Probes for INS

Adult hippocampal neurogenesis (AHN) is a well-studied phenomenon that involves the derivation of new neurons from neural progenitor cells in the dentate gyrus region of the hippocampus, an area responsible for cognitive functions such as learning and memory storage. Moreover, the hippocampus is known to be implicated in neurological conditions such as Alzheimer's disease. Although AHN has been extensively observed in animal models for twenty years, its existence and persistence in humans have been widely debated in academia, heavily based on post-mortem immunohistochemical markers. Using the search engines PubMed and Google Scholar for “Adult Human Neurogenesis,” 143 articles that were most relevant to the history of AHN discovery, detection in rodents, immunohistochemical studies on post-mortem human sections, and therapeutic development targeting AHN were reviewed. This review article highlights the current understanding of AHN in rodents and humans, its implications in neurodegenerative diseases and therapeutics, and the inconsistencies and methodological variabilities encountered in studying AHN in humans. Furthermore, the correlation between AHN and diseases such as mood disorders and Alzheimer's disease is still not well established, with conflicting findings reported. Standardization of transcriptomic methodologies and increased availability of post-mortem human brain samples are crucial in advancing AHN research. This review article attempts to discover the fascinating and controversial world of adult human neurogenesis and its potential implications in treating neurological disorders. Apart from the discussion on AHN existence, tackling devastating diseases with this supplemental knowledge can lead to therapeutic advancements which greatly rely on understanding not only the presence of AHN but the mechanisms mediating its availability.

Growth hormone secretagogue receptor 1a (GHSR1a)-the receptor for orexigenic hormone ghrelin-is a G protein-coupled receptor that is widely distributed in the brain, including the hippocampus. Studies have demonstrated that genetic deletion of GHSR1a affects memory, suggesting the importance of ghrelin/GHSR1a signaling in cognitive control. However, current reports are controversial, and the mechanism underlying GHSR1a modulation of memory is uncertain. Here, we first report that global GHSR1a knockout enhances hippocampus-dependent memory, facilitates initial LTP in dorsal hippocampal Schaffer Collateral-CA1 synapses, and downregulates Akt activity in the hippocampus. Moreover, we show that the intrinsic excitability of GAD67+ interneurons-rather than neighboring pyramidal neurons in the dCA1-is suppressed by GHSR1a deletion, an effect that is antagonized by acute application of the Akt activator SC79. In addition, the inhibitory postsynaptic currents (IPSCs) on dCA1 pyramidal neurons are selectively reduced in mice with a GHSR1a deficiency. Finally, we demonstrate that selectively increasing the excitability of parvalbumin-expressing interneurons by hM3Dq-DREADDs increases IPSCs on dCA1 pyramidal neurons and normalizes memory in Ghsr1a KO mice. Our findings thus reveal a novel mechanism underlying memory enhancement of GHSR1a deficiency and herein support an adverse effect of GHSR1a signaling in hippocampus-dependent memory processes.

Epigenetic mechanisms are altered in the striatum of HD patients and mouse models, but how they might contribute to pathogenesis, including cognitive deficits, is unclear. Epigenetic regulation is critical to learning and memory processes, through transcriptional control of gene program promoting neural plasticity. We asked whether memory-associated epigenetic and transcriptional responses were impaired in HD R6/1 mice. To this end, we trained R6/1 mice (and control mice) in an aquatic navigation task, the double H maze, which allows assessing striatum-dependent memory (e.g. egocentric spatial memory). We then generated ChIP-seq, 4C-seq and RNA-seq datasets on striatal tissue of HD and control mice during egocentric memory processing, including memory acquisition and consolidation/recall. Egocentric memory was altered since early symptomatic stage in R6/1 mice, which correlated with dramatic reduction of striatal epigenetic and transcriptional changes induced by memory process. More specifically, multi-omic analysis showed that, during memory acquisition, 3D chromatin re-organization and transcriptional induction at BDNF-related genes were diminished in R6/1 striatum. Moreover, we found that changes in H3K9 acetylation (H3K9ac), which accompanied memory process in normal striatum, were attenuated in R6/1 striatum. Functional enrichment analyses further indicated that altered H3K9ac regulation during late phase of egocentric memory process (e.g. consolidation/recall) contributed to impaired TGFβ-dependent cellular plasticity. Together, this study provides support to the hypothesis that epigenetic dysregulation in HD contributes to cognitive deficits, and shed light on new targets of striatal plasticity, particularly H3K9ac and TFGβ signaling.