[Show abstract][Hide abstract] ABSTRACT: Acid sphingomyelinase (ASM) converts sphingomyelin (SM) into ceramide. Mutations in the ASM gene cause the mental retardation syndrome Niemann Pick type A (NPA), characterized as a lysosomal disorder because of the SM accumulation in these organelles. We here report that neurons from mice lacking ASM (ASMKO) present increased plasma membrane SM levels evident in detergent-resistant membranes. Paralleling this lipidic alteration, GPI-anchored proteins show an aberrant distribution in both axons and dendrites instead of the axonal enrichment observed in neurons from wild-type mice. Trafficking analysis suggests that this is due to defective internalization from dendrites. Increasing the SM content in wild-type neurons mimics these defects, whereas SM reduction in ASMKO neurons prevents their occurrence. Moreover, expression of active RhoA, which membrane attachment is affected by SM accumulation, rescues internalization rates in ASMKO neurons. These data unveil an unexpected role for ASM in neuronal plasma membrane organization and trafficking providing insight on the molecular mechanisms involved. They also suggest that deficiencies in such processes could be key pathological events in NPA disease.
Molecular biology of the cell 03/2008; 19(2):509-22. DOI:10.1091/mbc.E07-05-0439 · 4.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Defects in cellular localization and trafficking seem to facilitate the conversion of PrP(C) into the disease-associated form, PrP(Sc). Still, it is not clear to which membrane compartments PrP(C) localizes in hippocampal neurons a population most affected in the prion disease. We here show that in developing hippocampal neurons in culture PrP(C) is equally distributed to all neurites yet enriched in growth cones. By contrast, in fully mature neurons PrP(C) is restricted to axons. The axonal distribution in mature stages is paralleled by the increased partitioning of PrP(C) into detergent-resistant cholesterol-sphingolipid-rich domains (DRMs). Consistent with a cause-effect mechanism, disruption of DRMs by sphingolipid or cholesterol depletion leads to the non-polarized distribution and impaired endocytosis of PrP(C). These results indicate that DRMs are essential for proper trafficking and distribution of PrP(C) at late stages of neuronal differentiation and that its function, at least in hippocampus, is restricted to the axonal domain.
[Show abstract][Hide abstract] ABSTRACT: Substantial recent evidence suggests that defects in amyloid peptide degradation can be at the base of cases of sporadic Alzheimer's disease (AD). Among the discovered brain enzymes with the capacity to degrade amyloid peptide, the serine protease plasmin acquires special physiological relevance because of its low levels in areas of AD human brains with a high susceptibility to amyloid plaque accumulation. In this article we comment on a series of observations supporting the fact that plasmin paucity in the brain is not simply a secondary event in the disease but rather a primary defect in certain cases of sporadic AD. We also refer to recent data pointing to alterations in raft membrane domains and diminished membrane cholesterol as the underlying cause. Finally, we discuss the possibility that plasmin deficiency in the brain could lead to AD symptomatology because of amyloid aggregation and the triggering of cell death signaling cascades.
[Show abstract][Hide abstract] ABSTRACT: The serine protease plasmin can efficiently degrade amyloid peptide in vitro, and is found at low levels in the hippocampus of patients with Alzheimer's disease (AD). The cause of such paucity remains unknown. We show here that the levels of total brain plasminogen and plasminogen-binding molecules are normal in these brain samples, yet plasminogen membrane binding is greatly reduced. Biochemical analysis reveals that the membranes of these brains have a mild, still significant, cholesterol reduction compared to age-matched controls, and anomalous raft microdomains. This was reflected by the loss of raft-enriched proteins, including plasminogen-binding and -activating molecules. Using hippocampal neurons in culture, we demonstrate that removal of a similar amount of membrane cholesterol is sufficient to induce raft disorganization, leading to reduced plasminogen membrane binding and low plasmin activity. These results suggest that brain raft alterations may contribute to AD by rendering the plasminogen system inefficient.
[Show abstract][Hide abstract] ABSTRACT: Parkin is a ubiquitin ligase that facilitates proteasomal protein degradation and is involved in a common autosomal recessive form of Parkinson's disease. Its expression is part of the unfolded protein response in cell lines where its overexpression protects against unfolded protein stress. How parkin expression is regulated in brain primary cells under stress situations is however, less well established. Here, the cellular and subcellular localization of parkin under basal conditions and during unfolded protein stress was investigated in primary cultures of rat astrocytes and hippocampal neurons. Immunofluorescense microscopy and biochemical analysis demonstrated that parkin is mainly associated with the endoplasmic reticulum (ER) in hippocampal neurons while it is associated with Golgi membranes, the nuclei and light vesicles in astrocytes. The constitutive parkin expression was high in neurons as compared with astrocytes. However, unfolded protein stress elicited a selective increase in astrocytic parkin expression and a change in distribution, whereas neuronal parkin remained largely unmodified. The cell specific differences argue in favour of different cellular binding sites and substrates for the protein and a pathogenic role for astrocytes in Parkinson's disease caused by parkin dysfunction.
Journal of Neurochemistry 01/2003; 83(6):1431-40. DOI:10.1046/j.1471-4159.2002.01253.x · 4.28 Impact Factor