Rab11a and myosin Vb regulate recycling of the M4 muscarinic acetylcholine receptor

Regulation of Angiotensin II Type 1A Receptor Intracellular Retention, Degradation, and Recycling by Rab5, Rab7, and Rab11 GTPases

Previous studies have demonstrated that the interaction of the angiotensin II type 1A receptor (AT(1A)R) carboxyl-terminal tail with Rab5a may modulate Rab5a activity, leading to the homotypic fusion of endocytic vesicles. Therefore, we have investigated whether AT(1A)R/Rab5a interactions mediate the retention of AT(1A)R.beta-arrestin complexes in early endosomes and whether the overexpression of Rab7 and Rab11 GTPases influences AT(1A)R lysosomal degradation and plasma membrane recycling. We found that internalized AT(1A)R was retained in Rab5a-positive early endosomes and was neither targeted to lysosomes nor recycled back to the cell surface, whereas a mutant defective in Rab5a binding, AT(1A)R-(1-349), was targeted to lysosomes for degradation. However, the loss of Rab5a binding to the AT(1A)R carboxyl-terminal tail did not promote AT(1A)R recycling. Rather, it was the stable binding of beta-arrestin to the AT(1A)R that prevented, at least in part, AT(1A)R recycling. The overexpression of wild-type Rab7 and Rab7-Q67L resulted in both increased AT(1A)R degradation and AT(1A)R targeting to lysosomes. The Rab7 expression-dependent transition of "putative" AT(1A)R.beta-arrestin complexes to late endosomes was blocked by the expression of dominant-negative Rab5a-S34N. Rab11 overexpression established AT(1A)R recycling and promoted the redistribution of AT(1A)R.beta-arrestin complexes from early to recycling endosomes. Taken together, our data suggest that Rab5, Rab7, and Rab11 work in concert with one another to regulate the intracellular trafficking patterns of the AT(1A)R.

Myosin function in nervous and sensory systems

Development of the nervous system requires remarkable changes in cell structure that are dependent upon the cytoskeleton. The importance of specific components of the neuronal cytoskeleton, such as microtubules and neurofilaments, to neuronal function and development has been well established. Recently, increasing focus has been put on understanding the functional role of the actin cytoskeleton in neurons. Important modulators of the actin cytoskeleton are the large family of myosins, many of which (classes I, II, III, V, VI, VII, IX, and XV; Fig. 1) are expressed in developing neurons or sensory cells. Myosins are force-producing proteins that have been implicated in a wide variety of cellular functions in the developing nervous system, including neuronal migration, process outgrowth, and growth cone motility, as well as other aspects of morphogenesis, axonal transport, and synaptic and sensory functions. We review the roles that neuronal myosins play in these functions with particular focus on the first three events listed above, as well as sensory function.

Chemical-genetic inhibition of a sensitized mutant myosin Vb demonstrates a role in peripheral-pericentriolar membrane traffic

Selective, in situ inhibition of individual unconventional myosins is a powerful approach to determine their specific physiological functions. Here, we report the engineering of a myosin Vb mutant that still hydrolyzes ATP, yet is selectively sensitized to an N(6)-substituted ADP analog that inhibits its activity, causing it to remain tightly bound to actin. Inhibition of the sensitized mutant causes inhibition of accumulation of transferrin in the cytoplasm and increases levels of plasma-membrane transferrin receptor, suggesting that myosin Vb functions in traffic between peripheral and pericentrosomal compartments.

Nuclear prostaglandin receptors: role in pregnancy and parturition?

The key regulatory role of prostanoids [prostaglandins (PGs) and thromboxanes (TXs)] in the maintenance of pregnancy and initiation of parturition has been established. However, our understanding of how these events are fine-tuned by the recruitment of specific signaling pathways remains unclear. Whereas, initial thoughts were that PGs were lipophilic and would easily cross cell membranes without specific receptors or transport processes, it has since been realized that PG signaling occurs via specific cell surface G-protein coupled receptors (GPCRs) coupled to classical adenylate cyclase or inositol phosphate signaling pathways. Furthermore, specific PG transporters have been identified and cloned adding a further level of complexity to the regulation of paracrine action of these potent bioactive molecules. It is now apparent that PGs also activate nuclear receptors, opening the possibility of novel intracrine signaling mechanisms. The existence of intracrine signaling pathways is further supported by accumulating evidence linking the perinuclear localization of PG synthesizing enzymes with intracellular PG synthesis. This review will focus on the evidence for a role of nuclear actions of PGs in the regulation of pregnancy and parturition.

Dynamics of the apical plasma membrane recycling system during cell division

The members of the family of Rab11 small GTPases are critical regulators of the plasma membrane vesicle recycling system. While previous studies have determined that the Golgi apparatus disperses during mitosis and reorganizes after cytokinesis, the fate of the recycling system during the cell cycle is more obscure. We have now studied in MDCK cells the fate during mitosis of an apical recycling system cargo, the polymeric IgA receptor (pIgAR), and regulators of the recycling system, Rab11a and its interacting proteins myosin Vb, Rab11-FIP1, Rab11-FIP2 and pp75/Rip11. Rab11a, pIgAR and myosin Vb containing vesicles dispersed into diffuse puncta in the cytosol during prophase and then became clustered near the spindle poles after metaphase, increasing in intensity throughout telophase. A similar pattern was observed for Rab11-FIP1 and Rab11-FIP2. However, Rab11-FIP1 lost colocalization with other recycling system markers during late prophase, relocating to the pericentriolar material. During telophase, Rab11-FIP1 returned to recycling system vesicles. Western blot analysis indicated that both Rab11a and pIgAR remained associated with membrane vesicles throughout the cell cycle. This behavior of the Rab11a-containing apical recycling endosome system during division was distinct from that of the Golgi apparatus. These results indicate that critical components of the apical recycling system remain associated on vesicles throughout the cell cycle and may provide a means for rapid re-establishment of plasma membrane components after mitosis.

The dopamine transporter constitutively internalizes and recycles in a protein kinase C-regulated manner in stably transfected PC12 cell lines

The dopamine transporter (DAT) removes dopamine from the extracellular milieu and is potently inhibited by number of psychoactive drugs, including cocaine, amphetamines, and methylphenidate (Ritalin). Multiple lines of evidence demonstrate that protein kinase C (PKC) down-regulates dopamine transport, primarily by redistributing DAT from the plasma membrane to endosomal compartments, although the mechanisms facilitating transporter sequestration are not defined. Here, we demonstrate that DAT constitutively internalizes and recycles in rat pheochromocytoma (PC12) cells. Temperature blockades demonstrated basal internalization and reliance on recycling to maintain DAT cell surface levels. In contrast, recycling blockade with bafilomycin A1 significantly decreased transferrin receptor (TfR) surface expression but had no effect on DAT surface levels, suggesting that DAT and TfR traffic via distinct endosomal mechanisms. Kinetic analyses reveal robust constitutive DAT cycling to and from the plasma membrane, independent of transporter expression levels. In contrast, phorbol ester-mediated PKC activation accelerated DAT endocytosis and attenuated transporter recycling in a manner sensitive to DAT expression levels. These data demonstrate constitutive DAT trafficking and that PKC-mediated DAT sequestration is achieved by a combination of accelerated internalization and reduced recycling. Additionally, the differential sensitivity to expression level exhibited by constitutive and regulated DAT trafficking suggests that these two processes are mediated by independent cellular mechanisms.

Cytoskeletal motors and cargo in membrane trafficking: opportunities for high specificity in drug intervention

Membrane trafficking comprises the directed transport of vesicle and/or organelle cargos to specific locations throughout the cell, and is primarily driven by molecular motors tracking along microtubules and microfilaments. The mechanisms by which specific motor complexes attach to their respective vesicular cargo is of great interest, and is only now starting to be unraveled. The proteins identified as links between the molecular motors and the vesicular cargo are viable drug targets and represent opportunities to regulate small groups of related proteins or even single proteins, such as receptors and transporters, at the cytosolic trafficking level. Ultimately, continued development in this area will lead to greater success in directing endocytosed drugs to the desired intracellular targets, such as the cell nucleus or the basolateral membrane.