Among these, the most intriguing

is the connection between mitochondria and ER. These two organelles are linked, both biochemically and physically (Csordás et al., 2006), via mitochondria-associated ER membranes (ER-MAM, or MAM) (Rusiñol et al., 1994). Located mainly in the perinuclear region of cells (Area-Gomez et al., 2009 and Schon and Area-Gomez, 2010), MAM has been reported to be enriched in more than 75 proteins, including those involved in calcium homeostasis (e.g., MK-2206 molecular weight inositol-1,4,5-triphosphate [IP3] receptors [IP3Rs] and ryanodine receptors), in lipid metabolism (e.g., phosphatidylethenolamine N-methyltransferase), in intermediate metabolism (e.g., glucose-6-phosphatase), in cholesterol metabolism (e.g., acyl-coenzyme A:cholesterol acyltransferase 1 [ACAT1]), in the transfer of lipids between the ER and mitochondria (e.g., fatty acid transfer proteins 1 and 4), and in ER stress (e.g., glucose-regulated proteins 75 and 78) (Hayashi et al., 2009b). Contacts between the two organelles are maintained by MAM-associated proteins, such as phosphofurin acidic cluster sorting protein-2

(Simmen et al., 2005) and mitofusin-2 (MFN2), which is also required CHIR-99021 in vitro for mitochondrial fusion (de Brito and Scorrano, 2008). Interestingly, fission-1 (FIS1), a protein required for mitochondrial fission, has recently also been localized to the MAM (Iwasawa et al., 2011). The relationship between MAM and calcium trafficking (Csordás et al., 2010) is

worthy of some elaboration. As alluded to above, two cargo adaptor proteins discovered initially in Drosophila—Miro and Milton—are implicated in the specific linkage of mitochondria to kinesin-1 in neurons. Miro is anchored to the mitochondrial outer membrane ( Guo et al., 2005), and binds to the mitochondrial-specific adaptor protein Milton, which CYTH4 is linked to the kinesin-1 heavy chain ( Brickley et al., 2005, Glater et al., 2006 and Koutsopoulos et al., 2010). Miro is a calcium-binding protein ( Fransson et al., 2003), and thus has the potential for being a regulator of mitochondrial motility in neurons, in essence operating as a sensor of local [Ca2+] and ATP. It has been proposed that in the Ca2+-unbound state, Miro binds Milton and mitochondria are attached to microtubules, whereas in the Ca2+-bound state, Miro cannot bind Milton and mitochondria are uncoupled from microtubules ( Rice and Gelfand, 2006). This model is consistent with the “saltatory movement” model proposed by Hajnóczky ( Liu and Hajnóczky, 2009 and Yi et al., 2004), in which mitochondria move only when local [Ca2+] is low, and stop when the local [Ca2+] is high. Notably, only Ca2+ mobilized via IP3Rs (or, in muscle, via the related ryanodine receptors) could generate this result. We note, however, that very few of the experiments supporting this model have been conducted in mammalian neurons.