Mitochondria are dynamic organelles that frequently move, divide, and fuse with one another to maintain their architecture and functions. However, the signaling mechanisms involved in these processes are still not well characterized. In this study, we analyze mitochondrial dynamics and morphology in neurons. Using time-lapse imaging, we find that Ca2+ influx through voltage-dependent Ca2+ channels (VDCCs) causes a rapid halt in mitochondrial movement and induces mitochondrial fission. VDCC-associated Ca2+ signaling stimulates phosphorylation of dynamin-related protein 1 (Drp1) at serine 600 via activation of Ca2+/calmodulin-dependent protein kinase Iα (CaMKIα). In neurons and HeLa cells, phosphorylation of Drp1 at serine 600 is associated with an increase in Drp1 translocation to mitochondria, whereas in vitro, phosphorylation of Drp1 results in an increase in its affinity for Fis1. CaMKIα is a widely expressed protein kinase, suggesting that Ca2+ is likely to be functionally important in the control of mitochondrial dynamics through regulation of Drp1 phosphorylation in neurons and other cell types.

Protein-protein interaction plays important roles in transducing signals elicited from receptors on the cell surface to the nucleus. The Src homology 2 (SH2) and SH3 domains have been shown to bind to tyrosine-phosphorylated proteins (33) and proline-rich motifs (54), respectively (reviewed in references 4, 27, 51, and 61). There are a number of signaling molecules involved in the tyrosine kinase cascade which have either the SH2 domain or the SH3 domain or both, including the GTPase-activating protein for Ras (Ras GAP), phospholipase C-␥, the p85 subunit of PI-3 kinase, and Src and related kinases. While these molecules have enzymatic activities, there is another group of molecules that consist mostly of SH domains without any enzymatic domains. Crk (34, 38), Grb2/Ash (30, 37), Shc (52), and Nck (28) belong to the latter group, the so-called adapter molecules. These multivalent adapter molecules may connect signaling molecules on them. Besides the SH2 and SH3 domains, recently the pleckstrin homology domain also has been identified in a number of signaling molecules (40).The stimulation of cells with various growth factors or cytokines activates their cognate receptor tyrosine kinases or nonreceptor tyrosine kinases associating with the receptors, which results in the tyrosine phosphorylation of various signaling molecules (60). Each tyrosine-phosphorylated molecule is recognized and bound by a specific SH2 domain (64, 65). Ras GAP, phospholipase C-␥, and the p85 subunit of PI-3 kinase have been shown to become tyrosine phosphorylated and bind to activated receptors through their SH2 domains (4,27,51,61). The interaction between SH3 domains and proline-rich motifs seems to be constitutive and not to depend on stimulation of the cells (54). These interactions by the SH2 and SH3 domains are responsible for the formation of multimolecular signaling complexes, some of which translocate to the plasma membrane.

The worldwide prevalence of type 2 diabetes (T2D), which is caused by a combination of environmental and genetic factors, is increasing. With regard to genetic factors, variations in the gene encoding Cdk5 regulatory associated protein 1-like 1 (Cdkal1) have been associated with an impaired insulin response and increased risk of T2D across different ethnic populations, but the molecular function of this protein has not been characterized. Here, we show that Cdkal1 is a mammalian methylthiotransferase that biosynthesizes 2-methylthio-N 6 -threonylcarbamoyladenosine (ms 2 t 6 A) in tRNA Lys (UUU) and that it is required for the accurate translation of AAA and AAG codons. Mice with pancreatic β cell-specific KO of Cdkal1 (referred to herein as β cell KO mice) showed pancreatic islet hypertrophy, a decrease in insulin secretion, and impaired blood glucose control. In Cdkal1-deficient β cells, misreading of Lys codon in proinsulin occurred, resulting in a reduction of glucosestimulated proinsulin synthesis. Moreover, expression of ER stress-related genes was upregulated in these cells, and abnormally structured ER was observed. Further, the β cell KO mice were hypersensitive to high fat diet-induced ER stress. These findings suggest that glucose-stimulated translation of proinsulin may require fully modified tRNA Lys (UUU), which could potentially explain the molecular pathogenesis of T2D in patients carrying cdkal1 risk alleles.

Oxytocin is an essential hormone for mammalian labor and lactation. Here, we show a new function of oxytocin in causing plastic changes in hippocampal synapses during motherhood. In oxytocin-perfused hippocampal slices, one-train tetanus stimulation induced long-lasting, long-term potentiation (L-LTP) and phosphorylation of cyclic AMP-responsive element binding protein (CREB), and MAP kinase inhibitors blocked these inductions. An increase in CREB phosphorylation and L-LTP induced by one-train tetanus were observed in the multiparous mouse hippocampus without oxytocin application. Furthermore, intracerebroventricular injection of oxytocin in virgin mice improved long-term spatial learning in vivo, whereas an injection of oxytocin antagonist in multiparous mice significantly inhibited the improved spatial memory, L-LTP and CREB phosphorylation. These findings indicate that oxytocin is critically involved in improving hippocampus-dependent learning and memory during motherhood in mice.