Putative roles of mitochondrial Voltage-Dependent Anion Channel, Bcl-2 family proteins and c-Jun N-terminal Kinases in ischemic stroke associated apoptosis

Abstract

There is a constant need for better stroke treatments. Neurons at the periphery of an ischemic stroke affected brain tissue remains metabolically active for several hours or days after stroke onset. They later undergo mitochondrion-mediated apoptosis. It has been found that inhibiting apoptosis in the peripheral ischemic neurons could be very effective in the prevention of stroke progression. During stroke associated apoptosis, cytosolic c-Jun N-terminal Kinases (JNKs) and Bcl-2 family proteins translocate towards mitochondria and promote cytochrome c release by interacting with the outer mitochondrion membrane associated proteins. This review provides an overview of the plausible interactions of the outer mitochondrial membrane Voltage Dependent Anion Channel, Bcl-2 family proteins and JNKs in cytochrome c release in the peripheral ischemic stroke associated apoptotic neurons. The review ends with a note on designing new anti-stroke treatments.

Keywords: Bif-1, Bax interacting factor-1; Ischemic penumbra; JNKs, c-Jun N-terminal Kinases; MPT pore, Mitochondrial Permeability Transition pore; Mitochondrion-mediated apoptosis; VDAC, Voltage Dependent Anion Channel; Voltage-Dependent Anion Channel; c-Jun N-terminal Kinases.

Fig. 1

Schematic representation of the brain tissue suffering from ischemia. Ischemic tissue also known as ischemic infarct has two regions a. Ischemic core in the centre and b. Ischemic penumbra at the periphery surrounding the core. Ischemic infarct involves cell death by necrosis in the core of the tissue and apoptosis in the penumbra. Up to certain time after stroke onset the neurons in the penumbra region remain metabolically active and after that they undergo death by apoptosis.

Fig. 2

Schematic representation of the mitochondrion in the normal and in the apoptotic cells present in the ischemic ‘penumbra’ region. Three different models have been proposed for the release of cytochrome c during penumbral cell apoptosis. First model is that during apoptosis mitochondrial permeability transition (MPT) pores are formed through which cytochrome c passes out into the cytosol. The exact composition of MPT pore is unknown till date. VDAC at the outer mitochondrial membrane, adenine nucleotide translocator (ANT) protein present at the inner mitochondrial membrane, inorganic phosphate carrier (PHC) of the inner mitochondrial membrane and cyclophilin-D (CYPD) protein present in the mitochondrial matrix were earlier believed to be essential molecular components of MPT pore but knock down experiments have shown that MPT is possible without them. Mitochondrial F₁-Fo-ATP synthase, in particular the c subunit of the Fo domain, and SPG7, an integral protein of the inner mitochondrial membrane with metalloprotease activity have been recently identified as MPT components. In particular, F₁-Fo-ATPase dimers have been proposed to constitute the long-sought pore-forming component of the MPT pore. Bcl-2 family proteins, mitochondrial creatinine kinase 1 (CKMT1), hexokinase isoforms 1, 2 (HXK1, HXK2), glycogen synthase kinase 3β (GSK3β), p53 protein, peripheral benzodiazepine receptor (PBR) also known as translocator protein (TSPO) and protein kinase C epsilon isoform (PRKCε) have been shown to modulate the activity of the MPT pore. Second model is that during penumbral cell apoptosis VDAC might form homo- or hetero-oliomers. VDAC1 monomers and lower-order homo-oligomers (dimers, trimers, tetramers) form higher-order homo-oligomers (hexamers, octamers) through inter-channel contacts which act as cytochrome c conducting channel or there is association of Bax and truncated Bid (t-Bid) pro-apoptotic proteins with mitochondrial VDAC and formation of Bax-tBid-VDAC complexes/hetero-oligomers on the outer mitochondrial membrane. Formation of large pore size Bax-tBid-VDAC complexes allows passage of cytochrome c into the cytosol through them. Third model is that VDAC closes down during apoptosis. In the normal cell VDAC transports ions, adenine nucleotides and energy related metabolites across the mitochondria and cytochrome c remains bound to the inner mitochondrial membrane. It is proposed that during ischemic stroke associated penumbral cell apoptosis, there might be closure of VDAC by different mechanisms such that all the transport through VDAC is blocked. Blockage of VDAC leads to accumulation of ions and water in the mitochondrial matrix leading to matrix swelling, rupture of the outer mitochondrial membrane and cytochrome c release.

Fig. 3

Schematic representation showing the plausible interaction of c-Jun N-terminal Kinases (JNKs) with mitochondrial VDAC in penumbral neurons undergoing apoptosis. In the normal cell VDAC transports ions, adenine nucleotides and energy related metabolites across the mitochondria and cytochrome c remains bound to the inner mitochondrial membrane. Upon induction of apoptosis, JNKs translocate towards mitochondria and it is proposed that they phosphorylate VDAC. Phosphorylation of VDAC by JNKs leads to partial closure of VDAC such that ion, nucleotide and metabolite transport through VDAC is blocked. Blockage of VDAC leads to accumulation of ions and water in the mitochondrial matrix leading to swelling, rupture of the outer mitochondrial membrane and cytochrome c release. Cytochrome c activates caspase proteases in the cytosol dismantling the cells.

Fig. 4

Schematic representation showing the plausible roles of BCl₂ family proteins in penumbral cell apoptosis. Upon induction of apoptosis Bax gets activated by phosphorylation by c-Jun N-terminal Kinases (JNKs) or by interaction with other proteins like t-Bid, ΔN BCl-xL or Bax interacting factor (Bif-1) leading to the formation of homo or hetero-oligomers. At the mitochondrial outer membrane these oligomers form pores through which cytochrome c leaks out into the cytosol and activates caspases leading to cell demise.