Fig. 3

Systematic analysis of key factors in the mitochondria-associated innate immunity pathway, using acute starvation of human SH-SY5Y neuroblastoma cells to reveal the transcriptional regulation of stress responses and their dependence on PINK1. Transcript levels were documented in control non-target knock-down (NT-KD) versus PINK1-knock-down (PINK1-KD) cells for the following inflammatory factors. a Summary scheme on the detection of pathogen DNA/RNA in the cytosol and the mitochondria-associated triggering of innate immunity, as altered during starvation by PINK1. Viral RNA (ssRNA or dsRNA) is recognized in the cytosol by helicase RIG-I and MDA5. This triggers the dimerization of MAVS in the outer mitochondrial membrane (OMM) which recruits transactivators (such as TRADD, TRAF, IKK family) leading to nuclear translocation of phosphorylated IRF3, IRF7, and NF-κB and promoting the transcription of IFN stimulated genes and pro-inflammatory cytokines, respectively. Effector molecules such as IFIT1, IFIT2, IFIT3, and RSAD2 (viperin) inhibit virus DNA/RNA replication. IFIT3 also functions as a scaffold to facilitate interaction between MAVS and TBK1 and represents a positive feedback of DDX58 (RIG-I) signaling through MAVS. Low ΔΨm or decreased ROS inhibit MAVS-mediated signaling. Mitochondria cooperate with the endoplasmic reticulum (ER) to regulate lipid synthesis and antiviral signaling at the mitochondria-associated membranes (MAM), possibly by interactions of MAVS/MFN1 with TMEM173 (STING). MFN1 leads to the redistribution of MAVS along mitochondria and a fusion of the mitochondrial network that promotes the interaction between MAVS and STING. Low energy induces the localization of PINK1 to the OMM and recruitment of PARKIN from the cytosol, which is the signal for dysfunctional mitochondria to be digested in the autophagosome. Abbreviations: 5′ ppp 5′ triphosphate, ΔΨm mitochondrial membrane potential, IκB inhibitor of κ light polypeptide gene enhancer in B cells, IKK IκB Kinase, LC3II phosphatidylethanolamine conjugate of the autophagy-related protein LC3 (MAP1LC3), MDA5 melanoma differentiation-associated gene 5, NF-κB nuclear factor κB, p62 sequestosome-1, adaptor between polyubiquitinated substrates and autophagic machinery, PARKIN ubiquitin ligase, its loss-of-function leads to the PARK2 variant of Parkinson’s disease, RIG-1 (DDX58) retinoic acid-inducible gene-1 (DExD/H-Box RNA Helicase 58), ROS reactive oxygen species, ssRNA/dsRNA single-stranded/double-stranded RNA, TMEM173 (STING) transmembrane Protein 173, TRADD tumor necrosis factor receptor type 1 associated death domain protein, TRAF TNF-receptor-associated factor. b PINK1 (PTEN induced kinase 1) as a known determinant of selective mitophagy and autosomal recessive Parkinson’s disease; HPRT1 (Hypoxanthine Phosphoribosyltransferase 1) as a loading control. c RSAD2 (= viperin, radical S-adenosyl methionine domain containing 2) as an interferon-inducible lipid-droplet associated virus inhibitory factor. d HEBP1 (heme binding protein 1) that contains a natural ligand for formyl peptide receptor-like receptor 2. e TBK1 (TANK-binding kinase 1) that phosphorylates interferon regulatory factors in response to toll-like receptor activation. f IRF3 (interferon regulatory factor 3) as a regulator of type I Interferon gene transcription. g MFN1 (mitofusin 1) as PARKIN-dependent factor in mitochondrial dynamics and mitochondria-associated anti-microbial signaling. h IFIT3 (interferon-induced protein with tetratricopeptide repeats 3) as a detector of pathogen DNA/RNA. i IFIT1 (interferon-induced protein with tetratricopeptide repeats 3) as a detector of pathogen DNA/RNA. j MAVS (mitochondria-associated viral sensor) as inducer of interferon-dependent long-term expression of defense factors. k–m The levels of known autophagy factors during this starvation time course exhibited progressive consumption of p62 in spite of its transcriptional induction, in parallel to a PINK1-modulated transcriptional induction of LRRK2. Four independent experiments compared their expression during a nutrient and trophic deprivation time course triggered by a culture switch from RPMI growth medium to HBSS starvation medium. The bar graphs show mean and standard error of the mean, illustrating the significance with asterisks (*p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001)