Dual inhibition of HSF1 and DYRK2 impedes cancer progression.

Preserving proteostasis is a major survival mechanism for cancer. Dual specificity tyrosine phosphorylation-regulated kinase 2 (DYRK2) is a key oncogenic kinase that directly activates the transcription factor heat-shock factor 1 (HSF1) and the 26S proteasome. Targeting DYRK2 has proven to be a tractable strategy to target cancers sensitive to proteotoxic stress; however, the development of HSF1 inhibitors remains in its infancy. Importantly, multiple other kinases have been shown to redundantly activate HSF1 that promoted ideas to directly target HSF1. The eventual development of direct HSF1 inhibitor KRIBB11 suggests that the transcription factor is indeed a druggable target. The current study establishes that concurrent targeting of HSF1 and DYRK2 can indeed impede cancer by inducing apoptosis faster than individual targetting. Furthermore, targeting the DYRK2-HSF1 axis induces death in proteasome inhibitor-resistant cells and reduces triple-negative breast cancer (TNBC) burden in ectopic and orthotopic xenograft models. Together the data indicate that cotargeting of kinase DYRK2 and its substrate HSF1 could prove to be a beneficial strategy in perturbing neoplastic malignancies.

Hexosamine pathway activation improves memory but does not extend lifespan in mice.

Glucosamine feeding and genetic activation of the hexosamine biosynthetic pathway (HBP) have been linked to improved protein quality control and lifespan extension. However, as an energy sensor, the HBP has been implicated in tumor progression and diabetes. Given these opposing outcomes, it is imperative to explore the long-term effects of chronic HBP activation in mammals. Thus, we asked if HBP activation affects metabolism, coordination, memory, and survival in mice. N-acetyl-D-glucosamine (GlcNAc) supplementation in the drinking water had no adverse effect on weight in males but increased weight in young females. Glucose or insulin tolerance was not affected up to 20 months of age. Of note, we observed improved memory in young male mice supplemented with GlcNAc. Survival was not changed by GlcNAc treatment. To assess the effects of genetic HBP activation, we overexpressed the pathway's key enzyme GFAT1 and a constitutively activated mutant form in all mouse tissues. We detected elevated levels of the HBP product UDP-GlcNAc in mouse brains, but did not find any effects on behavior, memory, or survival. Together, while dietary GlcNAc supplementation did not extend survival in mice, it positively affected memory and is generally well tolerated.

GFPT2/GFAT2 and AMDHD2 act in tandem to control the hexosamine pathway.

The hexosamine biosynthetic pathway (HBP) produces the essential metabolite UDP-GlcNAc and plays a key role in metabolism, health, and aging. The HBP is controlled by its rate-limiting enzyme glutamine fructose-6-phosphate amidotransferase (GFPT/GFAT) that is directly inhibited by UDP-GlcNAc in a feedback loop. HBP regulation by GFPT is well studied but other HBP regulators have remained obscure. Elevated UDP-GlcNAc levels counteract the glycosylation toxin tunicamycin (TM), and thus we screened for TM resistance in haploid mouse embryonic stem cells (mESCs) using random chemical mutagenesis to determine alternative HBP regulation. We identified the N-acetylglucosamine deacetylase AMDHD2 that catalyzes a reverse reaction in the HBP and its loss strongly elevated UDP-GlcNAc. To better understand AMDHD2, we solved the crystal structure and found that loss-of-function (LOF) is caused by protein destabilization or interference with its catalytic activity. Finally, we show that mESCs express AMDHD2 together with GFPT2 instead of the more common paralog GFPT1. Compared with GFPT1, GFPT2 had a much lower sensitivity to UDP-GlcNAc inhibition, explaining how AMDHD2 LOF resulted in HBP activation. This HBP configuration in which AMDHD2 serves to balance GFPT2 activity was also observed in other mESCs and, consistently, the GFPT2:GFPT1 ratio decreased with differentiation of human embryonic stem cells. Taken together, our data reveal a critical function of AMDHD2 in limiting UDP-GlcNAc production in cells that use GFPT2 for metabolite entry into the HBP.

The RATIOnal Role of Polyamines in Epidermal Differentiation.

Polyamines have been implicated in skin tumorigenesis; however, their role in epidermal homeostasis remains obscure. In a new article in the Journal of Investigative Dermatology, Rahim et al. (2021) report that keratinocyte differentiation requires a shift in polyamine ratios that is mediated by AMD1. Results suggest that targeting polyamine availability might be useful in the treatment of hyperproliferative skin disorders.

N1-acetylspermidine is a determinant of hair follicle stem cell fate.

Stem cell differentiation is accompanied by increased mRNA translation. The rate of protein biosynthesis is influenced by the polyamines putrescine, spermidine and spermine, which are essential for cell growth and stem cell maintenance. However, the role of polyamines as endogenous effectors of stem cell fate and whether they act through translational control remains obscure. Here, we investigate the function of polyamines in stem cell fate decisions using hair follicle stem cell (HFSC) organoids. Compared to progenitor cells, HFSCs showed lower translation rates, correlating with reduced polyamine levels. Surprisingly, overall polyamine depletion decreased translation but did not affect cell fate. In contrast, specific depletion of natural polyamines mediated by spermidine/spermine N1-acetyltransferase (SSAT; also known as SAT1) activation did not reduce translation but enhanced stemness. These results suggest a translation-independent role of polyamines in cell fate regulation. Indeed, we identified N1-acetylspermidine as a determinant of cell fate that acted through increasing self-renewal, and observed elevated N1-acetylspermidine levels upon depilation-mediated HFSC proliferation and differentiation in vivo. Overall, this study delineates the diverse routes of polyamine metabolism-mediated regulation of stem cell fate decisions. This article has an associated First Person interview with the first author of the paper.

Glutamine Metabolism Controls Stem Cell Fate Reversibility and Long-Term Maintenance in the Hair Follicle.

Stem cells reside in specialized niches that are critical for their function. Upon activation, hair follicle stem cells (HFSCs) exit their niche to generate the outer root sheath (ORS), but a subset of ORS progeny returns to the niche to resume an SC state. Mechanisms of this fate reversibility are unclear. We show that the ability of ORS cells to return to the SC state requires suppression of a metabolic switch from glycolysis to oxidative phosphorylation and glutamine metabolism that occurs during early HFSC lineage progression. HFSC fate reversibility and glutamine metabolism are regulated by the mammalian target of rapamycin complex 2 (mTORC2)-Akt signaling axis within the niche. Deletion of mTORC2 results in a failure to re-establish the HFSC niche, defective hair follicle regeneration, and compromised long-term maintenance of HFSCs. These findings highlight the importance of spatiotemporal control of SC metabolic states in organ homeostasis.

Hexosamine Pathway Activation Improves Protein Homeostasis through the Integrated Stress Response.

Activation of the hexosamine pathway (HP) through gain-of-function mutations in its rate-limiting enzyme glutamine fructose-6-phosphate amidotransferase (GFAT-1) ameliorates proteotoxicity and increases lifespan in Caenorhabditis elegans. Here, we investigate the role of the HP in mammalian protein quality control. In mouse neuronal cells, elevation of HP activity led to phosphorylation of both PERK and eIF2α as well as downstream ATF4 activation, identifying the HP as a modulator of the integrated stress response (ISR). Increasing uridine 5'-diphospho-N-acetyl-D-glucosamine (UDP-GlcNAc) levels through GFAT1 gain-of-function mutations or supplementation with the precursor GlcNAc reduces aggregation of the polyglutamine (polyQ) protein Ataxin-3. Blocking PERK signaling or autophagy suppresses this effect. In C. elegans, overexpression of gfat-1 likewise activates the ISR. Consistently, co-overexpression of gfat-1 and proteotoxic polyQ peptides in muscles reveals a strong protective cell-autonomous role of the HP. Thus, the HP has a conserved role in improving protein quality control through modulation of the ISR.

Loss of GFAT-1 feedback regulation activates the hexosamine pathway that modulates protein homeostasis.

Glutamine fructose-6-phosphate amidotransferase (GFAT) is the key enzyme in the hexosamine pathway (HP) that produces uridine 5'-diphospho-N-acetyl-D-glucosamine (UDP-GlcNAc), linking energy metabolism with posttranslational protein glycosylation. In Caenorhabditis elegans, we previously identified gfat-1 gain-of-function mutations that elevate UDP-GlcNAc levels, improve protein homeostasis, and extend lifespan. GFAT is highly conserved, but the gain-of-function mechanism and its relevance in mammalian cells remained unclear. Here, we present the full-length crystal structure of human GFAT-1 in complex with various ligands and with important mutations. UDP-GlcNAc directly interacts with GFAT-1, inhibiting catalytic activity. The longevity-associated G451E variant shows drastically reduced sensitivity to UDP-GlcNAc inhibition in enzyme activity assays. Our structural and functional data point to a critical role of the interdomain linker in UDP-GlcNAc inhibition. In mammalian cells, the G451E variant potently activates the HP. Therefore, GFAT-1 gain-of-function through loss of feedback inhibition constitutes a potential target for the treatment of age-related proteinopathies.

Unbiased compound-protein interface mapping and prediction of chemoresistance loci through forward genetics in haploid stem cells.

Forward genetic screens in haploid mammalian cells have recently emerged as powerful tools for the discovery and investigation of recessive traits. Use of the haploid system provides unique genetic tractability and resolution. Upon positive selection, these screens typically employ analysis of loss-of-function (LOF) alleles and are thus limited to non-essential genes. Many relevant compounds, including anti-cancer therapeutics, however, target essential genes, precluding positive selection of LOF alleles. Here, we asked whether the use of random and saturating chemical mutagenesis might enable screens that identify essential biological targets of toxic compounds. We compare and contrast chemical mutagenesis with insertional mutagenesis. Selecting mutagenized cells with thapsigargin, an inhibitor of the essential Ca2+ pump SERCA2, insertional mutagenesis retrieved cell clones overexpressing SERCA2. With chemical mutagenesis, we identify six single amino acid substitutions in the known SERCA2-thapsigargin binding interface that confer drug resistance. In a second screen, we used the anti-cancer drug MG132/bortezomib (Velcade), which inhibits proteasome activity. Using chemical mutagenesis, we found 7 point mutations in the essential subunit Psmb5 that map to the bortezomib binding surface. Importantly, 4 of these had previously been identified in human tumors with acquired bortezomib resistance. Insertional mutagenesis did not identify Psmb5 in this screen, demonstrating the unique ability of chemical mutagenesis to identify relevant point mutations in essential genes. Thus, chemical mutagenesis in haploid embryonic stem cells can define the interaction of toxic small molecules with essential proteins at amino acid resolution, fully mapping small molecule-protein binding interfaces.

Motor neuron intrinsic and extrinsic mechanisms contribute to the pathogenesis of FUS-associated amyotrophic lateral sclerosis.

Motor neuron-extrinsic mechanisms have been shown to participate in the pathogenesis of ALS-SOD1, one familial form of amyotrophic lateral sclerosis (ALS). It remains unclear whether such mechanisms contribute to other familial forms, such as TDP-43 and FUS-associated ALS. Here, we characterize a single-copy mouse model of ALS-FUS that conditionally expresses a disease-relevant truncating FUS mutant from the endogenous murine Fus gene. We show that these mice, but not mice heterozygous for a Fus null allele, develop similar pathology as ALS-FUS patients and a mild motor neuron phenotype. Most importantly, CRE-mediated rescue of the Fus mutation within motor neurons prevented degeneration of motor neuron cell bodies, but only delayed appearance of motor symptoms. Indeed, we observed downregulation of multiple myelin-related genes, and increased numbers of oligodendrocytes in the spinal cord supporting their contribution to behavioral deficits. In all, we show that mutant FUS triggers toxic events in both motor neurons and neighboring cells to elicit motor neuron disease.