Proper growth of dendrites is critical to the formation of neuronal circuits, but the cellular machinery that directs the addition of membrane components to generate dendritic architecture remains obscure. Here, we demonstrate that post-Golgi membrane trafficking is polarized toward longer dendrites of hippocampal pyramidal neurons in vitro and toward apical dendrites in vivo. Small Golgi outposts partition selectively into longer dendrites and are excluded from axons. In dendrites, Golgi outposts concentrate at branchpoints where they engage in post-Golgi trafficking. Within the cell body, the Golgi apparatus orients toward the longest dendrite, and this Golgi polarity precedes asymmetric dendrite growth. Manipulations that selectively block post-Golgi trafficking halt dendrite growth in developing neurons and cause a shrinkage of dendrites in mature pyramidal neurons. Further, disruption of Golgi polarity produces neurons with symmetric dendritic arbors lacking a single longest principal dendrite. These results define a novel polarized organization of neuronal secretory trafficking and demonstrate a mechanistic link between directed membrane trafficking and asymmetric dendrite growth.

Monodisperse, spherical, polyethylene glycol (PEG)-coated silica nanoparticles have been prepared at room temperature and characterized for the purpose of biomedical applications. The particles were synthesized by the hydrolysis of tetramethyl orthosilicate (TMOS) in alcohol media under catalysis by ammonia, and their size can range from about 50-350 nm in diameter. We studied the particle size and size distribution using a scanning electron microscope (SEM) and an asymmetric field-flow fractionation (AFFF) multiangle static light-scattering instrument. The chemical and/or physical binding of PEG to the silica nanoparticles was studied by infrared spectroscopy, and the weight percentage of PEG attached to the particles was quantified. The PEG-coated silica nanoparticles showed enhanced colloidal stability when redispersed into aqueous solutions from the dried state as a result of the steric stabilization function of the PEG polymer grafted on the surface of particles. A nonspecific protein-binding test was also carried out to show that the PEG coating can help reduce the protein adsorption onto the surface of the particles, relating to the biocompatibility of these PEG-coated particles. Also, the inclusion of magnetic nanoparticles into the silica particles was shown as an example of the possible applications of PEG-coated silica particles. These silica nanoparticles, as a matrix for encapsulation of certain reagents, have potential for applications to in vivo diagnosis, analysis, and measurements inside intact biologic systems.

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The development and characterisation of a fluorescent optical PEBBLE (Probe Encapsulated By Biologically Localised Embedding) nanosensor for the detection of zinc is detailed. A ratiometric sensor has been fabricated that incorporates two fluorescent dyes; one is sensitive to zinc and the other acts as a reference. The sensing components are entrapped within a polymer matrix by a microemulsion polymerisation process that produces spherical sensors that are in the size region of 20 to 200 nm. Cellular measurements are made possible by the small sensor size and the biocompatibility of the matrix. The effects of reversibility, photobleaching and leaching have been examined, as well as the selectivity towards zinc over other cellular ions such as Na+, Ca2+, K+, and Mg2+. The dynamic range of these sensors was found to be 4 to 50 microM Zn2+ with a linear range from 15 to 40 microM. The response time for the PEBBLE is less than 4 s and the sensor is reversible. In addition, the nanosensors are photostable and leaching from the matrix, determined using a novel method, is minimal. These sensors are capable of real-time inter- and intra-cellular imaging and are insensitive to interference from proteins.