The Caenorhabditis elegans germ line provides a model for understanding how signaling from a stem cell niche promotes continued mitotic divisions at the expense of differentiation. Here we report cellular analyses designed to identify germline stem cells within the germline mitotic region of adult hermaphrodites. Our results support several conclusions. First, all germ cells within the mitotic region are actively cycling, as visualized by bromodeoxyuridine (BrdU) labeling. No quiescent cells were found. Second, germ cells in the mitotic region lose BrdU label uniformly, either by movement of labeled cells into the meiotic region or by dilution, probably due to replication. No label-retaining cells were found in the mitotic region. Third, the distal tip cell niche extends processes that nearly encircle adjacent germ cells, a phenomenon that is likely to anchor the distal-most germ cells within the niche. Fourth, germline mitoses are not oriented reproducibly, even within the immediate confines of the niche. We propose that germ cells in the distal-most rows of the mitotic region serve as stem cells and more proximal germ cells embark on the path to differentiation. We also propose that C. elegans adult germline stem cells are maintained by proximity to the niche rather than by programmed asymmetric divisions. INTRODUCTIONStem cells are responsible for generating tissues during development and maintaining them during adulthood. To accomplish these tasks, stem cells must produce additional stem cells (self-renewal) as well as differentiated cells. Over the past few years, considerable progress has been made in the analysis of individual stem cells in several organisms, including hematopoietic stem cells (HSC) in mammals (Kiel et al., 2005;Shizuru et al., 2005) and germline stem cells (GSC) in Drosophila (Wong et al., 2005). In this article, we investigate the germline mitotic region in the Caenorhabditis elegans adult hermaphrodite, which contains GSC. Parallels between C. elegans GSC and other stem cell systems include use of Notch signaling to control both HSC and C. elegans GSC (Calvi et al., 2003;Kimble and Crittenden, 2005) and use of Puf proteins to control both Drosophila and C. elegans GSC .C. elegans adult GSC are found at the distal end of the gonadal arm within the "mitotic region," which is defined by the presence of mitotically dividing germ cells (see Figure 1, A and B). In adults, the single somatic distal tip cell (DTC) is located at the tip of the mitotic region and forms a stem cell niche (Kimble and White, 1981). The distal sheath cells are important for larval germline proliferation (Killian and Hubbard, 2005), but they have little or no contact with the mitotic region in adults (Hall et al., 1999;Killian and Hubbard, 2005). The DTC and the mitotic germline cells are encapsulated by a thin extracellular matrix, which separates them from neighboring organs (e.g., intestine; Hall et al., 1999;Lints and Hall, 2004). Proximal to the mitotic region, the "transition zone" contains germ cells in early...

Transport of synaptic components is a regulated process. Loss-of-function mutations in the C. elegans unc-16 gene result in the mislocalization of synaptic vesicle and glutamate receptor markers. unc-16 encodes a homolog of mouse JSAP1/JIP3 and Drosophila Sunday Driver. Like JSAP1/JIP3, UNC-16 physically interacts with JNK and JNK kinases. Deletion mutations in Caenorhabditis elegans JNK and JNK kinases result in similar mislocalization of synaptic vesicle markers and enhance weak unc-16 mutant phenotypes. unc-116 kinesin heavy chain mutants also mislocalize synaptic vesicle markers, as well as a functional UNC-16::GFP. Intriguingly, unc-16 mutations partially suppress the vesicle retention defect in unc-104 KIF1A kinesin mutants. Our results suggest that UNC-16 may regulate the localization of vesicular cargo by integrating JNK signaling and kinesin-1 transport.

Kinesin-1 is a heterotetramer composed of kinesin heavy chain (KHC) and kinesin light chain (KLC). The

The mesenchymal distal tip cell (DTC) provides the niche for Caenorhabditis elegans germline stem cells (GSCs). The DTC has a complex cellular architecture: its cell body caps the distal gonadal end and contacts germ cells extensively, but it also includes multiple cellular processes that extend along the germline tube and intercalate between germ cells. Here we use the lag-2 DTC promoter to drive expression of myristoylated GFP, which highlights DTC membranes and permits a more detailed view of DTC architecture. We find that short processes intercalating between germ cells contact more germ cells than seen previously. We define this region of extensive niche contact with germ cells as the DTC plexus. The extent of the DTC plexus corresponds well with the previously determined extent of the GSC pool. Moreover, expression of a differentiation marker increases as germ cells move out of the plexus. Maintenance of this DTC plexus depends on the presence of undifferentiated germ cells, suggesting that germ cell state can influence niche architecture. The roles of this DTC architecture remain an open question. One idea is that the DTC plexus delivers Notch signaling to the cluster of germ cells comprising the GSC pool; another idea is that the plexus anchors GSCs at the distal end.