Changes in ocular aquaporin-4 (AQP4) expression following retinal injury.

PURPOSE: Changes in the expression of water channels or aquaporins (AQP) have been reported in several diseases. However, such changes and mechanisms remain to be evaluated for retinal injury. This study was designed to analyze changes in the expression of AQP4 following elevation of intraocular pressure (IOP) and after intravitreal endothelin-1 injection and the potential involvement of the ubiquitin-dependent proteasome. METHODS: Retinal injuries were induced by the elevation of intraocular pressure in rat eyes using the Morrison model or following endothelin-1 intravitreal injection. Immunohistochemistry using a combination of glial fibrillary acidic protein (GFAP) and aquaporin-4 antibodies were employed to follow changes in the optic nerve head astrocytes. Real-time quantitative PCR (Q-PCR) was used for measuring changes in AQP4, ubiquitin hydrolase L1 (UCH-L1), and beta-actin messages. Changes in AQP4, caspase-3, thy-1, ubiquitination, and GFAP expression were also followed in total retinal extracts using western blotting. An S5a column was used to purify ubiquitinated proteins. RESULTS: In retinas of both injury models, there was an upregulation of GFAP (a marker of astrogliosis), caspase-3, and downregulation of thy-1, a marker for retinal ganglion cell stress, and decreased retinal AQP4 mRNA and protein levels as determined by Q-PCR, and western blotting, respectively. By contrast, IOP enhanced expression and co-localization of GFAP and AQP4 in optic nerve astrocytes. AQP4 was detected in affinity-purified ubiquitinated proteins using S5a column, suggesting that AQP4 is a target for degradation by the ubiquitin-dependent proteasome. While elevation of IOP induced an increase in ubiquitination in retinal extracts, it decreased ubiquitination in optic nerve extracts as detected by western blotting. Enhanced ubiquitination and decreased ubiquitination appear to correlate with AQP4 expression. IOP decreased UCH-L1 (or protein gene protein [PGP9.5]) in retinal extracts as judged by Q-PCR. CONCLUSIONS: The enhanced expression of AQP4 in optic nerve astrocytes following elevation of IOP may explain the astrocytic hypertrophy normally seen in glaucoma patients and may involve alteration in the activity of ubiquitin-dependent proteasomal degradation system. The decreased ubiquitination in the optic nerve may lead to increased levels of proapoptotic proteins known to be degraded by the proteasome, and thus to axonal degeneration in glaucoma.

Stress-induced changes in neuronal Aquaporin-9 (AQP9) in a retinal ganglion cell-line.

The water channel, Aquaporin-9 (AQP9) is enriched in selected neuronal populations and is unique its ability to act as a lactate-glycerol channel supplying neurons with alternative fuel under ischaemic conditions. AQP9 was detected in RGC-5 cells, a retinal ganglion cell-line, primary RGCs, and retina by Western blotting, real-time PCR (RT-PCR) and immunohistochemistry. RGC-5 cells subjected to a hypotonic stress increased their cell volume that was blocked by known inhibitor of AQP9 (phloretin (40 microM)). RGC-5 cells subjected to hypoxia, showed an up-regulation in AQP9 expression as judged by Western blotting and RT-PCR. Similarly, hypotonic shock (50%) increased AQP9 expression as determined by RT-PCR. AQP9 is involved in energy balance as a glycerol-lactate channel and also appears to regulate cell volume in retinal ganglion neurons. This water channel may play a key role in retinal ganglion pathology.

A ramble through the cell: how can we clear such a complicated trail?

The arrangement of course information in a logical sequence for molecular life science (MLS) courses remains a matter of some controversy, even within a single subdiscipline such as biochemistry. This is due to the explosion of knowledge, the latest bioinformatic revelations, and the observation that new discoveries sometimes reveal specific connections between previously disparate topics. However, the general outlines of biomedical information are in place, at least the knowledge that should be conveyed to undergraduates taking cell and molecular biology and biochemistry. Despite the increasing amount and complexity of the information to be presented, integration and unification are possible because the molecular reactions and interactions that underlie all life processes are coming into view: they are common to all cellular structural rearrangements, nucleic acid functions, and biochemical reactions, whether of plant or animal origin. Also, it is no longer possible to draw clear boundaries between cell biology, biochemistry, and molecular biology that would not violate the fundamental unity of our understanding. Therefore, an arrangement of content is proposed for a two-semester course that aims to present a unified portrait of upper-division undergraduate MLS.

Neuronal calcium sensor-1 facilitates neuronal exocytosis through phosphatidylinositol 4-kinase.

This work tested the theory that neuronal calcium sensor-1 (NCS-1) has effects on neurotransmitter release beyond its actions on membrane channels. We used nerve-ending preparations where membrane channels are bypassed through membrane permeabilization made by mechanical disruption or streptolysin-O. Nerve ending NCS-1 and phosphatidylinositol 4-kinase (PI4K) are largely or entirely particulate, so their concentrations in nerve endings remain constant after breaching the membrane. Exogenous, myristoylated NCS-1 stimulated nerve ending phosphatidylinositol 4-phosphate [PI(4)P] synthesis, but non-myristoylated-NCS-1 did not. The N-terminal peptide of NCS-1 interfered with PI(4)P synthesis, and with spontaneous and Ca(2+)-evoked release of both [(3)H]-norepinephrine (NA) and [(14)C]-glutamate (glu) in a concentration-dependent manner. An antibody raised against the N-terminal of NCS-1 inhibited perforated nerve ending PI(4)P synthesis, but the C-terminal antibody had no effects. Antibodies against the N- and C-termini of NCS-1 caused significant increases in mini/spontaneous/stimulation-independent release of [(3)H]-NA from perforated nerve endings, but had no effect on [(14)C]-glu release. These results support the idea that NCS-1 facilitates nerve ending neurotransmitter release and phosphoinositide production via PI4K and localizes these effects to the N-terminal of NCS-1. Combined with previous work on the regulation of channels by NCS-1, the data are consistent with the hypothesis that a NCS-1-PI4K (NP, neuropotentiator) complex may serve as an essential linker between lipid and protein metabolism to regulate membrane traffic and co-ordinate it with ion fluxes and plasticity in the nerve ending.

ADP-ribosylation factor6 regulates both [3H]-noradrenaline and [14C]-glutamate exocytosis through phosphatidylinositol 4,5-bisphosphate.

GTP phosphohydrolase (cell regulating) (EC 3.6.1.47, ADP-ribosylation factor6, ARF6) has been shown to play an important role in different steps of membrane trafficking. It also regulates chromaffin granule exocytosis through phosphatidylcholine phosphatidohydrolase (EC 3.1.4.14, PLD) activation. In this study, the role of ARF6 in neurotransmitter release from both dense-core granules (DCGs) and synaptic vesicles (SVs) in rat brain cortex nerve endings was investigated. We observed that synaptosomal ARF6 is largely particulate but moves to a less easily pelleted compartment upon nerve ending stimulation. We also found that direct inhibition of ARF6 by a specific antibody or interference with ARF6 downstream effects by a myristoylated N-terminal ARF6 peptide both significantly decreased both [3H]-noradrenaline and [14C]-glutamate exocytosis. Addition of phosphatidic acid (PA) and phosphatidylinositol 4,5-bisphosphate (PIP2) partially or completely restored exocytosis. These findings suggest that ARF6 plays important regulatory roles for both DCG and SV exocytosis by activating PLD and ATP:1-phosphatidyl-1D-myo-inositol 4-phosphate 5-phosphotransferase (EC 2.7.1.68, PI4P-5K) to enhance PIP2 synthesis and nerve ending membrane trafficking.

Incubation of nerve endings with a physiological concentration of Abeta1-42 activates CaV2.2(N-Type)-voltage operated calcium channels and acutely increases glutamate and noradrenaline release.

We wish to understand the normal function of amyloid-beta peptides (Abeta) and to see if they destabilize neuronal calcium homeostasis [Mattson et al., J. Neurosci. 12 (1992), 376-389]. We observed that a physiological concentration (10 nM) of Abeta1-42 increased both glutamate and noradrenaline exocytosis from rat cortical nerve endings at least in part by activation of N-type Ca2+ channels. Abeta oligomers rather than monomers or fibrils probably are the most active form. Three alternatively-proposed effects of Abeta (reactive oxygen species formation, membrane perforation, and disruption of Ca2+ stores) also were tested by incubating nerve endings with a relatively high (by this study's standards) concentration of Abeta1-42(100 nM). None of the three proposed effects were detected during these incubations. These results support the hypothesis that persistent elevations of Abeta, which normally operates as a modulator of N-type voltage gated calcium channels, could increase internal nerve ending Ca2+ and excitatory neurotransmitter release to produce the early neurotoxic effects that eventually lead to Alzheimer's disease.

Phosphatidylinositol 4,5-bisphosphate promotes both [3H]-noradrenaline and [14C]-glutamate exocytosis from nerve endings.

Inhibition of phosphatidylinositol 4-phosphate (PI4P) and phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) synthesis by phenylarsine oxide (PAO) inhibits both [3H]-noradrenaline ([3H]-NA) and [14C]-glutamate ([14C]-glu) exocytosis from streptolysin-O (SLO)-perforated synaptosomes. When PI4,5P(2) is blocked by an antibody or chelated by neomycin, neurotransmitter exocytosis again is inhibited. Also, when phosphoinositide (PI) synthesis is indirectly decreased by shunting phosphatidic acid (PA) synthesis into phosphatidylbutanol production, both [14C]-glutamate and [3H]-noradrenaline exocytosis are inhibited. All of these results indicate that PI4,5P(2) is necessary for exocytosis of both synaptic vesicles (SVs) and dense core vesicles (DCVs).