The β-site amyloid precursor protein cleaving enzyme 1 (BACE1) is necessary to generate the Aβ peptide, which is implicated in Alzheimer's disease pathology. Studies show that the expression of BACE1 ...and its protease activity are tightly regulated, but the physiological function of BACE1 remains poorly understood. Recently, numerous axon guidance proteins were identified as potential substrates of BACE1. Here, we examined the consequences of loss of BACE1 function in a well-defined in vivo model system of axon guidance, mouse olfactory sensory neurons (OSNs). The BACE1 protein resides predominantly in proximal segment and the termini of OSN axons, and the expression of BACE1 inversely correlates with odor-evoked neural activity. The precision of targeting of OSN axons is disturbed in both BACE1 null and, surprisingly, in BACE1 heterozygous mice. We propose that BACE1 cleavage of axon guidance proteins is essential to maintain the connectivity of OSNs in vivo.
Activity-dependent competition that operates on branch stability or formation plays a critical role in shaping the pattern and complexity of axonal terminal arbors. In the mammalian central nervous ...system (CNS), the effect of activity-dependent competition on axon arborization and on the assembly of sensory maps is well established. However, the molecular pathways that modulate axonal-branch stability or formation in competitive environments remain unknown.
We establish an in vivo axonal-competition paradigm in the mouse olfactory system by employing a genetic strategy that permits suppression of neurosecretory activity in random subsets of olfactory sensory neurons (OSNs). Long-term follow up confirmed that this genetic manipulation triggers competition by revealing a bias toward selective stabilization of active arbors and local degeneration of synaptically silent ones. By using a battery of genetically modified mouse models, we demonstrate that a decrease either in the total levels or the levels of activity-dependent secreted BDNF (due to a val66met substitution), rescues silent arbors from withering. We show that this effect may be mediated, at least in part, by p75(NTR).
We establish and experimentally validate a genetic in vivo axonal-competition paradigm in the mammalian CNS. By using this paradigm, we provide evidence for a specific effect of BDNF signaling on terminal-arbor pruning under competition in vivo. Our results have implications for the formation and refinement of the olfactory and other sensory maps, as well as for neuropsychiatric diseases and traits modulated by the BDNF val66met variant.
The BKCa channel, a potassium channel that is allosterically activated by voltage and calcium, is expressed in both excitable and non-excitable cells. The channel plays an important role in ...regulating membrane excitability. The channel activity can be modulated by post-translational modifications such as phosphorylation. Recently, hippocampal BKCa channels were shown to be directly modulated by assembly/disassembly of the submembranous actin cytoskeleton. Here, we report that the BKCa channel physically interacts with the light chain of microtubule associated protein 1A (MAP1A). The light chain was isolated in a yeast two-hybrid screen of a human brain cDNA library. The specificity of the interaction was demonstrated in biochemical experiments utilizing GST fusion protein pulldown assays and reciprocal co-immunoprecipitations from rat brain. Furthermore, utilizing immunofluorescence, the BKCa channel and MAP1A co-localize in the Purkinje cell layer of the cerebellum. These studies identify a novel interaction between the C-terminal tail of the BKCa channel and the light chain of MAP1A, which enables channel association with and modulation by the cytoskeleton.
Individuals with 22q11.2 microdeletions have cognitive deficits and a high risk of developing schizophrenia. Here we provide evidence that primary hippocampal neurons from a mouse model of 22q11.2 ...deletion (Df(16)A(+/-) mice) have decreased density of dendritic spines and glutamatergic synapses, as well as impaired dendritic growth. These deficits were prevented by introduction of the enzymatically active ZDHHC8 palmitoyltransferase encoded by a gene in the 22q11.2 locus, and they were also observed in primary cultures from Zdhhc8-deficient mice. Many of these deficits were also present in the hippocampi of adult Df(16)A(+/-) and Zdhhc8-deficient mice. Finally, we provide evidence that PSD95 is one of the substrates of ZDHHC8. Our analysis reveals that 22q11.2 microdeletion results in deficits in neuronal development and suggests that impaired neuronal protein palmitoylation contributes to many of these deficits.
Triggers of innate immune signaling in the CNS of patients with amyotrophic lateral sclerosis and frontotemporal degeneration (ALS/FTD) remain elusive. We report the presence of cytoplasmic ...double-stranded RNA (cdsRNA), an established trigger of innate immunity, in ALS-FTD brains carrying
intronic hexanucleotide expansions that included genomically encoded expansions of the G
C
repeat sequences. The presence of cdsRNA in human brains was coincident with cytoplasmic TAR DNA binding protein 43 (TDP-43) inclusions, a pathologic hallmark of ALS/FTD. Introducing cdsRNA into cultured human neural cells induced type I interferon (IFN-I) signaling and death that was rescued by FDA-approved JAK inhibitors. In mice, genomically encoded dsRNAs expressed exclusively in a neuronal class induced IFN-I and death in connected neurons non-cell-autonomously. Our findings establish that genomically encoded cdsRNAs trigger sterile, viral-mimetic IFN-I induction and propagated death within neural circuits and may drive neuroinflammation and neurodegeneration in patients with ALS/FTD.
Previous studies indicate that the gaseous messengers carbon monoxide (CO) and nitric oxide (NO) can interact to cause robust increases in intracellular cGMP levels in the retina. The purpose of the ...present study was to investigate the biochemical basis of the interactions between NO and CO for these increases. Turtle retinas were incubated in vitro with CO to stimulate cGMP production in the presence or absence of the nitric oxide synthase inhibitors
N-ω-nitro-
l-arginine methyl ester and
S-methyl-thiocitrulline. Cyclic GMP immunocytochemistry was then used to evaluate the changes in cGMP levels in response to these stimuli. The results indicated that CO itself stimulated increases in cGMP in bipolar and amacrine cells, and that the increases were completely blocked by SMTC and
l-NAME. We postulate that the increases of cGMP in response to CO might be mediated, at least partly, by CO displacing and releasing NO from its intracellular storage pool(s).
Recent studies imaging nitric oxide (NO) production in the retina have indicated a much wider distribution of NO production than would be suggested by previous light-microscopic localizations of ...neuronal nitric oxide synthase (nNOS). To help resolve this discrepancy, the present study analyzed the ultrastructural localization of nNOS-like immunoreactivity (-LI) in all layers of the retina. In the ellipsoids of rod photoreceptors and the accessory elements of double cones, nNOS-LI was associated with some atypical mitochondria. In the outer plexiform layer, nNOS-LI was in some postsynaptic horizontal and bipolar cell processes at photoreceptor ribbon synapses. In some amacrine and ganglion cell somata, nNOS-LI was diffusely localized in the cytoplasm and associated with the endoplasmic reticulum. In the inner plexiform layer, nNOS-LI diffusely filled some amacrine cell processes, while in other amacrine cells nNOS-LI was selectively localized at the presynaptic specializations of conventional synapses. Neuronal NOS-LI was also found at membrane specializations in bipolar cell terminals that were distinct from their normal ribbon synapses. Finally, some nNOS-LI was found in mitochondria in Muller cells. The diverse subcellular localizations of nNOS-LI indicates that NO may play distinct functional roles in many retinal cells, which correlates well with the widespread NO production found in previous NO imaging studies.
Heme oxygenase-2 (HO-2) synthesizes carbon monoxide (CO), a modulator of soluble guanylate cyclase (sGC). To examine this signal transduction pathway in the retina, we immunocytochemically localized ...HO-2, and investigated the effects of CO on cGMP levels. In turtle, HO-2-like immunoreactivity (-LI) was in all photoreceptors, some amacrine cells, and in numerous bipolar and ganglion cells. HO-2-LI colocalized with sGC activity in many cells. In rat, HO-LI was found only in the inner retina, in ganglion and amacrine cells. In turtle, stimulation with CO alone primarily increased cGMP-LI in bipolar cells in the visual streak. Stimulation with a combination of CO and nitric oxide (NO) dramatically increased cGMP-LI throughout the retina, in comparison to the smaller increases seen with NO or CO alone. These data suggest that CO is an endogenous modulator of the sGC/cGMP signaling pathway in many retinal neurons, and can dramatically amplify NO-stimulated increases in cGMP.
The amyloid beta peptide aggregates into amyloid plaques at presymptomatic stages of Alzheimer's disease, but the temporal relationship between plaque formation and neuronal dysfunction is poorly ...understood. Here we demonstrate that the connectivity of the peripheral olfactory neural circuit is perturbed in mice overexpressing human APPsw (Swedish mutation) before the onset of plaques. Expression of human APPsw exclusively in olfactory sensory neurons also perturbs connectivity with associated reductions in odour-evoked gene expression and olfactory acuity. By contrast, olfactory sensory neuron axons project correctly in mice overexpressing wild-type human amyloid precursor protein throughout the brain and in mice overexpressing M671V human APP, a missense mutation that reduces amyloid beta production, exclusively in olfactory sensory neurons. Furthermore, expression of Aβ40 or Aβ42 solely in the olfactory epithelium disrupts the olfactory sensory neuron axon targeting. Our data indicate that altering the structural connectivity and function of highly plastic neural circuits is one of the pleiotropic actions of soluble human amyloid beta.
Olfactory dysfunction is broadly associated with neurodevelopmental and neurodegenerative diseases and predicts increased mortality rates in healthy individuals. Conventional measurements of ...olfactory health assess odor processing pathways within the brain and provide a limited understanding of primary odor detection. Quantification of the olfactory sensory neurons (OSNs), which detect odors within the nasal cavity, would provide insight into the etiology of olfactory dysfunction associated with disease and mortality. Notably, OSNs are continually replenished by adult neurogenesis in mammals, including humans, so OSN measurements are primed to provide specialized insights into neurological disease. Here, we have evaluated a PET radiotracer, 11CGV1-57, that specifically binds mature OSNs and quantifies the mature OSN population in vivo. 11CGV1-57 monitored native OSN population dynamics in rodents, detecting OSN generation during postnatal development and aging-associated neurodegeneration. 11CGV1-57 additionally measured rates of neuron regeneration after acute injury and early-stage OSN deficits in a rodent tauopathy model of neurodegenerative disease. Preliminary assessment in nonhuman primates suggested maintained uptake and saturable binding of 18FGV1-57 in primate nasal epithelium, supporting its translational potential. Future applications for GV1-57 include monitoring additional diseases or conditions associated with olfactory dysregulation, including cognitive decline, as well as monitoring effects of neuroregenerative or neuroprotective therapeutics.
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