In vivo MRI reveals the dynamics of pathological changes in the brains of cathepsin D-deficient mice and correlates changes in manganese-enhanced MRI with microglial activation

Cathepsin D (CTSD; EC 3.4.23.5) is essential for normal development and/or maintenance of neurons in the central nervous system: its deficiency causes a devastating neurological disorder with severely shortened life span in man, sheep and mouse. Neuropathologically, the CTSD deficiencies are characterized by selective neuronal degeneration, gliosis and accumulation of autofluorescent proteinaceous storage material in neurons. Our aim was to study the dynamics behind the pathological alterations occurring in the brains of CTSD-deficient (CTSD−/−) mice by using in vivo magnetic resonance imaging (MRI) and histology. In order to do this, we measured T2 signal intensity (SI), apparent diffusion coefficient, area and volume of multiple brain structures from MR images acquired using T2-, T1- and diffusion-weighted sequences at three time points during disease progression. MRI revealed no differences in the brains between CTSD−/− and control mice at postnatal day 15±1 (P15±1), representing an initial stage of the disease. In the intermediate stage of the disease, P19(±1), SI alterations in the thalami of the affected mice became evident in both T1- and T2-weighted images. The terminal stage of the disease, P25, was characterized by marked alterations in the T2 SI, apparent diffusion coefficient and volume of multiple brain structures in CTSD−/− mice. In addition, manganese enhanced high-resolution T1-weighted 3D sequences (MEMRI) and histological stainings revealed that the hyperintense signal areas in MEMRI matched perfectly with areas of microglial activation in the brains of CTSD−/− mice at the terminal disease stage. In conclusion, the SI alterations in the thalami of CTSD−/− mice preceded other changes, and the degenerative process was greatly enhanced at the age P19(±1), leading to severely reduced brain volume in just 6 days.