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. 2014 Nov 20;21(11):1585-96.
doi: 10.1016/j.chembiol.2014.09.019.

PMI: a ΔΨm independent pharmacological regulator of mitophagy

Daniel A East, Francesca Fagiani, James Crosby, Nikolaos D Georgakopoulos, Hélène Bertrand, Marjolein Schaap, Adrian Fowkes, Geoff Wells, Michelangelo Campanella

PMI: a ΔΨm independent pharmacological regulator of mitophagy

Daniel A East et al. Chem Biol. .

Abstract

Mitophagy is central to mitochondrial and cellular homeostasis and operates via the PINK1/Parkin pathway _targeting mitochondria devoid of membrane potential (ΔΨm) to autophagosomes. Although mitophagy is recognized as a fundamental cellular process, selective pharmacologic modulators of mitophagy are almost nonexistent. We developed a compound that increases the expression and signaling of the autophagic adaptor molecule P62/SQSTM1 and forces mitochondria into autophagy. The compound, P62-mediated mitophagy inducer (PMI), activates mitophagy without recruiting Parkin or collapsing ΔΨm and retains activity in cells devoid of a fully functional PINK1/Parkin pathway. PMI drives mitochondria to a process of quality control without compromising the bio-energetic competence of the whole network while exposing just those organelles to be recycled. Thus, PMI circumvents the toxicity and some of the nonspecific effects associated with the abrupt dissipation of ΔΨm by ionophores routinely used to induce mitophagy and represents a prototype pharmacological tool to investigate the molecular mechanisms of mitophagy.

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Figures

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Graphical abstract
Figure 1
Figure 1
PMI Stabilizes Nrf2 and Upregulates P62 Expression Activating Mitophagy (A) Structures of compounds 1 (sulforaphane) and 2 (PMI); CD is the concentration of compound causing a doubling of the control level of NQO1 enzymatic activity. (B) Western blot to show induction of Nrf2-dependent gene products versus time in Hepa1c1c7 cells (cytoplasmic) exposed to 10 μM PMI. (C) Induction of NQO1 (NAD(P)H dependent quinone oxidoreductase-1) by compounds 1 (○) and 2 (●). (D) Western blots to demonstrate Nrf2 stabilization in cells treated with either PMI (10 μM) or, sulforaphane (1 μM) versus time (E) RT-PCR analysis for estimation of p62 mRNA levels in MEFs following treatment with PMI versus time. Values are presented as arbitrary units normalized to 18 s RNA levels for each sample, n ≥ 3. (F) Western blot to demonstrate P62 expression in MEF cells treated with DMSO vehicle control, 10 μM PMI, or 1 μM sulforaphane for 24 hr. Β-actin is shown as a loading control. (G) Graph shows P62:β-actin ratio band density analysis, n = 3. (H) Representative confocal images of β-subunit staining to highlight mitochondrial density in MEF cells treated with DMSO vehicle control or 10 μM PMI for 24 hr. (I) Graph shows average mitochondrial area as a percentage of cell size, n ≥ 50. (J) Western blot to demonstrate reduction in MTCO1 levels following 4 hr FCCP or 24 hr PMI exposure. (K) Graph shows MTCO1:Tubulin ratio band density analysis normalized to control, n = 3. All values are mean ± SEM, ∗p < 0.05, ∗∗p < 0.01, ∗∗∗p < 0.001.
Figure 2
Figure 2
PMI Drives LC3 Mitochondrial Recruitment (A and B) Western blot highlighting increased LC3-II in the mitochondrial fraction of WT MEFs treated with FCCP (A) and PMI quantified in (B); n = 3. (C and D) Western blot highlighting no increase in LC3-II in the mitochondrial fraction of p62−/−MEFs treated with FCCP (C) and PMI quantified in (D) n = 3. (E) Representative images of LC3 localization in MEF cells treated with DMSO vehicle control or 10 μM PMI for 24 hr, before and after treatment with FCCP (20 μM) for 4 hr. Scale bar represents 10 μm. A magnification of the merge images is shown in areas demarcated by the white box. (F) Quantification of the degree of LC3:β-subunit colocalization in MEF cells. All values are mean ± SEM, ∗p < 0.05, ∗∗p < 0.01, ∗∗∗p < 0.001.
Figure 3
Figure 3
PMI-Induced Mitochondrial Recruitment of P62 Is Parkin Independent (A) Representative images of P62 localization in MEF cells treated with DMSO vehicle control or 10 μM PMI for 24 hr, before and after treatment with FCCP (20 μM) for 4 hr. Scale bar represents 10 μm. A magnification of the merge images is shown in areas demarcated by the white box. Scale bar represents 10 μm. (B) Quantification of the degree of p62:β-subunit: colocalization in MEF cells, n > 30. (C) Representative images of Parkin localization in MEF cells treated with DMSO vehicle control or 10 μM PMI for 24 hr, before and after treatment with FCCP (20 μM) for 4 hr. Scale bar represents 10 μm. A magnification of the merge images is shown in areas demarcated by the white box. Scale bar represents 1 μm. (D) Quantification of the degree of parkin:β-subunit colocalization in MEF cells, n > 30. All values are mean ± SEM, ∗∗p < 0.01, ∗∗∗p < 0.001.
Figure 4
Figure 4
In Parkin Knockdown Cells, PMI Promotes LC3 Mitochondrial Accumulation (A) Representative images of LC3 localization in WT and Parkin knockdown MEF cells treated with DMSO vehicle control or 10 μM PMI for 24 hr. Scale bar represents 10 μm. (B) A magnification of the merged images is shown in areas demarcated by the white box. Quantification of mitochondrial LC3 localization in WT and Parkin knockdown MEF cells treated with DMSO vehicle control or 10 μM PMI for 24 hr, n > 30. All values are mean ± SEM, ∗∗p < 0.01.
Figure 5
Figure 5
In PINK1 Knockout Cells, LC3 Is Recruited to Mitochondria by PMI (A) Representative images of LC3 localization in WT and Pink1 knockout SH-SY5Y treated with DMSO vehicle control or 10 μM PMI for 24 hr. Scale bar represents 10 μm. A magnification of the merge images is shown in areas demarcated by the white box. (B) Quantification of mitochondrial LC3 localization in WT and Pink1 knockout SH-SY5Y cells treated with DMSO vehicle control or 10 μM PMI for 24 hr, n > 30. All values are mean ± SEM, ∗∗p < 0.01.
Figure 6
Figure 6
PMI Mediates Poly-Ubiquitination of Mitochondria and Increases ΔΨm (A) Western blot of mitochondrial fractions, highlighting mitochondrial ubiquitination in control or PMI-treated MEF cells before and after treatment with 20 mM FCCP. (B) The graph shows total ubiquitin band density analysis relative to β-subunit loading control, n = 3. (C) Representative confocal images depicting differences in ΔΨm when in MEF cells treated with DMSO vehicle control or 10 μM PMI for 24 hr and loaded with the cationic mitochondria selective probe TMRM (red) for 30 min. Scale bar represents 10 μm. (D and E) Mean basal TMRM fluorescence quantification n > 10 (D) and shows representative traces of the effects of FCCP on mitochondrial membrane potential in treated with DMSO vehicle control or 10 μM PMI for 24 hr (E). (F) Values of cytosolic ROS accumulation data collected in MEF cells by recording the rate of nuclear uptake of the O2− sensitive dye, dihydroethidium (DHE), n > 30. (G) Mitochondrial ROS generation collected in MEF cells by recording fluorescence intensity of O2− sensitive, mitochondrial-specific dye, MitoSOX, n > 20. All values are mean ± SEM, ∗p < 0.05 ∗∗p < 0.01.
Figure 7
Figure 7
The Proposed Working Model for PMI (A) In healthy WT cells, a proportion of mitochondria will be destined for destruction due to age, damage, or dysfunction. Parkin-mediated ubiquitination (orange spheres) primes these mitochondria to enter the autophagic pathway and available P62 links them to LC3 (green spheres) and the growing autophagosome. (B) In PMI-treated cells, P62 is more abundant therefore able to drive mitochondria into autophagy with increased efficiency. (C) In cells where Parkin expression is reduced, a reduced number of mitochondria are primed for autophagy, so although P62 is overexpressed, the efficiency of mitophagy is reduced. (D) Finally, in cells devoid of PINK1, Parkin is not recruited to mitochondria; however, redundant/alternative ubiquitin ligases may still be capable of ubiquitinating mitochondria, which can then be driven into autophagy by P62, albeit with reduced efficiency.
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