Assessing Lysosomal Pathology using Magnetic Resonance Imaging

Introduction: There are many neurodegenerative diseases that cause abnormalities in the lysosome including Alzheimer’s, Tay-Sachs, and Sandhoff disease. In these diseases, cellular irregularities such as oxidative stress and excessive macromolecule accumulation disrupt the membrane integrity of the lysosome, causing the organelle to lose its acidity. We hypothesize that magnetic resonance imaging (MRI) can be used to detect lysosome membrane permeabilization and loss of acidity in mouse models with lysosomal pathology. Shapiro and Koretsky showed that Mn3O is a convertible positive T1 contrast agent that is insoluble normally but dissolves in acid Mn3O4. Once in acid, it degrades into Mn that significantly shortens T1 recovery. By incubating lysosomes with Mn3O4, we propose that we can visually assess the acidity and intactness of the lysosomes in vitro and thereby detect lysosome pathology. If successful, the results would indicate that MRI could potentially be applied to assess lysosome abnormality in vivo, a possible method of improving the diagnosis of many neurological disorders such as Alzheimer’s disease. Methods: Lysosome Isolation: The animals used are the Sandhoff disease mouse model HexB-/-. HexB wildtype and HexB-/-, aged 3-5 months, were anesthesized with isofluorane and sacrificed. Brains were immediately extracted and washed with 1XPBS. After being homogenized in 1X Extraction Buffer (Sigma Inc.) with 1% protease inhibitor (1:4, w/v) using a Dounce homogenizer, the tissue was centrifuged at 1,000 x g. More 1X Extraction Buffer with 1% protease inhibitor (1:2, w/v) was added to the homogenate before the tissue was sonicated with 3 pulses. The supernatant from both centrifugations were further centrifuged at 20,000 x g. 1X Extraction Buffer with 1% protease inhibitor was added (1:0.8, w/v) to the pellet to yield the lysosome fraction. All procedures were done at 4 oC. Acid Phosphatase Test: The acid phosphatase test was used to determine the amount of lysosomes in each isolated fraction. Pnitrophenyl phosphate (Sigma Inc.) tablets were dissolved in citrate buffer, 0.09 M pH 4.8 (1:5, w/v), forming the substrate solution. The positive control of the acid phosphatase enzyme was made by dissolving acid phosphatase lyophilized powder (Sigma Inc.) in chilled water (1:4 w/v). The substrate solution was equilibrated at 37 oC for 10 minutes before 30 ul of lysosome fractions and 10 uL of the positive control were added. The samples were then incubated at 37 oC for 10 minutes while the positive control was incubated for 15 minutes. 2 ml of 0.5 NaOH was added to each sample. All samples were read at 405 nm on the spectrophotometer. The activity of the acid phosphatase was calculated using the procedure detailed by Sigma Inc. Mn3O4 Incubation and T1 Weighted Imaging: Mn3O4 (Sigma Inc.) was crushed using mortar and pestle for 30 minutes and suspended in 1X PBS. All lysosome fractions were incubated with 2.31 mg of Mn3O4 at 4 oC overnight. Before imaging, the lysosome fractions were all centrifuged at 20,000 x g for 5 minutes at 4 oC to remove excess Mn3O4. All pellets were then suspended in 300 ml of 1X Extraction Buffer with 1% protease inhibitor. All imaging protocols were completed using a 9.4 T, Bruker Avance Biospec Spectrometer, 21 cm bore horizontal imaging system (Bruker Biospin, Billerica, MA) with a 35 mm volume resonator. Data was collected using Paravision 4 (Bruker Biospin, Billerica, MA) on a Red Hat Linux computer. All phantoms were imaged utilizing a T1 weighted protocol, using a spin echo sequence with a dynamic T1 parameter: TR=326.38, 928.309, 1949.379, 7500 ms. Other parameters include TE=6.6 ms, FOV= 4.0 cm, slice thickness= 1 mm, averages=1, matrix=128 X 128. Results: