Ji Changyi, Kosman Daniel J
Department of Biochemistry, State University of New York, School of Medicine and Biomedical Sciences Buffalo, Buffalo, New York, USA.
J Neurochem. 2015 Jun;133(5):668-83. doi: 10.1111/jnc.13040. Epub 2015 Mar 10.
The molecular mechanisms of iron trafficking in neurons have not been elucidated. In this study, we characterized the expression and localization of ferrous iron transporters Zip8, Zip14 and divalent metal transporter 1 (DMT1), and ferrireductases Steap2 and stromal cell-derived receptor 2 in primary rat hippocampal neurons. Steap2 and Zip8 partially co-localize, indicating these two proteins may function in Fe(3+) reduction prior to Fe(2+) permeation. Zip8, DMT1, and Steap2 co-localize with the transferrin receptor/transferrin complex, suggesting they may be involved in transferrin receptor/transferrin-mediated iron assimilation. In brain interstitial fluid, transferring-bound iron (TBI) and non-transferrin-bound iron (NTBI) exist as potential iron sources. Primary hippocampal neurons exhibit significant iron uptake from TBI (Transferrin-(59) Fe(3+)) and NTBI, whether presented as (59) Fe(2+) -citrate or (59) Fe(3+) -citrate; reductase-independent (59) Fe(2+) uptake was the most efficient uptake pathway of the three. Kinetic analysis of Zn(2+) inhibition of Fe(2+) uptake indicated that DMT1 plays only a minor role in the uptake of NTBI. In contrast, localization and knockdown data indicate that Zip8 makes a major contribution. Data suggest also that cell accumulation of (59) Fe from TBI relies at least in part on an endocytosis-independent pathway. These data suggest that Zip8 and Steap2 play a major role in iron accumulation from NTBI and TBI by hippocampal neurons. Analysis of the expression and localization of known iron uptake transporters demonstrated that Zip8 makes a major contribution to iron accumulation in primary cultures of rat embryonic hippocampal neurons. These cells exhibit uptake pathways for ferrous and ferric iron (non-transferrin-bound iron, NTBI in figure) and for transferrin-bound iron; the ferrireductases Steap2 and SDR2 support the uptake of ferric iron substrates. Zip8 and Steap2 are strongly expressed in the plasma membrane of both soma and processes, implying a crucial role in iron accumulation from NTBI and transferrin-bound iron (TBI) by hippocampal neurons.
神经元中铁转运的分子机制尚未阐明。在本研究中，我们对原代大鼠海马神经元中二价铁转运蛋白Zip8、Zip14和二价金属转运蛋白1（DMT1）以及铁还原酶Steap2和基质细胞衍生受体2的表达和定位进行了表征。Steap2和Zip8部分共定位，表明这两种蛋白可能在Fe(2+)渗透之前的Fe(3+)还原过程中发挥作用。Zip8、DMT1和Steap2与转铁蛋白受体/转铁蛋白复合物共定位，表明它们可能参与转铁蛋白受体/转铁蛋白介导的铁吸收。在脑间质液中，转铁蛋白结合铁（TBI）和非转铁蛋白结合铁（NTBI）作为潜在的铁源存在。原代海马神经元无论以（59）Fe(2+)-柠檬酸盐还是（59）Fe(3+)-柠檬酸盐形式呈现，都表现出从TBI（转铁蛋白-（59）Fe(3+)）和NTBI中显著摄取铁；不依赖还原酶的（59）Fe(2+)摄取是这三种摄取途径中最有效的。Zn(2+)对Fe(2+)摄取抑制的动力学分析表明，DMT1在NTBI摄取中仅起次要作用。相比之下，定位和敲低数据表明Zip8起主要作用。数据还表明，TBI中（59）Fe的细胞积累至少部分依赖于不依赖内吞作用的途径。这些数据表明，Zip8和Steap2在海马神经元从NTBI和TBI中积累铁的过程中起主要作用。对已知铁摄取转运蛋白的表达和定位分析表明，Zip8在大鼠胚胎海马神经元原代培养物中的铁积累中起主要作用。这些细胞表现出对二价铁和三价铁（图中非转铁蛋白结合铁，NTBI）以及转铁蛋白结合铁的摄取途径；铁还原酶Steap2和SDR2支持三价铁底物的摄取。Zip8和Steap2在胞体和突起的质膜中均强烈表达，这意味着它们在海马神经元从NTBI和转铁蛋白结合铁（TBI）中积累铁的过程中起关键作用。
