Narbonne P et al. (2010) Development "Differential requirements for STRAD in LKB1-dependent functions in C. elegans."

Li S, Labbe JC, Hyenne V, Roy R, Narbonne P

[Development, 2010]

The protein kinase LKB1 is a crucial regulator of cell growth/proliferation and cell polarity and is the causative gene in the cancer-predisposing disease Peutz-Jeghers syndrome (PJS). The activity of LKB1 is greatly enhanced following its association with the Ste20-like adapter protein STRAD. Unlike LKB1 however, mutations in STRAD have not been identified in PJS patients and thus, the key tumour suppressive role(s) of LKB1 might be STRAD independent. Here, we report that Caenorhabditis elegans strd-1/STRAD mutants recapitulate many phenotypes typical of par-4/LKB1 loss of function, showing defects during early embryonic and dauer development. Interestingly, although the growth/proliferation defects in severe par-4 and strd-1 mutant dauers are comparable, strd-1 mutant embryos do not share the polarity defects of par-4 embryos. We demonstrate that most of par-4-dependent regulation of germline stem cell (GSC) quiescence occurs through AMPK, whereby PAR-4 requires STRD-1 to phosphorylate and activate AMPK. Consistent with this, even though AMPK plays a major role in the regulation of cell proliferation, like strd-1 it does not affect embryonic polarity. Instead, we found that the PAR-4-mediated phosphorylation of polarity regulators such as PAR-1 and MEX-5 in the early embryo occurs in the absence of STRD-1. Thus, PAR-4 requires STRD-1 to phosphorylate AMPK to regulate cell growth/proliferation under reduced insulin signalling conditions, whereas PAR-4 can promote phosphorylation of key proteins, including PAR-1 and MEX-5, to specify early embryonic polarity independently of STRD-1. Our results therefore identify a key strd-1/STRAD-independent function of par-4/LKB1 in polarity establishment that is likely to be important for tumour suppression in humans.

Narbonne P et al. (2006) Cell Div "Regulation of germline stem cell proliferation downstream of nutrient sensing."

Narbonne P, Roy R

[Cell Div, 2006]

Stem cells have recently attracted significant attention largely due to their potential therapeutic properties, but also because of their role in tumorigenesis and their resemblance, in many aspects, to cancerous cells. Understanding how stem cells are regulated, namely with respect to the control of their proliferation and differentiation within a functional organism, is thus primordial to safely profit from their therapeutic benefits. Here, we review recent advances in the understanding of germline stem cell proliferation control by factors that respond to the nutritional status and/or insulin signaling, through studies performed in C. elegans and Drosophila. Together, these data uncover some shared fundamental features that underlie the central control of cellular proliferation within a target stem cell population in an organism. These features may indeed be conserved in higher organisms and may apply to various other stem cell populations.

Narbonne P et al. (2009) Nature "Caenorhabditis elegans dauers need LKB1/AMPK to ration lipid reserves and ensure long-term survival."

Roy R, Narbonne P

[Nature, 2009]

Many organisms can enter a dormant state or diapause to survive harsh environmental conditions for extended durations. When Caenorhabditis elegans larvae enter dauer they arrest feeding but remain active and motile, yet become stress-resistant, extremely long-lived and non-ageing. Entry into dauer is associated with a reduction in insulin-like signalling, the accumulation of nutritive resources and a concomitant global change in metabolism, yet the precise molecular and physiological processes that enable long-term survival in the absence of caloric intake remain largely unknown. We show here that C. elegans larvae that lack LKB1/AMPK (AMP-activated protein kinase) signalling enter dauer normally, but then rapidly consume their stored energy and prematurely expire following vital organ failure. We found that this signalling pathway acts in adipose-like tissues to downregulate triglyceride hydrolysis so that these lipid reserves are rationed to last the entire duration of the arrest. Indeed, the downregulation of adipose triglyceride lipase (ATGL-1) activity suppresses both the rapid depletion of stored lipids and reduced life span of AMPK mutant dauers, while AMPK directly phosphorylates ATGL-1. Finally, we show that the slow release of energy during dauer is critical for appropriate long-term osmoregulation, which fails as triglyceride resources become depleted. These mechanisms may be essential for survival through diapause, hibernation, or long-term fasting in diverse organisms and may also underlie AMPK-dependent life span extension.

Narbonne P et al. (2006) Development "Inhibition of germline proliferation during C. elegans dauer development requires PTEN, LKB1 and AMPK signalling."

Narbonne P, Roy R

[Development, 2006]

In C. elegans, reduced insulin-like signalling induces developmental quiescence, reproductive delay and lifespan extension. We show here that the C. elegans orthologues of LKB1 and AMPK cooperate during conditions of reduced insulin-like signalling to establish cell cycle quiescence in the germline stem cell population, in addition to prolonging lifespan. The inactivation of either protein causes aberrant germline proliferation during diapause-like ''dauer'' development, whereas the loss of AMPK uncouples developmental arrest from lifespan extension. Reduced TGF-beta activity also triggers developmental quiescence independent of the insulin-like pathway. Our data suggest that these two signalling pathways converge on the C. elegans PTEN orthologue to coordinate germline proliferation with somatic development during dauer formation, via the regulation of AMPK and its upstream activator LKB1, rather than through the canonical insulin-like signalling cascade. In humans, germline mutations in TGF-beta family members, PTEN or LKB1 result in related tumour-predisposing syndromes. Our findings establish a developmental relationship that may underscore their shared, characteristic aetiology.

Narbonne P et al. (2015) Development "DAF-18/PTEN locally antagonizes insulin signalling to couple germline stem cell proliferation to oocyte needs in C. elegans."

Maddox PS, Narbonne P, Labbe JC

[Development, 2015]

During development, stem cell populations rapidly proliferate to populate the expanding tissues and organs. During this phase, nutrient status, by systemically affecting insulin/IGF-1 signalling, largely dictates stem cell proliferation rates. In adults, however, differentiated stem cell progeny requirements are generally reduced and vary according to the spatiotemporal needs of each tissue. We demonstrate here that differential regulation of germline stem cell proliferation rates in Caenorhabditis elegans adults is accomplished through localized neutralization of insulin/IGF-1 signalling, requiring DAF-18/PTEN, but not DAF-16/FOXO. Indeed, the specific accumulation of oocytes, the terminally differentiated stem cell progeny, triggers a feedback signal that locally antagonizes insulin/IGF-1 signalling outputs in the germ line, regardless of their systemic levels, to block germline stem cell proliferation. Thus, during adulthood, stem cells can differentially respond within tissues to otherwise equal insulin/IGF-1 signalling inputs, according to the needs for production of their immediate terminally differentiated progeny.

Narbonne P et al. (2017) PLoS Genet "DAF-18/PTEN signals through AAK-1/AMPK to inhibit MPK-1/MAPK in feedback control of germline stem cell proliferation."

Narbonne P, Maddox PS, Labbe JC

[PLoS Genet, 2017]

Under replete growth conditions, abundant nutrient uptake leads to the systemic activation of insulin/IGF-1 signalling (IIS) and the promotion of stem cell growth/proliferation. Activated IIS can stimulate the ERK/MAPK pathway, the activation of which also supports optimal stem cell proliferation in various systems. Stem cell proliferation rates can further be locally refined to meet the resident tissue's need for differentiated progeny. We have recently shown that the accumulation of mature oocytes in the C. elegans germ line, through DAF-18/PTEN, inhibits adult germline stem cell (GSC) proliferation, despite high systemic IIS activation. We show here that this feedback occurs through a novel cryptic signalling pathway that requires PAR-4/LKB1, AAK-1/AMPK and PAR-5/14-3-3 to inhibit the activity of MPK-1/MAPK, antagonize IIS, and inhibit both GSC proliferation and the production of additional oocytes. Interestingly, our results imply that DAF-18/PTEN, through PAR-4/LKB1, can activate AAK-1/AMPK in the absence of apparent energy stress. As all components are conserved, similar signalling cascades may regulate stem cell activities in other organisms and be widely implicated in cancer.

Cunningham KA et al. (2009) Cell Metab "Fat rationing in dauer times."

Ashrafi K, Cunningham KA

[Cell Metab, 2009]

The fundamental task of maintaining energy balance is complex when nutrient levels are plentiful, but it becomes even more challenging when nutrients are dynamic or scarce. A recent Nature report delineates a role of the AMP kinase pathway in rationing energy stores for the long-term survival of Caenorhabditis elegans dauers (Narbonne and Roy, 2009).

Loo TW et al. (2013) Biochemistry "A salt bridge in intracellular loop 2 is essential for folding of human p-glycoprotein."

Clarke DM, Loo TW

[Biochemistry, 2013]

There is no high-resolution structure of the human P-glycoprotein (P-gp, ABCB1) drug pump. Homology models based on the crystal structures of mouse and Caenorhabditis elegans P-gps show extensive contacts between intracellular loop 2 (ICL2, in the first transmembrane domain) and the second nucleotide-binding domain. Human P-gp modeled on these P-gp structures yields different ICL2 structures. Only the model based on the C. elegans P-gp structure predicts the presence of a salt bridge. We show that the Glu256-Arg276 salt bridge was critical for P-gp folding.

Roy, Vincent et al. MicroPubl Biol

Gagne, Olivier, Hamiche, Karim, Narbonne, Patrick, Labbe, Jean-Claude, Roy, Vincent

[MicroPubl Biol, 2018]

PAR-4/LKB1 is maternally expressed and required for the asymmetrical distribution of early embryonic determinants and viability in C. elegans (Morton et al. 1992; Watts et al. 2000; Tenlen et al. 2008). It is also implicated in a variety of postembryonic processes, including germline stem cell quiescence (Narbonne and Roy 2006; Narbonne et al. 2017), neuronal growth and polarity (Kim et al. 2010; Teichmann and Shen 2011), cytoskeletal rearrangements (Narbonne et al. 2010; Chartier et al. 2011), and metabolism (Narbonne and Roy 2009). Its expression pattern and subcellular localization has been determined in fixed animals by antibody staining (Watts et al. 2000). Here, we used CRISPR/Cas9-mediated genome editing to fluorescently label the two longest endogenous PAR-4 isoforms, PAR-4A and PAR-4C, with monomeric Neon Green (mNG) (Shaner et al. 2013), using the self-excising cassette (SEC) system (Dickinson et al. 2015) (Fig. 1A). We found ubiquitous mNG::par-4a & c expression and cytoplasmic and cortical enrichment of mNG::PAR-4A & C proteins in the germ line and early embryos (Fig. 1B-H), as previously described (Watts et al. 2000). Interestingly, we find that mNG::PAR-4A & C cortical enrichment is transiently lost in the pachytene area of the germ line (Fig. 1F), although it remains unclear whether this is functionally relevant. The intermediate strain that still contains the SEC (Dickinson et al. 2015) is predicted to be null for par-4a & c, potentially leaving the shorter par-4b isoform functional. To evaluate the requirement for par-4a & c we examined the embryonic lethality (at 25C) of the generated SEC-containing and SEC-excised strains. We found that the self-progeny of homozygous SEC-containing (e.g. par-4a & c null) animals is viable (Fig. 1H). This suggests that par-4b alone is sufficient to establish embryonic polarity and sustain the essential function of par-4. Consistent with this, to our knowledge, all existing par-4 alleles that impair embryonic development disrupt par-4b (Morton et al. 1992; Watts et al. 2000).

Simske JS et al. (1995) Nature "Sequential signalling during Caenorhabditis elegans vulval induction."

Simske JS, Kim SK

[Nature, 1995]

During the induction of the Caenorhabditis elegans vulva, cell signalling causes initially equipotent cells to express a reproducible pattern of cell fates(1,2). The position of the anchor cell determines the pattern of vulval precursor cell fates, such that the closest precursor cell (P6.p) expresses the primary cell fate, the next closest cells (P5.p and P7.p) both express the secondary cell fate, and each of the precursor cells located at a distance (P3.p, P4.p and P8.p) express the tertiary cell fate (Fig. 1a)(3-5). We present data indicating that this stereotypical pattern of cell fates can be generated by sequential signals. We identified genetic mosaic animals in which P5.p and P7.p were defective in the anchor-cell signal-transduction pathway and observed that these cells adopted the secondary cell fate, indicating that anchor-cell signal transduction is not required for the expression of the secondary cell fate. These results suggest that the anchor cell induces P6.p to express the primary cell fate, and that P6.p subsequently induces P5.p and P7.p to express the secondary cell fate.