Cell transplantation approaches to retinal ganglion cell neuroprotection in glaucoma

Glaucoma is a complex neurodegenerative disease that involves interactions among multiple signaling pathways, ultimately leading to progressive retinal ganglion cell (RGC) death. The development of neuroprotective approaches to glaucoma therapy could preserve vision by modulating these pathologic pathways or by acting directly on RGCs to attenuate cell death and maintain function. Intraocular cell transplantation is being evaluated as one approach to achieve sustained RGC neuroprotection. Unlike traditional pharmacological approaches, transplanted cells might be capable of simultaneously targeting multiple pro-survival pathways via local delivery of secreted factors and/or via modulation of the intraocular microenvironment. Elucidating the mechanisms by which different cell types attenuate RGC death in models of glaucoma may uncover additional novel mechanisms of neuroprotection. In this review, we will discuss the rationale for transplantation-based approaches to neuroprotection for glaucoma and explore the various mechanisms of action proposed to account for RGC neuroprotection achieved by two distinct cell classes that have been studied most extensively for this purpose: glial cells and mesenchymal stem cells.

Abstract 102: FOCUS: Appropriate Use of Cardiac Imaging

Introduction The American College of Cardiology has created a new initiative (FOCUS) to help reduce inappropriate use of clinical imaging in cardiology. In accord with the effort, the Sanger Heart and Vascular Institute (SHVI) has designed a program to enhance appropriate use of cardiac imaging.

Hypothesis: By implementing a quality improvement program to evaluate the appropriateness of cardiac imaging, there will be a 10% improvement in the percentage of studies ordered for appropriate indications and a corresponding decrease in the number of studies ordered for inappropriate indications.

Methods: The initiative utilized an integrated multifaceted approach to quality improvement: 1) Staff / physician education with video teleconferences: Presentations were given at SHVI’s Grand Round, focusing on the appropriate use criteria as outlined in the ACCF/ASE/ASNC documents. An important component was interactive case studies highlighting the principles of appropriate use criteria. 2) Peer review and audits: Each practitioner and site received individual performance feedback. This was a complete analysis which included testing appropriateness as an indication. 3) System support tools: A comprehensive order sheet with check boxes for all appropriate indications was designed. All ordering physicians were asked to use this form for ordering all cardiac imaging studies. For those studies in which the form was not utilized, the imaging technologist attempted to obtain the appropriate information. All information from the forms was entered into a database designed specifically for this study. A baseline survey of 30 consecutive studies was selected for analysis from each echocardiography and cardiac nuclear imaging site within SHVI for a total of 360 studies. The ongoing post intervention survey conducts a monthly review of 25% of all studies per site.

Results: At baseline, 77% of transthoracic echocardiographic studies were ordered for appropriate indications. Post intervention there was significant improvement: 91% (p<0.0013) of studies were categorized as appropriate and 1% of studies categorized as inappropriate (14% improvement in each). Of the remaining studies, 9% could not be categorized. With regards to stress cardiac imaging, 71% of studies were ordered for appropriate indications at baseline. Post intervention, there was significant improvement: 94% (p<0.0008) of studies categorized as appropriate (23% improvement), 16% categorized as inappropriate, and 6% could not be categorized.

Conclusion: Although there is limited data on a national basis, published literature would suggest 70-80% appropriate use of cardiac imaging as a baseline benchmark. The results of this initiative demonstrated a statistically significant benefit in the appropriate use of cardiac imaging for SHVI from this baseline. The data gathering and database capabilities established during this initiative will allow quarterly reviews to providers, which will help ensure continued compliance with the guidelines.

Use of an adult rat retinal explant model for screening of potential retinal ganglion cell neuroprotective therapies

PURPOSE. To validate an established adult organotypic retinal explant culture system for use as an efficient medium-throughput screening tool to investigate novel retinal ganglion cell (RGC) neuroprotective therapies. METHODS. Optimal culture conditions for detecting RGC neuroprotection in rat retinal explants were identified. Retinal explants were treated with various recognized, or purported, neuroprotective agents and cultured for either 4 or 7 days ex vivo. The number of cells surviving in the RGC layer (RGCL) was quantified using histologic and immunohistochemical techniques, and statistical analyses were applied to detect neuroprotective effects. RESULTS. The ability to replicate previously reported in vivo RGC neuroprotection in retinal explants was verified by demonstrating that caspase inhibition, brain-derived neurotrophic factor treatment, and stem cell transplantation all reduced RGCL cell loss in this model. Further screening of potential neuroprotective pharmacologic agents demonstrated that betaxolol, losartan, tafluprost, and simvastatin all alleviated RGCL cell loss in retinal explants, supporting previous reports. However, treatment with brimonidine did not protect RGCL neurons from death in retinal explant cultures. Explants cultured for 4 days ex vivo proved most sensitive for detecting neuroprotection. CONCLUSIONS. The current adult rat retinal explant culture model offers advantages over other models for screening potential neuroprotective drugs, including maintenance of neurons in situ, control of environmental conditions, and dissociation from other factors such as intraocular pressure. Verification that neuroprotection by previously identified RGC-protective therapies could be replicated in adult retinal explant cultures suggests that this model could be used for efficient medium-throughput screening of novel neuroprotective therapies for retinal neurodegenerative disease.

Thermal stability of bimatoprost, latanoprost, and travoprost under simulated daily use

PURPOSE

To determine the stability of bimatoprost, latanoprost, and travoprost under conditions of simulated daily use and varying degrees of thermal stress.

METHODS

Commercially available bimatoprost, latanoprost, and travoprost were obtained in their original bottles as distributed by the manufacturers. Bottles were stored in calibrated, nonhumidified, light-free incubators maintained at 27°C, 37°C, or 50°C for 3, 9, 15, or 30 days. Capped bottles were inverted and left uncapped for 1  min daily to simulate patient use; no drops were expelled. Bimatoprost concentration was analyzed using liquid chromatography with ultraviolet detection at 210  nm. Latanoprost and travoprost concentrations were analyzed by liquid chromatography/tandem mass spectrometry (MS/MS) using selected reaction monitoring.

RESULTS

Off-the-shelf control bottles of bimatoprost contained 102% of the labeled concentration. In all combinations of stress temperature and duration, mean bimatoprost concentration ranged from 100% to 116% of the labeled concentration with no measurable degradation. Off-the-shelf control bottles of latanoprost contained 115% of the labeled concentration. Mean latanoprost concentration ranged from 97% to 120% of the labeled concentration. Latanoprost was stable at 27°C. When stressed at 37°C or 50°C, latanoprost degraded at a rate of 0.15 or 0.29  μg/mL/day, respectively. Off-the-shelf control bottles of travoprost contained 120% of the labeled concentration. Mean travoprost concentration ranged from 83% to 142% of the labeled concentration. Travoprost was stable at 27°C and 37°C, although concentration measurements at 37°C exhibited high variability. When stressed at 50°C, travoprost degraded at a rate of 0.46  μg/mL/day.

CONCLUSIONS

Higher than expected concentrations for stressed drug samples are likely a result of evaporation. Under the conditions of thermal stress tested in this study, bimatoprost remained stable for all conditions tested. Latanoprost degradation was measurable only in samples stressed at 37°C and 50°C, whereas travoprost degradation was statistically significant only in samples stressed at 50°C.

Io: a surface evaporite deposit?

A model is suggested for Io's surface composition involving evaporite salt deposits, rich in sodium and sulfur. According to this model, these deposits were produced as a result of the migration of salt-saturated aqueous solutions to Io's surface from a warm or hot interior followed by loss of the water to space. This model satisfies cosmochemical constraints based on Io's initial composition, current density, and thermal history. Salt-rich assemblages are easily derivable from the leaching of carbonaceous chondritic material; the chemical and optical properties of such deposits, after modification by irradiation, can be used to explain Io's overall albedo and spectral reflectance, its dark reddish poles, and the observed sodium emission as well as or better than other currently suggested materials.

Asteroid vesta: spectral reflectivity and compositional implications

The spectral reflectivity (0.30 to 1.10 microns) of several asteroids has been measured for the first time. The reflection spectrum for Vesta contains a strong absorption band centered near 0.9 micron and a weaker absorption feature between 0.5 and 0.6 micron. The reflectivity decreases strongly in the ultraviolet. The reflection spectrum for the asteroid Pallas and probably for Ceres does not contain the 0.9-micron band. Vesta shows the strongest and best-defined absorption bands yet seen in the reflection spectrum for the solid surface of an object in the solar system. The strong 0.9-micron band arises from electronic absorptions in ferrous iron on the M2 site of a magnesian pyroxene. Comparison with laboratory measurements on meteorites and Apollo 11 samples indicates that the surface of Vesta has a composition very similar to that of certain basaltic achondrites.

Voyager 2 in the uranian system: imaging science results

Voyager 2 images of the southern hemisphere of Uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere. The alignment of the cloud bands is similar to that of bands on Jupiter and Saturn, but the zonal winds are nearly opposite. At mid-latitudes (-70 degrees to -27 degrees ), where winds were measured, the atmosphere rotates faster than the magnetic field; however, the rotation rate of the atmosphere decreases toward the equator, so that the two probably corotate at about -20 degrees . Voyager images confirm the extremely low albedo of the ring particles. High phase angle images reveal on the order of 10(2) new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings, as well as a broad, diffuse, low optical depth ring just inside the main rings system. Nine of the newly discovered small satellites (40 to 165 kilometers in diameter) orbit between the rings and Miranda; the tenth is within the ring system. Two of these small objects may gravitationally confine the e ring. Oberon and Umbriel have heavily cratered surfaces resembling the ancient cratered highlands of Earth's moon, although Umbriel is almost completely covered with uniform dark material, which perhaps indicates some ongoing process. Titania and Ariel show crater populations different from those on Oberon and Umbriel; these were probably generated by collisions with debris confined to their orbits. Titania and Ariel also show many extensional fault systems; Ariel shows strong evidence for the presence of extrusive material. About halfof Miranda's surface is relatively bland, old, cratered terrain. The remainder comprises three large regions of younger terrain, each rectangular to ovoid in plan, that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials, perhaps suggesting some exotic composition.

Neurotrophic factor delivery as a protective treatment for glaucoma

Glaucoma is a progressive optic neuropathy and a major cause of visual impairment worldwide. Neuroprotective therapies for glaucoma aim to ameliorate retinal ganglion cell degeneration through direct or indirect action on these neurons. Neurotrophic factor (NTF) delivery is a key target for the development of potential neuroprotective glaucoma treatments. This article will critically summarize the evidence that NTF deprivation and/or dysfunction plays a role in the pathogenesis of glaucoma. Experimental support for the neuroprotective potential of NTF supplementation in animal models of glaucoma will be reviewed, in particular for brain-derived neurotrophic factor, ciliary neurotrophic factor, and glial cell line-derived neurotrophic factor. Finally, the challenges of clinical translation will be considered with an emphasis on the most promising NTF delivery strategies including slow-release drug delivery, gene therapy, and cell transplantation.

Postoperative better than preoperative C-reactive protein at predicting outcome after potentially curative nephrectomy for renal cell carcinoma

OBJECTIVES

Preoperative C-reactive protein (CRP) predicts metastasis and mortality in localized renal cell carcinoma (RCC). However, the predictive potential of after resection of localized RCC remains unclear. Therefore, we assessed the absolute ability of postoperative CRP to predict metastases and mortality as a continuous variable.

METHODS

Patients with clinically localized (T1-T3N0M0) clear-cell RCC were followed for 1 year postoperatively. Metastases were identified radiologically and mortality by death certificate. Univariate and multivariate binary logistic regression analyses examined 1 year relapse-free survival (RFS) and overall survival (OS) across patient and disease characteristics.

RESULTS

Of the 110 patients in this study, 16.4% developed metastases and 6.4% died. Mean (SD) postoperative CRP for patients who did and did not develop metastases were 69.06 (73.55) mg/L and 5.27 (7.80), respectively. Mean (SD) postoperative CRP for patients who did and did not die were 89.31 (69.51) mg/L and 10.88 (30.32), respectively. In multivariate analysis, T-stage (OR: 12.452, 95% CI: 2.889-53.660) and postoperative CRP ((B: .080, SE: .025; P < .001) were significant predictors of RFS. T-Stage (OR: 11.715; 95% CI: 1.102-124.519) and postoperative CRP (B: .017; SE: .007; P < .001) were also significant predictors of OS. After adjusting for postoperative CRP, preoperative CRP was not predictive of these outcomes.

CONCLUSIONS

Postoperative, not preoperative, CRP is the better predictor of metastasis and mortality following nephrectomy for localized RCC. Clinicians should consider absolute postoperative CRP to identify high-risk patients for closer surveillance or additional therapy. Predictive algorithms should consider incorporating postoperative CRP as a continuous variable to maximize predictive ability.

Neuroprotective effects of intravitreal mesenchymal stem cell transplantation in experimental glaucoma

Purpose. Retrograde neurotrophic factor transport blockade has been implicated in the pathophysiology of glaucoma. Stem cell transplantation appears to ameliorate some neurodegenerative conditions in the brain and spinal cord, in part by neurotrophic factor secretion. The present study was conducted to determine whether local or systemic bone marrow-derived mesenchymal stem cell (MSC) transplantation can confer neuroprotection in a rat model of laser-induced ocular hypertensive glaucoma. Methods. MSCs were isolated from the bone marrow of adult wild-type and transgenic rats that ubiquitously express green fluorescent protein. MSCs were transplanted intravitreally 1 week before, or intravenously on the day of, ocular hypertension induction by laser photocoagulation of the trabecular meshwork. Ocular MSC localization and integration were determined by immunohistochemistry. Optic nerve damage was quantified by counting axons within optic nerve cross-sections 4 weeks after laser treatment. Results. After intravitreal transplantation, MSCs survived for at least 5 weeks. Cells were found mainly in the vitreous cavity, though a small proportion of discrete cells migrated into the host retina. Intravitreal MSC transplantation resulted in a statistically significant increase in overall RGC axon survival and a significant decrease in the rate of RGC axon loss normalized to cumulative intraocular pressure exposure. After intravenous transplantation, MSCs did not migrate to the injured eye. Intravenous transplantation had no effect on optic nerve damage. Conclusions. Local, but not systemic, transplantation of MSCs was neuroprotective in a rat glaucoma model. Autologous intravitreal transplantation of MSCs should be investigated further as a potential neuroprotective therapy for glaucoma.

Identification of barriers to retinal engraftment of transplanted stem cells

PURPOSE

Intraocular stem cell transplantation may be therapeutic for retinal neurodegenerative diseases such as glaucoma via neuronal replacement and/or neuroprotection. However, efficacy is hindered by extremely poor retinal graft integration. The purpose was to identify the major barrier to retinal integration of intravitreally transplanted stem cells, which was hypothesized to include the cellular and/or extracellular matrix (ECM) components of the inner limiting membrane (ILM).

METHODS

Mesenchymal stem cells (MSCs) were cocultured on the vitreal surface of retinal explants. Retinal MSC migration was compared between control explants and explants in which portions of the ILM were removed by mechanical peeling; the inner basal lamina was digested with collagenase; and glial cell reactivity was selectively modulated with alpha-aminoadipic acid (AAA). In vivo, the MSCs were transplanted after intravitreal AAA or saline injection into glaucomatous rat eyes.

RESULTS

Retinal MSC migration correlated positively with the amount of peeled ILM, whereas enzymatic digestion of the basal lamina was robust but did not enhance MSC entry. In contrast, AAA treatment suppressed glial cell reactivity and facilitated a >50-fold increase in MSC migration into retinal explants. In vivo analysis showed that AAA treatment led to a more than fourfold increase in retinal engraftment.

CONCLUSIONS

The results demonstrated that the ECM of the inner basal lamina is neither necessary nor sufficient to prevent migration of transplanted cells into the neural retina. In contrast, glial reactivity was associated with poor graft migration. Targeted disruption of glial reactivity dramatically improved the structural integration of intravitreally transplanted cells.

Rodent models of glaucoma

Glaucoma is a progressive, age-related optic neuropathy and a leading cause of irreversible blindness in the world. Animal models of glaucoma are essential to our continued efforts of elucidating the natural course of the disease and to developing therapeutic interventions to halt or reverse the progression of the condition. Over the past 10-15 years, rodents have become a popular model organism to study glaucoma, because of their high degree of availability, relatively low cost, short life-span, and amenability to experimental and genetic manipulation. In this review, we examine the numerous in vivo and in vitro rodent models of glaucoma, discuss the methods used to generate them, summarize some of the major findings obtained in these models, and identify individual strengths and weaknesses for the various systems.

Stem cells for neuroprotection in glaucoma

Stem cell transplantation is currently being explored as a therapy for many neurodegenerative diseases including glaucoma. Cellular therapies have the potential to provide chronic neuroprotection after a single treatment, and early results have been encouraging in models of spinal cord injury and Parkinson's disease. Stem cells may prove ideal for use in such treatments as they can accumulate at sites of injury in the central nervous system (CNS) and may also offer the possibility of targeted treatment delivery. Numerous stem cell sources exist, with embryonic and fetal stem cells liable to be superseded by adult-derived cells as techniques to modify the potency and differentiation of somatic cells improve. Possible neuroprotective mechanisms offered by stem cell transplantation include the supply of neurotrophic factors and the modulation of matrix metalloproteinases and other components of the CNS environment to facilitate endogenous repair. Though formidable challenges remain, stem cell transplantation remains a promising therapeutic approach in glaucoma. In addition, such studies may also provide important insights relevant to other neurodegenerative diseases.

Rebound tonometry in conscious, conditioned mice avoids the acute and profound effects of anesthesia on intraocular pressure

AIMS

The aims of this study were to evaluate the accuracy, repeatability, and safety of multiple intraocular pressure (IOP) measurements by a commercially available rebound tonometer in conscious, conditioned mice, and to characterize the acute and profound effects of anesthesia on IOP in mice.

METHODS

To test the accuracy of the tonometer, IOPs of CD-1 mice under ketamine/xylazine anesthesia were experimentally set and monitored with a water manometer/transducer system following transcorneal cannulation while simultaneously performing tonometry. The long- and short-term repeatability of the tonometer was tested in conscious, restrained mice, as measurements were taken once-daily in the afternoon for 4 consecutive days. On day 5, IOPs were measured in the same mice once every 4 min for 32 min. On 2 separate days, mice were administered ketamine/xylazine or 2,2,2-tribromoethanol anesthesia, in a crossover design, and IOPs were measured once every 2 min for 32 min. Rebound tonometry was performed in conscious mice before and 1 hour after 1 drop of timolol maleate (10 microL of 0.5%) application to 1 eye.

RESULTS

IOP measurements by rebound tonometry correlated well with manometry for pressures between 8 and 38 mmHg (y = 0.98x - 0.32, R(2) = 0.94; P < 0.001). The average tonometric IOP was invariant over 4 days (range, 11.7-13.2 mmHg). IOPs dropped significantly ( P < or = 0.05) within 6 min (ketamine/xylazine) or 10 min (2,2,2-tribromoethanol) postadministration of anesthesia but not with conscious restraint. Timolol significantly (P < 0.001) lowered IOP from 12.8 +/- 0.3 (mean +/- standard error of the mean) to 10.1 +/- 0.6 mmHg, as measured by the tonometer.

CONCLUSIONS

Rebound tonometry can be used to obtain accurate IOP measurements in conscious, restrained mice while avoiding the rapid and profound ocular hypotensive effects of general anesthesia. Small changes in IOP with an aqueous-flow suppressant are readily detectable with conscious restraint that may be missed with chemical restraint.

Development and characterization of an adult retinal explant organotypic tissue culture system as an in vitro intraocular stem cell transplantation model

PURPOSE

To develop and characterize a retinal explant culture system to facilitate investigation of novel methods of improving retinal stem cell therapy.

METHODS

Retinas explanted from adult rats were cultured in serum-free medium (B27/N2) or medium containing normal horse serum (NHS). Tissue viability was assessed by gross morphology, propidium iodide (PI) uptake, cell survival quantification, activated caspase-3 expression, and immunohistochemistry. Müller progenitor cells (hMIO-M1), or mesenchymal stem cells (MSC) were placed on explants, to model intravitreal cell transplantation. Explants were compared with whole eyes, with or without experimental glaucoma and/or intravitreal cell transplantation.

RESULTS

Explants cultured in B27/N2 medium were viable for at least 17 days, as assessed by the aforementioned parameters. NHS medium was associated with obvious tissue degradation, greater/more diffuse PI uptake, significant cell loss over time, and temporal increase in activated caspase-3(+) cells. Explants in B27/N2 medium strongly expressed beta-III-tubulin, neurofilament, NeuN, Brn3a, Thy-1, GFAP, vimentin, nestin, and glutamine synthetase, whereas immunoreactivity was weak in NHS medium and decreased further with time. Seven and 14 days after coculture or transplantation, glial reactivity (GFAP/vimentin expression) was highly upregulated in explants and eyes, respectively. Some grafted cells migrated into the retina, but most remained outside the inner limiting membrane.

CONCLUSIONS

Retinal explants prepared using the described techniques and cultured in B27/N2 medium are viable for at least 2 weeks and mimic in vivo glial reactivity to transplantation while allowing few grafted cells to integrate. This system may be a useful in vitro model for investigating methods of enhancing retinal stem cell therapy.

T cell independent mechanism for copolymer-1-induced neuroprotection

Despite active investigation of copolymer-1 (Cop-1) for nearly 40 years the mechanisms underlying its neuroprotective properties remain contentious. Nonetheless, current dogma for Cop-1 neuroprotective activities in autoimmune and neurodegenerative diseases include bystander suppression of autoimmune T cells and attenuation of microglial responses. In this report, we demonstrate that Cop-1 interacts directly with primary human neurons and decreases neuronal cell death induced by staurosporine or oxidative stress. This neuroprotection is mediated through protein kinase Calpha and brain-derived neurotrophic factor. Dendritic cells (DC) uptake Cop-1, deliver it to the injury site, and release it in an active form. Interactions between Cop-1 and DC enhance DC blood brain barrier migration. In a rat model with optic nerve crush injury, Cop-1-primed DC induce T cell independent neuroprotection. These findings may facilitate the development of neuroprotective approaches using DC-mediated Cop-1 delivery to diseased nervous tissue.

Bacterial DNA confers neuroprotection after optic nerve injury by suppressing CD4+CD25+ regulatory T-cell activity

PURPOSE

Protective autoimmunity attenuates secondary degeneration after central nervous system (CNS) injury. Such neuroprotection is achieved via activation of autoimmune CD4(+)CD25(-) effector T cells (Teffs) or suppression of naturally occurring CD4(+)CD25(+) regulatory T cells (Tregs). In this study the ability of bacterial DNA, characterized by unmethylated CpG islands, to downregulate Treg activity and therefore, to confer neuroprotection was investigated.

METHODS

The effects of CpG on suppressive activity of mouse Tregs were studied by coculturing Tregs with Teffs and measuring proliferation by radiolabeled thymidine. The neuroprotective effects of CpG-mediated Treg suppression was examined in rats after optic nerve crush.

RESULTS

Teff proliferation in response to T-cell receptor stimuli was significantly reduced when the Teffs were cocultured with Tregs, compared with Teff activation when cultured alone. Treating Tregs with CpG reduced their suppressive activity and restored Teff proliferation to baseline levels. CpG injection in rats with optic nerve crush conferred significant neuroprotection compared with that in untreated control rats (118 +/- 8 cells/mm(2) vs. 69 +/- 5 cells/mm(2), respectively; mean +/- SEM; P < 0.05). CpG-mediated neuroprotection was accompanied by significantly increased T-cell infiltration at the injury site. Similar CpG treatment of athymic nude rats yielded no neuroprotection, further suggesting a T-cell-dependent mechanism of CpG action.

CONCLUSIONS

These findings strongly support the notion that alleviation of Treg suppression after injury benefits neuronal survival. Bacterial DNA attenuation of Treg suppressive activity may represent an evolutionary adaptation that curbs the amplified infection risk after CNS trauma, due to blood-brain barrier breakdown. This study may prompt development of new neuroprotective therapies aimed at the immune system, to benefit the injured CNS.

Hyperion's sponge-like appearance

Hyperion is Saturn's largest known irregularly shaped satellite and the only moon observed to undergo chaotic rotation. Previous work has identified Hyperion's surface as distinct from other small icy objects but left the causes unsettled. Here we report high-resolution images that reveal a unique sponge-like appearance at scales of a few kilometres. Mapping shows a high surface density of relatively well-preserved craters two to ten kilometres across. We have also determined Hyperion's size and mass, and calculated the mean density as 544 +/- 50 kg m(-3), which indicates a porosity of >40 per cent. The high porosity may enhance preservation of craters by minimizing the amount of ejecta produced or retained, and accordingly may be the crucial factor in crafting this unusual surface.

Cassini Observes the Active South Pole of Enceladus

Cassini has identified a geologically active province at the south pole of Saturn's moon Enceladus. In images acquired by the Imaging Science Subsystem (ISS), this region is circumscribed by a chain of folded ridges and troughs at approximately 55 degrees S latitude. The terrain southward of this boundary is distinguished by its albedo and color contrasts, elevated temperatures, extreme geologic youth, and narrow tectonic rifts that exhibit coarse-grained ice and coincide with the hottest temperatures measured in the region. Jets of fine icy particles that supply Saturn's E ring emanate from this province, carried aloft by water vapor probably venting from subsurface reservoirs of liquid water. The shape of Enceladus suggests a possible intense heating epoch in the past by capture into a 1:4 secondary spin/orbit resonance.

Cassini Imaging Science: Initial Results on Phoebe and Iapetus

The Cassini Imaging Science Subsystem acquired high-resolution imaging data on the outer Saturnian moon, Phoebe, during Cassini's close flyby on 11 June 2004 and on Iapetus during a flyby on 31 December 2004. Phoebe has a heavily cratered and ancient surface, shows evidence of ice near the surface, has distinct layering of different materials, and has a mean density that is indicative of an ice-rock mixture. Iapetus's dark leading side (Cassini Regio) is ancient, heavily cratered terrain bisected by an equatorial ridge system that reaches 20 kilometers relief. Local albedo variations within and bordering Cassini Regio suggest mass wasting of ballistically deposited material, the origin of which remains unknown.

Cassini Imaging Science: Initial Results on Saturn's Atmosphere

The Cassini Imaging Science Subsystem (ISS) began observing Saturn in early February 2004. From analysis of cloud motions through early October 2004, we report vertical wind shear in Saturn's equatorial jet and a maximum wind speed of approximately 375 meters per second, a value that differs from both Hubble Space Telescope and Voyager values. We also report a particularly active narrow southern mid-latitude region in which dark ovals are observed both to merge with each other and to arise from the eruptions of large, bright storms. Bright storm eruptions are correlated with Saturn's electrostatic discharges, which are thought to originate from lightning.

Cassini Imaging Science: Initial Results on Saturn's Rings and Small Satellites

Images acquired of Saturn's rings and small moons by the Cassini Imaging Science Subsystem (ISS) during the first 9 months of Cassini operations at Saturn have produced many new findings. These include new saturnian moons; refined orbits of new and previously known moons; narrow diffuse rings in the F-ring region and embedded in gaps within the main rings; exceptionally fine-scale ring structure in moderate- to high-optical depth regions; new estimates for the masses of ring-region moons, as well as ring particle properties in the Cassini division, derived from the analysis of linear density waves; ring particle albedos in select ring regions; and never-before-seen phenomena within the rings.

Galileo at Io: results from high-resolution imaging

During late 1999/early 2000, the solid state imaging experiment on the Galileo spacecraft returned more than 100 high-resolution (5 to 500 meters per pixel) images of volcanically active Io. We observed an active lava lake, an active curtain of lava, active lava flows, calderas, mountains, plateaus, and plains. Several of the sulfur dioxide-rich plumes are erupting from distal flows, rather than from the source of silicate lava (caldera or fissure, often with red pyroclastic deposits). Most of the active flows in equatorial regions are being emplaced slowly beneath insulated crust, but rapidly emplaced channelized flows are also found at all latitudes. There is no evidence for high-viscosity lava, but some bright flows may consist of sulfur rather than mafic silicates. The mountains, plateaus, and calderas are strongly influenced by tectonics and gravitational collapse. Sapping channels and scarps suggest that many portions of the upper approximately 1 kilometer are rich in volatiles.