Date Published: May 28, 2019
Publisher: Public Library of Science
Author(s): Michael Solarski, Declan Williams, Mohadeseh Mehrabian, Hansen Wang, Holger Wille, Gerold Schmitt-Ulms, Hubert Vaudry.
Somatostatin (SST) is a cyclic peptide that is understood to inhibit the release of hormones and neurotransmitters from a variety of cells by binding to one of five canonical G protein-coupled SST receptors (SSTR1 to SSTR5). Recently, SST was also observed to interact with the amyloid beta (Aβ) peptide and affect its aggregation kinetics, raising the possibility that it may bind other brain proteins. Here we report on an SST interactome analysis that made use of human brain extracts as biological source material and incorporated advanced mass spectrometry workflows for the relative quantitation of SST binding proteins. The analysis revealed SST to predominantly bind several members of the P-type family of ATPases. Subsequent validation experiments confirmed an interaction between SST and the sodium-potassium pump (Na+/K+-ATPase) and identified a tryptophan residue within SST as critical for binding. Functional analyses in three different cell lines indicated that SST might negatively modulate the K+ uptake rate of the Na+/K+-ATPase.
Somatostatin (SST) is an inhibitory peptide hormone produced by specific cells, including somatostatinergic neurons in several brain regions and somatotropic cells, known as delta cells, in pancreatic islets, the pyloric antrum and the duodenum. SST was initially discovered as a factor that inhibits growth hormone (GH) release from the anterior pituitary . To date, SST is understood to also act as an inhibitor of synaptic transmission in the central nervous system, to regulate insulin and glucagon release from the pancreas , and to suppress digestive secretions . SST exists primarily in two functional forms, a canonical 14-amino acid peptide (SST14) and an N-terminally extended 28-amino acid version of this peptide (SST28) . Both versions of the peptide form cyclic structures, due to the presence of a highly conserved disulfide bridge, and are derived from the proteolytic cleavage of the 116-amino acid preprosomatostatin (PPSST), which is coded on Chromosome 3 in humans . Less prominent cleavage products of PPSST exist, including a peptide encompassing residues 31–43 of unknown function, named neuronostatin . SST is assumed to exert its influence primarily through interactions with five cognate G protein-coupled receptors (GPCRs), known as somatostatin receptors (SSTR1 to SSTR5), which are expressed widely throughout the body [6, 7]. Both SST14 and SST28, as well as the closely related paralog cortistatin (CST), activate these receptors through a shared four-amino acid binding epitope comprised of the single-letter amino acid sequence FWKT , albeit with different potencies .
The goal of the current study was to generate an inventory of human brain proteins that bind to SST. Biotin-SST affinity capture robustly enriched 88 proteins each with at least 3 confident peptide-spectrum matches (PSMs). Among the top SST candidate interactors were multiple members of the P-type ATPase superfamily. Follow-up experiments, which centered on the Na+/K+-ATPase, validated that both SST28 and SST14 can bind to this pump. We then observed that binding exhibits selectivity with regards to both the SST bait peptide when compared to similar peptides, and to the P-type ATPase prey when compared to other abundant brain proteins. Moreover, we identified a tryptophan residue within SST that appears to be critical for binding to the Na+/K+-ATPase. Interestingly, this tryptophan is embedded within the core FWKT sequence motif known to also mediate binding of SST to its cognate receptors. Finally, we observed that SST has a cell type-specific inhibitory effect on the activity of the Na+/K+-ATPase.