LGN Directs Interphase Endothelial Cell Behavior via the Microtubule Network

Nuclear SUN1 stabilizes endothelial cell junctions via microtubules to regulate blood vessel formation

Endothelial cells line all blood vessels, where they coordinate blood vessel formation and the blood-tissue barrier via regulation of cell-cell junctions. The nucleus also regulates endothelial cell behaviors, but it is unclear how the nucleus contributes to endothelial cell activities at the cell periphery. Here, we show that the nuclear-localized linker of the nucleoskeleton and cytoskeleton (LINC) complex protein SUN1 regulates vascular sprouting and endothelial cell-cell junction morphology and function. Loss of murine endothelial Sun1 impaired blood vessel formation and destabilized junctions, angiogenic sprouts formed but retracted in SUN1-depleted sprouts, and zebrafish vessels lacking Sun1b had aberrant junctions and defective cell-cell connections. At the cellular level, SUN1 stabilized endothelial cell-cell junctions, promoted junction function, and regulated contractility. Mechanistically, SUN1 depletion altered cell behaviors via the cytoskeleton without changing transcriptional profiles. Reduced peripheral microtubule density, fewer junction contacts, and increased catastrophes accompanied SUN1 loss, and microtubule depolymerization phenocopied effects on junctions. Depletion of GEF-H1, a microtubule-regulated Rho activator, or the LINC complex protein nesprin-1 rescued defective junctions of SUN1-depleted endothelial cells. Thus, endothelial SUN1 regulates peripheral cell-cell junctions from the nucleus via LINC complex-based microtubule interactions that affect peripheral microtubule dynamics and Rho-regulated contractility, and this long-range regulation is important for proper blood vessel sprouting and junction integrity.

Targeting Microtubules for the Treatment of Heart Disease

Heart disease remains the leading cause of morbidity and mortality worldwide. With the advancement of modern technology, the role(s) of microtubules in the pathogenesis of heart disease has become increasingly apparent, though currently there are limited treatments targeting microtubule-relevant mechanisms. Here, we review the functions of microtubules in the cardiovascular system and their specific adaptive and pathological phenotypes in cardiac disorders. We further explore the use of microtubule-targeting drugs and highlight promising druggable therapeutic targets for the future treatment of heart diseases.

Interaction between Discs large and Pins/LGN/GPSM2: a comparison across species

The orientation of the mitotic spindle determines the direction of cell division, and therefore contributes to tissue shape and cell fate. Interaction between the multifunctional scaffolding protein Discs large (Dlg) and the canonical spindle orienting factor GPSM2 (called Pins in Drosophila and LGN in vertebrates) has been established in bilaterian models, but its function remains unclear. We used a phylogenetic approach to test whether the interaction is obligate in animals, and in particular whether Pins/LGN/GPSM2 evolved in multicellular organisms as a Dlg-binding protein. We show that Dlg diverged in C. elegans and the syncytial sponge Opsacas minuta and propose that this divergence may correspond with differences in spindle orientation requirements between these organisms and the canonical pathways described in bilaterians. We also demonstrate that Pins/LGN/GPSM2 is present in basal animals, but the established Dlg-interaction site cannot be found in either Placozoa or Porifera. Our results suggest that the interaction between Pins/LGN/GPSM2 and Dlg appeared in Cnidaria, and we therefore speculate that it may have evolved to promote accurate division orientation in the nervous system. This work reveals the evolutionary history of the Pins/LGN/GPSM2-Dlg interaction and suggests new possibilities for its importance in spindle orientation during epithelial and neural tissue development.

Cell Biology: Pardon the Intrusion

Whilst tissues form during development, some cells are extruded from epithelial monolayers. Rather than dying or differentiating, a new study shows that displaced cells can reintegrate after dividing. Surprisingly, this 'intrusion' pathway shares common features with axon guidance.

Orientation of the Mitotic Spindle in Blood Vessel Development

Angiogenesis requires coordinated endothelial cell specification, proliferation, and collective migration. The orientation of endothelial cell division is tightly regulated during the earliest stages of blood vessel formation in response to morphogenetic cues and the controlled orientation of the mitotic spindle. Consequently, oriented cell division is a vital mechanism in vessel morphogenesis, and defective spindle orientation can perturb the spatial arrangement of daughter cells and consequently contribute to several diseases related to vascular development. Many factors affect endothelial cell proliferation and orientation and therefore blood vessel formation, with the relationship between improper spindle orientation in endothelial cells and various diseases extensively studied. Here we review the molecular mechanisms driving the orientation of endothelial cell division, particularly with respect to the mitotic spindle, and how these processes affect vascular development, disease pathogenesis, and their potential as novel targets.

Proteostasis is essential during cochlear development for neuron survival and hair cell polarity

Protein homeostasis is essential to cell function, and a compromised ability to reduce the load of misfolded and aggregated proteins is linked to numerous age-related diseases, including hearing loss. Here, we show that altered proteostasis consequent to Elongator complex deficiency also impacts the proper development of the cochlea and results in deafness. In the absence of the catalytic subunit Elp3, differentiating spiral ganglion neurons display large aggresome-like structures and undergo apoptosis before birth. The cochlear mechanosensory cells are able to survive proteostasis disruption but suffer defects in polarity and stereociliary bundle morphogenesis. We demonstrate that protein aggregates accumulate at the apical surface of hair cells, where they cause a local slowdown of microtubular trafficking, altering the distribution of intrinsic polarity proteins and affecting kinocilium position and length. Alleviation of protein misfolding using the chemical chaperone 4-phenylbutyric acid during embryonic development ameliorates hair cell polarity in Elp3-deficient animals. Our study highlights the importance of developmental proteostasis in the cochlea and unveils an unexpected link between proteome integrity and polarized organization of cellular components.

Quantification of adherens junction disruption and contiguous paracellular protein leak in human lung endothelial cells under septic conditions

OBJECTIVE

Sepsis is associated with dysfunction of MVEC resulting in organ edema and inflammation. VE-cadherin, a component of MVEC adherens junctions, may be disrupted in sepsis. However, the direct connection between individual MVEC VE-cadherin disruption and increased paracellular permeability is uncertain.

METHODS

Human pulmonary MVEC were cultured on a biotin matrix and treated with cytomix, as a model of sepsis, vs PBS. MVEC permeability was assessed by trans-MVEC monolayer leak of Oregon green 488-conjugated avidin, which bound subcellular biotin to localize sites of paracellular leak. Leak was correlated with individual cell-specific MVEC surface VE-cadherin continuity by fluorescence microscopy.

RESULTS

Cytomix treatment reduced total MVEC VE-cadherin density, disrupted surface VE-cadherin continuity, was associated with intercellular gap formation, and enhanced paracellular avidin leak. Cytomix-induced MVEC paracellular avidin leak was strongly correlated temporally and was highly contiguous with focal MVEC surface VE-cadherin disruption. Total cellular VE-cadherin density was less strongly correlated with MVEC paracellular avidin leak and individual cell-specific focal surface VE-cadherin discontinuity.

CONCLUSIONS

These data support a mechanistic link between septic human lung MVEC VE-cadherin disruption and contiguous paracellular protein leak, and will permit more detailed assessment of individual cell-specific mechanisms of septic MVEC barrier dysfunction.

Matrix Stiffness Modulates Mesenchymal Stem Cell Sensitivity to Geometric Asymmetry Signals

Human stem cells hold significant potential for the treatment of various diseases. However, their use as a therapy is hampered because of limited understanding of the mechanisms by which they respond to environmental stimuli. Efforts to understand extracellular biophysical cues have demonstrated the critical roles of geometrical and mechanical signals in determining the fate of stem cells. The goal of this study was to explore the interplay between cell polarity and matrix stiffness in stem cell lineage specification. We hypothesize that confining cells to asymmetric extracellular matrix islands will impart polarity at a single-cell level and will interact with mechanical signals to define the lineage of stem cells. To test these hypotheses, we employed microcontact printing to create patterned symmetric and asymmetric hydrogel islands of soft and hard surface stiffness. Human mesenchymal stem cells (hMSCs) were confined to these islands at the single-cell level and given the ability to differentiate along adipogenic or osteogenic routes. Our results demonstrated that cell polarity defines the lineage specification of hMSCs only on islands with low stiffness. Insight gained from this study provides a rational basis for designing stem cell cultures to enhance tissue engineering and regenerative medicine strategies.

CLASP1 regulates endothelial cell branching morphology and directed migration

Endothelial cell (EC) branching is critically dependent upon the dynamic nature of the microtubule (MT) cytoskeleton. Extracellular matrix (ECM) mechanosensing is a prominent mechanism by which cytoskeletal reorganization is achieved; yet how ECM-induced signaling is able to target cytoskeletal reorganization intracellularly to facilitate productive EC branching morphogenesis is not known. Here, we tested the hypothesis that the composition and density of the ECM drive the regulation of MT growth dynamics in ECs by targeting the MT stabilizing protein, cytoplasmic linker associated protein 1 (CLASP1). High-resolution fluorescent microscopy coupled with computational image analysis reveal that CLASP1 promotes slow MT growth on glass ECMs and promotes short-lived MT growth on high-density collagen-I and fibronectin ECMs. Within EC branches, engagement of either high-density collagen-I or high-density fibronectin ECMs results in reduced MT growth speeds, while CLASP1-dependent effects on MT dynamics promotes elevated numbers of short, branched protrusions that guide persistent and directed EC migration.

Summary: CLASP1 modulates microtubule dynamics with sub-cellular specificity in response to extracellular matrix density and composition. CLASP1 effects on microtubules promote short, branched protrusions that guide persistent and directional EC migration. This article has an associated First Person interview with the first author of the paper as part of the supplementary information.