Humanin Inactivates Pro‐Apoptotic BCL‐2 Proteins Through a Common Fibril‐Forming Mechanism

Abstract

Neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's are associated with accumulation of amyloid plaques or amyloid-like complexes that foster cellular dysregulation. For example, neurons with amyloid inclusions often undergo premature apoptosis resulting from excessive expression of p38 mitogen-activated protein kinase. The increased abundance of this enzyme leads to mass phosphorylation and activation of a protein from the B-cell lymphoma 2 (BCL-2) family, BAX. BAX is the central regulatory protein for mitochondrial outer membrane permeabilization (MOMP), a poration process that commits cells to apoptosis by releasing death-propagating factors from the mitochondria. Thus, it may be possible to prevent premature neuronal apoptosis by identifying endogenous factors that decrease the MOMP capacity of BAX and other pro-apoptotic BCL-2 proteins. Humanin (HN) is a mitochondria-derived peptide that is released by the organelle in response to several stress stimuli. In the cytosol or on the mitochondrial membrane HN can counteract the death signal by forming specific complexes with some of the BCL-2 proteins. We recently reported the in vitro amyloid-like fibrillation of BAX and a second pro-apoptotic family member, BID, with HN. We proposed this as a novel anti-apoptotic mechanism that inhibits pro-apoptotic BCL-2 proteins from initiating MOMP by sequestering them into fibrils, a heretofore unprecedented phenomenon that involves refolding globular BCL-2 proteins into fibrils where they undergo significant alpha-helix to beta-sheet fold-switching. These fibrils represent a new direction for the field of cell death and elucidating mechanisms of the BCL-2 family. The existence of this fold-switching mechanism and fibrillation insists that at least some pro-apoptotic members of the BCL-2 family have unappreciated, functional beta-sheet conformations. We hypothesize that in the membrane-bound conformations these pro-apoptotic BCL-2 proteins might adopt beta-sheet structures. Perhaps their fold-switching dynamics may play a role in the pore-forming process in the mitochondrial membrane, altering its membrane potential and leading up to the release of death factors into the cytosol. Therapeutics that can affect BCL-2 protein propensity for either secondary structure state may prove to be fruitful toward ameliorating disease states where BCL-2 family activity has become misregulated.