Akirin2 is essential for the formation of the cerebral cortex

Abstract

BackgroundThe proper spatial and temporal regulation of dorsal telencephalic progenitor behavior is a prerequisite for the formation of the highly-organized, six-layered cerebral cortex. Premature differentiation of cells, disruption of cell cycle timing, excessive apoptosis, and/or incorrect neuronal migration signals can have devastating effects, resulting in a number of neurodevelopmental disorders involving microcephaly and/or lissencephaly. Though genes encoding many key players in cortical development have been identified, our understanding remains incomplete. We show that the gene encoding Akirin2, a small nuclear protein, is expressed in the embryonic telencephalon. Converging evidence indicates that Akirin2 acts as a bridge between transcription factors (including Twist and NF-κB proteins) and the BAF (SWI/SNF) chromatin remodeling machinery to regulate patterns of gene expression. Constitutive knockout of Akirin2 is early embryonic lethal in mice, while restricted loss in B cells led to disrupted proliferation and cell survival.MethodsWe generated cortex-restricted Akirin2 knockouts by crossing mice harboring a floxed Akirin2 allele with the Emx1-Cre transgenic line and assessed the resulting embryos using in situ hybridization, EdU labeling, and immunohistochemistry.ResultsThe vast majority of Akirin2 mutants do not survive past birth, and exhibit extreme microcephaly, with little dorsal telencephalic tissue and no recognizable cortex. This is primarily due to massive cell death of early cortical progenitors, which begins at embryonic day (E)10, shortly after Emx1-Cre is active. Immunostaining and cell cycle analysis using EdU labeling indicate that Akirin2-null progenitors fail to proliferate normally, produce fewer neurons, and undergo extensive apoptosis. All of the neurons that are generated in Akirin2 mutants also undergo apoptosis by E12. In situ hybridization for Wnt3a and Wnt-responsive genes suggest defective formation and/or function of the cortical hem in Akirin2 null mice. Furthermore, the apical ventricular surface becomes disrupted, and Sox2-positive progenitors are found to “spill” into the lateral ventricle.ConclusionsOur data demonstrate a previously-unsuspected role for Akirin2 in early cortical development and, given its known nuclear roles, suggest that it may act to regulate gene expression patterns critical for early progenitor cell behavior and cortical neuron production.Electronic supplementary materialThe online version of this article (doi:10.1186/s13064-016-0076-8) contains supplementary material, which is available to authorized users.

A critical role for the nuclear protein Akirin2 in the formation of mammalian muscle in vivo

“…Together with prior studies in other cell types identifying Aki2 as a master regulator of gene expression, our data indicate that Aki2 controls the proper expression of key factors that drive mammalian myogenesis. Interestingly, a recent study reported that deletions in human chromosome 6q11‐15, which contains the Aki2 gene among several others, is associated with developmental delay, abnormal skull shape, dysplastic outer ear, and hypotonia (Engwerda et al, ), phenotypes potentially consistent with what we observe here and elsewhere (Bosch et al, ) in mice.…”

“…The loss of Aki2 in Aki2 SimKO mutants resulted in cell death, especially in the dermomyotome region in which Pax3 is expressed. This is a finding reminiscent of our prior work on corticogenesis (Bosch et al, ), where we showed that massive cell death occurred in the telencephalon within 12 hr after Emx1‐Cre ‐mediated Aki2 deletion. In addition, other studies suggest that an important functional role for Aki2 occurs during rapid proliferation of cells in response to a stimulus (Komiya, Akiyama, Sakumoto, & Tashiro, ; Komiya et al, ; Tartey et al, ), consistent with our observations.…”

“…Briefly, floxed Akirin 2 conditional mutant mice (Goto et al, ) that we have used previously (Bosch et al, ) were crossed with a BAC transgenic expressing Cre from the Sim1 promoter (Balthasar et al, ) to excise the floxed Akirin2 allele. We included a tdTomato reporter allele ( Ai14‐tdTomato , obtained from The Jackson Laboratory, stock #007914; (Madisen et al, ); containing a floxed stop cassette preceding the tdTomato gene in the ROSA26 locus.…”

“…Alcian Blue and Alizarin Red staining was performed on P0 Aki2 SimKO mice (plus littermate controls) according to previously reported protocols (Bosch et al, ; McLeod, ; Rigueur & Lyons, ). H&E staining was performed as described (Bosch et al, ) on E18 forelimbs from Aki2 SimKO and littermate control animals.…”

“…Drosophila and Caenorhabditis elegans each possess a single Akirin gene, whereas mammals have two homologues, termed Akirin1 (Aki1) and Akirin2 (Aki2) ; while Aki1 is dispensable for mouse development, Aki2 null embryos could not be recovered even as early as embryonic day (E)9.5 (Clemons et al, ; Goto et al, ; Macqueen & Johnston, ). Utilizing conditional Aki2 mutant mice and an Emx1‐Cre driver, we have shown that Aki2 is essential for the survival and proliferation of cerebral cortical progenitor cells (Bosch, Fuller, Sleeth, & Weiner, ), and also that it plays a role in facilitating interdigital tissue regression during limb development (Bosch, Fuller, & Weiner, ). Aki2 is also known to act in the mouse immune system, where it regulates gene expression in B cells and macrophages by coordinating the recruitment of NFκB and the BAF chromatin remodeling complex to gene promoters (Tartey et al, , ; Tartey & Takeuchi, ).…”