Rab GTPases as regulators of endocytosis, targets of disease and therapeutic opportunities

Abstract

Rab GTPases are well-recognized targets in human disease, although are underexplored therapeutically. Elucidation of how mutant or dysregulated Rab GTPases and accessory proteins contribute to organ specific and systemic disease remains an area of intensive study and an essential foundation for effective drug targeting. Mutation of Rab GTPases or associated regulatory proteins causes numerous human genetic diseases. Cancer, neurodegeneration and diabetes represent examples of acquired human diseases resulting from the up- or downregulation or aberrant function of Rab GTPases. The broad range of physiologic processes and organ systems affected by altered Rab GTPase activity is based on pivotal roles in responding to cell signaling and metabolic demand through the coordinated regulation of membrane trafficking. The Rab-regulated processes of cargo sorting, cytoskeletal translocation of vesicles and appropriate fusion with the target membranes control cell metabolism, viability, growth and differentiation. In this review, we focus on Rab GTPase roles in endocytosis to illustrate normal function and the consequences of dysregulation resulting in human disease. Selected examples are designed to illustrate how defects in Rab GTPase cascades alter endocytic trafficking that underlie neurologic, lipid storage, and metabolic bone disorders as well as cancer. Perspectives on potential therapeutic modulation of GTPase activity through small molecule interventions are provided.

Fig. 1

Rab GTPases in normal cellular and molecular functions vs alterations in human disease. (Left panel) Normal functions of endocytic Rab GTPases in (i) endocytosis and recycling, (ii) degradative pathways (including autophagic and phagocytic pathways), (iii) lysosome biogenesis and regulated secretion, (iv) ciliogenesis and ciliary trafficking. (Right panel) Disease-causing alterations in Rab GTPase function may be acquired or genetic leading to loss of function, increased or decreased expression or activity.

Fig. 2

Rab GTPases in endocytosis, recycling, and degradative pathways. Receptor mediated endocytosis occurs via clathrin-coated vesicles and is regulated by Rab5 and Rab21. Internalized cargo is delivered to early/sorting endosomes. From here molecules can return to the plasma membrane via fast or slow recycling routes through specialized recycling endosomes and the activities of distinct Rab GTPases (further detailed in the text). Newly synthesized plasma membrane proteins are delivered from the trans-Golgi network to recycling endosomes, while lysosomal hydrolases are delivered to early and late endosomes via two mannose 6-phosphate receptors. Recycling from early endosomes to the Golgi depends on Rab6, while Rab9 controls transport from the late endosome to the trans-Golgi. Rab7 is a critical Rab GTPase on multiple degradative pathways; promoting late endosome, phagosome and autophagosome fusion with lysosomes in cooperation with specialized Rab GTPases on each of these pathways. In conjunction with Rac1, Rab7 is also pivotal in cadherin degradation by epithelia and neurons, as well as in bone resorption by osteoclasts. Light green overlay encompasses endocytic and recycling circuits; light blue overlay encompasses degradative circuits; and neutral overlay encompasses the biosynthetic/exocytic routes.

Fig. 3

Rab GTPase regulation and points of therapeutic intervention. The Rab GTPase activation cycle entails lipid modification through prenylation, nucleotide binding and hydrolysis regulated by guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs), effector protein interactions and cytosolic recycling by guanine dissociation inhibitor (GDI). Possible modes of Rab GTPase inhibition include disruption of membrane association through prenylation inhibitors; GEF inhibitors to block activation; nucleotide binding inhibitors to block activation analogous to kinase inhibitors; and protein–protein interaction inhibitors.

Fig. 4

Genetic diseases disrupt endocytic recycling circuits and cause neuronal degeneration, glaucoma and retinal degeneration. The schematic depicts the normal functions and protein complexes of the Huntingtin (Htt) and optineurin (OPTN) proteins on the Golgi and various endosomal compartments. Mutant Htt (mHtt) causes the severe neurodegenerative Huntington's disease and mutant OPTN causes vision disorders. (a) Htt protein is associated with the trans-Golgi and post-Golgi vesicles where it binds OPTN, Fip2, Rab8 and myosin VI. mHtt expression causes OPTN and Rab8 dissociation and blocks lysosomal enzyme export to endosomes. (b) Htt increases guanine nucleotide exchange factor (GEF)-mediated activation of Rab11a on recycling endosomes. mHtt causes Rab11 inactivation and release from endosomes. (c) Htt binds HAP40 and Rab5 on early endosomes. mHtt causes early endosome transfer from microtubules to actin filaments and peripheral immobilization. (d) Htt forms a complex with HAP1, the p150dynein/dynactin motor complex that is important in trafficking brain-derived neurotrophic factor (BDNF) containing vesicles to late endosomes.

Fig. 5

Rab GTPase up- or downregulation can result in acquired disease. Rab25 serves as a well-studied example of how increased or decreased GTPase expression can impact trafficking of critical molecules involved in cell signaling and adhesion and adversely affect cell physiology in cancer by increasing proliferation, dedifferentiation, motility and metastasis. It is speculated that in cases where Rab25 expression is decreased (e.g. colon and some types of breast cancer) that additional mutations or factors may be in play that are either synergistic with Rab25 underexpression or result in independence from proteins/functions regulated by the Rab25 recycling route.

Fig. 6

Axon viability depends on coordinated endocytic trafficking and signaling. Nerve growth factor (NGF) binds to the TrkA receptor tyrosine kinase and stimulates phosphorylation and internalization. Endocytosed TrkA is transported from Rab5-positive early endosomes to Rab7-positive late endosomes. In peripheral neurons long distance transport of late endosomes to the cell body is critical for growth factor degradation and proper nuclear signaling to maintain cell viability and differentiation. Transport occurs on microtubules through Rab7, the Rab interacting lysosomal protein (RILP) effector and dynactin minus-end directed motor complex. Return transport to the synapse is mediated by Rab7 in conjunction with a plus-end directed kinesin motor, likely KIF3a. Inset illustrates endosomal membrane protein complexes involved in Rab conversion that allows transfer of cargo along the degradative pathway. Several conserved multimeric protein complexes first identified in yeast are thought to aid in Rab conversion, directed transport and fusion (RETROMER endosome to Golgi; CORVET early to late endosome; HOPS late endosome to lysosome).