Semiquantitative assessment of optic nerve injury using manganese-enhanced MRI

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.

Influence of retinal NMDA receptor activity during autoimmune optic neuritis

Autoimmune optic neuritis (AON), a model of multiple sclerosis-associated optic neuritis, is accompanied by degeneration of retinal ganglion cells (RGCs) and optic nerve demyelination and axonal loss. In order to investigate the role of N-methyl-d-aspartate (NMDA) receptors in mediating RGC degeneration, upstream changes in the optic nerve actin cytoskeleton and associated deterioration in visual function, we induced AON in Brown Norway rats by immunization with myelin oligodendrocyte glycoprotein. Subsequently, visual acuity was assessed by recording visual evoked potentials and electroretinograms prior to extraction of optic nerves for western blot analysis and retinas for quantification of RGCs. As previously reported, in Brown Norway rats RGC degeneration is observed prior to onset of immune cell infiltration and demyelination of the optic nerves. However, within the optic nerve, destabilization of the actin cytoskeleton could be seen as indicated by an increase in the globular to filamentous actin ratio. Interestingly, these changes could be mimicked by intravitreal injection of glutamate, and similarly blocked by application of the NMDA receptor blocker MK-801, leading us to propose that prior to optic nerve lesion formation, NMDA receptor activation within the retina leads to retinal calcium accumulation, actin destabilization within the optic nerve as well as a deterioration of visual acuity during AON.

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.

Eliminating Nox2 reactive oxygen species production protects dystrophic skeletal muscle from pathological calcium influx assessed in vivo by manganese-enhanced magnetic resonance imaging

KEY POINTS

Inhibiting Nox2 reactive oxygen species (ROS) production reduced in vivo calcium influx in dystrophic muscle. The lack of Nox2 ROS production protected against decreased in vivo muscle function in dystrophic mice. Manganese-enhanced magnetic resonance imaging (MEMRI) was able to detect alterations in basal calcium levels in skeletal muscle and differentiate disease status. Administration of Mn2+ did not affect muscle function or the health of the animal, and Mn2+ was cleared from skeletal muscle rapidly. We conclude that MEMRI may be a viable, non-invasive technique to monitor molecular alterations in disease progression and evaluate the effectiveness of potential therapies for Duchenne muscular dystrophy.

ABSTRACT

Duchenne muscular dystrophy (DMD) is an X-linked progressive degenerative disease resulting from a mutation in the gene that encodes dystrophin, leading to decreased muscle mechanical stability and force production. Increased Nox2 reactive oxygen species (ROS) production and sarcolemmal Ca2+ influx are early indicators of disease pathology, and eliminating Nox2 ROS production reduces aberrant Ca2+ influx in young mdx mice, a model of DMD. Various imaging modalities have been used to study dystrophic muscle in vivo; however, they are based upon alterations in muscle morphology or inflammation. Manganese has been used for indirect monitoring of calcium influx across the sarcolemma and may allow detection of molecular alterations in disease progression in vivo using manganese-enhanced magnetic resonance imaging (MEMRI). Therefore, we hypothesized that eliminating Nox2 ROS production would decrease calcium influx in adult mdx mice and that MEMRI would be able to monitor and differentiate disease status in dystrophic muscle. Both in vitro and in vivo data demonstrate that eliminating Nox2 ROS protected against aberrant Ca2+ influx and improved muscle function in dystrophic muscle. MEMRI was able to differentiate between different pathological states in vivo, with no long-term effects on animal health or muscle function. We conclude that MEMRI is a viable, non-invasive technique to differentiate disease status and might provide a means to monitor and evaluate the effectiveness of potential therapies in dystrophic muscle.