy degradation through interaction with Rab proteins. Dodson et al. found that LRRK interacts with Rab7, and co-expression of LRRK and Rab7 leads to a decrease in lysosomal localization in the peripancreatic cells in Drosophila follicular cells. Esteves et al also found that LRRK2 inhibitors reduced Rab7 levels in fibroblasts, suggesting that LRRK2 may negatively regulate the transport of Rab7 to lysosomes by acting on Rab7, thereby inhibiting autophagy degradation. The regulation of mitochondrial autophagy by LRRK2 may be mediated through modification of p62, B cell lymphoma/leukemia 2 protein (Bcl-2) and motor-associated protein 1 (Drp1). Su et al found that overexpression of wild-type LRRK2 or G2019S mutants in HeLa cells resulted in many p62 translocation to mitochondria, and this phenomenon was more pronounced in cells overexpressing the G2019S mutant, whereas knockdown of p62 significantly inhibited G2019S. The decrease in the number of mitochondria induced by the mutant suggests that mitochondrial hyper autophagy caused by the G2019S mutant may be mediated by p62.
The most important concern about the regulation of the LLRK2 signaling pathway in clinical is the effect of the mutant gene on the LLRK2 signaling pathway. Among them, the G2019S mutation is the most common. In some Europe and America countries, the mutation rate can reach 10%, but it is not considered to be a highly pathogenic gene in Chinese Parkinson's patients. G2019S is the exon 41 of the LRRK2 gene, and the 12020T mutation in G2019S is in the N-terminal tyrosine kinase domain. These mutations are auto phosphorylated by the increase of LRRK2 autophosphorylation level and substrate phosphorylation level, causing increased LRRK2 kinase activity or phosphorylation of the universal substrate myelin basic protein. Neuronal activity triggers the formation of intracellular asasr, which induces apoptosis through the mitochondrial pathway. It also increases the activity of phosphatase and thus affects intracellular transport, leading to axonal growth and development disorders. Increased LRRK2 kinase activity produces neurotoxicity that ultimately leads to cell death. Recent studies have shown significant changes in key protein phosphorylation of leukocyte signaling in MAPK~ and patients with abnormal protein phosphorylation mutations implicated in G2019S. G2019S mutations have a major impact on Parkinson's disease, studies suggest finding a protein downstream of G2019S that will benefit Parkinson's disease intervention. Zinprich et al. found that there are two mutations of R1441G and R1441C at the same site in the Roc domain of LRRK2 when analyzing the 6 autosomal dominant FD families. Some studies have shown that the activity of LRRK2 kinase is regulated by GTP with an intrinsic GTPase that regulates the ROC domain of LRRK2. The R1441C/G mutation will cause neuronal results and functional changes. The cytoskeleton is abnormal, which is also a prominent feature of the pathological changes of neurodegenerative diseases. G2385R is also an important gene mutation point. It has not been found to be associated with Parkinson's disease in Western case-control studies in Western countries, but significantly associated with Parkinson's disease in Taiwan and mainland China. This point mutation is a research hotspot of Asian Parkinson's disease. G2385R mutation causes mitochondrial dysfunction and apoptosis, and the LRR domain and WD-40 domain are involved in this process.
Relationship with diseases
In pathological conditions, LRRK2 causes abnormal function of macroautophagy at different stages (including autophagy induction stage and autophagy degradation stage) by acting on multiple proteins during autophagy. In addition, LRRK2 also inhibits CMA by interacting with LAMP2A. The regulation of LRRK2 on autophagy may be involved in the pathogenesis of PD, and in the further study of the mechanism of LRRK2 autophagy regulation, people will deepen their understanding of the pathogenesis of PD and provide guidance for the selection of new targets for the treatment of PD.
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