Loss of parkin causes endoplasmic reticulum calcium dyshomeostasis by upregulation of reticulocalbin 1

Increasing evidence suggests that astrocytes play an important role in the progression of Parkinson's disease (PD). Previous studies on our parkin knockout mouse demonstrated a higher accumulation of damaged mitochondria in astrocytes than in surrounding dopaminergic (DA) neurons, suggesting that Parkin plays a crucial role regarding their interaction during PD pathogenesis. In the current study, we examined primary mesencephalic astrocytes and neurons in a direct co‐culture system and discovered that the parkin deletion causes an impaired differentiation of mesencephalic neurons. This effect required the parkin mutation in astrocytes as well as in neurons. In Valinomycin‐treated parkin‐deficient astrocytes, ubiquitination of Mitofusin 2 was abolished, whereas there was no significant degradation of the outer mitochondrial membrane protein Tom70. This result may explain the accumulation of damaged mitochondria in parkin‐deficient astrocytes. We examined differential gene expression in the substantia nigra region of our parkin‐KO mouse by RNA sequencing and identified an upregulation of the endoplasmic reticulum (ER) Ca2+‐binding protein reticulocalbin 1 (RCN1) expression, which was validated using qPCR. Immunostaining of the SN brain region revealed RCN1 expression mainly in astrocytes. Our subcellular fractionation of brain extract has shown that RCN1 is located in the ER and in mitochondria‐associated membranes (MAM). Moreover, a loss of Parkin function reduced ATP‐stimulated calcium‐release in ER mesencephalic astrocytes that could be attenuated by siRNA‐mediated RCN1 knockdown. Our results indicate that RCN1 plays an important role in ER‐associated calcium dyshomeostasis caused by the loss of Parkin function in mesencephalic astrocytes, thereby highlighting the relevance of astrocyte function in PD pathomechanisms. The establishment of the C57/BL6 congenic parkin KO mouse strain (Padel) without the neomycin‐resistance‐gene enabled a precise genome‐wide RNA‐sequencing of the substantia nigra, leading to the identification of increased RCN1 expression, which acts as an endoplasmic reticulum Ca2+‐binding protein. RCN1 upregulation causes a decrease in ER‐mediated calcium release. Padel astrocytes exhibit an increase in ER–mitochondria contact sites, where RCN1 could serve as an important mediator of ER–mitochondria communication, parallel to (Mitofusin 2) Mfn2.