ATP6AP2, a regulator of LRP6/ß-catenin protein trafficking, promotes Wnt/ß-catenin signaling and bone formation in a cell type dependent manner.

Wnt/ß-catenin signaling is critical for various cellular processes in multiple cell types, including osteoblast (OB) differentiation and function. Exactly how Wnt/ß-catenin signaling is regulated in OBs remain elusive. ATP6AP2, an accessory subunit of V-ATPase, plays important roles in multiple cell types/organs and multiple signaling pathways. However, little is known whether and how ATP6AP2 in OBs regulates Wnt/ß-catenin signaling and bone formation. Here we provide evidence for ATP6AP2 in the OB-lineage cells to promote OB-mediated bone formation and bone homeostasis selectively in the trabecular bone regions. Conditionally knocking out (CKO) ATP6AP2 in the OB-lineage cells (Atp6ap2Ocn-Cre) reduced trabecular, but not cortical, bone formation and bone mass. Proteomic and cellular biochemical studies revealed that LRP6 and N-cadherin were reduced in ATP6AP2-KO BMSCs and OBs, but not osteocytes. Additional in vitro and in vivo studies revealed impaired ß-catenin signaling in ATP6AP2-KO BMSCs and OBs, but not osteocytes, under both basal and Wnt stimulated conditions, although LRP5 was decreased in ATP6AP2-KO osteocytes, but not BMSCs. Further cell biological studies uncovered that osteoblastic ATP6AP2 is not required for Wnt3a suppression of ß-catenin phosphorylation, but necessary for LRP6/ß-catenin and N-cadherin/ß-catenin protein complex distribution at the cell membrane, thus preventing their degradation. Expression of active ß-catenin diminished the OB differentiation deficit in ATP6AP2-KO BMSCs. Taken together, these results support the view for ATP6AP2 as a critical regulator of both LRP6 and N-cadherin protein trafficking and stability, and thus regulating ß-catenin levels, demonstrating an un-recognized function of osteoblastic ATP6AP2 in promoting Wnt/LRP6/ß-catenin signaling and trabecular bone formation.

Lactate Metabolism, Signaling, and Function in Brain Development, Synaptic Plasticity, Angiogenesis, and Neurodegenerative Diseases.

Neural tissue requires a great metabolic demand despite negligible intrinsic energy stores. As a result, the central nervous system (CNS) depends upon a continuous influx of metabolic substrates from the blood. Disruption of this process can lead to impairment of neurological functions, loss of consciousness, and coma within minutes. Intricate neurovascular networks permit both spatially and temporally appropriate metabolic substrate delivery. Lactate is the end product of anaerobic or aerobic glycolysis, converted from pyruvate by lactate dehydrogenase-5 (LDH-5). Although abundant in the brain, it was traditionally considered a byproduct or waste of glycolysis. However, recent evidence indicates lactate may be an important energy source as well as a metabolic signaling molecule for the brain and astrocytes-the most abundant glial cell-playing a crucial role in energy delivery, storage, production, and utilization. The astrocyte-neuron lactate-shuttle hypothesis states that lactate, once released into the extracellular space by astrocytes, can be up-taken and metabolized by neurons. This review focuses on this hypothesis, highlighting lactate's emerging role in the brain, with particular emphasis on its role during development, synaptic plasticity, angiogenesis, and disease.

APP in the Neuromuscular Junction for the Development of Sarcopenia and Alzheimer's Disease.

Sarcopenia, an illness condition usually characterized by a loss of skeletal muscle mass and muscle strength or function, is often associated with neurodegenerative diseases, such as Alzheimer's disease (AD), a common type of dementia, leading to memory loss and other cognitive impairment. However, the underlying mechanisms for their associations and relationships are less well understood. The App, a Mendelian gene for early-onset AD, encodes amyloid precursor protein (APP), a transmembrane protein enriched at both the neuromuscular junction (NMJ) and synapses in the central nervous system (CNS). Here, in this review, we highlight APP and its family members' physiological functions and Swedish mutant APP (APPswe)'s pathological roles in muscles and NMJ. Understanding APP's pathophysiological functions in muscles and NMJ is likely to uncover insights not only into neuromuscular diseases but also AD. We summarize key findings from the burgeoning literature, which may open new avenues to investigate the link between muscle cells and brain cells in the development and progression of AD and sarcopenia.

Multi-omic approach characterises the neuroprotective role of retromer in regulating lysosomal health.

Retromer controls cellular homeostasis through regulating integral membrane protein sorting and transport and by controlling maturation of the endo-lysosomal network. Retromer dysfunction, which is linked to neurodegenerative disorders including Parkinson's and Alzheimer's diseases, manifests in complex cellular phenotypes, though the precise nature of this dysfunction, and its relation to neurodegeneration, remain unclear. Here, we perform an integrated multi-omics approach to provide precise insight into the impact of Retromer dysfunction on endo-lysosomal health and homeostasis within a human neuroglioma cell model. We quantify widespread changes to the lysosomal proteome, indicative of broad lysosomal dysfunction and inefficient autophagic lysosome reformation, coupled with a reconfigured cell surface proteome and secretome reflective of increased lysosomal exocytosis. Through this global proteomic approach and parallel transcriptomic analysis, we provide a holistic view of Retromer function in regulating lysosomal homeostasis and emphasise its role in neuroprotection.

Hypothalamic Supramammillary Nucleus Selectively Excites Hippocampal CA3 Interneurons to Suppress CA3 Pyramidal Neuron Activity.

A key mode of neuronal communication between distant brain regions is through excitatory synaptic transmission mediated by long-range glutamatergic projections emitted from principal neurons. The long-range glutamatergic projection normally forms numerous en passant excitatory synapses onto both principal neurons and interneurons along its path. Under physiological conditions, the monosynaptic excitatory drive onto postsynaptic principal neurons outweighs disynaptic feedforward inhibition, with the net effect of depolarizing principal neurons. In contrast with this conventional doctrine, here we report that a glutamatergic projection from the hypothalamic supramammillary nucleus (SuM) largely evades postsynaptic pyramidal neurons (PNs), but preferentially target interneurons in the hippocampal CA3 region to predominantly provide feedforward inhibition. Using viral-based retrograde and anterograde tracing and ChannelRhodopsin2 (ChR2)-assisted patch-clamp recording in mice of either sex, we show that SuM projects sparsely to CA3 and provides minimal excitation onto CA3 PNs. Surprisingly, despite its sparse innervation, the SuM input inhibits all CA3 PNs along the transverse axis. Further, we find that SuM provides strong monosynaptic excitation onto CA3 parvalbumin-expressing interneurons evenly along the transverse axis, which likely mediates the SuM-driven feedforward inhibition. Together, our results demonstrate that a novel long-range glutamatergic pathway largely evades principal neurons, but rather preferentially innervates interneurons in a distant brain region to suppress principal neuron activity. Moreover, our findings reveal a new means by which SuM regulates hippocampal activity through SuM-to-CA3 circuit, independent of the previously focused projections from SuM to CA2 or dentate gyrus.SIGNIFICANCE STATEMENT The dominant mode of neuronal communication between brain regions is the excitatory synaptic transmission mediated by long-range glutamatergic projections, which form en passant excitatory synapses onto both pyramidal neurons and interneurons along its path. Under normal conditions, the excitation onto postsynaptic neurons outweighs feedforward inhibition, with the net effect of depolarization. In contrast with this conventional doctrine, here we report that a glutamatergic input from hypothalamic supramammillary nucleus (SuM) largely evades PNs but selectively targets interneurons to almost exclusively provide disynaptic feedforward inhibition onto hippocampal CA3 PNs. Thus, our findings reveal a novel subcortical-hippocampal circuit that enables SuM to regulate hippocampal activity via SuM-CA3 circuit, independent of its projections to CA2 or dentate gyrus.

Muscular Swedish mutant APP-to-Brain axis in the development of Alzheimer's disease.

Alzheimer's disease (AD) is the most common form of dementia. Notably, patients with AD often suffer from severe sarcopenia. However, their direct link and relationship remain poorly understood. Here, we generated a mouse line, TgAPPsweHSA, by crossing LSL (LoxP-STOP-LoxP)-APPswe with HSA-Cre mice, which express APPswe (Swedish mutant APP) selectively in skeletal muscles. Examining phenotypes in TgAPPsweHSA mice showed not only sarcopenia-like deficit, but also AD-relevant hippocampal inflammation, impairments in adult hippocampal neurogenesis and blood brain barrier (BBB), and depression-like behaviors. Further studies suggest that APPswe expression in skeletal muscles induces senescence and expressions of senescence-associated secretory phenotypes (SASPs), which include inflammatory cytokines and chemokines; but decreases growth factors, such as PDGF-BB and BDNF. These changes likely contribute to the systemic and hippocampal inflammation, deficits in neurogenesis and BBB, and depression-like behaviors, revealing a link of sarcopenia with AD, and uncovering an axis of muscular APPswe to brain in AD development.

Radiographic changes of mandibular cortical bone in bisphosphonate drug holiday.

Objectives: There have been few studies to date on the residual effect of bisphosphonate. This study investigated the radiographic changes of mandibular cortical thickness upon bisphosphonate drug holiday. Materials and Methods: This retrospective study includes 36 patients diagnosed with MRONJ (medication-related osteonecrosis of the jaw) at Ajou University Dental Hospital in 2010-2021. All patients stopped taking bisphosphonate under consultation with the prescribing physicians. Panoramic radiographs were taken at the start of discontinuation (T0), 12 months after (T1), and 18 months after (T2) discontinuation of bisphosphonate, respectively. Mental index and panoramic mandibular index were calculated using Ledgerton's method. Paired t-tests were used to analyze differences over time. Results: The difference in indices (mental index and panoramic mandibular index) between T0 and T1 was not statistically significant (paired t-test, P>0.05). However, the difference in these indices between T1 and T2 was statistically significant (paired t-test, P<0.05). Conclusion: The cortical thickness of the mandible decreased in the late stage (after 18 months) as observed by panoramic radiograph.

Critical Role of Neuronal Vps35 in Blood Vessel Branching and Maturation in Developing Mouse Brain.

Vps35 (vacuolar protein sorting 35), a key component of retromer, plays a crucial role in selective retrieval of transmembrane proteins from endosomes to trans-Golgi networks. Dysfunctional Vps35/retromer is a risk factor for the development of neurodegenerative diseases. Vps35 is highly expressed in developing pyramidal neurons, both in the mouse neocortex and hippocampus, Although embryonic neuronal Vps35's function in promoting neuronal terminal differentiation and survival is evident, it remains unclear whether and how neuronal Vps35 communicates with other types of brain cells, such as blood vessels (BVs), which are essential for supplying nutrients to neurons. Dysfunctional BVs contribute to the pathogenesis of various neurodegenerative disorders. Here, we provide evidence for embryonic neuronal Vps35 as critical for BV branching and maturation in the developing mouse brain. Selectively knocking out (KO) Vps35 in mouse embryonic, not postnatal, neurons results in reductions in BV branching and density, arteriole diameter, and BV-associated pericytes and microglia but an increase in BV-associated reactive astrocytes. Deletion of microglia by PLX3397 enhances these BV deficits in mutant mice. These results reveal the function of neuronal Vps35 in neurovascular coupling in the developing mouse brain and implicate BV-associated microglia as underlying this event.

Osteoblastic Swedish mutant APP expedites brain deficits by inducing endoplasmic reticulum stress-driven senescence.

Patients with Alzheimer's disease (AD) often have osteoporosis or osteopenia. However, their direct link and relationship remain largely unclear. Previous studies have detected osteoporotic deficits in young adult Tg2576 and TgAPPsweOCN mice, which express APPswe (Swedish mutant) ubiquitously and selectively in osteoblast (OB)-lineage cells. This raises the question, whether osteoblastic APPswe contributes to AD development. Here, we provide evidence that TgAPPsweOCN mice also exhibit AD-relevant brain pathologies and behavior phenotypes. Some brain pathologies include age-dependent and regional-selective increases in glial activation and pro-inflammatory cytokines, which are accompanied by behavioral phenotypes such as anxiety, depression, and altered learning and memory. Further cellular studies suggest that APPswe, but not APPwt or APPlon (London mutant), in OB-lineage cells induces endoplasmic reticulum-stress driven senescence, driving systemic and cortex inflammation as well as behavioral changes in 6-month-old TgAPPsweOCN mice. These results therefore reveal an unrecognized function of osteoblastic APPswe to brain axis in AD development.

Expression of Low Level of VPS35-mCherry Fusion Protein Diminishes Vps35 Depletion Induced Neuron Terminal Differentiation Deficits and Neurodegenerative Pathology, and Prevents Neonatal Death.

Vps35 (vacuolar protein sorting 35) is a key component of retromer that consists of Vps35, Vps26, and Vps29 trimers, and sortin nexin dimers. Dysfunctional Vps35/retromer is believed to be a risk factor for development of various neurodegenerative diseases. Vps35Neurod6 mice, which selectively knock out Vps35 in Neurod6-Cre+ pyramidal neurons, exhibit age-dependent impairments in terminal differentiation of dendrites and axons of cortical and hippocampal neurons, neuro-degenerative pathology (i.e., increases in P62 and Tdp43 (TAR DNA-binding protein 43) proteins, cell death, and reactive gliosis), and neonatal death. The relationships among these phenotypes and the underlying mechanisms remain largely unclear. Here, we provide evidence that expression of low level of VPS35-mCherry fusion protein in Vps35Neurod6 mice could diminish the phenotypes in an age-dependent manner. Specifically, we have generated a conditional transgenic mouse line, LSL-Vps35-mCherry, which expresses VPS35-mCherry fusion protein in a Cre-dependent manner. Crossing LSL-Vps35-mCherry with Vps35Neurod6 to obtain TgVPS35-mCherry, Vps35Neurod6 mice prevent the neonatal death and diminish the dendritic morphogenesis deficit and gliosis at the neonatal, but not the adult age. Further studies revealed that the Vps35-mCherry transgene expression was low, and the level of Vps35 mRNA comprised only ~5-7% of the Vps35 mRNA of control mice. Such low level of VPS35-mCherry could restore the amount of other retromer components (Vps26a and Vps29) at the neonatal age (P14). Importantly, the neurodegenerative pathology presented in the survived adult TgVps35-mCherry; Vps35Neurod6 mice. These results demonstrate the sufficiency of low level of VPS35-mCherry fusion protein to diminish the phenotypes in Vps35Neurod6 mice at the neonatal age, verifying a key role of neuronal Vps35 in stabilizing retromer complex proteins, and supporting the view for Vps35 as a potential therapeutic target for neurodegenerative diseases.

Hepcidin contributes to Swedish mutant APP-induced osteoclastogenesis and trabecular bone loss.

Patients with Alzheimer's disease (AD) often have lower bone mass than healthy individuals. However, the mechanisms underlying this change remain elusive. Previously, we found that Tg2576 mice, an AD animal model that ubiquitously expresses Swedish mutant amyloid precursor protein (APPswe), shows osteoporotic changes, reduced bone formation, and increased bone resorption. To understand how bone deficits develop in Tg2576 mice, we used a multiplex antibody array to screen for serum proteins that are altered in Tg2576 mice and identified hepcidin, a master regulator of iron homeostasis. We further investigated hepcidin's function in bone homeostasis and found that hepcidin levels were increased not only in the serum but also in the liver, muscle, and osteoblast (OB) lineage cells in Tg2576 mice at both the mRNA and protein levels. We then generated mice selectively expressing hepcidin in hepatocytes or OB lineage cells, which showed trabecular bone loss and increased osteoclast (OC)-mediated bone resorption. Further cell studies suggested that hepcidin increased OC precursor proliferation and differentiation by downregulating ferroportin (FPN) expression and increasing intracellular iron levels. In OB lineage cells, APPswe enhanced hepcidin expression by inducing ER stress and increasing OC formation, in part through hepcidin. Together, these results suggest that increased hepcidin expression in hepatocytes and OB lineage cells in Tg2576 mice contributes to enhanced osteoclastogenesis and trabecular bone loss, identifying the hepcidin-FPN-iron axis as a potential therapeutic target to prevent AD-associated bone loss.

Hippocampal astrocytic neogenin regulating glutamate uptake, a critical pathway for preventing epileptic response.

Epilepsy, a common neurological disorder, is featured with recurrent seizures. Its underlying pathological mechanisms remain elusive. Here, we provide evidence for loss of neogenin (NEO1), a coreceptor for multiple ligands, including netrins and bone morphological proteins, in the development of epilepsy. NEO1 is reduced in hippocampi from patients with epilepsy based on transcriptome and proteomic analyses. Neo1 knocking out (KO) in mouse brains displays elevated epileptiform spikes and seizure susceptibility. These phenotypes were undetectable in mice, with selectively depleted NEO1 in excitatory (NeuroD6-Cre+) or inhibitory (parvalbumin+) neurons, but present in mice with specific hippocampal astrocytic Neo1 KO. Additionally, neurons in hippocampal dentate gyrus, a vulnerable region in epilepsy, in mice with astrocyte-specific Neo1 KO show reductions in inhibitory synaptic vesicles and the frequency of miniature inhibitory postsynaptic current(mIPSC), but increase of the duration of miniature excitatory postsynaptic current and tonic NMDA receptor currents, suggesting impairments in both GABAergic transmission and extracellular glutamate clearance. Further proteomic and cell biological analyses of cell-surface proteins identified GLAST, a glutamate-aspartate transporter that is marked reduced in Neo1 KO astrocytes and the hippocampus. NEO1 interacts with GLAST and promotes GLAST surface distribution in astrocytes. Expressing NEO1 or GLAST in Neo1 KO astrocytes in the hippocampus abolishes the epileptic phenotype. Taken together, these results uncover an unrecognized pathway of NEO1-GLAST in hippocampal GFAP+ astrocytes, which is critical for GLAST surface distribution and function, and GABAergic transmission, unveiling NEO1 as a valuable therapeutic target to protect the brain from epilepsy.

Critical Roles of Embryonic Born Dorsal Dentate Granule Neurons for Activity-Dependent Increases in BDNF, Adult Hippocampal Neurogenesis, and Antianxiety-like Behaviors.

BACKGROUND: Dentate gyrus (DG), a "gate" that controls information flow into the hippocampus, plays important roles in regulating both cognitive (e.g., spatial learning and memory) and mood behaviors. Deficits in DG neurons contribute to the pathogenesis of not only neurological, but also psychiatric, disorders, such as anxiety disorder. Whereas DG's function in spatial learning and memory has been extensively investigated, its role in regulating anxiety remains elusive. METHODS: Using c-Fos to mark DG neuron activation, we identified a group of embryonic born dorsal DG (dDG) neurons, which were activated by anxiogenic stimuli and specifically express osteocalcin (Ocn)-Cre. We further investigated their functions in regulating anxiety and the underlying mechanisms by using a combination of chemogenetic, electrophysiological, and RNA-sequencing methods. RESULTS: The Ocn-Cre+ dDG neurons were highly active in response to anxiogenic environment but had lower excitability and fewer presynaptic inputs than those of Ocn-Cre- or adult born dDG neurons. Activating Ocn-Cre+ dDG neurons suppressed anxiety-like behaviors and increased adult DG neurogenesis, whereas ablating or chronically inhibiting Ocn-Cre+ dDG neurons exacerbated anxiety-like behaviors, impaired adult DG neurogenesis, and abolished activity (e.g., voluntary wheel running)-induced anxiolytic effect and adult DG neurogenesis. RNA-sequencing screening for factors induced by activation of Ocn-Cre+ dDG neurons identified BDNF, which was required for Ocn-Cre+ dDG neurons mediated antianxiety-like behaviors and adult DG neurogenesis. CONCLUSIONS: These results demonstrate critical functions of Ocn-Cre+ dDG neurons in suppressing anxiety-like behaviors but promoting adult DG neurogenesis, and both functions are likely through activation of BDNF.

Astrocytic neogenin/netrin-1 pathway promotes blood vessel homeostasis and function in mouse cortex.

Astrocytes have multiple functions in the brain, including affecting blood vessel (BV) homeostasis and function. However, the underlying mechanisms remain elusive. Here, we provide evidence that astrocytic neogenin (NEO1), a member of deleted in colorectal cancer (DCC) family netrin receptors, is involved in blood vessel homeostasis and function. Mice with Neo1 depletion in astrocytes exhibited clustered astrocyte distribution and increased BVs in their cortices. These BVs were leaky, with reduced blood flow, disrupted vascular basement membranes (vBMs), decreased pericytes, impaired endothelial cell (EC) barrier, and elevated tip EC proliferation. Increased proliferation was also detected in cultured ECs exposed to the conditioned medium (CM) of NEO1-depleted astrocytes. Further screening for angiogenetic factors in the CM identified netrin-1 (NTN1), whose expression was decreased in NEO1-depleted cortical astrocytes. Adding NTN1 into the CM of NEO1-depleted astrocytes attenuated EC proliferation. Expressing NTN1 in NEO1 mutant cortical astrocytes ameliorated phenotypes in blood-brain barrier (BBB), EC, and astrocyte distribution. NTN1 depletion in astrocytes resulted in BV/BBB deficits in the cortex similar to those in Neo1 mutant mice. In aggregate, these results uncovered an unrecognized pathway, astrocytic NEO1 to NTN1, not only regulating astrocyte distribution, but also promoting cortical BV homeostasis and function.

Ependymal Vps35 Promotes Ependymal Cell Differentiation and Survival, Suppresses Microglial Activation, and Prevents Neonatal Hydrocephalus.

Hydrocephalus is a pathologic condition associated with various brain diseases, including Alzheimer's disease (AD). Dysfunctional ependymal cells (EpCs) are believed to contribute to the development of hydrocephalus. It is thus of interest to investigate EpCs' development and function. Here, we report that vacuolar protein sorting-associated protein 35 (VPS35) is critical for EpC differentiation, ciliogenesis, and survival, and thus preventing neonatal hydrocephalus. VPS35 is abundantly expressed in EpCs. Mice with conditional knock-out (cKO) of Vps35 in embryonic (Vps35GFAP-Cre and Vps35Emx1-Cre) or postnatal (Vps35Foxj1-CreER) EpC progenitors exhibit enlarged lateral ventricles (LVs) and hydrocephalus-like pathology. Further studies reveal marked reductions in EpCs and their cilia in both Vps35GFAP-Cre and Vps35Foxj1-CreER mutant mice. The reduced EpCs appear to be due to impairments in EpC differentiation and survival. Additionally, both Vps35GFAP-Cre and Vps35Foxj1-CreER neonatal pups exhibit increased cell proliferation and death largely in a region close to LV-EpCs. Many microglia close to the mutant LV-EpC region become activated. Depletion of the microglia by PLX3397, an antagonist of colony-stimulating factor 1 receptor (CSF1R), restores LV-EpCs and diminishes the pathology of neonatal hydrocephalus in Vps35Foxj1-CreER mice. Taken together, these observations suggest unrecognized functions of Vps35 in EpC differentiation, ciliogenesis, and survival in neonatal LV, and reveal pathologic roles of locally activated microglia in EpC homeostasis and hydrocephalus development.SIGNIFICANCE STATEMENT This study reports critical functions of vacuolar protein sorting-associated protein 35 (VPS35) not only in promoting ependymal cell (EpC) differentiation, ciliogenesis, and survival, but also in preventing local microglial activation. The dysfunctional EpCs and activated microglia are likely to induce hydrocephalus.

Hippocampal Neurogenesis and Neural Circuit Formation in a Cuprizone-Induced Multiple Sclerosis Mouse Model.

Cognitive impairments are key features in multiple sclerosis (MS), a progressive disorder characterized by neuroinflammation-induced demyelination in the central nervous system. To understand the neural substrates that link demyelination to cognitive deficits in MS, we investigated hippocampal neurogenesis and synaptic connectivity of adult-born neurons, which play an essential role in cognitive function. The administration and withdrawal of the combination of cuprizone and rapamycin (Cup/Rap) in C57BL/6J male mice efficiently demyelinated and remyelinated the hippocampus, respectively. In the demyelinated hippocampus, neurogenesis was nearly absent in the dentate gyrus, which was due to inhibited proliferation of neural stem cells (NSCs). Specifically, radial glia-like type 1 NSCs were shifted from a proliferative state to a mitotically-quiescent state in the demyelinated hippocampus. In addition, dendritic spine densities of adult-born neurons were significantly decreased, indicating a reduction in synaptic connections between hippocampal newborn neurons and excitatory input neurons. Concomitant with hippocampal remyelination induced by withdrawal of Cup/Rap, proliferation of type 1 NSCs and dendritic spine densities of adult-born neurons reverted to normal in the hippocampus. Our study shows that proliferation of hippocampal NSCs and synaptic connectivity of adult-born neurons are inversely correlated with the level of demyelination, providing critical insight into hippocampal neurogenesis as a potential therapeutic target to treat cognitive deficits associated with MS.SIGNIFICANCE STATEMENT To identify the neural substrates that mediate cognitive dysfunctions associated with a majority of MS patients, we investigated hippocampal neurogenesis and structural development of adult-born neurons using a Cup/Rap model, which recapitulates the hippocampal demyelination that occurs in MS patients. A shift of NSCs from a proliferatively-active state to mitotically-quiescent state dramatically decreased neurogenesis in the demyelinated hippocampus. Formation of dendritic spines on newborn neurons was also impaired following demyelination. Interestingly, the altered neurogenesis and synaptic connectivity of newborn neurons were reversed to normal levels during remyelination. Thus, our study revealed reversible genesis and synaptic connectivity of adult-born neurons between the demyelinated and remyelinated hippocampus, suggesting hippocampal neurogenesis as a potential target to normalize cognitive impairments in MS patients.

Activity of hippocampal adult-born neurons regulates alcohol withdrawal seizures.

Alcohol withdrawal (AW) after chronic alcohol exposure produces a series of symptoms, with AW-associated seizures being among the most serious and dangerous. However, the mechanism underlying AW seizures has yet to be established. In our mouse model, a sudden AW produced 2 waves of seizures: the first wave includes a surge of multiple seizures that occurs within hours to days of AW, and the second wave consists of sustained expression of epileptiform spikes and wave discharges (SWDs) during a protracted period of abstinence. We revealed that the structural and functional adaptations in newborn dentate granule cells (DGCs) in the hippocampus underlie the second wave of seizures but not the first wave. While the general morphology of newborn DGCs remained unchanged, AW increased the dendritic spine density of newborn DGCs, suggesting that AW induced synaptic connectivity of newborn DGCs with excitatory afferent neurons and enhanced excitability of newborn DGCs. Indeed, specific activation and suppression of newborn DGCs by the chemogenetic DREADD method increased and decreased the expression of epileptiform SWDs, respectively, during abstinence. Thus, our study unveiled that the pathological plasticity of hippocampal newborn DGCs underlies AW seizures during a protracted period of abstinence, providing critical insight into hippocampal neural circuits as a foundation to understand and treat AW seizures.

Pten loss results in inappropriate excitatory connectivity.

Pten mutations are associated with autism spectrum disorder. Pten loss of function in neurons increases excitatory synaptic connectivity, contributing to an imbalance between excitation and inhibition. We aimed to determine whether Pten loss results in aberrant connectivity in neural circuits. We compared postnatally generated wild-type and Pten knockout granule neurons integrating into the dentate gyrus using a variety of methods to examine their connectivity. We found that postsynaptic Pten loss provides an advantage to dendritic spines in competition over a limited pool of presynaptic boutons. Retrograde monosynaptic tracing with rabies virus reveals that this results in synaptic contact with more presynaptic partners. Using independently excitable opsins to interrogate multiple inputs onto a single neuron, we found that excess connectivity is established indiscriminately from among glutamatergic afferents. Therefore, Pten loss results in inappropriate connectivity whereby neurons are coupled to a greater number of synaptic partners.

Setd5 haploinsufficiency alters neuronal network connectivity and leads to autistic-like behaviors in mice.

SETD5, a gene linked to intellectual disability (ID) and autism spectrum disorder (ASD), is a member of the SET-domain family and encodes a putative histone methyltransferase (HMT). To date, the mechanism by which SETD5 haploinsufficiency causes ASD/ID remains an unanswered question. Setd5 is the highly conserved mouse homolog, and although the Setd5 null mouse is embryonic lethal, the heterozygote is viable. Morphological tracing and multielectrode array was used on cultured cortical neurons. MRI was conducted of adult mouse brains and immunohistochemistry of juvenile mouse brains. RNA-Seq was used to investigate gene expression in the developing cortex. Behavioral assays were conducted on adult mice. Setd5+/- cortical neurons displayed significantly reduced synaptic density and neuritic outgrowth in vitro, with corresponding decreases in network activity and synchrony by electrophysiology. A specific subpopulation of fetal Setd5+/- cortical neurons showed altered gene expression of neurodevelopment-related genes. Setd5+/- animals manifested several autism-like behaviors, including hyperactivity, cognitive deficit, and altered social interactions. Anatomical differences were observed in Setd5+/- adult brains, accompanied by a deficit of deep-layer cortical neurons in the developing brain. Our data converge on a picture of abnormal neurodevelopment driven by Setd5 haploinsufficiency, consistent with a highly penetrant risk factor.

Chemogenetic silencing of hippocampal neurons suppresses epileptic neural circuits.

We investigated how pathological changes in newborn hippocampal dentate granule cells (DGCs) lead to epilepsy. Using a rabies virus-mediated retrograde tracing system and a designer receptors exclusively activated by designer drugs (DREADD) chemogenetic method, we demonstrated that newborn hippocampal DGCs are required for the formation of epileptic neural circuits and the induction of spontaneous recurrent seizures (SRS). A rabies virus-mediated mapping study revealed that aberrant circuit integration of hippocampal newborn DGCs formed excessive de novo excitatory connections as well as recurrent excitatory loops, allowing the hippocampus to produce, amplify, and propagate excessive recurrent excitatory signals. In epileptic mice, DREADD-mediated-specific suppression of hippocampal newborn DGCs dramatically reduced epileptic spikes and SRS in an inducible and reversible manner. Conversely, specific activation of hippocampal newborn DGCs increased both epileptic spikes and SRS. Our study reveals an essential role for hippocampal newborn DGCs in the formation and function of epileptic neural circuits, providing critical insights into DGCs as a potential therapeutic target for treating epilepsy.