Conditional expression of human β-hexosaminidase in the neurons of Sandhoff disease rescues mice from neurodegeneration but not neuroinflammation

In Vivo Targeting of the Neurovascular Unit: Challenges and Advancements

The blood-brain barrier (BBB) is essential for the homeostasis of the central nervous system (CNS). Functions of the BBB are performed by the neurovascular unit (NVU), which consists of endothelial cells, pericytes, astrocytes, microglia, basement membrane, and neurons. NVU cells interact closely and together are responsible for neurovascular coupling, BBB integrity, and transendothelial fluid transport. Studies have shown that NVU dysfunction is implicated in several acute and chronic neurological diseases, including Alzheimer's disease, multiple sclerosis, and stroke. The mechanisms of NVU disruption remain poorly understood, partially due to difficulties in selective targeting of NVU cells. In this review, we discuss the relative merits of available protein markers and drivers of the NVU along with recent advancements that have been made in the field to increase efficiency and specificity of NVU research.

Integrated methylome and phenome study of the circulating proteome reveals markers pertinent to brain health

Characterising associations between the methylome, proteome and phenome may provide insight into biological pathways governing brain health. Here, we report an integrated DNA methylation and phenotypic study of the circulating proteome in relation to brain health. Methylome-wide association studies of 4058 plasma proteins are performed (N = 774), identifying 2928 CpG-protein associations after adjustment for multiple testing. These are independent of known genetic protein quantitative trait loci (pQTLs) and common lifestyle effects. Phenome-wide association studies of each protein are then performed in relation to 15 neurological traits (N = 1,065), identifying 405 associations between the levels of 191 proteins and cognitive scores, brain imaging measures or APOE e4 status. We uncover 35 previously unreported DNA methylation signatures for 17 protein markers of brain health. The epigenetic and proteomic markers we identify are pertinent to understanding and stratifying brain health.

[Glial cells and pharmacological targets in Sandhoff disease]

Sandhoff disease (SD) is a genetic disorder caused by a mutation in the β-hexosaminidase B (HexB) gene in humans. This results in the massive accumulation of GM2 gangliosides in the nervous system, causing progressive neurodegeneration. The symptoms of SD include muscle weakness, seizures, and mental illness;along with loss of muscle coordination, vision, and hearing. In the most severe form, the onset begins during early infancy, and death usually occurs within 3-5 years of age. The established animal model, Hexb-deficient (Hexb^-/-) mouse, shows abnormalities that resemble the severe phenotype found in human infants. We have previously reported that activated microglia causes astrogliosis in Hexb^-/- mouse at the early stage of development that can be ameliorated via immunosuppression. Moreover, within the cerebral cortices of Hexb^-/- mouse, reactive astrocytes were found to express adenosine A_2A receptors in later inflammatory phases. Inhibiting this receptor with istradefylline decreases the number of activated microglial cells and inflammatory cytokines/chemokines. Thus, we underline the importance of the astrocytic A_2A receptor as a sensor, in regulating microglial activation in the late phase of inflammation.

Reprogramming oral epithelial keratinocytes into a pluripotent phenotype for tissue regeneration

Objectives

We set out to reprogram adult somatic oral epithelial keratinocytes into pluripotent cells for regenerative dentistry.

Setting and Sample population

Immortalized murine oral keratinocyte cell (IMOK) line raised from adult mouse mucosa were cultured in vitro in our studies.

Materials and Methods

Adult murine oral epithelial keratinocytes were chronically treated with TGF‐β1 in vitro, and the expression of Oct4, Nanog, Sox2 and Nestin, as well as specific homeobox Gata and Pax gene family members were investigated.

Results

We documented the induction of stem factors linked with pluripotency and/or the maintenance and regulation of stem‐cell self‐renewal in oral epithelial keratinocytes by TGFβ1. Moreover, we discovered that this TGF‐β1‐induced increase in Oct4, Nanog, Sox2 and Nestin was inhibited by SB431542, suggesting that TGF‐β1 signals via the TGF‐βRI receptor to induce pluripotency and stemness.

Conclusions

Adult oral epithelial keratinocytes treated chronically with TGF‐β1 acquired phenotypic characteristics consistent with pluripotent stem cells, highlighting the facileness of reprogramming adult oral keratinocytes into an unlimited supply of pluripotent stem cells.

Genetic defects in the sphingolipid degradation pathway and their effects on microglia in neurodegenerative disease

Sphingolipids, which function as plasma membrane lipids and signaling molecules, are highly enriched in neuronal and myelin membranes in the nervous system. They are degraded in lysosomes by a defined sequence of enzymatic steps. In the related group of disorders, the sphingolipidoses, mutations in the genes that encode the individual degradative enzymes cause lysosomal accumulation of sphingolipids and often result in severe neurodegenerative disease. Here we review the information indicating that microglia, which actively clear sphingolipid-rich membranes in the brain during development and homeostasis, are directly affected by these mutations and promote neurodegeneration in the sphingolipidoses. We also identify parallels between the sphingolipidoses and more common forms of neurodegeneration, which both exhibit evidence of defective sphingolipid clearance in the nervous system.

Brain endothelial specific gene therapy improves experimental Sandhoff disease

In Tay-Sachs and Sandhoff disease, a deficiency of the lysosomal enzyme β-hexosaminidase causes GM2 and other gangliosides to accumulate in neurons and triggers neurodegeneration. Although the pathology centers on neurons, β-hexosaminidase is mainly expressed outside of neurons, suggesting that gene therapy of these diseases should target non-neuronal cells to reconstitute physiological conditions. Here, we tested in Hexb^-/- mice, a model of Sandhoff disease, to determine whether endothelial expression of the genes for human β-hexosaminidase subunit A and B (HEXA, HEXB) is able to reduce disease symptoms and prolong survival of the affected mice. The brain endothelial selective vectors AAV-BR1-CAG-HEXA and AAV-BR1-CAG-HEXB transduced brain endothelial cells, which subsequently released β-hexosaminidase enzyme. In vivo intravenous administration of the gene vectors to adult and neonatal mice prolonged survival. They improved neurological function and reduced accumulation of the ganglioside GM2 and the glycolipid GA2 as well as astrocytic activation. Overall, the data demonstrate that endothelial cells are a suitable target for intravenous gene therapy of GM2 gangliosidoses and possibly other lysosomal storage disorders.

Hexb enzyme deficiency leads to lysosomal abnormalities in radial glia and microglia in zebrafish brain development

Sphingolipidoses are severe, mostly infantile lysosomal storage disorders (LSDs) caused by defective glycosphingolipid degradation. Two of these sphingolipidoses, Tay Sachs and Sandhoff diseases, are caused by β-Hexosaminidase (HEXB) enzyme deficiency, resulting in ganglioside (GM2) accumulation and neuronal loss. The precise sequence of cellular events preceding, and leading to, neuropathology remains unclear, but likely involves inflammation and lysosomal accumulation of GM2 in multiple cell types. We aimed to determine the consequences of Hexb activity loss for different brain cell types using zebrafish. Hexb deficient zebrafish (hexb-/- ) showed lysosomal abnormalities already early in development both in radial glia, which are the neuronal and glial progenitors, and in microglia. Additionally, at 5 days postfertilization, hexb-/- zebrafish showed reduced locomotor activity. Although specific oligosaccharides accumulate in the adult brain, hexb-/- ) zebrafish are viable and apparently resistant to Hexb deficiency. In all, we identified cellular consequences of loss of Hexb enzyme activity during embryonic brain development, showing early effects on glia, which possibly underlie the behavioral aberrations. Hereby, we identified clues into the contribution of non-neuronal lysosomal abnormalities in LSDs affecting the brain and provide a tool to further study what underlies the relative resistance to Hexb deficiency in vivo.

AAV-mediated gene delivery attenuates neuroinflammation in feline Sandhoff disease

Sandhoff disease (SD) is a lysosomal storage disorder characterized by the absence of hydrolytic enzyme β-N-acetylhexosaminidase (Hex), which results in storage of GM2 ganglioside in neurons and unremitting neurodegeneration. Neuron loss initially affects fine motor skills, but rapidly progresses to loss of all body faculties, a vegetative state, and death by five years of age in humans. A well-established feline model of SD allows characterization of the disease in a large animal model and provides a means to test the safety and efficacy of therapeutic interventions before initiating clinical trials. In this study, we demonstrate a robust central nervous system (CNS) inflammatory response in feline SD, primarily marked by expansion and activation of the microglial cell population. Quantification of major histocompatibility complex II (MHC-II) labeling revealed significant up-regulation throughout the CNS with areas rich in white matter most severely affected. Expression of the leukocyte chemokine macrophage inflammatory protein-1 alpha (MIP-1α) was also up-regulated in the brain. SD cats were treated with intracranial delivery of adeno-associated viral (AAV) vectors expressing feline Hex, with a study endpoint 16weeks post treatment. AAV-mediated gene delivery repressed the expansion and activation of microglia and normalized MHC-II and MIP-1α levels. These data reiterate the profound inflammatory response in SD and show that neuroinflammation is abrogated after AAV-mediated restoration of enzymatic activity.

TSPO in a murine model of Sandhoff disease: presymptomatic marker of neurodegeneration and disease pathophysiology

Translocator protein (18 kDa), formerly known as the peripheral benzodiazepine receptor (PBR), has been extensively used as a biomarker of active brain disease and neuroinflammation. TSPO expression increases dramatically in glial cells, particularly in microglia and astrocytes, as a result of brain injury, and this phenomenon is a component of the hallmark response of the brain to injury. In this study, we used a mouse model of Sandhoff disease (SD) to assess the longitudinal expression of TSPO as a function of disease progression and its relationship to behavioral and neuropathological endpoints. Focusing on the presymptomatic period of the disease, we used ex vivo [(3)H]DPA-713 quantitative autoradiography and in vivo [(125)I]IodoDPA-713 small animal SPECT imaging to show that brain TSPO levels markedly increase prior to physical and behavioral manifestation of disease. We further show that TSPO upregulation coincides with early neuronal GM2 ganglioside aggregation and is associated with ongoing neurodegeneration and activation of both microglia and astrocytes. In brain regions with increased TSPO levels, there is a differential pattern of glial cell activation with astrocytes being activated earlier than microglia during the progression of disease. Immunofluorescent confocal imaging confirmed that TSPO colocalizes with both microglia and astrocyte markers, but the glial source of the TSPO response differs by brain region and age in SD mice. Notably, TSPO colocalization with the astrocyte marker GFAP was greater than with the microglia marker, Mac-1. Taken together, our findings have significant implications for understanding TSPO glial cell biology and for detecting neurodegeneration prior to clinical expression of disease.

Monogenic neurological disorders of sphingolipid metabolism

Sphingolipids comprise a wide variety of molecules containing a sphingoid long-chain base that can be N-acylated. These lipids are particularly abundant in the central nervous system, being membrane components of neurons as well as non-neuronal cells. Direct evidence that these brain lipids play critical functions in brain physiology is illustrated by the dramatic consequences of genetic disturbances of their metabolism. Inherited defects of both synthesis and catabolism of sphingolipids are now identified in humans. These monogenic disorders are due to mutations in the genes encoding for the enzymes that catalyze either the formation or degradation of simple sphingolipids such as ceramides, or complex sphingolipids like glycolipids. They cause varying degrees of central nervous system dysfunction, quite similarly to the neurological disorders induced in mice by gene disruption of the corresponding enzymes. Herein, the enzyme deficiencies and metabolic alterations that underlie these diseases are reviewed. Their possible pathophysiological mechanisms and the functions played by sphingolipids one can deduce from these conditions are discussed. This article is part of a Special Issue entitled Brain Lipids.

Molecular events in the cell types of the olfactory epithelium during adult neurogenesis

Background

Adult neurogenesis, fundamental for cellular homeostasis in the mammalian olfactory epithelium, requires major shifts in gene expression to produce mature olfactory sensory neurons (OSNs) from multipotent progenitor cells. To understand these dynamic events requires identifying not only the genes involved but also the cell types that express each gene. Only then can the interrelationships of the encoded proteins reveal the sequences of molecular events that control the plasticity of the adult olfactory epithelium.

Results

Of 4,057 differentially abundant mRNAs at 5 days after lesion-induced OSN replacement in adult mice, 2,334 were decreased mRNAs expressed by mature OSNs. Of the 1,723 increased mRNAs, many were expressed by cell types other than OSNs and encoded proteins involved in cell proliferation and transcriptional regulation, consistent with increased basal cell proliferation. Others encoded fatty acid metabolism and lysosomal proteins expressed by infiltrating macrophages that help scavenge debris from the apoptosis of mature OSNs. The mRNAs of immature OSNs behaved dichotomously, increasing if they supported early events in OSN differentiation (axon initiation, vesicular trafficking, cytoskeletal organization and focal adhesions) but decreasing if they supported homeostatic processes that carry over into mature OSNs (energy production, axon maintenance and protein catabolism). The complexity of shifts in gene expression responsible for converting basal cells into neurons was evident in the increased abundance of 203 transcriptional regulators expressed by basal cells and immature OSNs.

Conclusions

Many of the molecular changes evoked during adult neurogenesis can now be ascribed to specific cellular events in the OSN cell lineage, thereby defining new stages in the development of these neurons. Most notably, the patterns of gene expression in immature OSNs changed in a characteristic fashion as these neurons differentiated. Initial patterns were consistent with the transition into a neuronal morphology (neuritogenesis) and later patterns with neuronal homeostasis. Overall, gene expression patterns during adult olfactory neurogenesis showed substantial similarity to those of embryonic brain.