Number of nucleoli and coiled bodies and distribution of fibrillar centres in differentiating Purkinje neurons of chick and rat cerebellum

The coilin N-terminus mediates multivalent interactions between coilin and Nopp140 to form and maintain Cajal bodies

Cajal bodies (CBs) are ubiquitous nuclear membraneless organelles (MLOs) that concentrate and promote efficient biogenesis of snRNA-protein complexes involved in splicing (snRNPs). Depletion of the CB scaffolding protein coilin disperses snRNPs, making CBs a model system for studying the structure and function of MLOs. Although it is assumed that CBs form through condensation, the biomolecular interactions responsible remain elusive. Here, we discover the unexpected capacity of coilin’s N-terminal domain (NTD) to form extensive fibrils in the cytoplasm and discrete nuclear puncta in vivo. Single amino acid mutational analysis reveals distinct molecular interactions between coilin NTD proteins to form fibrils and additional NTD interactions with the nuclear Nopp140 protein to form puncta. We provide evidence that Nopp140 has condensation capacity and is required for CB assembly. From these observations, we propose a model in which coilin NTD–NTD mediated assemblies make multivalent contacts with Nopp140 to achieve biomolecular condensation in the nucleus.

Cajal bodies are membraneless organelles scaffolded by coilin protein. Here, coilin–coilin and coilin–Nopp140 interaction sites are identified and perturbed, revealing coilin’s capacity to form long fibrils and be remodeled into spherical structures.

A Ribosomal Perspective on Neuronal Local Protein Synthesis

Continued mRNA translation and protein production are critical for various neuronal functions. In addition to the precise sorting of proteins from cell soma to distant locations, protein synthesis allows a dynamic remodeling of the local proteome in a spatially variable manner. This spatial heterogeneity of protein synthesis is shaped by several factors such as injury, guidance cues, developmental cues, neuromodulators, and synaptic activity. In matured neurons, thousands of synapses are non-uniformly distributed throughout the dendritic arbor. At any given moment, the activity of individual synapses varies over a wide range, giving rise to the variability in protein synthesis. While past studies have primarily focused on the translation factors or the identity of translated mRNAs to explain the source of this variation, the role of ribosomes in this regard continues to remain unclear. Here, we discuss how several stochastic mechanisms modulate ribosomal functions, contributing to the variability in neuronal protein expression. Also, we point out several underexplored factors such as local ion concentration, availability of tRNA or ATP during translation, and molecular composition and organization of a compartment that can influence protein synthesis and its variability in neurons.

Activity dependent LoNA regulates translation by coordinating rRNA transcription and methylation

The ribosome is indispensable for precisely controlling the capacity of protein synthesis. However, how translational machinery is coordinated to meet the translational demands remains elusive. Here, we identify a nucleolar-specific lncRNA (LoNA), its 5' portion binds and sequesters nucleolin to suppress rRNA transcription, and its snoRNA like 3' end recruits and diminishes fibrillarin activity to reduce rRNA methylation. Activity-dependent decrease of LoNA leads to elevated rRNA and ribosome levels, an increased proportion of polysomes, mRNA polysome loading, and protein translation. In addition, transport of ribosomes to synapses is particularly promoted, resulting in increased levels of AMPA/NMDA receptor, enhanced synaptic plasticity, long-term potentiation and consolidated memory. Strikingly, hippocampal LoNA deficiency not only enhances long-term memory in WT mice, but also restores impaired memory function in APP/PS1 transgenic mice. Together, these findings reveal the multifaceted role of LoNA in modulating ribosome biogenesis to meet the translational demands of long-term memory.

Cajal bodies in neurons

Cajal is commonly regarded as the father of modern neuroscience in recognition of his fundamental work on the structure of the nervous system. But Cajal also made seminal contributions to the knowledge of nuclear structure in the early 1900s, including the discovery of the "accessory body" later renamed "Cajal body" (CB). This important nuclear structure has emerged as a center for the assembly of ribonucleoproteins (RNPs) required for splicing, ribosome biogenesis and telomere maintenance. The modern era of CB research started in the 1990s with the discovery of coilin, now known as a scaffold protein of CBs, and specific probes for small nuclear RNAs (snRNAs). In this review, we summarize what we have learned in the recent decades concerning CBs in post-mitotic neurons, thereby ruling out dynamic changes in CB functions during the cell cycle. We show that CBs are particularly prominent in neurons, where they frequently associate with the nucleolus. Neuronal CBs are transcription-dependent nuclear organelles. Indeed, their number dynamically accommodates to support the high neuronal demand for splicing and ribosome biogenesis required for sustaining metabolic and bioelectrical activity. Mature neurons have canonical CBs enriched in coilin, survival motor neuron protein and snRNPs. Disruption and loss of neuronal CBs associate with severe neuronal dysfunctions in several neurological disorders such as motor neuron diseases. In particular, CB depletion in motor neurons seems to reflect a perturbation of transcription and splicing in spinal muscular atrophy, the most common genetic cause of infant mortality.

Nucleocytoplasmic protein shuttling: the direct route in synapse-to-nucleus signaling

In neurons multiple signaling pathways converge in the nucleus to regulate the expression of genes associated with long-term structural changes of synapto-dendritic input. Of pivotal importance for this type of transcriptional regulation is synapse-to-nucleus communication. Several studies suggest that the nuclear transport of proteins from synapses is involved in this signaling process, including evidence that synapses contain proteins with nuclear localization sequences and components of the nuclear import machinery. Here, we review the evidence for synapse-to-nucleus signaling by means of retrograde transport of proteins from distal processes. We discuss the mechanisms involved in their translocation and their role in the control of nuclear gene expression. Finally, we summarize the current thinking regarding the functional implications of nuclear signaling and address open questions in this evolving area of neuroscience.

Cajal's contribution to the knowledge of the neuronal cell nucleus

In 1906, the Spanish neurobiologist Santiago Ramón y Cajal was awarded the Nobel Prize in Physiology or Medicine in recognition of his work on the structure of neurons and their connections. Cajal is commonly regarded as the father of modern neuroscience. What is less well known is that Cajal also had a great interest in intracellular neuronal structures and developed the reduced silver nitrate method for the study of neurofibrils (neurofilaments) and nuclear subcompartments. It was in 1903 that Cajal discovered the "accessory body" ("Cajal body") and seven years later, published an article on the organization of the cell nucleus in mammalian neurons that represents a masterpiece of nuclear structure at the light microscopy level. In addition to the accessory body, it includes the analysis of several nuclear components currently recognized as fibrillar centers of the nucleolus, nuclear speckles of splicing factors, transcription foci, nuclear matrix, and the double nuclear membrane. The aim of this article is to revisit Cajal's contributions to the knowledge of the neuronal nucleus in light of our current understanding of nuclear structure and function.

Activity-dependent AIDA-1 nuclear signaling regulates nucleolar numbers and protein synthesis in neurons

Neuronal development, plasticity and survival require activity-dependent synapse-to-nucleus signaling. Most studies implicate an activity-dependent regulation of gene expression in this phenomenon. However, little is known about other nuclear functions that are regulated by synaptic activity. Here we show that a newly identified component of rat postsynaptic densities (PSDs), AIDA-1d, can regulate global protein synthesis by altering nucleolar numbers. AIDA-1d binds to the first two postsynaptic density-95/Discs large/zona occludens-1 (PDZ) domains of the scaffolding protein PSD-95 via its C-terminal three amino acids. Stimulation of NMDA receptors (NMDARs), which are also bound to PSD-95, results in a Ca2+-independent translocation of AIDA-1d to the nucleus, where it couples to Cajal bodies and induces Cajal body-nucleolar association. Long-term neuronal stimulation results in an AIDA-1-dependent increase in nucleolar numbers and protein synthesis. We propose that AIDA-1d mediates a link between synaptic activity and control of protein biosynthetic capacity by regulating nucleolar assembly.

Cajal body number and nucleolar size correlate with the cell body mass in human sensory ganglia neurons

This paper studies the cell size-dependent organization of the nucleolus and Cajal bodies (CBs) in dissociated human dorsal root ganglia (DRG) neurons from autopsy tissue samples of patients without neurological disease. The quantitative analysis of nucleoli with an anti-fibrillarin antibody showed that all neurons have only one nucleolus. However, the nucleolar volume and the number of fibrillar centers per nucleolus significantly increase as a function of cell body size. Immunostaining for coilin demonstrated the presence of numerous CBs in DRG neurons (up to 20 in large size neurons). The number of CBs per neuron correlated positively with the cell body volume. Light and electron microscopy immunocytochemical analysis revealed the concentration of coilin, snRNPs, SMN and fibrillarin in CBs of DRG neurons. CBs were frequently associated with the nucleolus, active chromatin domains and PML bodies, but not with telomeres. Our results support the view that the nucleolar volume and number of both fibrillar centers and CBs depend on the cell body mass, a parameter closely related to transcriptional and synaptic activity in mammalian neurons. Moreover, the unusual large number of CBs could facilitate the transfer of RNA processing components from CBs to nucleolar and nucleoplasmic sites of RNA processing.

Nuclear gems and Cajal (coiled) bodies in fetal tissues: nucleolar distribution of the spinal muscular atrophy protein, SMN

SMN, the affected protein in spinal muscular atrophy (SMA), is a cytoplasmic protein that also occurs in nuclear structures called "gems" and is involved in snRNP maturation. Coilin-p80 is a marker protein for nuclear Cajal bodies (coiled bodies; CBs) which are also involved in snRNP maturation, storage or transport. We now show that gems and CBs are present in all fetal tissues, even those that lack gems/CBs in the adult. Most gems and CBs occur as separate nuclear structures in fetal tissues, but their colocalization increases with fetal age and is almost complete in the adult. In adult tissues, up to half of all gems/CBs are inside the nucleolus, whereas in cultured cells they are almost exclusively nucleoplasmic. The nucleolar SMN is often more diffusely distributed, compared with nucleoplasmic gems. Up to 30% of cells in fetal tissues have SMN distributed throughout the nucleolus, instead of forming gems in the nucleoplasm. The results suggest a function for gems distinct from Cajal bodies in fetal nuclei and a nucleolar function for SMN. Spinal cord, the affected tissue in SMA, behaves differently in several respects. In both fetal and adult motor neurons, many gems/CBs occur as larger bodies closely associated with the nucleolar perimeter. Uniquely in motor neurons, gems/CBs are more numerous in adult than in fetal stages and colocalization of gems and CBs occurs earlier in development. These unusual features of motor neurons may relate to their special sensitivity to reduced SMN levels in SMA patients.

Cajal bodies: the first 100 years

Cajal bodies are small nuclear organelles first described nearly 100 years ago by Ramón y Cajal in vertebrate neural tissues. They have since been found in a variety of animal and plant nuclei, suggesting that they are involved in basic cellular processes. Cajal bodies contain a marker protein of unknown function, p80-coilin, and many components involved in transcription and processing of nuclear RNAs. Among these are the three eukaryotic RNA polymerases and factors required for transcribing and processing their respective nuclear transcripts: mRNA, rRNA, and pol III transcripts. A model is discussed in which Cajal bodies are the sites for preassembly of transcriptosomes, unitary particles involved in transcription and processing of RNA. A parallel is drawn to the nucleolus and the preassembly of ribosomes, which are unitary particles involved in translation of proteins.

Neuronal body size correlates with the number of nucleoli and Cajal bodies, and with the organization of the splicing machinery in rat trigeminal ganglion neurons

Trigeminal ganglion neurons comprise three main cell body-size types. This cell size heterogeneity provides an excellent neuronal model to study the cell size-dependent organization and dynamics of the nucleoli, Cajal (coiled) bodies (CBs), and nuclear speckles of pre-mRNA splicing factors, nuclear structures that play a key role in the normal neuronal physiology. We have analyzed the number of nucleoli and CBs and the structural and molecular organization of CBs and nuclear speckles in the three neuronal types by using immunofluorescence with antibodies that recognize nucleoli (fibrillarin), CBs (coilin), and nuclear speckles (snRNPs), confocal microscopy, and electron microscopy. Whereas the mean number of nucleoli per neuron decreases as a function of cell size, the number of CBs per cell significantly increases in large neurons in comparison with the small ones. In addition, large neurons have a higher proportion of CBs associated with the nucleolus. In all neuronal types, CBs concentrate coilin, fibrillarin, snRNPs, and the survival motor neuron protein (SMN). Immunostaining for snRNPs shows small speckle domains and extensive areas of diffuse nucleoplasmic signal in large neurons, in contrast with the large nuclear speckles found in small neurons. Furthermore, flow cytometric analysis shows that all neurons are in the range of diploid cells. These findings indicate that the fusion behavior of nucleoli, the formation of CBs and their relationships with the nucleolus, as well as the compartmentalization of the pre-mRNA splicing machinery, is related to cell body size in the trigeminal ganglion neurons. Because transcriptional activity is a basic determinant mechanism of cell size in diploid cells, we suggest that our findings reflect a distinct transcription-dependent organization of the nucleolus and splicing machinery in the three cell types of trigeminal ganglion neurons.

Coiled body numbers in the Arabidopsis root epidermis are regulated by cell type, developmental stage and cell cycle parameters

We have used whole mount immunofluorescence labelling with the antibody 4G3, raised against the human snRNP-specific protein U2B″, and whole mount in situ hybridization with an anti-sense probe to a conserved region of U2 snRNA, in combination with confocal microscopy, to examine the organization of spliceosomal components throughout the development of the Arabidopsis thaliana root epidermis. We show that the number of coiled bodies, nuclear organelles in which splicing snRNPs and snRNAs concentrate, is developmentally regulated in the Arabidopsis root epidermis. Firstly, there is a progression from a small number of coiled bodies in the quiescent centre and initial cells, to a larger number in the cell division zone, returning to a lower number in the cell elongation and differentiation zone. Secondly, trichoblasts (root-hair forming epidermal cells) have on average 1.5 times more and often smaller coiled bodies than atrichoblasts (hairless epidermal cells). Moreover, we have shown that these differences in coiled body numbers are related to differences in cell cycle stage, cell type and developmental stage, but are not due to differences in nucleolar or general metabolic activity per se. We discuss possible explanations, including a model in which coiled bodies coalesce during interphase, for the developmental dynamics of coiled bodies.

Changes in morphology and spatial position of coiled bodies during NGF-induced neuronal differentiation of PC12 cells

Interphase nuclei are organized into structural and functional domains. The coiled body, a nuclear organelle of unknown function, exhibits cell type-specific changes in number and morphology. Its association with nucleoli and with small nuclear ribonucleo-proteins (snRNPs) indicates that it functions in RNA processing. In cycling cells, coiled bodies are round structures not associated with nucleoli. In contrast, in neurons, they frequently present as nucleolar "caps." To test the hypothesis that neuronal differentiation is accompanied by changes in the spatial association of coiled bodies with nucleoli and in their morphology, PC12 cells were differentiated into a neuronal phenotype with nerve growth factor (NGF) and coiled bodies detected by immunocytochemical localization of p80-coilin and snRNPs. The fraction of cells that showed coiled bodies as nucleolar caps increased from 1.6 +/- 0.9% (mean +/- SEM) in controls to 16.5 +/- 1.6% in NGF-differentiated cultures. The fraction of cells with ring-like coiled bodies increased from 17.2 +/- 5.0% in controls to 57.8 +/- 4.4% in differentiated cells. This was accompanied by a decrease, from 81.2 +/- 5.7% to 25.7 +/- 3.1%, in the fraction of cells with small, round coiled bodies. SnRNPs remained associated with typical coiled bodies and with ring-like coiled bodies during NGF-induced recruitment of snRNPs to the nuclear periphery. Together with the observation that coiled bodies are also present as nucleolar caps in sensory neurons, the results indicate that coiled bodies alter their morphology and increase their association with nucleoli during NGF-induced neuronal differentiation.