Regulation of PP2A activity by Mid1 controls cranial neural crest speed and gangliogenesis

Morphometric analysis of the size-adjusted linear dimensions of the skull landmarks revealed craniofacial dysmorphology in Mid1-cKO mice

BACKGROUND

The early craniofacial development is a highly coordinated process involving neural crest cell migration, proliferation, epithelial apoptosis, and epithelial-mesenchymal transition (EMT). Both genetic defects and environmental factors can affect these processes and result in orofacial clefts. Mutations in MID1 gene cause X-linked Opitz Syndrome (OS), which is a congenital malformation characterized by craniofacial defects including cleft lip/palate (CLP). Previous studies demonstrated impaired neurological structure and function in Mid1 knockout mice, while no CLP was observed. However, given the highly variable severities of the facial manifestations observed in OS patients within the same family carrying identical genetic defects, subtle craniofacial malformations in Mid1 knockout mice could be overlooked in these studies. Therefore, we propose that a detailed morphometric analysis should be necessary to reveal mild craniofacial dysmorphologies that reflect the similar developmental defects seen in OS patients.

RESULTS

In this research, morphometric study of the P0 male Mid1-cKO mice were performed using Procrustes superimposition as well as EMDA analysis of the size-adjusted three-dimensional coordinates of 105 skull landmarks, which were collected on the bone surface reconstructed using microcomputed tomographic images. Our results revealed the craniofacial deformation such as the increased dimension of the frontal and nasal bone in Mid1-cKO mice, in line with the most prominent facial features such as hypertelorism, prominent forehead, broad and/or high nasal bridge seen in OS patients.

CONCLUSION

 While been extensively used in evolutionary biology and anthropology in the last decades, geometric morphometric analysis was much less used in developmental biology. Given the high interspecies variances in facial anatomy, the work presented in this research suggested the advantages of morphometric analysis in characterizing animal models of craniofacial developmental defects to reveal phenotypic variations and the underlining pathogenesis.

MID1 and MID2 regulate cell migration and epithelial-mesenchymal transition via modulating Wnt/β-catenin signaling

Background

The ubiquitin E3 ligase activity has been ascribed to MID1, the causative gene of X-linked OS, and its homologue, MID2. Both alpha4, the common MID protein partner, and PP2Ac in MID-alpha4-PP2Ac complexes can be ubiquitylated. Ubiquitylation of alpha4 converted its function toward PP2Ac from protective to destructive, while PP2A also affected MID protein phosphorylation and their subsequent trafficking on microtubules. It was believed that disruption of the function of MID1-alpha4-PP2A complex was vital to the pathogenesis of craniofacial malformation, the most prominent clinical manifestation of OS, although the detailed molecular mechanisms was not unravelled.

Methods

The cellular level of PP2A and phosphor-PP2A in cells overexpressing MID1/MID2 or in cells with siRNA mediated MID1/MID2 gene silencing was analyzed using Western blot. The Wnt signaling in these cells was further monitored using TCF/LEF luciferase reporter assay and the cellular level of β-catenin was also verified using western blot. Given the crosstalk of E-cadherin and Wnt via the common effector β-catenin, the potential influences of MID1/MID2 on the cell migration and epithelial-mesenchymal transition (EMT) were investigated using wound healing assay and immunofluorescence for E-cadherin and vimentin, respectively.

Results

Here, we presented the increased phosphorylation of PP2Ac in cells overexpressing MID1/MID2, and vice versa, in vitro, while the cellular level of total PP2Ac was unaffected. In addition, β-catenin, the effector of canonical Wnt signaling, was downregulated in cells overexpressing MID1/MID2 and upregulated in cells with siRNA mediated MID1/MID2 gene silencing. Down-regulated Wnt/β-catenin signaling by Okadaic acid, a specific inhibitor of PP2A, was partially rescued by siRNA mediated MID1/MID2 gene silencing. In consistent, an activated EMT and accelerated cell migration in cells with MID1/MID2 gene silencing were observed, and vice versa.

Conclusions

The results in this study indicated roles for MID1 and MID2 in regulating cell migration/EMT via modulating Wnt/β-catenin signaling, which might help to understand the molecular etiology of the facial abnormalities that are usually the consequences of defective neural crest cells migration and EMT at the early stage of craniofacial development.

The MID1 gene product in physiology and disease

MID1 is an E3 ubiquitin ligase of the Tripartite Motif (TRIM) subfamily of RING-containing proteins, hence also known as TRIM18. MID1 is a microtubule-binding protein found in complex with the catalytic subunit of PP2A (PP2Ac) and its regulatory subunit alpha 4 (α4). To date, several substrates and interactors of MID1 have been described, providing evidence for the involvement of MID1 in a plethora of essential biological processes, especially during embryonic development. Mutations in the MID1 gene are responsible of the X-linked form of Opitz syndrome (XLOS), a multiple congenital disease characterised by defects in the development of midline structures during embryogenesis. Here, we review MID1-related physiological mechanisms as well as the pathological implication of the MID1 gene in XLOS and in other clinical conditions.

Phenotypic spectrum associated with SPECC1L pathogenic variants: new families and critical review of the nosology of Teebi, Opitz GBBB, and Baraitser-Winter syndromes

The SPECC1L protein plays a role in adherens junctions involved in cell adhesion, actin cytoskeleton organization, microtubule stabilization, spindle organization and cytokinesis. It modulates PI3K-AKT signaling and controls cranial neural crest cell delamination during facial morphogenesis. SPECC1L causative variants were first identified in individuals with oblique facial clefts. Recently, causative variants in SPECC1L were reported in a pedigree reported in 1988 as atypical Opitz GBBB syndrome. Six families with SPECC1L variants have been reported thus far. We report here eight further pedigrees with SPECC1L variants, including a three-generation family, and a further individual of a previously published family. We discuss the nosology of Teebi and GBBB, and the syndromes related to SPECC1L variants. Although the phenotype of individuals with SPECC1L mutations shows overlap with Opitz syndrome in its craniofacial anomalies, the canonical laryngeal malformations and male genital anomalies are not observed. Instead, individuals with SPECCL1 variants have branchial fistulae, omphalocele, diaphragmatic hernias, and uterus didelphis. We also point to the clinical overlap of SPECC1L syndrome with mild Baraitser-Winter craniofrontofacial syndrome: they share similar dysmorphic features (wide, short nose with a large tip, cleft lip and palate, blepharoptosis, retrognathia, and craniosynostosis), although intellectual disability, neuronal migration defect, and muscular problems remain largely specific to Baraitser-Winter syndrome. In conclusion, we suggest that patients with pathogenic variants in SPECC1L should not be described as "dominant (or type 2) Opitz GBBB syndrome", and instead should be referred to as "SPECC1L syndrome" as both disorders show distinctive, non overlapping developmental anomalies beyond facial communalities.

Protein phosphatase 2A regulation of markers of extracellular matrix remodelling in hepatocellular carcinoma cells: functional consequences for tumour invasion

BACKGROUND AND PURPOSE

A hallmark of tumour invasion is breakdown of the extracellular matrix due to dysregulation of the matrix metalloproteinase (MMP) system. While our understanding of how this is regulated by kinase signalling pathways is well established, its counter-regulation by protein phosphatases (PP) is poorly understood. Therefore, we investigated the effect of PP inhibition on markers of extracellular remodelling and how PP2A activity modulated MMP-9 abundance and function of Hep3B cells.

EXPERIMENTAL APPROACH

Cells were exposed to okadaic acid (OA), tautomycetin and cyclosporin A, and the expression profile determined using PCR. Effects of OA and a protein inhibitor of PP2A, CIP2A, on MMP-9 abundance, PP2A activity and cell migration were investigated using ELISA, promoter constructs, siRNA knockdown and transwell migration assays.

KEY RESULTS

OA increased expression and abundance of MMP-9 and the tissue inhibitor of MMP, TIMP-1, without affecting other MMPs, TIMPs and ADAMs. The effect on MMP-9 was mimicked by CIP2A overexpression and knockdown of the PPP2CA catalytic, but not PPP2R1A scaffolding, subunit. Cyclosporin A and PPP1CA silencing did not alter MMP-9 expression, while tautomycetin transiently increased it. Mutation of AP-1, but not NF-κB, binding sites inhibited OA-mediated MMP-9 transcriptional activity. OA and CIP2A decreased PP2A activity and increased cell migration.

CONCLUSION AND IMPLICATIONS

OA increased MMP-9 by decreasing PP2A activity and PP2Ac, through AP-1 binding sites on the MMP-9 promoter. The functional consequence of this and CIP2A overexpression was increased cell migration. Hence, PP2A inhibition induced a metastatic phenotype through alterations in MMP-9 in Hep3B cells.

Brg1 coordinates multiple processes during retinogenesis and is a tumor suppressor in retinoblastoma

Retinal development requires precise temporal and spatial coordination of cell cycle exit, cell fate specification, cell migration and differentiation. When this process is disrupted, retinoblastoma, a developmental tumor of the retina, can form. Epigenetic modulators are central to precisely coordinating developmental events, and many epigenetic processes have been implicated in cancer. Studying epigenetic mechanisms in development is challenging because they often regulate multiple cellular processes; therefore, elucidating the primary molecular mechanisms involved can be difficult. Here we explore the role of Brg1 (Smarca4) in retinal development and retinoblastoma in mice using molecular and cellular approaches. Brg1 was found to regulate retinal size by controlling cell cycle length, cell cycle exit and cell survival during development. Brg1 was not required for cell fate specification but was required for photoreceptor differentiation and cell adhesion/polarity programs that contribute to proper retinal lamination during development. The combination of defective cell differentiation and lamination led to retinal degeneration in Brg1-deficient retinae. Despite the hypocellularity, premature cell cycle exit, increased cell death and extended cell cycle length, retinal progenitor cells persisted in Brg1-deficient retinae, making them more susceptible to retinoblastoma. ChIP-Seq analysis suggests that Brg1 might regulate gene expression through multiple mechanisms.

Summary: The SWI/SNF protein Brg1 controls cell cycle length, cell cycle exit and cell survival, and is required for cell differentiation and retinal lamination, in the developing mouse retina.

Protein phosphatase 2A plays an important role in migration of bone marrow stroma cells

Administration of bone marrow stroma cells (BMSCs) has the potential to ameliorate degenerative disorders and to repair injured sites. The homing of transplanted BMSCs to damaged tissues is a critical property of engraftment. Therefore, it is important to understand signal molecules controlling migration of BMSCs. Here, we demonstrate that serine-threonine protein phosphatase 2A (PP2A) is responsive to migration of BMSCs. Pharmacological Inhibition of PP2A, using okadaic acid (OA), leads to attenuated cell migration in rat primary BMSCs both in the absence or presence of stromal cell-derived factor-1 (SDF-1). Consistent with the above findings, knockdown of the main catalytic subunit PP2Acα using small interfering RNA also attenuates chemotaxis of BMSCs. On the other hand, cell viability of BMSCs remains unchanged with OA treatment or knockdown of PP2Acα subunit. Moreover, we observed an upregulation of PP2A-B55β in transcription level after SDF-1 treatment, indicating their potential role as the functioning regulatory subunit of PP2A phosphatase in BMSCs migration model. Collectively, these data provide first insight into the modulation of BMSCs migration by PP2A phosphatase activity and lay a foundation for exploring PP2A signaling as a modulating target for BMSCs transplantation.

Mid1/Mid2 expression in craniofacial development and a literature review of X-linked opitz syndrome

Background

Opitz syndrome (OS) is a genetic disorder that affects mainly the development of midline structures, including the craniofacial region, embryonic heart, and urogenital system. The manifestations of X‐linked OS are believed to be results of a malfunctioned gene, MID1, whose product has been shown to have ubiquitin E3 ligase activity and regulate the turnover of microtubular protein phosphatase 2Ac. MID2, a homolog of MID1, shares high structural and functional similarities with MID1. Identification of a missense mutation in MID2 in an Indian family causing overlapping phenotypes with OS provided the first evidence that MID2 might be involved in similar pathogenesis.

Methods

The clinic features and the genetic findings of all reported X‐linked OS were collectively summarized in this research. Real‐time RT‐PCR and in situ hybridization were used in the expression studies of Mid1/Mid2 in mouse embryos.

Results

Up‐to‐date, 88 different mutations have been identified in MID1 and most mutations occurred on the conserved amino acids of MID1 and MID2. Expression studies using real‐time RT‐PCR implicated a tendency of a mutually repressive expression pattern between Mid1 and Mid2 in mouse embryos. Further investigations using in situ hybridization revealed strong expressions of Mid1 and Mid2 in the epithelium of approaching facial prominences and downregulated expressions after fusion in mouse embryos.

Conclusions

Our results support the hypothesis of functional redundancy of Mid1/Mid2 and their potential roles in regulating tissue remodelling in early development.

The ubiquitin E3 ligase NOSIP modulates protein phosphatase 2A activity in craniofacial development

Holoprosencephaly is a common developmental disorder in humans characterised by incomplete brain hemisphere separation and midface anomalies. The etiology of holoprosencephaly is heterogeneous with environmental and genetic causes, but for a majority of holoprosencephaly cases the genes associated with the pathogenesis could not be identified so far. Here we report the generation of knockout mice for the ubiquitin E3 ligase NOSIP. The loss of NOSIP in mice causes holoprosencephaly and facial anomalies including cleft lip/palate, cyclopia and facial midline clefting. By a mass spectrometry based protein interaction screen we identified NOSIP as a novel interaction partner of protein phosphatase PP2A. NOSIP mediates the monoubiquitination of the PP2A catalytic subunit and the loss of NOSIP results in an increase in PP2A activity in craniofacial tissue in NOSIP knockout mice. We conclude, that NOSIP is a critical modulator of brain and craniofacial development in mice and a candidate gene for holoprosencephaly in humans.

Migratory neural crest cell αN-catenin impacts chick trigeminal ganglia formation

Neural crest cells are an embryonic cell population that is crucial for proper vertebrate development. Initially localized to the dorsal neural folds, premigratory neural crest cells undergo an epithelial-to-mesenchymal transition (EMT) and migrate to their final destinations in the developing embryo. Together with epidermally-derived placode cells, neural crest cells then form the cranial sensory ganglia of the peripheral nervous system. Our prior work has shown that αN-catenin, the neural subtype of the adherens junction α-catenin protein, regulates cranial neural crest cell EMT by controlling premigratory neural crest cell cadherin levels. Although αN-catenin down-regulation is critical for initial neural crest cell EMT, a potential role for αN-catenin in later neural crest cell migration, and formation of the cranial ganglia, has not been examined. In this study, we show for the first time that migratory neural crest cells that will give rise to the cranial trigeminal ganglia express αN-catenin and Cadherin-7. αN-catenin loss- and gain-of-function experiments reveal effects on the migratory neural crest cell population that include subsequent defects in trigeminal ganglia assembly. Moreover, αN-catenin perturbation in neural crest cells impacts the placode cell contribution to the trigeminal ganglia and also changes neural crest cell Cadherin-7 levels and localization. Together, these results highlight a novel function for αN-catenin in migratory neural crest cells that form the trigeminal ganglia.

Neural crest and placode interaction during the development of the cranial sensory system

In the vertebrate head, the peripheral components of the sensory nervous system are derived from two embryonic cell populations, the neural crest and cranial sensory placodes. Both arise in close proximity to each other at the border of the neural plate: neural crest precursors abut the future central nervous system, while placodes originate in a common preplacodal region slightly more lateral. During head morphogenesis, complex events organise these precursors into functional sensory structures, raising the question of how their development is coordinated. Here we review the evidence that neural crest and placode cells remain in close proximity throughout their development and interact repeatedly in a reciprocal manner. We also review recent controversies about the relative contribution of the neural crest and placodes to the otic and olfactory systems. We propose that a sequence of mutual interactions between the neural crest and placodes drives the coordinated morphogenesis that generates functional sensory systems within the head.

Metformin anti-tumor effect via disruption of the MID1 translational regulator complex and AR downregulation in prostate cancer cells

BACKGROUND

Metformin is an approved drug prescribed for diabetes. Its role as an anti-cancer agent has drawn significant attention because of its minimal side effects and low cost. However, its mechanism of anti-tumour action has not yet been fully clarified.

METHODS

The effect on cell growth was assessed by cell counting. Western blot was used for analysis of protein levels, Boyden chamber assays for analyses of cell migration and co-immunoprecipitation (CoIP) followed by western blot, PCR or qPCR for analysis of protein-protein and protein-mRNA interactions.

RESULTS

Metformin showed an anti-proliferative effect on a wide range of prostate cancer cells. It disrupted the AR translational MID1 regulator complex leading to release of the associated AR mRNA and subsequently to downregulation of AR protein in AR positive cell lines. Inhibition of AR positive and negative prostate cancer cells by metformin suggests involvement of additional targets. The inhibitory effect of metformin was mimicked by disruption of the MID1-α4/PP2A protein complex by siRNA knockdown of MID1 or α4 whereas AMPK activation was not required.

CONCLUSIONS

Findings reported herein uncover a mechanism for the anti-tumor activity of metformin in prostate cancer, which is independent of its anti-diabetic effects. These data provide a rationale for the use of metformin in the treatment of hormone naïve and castration-resistant prostate cancer and suggest AR is an important indirect target of metformin.

The E3 ubiquitin ligase midline 1 promotes allergen and rhinovirus-induced asthma by inhibiting protein phosphatase 2A activity

Allergic airway inflammation is associated with activation of innate immune pathways by allergens. Acute exacerbations of asthma are commonly associated with rhinovirus infection. Here we show that, after exposure to house dust mite (HDM) or rhinovirus infection, the E3 ubiquitin ligase midline 1 (MID1) is upregulated in mouse bronchial epithelium. HDM regulates MID1 expression in a Toll-like receptor 4 (TLR4)- and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-dependent manner. MID1 decreases protein phosphatase 2A (PP2A) activity through association with its catalytic subunit PP2Ac. siRNA-mediated knockdown of MID1 or pharmacological activation of PP2A using a nonphosphorylatable FTY720 analog in mice exposed to HDM reduces airway hyperreactivity and inflammation, including the expression of interleukin-25 (IL-25), IL-33 and CCL20, IL-5 and IL-13 release, nuclear factor (NF)κB activity, p38 mitogen-activated protein kinase (MAPK) phosphorylation, accumulation of eosinophils, T lymphocytes and myeloid dendritic cells, and the number of mucus-producing cells. MID1 inhibition also limited rhinovirus-induced exacerbation of allergic airway disease. We found that MID1 was upregulated in primary human bronchial epithelial cells upon HDM or rhinovirus exposure, and this correlated with TRAIL and CCL20 expression. Together, these findings identify a key role of MID1 in allergic airway inflammation and links innate immune pathway activation to the development and exacerbation of asthma.

Paladin is an antiphosphatase that regulates neural crest cell formation and migration

Although a network of transcription factors that specifies neural crest identity in the ectoderm has been defined, expression of neural crest transcription factors does not guarantee eventual migration as a neural crest cell. While much work has gone into determining regulatory relationships within the transcription factor network, the ability of protein modifications like phosphorylation to modulate the function of neural crest regulatory factors and determine when and where they are active also has crucial implications. Paladin, which was previously classified as a phosphatase based on sequence similarity, is expressed in chick neural crest precursors and is maintained throughout their epithelial to mesenchymal transition and migration. Loss of Paladin delays the expression of transcription factors Snail2 and Sox10 in premigratory neural crest cells, but does not affect accumulation of FoxD3, Cad6B or RhoB, indicating that Paladin differentially modulates the expression of genes previously thought to be coregulated within the neural crest gene regulatory network. Both gain and loss of Paladin function result in disrupted neural crest migration, reinforcing the importance of precisely regulated phosphorylation for neural crest migration. Mutation of critical, catalytic cysteine residues within Paladin's predicted phosphatase active site motifs did not abolish the function of Paladin in the neural crest. Collectively, these data indicate that Paladin is an antiphosphatase that modulates the activity of specific neural crest regulatory factors during neural crest development. Our work identifies a novel regulator of phosphorylation status that provides an additional layer of regulation in the neural crest.