Neuronal primary cilia regulate pyramidal cell positioning to the deep and superficial sublayers in the hippocampal CA1

It is well-recognized that primary cilia regulate embryonic neurodevelopment, but little is known about their roles in postnatal neurodevelopment. The striatum pyramidal (SP) of hippocampal CA1 consists of superficial and deep sublayers, however, it is not well understood how early- and late-born pyramidal neurons position to two sublayers postnatally. Here we show that neuronal primary cilia emerge after CA1 pyramidal cells have reached SP, but before final neuronal positioning. The axonemes of primary cilia of early-born neurons point to the stratum oriens (SO), whereas late-born neuronal cilia orient toward the stratum radiatum (SR), reflecting an inside-out lamination pattern. Neuronal primary cilia in SP undergo marked changes in morphology and orientation from postnatal day 5 (P5) to P14, concurrent with pyramidal cell positioning to the deep and superficial sublayers and with neuronal maturation. Transgenic overexpression of Arl13B, a protein regulating ciliogenesis, not only elongates primary cilia and promotes earlier cilia protrusion, but also affects centriole positioning and cilia orientation in SP. The centrioles of late-born neurons migrate excessively to cluster at SP bottom before primary cilia protrusion and a reverse movement back to the main SP. Similarly, this pull-back movement of centriole/cilia is also identified on late-born cortical pyramidal neurons, although early- and late-born cortical neurons display the same cilia orientation. Together, this study provides the first evidence demonstrating that late-born pyramidal neurons exhibit a reverse movement for cell positioning, and primary cilia regulate pyramidal neuronal positioning to the deep and superficial sublayers in the hippocampus.

Non-canonical Wnt signaling promotes directed migration of intestinal stem cells to sites of injury.

Tissue regeneration after injury requires coordinated regulation of stem cell activation, division, and daughter cell differentiation, processes that are increasingly well understood in many regenerating tissues. How accurate stem cell positioning and localized integration of new cells into the damaged epithelium are achieved, however, remains unclear. Here we show that enteroendocrine cells coordinate stem cell migration towards a wound in the Drosophila intestinal epithelium. In response to injury, EEs release the N-terminal domain of the PTK7 orthologue, Otk, which activates non-canonical Wnt signaling in ISCs, promoting actin-based protrusion formation and ISC migration towards a wound. We find that this migratory behavior is closely linked to ISC proliferation, and that it is required for efficient tissue repair during injury. Our findings highlight the role of non-canonical Wnt signaling in regeneration of the intestinal epithelium, and identify EE-released ligands as critical coordinators of ISC migration.

Spatial distribution of LTi-like cells in intestinal mucosa regulates type 3 innate immunity

Lymphoid tissue inducer (LTi)-like cells are tissue resident innate lymphocytes that rapidly secrete cytokines that promote gut epithelial integrity and protect against extracellular bacterial infections.Here, we report that the retention of LTi-like cells in conventional solitary intestinal lymphoid tissue (SILT) is essential for controlling LTi-like cell function and is maintained by expression of the chemokine receptor CXCR5. Deletion of Cxcr5 functionally unleashed LTi-like cells in a cell intrinsic manner, leading to uncontrolled IL-17 and IL-22 production. The elevated production of IL-22 in Cxcr5-deficient mice improved gut barrier integrity and protected mice during infection with the opportunistic pathogen Clostridium difficile. Interestingly, Cxcr5−/− mice developed LTi-like cell aggregates that were displaced from their typical niche at the intestinal crypt, and LTi-like cell hyperresponsiveness was associated with the local formation of this unconventional SILT. Thus, LTi-like cell positioning within mucosa controls their activity via niche-specific signals that temper cytokine production during homeostasis.

Physically asymmetric division of the C. elegans zygote ensures invariably successful embryogenesis

Asymmetric divisions that yield daughter cells of different sizes are frequent during early embryogenesis, but the importance of such a physical difference for successful development remains poorly understood. Here, we investigated this question using the first division ofCaenorhabditis elegansembryos, which yields a large AB cell and a small P1cell. We equalized AB and P1sizes using acute genetic inactivation or optogenetic manipulation of the spindle positioning protein LIN-5. We uncovered that only some embryos tolerated equalization, and that there was a size asymmetry threshold for viability. Cell lineage analysis of equalized embryos revealed an array of defects, including faster cell cycle progression in P1descendants, as well as defects in cell positioning, division orientation, and cell fate. Moreover, equalized embryos were more susceptible to external compression. Overall, we conclude that unequal first cleavage is essential for invariably successful embryonic development ofC. elegans.

ERECTA family signaling constrains CLAVATA3 and WUSCHEL to the center of the shoot apical meristem

The shoot apical meristem (SAM) is a reservoir of stem cells that gives rise to all post-embryonic aboveground plant organs. The size of the SAM remains stable over time due to a precise balance of stem cell replenishment versus cell incorporation into organ primordia. The WUSCHEL (WUS)/CLAVATA (CLV) negative feedback loop is central to SAM size regulation. Its correct function depends on accurate spatial expression of WUS and CLV3. A signaling pathway, consisting of ERECTA family (ERf) receptors and EPIDERMAL PATTERNING FACTOR LIKE (EPFL) ligands, restricts SAM width and promotes leaf initiation. While ERf receptors are expressed throughout the SAM, EPFL ligands are expressed in its periphery. Our genetic analysis demonstrated that ERfs and CLV3 synergistically regulate the size of the SAM, and wus is epistatic to erfs. Furthermore, activation of ERf signaling with exogenous EPFLs resulted in a rapid decrease of CLV3 and WUS expression. ERf-EPFL signaling inhibits expression of WUS and CLV3 in the periphery of the SAM, confining them to the center. These findings establish the molecular mechanism for stem cell positioning along the radial axis.

Physically asymmetric division of the C. elegans zygote ensures invariably successful embryogenesis

Asymmetric divisions that yield daughter cells of different sizes are frequent during early embryogenesis, but the importance of such a physical difference for successful development remains poorly understood. Here, we investigated this question using the first division of C. elegans embryos, which yields a large AB cell and a small P1 cell. We equalized AB and P1 sizes using acute genetic inactivation or optogenetic manipulation of the spindle positioning protein LIN-5. We uncovered that only some embryos tolerated equalization and that there was a size asymmetry threshold for viability. Cell lineage analysis of equalized embryos revealed an array of defects, including faster cell cycle progression in P1 descendants, as well as defects in cell positioning, division orientation, and cell fate. Moreover, equalized embryos were more susceptible to external compression. Overall, we conclude that unequal first cleavage is essential for invariably successful embryonic development of C. elegans.

Dia1 Coordinates Differentiation and Cell Sorting in a Stratified Epithelium

Although implicated in adhesion, few studies address how actin assembly factors guide cell positioning in multicellular tissue. The formin, Dia1, localizes to the proliferative basal layer of epidermis. In organotypic cultures, Dia1 depletion reduced basal cell density and resulted in stratified tissue with disorganized differentiation and proliferative markers. Since crowding induces differentiation in epidermal tissue, we hypothesized that Dia1 allows cells to reach densities amenable to differentiation prior to stratification. Consistent with this hypothesis, forced crowding of Dia1-deficient cells rescued transcriptional abnormalities. Dia1 promotes rapid growth of lateral adhesions, a behavior consistent with the ability of cells to remain monolayered when crowded. In aggregation assays, cells sorted into distinct layers based on Dia1 expression status. These results suggested that as basal cells proliferate, reintegration and packing of Dia1-positive daughter cells is favored while Dia1-negative cells tend to delaminate to a suprabasal compartment. These data demonstrate how formin expression patterns play a crucial role in constructing distinct domains within stratified epithelia.SummaryHarmon et al demonstrate that differential expression of an actin nucleator, the formin, Dia1, drives cell sorting and maintains distinct morphological domains within an epithelial tissue. This illuminates the possible utility of evolving a large formin family in orchestrating the compartmentalization and differentiation of complex tissues.

Planar cell polarity: moving from single cells to tissue-scale biology

ABSTRACTPlanar cell polarity (PCP) reflects cellular orientation within the plane of an epithelium. PCP is crucial during many biological patterning processes and for organ function. It is omnipresent, from convergent-extension mechanisms during early development through to terminal organogenesis, and it regulates many aspects of cell positioning and orientation during tissue morphogenesis, organ development and homeostasis. Suzanne Eaton used the power of Drosophila as a model system to study PCP, but her vision of, and impact on, PCP studies in flies translates to all animal models. As I highlight here, Suzanne's incorporation of quantitative biophysical studies of whole tissues, integrated with the detailed cell biology of PCP phenomena, completely changed how the field studies this intriguing feature. Moreover, Suzanne's impact on ongoing and future PCP studies is fundamental, long-lasting and transformative.

Morphogenesis of the islets of Langerhans is guided by extra-endocrine Slit2/3 signals

The spatial architecture of the islets of Langerhans is vitally important for their correct function, and alterations in islet morphogenesis often result in diabetes mellitus. We have previously reported that Roundabout (Robo) receptors are required for proper islet morphogenesis. As part of the Slit-Robo signaling pathway, Robo receptors function in conjunction with Slit ligands to mediate axon guidance, cell migration, and cell positioning in development. However, the role of Slit ligands in islet morphogenesis has not yet been determined. Here we report that Slit ligands are expressed in overlapping and distinct patterns in both endocrine and non-endocrine tissues in late pancreas development. We show that function of either Slit2 or Slit3, which are predominantly expressed in the pancreatic mesenchyme, is required and sufficient for islet morphogenesis, while Slit1, which is predominantly expressed in the β-cells, is dispensable for islet morphogenesis. We further show that Slit functions as a repellent signal to β-cells. These data suggest that clustering of endocrine cells during islet morphogenesis is guided, at least in part, by repelling Slit2/3 signals from the pancreatic mesenchyme.