Humanin Structural Versatility and Interaction with Model Cerebral Cortex Membranes

Mitochondrial-derived peptides: Antidiabetic functions and evolutionary perspectives

Mitochondrial-derived peptides (MDPs) are a novel class of bioactive microproteins encoded by short open-reading frames (sORF) in mitochondrial DNA (mtDNA). Currently, three types of MDPs have been identified: Humanin (HN), MOTS-c (Mitochondrial ORF within Twelve S rRNA type-c), and SHLP1-6 (small Humanin-like peptide, 1 to 6). The 12 S ribosomal RNA (MT-RNR1) gene harbors the sequence for MOTS-c, whereas HN and SHLP1-6 are encoded by the 16 S ribosomal RNA (MT-RNR2) gene. Special genetic codes are used in mtDNA as compared to nuclear DNA: (i) ATA and ATT are used as start codons in addition to the standard start codon ATG; (ii) AGA and AGG are used as stop codons instead of coding for arginine; (iii) the standard stop codon UGA is used to code for tryptophan. While HN, SHLP6, and MOTS-c are encoded by the H (heavy owing to high guanine + thymine base composition)-strand of the mtDNA, SHLP1-5 are encoded by the L (light owing to less guanine + thymine base composition)-strand. MDPs attenuate disease pathology including Type 1 diabetes (T1D), Type 2 diabetes (T2D), gestational diabetes, Alzheimer's disease (AD), cardiovascular diseases, prostate cancer, and macular degeneration. The current review will focus on the MDP regulation of T2D, T1D, and gestational diabetes along with an emphasis on the evolutionary pressures for conservation of the amino acid sequences of MDPs.

Inducible fold-switching as a mechanism to fibrillate pro-apoptotic BCL-2 proteins

Neurodegenerative diseases often are associated with cellular dysregulation that results in premature cell death or apoptosis. A common example is the accumulation of amyloid plaques that promotes the 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. Recent reports identify a naturally occurring peptide, Humanin (HN), that could block amyloid-beta-associated neuronal apoptosis by interacting with BCL-2 proteins. We recently showed humanin interaction leads to the amyloid-like fibrillation of BAX and a second BCL-2 family member, BID. 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 rapidly into fibrils where they undergo significant alpha-helix to beta-sheet fold-switching. Here we seek to further characterize the fibrillation and fold-switch in conditions that are known to induce amyloid fibrillation.

Mitochondrial-Derived Peptides in Diabetes and Its Complications

The changes of mitochondrial function are closely related to diabetes and its complications. Here we describe the effects of mitochondrial-derived peptides (MDPs), short peptides formed by transcription and translation of the open reading frame site in human mitochondrial DNA (mtDNA), on diabetes and its complications. We mainly focus on MDPs that have been discovered so far, such as Humanin (HN), mitochondrial open reading frame of the 12S rRNA-c (MOTS-c) and Small humanin-like peptides (SHLP 1-6), and elucidated the role of MDPs in diabetes and its major complications stroke and myocardial infarction by improving insulin resistance, inhibiting inflammatory response and anti-apoptosis. It provides more possibilities for the clinical application of mitochondrial derived peptides.

The humanin peptide mediates ELP nanoassembly and protects human retinal pigment epithelial cells from oxidative stress

Humanin (HN) is a hydrophobic 24-amino acid peptide derived from mitochondrial DNA that modulates cellular responses to oxidative stress and protects human retinal pigment epithelium (RPE) cells from apoptosis. To solubilize HN, this report describes two genetically-encoded fusions between HN and elastin-like polypeptides (ELP). ELPs provide steric stabilization and/or thermo-responsive phase separation. Fusions were designed to either remain soluble or phase separate at the physiological temperature of the retina. Interestingly, the soluble fusion assembles stable colloids with a hydrodynamic radius of 39.1 nm at 37°C. As intended, the thermo-responsive fusion forms large coacervates (>1,000 nm) at 37°C. Both fusions bind human RPE cells and protect against oxidative stress-induction of apoptosis (TUNEL, caspase-3 activation). Their activity is mediated through STAT3; furthermore, STAT3 inhibition eliminates their protection. These findings suggest that HN polypeptides may facilitate cellular delivery of biodegradable nanoparticles with potential protection against age-related diseases, including macular degeneration.

Humanin induces conformational changes in the apoptosis regulator BAX and sequesters it into fibers, preventing mitochondrial outer-membrane permeabilization

The mitochondrial, or intrinsic, apoptosis pathway is regulated mainly by members of the B-cell lymphoma 2 (BCL-2) protein family. BCL-2-associated X apoptosis regulator (BAX) plays a pivotal role in the initiation of mitochondria-mediated apoptosis as one of the factors causing mitochondrial outer-membrane permeabilization (MOMP). Of current interest are endogenous BAX ligands that inhibit its MOMP activity. Mitochondrial-derived peptides (MDPs) are a recently identified class of mitochondrial retrograde signaling molecules and are reported to be potent apoptosis inhibitors. Among them, humanin (HN) has been shown to suppress apoptosis by inhibiting BAX translocation to the mitochondrial outer membrane, but the molecular mechanism of this interaction is unknown. Here, using recombinant protein expression, along with light-scattering, CD, and fluorescence spectroscopy, we report that HN and BAX can form fibers together in vitro Results from negative stain EM experiments suggest that BAX undergoes secondary and tertiary structural rearrangements and incorporates into the fibers, and that its membrane-associating C-terminal helix is important for the fibrillation process. Additionally, HN mutations known to alter its anti-apoptotic activity affect fiber morphology. Our findings reveal for the first time a potential mechanism by which BAX can be sequestered by fibril formation, which can prevent it from initiating MOMP and committing the cell to apoptosis.

The Mitochondrial-Derived Peptides, HumaninS14G and Small Humanin-like Peptide 2, Exhibit Chaperone-like Activity

Mitochondrial-derived peptides (MDPs) and their analogs have emerged as wide-spectrum, stress response factors protective in amyloid disease models. MDP cytoprotective functions are generally attributed to anti-apoptotic activity, however, little is known about their capacity to facilitate the cell’s unfolded protein response via direct interactions with amyloidogenic proteins. Here, we explored the effects of the MDP-analog, humaninS14G (HNG), and the MDP, small humanin-like peptide 2 (SHLP2), on the misfolding of islet amyloid polypeptide (IAPP), a critical pathogenic step in type 2 diabetes mellitus (T2DM). Our thioflavin T fluorescence studies show that HNG inhibits IAPP misfolding at highly substoichiometric concentrations. Seeded fluorescence and co-sedimentation studies demonstrate MDPs block amyloid seeding and directly bind misfolded, seeding-capable IAPP species. Furthermore, our electron paramagnetic resonance spectroscopy and circular dichroism data indicate MDPs do not act by binding IAPP monomers. Taken together our results reveal a novel chaperone-like activity wherein these MDPs specifically target misfolded amyloid seeds to inhibit IAPP misfolding which, along with direct anti-apoptotic activity and beneficial metabolic effects, make HNG and SHLP2 exciting prospects as T2DM therapeutics. These data also suggest that other mitochondrial stress response factors within the MDP family may be amenable to development into therapeutics for protein-misfolding diseases.

A Spectroscopic Study of the Aggregation State of the Human Antimicrobial Peptide LL-37 in Bacterial versus Host Cell Model Membranes

The LL-37 antimicrobial peptide is the only cathelicidin peptide found in humans that has antimicrobial and immunomodulatory properties. Because it exerts also chemotactic and angiogenetic activity, LL-37 is involved in promoting wound healing, reducing inflammation, and strengthening the host immune response. The key to the effectiveness of antimicrobial peptides (AMPs) lies in the different compositions of bacterial versus host cell membranes. In this context, antimicrobial peptide LL-37 and two variants were studied in the presence of model membranes with different lipid compositions and charges. The investigation was performed using an experimental strategy that combines the site-directed spin labeling-electron paramagnetic resonance technique with circular dichroism and fluorescence emission spectroscopies. LL-37 interacts with negatively charged membranes forming a stable aggregate, which can likely produce toroidal pores until the amount of bound peptide exceeds a critical concentration. At the same time, we have clearly detected an aggregate with a higher oligomeric degree for interaction of LL-37 with neutral membranes. These data confirm the absence of cell selectivity of the peptide and a more complex role in stimulating host cells.

Inactive C8A‑humanin analog is as stable as a potent S14G‑humanin analog

We have previously shown that the structural stability of humanin (HN), a neuroprotective peptide ligand, is one of the attributes to the observed activity differences between HN analogs. It has been observed that the activity increased consecutively in the S7A‑HN analog, the parent HN and the S14G‑HN analog, consistent with the increased stability observed in that order. In the present study, the structure and stability of another inactive analog, C8A‑HN, was measured, which has been revealed to have no neuroprotective activity similar to that of the S7A‑HN analog and hence may have compromised stability. While all these analogs of HN demonstrated a similar disordered secondary structure in phosphate-buffered saline at 5˚C, as determined by circular dichroism spectroscopy, they revealed different structures at 37˚C. At 37˚C, less active HN and inactive S7A‑HN revealed a structure with a valley at ~217 nm, indicating a conversion from the disordered structure to a β‑sheet. Such a conversion was largely irreversible. By contrast, C8A‑HN and S14G‑HN demonstrated a similar structure at 37˚C and at 5˚C and remained largely disordered. The observed small structural changes of the C8A‑HN analog at 37˚C and its reversibility upon cooling do not support a hypothesis that the instability at 37˚C may have caused the reduced activity of this analog. Therefore an alternative explanation for its activity loss is required.