Wnt and Src signals converge on YAP-TEAD to drive intestinal regeneration.

Wnt signalling induces a gradient of stem/progenitor cell proliferation along the crypt-villus axis of the intestine, which becomes expanded during intestinal regeneration or tumour formation. The YAP transcriptional co-activator is known to be required for intestinal regeneration, but its mode of regulation remains controversial. Here we show that the YAP-TEAD transcription factor is a key downstream effector of Wnt signalling in the intestine. Loss of YAP activity by Yap/Taz conditional knockout results in sensitivity of crypt stem cells to apoptosis and reduced cell proliferation during regeneration. Gain of YAP activity by Lats1/2 conditional knockout is sufficient to drive a crypt hyperproliferation response. In particular, Wnt signalling acts transcriptionally to induce YAP and TEAD1/2/4 expression. YAP normally localises to the nucleus only in crypt base stem cells, but becomes nuclear in most intestinal epithelial cells during intestinal regeneration after irradiation, or during organoid growth, in a Src family kinase-dependent manner. YAP-driven crypt expansion during regeneration involves an elongation and flattening of the Wnt signalling gradient. Thus, Wnt and Src-YAP signals cooperate to drive intestinal regeneration.

Control of skeletal morphogenesis by the Hippo-YAP/TAZ pathway.

The Hippo-YAP/TAZ pathway is an important regulator of tissue growth, but can also control cell fate or tissue morphogenesis. Here, we investigate the function of the Hippo pathway during the development of cartilage, which forms the majority of the skeleton. Previously, YAP was proposed to inhibit skeletal size by repressing chondrocyte proliferation and differentiation. We find that, in vitro, Yap/Taz double knockout impairs murine chondrocyte proliferation, whereas constitutively nuclear nls-YAP5SA accelerates proliferation, in line with the canonical role of this pathway in most tissues. However, in vivo, cartilage-specific knockout of Yap/Taz does not prevent chondrocyte proliferation, differentiation or skeletal growth, but rather results in various skeletal deformities including cleft palate. Cartilage-specific expression of nls-YAP5SA or knockout of Lats1/2 do not increase cartilage growth, but instead lead to catastrophic malformations resembling chondrodysplasia or achondrogenesis. Physiological YAP target genes in cartilage include Ctgf, Cyr61 and several matrix remodelling enzymes. Thus, YAP/TAZ activity controls chondrocyte proliferation in vitro, possibly reflecting a regenerative response, but is dispensable for chondrocyte proliferation in vivo, and instead functions to control cartilage morphogenesis via regulation of the extracellular matrix.

Mask family proteins ANKHD1 and ANKRD17 regulate YAP nuclear import and stability.

Mask family proteins were discovered in Drosophila to promote the activity of the transcriptional coactivator Yorkie (Yki), the sole fly homolog of mammalian YAP (YAP1) and TAZ (WWTR1). The molecular function of Mask, or its mammalian homologs Mask1 (ANKHD1) and Mask2 (ANKRD17), remains unclear. Mask family proteins contain two ankyrin repeat domains that bind Yki/YAP as well as a conserved nuclear localisation sequence (NLS) and nuclear export sequence (NES), suggesting a role in nucleo-cytoplasmic transport. Here we show that Mask acts to promote nuclear import of Yki, and that addition of an ectopic NLS to Yki is sufficient to bypass the requirement for Mask in Yki-driven tissue growth. Mammalian Mask1/2 proteins also promote nuclear import of YAP, as well as stabilising YAP and driving formation of liquid droplets. Mask1/2 and YAP normally colocalise in a granular fashion in both nucleus and cytoplasm, and are co-regulated during mechanotransduction.

Structure and development of the subesophageal zone of the Drosophila brain. II. Sensory compartments.

The subesophageal zone (SEZ) of the Drosophila brain processes mechanosensory and gustatory sensory input from sensilla located on the head, mouth cavity and trunk. Motor output from the SEZ directly controls the movements involved in feeding behavior. In an accompanying paper (Hartenstein et al., ), we analyzed the systems of fiber tracts and secondary lineages to establish reliable criteria for defining boundaries between the four neuromeres of the SEZ, as well as discrete longitudinal neuropil domains within each SEZ neuromere. Here we use this anatomical framework to systematically map the sensory projections entering the SEZ throughout development. Our findings show continuity between larval and adult sensory neuropils. Gustatory axons from internal and external taste sensilla of the larva and adult form two closely related sensory projections, (a) the anterior central sensory center located deep in the ventromedial neuropil of the tritocerebrum and mandibular neuromere, and (b) the anterior ventral sensory center (AVSC), occupying a superficial layer within the ventromedial tritocerebrum. Additional, presumed mechanosensory terminal axons entering via the labial nerve define the ventromedial sensory center (VMSC) in the maxilla and labium. Mechanosensory afferents of the massive array of chordotonal organs (Johnston's organ) of the adult antenna project into the centrolateral neuropil column of the anterior SEZ, creating the antenno-mechanosensory and motor center (AMMC). Dendritic projections of dye back-filled motor neurons extend throughout a ventral layer of the SEZ, overlapping widely with the AVSC and VMSC. Our findings elucidate fundamental structural aspects of the developing sensory systems in Drosophila.

Characterization of tailless functions during Drosophila optic lobe formation.

Brain development goes through phases of proliferative growth and differentiation to ensure the formation of correct number and variety of neurons. How and when naïve neuroepithelial cells decide to enter a differentiation pathway remains poorly understood. In the Drosophila visual system, four optic ganglia emerge from neuroepithelia of the inner (IPC) and outer (OPC) proliferation centers. Here we demonstrate that the orphan nuclear receptor Tailless (Tll) is a key factor for the development of all optic ganglia. We describe tll expression during larval optic lobe development in unprecedented detail and find a spatiotemporally dynamic pattern. In the larval OPC, symmetrically dividing neuroepithelial cells transform into asymmetrically dividing medulla neuroblast and into lamina precursor cells in a precisely regulated fashion. Using genetic manipulations we found that tll is required for proper neuroepithelium morphology and neuroepithelial cell survival. We show that tll regulates the precise timing of the transition from neuroepithelial cells to medulla neuroblasts. In particular, however, we demonstrate that tll has a crucial role for the specification of lamina precursor cells. We propose that the Tll/Tlx transcription factors have an evolutionary conserved role in regulating neural precursor cell states in the Drosophila optic lobe and in the mammalian retina.

OPA1 loss of function affects in vitro neuronal maturation.

Mitochondrial dynamics control the organelle's morphology, with fusion leading to the formation of elongated tubules and fission leading to isolated puncta, as well as mitochondrial functions. Recent reports have shown that disruptions of mitochondrial dynamics contribute to neurodegenerative diseases. Mutations of the inner membrane GTPase OPA1 are responsible for type 1 dominant optic atrophy, by mechanisms not fully understood. We show here that in rodent cortical primary neurons, downregulation of the OPA1 protein leads to fragmented mitochondria that become less abundant along the dendrites. Furthermore, this inhibition results in reduced expression of mitochondrial respiratory complexes as well as mitochondrial DNA, decreased mitochondrial membrane potential, and diminished reactive oxygen species levels. The onset of synaptogenesis was markedly impaired through reductions in pre- and postsynaptic structural protein expression and synapse numbers without first affecting the dendritic arborization. With longer time in culture, OPA1 extinction led to a major restriction of dendritic growth, together with reduction of synaptic proteins. Furthermore, in maturing neurons we observed a transitory increase in mitochondrial filament length, associated with marked changes in the expression levels of OPA1, which occurred at the onset of synaptogenesis simultaneously with transitory increase in reactive oxygen species levels and NRF2/NFE2L2 nuclear translocation. This observation suggests that mitochondrial hyperfilamentation acts upstream of a reactive oxygen species-dependent NRF2 transcriptional activity, possibly impacting neuronal maturation, such a process being impaired by insufficient amount of OPA1. Our findings suggest a new role for OPA1 in synaptic maturation and dendritic growth through maintenance of proper mitochondrial oxidative metabolism and distribution, highlighting the role of mitochondrial dynamics in neuronal functioning and providing insights into dominant optic atrophy pathogenesis, as OPA1 loss affecting neuronal maturation could lead to early synaptic dysfunction.