Protocol for in vivo imaging and analysis of brainstem neuronal activity in the dorsal raphe nucleus of freely behaving mice.

In vivo brainstem imaging with miniature microscopy has been challenging due to surgical difficulty, high motion, and correlated activity between neurons. Here, we present a protocol for brainstem imaging in freely moving mice using the dorsal raphe nucleus as an example. We describe surgical procedures to inject a virus encoding GCaMP6m and securely implant a GRIN lens in the brainstem. We then detail motion correction and cell segmentation from the data to parse single-cell activity from correlated networks. For complete details on the use and execution of this protocol, please refer to Paquelet et al. (2022).1.

Single-cell activity and network properties of dorsal raphe nucleus serotonin neurons during emotionally salient behaviors.

The serotonin system modulates a wide variety of emotional behaviors and states, including reward processing, anxiety, and social interaction. To reveal the underlying patterns of neural activity, we visualized serotonergic neurons in the dorsal raphe nucleus (DRN5-HT) of mice using miniaturized microscopy during diverse emotional behaviors. We discovered ensembles of cells with highly correlated activity and found that DRN5-HT neurons are preferentially recruited by emotionally salient stimuli as opposed to neutral stimuli. Individual DRN5-HT neurons responded to diverse combinations of salient stimuli, with some preference for valence and sensory modality. Anatomically defined subpopulations projecting to either a reward-related structure (the ventral tegmental area) or an anxiety-related structure (the bed nucleus of the stria terminalis) contained all response types but were enriched in reward- and anxiety-responsive cells, respectively. Our results suggest that the DRN serotonin system responds to emotional salience using ensembles with mixed selectivity and biases in downstream connectivity.

An increase in spontaneous activity mediates visual habituation.

The cerebral cortex is spontaneously active, but the function of this ongoing activity remains unclear. To test whether spontaneous activity encodes learned experiences, we measured the response of neuronal populations in mouse primary visual cortex with chronic two-photon calcium imaging during visual habituation to a specific oriented stimulus. We find that, during habituation, spontaneous activity increases in neurons across the full range of orientation selectivity, eventually matching that of evoked levels. This increase in spontaneous activity robustly correlates with the degree of habituation. Moreover, boosting spontaneous activity with two-photon optogenetic stimulation to the levels of visually evoked activity accelerates habituation. Our study shows that cortical spontaneous activity is linked to habituation, and we propose that habituation unfolds by minimizing the difference between spontaneous and stimulus-evoked activity levels. We conclude that baseline spontaneous activity could gate incoming sensory information to the cortex based on the learned experience of the animal.

Design, Synthesis, and Biophysical Evaluation of Mechanism-Based Probes for Condensation Domains of Nonribosomal Peptide Synthetases.

Nonribosomal peptide synthetases (NRPSs) are remarkable modular enzymes that synthesize peptide natural products. The condensation (C) domain catalyzes the key amide bond-forming reaction, but structural characterization with bound donor and acceptor substrates has proven elusive. We describe the chemoenzymatic synthesis of condensation domain probes C1 and C2 designed to cross-link the donor and acceptor substrates within the condensation domain active site. These pantetheine probes contain nonhydrolyzable ketone and α,α-difluoroketone isosteres of the native thioester linkage. Using the bimodular NRPS responsible for synthesis of the siderophore enterobactin as a model system, probe C2 was shown by surface plasmon resonance (SPR) to stabilize an intermolecular interaction between the peptidyl carrier protein (PCP) and C domains in EntB and EntF, respectively, with a dissociation constant of 1-2 nM, whereas the unmodified holo-EntB showed no interaction with EntF. The described condensation domain chemical probes provide powerful tools to study dynamic multifunctional NRPS systems.

Structural insight into substrate and product binding in an archaeal mevalonate kinase.

Mevalonate kinase (MK) is a key enzyme of the mevalonate pathway, which produces the biosynthetic precursors for steroids, including cholesterol, and isoprenoids, the largest class of natural products. Currently available crystal structures of MK from different organisms depict the enzyme in its unbound, substrate-bound, and inhibitor-bound forms; however, until now no structure has yet been determined of MK bound to its product, 5-phosphomevalonate. Here, we present crystal structures of mevalonate-bound and 5-phosphomevalonate-bound MK from Methanosarcina mazei (MmMK), a methanogenic archaeon. In contrast to the prior structure of a eukaryotic MK bound with mevalonate, we find a striking lack of direct interactions between this archaeal MK and its substrate. Further, these two MmMK structures join the prior structure of the apoenzyme to complete the first suite of structural snapshots that depict unbound, substrate-bound, and product-bound forms of the same MK. With this collection of structures, we now provide additional insight into the catalytic mechanism of this biologically essential enzyme.

Structural Features and Domain Movements Controlling Substrate Binding and Cofactor Specificity in Class II HMG-CoA Reductase.

The key mevalonate pathway enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (HMGR) uses the cofactor NAD(P)H to reduce HMG-CoA to mevalonate in the production of countless metabolites and natural products. Although inhibition of HMGR by statin drugs is well-understood, several mechanistic details of HMGR catalysis remain unresolved, and the structural basis for the wide range of cofactor specificity for either NADH or NADPH among HMGRs from different organisms is also unknown. Here, we present crystal structures of HMGR from Streptococcus pneumoniae (SpHMGR) alongside kinetic data of the enzyme's cofactor preferences. Our structure of SpHMGR bound with its kinetically preferred NADPH cofactor suggests how NADPH-specific binding and recognition are achieved. In addition, our structure of HMG-CoA-bound SpHMGR reveals large, previously unknown conformational domain movements that may control HMGR substrate binding and enable cofactor exchange without intermediate release during the catalytic cycle. Taken together, this work provides critical new insights into both the HMGR reaction mechanism and the structural basis of cofactor specificity.

Structures of a Nonribosomal Peptide Synthetase Module Bound to MbtH-like Proteins Support a Highly Dynamic Domain Architecture.

Nonribosomal peptide synthetases (NRPSs) produce a wide variety of peptide natural products. During synthesis, the multidomain NRPSs act as an assembly line, passing the growing product from one module to the next. Each module generally consists of an integrated peptidyl carrier protein, an amino acid-loading adenylation domain, and a condensation domain that catalyzes peptide bond formation. Some adenylation domains interact with small partner proteins called MbtH-like proteins (MLPs) that enhance solubility or activity. A structure of an MLP bound to an adenylation domain has been previously reported using a truncated adenylation domain, precluding any insight that might be derived from understanding the influence of the MLP on the intact adenylation domain or on the dynamics of the entire NRPS module. Here, we present the structures of the full-length NRPS EntF bound to the MLPs from Escherichia coli and Pseudomonas aeruginosa These new structures, along with biochemical and bioinformatics support, further elaborate the residues that define the MLP-adenylation domain interface. Additionally, the structures highlight the dynamic behavior of NRPS modules, including the module core formed by the adenylation and condensation domains as well as the orientation of the mobile thioesterase domain.

Structural Biology of Nonribosomal Peptide Synthetases.

The nonribosomal peptide synthetases are modular enzymes that catalyze synthesis of important peptide products from a variety of standard and non-proteinogenic amino acid substrates. Within a single module are multiple catalytic domains that are responsible for incorporation of a single residue. After the amino acid is activated and covalently attached to an integrated carrier protein domain, the substrates and intermediates are delivered to neighboring catalytic domains for peptide bond formation or, in some modules, chemical modification. In the final module, the peptide is delivered to a terminal thioesterase domain that catalyzes release of the peptide product. This multi-domain modular architecture raises questions about the structural features that enable this assembly line synthesis in an efficient manner. The structures of the core component domains have been determined and demonstrate insights into the catalytic activity. More recently, multi-domain structures have been determined and are providing clues to the features of these enzyme systems that govern the functional interaction between multiple domains. This chapter describes the structures of NRPS proteins and the strategies that are being used to assist structural studies of these dynamic proteins, including careful consideration of domain boundaries for generation of truncated proteins and the use of mechanism-based inhibitors that trap interactions between the catalytic and carrier protein domains.

Structures of two distinct conformations of holo-non-ribosomal peptide synthetases.

Many important natural products are produced by multidomain non-ribosomal peptide synthetases (NRPSs). During synthesis, intermediates are covalently bound to integrated carrier domains and transported to neighbouring catalytic domains in an assembly line fashion. Understanding the structural basis for catalysis with non-ribosomal peptide synthetases will facilitate bioengineering to create novel products. Here we describe the structures of two different holo-non-ribosomal peptide synthetase modules, each revealing a distinct step in the catalytic cycle. One structure depicts the carrier domain cofactor bound to the peptide bond-forming condensation domain, whereas a second structure captures the installation of the amino acid onto the cofactor within the adenylation domain. These structures demonstrate that a conformational change within the adenylation domain guides transfer of intermediates between domains. Furthermore, one structure shows that the condensation and adenylation domains simultaneously adopt their catalytic conformations, increasing the overall efficiency in a revised structural cycle. These structures and the single-particle electron microscopy analysis demonstrate a highly dynamic domain architecture and provide the foundation for understanding the structural mechanisms that could enable engineering of novel non-ribosomal peptide synthetases.

The current state of the neurogenic theory of depression and anxiety.

Newborn neurons are continuously added to the adult hippocampus. Early studies found that adult neurogenesis is impaired in models of depression and anxiety and accelerated by antidepressant treatment. This led to the theory that depression results from impaired adult neurogenesis and restoration of adult neurogenesis leads to recovery. Follow up studies yielded a complex body of often inconsistent results, and the veracity of this theory is uncertain. We propose five criteria for acceptance of this theory, we review the recent evidence for each criterion, and we draw the following conclusions: Diverse animal models of depression and anxiety have impaired neurogenesis. Neurogenesis is consistently boosted by antidepressants in animal models only when animals are stressed. Ablation of neurogenesis in animal models impairs cognitive functions relevant to depression, but only a minority of studies find that ablation causes depression or anxiety. Recent human neuroimaging and postmortem studies are consistent with the neurogenic theory, but they are indirect. Finally, a novel drug developed based on the neurogenic theory is promising in animal models.

Analysis of the linker region joining the adenylation and carrier protein domains of the modular nonribosomal peptide synthetases.

Nonribosomal peptide synthetases (NRPSs) are multimodular proteins capable of producing important peptide natural products. Using an assembly line process, the amino acid substrate and peptide intermediates are passed between the active sites of different catalytic domains of the NRPS while bound covalently to a peptidyl carrier protein (PCP) domain. Examination of the linker sequences that join the NRPS adenylation and PCP domains identified several conserved proline residues that are not found in standalone adenylation domains. We examined the roles of these proline residues and neighboring conserved sequences through mutagenesis and biochemical analysis of the reaction catalyzed by the adenylation domain and the fully reconstituted NRPS pathway. In particular, we identified a conserved LPxP motif at the start of the adenylation-PCP linker. The LPxP motif interacts with a region on the adenylation domain to stabilize a critical catalytic lysine residue belonging to the A10 motif that immediately precedes the linker. Further, this interaction with the C-terminal subdomain of the adenylation domain may coordinate movement of the PCP with the conformational change of the adenylation domain. Through this work, we extend the conserved A10 motif of the adenylation domain and identify residues that enable proper adenylation domain function.

SCG10 is a JNK target in the axonal degeneration pathway.

Axons actively self-destruct following genetic, mechanical, metabolic, and toxic insults, but the mechanism of axonal degeneration is poorly understood. The JNK pathway promotes axonal degeneration shortly after axonal injury, hours before irreversible axon fragmentation ensues. Inhibition of JNK activity during this period delays axonal degeneration, but critical JNK substrates that facilitate axon degeneration are unknown. Here we show that superior cervical ganglion 10 (SCG10), an axonal JNK substrate, is lost rapidly from mouse dorsal root ganglion axons following axotomy. SCG10 loss precedes axon fragmentation and occurs selectively in the axon segments distal to transection that are destined to degenerate. Rapid SCG10 loss after injury requires JNK activity. The JNK phosphorylation sites on SCG10 are required for its rapid degradation, suggesting that direct JNK phosphorylation targets SCG10 for degradation. We present a mechanism for the selective loss of SCG10 distal to the injury site. In healthy axons, SCG10 undergoes rapid JNK-dependent degradation and is replenished by fast axonal transport. Injury blocks axonal transport and the delivery of SCG10, leading to the selective loss of the labile SCG10 distal to the injury site. SCG10 loss is functionally important: Knocking down SCG10 accelerates axon fragmentation, whereas experimentally maintaining SCG10 after injury promotes mitochondrial movement and delays axonal degeneration. Taken together, these data support the model that SCG10 is an axonal-maintenance factor whose loss is permissive for execution of the injury-induced axonal degeneration program.

Increased vesicular glutamate transporter expression causes excitotoxic neurodegeneration.

Increases in vesicular glutamate transporter (VGLUT) levels are observed after a variety of insults including hypoxic injury, stress, methamphetamine treatment, and in genetic seizure models. Such overexpression can cause an increase in the amount of glutamate released from each vesicle, but it is unknown whether this is sufficient to induce excitotoxic neurodegeneration. Here we show that overexpression of the Drosophila vesicular glutamate transporter (DVGLUT) leads to excess glutamate release, with some vesicles releasing several times the normal amount of glutamate. Increased DVGLUT expression also leads to an age-dependent loss of motor function and shortened lifespan, accompanied by a progressive neurodegeneration in the postsynaptic targets of the DVGLUT-overexpressing neurons. The early onset lethality, behavioral deficits, and neuronal pathology require overexpression of a functional DVGLUT transgene. Thus overexpression of DVGLUT is sufficient to generate excitotoxic neuropathological phenotypes and therefore reducing VGLUT levels after nervous system injury or stress may mitigate further damage.

Amyloid precursor protein cleavage-dependent and -independent axonal degeneration programs share a common nicotinamide mononucleotide adenylyltransferase 1-sensitive pathway.

Axonal degeneration is a hallmark of many debilitating neurological disorders and is thought to be regulated by mechanisms distinct from those governing cell body death. Recently, caspase 6 activation via amyloid precursor protein (APP) cleavage and activation of DR6 was discovered to induce axon degeneration after NGF withdrawal. We tested whether this pathway is involved in axonal degeneration caused by withdrawal of other trophic support, axotomy or vincristine exposure. Neurturin deprivation, like NGF withdrawal activated this APP/DR6/caspase 6 pathway and resulted in axonal degeneration, however, APP cleavage and caspase 6 activation were not involved in axonal degeneration induced by mechanical or toxic insults. However, loss of surface APP (sAPP) and caspase 6 activation were observed during axonal degeneration induced by dynactin 1(Dctn1) dysfunction, which disrupts axonal transport. Mutations in Dctn1 are associated with motor neuron disease and frontal temporal dementia, thus suggesting that the APP/caspase 6 pathway could be important in specific types of disease-associated axonal degeneration. The NGF deprivation paradigm, with its defined molecular pathway, was used to examine the context of Nmnat-mediated axonal protection. We found that although Nmnat blocks axonal degeneration after trophic factor withdrawal, it did not prevent loss of axon sAPP or caspase 6 activation within the axon, suggesting it acts downstream of caspase 6. These results indicate that diverse insults induce axonal degeneration via multiple pathways and that these degeneration signals converge on a common, Nmnat-sensitive program that is uniquely involved in axonal, but not cell body, degeneration.

Vesicular neurotransmitter transporter trafficking in vivo: moving from cells to flies.

During exocytosis, classical and amino acid neurotransmitters are released from the lumen of synaptic vesicles to allow signaling at the synapse. The storage of neurotransmitters in synaptic vesicles and other types of secretory vesicles requires the activity of specific vesicular transporters. Glutamate and monoamines such as dopamine are packaged by VGLUTs and VMATs respectively. Changes in the localization of either protein have the potential to up- or down regulate neurotransmitter release, and some of the mechanisms for sorting these proteins to secretory vesicles have been investigated in cultured cells in vitro. We have used Drosophila molecular genetic techniques to study vesicular transporter trafficking in an intact organism and have identified a motif required for localizing Drosophila VMAT (DVMAT) to synaptic vesicles in vivo. In contrast to DVMAT, large deletions of Drosophila VGLUT (DVGLUT) show relatively modest deficits in localizing to synaptic vesicles, suggesting that DVMAT and DVGLUT may undergo different modes of trafficking at the synapse. Further in vivo studies of DVMAT trafficking mutants will allow us to determine how changes in the localization of vesicular transporters affect the nervous system as a whole and complex behaviors mediated by aminergic circuits.

A tyrosine-based motif localizes a Drosophila vesicular transporter to synaptic vesicles in vivo.

Vesicular neurotransmitter transporters must localize to synaptic vesicles (SVs) to allow regulated neurotransmitter release at the synapse. However, the signals required to localize vesicular proteins to SVs in vivo remain unclear. To address this question we have tested the effects of mutating proposed trafficking domains in Drosophila orthologs of the vesicular monoamine and glutamate transporters, DVMAT-A and DVGLUT. We show that a tyrosine-based motif (YXXY) is important both for DVMAT-A internalization from the cell surface in vitro, and localization to SVs in vivo. In contrast, DVGLUT deletion mutants that lack a putative C-terminal trafficking domain show more modest defects in both internalization in vitro and trafficking to SVs in vivo. Our data show for the first time that mutation of a specific trafficking motif can disrupt localization to SVs in vivo and suggest possible differences in the sorting of VMATs versus VGLUTs to SVs at the synapse.

A dual leucine kinase-dependent axon self-destruction program promotes Wallerian degeneration.

Axon degeneration underlies many common neurological disorders, but the signaling pathways that orchestrate axon degeneration are unknown. We found that dual leucine kinase (DLK) [corrected to add (DLK) abbreviation] promoted degeneration of severed axons in Drosophila and mice, and that its target, c-Jun N-terminal kinase, promoted degeneration locally in axons as they committed to degenerate. This pathway also promoted degeneration after chemotherapy exposure and may be a component of a general axon self-destruction program.

The requirement for Phr1 in CNS axon tract formation reveals the corticostriatal boundary as a choice point for cortical axons.

Phr1 is the single well-conserved murine ortholog of the invertebrate ubiquitin ligase genes highwire (in Drosophila) and rpm-1 (in Caenorhabditis elegans). The function and mechanism of action of highwire and rpm-1 are similar--both cell-autonomously regulate synaptogenesis by down-regulating the ortholog of the mitogen-activated protein kinase kinase kinase dual leucine zipper kinase (MAPKKK DLK). Here, using a targeted conditional mutant, we demonstrate that Phr1 also plays essential roles in mammalian neural development. As in invertebrates, Phr1 functions cell-autonomously to sculpt motor nerve terminals. In addition, Phr1 plays essential roles in the formation of major CNS axon tracts including those of the internal capsule, in part via cell-nonautonomous mechanisms, and these results reveal a choice point for cortical axons at the corticostriatal boundary. Furthermore, whereas the neurite morphology phenotypes of highwire and rpm-1 are suppressed by loss of DLK in flies and worms, Phr1-dependent CNS defects persist in Phr1, DLK double mutants. Thus, in the mammalian nervous system Phr1 is required for formation of major CNS axon tracts via a mechanism that is both cell-nonautonomous and independent of DLK.

Characterization of biophysical and metabolic properties of cells labeled with superparamagnetic iron oxide nanoparticles and transfection agent for cellular MR imaging.

PURPOSE: To evaluate the effect of using the ferumoxides-poly-l-lysine (PLL) complex for magnetic cell labeling on the long-term viability, function, metabolism, and iron utilization of mammalian cells. MATERIALS AND METHODS: PLL was incubated with ferumoxides for 60 minutes, incompletely coating the superparamagnetic iron oxide (SPIO) through electrostatic interactions. Cells were coincubated overnight with the ferumoxides-PLL complex, and iron uptake, cell viability, apoptosis indexes, and reactive oxygen species formation were evaluated. The disappearance or the life span of the detectable iron nanoparticles in cells was also evaluated. The iron concentrations in the media also were assessed at different time points. Data were expressed as the mean +/- 1 SD, and one-way analysis of variance and the unpaired Student t test were used to test for significant differences. RESULTS: Intracytoplasmic nanoparticles were stained with Prussian blue when the ferumoxides-PLL complex had magnetically labeled the human mesenchymal stem and HeLa cells. The long-term viability, growth rate, and apoptotic indexes of the labeled cells were unaffected by the endosomal incorporation of SPIO, as compared with these characteristics of the nonlabeled cells. In nondividing human mesenchymal stem cells, endosomal iron nanoparticles could be detected after 7 weeks; however, in rapidly dividing cells, intracellular iron had disappeared by five to eight divisions. A nonsignificant transient increase in reactive oxygen species production was seen in the human mesenchymal stem and HeLa cell lines. Labeled human mesenchymal stem cells did not differentiate to other lineage. A significant increase in iron concentration was observed in both the human mesenchymal stem and HeLa cell media at day 7. CONCLUSION: Magnetic cellular labeling with the ferumoxides-PLL complex had no short- or long-term toxic effects on tumor or stem cells.