Forskolin blocks PINK1 and Parkin accumulation
To validate the results from Akabane et al.’s research, we tested whether an adenylate cyclase activator, Forskolin, reduces PINK1 and Parkin accumulation caused by CCCP. CCCP is a mitochondrial uncoupling agent inducing PINK1 stabilization and subsequent Parkin accumulation on mitochondria. As they demonstrated, acute Forskolin (100uM) treatment almost completely blocked Parkin accumulation induced by CCCP in HeLa cells stably expressing Parkin-YFP (Fig. 1A-B). As expected, we also successfully detected that Forskolin treatment blocked PINK1 accumulation significantly (Fig. 1C-D). These observations led us to examine the effects of Forskolin on CHCHD10S59L-induced mitochondrial toxicity.
Forskolin and Roflumilast mitigate the toxicity of CHCHD10 S59L in cell models.
When we treated Forskolin to HeLa cells transiently transfected with CHCHD10S59L, mitochondrial branch length increased significantly compared to that of DMSO-treated cells (Fig. 1E). Forskolin did not increase mitochondrial branch length in CHCHD10WT or empty vector transfected groups(Fig. 1E). Since Forskolin increases cyclic AMP levels by activating adenylate cyclases, we investigated whether phosphodiesterase inhibitors (PDE inhibitors) could induce the same effect by preventing cAMP degradation. First, we chose Roflumilast, a selective PDE4 inhibitor, that was approved and marketed for chronic obstructive pulmonary disease (COPD)(4) (5). When we treated Roflumilast (1 µM or 10 µM for 24hrs) to CHCHD10S59L-expressing HeLa cells, mitochondrial fragmentation caused by CHCHD10S59L was significantly reduced while aggregation of CHCHD10S59L was not affected (Fig. 1F,G,H).
Forskolin and Roflumilast reduce PINK1 accumulation caused by CHCHD10 S59L in HeLaPINK1−V5 cells.
We previously reported that CHCHD10S59L induced PINK1 accumulation on mitochondria and reduction of PINK1 expression or activity was beneficial to mitochondria morphologically and functionally(2). Therefore, we examined whether Forskolin and Roflumilast reduced CHCHD10S59L-induced PINK1 acculumation in HeLa cells. For this purpose, we used HeLa cells stably expressing V5 tagged PINK1 (HeLaPINK1−V5 cells)(6). When CHCHD10S59L was transfected and expressed, more than 80% of cells showed PINK1 accumulation on mitochondria and the PINK1 aggregation-positive cells were significantly decreased by Forskolin and Roflumilast treatment (Fig. 2A-B). Acute treatment of 50 µM Forskolin and 24 hour-treatment of 20 µM Forskolin showed similar effects (Fig. 2B). However, we did not observe significant dose-dependent effects of Roflumilast when it was treated with 1 µM or 10 µM concentrations in this experiment (Fig. 2B).
Forskolin and Roflumilast reduce Parkin accumulation caused by CHCHD10 S59L in HeLaParkin−YFP cells.
Mitochondria-localized PINK1 phosphorylates ubiquitins and the ubiquitin-like domain of Parkin, resulting in Parkin accumulation on mitochondria. Therefore, we investigated whether Forskolin and Roflumilast reduced Parkin acculumation in HeLa cells via reducing CHCHD10S59L-induced PINK1 accumulation. We used HeLa cells stably expressing yellow fluorescent protein (YFP)-tagged Parkin. When CHCHD10S59L was transfected and expressed, about 70% of cells had Parkin-YFP punctae colocalized with mitochondria (Fig. 2C). Although acute 50 µM Forskolin treatment reduced Parkin-YFP accumulation, 24 hour-treatment of 20 µM Forskolin treatment showed a more significant decrease of Parkin accumulation on mitochondria (Fig. 2D). Interestingly, a relatively low dose of Roflumilast (1 µM) significantly reduced Parkin-YFP accumulation more than 10 µM Roflumilast (Fig. 2D). This may be due to the dynamic regulation of cytosolic cAMP levels by cAMP-activated PKA and PKA-activated phosphodiesterase(7). Consistently, the seahorse analysis revealed that Forskolin and/or Roflumilast treatment increased basal mitochondrial respiration and ATP production with increased maximal respiration (Fig. 2E). The spare capacity was also increased minimally and not significantly in Roflumilast treated samples (Fig. 2E). The reduction of Parkin accumulation was also observed in a neuroblastoma cell line, SH-SY5Y, with transiently transfected mCherry-Parkin (Suppl Fig S1 A-B).
Forskolin and Roflumilast reduced LC3 accumulation on mitochondria.
The PINK1/Parkin pathway regulates mitophagy(8). We have demonstrated that CHCHD10S59L expression led to increased LC3 accumulation and subsequent mitophagy(2). Thus, we investigated whether Forskolin and Roflumilast could reduce LC3 accumulation caused by CHCHD10S59L. We observed that almost 80% of the CHCHD10S59L transfected HeLaYFP−Parkincells showed LC3 accumulation and Forskolin and Roflumilast treatment reduced the LC3 accumulation significantly (Fig. 3A-B).
Forskolin and Roflumilast ameliorated mitochondrial defects induced by C2C10H S81L in Drosophila.
Finally, we investigated the effect of Forskolin and Roflumilast treatment on mitochondrial defects in Drosophila expressing C2C10HS81L. For this, we expressed C2C10HS81L in muscle tissues with the MHC-GAL4 driver. The C2C10HS81L flies were reared in fly food containing Forskolin (50 uM), Roflumilast (40 uM), and DMSO. Indirect flight muscle sections of C2C10HS81L flies stained with a fluorophore conjugated streptavidin (green) and phalloidin (red) revealed fragmented mitochondria along with muscular degeneration (Fig. 3C, DMSO). This condition was ameliorated significantly in Forskolin and Roflumilast treated flies (Fig. 3C). ATP levels were measured as an indicator of mitochondrial dysfunction in the fly thoraxes which primarily consists of muscle tissues. We observed significantly improved ATP levels in the muscle tissues of flies expressing C2C10HS81L treated with Forskolin and Roflumilast, as compared to DMSO-fed C2C10HS81L flies (Fig. 3D). Therefore, Forskolin and Roflumilast imparted a rescue effect on the mitochondrial defects induced by toxic C2C10HS81L.
Other PDE4 inhibitors, Apremilast and Rolipram, are also effective.
To further validate the potential of PDE4 inhibitors against CHCHD10S59Lmediated toxicity, we also tested the first generation PDE4 inhibitor, Rolipram(9, 10), and another approved drug, Apremilast, which is prescribed for plaque psoriasis or psoriatic arthritis(9, 11, 12). Both Apremilast and Rolipram reduced Parkin-YFP accumulation in HeLaYFP−Parkin cells transiently transfected with CHCHD10S59L (Fig. 4A). 1 µM Apremilast reduced Parkin-YFP accumulation to a comparable level with 20µM Roflumilast. However, Rolipram only showed marginal efficacy though it was statistically significant in multiple independently repeated experiments (Fig. 4B). The seahorse analysis also verified that both Apremilast and Rolipram reduced the toxicity of CHCHD10S59L while Apremilast was more effective than Rolipram in rescuing defective mitochondrial function caused by CHCHD10S59L (Fig. 4C, S2 A-C). A similar trend in mitochondrial respiration was also reflected in the seahorse assay using SHSY5Y neuroblastoma cells (Fig S1 C).