Discrete Transport Phenomena in Endocytosis
Chinmay Pangarkar
Department of Chemical Engineering
UCSB Doctoral Candidate
Date:
Tuesday, Nov. 21, 2006
Time: 4:00 p.m.
Place: Engineering II, Room 3361
ABSTRACT
This work seeks to understand the transport phenomena associated with sub-micron (50nm - 1µm) particles in cells; for example organelles, viruses and synthetic drug and gene carriers. These phenomena form a special class of problems in that particle transport is determined not only by its thermal mobility, but more dominantly by its interactions with the cellular cytoskeleton (e.g. microtubules), motor-proteins and other organelles. The complex and multi-scale nature of these interactions makes it difficult to understand these transport phenomena.
We focus on the process of endocytosis. Endocytic vesicles called endosomes and lysosomes are sub-micron particles (100nm-500nm) responsible for basic cellular functions such as uptake of nutrients, signals and lipids. Endosomes receive material internalized from the surroundings and deliver it to lysosomes which, in turn, digest it. Using live-cell fluorescence microscopy, we measure the transport properties of single endosomes and lysosomes. Quantitative measurements combined with mathematical modeling, allow us to reveal simple physical principles which describe the transport of these particles at physiologically relevant time scales. The motor-driven transport of endosomes and lysosomes (as represented by a Peclet number); and their clustering/ de-clustering dynamics (as represented by a dimensionless cluster number) are related to their intracellular organization and finally, to the process of endosome-lysosome mass transfer. These results quantitatively explain, for the first time, how spatial organization and function of organelles emerge from their micro-scale transport properties. This spatial view of the endocytic pathway is central to the current endeavor of understanding the cell quantitatively. Further, this methodology is general and can be applied to diverse problems such as transport of viruses, intracellular bacteria and other organelles.
The same methodology is then applied to study the intracellular transport of synthetic gene vectors. The measured transport properties form the foundation of a mathematical model and also serve to validate it. The model explains the dependence of delivery efficiency upon cell-specific parameters such as cell size, shape, dimensionality (2D or 3D) and dividing ability. The model is used to predict the optimal intracellular itinerary of gene vectors. In general, it provides a platform to integrate and interpret experimental data, generate and test design strategies and guide future experimentation.