α-synucleinopathy associated calcium overload and autophagy failure is regulated by gain-of-function of Tousled-like kinase

Abstract

As a pathological hallmark in Parkinson’s disease (PD), α-synucleinopathy causes multiple cellular damages, including calcium overload, mitochondrial and autophagic dysfunction, and eventually dopamine neuron death. However, the hierarchy of these detrimental events is unclear. In Drosophila, we confirmed that overexpression of α-synuclein could induce all these cytotoxic events. To determine the specific cytotoxic events induced by calcium overload, we established a calcium overload model in Drosophila and performed genetic screens. We found that calcium overload caused mitochondrial damage and autophagy failure and cell death, and these cytotoxic processes could be strongly rescued by loss of Tousled-like kinase (TLK). Interestingly, loss of TLK also rescued defects induced by α-synuclein overexpression in Drosophila. This suggests that calcium overload acts as the crucial event upstream of mitochondrial and autophagy dysfunction. For TLK regulation of autophagy, our data indicated that a transcriptional factor REPTOR, which regulated the expression of several lysosomal genes, functioned downstream of TLK. In mammalian cells and mice, TLK2 (the homolog of Drosophila TLK) was phosphorylated under calcium overload. Upon phosphorylation, TLK2 increased its kinase activity. In addition, TLK2 could phosphorylate CREBRF (the human homolog of REPTOR) to cause its loss of transcription on the lysosomal genes. Moreover, TLK2 knockout mice rescued multi-aspect cytotoxicity induced by calcium overload and α-synuclein overexpression. Our research demonstrates that TLK2 acts as a key regulator to mediate cell death and dysfunctions of mitochondria and autophagy downstream of calcium overload.

Genetic modifiers of synucleinopathies—lessons from experimental models

“…This may be through its modulation of voltage-gated Ca 2+ channels, intracellular Ca 2+ channels, formation of Ca 2+ permeable pores, disruption in lipid membrane composition and packing, even ER or mitochondrial dysfunction [ 252 ]. As mentioned above, calcium overload due to α-syn overexpression leads to dysfunctions in mitochondria and autophagy, resulting in cell death, likely by acting through TLK2 [ 253 ]. Besides this, calcium was reported to bind and activate calcineurin and calmodulin, which dephosphorylates NFATC4, which then localizes to the nucleus to activate a toxic program [ 254 ].…”