MyoD phosphorylation on multiple C terminal sites regulates myogenic conversion activity

The Presence of Two MyoD Genes in a Subset of Acanthopterygii Fish Is Associated with a Polyserine Insert in MyoD1

The MyoD gene was duplicated during the teleost whole genome duplication and, while a second MyoD gene (MyoD2) was subsequently lost from the genomes of some lineages (including zebrafish), many fish lineages (including Alcolapia species) have retained both MyoD paralogues. Here we reveal the expression patterns of the two MyoD genes in Oreochromis (Alcolapia) alcalica using in situ hybridisation. We report our analysis of MyoD1 and MyoD2 protein sequences from 54 teleost species, and show that O. alcalica, along with some other teleosts, include a polyserine repeat between the amino terminal transactivation domains (TAD) and the cysteine-histidine rich region (H/C) in MyoD1. The evolutionary history of MyoD1 and MyoD2 is compared to the presence of this polyserine region using phylogenetics, and its functional relevance is tested using overexpression in a heterologous system to investigate subcellular localisation, stability, and activity of MyoD proteins that include and do not include the polyserine region.

Palmitic Acid Inhibits Myogenic Activity and Expression of Myosin Heavy Chain MHC IIb in Muscle Cells through Phosphorylation-Dependent MyoD Inactivation

Sarcopenia associated with aging and obesity is characterized by the atrophy of fast-twitch muscle fibers and an increase in intramuscular fat deposits. However, the mechanism of fast-twitch fiber-specific atrophy remains unclear. In this study, we aimed to assess the effect of palmitic acid (PA), the most common fatty acid component of human fat, on muscle fiber type, focusing on the expression of fiber-type-specific myosin heavy chain (MHC). Myotubes differentiated from C2C12 myoblasts were treated with PA. The PA treatment inhibited myotube formation and hypertrophy while reducing the gene expression of MHC IIb and IIx, specific isoforms of fast-twitch fibers. Consistent with this, a significant suppression of MHC IIb protein expression in PA-treated cells was observed. A reporter assay using plasmids containing the MHC IIb gene promoter revealed that the PA-induced reduction in MHC IIb gene expression was caused by the suppression of MyoD transcriptional activity through its phosphorylation. Treatment with a specific protein kinase C (PKC) inhibitor recovered the reduction in MHC IIb gene expression levels in PA-treated cells, suggesting the involvement of the PA-induced activation of PKC. Thus, PA selectively suppresses the mRNA and protein expression of fast-twitch MHC by modulating MyoD activity. This finding provides a potential pathogenic mechanism for age-related sarcopenia.

Commitment of Autologous Human Multipotent Stem Cells on Biomimetic Poly-L-lactic Acid-Based Scaffolds Is Strongly Influenced by Structure and Concentration of Carbon Nanomaterial

Nanocomposite scaffolds combining carbon nanomaterials (CNMs) with a biocompatible matrix are able to favor the neuronal differentiation and growth of a number of cell types, because they mimic neural-tissue nanotopography and/or conductivity. We performed comparative analysis of biomimetic scaffolds with poly-L-lactic acid (PLLA) matrix and three different p-methoxyphenyl functionalized carbon nanofillers, namely, carbon nanotubes (CNTs), carbon nanohorns (CNHs), and reduced graphene oxide (RGO), dispersed at varying concentrations. qRT-PCR analysis of the modulation of neuronal markers in human circulating multipotent cells cultured on nanocomposite scaffolds showed high variability in their expression patterns depending on the scaffolds’ inhomogeneities. Local stimuli variation could result in a multi- to oligopotency shift and commitment towards multiple cell lineages, which was assessed by the qRT-PCR profiling of markers for neural, adipogenic, and myogenic cell lineages. Less conductive scaffolds, i.e., bare poly-L-lactic acid (PLLA)-, CNH-, and RGO-based nanocomposites, appeared to boost the expression of myogenic-lineage marker genes. Moreover, scaffolds are much more effective on early commitment than in subsequent differentiation. This work suggests that biomimetic PLLA carbon-nanomaterial (PLLA-CNM) scaffolds combined with multipotent autologous cells can represent a powerful tool in the regenerative medicine of multiple tissue types, opening the route to next analyses with specific and standardized scaffold features.

Analysis of Phosphorylation Status of Ectopically Expressed Proteins in Early Xenopus Embryos

Xenopus embryos provide a rapid and accessible in vivo model, expressing a plethora of endogenous kinase and phosphatase enzymes that control protein phosphorylation and, in turn, affect physiological function. Traditionally, the detection of protein phosphorylation has been achieved by radioisotope phosphate labeling of proteins, sometimes with in vitro assays using recombinant proteins or with site-specific phospho-antibodies. However, the target phospho-sites and kinases responsible are often unknown, and the use of radioactive isotopes is not always desirable. Therefore, as a first step in determining the functional significance of potential phosphorylation, it is useful to show that a protein can be phosphorylated in vivo in Xenopus eggs and/or embryos. This protocol describes a nonradioactive method to visualize protein phosphorylation by exposing the protein to the egg/embryo kinase environment and then observing a difference in protein migration (as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis [SDS-PAGE] and western blot analysis) with and without treatment with the lambda phosphatase enzyme. Subsequent investigation of the ability of site-specific phospho-mutant proteins to recapitulate the effect of phosphatase treatment can be used to explore the identity of the phosphorylated sites. Moreover, the detection of multiple bands of the protein of interest even after phosphatase treatment points to the presence of other types of posttranslational modifications.

Analysis of Chromatin Binding of Ectopically Expressed Proteins in Early Xenopus Embryos

Xenopus embryos have long been used to show phenotypic effects following overexpression of proteins of interest such as transcription factors. Posttranslational modification of these proteins can dramatically alter the extent of the observed phenotype by inhibiting or enhancing protein activity. To determine the mechanisms controlling transcription factor activity, it is useful to compare relative levels of chromatin-bound protein, as this can reveal altered chromatin association in addition to changes in overall protein accumulation seen in the cytoplasm. Assaying protein binding to the bulk DNA described here compliments alternative assays such as electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) that measure site-specific DNA binding. This protocol describes a method to prepare and analyze chromatin and cytoplasmic extracts from embryos overexpressing proteins of interest, and it uses a robust fractionation procedure that results in clear separation of cytoplasmic tubulin from histone-H3 enriched chromatin. This assay for relative chromatin-bound protein is most suitable for comparing modified forms of a single protein (e.g., to investigate the effects of point mutations on chromatin association). Optimization is required for the specific protein of interest but guide ranges are provided.

Multi-site phosphorylation controls the neurogenic and myogenic activity of E47

The superfamily of basic-Helix-Loop-Helix (bHLH) transcription factors influence cell fate in all three embryonic germ layers, and the tissue-specific class II factors have received prominent attention for their potent ability to direct differentiation during development and in cellular reprogramming. The activity of many class II bHLH proteins driving differentiation, and the inhibitory class VI bHLH factor Hes1, is controlled by phosphorylation on multiple sites by Cyclin-dependent kinases (Cdks). As class II proteins are generally thought to be active through hetero-dimerisation with the ubiquitously expressed class I E proteins, regulation of class I transcription factors such as E47 may influence the activity of multiple tissue-specific bHLH proteins. Using differentiation of nerve and muscle in Xenopus frog embryos as a model system, we set out to explore whether with the ubiquitously expressed class I E protein E47 that hetero-dimerises with Class II bHLHs to control their activity, is also regulated by multi-site phosphorylation. We demonstrate that E47 can be readily phosphorylated by Cdks on multiple sites in vitro, while ectopically-expressed E47 exists in multiple phosphorylated forms in Xenopus embryos. Preventing multi-site phosphorylation using a phospho-mutant version of E47 enhances the neurogenic and myogenic activity of three different class II bHLH reprogramming factors, and also when E47 acts in hetero-dimerisation with endogenous proteins. Mechanistically, unlike phospho-regulation of class II bHLH factors, we find that preventing phosphorylation of E47 increases the amount of chromatin-bound E47 protein but without affecting its overall protein stability. Thus, multi-site phosphorylation is a conserved regulatory mechanism across the bHLH superfamily that can be manipulated to enhance cellular differentiation.

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E47 is phosphorylated in Xenopus embryos on up to 13 conserved SP/TP sites.

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In vitro, E47 protein can be phosphorylated by Cyclin-dependent-kinases.

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Under-phosphorylated E47 enhances activity of 3 bHLH reprogramming factors.

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Under-phosphorylated E47 enhances neuronal differentiation by hetero-dimerisation.

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E47 phospho-status alters chromatin binding but not overall protein stability.

N-terminal phosphorylation of xHes1 controls inhibition of primary neurogenesis in Xenopus

The processes of cell proliferation and differentiation are intimately linked during embryogenesis, and the superfamily of (basic) Helix-Loop-Helix (bHLH) transcription factors play critical roles in these events. For example, neuronal differentiation is promoted by class II bHLH proneural proteins such as Ngn2 and Ascl1, while class VI Hes proteins act to restrain differentiation and promote progenitor maintenance. We have previously described multi-site phosphorylation as a key regulator of tissue specific class II bHLH proteins in all three embryonic germ layers, and this enables coordination of differentiation with the cell cycle. Hes1 homologues also show analogous conserved proline directed kinase sites. Here we have used formation of Xenopus primary neurons to investigate the effects of xHes1 multi-site phosphorylation on both endogenous and ectopic proneural protein-induced neurogenesis. We find that xHes1 is phosphorylated in vivo, and preventing phosphorylation on three conserved SP/TP sites in the N terminus of the protein enhances xHes1 protein stability and repressor activity. Mechanistically, compared to wild-type xHes1, phospho-mutant xHes1 exhibits greater repression of Ngn2 transcription as well as producing a greater reduction in Ngn2 protein stability and chromatin binding. We propose that cell cycle dependent phosphorylation of class VI Hes proteins may act alongside similar regulation of class II bHLH proneural proteins to co-ordinate their activity.

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xHes1 is phosphorylated in Xenopus embryos on conserved N terminal SP/TP sites.

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In vitro, xHes1 protein can be phosphorylated by Cyclin-dependent-kinases.

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Under-phosphorylated xHes1 has increased protein stability relative to WT xHes1.

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Under-phosphorylated xHes1 has enhanced inhibitory activity during neurogenesis.

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xHes1 reduces Ngn2 expression, protein stability and chromatin binding.

Interaction between opposing modes of phospho-regulation of the proneural proteins Ascl1 and Ngn2

From the relatively simple nervous system of Drosophila to the elaborate mammalian cortex, neurogenesis requires exceptional spatial and temporal precision to co-ordinate progenitor cell proliferation and subsequent differentiation to a diverse range of neurons and glia. A limited number of transiently expressed proneural basic-helix-loop-helix (bHLH) transcription factors, for example achaete-scute-complex (as-c) and atonal (ato) in Drosophila and the vertebrate homologues Ascl1 and Neurogenin2 (Ngn2), are able to orchestrate the onset of neuronal determination, context-dependent subtype selection and even influence later aspects of neuronal migration and maturation. Within the last decade, two models have emerged to explain how the temporal activity of proneural determination factors is regulated by phosphorylation at distinct sites. One model describes how cell-cycle associated phosphorylation on multiple sites in the N and C termini of vertebrate proneural proteins limits neuronal differentiation in cycling progenitor cells. A second model describes phosphorylation on a single site in the bHLH domain of Drosophila atonal that acts as a binary switch, where phosphorylation terminates proneural activity. Here we combine activating mutations of phosphorylation sites in the N- and C- termini with an inhibitory phospho-mimetic mutation in the bHLH domain of Ascl1 and Ngn2 proteins, and test their functions in vivo using Xenopus embryos to determine which mode of phospho-regulation dominates. Enhancing activity by preventing N- and C terminal phosphorylation cannot overcome the inhibitory effect of mimicking phosphorylation of the bHLH domain. Thus we have established a hierarchy between these two modes of proneural protein control and suggest a model of temporal regulation for proneural protein activity.

Cell cycle-dependent phosphorylation and regulation of cellular differentiation

Embryogenesis requires an exquisite regulation of cell proliferation, cell cycle withdrawal and differentiation into a massively diverse range of cells at the correct time and place. Stem cells also remain to varying extents in different adult tissues, acting in tissue homeostasis and repair. Therefore, regulated proliferation and subsequent differentiation of stem and progenitor cells remains pivotal throughout life. Recent advances have characterised the cell cycle dynamics, epigenetics, transcriptome and proteome accompanying the transition from proliferation to differentiation, revealing multiple bidirectional interactions between the cell cycle machinery and factors driving differentiation. Here, we focus on a direct mechanistic link involving phosphorylation of differentiation-associated transcription factors by cell cycle-associated Cyclin-dependent kinases. We discuss examples from the three embryonic germ layers to illustrate this regulatory mechanism that co-ordinates the balance between cell proliferation and differentiation.

All in the family: proneural bHLH genes and neuronal diversity

Proneural basic Helix-Loop-Helix (bHLH) proteins are required for neuronal determination and the differentiation of most neural precursor cells. These transcription factors are expressed in vastly divergent organisms, ranging from sponges to primates. Here, we review proneural bHLH gene evolution and function in the Drosophila and vertebrate nervous systems, arguing that the Drosophila gene atonal provides a useful platform for understanding proneural gene structure and regulation. We also discuss how functional equivalency experiments using distinct proneural genes can reveal how proneural gene duplication and divergence are interwoven with neuronal complexity.