Christoph Köhler

Neuronal IGF-1 resistance reduces Aβ accumulation and protects against premature death in a model of Alzheimer’s disease

Alzheimers disease (AD) is characterized by progressive neurodegeneration leading to loss of cognitive abilities and ultimately to death. Postmortem investigations revealed decreased expression of cerebral insulin‐like growth factor (IGF)‐1 receptor (IGF‐1R) and insulin receptor substrate (IRS) proteins in patients with AD. To elucidate the role of insulin/IGF‐1 signaling in AD, we crossed mice expressing the Swedish mutation of amyloid precursor protein (APPSW, Tg2576 mice) as a model for AD with mice deficient for either IRS‐2, neuronal IGF‐1R (nIGF‐1R−/−), or neuronal insulin receptor (nIR−/−), and analyzed survival, glucose, and APP metabolism. In the present study, we show that IRS‐2 deficiency in Tg2576 mice completely reverses premature mortality in Tg2576 females and delays β‐amyloid (Aβ) accumulation. Analysis of APP metabolism suggested that delayed Aβ accumulation resulted from decreased APP processing. To delineate the upstream signal responsible for IRS‐2‐mediated disease protection, we analyzed mice with nIGF‐1R or nIR deficiency predominantly in the hippocampus. Interestingly, both male and female nIGF‐1R−/−Tg2576 mice were protected from premature death in the presence of decreased Aβ accumulation specifically in the hippocampus formation. However, neuronal IR deletion had no influence on lethality of Tg2576 mice. Thus, impaired IGF‐1/IRS‐2 signaling prevents premature death and delays amyloid accumulation in a model of AD.—Freude, S., Hettich, M. M., Schumann, C., Stohr, O., Koch, L., Kohler, C., Udelhoven, M., Leeser, U., Müller, M., Kubota, N., Kadowaki, T., Krone, W., Schroder, H., Bruning, J. C., Schubert, M. Neuronal IGF‐1 resistance reduces Aβ accumulation and protects against premature death in a model of Alzheimers disease. FASEB J. 23, 3315–3324 (2009). www.fasebj.org

Deficiency of the splicing factor Sfrs10 results in early embryonic lethality in mice and has no impact on full-length SMN /Smn splicing

The SR-like splicing factor SFRS10 (Htra2-beta1) is well known to influence various alternatively spliced exons without being an essential splicing factor. We have shown earlier that SFRS10 binds SMN1/SMN2 RNA and restores full-length (FL)-SMN2 mRNA levels in vitro. As SMN1 is absent in patients with spinal muscular atrophy (SMA), the level of FL-SMN2 determines the disease severity. Correct splicing of SMN2 can be facilitated by histone deacetylase inhibitors (HDACis) via upregulation of SFRS10. As HDACis are already used in SMA clinical trials, it is crucial to identify the spectrum of alternatively spliced transcripts modulated by SFRS10, because elevated SFRS10 levels may influence or misregulate also other biological processes. To address this issue, we generated a conditional Sfrs10 allele in mice using the Cre/loxP system. The ubiquitous homozygous deletion of Sfrs10, however, resulted in early embryonic lethality around E7.5, indicating an essential role of Sfrs10 during mouse embryogenesis. Deletion of Sfrs10 with recombinant Cre in murine embryonic fibroblasts (MEFs) derived from Sfrs10(fl/fl) embryos increased the low levels of SmnDelta7 3-4-fold, without affecting FL-Smn levels. The weak influence of Sfrs10 on Smn splicing was further proven by a Hb9-Cre driven motor neuron-specific deletion of Sfrs10 in mice, which developed normally without revealing any SMA phenotype. To assess the role of Sfrs10 on FL-SMN2 splicing, we established MEFs from Smn(-/-);SMN2(tg/tg);Sfrs10(fl/fl) embryos. Surprisingly, deletion of Sfrs10 by recombinant Cre showed no impact on SMN2 splicing but increased SMN levels. Our findings highlight the complexity by which alternatively spliced exons are regulated in vivo.

Allograft inflammatory factor-1/Ionized calcium-binding adapter molecule 1 is specifically expressed by most subpopulations of macrophages and spermatids in testis

Ionized calcium-binding adapter molecule 1 (Iba1) is a 147-amino-acid calcium-binding protein widely in use as a marker for microglia. It has actin-crosslinking activity and is involved in aspects of motility-associated rearrangement of the actin cytoskeleton. The Iba1 gene and protein are identical to allograft inflammatory factor-1 (AIF-1), a protein involved in various aspects of inflammation, which was investigated independently from Iba1. Although regarded to be monocyte/macrophage-specific, expression by germ cells in testis showed that AIF-1/Iba1 is not exclusively expressed by cells of the monocyte/macrophage lineage. Furthermore, AIF-1 was found in cells not belonging to the monocyte/macrophage lineage under pathological conditions. Here, the distribution of AIF-1/Iba1 in the normal mouse has been examined, by immunohistochemistry, to determine whether AIF-1/Iba1 expression is confined to macrophages and spermatids. Spermatids are the only cells not belonging to the monocyte/macrophage lineage found to express AIF-1/Iba1 in the normal mouse, by this method. This study has not demonstrated AIF-1/Iba1 expression in dendritic cells, although this protein might be expressed by subsets of dendritic cells. AIF-1/Iba1 can be regarded a “pan-macrophage marker” because, except for alveolar macrophages, all subpopulations of macrophages examined express AIF-1/Iba1.

IRS-2 branch of IGF-1 receptor signaling is essential for appropriate timing of myelination

Insulin‐like growth factor (IGF)‐1 increases proliferation, inhibits apoptosis and promotes differentiation of oligodendrocytes and their precursor cells, indicating an important function for IGF‐1 receptor (IGF‐1R) signaling in myelin development. The insulin receptor substrates (IRS), IRS‐1 and ‐2 serve as intracellular IGF‐1R adaptor proteins and are expressed in neurons, oligodendrocytes and their precursors. To address the role of IRS‐2 in myelination, we analyzed myelination in IRS‐2 deficient (IRS‐2−/−) mice and age‐matched controls during postnatal development. Interestingly, expression of the most abundant myelin proteins, myelin basic protein and proteolipid protein was reduced in IRS‐2−/− brains at postnatal day 10 (P10) as compared to controls. myelin basic protein immunostaining in P10‐IRS‐2−/− mice revealed a reduced immunostaining, but an unchanged regional distribution pattern. In cerebral myelin isolates at P10 unaltered relative expression of different myelin proteins was found, indicating quantitatively reduced but not qualitatively altered myelination. Interestingly, up‐regulation of IRS‐1 expression and increased IGF‐1R signaling were observed in IRS‐2−/− mice at P10‐14, indicating a compensatory mechanism to overcome IRS‐2 deficiency. Adult IRS‐2−/− mice showed unaltered myelination and motor function. Furthermore, in neuronal/brain‐specific insulin receptor knockout mice myelination was unchanged. Thus, our experiments reveal that IGF‐1R/IRS‐2 mediated signals are critical for appropriate timing of myelination in vivo.

Alzheimer's disease-like neuropathology of gene-targeted APP-SLxPS1mut mice expressing the amyloid precursor protein at endogenous levels.

Most transgenic mice used for preclinical evaluation of potential disease-modifying treatments of Alzheimers disease develop major histopathological features of this disease by several-fold overexpression of the human amyloid precursor protein. We studied the phenotype of three different strains of gene-targeted mice which express the amyloid precursor protein at endogenous levels. Only further crossing with transgenic mice overexpressing mutant human presenilin1 led to the deposition of extracellular amyloid, accompanied by the deposition of apolipoprotein E, an astrocyte and microglia reaction, and the occurrence of dilated cholinergic terminals in the cortex. Features of neurodegeneration, however, were absent. The pattern of plaque development and deposition in these mice was similar to that of amyloid precursor protein overproducing strains if crossed to presenilin1-transgenics. However, plaque development started much later and developed slowly until the age of 18 months but then increased more rapidly.

Active glycogen synthase kinase-3 and tau pathology-related tyrosine phosphorylation in pR5 human tau transgenic mice

We studied underlying pathomechanisms in tauopathies using pR5 mice that express the P301L tau mutation found in familial forms of frontotemporal dementia. In a longitudinal study we investigated the functional status of glycogen synthase kinase-3 and correlated it with the appearance of distinct tau phospho-epitopes. Neurons displaying increases in activating phosphorylation of glycogen synthase kinase-3α/β at tyrosine 279/216 also showed an intense rather than moderate AT8 (phospho-Ser202/Thr205 tau) immunoreactivity, and immunoreactivity for AT100 (phospho-Ser212/Thr214 tau) and phosphorylated Ser422, phospho-epitopes associated with fibrillar tau pathology. These neurons were rare in 8.5-month-old, but numerous in 18.5- and 28-month-old pR5 mice. Two antibodies that detect phosphotyrosine residues more generally only stained these neurons. In contrast, we did not find increased phosphotyrosine in neuronal perikarya of mice with an amyloid-β plaque pathology. Our results suggest a link between increased tyrosine phosphorylation and tau aggregation. They also reveal for the mouse models studied, that tau- rather than an amyloid-β peptide-induced pathology is associated with increased neuronal tyrosine phosphorylation.

Granulovacuolar degeneration and unfolded protein response in mouse models of tauopathy and Aβ amyloidosis.

Histopathological studies on the brains of tauopathy cases including cases with Alzheimers disease (AD) demonstrate that neurons with hyperphosphorylated protein tau display granulovacuolar degeneration (GVD), as evidenced by vacuolar lesions harboring a central granule, together with markers of the activated unfolded protein response (UPR). In order to examine whether this hallmark is reproduced in animal models we studied the presence of GVD and the activated UPR in two complementary mouse models, pR5 mice with a tau pathology and APPSLxPS1mut mice with an amyloid plaque pathology. Neither GVD nor a significant activation of the UPR was found in both APPSLxPS1mut mice and in those regions in the pR5 brain where only neurons with an early stage of tau hyperphosphorylation were present. In contrast, those neurons that displayed a tau phospho-epitope signature that only appeared in old pR5 mice and also correlated with Gallyas-positive tangle staining harbored granulovacuolar lesions that were labeled with the GVD markers casein kinases 1δ and 1ε. Granulovacuolar lesions in pR5 mice were also labeled with the UPR markers phosphorylated PKR-like endoplasmic reticulum kinase, phosphorylated inositol-requiring enzyme 1α and phosphorylated eukaryotic initiation factor 2α. However, GVD was rarely observed in neurons bearing mature neurofibrillary tangles as evidenced by Congo red staining. Our results suggest that NFT-formation activates the UPR in pR5 mice and that it is the early stages of neurofibrillary tangle formation that are accompanied by GVD, in line with observations from studies on human autopsy cases.

Hypercholesterolemia increases manganese superoxide dismutase immunoreactive macrophages in myocardium

The effect of hypercholesterolemia on manganese superoxide dismutase (MnSOD)-containing macrophages was investigated in male New Zealand white rabbits. Macrophages from control animals, which were marked with the RAM-11 antibody, demonstrated co-localization with MnSOD immunoreactivity, e.g. in the peri- and paravascular space within the myocardium, but not in the bone marrow. In rabbits fed a 0.5% cholesterol-enriched diet for 42 days, a significant increase (P<0.01) of MnSOD-immunoreactive macrophages within the myocardium was found concomitant to the drastic elevation of serum cholesterol level. In the bone marrow, MnSOD immunoreactivity did not change after cholesterol feeding. Thus in cholesterol-fed rabbits, the increase of MnSOD-containing macrophages seems to parallel that of lipoproteins. MnSOD is considered as being protective against the cytotoxic effects of those superoxide anions, possibly generated in macrophages, which are involved in the metabolism of modified lipoproteins.

The Actin-binding Protein Caldesmon Is in Spleen and Lymph Nodes Predominately Expressed by Smooth-muscle Cells, Reticular Cells, and Follicular Dendritic Cells

Reticular cells and follicular dendritic cells (FDCs) build up a framework that underlies the compartmentalization of spleens and lymph nodes. Subpopulations of reticular cells express the smooth-muscle isoform of actin, indicative of a specialized contractile apparatus. We have investigated the distribution of the actin-binding protein caldesmon in spleen and lymph nodes of mice and rats. Caldesmon modulates contraction and regulates cell motility. Alternative splicing of transcripts from a single gene results in high-molecular-mass isoforms (h-caldesmon) that are predominately expressed by smooth-muscle cells (SMCs), and low-molecular-mass isoforms (I-caldesmon) that are thought to be widely distributed in non-muscle tissues, but the distribution of caldesmon in spleen and lymph nodes has not been reported. We have performed Western blot analysis and immunohistochemistry using four different antibodies against caldesmon, among these a newly developed polyclonal antibody directed against recombinant mouse caldesmon. Western blot analysis showed the preponderance of I-caldesmon in spleen and lymph nodes. Our results from immunohistochemistry demonstrate caldesmon in SMCs, as expected, but also in reticular cells and FDCs, and suggest that the isoform highly expressed by reticular cells is I-caldesmon. In spleen of SCID mice, caldesmon was expressed by reticular cells in the absence of lymphocytes.

Analysis of the cholinergic pathology in the P301L tau transgenic pR5 model of tauopathy.

Cholinergic deafferentation of telencephalon is a major factor contributing to cognitive impairment in Alzheimers disease. There is evidence that the degeneration of cholinergic fibers which innervate the cortex and hippocampus is due to the development of neurofibrillary tangles in the perikarya of origin. Neurofibrillary tangle formation has been modeled in the transgenic pR5 mouse strain that overexpresses the longest human tau isoform together with the P301L mutation that has been previously identified in familial cases of frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). To test the suitability of the pR5 model as a model of Alzheimers disease concerning the cholinergic innervation of the telencephalon, we determined the expression of the human tau transgene and the presence of neurofibrillary changes in the basal nucleus of Meynert, the septal nuclei and the diagonal band of Broca, sources of cholinergic innervation of the cerebral cortex and hippocampus. We found that the cholinergic neurons of these nuclei, despite widespread expression of the human tau transgene, neither expressed human tau nor displayed immunoreactivity with antibodies AT8 and AT180 which recognize hyperphosphorylated tau. Immunoreactivity for choline-acetyl transferase did not reveal significant differences between pR5 mice and non-transgenic littermates in the basal forebrain, cortex and hippocampus. However, in the amygdala dystrophic cholinergic neurites were observed which were not present in non-transgenic mice. Our data show that although pR5 mice develop neurofibrillary lesions, they do not model the degeneration of basal forebrain cholinergic neurons observed in Alzheimers disease.