Hepatocellular carcinoma-associated depletion of the netrin-1 receptor Uncoordinated Phenotype-5A (UNC5A) skews the hepatic unfolded protein response towards prosurvival outcomes

Axon guidance molecules in liver pathology: Journeys on a damaged passport

BACKGROUND AND AIMS

The liver is an innervated organ that develops a variety of chronic liver disease (CLD). Axon guidance cues (AGCs), of which ephrins, netrins, semaphorins and slits are the main representative, are secreted or membrane-bound proteins that can attract or repel axons through interactions with their growth cones that contain receptors recognizing these messengers. While fundamentally implicated in the physiological development of the nervous system, the expression of AGCs can also be reinduced under acute or chronic conditions, such as CLD, that necessitate redeployment of neural networks.

METHODS

This review considers the ad hoc literature through the neglected canonical neural function of these proteins that is also applicable to the diseased liver (and not solely their observed parenchymal impact).

RESULTS

AGCs impact fibrosis regulation, immune functions, viral/host interactions, angiogenesis, and cell growth, both at the CLD and HCC levels. Special attention has been paid to distinguishing correlative and causal data in such datasets in order to streamline data interpretation. While hepatic mechanistic insights are to date limited, bioinformatic evidence for the identification of AGCs mRNAs positive cells, protein expression, quantitative regulation, and prognostic data have been provided. Liver-pertinent clinical studies based on the US Clinical Trials database are listed. Future research directions derived from AGC targeting are proposed.

CONCLUSION

This review highlights frequent implication of AGCs in CLD, linking traits of liver disorders and the local autonomic nervous system. Such data should contribute to diversifying current parameters of patient stratification and our understanding of CLD.

Axon guidance genes modulate neurotoxicity of ALS-associated UBQLN2

Mutations in the ubiquitin (Ub) chaperone Ubiquilin 2 (UBQLN2) cause X-linked forms of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) through unknown mechanisms. Here, we show that aggregation-prone, ALS-associated mutants of UBQLN2 (UBQLN2ALS) trigger heat stress-dependent neurodegeneration in Drosophila. A genetic modifier screen implicated endolysosomal and axon guidance genes, including the netrin receptor, Unc-5, as key modulators of UBQLN2 toxicity. Reduced gene dosage of Unc-5 or its coreceptor Dcc/frazzled diminished neurodegenerative phenotypes, including motor dysfunction, neuromuscular junction defects, and shortened lifespan, in flies expressing UBQLN2ALS alleles. Induced pluripotent stem cells (iPSCs) harboring UBQLN2ALS knockin mutations exhibited lysosomal defects while inducible motor neurons (iMNs) expressing UBQLN2ALS alleles exhibited cytosolic UBQLN2 inclusions, reduced neurite complexity, and growth cone defects that were partially reversed by silencing of UNC5B and DCC. The combined findings suggest that altered growth cone dynamics are a conserved pathomechanism in UBQLN2-associated ALS/FTD.

O-GlcNAcylation Links Nutrition to the Epigenetic Downregulation of UNC5A during Colon Carcinogenesis

Simple Summary

Nutritional disorders represent major risk factors for colorectal cancer according to mechanisms of action that are still insufficiently known. The aim of our study was to investigate the putative involvement of nutrition in the epigenetic downregulation of the tumor suppressor genes of the UNC5 (Uncoordinated 5) family during colonic carcinogenesis and to understand its molecular relays. Herein, we provided evidence that the consumption of a High Carbohydrate Diet worsens colon carcinogenesis in mice and is correlated with the downregulation of several members of the UNC5 family whose UNC5A (Uncoordinated 5A). Mechanistically, we identified the nutritional sensor O-GlcNAcylation as one of the molecular relays that regulate the recruitment of the PRC2 complex onto the UNC5A promoter to repress its transcription.

Abstract

While it is now accepted that nutrition can influence the epigenetic modifications occurring in colorectal cancer (CRC), the underlying mechanisms are not fully understood. Among the tumor suppressor genes frequently epigenetically downregulated in CRC, the four related genes of the UNC5 family: UNC5A, UNC5B, UNC5C and UNC5D encode dependence receptors that regulate the apoptosis/survival balance. Herein, in a mouse model of CRC, we found that the expression of UNC5A, UNC5B and UNC5C was diminished in tumors but only in mice subjected to a High Carbohydrate Diet (HCD) thus linking nutrition to their repression in CRC. O-GlcNAcylation is a nutritional sensor which has enhanced levels in CRC and regulates many cellular processes amongst epigenetics. We then investigated the putative involvement of O-GlcNAcylation in the epigenetic downregulation of the UNC5 family members. By a combination of pharmacological inhibition and RNA interference approaches coupled to RT-qPCR (Reverse Transcription-quantitative Polymerase Chain Reaction) analyses, promoter luciferase assay and CUT&RUN (Cleavage Under Target & Release Using Nuclease) experiments, we demonstrated that the O-GlcNAcylated form of the histone methyl transferase EZH2 (Enhancer of Zeste Homolog 2) represses the transcription of UNC5A in human colon cancer cells. Collectively, our data support the hypothesis that O-GlcNAcylation could represent one link between nutrition and epigenetic downregulation of key tumor suppressor genes governing colon carcinogenesis including UNC5A.