Identification of the chicken MARCKS phosphorylation site specific for differentiating neurons as Ser 25 using a monoclonal antibody and mass spectrometry

Functional Diversification of the Four MARCKS Family Members in Zebrafish Neural Development

Myristoylated alanin-rich C-kinase substrate (MARCKS) and MARCKS-like 1, each encoded by a different gene, comprise a very small family of actin-modulating proteins with essential roles in mammalian neural development. We show here that four genes (two marcks and two marcksl1) are present in teleosts including zebrafish, while ancient actinopterigians, sarcopterigian fishes, and chondrichtyans only have two. No marcks genes were found in agnaths or invertebrates. All four zebrafish genes are expressed during development, and we show here how their early knockdown causes defects in neural development, with some phenotypical differences. Knockdown of marcksa generated embryos with smaller brain and eyes, while marcksb caused different morphogenetic defects, such as larger hindbrain ventricle and folded retina. marcksl1a and marcksl1b morpholinos also caused smaller eyes and brain, although marcksl1a alone generated larger brain ventricles. At 24 hpf, marcksb caused a wider angle of the hindbrain walls, while marcksl1a showed a "T-shaped" neural tube and alterations in neuroepithelium organization. The double knockdown surprisingly produced new features, which included an increased neuroepithelial disorganization and partial neural tube duplications evident at 48 hpf, suggesting defects in convergent extension. This disorganization was also evident in the retina, although retinal ganglion cells were still able to differentiate. marcksl1b morphants presented a unique retinal phenotype characterized by the occurrence of sporadic ectopic neuronal differentiation. Although only marcksl1a morphant had a clear "ciliary phenotype," all presented significantly shorter cilia. Altogether, our data show that all marcks genes have functions in zebrafish neural development, with some differences that suggest the onset of protein diversification.

Searching for novel Cdk5 substrates in brain by comparative phosphoproteomics of wild type and Cdk5-/- mice

Protein phosphorylation is the most common post-translational modification that regulates several pivotal functions in cells. Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase which is mostly active in the nervous system. It regulates several biological processes such as neuronal migration, cytoskeletal dynamics, axonal guidance and synaptic plasticity among others. In search for novel substrates of Cdk5 in the brain we performed quantitative phosphoproteomics analysis, isolating phosphoproteins from whole brain derived from E18.5 Cdk5^+/+ and Cdk5^−/− embryos, using an Immobilized Metal-Ion Affinity Chromatography (IMAC), which specifically binds to phosphorylated proteins. The isolated phosphoproteins were eluted and isotopically labeled for relative and absolute quantitation (iTRAQ) and mass spectrometry identification. We found 40 proteins that showed decreased phosphorylation at Cdk5^−/− brains. In addition, out of these 40 hypophosphorylated proteins we characterized two proteins, :MARCKS (Myristoylated Alanine-Rich protein Kinase C substrate) and Grin1 (G protein regulated inducer of neurite outgrowth 1). MARCKS is known to be phosphorylated by Cdk5 in chick neural cells while Grin1 has not been reported to be phosphorylated by Cdk5. When these proteins were overexpressed in N2A neuroblastoma cell line along with p35, serine phosphorylation in their Cdk5 motifs was found to be increased. In contrast, treatments with roscovitine, the Cdk5 inhibitor, resulted in an opposite effect on serine phosphorylation in N2A cells and primary hippocampal neurons transfected with MARCKS. In summary, the results presented here identify Grin 1 as novel Cdk5 substrate and confirm previously identified MARCKS as a a bona fide Cdk5 substrate.

Structural characterization of a neuroblast-specific phosphorylated region of MARCKS

MARCKS (Myristoylated Alanine-Rich C Kinase substrate) is a natively unfolded protein that interacts with actin, Ca(2+)-Calmodulin, and some plasma membrane lipids. Such interactions occur at a highly conserved region that is specifically phosphorylated by PKC: the Effector Domain. There are two other conserved domains, MH1 (including a myristoylation site) and MH2, also located in the amino terminal region and whose structure and putative protein binding capabilities are currently unknown. MH2 sequence contains a serine that we described as being phosphorylated only in differentiating neurons (S25 in chick). Here, Circular Dichroism (CD) and Nuclear Magnetic Resonance (NMR) spectroscopy were used to characterize the phosphorylated and unphosphorylated forms of a peptide with the MARCKS sequence surrounding S25. The peptide phosphorylated at this residue is recognized by monoclonal antibody 3C3 (mAb 3C3). CD and NMR data indicated that S25 phosphorylation does not cause extensive modifications in the peptide structure. However, the sharper lines, the absence of multiple spin systems and relaxation dispersion data observed for the phosphorylated peptide suggested a more ordered structure. Surface Plasmon Resonance was employed to compare the binding properties of mAb 3C3 to MARCKS protein and peptide. SPR showed that mAb 3C3 binds to the whole protein and the peptide with a similar affinity, albeit different kinetics. The slightly ordered structure of the phosphorylated peptide might be at the origin of its ability to interact with mAb 3C3 antibody, but this binding did not noticeably modify the peptide structure.

A novel effect of MARCKS phosphorylation by activated PKC: the dephosphorylation of its serine 25 in chick neuroblasts

MARCKS (Myristoylated Alanine-Rich C Kinase Substrate) is a peripheral membrane protein, especially abundant in the nervous system, and functionally related to actin organization and Ca-calmodulin regulation depending on its phosphorylation by PKC. However, MARCKS is susceptible to be phosphorylated by several different kinases and the possible interactions between these phosphorylations have not been fully studied in intact cells. In differentiating neuroblasts, as well as some neurons, there is at least one cell-type specific phosphorylation site: serine 25 (S25) in the chick. We demonstrate here that S25 is included in a highly conserved protein sequence which is a Cdk phosphorylatable region, located far away from the PKC phosphorylation domain. S25 phosphorylation was inhibited by olomoucine and roscovitine in neuroblasts undergoing various states of cell differentiation in vitro. These results, considered in the known context of Cdks activity in neuroblasts, suggest that Cdk5 is the enzyme responsible for this phosphorylation. We find that the phosphorylation by PKC at the effector domain does not occur in the same molecules that are phosphorylated at serine 25. The in situ analysis of the subcellular distribution of these two phosphorylated MARCKS variants revealed that they are also segregated in different protein clusters. In addition, we find that a sustained stimulation of PKC by phorbol-12-myristate-13-acetate (PMA) provokes the progressive disappearance of phosphorylation at serine 25. Cells treated with PMA, but in the presence of several Ser/Thr phosphatase (PP1, PP2A and PP2B) inhibitors indicated that this dephosphorylation is achieved via a phosphatase 2A (PP2A) form. These results provide new evidence regarding the existence of a novel consequence of PKC stimulation upon the phosphorylated state of MARCKS in neural cells, and propose a link between PKC and PP2A activity on MARCKS.

Two myristoylated alanine-rich C-kinase substrate (MARCKS) paralogs are required for normal development in zebrafish

Myristoylated alanine-rich C-kinase substrate (MARCKS) is an actin binding protein substrate of protein kinase C (PKC) and critical for mouse and Xenopus development. Herein two MARCKS paralogs, marcksa and marcksb, are identified in zebrafish and the role of these genes in zebrafish development is evaluated. Morpholino-based targeting of either MARCKS protein resulted in increased mortality and a range of gross phenotypic abnormalities. Phenotypic abnormalities were classified as mild, moderate or severe, which is characterized by a slight curve of a full-length tail, a severe curve or twist of a full-length tail and a truncated tail, respectively. All three phenotypes displayed abnormal neural architecture. Histopathology of Marcks targeted embryos revealed abnormalities in retinal layering, gill formation and skeletal muscle morphology. These results demonstrate that Marcksa and Marcksb are required for normal zebrafish development and suggest that zebrafish are a suitable model to further study MARCKS function.

Homeostatic Signature of Anabolic Steroids in Cattle Using1H−13C HMBC NMR Metabonomics

We used metabonomics to discriminate the urinary signature of different anabolic steroid treatments in cattle having different physiological backgrounds (age, sex, and race). (1)H-(13)C heteronuclear multiple bonding connectivity NMR spectroscopy and multivariate statistical methods reveal that metabolites such as trimethylamine-N-oxide, dimethylamine, hippurate, creatine, creatinine, and citrate characterize the biological fingerprint of anabolic treatment. These urinary biomarkers suggest an overall homeostatic adaptation in nitrogen and energy metabolism. From results obtained in this study, it is now possible to consider metabonomics as a complementary method usable to improve doping control strategies to detect fraudulent anabolic treatment in cattle since the oriented global metabolic response provides helpful discrimination.

MARCKS in advanced stages of neural retina histogenesis

Myristoylated alanine-rich kinase C substrate (MARCKS), an actin-binding protein, is involved in several signal transduction pathways. It is susceptible to be phosphorylated by protein kinases as protein kinase C and some proline-directed kinases. These phosphorylations differently modulate its functions. We previously showed that a phosphorylation at its Ser25 (S25p-MARCKS) in chickens is a signature of this ubiquitous protein in neuron differentiation. To gain insight into the possible involvement of MARCKS in late retinal histogenesis, we compared the developmental expression patterns of the total protein and its S25p variants. Here we show that the most outstanding modifications occur at the outer retina, where S25p disappears at the end of embryonic development and where MARCKS is missing in adults. These results suggest diverse functional specializations in the different retinal layers.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.