Kidney International, Vol. 55 (1999), pp. 2524–2525

EDITORIAL

Regulation of endosomal acidification via Gi-type protein

The contribution by Codina, Gurich and DuBose in of the need to temporally couple the acidification processthis issue of Kidney International brings the regulation to the endocytosis of ligand-receptor complexes.of endosomal acidification into better focus [1]. It has There are several other examples whereby receptorslong been recognized that many cells take up and tran- undergoing endocytosis appear to signal the acidificationsiently store extracellular constituents (fluid, salts, hor- mechanism through the agency of G proteins that aremones, metabolites, viruses, etc.) in intracellular vesicles an integral part of the endosome. Polybasic regions intermed endosomes. These contents can then be delivered the cytosolic domains of the IGF II/mannose receptorto lysosomes for degradation, and some membrane con- and a2 receptor [7, 8], and now the transferrin receptorstituents can also be recycled back to the cell exterior. [1] appear to activate specific G proteins. This couplingAn important role in ligand-receptor interactions is well has been demonstrated by the use of cognate peptiderecognized. Acidification of the endosomal contents by sequences for the polybasic clusters found in the cyto-an H1-ATPase plays a role in the dissociation of ligands solic regions of these receptors. Relatively short peptidefrom receptors, and may even initiate proteolysis of the sequences can activate GTP binding by the G proteinsinternalized receptors and ligands [2, 3]. Several different and active their GTPase activity, actions thought to liefates await ligand-receptor complexes that have under- at the heart of a G-protein signaling mechanism. Codinagone endocytosis [reviewed in 1]: the ligand can dissoci- et al now demonstrate that peptides containing theate from its receptor and be metabolized while the recep- KPKR sequence of the transferrin receptor stimulatetor is recycled to the cell surface, or the entire complex endosomal acidification in the presence of GTP [1].can be recycled back to the cell surface, or the complex There was a concentration dependence and a maximalcan be targeted to lysosomes for degradation. Such inter- effect observed at a peptide concentration of 100 nm.actions may play a role in the processing of cell mem- The activation by this peptide was dependent upon GTP,brane surface proteins in addition to ligand-receptor was equivalent to that seen with GTPgS, a non-hydrolyz-complexes. For example, a cytoplasmic ubiquitin-protein able GTP analogue, and to the activation seen with mas-ligase, Nedd4, interacts with cytoplasmic proline-rich do- toparan, a peptide extracted from wasp venom that ismains of the epithelial sodium channel subunits. Follow- known to activate Gi proteins. Both the transferrin recep-ing ubiquitination, the channel complex undergoes endo- tor peptides containing the KPKR sequence, and masto-cytosis and lysosomal degradation [4]. paran activated [35S]GTPgS binding to purified mixture

The classical seven-membrane spanning G-protein of pig Gai2/3 incorporated into phospholipid vesicles.coupled receptors invoke specific cellular responses via The current studies do not specifically address the sitespecific messenger pathways that are characteristic of of interaction of the critical polybasic sequence (KPKR)the particular G proteins that are coupled to the receptor with the relevant G proteins, nor the effect of ligand[5]. While familiar from the perspective of the whole binding to the receptor on the ability of the KPKR se-cell, this concept may also apply to the immediate regula- quence to signal the G proteins. The site of interactiontion of the endocytotic process and acidification process. may not be specific since polylysine appears to have aEndosomal acidification may be a primary step in the similar effect, but there may be specificity with respectprocessing of a ligand-receptor complex. For example, to the G protein species as demonstrated by mastoparan.endosomal acidification dissociates low density lipopro- This protein shares a homologous sequence with thetein (LDL) from its receptor [3], followed by degradation transferrin receptor peptides (...KxxxxxxKR/K...; Fig. 3of the ligand, while the affinity of the transferrin receptor in [1]), but has a limited G protein repertoire since itfor its ligand is increased by endosomal acidification [6], activates Gi.Go.Gs...Gt [9]. The peptide sequencesand the ligand-receptor complex recycles back to the flanking the polybasic domain may play an importantplasma membrane. These prior studies emphasize the role in recognition and binding of the polybasic domainimportance of regulating endosomal acidification, and to the relevant G protein. Future studies can be antici-

pated that will further define the binding domains onboth the relevant receptors and the G proteins.Key words: ligand-receptor interaction, apical membrane, low density

lipoprotein, endocytosis. Finally, the current studies do not consider the mecha-nism by which endosomal acidification is stimulated. 1999 by the International Society of Nephrology

2524

Editorial 2525

mingham, Alabama 35294-0007, USA.While direct activation of the H1-ATPase could well beE-mail: dwarnock@nrtc.dom.uab.eduimagined, the mechanism of such activation has not been

defined. There must be direct kinetic effects on the as-REFERENCESsembled pump complex, since the activation is observed

1. Codina J, Gurich R, DuBose TD: Peptides derived from theimmediately with isolated endosomal vesicles, mitigatinghuman transferrin receptor stimulate endosomal acidification viaany effect on de novo synthesis of pump subunits. A a Gi-type protein. Kidney Int 55:2376–2382, 1999

direct regulatory effect on the parallel chloride channel, 2. Mellman I, Fuchs R, Helenius A: Acidification of the endocyticand exocytic pathways. Ann Review Biochem 55:663–700, 1986which is known to be co-expressed in the endosomal

3. Goldstein JL, Brown MS, Anderson RG, Russell DW, Schnei-system with the H1-ATPase, would be equally plausible. der WJ: Receptor-mediated endocytosis: Concepts emerging fromThis possibility is of topical interest with the recent local- the LDL receptor system. Ann Review Cell Biol 1:1–39, 1985

4. Staub O, Dho S, Henry PC, Correa J, Ishikawa T, McGlade J,ization of CIC-5, a voltage-gated chloride channel toRotin D: WW domains of Nedd4 bind to the proline-rich PYendosomes, and linkage of defects in this channel protein motifs in the epithelial Na1 channel deleted in Liddle’s syndrome.

to deranged endosomal function and low molecular EMBO J 15:2371–2380, 19965. Lefkowitz RJ, Caron MG: Adrenergic receptors. Models for theweight proteinuria [10]. The present results do demon-

study of receptors coupled to guanine nucleotide regulatory pro-strate that there are intrinsic G-proteins present on the teins. J Biol Chem 263:4993–4996, 1988cytosolic face of endosomes, and that these regulator 6. Dautry-Varsat A, Ciechanover A, Lodish HF: pH and the recy-

cling of transferrin during receptor-mediated endocytosis. Procproteins can affect the endosomal acidification process.Natl Acad Sci USA 80:2258–2262, 1983

In addition to regulation of the acidification mecha- 7. Okamoto T, Katada T, Murayama Y, Ui M, Ogata E, NishimotoI: A simple structure encodes G protein-activating function of thenism(s) by receptors that have undergone endocytosis,IGF-II/mannose 6-phosphate receptor. Cell 62:709–717, 1990cytosolic messengers could also affect these same acidi-

8. Okamoto T, Murayama Y, Hayashi Y, Inagaki M, Ogata E,fication processes, raising the possibility of coupling to Nishimoto I: Identification of a Gs activator region of the beta

2-adrenergic receptor that is autoregulated via protein kinaseand regulation by basolateral as well as apical membraneA-dependent phosphorylation. Cell 67:723–730, 1991ligand-receptor interactions. 9. Higashijima T, Uzu S, Nakajima T, Ross EM: Mastoparan, apeptide toxin from wasp venom, mimic receptors by activating

David G. Warnock GTP-binding regulatory proteins (G-proteins). J Biol ChemBirmingham, Alabama, USA 263:6491–6494, 1988

10. Scheinman SJ, Guay-Woodford LM, Thakker RV, WarnockReprint request to David G. Warnock, M.D., Division of Nephrol- DG: Genetic disorders of renal electrolyte transport. N Engl J

ogy, University of Alabama at Birmingham, UAB Station, Bir- Med 340:1177–1187, 1999