Increased transcription of transglutaminase 1 mediates neuronal death in in vitro models of neuronal stress and Aβ1–42-mediated toxicity

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The thermodynamic theory of action potential propagation: A sound basis for unification of the physics of nerve impulses

The thermodynamic theory of action potential propagation challenges the conventional understanding of the nerve signal as an exclusively electrical phenomenon. Often misunderstood as to its basic tenets and predictions, the thermodynamic theory is virtually ignored in mainstream neuroscience. Addressing a broad audience of neuroscientists, we here attempt to stimulate interest in the theory. We do this by providing a concise overview of its background, discussion of its intimate connection to Albert Einstein's treatment of the thermodynamics of interfaces and outlining its potential contribution to the building of a physical brain theory firmly grounded in first principles and the biophysical reality of individual nerve cells. As such, the paper does not attempt to advocate the superiority of the thermodynamic theory over any other approach to model the nerve impulse, but is meant as an open invitation to the neuroscience community to experimentally test the assumptions and predictions of the theory on their validity

Tissue Transglutaminase contributes to myelin phagocytosis in interleukin-4-treated human monocyte-derived macrophages

Macrophages exert either a detrimental or beneficial role in Multiple Sclerosis (MS) pathology, depending on their inflammatory environment. Tissue Transglutaminase (TG2), a calcium-dependent cross-linking enzyme, has been described as a novel marker for anti-inflammatory, interleukin-4 (IL-4) polarized macrophages (M(IL-4)), which represent a subpopulation of macrophages with phagocytic abilities. Since TG2 is expressed in macrophages in active human MS lesions, we questioned whether TG2 drives the differentiation of M(IL-4) into an anti-inflammatory phenotype and whether it plays a role in the phagocytosis of myelin by these cells. In macrophage-differentiated THP-1 monocytes, TG2 was increased upon IL-4 treatment. Reducing TG2 expression impairs the differentiation of M(IL-4) macrophages into an anti-inflammatory phenotype and drives them into a pro-inflammatory state. In addition, reduced TG2 expression resulted in increased presence of myelin basic protein in macrophages upon myelin exposure of M(IL-4) macrophages. Moreover, the elevated presence of an early endosome marker and equal expression of a lysosome marker compared to control macrophages, suggest that TG2 plays a role in phagosome maturation in M(IL-4) macrophages These data suggest that tuning macrophages into TG2 producing anti-inflammatory cells by IL-4 treatment may benefit effective myelin phagocytosis in e.g. demyelinating MS lesions and open avenues for successful regeneration

Absence of tissue transglutaminase reduces amyloid-beta pathology in APP23 mice

Aims: Alzheimer's disease (AD) is characterised by amyloid-beta (Aβ) aggregates in the brain. Targeting Aβ aggregates is a major approach for AD therapies, although attempts have had little to no success so far. A novel treatment option is to focus on blocking the actual formation of Aβ multimers. The enzyme tissue transglutaminase (TG2) is abundantly expressed in the human brain and plays a key role in post-translational modifications in Aβ resulting in covalently cross-linked, stable and neurotoxic Aβ oligomers. In vivo absence of TG2 in the APP23 mouse model may provide evidence that TG2 plays a key role in development and/or progression of Aβ-related pathology. Methods: Here, we compared the effects on Aβ pathology in the presence or absence of TG2 using 12-month-old wild type, APP23 and a crossbreed of the TG2−/− mouse model and APP23 mice (APP23/TG2−/−). Results: Using immunohistochemistry, we found that the number of Aβ deposits was significantly reduced in the absence of TG2 compared with age-matched APP23 mice. To pinpoint possible TG2-associated mechanisms involved in this observation, we analysed soluble brain Aβ1–40, Aβ1–42 and/or Aβ40/42 ratio, and mRNA levels of human APP and TG2 family members present in brain of the various mouse models. In addition, using immunohistochemistry, both beta-pleated sheet formation in Aβ deposits and the presence of reactive astrocytes associated with Aβ deposits were analysed. Conclusions: We found that absence of TG2 reduces the formation of Aβ pathology in the APP23 mouse model, suggesting that TG2 may be a suitable therapeutic target for reducing Aβ deposition in AD

Tissue transglutaminase modulates α-synuclein oligomerization

We have studied the interaction of the enzyme tissue transglutaminase (tTG), catalyzing cross-link formation between protein-bound glutamine residues and primary amines, with Parkinson's disease-associated α-synuclein protein variants at physiologically relevant concentrations. We have, for the first time, determined binding affinities of tTG for wild-type and mutant α-synucleins using surface plasmon resonance approaches, revealing high-affinity nanomolar equilibrium dissociation constants. Nanomolar tTG concentrations were sufficient for complete inhibition of fibrillization by effective α-synuclein cross-linking, resulting predominantly in intramolecularly cross-linked monomers accompanied by an oligomeric fraction. Since oligomeric species have a pathophysiological relevance we further investigated the properties of the tTG/α-synuclein oligomers. Atomic force microscopy revealed morphologically similar structures for oligomers from all α-synuclein variants; the extent of oligomer formation was found to correlate with tTG concentration. Unlike normal α-synuclein oligomers the resultant structures were extremely stable and resistant to GdnHCl and SDS. In contrast to normal β-sheet-containing oligomers, the tTG/α-synuclein oligomers appear to be unstructured and are unable to disrupt phospholipid vesicles. These data suggest that tTG binds equally effective to wild-type and disease mutant α-synuclein variants. We propose that tTG cross-linking imposes structural constraints on α-synuclein, preventing the assembly of structured oligomers required for disruption of membranes and for progression into fibrils. In general, cross-linking of amyloid forming proteins by tTG may prevent the progression into pathogenic species

Lipoprotein Receptor-Related Protein-1 Mediates Amyloid-β-Mediated Cell Death of Cerebrovascular Cells

Inefficient clearance of Aβ, caused by impaired blood-brain barrier crossing into the circulation, seems to be a major cause of Aβ accumulation in the brain of late-onset Alzheimer’s disease patients and hereditary cerebral hemorrhage with amyloidosis Dutch type. We observed association of receptor for advanced glycation end products, CD36, and low-density lipoprotein receptor (LDLR) with cerebral amyloid angiopathy in both Alzheimer’s disease and hereditary cerebral hemorrhage with amyloidosis Dutch type brains and increased low-density lipoprotein receptor-related protein-1 (LRP-1) expression by perivascular cells in cerebral amyloid angiopathy. We investigated if these Aβ receptors are involved in Aβ internalization and in Aβ-mediated cell death of human cerebrovascular cells and astrocytes. Expression of both the LRP-1 and LDLR by human brain pericytes and leptomeningeal smooth muscle cells, but not by astrocytes, increased on incubation with Aβ. Receptor-associated protein specifically inhibited Aβ-mediated up-regulation of LRP-1, but not of LDLR, and receptor-associated protein also decreased Aβ internalization and Aβ-mediated cell death. We conclude that especially LRP-1 and, to a minor extent, LDLR are involved in Aβ internalization by and Aβ-mediated cell death of cerebral perivascular cells. Although perivascular cells may adapt their Aβ internalization capacity to the levels of Aβ present, saturated LRP-1/LDLR-mediated uptake of Aβ results in degeneration of perivascular cells

Blockade of enzyme activity inhibits tissue transglutaminase-mediated transamidation of alpha-synuclein in a cellular model of Parkinson's disease

Transamidation of α-synuclein by the Ca(2+)-dependent enzyme tissue transglutaminase (tTG, EC 2.3.2.13) is implicated in Parkinson's disease (PD). tTG may therefore offer a novel therapeutic target to intervene in PD. Here we first evaluated the potency and efficacy of three recently developed irreversible active-site inhibitors of tTG (B003, Z006 and KCC009) to inhibit tTG activity in vitro and in living cells. In vitro, all compounds were found to be full inhibitors of tTG activity showing a rank order of potency (defined by IC-50 values) of Z006>B003>KCC009. Upon Ca(2+) ionophore (A23187) induced activation of cellular tTG (measured by incorporation of the tTG-specific amine substrate 5-(biotinamido)pentylamine (BAP) into cellular proteins) in neuroblastoma SH-SY5Y cells, only Z006 (0.3-30 μM) retained the capacity to completely inhibit tTG activity. Under these conditions B003 (3-300 μM) only partially blocked tTG activity whereas KCC009 (3-100 μM) failed to affect tTG activity at any of the concentrations used. Z006 (30 μM) also blocked the tTG mediated incorporation of BAP into α-synuclein monomers and SDS-resistant multimers in vitro and in α-synuclein overexpressing SHSY5Y cells exposed to A23187 or the PD mimetic 1-methyl-4-phenylpyridine (MPP(+)). Moreover, Z006 (30 μM) substantially reduced formation of SDS-resistant α-synuclein multimers in SH-SY5Y cells exposed to A23187 or MPP(+) in the absence of BAP. We conclude that α-synuclein is a cellular substrate for tTG under conditions mimicking PD and blockade of tTG activity counteracts α-synuclein transamidation and aggregation in vitro and in living cells. Moreover, our cell model appears an excellent readout to identify candidate inhibitors of intracellular tTG