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Determination of interactions between native proteins in cells is important for understanding function. Here the authors report MolBoolean as a method to detect interactions between endogenous proteins in subcellular compartments, using antibody-DNA conjugates for identification and signal amplification. Determining the levels of protein-protein interactions is essential for the analysis of signaling within the cell, characterization of mutation effects, protein function and activation in health and disease, among others. Herein, we describe MolBoolean - a method to detect interactions between endogenous proteins in various subcellular compartments, utilizing antibody-DNA conjugates for identification and signal amplification. In contrast to proximity ligation assays, MolBoolean simultaneously indicates the relative abundances of protein A and B not interacting with each other, as well as the pool of A and B proteins that are proximal enough to be considered an AB complex. MolBoolean is applicable both in fixed cells and tissue sections. The specific and quantifiable data that the method generates provide opportunities for both diagnostic use and medical research.

Many newly identified solute carriers (SLCs) and putative transporters have the possibility to be intricately involved in glucose metabolism. Here we show that many transporters of this type display a high degree of regulation at both mRNA and protein level following no or low glucose availability in mouse cortex cultures. We show that this is also the case in Drosophila melanogaster subjected to starvation or diets with different sugar content. Interestingly, re-introduction of glucose to media, or refeeding flies, normalized the gene expression of a number of the targets, indicating a fast and highly dynamic control. Our findings demonstrate high conservation of these transporters and how dependent both cell cultures and organisms are on gene and protein regulation during metabolic fluctuations. Several transporter genes were regulated simultaneously maybe to initiate alternative metabolic pathways as a response to low glucose levels, both in the cell cultures and in D. melanogaster. Our results display that newly identified SLCs of Major Facilitator Superfamily type, as well as the putative transporters included in our study, are regulated by glucose availability and could be involved in several cellular aspects dependent of glucose and/or its metabolites. Recently, a correlation between dysregulation of glucose in the central nervous system and numerous diseases such as obesity, type 2 diabetes mellitus as well as neurological disease such as Alzheimer’s and Parkinson’s diseases indicate a complex regulation and fine tuning of glucose levels in the brain. The fact that almost one third of transporters and transporter-related proteins remain orphans with unknown or contradictive substrate profile, location and function, pinpoint the need for further research about them to fully understand their mechanistic role and their impact on cellular metabolism.

Zinc finger BED domain containing protein 6 (Zbed6) has evolved from a domesticated DNA transposon and encodes a transcription factor unique to placental mammals. The aim of the present study was to investigate further the role of ZBED6 in insulin-producing cells, using mouse MIN6 cells, and to evaluate the effects of Zbed6 knockdown on basal -cell functions, such as morphology, transcriptional regulation, insulin content, and release. Zbed6-silenced cells and controls were characterized with a range of methods, including RNA sequencing, chromatin immunoprecipitation sequencing, insulin content and release, subplasma membrane Ca2+ measurements, cAMP determination, and morphologic studies. More than 700 genes showed differential expression in response to Zbed6 knockdown, which was paralleled by increased capacity to generate cAMP, as well as by augmented subplasmalemmal calcium concentration and insulin secretion in response to glucose stimulation. We identified >4000 putative ZBED6-binding sites in the MIN6 genome, with an enrichment of ZBED6 sites at upregulated genes, such as the -cell transcription factors v-maf musculoaponeurotic fibrosarcoma oncogene homolog A and Nk6 homeobox 1. We also observed altered morphology/growth patterns, as indicated by increased cell clustering, and in the appearance of axon-like Neurofilament, medium polypeptide and tubulin 3, class III-positive protrusions. We conclude that ZBED6 acts as a transcriptional regulator in MIN6 cells and that its activity suppresses insulin production, cell aggregation, and neuronal-like differentiation.Wang, X., Jiang, L., Wallerman, O., Younis, S., Yu, Q., Klaesson, A., Tengholm, A., Welsh, N., Andersson, L. ZBED6 negatively regulates insulin production, neuronal differentiation, and cell aggregation in MIN6 cells.

In situ proximity ligation assay (PLA) is a method for detection of protein interactions, post-translational modifications (PTMs) and individual proteins that allows information about their localization in a cell or tissue to be extracted. The method is based on oligonucleotide-conjugated antibodies (proximity probes) that upon binding of two epitopes in close proximity give rise to an amplifiable DNA circle. Rolling circle amplification (RCA) is used to create a DNA bundle of over a thousand repeats to which fluorescently labeled detection oligonucleotides are hybridized. This thesis is focused on improving the existing in situ PLA method and on developing new approaches for detection of proteins, protein-protein interactions and PTMs in situ in cells and tissues.

In paper I, a new enzyme-independent method capable of in situ detection of protein-protein interactions was developed. The method combined the proximity requirement of in situ PLA and the amplification of hybridization chain reaction (HCR) creating a proximity-dependent initiation of hybridization chain reaction (proxHCR). Circumventing the need for enzymes resulted in a cost-efficient method that is less sensitive to storing conditions.

Paper II addresses the problem of irregularly formed RCA products that can appear to be split into several fluorescent objects. A compaction oligonucleotide system was designed to crosslink the DNA bundle with itself and thereby reduce the size and increase the brightness of each individual RCA product.

In paper III, the conventional in situ PLA was redesigned to increase the detection efficiency of protein interactions and PTMs in situ. The new set of proximity probes was designed to have circularization oligonucleotides incorporated that were unfolded through enzymatic digestion. The UnFold in situ PLA was able to generate more signals and had a higher sensitivity than the conventional in situ PLA.

In paper IV, an oligonucleotide system able to generate signals for individual proteins (A or B) and their interaction (A and B) in a molecular Boolean (MolBoolean) protein analysis was designed. The MolBoolean design was able to generate signals detecting both individual proteins and their interaction in situ.  

We have redesigned probes for in situ proximity ligation assay (PLA), resulting in more efficient localized detection of target proteins. In situ PLA depends on recognition of target proteins by pairs of antibody-oligonucleotide conjugates (PLA probes), which jointly give rise to DNA circles that template localized rolling circle amplification reactions. The requirement for dual recognition of the target proteins improves selectivity by ignoring any cross-reactivity not shared by the antibodies, and it allows detection of protein-protein interactions and post-translational modifications. We herein describe an improved design of the PLA probes -UnFold probes - where all elements required for formation of circular DNA strands are incorporated in the probes. Premature interactions between the UnFold probes are prevented by including an enzymatic "unfolding" step in the detection reactions. This allows DNA circles to form by pairs of reagents only after excess reagents have been removed. We demonstrate the performance of UnFold probes for detection of protein-protein interactions and post-translational modifications in fixed cells and tissues, revealing considerably more efficient signal generation. We also apply the UnFold probes to detect IL-6 in solution phase after capture on solid supports, demonstrating increased sensitivity over both normal sandwich enzyme-linked immunosorbent assays and conventional PLA assays.

Solute carriers (SLCs) are vital as they are responsible for a major part of the molecular transport over lipid bilayers. At present, there are 430 identified SLCs, of which 28 are called atypical SLCs of major facilitator superfamily (MFS) type. These are MFSD1, 2A, 2B, 3, 4A, 4B, 5, 6, 6L, 7, 8, 9, 10, 11, 12, 13A, 14A, 14B; SV2A, SV2B, SV2C, SVOP, SVOPL; SPNS1, SPNS2, SPNS3; UNC93A and UNC93B1, and we studied their fundamental properties. We also included CLN3, an atypical SLC not yet belonging to any Pfam clan, because its involvement in the same neuronal degenerative disorders as MFSD8. With phylogenetic analyses and bioinformatic sequence comparisons, the proteins were divided into 15 families, denoted Atypical MFS Transporter Families (AMTF1-15). Hidden Markov models were used to identify orthologues from human to D.melanogaster and C.elegans. Topology predictions revealed 12 transmembrane segments (for all except CLN3), corresponding to the common MFS structure. With single-cell RNA sequencing and in situ proximity ligation assay on brain cells, co-expressions of several atypical SLC were identified. Finally, the transcription levels of all genes were analysed in the hypothalamic N25/2 cell line after complete amino acid starvation, showing altered expression levels for several atypical SLCs. 

Sensitive detection of protein interactions and post-translational modifications of native proteins is a challenge for research and diagnostic purposes. A method for this, which could be used in point-of-care devices and high-throughput screening, should be reliable, cost effective and robust. To achieve this, here we design a method (proxHCR) that combines the need for proximal binding with hybridization chain reaction (HCR) for signal amplification. When two oligonucleotide hairpins conjugated to antibodies bind in close proximity, they can be activated to reveal an initiator sequence. This starts a chain reaction of hybridization events between a pair of fluorophore-labelled oligonucleotide hairpins, generating a fluorescent product. In conclusion, we show the applicability of the proxHCR method for the detection of protein interactions and posttranslational modifications in microscopy and flow cytometry. As no enzymes are needed, proxHCR may be an inexpensive and robust alternative to proximity ligation assays.