Summary In budding yeast, the highly-conserved small GTPase Cdc42 localizes to the cortex at a cell pole and orchestrates the trafficking and deposition of cell surface materials required for building a bud or mating projection (shmoo). Using a combination of quantitative imaging and mathematical modeling, we elucidate mechanisms of dynamic recycling of Cdc42 that balance diffusion. Rdi1, a guanine nucleotide dissociation inhibitor (GDI), mediates a fast recycling pathway, while actin patch-mediated endocytosis accounts for a slower one. These recycling mechanisms are restricted to the same region of the nascent bud, as both are coupled to the Cdc42 GTPase cycle. We find that a single dynamic parameter, the rate of internalization inside the window of polarized delivery, is tuned to give rise to distinct shapes of Cdc42 distributions that correlate with distinct morphogenetic fates, such as the formation of a round bud or a pointed shmoo.

Cell-cell contact inhibition and the mechanical environment of cells have both been shown to regulate YAP nuclear localization to modulate cell proliferation. Changes in cellular contractility by genetic, pharmacological, and matrix stiffness perturbations regulate YAP nuclear localization. However, because contractility and F-actin organization are interconnected cytoskeletal properties, it remains unclear which of these distinctly regulates YAP localization. Here we show that in the absence of cell-cell contact, actomyosin contractility suppresses YAP phosphorylation at Ser 112 , however, neither loss of contractility nor increase in YAP phosphorylation is sufficient for its nuclear exclusion. We find that actin cytoskeletal integrity is essential for YAP nuclear localization, and can override phosphoregulation or contractility-mediated regulation of YAP nuclear localization. This actin-mediated regulation is conserved during mechanotransduction, as substrate compliance increased YAP phosphorylation and reduced cytoskeletal integrity leading to nuclear exclusion of both YAP and Ser(P) 112 -YAP. These data provide evidence for two actin-mediated pathways for YAP regulation; one in which actomyosin contractility regulates YAP phosphorylation, and a second that involves cytoskeletal integrity-mediated regulation of YAP nuclear localization independent of contractility. We suggest that in non-contact inhibited cells, this latter mechanism may be important in low stiffness regimes, such as may be encountered in physiological environments.

Lipid asymmetry at the plasma membrane is essential for such processes as cell polarity, cytokinesis and phagocytosis1-3. Here we identify the lipid flippase complex, composed of Lem3, Dnf1 or Dnf24, to play a role in the dynamic recycling of the Cdc42 GTPase, a key regulator of cell polarity5, in yeast. By using quantitative microscopy methods, we show that the flippase complex is required for fast dissociation of Cdc42 from the polar cortex by the guanine nucleotide dissociation inhibitor (GDI). A loss of flippase activity, or pharmacological blockage of the inward flipping of phosphatidylethanolamine (PE), a phospholipid with a neutral head group, disrupts Cdc42 polarity maintained by GDI-mediated recycling. PE flipping may reduce the charge interaction between a Cdc42 C-terminal cationic region with the plasma membrane inner leaflet, enriched for the negatively charged lipid phosphatidylserine (PS). Using a reconstituted system with supported lipid bilayers, we show that the relative composition of PE versus PS directly modulates Cdc42 extraction from the membrane by GDI.

Actin-based vesicular trafficking of Cdc42, leading to a polarized concentration of the GTPase, has been implicated in cell polarization, but it was recently debated whether this mechanism allows stable maintenance of cell polarity. Here we show that endocytosis and exocytosis are spatially segregated in the polar plasma membrane, with sites of exocytosis correlating with microdomains of higher concentration and slower diffusion of Cdc42 compared with surrounding regions. Numerical simulations using experimentally obtained diffusion coefficients and trafficking geometry revealed that non-uniform membrane diffusion of Cdc42 in fact enables temporally sustained cell polarity. We show further that phosphatidylserine, a phospholipid recently found to be crucial for cell polarity, is enriched in Cdc42 microdomains. Weakening a potential interaction between phosphatidylserine and Cdc42 enhances Cdc42 diffusion in the microdomains but impedes the strength of polarization. These findings demonstrate a critical role for membrane microdomains in vesicular trafficking-mediated cell polarity.

No abstract

ABSTRACTRetinal ganglion cell (RGC) degeneration is the root cause for vision loss in glaucoma as well as in other forms of optic neuropathies. Genetic analysis indicated abnormal mitochondrial quality control (MQC) as a major risk factor for optic neuropathies. However, nothing is known on how MQC regulates human retinal ganglion cell (hRGC) health and survival. Human pluripotent stem cells (hPSCs) provide opportunity to differentiate hRGCs and understand the abnormal MQC associated hRGC degeneration in great detail. Degradation of damaged mitochondria is a very critical step of MQC, here we have used stem cell derived hRGCs to understand the damaged mitochondrial degradation pathways for hRGC survival. Using pharmacological methods, we have investigated the role of the proteasomal and endo-lysosomal pathways in degrading damaged mitochondria in hRGCs and their precursor stem cells. We find that upon mitochondrial damage with the proton uncoupler carbonyl cyanide m-chlorophenyl hydrazone (CCCP), hRGCs more efficiently degraded mitochondria than their precursor stem cells. We further identified that for degrading damaged mitochondria, stem cells predominantly use the ubiquitine-proteasome system (UPS) while hRGCs use the endo-lysosomal pathway. UPS inhibition causes apoptosis in stem cells, while hRGC viability is dependent on the endo-lysosomal pathway but not on the UPS pathway. This suggests manipulation of the endo-lysosomal pathway could be therapeutically relevant for RGC protection in treating glaucoma. Endo-lysosome dependent cell survival is also conserved for other human neurons as differentiated human cerebral cortical neurons also degenerated upon endo-lysosomal inhibition but not for the proteasome inhibition.SIGNIFICANCE STATEMENTUsing human stem cells we have shown a switch in the mitochondrial degradation pathway during hRGC differentiation where endo-lysosomal pathway becomes the predominant pathway for cellular homeostasis and hRGC survival which is also true for human cortical neurons. These findings suggest manipulation of the endo-lysosomal pathway could be therapeutically relevant for RGC protection in treating glaucoma as well as for other neurodegenerative diseases.

Mitochondrial dysfunctions are widely afflicted in central nervous system (CNS) disorders with minimal understanding on how to improve mitochondrial homeostasis to promote neuroprotection. Here we have used human stem cell differentiated retinal ganglion cells (hRGCs) of the CNS, which are highly sensitive towards mitochondrial dysfunctions due to their unique structure and function, to identify mechanisms for improving mitochondrial quality control (MQC). We show that hRGCs are efficient in maintaining mitochondrial homeostasis through rapid degradation and biogenesis of mitochondria under acute damage. Using a glaucomatous Optineurin mutant (E50K) stem cell line, we show that at basal level mutant hRGCs possess less mitochondrial mass and suffer mitochondrial swelling due to excess ATP production load. Activation of mitochondrial biogenesis through pharmacological inhibition of the Tank binding kinase 1 (TBK1) restores energy homeostasis, mitigates mitochondrial swelling with neuroprotection against acute mitochondrial damage for glaucomatous E50K hRGCs, revealing a novel neuroprotection mechanism.

Inherited retinal degeneration is a group of blinding disorders afflicting more than 1 in 4000 worldwide. These disorders frequently cause the death of photoreceptor cells or retinal ganglion cells. In a subset of these disorders, photoreceptor cell death is a secondary consequence of retinal pigment epithelial cell dysfunction or degeneration. This manuscript reviews current efforts in identifying targets and developing small molecule-based therapies for these devastating neuronal degenerations, for which no cures exist. Photoreceptors and retinal ganglion cells are metabolically demanding owing to their unique structures and functional properties. Modulations of metabolic pathways, which are disrupted in most inherited retinal degenerations, serve as promising therapeutic strategies. In monogenic disorders, great insights were previously obtained regarding targets associated with the defective pathways, including phototransduction, visual cycle, and mitophagy. In addition to these target-based drug discoveries, we will discuss how phenotypic screening can be harnessed to discover beneficial molecules without prior knowledge of their mechanisms of action. Because of major anatomical and biological differences, it has frequently been challenging to model human inherited retinal degeneration conditions using small animals such as rodents. Recent advances in stem cell-based techniques are opening new avenues to obtain pure populations of human retinal ganglion cells and retinal organoids with photoreceptor cells. We will discuss concurrent ideas of utilizing stem-cell-based disease models for drug discovery and preclinical development.