Manganese uptake and distribution in the central nervous system (CNS)

Information about the nature of manganese (Mn)-binding ligands in plasma and serum, and its transport mechanism across the blood-brain barrier (BBB) is sparse. Most studies to date have focused on distribution, excretion, and accumulation of intravenous and intraperitoneal solutions of soluble divalent salts of Mn. Mn is transported in the blood primarily in the divalent oxidation state (Mn²⁺) and crosses the BBB via specific carriers at a rate far slower than in other tissues. Mn transport across the BBB occurs both in the 2+ and 3+ oxidation state. Within the CNS, Mn accumulates primarily within astrocytes, presumably because the astrocyte-specific enzyme, glutamine synthetase (CS), represents an important regulatory target of Mn. Compared to Mn²⁺, Mn³⁺ has a slower elimination rate and therefore, may have a greater tendency to accumulate in tissues. Furthermore, in view of the dependence of Mn accumulation within the CNS on iron (Fe) homeostasis, the oxidation state of Mn may represent a key determinant in the differential distribution, accumulation and secretion profiles of Mn, a fact that has received little attention in experimental biology toxicology. Accordingly, the distribution and membrane transport of Mn emphasizes the importance of: 1) the oxidation state of Mn, as it governs the affinity of Mn to endogenous ligands, and 2) the reaction of Mn³⁺ with transferrin, the plasma iron- carrying protein. This review will focus on transport kinetics of Mn across the BBB (both in the 2+ and 3+ oxidation state), the putative role of transferrin in the transport of Mn across the BBB, the transport of Mn by astrocytes, as well as the physiological significance of Mn to the function GS.