Endocytosis: Past,Present, and Future

Endocytosis may have been a driving force behind the evolution of eukaryotic cells. It plays critical roles in cell biology (e.g., signal transduction) and in organismal physiology (e.g., tissue morphogenesis).Endocytosis, the process of cellular ingestion, may have been the driving force behind evolution of the eucaryotic cell (de Duve 2007). Acquiring the ability to internalize macromolecules and digest them intracellularly would have allowed primordial cells to move out from their food sources and pursue a predatory existence; one that might have led to the development of endosymbiotic relationships with mitochondria and plastids. Thus, it is fitting that endocytosis was first discovered and named as the processes of cell “eating” and “drinking.” In 1883, the developmental biologist Ilya Metchnikoff coined the term phagocytosis, from the Greek “phagos” (to eat) and “cyte” (cell), after observing motile cells in transparent starfish larva surround and engulf small splinters that he had inserted (Tauber 2003). Decades later, in 1931, Warren H. Lewis, one of the earliest cell “cinematographers” coined the term pinocytosis, from the Greek “pinean” (to drink), after observing the uptake of surrounding media into large vesicles in cultured macrophages, sarcoma cells, and fibroblasts by time-lapse imaging (Lewis 1931; Corner 1967).Importantly, these pioneering studies also revealed that the function of endocytosis goes well beyond eating and drinking. Indeed, Metchnikoff, considered one of the founders of modern immunology, realized that the phagocytic behavior of the mesodermal amoeboid cells he had observed under the microscope could serve as a general defense system against invasive parasites, in the larva as in man. This revolutionary concept, termed the phagocytic theory, earned Metchnikoff the 1908 Nobel Prize in Physiology or Medicine for his work on phagocytic immunity, which he shared with Paul Ehrlich who discovered the complementary mechanisms of humoral immunity that led to the identification of antibodies (Vaughan 1965; Tauber 2003; Schmalstieg and Goldman 2008). The phagocytic theory was a milestone in immunology and cell biology, and formally gave birth to the field of endocytosis.Key discoveries over the intervening years, aided in large part by the advent of electron microscopy, revealed multiple pathways for endocytosis in mammalian cells that fulfill multiple critical cellular functions (Fig. 1). These mechanistically and morphologically distinct pathways, and their discoverers, include clathrin-mediated endocytosis (Roth and Porter 1964), caveolae uptake (Palade 1953; Yamada 1955), cholesterol-sensitive clathrin- and caveolae-independent pathways (Moya et al. 1985; Hansen et al. 1991; Lamaze et al. 2001), and, more recently, the large capacity CLIC/GEEC pathway (Kirkham et al. 2005). In place of Metchnikoff’s splinters, many of these discoveries resulted from the detection and tracking of internalized HRP-, ferritin-, or gold-conjugated ligands by electron microscopy. These electron-dense tracers allowed researchers to identify cellular structures associated with the uptake and intracellular sorting of receptor-bound ligands. A particularly striking example is the pioneering work of Roth and Porter, who in 1964 observed the uptake of yolk proteins into mosquito oocytes. To synchronize uptake, they killed female mosquitos at timed intervals after a blood feed and observed the sequential appearance of electron-dense yolk granules in coated pits, coated and uncoated vesicles, and progressively larger vesicles. Their remarkable observations accurately described coated vesicle budding, uncoating, homo- and heterotypic fusion events, as well as the emergence of tubules from early endosomes (Fig. 2), all of which are now known hallmarks of the early endocytic trafficking events.Open in a separate windowFigure 1.Time line for discoveries of endocytic pathways and their discoverers. Boxes are color-coded by pathway. *, Nobel laureate. HRP, horseradish peroxidase; CCVs, clathrin-coated vesicles; CCPs, clathrin-coated pits; EGFR, epidermal growth factor receptor; PM, plasma membrane; ER, endoplasmic reticulum; CLIC/GEEC, clathrin-independent carriers/GPI-enriched endocytic compartments.Open in a separate windowFigure 2.Fiftieth anniversary of the discovery of clathrin-mediated endocytosis by Roth and Porter (1964). The image is the hand-drawn summary of observations made by electron microscopic examination of the trafficking of yolk proteins in a mosquito oocyte. Note the many details, later confirmed and mechanistically studied over the intervening 50 years. These include the growth, invagination, and pinching off of coated pits (1,2), which concentrate cargo taken up by coated vesicles (3), the rapid uncoating of nascent-coated vesicles (4), homotypic fusion of nascent endocytic vesicles in the cell periphery (5), the formation of tubules from early endosomes (7), and changes in the intraluminal properties of larger endosomes (6). Finally, yolk proteins are stored in granules as crystalline bodies (8). (From Roth and Porter 1964; reprinted, with express permission, from Rockefeller University Press © 1964, The Journal of Cell Biology 20: 313–332, doi: 10.1083/jcb.20.2.313.)Another milestone in the field of endocytosis was the discovery of the lysosome by Christian de Duve (Appelmans et al. 1955). Whereas the finding of phagocytosis and other endocytic pathways was possible through microscopy, the discovery of lysosomes originated from a biochemical approach (Courtoy 2007), which benefited from the invention of the ultracentrifuge. de Duve and coworkers observed that preparations of acid phosphatase obtained by subcellular fractionation had an unusual behavior: contrary to most enzymatic activities, the activity of acid phosphatase increased rather than decreased with time, freezing–thawing of the fractions and the presence of detergents. Interestingly, the same was true for other hydrolases, which depended on acidic pH for their optimal activity. This led him to postulate that the acid hydrolases were contained in acidified membrane-bound vesicles. In collaboration with Alex Novikoff, he visualized these vesicles, the lysosomes, by electron microscopy (Beaufay et al. 1956) and later showed their content of acid phosphatase (Farquhar et al. 1972). In 1974, de Duve was awarded the Nobel Prize for Physiology or Medicine for his seminal finding of the lysosomes and peroxisomes. He shared it with Albert Claude and George E. Palade “for their discoveries concerning the structural and functional organization of the cell.” The importance of this work lies also in the significant therapeutic applications that followed. The discovery by Elizabeth Neufeld and collaborators of uptake of lysosomal enzymes by cells provided the foundation for enzyme replacement therapy for lysosomal storage disorders (Neufeld 2011).In the 1970s, research in endocytosis entered the molecular era. Using de Duve and Albert Claude-like methods of subcellular fractionation, Barbara M. Pearse purified clathrin-coated vesicles from pig brain (Pearse 1975). A year later, she isolated a major protein species of 180 kDa, which she named clathrin “to indicate the lattice-like structures which it forms” (Pearse 1976). It was a breakthrough that inaugurated the molecular dissection of clathrin-mediated endocytosis.Over the intervening years, the continued application of microscopy (which now spans from electron cryotomography to live cell, high-resolution fluorescence microscopy), genetics (in particular, in yeast, Caenorhabditis elegans and Drosophila melanogaster), biochemistry (including cell-free reconstitution of endocytic membrane trafficking events), as well as molecular and structural biology have revealed a great deal about the cellular machineries and mechanisms that govern trafficking along the endocytic pathway. A partial, and because of space limitations, necessarily incomplete list of milestones (

Integrons: Past,Present, and Future

SUMMARY

Integrons are versatile gene acquisition systems commonly found in bacterial genomes. They are ancient elements that are a hot spot for genomic complexity, generating phenotypic diversity and shaping adaptive responses. In recent times, they have had a major role in the acquisition, expression, and dissemination of antibiotic resistance genes. Assessing the ongoing threats posed by integrons requires an understanding of their origins and evolutionary history. This review examines the functions and activities of integrons before the antibiotic era. It shows how antibiotic use selected particular integrons from among the environmental pool of these elements, such that integrons carrying resistance genes are now present in the majority of Gram-negative pathogens. Finally, it examines the potential consequences of widespread pollution with the novel integrons that have been assembled via the agency of human antibiotic use and speculates on the potential uses of integrons as platforms for biotechnology.     

Bone Allografts: Past, Present and Future

Bone allograft transplantation has been performed in humans for more than one hundred and twenty years. During the first one hundred years (1880–1980), the major problem in bone allograft transplantation was availability. Most of the bone grafts used during this time were autografts. Allografts were not available due to a lack of legislation protecting procurers and processers. In addition, surgical procedures requiring allografts were not being performed. During the next twenty years (1980–2000), as allografis began to be used, the major issue was safety. Diseases transmitted during this period included AIDS and hepatitis. Avoidance of disease transmission became paramount. Sensitive blood tests and extensive efforts by bone banks to develop ways to clean. bone and clear it of infectious agents helped provide safe transplants. With concerns of availability and safety receding, the major issue in the future (2000–? ) will be the efficacy of the transplant. How allograft bone remodels in the host, how it incorporates and heals to host bone and how it integrates with the host skeleton will be the most important concerns of bone bankers and tissue transplant surgeons. Future research efforts will be applied to bone allograft transplantation to ensure that bone transplants heal quickly and sufficiently to be able to function as part of the weight-bearing skeletal system.     

Rice Research： Past, Present and Future

Rice （Oryza sativa L.） is a major crop in the world and provides the staple food for over half of the world＇s population. From thousands of years of cultivation and breeding to recent genomics, rice has been the focus of agriculture and plant research.     

Nitrogen Cycles: Past, Present, and Future

This paper contrasts the natural and anthropogenic controls on the conversion of unreactive N₂ to more reactive forms of nitrogen (Nr). A variety of data sets are used to construct global N budgets for 1860 and the early 1990s and to make projections for the global N budget in 2050. Regional N budgets for Asia, North America, and other major regions for the early 1990s, as well as the marine N budget, are presented to Highlight the dominant fluxes of nitrogen in each region. Important findings are that human activities increasingly dominate the N budget at the global and at most regional scales, the terrestrial and open ocean N budgets are essentially disconnected, and the fixed forms of N are accumulating in most environmental reservoirs. The largest uncertainties in our understanding of the N budget at most scales are the rates of natural biological nitrogen fixation, the amount of Nr storage in most environmental reservoirs, and the production rates of N₂ by denitrification.     

Human Adaptability: Past, Present, and Future

Human Adaptability: Past, Present, and Future. Stanley J. Ulijaszek and Rebecca A. Huss-Ashmore. eds. New York: Oxford University Press, 1997.326 pp.     

Viral Detection: Past,Present, and Future

Viruses are essentially composed of a nucleic acid (segmented or not, DNA, or RNA) and a protein coat. Despite their simplicity, these small pathogens are responsible for significant economic and humanitarian losses that have had dramatic consequences in the course of human history. Since their discovery, scientists have developed different strategies to efficiently detect viruses, using all possible viral features. Viruses shape, proteins, and nucleic acid are used in viral detection. In this review, the development of these techniques, especially for plant and mammalian viruses, their strengths and weaknesses as well as the latest cutting‐edge technologies that may be playing important roles in the years to come are described.     

Translational Bioinformatics:Past,Present, and Future

Though a relatively young discipline, translational bioinformatics (TBI) has become a key component of biomedical research in the era of precision medicine. Development of high-throughput technologies and electronic health records has caused a paradigm shift in both healthcare and biomedical research. Novel tools and methods are required to convert increasingly voluminous datasets into information and actionable knowledge. This review provides a definition and contex-tualization of the term TBI, describes the discipline’s brief history and past accomplishments, as well as current foci, and concludes with predictions of future directions in the field.     

Past,Present and Future of Human Chromaffin Cells: Role in Physiology and Therapeutics

Chromaffin cells are neuroendocrine cells mainly found in the medulla of the adrenal gland. Most existing knowledge of these cells has been the outcome of extensive research performed in animals, mainly in the cow, cat, mouse and rat. However, some insight into the physiology of this neuroendocrine cell in humans has been gained. This review summarizes the main findings reported in human chromaffin cells under physiological or disease conditions and discusses the clinical implications of these results.