Neurofilament content is correlated with branch length in developing collateral branches of Xenopus spinal cord neurons

Intermediate filaments in developing neurons: Beyond structure

Neuronal development relies on a highly choreographed progression of dynamic cellular processes by which newborn neurons migrate, extend axons and dendrites, innervate their targets, and make functional synapses. Many of these dynamic processes require coordinated changes in morphology, powered by the cell's cytoskeleton. Intermediate filaments (IFs) are the third major cytoskeletal elements in vertebrate cells, but are rarely considered when it comes to understanding axon and dendrite growth, pathfinding and synapse formation. In this review, we first introduce the many new and exciting concepts of IF function, discovered mostly in non-neuronal cells. These roles include dynamic rearrangements, crosstalk with microtubules and actin filaments, mechano-sensing and -transduction, and regulation of signaling cascades. We then discuss the understudied roles of neuronally expressed IFs, with a particular focus on IFs expressed during development, such as nestin, vimentin and α-internexin. Lastly, we illustrate how signaling modulation by the unconventional IF nestin shapes neuronal morphogenesis in unexpected and novel ways. Even though the first IF knockout mice were made over 20 years ago, the study of the cell biological functions of IFs in the brain still has much room for exciting new discoveries.

Xenopus laevis as a model system to study cytoskeletal dynamics during axon pathfinding

The model system, Xenopus laevis, has been used in innumerable research studies and has contributed to the understanding of multiple cytoskeletal components, including actin, microtubules, and neurofilaments, during axon pathfinding. Xenopus developmental stages have been widely characterized, and the Xenopus genome has been sequenced, allowing gene expression modifications through exogenous molecules. Xenopus cell cultures are ideal for long periods of live imaging because they are easily obtained and maintained, and they do not require special culture conditions. In addition, Xenopus have relatively large growth cones, compared to other vertebrates, thus providing a suitable system for imaging cytoskeletal components. Therefore, X. laevis is an ideal model organism in which to study cytoskeletal dynamics during axon pathfinding.

Microtubule-associated protein tau promotes neuronal class II β-tubulin microtubule formation and axon elongation in embryonic Xenopus laevis

Compared with its roles in neurodegeneration, much less is known about microtubule-associated protein tau's normal functions in vivo, especially during development. The external development and ease of manipulating gene expression of Xenopus laevis embryos make them especially useful for studying gene function during early development. To study tau's functions in axon outgrowth, we characterized the most prominent tau isoforms of Xenopus embryos and manipulated their expression. None of these four isoforms were strictly analogous to those commonly studied in mammals, as all constitutively contained exon 10, which is preferentially removed from mammalian fetal tau isoforms, as well as exon 8, which in mammals is rare. Nonetheless, like mammalian tau, Xenopus tau exhibited alternative splicing of exon 4a, which in mammals distinguishes 'big' tau of peripheral neurons, and exon 6. Strongly suppressing tau expression with antisense morpholino oligonucleotides only modestly compromised peripheral nerve outgrowth of intact tadpoles, but severely disrupted neuronal microtubules containing class II β-tubulins while leaving other microtubules largely unperturbed. Thus, the relatively mild dependence of axon development on tau likely resulted from having only a single class of microtubules disrupted by its loss. Also, consistent with its greater expression in long peripheral axons, boosting expression of 'big' tau increased neurite outgrowth significantly and enhanced tubulin acetylation more so than did the smaller isoform. These data demonstrate the utility of Xenopus as a tool to gain new insights into tau's functions in vivo.

The cytoskeletal and signaling mechanisms of axon collateral branching

During development, axons are guided to their appropriate targets by a variety of guidance factors. On arriving at their synaptic targets, or while en route, axons form branches. Branches generated de novo from the main axon are termed collateral branches. The generation of axon collateral branches allows individual neurons to make contacts with multiple neurons within a target and with multiple targets. In the adult nervous system, the formation of axon collateral branches is associated with injury and disease states and may contribute to normally occurring plasticity. Collateral branches are initiated by actin filament– based axonal protrusions that subsequently become invaded by microtubules, thereby allowing the branch to mature and continue extending. This article reviews the current knowledge of the cellular mechanisms of the formation of axon collateral branches. The major conclusions of this review are (1) the mechanisms of axon extension and branching are not identical; (2) active suppression of protrusive activity along the axon negatively regulates branching; (3) the earliest steps in the formation of axon branches involve focal activation of signaling pathways within axons, which in turn drive the formation of actin-based protrusions; and (4) regulation of the microtubule array by microtubule-associated and severing proteins underlies the development of branches. Linking the activation of signaling pathways to specific proteins that directly regulate the axonal cytoskeleton underlying the formation of collateral branches remains a frontier in the field.

hnRNP K post-transcriptionally co-regulates multiple cytoskeletal genes needed for axonogenesis

The RNA-binding protein, hnRNP K, is essential for axonogenesis. Suppressing its expression in Xenopus embryos yields terminally specified neurons with severely disorganized microtubules, microfilaments and neurofilaments, raising the hypothesis that hnRNP K post-transcriptionally regulates multiple transcripts of proteins that organize the axonal cytoskeleton. To identify downstream candidates for this regulation, RNAs that co-immunoprecipitated from juvenile brain with hnRNP K were identified on microarrays. A substantial number of these transcripts were linked to the cytoskeleton and to intracellular localization, trafficking and transport. Injection into embryos of a non-coding RNA bearing multiple copies of an hnRNP K RNA-binding consensus sequence found within these transcripts largely phenocopied hnRNP K knockdown, further supporting the idea that it regulates axonogenesis through its binding to downstream target RNAs. For further study of regulation by hnRNP K of the cytoskeleton during axon outgrowth, we focused on three validated RNAs representing elements associated with all three polymers - Arp2, tau and an α-internexin-like neurofilament. All three were co-regulated post-transcriptionally by hnRNP K, as hnRNP K knockdown yielded comparable defects in their nuclear export and translation but not transcription. Directly knocking down expression of all three together, but not each one individually, substantially reproduced the axonless phenotype, providing further evidence that regulation of axonogenesis by hnRNP K occurs largely through pleiotropic effects on cytoskeletal-associated targets. These experiments provide evidence that hnRNP K is the nexus of a novel post-transcriptional regulatory module controlling the synthesis of proteins that integrate all three cytoskeletal polymers to form the axon.

EBF factors drive expression of multiple classes of target genes governing neuronal development

Background

Early B cell factor (EBF) family members are transcription factors known to have important roles in several aspects of vertebrate neurogenesis, including commitment, migration and differentiation. Knowledge of how EBF family members contribute to neurogenesis is limited by a lack of detailed understanding of genes that are transcriptionally regulated by these factors.

Results

We performed a microarray screen in Xenopus animal caps to search for targets of EBF transcriptional activity, and identified candidate targets with multiple roles, including transcription factors of several classes. We determined that, among the most upregulated candidate genes with expected neuronal functions, most require EBF activity for some or all of their expression, and most have overlapping expression with ebf genes. We also found that the candidate target genes that had the most strongly overlapping expression patterns with ebf genes were predicted to be direct transcriptional targets of EBF transcriptional activity.

Conclusions

The identification of candidate targets that are transcription factor genes, including nscl-1, emx1 and aml1, improves our understanding of how EBF proteins participate in the hierarchy of transcription control during neuronal development, and suggests novel mechanisms by which EBF activity promotes migration and differentiation. Other candidate targets, including pcdh8 and kcnk5, expand our knowledge of the types of terminal differentiated neuronal functions that EBF proteins regulate.

Type III intermediate filament peripherin inhibits neuritogenesis in type II spiral ganglion neurons in vitro

Peripherin, a type III intermediate filament protein, forms part of the cytoskeleton in a subset of neurons, most of which have peripheral fibre projections. Studies suggest a role for peripherin in axon outgrowth and regeneration, but evidence for this in sensory and brain tissues is limited. The exclusive expression of peripherin in a sub-population of primary auditory neurons, the type II spiral ganglion neurons (SGN) prompted our investigation of the effect of peripherin gene deletion (pphKO) on these neurons. We used confocal immunofluorescence to examine the establishment of the innervation of the cochlear outer hair cells by the type II SGN neurites in vivo and in vitro, in wildtype (WT) and pphKO mice, in the first postnatal week. The distribution of the type II SGN nerve fibres was normal in pphKO cochleae. However, using P1 spiral ganglion explants under culture conditions where the majority of neurites were derived from type II SGN, pphKO resulted in increased numbers of neurites/explant compared to WT controls. Type II SGN neurites from pphKO explants extended approximately double the distance of WT neurites, and had reduced complexity based on greater distance between turning points. Addition of brain-derived neurotrophic factor (BDNF) to the culture media increased neurite number in WT and KO explants approximately 30-fold, but did not affect neurite length or distance between turning. These results indicate that peripherin may interact with other cytoskeletal elements to regulate outgrowth of the peripheral neurites of type II SGN, distinguishing these neurons from the type I SGN innervating the inner hair cells.

A crucial role for hnRNP K in axon development in Xenopus laevis

We report that hnRNP K, an RNA-binding protein implicated in multiple aspects of post-transcriptional gene control, is essential for axon outgrowth in Xenopus. Its intracellular localization was found to be consistent with one of its known roles as an mRNA shuttling protein. In early embryos, it was primarily nuclear, whereas later it occupied both the nucleus and cytoplasm to varying degrees in different neuronal subtypes. Antisense hnRNP K morpholino oligonucleotides (MOs) microinjected into blastomeres suppressed hnRNP K expression from neural plate stages through to at least stage 40. Differentiating neural cells in these embryos expressed several markers for terminally differentiated neurons but failed to make axons. Rescue experiments and the use of two separate hnRNP K MOs were carried out to confirm that these effects were specifically caused by knockdown of hnRNP K expression. For insights into the involvement of hnRNP K in neuronal post-transcriptional gene control at the molecular level, we compared effects on expression of the medium neurofilament protein (NF-M), the RNA for which binds hnRNP K, with that of peripherin, another intermediate filament protein, the RNA for which does not bind hnRNP K. hnRNP K knockdown compromised NF-M mRNA nucleocytoplasmic export and translation, but had no effect on peripherin. Because eliminating NF-M from Xenopus axons attenuates, but does not abolish, their outgrowth, hnRNP K must target additional RNAs needed for axon development. Our study supports the idea that translation of at least a subset of RNAs involved in axon development is controlled by post-transcriptional regulatory modules that have hnRNP K as an essential element.

Neuronal expression of peripherin, a type III intermediate filament protein, in the mouse hindbrain

Peripherin is a 57 kDa Type III intermediate filament protein associated with neurite extension, neuropathies such as amyotrophic lateral sclerosis, and cranial nerve and dorsal root projections. However, knowledge of peripherin expression in the CNS is limited. We have used immunoperoxidase histochemistry to characterise peripherin expression in the mouse hindbrain, including the inferior colliculus, pons, medulla and cerebellum. Peripherin immunolabelling was observed in the nerve fibres and nuclei that are associated with all cranial nerves [(CN) V-XII] in the hindbrain. Peripherin expression was prominent in the cell bodies and axons of the mesenchephalic trigeminal nucleus and the pars compacta region of nucleus ambiguus, and in the fibres that comprise the solitary tract, the descending spinal trigeminal tract and the trigeminal and facial nerves. A small proportion of peripherin positive fibres in CN VIII likely arise from cochlear type II spiral ganglion neurons. Peripherin positive fibres were also observed in the inferior cerebellar peduncle and folia in the intermediate zone of the cerebellum. Antibody specificity was confirmed by absence of labelling in hindbrain tissue from peripherin knockout mice. This study shows that in the adult mouse hindbrain, peripherin is expressed in discrete neuronal subpopulations that have sensory, motor and autonomic functions.