Apoptosis regulation at mitochondria : insights into the roles of CKMT1 and IkBα in apoptosis inhibition

Abstract

The point of no return in the intrinsic apoptosis pathway is the loss of outer mitochondrial membrane integrity. Protein complexes, such as the PT-pore, have been implicated in mediating mitochondrial disintegration via the induction of mitochondrial permeability transition (MPT). My study provides evidence that the mitochondrial creatine kinase CKMT1 is a key regulator of MPT through a complex distinct from the “classical” PT-pore. Overexpression of CKMT1 confers resistance against mitochondrial depolarization and apoptosis – an effect that is independent of its kinase activity. Furthermore, CKMT1 is a functionally necessary gatekeeper of MPT, as its depletion universally induces mitochondrial depolarization and apoptotic cell death in transformed and primary cells. This cannot be abrogated by the supplementation with phosphocreatine, the end product of the enzymatic reaction catalysed by CKMT1, but can efficiently be inhibited by bongkrekic acid – a pharmacological agent inhibiting mitochondrial membrane channels. My studies highlight that CKMT1 determines cellular fate and thus could be a promising therapeutic target as altered CKMT1 expression has been described in tumours and neurodegenerative disease. The second part of this thesis describes the intricate challenges of CKMT1 RNAi validation techniques and offers a detailed examination of the human CKMT1 locus. The third part addresses the effect of mitochondrial IκΒα in apoptosis. I reveal a fundamentally new function of the NF-κB target gene IκΒα, which is distinct from its well-established NF-κB inhibition. It potently represses apoptosis when targeted specifically to the outer mitochondrial membrane where its N-terminus interacts with and its anti-apoptotic property depends on VDAC1. Therefore, IκΒα regulates the intricate balance of cell survival and cell death via contributing to canonical NF-κB signalling and the “classical” PT-pore, respectively. Considering that NF-κB dysregulation is a major culprit in disease formation, the biological implications of my results could be crucial in understanding and targeting the underlying pathological mechanisms.