SLC6 transporter oligomerization.

Transporters of the solute carrier 6 (SLC6) family mediate the reuptake of neurotransmitters such as dopamine, norepinephrine, serotonin, GABA, and glycine. SLC6 family members are 12 transmembrane helix-spanning proteins that operate using the transmembrane sodium gradient for transport. These transporters assume various quaternary arrangements ranging from monomers to complex stoichiometries with multiple subunits. Dopamine and serotonin transporter oligomerization has been implicated in trafficking of newly formed proteins from the endoplasmic reticulum to the plasma membrane with a pre-fixed assembly. Once at the plasma membrane, oligomers are kept fixed in their quaternary assembly by interaction with phosphoinositides. While it remains unclear how oligomer formation precisely affects physiological transporter function, it has been shown that oligomerization supports the activity of release-type psychostimulants. Most recently, single molecule microscopy experiments unveiled that the stoichiometry differs between individual members of the SLC6 family. The present overview summarizes our understanding of the influence of plasma membrane constituents on transporter oligomerization, describes the known interfaces between protomers and discusses open questions.

An early folding contact between Phe19 and Leu34 is critical for amyloid-ß oligomer toxicity.

Small hydrophobic oligomers of aggregation-prone proteins are thought to be generically toxic. Here we examine this view by perturbing an early folding contact between Phe19 and Leu34 formed during the aggregation of Alzheimer's amyloid-ß (Aß40) peptide. We find that even conservative single mutations altering this interaction can abolish Aß40 toxicity. Significantly, the mutants are not distinguishable either by the oligomers size or by the end-state fibrillar structure from the wild type Aß40. We trace the change in their toxicity to a drastic lowering of membrane affinity. Therefore, nonlocal folding contacts play a key role in steering the oligomeric intermediates through specific conformations with very different properties and toxicity levels. Our results suggest that engineering the folding energy landscape may provide an alternative route to Alzheimer therapeutics.

Thermodynamically stable amyloid-ß monomers have much lower membrane affinity than the small oligomers.

Amyloid beta (Aß) is an extracellular 39-43 residue long peptide present in the mammalian cerebrospinal fluid, whose aggregation is associated with Alzheimer's disease (AD). Small oligomers of Aß are currently thought to be the key to toxicity. However, it is not clear why the monomers of Aß are non-toxic, and at what stage of aggregation toxicity emerges. Interactions of Aß with cell membranes is thought to be the initiator of toxicity, but membrane binding studies with different preparations of monomers and oligomers have not settled this issue. We have earlier found that thermodynamically stable Aß monomers emerge spontaneously from oligomeric mixtures upon long term incubation in physiological solutions (Nag et al., 2011). Here we show that the membrane-affinity of these stable Aß monomers is much lower than that of a mixture of monomers and small oligomers (containing dimers to decamers), providing a clue to the emergence of toxicity. Fluorescently labeled Aß40 monomers show negligible binding to cell membranes of a neuronal cell line (RN46A) at physiological concentrations (250 nM), while oligomers at the same concentrations show strong binding within 30 min of incubation. The increased affinity most likely does not require any specific neuronal receptor, since this difference in membrane-affinity was also observed in a somatic cell-line (HEK 293T). Similar results are also obtained for Aß42 monomers and oligomers. Minimal amount of cell death is observed at these concentrations even after 36 h of incubation. It is likely that membrane binding precedes subsequent slower toxic events induced by Aß. Our results (a) provide an explanation for the non-toxic nature of Aß monomers, (b) suggest that Aß toxicity emerges at the initial oligomeric phase, and (c) provide a quick assay for monitoring the benign-to-toxic transformation of Aß.