Cross-talk between canonical Wnt signaling and the sirtuin-FoxO longevity pathway to protect against muscular pathology induced by mutant PABPN1 expression in C. elegans.

Developmental pathways may be play a role in adult cell survival. However, whether they interact with longevity/cell survival pathways to confer protection against disease-associated proteotoxicity remains largely unknown. We previously reported that the inhibition of key longevity modulators such as the deacetylase sir-2.1/SIRT1 (Sir2) and its target daf-16/FoxO protects transgenics nematodes from muscle cell decline and abnormal motility produced by the expression of mutant (polyalanine-expanded) PABPN1, the oculopharyngeal muscular dystrophy (OPMD) protein. Here, we report that canonical Wnt signaling (i) modulates muscular pathology in mutant PABPN1 nematodes, and (ii) cooperates with the Sir2-FoxO longevity pathway to confer protection against mutant PABPN1 toxicity at the cellular and behavioral levels. Mutant PABPN1 toxicity was modified by genes along the canonical Wnt pathway, several of which depend on daf-16 for activity. ss-catenin and pop-1/TCF RNAi suppressed the protection from mutant PABPN1 confered by loss-of-function mutations in sir-2.1 and daf-16. Moreover, the aggravation of muscle cell pathology by increased sir-2.1 dosage was reversed by ss-catenin and pop-1 RNAi. The chemical inhibition of GSK-3ss, a repressor of ss-catenin activity, protected against mutant PABPN1 toxicity in a daf-16-dependent manner, which is consistent with a cross-talk between ss-catenin signaling and Sir2-FoxO signaling in protecting from mutant PABPN1 toxicity. Our data reveal that canonical Wnt signaling and Sir2-FoxO signaling interact to modulate diseased muscle survival, and indicate that GSK-3ss inhibitors and sirtuin inhibitors both have therapeutic potential for muscle protection in OPMD.

Sirtuin inhibition protects from the polyalanine muscular dystrophy protein PABPN1.

Oculopharyngeal muscular dystrophy (OPMD) is caused by polyalanine expansion in nuclear protein PABPN1 [poly(A) binding protein nuclear 1] and characterized by muscle degeneration. Druggable modifiers of proteotoxicity in degenerative diseases, notably the longevity modulators sirtuins, may constitute useful therapeutic targets. However, the modifiers of mutant PABPN1 are unknown. Here, we report that longevity and cell metabolism modifiers modulate mutant PABPN1 toxicity in the muscle cell. Using PABPN1 nematodes that show muscle cell degeneration and abnormal motility, we found that increased dosage of the sirtuin and deacetylase sir-2.1/SIRT1 exacerbated muscle pathology, an effect dependent on the transcription factor daf-16/FoxO and fuel sensor aak-2/AMPK (AMP-activated protein kinase), while null mutants of sir-2.1, daf-16 and aak-2 were protective. Consistently, the Sir2 inhibitor sirtinol was protective, whereas the Sir2 and AMPK activator resveratrol was detrimental. Furthermore, rescue by sirtinol was dependent on daf-16 and not aak-2, whereas aggravation by resveratrol was dependent on aak-2 and not daf-16. Finally, the survival of mammalian cells expressing mutant PABPN1 was promoted by sirtinol and decreased by resveratrol. Altogether, our data identify Sir2 and AMPK inhibition as therapeutic strategies for muscle protection in OPMD, extending the value of druggable proteins in cell maintenance networks to polyalanine diseases.

The cellular homeostasis of the gut: what the Drosophila model points out.

The digestive tract is subjected to many aggressions throughout animal life. Since disruptions of gut physiology impact on animal fitness and survival, maintenance of gut integrity and functionality is essential for the individual. Over the last 40 years, research on rodents has aimed at understanding how cellular homeostasis of the digestive tract is maintained when challenged with disruptions. Following the discovery of stem cells in the digestive tract of Drosophila, a flurry of studies made an important contribution to our understanding of how the proliferation and the differentiation of these cells are controlled and participate in the renewal of the digestive tract. Insights into these mechanisms in Drosophila have revealed many similarities with mammalian intestinal stem cells. For instance, the highly conserved EGFR, JAK/STAT, Wingless/Wnt, Hedgehog, Integrins, BMP/TGFß, Hippo and Insulin pathways all participate in adult intestinal cellular homeostasis. Here, we provide a literature review of recent advances in the field highlighting the adult Drosophila midgut as a convenient model for dissecting mechanisms involved in the maintenance of the cellular homeostasis of the digestive tract in conventionally reared conditions. In addition, we shed light on recently published data putting Drosophila forward as a genetic tool to decipher the mechanisms underlying intestinal diseases and intestinal tumour progression.

High sugar-induced insulin resistance in Drosophila relies on the lipocalin Neural Lazarillo.

In multicellular organisms, insulin/IGF signaling (IIS) plays a central role in matching energy needs with uptake and storage, participating in functions as diverse as metabolic homeostasis, growth, reproduction and ageing. In mammals, this pleiotropy of action relies in part on a dichotomy of action of insulin, IGF-I and their respective membrane-bound receptors. In organisms with simpler IIS, this functional separation is questionable. In Drosophila IIS consists of several insulin-like peptides called Dilps, activating a unique membrane receptor and its downstream signaling cascade. During larval development, IIS is involved in metabolic homeostasis and growth. We have used feeding conditions (high sugar diet, HSD) that induce an important change in metabolic homeostasis to monitor possible effects on growth. Unexpectedly we observed that HSD-fed animals exhibited severe growth inhibition as a consequence of peripheral Dilp resistance. Dilp-resistant animals present several metabolic disorders similar to those observed in type II diabetes (T2D) patients. By exploring the molecular mechanisms involved in Drosophila Dilp resistance, we found a major role for the lipocalin Neural Lazarillo (NLaz), a target of JNK signaling. NLaz expression is strongly increased upon HSD and animals heterozygous for an NLaz null mutation are fully protected from HSD-induced Dilp resistance. NLaz is a secreted protein homologous to the Retinol-Binding Protein 4 involved in the onset of T2D in human and mice. These results indicate that insulin resistance shares common molecular mechanisms in flies and human and that Drosophila could emerge as a powerful genetic system to study some aspects of this complex syndrome.

Characterization of sirtuin inhibitors in nematodes expressing a muscular dystrophy protein reveals muscle cell and behavioral protection by specific sirtinol analogues.

In oculopharyngeal muscular dystrophy (OPMD), a disease caused by polyalanine expansion in the nuclear protein PABPN1, the genetic inhibition of sirtuins and treatment with sirtuin inhibitors protect from mutant PABPN1 toxicity in transgenic nematodes. Here, we tested the SIRT1/2 inhibitors 1-12, bearing different degrees of inhibition, for protection against mutant PABPN1 toxicity in Caenorhabditis elegans. Compounds 2, 4, and 11 were the most efficient, revealing a potential therapeutic application for muscle cell protection in OPMD.

Deleterious and protective properties of an aggregate-prone protein with a polyalanine expansion.

Many aggregate-prone proteins, including proteins with long polyglutamine or polyalanine tracts, cause human diseases. Polyalanine proteins may also be present in the tissue of polyglutamine diseases as a result of frameshifting of the primary polyglutamine-encoding (CAG)n repeat mutation. We have generated a Drosophila model expressing green fluorescent protein tagged to 37 alanines that manifests both toxicity and inclusion formation in various tissues. Surprisingly, we show that this aggregate-prone protein with a polyalanine expansion can also protect against polyglutamine toxicity, which can be explained by induction of heat-shock response. A heat-shock response was also seen in an oculopharyngeal muscular dystrophy mouse model expressing an authentic polyalanine-expanded protein. We also show that long polyalanines can protect against a pro-apoptotic stimulus or the toxicity caused by the long polyalanines themselves. Thus, overexpression of an aggregate-prone protein without any normal functions can result in both pathogenic and protective effects in cell culture and in vivo.

Lithium rescues toxicity of aggregate-prone proteins in Drosophila by perturbing Wnt pathway.

We have previously shown that lithium can protect against the polyglutamine toxicity of the Huntington's disease mutation in cell models. Here, we demonstrate for the first time in vivo that lithium can protect against the toxicity caused by aggregate-prone proteins with either polyglutamine or polyalanine expansions in Drosophila. We also show that these protective effects can be partly accounted for by lithium acting through the Wnt/Wg pathway, as a GSK3beta-specific inhibitor and overexpression of dTCF also mediate protective effects. Our data suggest that lithium deserves serious consideration for further studies as a therapeutic for polyglutamine diseases, particularly as it is an established drug that has been used for several decades for chronic treatment of affective disorders.