Impact of repeated topical-loaded manganese-enhanced MRI on the mouse visual system

Axonal transport impairment and its relationship with diffusion tensor imaging metrics of a murine model of p301L tau induced tauopathy

Diffusion Tensor Imaging (DTI) and Manganese Enhanced MRI (MEMRI) are noninvasive tools to characterize neural fiber microstructure and axonal transport. A combination of both may provide novel insights into the progress of neurodegeneration. To investigate the relationship of DTI and MEMRI in white matter of tauopathy, twelve optic nerves of 11-month-old p301L tau mice were imaged and finished with postmortem immunohistochemistry. MEMRI was used to quantify Mn2+ accumulation rates in the optic nerve (ON, termed ONAR) and the Superior Colliculus (SC, termed SCAR), the primary terminal site of ON in mice. We found that both ONAR and SCAR revealed a significant linear correlation with mean diffusion (mD) and radial diffusion (rD) but not with other DTI quantities. Immunohistochemistry findings showed that ONAR, mD, and rD are significantly correlated with the myelin content (Myelin Basic Protein, p < 0.05) but not with the axonal density (SMI-31), tubulin density, or tau aggregates (AT8 staining). In summary, slower axonal transport appeared to have less myelinated axons and thinner remaining axons, associated with reduced rD and mD of in vivo DTI. A combination of in vivo MEMRI and DTI can provide critical information to delineate the progress of white matter deficits in neurodegenerative diseases.

Manganese-Enhanced Magnetic Resonance Imaging: Application in Central Nervous System Diseases

Manganese-enhanced magnetic resonance imaging (MEMRI) relies on the strong paramagnetism of Mn2+. Mn2+ is a calcium ion analog and can enter excitable cells through voltage-gated calcium channels. Mn2+ can be transported along the axons of neurons via microtubule-based fast axonal transport. Based on these properties, MEMRI is used to describe neuroanatomical structures, monitor neural activity, and evaluate axonal transport rates. The application of MEMRI in preclinical animal models of central nervous system (CNS) diseases can provide more information for the study of disease mechanisms. In this article, we provide a brief review of MEMRI use in CNS diseases ranging from neurodegenerative diseases to brain injury and spinal cord injury.

Radiation-induced impairment of optic nerve axonal transport in tree shrews and rats monitored by longitudinal manganese-enhanced MRI

PURPOSE

Radiation-induced optic neuropathy (RION) is a serious complication that occurs after radiation therapy of tumors in the vicinity of the optic nerve, yet its mechanism and imaging features are poorly understood. In this study, we employed manganese-enhanced MRI (MEMRI) to assess optic nerve axonal transport in tree shrews and rats after irradiation.

MATERIALS AND METHODS

A comparison of normal visual projections in tree shrews and rats was conducted by intravitreal MnCl₂ injection followed by MRI. Adult male tree shrews and rats received a total dose of 20 Gy delivered in two fractions (10 Gy per fraction) within 5 days. Longitudinal MEMRI was conducted 5, 10, 20 and 30 weeks after radiation. At the end of observation, motor proteins involved in axonal transport were detected by western blotting, and the axon cytoskeleton was assessed by immunofluorescence.

RESULTS

The eyeballs, lens sizes, vitreous volumes, optic nerves and superior colliculi of tree shrews were significantly larger than those of rats on MEMRI (P < 0.05). The Mn²⁺-enhancement of the optic nerve showed no significant changes at 5 and 10 weeks (P > 0.05) but decreased gradually from 20 to 30 weeks postirradiation (P < 0.05). The enhancement of the superior colliculus gradually decreased from 5 weeks to 30 weeks, and the decrease was most significant at 30 weeks (P < 0.05). The levels of the motor proteins cytoplasmic dynein-1, kinesin-1 and kinesin-2 in the experimental group were significantly decreased (P < 0.05). The immunofluorescence results showed that the α-tubulin, β-tubulin and SMI 31 levels in the experimental groups and control groups were not significantly different (P > 0.05).

CONCLUSION

Tree shrews show great advantages in visual neuroscience research involving MEMRI. The main cause of the decline in axonal transport in RION is an insufficient level of motor protein rather than damage to the axonal cytoskeletal structure. Longitudinal MEMRI can be used to detect changes in axonal transport function and to observe the relatively intact axon structure from the early to late stages after radiation administration.

Applications of Manganese-Enhanced Magnetic Resonance Imaging in Ophthalmology and Visual Neuroscience

Understanding the mechanisms of vision in health and disease requires knowledge of the anatomy and physiology of the eye and the neural pathways relevant to visual perception. As such, development of imaging techniques for the visual system is crucial for unveiling the neural basis of visual function or impairment. Magnetic resonance imaging (MRI) offers non-invasive probing of the structure and function of the neural circuits without depth limitation, and can help identify abnormalities in brain tissues in vivo. Among the advanced MRI techniques, manganese-enhanced MRI (MEMRI) involves the use of active manganese contrast agents that positively enhance brain tissue signals in T1-weighted imaging with respect to the levels of connectivity and activity. Depending on the routes of administration, accumulation of manganese ions in the eye and the visual pathways can be attributed to systemic distribution or their local transport across axons in an anterograde fashion, entering the neurons through voltage-gated calcium channels. The use of the paramagnetic manganese contrast in MRI has a wide range of applications in the visual system from imaging neurodevelopment to assessing and monitoring neurodegeneration, neuroplasticity, neuroprotection, and neuroregeneration. In this review, we present four major domains of scientific inquiry where MEMRI can be put to imperative use — deciphering neuroarchitecture, tracing neuronal tracts, detecting neuronal activity, and identifying or differentiating glial activity. We deliberate upon each category studies that have successfully employed MEMRI to examine the visual system, including the delivery protocols, spatiotemporal characteristics, and biophysical interpretation. Based on this literature, we have identified some critical challenges in the field in terms of toxicity, and sensitivity and specificity of manganese enhancement. We also discuss the pitfalls and alternatives of MEMRI which will provide new avenues to explore in the future.

Manganese-Enhanced Magnetic Resonance Imaging: Overview and Central Nervous System Applications With a Focus on Neurodegeneration

Manganese-enhanced magnetic resonance imaging (MEMRI) rose to prominence in the 1990s as a sensitive approach to high contrast imaging. Following the discovery of manganese conductance through calcium-permeable channels, MEMRI applications expanded to include functional imaging in the central nervous system (CNS) and other body systems. MEMRI has since been employed in the investigation of physiology in many animal models and in humans. Here, we review historical perspectives that follow the evolution of applied MRI research into MEMRI with particular focus on its potential toxicity. Furthermore, we discuss the more current in vivo investigative uses of MEMRI in CNS investigations and the brief but decorated clinical usage of chelated manganese compound mangafodipir in humans.

Ability of Mn(2+) to Permeate the Eye and Availability of Manganese-enhanced Magnetic Resonance Imaging for Visual Pathway Imaging via Topical Administration

Background:

Manganese-enhanced magnetic resonance imaging (MEMRI) for visual pathway imaging via topical administration requires further research. This study investigated the permeability of the corneal epithelium and corneal toxicity after topical administration of Mn²⁺ to understand the applicability of MEMRI.

Methods:

Forty New Zealand rabbits were divided into 0.05 mol/L, 0.10 mol/L, and 0.20 mol/L groups as well as a control group (n = 10 in each group). Each group was further subdivided into epithelium-removed and epithelium-intact subgroups (n = 5 in each subgroup). Rabbits were given 8 drops of MnCl₂ in 5 min intervals. The Mn²⁺ concentrations in the aqueous and vitreous humors were analyzed using inductively coupled plasma-mass spectrometry at different time points. MEMRI scanning was carried out to image the visual pathway after 24 h. The corneal toxicity of Mn²⁺ was evaluated with corneal imaging and pathology slices.

Results:

Between the aqueous and vitreous humors, there was a 10 h lag for the peak Mn²⁺ concentration times. The intraocular Mn²⁺ concentration increased with the concentration gradients of Mn²⁺ and was higher in the epithelium-removed subgroup than that in the epithelium-intact subgroup. The enhancement of the visual pathway was achieved in the 0.10 mol/L and 0.20 mol/L epithelium-removed subgroups. The corresponding peak concentrations of Mn²⁺ were 5087 ± 666 ng/ml, 22920 ± 1188 ng/ml in the aqueous humor and 884 ± 78 ng/ml, 2556 ± 492 ng/ml in the vitreous body, respectively. Corneal injury was evident in the epithelium-removed and 0.20 mol/L epithelium-intact subgroups.

Conclusions:

The corneal epithelium is a barrier to Mn²⁺, and the iris and lens septum might be another intraocular barrier to the permeation of Mn²⁺. An elevated Mn²⁺ concentration contributes to the increased permeation of Mn²⁺, higher MEMRI signal, and corneal toxicity. The enhancement of the visual pathway requires an effective Mn²⁺ concentration in the vitreous body.

[Contrast agents in MRI-diagnosis of multiple sclerosis]

Magnetic resonance imaging using contrast agents plays an important role in diagnosis and assessment of treatment efficacy in multiple sclerosis. The development of contrast agents on the basis of gadolinium or iron oxide nanoparticles has potential for diagnosis of pathological foci (tumors, amyloid plaques, inflammation and foci of demyelination or necrosis) in nervous system diseases. Newly developing types of diagnostic substances for visualization of pathological foci in multiple sclerosis are presented in this review.

The distribution of Mn2+ in rabbit eyes after topical administration for manganese-enhanced MRI

PURPOSE

To analyze the distribution of Mn(2+) in rabbit eyes after topical administration of Mncl2 for manganese-enhanced MRI.

METHODS

Forty-eight Chinese white rabbits were divided into three groups. In group 1 (n = 4), the baseline concentration of Mn(2+) in aqueous, vitreous and serum samples were analyzed. In group 2 and 3, the rabbits received one topical instillation (20 μL) of Mncl2 (1 mol • L(-1)). In group 2 (n = 40), aqueous, vitreous and serum samples were collected and analyzed at predetermined time points (0.5, 1, 2, 4, 6, 12, 24, 48, 72 and 168 hours postdose). Assays were performed using inductively coupled plasma-mass spectrometer (ICP-MS). In group 3 (n = 4), after topical administration of Mncl2, dynamic manganese-enhanced MRI (MEMRI) was performed at predetermined time points. The signal-to-noise ratio (SNR) was calculated to evaluate the enhancements of eyes.

RESULTS

After topical administration, the maximum concentrations of Mn(2+) in the aqueous and vitreous samples were 11.1641 ± 0.7202 (2 hours) and 1.5622 ± 0.1567 (12 hours). In group 3, the maximum enhancement of aqueous humor (SNR = 108.81 ± 10.65) appeared at 2 hours postdose, whereas, no significant changes were detected in vitreous.

CONCLUSION

Mn(2+) could distribute into aqueous humor rapidly after topical administration of Mncl2, whereas, the concentration of Mn(2+) in vitreous body fluctuated in a narrow range over the course. The uptake of Mn(2+) in retina may involve several different pathways.

Manganese-enhanced MRI (MEMRI) via topical loading of Mn(2+) significantly impairs mouse visual acuity: a comparison with intravitreal injection

Manganese-enhanced MRI (MEMRI) with topical loading of MnCl2 provides optic nerve enhancement comparable to that seen by intravitreal injection. However, the impact of this novel and non-invasive Mn(2+) loading method on visual function requires further assessments. The objective of this study is to determine the optimal topical Mn(2+) loading dosage for MEMRI and to assess visual function after MnCl2 loading. Intravitreal administration was performed to compare the two approaches of MnCl2 loading. Twenty-four hours after topical loading of 0, 0.5, 0.75, and 1 M MnCl2 , T1 -weighted, T2-weighted, diffusion tensor imaging and visual acuity (VA) assessments were performed to determine the best topical loading dosage for MEMRI measurements and to assess the integrity of retinas and optic nerves. Mice were perfusion fixed immediately after in vivo experiments for hematoxylin and eosin and immunohistochemistry staining. Topical loading of 1 M MnCl2 damaged the retinal photoreceptor layer with no detectable damage to retina ganglion cell layers or prechiasmatic optic nerves. For the topical loading, 0.75 M MnCl2 was required to see sufficient enhancement of the optic nerve. At this concentration the visual function was significantly affected, followed by a slow recovery. Intravitreal injection (0.25 μL of 0.2 M MnCl2 ) slightly affected VA, with full recovery a day later. To conclude, intravitreal MnCl2 injection provides more reproducible results with less adverse side-effects than topical loading.