The association of LRRK2 with a variety of membrane and vesicular structures, membrane-bound organelles, and microtubules suggests an affinity of LRRK2 for lipids or lipid-associated proteins and may suggest a potential role in the biogenesis and/or regulation of vesicular and membranous intracellular structures within the mammalian brain.

The water permeability of cell membranes differs by orders of magnitude, and most of this variability reflects the differential expression of aquaporin water channels. We have recently found that the CNS contains a member of the aquaporin family, aquaporin-4 (AQP4). As a prerequisite for understanding the cellular handling of water during neuronal activity, we have investigated the cellular and subcellular expression of AQP4 in the retina and optic nerve where activity-dependent ion fluxes have been studied in detail. In situ hybridization with digoxigenin-labeled riboprobes and immunogold labeling by a sensitive postembedding procedure demonstrated that AQP4 and AQP4 mRNA were restricted to glial cells, including Mü ller cells in the retina and fibrous astrocytes in the optic nerve. A quantitative immunogold analysis of the Mü ller cells showed that these cells exhibited three distinct membrane compartments with regard to AQP4 expression. End feet membranes (facing the vitreous body or blood vessels) were 10-15 times more intensely labeled than non-end feet membranes, whereas microvilli were devoid of AQP4. These data suggest that Mü ller cells play a prominent role in the water handling in the retina and that they direct osmotically driven water flux to the vitreous body and vessels rather than to the subretinal space. Fibrous astrocytes in the optic nerve similarly displayed a differential compartmentation of AQP4. The highest expression of AQP4 occurred in end feet membranes, whereas the membrane domain facing the nodal axolemma was associated with a lower level of immunoreactivity than the rest of the membrane. This arrangement may allow transcellular water redistribution to occur without inducing inappropriate volume changes in the perinodal extracellular space.

Neurofibrillary tangles (NFTs) are composed of abnormal aggregates of the cytoskeletal protein tau. Together with amyloid β (Aβ) plaques and neuronal and synaptic loss, NFTs constitute the primary pathological hallmarks of Alzheimer disease (AD). Recent evidence also suggests that caspases are activated early in the progression of AD and may play a role in neuronal loss and NFT pathology. Here we demonstrate that tau is cleaved at D421 (ΔTau) by executioner caspases. Following caspase-cleavage, ΔTau facilitates nucleation-dependent filament formation and readily adopts a conformational change recognized by the early pathological tau marker MC1. ΔTau can be phosphorylated by glycogen synthase kinase-3β and subsequently recognized by the NFT antibody PHF-1. In transgenic mice and AD brains, ΔTau associates with both early and late markers of NFTs and is correlated with cognitive decline. Additionally, ΔTau colocalizes with Aβ 1-42 and is induced by Aβ 1-42 in vitro. Collectively, our data imply that Aβ accumulation triggers caspase activation, leading to caspase-cleavage of tau, and that this is an early event that may precede hyperphosphorylation in the evolution of AD tangle pathology. These results suggest that therapeutics aimed at inhibiting tau caspase-cleavage may prove beneficial not only in preventing NFT formation, but also in slowing cognitive decline.

Both homozygous (L166P, M26I, deletion) and heterozygous mutations (D149A, A104T) in the DJ-1 gene have been identified in Parkinson's disease (PD) patients. The biochemical function and subcellular localization of DJ-1 protein have not been clarified. To date the localization of DJ-1 protein has largely been described in studies over-expressing tagged DJ-1 protein in vitro. It is not known whether the subcellular localization of over-expressed DJ-1 protein is identical to that of endogenously expressed DJ-1 protein both in vitro and in vivo. To clarify the subcellular localization and function of DJ-1, we generated three highly specific antibodies to DJ-1 protein and investigated the subcellular localization of endogenous DJ-1 protein in both mouse brain tissues and human neuroblastoma cells. We have found that DJ-1 is widely distributed and is highly expressed in the brain. By cell fractionation and immunogold electron microscopy, we have identified an endogenous pool of DJ-1 in mitochondrial matrix and inter-membrane space. To further investigate whether pathogenic mutations might prevent the distribution of DJ-1 to mitochondria, we generated human neuroblastoma cells stably transfected with wild-type (WT) or mutant (M26I, L166P, A104T, D149A) DJ-1 and performed mitochondrial fractionation and confocal co-localization imaging studies. When compared with WT and other mutants, L166P mutant exhibits largely reduced protein level. However, the pathogenic mutations do not alter the distribution of DJ-1 to mitochondria. Thus, DJ-1 is an integral mitochondrial protein that may have important functions in regulating mitochondrial physiology. Our findings of DJ-1's mitochondrial localization may have important implications for understanding the pathogenesis of PD.

Neurofibrillary tangles (NFTs) are composed of abnormal aggregates of the cytoskeletal protein tau. Together with amyloid β (Aβ) plaques and neuronal and synaptic loss, NFTs constitute the primary pathological hallmarks of Alzheimer disease (AD). Recent evidence also suggests that caspases are activated early in the progression of AD and may play a role in neuronal loss and NFT pathology. Here we demonstrate that tau is cleaved at D421 (ΔTau) by executioner caspases. Following caspase-cleavage, ΔTau facilitates nucleation-dependent filament formation and readily adopts a conformational change recognized by the early pathological tau marker MC1. ΔTau can be phosphorylated by glycogen synthase kinase-3β and subsequently recognized by the NFT antibody PHF-1. In transgenic mice and AD brains, ΔTau associates with both early and late markers of NFTs and is correlated with cognitive decline. Additionally, ΔTau colocalizes with Aβ 1-42 and is induced by Aβ 1-42 in vitro. Collectively, our data imply that Aβ accumulation triggers caspase activation, leading to caspase-cleavage of tau, and that this is an early event that may precede hyperphosphorylation in the evolution of AD tangle pathology. These results suggest that therapeutics aimed at inhibiting tau caspase-cleavage may prove beneficial not only in preventing NFT formation, but also in slowing cognitive decline.

Aquaporin-4 (AQP4) is the predominant water channel in brain and is selectively expressed in astrocytes. Astrocytic endfoot membranes exhibit tenfold higher densities of AQP4 than non-endfoot membranes, making AQP4 an excellent marker of astrocyte polarization. Loss of astrocyte polarization is known to compromise astrocytic function and to be associated with impaired water and K+ homeostasis. Here we investigate by a combination of light and electron microscopic immunocytochemistry whether amyloid deposition is associated with a loss of astrocyte polarization, using AQP4 as a marker. We used the tg-ArcSwe mouse model of Alzheimer's disease, as this model displays perivascular plaques as well as plaques confined to the neuropil. 3D reconstructions were done to establish the spatial relation between plaques and astrocytic endfeet, the latter known to contain the perivascular pool of AQP4. Changes in AQP4 expression emerge just after the appearance of the first plaques. Typically, there is a loss of AQP4 from endfoot membranes at sites of perivascular amyloid deposits, combined with an upregulation of AQP4 in the neuropil surrounding plaques. By electron microscopy it could be verified that the upregulation reflects an increased concentration of AQP4 in those delicate astrocytic processes that abound in synaptic regions. Thus, astrocytes exhibit a redistribution of AQP4 from endfoot membranes to non-endfoot membrane domains. The present data suggest that the development of amyloid deposits is associated with a loss of astrocyte polarization. The possible perturbation of water and K+ homeostasis could contribute to cognitive decline and seizure propensity in patients with Alzheimer's disease.

The distributions of the mRNAs encoding the L-glutamate transporters GLT1 and GLAST were examined in the rat brain by in situ hybridization using 35S-labelled oligonucleotide probes. Probes directed to GLT1 produced dense labelling in the hippocampus, neocortex and neostriatum, and weak labelling in the cerebellum. In contrast, GLAST mRNA appeared to be greatly enriched in the cerebellum compared to other brain regions. While the intensity of the labelling for GLAST and GLT1 varied among different regions, their cellular distributions appeared to coincide inasmuch as both mRNAs were mainly expressed by glial cells. Labelling occurred, inter alia, in glial cells throughout the hippocampus, and in Golgi epithelial cells in the Purkinje cell layer of the cerebellum.

Integrin class adhesion proteins are concentrated at adult brain synapses. Whether synaptic integrins engage kinase signaling cascades has not been determined, but is a question of importance to ideas about integrin involvement in functional synaptic plasticity. Accordingly, synaptoneurosomes from adult rat brain were used to test if matrix ligands activate integrin-associated tyrosine kinases, and if integrin signaling targets include NMDA-class glutamate neurotransmitter receptors. The integrin ligand peptide Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) induced rapid (within 5 min) and robust increases in tyrosine phosphorylation of focal adhesion kinase, prolinerich tyrosine kinase 2 and Src family kinases. Increases were similarly induced by the native ligand fibronectin, blocked with neutralizing antibodies to b1 integrin, and not obtained with control peptides, indicating that kinase activation was integrinmediated. Both GRGDSP and fibronectin caused rapid Src kinase-dependent increases in tyrosine phosphorylation of NMDA receptor subunits NR2A and NR2B in synaptoneurosomes and acute hippocampal slices. Tests of the physiological significance of the latter result showed that ligand treatment caused a rapid and b1 integrin-dependent increase in NMDA receptor-mediated synaptic responses. These results provide the first evidence that, in adult brain, synaptic integrins activate local kinase cascades with potent effects on the operation of nearby neurotransmitter receptors implicated in synaptic plasticity.