Neuroplasticity, Neuroregeneration, and Brain Repair

Abstracts

Keynote Address

Astrocytes in Central Nervous System Repair and Regeneration
Michael V. Sofroniew, MD, PhD, David Geffen School of Medicine at the University of California, Los Angeles, Los Angeles, California, United States

Astrocytes respond to all forms of central nervous system (CNS) damage. Molecular dissection in vivo shows that astrocyte reactivity is a broad spectrum of potential changes ranging from mild functional changes to overt scar formation. These changes are regulated by heterogeneous molecular signaling mechanisms in a context specific manner. Genetic loss-of-function studies are identifying diverse beneficial functions of reactive astrocytes, for example in regulating CNS inflammation or synaptic reorganization after CNS insults. Other studies indicate that contrary to long-standing dogma, newly-proliferated scarforming astrocytes aid, rather than inhibit, the regeneration of damaged CNS axons. Roles for astrocytes in sustaining and restoring neural circuit function after CNS injuries, including contributions to tissue repair, synaptic plasticity and regeneration are being explored and identified. The potential for astrocyte dysfunction to contribute to neural dysfunction is also being demonstrated. A conceptual framework is emerging that encompasses both ‘normal astrocyte reactivity’ that is critical for preservation of tissue and function after CNS insults, and ‘dysfunctional astrocyte reactivity’ which may be harmful through loss-of-functions due to genetic defects, genetic polymorphisms, autoimmune attack, or chronic exposure to inflammatory stimuli.
 
Anderson et al. (2016) Astrocyte scar formation aids central nervous system axon regeneration. Nature 532:195-200
Sofroniew MV (2015) Astrocyte barriers to neurotoxic inflammation. Nat Rev Neurosci 16:249-63
Burda JE, Sofroniew MV (2014) Reactive gliosis and the multicellular response to CNS damage and disease. Neuron 81:229-248

Regulation of Synapses and Synaptic Strength
Richard Tsien, DPhil, New York University School of Medicine, New York, New York, United States

Which brain disorders are truly diseases of the synapse – full blown synaptopathies? If dysfunctional synapses and abnormal excitability contribute, what are the other elements and how do they all knit together? These questions recur in disorders ranging from Alzheimer’s to autism. Here we address such generic issues in a specific case, by reporting our studies of autism spectrum disorder (ASD), a monogenic, dominant form called Timothy Syndrome (TiS). Individuals with TiS develop ASD with a >60% likelihood because of a point mutation in the pore-forming subunit of a calcium channel (CACNA1C, encoding CaV1.2). In a mouse model (TS2-neo), we found persistent and repetitive behavior, reduced social interaction, and altered vocalizations, impetus for clarifying the pathophysiology. Dysfunctional homeostatic plasticity is often suggested as a possible pathogenic mechanism for ASD. In studying homeostatic adaptation, we discovered intriguing differences in the autoregulation of synaptic weights and intrinsic excitability in TS2-neo cortical pyramidal cells subjected to action potential blockade by 24 hour tetrodotoxin (TTX) treatment. TS2-neo pyramidal neurons exhibited an exaggerated upregulation in amplitude and frequency of unitary events, accompanied by higher intensity of GluA1-containing AMPA receptors. Homeostatic adaptation of intrinsic excitability was similarly exaggerated. We observed similar electrophysiological alterations in layer 2/3 pyramidal neurons in primary visual cortex recorded ex vivo from visually deprived WT and TS2 animals. These results fit with notion that homeostatic autoregulatory effects can be abnormally large in ASD. They also provide novel perspective on how CaV1.2 operates as an activity reporter.
 
Coauthors: Simon D. Sun, BS, Ben S. Suutari, BS, Boxing Li, PhD
New York University School of Medicine, New York, New York, United States

Striatal Plasticity in Parkinson's Disease
D. James Surmeier, PhD, Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois, United States

Parkinson’s disease (PD) is the second most common neurodegenerative disease in the United States. The core motor symptoms of PD are a consequence of the degeneration of dopaminergic neurons that innervate the striatum. This innervation not only modulates the short-term excitability of striatal neurons but also shapes the strength of corticostriatal synapses in a way that regulates goal-directed and habitual actions. The loss of this innervation leads to an imbalance in the excitability of striatal circuits modulating basal ganglia circuits, leading to the hypokinetic features of the disease. It is widely appreciated that the striatal imbalance stems from the differential expression of dopamine receptors by direct pathway spiny projection neurons (dSPNs) and indirect pathway spiny projection neurons (iSPNs). What is less appreciated is the extent to which intrinsic and synaptic homeostatic plasticity, particularly in iSPNs, mitigate the consequences of declining dopaminergic signaling, but distort striatal connectivity with the cerebral cortex. Symptomatic therapies that restore basal levels of dopamine in the striatum can reverse alterations in intrinsic excitability but fail to correct distortions in synaptic strength. Moreover, with fluctuations in striatal dopamine levels, aberrant synaptic plasticity leads to the motor complications or dyskinesia. The presentation will summarize recent work in mouse models of PD exploring these adaptations in the striatal circuitry and strategies for reducing their impact on symptom severity.
 
Coauthors: Asami Tanimura, PhD, Steven M. Graves, PhD, and Weixing Shen, PhD, Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois, United States

Novel Mechanisms of Immune-mediated Nervous System Regeneration
Roman J. Giger, PhD, University of Michigan School of Medicine, Ann Arbor, Michigan, United States

Innate immunity can facilitate nervous system regeneration, yet the underlying cellular and molecular mechanisms are not well understood. We found that intraocular injection of lipopolysaccharide (LPS), a bacterial cell wall component, or the fungal cell wall extract zymosan both lead to rapid and comparable intravitreal accumulation of blood-derived myeloid cells. However, when combined with retro-orbital optic nerve crush injury, lengthy growth of severed retinal ganglion cell (RGC) axons occurs only in zymosan-injected mice, and not in LPS-injected mice. In mice deficient for the pattern recognition receptor dectin- 1 but not Toll-like receptor-2 (TLR2), zymosan-mediated RGC regeneration is greatly reduced. The combined loss of dectin-1 and TLR2 completely blocks the pro-regenerative effects of zymosan. In the retina, dectin-1 is expressed by microglia and dendritic cells, but not by RGCs. Dectin-1 is also present on blood-derived myeloid cells that accumulate in the vitreous. Intraocular injection of the dectin-1 ligand curdlan [a particulate form of β(1, 3)-glucan] promotes optic nerve regeneration comparable to zymosan in WT mice, but not in dectin-1(-/-) mice. Particulate β(1, 3)-glucan leads to increased Erk1/2 MAP-kinase signaling and cAMP response element-binding protein (CREB) activation in myeloid cells in vivo. Loss of the dectin-1 downstream effector caspase recruitment domain 9 (CARD9) blocks CREB activation and attenuates the axon-regenerative effects of β(1, 3)-glucan. Studies with dectin-1(-/-)/WT reciprocal bone marrow chimeric mice revealed a requirement for dectin-1 in both retina-resident immune cells and bone marrow-derived cells for β(1, 3)-glucan-elicited optic nerve regeneration. Collectively, these studies identify a molecular framework of how innate immunity enables repair of injured central nervous system neurons.

Subtype-specific Local Growth Cone Control over Circuit Development, Regeneration, and Degeneration
Jeffrey D. Macklis, MD, DHST, Department of Stem Cell and Regenerative Biology, and Center for Brain Science, Harvard University, Cambridge, Massachusetts, United States

The formation of circuits throughout the nervous system, and within and from the cerebral cortex in particular in this presentation, relies heavily on molecular machinery localized at the tips of growing axons in structures termed growth cones (GCs). Subsets of neurons’ transcriptomes and proteomes localize to GCs to implement growth and guidance of nascent axons toward their specific targets. Until now, these subcellular and likely subtypespecific RNA and protein networks have not been experimentally accessible directly from the brain. Here, I will report subtype-specific GC sorting and subcellular RNA-Proteome mapping as a generalizable approach to reveal and investigate local molecular machinery that drives brain circuit development. Applying this approach to the long-range axon projections of callosal projection neurons connecting the two hemispheres of the cerebral cortex, and to corticothalamic projection neurons, we identify that native GCs 1) possess remarkably deep and rich molecular constituents for local synthesis, folding, and turnover of select protein classes, suggesting function as "mini-cellular" and significantly ~autonomous units; 2) that each subtype contains both quite distinct, subtype-specific molecular machinery plus shared molecular machinery; 3) that hundreds of proteins and hundreds of RNAs (coding and not) are enriched orders of magnitude in GCs compared to their own parent somata, indicating subcellular polarity and isolation of functions; 4) that targeting motifs direct subtype-specific GC localization. As one example, we identify a hub molecule regulating cell growth found specifically in axon GCs rather than cell bodies, and that mRNA classes distribute within developing projection neurons based on their sensitivity for translation. Essentially this entire class of transcripts map specifically to GCs and not to their parent cell bodies. Given the importance of growth control for axon growth and regeneration, this subcellular organization might also have implications for future regenerative strategies in the nervous system. Subtype-specific GC sorting and differential subcellular RNA-proteome mapping is applicable to any labeled neurons in the nervous system, enabling identification of specific molecular substrates of circuit development, regeneration, and mis-wiring causing neuronal circuit pathology. The ability to directly compare multiple distinct GC-soma subtypes using multi-color sorting makes this approach broadly applicable to future studies in the fields of axon guidance, neurodevelopmental disorders, and neuron reprogramming. Subcellular RNA-proteome mapping enables identification of distinguishing molecular constituents of subtype-specific GCs with complex and unique trajectories that ultimately target distinct brain areas. Further, it enables direct molecular investigation of subtype-specific GCs compared with GCs from mutant, regenerative, non-regenerative, or reprogrammed neurons to discover molecular mechanisms of circuit development, mis-wiring, and regeneration.

Intermittent Bioenergetic Challenges Bolster Brain Resilience
Mark P. Mattson, PhD (1,2)
(1) Laboratory of Neurosciences, National Institute on Aging Intramural Research Program, Baltimore, Maryland, United States
(2) Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland, United States

Evolutionary considerations suggest that the brain should function well/optimally during periods of food deprivation/fasting and physical exertion. Brain-intrinsic pathways and peripheral signals by which fasting and exercise promote synaptic plasticity, neurogenesis and resilience under stressful conditions are being elucidated. Our findings suggest that running and fasting can stimulate mitochondrial biogenesis in neurons by mechanisms involving BDNF signaling and PGC-1α, a pathway critical for the formation and maintenance of synapses. The mitochondrial protein deacetylase SIRT3 mediates adaptive responses (stress resistance and modulation of neuronal network activity) of neurons to exercise and fasting. By mechanisms involving deacetylating SOD2 and cyclophilin D, SIRT3 protects neurons in animal models of acute brain injury and neurodegenerative disorders. Interestingly, behavioral adaptations to intermittent fasting (reduced anxiety and preservation of cognitive performance) involve SIRT3-dependent enhancement of GABAergic tone in the hippocampus, which may also protect against excitotoxicity. Regarding peripheral signals generated in response to fasting and vigorous exercise, we found that the ketone -hydroxybutyrate, which is produced from fatty acids during fasting and vigorous exercise, can stimulate BDNF production, which may mediate beneficial effects of these bioenergetic challenges on neuronal plasticity and stress resistance. Additional data suggest that combining energy restriction with exercise can elicit additive or synergistic beneficial effects on neuroplasticity and performance. Collectively, the emerging picture of bioenergetics and brain health reveals that intermittent energy restriction and exercise promote neuroplasticity and resistance to injury and disease, whereas a ‘couch potato’ lifestyle fosters suboptimal brain function and poor resilience. Supported by the NIA Intramural Research Program.

Malfunctioning Autophagy and its Pathway to Neurodegeneration
Ana Maria Cuervo, MD, PhD, Albert Einstein College of Medicine, Bronx, New York, United States

Autophagy is an essential cellular process that contributes to cellular quality control and to maintenance of the cellular energetic balance. Failure of different types of autophagy in neurons has been shown to result in major alterations in neuronal proteostasis, energetics and functioning and have been linked to severe neurodegenerative disorders. We have identified a dynamic cross-talk among different forms of autophagy that is used to protect cells from failure in any of these systems and to avoid toxic effects of pathogenic proteins on them. To gain a better understanding of the consequences of autophagy malfunctioning in neurons and on the rules that govern the autophagic cross-talk we have developed a series of mouse models with systemic or tissue-specific blockage of a selective form of autophagy known as chaperone-mediated autophagy (CMA). Analysis of these models has revealed that added to the previously known role of CMA as part of the cellular response to stress, this type of autophagy is also required in the regulation of important cellular processes such as metabolism of lipids and carbohydrates, cell cycle, cell reprograming and cellular differentiation. Phenotypic characterization of these mice is allowing us to link CMA deficiency with different age-related neurodegenerative diseases.

Mitochondrial-linked Mechanisms and Therapeutic Opportunities
Valina L. Dawson, PhD (1,2)
(1) Neuroregeneration and Stem Cell Programs, Institute for Cell Engineering, Departments of Physiology, Neurology and the Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland, United States
(2) Adrienne Helis Malvin Medical Research Foundation, New Orleans, Louisiana, United States

Parkinson's disease (PD) is a common neurodegenerative disease of complex etiology with roughly 90% of PD cases considered sporadic, and 10% attributable to familial mutations. Mutations in the parkin gene, which encodes the E3 ligase parkin, are the most common cause of autosomal recessive PD. To date, more than 100 pathogenic parkin mutations disrupt the protein's E3 ligase activity, either directly or by altering the solubility or stability of the protein, leading to dopaminergic cell death. Importantly, recent evidence from post-mortem PD brain samples and mouse models suggest that parkin is inactivated by post-translational modifications, including oxidation, nitrosylation, addition of dopamine and phosphorylation by c-Abl, an important stress-activated non-receptor tyrosine kinase that is activated in sporadic PD brains and in animal models of PD. Thus, inactivation of parkin maybe a common and critical feature of PD. PARIS (ZNF746) is a pathologic parkin substrate, which contributes to DA neuronal loss in animal models of parkin inactivation. PARIS accumulation plays a repressive function against the transcriptional coactivator, peroxisome proliferator-activated receptor gamma coactivator-1-alpha (PGC-1α) expression, thought to be critical for DA neuron survival. Consistent with its role in DA neuronal toxicity, PARIS accumulates in mouse brain with parkin inactivation and importantly in human sporadic and familial PD brains. Dysfunction of parkin has profound effects on mitochondrial quality control (autophagy, transport, fusion, fission and biogenesis), with PARIS playing an important role in mediating parkin’s effect in biogenesis. Disruption of the homeostatic mechanisms that controls the content of mitochondria in a cell through degradation (mitophagy) and synthesis (biogenesis), can lead to neurodegeneration and significantly contribute to the pathogenesis of PD.

Human Glial Progenitor Cell-based Treatment and Modeling of Neurological Disease
Steven A. Goldman, MD, PhD (1,2)
(1) University of Copenhagen Faculty of Medicine, Copenhagen, Denmark
(2) University of Rochester Medical Center, Rochester, New York, United States

The most abundant precursor cells of the adult human brain are glial progenitor cells (GPCs), which can give rise to both astrocytes and oligodendrocytes. As a result, diseases of glia may provide readily accessible targets for cell-based therapies. The glial diseases, which include the myelin disorders as well as those referable to astrocytic dysfunction, are among the most prevalent and disabling conditions in neurology, and may be particularly appropriate targets for progenitor cell-based therapy. This talk will focus on the potential utility of transplanting pluripotent stem cell-derived GPCs as a means of treating the diseases of myelin, as well as of those neurodegenerative disorders with significant astroglial involvement. I will also discuss the utility of the glial chimeric mice that result from the neonatal implantation of human GPCs into the mouse brain. In these mice, the human glial progenitors out-compete their murine counterparts to eventually dominate the glial population of the recipient brains. Human glial chimerization has significant effects on neurophysiology and behavior, which suggest the importance of human-specific glial attributes to neural network function. By generating human glial chimeric mice using patient-derived hiPSC-derived GPCs, we may now investigate the causal contributions of glia to human brain disease, by producing disease-specific human glial chimeras. These mice provide us a new model system by which to study not only the myelin disorders, but the entire range of neurodegenerative and neuropsychiatric diseases in which glia may causally participate.

Stem Cell-derived Astrocytes for the Treatment Neurodegenerative Diseases
Clive Svendsen, PhD, Cedars-Sinai Medical Center, Los Angeles, California, United States

Astrocytes and growth factors play a key role in brain health and functioning. We have developed a reliable way of expanding human neural progenitors from both fetal tissue and iPSCs as organoids without losing cell/cell contact. These organoids transition from making neurons to astrocytes at around passage 20. They can also be genetically modified to release powerful growth factors such as GDNF. We have been developing translational approaches to use these cells for treating amyotrophic lateral sclerosis (ALS), parkinson's disease (PD), huntington's disease (HD), alzheimer’s disease (AD), and retinal diseases. For ALS we have initiated a Phase 1/2a clinical trial in 18 patients to establish whether this approach can slow degeneration and maintain function.

Development of Functionally Heterogeneous Astrocytes in Mammalian Central Nervous System
David H Rowitch (1,2)

Astrocytes are heterogeneous in terms of morphology, gene expression and physiological properties. How is such diversity determined? This talk will focus on advances in understanding astrocyte development including the role of neural tube patterning in specification and developmental functions of astrocytes during synaptogenesis, axon path finding and motor neuron survival. We propose that a precise understanding of astrocyte development is critical to defining heterogeneity and could lead to a better appreciation for their roles in support of local-regional neural circuits. For example, we have shown that ventral spinal cord astrocytes express Sema3a, which is essential for synaptogenesis and motor neuron survival. New screening approaches could indicate diversification of cortical astrocytes conferring layer-specific functions. Such information leads to the proposal that optimized cell-based neuronal therapies would be augmented by co-transplant of regional specialized astrocytes.
 
Coauthors: Omer Bayrakartar (1,2), Anna V Molofsky (2), Kevin W. Kelley (2), Hui Hsin Tsai (2), Huiliang Li (3), Raquel Taveira Marques (3), Helin Zhuang (3), Arturo Alvarez-Buylla (2), Robert Krencick (2), Erik M Ullian (2), Nicola Allen (4), William D Richardson (3)
(1) University of Cambridge, Cambridge, United Kingdom
(2) University of California, San Francisco, San Francisco, California, United States
(3) University College London, London, United Kingdom
(4) Salk Institute, La Jolla, California, United States

Industry Perspective Lecture

Disease-modifying Drugs for Alzheimer's Disease — The Past and The Future
Eric Karran, BSc, PhD, Foundational Neuroscience Center, AbbVie, Cambridge, Masschusetts, United States

Alzheimer’s disease is a devastating neurodegenerative disease that follows an unremitting course leading to severe cognitive impairment. Increasing age represents the greatest risk factor and as the population shifts to an increasingly elderly demographic, the incidence and prevalence of this disorder will likely increase. No therapies are available that slow or prevent the progression of the disease, and recently, promising agents have failed in phase 3 clinical testing. I will review previous clinical trials to determine what might be learned, and consider what future trials may reveal.

Is Alzheimer's Disease Caused by Long-term Depression Gone Awry?
Graham Collingridge, FRS, FMedSci, FSB, FBPhS (1,2,3)
(1) Department of Physiology, University of Toronto, Toronto, Canada
(2) Lunenfeld-Tanenbaum Research Institute, Mount Sinai Hospital, Toronto, Canada
(3) School of Physiology, Pharmacology & Neuroscience, University of Bristol, Bristol, United Kingdom

There is growing evidence that dysregulated synaptic plasticity is a contributing factor in a wide variety of neurological and psychiatric disorders. We have proposed that one form of synaptic plasticity, N-methyl-D-aspartate (NMDA) receptor-dependent long-term depression (LTD), becomes overactive and triggers synapse elimination and the associated cognitive deficits in neurodegenerative conditions, such as Alzheimer’s disease. NMDAR-LTD is particularly prevalent early in development where it is likely to be involved in the physiological pruning of superfluous synaptic connections. It is then largely down regulated. We propose that a variety of genetic and environmental factors may lead to the inappropriate reactivation of NMDAR-LTD and that this constitutes an early and causal element in neurodegenerative conditions. In support of this hypothesis, we have found that a variety of molecules that are closely associated with Alzheimer's disease are integral components of the LTD process. Such a mechanism can explain the link between Abeta and tau and can, in theory, underlie both Abeta-dependent and Abeta-independent forms of neurodegenerative disorders.

Modeling Human Brain Development and Disorders using Human Induced Pluripotent Stem Cells (hiPSCs)
Guo-li Ming, MD, PhD, Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania

Three dimensional (3D) cerebral organoid cultures from human iPSCs have been recently developed to recapitulate the cytoarchitecture of the developing brain. This system offers unique advantages in understanding molecular and cellular mechanisms governing embryonic neural development and in modeling congenital neurodevelopmental disorders, such as microcephaly. We have improved the organoid technology and developed a robust protocol to produce forebrain-specific organoids derived from human iPSCs using a novel miniaturized spinning bioreactor that recapitulate the human embryonic cortical development. ZIKV, a mosquito-borne flavivirus, has re-emerged as a major public health concern globally because ZIKV causes congenital defects, including microcephaly, and is also associated with Guillain-Barré syndrome in infected adults. We found that ZIKV exhibit specific tropism towards human neural progenitor cells and results in cell death and defects in neural development. I will discuss our recent work in further dissecting the molecular mechanisms underlying the ZIKV pathogenesis and microcephaly.

Engineering of Neurogenesis via Lineage Reprogramming
Benedikt Berninger, PhD (1)

Engineering new neurons via lineage reprogramming provides new avenues for brain repair. We have previously found that reprogramming of glia into induced neurons in the adult mouse cerebral cortex was dependent on prior injury. One of the hallmarks of the injury response is increased glial proliferation. We reasoned that if proliferation is a property conducive for lineage conversion that glial populations that undergo expansive cell division during postnatal brain development may be endowed with a greater reprogramming competence. Indeed we found that forced expression of Ascl1 and Sox2 (AS) or Neurog2 and Bcl2 (NB) resulted in the generation of neurons in the absence of a prior injury. Interestingly NB- and AS-induced neurons differed markedly in their physiological properties, with the former developing in more mature neurons in terms of morphology and physiology. Choice of reprogramming factors is likely to impact the conversion trajectory. Using single cell RNA-sequencing we found that AS-induced reprogramming of adult human brain pericytes into induced GABA-positive neurons is not accomplished via immediate replacement of the pericyte transcriptional program by a terminal neuronal program, but involves the transition through a interneuron precursor-like state, albeit in the absence of cell division. Intriguingly, successful induction of the precursor state depends on the synergism between the two transcription factors as either factor alone results in the induction of gene expression profiles that markedly diverge from that induced when expressed conjointly. Our data reveal part of the molecular logic that drives lineage conversion of non-neural cells towards a GABA neuron identity.
 
Coauthors: Marisa Karow, PhD (1,2), Sophie Péron, PhD (1), Nicolás Marichal, PhD (1), Sven Falk, PhD (2), Gray Camp, PhD (3), Barbara Treutlein, PhD (3)
(1) University Medical Center Mainz of the Johannes Gutenberg University Mainz, Mainz, Germany
(2) Biomedical Center Munich, Ludwig-Maximilians University of Munich, Munich, Germany
(3) Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany

Rejuvenating and Re-engineering Aging Memory Circuits
Amar Sahay, PhD (1,2,3,4)
(1) Center for Regenerative Medicine, Massachusetts General Hospital, Boston, Massachusetts, United States
(2) Harvard Stem Cell Institute, Cambridge, Massachusetts, United States
(3) Department of Psychiatry, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts, United States
(4) BROAD Institute of Harvard and MIT, Cambridge, Massachusetts, United States

Episodic memory impairments and loss of memory precision are hallmarks of age-related cognitive decline and mild cognitive impairment (MCI). Studies by us and others have implicated adult hippocampal neurogenesis in resolution of memory interference through population based coding mechanisms that support pattern separation. Here, I will present two complimentary approaches, modulation of neuronal competition dynamics and molecular control of granule cell recruitment of inhibition, by which we can rejuvenate and re-engineer the dentate gyrus-CA3 circuit to improve memory precision in adulthood and aging.

In Vivo Two-photon Imaging of the Effects of Tauopathy and Amyloidopathy on Synapse Dynamics
Johanna Jackson (1)

A progressive loss of synapses occurs at the early clinical stages of Alzheimer’s disease (AD) and has been correlated with cognitive deficits in patients. However, it is relatively unknown how synapse dynamics are affected by the two main pathological hallmarks of AD; the accumulation of tau and beta-amyloid. Here we used in vivo two-photon microscopy to assess the temporal dynamics of axonal boutons and dendritic spines in mouse models of human tauopathy (rTg4510) and amyloidopathy (J20). Following a craniotomy, adeno-associated virus expressing green fluorescent protein was injected into the somatosensory cortex to enable the visualisation of neurons and a cranial window was implanted for long-term imaging. GFP-labelled neurons were imaged in both models during a time period which spanned the onset of pathology. The gross morphology of neurites and the dynamics of their synaptic structures were assessed as the pathology progressed. In rTg4510s, gross morphological changes such as the presence of dystrophic neurites were visible as the tauopathy progressed and these were found to have a distinctive morphological phenotype prior to neurite degeneration. Alongside this, synapse instability and loss were also observed and could be prevented by suppressing the P301L transgene. In J20 animals, amyloid plaques increased in size and density over time and, whilst spine density was unaffected, spine turnover was increased in the transgenic animals. Both tauopathy and amyloidopathy had effects on synapse dynamics as the respective pathology progressed. These results will inform subsequent drug discovery studies to identify novel therapies to stabilize synapses in AD.
 
Coauthors: Francesco Tamagnini(2), James Johnson (1,3), Terri-Leigh Stephen (1), Zeshan Ahmed (1), Alice Oliver-Evans (1), Soraya Meftah (1), Michael L. Hutton (1), John T. Isaac (1), Andrew Randall (2), Michael C. Ashby (3), and Michael J. O’Neill (1) (1) Lilly UK, Surrey, United Kingdom
(2) University of Exeter, Exeter, United Kingdom
(3) University of Bristol, Bristol, United Kingdom

Integrative Bioinformatics Approach to Understand the Role of Plasticity in Neurodegenerative Disease
Milo Robert Smith, Icahn School of Medicine at Mount Sinai, New York, New York, United States

Neuroplasticity is essential to normal brain function throughout life. Identifying brain diseases wherein neuroplasticity is disrupted will uncover novel disease pathophysiology and identify therapeutic targets. Using the well-characterized ocular dominance model of experience-dependent cortical plasticity, we generated transcriptional signatures of plasticity, which we matched to 436 disease signatures using a molecular matching algorithm. We identified diverse diseases associated to plasticity, and implementing a novel Disease Leverage Analysis, found inflammatory pathways as a putative common pathology to suppress experience-dependent plasticity, which we validated in vivo using the lipopolysaccharide model of inflammation (Smith, eNeuro 2016). Inflammation is a hallmark biological pathway common to all neurodegenerative diseases. Consistent with a relationship between plasticity and inflammation, we identified a range of neurodegenerative diseases as strongly associated with plasticity signatures, including Alzheimer's disease, Huntington's disease (HD), Parkinson's disease, and Amyotrophic Lateral Sclerosis, suggesting that experience-dependent plasticity may be disrupted in neurodegeneration. Interestingly, the expression patterns of genes in neurodegenerative disease signatures were heterogeneously correlated with plasticity signatures; independent signatures of the same disease (e.g. HD) indicated both anti-correlated and correlated expression patterns. Given that pre-symptomatic HD individuals have heightened plasticity-dependent perceptual learning (Beste, Current Biology 2012), the finding that HD has both correlated and anti-correlated expression relative to plasticity suggests a dynamic transcriptional landscape across the natural history of the disease. These findings call for additional work to fully elucidate the transcriptional trajectory of HD and other neurodegenerative diseases to facilitate discovery of the right treatment at the right time to restore plasticity and cognition.
 
Coauthors: Joel T. Dudley, PhD, and Hirofumi Morishita, MD, PhD
Icahn School of Medicine at Mount Sinai, New York, New York, United States

Testing Hereditary Spastic Paraplegia Genes for Roles in Modeling Axonal Endoplasmic Reticulum in Drosophila
Eliška Zlámalová, University of Cambridge, Department of Genetics, Cambridge, United Kingdom

The hereditary spastic paraplegias (HSPs) are a group of currently untreatable inherited neurologic disorders exhibiting progressively increasing spasticity and weakness of the lower limbs. The prevalence ranges from 3 to 18 in 100,000; hence, there are hundreds of thousands of HSPs sufferers worldwide. The diseases are accompanied by degeneration of corticospinal upper motor neuron axons, which are among the longest in the body. The cellular pathways responsible for causing HSPs are not fully described. The incomplete understanding of mechanisms of axonopathy poses a bottleneck in developing treatments for HSP and related neurodegenerative disorders. Moreover, the known genetic bases for the HSPs can be used to probe basic biological questions of axon formation and maintenance. Some of the most commonly mutated HSP genes encode proteins that model endoplasmic reticulum (ER), suggesting that ER is essential for axon formation and maintenance, and is potentially a location for HSP pathology. We have found that loss of reticulon and REEP proteins in Drosophila results in fewer and larger axonal ER tubules, with occasional gaps in an otherwise continuous axonal ER network. Nevertheless, ER remains present throughout the axon in these mutants, implying that additional proteins help shaping it. Therefore, my aim is to identify other ER-shaping proteins, particularly among HSP gene products, that play roles in axonal ER morphology and function. My strategy involves a small RNAi-based and null mutationbased screen of Drosophila HSP gene orthologs, and combination of confocal and electron microscopy of axonal ER in intact Drosophila knockdown and knockout animals.
 
Coauthors: Belgin Yalçın PhD, Lu Zhao PhD, and Cahir J. O’Kane PhD University of Cambridge, Department of Genetics, Cambridge, United Kingdom

Neuroimaging Studies for the Early Detection of Alzheimer's Disease
Reisa Sperling, MD, MMSc (1,2)

One of the continued dilemmas in the field is how best to identify individuals who are clearly on the Alzheimer’s disease (AD) trajectory but at an early enough stage of the AD pathophysiologic process to be maximally responsive to disease-modifying therapies. Converging data from Amyloid PET imaging, cerebrospinal fluid studies and large autopsy series suggest that one-third of clinically normal older individuals harbor a substantial burden of cerebral amyloid-beta (Aß). Accumulating evidence from these “Aß-positive normals” show aberrant functional network activity, cortical thinning and hippocampal atrophy, increased neocortical tau on Tau PET imaging, and other “AD-like” abnormalities on multi-modality imaging. Clinical studies have demonstrated an association between Aß burden and memory performance, greater subjective cognitive concerns, and an increased risk of cognitive decline, particularly among older individuals with markers of both Aß accumulation and neurodegeneration. Recent Amyloid and Tau data acquired in clinically normal older individuals from the Harvard Aging Brain Study and the Anti-Amyloid Treatment in Asymptomatic AD (A4) Study demonstrate a strong relationship between levels of Aß and Tau deposition, as well as an association with memory performance, even prior to clinical symptoms. While Amyloid PET is now increasingly being utilized to select participants for clinical trials in both preclinical (asymptomatic) and clinical (symptomatic) AD populations, Tau PET has particular potential to serve as a theragnostic biomarker outcome.
 
Coauthor: Keith Johnson, MD (2)
(1) Center for Alzheimer Research and Treatment, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts, United States
(2) Harvard Aging Brain Study, Departments of Neurology and Radiology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, United States

Neuronal Dysfunction in Mouse Models of Alzheimer's Disease In Vivo
Arthur Konnerth, PhD, Technical University of Munich, Munich, Germany

The focus of our research is a better understanding of the impairments of neuronal function in Alzheimer’s disease (AD). Because such analyses of single brain cells are not feasible in patients with AD, we perform most of our experimental work in appropriate mouse models of the disease. An essential feature of AD is the accumulation of amyloid-b (Ab) peptides in the brain, many years to decades before the onset of overt cognitive symptoms. Our results, involving in vivo two photon calcium imaging, suggest that during this very extended early phase of the disease, soluble Ab oligomers and amyloid plaques alter the function of a fraction of brain neurons and consequently that of large-scale networks by disrupting the balance of synaptic excitation and inhibition (E/I balance) in the brain. The analysis of mouse models of AD revealed that an Ab-induced change of the E/I balance caused hyperactivity in cortical and hippocampal neurons, a breakdown of slow-wave oscillations, as well as network hypersynchrony. Remarkably, hyperactivity of hippocampal neurons precedes amyloid plaque formation, suggesting that hyperactivity is one of the earliest dysfunctions in the pathophysiological cascade initiated by abnormal Ab accumulation. Therapeutics that repair the E/I balance in early AD may prevent neuronal dysfunction, widespread cell loss, and cognitive impairments associated with later stages of the disease.

The Ups and Downs of Translational Research in Alzheimer's Disease

Ralph A. Nixon, MD, PhD (1,2)
(1) Center of Dementia, Nathan S. Kline Institute, Orangeburg, New York, United States
(2) Departments of Psychiatry and Cell Biology, New York University Langone Medical Center, New York, New York, United States

When discovered, the amyloid precursor protein and its varied cleaved derivatives were initially investigated as modulators of cell growth, homeostasis, and intercellular communication and were viewed as contributors to a multifactorial disease process involving aging, repair, protective, and toxic influences. As AD research shifted strongly toward a paradigm of abeta neurotoxicity, the disease relevance and therapeutic promise of new findings were viewed mainly through the narrow lens of how abeta/ß-amyloid levels were affected. More recently, however, uncertainty as to which of the many multimeric forms of abeta, if any, is a critical driving factor in AD, combined with the less than hoped for results of anti-amyloid clinical trials, has encouraged a return to the multifactorial perspective of AD initiation and progression. Further fueling this return are the varied physiological roles now being identified for causative and risk genes and evidence that perturbations of these genes have disease-related impact well beyond that attributable to abeta. Increasingly, disruptions of processes linked to AD pathogenesis are being recognized in other neurodegenerative diseases of aging suggesting certain shared degenerative mechanisms and therapeutic targets. Strong genetic evidence, for example, implicates disruptions of proteostasis and endolysosomal trafficking and signaling as primary causes of proteinopathy, synaptic dysfunction, and degeneration irrespective of roles played by a particular proteotoxin. The final panel of this meeting will consider how anomalies known in AD and related disorders, such as inflammation, lipid handling/signaling, mitochondrial function, cell cycle, calcium dysregulation, etc – can be integrated into a more comprehensive notion of pathogenesis to yield more predictive disease models and efficacious therapies.