Studying Protein Function and the Role of Altered Protein Expression by Antibody Interference and Three-dimensional Reconstructions

A strict management of protein expression is not only essential to every organism alive, but also an important strategy to investigate protein functions ...

University of Erlangen-Nuremberg

A strict regulation of protein expression during developmental stages and in response to environmental signals is essential to every cell and organism. Recent research has shown that the mammalian brain is particularly sensitive to alterations in expression patterns of specific proteins and cognitive deficits as well as autistic behaviours have been linked to dysregulated protein expression. An intellectual disability characterised by changes in the expression of a variety of proteins is the fragile X syndrome. Due to the loss of a single mRNA binding protein, the Fragile X Mental Retardation Protein FMRP, vast misregulation of the mRNA metabolism is taking place in the disease. Here, we present the identification and characterisation of a novel protein named Simiate, whose mRNA contains several FMRP recognition motifs and associates with FMRP upon co-precipitation. Sequence analysis revealed that the protein evolved app. 1.7 billion years ago when eukaryotes developed. Applying antibodies generated against Simiate, the protein is detected in a variety of tissues, including the mammalian brain. On the subcellular level, Simiate localises to somata and nuclear speckles. We show that Simiate and nuclear speckles experience specific alterations in FMR1(-/-) mice. An antibody-based block of endogenous Simiate revealed that the protein is essential for cell survival. These findings suggest not only an important role for Simiate in gene transcription and/or RNA splicing, but also provide evidence for a function of nuclear speckles in the fragile X syndrome. Indeed, transcription and splicing are two fundamental mechanisms to control protein expression, that underlie not only synaptic plasticity and memory formation, but are also affected in several diseases associated with mental disabilities.

Identification and characterisation of Simiate, a novel protein linked to the fragile X syndrome.

The Actin cytoskeleton constitutes the functional base for a multitude of cellular processes extending from motility and migration to cell mechanics and morphogenesis. The latter is particularly important to neuronal cells since the accurate functioning of the brain crucially depends on the correct arborization of neurons, a process that requires the formation of several dozens to hundreds of dendritic branches. Recently, a model was proposed where different transcription factors are detailed to distinct facets and phases of dendritogenesis and exert their function by acting on the Actin cytoskeleton, however, the proteins involved as well as the underlying molecular mechanisms are largely unknown. Here, we demonstrate that Simiate, a protein previously indicated to activate transcription, directly associates with both, G- and F-Actin and in doing so, affects Actin polymerization and Actin turnover in living cells. Imaging studies illustrate that Simiate particularly influences filopodia dynamics and specifically increases the branching of proximal, but not distal dendrites of developing neurons. The data suggests that Simiate functions as a direct molecular link between transcription regulation on one side, and dendritogenesis on the other, wherein Simiate serves to coordinate the development of proximal and distal dendrites by acting on the Actin cytoskeleton of filopodia and on transcription regulation, hence supporting the novel model.

Simiate is an Actin binding protein involved in filopodia dynamics and arborization of neurons.

Chromatin organization and the management of transcription and splicing are fundamental to the correct functioning of every cell but, in particular, for highly active cells such as photoreceptors, the sensory neurons of the retina. Rod photoreceptor cells of nocturnal animals have recently been shown to have an inverted chromatin architecture compared with rod photoreceptor cells of diurnal animals. The heterochromatin is concentrated in the center of the nucleus, whereas the genetically active euchromatin is positioned close to the nuclear membrane. This unique chromatin architecture suggests that the transcription and splicing machinery is also subject to specific adaptations in these cells. Recently, we described the protein Simiate, which is enriched in nuclear speckles and seems to be involved in transcription and splicing processes. Here, we examine the distribution of Simiate and nuclear speckles in neurons of mouse retinae. In retinal neurons of the inner nuclear and ganglion cell layer, Simiate is concentrated in a clustered pattern in the nuclear interior, whereas in rod and cone photoreceptor cells, Simiate is present at the nuclear periphery. Further staining with markers for the transcription and splicing machinery has confirmed the localization of nuclear speckle components at the periphery. Comparing the distribution of nuclear speckles in retinae of the nocturnal mouse with the diurnal degu, we found no differences in the arrangement of the transcription and splicing machinery in their photoreceptor cells, thus suggesting that the organization of these machineries is not related to the animal's lifestyle but rather represents a general characteristic of photoreceptor organization and function.

Kristin Derlig

Institute for Biochemistry,
Emil-Fischer Centre,
Institute for Biochemistry, Emil-Fischer Centre

University of Erlangen-Nuremberg

Studying Protein Function and the Role of Altered Protein Expression by Antibody Interference and Three-dimensional Reconstructions

Kristin Derlig

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Institute for Biochemistry,Emil-Fischer Centre,Institute for Biochemistry, Emil-Fischer Centre

University of Erlangen-Nuremberg

Studying Protein Function and the Role of Altered Protein Expression by Antibody Interference and Three-dimensional Reconstructions

Institute for Biochemistry,
Emil-Fischer Centre,
Institute for Biochemistry, Emil-Fischer Centre