Pathogenic LRRK2 control of primary cilia and Hedgehog signaling in neurons and astrocytes of mouse brain

Abstract

Activating LRRK2 mutations cause Parkinson's disease, and pathogenic LRRK2 kinase interferes with ciliogenesis. Previously, we showed that cholinergic interneurons of the dorsal striatum lose their cilia in R1441C LRRK2 mutant mice (Dhekne et al., 2018). Here, we show that cilia loss is seen as early as 10 weeks of age in these mice and also in two other mouse strains carrying the most common human G2019S LRRK2 mutation. Loss of the PPM1H phosphatase that is specific for LRRK2-phosphorylated Rab GTPases yields the same cilia loss phenotype seen in mice expressing pathogenic LRRK2 kinase, strongly supporting a connection between Rab GTPase phosphorylation and cilia loss. Moreover, astrocytes throughout the striatum show a ciliation defect in all LRRK2 and PPM1H mutant models examined. Hedgehog signaling requires cilia, and loss of cilia in LRRK2 mutant rodents correlates with dysregulation of Hedgehog signaling as monitored by in situ hybridization of Gli1 and Gdnf transcripts. Dopaminergic neurons of the substantia nigra secrete a Hedgehog signal that is sensed in the striatum to trigger neuroprotection; our data support a model in which LRRK2 and PPM1H mutant mice show altered responses to critical Hedgehog signals in the nigrostriatal pathway.

LRRK2 at Striatal Synapses: Cell-Type Specificity and Mechanistic Insights

“…In mice carrying a germline R1441C LRRK2 mutation, CINs, but not other types of striatal neurons, lack primary cilia [ 64 ]. Phosphorylation of the LRRK2 substrate Rab10 impairs primary cilia formation [ 64 , 65 ]. Among other functions, primary cilia serve as specialized structures for the sonic hedgehog (Shh) signaling reception.…”

“…Among other functions, primary cilia serve as specialized structures for the sonic hedgehog (Shh) signaling reception. Shh released from dopaminergic and striatal neurons stimulates the release of trophic factors that provide essential support to dopaminergic axons [ 65 , 66 ]. Thus, the maintenance of primary cilia represents a functional role for LRRK2 that is both specific to cholinergic neurons and mechanistically linked to the integrity of dopaminergic axons.…”

“…The interaction between LRRK2 and Rab5 at postsynaptic sites may be similar to these two proteins at endocytic vesicles in axon terminals. Meanwhile, LRRK2 phosphorylation of Rab8 has been studied mechanistically and functionally in the context of ciliogenesis, where the phosphorylation of Rab8 or Rab10 at a conserved residue regulates their binding with the effector proteins RILPL1 and RILPL2, and thereby the formation and maintenance of cilia [ 64 , 65 , 114 ]. Presumably, these, or other effector proteins, could interact with Rab8 to regulate the delivery of AMPAR receptors into the synapse, an attractive potential explanation for the abnormalities at glutamatergic synapses observed in LRRK2 mutant models.…”

“…Meanwhile, PPM1H also dephosphorylates Rab7a, a substrate of LRRK1, but not LRRK2 [ 122 ]. As evidence for a functional role of PPM1H antagonizing LRRK2 phosphatase activity, a PPM1H knockout increases the endogenous phosphorylation of Rabs in cultured lung epithelial cells, and phenocopies the ciliary deficit caused by LRRK2 mutation in cultured murine embryonic fibroblasts as well as in vivo in striatal CINs and astrocytes [ 65 , 120 ]. Differential expression or activity of PPM1H across striatal cell types has not been investigated in detail.…”

The NRF2-Dependent Transcriptional Regulation of Antioxidant Defense Pathways: Relevance for Cell Type-Specific Vulnerability to Neurodegeneration and Therapeutic Intervention

“…LRRK2 G2019S astrocytes show an impaired internalization and clearance of α-synuclein, potentially through the loss-of-function of annexin A2 [ 211 ], the depletion of calcium in the endoplasmic reticulum, and mitochondrial dysfunction [ 212 ]. In vivo, LRRK2 mutations cause a loss of primary cilia in cholinergic neurons and astrocytes [ 213 ] and a decrease in astrocyte numbers in the striatum [ 214 ]. Interestingly, LRRK2 inhibition is capable of rescuing inflammatory and lysosomal defects in astrocytes derived from mice expressing the D409V mutant of GBA [ 215 ].…”