Phagocytosis is a principal component of the body’s innate immunity in which macrophages internalize targets in an actin-dependent manner. Targets vary widely in shape and size and include particles such as pathogens and senescent cells. Despite considerable progress in understanding this complicated process, the role of target geometry in phagocytosis has remained elusive. Previous studies on phagocytosis have been performed using spherical targets, thereby overlooking the role of particle shape. Using polystyrene particles of various sizes and shapes, we studied phagocytosis by alveolar macrophages. We report a surprising finding that particle shape, not size, plays a dominant role in phagocytosis. All shapes were capable of initiating phagocytosis in at least one orientation. However, the local particle shape, measured by tangent angles, at the point of initial contact dictates whether macrophages initiate phagocytosis or simply spread on particles. The local shape determines the complexity of the actin structure that must be created to initiate phagocytosis and allow the membrane to move over the particle. Failure to create the required actin structure results in simple spreading and not internalization. Particle size primarily impacts the completion of phagocytosis in cases where particle volume exceeds the cell volume.

https://www.pnas.org/content/pnas/103/13/4930.full.pdf

Role of target geometry in phagocytosis

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite "sensor" that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

https://www.physiology.org/doi/pdf/10.1152/physrev.00012.2005

Signaling Mechanisms Regulating Endothelial Permeability

Nanocarriers offer unique possibilities to overcome cellular barriers in order to improve the delivery of various drugs and drug candidates, including the promising therapeutic biomacromolecules (i.e., nucleic acids, proteins). There are various mechanisms of nanocarrier cell internalization that are dramatically influenced by nanoparticles' physicochemical properties. Depending on the cellular uptake and intracellular trafficking, different pharmacological applications may be considered. This review will discuss these opportunities, starting with the phagocytosis pathway, which, being increasingly well characterized and understood, has allowed several successes in the treatment of certain cancers and infectious diseases. On the other hand, the non-phagocytic pathways encompass various complicated mechanisms, such as clathrin-mediated endocytosis, caveolae-mediated endocytosis and macropinocytosis, which are more challenging to control for pharmaceutical drug delivery applications. Nevertheless, various strategies are being actively investigated in order to tailor nanocarriers able to deliver anticancer agents, nucleic acids, proteins and peptides for therapeutic applications by these non-phagocytic routes.

Nanocarriers’ entry into the cell: relevance to drug delivery

Intracellular membrane traffic defines a complex network of pathways that connects many of the membrane-bound organelles of eukaryotic cells. Although each pathway is governed by its own set of factors, they all contain Rab GTPases that serve as master regulators. In this review, we discuss how Rabs can regulate virtually all steps of membrane traffic from the formation of the transport vesicle at the donor membrane to its fusion at the target membrane. Some of the many regulatory functions performed by Rabs include interacting with diverse effector proteins that select cargo, promoting vesicle movement, and verifying the correct site of fusion. We describe cascade mechanisms that may define directionality in traffic and ensure that different Rabs do not overlap in the pathways that they regulate. Throughout this review we highlight how Rab dysfunction leads to a variety of disease states ranging from infectious diseases to cancer.

Role of Rab GTPases in membrane traffic and cell physiology.

Brucellosis remains a major zoonosis worldwide Although many countries have eradicated Brucella abortus from cattle, in some areas Brucella melitensis has emerged as a cause of infection in this species as well as in sheep and goats Despite vaccination campaigns with the Rev 1 strain, B melitensis remains the principal cause of human brucellosis Brucella suis is also emerging as an agent of infection in cattle, thus extending its opportunities to infect humans The recent isolation of distinctive strains of Brucella from marine mammals has extended its ecologic range Molecular genetic studies have demonstrated phylogenetic affiliation to Agrobacterium, Phyllobacterium, Ochrobactrum, and Rhizobium Polymerase chain reaction and gene probe development may provide more effective typing methods Pathogenicity is related to production of lipopolysaccharides containing a poly N-formyl perosamine O chain, CuZn superoxide dismutase, erythrlose phosphate dehydrogenase, stress-induced proteins related to intracellular survival, and adenine and guanine monophosphate inhibitors of phagocyte functions Protective immunity is conferred by antibody to lipopolysaccharide and T-cell-mediated macrophage activation triggered by protein antigens Diagnosis still centers on isolation of the organism and serologic test results, especially enzyme immunoassay, which is replacing other methods Polymerase chain reaction is also under evaluation Therapy is based on tetracyclines with or without rifampicin, aminoglycosides, or quinolones No satisfactory vaccines against human brucellosis are available, although attenuated purE mutants appear promising

/pdf/brucellosis-an-overview-14k0fkto7f.pdf

Brucellosis: an overview.

Dectin-1 uses novel mechanisms for yeast phagocytosis in macrophages

Phagocytosis is a highly conserved, complex process that has evolved to counter the constant threat posed by pathogens, effete cells and debris. Classically defined as a mechanism for internalising and destroy- ing particles greater than 0.5 mm in size, it is a receptor- mediated, actin-driven process. The best-studied phagocytic receptors are the opsono-receptors, FcgR and CR3. Phagocytic uptake involves actin dynamics including polymerisation, bundling, contraction, se- vering and depolymerisation of actin filaments. Re- cent evidence points to the importance of membrane remodelling during phagocytosis, both in terms of changes in lipid composition and delivery of new membrane to the sites of particle binding. Here we review the molecular mechanisms of phagocytic uptake and some of the strategies developed by microbial pathogens to manipulate this process.

Molecular mechanisms of phagocytic uptake in mammalian cells

By linking actin dynamics to extracellular components, integrins are involved in a wide range of cellular processes that are associated with or require cytoskeletal remodelling and cell-shape changes. One such function is integrin-dependent phagocytosis, a process that several integrins are capable of mediating and that allows the binding and clearance of particles. Integrin-dependent phagocytosis is involved in a wide range of physiological processes, from the clearance of microorganisms and apoptotic-cell removal to extracellular-matrix remodelling. Integrin signalling is also exploited by microbial pathogens for entry into host cells. Far from being a particular property of specific integrins and specialised cells, integrin-dependent uptake is emerging as a general, intrinsic ability of most integrins that is associated with their capacity to signal to the actin cytoskeleton. Integrin-mediated phagocytosis can therefore be used as a robust model in which to study integrin regulation and signalling.

/pdf/integrin-dependent-phagocytosis-spreading-from-microadhesion-5gjsq7ng4e.pdf

Integrin-dependent phagocytosis: spreading from microadhesion to new concepts.

Rap1 belongs to the Ras subgroup of small GTP-binding proteins. Whereas its early history has focused on its biochemical homology to Ras and the alleged functional antagonism between these two small GTPases, recent cellular evidence suggests that endogenous Rap1 plays a unique, Ras-independent role in eukaryotic cells. Activated by virtually all receptor types and second messengers, Rap1 controls adhesion-related functions such as phagocytosis, cell-cell contacts and functional activation of integrins through inside-out signalling. Whereas the precise mechanism by which its downstream effectors exert these diverse functions is unknown, Rap1 seems to fulfil the evolutionarily conserved function of patterning the eukaryotic cell, thus enabling it to respond to its environment, in particular through cytoskeletal remodelling.

/pdf/cellular-functions-of-the-rap1-gtp-binding-protein-a-pattern-18u3iwecqc.pdf

Cellular functions of the Rap1 GTP-binding protein: a pattern emerges

Despite their homology, the regulation of WASP and WAVE, activators of Arp2/3-dependent actin polymerization, has always been thought to be different. Several recent studies have revealed new aspects of their regulation, highlighting its complexity and the crucial role of post-translational modifications. New data also suggest additional functions for WASP family proteins, pushing us to reconsider existing models.

https://rupress.org/jcb/article-pdf/166/7/957/1314954/jcb1667957.pdf

Emmanuelle Caron

Papers

Dectin-1 uses novel mechanisms for yeast phagocytosis in macrophages

Molecular mechanisms of phagocytic uptake in mammalian cells

Integrin-dependent phagocytosis: spreading from microadhesion to new concepts.

Cellular functions of the Rap1 GTP-binding protein: a pattern emerges

Regulation of WASP/WAVE proteins: making a long story short.