Mechanisms of membrane trafficking in endocytic and non-endocytic pathways

PROJECT SUMMARY/ABSTRACT
The internalization and recycling of receptors is a key biological process in all eukaryotic cells. The
early/sorting endosome is the initial destination of receptors internalized from the plasma membrane (PM). This
endosome serves as a major sorting station from which receptors are shunted to late endosomes and
lysosomes for degradation, or are recycled back to the PM through a transitory network of vesicular and
tubular recycling endosomes. Whereas a decade ago most researchers thought that active sorting directed
proteins to the degradation pathways, targeting to the recycling pathway was thought to be largely a passive
process that occurs by default. However, recent evidence supports active sorting to the recycling pathways by
specific sorting nexin (SNX) and other proteins that bind to the cytoplasmic tails of receptors and specifically
target them for recycling. Although recycling is an essential process for all mammalian cells, its complex
regulation is poorly understood including the sorting of receptors on endosomal membranes, the budding and
fission of vesicles and tubules from the endosome, and the transport of receptors back to the PM. As such, our
knowledge of endosomal function lags substantially behind that of receptor internalization mechanisms. A key
group of regulatory proteins that controls sorting and trafficking at the endosome is the retromer complex.
Originally identified in the retrieval of biosynthetic cargo from endosomes to the Golgi complex, the retromer
has recently been implicated in the regulation of a variety of key cellular pathways both within and beyond the
scope of endocytic trafficking including endocytic recycling, mitochondrial homeostasis, the centrosome cycle
and ciliogenesis. The retromer complex also interacts with other key endocytic regulatory proteins, including
the tubular endosome scaffold MICAL-L1; its interaction partner and endosomal fission modulator, EH domain
containing 1 (EHD1); and a host of SNX proteins that mediate endosomal cargo sorting. The retromer also
links to the actin cytoskeleton via the WASH complex. Our laboratory has been focusing on an overall
understanding of the mechanisms by which endocytic regulatory proteins function both in endocytic pathways
and in non-endocytic trafficking. Our primary expertise is in biochemistry and molecular cell biology coupled
with advanced light microscopy, but we recognize the need to incorporate in vivo components into our
approach and have ongoing collaborations with other groups to examine these processes in whole organisms,
including zebrafish and worms. In our studies, we will address significant and as-yet-unresolved biological
problems such as: 1) how endosomal fission is regulated and linked to sorting and recycling and 2) how key
endocytic proteins mediate the biogenesis of the primary cilium.

Grant Number: 5R35GM144102-04
NIH Institute/Center: NIH
Principal Investigator: Steven Caplan