TIG1 Inhibits the mTOR Signaling Pathway in Malignant Melanoma Through the VAC14 Protein

BACKGROUND/AIM

Currently, there are few drug options available to treat malignant melanoma. Tazarotene-inducible gene 1 (TIG1) was originally isolated from skin tissue, but its function in skin tissue has not been clarified. The aim of this study was to elucidate the effect of TIG1 and mTOR signaling pathways associated with VAC14 on melanoma.

MATERIALS AND METHODS

The expression of TIG1 and VAC14 in melanoma tissue was analyzed using a melanoma tissue cDNA array. The interaction between TIG1 and VAC14 was analyzed using immunoprecipitation and immunostaining. Western blot was used to investigate the molecular targets of TIG1 and VAC14 in melanoma cells.

RESULTS

TIG1 was highly expressed in normal skin tissue but was low in malignant melanoma, while VAC14 showed the opposite trend. TIG1 inhibited insulin-induced cell proliferation and insulin-activated mammalian target of rapamycin complex 1 (mTORC1)-p70 S6 kinase but did not affect the level of phospho-AKT in A2058 melanoma cells. This suggests that the main target of TIG1 regulating cell growth is phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2] rather than the PI(4,5)P2 signaling pathway. Additional TIG1 showed no additive effect on the inhibition of mTOR signaling in the absence of VAC14 expression, suggesting that TIG1 inhibited the activation of mTOR mainly by inhibiting VAC14.

CONCLUSION

TIG1 may play an important role in preventing malignant melanoma through retinoic acid via VAC14.

The vacuolar morphology protein VAC14 plays an important role in sexual development in the filamentous ascomycete Sordaria macrospora

The multiprotein Fab1p/PIKfyve-complex regulating the abundance of the phospholipid phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P₂) is highly conserved among eukaryotes. In yeast/mammals, it is composed of the phosphatidylinositol 3-phosphate 5-kinase Fab1p/PIKfyve, the PtdIns(3,5)P₂ phosphatase Fig4p/Sac3 and the scaffolding subunit Vac14p/ArPIKfyve. The complex is located to vacuolar membranes in yeast and to endosomal membranes in mammals, where it controls the synthesis and turnover of PtdIns(3,5)P₂. In this study, we analyzed the role and function of the Fab1p/PIKfyve-complex scaffold protein SmVAC14 in the filamentous ascomycete Sordaria macrospora (Sm). We generated the Smvac14 deletion strain ∆vac14 and performed phenotypic analysis of the mutant. Furthermore, we conducted fluorescence microscopic localization studies of fluorescently labeled SmVAC14 with vacuolar and late endosomal marker proteins. Our results revealed that SmVAC14 is important for maintaining vacuolar size and appearance as well as proper sexual development in S. macrospora. In addition, SmVAC14 plays an important role in starvation stress response. Accordingly, our results propose that the turnover of PtdIns(3,5)P₂ is of great significance for developmental processes in filamentous fungi.

Altered homodimer formation and increased iron accumulation in VAC14-related disease: Case report and review of the literature

BACKGROUND

Pathogenic variants in the VAC14 component of PIKFYVE complex (VAC14) gene have been identified as a cause of a childhood-onset complex dystonia with striato-nigral degeneration. VAC14 is a scaffold protein relevant for the regulation of phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂) and is known to form homodimers.

METHODS

Whole exome sequencing was performed in a 32-year-old patient with adolescence-onset complex dystonia and his unaffected mother. We established primary fibroblast cultures from the patient and used stably transfected SH-SY5Y cells overexpressing wildtype or mutant VAC14 to investigate the influence of VAC14 variants on the homodimer formation. Furthermore, the current literature on VAC14-related disorders was reviewed.

RESULTS

Our patient presented with progressive, complex dystonia with anarthria, dysphagia, sensorineural deafness, spasticity and nigral and pallidal iron deposition and striatal hyperintensities upon MRI. We identified two rare compound-heterozygous VAC14 variants (p.Leu648Phe and p.Arg623His), both located at the C-terminus in the predicted homodimerization domain. Enhanced VAC14 homodimer formation was observed for two missense variants (p.Leu648Phe and p.Ala562Val, a published mutation), but not for p.Arg623His, compared to wildtype VAC14. In contrast to previous reports, no enlarged vacuoles were detected in fibroblasts of our patient.

CONCLUSIONS

We report a novel patient with a VAC14-related disorder and provide first evidence of an enhanced VAC14 homodimerization as a possible disease mechanism. Due to the increased iron deposition and the clinical overlap, this disorder should be discussed as a new form of neurodegeneration with brain iron accumulation (NBIA). We suggest that VAC14 should be implemented in NBIA gene panels.

Novel VAC14 variants identified in two Chinese siblings with childhood-onset striatonigral degeneration

Background

VAC14 is a component of a trimolecular complex that tightly regulates the level of phosphatidylinositol 3,5‐bisphosphate [PI (3,5) P2]. VAC14 pathogenic variants cause prominent vacuolation of neurons in basal ganglia of patients with childhood‐onset striatonigral degeneration (SNDC).

Methods

We identified two siblings with SNDC. Whole‐exome sequencing was performed for genetic molecular analysis in these probands.

Results

The patients were compound heterozygotes for two novel variants in the VAC14 gene, p.Ala582Thr and p.Arg681His. The pathogenicity of these variants was indicated by a bioinformatic study and protein three‐dimensional modeling. Eight previously reported SNDC cases and a Yunis–Varón syndrome caused by VAC14 mutations were summarized and compared.

Conclusion

We present novel compound heterozygous variants (c.1744G>A/c.2042G>A) in our proband, and these novel variants were predicted to be likely pathogenic. The affected siblings were clinically severe and lethal; their phenotypes were similar to the majority of previously reported SNDC cases, with the exception of two cases that showed mild clinical manifestations. VAC14 pathogenic variants may be associated with various phenotypes. Herein, we report the Chinese siblings with SNDC, they are the first Asian cases. Our results expanded the spectrum of VAC14 pathogenic variants and the ethnic backgrounds of the affected cases.

Genetic Overlap Between Alzheimer's Disease and Bipolar Disorder Implicates the MARK2 and VAC14 Genes

Background: Alzheimer's disease (AD) and bipolar disorder (BIP) are complex traits influenced by numerous common genetic variants, most of which remain to be detected. Clinical and epidemiological evidence suggest that AD and BIP are related. However, it is not established if this relation is of genetic origin. Here, we applied statistical methods based on the conditional false discovery rate (FDR) framework to detect genetic overlap between AD and BIP and utilized this overlap to increase the power to identify common genetic variants associated with either or both traits. Methods: We obtained genome wide association studies data from the International Genomics of Alzheimer's Project part 1 (17,008 AD cases and 37,154 controls) and the Psychiatric Genetic Consortium Bipolar Disorder Working Group (20,352 BIP cases and 31,358 controls). We used conditional QQ-plots to assess overlap in common genetic variants between AD and BIP. We exploited the genetic overlap to re-rank test-statistics for AD and BIP and improve detection of genetic variants using the conditional FDR framework. Results: Conditional QQ-plots demonstrated a polygenic overlap between AD and BIP. Using conditional FDR, we identified one novel genomic locus associated with AD, and nine novel loci associated with BIP. Further, we identified two novel loci jointly associated with AD and BIP implicating the MARK2 gene (lead SNP rs10792421, conjunctional FDR = 0.030, same direction of effect) and the VAC14 gene (lead SNP rs11649476, conjunctional FDR = 0.022, opposite direction of effect). Conclusion: We found polygenic overlap between AD and BIP and identified novel loci for each trait and two jointly associated loci. Further studies should examine if the shared loci implicating the MARK2 and VAC14 genes could explain parts of the shared and distinct features of AD and BIP.

Cerebral hypomyelination associated with biallelic variants of FIG4

The lipid phosphatase gene FIG4 is responsible for Yunis-Varón syndrome and Charcot-Marie-Tooth disease Type 4J, a peripheral neuropathy. We now describe four families with FIG4 variants and prominent abnormalities of central nervous system (CNS) white matter (leukoencephalopathy), with onset in early childhood, ranging from severe hypomyelination to mild undermyelination, in addition to peripheral neuropathy. Affected individuals inherited biallelic FIG4 variants from heterozygous parents. Cultured fibroblasts exhibit enlarged vacuoles characteristic of FIG4 dysfunction. Two unrelated families segregate the same G > A variant in the +1 position of intron 21 in the homozygous state in one family and compound heterozygous in the other. This mutation in the splice donor site of exon 21 results in read-through from exon 20 into intron 20 and truncation of the final 115 C-terminal amino acids of FIG4, with retention of partial function. The observed CNS white matter disorder in these families is consistent with the myelination defects in the FIG4 null mouse and the known role of FIG4 in oligodendrocyte maturation. The families described here the expanded clinical spectrum of FIG4 deficiency to include leukoencephalopathy.

The PIKfyve complex regulates the early melanosome homeostasis required for physiological amyloid formation

The metabolism of PI(3,5)P2 is regulated by the PIKfyve, VAC14 and FIG4 complex, mutations in which are associated with hypopigmentation in mice. These pigmentation defects indicate a key, but as yet unexplored, physiological relevance of this complex in the biogenesis of melanosomes. Here, we show that PIKfyve activity regulates formation of amyloid matrix composed of PMEL protein within the early endosomes in melanocytes, called stage I melanosomes. PIKfyve activity controls the membrane remodeling of stage I melanosomes, which regulates PMEL abundance, sorting and processing. PIKfyve activity also affects stage I melanosome kiss-and-run interactions with lysosomes, which are required for PMEL amyloidogenesis and the establishment of melanosome identity. Mechanistically, PIKfyve activity promotes both the formation of membrane tubules from stage I melanosomes and their release by modulating endosomal actin branching. Taken together, our data indicate that PIKfyve activity is a key regulator of the melanosomal import-export machinery that fine tunes the formation of functional amyloid fibrils in melanosomes and the maintenance of melanosome identity.This article has an associated First Person interview with the first author of the paper.

A cell-permeable tool for analysing APP intracellular domain function and manipulation of PIKfyve activity

The mechanisms for regulating PIKfyve complex activity are currently emerging. The PIKfyve complex, consisting of the phosphoinositide kinase PIKfyve (also known as FAB1), VAC14 and FIG4, is required for the production of phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2]. PIKfyve function is required for homoeostasis of the endo/lysosomal system and is crucially implicated in neuronal function and integrity, as loss of function mutations in the PIKfyve complex lead to neurodegeneration in mouse models and human patients. Our recent work has shown that the intracellular domain of the amyloid precursor protein (APP), a molecule central to the aetiology of Alzheimer's disease binds to VAC14 and enhances PIKfyve function. In the present study, we utilize this recent advance to create an easy-to-use tool for increasing PIKfyve activity in cells. We fused APP intracellular domain (AICD) to the HIV TAT domain, a cell-permeable peptide allowing proteins to penetrate cells. The resultant TAT-AICD fusion protein is cell permeable and triggers an increase in PI(3,5)P2 Using the PI(3,5)P2 specific GFP-ML1Nx2 probe, we show that cell-permeable AICD alters PI(3,5)P2 dynamics. TAT-AICD also provides partial protection from pharmacological inhibition of PIKfyve. All three lines of evidence show that the AICD activates the PIKfyve complex in cells, a finding that is important for our understanding of the mechanism of neurodegeneration in Alzheimer's disease.

Mouse models of PI(3,5)P2 deficiency with impaired lysosome function

The endolysosomal system and autophagy are essential components of macromolecular turnover in eukaryotic cells. The low-abundance signaling lipid PI(3,5)P2 is a key regulator of this pathway. Analysis of mouse models with defects in PI(3,5)P2 biosynthesis has revealed the unique dependence of the mammalian nervous system on this signaling pathway. This insight led to the discovery of the molecular basis for several human neurological disorders, including Charcot-Marie-Tooth disease and Yunis-Varon syndrome. Spontaneous mutants, conditional knockouts, transgenic lines, and gene-trap alleles of Fig4, Vac14, and Pikfyve (Fab1) in the mouse have provided novel information regarding the role of PI(3,5)P2in vivo. This review summarizes what has been learned from mouse models and highlights the utility of manipulating complex signaling pathways in vivo.