Synaptic changes in the thalamocortical system of cathepsin D-deficient mice: A model of human congenital neuronal ceroid-lipofuscinosis

Cathepsin D (CTSD; EC 3.4.23.5) is a lysosomal aspartic protease, the deficiency of which causes early-onset and particularly aggressive forms of neuronal ceroid-lipofuscinosis in infants, sheep, and mice. Cathepsin D deficiencies are characterized by severe neurodegeneration, but the molecular mechanisms behind the neuronal death remain poorly understood. In this study, we have systematically mapped the distribution of neuropathologic changes in CTSD-deficient mouse brains by stereologic, immunologic, and electron microscopic methods. We report highly accentuated neuropathologic changes within the ventral posterior nucleus (ventral posteromedial [VPM]/ventral posterolateral [VPL]) of thalamus and in neuronal laminae IV and VI of the somatosensory cortex (S1BF), which receive and send information to the thalamic VPM/VPL. These changes included pronounced astrocytosis and microglial activation that begin in the VPM/VPL thalamic nucleus of CTSD-deficient mice and are associated with reduced neuronal number and redistribution of presynaptic markers. In addition, loss of synapses, axonal pathology, and aggregation of synaptophysin and synaptobrevin were observed in the VPM/VPL. These synaptic alterations are accompanied by changes in the amount of synaptophysin/ synaptobrevin heterodimer, which regulates formation of the SNARE complex at the synapse. Taken together, these data reveal the somatosensory thalamocortical circuitry as a particular focus of pathologic changes and provide the first evidence for synaptic alterations at the molecular and ultrastructural levels in CTSD deficiency.