Molecular and Functional Characterization of TrkC Alternative Splicing Regulation and its Impact on the Fate Acquisition of Cortical Projection Neurons

The astonishing cellular diversity and finely tuned neuroanatomy of the cerebral cortex motivates sustained research efforts on the mechanisms that give rise to this intricate tissue. To enable the higher cognitive functions of mammals, various subtypes of cortical projection neurons, distributed in precise ratios and at specific positions in the cortex, participate in the neuronal circuitry of the brain. These positions and ratios are established during embryonic development. Including the neuron’s targets and input sources, the subtype identity of a neuron depends on its transcriptomic signature. Broadly speaking, cortical neuron subtypes project either to the contralateral hemisphere, such as callosal projection neurons (CPN) or outside of the cortex, such as corticofugal projection neurons (CFuPN). Both of these subtypes are generated by neural progenitor cells (NPCs) in the forebrain. During embryonic development, NPCs divide sequentially to generate other NPCs and various neuron subtypes, as dictated by the identity of the NPC itself. The fate decisions of an NPC lineage that determine how many neurons of each subtype it will produce are influenced by a spatially and temporally specified combination of molecular signals. These intra- and extracellular cues jointly intervene in cellular mechanisms, such as division speed, inheritance of fate-determining factors, and many more, ultimately shaping the ratio of neuron subtypes that the lineage produces during differentiation. One of the mechanisms that can control differentiation is the alternative splicing (AS) of primary transcripts, which can be regulated in a cell type- and stage-specific manner, ensuring adaptation to changing developmental requirements. AS is a way of generating molecular diversity by the context-dependent inclusion or exclusion of exons, introns or parts of exons from pre-mRNAs to form distinct mRNAs. At its simplest, AS is regulated by distinct sequence elements in the pre-mRNA, which are bound by splicing-regulatory proteins, termed splicing factors (SFs). SFs themselves respond to intra- and extracellular cues by being expressed or activated according to the needs of the developing tissue. Examples of AS regulating cell differentiation in other biological systems abound, but, even though AS is implicated in an increasing number of processes in the adult brain, its functions in cortex development are largely unexplored. The focus of this work are two isoforms of the neurotrophin-3 receptor, TrkC, which result from alternative splicing. The better studied of the two isoforms, the kinase-active TrkC-TK+, has been shown to act as a recipient of survival signals in neurons. Our reseach group previously showed that the less studied isoform, TrkC-T1, is a determinant of CFuPN fate, present in the development of the cortex in specific cell types and time windows. As the levels of TrkC-T1 in NPCs coordinate the numbers of CFuPN that are produced at the expense of CPN, the aim of this project was to explore how the AS of TrkC is regulated, in order to further our understanding of the factors that shape the NPC transcriptome and hence fate. In this work, we show that the balance between TrkC-T1 and TrkC-TK+ is cell type-specific in the developing cortex and primarily established at the level of AS regulation. We find that the splicing factors Srsf1 and Elavl1 regulate this isoform balance in an antagonistic manner. To our knowledge, this is the first described instance of these splicing factors co-regulating an AS event. In addition to this, we bring direct in vivo evidence that implicates both of these SFs in the CFuPN-CPN fate choice, a biological process they had not been previously linked to. We also find that Srsf1 and Elavl1 have different expression patterns in the developing cortex, a feature that contributes to the different cell type-specific splicing-regulatory environments that give rise to distinct ratios of TrkC-T1 to TrkC-TK+. Taken together, these findings further our knowledge of how cortical projection neuron fate is regulated at the posttranscriptional level.

Author dc:creator

• Weber, Andreea Ioana

• alternative splicing
• cortex development
• cell fate acquisition
• neuroscience
• neurotrophic receptor tyrosine kinases

Licence dc:rights.uri

• http://www.fu-berlin.de/sites/refubium/rechtliches/Nutzungsbedingungen

Language dc:language

eng

Identifier URI

http://dx.doi.org/10.17169/refubium-34750

OAI record oai:identifier

https://refubium.fu-berlin.de/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Arefubium.fu-berlin.de%3Afub188%2F35034