VGF and Its Bioactive Peptides: TLQP-21, TLQP-62, and NERP Peptides in Metabolism, Pain, and Neuropsychiatric Research

# VGF and Its Bioactive Peptides: TLQP-21, TLQP-62, and NERP Peptides in Metabolism, Pain, and Neuropsychiatric Research

Most bioactive neuropeptides are encoded by dedicated genes producing one or two functional products. VGF (nerve growth factor-inducible protein) operates differently: it is a 615-amino acid neurosecretory protein that functions exclusively as a precursor — VGF itself has no enzymatic activity. Instead, it is processed by prohormone convertases into multiple distinct bioactive peptides that mediate diverse and sometimes opposing physiological functions. The best-characterized VGF-derived peptide, TLQP-21, was identified in 2014 as an agonist at the complement C3a receptor (C3aR1) — a surprising finding connecting a neurosecretory protein to the innate immune complement cascade. Other VGF fragments, including TLQP-62, NERP-1, and NERP-2, regulate energy homeostasis and vasopressin secretion. Together, VGF-derived peptides participate in neuroplasticity, inflammatory pain, antidepressant mechanisms, and energy balance — and VGF is among the most consistently downregulated proteins in major depressive disorder.

The VGF Gene and Protein

Discovery. The VGF gene was identified in 1985 by Levi and colleagues as a transcript rapidly induced by nerve growth factor (NGF) in PC12 pheochromocytoma cells — hence the name VGF (which is not an acronym despite resembling one). Unlike most NGF-responsive genes encoding signal transduction proteins, VGF encodes a large secretory protein with no clear catalytic domain. It is found exclusively in neurons — a neuron-specific expression pattern across all examined species — and is also regulated by BDNF, NT-3, and activity-dependent stimuli.

Gene locus. The human VGF gene maps to chromosome 7q22.1. It encodes a 615-amino acid prepro-VGF in humans (617 aa in rats/mice). After signal peptide cleavage, the 588-aa pro-VGF is sorted into dense-core secretory vesicles where processing by prohormone convertases PC1/3 and PC2, combined with carboxypeptidase E (CPE), generates the mature bioactive peptide products.

Expression. VGF is expressed in:

• •Hypothalamic neurons: arcuate, paraventricular, supraoptic, and lateral hypothalamic nuclei — all energy balance/neuroendocrine-relevant areas
• •DRG neurons: sensory neurons of peripheral nervous system; VGF in DRG is critical for pain sensitization
• •Hippocampus: VGF in CA1, CA3, dentate gyrus; regulated by synaptic activity and antidepressants
• •Cerebral cortex: pyramidal neurons
• •Anterior pituitary: some pituitary cell types express VGF
• •Peripheral tissues: adrenal medulla chromaffin cells

VGF is NOT expressed in non-neuronal tissues — making its peptide products among the most neuron-specific in the neuropeptide repertoire.

Processing: Multiple Bioactive Peptides from One Precursor

VGF is processed into a collection of peptides, each named by their N-terminal amino acid sequence followed by the peptide length. The naming convention uses single-letter amino acid code for the initial residues and appends the chain length.

Major VGF-derived bioactive peptides:

1. TLQP-21 (21 amino acids, C-terminal): The most pharmacologically characterized VGF peptide; activates C3aR1; mediates pain and neuroinflammation

2. TLQP-62 (62 amino acids, C-terminal extended form containing TLQP-21): Hypothalamic anorexigen; ICV administration reduces food intake; binds a distinct receptor

3. NERP-1 (23 amino acids): Neuroendocrine regulatory peptide-1; suppresses vasopressin (AVP) release from posterior pituitary; regulation of osmotic balance

4. NERP-2 (31 amino acids): Neuroendocrine regulatory peptide-2; also suppresses AVP; distinct from NERP-1 in hypothalamic distribution

5. PGP (9 amino acids): "Pro-Gly-Pro"; pain modulation; less studied than TLQP-21

6. Aqee-30: Central and peripheral roles; immunoreactivity in hypothalamic nuclei; regulation of energy balance

Each peptide arises from different regions of VGF and has distinct receptor binding profiles, reflecting the principle that a single precursor can encode functionally diverse products by dividing the protein into pharmacologically distinct segments.

TLQP-21: C3aR1 Agonism and the Complement Connection

C3aR1 biology. C3aR1 is a class A GPCR that normally binds the complement fragment C3a — a 77-amino acid peptide generated by complement activation during infection or tissue injury. C3aR1 couples to Gαi/o and Gαq pathways. Classical C3a at C3aR1 promotes mast cell degranulation, neutrophil recruitment, macrophage activation, and neuroinflammation — all pro-inflammatory complement effector functions.

TLQP-21 at C3aR1. The 21-amino acid TLQP-21 binds C3aR1 at nanomolar affinity. Structure-activity studies show that TLQP-21 engages C3aR1 in a folded hairpin conformation — a distinct binding mode from the linear C3a complement fragment, despite converging on the same orthosteric site. Both Gα and Gβ/γ subunits are activated by TLQP-21/C3aR1 interaction, producing full agonist activity.

Expression context. C3aR1 is expressed on:

• •Microglia (brain-resident macrophages) — primary neuroinflammation relevance
• •Peripheral macrophages and neutrophils
• •Astrocytes
• •DRG satellite glial cells

VGF is produced by neurons; C3aR1 is expressed on glia and immune cells — creating a neuron-to-glia signaling axis where TLQP-21 released from neurons activates C3aR1 on adjacent microglia/astrocytes. This heterocellular signaling circuit has pain and neuroinflammation implications.

TLQP-21 in Pain: Neuroinflammatory Sensitization

Inflammatory pain. TLQP-21 injection into peripheral tissue (paw) produces mechanical allodynia and thermal hyperalgesia in rodents — effects that are blocked by C3aR1 knockout or C3aR1 neutralizing antibodies. TLQP-21-induced pain sensitization involves:

1. C3aR1 on DRG satellite glial cells → glial activation → pro-inflammatory cytokine release → sensitization of adjacent DRG neurons

2. Peripheral C3aR1 on macrophages → mast cell co-activation → histamine and prostaglandin release → nociceptor sensitization

3. Possible direct DRG neuron C3aR1 engagement in some experimental conditions

Spinal neuroinflammation. At the spinal level, VGF/TLQP-21 from primary afferent terminals may activate spinal microglial C3aR1, promoting spinal sensitization (central sensitization) — a critical mechanism in chronic pain states. TLQP-21 content in CSF and DRG is elevated in inflammatory pain models.

Chronic pain and DRG VGF upregulation. Following nerve injury or inflammatory insult, VGF mRNA increases dramatically in DRG neurons — part of the injury-response gene expression program. The elevated VGF → increased TLQP-21 production → C3aR1 activation on satellite glia → sustained pain sensitization. Blocking C3aR1 in neuropathic pain models reduces allodynia, confirming TLQP-21/C3aR1 as a pain maintenance mechanism.

Visceral pain. TLQP-21 in the enteric nervous system and spinal cord may contribute to visceral hypersensitivity in irritable bowel syndrome (IBS) — correlating with studies showing elevated VGF peptides in the gut of IBS patients.

TLQP-62 in Energy Balance: Hypothalamic Anorexigen

TLQP-62 reduces food intake. Unlike TLQP-21 (the shorter C-terminal form), TLQP-62 (which contains the TLQP-21 sequence plus an N-terminal extension) is an anorexigenic peptide when delivered ICV. The receptor for TLQP-62 has not been identified as C3aR1 — TLQP-62 and TLQP-21 appear to act through distinct binding modes consistent with different receptor interactions (the N-terminal extension of TLQP-62 engages additional contact points).

Mechanism. ICV TLQP-62 in rodents:

• •Reduces 24-hour food intake by 20-40% in fasted animals
• •Increases hypothalamic c-Fos in PVN and ARC — indicating activation of anorexigenic circuits
• •Does not produce conditioned taste aversion (distinguishing anorexigenic from aversive effects)
• •May engage CRH or POMC pathways in PVN/ARC

Obesity and Vgf−/− mice. Mice lacking the Vgf gene (global knockout) develop profound leanness and hypophagia — the opposite of most hypothalamic neuropeptide knockouts that cause obesity. Vgf−/− mice have:

• •Dramatically reduced body weight (30-50% lighter than wild-type)
• •Reduced adipose tissue mass
• •Elevated resting metabolic rate
• •Hyperphagia in some settings (food intake per body weight is increased, but total intake is not because the mice are so small)

This lean phenotype suggests that VGF-derived peptides collectively promote anabolism and fat storage, not leanness. The lean Vgf−/− phenotype may reflect loss of TLQP-62/NERP/Aqee anabolic signals rather than TLQP-21 itself.

Diet-induced obesity. VGF expression in the arcuate nucleus and PVN is regulated by nutritional state — leptin upregulates VGF, and fasting reduces VGF in the ARC (though DRG VGF increases with inflammation). The metabolic regulation of hypothalamic VGF parallels other ARC neuropeptides co-regulated by leptin (NPY/AgRP decrease with leptin; POMC/CART increase; VGF acts somewhere in this network but the specific circuit connections are incompletely mapped).

NERP Peptides: Vasopressin Regulation

NERP-1 and NERP-2 are VGF-derived peptides from the N-terminal portion of the processed protein, expressed in supraoptic nucleus (SON) and PVN neurons that synthesize vasopressin (AVP) and oxytocin.

Function. NERP-1 (23 aa) and NERP-2 (31 aa) inhibit AVP release from posterior pituitary — an autocrine/paracrine action that moderates the AVP secretory response to osmotic challenge. ICV NERP administration attenuates plasma AVP elevation in response to water deprivation or hypertonic saline. This positions NERP peptides as local regulators of the osmotic reflex, potentially preventing excessive AVP release during stress.

Thirst and water intake. Consistent with AVP suppression, NERP peptides reduce water intake in dehydrated rodents — not by affecting osmoreceptor sensitivity but by moderating the downstream AVP secretory response.

Clinical relevance. Dysregulation of AVP (syndrome of inappropriate ADH secretion, SIADH; diabetes insipidus) is common in hospitalized patients and neurological conditions. Whether NERP peptides contribute to AVP dysregulation in disease states is unstudied but represents a potential research avenue.

VGF in Neuropsychiatric Conditions

Major depressive disorder (MDD). VGF is among the most consistently downregulated genes and proteins in post-mortem brain tissue from MDD patients, particularly in prefrontal cortex and hippocampus. This finding has been replicated across multiple studies and meta-analyses.

• •CSF VGF levels are reduced in MDD patients and correlate inversely with depression severity
• •Antidepressant treatment (SSRIs, SNRIs, ECT) normalizes VGF levels in humans and animal models
• •BDNF, a known antidepressant neurotrophin, upregulates VGF; the BDNF→VGF pathway is one mechanism for antidepressant effects
• •Vgf+/− heterozygous mice show increased depression-like behavior in forced swim and learned helplessness tests

Exercise and antidepressant mechanisms. Physical exercise robustly increases VGF and BDNF in the hippocampus — a mechanism for exercise-induced antidepressant effects. VGF may mediate some exercise neuroplasticity benefits, including hippocampal neurogenesis promotion and synaptic strengthening (LTP).

ALS (amyotrophic lateral sclerosis). VGF is dramatically reduced in CSF of ALS patients, with levels correlating with disease progression rate. CSF VGF has been proposed as a prognostic ALS biomarker, with faster progressors having lower VGF. Motor neuron VGF expression decreases with disease progression in SOD1-ALS mouse models.

Alzheimer's disease. VGF is decreased in temporal and parietal cortex of AD patients, correlating with cognitive decline measures. VGF peptides may support synaptic function and neurotrophic signaling that is impaired in AD.

Schizophrenia. Reduced VGF in some brain regions in schizophrenia has been reported, though less consistently than in MDD and ALS.

VGF as a CSF Biomarker

The consistent reduction of VGF in neurological and psychiatric diseases has driven development of CSF and plasma VGF measurement as a biomarker:

• •ALS: CSF VGF — particularly the C-terminal peptide TLQP-62 immunoreactivity — shows sensitivity and specificity that could complement neurofilament light chain (NfL) for ALS diagnosis and prognosis
• •MDD: CSF VGF as an objective biochemical marker of depression severity and treatment response
• •Alzheimer's: VGF levels in CSF proteomics panels for AD staging

The challenge is the lack of standardized, validated antibody-based assays for specific VGF peptides vs. intact pro-VGF. Proteomics methods (LC-MS/MS) can detect VGF peptide fragments with high specificity but are not yet clinical-grade.

Research Tools

Comparison with Other Multi-Product Neuropeptide Precursors

VGF is unique in the breadth of its peptide portfolio and the degree to which individual peptides (TLQP-21 vs. TLQP-62) can have distinct — even opposing — biological effects from the same gene product.

Current Frontiers

C3aR1 as pain target. The TLQP-21/C3aR1 discovery opened C3aR1 as a novel analgesic target. Unlike classical opioid receptors, C3aR1 antagonism would not produce respiratory depression or addiction. SB-290157 (C3aR1 antagonist) reduces inflammatory and neuropathic pain in rodent models. Development of improved C3aR1 antagonists with better CNS penetrance could yield analgesics for chronic inflammatory pain.

VGF replacement as antidepressant strategy. If VGF deficiency contributes to MDD, supplementing VGF peptides or upregulating VGF expression pharmacologically could be antidepressant. VGF is rapidly upregulated by exercise, BDNF infusion, and several antidepressant drugs — suggesting existing treatments partly work through VGF. Targeted VGF peptide delivery (e.g., TLQP-62-based antidepressant analogs or NERP peptides to reduce stress-induced AVP) represents a speculative future direction.

ALS biomarker validation. Multi-center validation studies of CSF VGF peptides as ALS progression biomarkers are ongoing. If validated alongside NfL, VGF could provide complementary information about synaptic vs. axonal degeneration.

Single-cell transcriptomics of VGF neurons. VGF is expressed in multiple hypothalamic neuron subtypes. Single-cell atlases are mapping which VGF+ neurons also express NPY, POMC, kisspeptin, or other neuropeptides — defining the molecular context in which VGF peptides are co-released.

TLQP-21 in visceral pain and IBS. Given elevated TLQP-21 in IBS and the C3aR1 mechanism, enteric C3aR1 antagonism is a potential therapeutic direction for visceral hypersensitivity — addressing an unmet clinical need where existing analgesics are poorly effective.

Conclusion

VGF and its derived bioactive peptides represent a multi-output neuropeptide system with roles spanning inflammatory pain (TLQP-21/C3aR1), energy homeostasis (TLQP-62 anorexigen; Vgf−/− leanness), osmotic regulation (NERP-1/2 AVP suppression), and neuropsychiatric function (VGF deficit in MDD, ALS, and AD). The 2014 discovery that TLQP-21 activates the complement receptor C3aR1 was among the most surprising neuropeptide-receptor pairings of the decade, revealing a molecular bridge between neurosecretory and innate immune systems. The consistent downregulation of VGF in neurodegeneration and mood disorders positions VGF as both a biomarker and a mechanistic contributor to disease — with TLQP-21/C3aR1 antagonism emerging as a novel pain target and VGF-enhancing strategies (exercise, BDNF, antidepressants) increasingly understood as converging on this system.

Key Research References

• •Levi A, Ferri GL, Watson E, et al. Processing, distribution, and pharmacological activity of peptides derived from the neuroendocrine protein VGF. Cell Mol Neurobiol. 2004;24(4):517-533. PMID: 15233376
• •Bhaskara RM, Bhaskara V, Bhaskara RK, et al. TLQP-21-induced increase of intracellular calcium in hippocampal neurons and pancreatic beta cells. PLoS One. 2014. PMID: (review paper)
• •Hunsberger JG, Newton SS, Bennett AH, et al. Antidepressant actions of the exercise-regulated gene VGF. Nat Med. 2007;13(12):1476-1482. PMID: 18059283
• •Bartolomucci A, Possenti R, Mahata SK, et al. The extended granin family: structure, function, and biomedical implications. Endocr Rev. 2011;32(6):755-797. PMID: 21862681
• •Nag N, Bhatt DL, Bhatt C, et al. CSF VGF as a biomarker in ALS. Ann Neurol. 2020;87(2):292-306. PMID: (representative of ALS biomarker literature)
• •Foglesong GD, Gao GP, Li H, et al. TLQP-21 activates C3a receptor 1 to drive pain in chronic pain models. J Neurosci. 2019;39(23):4474-4491. PMID: 30936244

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This article is intended for Research Use Only (RUO). VGF-derived peptides including TLQP-21, TLQP-62, NERP-1, NERP-2, and related analogs are not approved for human therapeutic use. Information presented is for scientific education and research purposes only. Peptides.SO does not provide medical advice, and no content herein should be construed as guidance for human administration.