Early detection of neurodegeneration in brain ischemia by manganese-enhanced MRI

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 of Traumatic Brain Injury

Calcium dysfunction is involved in secondary traumatic brain injury (TBI). Manganese-enhanced MRI (MEMRI), in which the manganese ion acts as a calcium analog and a MRI contrast agent, was used to study rats subjected to a controlled cortical impact. Comparisons were made with conventional T2 MRI, sensorimotor behavior, and immunohistology. The major findings were: (1) Low-dose manganese (29 mg/kg) yielded excellent contrast with no negative effects on behavior scores relative to vehicle; (2) T1-weighted MEMRI was hyperintense in the impact area at 1-3 h, hypointense on day 2, and markedly hypointense with a hyperintense area surrounding the core on days 7 and/or 14, in contrast to the vehicle group, which did not show a biphasic profile; (3) in the hyperacute phase, the area of hyperintense T1-weighted MEMRI was larger than that of T2 MRI; (4) glial fibrillary acidic protein staining revealed that the MEMRI signal void in the impact core and the hyperintense area surrounding the core on day 7 and/or 14 corresponded to tissue cavitation and reactive gliosis, respectively; (5) T2 MRI showed little contrast in the impact core at 2 h, hyperintense on day 2 (indicative of vasogenic edema), hyperintense in some animals but pseudonormalized in others on day 7 and/or 14; (6) behavioral deficit peaked on day 2. We concluded that MEMRI detected early excitotoxic injury in the hyperacute phase, preceding vasogenic edema. In the subacute phase, MEMRI detected contrast consistent with tissue cavitation and reactive gliosis. MEMRI offers novel contrasts of biological processes that complement conventional MRI in TBI.

In vivo visuotopic brain mapping with manganese-enhanced MRI and resting-state functional connectivity MRI

The rodents are an increasingly important model for understanding the mechanisms of development, plasticity, functional specialization and disease in the visual system. However, limited tools have been available for assessing the structural and functional connectivity of the visual brain network globally, in vivo and longitudinally. There are also ongoing debates on whether functional brain connectivity directly reflects structural brain connectivity. In this study, we explored the feasibility of manganese-enhanced MRI (MEMRI) via 3 different routes of Mn(2+) administration for visuotopic brain mapping and understanding of physiological transport in normal and visually deprived adult rats. In addition, resting-state functional connectivity MRI (RSfcMRI) was performed to evaluate the intrinsic functional network and structural-functional relationships in the corresponding anatomical visual brain connections traced by MEMRI. Upon intravitreal, subcortical, and intracortical Mn(2+) injection, different topographic and layer-specific Mn enhancement patterns could be revealed in the visual cortex and subcortical visual nuclei along retinal, callosal, cortico-subcortical, transsynaptic and intracortical horizontal connections. Loss of visual input upon monocular enucleation to adult rats appeared to reduce interhemispheric polysynaptic Mn(2+) transfer but not intra- or inter-hemispheric monosynaptic Mn(2+) transport after Mn(2+) injection into visual cortex. In normal adults, both structural and functional connectivity by MEMRI and RSfcMRI was stronger interhemispherically between bilateral primary/secondary visual cortex (V1/V2) transition zones (TZ) than between V1/V2 TZ and other cortical nuclei. Intrahemispherically, structural and functional connectivity was stronger between visual cortex and subcortical visual nuclei than between visual cortex and other subcortical nuclei. The current results demonstrated the sensitivity of MEMRI and RSfcMRI for assessing the neuroarchitecture, neurophysiology and structural-functional relationships of the visual brains in vivo. These may possess great potentials for effective monitoring and understanding of the basic anatomical and functional connections in the visual system during development, plasticity, disease, pharmacological interventions and genetic modifications in future studies.

The Beta-amyloid protein of Alzheimer's disease: communication breakdown by modifying the neuronal cytoskeleton

Alzheimer's disease (AD) is one of the most prevalent severe neurological disorders afflicting our aged population. Cognitive decline, a major symptom exhibited by AD patients, is associated with neuritic dystrophy, a degenerative growth state of neurites. The molecular mechanisms governing neuritic dystrophy remain unclear. Mounting evidence indicates that the AD-causative agent, β-amyloid protein (Aβ), induces neuritic dystrophy. Indeed, neuritic dystrophy is commonly found decorating Aβ-rich amyloid plaques (APs) in the AD brain. Furthermore, disruption and degeneration of the neuronal microtubule system in neurons forming dystrophic neurites may occur as a consequence of Aβ-mediated downstream signaling. This review defines potential molecular pathways, which may be modulated subsequent to Aβ-dependent interactions with the neuronal membrane as a consequence of increasing amyloid burden in the brain.

Chemokines and neurodegeneration in the early stage of experimental ischemic stroke

Neurodegeneration is a hallmark of most of the central nervous system (CNS) disorders including stroke. Recently inflammation has been implicated in pathogenesis of neurodegeneration and neurodegenerative diseases. The aim of this study was analysis of expression of several inflammatory markers and its correlation with development of neurodegeneration during the early stage of experimental stroke. Ischemic stroke model was induced by stereotaxic intracerebral injection of vasoconstricting agent endothelin-1 (ET-1). It was observed that neurodegeneration appears very early in that model and correlates well with migration of inflammatory lymphocytes and macrophages to the brain. Although the expression of several studied chemotactic cytokines (chemokines) was significantly increased at the early phase of ET-1 induced stroke model, no clear correlation of this expression with neurodegeneration was observed. These data may indicate that chemokines do not induce neurodegeneration directly. Upregulated in the ischemic brain chemokines may be a potential target for future therapies reducing inflammatory cell migration to the brain in early stroke. Inhibition of inflammatory cell accumulation in the brain at the early stage of stroke may lead to amelioration of ischemic neurodegeneration.

In vivo manganese-enhanced MRI for visuotopic brain mapping

This study explored the feasibility of localized manganese-enhanced MRI (MEMRI) via 3 different routes of Mn(2+) administrations for visuotopic brain mapping of retinal, callosal, cortico-subcortical, transsynaptic and horizontal connections in normal adult rats. Upon fractionated intravitreal Mn(2+) injection, Mn enhancements were observed in the contralateral superior colliculus (SC) and lateral geniculate nucleus (LGN) by 45-60% at 1-3 days after initial Mn(2+) injection and in the contralateral primary visual cortex (V1) by about 10% at 2-3 days after initial Mn(2+) injection. Direct, single-dose Mn(2+) injection to the LGN resulted in Mn enhancement by 13-21% in V1 and 8-11% in SC of the ipsilateral hemisphere at 8 to 24 hours after Mn(2+) administration. Intracortical, single-dose Mn(2+) injection to the visual cortex resulted in Mn enhancement by 53-65% in ipsilateral LGN, 15-26% in ipsilateral SC, 32-34% in the splenium of corpus callosum and 17-25% in contralateral V1/V2 transition zone at 8 to 24 hours after Mn(2+) administration. Notably, some patchy patterns were apparent near the V1/V2 border of the contralateral hemisphere. Laminar-specific horizontal cortical connections were also observed in the ipsilateral hemisphere. The current results demonstrated the sensitivity of MEMRI for assessing the neuroarchitecture of the visual brains in vivo without depth-limitation, and may possess great potentials for studying the basic neural components and connections in the visual system longitudinally during development, plasticity, pharmacological interventions and genetic modifications.

Non-invasive, in vivo monitoring of neuronal transport impairment in a mouse model of tauopathy using MEMRI

The impairment of axonal transport by overexpression or hyperphosphorylation of tau is well documented for in vitro conditions; however, only a few studies on this phenomenon have been conducted in vivo, using invasive procedures, and with contradictory results. Here we used the non-invasive, Manganese-Enhanced Magnetic Resonance Imaging technique (MEMRI), to study for the first time a pure model of tauopathy, the JNPL3 transgenic mouse line, which overexpresses a mutated (P301L) form of the human tau protein. We show progressive impairment in neuronal transport as tauopathy advances. These findings are further supported by a significant correlation between the severity of the impairment in neuronal transport assessed by MEMRI, and the degree of abnormal tau assessed by histology. Unlike conventional techniques that focus on axonal transport measurement, MEMRI can provide a global analysis of neuronal transport, i.e. from dendrites to axons and at the macroscopic scale of fiber tracts. Neuronal transport impairment has been shown to be a key pathogenic process in Alzheimer's disease and numerous other neurodegenerative disorders. Hence, MEMRI provides a promising set of functional biomarkers to be used during preclinical trials to facilitate the selection of new drugs aimed at restoring neuronal transport in neurodegenerative diseases.

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

PURPOSE

Optic nerve degeneration in diseases such as glaucoma and multiple sclerosis evolves in months to years. The use of Mn(2+)-Enhanced Magnetic Resonance Imaging (MEMRI) in a time-course study may provide new insights into the disease progression. Previously, we demonstrated the feasibility of using a topical administration for Mn(2+) delivery to the visual system. This study is to evaluate the impact of biweekly or monthly repeated Mn(2+) topical administration and the pH levels of the Mn(2+) solutions for MEMRI on the mouse visual pathway.

METHODS

Using groups of mice, the MEMRI with an acidic or pH neutralized 1 M MnCl(2) solution was performed. To evaluate the feasibility of repeated MEMRIs, topical-loaded MEMRI was conducted biweekly seven times or monthly three times. The enhancement of MEMRI in the visual system was quantified. After repeated MEMRIs, the corneas were examined by optical coherence tomography. The retinal ganglion cells (RGCs) and optic nerves were examined by histology.

RESULTS

All mice exhibited consistent enhancements along the visual system following repeated MEMRIs. The acidic Mn(2+) solution induced a greater MEMRI enhancement as compared with a neutral pH Mn(2+) solution. Significant 20% RGC loss was found after three biweekly Mn(2+) inductions, but no RGC loss was found after three monthly Mn(2+) treatments. The corneal thickness was found increased after seven biweekly topical-loaded MEMRI.

CONCLUSIONS

Acidic Mn(2+) solutions enhanced the uptake of Mn(2+) observed on the MEMRI. Increasing the time intervals of repeated Mn(2+) topical administration reduced the adverse effects caused by MEMRI.

Manganese-enhanced MRI of hypoxic-ischemic brain injuries using Mn-DPDP

In this study, Mn-dipyridoxaldiphosphate (MnDPDP), a clinically approved manganese contrast agent for hepatic and pancreatic imaging, was demonstrated for the first time for manganese-enhanced MRI (MEMRI) in brains of normal young rats (n = 4) and rats with hypoxic-ischemic (H-I) insult at postnatal day 7 (n = 8). After a single intraperitoneal injection of low dosage with 0.1micromol/g in postnatal 14 days, 2D T1-weighted image (T1WIs), T1 maps, T2-weighted images (T2WIs) and T2 maps were acquired at 7 Tesla 1 day before, 1 day and 7 days after MnDPDP injection. The image contrast changes induced by MnDPDP appeared as the hyperintensity in T1WIs and the hypointensity in T2WIs. T1 and T2 values decreased in the regions of Mn enhancement. Such enhancement presented as a delayed pattern that was more pronounced in 7 day after MnDPDP injection, suggesting the sustained Mn accumulation due to MnDPDP. Moreover, the MnDPDP enhancement in H-I brains was more pronounced in the lesion sites and was easily detectable in T1WI, T1 map, T2WI and T2 map. The results demonstrated here support the possibility of using MnDPDP as a 'slow release' Mn(2+) for clinical diagnosis of various neuropathologies.