An internally eGFP-tagged α-adaptin is a fully functional and improved fiduciary marker for clathrin-coated pit dynamics

FACI is a novel clathrin adaptor protein 2-binding protein that facilitates low-density lipoprotein endocytosis

BACKGROUND

Cholesterol plays a vital role in multiple physiological processes. Cellular uptake of cholesterol is mediated primarily through endocytosis of low-density lipoprotein (LDL) receptor. New modifiers of this process remain to be characterized. Particularly, the role of fasting- and CREB-H-induced (FACI) protein in cholesterol homeostasis merits further investigation.

METHODS

Interactome profiling by proximity labeling and affinity purification - mass spectrometry was performed. Total internal reflection fluorescence microscopy and confocal immunofluorescence microscopy were used to analyze protein co-localization and interaction. Mutational analysis was carried out to define the domain and residues required for FACI localization and function. Endocytosis was traced by fluorescent cargos. LDL uptake in cultured cells and diet-induced hypercholesterolemia in mice were assessed.

RESULTS

FACI interacted with proteins critically involved in clathrin-mediated endocytosis, vesicle trafficking, and membrane cytoskeleton. FACI localized to clathrin-coated pits (CCP) on plasma membranes. FACI contains a conserved DxxxLI motif, which mediates its binding with the adaptor protein 2 (AP2) complex. Disruption of this motif of FACI abolished its CCP localization but didn't affect its association with plasma membrane. Cholesterol was found to facilitate FACI transport from plasma membrane to endocytic recycling compartment in a clathrin- and cytoskeleton-dependent manner. LDL endocytosis was enhanced in FACI-overexpressed AML12 cells but impaired in FACI-depleted HeLa cells. In vivo study indicated that hepatic FACI overexpression alleviated diet-induced hypercholesterolemia in mice.

CONCLUSIONS

FACI facilitates LDL endocytosis through its interaction with the AP2 complex.

Imaging nanoscale axial dynamics at the basal plasma membrane

Understanding of how energetically unfavorable plasma membrane shapes form, especially in the context of dynamic processes in living cells or tissues like clathrin-mediated endocytosis is in its infancy. Even though cutting-edge microscopy techniques that bridge this gap exist, they remain underused in biomedical sciences. Here, we demystify the perceived complexity of these advanced microscopy approaches and demonstrate their power in resolving nanometer axial dynamics in living cells. Total internal reflection fluorescence microscopy based approaches are the main focus of this review. We present clathrin-mediated endocytosis as a model system when describing the principles, data acquisition requirements, data interpretation strategies, and limitations of the described techniques. We hope this standardized description will bring the approaches for measuring nanoscale axial dynamics closer to the potential users and help in choosing the right approach to the right question.

Resolution doubling in light-sheet microscopy via oblique plane structured illumination

Structured illumination microscopy (SIM) doubles the spatial resolution of a fluorescence microscope without requiring high laser powers or specialized fluorophores. However, the excitation of out-of-focus fluorescence can accelerate photobleaching and phototoxicity. In contrast, light-sheet fluorescence microscopy (LSFM) largely avoids exciting out-of-focus fluorescence, thereby enabling volumetric imaging with low photobleaching and intrinsic optical sectioning. Combining SIM with LSFM would enable gentle three-dimensional (3D) imaging at doubled resolution. However, multiple orientations of the illumination pattern, which are needed for isotropic resolution doubling in SIM, are challenging to implement in a light-sheet format. Here we show that multidirectional structured illumination can be implemented in oblique plane microscopy, an LSFM technique that uses a single objective for excitation and detection, in a straightforward manner. We demonstrate isotropic lateral resolution below 150 nm, combined with lower phototoxicity compared to traditional SIM systems and volumetric acquisition speed exceeding 1 Hz.

Increasing the field-of-view in oblique plane microscopy via optical tiling

Fast volumetric imaging of large fluorescent samples with high-resolution is required for many biological applications. Oblique plane microscopy (OPM) provides high spatiotemporal resolution, but the field of view is typically limited by its optical train and the pixel number of the camera. Mechanically scanning the sample or decreasing the overall magnification of the imaging system can partially address this challenge, albeit by reducing the volumetric imaging speed or spatial resolution, respectively. Here, we introduce a novel dual-axis scan unit for OPM that facilitates rapid and high-resolution volumetric imaging throughout a volume of 800 × 500 × 200 microns. This enables us to perform volumetric imaging of cell monolayers, spheroids and zebrafish embryos with subcellular resolution. Furthermore, we combined this microscope with a multi-perspective projection imaging technique that increases the volumetric interrogation rate to more than 10 Hz. This allows us to rapidly probe a large field of view in a dimensionality reduced format, identify features of interest, and volumetrically image these regions with high spatiotemporal resolution.

FCHO controls AP2's initiating role in endocytosis through a PtdIns(4,5)P2-dependent switch

Clathrin-mediated endocytosis (CME) is the main mechanism by which mammalian cells control their cell surface proteome. Proper operation of the pivotal CME cargo adaptor AP2 requires membrane-localized Fer/Cip4 homology domain-only proteins (FCHO). Here, live-cell enhanced total internal reflection fluorescence-structured illumination microscopy shows that FCHO marks sites of clathrin-coated pit (CCP) initiation, which mature into uniform-sized CCPs comprising a central patch of AP2 and clathrin corralled by an FCHO/Epidermal growth factor potential receptor substrate number 15 (Eps15) ring. We dissect the network of interactions between the FCHO interdomain linker and AP2, which concentrates, orients, tethers, and partially destabilizes closed AP2 at the plasma membrane. AP2's subsequent membrane deposition drives its opening, which triggers FCHO displacement through steric competition with phosphatidylinositol 4,5-bisphosphate, clathrin, cargo, and CME accessory factors. FCHO can now relocate toward a CCP's outer edge to engage and activate further AP2s to drive CCP growth/maturation.

Imaging vesicle formation dynamics supports the flexible model of clathrin-mediated endocytosis

Clathrin polymerization and changes in plasma membrane architecture are necessary steps in forming vesicles to internalize cargo during clathrin-mediated endocytosis (CME). Simultaneous analysis of clathrin dynamics and membrane structure is challenging due to the limited axial resolution of fluorescence microscopes and the heterogeneity of CME. This has fueled conflicting models of vesicle assembly and obscured the roles of flat clathrin assemblies. Here, using Simultaneous Two-wavelength Axial Ratiometry (STAR) microscopy, we bridge this critical knowledge gap by quantifying the nanoscale dynamics of clathrin-coat shape change during vesicle assembly. We find that de novo clathrin accumulations generate both flat and curved structures. High-throughput analysis reveals that the initiation of vesicle curvature does not directly correlate with clathrin accumulation. We show clathrin accumulation is preferentially simultaneous with curvature formation at shorter-lived clathrin-coated vesicles (CCVs), but favors a flat-to-curved transition at longer-lived CCVs. The broad spectrum of curvature initiation dynamics revealed by STAR microscopy supports multiple productive mechanisms of vesicle formation and advocates for the flexible model of CME.

Despite decades of research, the dynamics of clathrin-coated vesicle formation is ambiguous. Here, authors use STAR microscopy to quantify the nanoscale dynamics of vesicle formation, supporting the flexible model of clathrin-mediated endocytosis.

CALM supports clathrin-coated vesicle completion upon membrane tension increase

The most represented components of clathrin-coated vesicles (CCVs) are clathrin triskelia and the adaptors clathrin assembly lymphoid myeloid leukemia protein (CALM) and the heterotetrameric complex AP2. Investigation of the dynamics of AP180-amino-terminal-homology (ANTH) recruitment during CCV formation has been hampered by CALM toxicity upon overexpression. We used knock-in gene editing to express a C-terminal-attached fluorescent version of CALM, while preserving its endogenous expression levels, and cutting-edge live-cell microscopy approaches to study CALM recruitment at forming CCVs. Our results demonstrate that CALM promotes vesicle completion upon membrane tension increase as a function of the amount of this adaptor present. Since the expression of adaptors, including CALM, differs among cells, our data support a model in which the efficiency of clathrin-mediated endocytosis is tissue specific and explain why CALM is essential during embryogenesis and red blood cell development.

An internally eGFP-tagged α-adaptin is a fully functional and improved fiduciary marker for clathrin-coated pit dynamics

Clathrin mediated endocytosis (CME) has been extensively studied in living cells by quantitative total internal reflection fluorescence microscopy (TIRFM). Fluorescent protein fusions to subunits of the major coat proteins, clathrin light chains or the heterotetrameric adaptor protein (AP2) complexes, have been used as fiduciary markers of clathrin coated pits (CCPs). However, the functionality of these fusion proteins has not been rigorously compared. Here, we generated stable cells lines overexpressing mRuby‐CLCa and/or μ2‐eGFP, σ2‐eGFP, two markers currently in use, or a novel marker generated by inserting eGFP into the unstructured hinge region of the α subunit (α‐eGFP). Using biochemical and TIRFM‐based assays, we compared the functionality of the AP2 markers. All of the eGFP‐tagged subunits were efficiently incorporated into AP2 and displayed greater accuracy in image‐based CCP analyses than mRuby‐CLCa. However, overexpression of either μ2‐eGFP or σ2‐eGFP impaired transferrin receptor uptake. In addition, μ2‐eGFP reduced the rates of CCP initiation and σ2‐eGFP perturbed AP2 incorporation into CCPs and CCP maturation. In contrast, CME and CCP dynamics were unperturbed in cells overexpressing α‐eGFP. Moreover, α‐eGFP was a more sensitive and accurate marker of CCP dynamics than mRuby‐CLCa. Thus, our work establishes α‐eGFP as a robust, fully functional marker for CME.