Cell and Molecular Biology, Imperial College London
Doi
Abstract
The VGF neurosecretory protein, first identified as a nerve growth factor (NGF)
inducible gene product, is selectively synthesised predominantly in neuronal and
neuroendocrine cells. The ~68 kDa VGF protein sequence is rich in paired basic
amino acids, and thus the protein undergoes endoproteolytic cleavage to produce
smaller peptides, which are stored in dense core vesicles and released upon
stimulation via the regulated secretory pathway both in vitro and in vivo. Several of
these VGF-derived peptides have been characterised and are involved in energy
homeostasis, reproductive processes, synaptic plasticity as well as pain modulation. A
number of studies have observed an increase in VGF gene expression in various pain
models and more recently the VGF-derived peptides, TLQP-21, LQEQ-19 and
TLQP-62 showed direct modulation of inflammatory and neuropathic pain when
applied in vivo. The molecular mechanisms of action of VGF-derived peptides are not
well understood and were investigated in this study. The TLQP-21 peptide, but not
LQEQ-19, was shown to dose-dependently induce an increase in intracellular Ca2+
levels from cellular internal stores in brain- and spinal cord-derived primary
microglia, in >65 % of the cell population in vitro. Three hour treatment of primary
microglia with TLQP-21 (100 nM) induced a 2.78 fold increase in Ccl11 and a 2.28
fold decrease in Cxcl9 gene expression levels relative to the vehicle control (Student's
t-test; p ≤ 0.05). Biochemical analysis using affinity chromatography and LC-MS/MS
techniques identified the gC1q-R protein as a potential binding partner / receptor for
TLQP-21. The gC1q-R protein is a ubiquitously expressed, multi-compartmental
protein involved in complement activation, inflammatory processes and the plasma
bradykinin formation pathway. These results tentatively suggest that TLQP-21 may
contribute to the modulation of pain through activation of primary microglia and
potentially involve interactions with components of the complement system. The
findings highlight the importance of VGF-derived peptides in pain research and could
lead to new perspectives and targets for pain therapeutics