Intranasal iron administration induces iron deposition, immunoactivation and cell-specific vulnerability in the olfactory bulb of C57BL/6 mice

Iron is the most abundant transition metal in the brain, essential for brain development and neuronal function. Whereas, iron deposition is associated with neurological disorders. The olfactory bulb (OB) serves as an entrance for environmental toxins and is early affected in neurodegenerative disease. It contains diverse neuronal populations with distinct characteristics and functions. Here, we explored cell-specific vulnerability to iron in the OB by establishing a mouse model of intranasal ferric ammonium citrate (FAC) administration. Olfactory discrimination tests assessed olfactory function. ICP-MS, Perl's-DAB/immunohistochemical double staining and an iron assay kit were utilized to evaluate iron levels in the OB tissues, cerebrospinal fluid (CSF) and serum. RNA-sequencing and immune infiltration analysis were performed to analyze the OB transcriptome and immune response. Results showed intranasal FAC administration caused olfactory dysfunction. Olfactory mucosa iron deposition, olfactory sensory neurons damage and OB iron deposition were observed without increasing CSF or blood iron levels. OB iron deposition activated multiple immune cells, microglia and astrocytes, without inducing ferroptosis. Neuronal and glial identities were retrieved by spatial transcriptome sequencing dataset of a healthy adult mouse OB. Cellular heterogeneity was observed in the mouse OB, with abundant neuroglia and neurons; in neurons, GABAergic neurons were most abundant cell type, followed by glutamatergic neurons and then dopaminergic neurons, while cholinergic and serotonergic neurons were sparsely populated. Damage to oligodendrocytes, dopaminergic and glutamatergic neurons was observed, while GABAergic neurons remained unchanged. These results provide new perspectives for understanding the loss of specific neurons and oligodendrocytes under iron stressed conditions.