A new model of axon degeneration in the mouse optic nerve using repeat intraocular pressure challenge

We characterize a new experimental model for inducing retinal ganglion cell (RGC) dysfunction and degeneration in mice. C57BL/6J mice were subjected to two acute periods of intraocular pressure (IOP) elevation (50 mmHg for 30 min) by cannulation of the anterior chamber. We used full-field electroretinography and visual evoked potentials (VEPs) to measure subsequent changes in retina and optic nerve function, and histochemical techniques to assess RGC survival and optic nerve structure. In 12 month old mice, a single IOP challenge caused loss and subsequent recovery of RGC function over the following 28 days with minimal cell death and no observed axonal damage. A second identical IOP challenge resulted in persistent RGC dysfunction and significant (36%) loss of RGC somas. This was accompanied by a 16.7% delay in the latency and a 27.6% decrease in the amplitude of the VEP. Severe axonal damage was seen histologically with enlargement of axons, myelin disruption, reduced axon density, and the presence of glial scarring. In contrast, younger 3 month old mice when exposed to a single or repeat IOP challenge showed quicker RGC functional recovery after a single challenge and full functional recovery after a repeat challenge with no detectable optic nerve dysfunction. These data demonstrate a highly reproducible and minimally invasive method for inducing RGC degeneration and axonal damage in mice. Resilience of the optic nerve to damage is highly dependent on animal age. The time-defined nature of functional versus structural loss seen in this model stands to facilitate investigation of neuroglial responses in the retina after IOP injury and the associated evaluation of neuroprotective treatment strategies. Further, the model may be used to investigate the impact of aging and the cellular switch between neurorecovery and neurodegeneration.

Non-optical coherence tomography modalities for assessment of angle closure

Primary angle closure glaucoma is a leading cause of irreversible blindness, particularly in Asia. Its pathophysiology is based in the closure of the anterior chamber angle (ACA). In addition to gonioscopy (current reference standard), in the past decade, anterior segment optical coherence tomography (AS-OCT) has been incorporated in routine ophthalmic practice to help assess the configuration of the ACA. Especially in nonspecialist ophthalmology practice, gonioscopy may be less frequently performed and AS-OCT may not be available, leading to the need of other anterior segment evaluation methods. Evaluating the anterior chamber depth (ACD) has long been recognized as screening tool for primary angle-closure glaucoma. It can be measured with several devices, such as Scheimpflug photography and the scanning peripheral ACD analyzer. It can also be estimated with the oblique flashlight test and van Herick technique (limbal ACD assessment). More recently, goniophotographic systems have been developed to produce images of the ACA similar to those seen with manual gonioscopy. NGS-1 automated gonioscope (NIDEK Co, Gamagori, Japan) and the RetCam (Natus Medical Incorporated, Pleasanton, CA) are commercially available. However, NGS-1 is the only one with a specialized software for ACA imaging. Several prototype devices are currently being developed, such as the GonioPEN and axicon lens assisted gonioscopy. This article aims to review different modalities of ACA assessment, beyond AS-OCT, and compare their relative advantages and disadvantages.

Association of the CYP39A1 G204E genetic variant with increased risk of glaucoma and blindness in patients with exfoliation syndrome

PURPOSE

Carriers of functionally deficient mutations in the CYP39A1 gene have been recently reported to have a 2-fold increased risk of exfoliation syndrome (XFS). The aim of this study was to evaluate the risk of blindness and related clinical phenotypes of XFS patients carrying the loss-of-function CYP39A1 G204E mutation in comparison to XFS patients without any CYP39A1 mutation.

DESIGN

Retrospective case study PARTICIPANTS: 35 patients diagnosed with XFS carrying the CYP39A1 G204E mutation and 150 XFS patients without any CYP39A1 mutation, who were randomly selected from the Japanese XFS cohort.

METHODS

Two-sided Fisher's Exact Test with an α-level <0.05 was used to estimate the significance of the calculated Odds Ratio (OR) for all categorical measures. Comparisons between groups of subjects were performed using linear mixed effect models with group as random effect and taking possible dependence between eyes within a subject into account.

MAIN OUTCOME MEASURES

Primary analysis compared the incidence of blindness (defined as visual acuity [VA]<0.05 decimal), prevalence of exfoliation glaucoma (XFG), history of glaucoma surgery, and indices of glaucoma severity such as visual field mean deviation (MD), intraocular pressure (IOP) and vertical cup-disc ratio (CDR), between CYP39A1 G204E carriers and those without any CYP39A1 mutation.

RESULTS

The overall risk for blindness was significantly higher in XFS patients carrying the CYP39A1 G204E variant (10/35 [28.6%]) compared to XFS patients without any CYP39A1 mutations (8/150 [5.3%]; OR7.1 [95%CI:2.7-20.2]; p<0.001). A higher proportion of XFS patients with the CYP39A1 G204E mutation (23/35 [65.7%]) had evidence of XFG in at least one eye compared to the comparison group (41/150 [27.3%]; OR5.1 [95%CI:2.4-11.4]; p<0.0001). Significantly higher peak IOP, larger vertical CDR and worse visual field MD were also found in CYP39A1 G204E variant carriers (p<0.001). Additionally, patients with the CYP39A1 G204E mutation (18/35 [51.4%]) required more laser or glaucoma surgical interventions compared to those without any CYP39A1 mutation (32/150 [21.3%], p<0.001).

CONCLUSIONS

Patients with XFS carrying the CYP39A1 G204E mutation had significantly increased risk of blindness, higher occurrence of XFG and more severe glaucoma compared to patients with XFS without any CYP39A1 mutation.

Divergence with gene flow within the recent chipmunk radiation (Tamias)

Increasing data have supported the importance of divergence with gene flow (DGF) in the generation of biological diversity. In such cases, lineage divergence occurs on a shorter timescale than does the completion of reproductive isolation. Although it is critical to explore the mechanisms driving divergence and preventing homogenization by hybridization, it is equally important to document cases of DGF in nature. Here we synthesize data that have accumulated over the last dozen or so years on DGF in the chipmunk (Tamias) radiation with new data that quantify very high rates of mitochondrial DNA (mtDNA) introgression among para- and sympatric species in the T. quadrivittatus group in the central and southern Rocky Mountains. These new data (188 cytochrome b sequences) bring the total number of sequences up to 1871; roughly 16% (298) of the chipmunks we have sequenced exhibit introgressed mtDNA. This includes ongoing introgression between subspecies and between both closely related and distantly related taxa. In addition, we have identified several taxa that are apparently fixed for ancient introgressions and in which there is no evidence of ongoing introgression. A recurrent observation is that these introgressions occur between ecologically and morphologically diverged, sometimes non-sister taxa that engage in well-documented niche partitioning. Thus, the chipmunk radiation in western North America represents an excellent mammalian example of speciation in the face of recurrent gene flow among lineages and where biogeography, habitat differentiation and mating systems suggest important roles for both ecological and sexual selection.

A comparative study of diagnostic tests for the assessment of herd magnesium status in cattle

In nine of ten pre-calving dairy herds we found that herd magnesium (Mg) status could be determined with 95% confidence by measurements of serum Mg (S-Mg), urinary Mg fractional clearance ratio (FCR) or creatinine corrected urinary Mg concentration (CUM) from 10 representative cows. Our findings support clinical impressions that FCR and CUM are more sensitive than S-Mg as predictors of a positive production response to Mg supplementation of dairy cows. The available clinical data is not adequately controlled and there is a need for controlled production-response trials to refine interpretation and to confirm the predictive value of these tests. Pending such trials we suggest that mean FCR >10% or CUM >1.0 mmol/l indicate that a positive response is likely. In seven lactating herds the act of milking was not associated with depression of S-Mg and we conclude that sampling cows immediately after milking Gould not result in spurious elevations of FCR. FCR and CUM show close correlation with each other but not with S-Mg.