E3 ubiquitin ligases play a crucial role in modulating receptor stability and signaling at the cell surface, yet the mechanisms governing their substrate specificity remain incompletely understood. Mahogunin Ring Finger 1 (MGRN1) is a membrane-tethered E3 ligase that fine-tunes signaling sensitivity by targeting surface receptors for ubiquitination and degradation. Unlike cytosolic E3 ligases, membrane-tethered E3s require transmembrane adapters to selectively recognize and regulate surface receptors, yet few such ligases have been studied in detail. While MGRN1 is known to regulate the receptor Smoothened (SMO) within the Hedgehog pathway through its interaction with the transmembrane adapter Multiple Epidermal Growth Factor-like 8 (MEGF8), the broader scope of its regulatory network has been speculative. Here, we identify Attractin (ATRN) and Attractin-like 1 (ATRNL1) as additional transmembrane adapters that recruit MGRN1 and regulate cell surface receptor turnover. Through co-immunoprecipitation, we show that ATRN and ATRNL1 likely interact with the RING domain of MGRN1. Functional assays reveal that MGRN1 requires these transmembrane adapters to ubiquitinate and degrade the melanocortin receptors MC1R and MC4R, in a process analogous to its regulation of SMO. Loss of MGRN1 leads to increased surface and ciliary localization of MC4R in fibroblasts and elevated MC1R levels in melanocytes, with the latter resulting in enhanced eumelanin production. These findings expand the repertoire of MGRN1-regulated receptors and provide new insight into a shared mechanism by which membrane-tethered E3 ligases utilize transmembrane adapters to dictate substrate receptor specificity. By elucidating how MGRN1 selectively engages with surface receptors, this work establishes a broader framework for understanding how this unique class of E3 ligases fine-tunes receptor homeostasis and signaling output.

Myelinating Schwann cells are particularly susceptible to defects in endosomal trafficking. TSG101 is a component of the endosomal trafficking machinery that mediates the sorting of ubiquitinated receptors into multivesicular bodies. We previously demonstrated that deleting Tsg101 from mouse oligodendrocytes in the central nervous system causes rapid onset de/dys-myelination and vacuolation of white matter, suggesting an important role for TSG101-dependent trafficking in myelination. Here, we show that TSG101 is also required for normal myelination in the peripheral nervous system.

The laboratory mouse represents a powerful mammalian system to study the genetic basis of human disease. This chapter describes techniques to create mice that carry specific genome modifications, based on known human disease-associated variants, or to induce random mutations that identify novel genes/pathways. Advantages and disadvantages of each method and identification of suitable control animals are discussed. Choosing appropriate phenotypic assays and executing them properly are also critical if high-quality information is to be gained from new mouse models. Assays discussed here include behavioral screens (which must be relevant to the disease being modeled), imaging studies (which may incorporate fluorescent or bioluminescent reporters), and histopathology (including immunohistochemistry or immunofluorescence). As new methods for creating and analyzing mouse mutants continue to be developed, their value for understanding disease mechanisms and testing therapeutic strategies will remain strong.

FOXG1 syndrome is a severe neurodevelopmental disorder characterized by microcephaly, profound intellectual disability with communication deficits including lack of speech, impaired social interaction, increased anxiety, hyperkinetic/dyskinetic movements, seizures and abnormal sleep patterns. Mutations in a single allele of the FOXG1 gene cause disease, likely due to loss-of-function. However, current therapies do not target this root cause of FOXG1 syndrome and have little to modest therapeutic benefit on only a small subset of symptoms. To date, the therapeutic potential of restoring FOXG1 levels in the brain with adeno-associated virus (AAV) FOXG1 gene replacement therapy has only been reported in a Foxg1fl/+;NexCre mouse model that lacks one Foxg1 allele but does not express mutant FOXG1, and with only neuroanatomical endpoints evaluated. Here, in a FOXG1 mouse model that contains a highly prevalent, patient-specific Q84P mutation, we describe the beneficial effects of AAV human FOXG1 gene replacement therapy administered by intracerebroventricular (ICV) injection at postnatal day 6 (P6) on several behavioral deficits that are relevant to key features of human FOXG1 syndrome. Our studies demonstrate that AAV FOXG1 gene replacement therapy is a promising approach for the treatment of a subset of functional deficits in human FOXG1 syndrome. ### Competing Interest Statement The authors declare competing financial interests in the form of funding from Believe In A Cure (P.A., B.B., C.D., A.D., K.F., G.G., J.G., T.M.G., F.H., K.K., S.Ra., D.W.Y.S., C.L.T., D.W.). S.Re. is chief executive officer of Believe In A Cure.

Myelinating Schwann cells are particularly susceptible to defects in endosomal trafficking. TSG101 is a component of the endosomal trafficking machinery that mediates the sorting of ubiquitinated receptors into multivesicular bodies. We previously demonstrated that deleting Tsg101 from mouse oligodendrocytes in the central nervous system causes rapid onset de/dys-myelination and vacuolation of white matter, suggesting an important role for TSG101-dependent trafficking in myelination. Here, we show that TSG101 is also required for normal myelination in the peripheral nervous system.

The mammalian Tumor Susceptibility Gene 101 (TSG101) encodes a protein with diverse functions that control the proliferation and survival of cells, but its role in malignant transformation and cancer development has remained enigmatic. To study the pro-tumorigenic functions of TSG101, we developed a bi-transgenic mouse model that expresses exogenous TSG101 along with a luciferase reporter in a ligand-controlled manner in the mammary gland epithelium. We performed a comprehensive histopathologic, biochemical, and molecular characterization of ductal hyperplasia and mammary tumors. Unsupervised hierarchical clustering based on 1,723 intrinsic genes of ten TSG101-overexpressing cancers alongside 251 tissue samples representing 31 reference mammary tumor models and normal mammary glands was conducted. Females overexpressing TSG101 develop ductal hyperplasia, adenomyoepitheliomas, and palpable adenosquamous carcinomas at an average latency of approximately ten months. These metaplastic mammary tumors are comprised of transforming basal and luminal epithelial cells. Using a GFP reporter strain to monitor the transgene activation at the single-cell level, we determined that the epithelial heterogeneity within transforming ducts and ensuing carcinomas originated from the luminal epithelium. At the molecular level, TSG101-induced mammary tumors are triple-negative and exhibit gene expression signatures of Wnt and inflammatory cytokine signaling, which are key regulators of epithelial cell fate. The ligand-controlled downregulation of exogenous TSG101 in established carcinomas led to tumor regression. We demonstrated that the TSG101-mediated activation of PI3K/AKT signaling, as well as upregulation of Cyclin D1 and MDM2, are dependent on the perpetual expression of the TSG101 oncoprotein. The collective findings of this study provide in vivo evidence that TSG101 possesses pro-tumorigenic properties that extend to cancer progression and maintenance, suggesting that this protein could be a rational molecular target to prevent and treat a subset of mammary tumors.

Endosomal dysfunction is one of the earliest cellular signs in Alzheimer’s disease. Tumor susceptibility gene 101 protein (TSG101) is a component of the endosomal sorting complex required for transport (ESCRT)-I, which plays a key role in sorting ubiquitinated cell surface proteins and lipids onto intraluminal vesicles of multivesicular bodies for trafficking to lysosomes or autophagosomes for degradation, or to the plasma membrane for exosomal secretion. TSG101-dependent trafficking has been implicated in the propagation and spread of misfolded proteins associated with neurodegenerative diseases. We used transgenesis mice to study the in vivo consequences of disrupting TSG101-dependent trafficking in adult neurons. Mice lacking Tsg101 in forebrain neurons (Tsg101ck2-null) showed rapid loss of hippocampal neurons and progressive forebrain atrophy. Astrogliosis was apparent in the dentate gyrus within 1 week of deleting Tsg101, followed by apoptosis of hippocampal CA3 neurons and accumulation of the autophagy adapter P62/SQSTM1 and ubiquitinated proteins. Failure to detect lipidated LC3 indicated autophagy was impaired rather than upregulated. Endosomal markers (RAB5 and RAB7) and amyloid protein also accumulated in hippocampal neurons of Tsg101ck2-null mice. Our data establish a critical role for TSG101 in neuronal survival and demonstrate the importance of the in vivo assessment of gene and protein functions.

Mahogunin ring finger 1 (MGRN1) is a membrane-tethered E3 ligase that fine-tunes signaling sensitivity by targeting surface receptors for ubiquitylation and degradation. Although MGRN1 is known to regulate the Hedgehog signaling effector Smoothened (SMO) via the transmembrane adapter multiple epidermal growth factor-like 8 (MEGF8), the broader scope of its regulatory network has been speculative. Here, we identify attractin (ATRN) and attractin-like 1 (ATRNL1) as additional transmembrane adapters that recruit MGRN1 and regulate cell surface receptor turnover. Through coimmunoprecipitation, we show that ATRN interacts with the RING domain of MGRN1. Functional assays suggest that ATRN and ATRNL1 work with MGRN1 to promote the ubiquitylation and degradation of the melanocortin receptors MC1R and MC4R, in a process analogous to its regulation of SMO. Loss of MGRN1 or ATRN leads to increased surface and ciliary localization of MC4R in fibroblasts and elevated MC1R levels in melanocytes, resulting in enhanced eumelanin production. These findings expand the known repertoire of MGRN1-regulated receptors and provide new insight into a shared mechanism by which membrane-tethered E3 ligases utilize transmembrane adapters to facilitate substrate receptor specificity.

PURPOSE:This minireview discusses desmosome and hemidesmosome disassembly and/or internalization and subsequent release via exosomes in retinal pigmented epithelium (RPE) under oxidative stress conditions, and whether it may be a precursor to epithelial-mesenchymal transition in early Age-related Macular Degeneration (AMD). METHODS:Literature review and discussion of novel findings relevant to the focus of the review. RESULTS:The RPE forms the outer blood-retinal barrier, and like other epithelia it has several different types of cell-cell junctions, such as desmosomes. The RPE provides key metabolic and nutrient support to photoreceptors and the function of normal vision. The RPE is a principal location of disease-associated changes in AMD, due to its essential role in visual homeostasis. Exosomes are lipid bilayer membrane vesicles of nanometer sizes that are released via a dedicated machinery by all cells and carry out a multitude of functions related to cellular signaling and waste management. In the RPE they are released from both the apical and basal sides, and the cargo composition reflects this polarization. We have recently showed that exosomes released from the basolateral side of RPE cells under chronic oxidative stress conditions, contain desmosome and hemidesmosome proteins. Here we discuss the composition of desmosomes and hemidesmosomes in the RPE, the role of exosomes and ubiquitination pathways in their disassembly, and whether this dismantling is a precursor to epithelial-mesenchymal transition. Further considerations include how the exosome-mediated shedding of desmosome and hemidesmosome components is related to lysosomal and/or proteasomal overload, and how these pathways can be modulated to intervene in early stages of AMD. CONCLUSIONS:This review provides an overview of the current knowledge about desmosome and hemidesmosome disassembly in RPE, its intersection with the exosome pathway, and potential role in epithelial-mesenchymal transition. We discuss several potential targets for therapeutic intervention in pre-symptomatic or early-stage AMD in these pathways.

Solute carrier family 1 member 4 (SLC1A4), also referred to as Alanine/Serine/Cysteine/Threonine-preferring Transporter 1 (ASCT1), is a sodium-dependent neutral amino acid transporter. It is expressed in many tissues, including the brain, where it is expressed primarily on astrocytes and plays key roles in neuronal differentiation and development, maintaining neurotransmitter homeostasis, and N-methyl-D-aspartate neurotransmission, through regulation of L- and D-serine. Mutations in SLC1A4 are associated with the rare autosomal recessive neurodevelopmental disorder spastic tetraplegia, thin corpus callosum, and progressive microcephaly (SPATCCM, OMIM 616657). Psychomotor development and speech are significantly impaired in these patients, and many develop seizures. We generated and characterized a knock-in mouse model for the most common mutant allele, which results in a single amino acid change (p.Glu256Lys, or E256K). Homozygous mutants had increased D-serine uptake in the brain, microcephaly, and thin corpus callosum and cortex layer 1. While p.E256K homozygotes showed some significant differences in exploratory behavior relative to wildtype mice, their performance in assays for motor coordination, endurance, learning, and memory was normal, and they showed no significant differences in long-term potentiation. Taken together, these results indicate that the impact of the p.E256K mutation on cognition and motor function is minimal in mice, but other aspects of SLC1A4 function in the brain are conserved. Mice homozygous for p.E256K may be a good model for understanding the developmental basis of the corpus callosum and microcephaly phenotypes observed in SPATCCM patients and assessing whether they are rescued by serine supplementation.

The 35th International Mammalian Genome Conference (IMGC) was held on July 17-20, 2022 in Vancouver, British Columbia; this conference marked the first time the International Mammalian Genome Society (IMGS) hosted a meeting in Canada. Scientists from around the world participated to share advances in genetics and genomics research across mammalian species. A diverse attendance of pre-doctoral and post-doctoral trainees, young investigators, established researchers, clinicians, bioinformaticians, and computational biologists enjoyed a rich scientific program selected from 88 abstracts in the fields of cancer, conservation genetics, developmental biology, epigenetics, human disease modeling, immunology, infectious diseases, systems genetics, translational biology, and technological advances.

Birth defects result from interactions between genetic and environmental factors, but the mechanisms remain poorly understood. We find that mutations and teratogens interact in predictable ways to cause birth defects by changing target cell sensitivity to Hedgehog (Hh) ligands. These interactions converge on a membrane protein complex, the MMM complex, that promotes degradation of the Hh transducer Smoothened (SMO). Deficiency of the MMM component MOSMO results in elevated SMO and increased Hh signaling, causing multiple birth defects. In utero exposure to a teratogen that directly inhibits SMO reduces the penetrance and expressivity of birth defects in Mosmo-/- embryos. Additionally, tissues that develop normally in Mosmo-/- embryos are refractory to the teratogen. Thus, changes in the abundance of the protein target of a teratogen can change birth defect outcomes by quantitative shifts in Hh signaling. Consequently, small molecules that re-calibrate signaling strength could be harnessed to rescue structural birth defects.

The etiology of congenital heart defects (CHDs), which are among the most common human birth defects, is poorly understood because of its complex genetic architecture. Here, we show that two genes implicated in CHDs, Megf8 and Mgrn1, interact genetically and biochemically to regulate the strength of Hedgehog signaling in target cells. MEGF8, a transmembrane protein, and MGRN1, a RING superfamily E3 ligase, assemble to form a receptor-like ubiquitin ligase complex that catalyzes the ubiquitination and degradation of the Hedgehog pathway transducer Smoothened. Homozygous Megf8 and Mgrn1 mutations increased Smoothened abundance and elevated sensitivity to Hedgehog ligands. While mice heterozygous for loss-of-function Megf8 or Mgrn1 mutations were normal, double heterozygous embryos exhibited an incompletely penetrant syndrome of CHDs with heterotaxy. Thus, genetic interactions can arise from biochemical mechanisms that calibrate morphogen signaling strength, a conclusion broadly relevant for the many human diseases in which oligogenic inheritance is emerging as a mechanism for heritability.

The etiology of congenital heart defects (CHDs), amongst the most common human birth defects, is poorly understood partly because of its complex genetic architecture. Here we show that two genes previously implicated in CHDs, Megf8 and Mgrn1 , interact genetically and biochemically to regulate the strength of Hedgehog signaling in target cells. MEGF8, a single-pass transmembrane protein, and MGRN1, a RING superfamily E3 ligase, assemble to form a transmembrane ubiquitin ligase complex that catalyzes the ubiquitination and degradation of the Hedgehog pathway transducer Smoothened. Homozygous Megf8 and Mgrn1 mutations increased Smoothened abundance and elevated sensitivity to Hedgehog ligands. While mice heterozygous for loss-of-function Megf8 or Mgrn1 mutations were normal, double heterozygous embryos exhibited an incompletely penetrant syndrome of CHDs with heterotaxy. Thus, genetic interactions between components of a receptor-like ubiquitin ligase complex that tunes morphogen signaling strength can cause a birth defect syndrome inherited in an oligogenic pattern.### Competing Interest StatementThe authors have declared no competing interest.

Spongiform encephalopathy is an intriguing yet poorly understood neuropathology characterized by vacuoles, demyelination, and gliosis. It is observed in patients with prion disease, primary mitochondrial disease, HIV-1 infection of the brain, and some inherited disorders, but the underlying mechanism of disease remains unclear. The brains of mice lacking the MGRN1 E3 ubiquitin ligase develop vacuoles by 9months of age. MGRN1-dependent ubiquitination has been reported to regulate mitofusin 1 and GP78, suggesting MGRN1 may have a direct effect on mitochondrial homeostasis. Here, we demonstrate that some MGRN1 localizes to mitochondria, most likely due to N-myristoylation, and mitochondria in cells from Mgrn1 null mutant mice display fragmentation and depolarization without recruitment of the parkin E3 ubiquitin ligase. The late onset of pathology in the brains of Mgrn1 null mutant mice suggests that a further, age-dependent effect on mitochondrial homeostasis may be required to trigger vacuolation. Parkin protein and mRNA levels showed a significant decline in the brains of Mgrn1 null mutant mice by 12months of age. To test whether loss of parkin triggers vacuolation through a synergistic effect, we generated Mgrn1; parkin double mutant mice. By 1month of age, their brains demonstrated more severe mitochondrial dysfunction than Mgrn1 null mutants, but there was no effect on the age-of-onset of spongiform neurodegeneration. Expression of the ATF4 transcription factor, a key regulator of the mitochondrial stress response, also declined in the brains of aged Mgrn1 null mutant mice. Together, the data presented here indicate that loss of MGRN1has early, direct effects on mitochondrial homeostasis and late, indirect effects on the ability of cells to respond to mitochondrial stress.

High on the Heidelberg hills, inside the Advanced Training Centre of the European Molecular Biology Laboratory (EMBL) campus with its unique double-helix staircase, scientists gathered for the EMBL conference “ Mammalian Genetics and Genomics: From Molecular Mechanisms to Translational Applications ,” organized in cooperation with the International Mammalian Genome Society (IMGS) and the Mouse Molecular Genetics (MMG) group. The conference attracted 205 participants from 30 countries, representing 6 of the 7 continents—all except Antarctica. It was a richly diverse group of geneticists, clinicians, and bioinformaticians, with presentations by established and junior investigators, including many trainees. From the 24th–27th of October 2017, they shared exciting advances in mammalian genetics and genomics research, from the introduction of cutting-edge technologies to descriptions of translational studies involving highly relevant models of human disease.

BACKGROUND INFORMATION:Vacuolation of the central nervous system (CNS) is observed in patients with transmissible spongiform encephalopathy, HIV-related encephalopathy and some inherited diseases, but the underlying cellular mechanisms remain poorly understood. Mice lacking the mahogunin ring finger-1 (MGRN1) E3 ubiquitin ligase develop progressive, widespread spongiform degeneration of the CNS. MGRN1 ubiquitinates and regulates tumour susceptibility gene 101 (TSG101), a central component of the endosomal trafficking machinery. As loss of MGRN1 is predicted to cause partial TSG101 loss-of-function, we hypothesised that CNS vacuolation in Mgrn1 null mice may be caused by the accumulation of multi-cisternal endosome-like 'class E' vacuolar protein sorting (vps) compartments similar to those observed in Tsg101-depleted cells in culture.RESULTS:To test this hypothesis, Tsg101 was deleted from mature oligodendroglia in vivo. This resulted in severe spongiform encephalopathy, histopathologically similar to that observed in Mgrn1 null mutant mice but with a more rapid onset. Vacuoles in the brains of Tsg101-deleted and Mgrn1 mutant mice labelled with endosomal markers, consistent with an endosomal origin. Vacuoles in the brains of mice inoculated with Rocky Mountain Laboratory (RML) prions did not label with these markers, indicating a different origin, consistent with previously published studies that indicate RML prions have a primary effect on neurons and cause vacuolation in an MGRN1-independent manner. Oligodendroglial deletion of Rab7, which mediates late endosome-to-lysosome trafficking and autophagosome-lysosome fusion, did not cause spongiform change.CONCLUSIONS:Our data suggest that the formation of multi-cisternal 'class E' vps endosomal structures in oligodendroglia leads to vacuolation.SIGNIFICANCE:This work provides the first evidence that disrupting multi-vesicular body formation in oligodendroglia can cause white matter vacuolation and demyelination. HIV is known to hijack the endosomal sorting machinery, suggesting that HIV infection of the CNS may also act through this pathway to cause encephalopathy.

The laboratory mouse represents a powerful mammalian system to study the genetic basis of human disease. This chapter describes techniques to create mice that carry specific genome modifications, based on known human ­disease-associated variants, or to induce random mutations that identify novel genes/pathways. Advantages and disadvantages of each method and identification of suitable control animals are discussed. Choosing appropriate phenotypic assays and executing them properly is critical if high-quality information is to be gained from new mouse models. Assays discussed here include behavioral screens (which must be relevant to the disease being modeled), imaging studies (which may incorporate fluorescent or bioluminescent reporters), and histopathology (including immunohistochemistry or immunofluorescence). As new methods for creating and analyzing mouse mutants continue to be developed, their value for understanding disease mechanisms and testing therapeutic strategies will remain strong.

Mice homozygous for the gray tremor (gt) mutation have a pleiotropic phenotype that includes pigmentation defects, megacolon, whole body tremors, sporadic seizures, hypo- and dys-myelination of the central nervous system (CNS) and peripheral nervous system, vacuolation of the CNS, and early death. Vacuolation similar to that caused by prions was originally reported to be transmissible, but subsequent studies showed the inherited disease was not infectious. The gt mutation mapped to distal mouse chromosome 15, to the same region as Sox10, which encodes a transcription factor with essential roles in neural crest survival and differentiation. As dominant mutations in mouse or human SOX10 cause white spotting and intestinal aganglionosis, we screened the Sox10 coding region for mutations in gt/gt DNA. An adenosine to guanine transversion was identified in exon 2 that changes a highly conserved glutamic acid residue in the SOX10 DNA binding domain to glycine. This mutant allele was not seen in wildtype mice, including the related GT/Le strain, and failed to complement a Sox10 null allele. Gene expression analysis revealed significant down-regulation of genes involved in myelin lipid biosynthesis pathways in gt/gt brains. Knockout mice for some of these genes develop CNS vacuolation and/or myelination defects, suggesting that their down-regulation may contribute to these phenotypes in gt mutants and could underlie the neurological phenotypes associated with peripheral demyelinating neuropathy–central dysmyelinating leukodystrophy–Waardenburg syndrome–Hirschsprung disease, caused by mutations in human SOX10.

The degradation of the main fibrillar collagens, collagens I and II, is a crucial process for skeletal development. The most abundant dipeptides generated from the catabolism of collagens contain proline and hydroxyproline. In humans, prolidase is the only enzyme able to hydrolyze dipeptides containing these amino acids at their C-terminal end, thus being a key player in collagen synthesis and turnover. Mutations in the prolidase gene cause prolidase deficiency (PD), a rare recessive disorder. Here we describe 12 PD patients, 9 of whom were molecularly characterized in this study. Following a retrospective analysis of all of them a skeletal phenotype associated with short stature, hypertelorism, nose abnormalities, microcephaly, osteopenia and genu valgum, independent of both the type of mutation and the presence of the mutant protein was identified. In order to understand the molecular basis of the bone phenotype associated with PD, we analyzed a recently identified mouse model for the disease, the dark-like (dal) mutant. The dal/dal mice showed a short snout, they were smaller than controls, their femurs were significantly shorter and pQCT and μCT analyses of long bones revealed compromised bone properties at the cortical and at the trabecular level in both male and female animals. The differences were more pronounce at 1 month being the most parameters normalized by 2 months of age. A delay in the formation of the second ossification center was evident at postnatal day 10. Our work reveals that reduced bone growth was due to impaired chondrocyte proliferation and increased apoptosis rate in the proliferative zone associated with reduced hyperthrophic zone height. These data suggest that lack of prolidase, a cytosolic enzyme involved in the final stage of protein catabolism, is required for normal skeletogenesis especially at early age when the requirement for collagen synthesis and degradation is the highest.