Manganese ion (Mn2+) is a calcium (Ca2+) analog that can enter neurons and other excitable cells through voltage gated Ca2+ channels. Mn2+ is also a paramagnetic that shortens the spin-lattice relaxation time constant (T1) of tissues where it has accumulated, resulting in positive contrast enhancement. Mn2+ was first investigated as a magnetic resonance imaging (MRI) contrast agent approximately 20 years ago to assess the toxicity of the metal in rats. In the late 1990s, Alan Koretsky and colleagues pioneered the use of manganese enhanced MRI (MEMRI) towards studying brain activity, tract tracing and enhancing anatomical detail. This review will describe the methodologies and applications of MEMRI in the following areas: monitoring brain activity in animal models, in vivo neuronal tract tracing and using MEMRI to assess in vivo axonal transport rates.

BackgroundAxonal transport is vital for neurons and deficits in this process have been previously reported in a few mouse models of Alzheimer's disease prior to the appearance of plaques and tangles. However, it remains to be determined whether axonal transport is defective prior to the onset of neurodegeneration. The rTg4510 mouse, a fronto-temporal dementia and parkinsonism-17 (FTDP-17) tauopathy model, over-express tau-P301L mutation found in familial forms of FTDP-17, in the forebrain driven by the calcium–calmodulin kinase II promoter. This mouse model exhibits tau pathology, neurodegeneration in the forebrain, and associated behavioral deficits beginning at 4–5 months of age.Animal modelrTg4510 transgenic mice were used in these studies. Mice were given 2 μL of MnCl2 in each nostril 1 h prior to Magnetic Resonance Imaging (MRI). Following MnCl2 nasal lavage, mice were imaged using Manganese enhanced Magnetic Resonance Imaging (MEMRI) Protocol with TE = 8.5 ms, TR = 504 ms, FOV = 3.0 cm, matrix size = 128 × 128 × 128, number of cycles = 15 with each cycle taking approximately 2 min, 9 s, and 24 ms using Paravision software (BrukerBioSpin, Billerica, MA). During imaging, body temperature was maintained at 37.0 °C using an animal heating system (SA Instruments, Stony Brook, NY).Data analysisResulting images were analyzed using Paravision software. Regions of interest (ROI) within the olfactory neuronal layer (ONL) and the water phantom consisting of one pixel (ONL) and 9 pixels (water) were selected and copied across each of the 15 cycles. Signal intensities (SI) of ONL and water phantom ROIs were measured. SI values obtained for ONL were then normalized the water phantom SI values. The correlation between normalized signal intensity in the ONL and time were assessed using Prism (GraphPad Software, San Diego, CA).ResultsUsing the MEMRI technique on 1.5, 3, 5, and 10-month old rTg4510 mice and littermate controls, we found significant axonal transport deficits present in the rTg4510 mice beginning at 3 months of age in an age-dependent manner. Using linear regression analysis, we measured rates of axonal transport at 1.5, 3, 5, and 10 months of age in rTg4510 and WT mice. Axonal transport rates were observed in rTg4510 mice at 48% of WT levels at 3 months, 40% of WT levels at 5 months, and 30% of WT levels at 10 months of age. In order to determine the point at which tau appears in the cortex, we probed for phosphorylated tau levels, and found that pSer262 is present at 3 months of age, not earlier at 1.5 months of age, but observed no pathological tau species until 6 months of age, months after the onset of the transport deficits. In addition, we saw localization of tau in the ONL at 6 months of age.DiscussionIn our study, we identified the presence of age-dependent axonal transport deficits beginning at 3 months of age in rTg4510 mice. We correlated these deficits at 3 months to the presence of hyperphosphorylated tau in the brain and the presence within the olfactory epithelium. We observed tau pathology not...

IntroductionCurrent therapy for Alzheimer's disease (AD) focuses on delaying progression, illustrating the need for more effective therapeutic targets. Histone deacetylase 6 (HDAC6) modulates tubulin acetylation and has been implicated as an attractive target. HDAC6 is also elevated in postmortem tissue samples from patients. However, HDAC6 inhibitors have had limited success preclinically due to low blood-brain barrier penetration.MethodWe investigated a specific, potent HDAC6 inhibitor (ACY-738) in a mouse model of AD. We determined the effects of ACY-738 treatment on axonal transport, behavior, and pathology in amyloid precursor protein/presenilin 1 mice.ResultsWe demonstrated improvements in in vivo axonal transport in two treatment groups as a result of ACY-738 brain levels. We also demonstrated recovery of short-term learning and memory deficits, hyperactivity, and modifications of tau and tubulin.DiscussionOur findings implicate specific, targeted HDAC6 inhibitors as potential therapeutics and demonstrate that further investigations are warranted into effects of HDAC6 inhibitors in AD.

In many human diseases, the presence of inflammation is associated with an increase in the level of reactive oxygen species (ROS). The resulting state of oxidative stress is highly detrimental and can initiate a cascade of events that ultimately lead to cell death. Thus, many therapeutic attempts have been focused on either modulating the immune system to lower inflammation or reducing the damaging caused by ROS. Berlin et al. reported the development of a novel nanoantioxidant known as poly(ethylene glycol)‐functionalized‐hydrophilic carbon clusters (PEG‐HCCs). They showed that PEG‐HCCs could be targeted to cancer cells, utilized as a drug delivery vector, and can even be visualized ex vivo. Our work here furthers this work and characterizes Gd‐DTPA conjugated PEG‐HCCs and explores the potential for in vivo tracking of T cells in live mice. We utilized a mouse model of delayed‐type hypersensitivity (DTH) to assess the immunomodulatory effects of PEG‐HCCs. The T 1‐agent Gd‐DTPA was then conjugated to the PEG‐HCCs and T 1 measurements, and T 1‐weighted MRI of the modified PEG‐HCCs was done to assess their relaxivity. We then assessed if PEG‐HCCs could be visualized both ex vivo and in vivo within the mouse lymph node and spleen. Mice treated with PEG‐HCCs showed significant improvements in the DTH assay as compared to the vehicle (saline)‐treated control. Flow cytometry demonstrated that splenic T cells are capable of internalizing PEG‐HCCs whereas fluorescent immunohistochemistry showed that PEG‐HCCs are detectable within the cortex of lymph nodes. Finally, our nanoantioxidants can be visualized in vivo within the lymph nodes and spleen of a mouse after addition of the Gd‐DTPA. PEG‐HCCs are internalized by T cells in the spleen and can reduce inflammation by suppression of a recall immune response. PEG‐HCCs can be modified to allow for both in vitro and in vivo visualization using MRI. © 2016 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.

No abstract