InteractiveFly: GeneBrief

Mitf: Biological Overview | References

Gene name - Mitf

Synonyms -

Cytological map position - 102F8-102F8

Function - bHLH transcription factor

Keywords - eye development midgut - controls transcription of all 15 v-ATPase components - modulator of metabolism for cellular homeostasis - Mitf, vacuolar-ATPase and TORC1 form a negative regulatory loop that maintains each of these important metabolic regulators in relative balance - control of lysosomal-autophagy pathway

Symbol - Mitf

FlyBase ID: FBgn0263112

Genetic map position - chr4:1,198,852-1,224,467

NCBI classification - Helix-loop-helix domain, MITF/TFEB/TFEC/TFE3 N-terminus

Cellular location - Nuclear

NCBI links: EntrezGene
Mitf orthologs: Biolitmine
Recent literature
Wang, Y., Huang, Y., Liu, J., Zhang, J., Xu, M., You, Z., Peng, C., Gong, Z. and Liu, W. (2019). Acetyltransferase GCN5 regulates autophagy and lysosome biogenesis by targeting TFEB. EMBO Rep: e48335. PubMed ID: 31750630
Accumulating evidence highlights the role of histone acetyltransferase GCN5 in the regulation of cell metabolism in metazoans. This study reports that GCN5 is a negative regulator of autophagy, a lysosome-dependent catabolic mechanism. In animal cells and Drosophila, GCN5 inhibits the biogenesis of autophagosomes and lysosomes by targeting TFEB, the master transcription factor for autophagy- and lysosome-related gene expression. GCN5 is a specific TFEB acetyltransferase, and acetylation by GCN5 results in the decrease in TFEB transcriptional activity. Induction of autophagy inactivates GCN5, accompanied by reduced TFEB acetylation and increased lysosome formation. It was further demonstrated that acetylation at K274 and K279 disrupts the dimerization of TFEB and the binding of TFEB to its target gene promoters. In a Tau-based neurodegenerative Drosophila model, deletion of dGcn5 improves the clearance of Tau protein aggregates and ameliorates the neurodegenerative phenotypes. Together, these results reveal GCN5 as a novel conserved TFEB regulator, and the regulatory mechanisms may be involved in autophagy- and lysosome-related physiological and pathological processes.
Cunningham, K. M., Maulding, K., Ruan, K., Senturk, M., Grima, J. C., Sung, H., Zuo, Z., Song, H., Gao, J., Dubey, S., Rothstein, J. D., Zhang, K., Bellen, H. J. and Lloyd, T. E. (2020). TFEB/Mitf links impaired nuclear import to autophagolysosomal dysfunction in C9-ALS. Elife 9. PubMed ID: 33300868
Disrupted nucleocytoplasmic transport (NCT) has been implicated in neurodegenerative disease pathogenesis; however, the mechanisms by which disrupted NCT causes neurodegeneration remain unclear. A Drosophila screen identified ref(2)P/p62, a key regulator of autophagy, as a potent suppressor of neurodegeneration caused by the GGGGCC hexanucleotide repeat expansion (G4C2 HRE) in C9orf72 that causes amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). p62 is increased and forms ubiquitinated aggregates due to decreased autophagic cargo degradation. Immunofluorescence and electron microscopy of Drosophila tissues demonstrate an accumulation of lysosome-like organelles that precedes neurodegeneration. These phenotypes are partially caused by cytoplasmic mislocalization of Mitf/TFEB, a key transcriptional regulator of autophagolysosomal function. Additionally, TFEB is mislocalized and downregulated in human cells expressing GGGGCC repeats and in C9-ALS patient motor cortex. These data suggest that the C9orf72-HRE impairs Mitf/TFEB nuclear import, thereby disrupting autophagy and exacerbating proteostasis defects in C9-ALS/FTD.

The v-ATPase is a fundamental eukaryotic enzyme that is central to cellular homeostasis. Although its impact on key metabolic regulators such as TORC1 is well documented, knowledge of mechanisms that regulate v-ATPase activity is limited. This study reports that the Drosophila transcription factor Mitf is a master regulator of the v-ATPase holoenzyme. Mitf directly controls transcription of all 15 v-ATPase components through M-box cis-sites and this coordinated regulation affects holoenzyme activity in vivo. In addition, through the v-ATPase, Mitf promotes the activity of TORC1, which in turn negatively regulates Mitf. Evidence is provided that Mitf, v-ATPase and TORC1 form a negative regulatory loop that maintains each of these important metabolic regulators in relative balance. Interestingly, direct regulation of v-ATPase genes by human MITF also occurs in cells of the melanocytic lineage, showing mechanistic conservation in the regulation of the v-ATPase by MITF family proteins in fly and mammals. Collectively, this evidence points to an ancient module comprising Mitf, v-ATPase and TORC1 that serves as a dynamic modulator of metabolism for cellular homeostasis (Zhang, 2015).

The vacuolar (H+)-ATPase (v-ATPase) is an evolutionary-conserved holoenzyme that controls basic cellular processes in eukaryotic cells. As an ATP-dependent proton pump, it acidifies intracellular or extracellular compartments and generates electrochemical gradients, with profound consequences on lysosomal degradation, the transport of metabolites across gut epithelia and many other cellular processes. In lysosomal metabolism, the v-ATPase is a dual player; its proton pumping ability establishes the low pH required by degradative enzymes, whereas its ATPase activity is essential for the amino acid-dependent activation of TORC1 (the Target Of Rapamycin Complex 1 kinase that links lysosomal degradation to the nutritional state of the cell). Interestingly, in mammalian cell lines, both negative (Settembre, 2011) and positive (Pena-Llopis, 2011) correlation of TORC1 activity with v-ATPase gene expression (ATP6 genes) has been reported. Thus, the effect of TORC1 on the v-ATPase is unclear. However, in both cases, members of the MiT/MITF-family of transcription factors were implicated as mediators of positive or negative regulation by TORC1 (Zhang, 2015).

The four MiT-family genes of mammals, MITF, TFEB, TFE3 and TFEC, encode bHLH-Zip transcription factors that control basic cellular processes in eukaryotes as well as tissue identity and differentiation in animal development. Recent studies in mammalian cell lines have implicated MITF, TFEB and TFE3 in the regulation of degradation pathways. Expression profiling showed induction of lysosomal and autophagy genes by these factors, with most of the targets containing the CLEAR element, a binding sites for TFEB. Interestingly, the nuclear versus cytoplasmic localization of MITF, TFEB, and TFE3 is controlled by the TORC1 kinase through phosphorylation. The mTOR-associated Rag GTPases can interact at an N-terminal motif present in all three MiT factors and promote localization at the lysosome, where phosphorylation of the transcription factors by TORC1 then leads to their cytoplasmic sequestration by the 14-3-3 anchor protein . Alternatively, phosphorylation of TFEB by active TORC1 at a C-terminal serine-rich motif has been proposed to promote its nuclear translocation and activation. Different cell culture conditions and the complication of dealing with multiple MiT family members may have contributed to this discrepancy. Nonetheless, the nature of this TORC1 regulation needs further study (Zhang, 2015).

The invertebrate model organism D. melanogaster offers two advantages. First, it provides a sophisticated genetic model to address questions in vivo, and second, it has a single MiT-family factor. The gene Mitf (CG43369) is expressed broadly at a low level throughout the Drosophila life cycle, but is particularly enriched in the digestive system (Hallsson, 2004). Its physiological roles are unknown, due to a lack of loss-of-function analyses. This study identifies Mitf as a master regulator of the major cellular v-ATPase through transcriptional control of all 15 subunits of the holoenzyme. Modulation of gene expression is direct and impacts holoenzyme activity, with profound consequences on all three metabolic regulators. Mitf, the v-ATPase and TORC1 form a regulatory module that maintains the three factors in dynamic balance and may provide an adaptive feature to its regulation of metabolism. Interestingly, these Mitf functions appear to be conserved in human cells, pointing to an ancient MiT/v-ATPase/TORC1 module for cellular homeostasis (Zhang, 2015).

The role of the v-ATPase as a fundamental regulator of metabolism is well documented and is underscored by its requirement in all eukaryotic cells (Marshansky, 2014). Understanding its regulation and how this ties to major metabolic pathways is critical to decoding the complex mechanisms of cellular homeostasis. This study shows that Drosophila Mitf plays a major role in regulating v-ATPase activity. Regulation is at the transcriptional level and direct, through cis sites generally located just upstream of the promoter or in a large early intron of each subunit-encoding Vha locus. Strikingly, fifteen Vha genes appear to be organized into an Mitf-regulated synexpression group that ensures co-production of all components of the major vATPase. Through this mechanism, Mitf functions as a master regulator of the holoenzyme in the digestive system and other fly tissues (Zhang, 2015).

Concerted expression of v-ATPase loci has also been observed in vertebrates but the genetic and molecular mechanisms mediating this synexpression are largely unknown. Whereas the fly has a single Mitf gene, the situation in mammals is more complex due to the presence of TFEB and TFE3 as well. Nonetheless, in human melanoma cells and most likely in melanocytes, MITF appears to directly regulate a set of ATP6 genes for all main v-ATPase subunits. Fifteen ATP6 genes (encoding the 13 holoenzyme subunits and 1 accessory protein) are bound in both melanoma cells and primary melanocytes; among these, all show expression correlation with MITF in cell lines and most are downregulated in response to the partial silencing of MITF in cell culture. These 15 ATP6 genes are considered to be the most likely targets of direct regulation by MITF in melanoma and melanocytes (Zhang, 2015).

The remaining 10 loci show variable effects (with only one bound in melanocytes). These may include genes that are targets of other MiT factors in other tissues and can respond to MITF when it is overexpressed (as is often the case in melanoma tumors). In fact, it is likely that TFEB and TFE3 play a similar role as MITF in controlling most of the ATP6 loci in Hela cells and ARPE-19 cells, respectively. Further analyses of these TFE factors and ATP6 genes in these cell lines will show if this is the case (Zhang, 2015).

In Drosophila and other insects, the v-ATPase works at the plasma membrane of cells lining gut and Malpighian tubules to regulate pH, energize ion transport and modulate fluid secretion (Wieczorek, 2009). In the developing epithelia of eye, wing and in the ovary, pH modulates the activity of internalized receptors such as Notch; hence, v-ATPase activity has repercussions on signaling. In wing discs, mutations in VhaM8.9 can cause planar cell polarity defects, in addition to disrupting endosomal trafficking (Hermle, 2013). Whereas most of these functions would likely be affected in Mitf mutants,some v-ATPase subunits also fulfill specialized roles (Hiesinger, 2005). In the latter case, the influence of Mitf would depend on whether the specific subunit is under Mitf control and, if not, on whether the holoenzyme is the critical agent. Ultimately, many of these functions are essential for life and thus explain the lethality of Mitf mutant alleles (Zhang, 2015).

In mammals, the v-ATPase plays essential roles in a broad range of processes that are regulated by one or other MiT family member. The v-ATPase is essential for the proper function of melanosomes and many melanosome genes, in addition to ATP6 genes, are under MITF control. The v-ATPase also contributes to bone remodeling in osteoclasts, a cell type that expresses, and depends on MITF, TFEB and TFE3 for normal function. Interestingly, double knock-out of the Tfe3 and Mitf genes leads to osteopetrosis. The v-ATPase has also been found at the plasma membrane of cancer cells, from where it promotes alkalization of the cytoplasm and acidification of the tumour micro-environment, and this activity was recently linked to the emergence of distant metastases in melanoma. Further studies will elucidate the exact relationship between MiT factors and the v-ATpase in these contexts (Zhang, 2015).

Importantly, in both vertebrates and Drosophila, the v-ATPase mediates the activation of TORC1 at the lysosomal membrane in response to amino acids (Zoncu, 2011), thereby downregulating lysosomal metabolism. Hence, the v-ATPase can have a negative influence on the lysosome even though it promotes lysosomal function through acidification. In Drosophila, exogenous Mitf leads to increased TORC1 activity and promotes sequestration of Mitf back to the cytoplasm, whereas decreased v-ATPase gene dosage results in more nuclear Mitf as well as lower TORC1 function. In 501mel cell, exogenous MITF can also increase TORC1 activity. Although the predominant isoform of MITF in melanocytes does not have the Rag-binding sites, other isoforms do, as do also TFEB and TFE3. Hence, the regulatory loop is most likely conserved in mammals and functions in many cell types (Zhang, 2015).

Most importantly, Mitf does not merely execute a pro-lysosomal program when freed from TORC1-induced sequestration. Through the v-ATPase, Mitf feeds back onto TORC1 to promote and limit the activity of these important metabolic regulators. The Mitf/v-ATPase/TORC1 regulatory loop adjusts the activity of all three players offering a mechanism for continuously balancing metabolic pathways as the nutritional state of the cell fluctuates. In addition, it may confer an adaptive feature to the module. In this model, the level of v-ATPase, present at the lysosome, would sensitize or desensitize the nutritional sensing mechanism to changes in aa levels, thereby priming the system to reset at a new normal through TORC1 reactivation or inactivation. Such mechanism would impose a limit on upregulation of catabolism under lower nutrient conditions, and an upper limit on active TORC1 and its promotion of anabolic pathways when nutrients are abundant. Interestingly, cell culture experiments show that prolonged starvation reactivates TORC1; here, the loop offers a potential molecular mechanism for this effect. Evolutionarily, the Mitf/v-ATPase/TORC1 regulatory module may have conferred a selective advantage by fine-tuning the nutrient sensing mechanism to maintain metabolism within an optimal range in an ever-changing environment; an advantage particularly important for unicellular organisms or for cell types that require more precise metabolic regulation in multicellular ones (Zhang, 2015).

Drosophila provides an excellent metazoan model to investigate the molecular mechanisms for co-regulation of v-ATPase subunits as well as Mitf's contribution to the maintenance of cellular homeostasis. It will be also important to investigate how different members of the MiT family participate in these processes in different cell types under different physiological conditions and what impact they have on cellular homeostasis in health and disease (Zhang, 2015).

Drosophila Mitf regulates the V-ATPase and the lysosomal-autophagic pathway

An evolutionary conserved gene network regulates the expression of genes involved in lysosome biogenesis, autophagy and lipid metabolism. This study reports that the lysosomal-autophagy pathway is controlled by Mitf gene in Drosophila. Mitf regulates the expression of genes encoding V-ATPase subunits as well as many additional genes involved in the lysosomal-autophagy pathway. Reduction of Mitf function leads to abnormal lysosomes and impairs autophagosome fusion and lipid breakdown during the response to starvation. In contrast, elevated Mitf levels increase the number of lysosomes, autophagosomes and autolysosomes, and decrease the size of lipid droplets. Inhibition of Drosophila MTORC1 induces Mitf translocation to the nucleus, underscoring conserved regulatory mechanisms between Drosophila and mammalian systems. Furthermore, Mitf-mediated clearance of cytosolic and nuclear expanded ATXN1 (ataxin 1) was demonstrated in a cellular model of spinocerebellar ataxia type 1 (SCA1). This remarkable observation illustrates the potential of the lysosomal-autophagy system to prevent toxic protein aggregation in both the cytoplasmic and nuclear compartments. It is anticipated that the genetics of the Drosophila model and the absence of redundant MIT transcription factors will be exploited to investigate the regulation and function of the lysosomal-autophagy gene network (Bouche, 2016).

Control of lysosomal biogenesis and Notch-dependent tissue patterning by components of the TFEB-V-ATPase axis in Drosophila melanogaster

In vertebrates, TFEB (transcription factor EB) and MITF (microphthalmia-associated transcription factor) family of basic Helix-Loop-Helix (bHLH) transcription factors regulate both lysosomal function and organ development. However, it is not clear whether these 2 processes are interconnected. This study shows that Mitf, the single TFEB and MITF ortholog in Drosophila, controls expression of vacuolar-type H+-ATPase pump (V-ATPase) subunits. Remarkably, it was also found that expression of Vha16-1 and Vha13, encoding 2 key components of V-ATPase, is patterned in the wing imaginal disc. In particular, Vha16-1 expression follows differentiation of proneural regions of the disc. These regions, that will form sensory organs in the adult, appear to possess a distinctive endo-lysosomal compartment and Notch (N) localization. Modulation of Mitf activity in the disc in vivo alters endo-lysosomal function and disrupts proneural patterning. Similar to these findings in Drosophila, in human breast epithelial cells, it was observed that the impairment of the Vha16-1 human ortholog ATP6V0C changes the size and function of the endo-lysosomal compartment and depletion of TFEB reduces ligand-independent N signaling activity. These data suggest that lysosomal-associated functions regulated by the TFEB-V-ATPase axis might play a conserved role in shaping cell fate (Tognon, 2016).

Drosophila model of Neuronopathic Gaucher Disease demonstrates lysosomal-autophagic defects and altered mTOR signalling and is functionally rescued by rapamycin

Glucocerebrosidase (GBA1) mutations are associated with Gaucher disease (GD), an autosomal recessive disorder caused by functional deficiency of glucocerebrosidase (GBA), a lysosomal enzyme that hydrolyzes glucosylceramide to ceramide and glucose. Neuronopathic forms of GD can be associated with rapid neurological decline (Type II) or manifest as a chronic form (Type III) with a wide spectrum of neurological signs. Furthermore, there is now a well-established link between GBA1 mutations and Parkinson's disease (PD), with heterozygote mutations in GBA1 considered the commonest genetic defect in PD. This study describes a novel Drosophila model of GD that lacks the two fly GBA1 orthologs (Gba1a and Gba1b). This knock-out model recapitulates the main features of GD at the cellular level with severe lysosomal defects and accumulation of glucosylceramide in the fly brain. A block in autophagy flux was demonstrated in association with reduced lifespan, age-dependent locomotor deficits and accumulation of autophagy substrates in dGBA-deficient fly brains. Furthermore, mechanistic target of rapamycin (mTOR) signaling is downregulated in dGBA knock-out flies, with a concomitant upregulation of Mitf gene expression, the fly ortholog of mammalian TFEB, likely as a compensatory response to the autophagy block. Moreover, the mTOR inhibitor rapamycin is able to partially ameliorate the lifespan, locomotor, and oxidative stress phenotypes. Together, these results demonstrate that this dGBA1-deficient fly model is a useful platform for the further study of the role of lysosomal-autophagic impairment and the potential therapeutic benefits of rapamycin in neuronopathic GD. These results also have important implications for the role of autophagy and mTOR signaling in GBA1-associated PD (Kinghorn, 2016).

The basic helix-loop-helix leucine zipper transcription factor Mitf is conserved in Drosophila and functions in eye development

The MITF protein is a member of the MYC family of basic helix-loop-helix leucine zipper (bHLH-Zip) transcription factors and is most closely related to the TFE3, TFEC, and TFEB proteins. In the mouse, MITF is required for the development of several different cell types, including the retinal pigment epithelial (RPE) cells of the eye. In Mitf mutant mice, the presumptive RPE cells hyperproliferate, abnormally express the retinal transcriptional regulator Pax6, and form an ectopic neural retina. This study reports the structure of the Mitf gene in Drosophila and demonstrate expression during embryonic development and in the eye- antennal imaginal disc. In vitro, transcriptional regulation by Drosophila Mitf, like its mouse counterpart, is modified by the Eyeless (Drosophila Pax6) transcription factor. In vivo, targeted expression of wild-type or dominant-negative Drosophila Mitf results in developmental abnormalities reminiscent of Mitf function in mouse eye development. These results suggest that the Mitf gene is the original member of the Mitf-Tfe subfamily of bHLH-Zip proteins and that its developmental function is at least partially conserved between vertebrates and invertebrates. These findings further support the common origin of the vertebrate and invertebrate eyes (Hallsson, 2004).

This study describe the identification and initial characterization of Dmel/Mitf, the Drosophila homolog of the vertebrate bHLH-Zip transcription factor gene Mitf. Like its vertebrate counterpart, Dmel/Mitf can activate a known Mitf reporter in vitro and this transcriptional activation is sensitive to regulation by Eyeless/Pax6. Targeted expression of wild-type or dominant-negative forms of Dmel/Mitf results in opposite effects on the development of the eye-disc region and suggests that Mitf's role in eye development is at least partially conserved between fly and vertebrates (Hallsson, 2016).

Despite extensive genome-wide searches for basic-helix-loop-helix proteins in the Drosophila genome, no Mitf or Mitf-related genes were found in previous analyses. This suggests that the Dmel/Mitf gene described here is the only family member found in the Drosophila genome. This is in sharp contrast to vertebrate genomes, which, in addition to Mitf, contain the three other closely related genes Tfeb, Tfe3, and TfeC. Furthermore, the zebrafish genome contains two Mitf genes (nacre/Mitfa and Mitfb) in addition to a presumed unknown number of Tfe genes. Other fish species, including Xiphophorus, Fugu rubripes, and Tetraodon nigroviridis, also contain two Mitf genes in their genomes, suggesting a gene duplication event in teleost fish after their separation from the bird/mammalian lineage. Studies on Mitf function should therefore be greatly simplified in Drosophila as compared to studies in vertebrate species (Hallsson, 2004).

Within the MYC supergene family of basic bHLH-Zip transcription factors, the Mitf gene is most closely related to the Tfeb, Tfec, and Tfe3 genes. Together these four proteins form the Mitf-Tfe subfamily of bHLH-Zip proteins. All four proteins share almost identical basic regions and very similar HLH and Zip regions; the sequence is quite divergent outside these domains. It is likely that the Mitf gene is most closely related to the ancestral form of the Mitf-Tfe family of proteins since there are more similarities between Dmel/Mitf and the mouse Mitf genes than between Dmel/Mitf and any of the three Tfe genes. For example, the mouse Tfec and Mitf genes differ at two positions in the helix 1 domain (YNINY in Tfec vs. FNIND in Mitf ) whereas the mouse and fly Mitf genes are identical. Similarly, the mouse Mitf and Tfe3 genes are different in one position in the basic domain (LLKE in Tfe3 vs. LAKE in Mitf ) and the Mitf and Tfeb genes are different in one position in helix 1 (LGML in Tfeb vs. LGTL in Mitf). All these residues are conserved in the mouse and fly Mitf genes, suggesting that the Mitf gene is the common ancestor and that the Tfe3, Tfeb, and Tfec genes arose from the ancestral gene after the separation of the vertebrate and invertebrate lineages (Hallsson, 2004).

The Drosophila Mitf protein is considerably larger than its vertebrate counterpart. In addition to the highly conserved bHLH-Zip domains, several other conserved regions were identified, suggesting that they represent regions of functional importance. These include a glutamine-rich region at the amino terminus, an amphipathic helical region with a transcription activation function, and a stretch of six amino acids at the carboxy end. In addition, a serine amino acid-which in the mouse MITF protein is phosphorylated by the MAP kinase pathway-is also conserved in Dmel/Mitf. Thus, regulation of Dmel/Mitf function may involve phosphorylation at this site (Hallsson, 2004).

Significant differences also exist between vertebrate and fly Mitf. Most notably, in the mouse, an additional first exon (1M) codes for 11 amino acids and is included in a melanocyte-specific form of the Mitf mRNA, the M form. No sequences have been found corresponding to exon 1M near the Dmel/Mitf gene. If it is assumed that the order of exons in the gene is conserved between mouse and Drosophila, then exon 1M would be situated between exons 2 and 3. However, the intron between exons 2 and 3 in Drosophila is only 51 nucleotides long and does not include an ATG. This lack of conservation is not unexpected. Vertebrate melanocytes originate from the neural crest, a cell lineage with no counterpart in Drosophila. Although the Drosophila eye does contain pigment cells, these arise from the eye-antennal epithelium and their pigment granules (ommochromes and drosopterins) are chemically distinct from the melanosomes (melanins) of vertebrates. Hence, fly pigment cells are not evolutionarily related to melanocytes. In this respect, exon 1M may reflect an evolutionary modification of the ancestral Mitf gene that arose specifically in the vertebrate lineage. Consistent with that, none of the related Tfe genes have an M-like exon, suggesting that this exon arose after the Tfe genes had arisen from the ancestral Mitf gene. Although the recently characterized Mitf gene of the ascidian Halocynthia roretzi is expressed in pigment lineage blastomeres, it does not appear to contain sequences that resemble exon 1M . Interestingly, the ascidian Mitf gene is expressed maternally, like its Drosophila counterpart (Hallsson, 2004).

The Mitf gene is expressed in the mouse eye during the optic vesicle and optic cup stages of eye development and is required for the normal formation and maintenance of the RPE. The RPE is a single layer of cuboidal cells, which basally displays numerous infoldings while apically abundant microvilli enclose and interdigitate with rod outer segments. In Mitf mi/mi mutant mice, the RPE apical microvilli are absent and elongated rod outer segments do not develop. During development, the retina and RPE cell layers are closely juxtaposed and recent evidence supports an early role for the RPE in morphogenesis of the neural retina. Genetic ablation of the RPE cells early during eye formation prevents lamination of the retina, and later ablation results in loss of laminar organization. In addition, factors secreted by the RPE have been shown to positively influence the development and maintenance of normal retinal morphology. Thus, the RPE is thought to be a source of signaling molecules that lead to proper patterning and maintenance of the vertebrate retina (Hallsson, 2004).

The Drosophila eye, albeit structurally very different from the mouse eye, also develops from a bilayered epithelial structure. The progenitor epithelium that gives rise to the adult fly eye and associated head cuticle consists of a flattened sac with a columnar 'disc proper' cell layer (from which the retina develops) and a noncolumnar 'peripodial' cell layer (from which mostly cuticle, or epidermis, will form). Until recently the peripodial cell layer was not thought to be directly involved in retinal morphogenesis and it does not in fact contribute directly to any part of the adult eye (as mentioned above, these cells give rise to head cuticle). However, two groups have recently shown that peripodial cells are in fact required for proper development of the retina. In addition, to other groups have shown that cellular projections, named 'transluminal' projections, extend from one layer to the other and provide a mechanism for direct interactions between these two layers. Thus, it is now thought that signaling occurs between cells of peripodial and disc proper layers and that these interactions are essential for proper retinal development. The expression of Dmel/Mitf in the peripodial cell layer, specifically in the portion of the peripodial membrane that overlooks the site of photoreceptor neuron formation (MF), suggests that Mitf may be involved in this process (Hallsson, 2004).

To investigate the potential role of Dmel/Mitf in eye development, the wild type and dominant- negative Mitf EA mutant were expressed in the developing eye-antennal disc. Discs expressing wild-type Dmel/Mitf were variably reduced in size and neuronal morphogenesis was always reduced and occasionally absent. On the contrary, discs expressing the Mitf EA mutant version were larger than wild-type discs and the developing photoreceptor field appeared correspondingly expanded. The striking effect on disc size likely reflects changes in proliferation, whereas the variation in neuronal field size may be secondary to this or result from effects on primordia formation (cuticle/peripodial vs. eye) within the epithelium. As vertebrate Mitf has been implicated in proliferation and RPE specification (Nguyen and Arnheiter 2000), these observations strongly suggest significant conservation of Mitf's role in eye development. Further investigation of Dmel/Mitf function awaits the generation of Dmel/Mitf mutant alleles and better peripodial-specific drivers. Nonetheless, the similarities this study has uncovered between the peripodial membrane of the fly eye-antennal disc and the RPE of the vertebrate optic vesicle/ cup are very intriguing. The expression of Mitf in both epithelia raises the possibility that these tissues are evolutionarily related. In such a scenario, the ancestral tissue from which the eye eventually formed may have already displayed a partition into two fields: a nonneural Mitf/Pax6 field and a neural Mitf/Pax6 field. Moreover, development of these two fields may have already involved inductive events between juxtaposed cell layers. Parallel investigations of Mitf function in mouse and fly will provide useful insights in evaluating these hypotheses (Hallsson, 2004).


Search PubMed for articles about Drosophila Mitf

Bouche, V., Perez Espinosa, A., Leone, L., Sardiello, M., Ballabio, A. and Botas, J. (2016). Drosophila Mitf regulates the V-ATPase and the lysosomal-autophagic pathway. Autophagy 12(3):484-98. PubMed ID: 26761346

Hallsson, J. H., Haflidadöttir, B. S., Stivers, C., Odenwald, W., Arnheiter, H., Pignoni, F. and Steingrïmsson, E. (2004). The basic helix-loop-helix leucine zipper transcription factor Mitf is conserved in Drosophila and functions in eye development. Genetics 167: 233-241. PubMed ID: 15166150

Hermle, T., Guida, M. C., Beck, S., Helmstädter, S. and Simons, M. (2013). Drosophila ATP6AP2/VhaPRR functions both as a novel planar cell polarity core protein and a regulator of endosomal trafficking. EMBO J. 32: 245-259. PubMed ID: 23292348

Hiesinger, P. R., Fayyazuddin, A., Mehta, S. Q., Rosenmund, T., Schulze, K. L., Zhai, R. G., Verstreken, P., Cao, Y., Zhou, Y., Kunz, J. et al. (2005). The v-ATPase V0 subunit a1 is required for a late step in synaptic vesicle exocytosis in Drosophila. Cell 121: 607-620. PubMed ID: 15907473

Kinghorn, K. J., Gronke, S., Castillo-Quan, J. I., Woodling, N. S., Li, L., Sirka, E., Gegg, M., Mills, K., Hardy, J., Bjedov, I. and Partridge, L. (2016). A Drosophila model of Neuronopathic Gaucher Disease demonstrates lysosomal-autophagic defects and altered mTOR signalling and is functionally rescued by rapamycin. J Neurosci 36: 11654-11670. PubMed ID: 27852774

Marshansky, V., Rubinstein, J. L. and Grüber, G.(2014). Eukaryotic V-ATPase: novel structural findings and finctional insights. Biochim. Biophys. Acta. 1837: 857-879. PubMed ID: 24508215

Nguyen, M. and Arnheiter, H. (2000). Signaling and transcriptional regulation in early mammalian eye development: a link between FGF and MITF. Development 127(16): 3581-3591. PubMed ID: 10903182

Peña-Llopis, S., Vega-Rubin-de-Celis, S., Schwartz, J. C., Wolff, N. C., Tran, T. A. T., Zou, L., Xie, X.-J., Corey, D. R. and Brugarolas, J. (2011). Regulation of TFEB and V-ATPases by mTORC1. EMBO J. 30: 3242-3258. PubMed ID: 21804531

Settembre, C., et al. (2011). TFEB links autophagy to lysosomal biogenesis. Science 332: 1429-1433. PubMed ID: 21617040

Tognon, E., Kobia, F., Busi, I., Fumagalli, A., De Masi, F. and Vaccari, T. (2016). Control of lysosomal biogenesis and Notch-dependent tissue patterning by components of the TFEB-V-ATPase axis in Drosophila melanogaster. Autophagy [Epub ahead of print]. PubMed ID: 26727288

Wieczorek, H., Beyenbach, K. W., Huss, M. and Vitavska, O. (2009). Vacuolar-type proton pumps in insect epithelia. J. Exp. Biol. 212: 1611-1619. PubMed ID: 19448071

Zhang, T., Zhou, Q., Ogmundsdottir, M. H., Moller, K., Siddaway, R., Larue, L., Hsing, M., Kong, S. W., Goding, C. R., Palsson, A., Steingrimsson, E. and Pignoni, F. (2015). Mitf is a master regulator of the v-ATPase, forming a control module for cellular homeostasis with v-ATPase and TORC1. J Cell Sci 128: 2938-2950. PubMed ID: 26092939

Zoncu, R., Bar-Peled, L., Efeyan, A., Wang, S., Sancak, Y. and Sabatini, D. M. (2011). mTORC1 senses lysosomal amino acids through an inside-out mechanism that requires the vacuolar H+-ATPase. Science 334: 678-683. PubMed ID: 22053050

date revised: 26 April 2021

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