The Rac1 homolog CED-10 is a component of the MES-1/SRC-1 pathway for asymmetric division of the C. elegans EMS blastomere

Asymmetric cell division is essential for the creation of cell types with different identities and functions. The EMS blastomere of the four-cell Caenorhabditis elegans embryo undergoes an asymmetric division in response to partially redundant signaling pathways. One pathway involves a Wnt signal emanating from the neighboring P2 cell, while the other pathway is defined by the receptor-like MES-1 protein localized at the EMS/P2 cell contact, and the cytoplasmic kinase SRC-1. In response to these pathways, the EMS nuclear-centrosome complex rotates so that the spindle forms on the anterior-posterior axis; after division, the daughter cell contacting P2 becomes the endodermal precursor cell. Here we identify the Rac1 homolog, CED-10, as a new component of the MES-1/SRC-1 pathway. Loss of CED-10 affects both spindle positioning and endoderm specification. Although MES-1 is still present at the EMS/P2 contact in ced-10 embryos, SRC-1 dependent phosphorylation is reduced. These and other results suggest that CED-10 acts downstream of MES-1 and upstream or at the level of SRC-1 activity. In addition, we find that the branched actin regulator ARX-2 is enriched at the EMS/P2 cell contact site, in a CED-10 dependent manner. Loss of ARX-2 results in spindle positioning defects, suggesting that CED-10 acts through branched actin to promote the asymmetric division of the EMS cell.

Multiple pathways for reestablishing PAR polarity in C. elegans embryo

Asymmetric cell divisions, where cells divide with respect to a polarized axis and give rise to daughter cells with different fates, are critically important for development. In many such divisions, the conserved PAR polarity proteins accumulate in distinct cortical domains in response to a symmetry breaking cue. The one-cell C. elegans embryo is a paradigm for understanding mechanisms of PAR polarization, but much less is known about polarity in subsequent divisions. Here, we investigate the polarization of the P₁ cell of the two-cell embryo. A posterior PAR-2 domain forms in the first 4 min, and polarization becomes stronger over time. Initial polarization depends on the PAR-1 and PKC-3 kinases, and the downstream polarity regulators MEX-5 and PLK-1. However, par-1 and plk-1 mutants exhibit delayed polarization. This late polarization correlated with the time of centrosome maturation and actomyosin flow, and loss of centrosome maturation or myosin in par-1 mutant embryos caused an even stronger polarity phenotype. Based on these and other results, we propose that PAR polarity in the P₁ cell is generated by at least two redundant mechanisms: There is a novel early pathway dependent on PAR-1, PKC-3 and cytoplasmic polarity, and a late pathway that resembles symmetry breaking in the one-cell embryo and requires PKC-3, centrosome associated AIR-1 and myosin flow.

The kinases PIG-1 and PAR-1 act in redundant pathways to regulate asymmetric division in the EMS blastomere of C. elegans

The PAR-1 kinase of C. elegans is localized to the posterior of the one-cell embryo and its mutations affect asymmetric spindle placement and partitioning of cytoplasmic components in the first cell cycle. However, par-1 mutations do not cause failure to restrict the anterior PAR polarity complex to the same extent as mutations in the posteriorly localized PAR-2 protein. Further, it has been difficult to examine the role of PAR-1 in subsequent divisions due to the early defects in par-1 mutant embryos. Here we show that the PIG-1 kinase acts redundantly with PAR-1 to restrict the anterior PAR-3 protein for normal polarity in the one-cell embryo. By using a temperature sensitive allele of par-1, which exhibits enhanced lethality when combined with a pig-1 mutation, we have further explored roles for these genes in subsequent divisions. We find that both PIG-1 and PAR-1 regulate spindle orientation in the EMS blastomere of the four-cell stage embryo to ensure that it undergoes an asymmetric division. In this cell, PIG-1 and PAR-1 act in parallel pathways for spindle positioning, PIG-1 in the MES-1/SRC-1 pathway and PAR-1 in the Wnt pathway.

LET-99 functions in the astral furrowing pathway, where it is required for myosin enrichment in the contractile ring

LET-99 is required for furrowing during cytokinesis in both symmetrically and asymmetrically dividing cells. This function is distinct from the role of LET-99 in spindle positioning with Gα signaling. LET-99 is localized to the furrow, where it acts to promote myosin enrichment.

The anaphase spindle determines the position of the cytokinesis furrow, such that the contractile ring assembles in an equatorial zone between the two spindle poles. Contractile ring formation is mediated by RhoA activation at the equator by the centralspindlin complex and midzone microtubules. Astral microtubules also inhibit RhoA accumulation at the poles. In the Caenorhabditis elegans one-cell embryo, the astral microtubule–dependent pathway requires anillin, NOP-1, and LET-99. LET-99 is well characterized for generating the asymmetric cortical localization of the Gα-dependent force-generating complex that positions the spindle during asymmetric division. However, whether the role of LET-99 in cytokinesis is specific to asymmetric division and whether it acts through Gα to promote furrowing are unclear. Here we show that LET-99 contributes to furrowing in both asymmetrically and symmetrically dividing cells, independent of its function in spindle positioning and Gα regulation. LET-99 acts in a pathway parallel to anillin and is required for myosin enrichment into the contractile ring. These and other results suggest a positive feedback model in which LET-99 localizes to the presumptive cleavage furrow in response to the spindle and myosin. Once positioned there, LET-99 enhances myosin accumulation to promote furrowing in both symmetrically and asymmetrically dividing cells.

TorsinA regulates the LINC to moving nuclei

Starr and Rose discuss work by Saunders et al. demonstrating that torsinA and LAP1 regulate nuclear movement during fibroblast polarization.

How LINC complexes are regulated to connect nuclei to the cytoskeleton during nuclear migration is unknown. Saunders et al. (2017. J. Cell Biol.https://doi.org/10.1083/jcb.201507113) show that the AAA+ ATPase torsinA and its partner LAP1 are required for nuclear migration during fibroblast polarization by mediating the dynamics of LINC complexes.

The 14-3-3 protein PAR-5 regulates the asymmetric localization of the LET-99 spindle positioning protein

PAR proteins play important roles in establishing cytoplasmic polarity as well as regulating spindle positioning during asymmetric division. However, the molecular mechanisms by which the PAR proteins generate asymmetry in different cell types are still being elucidated. Previous studies in Caenorhabditis elegans revealed that PAR-3 and PAR-1 regulate the asymmetric localization of LET-99, which in turn controls spindle positioning by affecting the distribution of the conserved force generating complex. In wild-type embryos, LET-99 is localized in a lateral cortical band pattern, via inhibition at the anterior by PAR-3 and at the posterior by PAR-1. In this report, we show that the 14-3-3 protein PAR-5 is also required for cortical LET-99 asymmetry. PAR-5 associated with LET-99 in pull-down assays, and two PAR-5 binding sites were identified in LET-99 using the yeast two-hybrid assay. Mutation of these sites abolished binding in yeast and altered LET-99 localization in vivo: LET-99 was present at the highest levels at the posterior pole of the embryo instead of a band in par-5 embryos. Together the results indicate that PAR-5 acts in a mechanism with PAR-1 to regulate LET-99 cortical localization.

A novel function for the Caenorhabditis elegans torsin OOC-5 in nucleoporin localization and nuclear import

Mutation in the human AAA+ protein torsinA leads to DYT1 dystonia. Loss of a Caenorhabditis elegans torsin, OOC-5, leads to defects in nucleoporin localization and nuclear import, a novel phenotype for a torsin mutant. NE ultrastructural defects similar to those in mouse and fly torsin mutants are also found, showing conservation of function.

Torsin proteins are AAA+ ATPases that localize to the endoplasmic reticular/nuclear envelope (ER/NE) lumen. A mutation that markedly impairs torsinA function causes the CNS disorder DYT1 dystonia. Abnormalities of NE membranes have been linked to torsinA loss of function and the pathogenesis of DYT1 dystonia, leading us to investigate the role of the Caenorhabditis elegans torsinA homologue OOC-5 at the NE. We report a novel role for torsin in nuclear pore biology. In ooc-5–mutant germ cell nuclei, nucleoporins (Nups) were mislocalized in large plaques beginning at meiotic entry and persisted throughout meiosis. Moreover, the KASH protein ZYG-12 was mislocalized in ooc-5 gonads. Nups were mislocalized in adult intestinal nuclei and in embryos from mutant mothers. EM analysis revealed vesicle-like structures in the perinuclear space of intestinal and germ cell nuclei, similar to defects reported in torsin-mutant flies and mice. Consistent with a functional disruption of Nups, ooc-5–mutant embryos displayed impaired nuclear import kinetics, although the nuclear pore-size exclusion barrier was maintained. Our data are the first to demonstrate a requirement for a torsin for normal Nup localization and function and suggest that these functions are likely conserved.

A specialist seating assessment clinic: changing pressure relief practice

STUDY DESIGN

Description of a clinical service, evaluation of pressure relief practices.

OBJECTIVES

To describe a specialist seating assessment clinic and a change in clinical practice arising from its work.

SETTING

National Spinal Injuries Centre, Stoke Mandeville Hospital, UK.

METHODS

Retrospective review of the ischial transcutaneous oxygen measurements of 50 newly injured and chronic spinal cord-injured (SCI) individuals seen in a specialist seating assessment clinic. Tissue oxygenation was measured in the sitting position (loaded) and during pressure relief (unloaded).

RESULTS

Mean duration of pressure relief required to raise tissue oxygen to unloaded levels was 1 min 51 s (range 42 s-3 min 30 s).

CONCLUSION

These results confirmed the clinical perception that brief pressure lifts of 15-30 s are ineffective in raising transcutaneous oxygen tension (TcPO(2)) to the unloaded level for most individuals. Sustaining the traditional pressure relief by lifting up from the seat for the necessary extended duration is neither practical nor desirable for the majority of clients. It was found that alternative methods of pressure relief were more easily sustainable and very efficient.

CLASPs function redundantly to regulate astral microtubules in the C. elegans embryo

Microtubule dynamics are thought to play an important role in regulating microtubule interactions with cortical force generating motor proteins that position the spindle during asymmetric cell division. CLASPs are microtubule-associated proteins that have a conserved role in regulating microtubule dynamics in diverse cell types. Caenorhabditis elegans has three CLASP homologs in its genome. CLS-2 is known to localize to kinetochores and is needed for chromosome segregation at meiosis and mitosis; however CLS-1 and CLS-3 have not been reported to have any role in embryonic development. Here, we show that depletion of CLS-2 in combination with either CLS-1 or CLS-3 results in defects in nuclear rotation, maintenance of spindle length, and spindle displacement in the one-cell embryo. Polarity is normal in these embryos, but reduced numbers of astral microtubules reach all regions of the cortex at the time of spindle positioning. Analysis of the microtubule plus-end tracker EB1 also revealed a reduced number of growing microtubules reaching the cortex in CLASP depleted embryos, but the polymerization rate of astral microtubules was not slower than in wild type. These results indicate that C. elegans CLASPs act partially redundantly to regulate astral microtubules and position the spindle during asymmetric cell division. Further, we show that these spindle pole-positioning roles are independent of the CLS-2 binding proteins HCP-1 and HCP-2.

Control of asymmetric cell division in early C. elegans embryogenesis: teaming-up translational repression and protein degradation

Asymmetric cell division is a fundamental mechanism for the generation of body axes and cell diversity during early embryogenesis in many organisms. During intrinsically asymmetric divisions, an axis of polarity is established within the cell and the division plane is oriented to ensure the differential segregation of developmental determinants to the daughter cells. Studies in the nematode Caenorhabditis elegans have contributed greatly to our understanding of the regulatory mechanisms underlying cell polarity and asymmetric division. However, much remains to be elucidated about the molecular machinery controlling the spatiotemporal distribution of key components. In this review we discuss recent findings that reveal intricate interactions between translational control and targeted proteolysis. These two mechanisms of regulation serve to carefully modulate protein levels and reinforce asymmetries, or to eliminate proteins from certain cells.

LET-99 inhibits lateral posterior pulling forces during asymmetric spindle elongation in C. elegans embryos

Cortical pulling on astral microtubules positions the mitotic spindle in response to PAR polarity cues and G protein signaling in many systems. In Caenorhabditis elegans single-cell embryos, posterior spindle displacement depends on Galpha and its regulators GPR-1/2 and LIN-5. GPR-1/2 and LIN-5 are necessary for cortical pulling forces and become enriched at the posterior cortex, which suggests that higher forces act on the posterior spindle pole compared with the anterior pole. However, the precise distribution of cortical forces and how they are regulated remains to be determined. Using spindle severing, single centrosome assays, and centrosome fragmentation, we show that both the anterior and posterior cortices generate more pulling force than the lateral-posterior region. Lateral inhibition depends on LET-99, which inhibits GPR-1/2 localization to produce a bipolar GPR-1/2 pattern. Thus, rather than two domains of cortical force, there are three. We propose that the attenuation of lateral forces prevents counterproductive pulling, resulting in a higher net force toward the posterior that contributes to spindle elongation and displacement.

An eIF4E-binding protein regulates katanin protein levels in C. elegans embryos

In Caenorhabditis elegans, the MEI-1-katanin microtubule-severing complex is required for meiosis, but must be down-regulated during the transition to embryogenesis to prevent defects in mitosis. A cullin-dependent degradation pathway for MEI-1 protein has been well documented. In this paper, we report that translational repression may also play a role in MEI-1 down-regulation. Reduction of spn-2 function results in spindle orientation defects due to ectopic MEI-1 expression during embryonic mitosis. MEL-26, which is both required for MEI-1 degradation and is itself a target of the cullin degradation pathway, is present at normal levels in spn-2 mutant embryos, suggesting that the degradation pathway is functional. Cloning of spn-2 reveals that it encodes an eIF4E-binding protein that localizes to the cytoplasm and to ribonucleoprotein particles called P granules. SPN-2 binds to the RNA-binding protein OMA-1, which in turn binds to the mei-1 3' untranslated region. Thus, our results suggest that SPN-2 functions as an eIF4E-binding protein to negatively regulate translation of mei-1.

The torsin-family AAA+ protein OOC-5 contains a critical disulfide adjacent to Sensor-II that couples redox state to nucleotide binding

A subgroup of the AAA+ proteins that reside in the endoplasmic reticulum and the nuclear envelope including human torsinA, a protein mutated in hereditary dystonia, is called the torsin family of AAA+ proteins. A multiple-sequence alignment of this family with Hsp100 proteins of known structure reveals a conserved cysteine in the C-terminus of torsin proteins within the Sensor-II motif. A structural model predicts this cysteine to be a part of an intramolecular disulfide bond, suggesting that it may function as a redox sensor to regulate ATPase activity. In vitro experiments with OOC-5, a torsinA homolog from Caenorhabditis elegans, demonstrate that redox changes that reduce this disulfide bond affect the binding of ATP and ADP and cause an attendant local conformational change detected by limited proteolysis. Transgenic worms expressing an ooc-5 gene with cysteine-to-serine mutations that disrupt the disulfide bond have a very low embryo hatch rate compared with wild-type controls, indicating these two cysteines are essential for OOC-5 function. We propose that the Sensor-II in torsin family proteins is a redox-regulated sensor. This regulatory mechanism may be central to the function of OOC-5 and human torsinA.

Dynamic localization of LIN-5 and GPR-1/2 to cortical force generation domains during spindle positioning

G protein signaling pathways regulate mitotic spindle positioning during cell division in many systems. In Caenorhabditis elegans embryos, G alpha subunits act with the positive regulators GPR-1/2 and LIN-5 to generate cortical pulling forces for posterior spindle displacement during the first asymmetric division. GPR-1/2 are asymmetrically localized at the posterior cortex by PAR polarity cues at this time. Here we show that LIN-5 colocalizes with GPR-1/2 in one-cell embryos during spindle displacement. Significantly, we also find that LIN-5 and GPR-1/2 are localized to the opposite, anterior cortex in a polarity-dependent manner during the nuclear centration and rotation movements that orient the forming spindle onto the polarity axis. The depletion of LIN-5 or GPR-1/2 results in decreased centration and rotation rates, indicating a role in force generation at this stage. The localization of LIN-5 and GPR-1/2 is largely interdependent and requires G alpha. Further, LIN-5 immunoprecipitates with G alpha in vivo, and this association is GPR-1/2 dependent. These results suggest that a complex of G alpha/GPR-1/2/LIN-5 is asymmetrically localized in response to polarity cues, and this may be the active signaling complex that transmits asymmetries to the force generation machinery during both nuclear rotation and spindle displacement.

PAR-3 and PAR-1 inhibit LET-99 localization to generate a cortical band important for spindle positioning in Caenorhabditis elegans embryos

The conserved PAR proteins are localized in asymmetric cortical domains and are required for the polarized localization of cell fate determinants in many organisms. In Caenorhabditis elegans embryos, LET-99 and G protein signaling act downstream of the PARs to regulate spindle positioning and ensure asymmetric division. PAR-3 and PAR-2 localize LET-99 to a posterior cortical band through an unknown mechanism. Here we report that LET-99 asymmetry depends on cortically localized PAR-1 and PAR-4 but not on cytoplasmic polarity effectors. In par-1 and par-4 embryos, LET-99 accumulates at the entire posterior cortex, but remains at low levels at the anterior cortex occupied by PAR-3. Further, PAR-3 and PAR-1 have graded cortical distributions with the highest levels at the anterior and posterior poles, respectively, and the lowest levels of these proteins correlate with high LET-99 accumulation. These results suggest that PAR-3 and PAR-1 inhibit the localization of LET-99 to generate a band pattern. In addition, PAR-1 kinase activity is required for the inhibition of LET-99 localization, and PAR-1 associates with LET-99. Finally, examination of par-1 embryos suggests that the banded pattern of LET-99 is critical for normal posterior spindle displacement and to prevent spindle misorientation caused by cell shape constraints.

LET-711, the Caenorhabditis elegans NOT1 ortholog, is required for spindle positioning and regulation of microtubule length in embryos

Spindle positioning is essential for the segregation of cell fate determinants during asymmetric division, as well as for proper cellular arrangements during development. In Caenorhabditis elegans embryos, spindle positioning depends on interactions between the astral microtubules and the cell cortex. Here we show that let-711 is required for spindle positioning in the early embryo. Strong loss of let-711 function leads to sterility, whereas partial loss of function results in embryos with defects in the centration and rotation movements that position the first mitotic spindle. let-711 mutant embryos have longer microtubules that are more cold-stable than in wild type, a phenotype opposite to the short microtubule phenotype caused by mutations in the C. elegans XMAP215 homolog ZYG-9. Simultaneous reduction of both ZYG-9 and LET-711 can rescue the centration and rotation defects of both single mutants. let-711 mutant embryos also have larger than wild-type centrosomes at which higher levels of ZYG-9 accumulate compared with wild type. Molecular identification of LET-711 shows it to be an ortholog of NOT1, the core component of the CCR4/NOT complex, which plays roles in the negative regulation of gene expression at transcriptional and post-transcriptional levels in yeast, flies, and mammals. We therefore propose that LET-711 inhibits the expression of ZYG-9 and potentially other centrosome-associated proteins, in order to maintain normal centrosome size and microtubule dynamics during early embryonic divisions.

Asymmetric cell division and axis formation in the embryo

Asymmetric cell divisions play an important role in generating diversity during metazoan development. In the early C. elegans embryo, a series of asymmetric divisions are crucial for establishing the three principal axes of the body plan (AP, DV, LR) and for segregating determinants that specify cell fates. In this review, we focus on events in the one-cell embryo that result in the establishment of the AP axis and the first asymmetric division. We first describe how the sperm-derived centrosome initiates movements of the cortical actomyosin network that result in the polarized distribution of PAR proteins. We then briefly discuss how components acting downstream of the PAR proteins mediate unequal segregation of cell fate determinants to the anterior blastomere AB and the posterior blastomere P1. We also review how a heterotrimeric G protein pathway generates cortically based pulling forces acting on astral microtubules, thus mediating centrosome and spindle positioning in response to AP polarity cues. In addition, we briefly highlight events involved in establishing the DV and LR axes. The DV axis is established at the four-cell stage, following specific cell-cell interactions that occur between P2 and EMS, the two daughters of P1, as well as between P2 and ABp, a daughter of AB. The LR axis is established shortly thereafter by the division pattern of ABa and ABp. We conclude by mentioning how findings made in early C. elegans embryos are relevant to understanding asymmetric cell division and pattern formation across metazoan evolution.

LET-99 opposes Galpha/GPR signaling to generate asymmetry for spindle positioning in response to PAR and MES-1/SRC-1 signaling

G-protein signaling plays important roles in asymmetric cell division. In C. elegans embryos, homologs of receptor-independent G protein activators, GPR-1 and GPR-2 (GPR-1/2), function together with Galpha (GOA-1 and GPA-16) to generate asymmetric spindle pole elongation during divisions in the P lineage. Although Galpha is uniformly localized at the cell cortex, the cortical localization of GPR-1/2 is asymmetric in dividing P cells. In this report, we show that the asymmetry of GPR-1/2 localization depends on PAR-3 and its downstream intermediate LET-99. Furthermore, in addition to its involvement in spindle elongation, Galpha is required for the intrinsically programmed nuclear rotation event that orients the spindle in the one-cell. LET-99 functions antagonistically to the Galpha/GPR-1/2 signaling pathway, providing an explanation for how Galpha-dependent force is regulated asymmetrically by PAR polarity cues during both nuclear rotation and anaphase spindle elongation. In addition, Galpha and LET-99 are required for spindle orientation during the extrinsically polarized division of EMS cells. In this cell, both GPR-1/2 and LET-99 are asymmetrically localized in response to the MES-1/SRC-1 signaling pathway. Their localization patterns at the EMS/P2 cell boundary are complementary, suggesting that LET-99 and Galpha/GPR-1/2 signaling function in opposite ways during this cell division as well. These results provide insight into how polarity cues are transmitted into specific spindle positions in both extrinsic and intrinsic pathways of asymmetric cell division.

Embryonic handedness choice in C. elegans involves the Galpha protein GPA-16

The mechanism by which polarity of the left-right (LR) axis is initially established with the correct handedness is not understood for any embryo. C. elegans embryos exhibit LR asymmetry with an invariant handedness that is first apparent at the six-cell stage and persists throughout development. We show here that a strong loss-of-function mutation in a gene originally designated spn-1 affects early spindle orientations and results in near randomization of handedness choice. This mutation interacts genetically with mutations in three par genes that encode localized cortical components. We show that the spn-1 gene encodes the Galpha protein GPA-16, which appears to be required for centrosomal association of a Gbeta protein. We will henceforth refer to this gene as gpa-16. These results demonstrate for the first time involvement of heterotrimeric G proteins in establishment of embryonic LR asymmetry and suggest how they might act.

PAR-dependent and geometry-dependent mechanisms of spindle positioning

During intrinsically asymmetric division, the spindle is oriented onto a polarized axis specified by a group of conserved PAR proteins. Extrinsic geometric asymmetry generated by cell shape also affects spindle orientation in some systems, but how intrinsic and extrinsic mechanisms coexist without interfering with each other is unknown. In some asymmetrically dividing cells of the wild-type Caenorhabditis elegans embryo, nuclear rotation directed toward the anterior cortex orients the forming spindle. We find that in such cells, a PAR-dependent mechanism dominates and causes rotation onto the polarized axis, regardless of cell shape. However, when geometric asymmetry is removed, free nuclear rotation in the center of the cell is observed, indicating that the anterior-directed nature of rotation in unaltered embryos is an effect of cell shape. This free rotation is inconsistent with the prevailing model for nuclear rotation, the specialized cortical site model. In contrast, in par-3 mutant embryos, a geometry-dependent mechanism becomes active and causes directed nuclear rotation. These results lead to the model that in wild-type embryos both PAR-3 and PAR-2 are essential for nuclear rotation in asymmetrically dividing cells, but that PAR-3 inhibits geometry-dependent rotation in nonpolarized cells, thus preventing cell shape from interfering with spindle orientation.

LET-99 determines spindle position and is asymmetrically enriched in response to PAR polarity cues inC. elegansembryos

Asymmetric cell division depends on coordinating the position of the mitotic spindle with the axis of cellular polarity. We provide evidence that LET-99 is a link between polarity cues and the downstream machinery that determines spindle positioning in C. elegans embryos. In let-99 one-cell embryos, the nuclear-centrosome complex exhibits a hyperactive oscillation that is dynein dependent, instead of the normal anteriorly directed migration and rotation of the nuclear-centrosome complex. Furthermore, at anaphase in let-99 embryos the spindle poles do not show the characteristic asymmetric movements typical of wild type animals. LET-99 is a DEP domain protein that is asymmetrically enriched in a band that encircles P lineage cells. The LET-99 localization pattern is dependent on PAR polarity cues and correlates with nuclear rotation and anaphase spindle pole movements in wild-type embryos, as well as with changes in these movements in par mutant embryos. In particular, LET-99 is uniformly localized in one-cell par-3 embryos at the time of nuclear rotation. Rotation fails in spherical par-3 embryos in which the eggshell has been removed, but rotation occurs normally in spherical wild-type embryos. The latter results indicate that nuclear rotation in intact par-3 embryos is dictated by the geometry of the oblong egg and are consistent with the model that the LET-99 band is important for rotation in wild-type embryos. Together, the data indicate that LET-99 acts downstream of PAR-3 and PAR-2 to determine spindle positioning, potentially through the asymmetric regulation of forces on the spindle.

The Caenorhabditis elegans polarity gene ooc-5 encodes a Torsin-related protein of the AAA ATPase superfamily

The PAR proteins are required for polarity and asymmetric localization of cell fate determinants in C. elegans embryos. In addition, several of the PAR proteins are conserved and localized asymmetrically in polarized cells in Drosophila, Xenopus and mammals. We have previously shown that ooc-5 and ooc-3 mutations result in defects in spindle orientation and polarity in early C. elegans embryos. In particular, mutations in these genes affect the re-establishment of PAR protein asymmetry in the P1 cell of two-cell embryos. We now report that ooc-5 encodes a putative ATPase of the Clp/Hsp100 and AAA superfamilies of proteins, with highest sequence similarity to Torsin proteins; the gene for human Torsin A is mutated in individuals with early-onset torsion dystonia, a neuromuscular disease. Although Clp/Hsp100 and AAA family proteins have roles in diverse cellular activities, many are involved in the assembly or disassembly of proteins or protein complexes; thus, OOC-5 may function as a chaperone. OOC-5 protein co-localizes with a marker of the endoplasmic reticulum in all blastomeres of the early C. elegans embryo, in a pattern indistinguishable from that of OOC-3 protein. Furthermore, OOC-5 localization depends on the normal function of the ooc-3 gene. These results suggest that OOC-3 and OOC-5 function in the secretion of proteins required for the localization of PAR proteins in the P1 cell, and may have implications for the study of torsion dystonia.

Analysis of the roles of kinesin and dynein motors in microtubule-based transport in the Caenorhabditis elegans nervous system

The heteromeric kinesins constitute a subfamily of kinesin-related motor complexes that function in several distinct intracellular transport events. The founding member of this subfamily, heterotrimeric kinesin II, has been purified and characterized from early sea urchin embryos, where it was shown using antibody perturbation to be required for the synthesis of motile cilia, presumably by driving the anterograde transport of raft complexes. To further characterize heteromeric kinesin transport pathways, and to attempt to identify cargo molecules, we are using the model organism Caenorhabditis elegans to exploit its well-characterized nervous system and simple genetics. Here we describe methods for large-scale nematode growth and partial purification of kinesin-related holoenzymes from C. elegans, and an in vivo transport assay that allows the direct labeling and visualization of motor complexes and putative cargo molecules moving in living C. elegans neurons. This transport assay is being used to characterize the in vivo transport properties of motor enzymes in living cells, and to exploit a number of existing mutations in C. elegans that may represent constituents of heteromeric kinesin-driven transport pathways, for example, the retrograde intraflagellar transport motor CHE-3 dynein, as well as cargo molecules and/or regulatory molecules.

Mutations in ooc-5 and ooc-3 disrupt oocyte formation and the reestablishment of asymmetric PAR protein localization in two-cell Caenorhabditis elegans embryos

The early development of Caenorhabditis elegans embryos is characterized by a series of asymmetric divisions in which the mitotic spindle is repeatedly oriented on the same axis due to a rotation of the nuclear-centrosome complex. To identify genes involved in the control of spindle orientation, we have screened maternal-effect lethal mutants for alterations in cleavage pattern. Here we describe mutations in ooc-5 and ooc-3, which were isolated on the basis of a nuclear rotation defect in the P(1) cell of two-cell embryos. These mutations are novel in that they affect the asymmetric localization of PAR proteins at the two-cell stage, but not at the one-cell stage. In wild-type two-cell embryos, PAR-3 protein is present around the entire periphery of the AB cell and prevents nuclear rotation in this cell. In contrast, PAR-2 functions to allow nuclear rotation in the P(1) cell by restricting PAR-3 localization to the anterior periphery of P(1). In ooc-5 and ooc-3 mutant embryos, PAR-3 was mislocalized around the periphery of P(1), while PAR-2 was reduced or absent. The germ-line-specific P granules were also mislocalized at the two-cell stage. Mutations in ooc-5 and ooc-3 also result in reduced-size oocytes and embryos. However, par-3 ooc double-mutant embryos can exhibit nuclear rotation, indicating that small size per se does not prevent rotation and that PAR-3 mislocalization contributes to the failure of rotation in ooc mutants. We therefore postulate that wild-type ooc-5 and ooc-3 function in oogenesis and in the reestablishment of asymmetric domains of PAR proteins at the two-cell stage.

Role of a class DHC1b dynein in retrograde transport of IFT motors and IFT raft particles along cilia, but not dendrites, in chemosensory neurons of living Caenorhabditis elegans

The heterotrimeric motor protein, kinesin-II, and its presumptive cargo, can be observed moving anterogradely at 0.7 microm/s by intraflagellar transport (IFT) within sensory cilia of chemosensory neurons of living Caenorhabditis elegans, using a fluorescence microscope-based transport assay (Orozco, J.T., K.P. Wedaman, D. Signor, H. Brown, L. Rose, and J.M. Scholey. 1999. Nature. 398:674). Here, we report that kinesin-II, and two of its presumptive cargo molecules, OSM-1 and OSM-6, all move at approximately 1.1 microm/s in the retrograde direction along cilia and dendrites, which is consistent with the hypothesis that these proteins are retrieved from the distal endings of the cilia by a retrograde transport pathway that moves them along cilia and then dendrites, back to the neuronal cell body. To test the hypothesis that the minus end-directed microtubule motor protein, cytoplasmic dynein, drives this retrograde transport pathway, we visualized movement of kinesin-II and its cargo along dendrites and cilia in a che-3 cytoplasmic dynein mutant background, and observed an inhibition of retrograde transport in cilia but not in dendrites. In contrast, anterograde IFT proceeds normally in che-3 mutants. Thus, we propose that the class DHC1b cytoplasmic dynein, CHE-3, is specifically responsible for the retrograde transport of the anterograde motor, kinesin-II, and its cargo within sensory cilia, but not within dendrites.

Early patterning of the C. elegans embryo

Studies of about 20 maternally expressed genes are providing an understanding of mechanisms of patterning and cell-fate determination in the early Caenorhabditis elegans embryo. The analyses have revealed that fates of the early blastomeres are specified by a combination of intrinsically asymmetric cell divisions and two types of cell-cell interactions: inductions and polarizing interactions. In this review we summarize the current level of understanding of the molecular mechanisms underlying these processes in the specification of cell fates in the pregastrulation embryo.

Two heteromeric kinesin complexes in chemosensory neurons and sensory cilia of Caenorhabditis elegans

Chemosensation in the nervous system of the nematode Caenorhabditis elegans depends on sensory cilia, whose assembly and maintenance requires the transport of components such as axonemal proteins and signal transduction machinery to their site of incorporation into ciliary structures. Members of the heteromeric kinesin family of microtubule motors are prime candidates for playing key roles in these transport events. Here we describe the molecular characterization and partial purification of two heteromeric kinesin complexes from C. elegans, heterotrimeric CeKinesin-II and dimeric CeOsm-3. Transgenic worms expressing green fluorescent protein driven by endogenous heteromeric kinesin promoters reveal that both CeKinesin-II and CeOsm-3 are expressed in amphid, inner labial, and phasmid chemosensory neurons. Additionally, immunolocalization experiments on fixed worms show an intense concentration of CeKinesin-II and CeOsm-3 polypeptides in the ciliated endings of these chemosensory neurons and a punctate localization pattern in the corresponding cell bodies and dendrites. These results, together with the phenotypes of known mutants in the pathway of sensory ciliary assembly, suggest that CeKinesin-II and CeOsm-3 drive the transport of ciliary components required for sequential steps in the assembly of chemosensory cilia.

The let-99 gene is required for proper spindle orientation during cleavage of the C. elegans embryo

The orientation of cell division is a critical aspect of development. In 2-cell C. elegans embryos, the spindle in the posterior cell is aligned along the long axis of the embryo and contributes to the unequal partitioning of cytoplasm, while the spindle in the anterior cell is oriented transverse to the long axis. Differing spindle alignments arise from blastomere-specific rotations of the nuclear-centrosome complex at prophase. We have found that mutations in the maternally expressed gene let-99 affect spindle orientation in all cells during the first three cleavages. During these divisions, the nuclear-centrosome complex appears unstable in position. In addition, in almost half of the mutant embryos, there are reversals of the normal pattern of spindle orientations at second cleavage: the spindle of the anterior cell is aligned with the long axis of the embryo and nuclear rotation fails in the posterior cell causing the spindle to form transverse to the long axis. In most of the remaining embryos, spindles in both cells are transverse at second cleavage. The distributions of several asymmetrically localized proteins, including P granules and PAR-3, are normal in early let-99 embryos, but are perturbed by the abnormal cell division orientations at second cleavage. The accumulation of actin and actin capping protein, which marks the site involved in nuclear rotation in 2-cell wild-type embryos, is abnormal but is not reversed in let-99 mutant embryos. Based on these data, we conclude that let-99(+) is required for the proper orientation of spindles after the establishment of polarity, and we postulate that let-99(+) plays a role in interactions between the astral microtubules and the cortical cytoskeleton.

Pseudocleavage is dispensable for polarity and development in C. elegans embryos

The first cleavage of the Caenorhabditis elegans embryo is asymmetrical, producing daughters with different cell fates. During the first cell cycle, P granules, cytoplasmic components that are segregated to the germ-line, are localized to the posterior of the embryo. It has been hypothesized that the asymmetrical behavior of the daughters of the first division results from a similar localization of developmental determinants. A process called pseudocleavage also occurs during the first cell cycle: Anterior cortical contractions culminate in a single partial constriction of the embryo called the pseudocleavage furrow. Coincident with pseudocleavage, there is an anteriorly directed flow of cortical cytoplasm and a posteriorly directed flow of internal cytoplasm. Foci of filamentous cortical actin become asymmetrically distributed into an anterior cap. Roles for these various first cell cycle events in cytoplasmic localization and development have been suggested but remain unclear. We have isolated a maternal effect mutation, nop-1(it142), which abolishes the anterior cortical contractions and the pseudocleavage furrow. In addition, cortical actin foci remain uniformly distributed in most embryos. Despite these defects, cytoplasmic and cortical streaming is present and P granules are localized to the posterior of early embryos. In most embryos from mutant mothers, development proceeds normally and the embryos hatch and grow into fertile adults. We conclude that the pseudocleavage contractions and furrow are dispensable for the development of C. elegans.

The Drosophila cellularization gene nullo produces a blastoderm-specific transcript whose levels respond to the nucleocytoplasmic ratio

The initial development of the Drosophila embryo is characterized by rapid nuclear mitosis without cytokinesis. After 13 such mitoses, a coordinated cell division process called cellularization occurs, during which membranes simultaneously enclose each nucleus in a cell. Cellularization requires the establishment of a hexagonal network of actin and myosin filaments in the cortex of the embryo; the filaments are located on the cytoplasmic face of the invaginating membrane furrows. Zygotic expression of the nullo gene is essential for the maintenance of an intact actin-myosin network. We have cloned the nullo gene and present its sequence as well as a characterization of nullo transcript levels in wild-type and mutant embryos. The nullo gene encodes a predicted protein of 213 amino acids, a large proportion of which is basic. nullo transcripts are first detectable at nuclear cell cycle 11, peak in accumulation at the end of cycle 13, and disappear rapidly as cellularization begins. The gene does not appear to be expressed at any other time in the life of the organism. The normal accumulation of nullo transcripts does not require gene activity of other zygotic cellularization genes. The regulation of nullo RNA levels during cycle 14, however, is coupled to the nucleocytoplasmic ratio, which also controls the cessation of rapid, synchronous mitosis just before cellularization.

Effect of abdominal binders on breathing in tetraplegic patients

We studied the effect on breathing of a conventional and a newly designed abdominal binder in seven patients with complete tetraplegia. The indices of respiratory ability used were the transdiaphragmatic pressure on maximal sniff (sniff Pdi), the maximum static inspiratory mouth pressure (PImax), and the vital capacity (VC). These were measured in patients with and without binders, in the supine position, raised up to 70 degrees on a tilt table, and seated upright. When patients were raised from the supine to the 70 degrees tilt and to the seated posture, sniff Pdi and VC decreased. Both binders improved VC in the seated position and at 70 degrees tilt, and sniff Pdi at 70 degrees tilt. The new binder was as effective as but no better than the conventional binder. PImax was too variable to be a valuable index of inspiratory power. These findings support the view that abdominal binders assist breathing in tetraplegic patients who are seated or raised to near vertical positions.

Measurement of abdominal wall compliance in normal subjects and tetraplegic patients

On inspiration descent of the diaphragm is opposed by the passive properties of the abdominal wall, the tone of its muscles, and the inertia of the abdominal contents. As a result, intra-abdominal pressure rises and promotes rib cage expansion. In patients with high spinal injury the diaphragm is the most important muscle of inspiration and abdominal wall displacement is more evident than in normal subjects. Abdominal wall compliance has been measured by relating gastric pressure to abdominal wall displacement, which was determined by means of an optical contour mapping system. Six normal subjects and six tetraplegic patients were studied in the supine posture, during passive expiration from total lung capacity to functional residual capacity. Over this lung volume range the normal subjects partitioned an average of 31% of expired volume to the abdominal compartment, while the corresponding average figure in the patients was 77% of expired volume. Since the range of gastric pressure was similar in the two groups, it is concluded that abdominal wall compliance is greater in tetraplegic patients. This high compliance could have a detrimental effect on lower rib cage expansion.

Physical ability in relation to anthropometric measurements in persons with complete spinal cord lesion below the sixth cervical segment

A study was undertaken to determine the ability of patients with complete tetraplegia below cervical sixth segment to transfer in relation to their anthropometric characteristics. Thirty-six chronic patients were assessed and spasticity was measured. A discriminant function analysis was carried out to assess the extent to which a number of anthropometric and anatomical variables could predict the patients' final ability to effect a transfer. Using nine of the original 23 predictor variables it is possible to correctly classify a patient's eventual ability to transfer in 92% of cases.