Local cytoskeletal and organelle interactions impact molecular-motor- driven early endosomal trafficking

Digitally Assisted Single-Particle Tracking for Accurate Analysis of Complicated Cargo Transport Dynamics in Microtubule Networks

Intracellular transport is a fundamental process crucial for cellular function, driven by the coordinated action of motor proteins that move cargo along microtubule tracks. Traditional tracking methods primarily focus on cargo trajectories, often overlooking rotational dynamics and their impact on cargo interactions with the complex microtubule network. To address this limitation, we introduced a digitally assisted single-particle tracking (dSPT) method that significantly advances the angular resolution of intracellular cargo dynamics. By integrating intensity measurements with advanced digital classification algorithms to process defocused half-plane image patterns captured through bifocal parallax microscopy, this approach extends the angular resolution range from the conventional method to a full 0-360° range, even in heterogeneous cellular environments, while maintaining high spatial and temporal resolutions. In intracellular transport events, we directly observed the accurate determination of the chiral rotational directions and precise calculation of the step angles. When combined with super-resolution radial fluctuation (SRRF) imaging to achieve higher-resolution microtubule imaging, our dSPT technique enables in vivo investigations of cargo dynamics during intracellular transport. To validate this, we studied the rotational dynamics of the cargo in microtubule confinement. Furthermore, we identified characteristic patch-searching patterns in the microtubule network, where cargo exhibited a combined motion pattern of confined and hopping diffusion to navigate through the constraints imposed by the microtubules.

Microtubule acetylation is a biomarker of cytoplasmic health during cellular senescence

Cellular senescence is marked by cytoskeletal dysfunction, yet the role of microtubule post-translational modifications (PTMs) remains unclear. We demonstrate that microtubule acetylation increases during drug-induced senescence in human cells and during natural aging in Drosophila . Elevating acetylation via HDAC6 inhibition or α TAT1 overexpression in BEAS-2B cells disrupts anterograde Rab6A vesicle transport, but spares retrograde transport of Rab5 endosomes. Hyperacetylation results in slowed microtubule polymerization and decreased cytoplasmic fluidity, impeding diffusion of micron-sized condensates. These effects are distinct from enhanced detyrosination, and correlate with altered viscoelasticity and resistance to osmotic stress. Modulating cytoplasmic viscosity reciprocally perturbs microtubule dynamics, revealing bidirectional mechanical regulation. Senescent cells phenocopy hyperacetylated cells, exhibiting analogous effects on transport and microtubule polymerization. Our findings establish acetylation as a biomarker for cytoplasmic health and a potential driver of age-related cytoplasmic densification and organelle transport decline, linking microtubule PTMs to biomechanical feedback loops that exacerbate senescence. This work highlights the role of acetylation in bridging cytoskeletal changes to broader aging hallmarks.

A novel bacterial effector protein mediates ER-LD membrane contacts to regulate host lipid droplets

Effective intracellular communication between cellular organelles occurs at dedicated membrane contact sites (MCSs). Tether proteins are responsible for the establishment of MCSs, enabling direct communication between organelles to ensure organelle function and host cell homeostasis. While recent research has identified tether proteins in several bacterial pathogens, their functions have predominantly been associated with mediating inter-organelle communication between the bacteria containing vacuole (BCV) and the host endoplasmic reticulum (ER). Here, we identify a novel bacterial effector protein, CbEPF1, which acts as a molecular tether beyond the confines of the BCV and facilitates interactions between host cell organelles. Coxiella burnetii, an obligate intracellular bacterial pathogen, encodes the FFAT motif-containing protein CbEPF1 which localizes to host lipid droplets (LDs). CbEPF1 establishes inter-organelle contact sites between host LDs and the ER through its interactions with VAP family proteins. Intriguingly, CbEPF1 modulates growth of host LDs in a FFAT motif-dependent manner. These findings highlight the potential for bacterial effector proteins to impact host cellular homeostasis by manipulating inter-organelle communication beyond conventional BCVs.

Systems mapping of bidirectional endosomal transport through the crowded cell

Kinesin and dynein-dynactin motors move endosomes and other vesicles bidirectionally along microtubules, a process mainly studied under in vitro conditions. Here, we provide a physiological bidirectional transport model following color-coded, endogenously tagged transport-related proteins as they move through a crowded cellular environment. Late endosomes (LEs) surf bidirectionally on Protrudin-enriched endoplasmic reticulum (ER) membrane contact sites, while hopping and gliding along microtubules and bypassing cellular obstacles, such as mitochondria. During bidirectional transport, late endosomes do not switch between opposing Rab7 GTPase effectors, RILP and FYCO1, or their associated dynein and KIF5B motor proteins, respectively. In the endogenous setting, far fewer motors associate with endosomal membranes relative to effectors, implying coordination of transport with other aspects of endosome physiology through GTPase-regulated mechanisms. We find that directionality of transport is provided in part by various microtubule-associated proteins (MAPs), including MID1, EB1, and CEP169, which recruit Lis1-activated dynein motors to microtubule plus ends for transport of early and late endosomal populations. At these microtubule plus ends, activated dynein motors encounter the dynactin subunit p150glued and become competent for endosomal capture and minus-end movement in collaboration with membrane-associated Rab7-RILP. We show that endosomes surf over the ER through the crowded cell and move bidirectionally under the control of MAPs for motor activation and through motor replacement and capture by endosomal anchors.

Microtubule-associated protein MAP7 promotes tubulin posttranslational modifications and cargo transport to enable osmotic adaptation

Cells remodel their cytoskeletal networks to adapt to their environment. Here, we analyze the mechanisms utilized by the cell to tailor its microtubule landscape in response to changes in osmolarity that alter macromolecular crowding. By integrating live-cell imaging, ex vivo enzymatic assays, and in vitro reconstitution, we probe the impact of cytoplasmic density on microtubule-associated proteins (MAPs) and tubulin posttranslational modifications (PTMs). We find that human epithelial cells respond to fluctuations in cytoplasmic density by modulating microtubule acetylation, detyrosination, or MAP7 association without differentially affecting polyglutamylation, tyrosination, or MAP4 association. These MAP-PTM combinations alter intracellular cargo transport, enabling the cell to respond to osmotic challenges. We further dissect the molecular mechanisms governing tubulin PTM specification and find that MAP7 promotes acetylation and inhibits detyrosination. Our data identify MAP7 in modulating the tubulin code, resulting in microtubule cytoskeleton remodeling and alteration of intracellular transport as an integrated mechanism of cellular adaptation.

Using an ER-specific optogenetic mechanostimulator to understand the mechanosensitivity of the endoplasmic reticulum

The ability of cells to perceive and respond to mechanical cues is essential for numerous biological activities. Emerging evidence indicates important contributions of organelles to cellular mechanosensitivity and mechanotransduction. However, whether and how the endoplasmic reticulum (ER) senses and reacts to mechanical forces remains elusive. To fill the knowledge gap, after developing a light-inducible ER-specific mechanostimulator (LIMER), we identify that mechanostimulation of ER elicits a transient, rapid efflux of Ca2+ from ER in monkey kidney COS-7 cells, which is dependent on the cation channels transient receptor potential cation channel, subfamily V, member 1 (TRPV1) and polycystin-2 (PKD2) in an additive manner. This ER Ca2+ release can be repeatedly stimulated and tuned by varying the intensity and duration of force application. Moreover, ER-specific mechanostimulation inhibits ER-to-Golgi trafficking. Sustained mechanostimuli increase the levels of binding-immunoglobulin protein (BiP) expression and phosphorylated eIF2α, two markers for ER stress. Our results provide direct evidence for ER mechanosensitivity and tight mechanoregulation of ER functions, placing ER as an important player on the intricate map of cellular mechanotransduction.

Principles of organelle positioning in motile and non-motile cells

Cells are equipped with asymmetrically localised and functionally specialised components, including cytoskeletal structures and organelles. Positioning these components to specific intracellular locations in an asymmetric manner is critical for their functionality and affects processes like immune responses, tissue maintenance, muscle functionality, and neurobiology. Here, we provide an overview of strategies to actively move, position, and anchor organelles to specific locations. By conceptualizing the cytoskeletal forces and the organelle-to-cytoskeleton connectivity, we present a framework of active positioning of both membrane-enclosed and membrane-less organelles. Using this framework, we discuss how different principles of force generation and organelle anchorage are utilised by different cells, such as mesenchymal and amoeboid cells, and how the microenvironment influences the plasticity of organelle positioning. Given that motile cells face the challenge of coordinating the positioning of their content with cellular motion, we particularly focus on principles of organelle positioning during migration. In this context, we discuss novel findings on organelle positioning by anchorage-independent mechanisms and their advantages and disadvantages in motile as well as stationary cells.

Single-molecule imaging of stochastic interactions that drive dynein activation and cargo movement in cells

Cytoplasmic dynein 1 (dynein) is the primary minus end-directed motor protein in most eukaryotic cells. Dynein remains in an inactive conformation until the formation of a tripartite complex comprising dynein, its regulator dynactin, and a cargo adaptor. How this process of dynein activation occurs is unclear since it entails the formation of a three-protein complex inside the crowded environs of a cell. Here, we employed live-cell, single-molecule imaging to visualize and track fluorescently tagged dynein. First, we observed that only ∼30% of dynein molecules that bound to the microtubule (MT) engaged in minus end-directed movement, and that too for a short duration of ∼0.6 s. Next, using high-resolution imaging in live and fixed cells and using correlative light and electron microscopy, we discovered that dynactin and endosomal cargo remained in proximity to each other and to MTs. We then employed two-color imaging to visualize cargo movement effected by single motor binding. Finally, we performed long-term imaging to show that short movements are sufficient to drive cargo to the perinuclear region of the cell. Taken together, we discovered a search mechanism that is facilitated by dynein's frequent MT binding-unbinding kinetics: (i) in a futile event when dynein does not encounter cargo anchored in proximity to the MT, dynein dissociates and diffuses into the cytoplasm, (ii) when dynein encounters cargo and dynactin upon MT binding, it moves cargo in a short run. Several of these short runs are undertaken in succession for long-range directed movement. In conclusion, we demonstrate that dynein activation and cargo capture are coupled in a step that relies on the reduction of dimensionality to enable minus end-directed transport in cellulo and that complex cargo behavior emerges from stochastic motor-cargo interactions.

Coordination, cooperation, competition, crowding and congestion of molecular motors: Theoretical models and computer simulations

Cytoskeletal motor proteins are biological nanomachines that convert chemical energy into mechanical work to carry out various functions such as cell division, cell motility, cargo transport, muscle contraction, beating of cilia and flagella, and ciliogenesis. Most of these processes are driven by the collective operation of several motors in the crowded viscous intracellular environment. Imaging and manipulation of the motors with powerful experimental probes have been complemented by mathematical analysis and computer simulations of the corresponding theoretical models. In this article, we illustrate some of the key theoretical approaches used to understand how coordination, cooperation and competition of multiple motors in the crowded intra-cellular environment drive the processes that are essential for biological function of a cell. In spite of the focus on theory, experimentalists will also find this article as an useful summary of the progress made so far in understanding multiple motor systems.

Making the connection: How membrane contact sites have changed our view of organelle biology

The view of organelles and how they operate together has changed dramatically over the last two decades. The textbook view of organelles was that they operated largely independently and were connected by vesicular trafficking and the diffusion of signals through the cytoplasm. We now know that all organelles make functional close contacts with one another, often called membrane contact sites. The study of these sites has moved to center stage in cell biology as it has become clear that they play critical roles in healthy and developing cells and during cell stress and disease states. Contact sites have important roles in intracellular signaling, lipid metabolism, motor-protein-mediated membrane dynamics, organelle division, and organelle biogenesis. Here, we summarize the major conceptual changes that have occurred in cell biology as we have come to appreciate how contact sites integrate the activities of organelles.

Single-molecule analysis of intracellular insulin granule behavior and its application to analyzing cytoskeletal dependence and pathophysiological implications

Introduction: Mobilization of intracellular insulin granules to the plasma membrane plays a crucial role in regulating insulin secretion. However, the regulatory mechanisms of this mobilization process have been poorly understood due to technical limitations. In this study, we propose a convenient approach for assessing intracellular insulin granule behavior based on single-molecule analysis of insulin granule membrane proteins labeled with Quantum dot fluorescent nanocrystals. Methods: This approach allows us to analyze intracellular insulin granule movement with subpixel accuracy at 33 fps. We tracked two insulin granule membrane proteins, phogrin and zinc transporter 8, fused to HaloTag in rat insulinoma INS-1 cells and, by evaluating the tracks with mean-square displacement, demonstrated the characteristic behavior of insulin granules. Results and discussion: Pharmacological perturbations of microtubules and F-actin affected insulin granule behavior on distinct modalities. Specifically, microtubule dynamics and F-actin positively and negatively regulate insulin granule behavior, respectively, presumably by modulating each different behavioral mode. Furthermore, we observed impaired insulin granule behavior and cytoskeletal architecture under chronic treatment of high concentrations of glucose and palmitate. Our approach provides detailed information regarding intracellular insulin granule mobilization and its pathophysiological implications. This study sheds new light on the regulatory mechanisms of intracellular insulin granule mobilization and has important implications for understanding the pathogenesis of diabetes.

HEATR5B associates with dynein-dynactin and promotes motility of AP1-bound endosomal membranes

The microtubule motor dynein mediates polarised trafficking of a wide variety of organelles, vesicles and macromolecules. These functions are dependent on the dynactin complex, which helps recruit cargoes to dynein's tail and activates motor movement. How the dynein-dynactin complex orchestrates trafficking of diverse cargoes is unclear. Here, we identify HEATR5B, an interactor of the adaptor protein-1 (AP1) clathrin adaptor complex, as a novel player in dynein-dynactin function. HEATR5B was recovered in a biochemical screen for proteins whose association with the dynein tail is augmented by dynactin. We show that HEATR5B binds directly to the dynein tail and dynactin and stimulates motility of AP1-associated endosomal membranes in human cells. We also demonstrate that the Drosophila HEATR5B homologue is an essential gene that selectively promotes dynein-based transport of AP1-bound membranes to the Golgi apparatus. As HEATR5B lacks the coiled-coil architecture typical of dynein adaptors, our data point to a non-canonical process orchestrating motor function on a specific cargo. We additionally show that HEATR5B promotes association of AP1 with endosomal membranes independently of dynein. Thus, HEATR5B co-ordinates multiple events in AP1-based trafficking.

The kinesin-5 tail and bipolar minifilament domains are the origin of its microtubule crosslinking and sliding activity

Kinesin-5 crosslinks and slides apart microtubules to assemble, elongate, and maintain the mitotic spindle. Kinesin-5 is a tetramer, where two N-terminal motor domains are positioned at each end of the motor, and the coiled-coil stalk domains are organized into a tetrameric bundle through the bipolar assembly (BASS) domain. To dissect the function of the individual structural elements of the motor, we constructed a minimal kinesin-5 tetramer (mini-tetramer). We determined the x-ray structure of the extended, 34-nm BASS domain. Guided by these structural studies, we generated active bipolar kinesin-5 mini-tetramer motors from Drosophila melanogastor and human orthologues which are half the length of native kinesin-5. We then used these kinesin-5 mini-tetramers to examine the role of two unique structural adaptations of kinesin-5: 1) the length and flexibility of the tetramer, and 2) the C-terminal tails which interact with the motor domains to coordinate their ATPase activity. The C-terminal domain causes frequent pausing and clustering of kinesin-5. By comparing microtubule crosslinking and sliding by mini-tetramer and full-length kinesin-5, we find that both the length and flexibility of kinesin-5 and the C-terminal tails govern its ability to crosslink microtubules. Once crosslinked, stiffer mini-tetramers slide antiparallel microtubules more efficiently than full-length motors.

Deterministic early endosomal maturations emerge from a stochastic trigger-and-convert mechanism

Endosomal maturation is critical for robust and timely cargo transport to specific cellular compartments. The most prominent model of early endosomal maturation involves a phosphoinositide-driven gain or loss of specific proteins on individual endosomes, emphasising an autonomous and stochastic description. However, limitations in fast, volumetric imaging long hindered direct whole cell-level measurements of absolute numbers of maturation events. Here, we use lattice light-sheet imaging and bespoke automated analysis to track individual very early (APPL1-positive) and early (EEA1-positive) endosomes over the entire population, demonstrating that direct inter-endosomal contact drives maturation between these populations. Using fluorescence lifetime, we show that this endosomal interaction is underpinned by asymmetric binding of EEA1 to very early and early endosomes through its N- and C-termini, respectively. In combination with agent-based simulation which supports a 'trigger-and-convert' model, our findings indicate that APPL1- to EEA1-positive maturation is driven not by autonomous events but by heterotypic EEA1-mediated interactions, providing a mechanism for temporal and population-level control of maturation.

Macromolecular Crowding Tailors the Microtubule Cytoskeleton Through Tubulin Modifications and Microtubule-Associated Proteins

Cells remodel their cytoskeletal networks to adapt to their environment. Here, we analyze the mechanisms utilized by the cell to tailor its microtubule landscape in response to changes in osmolarity that alter macromolecular crowding. By integrating live cell imaging, ex vivo enzymatic assays, and in vitro reconstitution, we probe the impact of acute perturbations in cytoplasmic density on microtubule-associated proteins (MAPs) and tubulin posttranslational modifications (PTMs), unraveling the molecular underpinnings of cellular adaptation via the microtubule cytoskeleton. We find that cells respond to fluctuations in cytoplasmic density by modulating microtubule acetylation, detyrosination, or MAP7 association, without differentially affecting polyglutamylation, tyrosination, or MAP4 association. These MAP-PTM combinations alter intracellular cargo transport, enabling the cell to respond to osmotic challenges. We further dissect the molecular mechanisms governing tubulin PTM specification, and find that MAP7 promotes acetylation by biasing the conformation of the microtubule lattice, and directly inhibits detyrosination. Acetylation and detyrosination can therefore be decoupled and utilized for distinct cellular purposes. Our data reveal that the MAP code dictates the tubulin code, resulting in remodeling of the microtubule cytoskeleton and alteration of intracellular transport as an integrated mechanism of cellular adaptation.

Paxillin regulates Rab5-mediated vesicle motility through modulating microtubule acetylation

Rab GTPase-mediated vesicle trafficking of cell surface proteins, including integrins, through endocytic and recycling pathways is important in controlling cell-extracellular matrix interactions during cell migration. The focal adhesion adaptor protein, paxillin, plays a central role in regulating adhesion dynamics and was previously shown to promote anterograde vesicle trafficking through modulation of microtubule acetylation via its inhibition of the deacetylase HDAC6. The role of paxillin in retrograde trafficking is unknown. Herein, we identified a role for paxillin in the modulation of the Rab5 GTPase, which is necessary for regulating early endosome dynamics and focal adhesion turnover. Using MDA-MB-231 breast cancer cells and paxillin (-/-) fibroblasts, paxillin was shown to impact Rab5-associated vesicle size and distribution, as well as Rab5 GTPase activity, through its modulation of HDAC6. Using a combination of real-time imaging and particle tracking analysis, paxillin was shown to promote Rab5-associated vesicle motility through inhibition of HDAC6-mediated micro-tubule deacetylation, along with the localization of active integrin to focal adhesions.

Ensemble heterogeneity mimics ageing for endosomal dynamics within eukaryotic cells

Transport processes of many structures inside living cells display anomalous diffusion, such as endosomes in eukaryotic cells. They are also heterogeneous in space and time. Large ensembles of single particle trajectories allow the heterogeneities to be quantified in detail and provide insights for mathematical modelling. The development of accurate mathematical models for heterogeneous dynamics has the potential to enable the design and optimization of various technological applications, for example, the design of effective drug delivery systems. Central questions in the analysis of anomalous dynamics are ergodicity and statistical ageing which allow for selecting the proper model for the description. It is believed that non-ergodicity and ageing occur concurrently. However, we found that the anomalous dynamics of endosomes is paradoxical since it is ergodic but shows ageing. We show that this behaviour is caused by ensemble heterogeneity that, in addition to space-time heterogeneity within a single trajectory, is an inherent property of endosomal motion. Our work introduces novel approaches for the analysis and modelling of heterogeneous dynamics.

Kazrin promotes dynein/dynactin-dependent traffic from early to recycling endosomes

Kazrin is a protein widely expressed in vertebrates whose depletion causes a myriad of developmental defects, in part derived from altered cell adhesion and migration, as well as failure to undergo epidermal to mesenchymal transition. However, the primary molecular role of kazrin, which might contribute to all these functions, has not been elucidated yet. We previously identified one of its isoforms, kazrin C, as a protein that potently inhibits clathrin-mediated endocytosis when overexpressed. We now generated kazrin knock-out mouse embryonic fibroblasts to investigate its endocytic function. We found that kazrin depletion delays juxtanuclear enrichment of internalized material, indicating a role in endocytic traffic from early to recycling endosomes. Consistently, we found that the C-terminal domain of kazrin C, predicted to be an intrinsically disordered region, directly interacts with several early endosome (EE) components, and that kazrin depletion impairs retrograde motility of these organelles. Further, we noticed that the N-terminus of kazrin C shares homology with dynein/dynactin adaptors and that it directly interacts with the dynactin complex and the dynein light intermediate chain 1. Altogether, the data indicate that one of the primary kazrin functions is to facilitate endocytic recycling by promoting dynein/dynactin-dependent transport of EEs or EE-derived transport intermediates to the recycling endosomes.

Huntingtin S421 phosphorylation increases kinesin and dynein engagement on early endosomes and lysosomes

Huntingtin (HTT) is a scaffolding protein that recruits motor proteins to vesicular cargoes, enabling it to regulate kinesin-1, dynein, and myosin-VI-dependent transport. To maintain the native stoichiometry of HTT with its interacting partners, we used CRISPR/Cas9 to induce a phosphomimetic mutation of the endogenous HTT at S421 (HTT-S421D). Using single-particle tracking, optical tweezers, and immunofluorescence, we examined the effects of this mutation on the motility of early endosomes and lysosomes. In HTT-S421D cells, lysosomes exhibit longer displacements and higher processive fractions compared with wild-type (HTT-WT) cells. Kinesins and dyneins exert greater forces on early endosomes and lysosomes in cells expressing HTT-S421D. In addition, endosomes bind to microtubules faster and are more resistant to detachment under load. The recruitment of kinesins and dyneins to microtubules is enhanced in HTT-S421D cells. In contrast, overexpression of HTT had variable effects on the processivity, displacement, and directional bias of both early endosomes and lysosomes. These data indicate that phosphorylation of the endogenous HTT causes early endosomes and lysosomes to move longer distances and more processively by recruiting and activating both kinesin-1 and dynein.

Activity-assisted barrier crossing of self-propelled colloids over parallel microgrooves

We report a systematic study of the dynamics of self-propelled particles (SPPs) over a one-dimensional periodic potential landscape U_{0}(x), which is fabricated on a microgroove-patterned polydimethylsiloxane (PDMS) substrate. From the measured nonequilibrium probability density function P(x;F_{0}) of the SPPs, we find that the escape dynamics of the slow rotating SPPs across the potential landscape can be described by an effective potential U_{eff}(x;F_{0}), once the self-propulsion force F_{0} is included into the potential under the fixed angle approximation. This work demonstrates that the parallel microgrooves provide a versatile platform for a quantitative understanding of the interplay among the self-propulsion force F_{0}, spatial confinement by U_{0}(x), and thermal noise, as well as its effects on activity-assisted escape dynamics and transport of the SPPs.

Endosomal sorting sorted - motors, adaptors and lessons from in vitro and cellular studies

Motor proteins are key players in exerting spatiotemporal control over the intracellular location of membrane-bound compartments, including endosomes containing cargo. In this Review, we focus on how motors and their cargo adaptors regulate positioning of cargoes from the earliest stages of endocytosis and through the two main intracellular itineraries: (1) degradation at the lysosome or (2) recycling back to the plasma membrane. In vitro and cellular (in vivo) studies on cargo transport thus far have typically focussed independently on either the motor proteins and adaptors, or membrane trafficking. Here, we will discuss recent studies to highlight what is known about the regulation of endosomal vesicle positioning and transport by motors and cargo adaptors. We also emphasise that in vitro and cellular studies are often performed at different scales, from single molecules to whole organelles, with the aim to provide a perspective on the unified principles of motor-driven cargo trafficking in living cells that can be learned from these differing scales.

Endosome biogenesis is controlled by ER and the cytoskeleton at tripartite junctions

The plasma membrane (PM) and its associated cargo are internalized into small vesicles via endocytosis funneling cargo into endosomes. The endosomal system must efficiently deliver cargos, as well as recycle cargo receptors and membrane to maintain homeostasis. In animal cells, endosome trafficking, maturation, and cargo recycling rely on the actin and microtubule cytoskeleton. Microtubules and their associated motor proteins provide the roads on which endosomes move and fuse during cargo sorting and delivery. In addition, highly dynamic assemblies of actin adjust the shape of the endosomal membrane to promote cargo segregation into budding domains allowing for receptor recycling. Recent work has revealed that the endoplasmic reticulum (ER) frequently acts as an intermediary between endosomes and their cytoskeletal regulators via membrane contact sites (MCSs). This review will discuss the factors which form these tripartite junction between the ER, endosomes, and the cytoskeleton as well as their function.

Tex2 is required for lysosomal functions at TMEM55-dependent ER membrane contact sites

ER tubules form and maintain membrane contact sites (MCSs) with late endosomes/lysosomes (LE/lys). The molecular composition and cellular functions of these MCSs are poorly understood. Here, we find that Tex2, an SMP domain-containing lipid transfer protein conserved in metazoan and yeast, is a tubular ER protein and is recruited to ER-LE/lys MCSs by TMEM55, phosphatases that convert PI(4,5)P2 to PI5P on LE/lys. We show that the Tex2-TMEM55 interaction occurs between an N-terminal region of Tex2 and a catalytic motif in the PTase domain of TMEM55. The Tex2-TMEM55 interaction can be regulated by endosome-resident type 2 PI4K activities. Functionally, Tex2 knockout results in defects in lysosomal trafficking, digestive capacity, and lipid composition of LE/lys membranes. Together, our data identify Tex2 as a tubular ER protein that resides at TMEM55-dependent ER-LE/lys MCSs required for lysosomal functions.

Intracellular Dynamics of Extracellular Vesicles by Segmented Trajectory Analysis

The analysis of nanoparticle (NP) dynamics in live cell studies by video tracking provides detailed information on their interactions and trafficking in the cells. Although the video analysis is not yet routinely used in NP studies, the equipment suitable for the experiments is already available in most laboratories. Here, we compare trajectory patterns, diffusion coefficients, and particle velocities of NPs in A549 cells with a rather simple experimental setup consisting of a fluorescence microscope and openly available trajectory analysis software. The studied NPs include commercial fluorescent polymeric particles and two subpopulations of PC-3 cell-derived extracellular vesicles (EVs). As bioderived natural nanoparticles, the fluorescence intensities of the EVs limited the recording speed. Therefore, we studied the effect of the recording frame rate and analysis parameters to the trajectory results with bright fluorescent commercial NPs. We show that the trajectory classification and the apparent particle velocities are affected by the recording frame rate, while the diffusion constants stay comparable. The NP trajectory patterns were similar for all NP types and resembled intracellular vesicular transport. Interestingly, the EV movements were faster than the commercial NPs, which contrasts with their physical sizes and may indicate a greater role of the motor proteins in their intracellular transports.

Choreographing the motor-driven endosomal dance

The endosomal system orchestrates the transport of lipids, proteins and nutrients across the entire cell. Along their journey, endosomes mature, change shape via fusion and fission, and communicate with other organelles. This intriguing endosomal choreography, which includes bidirectional and stop-and-go motions, is coordinated by the microtubule-based motor proteins dynein and kinesin. These motors bridge various endosomal subtypes to the microtubule tracks thanks to their cargo-binding domain interacting with endosome-associated proteins, and their motor domain interacting with microtubules and associated proteins. Together, these interactions determine the mobility of different endosomal structures. In this Review, we provide a comprehensive overview of the factors regulating the different interactions to tune the fascinating dance of endosomes along microtubules.

Tau differentially regulates the transport of early endosomes and lysosomes

Microtubule-associated proteins (MAPs) modulate the motility of kinesin and dynein along microtubules to control the transport of vesicles and organelles. The neuronal MAP tau inhibits kinesin-dependent transport. Phosphorylation of tau at Tyr-18 by fyn kinase results in weakened inhibition of kinesin-1. We examined the motility of early endosomes and lysosomes in cells expressing wild-type (WT) tau and phosphomimetic Y18E tau. We quantified the effects on motility as a function of the tau expression level. Lysosome motility is strongly inhibited by tau. Y18E tau preferentially inhibits lysosomes in the cell periphery, while centrally located lysosomes are less affected. Early endosomes are more sensitive to tau than lysosomes and are inhibited by both WT and Y18E tau. Our results show that different cargoes have disparate responses to tau, likely governed by the types of kinesin motors driving their transport. In support of this model, kinesin-1 and -3 are strongly inhibited by tau while kinesin-2 and dynein are less affected. In contrast to kinesin-1, we find that kinesin-3 is strongly inhibited by phosphorylated tau.

Hitchhiking Across Kingdoms: Cotransport of Cargos in Fungal, Animal, and Plant Cells

Eukaryotic cells across the tree of life organize their subcellular components via intracellular transport mechanisms. In canonical transport, myosin, kinesin, and dynein motor proteins interact with cargos via adaptor proteins and move along filamentous actin or microtubule tracks. In contrast to this canonical mode, hitchhiking is a newly discovered mode of intracellular transport in which a cargo attaches itself to an already-motile cargo rather than directly associating with a motor protein itself. Many cargos including messenger RNAs, protein complexes, and organelles hitchhike on membrane-bound cargos. Hitchhiking-like behaviors have been shown to impact cellular processes including local protein translation, long-distance signaling, and organelle network reorganization. Here, we review instances of cargo hitchhiking in fungal, animal, and plant cells and discuss the potential cellular and evolutionary importance of hitchhiking in these different contexts.

Spatiotemporal tracking of the transport of RNA nano-drugs: from transmembrane to intracellular delivery

The popularity of RNA nanoparticles (RNPs) has risen rapidly during the past decade due to the development of RNA nanotechnology. Understanding the fast dynamic process of cell entry and intracellular delivery of RNPs is essential for the design of intelligent therapeutic RNA nano-drugs and mRNA vaccines.How the interaction between the membrane and target ligand of RNPs influences the cell entry, and how the dynamic mechanism of RNPs takes place in different organelles remain ill-defined. Herein, the cell entry of Antimir21-RNP-Apt is monitored using a force tracing technique with a high spatiotemporal resolution at the single particle level, the specific interaction of Apt and EGFR promotes the cell entry efficiency and achieves long-lasting curative effects. Furthermore, the intracellular delivery pathway through different organelles is discovered using fluorescence tracking, and the low motility in early endosomes and the high motility in late endosomes are analyzed. This report provides key strategies for engineering RNA nanomedicines and facilitating clinical translation.

PKAN hiPS-Derived Astrocytes Show Impairment of Endosomal Trafficking: A Potential Mechanism Underlying Iron Accumulation

PKAN disease is caused by mutations in the PANK2 gene, encoding the mitochondrial enzyme pantothenate kinase 2, catalyzing the first and key reaction in Coenzyme A (CoA) biosynthetic process. This disorder is characterized by progressive neurodegeneration and excessive iron deposition in the brain. The pathogenic mechanisms of PKAN are still unclear, and the available therapies are only symptomatic. Although iron accumulation is a hallmark of PKAN, its relationship with CoA dysfunction is not clear. We have previously developed hiPS-derived astrocytes from PKAN patients showing iron overload, thus recapitulating the human phenotype. In this work, we demonstrated that PKAN astrocytes presented an increase in transferrin uptake, a key route for cellular iron intake via transferrin receptor-mediated endocytosis of transferrin-bound iron. Investigation of constitutive exo-endocytosis and vesicular dynamics, exploiting the activity-enriching biosensor SynaptoZip, led to the finding of a general impairment in the constitutive endosomal trafficking in PKAN astrocytes. CoA and 4-phenylbutyric acid treatments were found to be effective in partially rescuing the aberrant vesicular behavior and iron intake. Our results demonstrate that the impairment of CoA biosynthesis could interfere with pivotal intracellular mechanisms involved in membrane fusions and vesicular trafficking, leading to an aberrant transferrin receptor-mediated iron uptake.

The sodium proton exchanger NHE9 regulates phagosome maturation and bactericidal activity in macrophages

Acidification of phagosomes is essential for the bactericidal activity of macrophages. Targeting machinery that regulates pH within the phagosomes is a prominent strategy employed by various pathogens that have emerged as major threats to public health. Nascent phagosomes acquire the machinery for pH regulation through a graded maturation process involving fusion with endolysosomes. In addition, meticulous coordination between proton pumping and leakage mechanisms is crucial for maintaining optimal pH within the phagosome. However, relative to mechanisms involved in acidifying the phagosome lumen, little is known about proton leakage pathways in this organelle. Sodium proton transporter NHE9 is a known proton leakage pathway located on the endosomes. As phagosomes acquire proteins through fusions with endosomes during maturation, NHE9 seemed a promising candidate for regulating proton fluxes on the phagosome. Here, using genetic and biophysical approaches, we show NHE9 is an important proton leakage pathway associated with the maturing phagosome. NHE9 is highly expressed in immune cells, specifically macrophages; however, NHE9 expression is strongly downregulated upon bacterial infection. We show that compensatory ectopic NHE9 expression hinders the directed motion of phagosomes along microtubules and promotes early detachment from the microtubule tracks. As a result, these phagosomes have shorter run lengths and are not successful in reaching the lysosome. In accordance with this observation, we demonstrate that NHE9 expression levels negatively correlate with bacterial survival. Together, our findings show that NHE9 regulates lumenal pH to affect phagosome maturation, and consequently, microbicidal activity in macrophages.

Spatiotemporal three-dimensional transport dynamics of endocytic cargos and their physical regulations in cells

Intracellular transport, regulated by complex cytoarchitectures and active driving forces, is crucial for biomolecule translocations and relates to many cellular functions. Despite extensive knowledge obtained from two-dimensional (2D) experiments, the real three-dimensional (3D) spatiotemporal characteristics of intracellular transport is still unclear. With 3D single-particle tracking, we comprehensively studied the transport dynamics of endocytic cargos. With varying timescale, the intracellular transport changes from thermal-dominated 3D-constrained motion to active-dominated quasi-2D motion. Spatially, the lateral motion is heterogeneous with peripheral regions being faster than perinuclear regions, while the axial motion is homogeneous across the cells. We further confirmed that such anisotropy and heterogeneity of vesicle transport result from actively directed motion on microtubules. Strikingly, inside the vesicles, we observed endocytic nanoparticles make diffusive motions on their inner membranes when microtubules are absent, suggesting endocytic cargos are normally localized at the inner vesicle membranes through a physical connection to the microtubules outside during transport.

Perinuclear positioning of endosomes can affect PS-ASO activities

Phosphorothioate (PS) modified antisense oligonucleotide (ASO) drugs that act on cellular RNAs must enter cells and be released from endocytic organelles to elicit antisense activity. It has been shown that PS-ASOs are mainly released by late endosomes. However, it is unclear how endosome movement in cells contributes to PS-ASO activity. Here, we show that PS-ASOs in early endosomes display Brownian type motion and migrate only short distances, whereas PS-ASOs in late endosomes (LEs) move linearly along microtubules with substantial distances. In cells with normal microtubules and LE movement, PS-ASO-loaded LEs tend to congregate perinuclearly. Disruption of perinuclear positioning of LEs by reduction of dynein 1 decreased PS-ASO activity, without affecting PS-ASO cellular uptake. Similarly, disruption of perinuclear positioning of PS-ASO-LE foci by reduction of ER tethering proteins RNF26, SQSTM1 and UBE2J1, or by overexpression of P50 all decreased PS-ASO activity. However, enhancing perinuclear positioning through reduction of USP15 or over-expression of RNF26 modestly increased PS-ASO activity, indicating that LE perinuclear positioning is required for ensuring efficient PS-ASO release. Together, these observations suggest that LE movement along microtubules and perinuclear positioning affect PS-ASO productive release.

A two-photon fluorescence probe for cell membrane imaging under temporal-focusing multiphoton excitation microscopy (TFMPEM)

A small-sized chromophore, BTTA-2OH, manifesting favorable solubility, large two-photon excitation efficiency, and good fluorescence photostability was synthesized to label the membrane of living cells for visualizing the dynamic movement of membrane-related vesicles via a two-photon fluorescence imaging technique based on wavelength-tunable temporal-focusing multiphoton excitation microscopy.

Intertwined and Finely Balanced: Endoplasmic Reticulum Morphology, Dynamics, Function, and Diseases

The endoplasmic reticulum (ER) is an organelle that is responsible for many essential subcellular processes. Interconnected narrow tubules at the periphery and thicker sheet-like regions in the perinuclear region are linked to the nuclear envelope. It is becoming apparent that the complex morphology and dynamics of the ER are linked to its function. Mutations in the proteins involved in regulating ER structure and movement are implicated in many diseases including neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and amyotrophic lateral sclerosis (ALS). The ER is also hijacked by pathogens to promote their replication. Bacteria such as Legionella pneumophila and Chlamydia trachomatis, as well as the Zika virus, bind to ER morphology and dynamics-regulating proteins to exploit the functions of the ER to their advantage. This review covers our understanding of ER morphology, including the functional subdomains and membrane contact sites that the organelle forms. We also focus on ER dynamics and the current efforts to quantify ER motion and discuss the diseases related to ER morphology and dynamics.

Local Analysis of Heterogeneous Intracellular Transport: Slow and Fast Moving Endosomes

Trajectories of endosomes inside living eukaryotic cells are highly heterogeneous in space and time and diffuse anomalously due to a combination of viscoelasticity, caging, aggregation and active transport. Some of the trajectories display switching between persistent and anti-persistent motion, while others jiggle around in one position for the whole measurement time. By splitting the ensemble of endosome trajectories into slow moving subdiffusive and fast moving superdiffusive endosomes, we analyzed them separately. The mean squared displacements and velocity auto-correlation functions confirm the effectiveness of the splitting methods. Applying the local analysis, we show that both ensembles are characterized by a spectrum of local anomalous exponents and local generalized diffusion coefficients. Slow and fast endosomes have exponential distributions of local anomalous exponents and power law distributions of generalized diffusion coefficients. This suggests that heterogeneous fractional Brownian motion is an appropriate model for both fast and slow moving endosomes. This article is part of a Special Issue entitled: "Recent Advances In Single-Particle Tracking: Experiment and Analysis" edited by Janusz Szwabiński and Aleksander Weron.

Axon terminals control endolysosome diffusion to support synaptic remodelling

Endolysosomes present in the presynaptic terminal move by diffusion constrained by F-actin and increase their mobility during the remodelling of synaptic connectivity to support a local degradative activity.

Endolysosomes are acidic organelles formed by the fusion of endosomes with lysosomes. In the presynaptic compartment they contribute to protein homeostasis, the maintenance of vesicle pools and synaptic stability. Here, we evaluated the mobility of endolysosomes found in axon terminals of olfactory sensory neurons of Xenopus tropicalis tadpoles. F-actin restricts the motion of these presynaptic acidic organelles which is characterized by a diffusion coefficient of 6.7 × 10⁻³ μm²·s⁻¹. Local injection of secreted protein acidic and rich in cysteine (SPARC) in the glomerular layer of the olfactory bulb disrupts the structure of synaptic F-actin patches and increases the presence and mobility of endolysosomal organelles found in axon terminals. The increased motion of endolysosomes is localized to the presynaptic compartment and does not promote their access to axonal regions for retrograde transportation to the cell body. Local activation of synaptic degradation mechanisms mediated by SPARC coincides with a loss of the ability of tadpoles to detect waterborne odorants. Together, these observations show that the diffusion of presynaptic endolysosomes increases during conditions of synaptic remodelling to support their local degradative activity.

CaV β controls the endocytic turnover of CaV 1.2 L-type calcium channel

Membrane depolarization activates the multisubunit Ca_V 1.2 L-type calcium channel initiating various excitation coupling responses. Intracellular trafficking into and out of the plasma membrane regulates the channel's surface expression and stability, and thus, the strength of Ca_V 1.2-mediated Ca²⁺ signals. The mechanisms regulating the residency time of the channel at the cell membrane are unclear. Here, we coexpressed the channel core complex Ca_V 1.2α₁ pore-forming and auxiliary Ca_V β subunits and analyzed their trafficking dynamics from single-particle-tracking trajectories. Speed histograms obtained for each subunit were best fitted to a sum of diffusive and directed motion terms. The same mean speed for the highest-mobility state underlying directed motion was found for all subunits. The frequency of this component increased by covalent linkage of Ca_V β to Ca_V 1.2α₁ suggesting that high-speed transport occurs in association with Ca_V β. Selective tracking of Ca_V 1.2α₁ along the postendocytic pathway failed to show the highly mobile state, implying Ca_V β-independent retrograde transport. Retrograde speeds of Ca_V 1.2α₁ are compatible with myosin VI-mediated backward transport. Moreover, residency time at the cell surface was significantly prolonged when Ca_V 1.2α₁ was covalently linked to Ca_V β. Thus, Ca_V β promotes fast transport speed along anterograde trafficking and acts as a molecular switch controlling the endocytic turnover of L-type calcium channels.

PDZD8-mediated lipid transfer at contacts between the ER and late endosomes/lysosomes is required for neurite outgrowth

Membrane contact sites (MCSs) between the endoplasmic reticulum (ER) and late endosomes/lysosomes (LE/lys) are emerging as critical hubs for diverse cellular events, and changes in their extents are linked to severe neurological diseases. While recent studies show that the synaptotagmin-like mitochondrial-lipid-binding (SMP) domain-containing protein PDZD8 may mediate the formation of ER-LE/lys MCSs, the cellular functions of PDZD8 remain largely elusive. Here, we attempt to investigate the lipid transfer activities of PDZD8 and the extent to which its cellular functions depend on its lipid transfer activities. In accordance with recent studies, we demonstrate that PDZD8 is a protrudin (ZFYVE27)-interacting protein and that PDZD8 acts as a tether at ER-LE/lys MCSs. Furthermore, we discover that the SMP domain of PDZD8 binds glycerophospholipids and ceramides both in vivo and in vitro, and that the SMP domain can transport lipids between membranes in vitro. Functionally, PDZD8 is required for LE/lys positioning and neurite outgrowth, which is dependent on the lipid transfer activity of the SMP domain.

Revealing Microtubule-Dependent Slow-Directed Motility by Single-Particle Tracking

Microtubules (MTs) are the main component of cytoskeletons, providing long tracks for cargo trafficking across the cytoplasm. In the past years, transport along MTs was frequently reported to be rapid directed motions with speeds of several micrometers per second, but is that all the truth? Using single-particle tracking, we roundly and precisely analyzed the dynamic behaviors of three kinds of cargoes transported along MTs in two types of cells. It was found that during the transport processes, the directed motions of the cargoes were frequently interrupted by nondirected motions which greatly reduced the translocation rate toward the nucleus. What is more, in addition to the widely reported rapid directed motions, a type of directed motions with most speeds below 0.5 μm/s occurred more frequently. On the whole, these slow directed motions took longer than the rapid directed motions and resulted in displacements same as those of the rapid ones. To sum up, while travelling along MTs toward the cell interior, endocytosed cargoes moved alternately in rapid-directed, slow-directed and nondirected modes. In this process, the rapid- and the slow-directed motions contributed almost equally to the cargoes' translocation. This work provides original insights into the transport on MTs, facilitating a more comprehensive understanding of intracellular trafficking.

Cytosolic delivery of membrane-penetrating QDs into T cell lymphocytes: implications in immunotherapy and drug delivery

We report single-particle characterization of membrane-penetrating semiconductor quantum dots (QDs) in T cell lymphocytes. We functionalized water-soluble CdSe/CdZnS QDs with a cell-penetrating peptide composed of an Asp-Ser-Ser (DSS) repeat sequence. DSS and peptide-free control QDs displayed concentration-dependent internalization. Intensity profiles from single-particle imaging revealed a propensity of DSS-QDs to maintain a monomeric state in the T cell cytosol, whereas control QDs formed pronounced clusters. Single-particle tracking showed a direct correlation between individual QD clusters' mobility and aggregation state. A significant portion of control QDs colocalized with an endosome marker inside the T cells, while the percentage of DSS-QDs colocalized dropped to 9%. Endocytosis inhibition abrogated the internalization of control QDs, while DSS-QD internalization only mildly decreased, suggesting an alternative cell-entry mechanism. Using 3D single-particle tracking, we captured the rapid membrane-penetrating activity of a DSS-QD. The ability to characterize membrane penetrating activities in live T cells creates inroads for the optimization of gene therapy and drug delivery through the use of novel nanomaterials.

In the Right Place at the Right Time: Regulation of Cell Metabolism by IP3R-Mediated Inter-Organelle Ca2+ Fluxes

In the last few years, metabolism has been shown to be controlled by cross-organelle communication. The relationship between the endoplasmic reticulum and mitochondria/lysosomes is the most studied; here, inositol 1,4,5-triphosphate (IP3) receptor (IP3R)-mediated calcium (Ca²⁺) release plays a central role. Recent evidence suggests that IP3R isoforms participate in synthesis and degradation pathways. This minireview will summarize the current findings in this area, emphasizing the critical role of Ca²⁺ communication on organelle function as well as catabolism and anabolism, particularly in cancer.

Rapid whole cell imaging reveals a calcium-APPL1-dynein nexus that regulates cohort trafficking of stimulated EGF receptors

The endosomal system provides rich signal processing capabilities for responses elicited by growth factor receptors and their ligands. At the single cell level, endosomal trafficking becomes a critical component of signal processing, as exemplified by the epidermal growth factor (EGF) receptors. Activated EGFRs are trafficked to the phosphatase-enriched peri-nuclear region (PNR), where they are dephosphorylated and degraded. The details of the mechanisms that govern the movements of stimulated EGFRs towards the PNR, are not completely known. Here, exploiting the advantages of lattice light-sheet microscopy, we show that EGFR activation by EGF triggers a transient calcium increase causing a whole-cell level redistribution of Adaptor Protein, Phosphotyrosine Interacting with PH Domain And Leucine Zipper 1 (APPL1) from pre-existing endosomes within one minute, the rebinding of liberated APPL1 directly to EGFR, and the dynein-dependent translocation of APPL1-EGF-bearing endosomes to the PNR within ten minutes. The cell spanning, fast acting network that we reveal integrates a cascade of events dedicated to the cohort movement of activated EGF receptors. Our findings support the intriguing proposal that certain endosomal pathways have shed some of the stochastic strategies of traditional trafficking and have evolved processes that provide the temporal predictability that typify canonical signaling.

Mobile late endosomes modulate peripheral endoplasmic reticulum network architecture

The endoplasmic reticulum (ER) is the largest organelle contacting virtually every other organelle for information exchange and control of processes such as transport, fusion, and fission. Here, we studied the role of the other organelles on ER network architecture in the cell periphery. We show that the co‐migration of the ER with other organelles, called ER hitchhiking facilitated by late endosomes and lysosomes is a major mechanism controlling ER network architecture. When hitchhiking occurs, emerging ER structures may fuse with the existing ER tubules to alter the local ER architecture. This couples late endosomal/lysosomal positioning and mobility to ER network architecture. Conditions restricting late endosomal movement—including cell starvation—or the depletion of tether proteins that link the ER to late endosomes reduce ER dynamics and limit the complexity of the peripheral ER network architecture. This indicates that among many factors, the ER is controlled by late endosomal movement resulting in an alteration of the ER network architecture.

An optimized toolbox for the optogenetic control of intracellular transport

Cellular functioning relies on active transport of organelles by molecular motors. To explore how intracellular organelle distributions affect cellular functions, several optogenetic approaches enable organelle repositioning through light-inducible recruitment of motors to specific organelles. Nonetheless, robust application of these methods in cellular populations without side effects has remained challenging. Here, we introduce an improved toolbox for optogenetic control of intracellular transport that optimizes cellular responsiveness and limits adverse effects. To improve dynamic range, we employed improved optogenetic heterodimerization modules and engineered a photosensitive kinesin-3, which is activated upon blue light–sensitive homodimerization. This opto-kinesin prevented motor activation before experimental onset, limited dark-state activation, and improved responsiveness. In addition, we adopted moss kinesin-14 for efficient retrograde transport with minimal adverse effects on endogenous transport. Using this optimized toolbox, we demonstrate robust reversible repositioning of (endogenously tagged) organelles within cellular populations. More robust control over organelle motility will aid in dissecting spatial cell biology and transport-related diseases.

Resolving cargo-motor-track interactions with bifocal parallax single-particle tracking

Resolving coordinated biomolecular interactions in living cellular environments is vital for understanding the mechanisms of molecular nanomachines. The conventional approach relies on localizing and tracking target biomolecules and/or subcellular organelles labeled with imaging probes. However, it is challenging to gain information on rotational dynamics, which can be more indicative of the work done by molecular motors and their dynamic binding status. Herein, a bifocal parallax single-particle tracking method using half-plane point spread functions has been developed to resolve the full-range azimuth angle (0-360°), polar angle, and three-dimensional (3D) displacement in real time under complex living cell conditions. Using this method, quantitative rotational and translational motion of the cargo in a 3D cell cytoskeleton was obtained. Not only were well-known active intracellular transport and free diffusion observed, but new interactions (tight attachment and tethered rotation) were also discovered for better interpretation of the dynamics of cargo-motor-track interactions at various types of microtubule intersections.

The structure and global distribution of the endoplasmic reticulum network are actively regulated by lysosomes

The endoplasmic reticulum (ER) comprises morphologically and functionally distinct domains: sheets and interconnected tubules. These domains undergo dynamic reshaping in response to changes in the cellular environment. However, the mechanisms behind this rapid remodeling are largely unknown. Here, we report that ER remodeling is actively driven by lysosomes, following lysosome repositioning in response to changes in nutritional status: The anchorage of lysosomes to ER growth tips is critical for ER tubule elongation and connection. We validate this causal link via the chemo- and optogenetically driven repositioning of lysosomes, which leads to both a redistribution of the ER tubules and a change of its global morphology. Therefore, lysosomes sense metabolic change in the cell and regulate ER tubule distribution accordingly. Dysfunction in this mechanism during axonal extension may lead to axonal growth defects. Our results demonstrate a critical role of lysosome-regulated ER dynamics and reshaping in nutrient responses and neuronal development.

A central role of the endoplasmic reticulum in the cell emerges from its functional contact sites with multiple organelles

Early eukaryotic cells emerged from the compartmentalization of metabolic processes into specific organelles through the development of an endomembrane system (ES), a precursor of the endoplasmic reticulum (ER), which was essential for their survival. Recently, substantial evidence emerged on how organelles communicate among themselves and with the plasma membrane (PM) through contact sites (CSs). From these studies, the ER-the largest single structure in eukaryotic cells-emerges as a central player communicating with all organelles to coordinate cell functions and respond to external stimuli to maintain cellular homeostasis. Herein we review the functional insights into the ER-CSs with other organelles in a physiological perspective. We hypothesize that, in addition to the primitive role by the ES in the appearance of proto-eukaryotes, its successor-the ER-emerges as the key coordinator of inter-organelle/PM communication. The ER thus appears to be the 'maestro' driving eukaryotic cell evolution by incorporating new functions/organelles, while remaining the real coordinator overarching cellular functions and orchestrating them with the external milieu.

Getting around the cell: physical transport in the intracellular world

Eukaryotic cells face the challenging task of transporting a variety of particles through the complex intracellular milieu in order to deliver, distribute, and mix the many components that support cell function. In this review, we explore the biological objectives and physical mechanisms of intracellular transport. Our focus is on cytoplasmic and intra-organelle transport at the whole-cell scale. We outline several key biological functions that depend on physically transporting components across the cell, including the delivery of secreted proteins, support of cell growth and repair, propagation of intracellular signals, establishment of organelle contacts, and spatial organization of metabolic gradients. We then review the three primary physical modes of transport in eukaryotic cells: diffusive motion, motor-driven transport, and advection by cytoplasmic flow. For each mechanism, we identify the main factors that determine speed and directionality. We also highlight the efficiency of each transport mode in fulfilling various key objectives of transport, such as particle mixing, directed delivery, and rapid target search. Taken together, the interplay of diffusion, molecular motors, and flows supports the intracellular transport needs that underlie a broad variety of biological phenomena.