Immunohistochemical localization of MD2, a co-receptor of TLR4, in the adult mouse brain

Abstract

Background: MD2, a co-receptor of a classical proinflammatory protein TLR4 whose activation leads to neuroinflammation. It is widely accepted that TLR4 is expressed on the cell surface of microglia and astrocytes, and MD2 is expected to be expressed by these cells as well. However, our previous study showed that neurons from certain nuclei also expressed MD2. Whether MD2 is expressed by other brain nuclei is still unknown. It is the aim of the present study to map the distribution of MD2 positive cells in the adult mouse brain.Methods: Immunohistochemical staining against MD2 was completed to localize MD2 positive cells in the mouse brain by comparing the location of positive cells with the mouse brain atlas. Retrograde tracing and double immunofluorescence staining with both MD2 and TH were conducted to examine whether dopaminergic neurons in the substantia nigra projecting to the striatum were MD2 positive.Results: MD2 positive cells were found in the majority of mouse brain nuclei with clusters of cells in the olfactory bulb, cortices, the red nucleus, and cranial nuclei. Subcortical nuclei had heterogenous staining of MD2 with more prominent cells in the basolateral and the central amygdaloid nuclei. The ventral pallidum and the diagonal bands had positive cells with similar density and shape. Prominent cells were present in thalamic nuclei which were nearly homogenous, reticular formation of the brainstem where cells were dispersed with similar density. The hypothalamus had fewer outstanding cells compared with the thalamus. The red nucleus, the substantia nigra, and the ventral tegmental area in the pretectum had outstanding cells with the latter two containing dopaminergic neurons projecting to the striatum as shown in retrograde tracing and double fluorescence staining. Raphe, sensory cranial, and deep cerebellar nuclei also had MD2 positive cells with moderate density.Conclusion: MD2 is present not only in glial cells, but also in neurons. Some MD2 positive dopaminergic neurons in SNC and VTA projected to the striatum.