Strome S (1993) Cell "Determination of cleavage planes."

Strome S

[Cell, 1993]

Most biologists appreciate the precision with which the mitotic spindle separates the duplicated chromosomes into two identical sets. Not as many realize that precision in the subsequent cleavage of the cell into two daughter cells is also important. Selection of the proper orientation and placement of the cleavage plane determines the relative positions of the daughter cells and ensures that each will receive the proper complement of cytoplasmic and membrane material, along with a nucleus.

Strome S (2011) Science "Neuroscience. Toward reprogramming gonads to brains."

Strome S

[Science, 2011]

Strome S (1982) Worm Breeder's Gazette "more antibodies to germ-line-specific granules in C elegans."

Strome S

[Worm Breeder's Gazette, 1982]

In the last newsletter I reported the finding of several commercial preparations of rabbit and goat IgG that fortuitously stain granules unique to the germ line (P granules) in C. elegans. Several additional antibody preparations have now been generated. From a screen of 26 rabbits from the wilds of Arvada, Colorado, the serum from 2 rabbits contains anti-P-granule antibodies. A hybridoma cell line that secretes IgG directed against P granules was obtained by immunizing mice with C. elegans egg homogenate and screening the resulting antibodies by immunofluorescence microscopy on fixed C. elegans eggs. Several new hybridomas that synthesize anti-P-granule antibodies are presently being cloned. Hopefully, at least some of these antibodies are directed against different determinants on P granules. Mutants that show defects in the early embryonic clevage patterns are being used to determine whether the position and orientation of the mitotic spindle and cleavage furrow are involved in granule segregation. A wild-type zygote divides transversely and asymmetrically. Zyg-9 (b244) zygotes divide first longitudinally and then transversely (see below). Despite the altered orientation of the spindle and cleavage furrow, P granules are localized at the posterior pole of the embryo and thus are distributed to both of the resulting blastomeres. This result suggests both that P granules do not segregate with one nucleus or one half-spindle and that the positional information responsible for P-granule segregation is separate from the positional information that orients the spindle and cleavage furrow. Zyg-11 (b2) zygotes exhibit variable positioning of the first cleavage plane, as shown below. The localization of P granules in zgy- 11 embryos is also variable and difficult to interpret. In a strain carrying mn40, an amber allele of zgy-11, a large percentage of early embryos have P granules localized at the end of the embryo next to the polar body, which in wild-type embryos is almost always anterior. It is not known whether in these zgy-11 embryos the polarity of the whole embryo is reversed relative to the polar body, or whether P granules are segregated to the wrong end of the embryo. In many of the symmetrical 2- and 4-cell zgy-11 embryos, P granules are found in all of the cells. These results support the suggestion that this mutant is defective in the establishment of polarity (see Wolf, Kemphues, Wood & Hirsh, this issue). To complement these mutational manipulations, C. elegans embryos are being treated with inhibitors of microtubule- and microfilament- mediated processes to investigate the possible roles of these cytoskeletal components in P-granule movement. [See Figure 1]

Strome S (1986) "Establishment of asymmetry in early C. elegans embryos: Visualization with antibodies to germ cell components."

Strome S

[1986]

Strome S (1986) J Embryol Exp Morphol "Asymmetric movements of cytoplasmic components in Caenorhabditis elegans zygotes."

Strome S

[J Embryol Exp Morphol, 1986]

One of the central problems facing developmental biologists is understanding how the unicellular zygote develops into a multicellular embryo composed of different tissue types. It is now clear that differentiated cell types differ because they express different sets of genes. However, how cells become instructed to express different sets of genes remains a mystery. One popular model for how cell fates are determined invokes the existence and asymmetric distribution of cytoplasmic 'determinants' of cell fate. According to this model, the developmental programmes of embryonic blastomeres are specified by internal factors that are differentially segregated to different blastomeres during the early cleavages of the zygote. Alternatively, cells may be instructed by extrinsic signals, in which case the positions of the cells in the embryo and cell-cell interactions would be important. Observation and manipulation of embryos that show 'mosaic' development provide indirect support for the cell determinant theory. Individual blastomeres isolated from these embryos divide and express the differentiated phenotyes that they would have expressed in the intact embryo. Conversely removal of individual blastomeres from these embryos leads to juveniles or adults missing the tissues derived from the removed blastomeres. The autonomy of blastomere development suggests that, at least in some embryos, instructions for development are internal and that external cues are not required. Despite many suggestive experiments like those described above, a determinant of cell fate has yet to be identified. Perhaps the strongest candidates are germ granules. These electron-dense structures are found uniquely in germ cells, in almost every metazoan that has been examined. The experiments of Illmensee & Mahowald demonstrated that the cytoplasm containing these germ or polar granules is determinative for the germline of Drosophila, although the structures themselves have not been proven to be the determinants. The questions that need to be answered are: (1) are germ granules required for germ cell development?; (2) do determinants of somatic cell fates exist?; (3) if such determinants exist, what are they composed of?; and (4) how are cytoplasmic components differentially partitioned to different embryonic blastomeres? Basically, how are cell differences

Strome S et al. (1984) "Segregation of germ-line-specific antigens during embryogenesis in C. elegans."

Wood WB, Strome S

[1984]

Germ cells in a wide variety of invertebrate and vertebrate species contain distinctive cytoplasmic organelles that have been visualized by electron microscopy. The ubiliquity of such structures suggests that they play some role in germ-line determination or differentiation, or both. However, the nature and function of these structures remain unknown. We describe experiments with two types of immunologic probes, rabbit sera and mouse monoclonal antibodies, directed against ctyoplamsic granules that are unique to germ-line cells in the nematode, Caenorhabditis elegans, and that may correspond to the germ-line-specific structures seen by electron microscopy in C. elegans embryos. The antibodies have been used to follow the granules, termed P granules, during early embryonic cleavage stages and throughout larval and adult development. P granules become progressively localized to the germ-line precursor cells during early embryogenesis. We are using conditionally lethal maternal-effect mutations to study this localization process. In addition to providing a rapid assay for P granules in wild-type, mutant, and experimentally maipulated embryos, the antibodies also promise to be useful in biochemically characterizing the granules and in investigating their

Strome S (1985) Worm Breeder's Gazette "visualization of microfilaments in C elegans oocytes, zygotes, and gonadal tissue."

Strome S

[Worm Breeder's Gazette, 1985]

Several intracellular motility events in the C. elegans zygote ( pseudocleavage, segregation of germ-line-specific-F granules, and generation of an asymmetric spindle) appear to depend on microfilaments (MFs). As a first step toward trying to understand the participation of MFs in these manifestations of zygotic asymmetry, I have characterized the distribution of MFs in oocytes and early embryos, using the F-actin-specific probe phalloidin and also antiactin antibodies to stain fixed specimens. In oocytes and newly fertilized zygotes, MFs are found in a uniform cortical meshwork of fine fibers and dots or foci. In the zygote, concomitant with the intracellular movements that are believed to be MF-mediated, MFs also become asymmetrically rearranged; as the zygote undergoes pseudocleavage and as P granules become localized in the posterior of the zygote, the foci of F-actin become progressively more concentrated in the anterior hemisphere. The foci remain anterior as the spindle becomes asymmetric and the zygote undergoes first mitosis, at which time fibers align circumferentially around the zygote where the cleavage furrow will form. The distribution of MFs seen by fluorescence suggests that the anterior foci of MFs may be directly responsible for the contractions of the anterior membrane seen during pseudocleavage. Contraction of MFs at the anterior end of the zygote may push or squeeze P granules posterior. The anterior MF foci may also participate in movement of the egg pronucleus toward the sperm pronucleus and then the asymmetric movements of the mitotic spindle. We are currently testing several predictions of this model. Phalloidin and anti-actin antibodies have also been used to visualize MFs in the somatic gonad. The sheath cells that surround maturing oocytes are visibly contractile and contain an unusual array of MFs; the MFs run roughly longitudinally from the loop of the gonad to the spermatheca and are interdigitated with myosin thick filaments, but not in a sarcomeric configuration. These actin and myosin filaments probably interact to cause sheath cell contraction.

Strome S et al. (1984) Worm Breeder's Gazette "genetic approach to studying P granule function."

Wood WB, Strome S

[Worm Breeder's Gazette, 1984]

One approach to analyzing the function of germline granules (P granules) in C. elegans is to isolate and characterize mutants in which they are altered or lacking. We predict that if they are required during development, worms lacking functional P granules will be sterile. To test this, we are using a genetic scheme devised by Jim Preiss to obtain sterile mutants. [Mutations at the lin-2 locus result in defective vulvae. Fertile lin-2/lin-2 hermaphrodites cannot lay their eggs and as a result are consumed by their progeny. However, sterile lin-2/lin-2 worms do not produce progeny and therefore survive.] We are examining these sterile worms for alteration or absence of P granules, by immunofluorescence staining with an anti-P- granule monoclonal antibody (MApK76). From the first mutagenesis of lin-2/lin-2 worms, done in collaboration with Ken Kemphues and Nurit Wolf, a P-granule mutant was isolated. The mutant has been characterized as follows: 1) The P-granule defect was separable from the mutation(s) causing sterility. Mutant worms that do not stain with MAbK76 appear morphologically normal and are fertile. 2) Although P-granule staining is not detected with MAbK76, two other anti-P-granule MAb's (M629 and L416) do stain P granules, as do some anti-P-granule sera. Thus the mutation seems to result in loss or alteration of the antigenic determinant or the P- granule component recognized by MAbK76. These results indicate that at least some of the anti-P-granule antibodies recognize different antigenic determinants and that the K76 determinant in its normal state is not required for germ-cell development. Immunoprecipitations from wild-type vs. mutant worm homogenates are in progress to determine whether any of the putative P-granule polypeptides are altered or missing in mutant worms. 3) The mutation is recessive; worms heterozygous for the mutation stain positively with MAbK76. 4) The mutation has been outcrossed and assigned to linkage group IV; further crosses are in progress to map the mutation. We have used the P-granule mutant to determine how long maternal P- granule material (specifically K76 antigen) present in oocytes persists in embryos. This was done by examining homozygous mutant embryos produced by hermaphrodites heterozygous for the mutation. Maternal P granules persist throughout embryogenesis and into the first larval stage (L1). New P-granule material (K76 antigen) begins to be synthesized in L1's, when the two germ-line precursor cells start to proliferate. This was determined by examining heterozygous embryos and larvae produced by mating homozygous mutant hermaphrodites with wild-type males.

Strome S et al. (2007) Science "Germ versus soma decisions: lessons from flies and worms."

Strome S, Lehmann R

[Science, 2007]

The early embryo is formed by the fusion of two germ cells that must generate not only all of the nonreproductive somatic cell types of its body but also the germ cells for the next generation. Therefore, embryo cells face a crucial decision: whether to develop as germ or soma. How is this fundamental decision made and germ cell fate maintained during development? Studies in the nematode worm Caenorhabditis elegans and fruit fly Drosophila identify some of the decision-making strategies, including segregation of a specialized germ plasm and global transcriptional regulation.

Strome S et al. (2015) Nat Rev Mol Cell Biol "Specifying and protecting germ cell fate."

Updike D, Strome S

[Nat Rev Mol Cell Biol, 2015]

Germ cells are the special cells in the body that undergo meiosis to generate gametes and subsequently entire new organisms after fertilization, a process that continues generation after generation. Recent studies have expanded our understanding of the factors and mechanisms that specify germ cell fate, including the partitioning of maternally supplied 'germ plasm', inheritance of epigenetic memory and expression of transcription factors crucial for primordial germ cell (PGC) development. Even after PGCs are specified, germline fate is labile and thus requires protective mechanisms, such as global transcriptional repression, chromatin state alteration and translation of only germline-appropriate transcripts. Findings from diverse species continue to provide insights into the shared and divergent needs of these special reproductive cells.