Spexin

Overview

Spexin (SPX, also known as neuropeptide Q or NPQ) is a 14-amino acid neuropeptide identified in 2007 by Mirabeau and colleagues using a bioinformatics pipeline that searched the human genome for conserved prohormone-like sequences. Like phoenixin, spexin emerged from computational genomics rather than traditional biochemical isolation, reflecting the modern era of peptide discovery.

Spexin attracted significant metabolic research interest when it was found to be among the most dramatically downregulated genes in obese human adipose tissue compared to lean controls. Circulating spexin levels are reduced in obese individuals, correlate negatively with BMI, and decline further with increasing metabolic dysfunction. This pattern — an appetite-suppressing peptide that is deficient precisely when it is most needed — mirrors the conceptual framework of leptin resistance, though through a distinct mechanism.

In 2014, spexin's receptors were identified as galanin receptor subtypes GalR2 and GalR3. This was surprising because spexin shares minimal sequence homology with galanin itself, yet it binds these receptors with nanomolar affinity. Spexin does not bind GalR1, making it the first known endogenous GalR2/GalR3-selective ligand and distinguishing its pharmacological profile from galanin.

Amino Acid Sequence

Human spexin: NWTPQAMLYLKGAQ-NH₂

• Molecular weight: ~1,577 g/mol
• Gene: C12orf39 / SPX (chromosome 12p12.1)
• CAS Number: 1258003-90-3
• Receptors: GalR2, GalR3

Structural features:

• C-terminal amidation — essential for receptor binding and biological activity
• 14 amino acids — compact neuropeptide
• High evolutionary conservation — sequence is identical across mammals and conserved in fish, suggesting ancient origin
• No significant sequence homology with galanin — despite sharing receptors; implies convergent receptor binding through different structural mechanisms
• Amphipathic character — has both hydrophobic and hydrophilic regions, potentially relevant to membrane interactions

Mechanism of Action

Galanin Receptor Signaling

Spexin signals through two of the three galanin receptor subtypes:

GalR2:

• Couples to Gq/11 and Gi/o
• Activates phospholipase C, increasing IP3 and diacylglycerol
• Also activates MAPK/ERK pathway
• Expressed in hypothalamus, hippocampus, adipose tissue, GI tract

GalR3:

• Couples primarily to Gi/o
• Inhibits adenylyl cyclase, reducing cAMP
• Opens GIRK channels
• Expressed in hypothalamus, testis, pituitary

Spexin does not bind GalR1, which is the primary receptor for galanin's appetite-stimulating effects. This selectivity means spexin and galanin have overlapping but distinct pharmacological profiles.

Appetite and Weight Regulation

• Spexin suppresses food intake when administered centrally or peripherally in rodent and fish models
• Reduces fat preference and high-fat diet consumption
• Mechanism involves GalR2/GalR3 signaling in hypothalamic appetite circuits
• Spexin gene expression in adipose tissue and hypothalamus is suppressed by obesity and high-fat diet
• Spexin administration to obese animals reduces body weight, fat mass, and food intake
• Interacts with leptin signaling: leptin stimulates spexin expression, suggesting spexin may be a downstream mediator of some leptin effects

Lipid Metabolism

• Reduces long-chain fatty acid uptake in adipocytes by decreasing fatty acid transport proteins
• Inhibits hepatic lipogenesis
• These effects are primarily mediated through GalR2

Reproduction

• Spexin modulates gonadotropin release, with effects on LH secretion
• Expressed in the ovary and testis
• May interact with kisspeptin/GnRH signaling in reproductive regulation
• Spexin levels vary across the menstrual cycle

Research Summary

Pharmacokinetics

• Half-life: Not precisely determined; estimated in the minutes range based on peptide characteristics
• Circulating levels: Detectable in human serum; significantly reduced in obese individuals (ranges vary by assay)
• Expression: Widespread — hypothalamus, adipose tissue (both white and brown), liver, stomach, intestine, ovary, testis, adrenal gland
• Regulation: Downregulated by obesity, high-fat diet, caloric excess; upregulated by leptin, weight loss, caloric restriction
• Route (research): Intraperitoneal, intracerebroventricular (animal studies)

Common Discussion Topics

1. Spexin deficiency in obesity — The dramatic reduction of spexin in obesity makes it a candidate for "missing signal" hypothesis of metabolic dysfunction, analogous to leptin deficiency but through a different receptor system. Whether spexin replacement could treat obesity is an active question.

2. Galanin receptor selectivity — Spexin's selectivity for GalR2/GalR3 over GalR1 is pharmacologically significant because galanin itself activates all three receptors. This means spexin provides a subset of galanin's effects without GalR1-mediated appetite stimulation.

3. Bioinformatics discovery wave — Spexin and phoenixin represent a wave of peptides discovered through computational genomics. The approach of mining genomes for conserved prohormone features has revealed that the neuropeptide catalog is larger than previously appreciated.

4. Leptin-spexin axis — Leptin stimulates spexin expression, and spexin levels fall in leptin-resistant states. This suggests spexin may be a downstream effector of leptin signaling, potentially explaining some aspects of the metabolic dysfunction that accompanies leptin resistance.

5. Fish model advantages — Much spexin research uses zebrafish, where the peptide's effects on feeding, metabolism, and reproduction are robust and amenable to genetic manipulation. Fish models have been particularly useful for establishing spexin's conserved functions across vertebrates.

Related Compounds

• Galanin — shares GalR2/GalR3 receptors but also activates GalR1; broader neuropeptide roles
• Leptin — upstream regulator of spexin expression; complementary satiety signal
• Neuropeptide Y — appetite-stimulating peptide with opposing effects on food intake
• Phoenixin — fellow bioinformatics-discovered neuropeptide with reproductive roles
• Kisspeptin — reproductive neuropeptide with potential spexin interactions