Fish-hunting cone snail disrupts prey’s glucose homeostasis with weaponized mimetics of somatostatin and insulin

The study investigates how the fish-hunting cone snail, Conus geographus, disrupts prey glucose homeostasis using hormone-mimicking venom peptides. Prior work revealed that cone snails deploy neurotoxins targeting ion channels at the neuromuscular junction and that some species weaponize insulin (con-insulins) to induce rapid hypoglycemia in fish prey, impairing locomotion. Given that vertebrate glycemia is tightly regulated by the antagonistic actions of insulin and glucagon, and that somatostatin (SST) modulates pancreatic hormone secretion via SSTRs (notably SSTR2 suppressing glucagon from alpha cells), the authors hypothesized that C. geographus venom also contains somatostatin-like peptides that inhibit glucagon release. The research aims to identify, structurally characterize, and functionally evaluate somatostatin-mimicking venom peptides (consomatins) from C. geographus, assess their receptor selectivity and signaling bias, and determine their impact on glucagon secretion in mammalian and fish systems, thereby elucidating a combinatorial strategy targeting glucose homeostasis for prey capture.

Background literature outlines cone snail venom strategies, including the 'motor cabal' neurotoxins and the emerging class of 'doppelganger' peptides that mimic endogenous hormones. Prior discoveries showed C. geographus uses con-insulins to provoke hypoglycemic shock in fish. Somatostatin signaling via five GPCR subtypes regulates pancreatic hormone secretion; in mammals and fish, SSTR2 on alpha cells inhibits glucagon release. Previous transcriptomic and proteomic surveys suggested SST-like toxins in cone snails and that fish express duplicated SSTR2 isoforms. Clinically, stable SST analogs like octreotide are SSTR2-selective and used therapeutically, providing a pharmacological benchmark. This study builds on those insights to probe venom SST mimetics, their evolutionary derivation from endogenous SSRP-like peptides, and their potential to synergize with venom insulins.

• Venom gland transcriptomics and spatial expression: RNA-seq profiling of whole and quartered venom glands (segments B1–B4) from C. geographus to quantify consomatin and con-insulin transcripts and assess co-expression, with previous 454 datasets reanalyzed for validation.
• Heterologous receptor assays: Synthetic Consomatin pG1 was tested on human SSTR subtypes using PRESTO-Tango β-arrestin recruitment assays to determine potency and selectivity. G protein signaling was profiled using BRET-based biosensors to monitor Gαi/o subunit dissociation (including Gαo bias).
• Fish receptor pharmacology: Zebrafish SSTR2 isoforms (Dr-sstr2a, Dr-sstr2b) were cloned into Tango constructs. PRESTO-Tango and G protein dissociation assays quantified activation by SS-14 and pG1, establishing isoform selectivity and potency.
• Islet and pancreas physiology: Mouse islets were used to measure glucagon secretion under low (1–2 mM) and high (10 mM) glucose with or without 1 nM SS-14 or pG1. A perfused rat pancreas preparation under low glucose (3.5 mM) assessed dynamic glucagon output during brief applications of pG1 (0.1–10 nM) and washouts; L-arginine served as positive control.
• Activity-guided fractionation: Crude C. geographus venom was fractionated by RP-HPLC into pools and fractions, screened for SSTR activity (G protein and PRESTO-Tango assays). The most active fraction (#32) and subfraction (#32-20) were isolated.
• Structural elucidation and glycomics: Active subfraction underwent reduction/alkylation, tryptic digestion, and high-resolution MS/MS (HCD and ETD) to map peptide sequence and assign O-glycans. Intact mass profiling identified multiple glyco-proteoforms. O-glycan release and MS confirmed deoxyhexose-containing core-1 structures. Edman sequencing supported residue assignments, including glycosylated threonines.
• Comparative pharmacology of native fraction: The active venom subfraction was tested on human SSTRs and zebrafish Dr-sstr2 isoforms to compare activity with synthetic pG1.
• Evolutionary analyses: Phylogenetics of SSTRs across human, mouse, and teleosts; gene structure inference (exonerate) revealing an additional exon encoding the extended glycosylated N-terminus in the venom consomatin gene; sequence alignments showing mimicry of fish SS4 peptides, including a channel catfish pancreatic glycosylated SST.
• Statistics: Concentration-response curves fitted with 4-parameter logistic models; two-way repeated measures ANOVA with Sidak or Dunnett’s tests where appropriate; operational model analyses for ligand bias relative to SS-14.

• Venom strategy targeting glucose homeostasis: In addition to con-insulins, C. geographus expresses somatostatin-like toxins (consomatins) highly in the distal venom gland (segment B4), indicating potential co-release to induce and sustain hypoglycemia.
• Receptor selectivity and potency: Consomatin pG1 is a potent, selective agonist for SSTR2, displaying robust PRESTO-Tango activation and G protein signaling with notable Gαo bias relative to SS-14 and octreotide. Reported reference potencies include: octreotide at human SSTR2 EC50 ≈ 0.90 nM (pEC50 9.05 ± 0.04) vs SS-14 EC50 ≈ 18.5 nM (pEC50 7.73 ± 0.04). Consomatin pG1 showed low-nanomolar potency at SSTR2 in Tango assays (EC50 ≈ 3.6 nM reported) and preferential Gαo coupling in BRET assays.
• Fish receptor isoform specificity: Zebrafish SS-14 activated both Dr-sstr2a and Dr-sstr2b (EC50 ~28.7 nM and ~13.9 nM, respectively), whereas Consomatin pG1 selectively activated Dr-sstr2b (EC50 ~2.53 nM) with minimal activity at Dr-sstr2a, indicating prey-relevant isoform selectivity.
• Suppression of glucagon secretion: In isolated mouse islets under low glucose, 1 nM pG1 or SS-14 reduced glucagon secretion by ~33% (p < 0.01). In perfused rat pancreas at low glucose, 0.1 nM pG1 decreased glucagon output by ~24% within 5 minutes, with incomplete recovery after 15-minute washout, suggesting high-affinity or slow-dissociation receptor engagement.
• Identification of native active peptide: Activity-guided fractionation pinpointed venom fraction #32 and subfraction #32-20 as highly active at SSTRs (human and zebrafish) and capable of suppressing glucagon in perfused pancreas. MS/MS and Edman sequencing revealed that native Consomatin G1 possesses an extended N-terminal tail bearing multiple O-linked glycans (including deoxyhexose-containing core-1 structures), with glycosylation mapped to Thr12/Thr13.
• Glycosylation is functionally critical: A synthetic des-glycosylated analog did not recapitulate the native venom subfraction’s potency/selectivity at fish SSTR2 isoforms, indicating the essential role of the glycosylated N-terminus for activity.
• Chemical mimicry of fish pancreatic SST: The glycosylated N-terminal tail of Consomatin G1 closely mimics teleost SS4 peptides, including a glycosylated somatostatin from channel catfish pancreas, supporting convergent evolution to target fish SSTR2.
• Evolutionary origin: Consomatins derive from endogenous SSRP-like signaling genes; in C. geographus, the venom gene acquired an additional exon encoding the glycosylated N-terminal extension, consistent with directional selection for prey SSTR2 activation.
• Crude venom activity: Whole C. geographus venom activated both zebrafish SSTR2 isoforms and retained selectivity for human SSTR2, supporting the presence of multiple active glyco-proteoforms.

The findings demonstrate that C. geographus employs a combinatorial endocrine strategy to subdue prey: con-insulins acutely lower blood glucose, while SSTR2-selective consomatins suppress glucagon, limiting counter-regulatory responses and prolonging hypoglycemia. The selective potency of pG1 at human SSTR2 and zebrafish Dr-sstr2b, alongside biased Gαo signaling, aligns with the physiological role of SSTR2 in inhibiting alpha-cell glucagon release. Native venom consomatin glyco-proteoforms possess complex O-glycans essential for high activity at fish receptors, indicating that glycosylation fine-tunes receptor engagement and may enhance stability and binding kinetics, explaining the slow washout in perfused pancreas. Structural and phylogenetic analyses support an evolutionary trajectory from endogenous SSRP-like peptides to venom constituents with added exons encoding glycosylated N-termini, converging on the structural motifs of fish SS4 peptides. This exemplifies chemical mimicry optimized for prey physiology. The work underscores glucose homeostasis as an effective and previously underappreciated venom target and highlights doppelganger peptides as sources of pharmacological innovation, mirroring features of clinically used SSTR2-targeting analogs.

C. geographus leverages dual hormone-mimicking toxins—insulin and a somatostatin SSTR2-selective glycopeptide—to disrupt prey glucose homeostasis. The study identifies and characterizes native consomatin glyco-proteoforms, reveals their potency and Gαo-biased signaling at SSTR2, demonstrates suppression of glucagon in mammalian islets and perfused pancreas, and shows selective activation of a fish SSTR2 isoform. The glycosylated N-terminal tail is crucial for activity and mimics fish pancreatic somatostatin. Evolutionary analyses indicate recruitment and diversification of an endogenous SSRP-like gene with acquisition of a glycosylated extension. These insights establish glucose regulation as a key envenomation target and suggest venom-derived SST mimetics as templates for designing selective, biased SSTR2 agonists. Future work should clarify in vivo synergy with con-insulins in fish, resolve precise glycan structures/linkages, map receptor interactions structurally, and survey additional venom peptides that modulate glucose control (e.g., incretin-like components).

• Direct in vivo demonstration of combined con-insulin and consomatin action in prey was not achieved; severe hypoglycemia complicates in vivo assessment of additive effects.
• The complex, heterogeneous glycosylation of native consomatin proteoforms limited de novo sequencing and necessitated activity-guided purification; exact glycan compositions and linkages remain to be fully defined.
• Some pharmacological observations (e.g., variable receptor subtype activity across assay formats) suggest assay-dependent differences; comprehensive structure–activity mapping, including glyco-variants, is needed.
• Fish receptor analyses were limited to zebrafish SSTR2 isoforms; species-specific differences in SSTR repertoires and pharmacology across teleost prey remain to be explored.