Chromatin architecture at susceptible gene loci in cerebellar Purkinje cells characterizes DNA damage-induced neurodegeneration

The pathogenesis of inherited genome instability neurodegenerative syndromes remains largely unknown. Here, we report new disease-relevant murine models of genome instability–driven neurodegeneration involving disabled ATM and APTX that develop debilitating ataxia. We show that neurodegeneration and ataxia result from transcriptional interference in the cerebellum via aberrant messenger RNA splicing. Unexpectedly, these splicing defects were restricted to only Purkinje cells, disrupting the expression of critical homeostatic regulators including ITPR1, GRID2, and CA8. Abundant genotoxic R loops were also found at these Purkinje cell gene loci, further exacerbating DNA damage and transcriptional disruption. Using ATAC-seq to profile global chromatin accessibility in the cerebellum, we found a notably unique chromatin conformation specifically in Purkinje chromatin at the affected gene loci, thereby promoting susceptibility to DNA damage. These data reveal the pathogenic basis of DNA damage in the nervous system and suggest chromatin conformation as a feature in directing genome instability–associated neuropathology.

Genome instability independent of type I interferon signaling drives neuropathology caused by impaired ribonucleotide excision repair

Aicardi-Goutières syndrome (AGS) is a monogenic type I interferonopathy characterized by neurodevelopmental defects and upregulation of type I interferon signaling and neuroinflammation. Mutations in genes that function in nucleic acid metabolism, including RNASEH2, are linked to AGS. Ribonuclease H2 (RNASEH2) is a genome surveillance factor critical for DNA integrity by removing ribonucleotides incorporated into replicating DNA. Here we show that RNASEH2 is necessary for neurogenesis and to avoid activation of interferon-responsive genes and neuroinflammation. Cerebellar defects after RNASEH2B inactivation are rescued by p53 but not cGAS deletion, suggesting that DNA damage signaling, not neuroinflammation, accounts for neuropathology. Coincident inactivation of Atm and Rnaseh2 further affected cerebellar development causing ataxia, which was dependent upon aberrant activation of non-homologous end-joining (NHEJ). The loss of ATM also markedly exacerbates cGAS-dependent type I interferon signaling. Thus, DNA damage-dependent signaling rather than type I interferon signaling underlies neurodegeneration in this class of neurodevelopmental/neuroinflammatory disease.

Practical issues in stem cell therapy for Alzheimer's disease

We have demonstrated that aged animals show significant improvements in cognitive function and neurogenesis after brain transplantation of human neural stem cells or of human adult mesenchymal stem cells that have been dedifferentiated by transfection of the embryonic stem cell gene. We have also demonstrated that peripheral administration of a pyrimidine derivative increased cognition, endogenous brain stem cell proliferation and neurogenesis. These results indicate a bright future for stem cell therapies in Alzheimer's disease (AD). Before this is realized, however, we need to consider the affect of AD pathology on stem cell biology to establish an effective stem cell therapy for this disease. Although amyloid-beta (Abeta) deposition is a hallmark of AD, an absence of a phenotype in the beta-amyloid precursor protein (APP) knockout mouse, might lead one to underestimate the potential physiological functions of APP and suggest that it is unessential or can be compensated for. We have found, however, that APP is needed for differentiation of neural stem cells (NSCs) in vitro, and that NSCs transplanted into a APP-knockout mouse did not migrate or differentiate -- indicating that APP plays an important role in differentiation or migration process of NSCs in the brain. Then again, treatment with high a concentration of APP or its over-expression increased glial differentiation of NSCs. Human NSCs transplanted into APP-transgenic mouse brain exhibited less neurogenesis and active gliosis around the plaque like formations. Treatment of such animals with the compound, (+)-phenserine, that is known to reduce APP protein levels, increased neurogenesis and suppressed gliosis. These results suggest APP levels can regulate NSC biology in the adult brain, that altered APP metabolism in Down syndrome or AD may have implications for the pathophysiology of these diseases, and that a combination of stem cell therapy and regulation of APP levels could provide a treatment strategy for these disorders.

Stem cell strategies for Alzheimer's disease therapy

We have found much evidence that the brain is capable of regenerating neurons after maturation. In our previous study, human neural stem cells (HNSCs) transplanted into aged rat brains differentiated into neural cells and significantly improved the cognitive functions of the animals, indicating that HNSCs may be a promising candidate for cell-replacement therapies for neurodegenerative diseases including Alzheimer's disease (AD). However, ethical and practical issues associated with HNSCs compel us to explore alternative strategies. Here, we report novel technologies to differentiate adult human mesenchymal stem cells, a subset of stromal cells in the bone marrow, into neural cells by modifying DNA methylation or over expression of nanog, a homeobox gene expressed in embryonic stem cells. We also report peripheral administrations of a pyrimidine derivative that increases endogenous stem cell proliferation improves cognitive function of the aged animal. Although these results may promise a bright future for clinical applications used towards stem cell strategies in AD therapy, we must acknowledge the complexity of AD. We found that glial differentiation takes place in stem cells transplanted into amyloid-( precursor protein (APP) transgenic mice. We also found that over expression of APP gene or recombinant APP treatment causes glial differentiation of stem cells. Although further detailed mechanistic studies may be required, RNA interference of APP or reduction of APP levels in the brain can significantly reduced glial differentiation of stem cells and may be useful in promoting neurogenesis after stem cell transplantation.

Cell surface display of human immunodeficiency virus type 1 gp120 on Escherichia coli by using ice nucleation protein

A new system designed for cell surface display of recombinant proteins on Escherichia coli has been evaluated for expression of eukaryotic viral proteins. Human immunodeficiency virus type 1 (HIV-1) gp120 was fused to the C terminus of ice nucleation protein (INP), an outer membrane protein of Pseudomonas syringae. Western blotting, immunofluorescence microscopy, fluorescence-activated cell-sorting analysis, whole-cell enzyme-linked immunosorbent assay, and ice nucleation activity assay confirmed the successful expression of HIV-1 gp120 on the surface of Escherichia coli. This study shows that the INP system can be used for the expression of eukaryotic viral proteins. There is also a possibility that the INP system can be used as an AIDS diagnostic system, an oral vaccine delivery system, and an expression system for various heterologous higher-molecular-weight proteins.