Sara phosphorylation state controls the dispatch of endosomes from the central spindle during asymmetric division

The Evolution of Extreme Genetic Variability in a Parasite-Resistance Complex

Genomic regions that play a role in parasite defense are often found to be highly variable, with the major histocompatibility complex serving as an iconic example. Single nucleotide polymorphisms may represent only a small portion of this variability, with Indel polymorphisms and copy number variation further contributing. In extreme cases, haplotypes may no longer be recognized as orthologous. Understanding the evolution of such highly divergent regions is challenging because the most extreme variation is not visible using reference-assisted genomic approaches. Here we analyze the case of the Pasteuria Resistance Complex in the crustacean Daphnia magna, a defense complex in the host against the common and virulent bacterium Pasteuria ramosa. Two haplotypes of this region have been previously described, with parts of it being nonhomologous, and the region has been shown to be under balancing selection. Using pan-genome analysis and tree reconciliation methods to explore the evolution of the Pasteuria Resistance Complex and its characteristics within and between species of Daphnia and other Cladoceran species, our analysis revealed a remarkable diversity in this region even among host species, with many nonhomologous hyper-divergent haplotypes. The Pasteuria Resistance Complex is characterized by extensive duplication and losses of Fucosyltransferase (FuT) and Galactosyltransferase (GalT) genes that are believed to play a role in parasite defense. The Pasteuria Resistance Complex region can be traced back to common ancestors over 250 million years. The unique combination of an ancient resistance complex and a dynamic, hyper-divergent genomic environment presents a fascinating opportunity to investigate the role of such regions in the evolution and long-term maintenance of resistance polymorphisms. Our findings offer valuable insights into the evolutionary forces shaping disease resistance and adaptation, not only in the genus Daphnia, but potentially across the entire Cladocera class.

Cellular and molecular mechanisms of asymmetric stem cell division in tissue homeostasis

The asymmetric cell division determines cell diversity and distinct sibling cell fates by mechanisms linked to mitosis. Many adult stem cells divide asymmetrically to balance self-renewal and differentiation. The process of asymmetric cell division involves an axis of polarity and, second, the localization of cell fate determinants at the cell poles. Asymmetric division of stem cells is achieved by intrinsic and extrinsic fate determinants such as signaling molecules, epigenetics factors, molecules regulating gene expression, and polarized organelles. At least some stem cells perform asymmetric and symmetric cell divisions during development. Asymmetric division ensures that the number of stem cells remains constant throughout life. The asymmetric division of stem cells plays an important role in biological events such as embryogenesis, tissue regeneration and carcinogenesis. This review summarizes recent advances in the regulation of asymmetric stem cell division in model organisms.

Microtubule polarity determines the lineage of embryonic neural precursor in zebrafish spinal cord

The phenomenal diversity of neuronal types in the central nervous system is achieved in part by the asymmetric division of neural precursors. In zebrafish neural precursors, asymmetric dispatch of Sara endosomes (with its Notch signaling cargo) functions as fate determinant which mediates asymmetric division. Here, we found two distinct pools of neural precursors based on Sara endosome inheritance and spindle-microtubule enrichment. Symmetric or asymmetric levels of spindle-microtubules drive differently Sara endosomes inheritance and predict neural precursor lineage. We uncover that CAMSAP2a/CAMSAP3a and KIF16Ba govern microtubule asymmetry and endosome motility, unveiling the heterogeneity of neural precursors. Using a plethora of physical and cell biological assays, we determined the physical parameters and molecular mechanisms behind microtubule asymmetries and biased endosome motility. Evolutionarily, the values of those parameters explain why all sensory organ precursor cells are asymmetric in flies while, in zebrafish spinal cord, two populations of neural precursors (symmetric vs asymmetric) are possible.

Elongator stabilizes microtubules to control central spindle asymmetry and polarized trafficking of cell fate determinants

Asymmetric cell division gives rise to two daughter cells that inherit different determinants, thereby acquiring different fates. Polarized trafficking of endosomes containing fate determinants recently emerged as an evolutionarily conserved feature of asymmetric cell division to enhance the robustness of asymmetric cell fate determination in flies, fish and mammals. In particular, polarized sorting of signalling endosomes by an asymmetric central spindle contributes to asymmetric cell division in Drosophila melanogaster. However, how central spindle asymmetry arises remains elusive. Here we identify a moonlighting function of the Elongator complex-an established protein acetylase and tRNA methylase involved in the fidelity of protein translation-as a key factor for central spindle asymmetry. Elongator controls spindle asymmetry by stabilizing microtubules differentially on the anterior side of the central spindle. Accordingly, lowering the activity of Elongator on the anterior side using nanobodies mistargets endosomes to the wrong cell. Molecularly, Elongator regulates microtubule dynamics independently of its acetylation and methylation enzymatic activities. Instead, Elongator directly binds to microtubules and increases their polymerization speed while decreasing their catastrophe frequency. Our data establish a non-canonical role of Elongator at the core of cytoskeleton polarity and asymmetric signalling.

Asymmetric Notch activity by differential inheritance of lysosomes in human neural stem cells

Symmetric and asymmetric cell divisions are conserved strategies for stem cell expansion and the generation of more committed progeny, respectively. Here, we demonstrate that in human neural stem cells (NSCs), lysosomes are asymmetrically inherited during mitosis. We show that lysosomes contain Notch receptors and that Notch activation occurs the acidic lysosome environment. The lysosome asymmetry correlates with the expression of the Notch target gene HES1 and the activity of Notch signaling in the daughter cells. Furthermore, an asymmetry of lysosomes and Notch receptors was also observed in a human organoid model of brain development with mitotic figures showing preferential inheritance of lysosomes and Notch receptor in that daughter cell remaining attached to the apical membrane. Thus, this study suggests a previously unknown function of lysosomes as a signaling hub to establish a bias in Notch signaling activity between daughter cells after an asymmetric cell division of human NSCs.

Protein phosphatase 1: life-course regulation by SDS22 and Inhibitor-3

Protein phosphatase 1 (PP1) is expressed in all eukaryotic cells and catalyzes a sizable fraction of protein Ser/Thr dephosphorylation events. It is tightly regulated in space and time through association with a wide array of regulatory interactors of protein phosphatase one (RIPPOs). Suppressor-of-Dis2-number 2 (SDS22) and Inhibitor-3 (I3), which form a ternary complex with PP1, are the first two evolved and most widely expressed RIPPOs. Their deletion causes mitotic-arrest phenotypes and is lethal in some organisms. The role of SDS22 and I3 in PP1 regulation has been a mystery for decades as they were independently identified as both activators and inhibitors of PP1. This conundrum has largely been solved by recent reports showing that SDS22 and I3 control multiple steps of the life course of PP1. Indeed, they contribute to (a) the stabilization and activation of newly translated PP1, (b) the translocation of PP1 to the nucleus, and (c) the storage of PP1 as a reserve for holoenzyme assembly. Preliminary evidence suggests that SDS22 and I3 may also function as scavengers of released or aged PP1 for re-use in holoenzyme assembly or proteolytical degradation, respectively. Hence, SDS22 and I3 are emerging as master regulators of the life course of PP1.

Fine-Tuning the TGFβ Signaling Pathway by SARA During Neuronal Development

Neural development is a complex process that involves critical events, including cytoskeleton dynamics and selective trafficking of proteins to defined cellular destinations. In this regard, Smad Anchor for Receptor Activation (SARA) is an early endosome resident protein, where perform trafficking- associated functions. In addition, SARA is also involved in cell signaling, including the TGFβ-dependent pathway. Accordingly, SARA, and TGFβ signaling are required for proper axonal specification and migration of cortical neurons, unveiling a critical role for neuronal development. However, the cooperative action between the TGFβ pathway and SARA to this process has remained understudied. In this work, we show novel evidence suggesting a cross-talk between SARA and TGFβ pathway needed for proper polarization, axonal specification, growth and cortical migration of central neurons both in vitro and in vivo. Using microscopy tools and cultured hippocampal neurons, we show a local interaction between SARA and TβRI (TGFβ I receptor) at endosomes. In addition, SARA loss of function, induced by the expression of the dominant-negative SARA-F728A, over-activates the TGFβ pathway, most likely by preserving phosphorylated TβRI. Consequently, SARA-mediated activation of TGFβ pathway impacts on neuronal development, promoting axonal growth and cortical migration of neurons during brain development. Moreover, our data suggests that SARA basally prevents the activation of TβRI through the recruitment of the inhibitory complex PP1c/GADD34 in polarizing neurons. Together, these results propose that SARA is a negative regulator of the TGFβ pathway, being critical for a proper orchestration for neuronal development.

The exocyst complex and Rab5 are required for abscission by localizing ESCRT III subunits to the cytokinetic bridge

Cytokinesis is the final step of cell division following chromosome segregation that generates two daughter cells. The conserved exocyst complex is required for scission of the intercellular cytokinetic bridge, although the molecular mechanisms it employs in this process are unclear. We identify and validate the early endocytic GTPase Rab5 as interacting with the exocyst complex in mammalian cells. Rab5 localizes in the cytokinetic bridge and on the midbody ring in a manner similar to the exocyst complex. Depletion of Rab5 led to delayed abscission. Caenorhabditis elegans orthologs of both exocyst complex subunits and Rab5 localize along the cleavage furrow and are required for cytokinesis in early embryos. Cytokinetic cells depleted of either Rab5 or the exocyst subunits Exoc3 and Exoc4 showed impaired deposition of the endosomal sorting complexes required for transport (ESCRT) III subunits CHMP2B and/or CHMP4B near the midbody ring. The study reveals an evolutionarily conserved role for the early endocytic marker Rab5 in cytokinetic abscission. In addition, it uncovers a key requirement of the exocyst and Rab5 for the delivery of components of the membrane-severing ESCRT III machinery to complete cytokinesis.

New Player in Endosomal Trafficking: Differential Roles of Smad Anchor for Receptor Activation (SARA) Protein

The development and maintenance of multicellular organisms require specialized coordination between external cellular signals and the proteins receiving stimuli and regulating responses. A critical role in the proper functioning of these processes is played by endosomal trafficking, which enables the transport of proteins to targeted sites as well as their return to the plasma membrane through its essential components, the endosomes. During this trafficking, signaling pathways controlling functions related to the endosomal system are activated both directly and indirectly. Although there are a considerable number of molecules participating in these processes, some are more known than others for their specific functions. Toward the end of the 1990s, Smad anchor for receptor activation (SARA) protein was described to be controlling and to facilitate the localization of Smads to transforming growth factor β (TGF-β) receptors during TGF-β signaling activation, and, strikingly, SARA was also identified to be one of the proteins that bind to early endosomes (EEs) participating in membrane trafficking in several cell models. The purpose of this review is to analyze the state of the art of the contribution of SARA in different cell types and cellular contexts, focusing on the biological role of SARA in two main processes, trafficking and cellular signaling, both of which are necessary for intercellular coordination, communication, and development.

Emerging mechanisms of asymmetric stem cell division

Venkei and Yamashita summarize recent advances in our understanding of asymmetric stem cell division in tissue homeostasis.

The asymmetric cell division of stem cells, which produces one stem cell and one differentiating cell, has emerged as a mechanism to balance stem cell self-renewal and differentiation. Elaborate cellular mechanisms that orchestrate the processes required for asymmetric cell divisions are often shared between stem cells and other asymmetrically dividing cells. During asymmetric cell division, cells must establish asymmetry/polarity, which is guided by varying degrees of intrinsic versus extrinsic cues, and use intracellular machineries to divide in a desired orientation in the context of the asymmetry/polarity. Recent studies have expanded our knowledge on the mechanisms of asymmetric cell divisions, revealing the previously unappreciated complexity in setting up the cellular and/or environmental asymmetry, ensuring binary outcomes of the fate determination. In this review, we summarize recent progress in understanding the mechanisms and regulations of asymmetric stem cell division.