Distribution and Levels of Insulin-like Growth Factor 2 Receptor Across Mouse Brain Cell Types.

Abstract

Background: The insulin-like growth factor 2 receptor (IGF-2R), also known as the cation-independent mannose 6-phosphate receptor (CI-M6PR), is emerging as a critical receptor for brain function and disease. IGF-2R, in fact, plays a key role in long-term memory, and its activation by several ligands shows beneficial effects in multiple neurodevelopmental and neurodegenerative disease models. Thus, its targeting is very promising for neuropsychiatric therapeutic interventions. IGF-2R's main known functions are transport of lysosomal enzymes and regulation of developmental tissue growth, but in the brain, it also controls learning-dependent protein synthesis underlying long-term memory. However, little is known about this receptor in brain cells, including its cell-type-specific and subcellular expression.

Methods: We conducted a comprehensive investigation to comparatively assess IGF-2R protein levels in different brain cell types across various brain regions in adult male C57BL/6J mice using dual and multiplex immunofluorescent staining with cell-type-specific markers. The IGF-2R protein distribution was also compared with Igf2r mRNA expression in publicly available single-cell RNA sequencing databases.

Results: A ranking of IGF-2R levels in the soma of various cell types in the hippocampus and cortical regions revealed that the highest enrichment is, by far, in excitatory and inhibitory neurons, followed by vascular mural cells and subpopulations of oligodendrocyte lineage cells, with low to undetectable levels in astrocytes, microglia, vascular endothelial cells, and perivascular fibroblasts. High levels of IGF-2R were also found in ependymal cells, choroid plexus epithelial cells, and a subpopulation of meningeal fibroblast-like cells. IGF-2R was found in dendritic and putative axonal compartments throughout the brain, with particularly high levels in the stratum lucidum. The receptor's protein distribution aligned with that of the mRNA in mouse brain databases.

Conclusions: These results suggest that IGF-2R-mediated functions in the brain vary across different cell types and subcellular compartments, with the most active roles in specific subpopulations of neurons, mural cells, ependymal cells, meningeal cells, and cells of the oligodendrocyte lineage. This study advances our understanding of IGF-2R's distribution in the brain, which is essential for formulating new hypotheses about its functions and therapeutic targeting.