Alzheimer's disease (AD) is characterized by a progressive dysfunction of central neurons. Recent experimental evidence indicates that in the cortex, in addition to the silencing of a fraction of neurons, other neurons are hyperactive in amyloid-β (Aβ) plaqueenriched regions. However, it has remained unknown what comes first, neuronal silencing or hyperactivity, and what mechanisms might underlie the primary neuronal dysfunction. Here we examine the activity patterns of hippocampal CA1 neurons in a mouse model of AD in vivo using two-photon Ca 2+ imaging. We found that neuronal activity in the plaque-bearing CA1 region of older mice is profoundly altered. There was a marked increase in the fractions of both silent and hyperactive neurons, as previously also found in the cortex. Remarkably, in the hippocampus of young mice, we observed a selective increase in hyperactive neurons already before the formation of plaques, suggesting that soluble species of Aβ may underlie this impairment. Indeed, we found that acute treatment with the γ-secretase inhibitor LY-411575 reduces soluble Aβ levels and rescues the neuronal dysfunction. Furthermore, we demonstrate that direct application of soluble Aβ can induce neuronal hyperactivity in wild-type mice. Thus, our study identifies hippocampal hyperactivity as a very early functional impairment in AD transgenic mice and provides direct evidence that soluble Aβ is crucial for hippocampal hyperactivity.brain disease | in vivo imaging A lzheimer's disease (AD) is associated with multiple neuronal dysfunctions, including impairments underlying the storage and processing of information in the brain (1). One of the major functional defects in AD is a massive decrease in neuronal activity (2, 3). This generalized silencing of brain circuits generated the synaptic failure hypothesis (4). Interestingly, more recent studies revealed a more complex picture of the neuronal defects in AD, demonstrating a mix of both hypoactivity and hyperactivity in various brain regions. For example, in transgenic mice overexpressing both mutant human amyloid precursor protein (APP) and mutant human presenilin 1 (PS1), half of the neurons in layer 2/3 of the cortex were functionally impaired, with a decrease in neuronal activity in 29% of the neurons (termed "silent" neurons) and a profound increase in more than 20% of neurons (termed "hyperactive" neurons) (5). Interestingly, the increase in hyperactive neurons was most prominent in the vicinity of plaques. Such alterations of cortical activity were not observed in predepositing transgenic mice or in wild-type mice, indicating that the changes in neuronal activity were temporally correlated with the histological pathology. Consistently, resting Ca 2+ levels in cortical dendrites of APP/PS1 transgenic mice were substantially increased in the area surrounding plaques (6). Furthermore, APP transgenic mice exhibited nonconvulsive seizure activity in cortex and hippocampus, which was associated with GABAergic sprouting, enhanced synaptic inhibition, and syna...

The stem cells that maintain and repair the postnatal skeleton remain undefined. One model suggests that perisinusoidal mesenchymal stem cells (MSCs) give rise to osteoblasts, chondrocytes, marrow stromal cells, and adipocytes, although the existence of these cells has not been proven through fate-mapping experiments. We demonstrate here that expression of the bone morphogenetic protein (BMP) antagonist gremlin 1 defines a population of osteochondroreticular (OCR) stem cells in the bone marrow. OCR stem cells self-renew and generate osteoblasts, chondrocytes, and reticular marrow stromal cells, but not adipocytes. OCR stem cells are concentrated within the metaphysis of long bones not in the perisinusoidal space and are needed for bone development, bone remodeling, and fracture repair. Grem1 expression also identifies intestinal reticular stem cells (iRSCs) that are cells of origin for the periepithelial intestinal mesenchymal sheath. Grem1 expression identifies distinct connective tissue stem cells in both the bone (OCR stem cells) and the intestine (iRSCs).

SUMMARY Gene expression can be post-transcriptionally regulated via dynamic and reversible RNA modifications. N1-methyladenosine (m1A) is a recently identified mRNA modification; however, little is known about its precise location and biogenesis. Here, we develop a base-resolution m1A profiling method, based on m1A-induced misincorporation during reverse transcription, and report distinct classes of m1A methylome in the human transcriptome. m1A in 5′-UTR, particularly those at the mRNA cap, associate with increased translation efficiency. A different, small subset of m1A exhibit a GUUCRA tRNA-like motif, are evenly distributed in the transcriptome and are dependent on the methyltransferase TRMT6/61A. Additionally, we show that m1A is prevalent in the mitochondrial-encoded transcripts. Manipulation of m1A level via TRMT61B, a mitochondria-localizing m1A methyltransferase, demonstrates that m1A in mitochondrial mRNA interferes with translation. Collectively, our approaches reveal distinct classes of m1A methylome and provide a resource for functional studies of m1A-mediated epitranscriptomic regulation.