The internal pH of synaptic-like microvesicles in rat pinealocytes in culture

Cresyl violet: a superior fluorescent lysosomal marker

We have characterized cresyl violet as a membrane-permeant fluorophore that localizes to lysosomes and acidic vacuoles of budding yeast, Drosophila, human, murine and canine cells. An acidotropic weak base, cresyl violet is shown to be virtually insensitive to physiological alkali and divalent cations. Because of its unique spectral properties, it can be used in combination with green, red and far-red fluorophores, is less susceptible to photobleaching than alternative acidotropic probes, and does not undergo photoconversion. At concentrations that yield bright labeling of acidic compartments, cresyl violet does not alter the organellar pH nor does it affect the buffering capacity. Its affordability, together with its chemical and spectral properties, make cresyl violet a superior lysosomal marker devoid of many of the negative characteristics associated with other lysosomal probes.

The ClC-3 Cl-/H+ antiporter becomes uncoupled at low extracellular pH

Adenovirus expressing ClC-3 (Ad-ClC-3) induces Cl(-)/H(+) antiport current (I(ClC-3)) in HEK293 cells. The outward rectification and time dependence of I(ClC-3) closely resemble an endogenous HEK293 cell acid-activated Cl(-) current (ICl(acid)) seen at extracellular pH <or= 5.5. ICl(acid) was present in smooth muscle cells from wild-type but not ClC-3 null mice. We therefore sought to determine whether these currents were related. ICl(acid) was larger in cells expressing Ad-ClC-3. Protons shifted the reversal potential (E(rev)) of I(ClC-3) between pH 8.2 and 6.2, but not pH 6.2 and 5.2, suggesting that Cl(-) and H(+) transport become uncoupled at low pH. At pH 4.0 E(rev) was completely Cl(-) dependent (55.8 +/- 2.3 mV/decade). Several findings linked ClC-3 with native ICl(acid); 1) RNA interference directed at ClC-3 message reduced native ICl(acid); 2) removal of the extracellular "fast gate" (E224A) produced large currents that were pH-insensitive; and 3) wild-type I(ClC-3) and ICl(acid) were both inhibited by (2-sulfonatoethyl)methanethiosulfonate (MTSES; 10-500 microm)-induced alkanethiolation at exposed cysteine residues. However, a ClC-3 mutant lacking four extracellular cysteine residues (C103_P130del) was completely resistant to MTSES. C103_P130del currents were still acid-activated, but could be distinguished from wild-type I(ClC-3) and from native ICl(acid) by a much slower response to low pH. Thus, ClC-3 currents are activated by protons and ClC-3 protein may account for native ICl(acid). Low pH uncouples Cl(-)/H(+) transport so that at pH 4.0 ClC-3 behaves as an anion-selective channel. These findings have important implications for the biology of Cl(-)/H(+) antiporters and perhaps for pH regulation in highly acidic intracellular compartments.

Molecular probes for sensing the cholesterol composition of subcellular organelle membranes

Neuroendocrine cells contain two types of secretagogue-regulated acidic compartments: secretory granules (SGs) and synaptic-like microvesicles (SLMVs), which can be identified by acidotropic probes such as acridine orange (AO) and DAMP. We investigated the accumulation of these probes in SGs and SLMVs as a function of glucose levels in the culture media using a pancreatic beta-cell line MIN6. AO was accumulated in the low-glucose condition, but not in the high-glucose condition. The AO accumulation correlated well with the SLMV dynamics by glucose and DAMP was localized in the SGs. Because SG membranes are reportedly high in cholesterol, we prepared liposomes with increasing cholesterol levels. AO is well incorporated into liposomes having a 20 to 40 mol% cholesterol composition, whereas DAMP was so in those having over 40 mol% cholesterol levels. Indeed, when cholesterol was depleted from MIN6 SG membranes, DAMP incorporation decreased, instead AO was incorporated. In PC12 cells, AO incorporation into SGs was significant but DAMP incorporation was limited. Consistently, the cholesterol composition was found 37 to 39 mol% in the SG membrane of PC12 cells. We suggest that cholesterol-sensing probes, AO and DAMP, are useful tools for investigating cholesterol compositions in acidic organelle membranes.

Inhibition of the vacuolar H(+)-pump with bafilomycin A1 does not induce acrosome reaction or activate proacrosin in mouse spermatozoa

Acrosomal protease activation is regarded as an important event triggered by acrosomal reaction and leading to sperm passage through zona pellucida. Mammalian acrosome has an internal acid pH that probably helps to maintain inactive proenzymes that otherwise could be precociously activated and prevent normal fertilization. In this work, we have studied the effect of bafilomycin A1, a potent and specific inhibitor of vacuolar H(+)-pump (V-ATPase), on acrosome reaction and proacrosin activation. We used the pH-sensitive probe Lysotracker Green DND-26 to monitor qualitatively intra-acrosomal pH in cauda epididymal mouse spermatozoa. Our results showed that loss of Lysotracker label induced by bafilomycin A1 (acrosome alkalinization) did not induce acrosome reaction or proacrosin activation. We also developed a new technique for imaging the acrosome, and for evaluating the acrosome reaction, in live mouse spermatozoa using Lysotracker DND-26. These results showed that the V-ATPase is a key regulator of mammalian acrosome pH, and that acrosome alkalinization is not the only prerequisite to activate proacrosin under in vivo conditions. Our results suggest that acrosome alkalinization and acrosome reaction are two processes that could be independently regulated during mammalian sperm capacitation.

Phosphatidylinositol 3-kinase-mediated effects of glucose on vacuolar H+-ATPase assembly, translocation, and acidification of intracellular compartments in renal epithelial cells

Vacuolar H+-ATPases (V-ATPases) are a family of ATP-driven proton pumps. They maintain pH gradients between intracellular compartments and are required for proton secretion out of the cytoplasm. Mechanisms of extrinsic control of V-ATPase are poorly understood. Previous studies showed that glucose is an important regulator of V-ATPase assembly in Saccharomyces cerevisiae. Human V-ATPase directly interacts with aldolase, providing a coupling mechanism for glucose metabolism and V-ATPase function. Here we show that glucose is a crucial regulator of V-ATPase in renal epithelial cells and that the effect of glucose is mediated by phosphatidylinositol 3-kinase (PI3K). Glucose stimulates V-ATPase-dependent acidification of the intracellular compartments in human proximal tubular cells HK-2 and porcine renal epithelial cells LLC-PK1. Glucose induces rapid ATP-independent assembly of the V1 and Vo domains of V-ATPase and extensive translocation of the V-ATPase V1 and Vo domains between different membrane pools and between membranes and the cytoplasm. In HK-2 cells, glucose stimulates polarized translocation of V-ATPase to the apical plasma membrane. The effects of glucose on V-ATPase trafficking and assembly can be abolished by pretreatment with the PI3K inhibitor LY294002 and can be reproduced in glucose-deprived cells by adenoviral expression of the constitutively active catalytic subunit p110alpha of PI3K. Taken together these data provide evidence that, in renal epithelial cells, glucose plays an important role in the control of V-ATPase-dependent acidification of intracellular compartments and V-ATPase assembly and trafficking and that the effects of glucose are mediated by PI3K-dependent signaling.

Glutamatergic Chemical Transmission: Look! Here, There, and Anywhere

Vesicular glutamate transporter (VGLUT) is responsible for the active transport of L-glutamate in synaptic vesicles and thus plays an essential role in the glutamatergic chemical transmission in the central nervous system. VGLUT comprises three isoforms, VGLUT1, 2, and 3, and is a potential marker for the glutamatergic phenotype. Recent studies indicated that VGLUT is also expressed in non-neuronal cells, and localized with various organelles such as synaptic-like microvesicles in the pineal gland, and hormone-containing secretory granules in endocrine cells. L-Glutamate is stored in these organelles, secreted upon various forms of stimulation, and then acts as a paracrine-like modulator. Thus, VGLUTs highlight a novel framework of glutamatergic signaling and reveal its diverse modes of action.

Co-expression of vesicular glutamate transporters (VGLUT1 and VGLUT2) and their association with synaptic-like microvesicles in rat pinealocytes

A vesicular glutamate transporter (VGLUT) is responsible for the accumulation of l-glutamate in synaptic vesicles in glutamatergic neurons. Two isoforms, VGLUT1 and VGLUT2, have been identified, which are complementarily expressed in these neurons. Mammalian pinealocytes, endocrine cells for melatonin, are also glutamatergic in nature, accumulate l-glutamate in synaptic-like microvesicles (SLMVs), and secrete it through exocytosis. Although the storage of l-glutamate in SLMVs is mediated through a VGLUT, the molecular nature of the transporter is less understood. We recently observed that VGLUT2 is expressed in pinealocytes. In the present study, we show that pinealocytes also express VGLUT1. RT-PCR and northern blot analyses indicated expression of the VGLUT1 gene in pineal gland. Western blotting with specific antibodies against VGLUT1 indicated the presence of VGLUT1 in pineal gland. Indirect immunofluorescence microscopy with a section of pineal gland and cultured cells indicated that VGLUT1 and VGLUT2 are co-localized with process terminal regions of pinealocytes. Furthermore, immunoelectronmicroscopy as well as subcellular fractionation studies revealed that both VGLUT1 and VGLUT2 are specifically associated with SLMVs. These results indicate that both VGLUTs are responsible for storage of l-glutamate in SLMVs in pinealocytes. Pinealocytes are the first exception as to complementary expression of VGLUT1 and VGLUT2.