A Peptide Fusion a Day Keeps the Aggregates Away
Authors: Alain Prochiantz
Journal: Molecular Therapy (2007)

Many neurodegenerative diseases are associated with the formation of insoluble structures composed primarily of protein aggregates. Examples include Alzheimer disease (neurofibrillary tangles and amyloid plaques), Parkinson disease (Lewy bodies), and a class of diseases provoked by extension of the length of polyglutamine (polyQ) sequences in particular proteins. The extended polyQ sequences result from abnormal expansion of CAG repeats in the genes encoding the affected proteins, and they are thus inherited. In this issue, Popiel et al.1Popiel HA Nagai Y Fujikake N Toda T Protein transduction domain-mediated delivery of the aggregate inhibitor peptide QBP1 suppresses polyglutamine-induced neurodegeneration in vivo.Mol Ther. 2006; 15: 303-309Abstract Full Text Full Text PDF Scopus (50) Google Scholar tested a new therapeutic strategy to treat a Drosophila model of polyQ disease that involves feeding to the flies a fusion peptide designed to disrupt polyQ aggregates. The major polyQ diseases are spinocerebellar ataxias, dentatorubral-pallidoluysian atrophy, spinobulbar muscular atrophy, and Huntington disease.2Berman SB Greenamyre JT Update on Huntington's disease.Curr Neurol Neurosci Rep. 2006; 6: 281-286Crossref PubMed Scopus (5) Google Scholar The role of the polyQ aggregates in disease pathology is not clear. Some investigators have proposed that aggregates are beneficial because they provide “hot spots” for the accumulation of potentially deleterious factors, including the mutated protein itself (polyQ may confer adverse properties to the soluble proteins), so as to prevent it from intermingling with and affecting the activity of other proteins.3Gispert-Sanchez S Auburger G The role of protein aggregates in neuronal pathology: guilty, innocent, or just trying to help?.J Neural Transm Suppl. 2006; 70: 111-117Crossref PubMed Scopus (24) Google Scholar However, a majority of investigators consider aggregates detrimental, in particular because they trap several factors indispensable for proper cell metabolism or function. The concept of aggregate toxicity forms the basis for the search for molecules capable of disrupting the aggregates or blocking their formation.4Weydt P La Spada AR Targeting protein aggregation in neurodegeneration—lessons from polyglutamine disorders.Expert Opin Ther Targets. 2006; 10: 505-513Crossref PubMed Scopus (19) Google Scholar For example, some investigators have proposed the development of “β-breakers,” because it often happens (as in the case of prions) that insolubility is a consequence of conformational alteration of α-helices into β-sheets.5Bieler S Soto C Beta-sheet breakers for Alzheimer's disease therapy.Curr Drug Targets. 2004; 5: 553-558Crossref PubMed Scopus (50) Google Scholar Nagai and colleagues6Nagai Y Fujikake N Ohno K Higashiyama H Popiel HA Rahadian J et al.Prevention of polyglutamine oligomerization and neurodegeneration by the peptide inhibitor QBP1 in Drosophila.Hum Mol Genet. 2003; 12: 1253-1259Crossref PubMed Scopus (120) Google Scholar, 7Nagai Y Tucker T Ren H Kenan DJ Henderson BS Keene JD et al.Inhibition of polyglutamine protein aggregation and cell death by novel peptides identified by phage display screening.J Biol Chem. 2000; 275: 10437-10442Crossref PubMed Scopus (168) Google Scholar recently adopted a similar strategy, using phage display to identify six short polyQ binding peptides (PQBP1–PQBP6) that could inhibit polyQ polymerization in vitro. One of their peptides, PQBP1, could antagonize polyQ-induced formation of inclusion bodies, neuronal degeneration, and premature death when expressed as a dimer, (PQBP1)2, in a Drosophila model of polyQ disease. These studies suggested the possibility of using gene therapy to express the inhibitory peptide from a viral vector to treat these diseases. However, the group decided to explore an alternative strategy based on the use of transduction peptides. Transduction peptides constitute a class of short peptides that can gain access to the cytoplasm and nucleus after internalization by living cells.8Joliot A Prochiantz A Transduction peptides: from technology to physiology.Nat Cell Biol. 2004; 6: 189-196Crossref PubMed Scopus (449) Google Scholar, 9Wadia JS Stan RV Dowdy SF Transducible TAT-HA fusogenic peptide enhances escape of TAT-fusion proteins after lipid raft macropinocytosis.Nat Med. 2004; 10: 310-315Crossref PubMed Scopus (1422) Google Scholar Such peptides can be fused to hydrophilic cargoes that can then be delivered into cells both in vitro and in vivo. Common examples include Penetratin and Tat. Penetratin is also called Antennapedia (Antp), because it corresponds to a 16 amino acid long sequence that is present in the Drosophila transcription factor, Antennapedia. Tat is an arginine-rich peptide found in the sequence of the HIV transcription factor, TAT. Many other peptides have been found that share vector properties similar to those of Antp and Tat, including poly-Arg and Transportan.10Rothbard JB Kreider E VanDeusen CL Wright L Wylie BL Wender PA Arginine-rich molecular transporters for drug delivery: role of backbone spacing in cellular uptake.J Med Chem. 2002; 45: 3612-3618Crossref PubMed Scopus (285) Google Scholar, 11Mae M Langel U Cell-penetrating peptides as vectors for peptide, protein and oligonucleotide delivery.Curr Opin Pharmacol. 2006; 6: 509-514Crossref PubMed Scopus (289) Google Scholar Importantly, transduction peptides are not internalized through identical routes. For example, internalization of Tat requires endocytosis and is energy dependent, whereas Antp internalization takes place at 4 °C. Regardless of the mechanism of entry, all such peptides must cross the lipid bilayer, either at the level of the plasma membrane, as is the case for Antp, or after endocytosis, as for Tat.8Joliot A Prochiantz A Transduction peptides: from technology to physiology.Nat Cell Biol. 2004; 6: 189-196Crossref PubMed Scopus (449) Google Scholar, 9Wadia JS Stan RV Dowdy SF Transducible TAT-HA fusogenic peptide enhances escape of TAT-fusion proteins after lipid raft macropinocytosis.Nat Med. 2004; 10: 310-315Crossref PubMed Scopus (1422) Google Scholar It must be underscored that attaching a cargo to a transduction peptide can modify the properties of the cargo and/or the vector. Each chimeric cargo-vector (or vector-cargo) construct must therefore be viewed as a unique entity that must be tested for entry and for activity. In the new work, Popiel and colleagues used Antp and Tat to internalize PQBP1. They first demonstrated that the fusion peptides prevented polyQ-induced polymerization in vitro, establishing that the biological activity of PQBP1 was not impeded by the vector. They then tested the internalization of Tat-PQBP1 and Antp-PQBP1 by COS cells and found that both fusions were active and exhibited little toxicity, and that their internalization decreased the formation of inclusion bodies after expression of polyQ constructs. Because Antp seemed more efficient and less toxic than Tat in their model, the authors selected the Antp fusion for in vivo experiments that made use of the same model used to demonstrate that genetic co-expression of polyQ and (PQBP1)2 could antagonize the formation of inclusion bodies, block neuronal death, and prevent death of the flies. However, instead of expressing PQBP1 genetically, they fed the flies Drosophila food containing 0.2 mM Antp-PQBP1. The authors again observed decreased inclusion body formation, decreased neuronal death and an extension of life. This is not the first study to show that a transduction peptide can be used to deliver a cargo with physiological activity in vivo. Outside of the central nervous system (CNS), both Antp and Tat have been shown to exhibit internalization activity when injected intraperitoneally or into the circulation. However, in the CNS, most experiments thus far have involved injection of Tat fusions into the brain parenchyme or in peritoneum of the rodent. Injection into the peritoneum resulted in strong physiological effects, suggesting that the peptide-cargoes could cross the blood–brain barrier.12Murriel CL Dowdy SF Influence of protein transduction domains on intracellular delivery of macromolecules.Expert Opin Drug Deliv. 2006; 3: 739-746Crossref PubMed Scopus (96) Google Scholar The article by Popiel and colleagues presents the first case of oral ingestion of a transduction peptide with an activity in the nervous system. Although these results are of great interest, an important caveat is that it is quite difficult to make the jump from flies to mammals with such technology. Even if passage across the intestinal epithelium and into the brain could be reproduced in rodents, many points would have to be resolved before transduction peptides could be used as pharmacological tools. There have been no studies of the possible toxic or mutagenic activities of these peptides. In addition, one must also be able to target this new class of pharmacological agents to the right cells and, once in the cells, to the right subcellular compartment. There is thus plenty of work ahead. However, considering the results gathered in only a few years by a small number of research groups, it is obvious that this new technology is very promising. A main advantage of transduction peptides is that they provide access to intracellular targets, as viral vectors do, but that they remain very classical pharmacological agents. If proteins such as interferon or insulin can be used therapeutically, there is no reason to doubt the interest of approaches using transduction peptides or nonpeptidic compounds modeled after these peptides.