Evidence for the presence of phosphoinositide cycle and its involvement in cellular signal transduction in the rabbit lens

Evaluation of Systemic Medications Associated With Surgically Treated Cataract Among US Adults

PURPOSE

To comprehensively determine the associations between systemic medications and surgically treated cataract in the US population.

DESIGNS

Retrospective cross-sectional study.

METHODS

Participants aged ≥40 years from the 1999-2008 National Health and Nutrition Examination Survey (NHANES) were included. Surgically treated cataract was defined as cataract requiring a procedure. Data on prescription drug use over the past 30 days were collected via home interviews. Drug categories for ophthalmic indications and those prescribed in less than 0.5% of the participants were excluded from the analysis. Separate logistic regression models were used to explore associations between each drug category and surgically treated cataract. The Benjamin-Hochberg procedure was used to control the false discovery rate.

RESULTS

A total of 14,931 were included in the present analysis. The weighted prevalence of surgically treated cataract was 9.6% (n=2010). We identified 20 drug categories that had significant associations with surgically treated cataract, of which 8 associations remained statistically significant after further adjustment for pertinent comorbidities. The 3 drug categories with the highest odds ratio (OR) values were tricyclic antidepressants (OR, 2.21; 95% CI, 1.38-3.51; P = .001), insulin (OR, 2.13; 95% CI, 1.48-3.07; P = 9.41×10^-5) and group III antiarrhythmic agents (OR, 2.00; 95% CI, 1.25-3.19; P = .004). The use of sex hormone combinations among women reduced the risk of having surgically treated cataract (OR, 0.011; 95% CI, 0.001-0.089; P = 5.98×10^-5). Dose-response relationships were observed for all 8 drug categories.

CONCLUSIONS

Our comprehensive evaluation provides new knowledge on the complex relationships between systemic medications and surgically treated cataract.

Drugs associated with cataract formation represent an unmet need in cataract research

Decreased light transmittance through the ocular lens, termed cataract, is a leading cause of low vision and blindness worldwide. Cataract causes significantly decreased quality of life, particularly in the elderly. Environmental risk factors, including aging, UV exposure, diabetes, smoking and some prescription drugs, are all contributors to cataract formation. In particular, drug-induced cataract represents a poorly-addressed source of cataract. To better understand the potential impact of prescription drugs on cataract, we analyzed publicly-available drug prescriptions data from the Australian Pharmaceutical Benefits Scheme. The data was analyzed for the 5-year period from July 2014 to June 2019. Analyses included the number of prescriptions for each drug, as well as the associated government and total prescription costs. The drugs chosen for analysis belonged to any of four broad categories—those with known, probable, possible or uncertain association with cataract in patients. The analyses revealed high prescription rates and costs for drugs in the Known category (e.g., steroids) and Possible category (e.g., psychotropic drugs). Collectively, these data provide valuable insights into specific prescription drugs that likely contribute to the increasing annual burden of new cataract cases. These data highlight the need—as well as new, stem cell-based opportunities—to elucidate molecular mechanisms of drug-induced cataract formation.

Rapid and efficient cataract gene evaluation in F0 zebrafish using CRISPR-Cas9 ribonucleoprotein complexes

Cataract is the leading cause of blindness worldwide. Congenital or paediatric cataract can result in permanent visual impairment or blindness even with best attempts at treatment. A significant proportion of paediatric cataract has a genetic cause. Therefore, identifying the genes that lead to cataract formation is essential for understanding the pathological process of inherited paediatric cataract as well as to the development of new therapies. Despite clear progress in genomics technologies, verification of the biological effects of newly identified candidate genes and variants is still challenging. Here, we provide a step-by-step pipeline to evaluate cataract candidate genes in F0 zebrafish using CRISPR-Cas9 ribonucleoprotein complexes (RNP). Detailed descriptions of CRISPR-Cas9 RNP design and formulation, microinjection, optimization of CRISPR-Cas9 RNP reagent dose and delivery route, editing efficacy analysis as well as cataract formation evaluation are included. Following this protocol, any cataract candidates can be readily and efficiently evaluated within 2 weeks using basic laboratory supplies.

Lens Connexin Channels Show Differential Permeability to Signaling Molecules

Gap junction channels mediate the direct intercellular passage of small ions as well as larger solutes such as second messengers. A family of proteins called connexins make up the subunits of gap junction channels in chordate animals. Each individual connexin forms channels that exhibit distinct permeability to molecules that influence cellular signaling, such as calcium ions, cyclic nucleotides, or inositol phosphates. In this review, we examine the permeability of connexin channels containing Cx43, Cx46, and Cx50 to signaling molecules and attempt to relate the observed differences in permeability to possible in vivo consequences that were revealed by studies of transgenic animals where these connexin genes have been manipulated. Taken together, these data suggest that differences in the permeability of individual connexin channels to larger solutes like 3',5'-cyclic adenosine monophosphate (cAMP) and inositol 1,4,5-trisphosphate (IP₃) could play a role in regulating epithelial cell division, differentiation, and homeostasis in organs like the ocular lens.

Connexin43 and connexin50 channels exhibit different permeability to the second messenger inositol triphosphate

Gap junction channels made of different connexins have distinct permeability to second messengers, which could affect many cell processes, including lens epithelial cell division. Here, we have compared the permeability of IP₃ and Ca²⁺ through channels made from two connexins, Cx43 and Cx50, that are highly expressed in vertebrate lens epithelial cells. Solute transfer was measured while simultaneously monitoring junctional conductance via dual whole-cell/perforated patch clamp. HeLa cells expressing Cx43 or Cx50 were loaded with Fluo-8, and IP₃ or Ca²⁺ were delivered via patch pipette to one cell of a pair, or to a monolayer while fluorescence intensity changes were recorded. Cx43 channels were permeable to IP₃ and Ca²⁺. Conversely, Cx50 channels were impermeable to IP₃, while exhibiting high permeation of Ca²⁺. Reduced Cx50 permeability to IP₃ could play a role in regulating cell division and homeostasis in the lens.

Antidepressants use and risk of cataract development: a systematic review and meta-analysis

Background

Epidemiological studies suggest that antidepressants use may increase the risk of cataract, but the results are inconclusive. We aimed to examine this association by performing a systematic review and meta-analysis.

Methods

Relevant studies were identified by searching PubMed and Web of Science databases through June 2017. We included studies that reported risk estimates for the association between antidepressants use and cataract risk. A random-effects model was used to calculate the summary odds ratio (OR) with its 95% confidence interval (CI).

Results

We identified seven studies of antidepressants use and risk of cataract involving 447,672 cases and 1,510,391 controls. Overall, the combined ORs (95% CIs) of cataract for selective serotonin reuptake inhibitors (SSRIs), serotonin noradrenalin reuptake inhibitors (SNRIs), and tricyclic antidepressants (TCAs) were 1.12 (1.06–1.19), 1.13 (1.04–1.24), and 1.19 (1.11–1.28), respectively. A certain degree of heterogeneity was observed across studies (P < 0.001, I² = 92.2% for SSRIs, P = 0.026, I² = 67.5% for SNRIs, and P = 0.092, I² = 58.0% for TCAs).

Conclusion

This meta-analysis provides evidence of a significant positive association between antidepressants use and risk of cataract. Because of the heterogeneity and limited eligible studies, further prospective studies are warranted to confirm the preliminary findings of our study.

Antidepressants and risk of cataract development: A population-based, nested case-control study

BACKGROUND

Previous studies demonstrated increased risk of cataract development among users of selective serotonin reuptake inhibitors (SSRIs). However, it remains unknown whether this risk also prevails with the use of other types of antidepressants. The aim of this study was to investigate whether use of antidepressants is associated with an increased risk of cataract development. Moreover, the relationship between binding affinities of serotonin transporter (SERT) of antidepressant and the risk of cataracts is examined.

METHODS

We conducted a nested case-control study using National Health Insurance Research Database in Taiwan. A total of 14,288 patients were included; 7651 in the cataract group and 6637 in the control group. Antidepressant exposure was categorized by type, duration of use, and binding affinities of SERT. The association between antidepressant exposure and cataract development was assessed using conditional logistic regression analysis.

RESULTS

The adjusted odds ratios (AORs) for developing cataracts among continuous users of SSRIs, serotonin norepinephrine reuptake inhibitors (SNRIs), and other antidepressants were 1.26 (95% confidence interval (CI): 1.12-1.41, p<0.001), 1.21 (95% CI: 1.02-1.43, p=0.027), and 1.18 (95% CI: 1.04-1.34, p=0.009) respectively. Specifically, continuous uses of fluoxetine (AOR: 1.21; 95% CI: 1.01-1.46, p=0.042), fluvoxamine (AOR: 1.47; 95% CI: 1.01-2.12, p=0.043), venlafaxine (AOR: 1.44; 95% CI: 1.19-1.74, p<0.001) significantly increased the risk of cataract development. Moreover, continuous users of antidepressants with intermediate SERT binding affinities (AOR: 1.68; 95% CI: 1.10-2.56, p=0.017) were significantly associated with increased risks of cataract development.

LIMITATIONS

Several confounding factors such as obesity, multiple drug users, family history of cataracts, substance use, and environmental factors (such as sunlight or radiation exposure) were acquired.

CONCLUSIONS

We found increased risk of cataract development in patients continuously using antidepressants. Regular ocular evaluations in these patients are warranted.

Use of atypical antipsychotics and risks of cataract development in patients with schizophrenia: A population-based, nested case-control study

OBJECTIVE

Previous research has suggested a link between typical antipsychotic use and the development of cataracts, but the association between atypical antipsychotics and cataracts remains unclear in schizophrenia (SZ).

METHODS

A retrospective nested case-control study was conducted using data from the National Health Insurance Database of Taiwan between the year of 2000 and 2011. A total of 2144 SZ patients with cataracts and 2222 controls matched for age, sex, and index date were included. Antipsychotic exposure was categorized by type and duration, and the association between antipsychotic exposure and cataract development was assessed using a conditional logistic regression analysis.

RESULTS

We found that the severity of physical comorbidities, concurrent antidepressant use, and comorbidity with glaucoma or other retinal disorders were associated with an increased risk for cataract development. Alternatively, we did not find significant associations between continuous use of clozapine, risperidone, paliperidone, ziprasidone, olanzapine, quetiapine, amisulpride, zotepine or aripiprazole and risk of cataract development in SZ patients.

CONCLUSIONS

We did not detect any association between atypical antipsychotic use and risk of cataract development in SZ patients. Future studies with longitudinal ocular evaluations in patients using atypical antipsychotics are warranted to confirm our findings.

Selective serotonin reuptake inhibitor use and increased risk of cataract surgery: a population-based, case-control study

PURPOSE

To investigate whether selective serotonin reuptake inhibitor use is associated with an increased risk of cataract surgery.

DESIGN

Population-based case-control study.

METHODS

setting: Olmsted County, Minnesota. patient population: Eligible patients were county residents in the Rochester Epidemiology Project. Cases included 6024 county residents aged 50+ years who underwent first-eye cataract surgery between January 1, 2004 and December 31, 2011. Controls included 6024 residents who never had cataract surgery and were matched to cases by age, sex, and date of surgery. Logistic regression models were used to compute odds ratios for differences in selective serotonin reuptake inhibitor use between cases and controls, and to adjust for confounding variables. observation procedure: Rochester Epidemiology Project databases were used to assess cataract surgery and selective serotonin reuptake inhibitor treatment. main outcome measure: Selective serotonin reuptake inhibitor use.

RESULTS

In the cataract surgery cohort of 6024 residents, 1024 (17%) were selective serotonin reuptake inhibitor users compared to 788 (13%) in the matched cohort of 6024 residents never having cataract surgery (P < .001). Selective serotonin reuptake inhibitor use of 1 or more years was associated with an increased risk of cataract surgery (odds ratio [OR] = 1.36; 95% confidence interval [CI], 1.23-1.51; P < .001). The associations were similar in women (OR = 1.37; 95% CI, 1.22-1.55; P < .001) and men (OR = 1.34; 95% CI, 1.12-1.61; P = .002). The risk of cataract surgery was highest with citalopram use (OR = 1.53; 95% CI, 1.33-1.77; P < .001).

CONCLUSION

Selective serotonin reuptake inhibitor use of 1 or more years in people aged 50+ years is associated with an increased risk of cataract surgery.

The association between cataract surgery and atypical antipsychotic use: a nested case-control study

PURPOSE

To investigate cataract risk associated with the use of atypical antipsychotics.

DESIGN

Retrospective, nested case-control study.

METHODS

A large health claims database (The British Columbia Ministry of Health Databases) from British Columbia, Canada, was used from January 2000 through December 2007. Cases were defined as clinically significant cataracts requiring surgery and were identified using cataract surgery procedure codes. For each case, 4 to 10 controls were selected randomly using a density-based sampling approach and were matched to cases by age and calendar time. Rate ratios were calculated for users of atypical and typical antipsychotics adjusting for known cataractogenic factors.

RESULTS

One hundred sixty-two thousand five hundred one cases of cataract surgery and 650 004 controls were included. The adjusted rate ratio for current users of atypical antipsychotics was 0.84 (95% confidence interval, 0.80 to 0.89) compared with nonusers. A greater number of prescriptions filled in the year before cataract surgery compared with the median number of filled prescriptions was associated with a lower cataract surgery rate (adjusted rate ratio, 0.70; 95% confidence interval, 0.65 to 0.75) than those with fewer prescriptions filled (adjusted rate ratio, 0.85; 95% confidence interval, 0.79 to 0.91).

CONCLUSIONS

A protective association between the use of atypical antipsychotics and risk of clinically significant cataracts requiring surgery was established. Potential biochemical and neurochemical mechanisms for this protective effect are discussed.

Selective serotonin reuptake inhibitors and the risk of cataracts: a nested case-control study

OBJECTIVE

Older-generation antidepressants have been associated with increasing the risk of cataracts. Although animal studies have alluded to a potential link between selective serotonin reuptake inhibitors (SSRIs) and the development of cataracts, no large population based-study has addressed this potential association. This study sought to quantify the risk of cataracts with SSRIs by conducting a pharmacoepidemiologic study using the linked administrative databases in the province of Quebec, Canada.

DESIGN

Nested case-control study.

PARTICIPANTS

A cohort of subjects who had received a coronary revascularization procedure from 1995 through 2004 in the province of Quebec, Canada.

METHODS

Using an administrative data set, a case-control study was conducted within a cohort of Quebec residents who had received a coronary revascularization procedure from 1995 through 2004. Cases were defined as those with the first diagnosis of a cataract diagnosed by an ophthalmologist. For each case, 10 controls were selected and matched to the cases by index date, age, and cohort entry. Crude and adjusted rate ratios (RRs) and corresponding confidence intervals (CIs) were computed for current use of SSRIs. Rate ratios were adjusted for gender, corticosteroid use, statins, high blood pressure, antihypertensives, and antidiabetics.

MAIN OUTCOME MEASURES

First International Classification for Disease (Ninth Revision) code for a cataract diagnosed by an ophthalmologist.

RESULTS

Eighteen thousand seven hundred eighty-four cases and 187 840 controls met our study inclusion criteria. The adjusted RR for cataracts among current users of SSRIs was 1.15 (95% CI, 1.08-1.23). The risk of cataracts was highest with fluvoxamine (RR, 1.39; 95% CI, 1.07-1.80), followed by venlafaxine (RR, 1.33; 95% CI, 1.14-1.55) and paroxetine for cataract surgery (RR, 1.23; 95% CI, 1.05-1.45). The average time to diagnosis of cataracts while on SSRI therapy was 656 days.

CONCLUSIONS

A possible association was found between current exposure to SSRIs, especially fluvoxamine and venlafaxine, and a future diagnosis of cataracts. The possibility that this observation may be the result of the effect of smoking, which could not be controlled for in the study, cannot be excluded. Future studies are needed to confirm this association in other populations.

Selective serotonin reuptake inhibitors: a review of its effects on intraocular pressure

The increase in serotonin (5-HT) neurotransmission is considered to be one of the most efficacious medical approach to depression and its related disorders. The selective serotonin reuptake inhibitors (SSRIs) represent the most widely antidepressive drugs utilized in the medical treatment of depressed patients. Currently available SSRIs include fluoxetine, sertraline, paroxetine, fluvoxamine, citalopram and escitalopram. The primary SSRIs pharmacological action's mechanism consists in the presynaptic inhibition on the serotonin reuptake, with an increased availability of this amine into the synaptic cleft. Serotonin produces its effects as a consequence of interactions with appropriate receptors. Seven distinct families of 5-HT receptors have been identified (5-HT(1) to 5-HT(7)), and subpopulations have been described for several of these. The interaction between serotonin and post-synaptic receptors mediates a wide range of functions. The SSRIs have a very favorable safety profile, although clinical signs of several unexpected pathologic events are often misdiagnosed, in particular, those regarding the eye. In all cases reported in the literature the angle-closure glaucoma represents the most important SSRIs-related ocular adverse event. Thus, it is not quite hazardous to hypothesize that also the other reported and unspecified visual disturbances could be attributed - at least in some cases - to IOP modifications. The knowledge of SSRIs individual tolerability, angle-closure predisposition and critical IOP could be important goals able to avoid further and more dangerous ocular side effects.

The mechanisms of calcium homeostasis and signalling in the lens

Excessive Ca(2+) can be detrimental to cells and raised levels of Ca(2+) in human lenses with cortical cataract have been found to play a major role in the opacification process. Ca(2+) homeostasis is therefore, recognised as having fundamental importance in lens pathophysiology. Furthermore, Ca(2+) plays a central role as a second messenger in cell signalling and mechanisms have evolved which give cells exquisite control over intracellular Ca(2+) ([Ca(2+)](i)) via an array of specialised regulatory and signalling proteins. In this review we discuss these mechanisms as they apply to the lens. Ca(2+) levels in human aqueous humour are approximately 1 mM and there is a large, 10,000 fold, inwardly directed gradient across the plasma membrane. In the face of such a large gradient highly efficient mechanisms are needed to maintain low [Ca(2+)](i). The Na(+)/Ca(2+) exchanger (NCX) and plasma membrane Ca(2+)-ATPase (PMCA) actively remove Ca(2+) from the cells, whereas the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) sequesters Ca(2+) in the endoplasmic reticulum (ER) Ca(2+) store. In lens epithelial cells the dominant role is played by the ATPases, whilst in the fibre cells NCX activity appears to be more important. Usually, [Ca(2+)](i) can be increased in a number of ways. Ca(2+) influx through the plasma membrane, for example, is mediated by an array of channels with evidence in the lens for the presence of voltage-operated Ca(2+) channels (VOCCs), receptor-operated Ca(2+) channels (ROCCs) and channels mediating store-operated Ca(2+) entry (SOCE). Ca(2+) signalling is initiated via activation of G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTK) of which the lens expresses a surprisingly diverse array responding to various neurotransmitters, hormones, growth factors, autocoids and proteases. Downstream of plasma membrane receptors are IP(3)-gated channels (IP(3)Rs) and ryanodine receptors (RYRs) located in the ER, which when activated cause a rapid increase in [Ca(2+)](i) and these have also been identified in the lens. Through an appreciation of the diversity and complexity of the mechanisms involved in Ca(2+) homeostasis in normal lens cells we move closer to an understanding of the mechanisms which mediate pathological Ca(2+) overload as occurs in the process of cataract formation.

Nucleotides in ocular secretions: their role in ocular physiology

The eye is the sense organ that permits the detection of light owing to the existence of a sophisticated neuronal array, called the retina, which is responsive to photons. The correct functioning of this complex system requires the coordination of several intraocular structures that ultimately permit the perfect focusing of images on the neural retina. Light has to pass through different media: the tear, the cornea, aqueous humour, lens, and vitreous humour before it reaches the retina. Moreover, the composition and structure of some of these media can change due to several physiological mechanisms. Nucleotides are active components of the humours bathing relevant ocular structures. The tear contains nucleotides and dinucleotides that control the process of tearing, wound healing and protects of superficial infections. In the inner eye, the aqueous humour also presents a collection of mono and dinucleotides that affect pupil contraction, aqueous humour production and accommodation. Behind the lens and between this structure and the retina the vitreous humour can modify the physiology of the retinal cells, mostly the ganglion cells. By investigating the actions of nucleotides and dinucleotide present in the ocular humours we will be able not only to understand the functioning of the ocular structures but also to develop new pharmacological therapies for pathologies such as dry eye, glaucoma or retinal detachment.

Sphingolipids in human lens membranes: an update on their composition and possible biological implications

The unique nature of the most abundant phospholipids in human lens membranes remained overlooked until the 1990s when it was possible to discern dihydrosphingomyelins (DHSMs) from the more common sphingomyelins (SMs). Unlike in other mammalian membranes, DHSMs comprise nearly half of the phospholipids in adult human lenses. Compared to SMs with a trans double bond between carbons 4 and 5 of the sphingoid backbone, the absence of this unsaturation site in DHSMs allows the participation of the OH group on C3 in intermolecular H-bonds and leads to stronger interlipid interactions with both neighboring DHSMs and cholesterol. Phospholipid compositional changes with age and lens region observed in mammals with various life spans and lens growth rates, suggest that the highest levels of DHSMs along with the lowest amounts of phosphatidylcholines and SMs are found in lenses with the lowest growth rate, namely human lenses. The participation of phospholipid metabolites in the control of mitosis and elongation of lens cells is plausible and deserves investigation.

Diabetes can alter the signal transduction pathways in the lens of rats

Diabetes is known to affect cataract formation by means of osmotic stress induced by activated aldose reductase in the sorbitol pathway. In addition, alterations in the bioavailability of numerous extralenticular growth factors has been reported and shown to result in various consequences. We have found that the basic fibroblast growth factor (bFGF) accumulates in the vitreous humor of 3- and 8-week diabetic rats. Consequently, the associating signal transduction cascades were severely disrupted, including upregulated phosphorylation of extracellular signal-regulated kinase (ERK) and the common stress-associated mitogen-activated protein kinases p38 and SAPK/JNK. Conversely, under diabetic condition, we observed a dramatic inhibition of phosphatidylinositol-3 kinase activity in lenses obtained from the same animal. Rats treated with the aldose reductase inhibitor AL01576 for the duration of the diabetic condition showed that the diabetes-induced lenticular signaling alterations were normalized, comparable to controls. However, treatment of AL01576 in vitro was ineffective at normalizing the altered constituents in extracted diabetic vitreous after the onset of diabetes. The effect of AL01576 in the high galactose-induced cataract model in vitro was also examined. Administration of AL01576 to lens organ culture normalized the aberrant signaling effects and morphological characteristics associated with in vitro sugar cataract formation. In conclusion, our findings demonstrate diabetes-associated alterations in the lens signal transduction parameters and the effectiveness of AL01576 at normalizing such alterations. The causes for these alterations can be attributed to elevated vitreal bFGF in conjunction with osmotic stress and associated attenuation in redox status of the lens.

Ca(2+) regulation in differentiating lens cells in culture

In the ocular lens, cataract formation is associated with an elevated intracellular Ca(2+) concentration (Ca(2+)(i)) resulting from the loss of lens cell Ca(2+) regulation. The mechanisms regulating Ca(2+)(i) have been characterized previously in lens epithelial cells, but have not been well characterized in the more differentiated lens fiber cells. The mechanisms regulating Ca(2+)(i) in clusters of fiber-like cells (lentoids) in a sheep lens primary cell culture system in which the epithelial cells differentiate into enlarged fiber-like cells were investigated. Only approximately 50% of the lentoids responded to thapsigargin and/or agonists (ATP and epinephrine), compared to>95% of the epithelial cells. Remarkably, most (90%) lentoids exhibited a resting cytosolic Ca(2+)(i) that was approximately three-fold greater than that in epithelial cells (approximately 100n M). This elevated resting cytosolic Ca(2+)(i) was not affected by thapsigargin treatment, but decreased upon removal of extracellular Ca(2+) or addition of the Ca(2+) channel blocker Gd(3+) (5mM ). These results suggest that a plasma membrane Ca(2+) channel is more active in lentoids than in epithelial cells. Indeed, when plasma membrane cation channel activity was monitored by Mn(2+) influx and quenching of fura-2 fluorescence, quenching was faster in lentoids than epithelial cells. Following thapsigargin treatment, capacitative Ca(2+) entry was activated in epithelial cells but not lentoids. In conclusion, during differentiation in primary cell culture, lens cells lose their ability to respond to agonists and exhibit an elevated resting Ca(2+)(i) that was dependent on the activation of a Ca(2+) influx pathway. The results of this study support the possibility that a sustained elevation in resting Ca(2+)(i) is one of the factors controlling lens cell differentiation, possibly by triggering events such as calpain activation.

Studies of the mitogen-activated protein kinases and phosphatidylinositol-3 kinase in the lens. 1. The mitogenic and stress responses

The current understanding of the cellular signal transduction system is that cells initially respond to outside stimuli, such as growth factors or neurotransmitters, through ligand binding to the respective growth factor receptors or the G-protein-coupled receptors, to initiate transduction of the stimulus. This is followed by a series of association-dissociation and phosphorylation-dephosphorylation processes among the components of a well-defined and intricate infrastructure between the cell membrane and the cytosolic protein kinases to activate and initiate nuclear target genes for cell proliferation, differentiation and other cellular functions. Although some past reports have indicated this signaling machinery is present in the lens, certain pathways, namely the mitogen-response pathway (Raf-MEK-ERK cascade), the stress-response pathways (p38 and SAPK/JNK cascades) and the survival pathway (PI-3K-Akt), have not been thoroughly explored in an intact lens. These pathways were studied using porcine lenses cultured under mitogenic (10 ngml(-1) growth factor) or osmotic stress (30 mM galactose) conditions to examine the cellular response in the epithelial layer, using unstimulated lenses as controls. It was found that all the key members in the Raf-MEK-ERK cascade and PI-3K-Akt cascade were present and that growth factors had a differential stimulatory effect on them. Basic-FGF was the most potent stimulator for ERK followed by EGF and IGF-1, while PDGFab and VEGF were less active. The opposite was true for their stimulatory effect on PI-3K. Hyperglycemic-induced osmotic stress stimulated p38 but not SAPK/JNK, while bFGF could stimulate SAPK/JNK but not p38. Both stimuli activated the Raf-MEK-ERK and PI-3K-Akt pathways. Osmotic-induced activation could be normalized using an aldose reductase inhibitor.

Calcium cell signalling and cataract: role of the endoplasmic reticulum

The lens possesses an impressive array of G-protein receptors that are coupled to the release of intracellular calcium. They include members of the muscarinic, adrenergic and purinergic families and activation of the former has been implicated in cataract for some time. There are several possible mechanisms whereby activation of such receptors could give rise to cataract. A prolonged increase in intracellular calcium would be expected to activate proteases such as calpain and so could induce unscheduled and irreversible breakdown of important structural proteins. It has recently been shown that activation of G-protein receptors also modulates lens cell growth, and any interference with the highly controlled pattern of cell growth and development within the lens is also likely to have catastrophic consequences. If the calcium store is totally inactivated in lens cells, for example by exposure to thapsigargin, then growth ceases. This finding provides a means of inhibiting the lens cell growth which leads to posterior capsular opacification (PCO). For example, it has been shown that thapsigargin-coated intraocular lenses totally inhibit lens cell growth within cultured capsular bags, and if this technology could be transferred to the clinic then it could provide a simple and relatively inexpensive means of preventing PCO.

Localization of 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in rabbit ocular and brain tissues

Serotonin is thought to play a physiological role in various tissues of the rabbit eye, yet little is known about the relative distribution of the different serotonin receptors. Demonstration of the receptor subtypes present in the various ocular tissues is essential in order to understand the function of serotonin in the eye. Using a combination of in situ hybridization histochemistry, in vitro receptor autoradiography and polymerase chain reaction studies, we have explored the distribution of the 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in the rabbit eye. As these receptors have not been sequenced in the rabbit, we initially established the suitability of the oligonucleotide probes by analysis of brain tissue. The distributions of 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptor messenger RNAs in rabbit brain correlated well with those in other species, confirming the specificity of the probes for detection of the messenger RNAs in rabbit tissues. In the eye, the expression of 5-hydroxytryptamine1A receptors appears to be restricted to the epithelial cell layer of the ciliary processes, although very low levels may appear in the retina. In contrast, the expression of 5-hydroxytryptamine7 receptor messenger RNA is more widespread with positive signals evident in the ciliary processes, retina and iris. The results confirm the existence of 5-hydroxytryptamine1A receptors in the ciliary body and their localization in the ciliary epithelium supports the hypothesis that they are involved in the secretion of aqueous humour. Unexpectedly, there was little evidence to support the idea that 5-hydroxytryptamine1A receptors are present in the retina and iris sphincter. However, the subsequent finding of 5-hydroxytryptamine7 receptor messenger RNA in the retina and iris may explain the apparent absence of 5-hydroxytryptamine1A receptors in these tissues. The presence of both 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in the ciliary processes may account for the complex intraocular pressure response of the rabbit to serotonin.

Detection of different metabolites in the rabbit lens by high resolution 1H NMR spectroscopy

PURPOSE

To investigate the metabolic profile of the rabbit lens using high resolution 1H NMR spectroscopy including two-dimensional shift correlated (COSY) technique.

METHODS

Perchloric acid extracts of the rabbit lens were analysed with a Bruker AM-500 spectrometer and the metabolites were assigned in the spectra. Some of these were also quantified.

RESULTS

More than 20 metabolites were detected in the perchloric acid extract of a single lens, including amino acids, nucleotides and other related compounds. Of particular importance is the ability to detect and identify glutathione, myoinositol, scyllo-inositol and taurine.

CONCLUSIONS

The study demonstrated the potential of 1H NMR spectroscopy for monitoring the metabolic profile of the lens in normal and pathologic conditions.

Mechanical stimulation initiates cell-to-cell calcium signaling in ovine lens epithelial cells

Although abnormalities in calcium regulation have been implicated in the development of most forms of cataract, the mechanisms by which Ca2+ is regulated in the cells of the ocular lens remain poorly defined. Cell-to-cell Ca2+ signaling was investigated in primary cultures of ovine epithelial cells using the Ca(2+)-reporter dye fura-2 and fluorescence microscopy. Mechanical stimulation of a single cell with a micropipette initiated a propagated increase in cytosolic free Ca2+ that spread from the stimulated cell through 2–8 tiers of surrounding cells. During this intercellular Ca2+ wave, cytosolic Ca2+ increased 2- to 12-fold from resting levels of approximately 100 nM. Nanomolar extracellular Ca2+ did not affect the cell-to-cell propagation of the Ca2+ wave, but reduced the magnitude of the cytosolic Ca2+ increases, which was most evident in the mechanically-stimulated cell. Depletion of intracellular Ca2+ stores with thapsigargin eliminated the propagated intercellular Ca2+ wave, but did not prevent the cytosolic Ca2+ increase in the mechanically-stimulated cell, which required extracellular Ca2+ and was attenuated by the addition of the Ca2+ channel blockers Ni2+, Gd3+ and La3+ to the medium. These results are most easily explained by a mechanically-activated channel in the plasma membrane of the stimulated cell. The propagated increase in cytosolic Ca2+ appeared to be communicated to adjacent cells by the passage of an intracellular messenger other than Ca2+ through gap junction channels. However, if the plasma membrane of the mechanically-stimulated cell was ruptured such that there was loss of cytosolic contents, the increase in cytosolic Ca2+ in the surrounding cells was elicited by both a messenger passing through gap junction channels and by a cytosolic factor(s) diffusing through the extracellular medium. These results demonstrate the existence of intercellular Ca2+ signaling in lens cells, which may play a role in regulating cytosolic Ca2+ in the intact lens.

Histamine and ATP mobilize calcium by activation of H1 and P2u receptors in human lens epithelial cells

1. Cytoplasmic calcium concentration ([Ca2+]1) of single superfused tissue-cultured human lens epithelial cells (HLEC) was monitored using the fluorescent dye fura-2; the resting values were low and stable for several hours ([Ca2+]i = 96 +/- 20 nM; mean +/- S.D., n = 16). 2. Continuous superfusion with either ATP or histamine (0.1-10 microM) produced regular oscillations in [Ca2+]i that could be maintained for a short time in the absence of external calcium. 3. Short (30 s) pulses of histamine (0.1-100 microM) induced a transient rise in [Ca2+]i, the time course of which was insensitive to the removal of external calcium. The rate of rise and the amplitude of the response were very sensitive to agonist concentration, whereas the rate of recovery was relatively constant. 4. The responses to long pulses of histamine (> 100 s) consisted of an initial transient followed by a maintained [Ca2+]i which returned to baseline on removal of external calcium. 5. The kinetics of the responses to short and long pulses of ATP (0.1-100 microM) were very similar to those of histamine and showed a similar sensitivity to the presence or absence of external calcium. 6. The histamine responses were abolished by triprolidine (1 microM), but unaffected by ranitidine (1 microM), indicating that an Hi receptor subtype is activated by histamine. 7. The ATP responses were reversibly inhibited by suramin and the potency sequence for a range of agonists was ATP = UTP = ATP gamma S > ADP = GTP >> AMP = adenosine, indicating that activation of a P2u receptor subtype was responsible for the increase in [Ca2+]i. 8. Both histamine and ATP responses were abolished by thapsigargin (100 nM), confirming that calcium release from intracellular stores was responsible for the initial peak of the response. Application of either agonist during the plateau phase of the thapsigargin response often led to a marked, but reversible, decline in [Ca2+]i, indicating the presence of a further, normally hidden, calcium regulatory factor associated with the presence of the agonist. 9. Maximal concentrations of either histamine or ATP totally emptied the calcium store as a subsequent application of the other agonist (or thapsigargin), in the absence of external calcium, failed to induce a further increase in the calcium signal.

Kinetics of myo-inositol transport in rat ocular lens

Myo-inositol (MI) influx as a function of concentration in rat lens consisted of a saturable component, fit by a rectangular hyperbola, and a linear component which was more distinct at high myo-inositol concentrations suggesting passive diffusion. The hyperbolic component was half-maximally saturated (Kt) at 61.3 microM and had a maximal transport rate (Jmax) of 44.6 mumol/kg wet wt/h. The linear component had an apparent permeability coefficient of 1.44 x 10(-6) s-1. Sorbitol, which distributed rapidly in the extracellular space (6.83 ml/100 g wet wt), also appeared to enter the intracellular space with a permeability coefficient of 1.37 x 10(-6) s-1, similar to that of myo-inositol. The influx of myo-inositol was critically dependent on the concentration of extracellular sodium consistent with a sodium-myo-inositol cotransport. The kinetics of influx activation by sodium suggested an apparent 2:1 coupling ratio for sodium and myo-inositol. When potassium was used as sodium substitute, a significantly stronger influx inhibition was observed than with nondepolarizing sodium substitutes, indicating that myo-inositol was driven by the electrochemical gradient of sodium rather than the chemical gradient only. Reducing the extracellular Na concentration increased the MI concentration at which transport was half-maximally activated, suggesting an ordered binding sequence of Na followed by MI. Myo-inositol influx was competitively inhibited by phlorizin with an inhibitory coefficient (Ki) of 35 microM. Phloretin also was capable of inhibition but with a much lesser efficacy. Myo-inositol desaturates from the lens at a rate of 0.00862 h-1. Approximately 19% of the efflux can be inhibited with phlorizin, suggesting that it represents carrier-mediated flux. The phlorizin insensitive flux has a rate of 0.00695 h-1 or 1.93 x 10(-6) s-1, similar to the Na-independent passive influx. MI influx is due to a Na-dependent, phlorizin-sensitive active transport while the efflux consists largely of a phlorizin-independent passive leakage.

Immunocytochemical localization of prostaglandin E2 receptor subtypes in porcine ocular tissues. II. Nonuveal tissues

The localization of PGE2 receptor subtypes (EP1, EP2 and EP3) in nonuveal tissues of porcine eye was investigated by immunocytochemical methods with polyclonal antibodies directed against conserved peptide sequences in the carboxyl terminal region of the receptor subtypes. EP1 was found only in corneal epithelium, while EP2 immunoreactivity was associated with corneal epithelium and endothelium and lens epithelium. EP3 receptor subtype was localized to all ocular tissues examined (except lens fiber cells), i.e. corneal epithelium and endothelium, lens epithelium, retinal photoreceptor and pigmented epithelium, Bipolar cell, horizontal cell, amacrine cell, ganglion cell and Muller cell.

Myo-inositol transport in the lens of galactose-maintained rats

Lens myo-inositol (MI) content is regulated by a pump-leak system consisting of an active Na-dependent MI transport and its passive permeability through the membrane. We measured the active MI uptake and membrane permeability in lenses of rats maintained on a 50% galactose diet for 1, 3 and 7 days. After only 1 day of galactose feeding, active MI uptake in the lens was reduced dramatically by 74% compared to age-matched control lenses; by day 3, active MI transport was decreased by 89% and it was undetectable by day 7. The passive membrane permeability was determined by measuring (a) the passive MI influx and (b) the 3H-sorbitol flux. After 1 day of galactose feeding, the membrane permeability increased such that within 3 days it increased to 5-6 fold. Galactose feeding also led to a rapid increase in lens polyol content. After 1 day, lens polyol increased to 53 mumol/g wet wt compared to a control value of 0.35 mumol/g wet wt and increased further to 65 and 72 mumol/g wet wt after 3 and 7 days of galactose feeding respectively. Lens galactose accumulation was low (3 mumol/g wet wt) up to 7 days; however, it was rapidly increased after 7 days. Our results indicate that galactose feeding rapidly interfered with MI homeostasis by a severe depression of active MI transport and a rapid increase in membrane permeability. These interferences of MI homeostasis correlate with the appearance of high polyol levels.

Sorbinil prevents the hypergalactosemic-induced reduction in [3H]-myo-inositol uptake and decreased [3H]-myo-inositol incorporation into the phosphoinositide cycle in bovine lens epithelial cells in vitro

The synthesis of phosphatidylinositol, phosphatidylinositol-4-phosphate and phosphatidylinositol-4-5-bisphosphate was studied using 3H-myo-inositol (3H-MI) as precursor in cultured bovine lens epithelial cells (BLECs) maintained in galactose-free, physiological medium or 40 mM galactose (Gal) +/- sorbinil for six days. The formation of inositol polyphosphates from phosphoinositides was also shown. Galactitol did not exceed 2mM in Gal-incubated cells after six days of exposure; no galactitol was observed in BLECs maintained in galactose-free, physiological medium or Gal supplemented with sorbinil. Uptake of 3H-myo-inositol(3H-MI) into BLECs was significantly reduced in cells exposed to Gal. A concomitant reduction in 3H-MI incorporation was observed in the phosphoinositides, as well as with the released inositol phosphates. The simultaneous addition of sorbinil to the Gal medium corrected the drop in 3H-MI uptake and normalized 3H-MI incorporation into the phosphoinositides and inositol phosphates. While an apparent decrease in the three inositol-containing lipids was observed with the Gal-incubated cells, based on 3H-MI incorporation, there was no change in total membrane phosphatidylinositol content when compared to cells maintained in physiological medium as determined by the microgram Pl PO4 per microgram total membrane PO4. The apparent loss of radiolabeled phosphoinositides was attributed to the decreased specific activity resulting from the lower internal pool of 3H-MI in the Gal-exposed cells available for incorporation into the phosphoinositides.(ABSTRACT TRUNCATED AT 250 WORDS)

Micro-quantitation of lens myo-inositol by anion exchange chromatography

A method capable of the determination of pmole quantities of myo-inositol in mgr amounts of tissue by anion exchange chromatography is detailed for use in lens and potentially other tissues. Samples were rendered protein-free through ZnSO4/Ba(OH)2 precipitation, lyophilized and reconstituted in water just prior to analysis. An aliquot of sample was injected onto an anion exchange column and eluted with a 0.045 M sodium hydroxide mobile phase. Each analysis requires 30 minutes to complete. Average recovery of myoinositol added to lens sample prior to injection was 100.6%. The coefficient of variation for repeated sample injections was 2.9%. Rabbit lens averaged 11.4 mumol/gr wet weight with epithelium containing 8.5 mumol/gr wet weight while human lens contained 20.1 mumol/gr wet weight and human epithelial cells had 17.5 nmol/mgr protein.