Synaptic architecture of a memory engram in the mouse hippocampus

Memory engrams are formed through experience-dependent remodeling of neural circuits, but their detailed architectures have remained unresolved. Using 3D electron microscopy, we performed nanoscale reconstructions of the hippocampal CA3-CA1 pathway following chemogenetic labeling of cellular ensembles with a remote history of correlated excitation during associative learning. Projection neurons involved in memory acquisition expanded their connectomes via multi-synaptic boutons without altering the numbers and spatial arrangements of individual axonal terminals and dendritic spines. This expansion was driven by presynaptic activity elicited by specific negative valence stimuli, regardless of the co-activation state of postsynaptic partners. The rewiring of initial ensembles representing an engram coincided with local, input-specific changes in the shapes and organelle composition of glutamatergic synapses, reflecting their weights and potential for further modifications. Our findings challenge the view that the connectivity among neuronal substrates of memory traces is governed by Hebbian mechanisms, and offer a structural basis for representational drifts.

Retrolaminar Demyelination of Structurally Intact Axons in Nonhuman Primate Experimental Glaucoma

Purpose

To determine if structurally intact, retrolaminar optic nerve (RON) axons are demyelinated in nonhuman primate (NHP) experimental glaucoma (EG).

Methods

Unilateral EG NHPs (n = 3) were perfusion fixed, EG and control eyes were enucleated, and foveal Bruch's membrane opening (FoBMO) 30° sectoral axon counts were estimated. Optic nerve heads were trephined; serial vibratome sections (VSs) were imaged and colocalized to a fundus photograph establishing their FoBMO location. The peripheral neural canal region within n = 5 EG versus control eye VS comparisons was targeted for scanning block-face electron microscopic reconstruction (SBEMR) using micro-computed tomographic reconstructions (µCTRs) of each VS. Posterior laminar beams within each µCTR were segmented, allowing a best-fit posterior laminar surface (PLS) to be colocalized into its respective SBEMR. Within each SBEMR, up to 300 axons were randomly traced until they ended (nonintact) or left the block (intact). For each intact axon, myelin onset was identified and myelin onset distance (MOD) was measured relative to the PLS. For each EG versus control SBEMR comparison, survival analyses compared EG and control MOD.

Results

MOD calculations were successful in three EG and five control eye SBEMRs. Within each SBEMR comparison, EG versus control eye axon loss was -32.9%, -8.3%, and -15.2% (respectively), and MOD was increased in the EG versus control SBEMR (P < 0.0001 for each EG versus control SBEMR comparison). When data from all three EG eye SBEMRs were compared to all five control eye SBEMRs, MOD was increased within the EG eyes.

Conclusions

Structurally intact, RON axons are demyelinated in NHP early to moderate EG. Studies to determine their functional status are indicated.

Abstract 2818: In vivo imaging of mitochondrial bioenergetics in lung cancer

Non-small cell lung cancer (NSCLC) is a histologically, genetically and metabolically heterogeneous disease. The mitochondria are essential regulators of cellular energy and metabolism and they play a critical role in sustaining growth and survival of lung tumor cells. However, our understanding of mitochondrial metabolism in cancer at an in vivo level has been limited thus leaving a large gap in our knowledge of how mitochondrial bioenergetics support tumor growth. To better study mitochondrial bioenergetics in lung tumors, we recently developed and validated a voltage sensitive, positron emission tomography (PET) tracer known as 4-[18F]fluorobenzyl triphenylphosphonium (18FBnTP) that we used to profile mitochondrial bioenergetics in autochthonous K-Ras driven mouse models of lung cancer (Momcilovic et al., (2019) Nature). The use of 18FBnTP PET imaging enabled us to functionally profile mitochondrial bioenergetics in live tumors and discover distinct functional mitochondrial heterogeneity conserved across different NSCLC tumor subtypes. In order to study mitochondria at the level of ultrastructure we coupled 18FBnTP PET with 3D serial block-face scanning electron microscopy (3D SBEM). By coupling these two techniques we are able to image and quantify mitochondria heterogeneity from whole tumors down to the ultrastructures of individual mitochondria within tumor cells. Our study reveals distinct organization of mitochondrial structure and function as lung tumors adapt during tumorigenesis. We anticipate that coupling 18FBnTP PET imaging with 3D SBEM will have dynamic applications beyond that of lung cancer and enrich our understanding how mitochondria impact human disease.

Citation Format: Mingqi Han, Milica Momcilovic, Eric Bushong, Linsey Stiles, Orian Shirihai, Carla Koehler, Saman Sadeghi, Mark Ellisman, David B. Shackelford. In vivo imaging of mitochondrial bioenergetics in lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2818.

Nanoscale 3D EM reconstructions reveal intrinsic mechanisms of structural diversity of chemical synapses

Chemical synapses of shared cellular origins have remarkably heterogeneous structures, but how this diversity is generated is unclear. Here, we use three-dimensional (3D) electron microscopy and artificial intelligence algorithms for image processing to reconstruct functional excitatory microcircuits in the mouse hippocampus and microcircuits in which neurotransmitter signaling is permanently suppressed with genetic tools throughout the lifespan. These nanoscale analyses reveal that experience is dispensable for morphogenesis of synapses with different geometric shapes and contents of membrane organelles and that arrangement of morphologically distinct connections in local networks is stochastic. Moreover, loss of activity increases the variability in sizes of opposed pre- and postsynaptic structures without disrupting their alignments, suggesting that inherently variable weights of naive connections become progressively matched with repetitive use. These results demonstrate that mechanisms for the structural diversity of neuronal synapses are intrinsic and provide insights into how circuits essential for memory storage assemble and integrate information.

Age Mosaicism across Multiple Scales in Adult Tissues

Most neurons are not replaced during an animal's lifetime. This nondividing state is characterized by extreme longevity and age-dependent decline of key regulatory proteins. To study the lifespans of cells and proteins in adult tissues, we combined isotope labeling of mice with a hybrid imaging method (MIMS-EM). Using ¹⁵N mapping, we show that liver and pancreas are composed of cells with vastly different ages, many as old as the animal. Strikingly, we also found that a subset of fibroblasts and endothelial cells, both known for their replicative potential, are characterized by the absence of cell division during adulthood. In addition, we show that the primary cilia of beta cells and neurons contains different structural regions with vastly different lifespans. Based on these results, we propose that age mosaicism across multiple scales is a fundamental principle of adult tissue, cell, and protein complex organization.

Deceleration of probe beam by stage bias potential improves resolution of serial block-face scanning electron microscopic images

Serial block-face scanning electron microscopy (SBEM) is quickly becoming an important imaging tool to explore three-dimensional biological structure across spatial scales. At probe-beam-electron energies of 2.0 keV or lower, the axial resolution should improve, because there is less primary electron penetration into the block face. More specifically, at these lower energies, the interaction volume is much smaller, and therefore, surface detail is more highly resolved. However, the backscattered electron yield for metal contrast agents and the backscattered electron detector sensitivity are both sub-optimal at these lower energies, thus negating the gain in axial resolution. We found that the application of a negative voltage (reversal potential) applied to a modified SBEM stage creates a tunable electric field at the sample. This field can be used to decrease the probe-beam-landing energy and, at the same time, alter the trajectory of the signal to increase the signal collected by the detector. With decelerated low landing-energy electrons, we observed that the probe-beam-electron-penetration depth was reduced to less than 30 nm in epoxy-embedded biological specimens. Concurrently, a large increase in recorded signal occurred due to the re-acceleration of BSEs in the bias field towards the objective pole piece where the detector is located. By tuning the bias field, we were able to manipulate the trajectories of the  primary and secondary electrons, enabling the spatial discrimination of these signals using an advanced ring-type BSE detector configuration or a standard monolithic BSE detector coupled with a blocking aperture.

Visualizing viral protein structures in cells using genetic probes for correlated light and electron microscopy

Structural studies of viral proteins most often use high-resolution techniques such as X-ray crystallography, nuclear magnetic resonance, single particle negative stain, or cryo-electron microscopy (EM) to reveal atomic interactions of soluble, homogeneous viral proteins or viral protein complexes. Once viral proteins or complexes are separated from their host's cellular environment, their natural in situ structure and details of how they interact with other cellular components may be lost. EM has been an invaluable tool in virology since its introduction in the late 1940's and subsequent application to cells in the 1950's. EM studies have expanded our knowledge of viral entry, viral replication, alteration of cellular components, and viral lysis. Most of these early studies were focused on conspicuous morphological cellular changes, because classic EM metal stains were designed to highlight classes of cellular structures rather than specific molecular structures. Much later, to identify viral proteins inducing specific structural configurations at the cellular level, immunostaining with a primary antibody followed by colloidal gold secondary antibody was employed to mark the location of specific viral proteins. This technique can suffer from artifacts in cellular ultrastructure due to compromises required to provide access to the immuno-reagents. Immunolocalization methods also require the generation of highly specific antibodies, which may not be available for every viral protein. Here we discuss new methods to visualize viral proteins and structures at high resolutions in situ using correlated light and electron microscopy (CLEM). We discuss the use of genetically encoded protein fusions that oxidize diaminobenzidine (DAB) into an osmiophilic polymer that can be visualized by EM. Detailed protocols for applying the genetically encoded photo-oxidizing protein MiniSOG to a viral protein, photo-oxidation of the fusion protein to yield DAB polymer staining, and preparation of photo-oxidized samples for TEM and serial block-face scanning EM (SBEM) for large-scale volume EM data acquisition are also presented. As an example, we discuss the recent multi-scale analysis of Adenoviral protein E4-ORF3 that reveals a new type of multi-functional polymer that disrupts multiple cellular proteins. This new capability to visualize unambiguously specific viral protein structures at high resolutions in the native cellular environment is revealing new insights into how they usurp host proteins and functions to drive pathological viral replication.

DRP1 inhibition rescues retinal ganglion cells and their axons by preserving mitochondrial integrity in a mouse model of glaucoma

Glaucoma is the leading cause of irreversible blindness and is characterized by slow and progressive degeneration of the optic nerve head axons and retinal ganglion cell (RGC), leading to loss of visual function. Although oxidative stress and/or alteration of mitochondrial (mt) dynamics induced by elevated intraocular pressure (IOP) are associated with this neurodegenerative disease, the mechanisms that regulate mt dysfunction-mediated glaucomatous neurodegeneration are poorly understood. Using a mouse model of glaucoma, DBA/2J (D2), which spontaneously develops elevated IOP, as well as an in vitro RGC culture system, we show here that oxidative stress, as evidenced by increasing superoxide dismutase 2 (SOD2) and mt transcription factor A (Tfam) protein expression, triggers mt fission and loss by increasing dynamin-related protein 1 (DRP1) in the retina of glaucomatous D2 mice as well as in cultured RGCs exposed to elevated hydrostatic pressure in vitro. DRP1 inhibition by overexpressing DRP1 K38A mutant blocks mt fission and triggers a subsequent reduction of oxidative stress, as evidenced by decreasing SOD2 and Tfam protein expression. DRP1 inhibition promotes RGC survival by increasing phosphorylation of Bad at serine 112 in the retina and preserves RGC axons by maintaining mt integrity in the glial lamina of glaucomatous D2 mice. These findings demonstrate an important vicious cycle involved in glaucomatous neurodegeneration that starts with elevated IOP producing oxidative stress; the oxidative stress then leads to mt fission and a specific form of mt dysfunction that generates further oxidative stress, thus perpetuating the cycle. Our findings suggest that DRP1 is a potential therapeutic target for ameliorating oxidative stress-mediated mt fission and dysfunction in RGC and its axons during glaucomatous neurodegeneration. Thus, DRP1 inhibition may provide a new therapeutic strategy for protecting both RGCs and their axons in glaucoma and other optic neuropathies.

Improving signal to noise in labeled biological specimens using energy-filtered TEM of sections with a drift correction strategy and a direct detection device

Energy filtered transmission electron microscopy techniques are regularly used to build elemental maps of spatially distributed nanoparticles in materials and biological specimens. When working with thick biological sections, electron energy loss spectroscopy techniques involving core-loss electrons often require exposures exceeding several minutes to provide sufficient signal to noise. Image quality with these long exposures is often compromised by specimen drift, which results in blurring and reduced resolution. To mitigate drift artifacts, a series of short exposure images can be acquired, aligned, and merged to form a single image. For samples where the target elements have extremely low signal yields, the use of charge coupled device (CCD)-based detectors for this purpose can be problematic. At short acquisition times, the images produced by CCDs can be noisy and may contain fixed pattern artifacts that impact subsequent correlative alignment. Here we report on the use of direct electron detection devices (DDD's) to increase the signal to noise as compared with CCD's. A 3× improvement in signal is reported with a DDD versus a comparably formatted CCD, with equivalent dose on each detector. With the fast rolling-readout design of the DDD, the duty cycle provides a major benefit, as there is no dead time between successive frames.

Abstract LB-191: Glioblastomas can originate from neurons in the CNS

Malignant gliomas remain one of the most aggressive tumors of the central nervous system. Different interpretations have been proposed about the nature of the neural cell type that is targeted by transformation and results in tumorigenesis. The identification of the cellular origin of gliomas presents an opportunity for improving our understanding of this type of cancer. We recently developed a mouse glioma model using Cre-inducible lentiviral vectors that faithfully recapitulate the pathophysiology of human glioblastoma multiforme (GBM). Injection of a single lentiviral vector expressing H-RasV12-shp53 in the cortex of Synapsin I-Cre (SynI-Cre) mice led to tumor formation after 6-8 weeks of injection. SynI-Cre mice primarily and specifically express the Cre recombinase transgene in differentiated neurons. Tumors were also obtained when CamK2a-Cre mice, also expressing Cre specifically in neurons, were injected with the same virus. We also aim to target astrocytes by injecting the virus either in the cortex or the stratium of GFAP-Cre mice, and tumors presenting the classical characteristics of GBM developed, suggesting that astrocytes can also serve as the glioma cell of origin. We made sections of these brains at various time points following injection of the lentiviral vector and, using high resolution large-scale mosaic imaging, we examined the expression of different markers. Notably, tumors start out to be GFAP+, but by eight weeks are largely Nestin+ and Sox2+. We believe that either astrocytes or neurons can be reprogrammed by the introduction of oncogenes/tumor suppressors to form cancer iPS-like stem cells that can give rise to all the cell lineages and heterogeneity observed in GBM. To further explore this hypothesis, we transduced primary cortical astrocytes and neurons obtained from GFAP-Cre and SynI-Cre mice, respectively. The transduced cells when switched to neural stem cell (NSC) media displayed: i) neurosphere-like structures, ii) robust NSC marker expression (Nestin and Sox2), iii) self-renewal capacity, iv) strong tumor initiating capacity, v) expression of reprogramming factors, and vi) capacity to differentiate into different lineages. Finally, we assessed the human relevance of our findings by comparing the transcriptome profile of tumors in our model with the molecular signatures of human glioma samples. The data from the molecular signatures and histopathology of tumors originating in the cortex where the primary target is astrocytes in the GFAP cre mice and Neurons in the Synapsin Cre mice show both are mesenchymal GBM subtype. We obtained Neural subtype mostly when the virus was injected in the hippocampus of Nestin-Cre mice (aim to target NSC/progenitor cells). Together, our results suggest that any cell in the brain, whether terminally differentiated or neural stem cell, can be the glioma cell of origin and the biological behavior of these tumors depends on the dysregulation of specific genetic elements.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-191. doi:1538-7445.AM2012-LB-191

Abstract 975: Glioma cell of origin: Reprogramming and cancer stem cells

Glioblastomas are the most common and lethal form of intracranial tumors. In the last century we have accumulated tremendous amounts of data on this type of cancer, but we have achieved very little improvement in its treatment. This inadequate progress led us to reexamine the gliomagenesis theory and reconsider the cell of origin of this deadly disease. We recently developed a mouse glioma model using Cre-inducible lentiviral vectors that faithfully recapitulate the pathophysiology of human glioblastoma multiforme (GBM) (1). Injection of a single lentiviral vector expressing H-RasV12 and si_p53 into the brain of GFAP-Cre or Nestin-Cre mice led to tumor formation after 6-8 weeks of injection. Tumors were also obtained when the virus was injected either in the cortex or the stratium in the GFAP-Cre mice, suggesting that astrocytes can also serve as the glioma cell of origin. We made sections of these brains at various time points following injection of the lentiviral vector and, using high resolution large-scale mosaic imaging (2), we examined the expression of different markers. Notably, tumors start out to be GFAP+, but by eight weeks are largely Nestin+ and Sox2+. We believe that astrocytes can be reprogrammed by the introduction of oncogenes/tumor suppressors to form cancer iPS-like stem cells that can give rise to all the cell lineages and heterogeneity observed in GBM. To further explore this hypothesis, we transduced primary cortical astrocytes obtained from GFAP-Cre P2 mice. Only when both H-Ras and si_p53 were in the viral vector were the cells able to reprogram and give rise to neurospheres, a property ascribed to neural stem cells. We were also able to obtain tumors when we aimed to transduce neurons by injecting our lentiviral vector in Syn-Cre mice, suggesting that the reprogramming process is not restricted to glial cells. So far we have used H-Ras as surrogate for EGFR amplification and loss of NF1 observed in patients, but recently we have generated a lenti vector that combines both tumor suppressor genes si_NF1 and si_p53, and the preliminary results confirmed our findings using H-Ras-si_p53 lentivector.

Together, our results suggest that any cell in the brain, whether terminally differentiated or neural stem cell, can be the glioma cell of origin and the biological behavior of these tumors depends on the dysregulation of specific genetic elements.

(1) Marumoto T, Tashiro A, Friedmann-Morvinski D, Scadeng M, Soda Y, Gage FH, Verma IM. Development of a novel mouse glioma model using lentiviral vectors. Nat Med 15 (1), 110-116 (2009).

(2) Price DL, Chow SK, Maclean NA, Hakozaki H, Peltier S, Martone ME, Ellisman MH. High-resolution large-scale mosaic imaging using multiphoton microscopy to characterize transgenic mouse models of human neurological disorders. Neuroinformatics. 2006 Winter;4(1):65-80.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 975. doi:10.1158/1538-7445.AM2011-975

Absence of glial fibrillary acidic protein and vimentin prevents hypertrophy of astrocytic processes and improves post-traumatic regeneration

The regenerative capacity of the CNS is extremely limited. The reason for this is unclear, but glial cell involvement has been suspected, and oligodendrocytes have been implicated as inhibitors of neuroregeneration (Chen et al., 2000, GrandPre et al., 2000; Fournier et al., 2001). The role of astrocytes in this process was proposed but remains incompletely understood (Silver and Miller, 2004). Astrocyte activation (reactive gliosis) accompanies neurotrauma, stroke, neurodegenerative diseases, or tumors. Two prominent hallmarks of reactive gliosis are hypertrophy of astrocytic processes and upregulation of intermediate filaments. Using the entorhinal cortex lesion model in mice, we found that reactive astrocytes devoid of the intermediate filament proteins glial fibrillary acidic protein and vimentin (GFAP-/-Vim-/-), and consequently lacking intermediate filaments (Colucci-Guyon et al., 1994; Pekny et al., 1995; Eliasson et al., 1999), showed only a limited hypertrophy of cell processes. Instead, many processes were shorter and not straight, albeit the volume of neuropil reached by a single astrocyte was the same as in wild-type mice. This was accompanied by remarkable synaptic regeneration in the hippocampus. On a molecular level, GFAP-/-Vim-/- reactive astrocytes could not upregulate endothelin B receptors, suggesting that the upregulation is intermediate filament dependent. These findings show a novel role for intermediate filaments in astrocytes and implicate reactive astrocytes as potent inhibitors of neuroregeneration.

Phalloidin-eosin followed by photo-oxidation: a novel method for localizing F-actin at the light and electron microscopic levels

We describe a novel high-resolution method to detect F-actin at the light and electron microscopic levels through the use of the actin-binding protein phalloidin conjugated to the fluorophore eosin, followed by photo-oxidation of diaminobenzidine. This method possesses several key advantages over antibody-based labeling and structural methods. First, phalloidin binding to F-actin can tolerate relatively high concentrations of glutaraldehyde (up to 1%) in the primary fixative, resulting in good ultrastructural preservation. Second, because both eosin and phalloidin are relatively small molecules, considerable penetration of reagents into aldehyde-fixed tissue was obtained without any permeabilization steps, allowing 3D reconstructions at the electron microscopic level. By employing a secondary fixation with tannic acid combined with low pH osmication, conditions known to stabilize actin filaments during preparation for electron microscopy, we were able to visualize individual actin filaments in some structures. Finally, we show that fluorescent phalloidin can be directly injected into neurons to label actin-rich structures such as dendritic spines. These results suggest that the fluorescent phalloidin is an excellent tool for the study of actin networks at high resolution.