Immune hyporesponsiveness to amyloid beta-peptide in amyloid precursor protein transgenic mice: implications for the pathogenesis and treatment of Alzheimer's disease

Alzheimer's disease is a dementia that involves progressive deposition of amyloid beta-protein (Abeta) in brain regions important for memory and cognition, followed by secondary inflammation that contributes to the neuropathologic process. Immunization with Abeta can reduce cerebral Abeta burden and consequent neuropathologic changes in the brains of mice transgenic for the beta-amyloid precursor protein (APP). We found that transgenic expression of human APP in B6SJL mice, under the prion promoter, results in immune hyporesponsiveness to human Abeta, in terms of both antibody and cellular immune responses. The decreased antibody responses were related not to B cell tolerance but rather to the inability of Abeta-specific T cells to provide help for antibody production. The immune hyporesponsiveness could be overcome if T cell help was provided by coupling an Abeta B cell epitope to BSA. Our results suggest that expression of APP in transgenic mice is associated with an Abeta-specific impaired adaptive immune response that may contribute to the neuropathology. Moreover, humans with life-long elevation of brain and peripheral Abeta (e.g., patients with presenilin mutations or Down syndrome) could have reduced immune responses to Abeta vaccination.

Differential T cell response in central and peripheral nerve injury: connection with immune privilege

The central nervous system (CNS), unlike the peripheral nervous system (PNS), is an immune-privileged site in which local immune responses are restricted. Whereas immune privilege in the intact CNS has been studied intensively, little is known about its effects after trauma. In this study, we examined the influence of CNS immune privilege on T cell response to central nerve injury. Immunocytochemistry revealed a significantly greater accumulation of endogenous T cells in the injured rat sciatic nerve than in the injured rat optic nerve (representing PNS and CNS white matter trauma, respectively). Use of the in situ terminal deoxytransferase-catalyzed DNA nick end labeling (TUNEL) procedure revealed extensive death of accumulating T cells in injured CNS nerves as well as in CNS nerves of rats with acute experimental autoimmune encephalomyelitis, but not in injured PNS nerves. Although Fas ligand (FasL) protein was expressed in white matter tissue of both systems, it was more pronounced in the CNS. Expression of major histocompatibility complex (MHC) class II antigens was found to be constitutive in the PNS, but in the CNS was induced only after injury. Our findings suggest that the T cell response to central nerve injury is restricted by the reduced expression of MHC class II antigens, the pronounced FasL expression, and the elimination of infiltrating lymphocytes through cell death.

GTP-dependent conformational changes associated with the functional switch between Galpha and cross-linking activities in brain-derived tissue transglutaminase

GTP and Ca2+, two well-known modulators of intracellular signaling pathways, control a structural/functional switch between two vital and mutually exclusive activities, cross-linking and Galpha activity, in the same enzyme. The enzyme, a brain-derived tissue-type transglutaminase (TGase), was recently cloned by us in two forms, one of which (s-TGN) lacks a C-terminal region that is present in the other (l-TGN). Immunoreaction with antibodies directed against a peptide present in the C-terminus of l-TGN but missing in s-TGN suggested that this site, which is located in the C-terminal fourth domain, undergoes conformational changes as a result of interaction between l-TGN and GTP. Site-directed mutagenesis suggested that the third domain is involved in mediating the inhibition of the cross-linking activity. These results were supported by molecular modeling, which further suggested that domains III and IV both participate in conformational changes leading to the functional switch between the Ca2+-dependent cross-linking activity and the Galpha activity.

Factor XIIIa as a nerve-associated transglutaminase

Recent findings have led to changes in the traditional concept of nerve recovery, including the realization that injured nerves, like any other injured tissue, need the assistance of blood-derived cells and factors in order to heal. We show that factor XIIIa (FXIIIa, the potentially active a2subunit of factor XIII), an enzyme that participates in blood coagulation by stabilizing the fibrin clot, is also active in the nervous system where it may play a key role in the healing of injured tissue. We demonstrate that the plasma, macrophages and nerves of fish contain a 55 kDa form of transglutaminase that cross-reacts immunologically with the a-subunit of FXIII in mammals (80 kDa). The fish enzyme in the plasma, unlike its mammalian counterpart, is active, pointing to a difference in control of the coagulation pathway in the two species. Analysis of FXIIIa expression in mammalian neural tissues and their response to injury revealed high levels of the enzyme in media conditioned by peripheral nerves as compared with medium conditioned by nerves of the central nervous system. Furthermore, similarity was observed in the postinjury behavior of FXIIIa in regenerating nerve tissues (peripheral nervous system of mammals and the central nervous system of fish). We suggest that the postinjury level of factor XIIIa in the nervous system may be related to the tissue's regenerative capacity, and that FXIIIa may therefore be a link underlying a possible association between the processes of blood coagulation and nerve healing.

Expression of GTP-dependent and GTP-independent tissue-type transglutaminase in cytokine-treated rat brain astrocytes

Tissue-type transglutaminases (TGases) were recently shown to exert dual enzymatic activities; they catalyze the posttranslational modification of proteins by transamidation, and they also act as guanosine triphosphatase (GTPase). Here we show that a tissue-type TGase is expressed in rat brain astrocytes in vitro, and is induced by the inflammation-associated cytokines interleukin-1beta and to a lesser extent by tumor necrosis factor-alpha. Induction is accompanied by overexpression and appearance of an additional shorter clone, which does not contain the long 3'-untranslated region and encodes for a novel TGase enzyme whose C terminus lacks a site that affects the enzyme's interaction with guanosine triphosphate (GTP). Expression of two clones revealed that the long form is inhibited noncompetitively by GTP, but the short form significantly less so. The different affinities for GTP may account for the difference in physiological function between these two enzymes.

Complete transection of rat optic nerve while sparing the meninges and the vasculature: an experimental model for optic nerve neuropathy and trauma

In this study we present a method to achieve a complete transection of optic nerve axons in adult rat, while preserving the vasculature and retaining the continuity of the meninges. Under deep anesthesia, the optic nerve of adult rat is exposed. Using specially designed instruments built from disposable glass microsampling pipettes, a small opening is created in the meninges of the optic nerve, 2-3 mm behind the eye globe. A glass dissector is introduced through the opening and is used to cut all the axons through the whole width of the nerve. Complete transection of the optic nerve axons was achieved, while retaining the continuity of the meninges and avoiding damage to the nerve's vascular supply. Transection was confirmed by transillumination showing a complete gap in the continuity of the nerve axons, and by both morphological and electrophysiological criteria. Nerve transection performed by the conventional technique leads to neuroma formation and hampers regeneration. Crush injury may cause nerve ischemia, which is detrimental to axonal recovery. Both of these disadvantages are avoided by the method of transection presented here. The opening created in the 'meningeal tube' can be used to inject substances that may be of benefit in recovery, rescue and/or regeneration of the injured axons. The model is particularly suitable for in vivo studies on nerve regeneration, and especially for screening of putative therapeutic agents.

The fumR gene encoding fumarase in the filamentous fungus Rhizopus oryzae: cloning, structure and expression

The filamentous fungus Rhizopus oryzae (Ro) is known for its ability to overproduce and accumulate high levels of fumaric acid (FA) under stress conditions. In order to study the molecular mechanisms involved in the increased biosynthesis of FA, the gene (designated fumR) encoding Ro fumarase was cloned and analysed for its structure and expression. Nucleotide (nt) sequence and comparison of the fumR product with fumarases from various sources established that fumR contains nine introns and encodes a deduced product of 494 amino acids (aa), related to class-II fumarases. A fumarase protein of 50 kDa was immuno-detected in crude Ro extracts. Primer extension experiments mapped the 5' end of the fumR RNA 159 nt upstream from the putative translation start codon. Both primer extension and Northern analysis showed the existence of one transcript of fumR. The level of fumR RNA increased in cells producing FA under stress conditions (high carbon and low nitrogen levels in the medium), suggesting that transcriptional regulation of fumR might be involved in the overproduction and accumulation of FA by Ro cells under stress conditions. The possibility that additional mechanisms are responsible for this phenomenon is discussed.