We report how rotational variations in transmembrane (TM) helix interactions participate in the activity states of the thrombopoietin receptor (TpoR), a type 1 cytokine receptor that controls the production of blood platelets. We also explore the mechanism of small-molecule agonists that do not mimic the natural ligand. We show, by a combination of cysteine cross-linking, alanine-scanning mutagenesis, and computational simulations, that the TpoR TM dimerizes strongly and can adopt 3 different stable, rotationally related conformations, which may correspond to specific states of the full-length receptor (active, inactive, and partially active). Thus, our data suggest that signaling and inactive states of the receptor are related by receptor subunit rotations, rather than a simple monomer-dimer transition. Moreover, results from experiments with and without agonists in vitro and in cells allow us to propose a novel allosteric mechanism of action for a class of small molecules, in which they activate TpoR by binding to the TM region and by exploiting the rotational states of the dimeric receptor. Overall, our results support the emerging view of the participation of mutual rotations of the TM domains in cytokine receptor activation.

SUMMARY Our goal is to define the properties of cell-impermeable cargo molecules that can be delivered into cells by pHLIP (pH (Low) Insertion Peptide), which can selectively target tumors in vivo based on their acidity. Using biophysical methods and fluorescence microscopy, we show that pHLIP can successfully deliver polar and membrane-impermeable cyclic peptides linked to its C-terminus through the membranes of lipid vesicles and cancer cells. Our results also indicate that the translocation of these cargo molecules is pH-dependent and mediated by transmembrane helix formation. Since a broad range of cell-impermeable molecules is excluded from discovery efforts because they cannot traverse membranes on their own, we believe that pHLIP has the potential to expand therapeutic options for acidic tissues such as tumors and sites of inflammation.

Localized delivery is vital for the successful development of novel and effective therapeutics for the treatment of cancer. The targeting and delivery described herein is based on the pH(Low) Insertion Peptide (pHLIP), a unique delivery peptide that can selectively target tumors in mice and translocate and release cargo molecules intra-cellularly based solely on the low extracellular pH intrinsic to cancer cells. In this study, we investigate the efficacy of pHLIP to target and deliver the highly potent and clinically validated microtubule inhibitor monomethyl auristatin E (MMAE) to cancer cells and breast tumors. We show that pHLIP-MMAE conjugates induce a potent cytotoxic effect (> 90% inhibition of cell growth) in a concentration- and pH-dependent manner after only 2-hour incubation without any apparent disruption of the plasma membrane. pHLIP-MMAE conjugates exhibit between an 11 and 144-fold higher anti-proliferative effect at low pH than at physiological pH, and a pronounced pH-dependent cytotoxicity as compared to free drug. Furthermore, we demonstrate that a pHLIP-MMAE drug conjugate effectively targets triple negative breast tumor xenografts in mice. These results indicate pHLIP-based auristatin conjugates may have an enhanced therapeutic window as compared to free drug, providing a targeting mechanism to attenuate systemic toxicity.

Positively charged antimicrobial peptides have become promising agents for the treatment of cancer by inducing apoptosis though their preferential binding and disruption of negatively charged membranes, such as the mitochondrial membrane. (KLAKLAK)2 is such a peptide but due to its polarity, it cannot cross the cellular membrane and therefore relies on the use of a delivery agent. For targeted delivery, previous studies have relied on cell penetrating peptides, nanoparticles or specific biomarkers. Herein, we investigated the first use of pHLIP to selectively target and directly translocate (KLAKLAK)2 into the cytoplasm of breast cancer cells, based on the acidic tumor micro-environment. With the goal of identifying a lead conjugate with optimized selective cytotoxicity towards cancer cells, we analyzed a family of (KLAKLAK)2 analogs with varying size, polarity and charge. We present a highly efficacious pHLIP conjugate that selectively induces concentration- and pH-dependent toxicity in breast cancer cells.

The human adenosine A 2A receptor (A 2A R) belongs to one of the largest family of membrane proteins, the G-protein coupled receptors (GPCRs), characterized by seven transmembrane (TM) helices. Little is known about the determinants of their structures, folding, assembly, activation mechanisms, and oligomeric states. Previous studies in our group showed that peptides corresponding to all seven TM domains form stable helical structures in detergent micelles and lipid vesicles. However, the peptides behave differently; TM5 is the only peptide to have a ratio [u] 222 /[u] 208 obtained by circular dichroism (CD) spectroscopy >1. This finding suggested to us that TM5 might self-associate. In the present study, we investigate the unique properties of the TM5 domain. We performed detailed analyses of TM5 peptide behavior in membrane-mimetic environments using CD spectroscopy, fluorescence spectroscopy and Fo¨rster resonance energy transfer, and gel electrophoresis. We find that TM5 peptide has the ability to self-associate to form oligomeric structures in various hydrophobic milieus and that these oligomers are highly resistant to temperature and chemical denaturation. We also find that mutation of the full-length A 2A R at position M193, which is located in the fifth TM domain, noticeably alters A 2A R monomer:dimer ratio as observed on SDS-PAGE. Our results suggest that parallel association of TM5 dimers may play a role in the known adenosine A 2A receptor dimerization. This study represents the first evidence of an individual GPCR transmembrane domain self-association.Keywords: membrane proteins; GPCR dimerization; helix association; transmembrane peptide; FRET The human adenosine A 2A receptor (A 2A R) is a member of the G-protein coupled receptor (GPCR) superfamily. GPCRs are integral membrane proteins characterized by seven transmembrane (TM) helices that mediate a plethora of cellular signals across the plasma membrane via coupling to G-proteins. They modulate many physiological processes and are linked to numerous human diseases (Shichida and Imai 1998; Gether 2000; Gurrath 2001), and consequently, are the targets of an increasingly large number of drugs (Gurrath 2001). Until recently, GPCRs were believed to work as monomeric entities, activating G proteins in a 1:1 stoichiometric ratio. However, this classical model of coupling may be oversimplified, since a large body of evidence has shown that many GPCRs exist as homodimers, heterodimers, or even as higher order oligomers (Jones et al. 1998;Jordan and Devi 1999;Bai 2004;Fotiadis et al. 2004). Recent studies have also demonstrated that oligomerization has important effects on GPCRs' functions, including ligand binding, receptor Abbreviations: GPCR, G-protein coupled receptor; TM, transmembrane; A 2A R, adenosine A 2A receptor; SDS, sodium dodecyl sulfate; DMPC, dimyristoyl phosphatidylcholine; CD, circular dichroism; PAGE, Polyacrylamide gel electrophoresis; FRET, Fo¨rster resonance energy transfer; PFO, perfluoro-octanoic acid; T m , temperature of ...