Miratul M. K. Muquit

Haem-Regulated Inhibitor (HRI, aka EIF2AK1) is one of four stress-sensing kinases that phosphorylate eIF2α as part of the integrated stress response (ISR). HRI was first characterized as a haem-sensing kinase where it is inhibited when bound to haem. Recent studies have revealed another role for HRI in sensing mitochondrial stress via the mitochondrial protein DELE1. Upon stress, DELE1 is cleaved, and its C-terminal fragment (DELE1 ^CTD ) is released from the mitochondria to the cytoplasm, where it interacts with HRI, to trigger the ISR. This pathway is critical for mitochondrial quality control and neuronal health. Here, we perform biophysical analysis to demonstrate that purified recombinant DELE1 ^CTD binds with high affinity to the N-terminal region of HRI, competing with and displacing haem to activate HRI and pointing to a shared binding site. Using Hydrogen Deuterium Exchange Mass Spectrometry (HDX-MS), we map the binding footprint of DELE1 ^CTD on HRI and confirm that it overlaps with the haem-binding site. These findings support a simple activation mechanism: DELE1 ^CTD activates HRI by excluding inhibitory haem from its binding site.

This protocol is intended for the extraction of DNA from the mouse tissue homogenate after Organellar IP. Following DNA extraction, a PCR for mtDNA genes is performed.

Cells have evolved organelle-specific responses to maintain protein homeostasis (proteostasis). During proteostatic stress, mitochondria down-regulate translation and enhance protein folding, yet the underlying mechanisms remain poorly defined. Here, we used cryo–electron tomography to observe the structural consequences of mitochondrial proteostatic stress within human cells. We detected protein aggregates within the mitochondrial matrix, accompanied by a marked remodeling of cristae architecture. Concomitantly, the number of mitochondrial ribosome complexes was significantly reduced. Mitochondrial Hsp60 (mHsp60), a key protein folding machine, underwent major conformational changes to favor complexes with its co-chaperone mHsp10. We visualized the interactions of mHsp60 with native substrate proteins and determined in vitro mHsp60 cryo–electron microscopy structures enabling nucleotide state assignment of the in situ structures. These data converge on a model of the mHsp60 functional cycle and its essential role in mitochondrial proteostasis. More broadly, our findings reveal structural mechanisms governing mitochondrial protein biosynthesis and their remodeling under proteostatic stress.

Cells have evolved organelle-specific responses to maintain protein homeostasis (proteostasis). During proteostatic stress, mitochondria downregulate translation and enhance protein folding, yet the underlying mechanisms remain poorly defined. Here, we employed cryo-electron tomography to observe the structural consequences of mitochondrial proteostatic stress within human cells. We detected protein aggregates within the mitochondrial matrix, accompanied by a marked remodeling of cristae architecture. Concomitantly, the number of mitochondrial ribosome complexes was significantly reduced. Mitochondrial Hsp60 (mHsp60), a key protein folding machine, underwent major conformational changes to favor complexes with its co-chaperone mHsp10. We visualized the interactions of mHsp60 with native substrate proteins, and determined in vitro mHsp60 cryo- EM structures enabling nucleotide state assignment of the in situ structures. These data converge on a model of the mHsp60 functional cycle and its essential role in mitochondrial proteostasis. More broadly, our findings reveal structural mechanisms governing mitochondrial protein biosynthesis and their remodeling under proteostatic stress.

A CRISPR/Cas9-based strategy is presented for generating U-2 OS cells with a PINK1-GFP knock-in at the N-terminus of the endogenous PINK1 locus (replacing the endogenous PINK1 gene with PINK1-GFP cDNA). The method uses Cas9 nickase and paired sgRNAs to introduce a double-stranded break in exon 1 of PINK1, enabling homology-directed repair using a donor plasmid encoding PINK1-GFP cDNA flanked by approximately 500 bp homology arms as the template. Following plasmid transfection, cells undergo a 10 day recovery period to allow depletion of transiently expressed PINK1-GFP. Mitochondrial depolarisation is then induced using antimycin and oligomycin to increase PINK1-GFP expression levels and therefore enhance GFP signal detection. GFP positive cells were isolated by fluorescence-activated cell sorting into 96-well plates using a low threshold gate to capture clones with physiologically relevant expression and were subsequently expanded and validated by immunoblotting.

Heme-regulated inhibitor (HRI) is one of the four mammalian kinases that phosphorylate eIF2α, facilitating a cellular response to stress through the regulation of mRNA translation. Originally identified as a heme sensor in erythroid progenitor cells, HRI has since emerged as a potential therapeutic target in both cancer and neurodegeneration. Here, we characterise two modes of HRI inhibition using structural mass spectrometry, biochemistry, and biophysics. We examined several competitive ATP-mimetic inhibitors – dabrafenib, encorafenib, and GCN2iB – and compared them with the heme-mimetic allosteric inhibitor, hemin. By combining hydrogen–deuterium exchange mass spectrometry with protein models generated by AlphaFold 3, we investigated the structural basis of inhibition by dabrafenib and hemin. Our analysis revealed that hemin inhibition induces large-scale structural rearrangements in HRI, which are not observed with ATP-mimetic inhibitors. Our results suggest that HRI may be inhibited using two distinctly different modalities, which may guide future drug development.

Loss-of-function mutations in the PINK1 kinase lead to early-onset Parkinson’s disease (PD). PINK1 is activated by mitochondrial damage to phosphorylate ubiquitin and Parkin, triggering mitophagy. PINK1 also indirectly phosphorylates Rab GTPases, such as Rab8A. Using an siRNA library targeting human Ser/Thr kinases in HeLa cells, we identified EIF2AK1 [heme-regulated inhibitor (HRI) kinase], a branch of the integrated stress response (ISR), as a negative regulator of PINK1. EIF2AK1 knockdown enhances mitochondrial depolarization–induced PINK1 stabilization and phosphorylation of ubiquitin and Rab8A. These results were confirmed in SK-OV-3, U2OS, and ARPE-19 cells. Knockdown of DELE1, an activator of EIF2AK1, produced similar effects. Notably, the ISR inhibitor ISRIB also enhanced PINK1 activation. In human cells with mito-QC mitophagy reporters, EIF2AK1 knockdown or ISRIB treatment increased PINK1-dependent mitophagy without affecting deferiprone-induced mitophagy. These findings suggest that the DELE1-EIF2AK1 ISR pathway is a negative regulator of PINK1-dependent mitophagy. Further evaluation in PD-relevant models is needed to assess the therapeutic potential of targeting this pathway.

We present a method for assessing the enzyme activity of lysosomal glucocerebrosidase (GBA1, GCase) in protein lysates by monitoring the hydrolysis of the fluorescent substrate Resorufin β-D-glucopyranoside. The assay is conducted at low pH, where non-lysosomal glucocerebrosidase activity is expected to be minimal. This approach is supported by the complete loss of the fluorescent signal in GBA1 knockout (KO) cells. The assay is compatible with cell and tissue lysates, as well as organellar immunoprecipitations. Given the variable expression of GBA1 across different tissues and cell types, the amount of protein used in the assay can be adjusted accordingly.

Mutations in Leucine-rich repeat kinase 2 (LRRK2) and PTEN-induced kinase 1 (PINK1) are associated with familial Parkinson’s disease (PD). LRRK2 phosphorylates Rab guanosine triphosphatase (GTPases) within the Switch II domain while PINK1 directly phosphorylates Parkin and ubiquitin (Ub) and indirectly induces phosphorylation of a subset of Rab GTPases. Herein we have crossed LRRK2 [R1441C] mutant knock-in mice with PINK1 knock-out (KO) mice and report that loss of PINK1 does not impact endogenous LRRK2-mediated Rab phosphorylation nor do we see significant effect of mutant LRRK2 on PINK1-mediated Rab and Ub phosphorylation. In addition, we observe that a pool of the Rab-specific, protein phosphatase family member 1H phosphatase, is transcriptionally up-regulated and recruited to damaged mitochondria, independent of PINK1 or LRRK2 activity. Parallel signaling of LRRK2 and PINK1 pathways is supported by assessment of motor behavioral studies that show no evidence of genetic interaction in crossed mouse lines. Previously we showed loss of cilia in LRRK2 R1441C mice and herein we show that PINK1 KO mice exhibit a ciliogenesis defect in striatal cholinergic interneurons and astrocytes that interferes with Hedgehog induction of glial derived-neurotrophic factor transcription. This is not exacerbated in double-mutant LRRK2 and PINK1 mice. Overall, our analysis indicates that LRRK2 activation and/or loss of PINK1 function along parallel pathways to impair ciliogenesis, suggesting a convergent mechanism toward PD. Our data suggest that reversal of defects downstream of ciliogenesis offers a common therapeutic strategy for LRRK2 or PINK1 PD patients, whereas LRRK2 inhibitors that are currently in clinical trials are unlikely to benefit PINK1 PD patients.

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