Next generation organelles: structure and role of germ granules in the germline

Abstract

Germ cells belong to a unique class of stem cells that gives rise to eggs and sperm, and ultimately to an entire organism after gamete fusion. In many organisms, germ cells contain electron-dense structures that are also known as nuage or germ granules. Although germ granules were discovered more than 100 years ago, their composition, structure, assembly, and function are not fully understood. Germ granules contain non-coding RNAs, mRNAs, and proteins required for germline development. Here we review recent studies that highlight the importance of several protein families in germ granule assembly and function, including germ granule inducers, which initiate the granule formation, and downstream components, such as RNA helicases and Tudor domain-Piwi protein-piRNA complexes. Assembly of these components into one granule is likely to result in a highly efficient molecular machine that ensures translational control and protects germline DNA from mutations caused by mobile genetic elements. Furthermore, recent studies have shown that different somatic cells, including stem cells and neurons, produce germ granule components that play a crucial role in stem cell maintenance and memory formation, indicating a much more diverse functional repertoire for these organelles than previously thought.

Figure 1

Illustration of microtubule activity, the localization of osk mRNA and Baz protein during initiation of germ plasm assembly in Drosophila oocyte. During oogenesis stage 2 to 6, microtubules group the minus-ends in the oocyte and reach the plus-ends into nurse cells. In stage 6 to 7, microtubules appear from the entire oocyte cortex and rearrange so that the plus-ends cluster towards the center of the oocyte, along with the disassembly of early-stage microtubule organizing center (MTOC). At stage 8 to 9, microtubules at the posterior pole disassemble, followed by the relocation of microtubule plus-ends to the posterior pole. This microtubule reorganization during mid-oogenesis requires the functionality of Baz, which localizes to the anterior and lateral cortex. At stage 9, osk mRNA is transported to the posterior pole and its translation derepression is initiated by the accumulation of microtubule plus-ends. Then, the resulted Osk protein starts the germ plasm assembly by recruiting other germ plasm components such as Vas and Tud (Becalska and Gavis, 2010; Breitwieser et al., 1996; Ephrussi and Lehmann, 1992; Steinhauer and Kalderon, 2006). (Part of the figure is modified from Steinhauer and Kalderon, 2006).

Figure 2

Buc protein activity during germ plasm assembly in zebrafish oocyte. Buc (indicated in red) localizes to the Balbiani body in the oocyte and functions to recruit germ plasm RNA components including dazl, vas, and nos. During development, Buc gradually spreads within the vegetal pole (Bontems et al., 2009).

Figure 3

PGL proteins utilize the two types of functional domains to start formation of P granule-like aggregates. In step 1, RNAs and RNA binding proteins exist as small RNPs. In step 2, PGL proteins use the RGG box to bind to the RNA components in the RNPs. In step 3, PGL proteins associated with RNPs begin self-aggregation through the self-interacting domains. In step 4, PGL protein self-aggregation continues to build P granules (Hanazawa et al., 2011).

Figure 4

Germ granule assembly through Tudor-Piwi interaction. The Tudor-Piwi interactions may exhibit a combination of several possible binding patterns as follows. First, a Piwi protein contains one sDMA which binds to one Tud domain. Second, a Piwi protein contains multiple sDMAs each of which binds to one Tud domain on the same Tud domain-containing protein. Third, a Piwi protein contains multiple sDMAs each of which binds to one Tud domain on different Tud domain-containing proteins (Chen et al., 2011).

Figure 5

Prion-like germ granule assembly mechanisms exemplified by RNA binding proteins containing Low Complexity (LC) sequences and RNA binding domains. The LC sequences enable the RNA binding proteins to exist in one of the following three states: a monomeric, soluble state; a polymeric, amyloid-like fiber state; or a pathogenic aggregate state. The transition between the first two states is reversible, allowing RNA binding proteins to enter or exit the prion-like aggregated cellular structure. However, the conversion to the third state is irreversible and pathogenic (Han et al., 2012; Kato et al., 2012).