Structure of MPTP Causes and consequences of MPTP opening Effectors of MPTP
In 1976, Hunter and Haworth first characterized the mitochondrial permeability transition in beef heart mitochondria. In 1987, Martin Crompton et al. implied that the pore is a unique molecular entity that allows the passage of any molecule of <1500daltons across the inner mitochondrial membrane.
Mitochondrial permeability transition pore (MPTP) is a non-specific pore which spans the inner and outer membrane. Two hypotheses about MPTP structure Models of MPTP structure
The first: Three core components: VDAC (voltage-dependent anion channel) ANT (adenine nucleotide translocator) CypD (cyclophilin D) exhibits peptidyl-prolyl cis-trans isomerase(PIPase) activity.
Two conformations of ANT According to ATP/ADP-binding site m-conformation: on the matrix side of the inner membrane c-conformation: on the cytoplasmic side
The second: Misfolding and clustering of mitochondrial membrane proteins and regulated by chaperones.
MPTP has two open conductance modes: Regulated and unregulated Regulated MPTP opening requires several inducers and the loading of Ca 2+ into matrix space and is inhibited by Ca 2+ chelators, Mg 2+,and cyclosporin A (CsA). Types of MPTP
Unregulated MPTP opening occurs with high concentration of various inducers. It is Ca 2+ -independent and insensitive to Ca 2+ chelators, Mg 2+,and CsA. Types of MPTP
Opening of regulated and unregulated PT pores by HgCl 2. After 2-3 min pre-incubation, 5 micromol/L HgCl 2 plus 50 micromol/LCaCl 2 (A), 20 micromol/L HgCl 2 (B)were added in rat liver mitochondria (0.5 mg protein/ml). From the beginning of the incubations, 1micromol/L CsA (traces b), 0.5 millimol/L EGTA (traces c), or 5 millimol/L MgCl 2 (traces d) was present or no additions were made (traces a).
Consquences of MPTP opening mitochondria swell and outer membrane rupture protein release (eg. Cytochrome c) MPTP opening small molecular across MPTP freely mitochondria swell and outer membrane rupture proton enter into mitochondria proton translocate ATPase hydrolyze ATP
Effectors of MPTP There are many effectors influencing MPTP. They bond to various sites.
Effect via change in CyP-D binding to the ANT Effect via change in nucleotide binding to the ANT Direct effect on Ca 2+ binding to the ANT Activator Oxidative stress Increased matrix volume Inhibitor CsA and some analogues SfA (inhibits PPIase activity of CyP-D but not binding) Oxidative stress ‘‘c’’ conformation of ANT (atractyloside) Adenine nucleotide depletion High matrix [P i ] and [PP i ] Depolarization Increase membrane potential “m’’conformation of ANT (bongkrekic acid) High pH Low pH Mg 2+, Mn 2+, Sr 2+, Ba 2+
Proposed scheme for the mechanism of pore opening ATP ADP Cytosol Matrix Normal Impermeable State Adenine nucleotide translocase Cyclosporin A Activated by oxidative stress which decrease ADP/ATP binding Inhibited by [Mg 2+ ], low pH, adenine nucleotides Ca Triggered by low [Ca 2+] Pathological Non-specific Pore Ca 2+ Binding increased by oxidative stress. Cyp-D binding increases sensitivity to [Ca 2+ ]. ATP ADP Impermeable State Cyclophilin D Sanglifehrin A is a novel immuno- suppressant that binds to CyP-D and inhibits its PPIase activity SfA X
MPTP opens during reperfusion but not ischemia In ischemia phase, many factors which can induce MPTP opening is developed, but the pH is low(<7), MPTP will not open Upon reperfusion, mitochondria are again able to respire and generate a membrane potential to drive ATP synthesis,pH can recover, while inducers are also present, so MPTP opens.