C. elegansSUR-6/PR55 cooperates with LET-92/protein phosphatase 2A and promotes Raf activity independently of inhibitory Akt phosphorylation sites

Emerging insights into serine/threonine-specific phosphoprotein phosphatase function and selectivity

Protein phosphorylation on serine and threonine residues is a widely distributed post-translational modification on proteins that acts to regulate their function. Phosphoprotein phosphatases (PPPs) contribute significantly to a plethora of cellular functions through the accurate dephosphorylation of phosphorylated residues. Most PPPs accomplish their purpose through the formation of complex holoenzymes composed of a catalytic subunit with various regulatory subunits. PPP holoenzymes then bind and dephosphorylate substrates in a highly specific manner. Despite the high prevalence of PPPs and their important role for cellular function, their mechanisms of action in the cell are still not well understood. Nevertheless, substantial experimental advancements in (phospho-)proteomics, structural and computational biology have contributed significantly to a better understanding of PPP biology in recent years. This Review focuses on recent approaches and provides an overview of substantial new insights into the complex mechanism of PPP holoenzyme regulation and substrate selectivity.

Impairment of the neurotrophic signaling hub B-Raf contributes to motoneuron degeneration in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a motoneuron disease caused by deletions of the Survival of Motoneuron 1 gene (SMN1) and low SMN protein levels. SMN restoration is the concept behind a number of recently approved drugs which result in impressive yet limited effects. Since SMN has already been enhanced in treated patients, complementary SMN-independent approaches are needed. Previously, a number of altered signaling pathways which regulate motoneuron degeneration have been identified as candidate targets. However, signaling pathways form networks, and their connectivity is still unknown in SMA. Here, we used presymptomatic SMA mice to elucidate the network of altered signaling in SMA. The SMA network is structured in two clusters with AKT and 14-3-3 ζ/δ in their centers. Both clusters are connected by B-Raf as a major signaling hub. The direct interaction of B-Raf with 14-3-3 ζ/δ is important for an efficient neurotrophic activation of the MEK/ERK pathway and crucial for motoneuron survival. Further analyses in SMA mice revealed that both proteins were down-regulated in motoneurons and the spinal cord with B-Raf being reduced at presymptomatic stages. Primary fibroblasts and iPSC-derived motoneurons from SMA patients both showed the same pattern of down-regulation. This mechanism is conserved across species since a Caenorhabditis elegans SMA model showed less expression of the B-Raf homolog lin-45 Accordingly, motoneuron survival was rescued by a cell autonomous lin-45 expression in a C. elegans SMA model resulting in improved motor functions. This rescue was effective even after the onset of motoneuron degeneration and mediated by the MEK/ERK pathway.

The PP2A/4/6 subfamily of phosphoprotein phosphatases regulates DAF-16 and confers resistance to environmental stress in postreproductive adult C. elegans

Insulin and insulin-like growth factors are longevity determinants that negatively regulate Forkhead box class O (FoxO) transcription factors. In C. elegans mutations that constitutively activate DAF-16, the ortholog of mammalian FoxO3a, extend lifespan by two-fold. While environmental insults induce DAF-16 activity in younger animals, it also becomes activated in an age-dependent manner in the absence of stress, modulating gene expression well into late adulthood. The mechanism by which DAF-16 activity is regulated during aging has not been defined. Since phosphorylation of DAF-16 generally leads to its inhibition, we asked whether phosphatases might be necessary for its increased transcriptional activity in adult C. elegans. We focused on the PP2A/4/6 subfamily of phosphoprotein phosphatases, members of which had been implicated to regulate DAF-16 under low insulin signaling conditions but had not been investigated during aging in wildtype animals. Using reverse genetics, we functionally characterized all C. elegans orthologs of human catalytic, regulatory, and scaffolding subunits of PP2A/4/6 holoenzymes in postreproductive adults. We found that PP2A complex constituents PAA-1 and PPTR-1 regulate DAF-16 transcriptional activity during aging and that they cooperate with the catalytic subunit LET-92 to protect adult animals from ultraviolet radiation. PP4 complex members PPH-4.1/4.2, and SMK-1 also appear to regulate DAF-16 in an age-dependent manner, and together with PPFR-2 they contribute to innate immunity. Interestingly, SUR-6 but no other subunit of the PP2A complex was necessary for the survival of pathogen-infected animals. Finally, we found that PP6 complex constituents PPH-6 and SAPS-1 contribute to host defense during aging, apparently without affecting DAF-16 transcriptional activity. Our studies indicate that a set of PP2A/4/6 complexes protect adult C. elegans from environmental stress, thus preserving healthspan. Therefore, along with their functions in cell division and development, the PP2A/4/6 phosphatases also appear to play critical roles later in life.

Inhibition of PAR-1 delays aging via activating AMPK in C. elegans

The antagonistic pleiotropy theory of aging suggests that genes essential for growth and development are likely to modulate aging later in life. Previous studies in C. elegans demonstrate that inhibition of certain developmentally essential genes during adulthood leads to significant lifespan extension. PAR-1, a highly conserved serine/threonine kinase, functions as a key cellular polarity regulator during the embryonic development. However, the role of PAR-1 during adulthood remains unknown. Here we show that inhibition of par-1 either by a temperature-sensitive mutant or by RNAi knockdown only during adulthood is sufficient to extend lifespan in C. elegans. Inhibition of par-1 also improves healthspan, as indicated by increased stress resistance, enhanced proteotoxicity resistance, as well as reduced muscular function decline over time. Additionally, tissue-enriched RNAi knockdown analysis reveals that PAR-1 mainly functions in the epidermis to regulate lifespan. Further genetic epistatic and molecular studies demonstrate that the effect of par-1 on lifespan requires the AMP-activated protein kinase (AMPK), and RNAi knockdown of par-1 results in age-dependent AMPK activation and reduced lipid accumulation in the metabolic tissue. Taken together, our findings reveal a previously undescribed function of PAR-1 in adulthood, which will help to understand the molecular links between development and aging.

Protein Serine/Threonine Phosphatases: Keys to Unlocking Regulators and Substrates

Protein serine/threonine phosphatases (PPPs) are ancient enzymes, with distinct types conserved across eukaryotic evolution. PPPs are segregated into types primarily on the basis of the unique interactions of PPP catalytic subunits with regulatory proteins. The resulting holoenzymes dock substrates distal to the active site to enhance specificity. This review focuses on the subunit and substrate interactions for PPP that depend on short linear motifs. Insights about these motifs from structures of holoenzymes open new opportunities for computational biology approaches to elucidate PPP networks. There is an expanding knowledge base of posttranslational modifications of PPP catalytic and regulatory subunits, as well as of their substrates, including phosphorylation, acetylation, and ubiquitination. Cross talk between these posttranslational modifications creates PPP-based signaling. Knowledge of PPP complexes, signaling clusters, as well as how PPPs communicate with each other in response to cellular signals should unlock the doors to PPP networks and signaling "clouds" that orchestrate and coordinate different aspects of cell physiology.

C. elegans PTEN and AMPK block neuroblast divisions by inhibiting a BMP-insulin-PP2A-MAPK pathway

Caenorhabditis elegans that hatch in the absence of food stop their postembryonic development in a process called L1 arrest. Intriguingly, we find that the postembryonic Q neuroblasts divide and migrate during L1 arrest in mutants that have lost the energy sensor AMP-activated protein kinase (AMPK) or the insulin/IGF-1 signaling (IIS) negative regulator DAF-18/PTEN. We report that DBL-1/BMP works upstream of IIS to promote agonistic insulin-like peptides during L1 arrest. However, the abnormal Q cell divisions that occur during L1 arrest use a novel branch of the IIS pathway that is independent of the terminal transcription factor DAF-16/FOXO. Using genetic epistasis and drug interactions we show that AMPK functions downstream of, or in parallel with DAF-18/PTEN and IIS to inhibit PP2A function. Further, we show that PP2A regulates the abnormal Q cell divisions by activating the MPK-1/ERK signaling pathway via LIN-45/RAF, independently of LET-60/RAS. PP2A acts as a tumor suppressor in many oncogenic signaling cascades. Our work demonstrates a new role for PP2A that is needed to induce neuroblast divisions during starvation and is regulated by both insulin and AMPK.

Protein phosphatase 2A is crucial for sarcomere organization in Caenorhabditis elegans striated muscle

Protein phosphatase 2A (PP2A) is a heterotrimer composed of single catalytic and scaffolding subunits and one of several possible regulatory subunits. We identified PPTR-2, a regulatory subunit of PP2A, as a binding partner for the giant muscle protein UNC-89 (obscurin) in Caenorhabditis elegans. PPTR-2 is required for sarcomere organization when its paralogue, PPTR-1, is deficient. PPTR-2 localizes to the sarcomere at dense bodies and M-lines, colocalizing with UNC-89 at M-lines. PP2A components in C. elegans include one catalytic subunit LET-92, one scaffolding subunit (PAA-1), and five regulatory subunits (SUR-6, PPTR-1, PPTR-2, RSA-1, and CASH-1). In adult muscle, loss of function in any of these subunits results in sarcomere disorganization. rsa-1 mutants show an interesting phenotype: one of the two myosin heavy chains, MHC A, localizes as closely spaced double lines rather than single lines. This "double line" phenotype is found in rare missense mutants of the head domain of MHC B myosin, such as unc-54(s74). Analysis of phosphoproteins in the unc-54(s74) mutant revealed two additional phosphoserines in the nonhelical tailpiece of MHC A. Antibodies localize PPTR-1, PAA-1, and SUR-6 to I-bands and RSA-1 to M-lines and I-bands. Therefore, PP2A localizes to sarcomeres and functions in the assembly or maintenance of sarcomeres.

Phosphatase PP2A and microtubule-mediated pulling forces disassemble centrosomes during mitotic exit

Centrosomes are microtubule-nucleating organelles that facilitate chromosome segregation and cell division in metazoans. Centrosomes comprise centrioles that organize a micron-scale mass of protein called pericentriolar material (PCM) from which microtubules nucleate. During each cell cycle, PCM accumulates around centrioles through phosphorylation-mediated assembly of PCM scaffold proteins. During mitotic exit, PCM swiftly disassembles by an unknown mechanism. Here, we used Caenorhabditis elegans embryos to determine the mechanism and importance of PCM disassembly in dividing cells. We found that the phosphatase PP2A and its regulatory subunit SUR-6 (PP2A^SUR-6), together with cortically directed microtubule pulling forces, actively disassemble PCM. In embryos depleted of these activities, ∼25% of PCM persisted from one cell cycle into the next. Purified PP2A^SUR-6 could dephosphorylate the major PCM scaffold protein SPD-5 in vitro. Our data suggest that PCM disassembly occurs through a combination of dephosphorylation of PCM components and force-driven fragmentation of the PCM scaffold.

Summary: Centrosomes acquire and lose pericentriolar material during each cell cycle. We show that dephosphorylation and cortically directed forces disassemble pericentriolar material at the end of mitosis.

Functions of protein phosphatase-6 in NF-κB signaling and in lymphocytes

Protein phosphatase-6 (PP6) is a member of the PPP family of Ser/Thr phosphatases involved in intracellular signaling. PP6 is conserved among all eukaryotes, and genetics in model organisms indicates it has non-redundant functions relative to other PPP phosphatases. PP6 functions in association with conserved SAPS subunits and, in vertebrate species, forms heterotrimers with Ankrd subunits. Multiple studies have demonstrated how PP6 exerts negative control at different steps of nuclear factor kappaB signaling. Expression of PP6 catalytic subunit and the PPP6R1 subunit is especially high in hematopoietic cells and lymphoid tissues. Recent efforts at conditionally knocking out genes for PP6c or PP6R1 (SAPS1) have revealed distinctive effects on development of and signaling in lymphocytes.

The metastasis suppressor Nm23 as a modulator of Ras/ERK signaling

NM23-H1 (also known as NME1) was the first identified metastasis suppressor, which displays a nucleoside diphosphate kinase (NDPK) and histidine protein kinase activity. NDPKs are linked to many processes, such as cell migration, proliferation, differentiation, but the exact mechanism whereby NM23-H1 inhibits the metastatic potential of cancer cells remains elusive. However, some recent data suggest that NM23-H1 may exert its anti-metastatic effect by blocking Ras/ERK signaling. In mammalian cell lines NDPK-mediated attenuation of Ras/ERK signaling occurs through phosphorylation (thus inactivation) of KSR (kinase suppressor of Ras) scaffolds. In this review I summarize our knowledge about KSR's function and its regulation in mammals and in C. elegans. Genetic studies in the nematode contributed substantially to our understanding of the function and regulation of the Ras pathway (i.e. KSR's discovery is also linked to the nematode). Components of the RTK/Ras/ERK pathway seem to be highly conserved between mammals and worms. NDK-1, the worm homolog of NM23-H1 affects Ras/MAPK signaling at the level of KSRs, and a functional interaction between NDK-1/NDPK and KSRs was first demonstrated in the worm in vivo. However, NDK-1 is a factor, which is necessary for proper MAPK activation, thus it activates rather than suppresses Ras/MAPK signaling in the worm. The contradiction between results in mammalian cell lines and in the worm regarding NDPKs' effect exerted on the outcome of Ras signaling might be resolved, if we better understand the function, structure and regulation of KSR scaffolds.

par-1, atypical pkc, and PP2A/B55 sur-6 are implicated in the regulation of exocyst-mediated membrane trafficking in Caenorhabditis elegans

The exocyst is a conserved protein complex that is involved in tethering secretory vesicles to the plasma membrane and regulating cell polarity. Despite a large body of work, little is known how exocyst function is controlled. To identify regulators for exocyst function, we performed a targeted RNA interference (RNAi) screen in Caenorhabditis elegans to uncover kinases and phosphatases that genetically interact with the exocyst. We identified seven kinase and seven phosphatase genes that display enhanced phenotypes when combined with hypomorphic alleles of exoc-7 (exo70), exoc-8 (exo84), or an exoc-7;exoc-8 double mutant. We show that in line with its reported role in exocytotic membrane trafficking, a defective exoc-8 caused accumulation of exocytotic soluble NSF attachment protein receptor (SNARE) proteins in both intestinal and neuronal cells in C. elegans. Down-regulation of the phosphatase protein phosphatase 2A (PP2A) phosphatase regulatory subunit sur-6/B55 gene resulted in accumulation of exocytic SNARE proteins SNB-1 and SNAP-29 in wild-type and in exoc-8 mutant animals. In contrast, RNAi of the kinase par-1 caused reduced intracellular green fluorescent protein signal for the same proteins. Double RNAi experiments for par-1, pkc-3, and sur-6/B55 in C. elegans suggest a possible cooperation and involvement in postembryo lethality, developmental timing, as well as SNARE protein trafficking. Functional analysis of the homologous kinases and phosphatases in Drosophila median neurosecretory cells showed that atypical protein kinase C kinase and phosphatase PP2A regulate exocyst-dependent, insulin-like peptide secretion. Collectively, these results characterize kinases and phosphatases implicated in the regulation of exocyst function, and suggest the possibility for interplay between the par-1 and pkc-3 kinases and the PP2A phosphatase regulatory subunit sur-6 in this process.

Canonical RTK-Ras-ERK signaling and related alternative pathways

Receptor Tyrosine Kinase (RTK)-Ras-Extracellular signal-regulated kinase (ERK) signaling pathways control many aspects of C. elegans development and behavior. Studies in C. elegans helped elucidate the basic framework of the RTK-Ras-ERK pathway and continue to provide insights into its complex regulation, its biological roles, how it elicits cell-type appropriate responses, and how it interacts with other signaling pathways to do so. C. elegans studies have also revealed biological contexts in which alternative RTK- or Ras-dependent pathways are used instead of the canonical pathway.

SEL-10/Fbw7-dependent negative feedback regulation of LIN-45/Braf signaling in C. elegans via a conserved phosphodegron

The conserved E3 ubiquitin ligase component named SEL-10 in Caenorhabditis elegans and Fbw7 in mammals targets substrates for ubiquitin-mediated degradation through a high-affinity binding site called a Cdc4 phosphodegron (CPD). As many known substrates of Fbw7 are oncoproteins, the identification of new substrates may offer insight into cancer biology as well as aspects of proteome regulation. Here, we evaluated whether the presence of an evolutionarily conserved CPD would be a feasible complement to proteomics-based approaches for identifying new potential substrates. For functional assessments, we focused on LIN-45, a component of the signal transduction pathway underlying vulval induction and the ortholog of human Braf, an effector of Ras in numerous cancers. Our analysis demonstrates that LIN-45 behaves as a bona fide substrate of SEL-10, with mutation of the CPD or loss of sel-10 resulting in increased activity and protein stability in vivo. Furthermore, during vulval induction, the downstream kinase MPK-1/ERK is also required for LIN-45 protein degradation in a negative feedback loop, resulting in degradation of LIN-45 where ERK is highly active. As the CPD consensus sequence is conserved in human Braf, we propose that Fbw7 may also regulate Braf stability in some cell contexts. We discuss the implications of our findings for vulval development in C. elegans, the potential applicability to human Braf, and the value of a CPD-based predictive approach for human Fbw7 substrates.

Bypassing the Greatwall-Endosulfine pathway: plasticity of a pivotal cell-cycle regulatory module in Drosophila melanogaster and Caenorhabditis elegans

In vertebrates, mitotic and meiotic M phase is facilitated by the kinase Greatwall (Gwl), which phosphorylates a conserved sequence in the effector Endosulfine (Endos). Phosphorylated Endos inactivates the phosphatase PP2A/B55 to stabilize M-phase-specific phosphorylations added to many proteins by cyclin-dependent kinases (CDKs). We show here that this module functions essentially identically in Drosophila melanogaster and is necessary for proper mitotic and meiotic cell division in a wide variety of tissues. Despite the importance and evolutionary conservation of this pathway between insects and vertebrates, it can be bypassed in at least two situations. First, heterozygosity for loss-of-function mutations of twins, which encodes the Drosophila B55 protein, suppresses the effects of endos or gwl mutations. Several types of cell division occur normally in twins heterozygotes in the complete absence of Endos or the near absence of Gwl. Second, this module is nonessential in the nematode Caenorhaditis elegans. The worm genome does not contain an obvious ortholog of gwl, although it encodes a single Endos protein with a surprisingly well-conserved Gwl target site. Deletion of this site from worm Endos has no obvious effects on cell divisions involved in viability or reproduction under normal laboratory conditions. In contrast to these situations, removal of one copy of twins does not completely bypass the requirement for endos or gwl for Drosophila female fertility, although reducing twins dosage reverses the meiotic maturation defects of hypomorphic gwl mutants. These results have interesting implications for the function and evolution of the mechanisms modulating removal of CDK-directed phosphorylations.

Regulation of the MEX-5 gradient by a spatially segregated kinase/phosphatase cycle

Protein concentration gradients encode spatial information across cells and tissues and often depend on spatially localized protein synthesis. Here, we report that a different mechanism underlies the MEX-5 gradient. MEX-5 is an RNA-binding protein that becomes distributed in a cytoplasmic gradient along the anterior-to-posterior axis of the one-cell C. elegans embryo. We demonstrate that the MEX-5 gradient is a direct consequence of an underlying gradient in MEX-5 diffusivity. The MEX-5 diffusion gradient arises when the PAR-1 kinase stimulates the release of MEX-5 from slow-diffusive, RNA-containing complexes in the posterior cytoplasm. PAR-1 directly phosphorylates MEX-5 and is antagonized by the spatially uniform phosphatase PP2A. Mathematical modeling and in vivo observations demonstrate that spatially segregated phosphorylation and dephosphorylation reactions are sufficient to generate stable protein concentration gradients in the cytoplasm. The principles demonstrated here apply to any spatially segregated modification cycle that affects protein diffusion and do not require protein synthesis or degradation.

Enhanced phosphatase activity attenuates α-synucleinopathy in a mouse model

α-Synuclein (α-Syn) is a key protein that accumulates as hyperphosphorylated aggregates in pathologic hallmark features of Parkinson's disease (PD) and other neurodegenerative disorders. Phosphorylation of this protein at serine 129 is believed to promote its aggregation and neurotoxicity, suggesting that this post-translational modification could be a therapeutic target. Here, we demonstrate that phosphoprotein phosphatase 2A (PP2A) dephosphorylates α-Syn at serine 129 and that this activity is greatly enhanced by carboxyl methylation of the catalytic C subunit of PP2A. α-Syn-transgenic mice raised on a diet supplemented with eicosanoyl-5-hydroxytryptamide, an agent that enhances PP2A methylation, dramatically reduced both α-Syn phosphorylation at Serine 129 and α-Syn aggregation in the brain. These biochemical changes were associated with enhanced neuronal activity, increased dendritic arborizations, and reduced astroglial and microglial activation, as well as improved motor performance. These findings support the notion that serine 129 phosphorylation of α-Syn is of pathogenetic significance and that promoting PP2A activity is a viable disease-modifying therapeutic strategy for α-synucleinopathies such as PD.

PP2A phosphatase acts upon SAS-5 to ensure centriole formation in C. elegans embryos

Centrosome duplication occurs once per cell cycle and ensures that the two resulting centrosomes assemble a bipolar mitotic spindle. Centriole formation is fundamental for centrosome duplication. In Caenorhabditis elegans, the evolutionarily conserved proteins SPD-2, ZYG-1, SAS-6, SAS-5, and SAS-4 are essential for centriole formation, but how they function is not fully understood. Here, we demonstrate that Protein Phosphatase 2A (PP2A) is also critical for centriole formation in C. elegans embryos. We find that PP2A subunits genetically and physically interact with the SAS-5/SAS-6 complex. Furthermore, we show that PP2A-mediated dephosphorylation promotes centriolar targeting of SAS-5 and ensures SAS-6 delivery to the site of centriole assembly. We find that PP2A is similarly needed for the presence of HsSAS-6 at centrioles and for centriole formation in human cells. These findings lead us to propose that PP2A-mediated loading of SAS-6 proteins is critical at the onset of centriole formation.

Protein phosphatase 2A-SUR-6/B55 regulates centriole duplication in C. elegans by controlling the levels of centriole assembly factors

Centrioles play a crucial role in mitotic spindle assembly and duplicate precisely once per cell cycle. In worms, flies, and humans, centriole assembly is dependent upon a key regulatory kinase (ZYG-1/Sak/Plk4) and its downstream effectors SAS-5 and SAS-6. Here we report a role for protein phosphatase 2A (PP2A) in centriole duplication. We find that the PP2A catalytic subunit LET-92, the scaffolding subunit PAA-1, and the B55 regulatory subunit SUR-6 function together to positively regulate centriole assembly. In PP2A-SUR-6-depleted embryos, the levels of ZYG-1 and SAS-5 are reduced and the ZYG-1- and SAS-5-dependent recruitment of SAS-6 to the nascent centriole fails. We show that PP2A physically associates with SAS-5 in vivo and that inhibiting proteolysis can rescue SAS-5 levels and the centriole duplication defect of PP2A-depleted embryos. Together, our findings indicate that PP2A-SUR-6 promotes centriole assembly by protecting ZYG-1 and SAS-5 from degradation.

A survey of new temperature-sensitive, embryonic-lethal mutations in C. elegans: 24 alleles of thirteen genes

To study essential maternal gene requirements in the early C. elegans embryo, we have screened for temperature-sensitive, embryonic lethal mutations in an effort to bypass essential zygotic requirements for such genes during larval and adult germline development. With conditional alleles, multiple essential requirements can be examined by shifting at different times from the permissive temperature of 15°C to the restrictive temperature of 26°C. Here we describe 24 conditional mutations that affect 13 different loci and report the identity of the gene mutations responsible for the conditional lethality in 22 of the mutants. All but four are mis-sense mutations, with two mutations affecting splice sites, another creating an in-frame deletion, and one creating a premature stop codon. Almost all of the mis-sense mutations affect residues conserved in orthologs, and thus may be useful for engineering conditional mutations in other organisms. We find that 62% of the mutants display additional phenotypes when shifted to the restrictive temperature as L1 larvae, in addition to causing embryonic lethality after L4 upshifts. Remarkably, we also found that 13 out of the 24 mutations appear to be fast-acting, making them particularly useful for careful dissection of multiple essential requirements. Our findings highlight the value of C. elegans for identifying useful temperature-sensitive mutations in essential genes, and provide new insights into the requirements for some of the affected loci.

Atypical cadherins Dachsous and Fat control dynamics of noncentrosomal microtubules in planar cell polarity

How global organ asymmetry and individual cell polarity are connected to each other is a central question in studying planar cell polarity (PCP). In the Drosophila wing, which develops PCP along its proximal-distal (P-D) axis, we previously proposed that the core PCP mediator Frizzled redistributes distally in a microtubule (MT)-dependent manner. Here, we performed organ-wide analysis of MT dynamics by introducing quantitative in vivo imaging. We observed MTs aligning along the P-D axis at the onset of redistribution and a small but significant excess of + ends-distal MTs in the proximal region of the wing. This characteristic alignment and asymmetry of MT growth was controlled by atypical cadherins Dachsous (Ds) and Fat (Ft). Furthermore, the action of Ft was mediated in part by PAR-1. All these data support the idea that the active reorientation of MT growth adjusts cell polarity along the organ axis.

TBC-2 regulates RAB-5/RAB-7-mediated endosomal trafficking in Caenorhabditis elegans

The RAB-5 and RAB-7 GTPases regulate endosome to lysosome trafficking. Here, we show that Caenorhabditis elegans TBC-2 functions as a RAB-5 GAP. TBC-2 colocalizes with RAB-7 on late endosomes, and requires RAB-7 for membrane localization where TBC-2 could function to antagonize RAB-5 activity during early to late endosome maturation.

During endosome maturation the early endosomal Rab5 GTPase is replaced with the late endosomal Rab7 GTPase. It has been proposed that active Rab5 can recruit and activate Rab7, which in turn could inactivate and remove Rab5. However, many of the Rab5 and Rab7 regulators that mediate endosome maturation are not known. Here, we identify Caenorhabditis elegans TBC-2, a conserved putative Rab GTPase-activating protein (GAP), as a regulator of endosome to lysosome trafficking in several tissues. We show that tbc-2 mutant animals accumulate enormous RAB-7–positive late endosomes in the intestine containing refractile material. RAB-5, RAB-7, and components of the homotypic fusion and vacuole protein sorting (HOPS) complex, a RAB-7 effector/putative guanine nucleotide exchange factor (GEF), are required for the tbc-2(−) intestinal phenotype. Expression of activated RAB-5 Q78L in the intestine phenocopies the tbc-2(−) large late endosome phenotype in a RAB-7 and HOPS complex-dependent manner. TBC-2 requires the catalytic arginine-finger for function in vivo and displays the strongest GAP activity on RAB-5 in vitro. However, TBC-2 colocalizes primarily with RAB-7 on late endosomes and requires RAB-7 for membrane localization. Our data suggest that TBC-2 functions on late endosomes to inactivate RAB-5 during endosome maturation.

Protein phosphatase 2A cooperates with the autophagy-related kinase UNC-51 to regulate axon guidance in Caenorhabditis elegans

UNC-51 is a serine/threonine protein kinase conserved from yeast to humans. The yeast homolog Atg1 regulates autophagy (catabolic membrane trafficking) required for surviving starvation. In C. elegans, UNC-51 regulates the axon guidance of many neurons by a different mechanism than it and its homologs use for autophagy. UNC-51 regulates the subcellular localization (trafficking) of UNC-5, a receptor for the axon guidance molecule UNC-6/Netrin; however, the molecular details of the role for UNC-51 are largely unknown. Here, we report that UNC-51 physically interacts with LET-92, the catalytic subunit of serine/threonine protein phosphatase 2A (PP2A-C), which plays important roles in many cellular functions. A low allelic dose of LET-92 partially suppressed axon guidance defects of weak, but not severe, unc-51 mutants, and a low allelic dose of PP2A regulatory subunits A (PAA-1/PP2A-A) and B (SUR-6/PP2A-B) partially enhanced the weak unc-51 mutants. We also found that LET-92 can work cell-non-autonomously on axon guidance in neurons, and that LET-92 colocalized with UNC-51 in neurons. In addition, PP2A dephosphorylated phosphoproteins that had been phosphorylated by UNC-51. These results suggest that, by forming a complex, PP2A cooperates with UNC-51 to regulate axon guidance by regulating phosphorylation. This is the first report of a serine/threonine protein phosphatase functioning in axon guidance in vivo.

Protein phosphatase 2A subunit gene haplotypes and proliferative breast disease modify breast cancer risk

BACKGROUND

Protein phosphatase 2A (PP2A) is a major cellular phosphatase and plays key regulatory roles in growth, differentiation, and apoptosis. Women who are diagnosed with benign proliferative breast disease are at increased risk for the subsequent development of breast cancer.

METHODS

The authors evaluated genetic variation of PP2A holoenzyme subunits for their potential contribution to breast cancer risk. A nested case-control investigation was performed on a cohort of women who had a history of benign breast disease. The women were followed for an average of 18 years, and DNA prepared from the original archival benign breast biopsy (1954-1995) was available for 450 women who were diagnosed with breast cancer on follow-up and for 890 of 900 women in a control group who were matched on race, age, and year of entry biopsy.

RESULTS

Single allele-based and haplotype-based tests of association were conducted with assessment of significance by permutation testing. Significant risk and protective haplotypes of the PP2A structural/regulatory subunit A alpha isoform (PPP2R1A) were identified and had odds ratios of 1.63 (95% confidence interval [CI], 1.3-2.1) and 0.55 (95% CI, 0.41-0.76), respectively. These odds ratios remained significant after the analysis was adjusted for multiple comparisons. Women who had both the PPP2R1A risk haplotype and a history of proliferative breast disease had an odds ratio of 2.44 (95% CI, 1.7-3.5) for the subsequent development of breast cancer. The effects of haplotypes for 2 PP2A regulatory subunit genes, PP2 regulatory subunit B alpha isoform (PPP2R2A) and PP2A regulatory subunit B' epsilon isoform (PPP2R5E) on breast cancer risk were nominally significant but did not remain significant after the analysis was adjusted for multiple comparisons.

CONCLUSIONS

The current findings supported the previously hypothesized role of PP2A as a tumor suppressor gene in breast cancer.

MAP kinase signaling antagonizes PAR-1 function during polarization of the early Caenorhabditis elegans embryo

PAR proteins (partitioning defective) are major regulators of cell polarity and asymmetric cell division. One of the par genes, par-1, encodes a Ser/Thr kinase that is conserved from yeast to mammals. In Caenorhabditis elegans, par-1 governs asymmetric cell division by ensuring the polar distribution of cell fate determinants. However the precise mechanisms by which PAR-1 regulates asymmetric cell division in C. elegans remain to be elucidated. We performed a genomewide RNAi screen and identified six genes that specifically suppress the embryonic lethal phenotype associated with mutations in par-1. One of these suppressors is mpk-1, the C. elegans homolog of the conserved mitogen activated protein (MAP) kinase ERK. Loss of function of mpk-1 restored embryonic viability, asynchronous cell divisions, the asymmetric distribution of cell fate specification markers, and the distribution of PAR-1 protein in par-1 mutant embryos, indicating that this genetic interaction is functionally relevant for embryonic development. Furthermore, disrupting the function of other components of the MAPK signaling pathway resulted in suppression of par-1 embryonic lethality. Our data therefore indicates that MAP kinase signaling antagonizes PAR-1 signaling during early C. elegans embryonic polarization.

Raf-independent, PP2A-dependent MEK activation in response to ERK silencing

Biological roles of ERK and MEK in signal transduction have been controversial. The aim of the current study was to determine the role of ERK1/2 in signaling through the ERK-MAPK cascade by using RNAi methodology. Transient transfection of erk1 or erk2 siRNA decreased the respective protein level to 3-8% in human lung fibroblasts. Interestingly, individual ERK isoform silencing resulted in a 2-fold reciprocal increase in phosphorylation of the alternate ERK isoform, with no change in respective total protein expression. Moreover, MEK was hyperphosphorylated as a result of combined ERK1 and ERK2 silencing, but was unaffected in individual ERK1 or ERK2 silenced cells. This hyperactivation of MEK was not due to activation of Raf family members, but rather was associated with PP2A downregulation. These data highlight the existence of a feedback loop in normal cells whereby ERK silencing is associated with decreased PP2A activity and consequent MEK activation.

Protein phosphatase 2A regulatory subunits and cancer

The serine/threonine protein phosphatase (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of a number of major signaling pathways whose deregulation can contribute to cancer. The specificity and activity of PP2A are highly regulated through the interaction of a family of regulatory B subunits with the substrates. Accumulating evidence indicates that PP2A acts as a tumor suppressor. In this review we summarize the known effects of specific PP2A holoenzymes and their roles in cancer relevant pathways. In particular we highlight PP2A function in the regulation of MAPK and Wnt signaling.

A whole-genome RNAi Screen for C. elegans miRNA pathway genes

BACKGROUND

miRNAs are an abundant class of small, endogenous regulatory RNAs. Although it is now appreciated that miRNAs are involved in a broad range of biological processes, relatively little is known about the actual mechanism by which miRNAs downregulate target gene expression. An exploration of which protein cofactors are necessary for a miRNA to downregulate a target gene should reveal more fully the molecular mechanisms by which miRNAs are processed, trafficked, and regulate their target genes.

RESULTS

A weak allele of the C. elegans miRNA gene let-7 was used as a sensitized genetic background for a whole-genome RNAi screen to detect miRNA pathway genes, and 213 candidate miRNA pathway genes were identified. About 2/3 of the 61 candidates with the strongest phenotype were validated through genetic tests examining the dependence of the let-7 phenotype on target genes known to function in the let-7 pathway. Biochemical tests for let-7 miRNA production place the function of nearly all of these new miRNA pathway genes downstream of let-7 expression and processing. By monitoring the downregulation of the protein product of the lin-14 mRNA, which is the target of the lin-4 miRNA, we have identified 19 general miRNA pathway genes.

CONCLUSIONS

The 213 candidate miRNA pathway genes identified could act at steps that produce and traffic miRNAs or in downstream steps that detect miRNA::mRNA duplexes to regulate mRNA translation. The 19 validated general miRNA pathway genes are good candidates for genes that may define protein cofactors for sorting or targeting miRNA::mRNA duplexes, or for recognizing the miRNA base-paired to the target mRNA to downregulate translation.

Cryptic quantitative evolution of the vulva intercellular signaling network in Caenorhabditis

BACKGROUND

The Caenorhabditis vulva is formed from a row of Pn.p precursor cells, which adopt a spatial cell-fate pattern-3 degrees 3 degrees 2 degrees 1 degrees 2 degrees 3 degrees -centered on the gonadal anchor cell. This pattern is robustly specified by an intercellular signaling network including EGF/Ras induction from the anchor cell and Delta/Notch signaling between the precursor cells. It is unknown how the roles and quantitative contributions of these signaling pathways have evolved in closely related Caenorhabditis species.

RESULTS

Cryptic evolution in the network is uncovered by quantification of cell-fate-pattern frequencies obtained after displacement of the system out of its normal range, either by anchor-cell ablations or through LIN-3/EGF overexpression. Silent evolution in the Caenorhabditis genus covers a large neutral space of cell-fate patterns. Direct induction of the 1 degrees fate as in C. elegans appeared within the genus. C. briggsae displays a graded induction of 1 degrees and 2 degrees fates, with 1 degrees fate induction requiring a longer time than in C. elegans, and a reduced lateral inhibition of adjacent 1 degrees fates. C. remanei displays a strong lateral induction of 2 degrees fates relative to vulval-fate activation in the central cell. This evolution in cell-fate pattern space can be experimentally reconstituted by mild variations of Ras, Wnt, and Notch pathway activities in C. elegans and C. briggsae.

CONCLUSIONS

Quantitative evolution in the roles of graded induction by LIN-3/EGF and Notch signaling is demonstrated for the Caenorhabditis vulva signaling network. This evolutionary system biology approach provides a quantitative view of the variational properties of this biological system.

KSR and CNK: two scaffolds regulating RAS-mediated RAF activation

The RAS-RAF-MEK-extracellular-regulated kinase (RAS/ERK) pathway is a major intracellular route used by metazoan cells to channel to downstream targets a diverse array of signals, including those controlling cell proliferation and survival. Recent findings suggest that the pathway is assembled by specific scaffolding proteins that in turn regulate the efficiency, the location and/or the duration of signal transmission. Here, through the angle of studies conducted in Drosophila and C. elegans, we present two such proteins, the kinase suppressor of RAS (KSR) and connector enhancer of KSR (CNK) scaffolds, and highlight their implication in a novel mechanism regulating RAS-mediated RAF activation. Based on recent findings, we discuss the possibility that KSR, a RAF-like protein, does not solely act as a scaffold, but directly induces RAF catalytic function by a kinase-independent mechanism apparently shared by RAF-like proteins.

ASNA-1 positively regulates insulin secretion in C. elegans and mammalian cells

C. elegans worms hatching in the absence of food show growth arrest during the first larval stage (L1). While much has been learned about the later diapause, dauer, which worms enter under adverse conditions, much less is known about the mechanisms governing L1 arrest. Here we show that worms lacking activity of the asna-1 gene arrest growth reversibly at the L1 stage even when food is abundant. asna-1 encodes an ATPase that functions nonautonomously to regulate growth. asna-1 is expressed in a restricted set of sensory neurons and in insulin-producing intestinal cells. asna-1 mutants are reduced in insulin secretion while overexpression of asna-1 mimics the effects of insulin overexpression. Human ASNA1 is highly expressed in pancreatic beta cells, but not in other pancreatic endocrine cell types, and regulates insulin secretion in cultured cells. We propose that ASNA1 is an evolutionarily conserved modulator of insulin signaling.

The C. elegans RSA complex localizes protein phosphatase 2A to centrosomes and regulates mitotic spindle assembly

Microtubule behavior changes during the cell cycle and during spindle assembly. However, it remains unclear how these changes are regulated and coordinated. We describe a complex that targets the Protein Phosphatase 2A holoenzyme (PP2A) to centrosomes in C. elegans embryos. This complex includes Regulator of Spindle Assembly 1 (RSA-1), a targeting subunit for PP2A, and RSA-2, a protein that binds and recruits RSA-1 to centrosomes. In contrast to the multiple functions of the PP2A catalytic subunit, RSA-1 and RSA-2 are specifically required for microtubule outgrowth from centrosomes and for spindle assembly. The centrosomally localized RSA-PP2A complex mediates these functions in part by regulating two critical mitotic effectors: the microtubule destabilizer KLP-7 and the C. elegans regulator of spindle assembly TPXL-1. By regulating a subset of PP2A functions at the centrosome, the RSA complex could therefore provide a means of coordinating microtubule outgrowth from centrosomes and kinetochore microtubule stability during mitotic spindle assembly.

Robustness and evolution: concepts, insights and challenges from a developmental model system

Robustness, the persistence of an organismal trait under perturbations, is a ubiquitous property of complex living systems. We here discuss key concepts related to robustness with examples from vulva development in the nematode Caenorhabditis elegans. We emphasize the need to be clear about the perturbations a trait is (or is not) robust to. We discuss two prominent mechanistic causes of robustness, namely redundancy and distributed robustness. We also discuss possible evolutionary causes of robustness, one of which does not involve natural selection. To better understand robustness is of paramount importance for understanding organismal evolution. Part of the reason is that highly robust systems can accumulate cryptic variation that can serve as a source of new adaptations and evolutionary innovations. We point to some key challenges in improving our understanding of robustness.

Extending from PARs in Caenorhabditis elegans to homologues in Haemonchus contortus and other parasitic nematodes

Signal transduction molecules play key roles in the regulation of developmental processes, such as morphogenesis, organogenesis and cell differentiation in all organisms. They are organized into 'pathways' that represent a coordinated network of cell-surface receptors and intracellular molecules, being involved in sensing environmental stimuli and transducing signals to regulate or modulate cellular processes, such as gene expression and cytoskeletal dynamics. A particularly important group of molecules implicated in the regulation of the cytoskeleton for the establishment and maintenance of cell polarity is the PAR proteins (derived from partition defective in asymmetric cell division). The present article reviews salient aspects of PAR proteins involved in the early embryonic development and morphogenesis of the free-living nematode Caenorhabditis elegans and some other organisms, with an emphasis on the molecule PAR-1. Recent advances in the knowledge and understanding of PAR-1 homologues from the economically important parasitic nematode, Haemonchus contortus, of small ruminants is summarized and discussed in the context of exploring avenues for future research in this area for parasitic nematodes.

A Phosphatase Holoenzyme Comprised of Shoc2/Sur8 and the Catalytic Subunit of PP1 Functions as an M-Ras Effector to Modulate Raf Activity

Ras family GTPases (RFGs) are known to share many regulatory and effector proteins. How signaling and biological specificity is achieved is poorly understood. Using a proteomics approach, we have identified a complex comprised of Shoc2/Sur-8 and the catalytic subunit of protein phosphatase 1 (PP1c) as a highly specific M-Ras effector. M-Ras targets Shoc2-PP1c to stimulate Raf activity by dephosphorylating the S259 inhibitory site of Raf proteins bound to other molecules of M-Ras or Ras. Therefore, distinct RFGs, through independent effectors, can regulate different steps in the activation of Raf kinases. Shoc2 function is essential for activation of the MAPK pathway by growth factors. Furthermore, in tumor cells with Ras gene mutations, inhibition of Shoc2 expression inhibits MAPK, but not PI3K activity. We propose that the Shoc2-PP1c holoenzyme provides an attractive therapeutic target for inhibition of the MAPK pathway in cancer.

Positive regulation of Raf1-MEK1/2-ERK1/2 signaling by protein serine/threonine phosphatase 2A holoenzymes

Protein serine/threonine phosphatase 2A (PP2A) regulates a wide variety of cellular signal transduction pathways. The predominant form of PP2A in cells is a heterotrimeric holoenzyme consisting of a scaffolding (A) subunit, a regulatory (B) subunit, and a catalytic (C) subunit. Although PP2A is known to regulate Raf1-MEK1/2-ERK1/2 signaling at multiple steps in this pathway, the specific PP2A holoenzymes involved remain unclear. To address this question, we established tetracycline-inducible human embryonic kidney 293 cell lines for overexpression of FLAG-tagged Balpha/delta regulatory subunits by approximately 3-fold or knock-down of Balpha by greater than 70% compared with endogenous levels. The expression of functional epitope-tagged B subunits was confirmed by the detection of A and C subunits as well as phosphatase activity in FLAG immune complexes from extracts of cells overexpressing the FLAG-Balpha/delta subunit. Western analysis of the cell extracts using phosphospecific antibodies for MEK1/2 and ERK1/2 demonstrated that activation of these kinases in response to epidermal growth factor was markedly diminished in Balpha knock-down cells but elevated in Balpha- and Bdelta-overexpressing cells as compared with control cells. In parallel with the activation of MEK1/2 and ERK1/2, the inhibitory phosphorylation site of Raf1 (Ser-259) was dephosphorylated in cells overexpressing Balpha or Bdelta. Pharmacological inhibitor studies as well as reporter assays for ERK-dependent activation of the transcription factor Elk1 revealed that the PP2A holoenzymes ABalphaC and ABdeltaC act downstream of Ras and upstream of MEK1 to promote activation of this MAPK signaling cascade. Furthermore both PP2A holoenzymes were found to associate with Raf1 and catalyze dephosphorylation of inhibitory phospho-Ser-259. Together these findings indicate that PP2A ABalphaC and ABdeltaC holoenzymes function as positive regulators of Raf1-MEK1/2-ERK1/2 signaling by targeting Raf1.

Distinct protein phosphatase 2A heterotrimers modulate growth factor signaling to extracellular signal-regulated kinases and Akt

A key regulator of many kinase cascades, heterotrimeric protein serine/threonine phosphatase 2A (PP2A), is composed of catalytic (C), scaffold (A), and variable regulatory subunits (B, B', B'' gene families). In neuronal PC12 cells, PP2A acts predominantly as a gatekeeper of extracellular signal-regulated kinase (ERK) activity, as shown by inducible RNA interference of the Aalpha scaffolding subunit and PP2A inhibition by okadaic acid. Although okadaic acid potentiates Akt/protein kinase B and ERK phosphorylation in response to epidermal, basic fibroblast, or nerve growth factor, silencing of Aalpha paradoxically has the opposite effect. Epidermal growth factor receptor Tyr phosphorylation was unchanged following Aalpha knockdown, suggesting that chronic Akt and ERK hyperphosphorylation leads to compensatory down-regulation of signaling molecules upstream of Ras and blunted growth factor responses. Inducible exchange of wild-type Aalpha with a mutant with selective B' subunit binding deficiency implicated PP2A/B' heterotrimers as Akt modulators. Conversely, silencing of the B-family regulatory subunits Balpha and Bdelta led to hyperactivation of ERK stimulated by constitutively active MEK1. In vitro dephosphorylation assays further support a role for Balpha and Bdelta in targeting the PP2A heterotrimer to dephosphorylate and inactivate ERKs. Thus, receptor tyrosine kinase signaling cascades leading to Akt and ERK activation are modulated by PP2A holoenzymes with distinct regulatory properties.

Caenorhabditis elegans CNK-1 promotes Raf activation but is not essential for Ras/Raf signaling

Connector enhancer of Ksr (CNK) is a conserved multidomain protein essential for Ras signaling in Drosophila melanogaster and thought to be involved in Raf kinase activation. However, the precise role of CNK in Ras signaling is not known, and mammalian CNKs are proposed to have distinct functions. Caenorhabditis elegans has a single CNK homologue, cnk-1. Here, we describe the role of cnk-1 in C. elegans Ras signaling and its requirements for LIN-45 Raf activation. We find that cnk-1 positively regulates multiple Ras signaling events during development, but, unlike Drosophila CNK, cnk-1 does not appear to be essential for signaling. cnk-1 mutants appear to be normal but show cell-type-specific genetic interactions with mutations in two other Ras pathway scaffolds/adaptors ksr-1 and sur-8. Genetic epistasis using various activated LIN-45 Raf transgenes shows that CNK-1 promotes LIN-45 Raf activation at a step between the dephosphorylation of inhibitory sites in the regulatory domain and activating phosphorylation in the kinase domain. Our data are consistent with a model in which CNK promotes Raf phosphorylation/activation through membrane localization, oligomerization, or association with an activating kinase.

Protein Phosphatase 2A Regulatory Subunit Bβ Promotes MAP Kinase-mediated Migration of A431 Cells

BACKGROUND/AIMS

Phosphatases are involved in regulation of MAP kinase (MAPK). A431 cells migrate on collagen after EGF stimulation using MAPK. To clarify the involvement of PP2A in this MAPK-dependent migration, the expression of an isoform of the B regulatory subunit was inhibited.

METHODS

An antisense sequence corresponding to Bbeta cDNA was transfected into A431 cells. Their migratory activity on collagen was examined using Transwell, and MAPK phosphorylation and phosphatase activity were measured, and the results were compared with those obtained with mock-transfected cells.

RESULTS

Antisense-transfected cells showed less Bbeta protein and phosphatase activity than mock-transfected controls. Migration of antisense-transfected cells showed a low response to EGF. The response of MAPK phosphorylation of antisense-transfected cells to EGF stimulation and adhesion to collagen in the presence or absence of EGF were markedly decreased. Phosphatase activity of PP2A-Bbeta also did not respond to EGF, collagen or EGF plus collagen, and remained at low levels.

CONCLUSION

These results suggested that PP2A-Bbeta promotes cell migration through the MAPK cascade.