Cdc which is involved in filopodium formation
Cdc42, which is involved in filopodium formation, and Rac1, which is involved in lamellipodium formation, engage in cross-talk with one another [25,26,30,31]. In general, filopodium formation precedes lamellipodium formation, and filopodium formation (Cdc42 activity) suppresses lamellipodium formation (Rac1 activity), and vice versa. Rac1 activity is inhibited by specific GAPs, including chimaerins, which thereby act as inducers of filopodium formation by reducing the suppression of Cdc42. In the present study, we demonstrated for the first time that DGKβ selectively interacts with β2-chimaerin and, to lesser extent, α2-chimaerin, but not with α1- and β1-chimaerins (Fig. 4). Because PA activates β2-chimaerin [27,28], PA produced by DGKβ would augment β2-chimaerin activity. In addition, the induction of the translocation of β2-chimaerin to the plasma membrane induced by DGKβ (Fig. 4) may also enhance the activation of the Rac1-GAP.
DGKγ has a highly similar amino Cisplatin sequence to that of DGKβ . We previously reported that DGKγ serves as an upstream suppressor of Rac1 and lamellipodium formation and interacts with α2- and β2-chimaerin [24,33,34]. Moreover, overexpression of DGKγ induced filopodium formation in neuroblastoma cells . Therefore, DGKβ and DGKγ likely have similar functions in neurons. Although DGKβ and DGKγ are both strongly expressed in the brain, their localization within the brain differs [9,36], indicating that they exert specialized physiological functions in distinct regions.
Acknowledgments This study was supported in part by grants from MEXT/JSPS (Grant Numbers: 26291017, 15K14470, and 17H03650), the Futaba Electronic Memorial Foundation, and the Ono Medical Research Foundation (FS).
Article Interest in diacylglycerol (DAG) was initially raised by pioneering studies on the tumorigenic activity of DAG and analogs such as phorbol esters (Castagna et al., 1982, Sivak and Van Duuren, 1967), as well as by the seminal observation on agonist-induced DAG production in cells (Berridge, 1983). The classical and the novel isoforms of protein kinase C (PKCs) are the prototypical targets of DAG signaling, which bind DAG through their C1 domains. Although several other intracellular proteins containing C1 domains are regulated by DAG (e.g. Ras-GRP and Munc13), other proteins that lack C1-like domains can also be regulated by DAG, such as Transient receptor potential canonical (TRPC) protein (Carrasco and Mérida, 2007). These studies established DAG as the prototype of lipid second messengers, raising interest in its production, localization and metabolism.
DAG production at multiple subcellular sites Biochemically, DAG is lipid characterized by fast lateral (Prieto et al., 1994) and transbilayer (Bai and Pagano, 1997) diffusion rates, due to its small dimensions with an inverted cone profile. Additionally DAG has a low abundance in membranes, ranging from 2 mol% in Ras-transformed kidney cells (Preiss et al., 1986) to 0.8 mol% in fibroblasts (Chang and Huang, 1994). Fast diffusion rates and low abundance in resting cells allows for marked and rapid local variation in its concentration, controlled by the balance between localized synthesis and metabolism. This dynamic lipid is thus multifunctional, serving as a structural component defining membrane shape, a key signaling lipid at the plasma and intracellular membranes, and also a precursor for triglycerides and phospholipids at the endoplasmic reticulum (Fig. 1). Such plurality of functions justifies the existence of multiple “pools” of DAG localized to the plasma membrane, as well as intracellular membranes and nuclei, which are subject to strict control from extracellular cues. Indeed, agonist-induced DAG accumulation has been reported at the plasma membrane, the Golgi and in the nucleus using DAG reporters or biochemical assays (Divecha et al., 1991, Gallegos et al., 2006, Laurin et al., 1998). When just the plasma membrane is considered, localized concentrations of DAG can be observed in specific regions such as the leading edge of migrating cells (Nishioka et al., 2008) or at the immunological synapse (Spitaler et al., 2006). These localized DAG pools are maintained through a balance in its rate of synthesis and degradation by multiple metabolic pathways which are subject to strict regulatory constraints in response to intracellular and extracellular cues (Carrasco and Mérida, 2007).