Nereo Kalebic

Genetically Separable Functions of the MEC-17 Tubulin Acetyltransferase Affect Microtubule Organization

BACKGROUND Microtubules (MTs) are formed from the lateral association of 11-16 protofilament chains of tubulin dimers, with most cells containing 13-protofilament (13-p) MTs. How these different MTs are formed is unknown, although the number of protofilaments may depend on the nature of the α- and β-tubulins. RESULTS Here we show that the enzymatic activity of the Caenorhabiditis elegans α-tubulin acetyltransferase (α-TAT) MEC-17 allows the production of 15-p MTs in the touch receptor neurons (TRNs) MTs. Without MEC-17, MTs with between 11 and 15 protofilaments are seen. Loss of this enzymatic activity also changes the number and organization of the TRN MTs and affects TRN axonal morphology. In contrast, enzymatically inactive MEC-17 is sufficient for touch sensitivity and proper process outgrowth without correcting the MT defects. Thus, in addition to demonstrating that MEC-17 is required for MT structure and organization, our results suggest that the large number of 15-p MTs, normally found in the TRNs, is not essential for mechanosensation. CONCLUSION These experiments reveal a specific role for α-TAT in the formation of MTs and in the production of higher order MTs arrays. In addition, our results indicate that the α-TAT protein has functions that require acetyltransferase activity (such as the determination of protofilament number) and others that do not (presence of internal MT structures).

αTAT1 is the major α-tubulin acetyltransferase in mice

Post-translational modification of tubulin serves as a powerful means for rapidly adjusting the functional diversity of microtubules. Acetylation of the ε-amino group of K40 in α-tubulin is one such modification that is highly conserved in ciliated organisms. Recently, αTAT1, a Gcn5-related N-acetyltransferase, was identified as an α-tubulin acetyltransferase in Tetrahymena and C. elegans. Here we generate mice with a targeted deletion of Atat1 to determine its function in mammals. Remarkably, we observe a loss of detectable K40 α-tubulin acetylation in these mice across multiple tissues and in cellular structures such as cilia and axons where acetylation is normally enriched. Mice are viable and develop normally, however, the absence of Atat1 impacts upon sperm motility and male mouse fertility, and increases microtubule stability. Thus, αTAT1 has a conserved function as the major α-tubulin acetyltransferase in ciliated organisms and has an important role in regulating subcellular specialization of subsets of microtubules.

Tubulin Acetyltransferase αTAT1 Destabilizes Microtubules Independently of Its Acetylation Activity

ABSTRACT Acetylation of α-tubulin at lysine 40 (K40) is a well-conserved posttranslational modification that marks long-lived microtubules but has poorly understood functional significance. Recently, αTAT1, a member of the Gcn5-related N-acetyltransferase superfamily, has been identified as an α-tubulin acetyltransferase in ciliated organisms. Here, we explored the function of αTAT1 with the aim of understanding the consequences of αTAT1-mediated microtubule acetylation. We demonstrate that α-tubulin is the major target of αTAT1 but that αTAT1 also acetylates itself in a regulatory mechanism that is required for effective modification of tubulin. We further show that in mammalian cells, αTAT1 promotes microtubule destabilization and accelerates microtubule dynamics. Intriguingly, this effect persists in an αTAT1 mutant with no acetyltransferase activity, suggesting that interaction of αTAT1 with microtubules, rather than acetylation per se, is the critical factor regulating microtubule stability. Our data demonstrate that αTAT1 has cellular functions that extend beyond its classical enzymatic activity as an α-tubulin acetyltransferase.

CRISPR/Cas9‐induced disruption of gene expression in mouse embryonic brain and single neural stem cells in vivo

We have applied the CRISPR/Cas9 system in vivo to disrupt gene expression in neural stem cells in the developing mammalian brain. Two days after in utero electroporation of a single plasmid encoding Cas9 and an appropriate guide RNA (gRNA) into the embryonic neocortex of Tis21::GFP knock‐in mice, expression of GFP, which occurs specifically in neural stem cells committed to neurogenesis, was found to be nearly completely (≈90%) abolished in the progeny of the targeted cells. Importantly, upon in utero electroporation directly of recombinant Cas9/gRNA complex, near‐maximal efficiency of disruption of GFP expression was achieved already after 24 h. Furthermore, by using microinjection of the Cas9 protein/gRNA complex into neural stem cells in organotypic slice culture, we obtained disruption of GFP expression within a single cell cycle. Finally, we used either Cas9 plasmid in utero electroporation or Cas9 protein complex microinjection to disrupt the expression of Eomes/Tbr2, a gene fundamental for neocortical neurogenesis. This resulted in a reduction in basal progenitors and an increase in neuronal differentiation. Thus, the present in vivo application of the CRISPR/Cas9 system in neural stem cells provides a rapid, efficient and enduring disruption of expression of specific genes to dissect their role in mammalian brain development.

Epigenome profiling and editing of neocortical progenitor cells during development

The generation of neocortical neurons from neural progenitor cells (NPCs) is primarily controlled by transcription factors binding to DNA in the context of chromatin. To understand the complex layer of regulation that orchestrates different NPC types from the same DNA sequence, epigenome maps with cell type resolution are required. Here, we present genomewide histone methylation maps for distinct neural cell populations in the developing mouse neocortex. Using different chromatin features, we identify potential novel regulators of cortical NPCs. Moreover, we identify extensive H3K27me3 changes between NPC subtypes coinciding with major developmental and cell biological transitions. Interestingly, we detect dynamic H3K27me3 changes on promoters of several crucial transcription factors, including the basal progenitor regulator Eomes. We use catalytically inactive Cas9 fused with the histone methyltransferase Ezh2 to edit H3K27me3 at the Eomes locus in vivo, which results in reduced Tbr2 expression and lower basal progenitor abundance, underscoring the relevance of dynamic H3K27me3 changes during neocortex development. Taken together, we provide a rich resource of neocortical histone methylation data and outline an approach to investigate its contribution to the regulation of selected genes during neocortical development.

Acetylated tubulin is essential for touch sensation in mice.

At its most fundamental level, touch sensation requires the translation of mechanical energy into mechanosensitive ion channel opening, thereby generating electro-chemical signals. Our understanding of this process, especially how the cytoskeleton influences it, remains unknown. Here we demonstrate that mice lacking the α-tubulin acetyltransferase Atat1 in sensory neurons display profound deficits in their ability to detect mechanical stimuli. We show that all cutaneous afferent subtypes, including nociceptors have strongly reduced mechanosensitivity upon Atat1 deletion, and that consequently, mice are largely insensitive to mechanical touch and pain. We establish that this broad loss of mechanosensitivity is dependent upon the acetyltransferase activity of Atat1, which when absent leads to a decrease in cellular elasticity. By mimicking α-tubulin acetylation genetically, we show both cellular rigidity and mechanosensitivity can be restored in Atat1 deficient sensory neurons. Hence, our results indicate that by influencing cellular stiffness, α-tubulin acetylation sets the force required for touch. DOI: http://dx.doi.org/10.7554/eLife.20813.001

Insm1 Induces Neural Progenitor Delamination in Developing Neocortex via Downregulation of the Adherens Junction Belt-Specific Protein Plekha7

Delamination of neural progenitor cells (NPCs) from the ventricular surface is a crucial prerequisite to form the subventricular zone, the germinal layer linked to the expansion of the mammalian neocortex in development and evolution. Here, we dissect the molecular mechanism by which the transcription factor Insm1 promotes the generation of basal progenitors (BPs). Insm1 protein is most highly expressed in newborn BPs in mouse and human developing neocortex. Forced Insm1 expression in embryonic mouse neocortex causes NPC delamination, converting apical to basal radial glia. Insm1 represses the expression of the apical adherens junction belt-specific protein Plekha7. CRISPR/Cas9-mediated disruption of Plekha7 expression suffices to cause NPC delamination. Plekha7 overexpression impedes the intrinsic and counteracts the Insm1-induced, NPC delamination. Our findings uncover a novel molecular mechanism underlying NPC delamination in which a BP-genic transcription factor specifically targets the integrity of the apical adherens junction belt, rather than adherens junction components as such.

Extracellular Matrix Components HAPLN1, Lumican, and Collagen I Cause Hyaluronic Acid-Dependent Folding of the Developing Human Neocortex

Neocortical expansion, thought to underlie the cognitive traits unique to humans, is accompanied by cortical folding. This folding starts around gestational week (GW) 20, but what causes it remains largely unknown. Extracellular matrix (ECM) has been previously implicated in neocortical expansion and here we investigate the potential role of ECM in the formation of neocortical folds. We focus on three specific ECM components localized in the human fetal cortical plate (CP): hyaluronan and proteoglycan link protein 1 (HAPLN1), lumican and collagen I (collectively, HLC). Addition of HLC to cultures of human fetal neocortex (11-22 GW) caused local changes in tissue stiffness, induced CP folding, increased CP hyaluronic acid (HA), and required the HA-receptor CD168 and downstream ERK signaling. Importantly, loss of HA reduced HLC-induced and 22 GW physiological nascent folds. This was altered in samples with neurodevelopmental disorders, indicating it may be a useful system to study such disorders.

YAP activity is necessary and sufficient for basal progenitor abundance and proliferation in the developing neocortex

The expansion of the neocortex during mammalian evolution has been linked to an enlargement of the subventricular zone during cortical development and an increase in the proliferation of the basal progenitors residing therein. Here, we explored a potential role of YAP, the major downstream effector of the Hippo signaling pathway, in proliferation of basal progenitors. We show that YAP expression and activity are high in ferret and human basal progenitors, which are known to exhibit high proliferative capacity, but low in mouse basal progenitors, which lack such capacity. To induce YAP activity in mouse basal progenitors, we expressed a constitutively active YAP (CA-YAP). This resulted in an increase in proliferation of basal progenitor. In addition, CA-YAP expressing mouse basal progenitors promoted the production of upper-layer neurons. To investigate if YAP is required for the proliferation of basal progenitors, we pharmacologically interfered with the function of YAP in the developing ferret and human neocortex. This resulted in a decrease of cycling basal progenitors. In concert, genetical interference with the function of YAP in ferret developing neocortex resulted in decreased abundance of basal progenitors. Together, our data indicate that YAP promotes the proliferation of basal progenitors and suggest that changes in YAP activity levels contributed to the evolutionary expansion of the neocortex.

Human-specific ARHGAP11B induces hallmarks of neocortical expansion in developing ferret neocortex

The evolutionary increase in size and complexity of the primate neocortex is thought to underlie the higher cognitive abilities of humans. ARHGAP11B is a human-specific gene that, based on its expression pattern in fetal human neocortex and progenitor effects in embryonic mouse neocortex, has been proposed to have a key function in the evolutionary expansion of the neocortex. Here, we study the effects of ARHGAP11B expression in the developing neocortex of the gyrencephalic ferret. In contrast to its effects in mouse, ARHGAP11B markedly increases proliferative basal radial glia, a progenitor cell type thought to be instrumental for neocortical expansion, and results in extension of the neurogenic period and an increase in upper-layer neurons. As a consequence, the postnatal ferret neocortex exhibits an increased neuron density in the upper cortical layers and expands in the radial dimension. Thus, human-specific ARHGAP11B can elicit hallmarks of neocortical expansion in developing ferret neocortex.