INTRODUCTION: Insulin-like growth factor 1 (IGF1) and its receptor (IGF1R) constitute as a crucial regulator in dendritic spine structural plasticity. However, extracellular IGF1 is mainly bound to its binding protein 2 (IGFBP2), which protects IGF1 from degradation but restricts its receptor interaction. High impact publications has shown IGFBP2 as an astrocyte-derived protein crucial for plasticity but its precise cellular origin, localization and role in plasticity remain unclear.

AIM(S): We aim to determine the cellular source and synaptic localization of IGFBP2 and evaluate its role in IGF1-dependent structural plasticity.

METHOD(S): Immunofluorescence was performed in dissociated neuronal cultures, hippocampal slices and organotypic hippocampal cultures to determine IGFBP2 cellular origin. Colocalization with synaptic markers was analyzed to assess enrichment at excitatory synapses. Synaptoneurosomes were isolated and analyzed by Western blotting to show IGFBP2 presence within synapses as well as astrocytic perisynaptic processes. Proteolytic processing of IGFBP2 by MMP-9 was examined using stimulated synaptoneurosomes in the presence or absence of MMP-9 inhibitors. Two-photon imaging combined with glutamate uncaging was used to assess structural long-term potentiation (sLTP) and to test involvement of IGFBP2 and MMP-9 in IGF1R activation.

RESULTS: We show that IGFBP2 is astrocytic protein and localizes in proximity to excitatory synapses. We identify IGFBP2 as crucial for spine volume enlargement and IGF1R activation during sLTP. Moreover, we confirm IGFBP2 cleavage by MMP-9 and their involvement in described extracellular system.

CONCLUSIONS: Astrocyte-derived IGFBP2 localizes on synapses and is required for IGF1R-dependent structural plasticity. MMP-9-mediated processing of IGFBP2 regulates local IGF1 availability at synapses.

FINANCIAL SUPPORT: National Science Centre, Poland grant: 2023/51/B/NZ4/02135