RIFLE: a novel ring zinc finger-leucine-rich repeat containing protein, regulates select cell adhesion molecules in PC12 cells

LRSAM1 E3 Ubiquitin Ligase Promotes Choriocarcinoma Progression and Metastasis via p53/p21 Signaling Impediment

Objective

The E3 ubiquitin ligase LRSAM1 (LRSAM1) was involved in many cancers, but whether it exerts anti- or protumor efficacies on choriocarcinoma cellular structures remains unknown. We wanted to explore the efficacies of aberrant LRSAM1 expression on human choriocarcinoma cellular structures and the underlying mechanisms.

Methods

LRSAM1 mRNA expressions in choriocarcinoma lines of cells JEG-3 and JAR cellular structures, as well as HTR8/sev8 human trophoblastic cell line cellular structures, were assessed using assay analysis of quantitative real-time polymerase chain reactions. We compared cell proliferation, migratory flow, invasive force, adhesion, and apoptotic process between cellular structures infected with si-LRSAM1 plasmids versus negative controls using CCK-8, clone formation, Transwell, adhesion, and flow cytometry assays. Protein expressions of LRSAM1, E-cadherin, and N-cadherin (indicators of epithelial-mesenchymal transformation) and p53/p21 pathway components were quantitated using a Western blot assay. The morphology of tumor lesions was observed in xenografted nude mice using immunohistochemistry (IHC) analyses.

Results

LRSAM1 was markedly overexpressed within JEG-3 and JAR choriocarcinoma cellular structures compared to HTR8/sev8 trophoblast cellular structures. Compared to si-NC, LRSAM1 knockdown robustly restricted cell proliferating, migratory flow, invasive force, and adhesion and fueled apoptotic cell process in JEG-3 as well as JAR cellular structures and suppressed tumor growth, as evidenced by the reduction in tumor volume and weight in naked mice inoculated with transfected cellular structures. Compared to si-negative control (si-NC), si-LRSAM1 significantly decreased Ki67 (a proliferating indicator) and N-cadherin expressions but reduced E-cadherin expression in JEG-3 and JAR cellular structures. Blocking the p53/p21 pathway by pifithrin-a (a p53 restrictor) successfully reversed the anti-inhibitory effect of LRSAM1 depletion, resulting in enhanced proliferating and metastasis in JEG-3 and JAR cellular structures.

Conclusion

LRSAM1 exerts tumorigenic roles in choriocarcinoma. Via the activating of the p53/p21 pathway of signaling and impediment of choriocarcinoma cell proliferating, migratory flow, and invasive force, LRSAM1 knockdown slows the course of the disease. For choriocarcinoma diagnosis and treatment, it serves as a new therapeutic target.

LRSAM1 and the RING domain: Charcot-Marie-Tooth disease and beyond

In the past decade, mutations in LRSAM1 were identified as the genetic cause of both dominant and recessive forms of axonal CMT type 2P (CMT2P). Despite demonstrating different inheritance patterns, dominant CMT2P is usually characterized by relatively mild, slowly progressive axonal neuropathy, mainly involving lower limbs, with age of onset between the second and fifth decades of life. Asymptomatic individuals were identified in several pedigrees exemplifying the strong phenotypic variability of these patients requiring serial clinical evaluation to establish correct diagnosis; in this respect, magnetic resonance imaging of lower-limb musculature showing fatty atrophy might be helpful in detecting subclinical gene mutation carriers. LRSAM1 is a universally expressed RING-type E3 ubiquitin protein ligase catalysing the final step in the ubiquitination cascade. Strikingly, TSG101 remains the only known ubiquitination target hampering our mechanistic understanding of the role of LRSAM1 in the cell. The recessive CMT mutations lead to complete loss of LRSAM1, contrary to the heterozygous dominant variants. These tightly cluster in the C-terminal RING domain highlighting its importance in governing the CMT disease. The domain is crucial for the ubiquitination function of LRSAM1 and CMT mutations disrupt its function, however it remains unknown how this leads to the peripheral neuropathy. Additionally, recent studies have linked LRSAM1 with other neurodegenerative diseases of peripheral and central nervous systems. In this review we share our experience with the challenging clinical diagnosis of CMT2P and summarize the mechanistic insights about the LRSAM1 dysfunction that might be helpful for the neurodegenerative field at large.

LRSAM1 E3 ubiquitin ligase promotes proteasomal clearance of E6-AP protein

Numerous proteins participate and actively contribute to the various cellular mechanisms, where several of them are crucial for regular metabolism, including survival. Thus, to maintain optimal cellular physiology, cells govern protein quality control functions with the assistance of comprehensive actions of molecular chaperones, the ubiquitin-proteasome system, and autophagy. In the ubiquitin-proteasome pathway, few quality control E3 ubiquitin ligases actively participate against misfolded protein aggregation generated via stress conditions. But how these quality control E3s active expression levels returned to basal levels when cells achieved re-establishment of proteostasis is still poorly understood. Our current study demonstrated that LRSAM1 E3 ubiquitin ligase promotes the proteasomal degradation of quality control E3 ubiquitin ligase E6-AP. We have observed the co-localization and recruitment of LRSAM1 with E6-AP protein and noticed that LRSAM1 induces the endogenous turnover of E6-AP. Partial depletion of LRSAM1 elevates the levels of E6-AP and affects overall cell cycle regulatory proteins (p53 and p27) expression, including the rate of cellular proliferation. The current finding also provides an excellent opportunity to better understand the basis of the E6-AP associated pathomechanism of Angelman Syndrome disorder. Additionally, this study touches upon the novel potential molecular strategy to regulate the levels of one quality control E3 ubiquitin ligase with another E3 ubiquitin ligase and restore proteostasis and provide a possible therapeutic approach against abnormal protein aggregation diseases.

Leucine-rich repeat and sterile alpha motif containing 1 promotes the oncogenic growth of human hepatocellular carcinoma cells

Background

Hepatocellular carcinoma (HCC), the most common primary cancer of the liver, is one of the most common malignancies and the leading cause of cancer-related death worldwide. Leucine-rich repeat and sterile alpha motif containing 1 (LRSAM1) is an E3 ubiquitin ligase involved in diverse cellular activities, including the regulation of cargo sorting, cell adhesion and antibacterial autophagy. The role of LRSAM1 in HCC remains unknown.

Methods

In this study, we reviewed the TCGA database and then performed gain-of-function and loss-of-function analyses of LRSAM1 in HCC cell lines.

Results

We found that the mRNA expression level of LRSAM1 was significantly increased in clinical HCC tissues in the TCGA database. Transient LRSAM1 knockdown in several human HCC cell lines led to reduced growth in conventional culture conditions. Stable LRSAM1 knockdown in HepG2 cells led to impaired anchorage-independent growth whereas its stable ectopic overexpression yielded the opposite effects. LRSAM1 overexpression in HepG2 cells enhanced in vivo tumorigenicity, whereas LRSAM1 knockdown in this cell line significantly impaired tumor growth.

Conclusions

Our data suggest that LRSAM1 promotes the oncogenic growth of human HCC cells, although the underlying mechanisms remain to be explored.

LRSAM1 E3 ubiquitin ligase: molecular neurobiological perspectives linked with brain diseases

Cellular protein quality control (PQC) plays a significant role in the maintenance of cellular homeostasis. Failure of PQC mechanism may lead to various neurodegenerative diseases due to accumulation of aberrant proteins. To avoid such fatal neuronal conditions PQC employs autophagy and ubiquitin proteasome system (UPS) to degrade misfolded proteins. Few quality control (QC) E3 ubiquitin ligases interplay an important role to specifically recognize misfolded proteins for their intracellular degradation. Leucine-rich repeat and sterile alpha motif-containing 1 (LRSAM1) is a really interesting new gene (RING) class protein that possesses E3 ubiquitin ligase activity with promising applications in PQC. LRSAM1 is also known as RING finger leucine repeat rich (RIFLE) or TSG 101-associated ligase (TAL). LRSAM1 has various cellular functions as it modulates the protein aggregation, endosomal sorting machinery and virus egress from the cells. Thus, this makes LRSAM1 interesting to study not only in protein conformational disorders such as neurodegeneration but also in immunological and other cancerous disorders. Furthermore, LRSAM1 interacts with both cellular protein degradation machineries and hence it can participate in maintenance of overall cellular proteostasis. Still, more research work on the quality control molecular functions of LRSAM1 is needed to comprehend its roles in various protein aggregatory diseases. Earlier findings suggest that in a mouse model of Charcot-Marie-Tooth (CMT) disease, lack of LRSAM1 functions sensitizes peripheral axons to degeneration. It has been observed that in CMT the patients retain dominant and recessive mutations of LRSAM1 gene, which encodes most likely a defective protein. However, still the comprehensive molecular pathomechanism of LRSAM1 in neuronal functions and neurodegenerative diseases is not known. The current article systematically represents the molecular functions, nature and detailed characterization of LRSAM1 E3 ubiquitin ligase. Here, we review emerging molecular mechanisms of LRSAM1 linked with neurobiological functions, with a clear focus on the mechanism of neurodegeneration and also on other diseases. Better understanding of LRSAM1 neurobiological and intracellular functions may contribute to develop promising novel therapeutic approaches, which can also propose new lines of molecular beneficial targets for various neurodegenerative diseases.

Deregulation of LRSAM1 expression impairs the levels of TSG101, UBE2N, VPS28, MDM2 and EGFR

CMT is the most common hereditary neuromuscular disorder of the peripheral nervous system with a prevalence of 1/2500 individuals and it is caused by mutations in more than 80 genes. LRSAM1, a RING finger ubiquitin ligase also known as TSG101-associated ligase (TAL), has been associated with Charcot-Marie-Tooth disease type 2P (CMT2P) and to date eight causative mutations have been identified. Little is currently known on the pathogenetic mechanisms that lead to the disease. We investigated the effect of LRSAM1 deregulation on possible LRSAM1 interacting molecules in cell based models. Possible LRSAM1 interacting molecules were identified using protein-protein interaction databases and literature data. Expression analysis of these molecules was performed in both CMT2P patient and control lymphoblastoid cell lines as well as in LRSAM1 and TSG101 downregulated SH-SY5Y cells.TSG101, UBE2N, VPS28, EGFR and MDM2 levels were significantly decreased in the CMT2P patient lymphoblastoid cell line as well as in LRSAM1 downregulated cells. TSG101 downregulation had a significant effect only on the expression of VPS28 and MDM2 and it did not affect the levels of LRSAM1. This study confirms that LRSAM1 is a regulator of TSG101 expression. Furthermore, deregulation of LRSAM1 significantly affects the levels of UBE2N, VPS28, EGFR and MDM2.

LRSAM1 Depletion Affects Neuroblastoma SH-SY5Y Cell Growth and Morphology: The LRSAM1 c.2047-1G>A Loss-of-Function Variant Fails to Rescue The Phenotype

Objective

Deleterious variants in LRSAM1, a RING finger ubiquitin ligase which is also known as TSG101-associated ligase (TAL), have recently been associated with Charcot-Marie-Tooth disease type 2P (CMT2P). The mechanism by which mutant LRSAM1 contributes to the development of neuropathy is currently unclear. The aim of this study was to induce LRSAM1 deficiency in a neuronal cell model, observe its effect on cell growth and morphology and attempt to rescue the phenotype with ancestral and mutant LRSAM1 transfections.

Materials and Methods

In this experimental study, we investigated the effect of LRSAM1 downregulation on neuroblastoma SH-SY5Y cells by siRNA technology where cells were transfected with siRNA against LRSAM1. The effects on the expression levels of TSG101, the only currently known LRSAM1 interacting molecule, were also examined. An equal dosage of ancestral or mutant LRSAM1 construct was transfected in LRSAM1-downregulated cells to investigate its effect on the phenotype of the cells and whether cell proliferation and morphology could be rescued.

Results

A significant reduction in TSG101 levels was observed with the downregulation of LRSAM1. In addition, LRSAM1 knockdown significantly decreased the growth rate of SH-SY5Y cells which is caused by a decrease in cell proliferation. An effect on cell morphology was also observed. Furthermore, we overexpressed the ancestral and the c.2047-1G>A mutant LRSAM1 in knocked down cells. Ancestral LRSAM1 recovered cell proliferation and partly the morphology, however, the c.2047-1G>A mutant did not recover cell proliferation and further aggravated the observed changes in cell morphology.

Conclusion

Our findings suggest that depletion of LRSAM1 affects neuroblastoma cells growth and morphology and that overexpression of the c.2047-1G>A mutant form, unlike the ancestral LRSAM1, fails to rescue the phenotype.

The self-association and activity regulation of LRSAM1 E3 ligase

LRSAM1, a RING-type E3 ubiquitin ligase, is essential for regulating cargo sorting, signaling pathways, cell adhesion and anti-bacterial autophagy. It is important to elucidate the mechanism that underlies the regulation of LRSAM1 E3 ligase activity. Here, we reported that LRSAM1 exhibited self-association in vitro and in vivo. We found the self-association of LRSAM1 promotes intermolecular ubiquitination and proved a potential N-terminal ubiquitination. The E3 activity of LRSAM1 is amplified when the RING domain is present in tandem with its N-terminal domain(s). Furthermore, we found that the CC2-SAM domain had a strong inhibitory effect on the E3 activity of LRSAM1 in vitro and blocked ubiquitination of TSG101 in vivo; the tandem CC1 domain, but not the individual CC1 domain, could counteract this inhibition. Collectively, our data characterized the self-association of LRSAM1 and showed how its domains may contribute to its overall activity.

Advances in the molecular diagnosis of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth (CMT) disease or hereditary motor and sensory neuropathy is the most common inherited neuromuscular disorder affecting at least 1 in 2500. CMT disease is pathologically and genetically heterogeneous and is characterized by a variable age of onset, slowly progressive weakness and muscle atrophy, starting in the lower limbs and subsequently affecting the upper extremities. Symptoms are usually slowly progressive, especially for the classic and late-onset phenotypes, but can be rather severe in early-onset forms. CMT is grouped into demyelinating, axonal and intermediate forms, based on electrophysiological and pathological findings. The demyelinating types are characterized by severely reduced motor nerve conduction velocities (MNCVs) and mainly by myelin abnormalities. The axonal types are characterized by normal or slightly reduced MNCVs and mainly axonal abnormalities. The intermediate types are characterized by MNCVs between 25 m/s and 45 m/s and they have features of both demyelination and axonopathy. Inheritance can be autosomal dominant, X-linked, or autosomal recessive. Mutations in more than 30 genes have been associated with the different forms of CMT, leading to major advancements in molecular diagnostics of the disease, as well as in the understanding of pathogenetic mechanisms. This editorial aims to provide an account that is practicable and efficient on the current molecular diagnostic procedures for CMT, in correlation with the clinical, pathological and electrophysiological findings. The most frequent causative mutations of CMT will also be outlined.

XLMR protein related to neurite extension (Xpn/KIAA2022) regulates cell-cell and cell-matrix adhesion and migration

X-linked mental retardation (XLMR) is a common cause of moderate to severe intellectual disability in males. XLMR protein related to neurite extension (Xpn, also known as KIAA2022) has been implicated as a gene responsible for XLMR in humans. Although Xpn is highly expressed in the developing brain and is involved in neurite outgrowth in PC12 cells and neurons, little is known about the functional role of Xpn. Here, we show that Xpn regulates cell-cell and cell-matrix adhesion and migration in PC12 cells. Xpn knockdown enhanced cell-cell and cell-matrix adhesion mediated by N-cadherin and β1-integrin, respectively. N-Cadherin and β1-integrin expression at the mRNA and protein levels was significantly increased in Xpn knockdown PC12 cells. Furthermore, overexpressed Xpn protein was strongly expressed in the nuclei of PC12 and 293T cells. Finally, depletion of Xpn perturbed cellular migration by enhancing N-cadherin and β1-integrin expression in a PC12 cell wound healing assay. We conclude that Xpn regulates cell-cell and cell-matrix adhesion and cellular migration by regulating the expression of adhesion molecules.

The LRR and RING domain protein LRSAM1 is an E3 ligase crucial for ubiquitin-dependent autophagy of intracellular Salmonella Typhimurium

Several species of pathogenic bacteria replicate within an intracellular vacuolar niche. Bacteria that escape into the cytosol are captured by the autophagic pathway and targeted for lysosomal degradation, representing a defense against bacterial exploitation of the host cytosol. Autophagic capture of Salmonella Typhimurium occurs predominantly via generation of a polyubiquitin signal around cytosolic bacteria, binding of adaptor proteins, and recruitment of autophagic machinery. However, the components mediating bacterial target selection and ubiquitination remain obscure. We identify LRSAM1 as the E3 ligase responsible for anti-Salmonella autophagy-associated ubiquitination. LRSAM1 localizes to several intracellular bacterial pathogens and generates the bacteria-associated ubiquitin signal; these functions require LRSAM1's leucine-rich repeat and RING domains, respectively. Using cells from LRSAM1-deficient individuals, we confirm that LRSAM1 is required for ubiquitination associated with intracellular bacteria but dispensable for ubiquitination of aggregated proteins. LRSAM1 is therefore a bacterial recognition protein and ubiquitin ligase that defends the cytoplasm from invasive pathogens.

Loss of the E3 ubiquitin ligase LRSAM1 sensitizes peripheral axons to degeneration in a mouse model of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous condition characterized by peripheral axon degeneration with subsequent motor and sensory deficits. Several CMT gene products function in endosomal sorting and trafficking to the lysosome, suggesting that defects in this cellular pathway might present a common pathogenic mechanism for these conditions. LRSAM1 is an E3 ubiquitin ligase that is implicated in this process, and mutations in LRSAM1 have recently been shown to cause CMT. We have generated mouse mutations in Lrsam1 to create an animal model of this form of CMT (CMT2P). Mouse Lrsam1 is abundantly expressed in the motor and sensory neurons of the peripheral nervous system. Both homozygous and heterozygous mice have largely normal neuromuscular performance and only a very mild neuropathy phenotype with age. However, Lrsam1 mutant mice are more sensitive to challenge with acrylamide, a neurotoxic agent that causes axon degeneration, indicating that the axons in the mutant mice are indeed compromised. In transfected cells, LRSAM1 primarily localizes in a perinuclear compartment immediately beyond the Golgi and shows little colocalization with components of the endosome to lysosome trafficking pathway, suggesting that other cellular mechanisms also merit consideration.

Charcot-Marie-Tooth disease and intracellular traffic

Mutations of genes whose primary function is the regulation of membrane traffic are increasingly being identified as the underlying causes of various important human disorders. Intriguingly, mutations in ubiquitously expressed membrane traffic genes often lead to cell type- or organ-specific disorders. This is particularly true for neuronal diseases, identifying the nervous system as the most sensitive tissue to alterations of membrane traffic. Charcot-Marie-Tooth (CMT) disease is one of the most common inherited peripheral neuropathies. It is also known as hereditary motor and sensory neuropathy (HMSN), which comprises a group of disorders specifically affecting peripheral nerves. This peripheral neuropathy, highly heterogeneous both clinically and genetically, is characterized by a slowly progressive degeneration of the muscle of the foot, lower leg, hand and forearm, accompanied by sensory loss in the toes, fingers and limbs. More than 30 genes have been identified as targets of mutations that cause CMT neuropathy. A number of these genes encode proteins directly or indirectly involved in the regulation of intracellular traffic. Indeed, the list of genes linked to CMT disease includes genes important for vesicle formation, phosphoinositide metabolism, lysosomal degradation, mitochondrial fission and fusion, and also genes encoding endosomal and cytoskeletal proteins. This review focuses on the link between intracellular transport and CMT disease, highlighting the molecular mechanisms that underlie the different forms of this peripheral neuropathy and discussing the pathophysiological impact of membrane transport genetic defects as well as possible future ways to counteract these defects.

LTP inhibits LTD in the hippocampus via regulation of GSK3beta

Glycogen synthase kinase-3 (GSK3) has been implicated in major neurological disorders, but its role in normal neuronal function is largely unknown. Here we show that GSK3beta mediates an interaction between two major forms of synaptic plasticity in the brain, N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) and NMDA receptor-dependent long-term depression (LTD). In rat hippocampal slices, GSK3beta inhibitors block the induction of LTD. Furthermore, the activity of GSK3beta is enhanced during LTD via activation of PP1. Conversely, following the induction of LTP, there is inhibition of GSK3beta activity. This regulation of GSK3beta during LTP involves activation of NMDA receptors and the PI3K-Akt pathway and disrupts the ability of synapses to undergo LTD for up to 1 hr. We conclude that the regulation of GSK3beta activity provides a powerful mechanism to preserve information encoded during LTP from erasure by subsequent LTD, perhaps thereby permitting the initial consolidation of learnt information.

Spongiform neurodegeneration-associated E3 ligase Mahogunin ubiquitylates TSG101 and regulates endosomal trafficking

A null mutation in the gene encoding the putative E3 ubiquitin-protein ligase Mahogunin causes spongiform neurodegeneration, a recessively transmitted prion-like disease in mice. However, no substrates of Mahogunin have been identified, and the cellular role of Mahogunin is unknown. Here, we report the identification of TSG101, a key component of the endosomal sorting complex required for transport (ESCRT)-I, as a specific Mahogunin substrate. We find that Mahogunin interacts with the ubiquitin E2 variant (UEV) domain of TSG101 via its PSAP motif and that it catalyzes monoubiquitylation of TSG101 both in vivo and in vitro. Depletion of Mahogunin by small interfering RNAs in mammalian cells disrupts endosome-to-lysosome trafficking of epidermal growth factor receptor, resulting in prolonged activation of a downstream signaling cascade. Our findings support a role for Mahogunin in a proteasome-independent ubiquitylation pathway and suggest a link between dysregulation of endosomal trafficking and spongiform neurodegeneration.