Niemann-Pick disease type C (NPC), caused by mutations in the Npc1 or Npc2 genes, is a progressive neurodegenerative disorder characterized by intracellular accumulation/redistribution of cholesterol in a number of tissues including the brain. This is accompanied by a severe loss of neurons in selected brain regions. In this study, we evaluated the role of lysosomal enzymes, cathepsins B and D, in determining neuronal vulnerability in NPC1-deficient (Npc1 ؊/؊ ) mouse brains. Our results showed that Npc1 ؊/؊ mice exhibit an age-dependent degeneration of neurons in the cerebellum but not in the hippocampus. The cellular level/expression and activity of cathepsins B and D are increased more predominantly in the cerebellum than in the hippocampus of Npc1 ؊/؊ mice. In addition, the cytosolic levels of cathepsins, cytochrome c, and Bax2 are higher in the cerebellum than in the hippocampus of Npc1 ؊/؊ mice, suggesting a role for these enzymes in the degeneration of neurons. This suggestion is supported by our observation that degeneration of cultured cortical neurons treated with U18666A, which induces an NPC1-like phenotype at the cellular level, can be attenuated by inhibition of cathepsin B or D enzyme activity. These results suggest that the increased level/activity and altered subcellular distribution of cathepsins may be associated with the underlying cause of neuronal vulnerability in Npc1 ؊/؊ brains. Therefore , their inhibitors may have therapeutic potential in attenuating NPC

Niemann-Pick type C (NPC) disease is an autosomal recessive neurodegenerative disorder characterized by intracellular accumulation of cholesterol and glycosphingolipids in many tissues including the brain. The disease is caused by mutations of either NPC1 or NPC2 gene and is accompanied by a severe loss of neurons in the cerebellum, but not in the hippocampus. NPC pathology exhibits some similarities with Alzheimer’s disease, including increased levels of amyloid β (Aβ)-related peptides in vulnerable brain regions, but very little is known about the expression of amyloid precursor protein (APP) or APP secretases in NPC disease. In the present study, we evaluated age-related alterations in the level/distribution of APP and its processing enzymes, β- and γ-secretases, in the hippocampus and cerebellum of Npc1−/− mice, a well-established model of NPC pathology. Our results show that levels and expression of APP and β-secretase are elevated in the cerebellum prior to changes in the hippocampus, whereas γ-secretase components are enhanced in both brain regions at the same time in Npc1−/− mice. Interestingly, a subset of reactive astrocytes in Npc1−/− mouse brains expresses high levels of APP as well as β- and γ-secretase components. Additionally, the activity of β-secretase is enhanced in both the hippocampus and cerebellum of Npc1−/− mice at all ages, while the level of C-terminal APP fragments is increased in the cerebellum of 10-week-old Npc1−/− mice. These results, taken together, suggest that increased level and processing of APP may be associated with the development of pathology and/or degenerative events observed in Npc1−/− mouse brains.

Sequential processing of amyloid precursor protein by β- and γ-secretases generates Alzheimer’s disease (AD)-associated β-amyloid peptides. Recently it was reported that the transmembrane protein p23/TMP21 associates with γ-secretase, and negatively regulates β-amyloid production. Despite the link between p23 function and AD pathogenesis, the expression of p23 has not been examined in the brain. Here, we describe the detailed immunohistochemical characterization of p23 expression in rodent and human brain. We report that p23 is co-expressed with γ-secretase subunits in select neuronal cell populations in rodent brain. Interestingly, the steady-state level of p23 in the brain is high during embryonic development and then declines after birth. Furthermore, the steady-state p23 levels are reduced in the brains of individuals with AD. We conclude that p23 is expressed in neurons throughout the brain and the decline in p23 expression during postnatal development may significantly contribute to enhanced β-amyloid production in the adult brain.

Nicastrin and presenilin 1 are integral components of the high molecular weight γ-secretase complexes that regulate proteolytic processing of various type I membrane proteins including amyloid precursor protein and Notch. At present, there is little information regarding the cellular distribution of nicastrin in the developing or adult rat brain. We report here, using immunoblotting and immunohistochemical methods, that nicastrin in the adult rat brain is widely expressed and colocalized with presenilin 1 in select neuronal populations within all major areas, including the basal forebrain, striatum, cortex, hippocampus, amygdala, thalamus, hypothalamus, cerebellum and brainstem. We also observed dense neuropil labeling in many regions in the brain, suggesting that nicastrin gets transported to dendrites and/or axon terminals in the central nervous system. The levels of nicastrin are found to be relatively high at the early stages of postnatal development and then declined gradually to reach the adult profile. At the cellular level, nicastrin is localized predominantly in neuronal cell bodies at early postanatal stages, but is apparent both in cell bodies and dendrites/ neuropil in all brain regions at the later stages. The regulation of nicastrin expression and localization during development and its distribution in a wide spectrum of neurons in the postnatal and adult rat brains provide an anatomical basis to suggest a multifunctional role for the γ-secretase complex in the developing and adult rat brains.

Kainic acid, an analogue of the excitatory neurotransmitter glutamate, can trigger seizures and neurotoxicity in the hippocampus and other limbic structures in a manner that mirrors the neuropathology of human temporal lobe epilepsy (TLE). However, the underlying mechanisms associated with the neurotoxicity remain unclear. Since amyloid-β (Aβ) peptides, which are critical in the development of Alzheimer's disease, can mediate toxicity by activating glutamatergic NMDA receptors, it is likely that the enhanced glutamatergic transmission that renders hippocampal neurons vulnerable to kainic acid treatment may involve Aβ peptides. Thus, we seek to establish what role Aβ plays in kainic acid-induced toxicity using in vivo and in vitro paradigms. Our results show that systemic injection of kainic acid to adult rats triggers seizures, gliosis and loss of hippocampal neurons, along with increased levels/processing of amyloid precursor protein (APP), resulting in the enhanced production of Aβ-related peptides. The changes in APP levels/processing were evident primarily in activated astrocytes, implying a role for astrocytic Aβ in kainic acid-induced toxicity. Accordingly, we showed that treating rat primary cultured astrocytes with kainic acid can lead to increased Aβ production/secretion without any compromise in cell viability. Additionally, we revealed that kainic acid reduces neuronal viability more in neuronal/astrocyte co-cultures than in pure neuronal culture, and this is attenuated by precluding Aβ production. Collectively, these results indicate that increased production/secretion of Aβ-related peptides from activated astrocytes can contribute to neurotoxicity in kainic acid-treated rats. Since kainic acid administration can lead to neuropathological changes resembling TLE, it is likely that APP/Aβ peptides derived from astrocytes may have a role in TLE pathogenesis.