Selective neuronal vulnerability in neurodegenerative diseases: from stressor thresholds to degeneration

Modeling neuronal vulnerability in ALS

Using computational models of motor neuron ion fluxes, firing properties, and energy requirements, Le Masson et al. (2014) reveal how local imbalances in energy homeostasis may self-amplify and contribute to neurodegeneration in ALS.

The hubs of the human connectome are generally implicated in the anatomy of brain disorders

Brain networks or 'connectomes' include a minority of highly connected hub nodes that are functionally valuable, because their topological centrality supports integrative processing and adaptive behaviours. Recent studies also suggest that hubs have higher metabolic demands and longer-distance connections than other brain regions, and therefore could be considered biologically costly. Assuming that hubs thus normally combine both high topological value and high biological cost, we predicted that pathological brain lesions would be concentrated in hub regions. To test this general hypothesis, we first identified the hubs of brain anatomical networks estimated from diffusion tensor imaging data on healthy volunteers (n = 56), and showed that computational attacks targeted on hubs disproportionally degraded the efficiency of brain networks compared to random attacks. We then prepared grey matter lesion maps, based on meta-analyses of published magnetic resonance imaging data on more than 20 000 subjects and 26 different brain disorders. Magnetic resonance imaging lesions that were common across all brain disorders were more likely to be located in hubs of the normal brain connectome (P < 10(-4), permutation test). Specifically, nine brain disorders had lesions that were significantly more likely to be located in hubs (P < 0.05, permutation test), including schizophrenia and Alzheimer's disease. Both these disorders had significantly hub-concentrated lesion distributions, although (almost completely) distinct subsets of cortical hubs were lesioned in each disorder: temporal lobe hubs specifically were associated with higher lesion probability in Alzheimer's disease, whereas in schizophrenia lesions were concentrated in both frontal and temporal cortical hubs. These results linking pathological lesions to the topological centrality of nodes in the normal diffusion tensor imaging connectome were generally replicated when hubs were defined instead by the meta-analysis of more than 1500 task-related functional neuroimaging studies of healthy volunteers to create a normative functional co-activation network. We conclude that the high cost/high value hubs of human brain networks are more likely to be anatomically abnormal than non-hubs in many (if not all) brain disorders.

Unaltered striatal dopamine release levels in young Parkin knockout, Pink1 knockout, DJ-1 knockout and LRRK2 R1441G transgenic mice

Parkinson's disease (PD) is one of the most prevalent neurodegenerative brain diseases; it is accompanied by extensive loss of dopamine (DA) neurons of the substantia nigra that project to the putamen, leading to impaired motor functions. Several genes have been associated with hereditary forms of the disease and transgenic mice have been developed by a number of groups to produce animal models of PD and to explore the basic functions of these genes. Surprisingly, most of the various mouse lines generated such as Parkin KO, Pink1 KO, DJ-1 KO and LRRK2 transgenic have been reported to lack degeneration of nigral DA neuron, one of the hallmarks of PD. However, modest impairments of motor behavior have been reported, suggesting the possibility that the models recapitulate at least some of the early stages of PD, including early dysfunction of DA axon terminals. To further evaluate this possibility, here we provide for the first time a systematic comparison of DA release in four different mouse lines, examined at a young age range, prior to potential age-dependent compensations. Using fast scan cyclic voltammetry in striatal sections prepared from young, 6–8 weeks old mice, we examined sub-second DA overflow evoked by single pulses and action potential trains. Unexpectedly, none of the models displayed any dysfunction of DA overflow or reuptake. These results, compatible with the lack of DA neuron loss in these models, suggest that molecular dysfunctions caused by the absence or mutation of these individual genes are not sufficient to perturb the function and survival of mouse DA neurons.

Golgi fragmentation precedes neuromuscular denervation and is associated with endosome abnormalities in SOD1-ALS mouse motor neurons

Background

Fragmentation of stacked cisterns of the Golgi apparatus into dispersed smaller elements is a feature associated with degeneration of neurons in amyotrophic lateral sclerosis (ALS) and some other neurodegenerative disorders. However, the role of Golgi fragmentation in motor neuron degeneration is not well understood.

Results

Here we use a SOD1-ALS mouse model (low-copy Gurney G93A-SOD1 mouse) to show that motor neurons with Golgi fragmentation are retrogradely labeled by intramuscularly injected CTB (beta subunit of cholera toxin), indicating that Golgi fragmentation precedes neuromuscular denervation and axon retraction. We further show that Golgi fragmentation may occur in the absence of and precede two other pathological markers, i.e. somatodendritic SOD1 inclusions, and the induction of ATF3 expression. In addition, we show that Golgi fragmentation is associated with an altered dendritic organization of the Golgi apparatus, does not depend on intact apoptotic machinery, and is facilitated in transgenic mice with impaired retrograde dynein-dependent transport (BICD2-N mice). A connection to altered dynein-dependent transport also is suggested by reduced expression of endosomal markers in neurons with Golgi fragmentation, which also occurs in neurons with impaired dynein function.

Conclusions

Together the data indicate that Golgi fragmentation is a very early event in the pathological cascade in ALS that is associated with altered organization of intracellular trafficking.

Pathways disrupted in human ALS motor neurons identified through genetic correction of mutant SOD1

Although many distinct mutations in a variety of genes are known to cause Amyotrophic Lateral Sclerosis (ALS), it remains poorly understood how they selectively impact motor neuron biology and whether they converge on common pathways to cause neuronal degeneration. Here, we have combined reprogramming and stem cell differentiation approaches with genome engineering and RNA sequencing to define the transcriptional and functional changes that are induced in human motor neurons by mutant SOD1. Mutant SOD1 protein induced a transcriptional signature indicative of increased oxidative stress, reduced mitochondrial function, altered subcellular transport, and activation of the ER stress and unfolded protein response pathways. Functional studies demonstrated that these pathways were perturbed in a manner dependent on the SOD1 mutation. Finally, interrogation of stem-cell-derived motor neurons produced from ALS patients harboring a repeat expansion in C9orf72 indicates that at least a subset of these changes are more broadly conserved in ALS.

Intrinsic membrane hyperexcitability of amyotrophic lateral sclerosis patient-derived motor neurons

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease of the motor nervous system. We show using multielectrode array and patch-clamp recordings that hyperexcitability detected by clinical neurophysiological studies of ALS patients is recapitulated in induced pluripotent stem cell-derived motor neurons from ALS patients harboring superoxide dismutase 1 (SOD1), C9orf72, and fused-in-sarcoma mutations. Motor neurons produced from a genetically corrected but otherwise isogenic SOD1(+/+) stem cell line do not display the hyperexcitability phenotype. SOD1(A4V/+) ALS patient-derived motor neurons have reduced delayed-rectifier potassium current amplitudes relative to control-derived motor neurons, a deficit that may underlie their hyperexcitability. The Kv7 channel activator retigabine both blocks the hyperexcitability and improves motor neuron survival in vitro when tested in SOD1 mutant ALS cases. Therefore, electrophysiological characterization of human stem cell-derived neurons can reveal disease-related mechanisms and identify therapeutic candidates.

Human superoxide dismutase 1 overexpression in motor neurons of Caenorhabditis elegans causes axon guidance defect and neurodegeneration

Strong evidence indicates that mutant Cu, Zn-superoxide dismutase 1 (SOD1) exerts toxic effect on motor neurons in amyotrophic lateral sclerosis (ALS). However, the nature of mutant SOD1-mediated motor neuron degeneration is poorly understood. To provide new insight into the mechanism by which mutant SOD1 induces motor neuron injury, we developed novel Caenorhabditis elegans models of ALS. Expression of human wild type or G93A SOD1 specifically in motor neurons of C. elegans caused progressive locomotion defect and paralytic phenotype, which recapitulate some characteristic features of ALS including age-dependent motor dysfunction and degeneration of motor neurons associated with SOD1 aggregation. In addition, the motor neuron loss is independent of cell death protein 3 (CED-3)/cell death protein 4 (CED-4) caspase pathway. We also found that before motor neurons began to die in adulthood, axon guidance defect of motor neuron appeared during the development stages. When green fluorescent protein (GFP)-tagged proteins related to axon guidance were examined in motor neurons, a significantly decreased density and number of GFP-tagged puncta were observed in the transgenic worms. Our models mimic axon developmental defect and the adult-onset degeneration of motor neurons in ALS. Using this model, we uncovered the cell-autonomous damage caused by human SOD1 to motor neurons in vivo, and provided a new insight into the developmental defect mechanism that may contribute to motor neuron degeneration in ALS.

Neuronal matrix metalloproteinase-9 is a determinant of selective neurodegeneration

Selective neuronal loss is the hallmark of neurodegenerative diseases. In patients with amyotrophic lateral sclerosis (ALS), most motor neurons die but those innervating extraocular, pelvic sphincter, and slow limb muscles exhibit selective resistance. We identified 18 genes that show >10-fold differential expression between resistant and vulnerable motor neurons. One of these, matrix metalloproteinase-9 (MMP-9), is expressed only by fast motor neurons, which are selectively vulnerable. In ALS model mice expressing mutant superoxide dismutase (SOD1), reduction of MMP-9 function using gene ablation, viral gene therapy, or pharmacological inhibition significantly delayed muscle denervation. In the presence of mutant SOD1, MMP-9 expressed by fast motor neurons themselves enhances activation of ER stress and is sufficient to trigger axonal die-back. These findings define MMP-9 as a candidate therapeutic target for ALS. The molecular basis of neuronal diversity thus provides significant insights into mechanisms of selective vulnerability to neurodegeneration.

Mechanisms of neurodegeneration and axonal dysfunction in multiple sclerosis

Multiple sclerosis (MS) is the most frequent chronic inflammatory disease of the CNS, and imposes major burdens on young lives. Great progress has been made in understanding and moderating the acute inflammatory components of MS, but the pathophysiological mechanisms of the concomitant neurodegeneration--which causes irreversible disability--are still not understood. Chronic inflammatory processes that continuously disturb neuroaxonal homeostasis drive neurodegeneration, so the clinical outcome probably depends on the balance of stressor load (inflammation) and any remaining capacity for neuronal self-protection. Hence, suitable drugs that promote the latter state are sorely needed. With the aim of identifying potential novel therapeutic targets in MS, we review research on the pathological mechanisms of neuroaxonal dysfunction and injury, such as altered ion channel activity, and the endogenous neuroprotective pathways that counteract oxidative stress and mitochondrial dysfunction. We focus on mechanisms inherent to neurons and their axons, which are separable from those acting on inflammatory responses and might, therefore, represent bona fide neuroprotective drug targets with the capability to halt MS progression.

Neuropathologic assessment of dementia markers in identical and fraternal twins

Twin studies are an incomparable source of investigation to shed light on genetic and non-genetic components of neurodegenerative diseases, as Alzheimer's disease (AD). Detailed clinicopathologic correlations using twin longitudinal data and post-mortem examinations are mostly missing. We describe clinical and pathologic findings of seven monozygotic (MZ) and dizygotic (DZ) twin pairs. Our findings show good agreement between clinical and pathologic diagnoses in the majority of the twin pairs, with greater neuropathologic concordance in MZ than DZ twins. Greater neuropathologic concordance was found for β-amyloid than tau pathology within the pairs. ApoE4 was associated with higher β-amyloid and earlier dementia onset, and importantly, higher frequency of other co-occurring brain pathologies, regardless of the zygosity. Dementia onset, dementia duration, difference between twins in age at dementia onset and at death, did not correlate with AD pathology. These clinicopathologic correlations of older identical and fraternal twins support the relevance of genetic factors in AD, but not their sufficiency to determine the pathology, and consequently the disease, even in monozygotic twins. It is the interaction among genetic and non-genetic risks which plays a major role in influencing, or probably determining, the degeneration of those brain circuits associated with pathology and cognitive deficits in AD.

FUS-regulated region- and cell-type-specific transcriptome is associated with cell selectivity in ALS/FTLD

FUS is genetically and pathologically linked to amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). To clarify the RNA metabolism cascade regulated by FUS in ALS/FTLD, we compared the FUS-regulated transcriptome profiles in different lineages of primary cells from the central nervous system. The profiles of FUS-mediated gene expression and alternative splicing in motor neurons were similar to those of cortical neurons, but not to those in cerebellar neurons despite the similarity of innate transcriptome signature. The gene expression profiles in glial cells were similar to those in motor and cortical neurons. We identified certain neurological diseases-associated genes, including Mapt, Stx1a, and Scn8a, among the profiles of gene expression and alternative splicing events regulated by FUS. Thus, FUS-regulated transcriptome profiles in each cell-type may determine cellular fate in association with FUS-mediated ALS/FTLD, and identified RNA targets for FUS could be therapeutic targets for ALS/FTLD.

Human variants in the neuronal basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) transcription factor complex NPAS4/ARNT2 disrupt function

Neuronal Per-Arnt-Sim homology (PAS) Factor 4 (NPAS4) is a neuronal activity-dependent transcription factor which heterodimerises with ARNT2 to regulate genes involved in inhibitory synapse formation. NPAS4 functions to maintain excitatory/inhibitory balance in neurons, while mouse models have shown it to play roles in memory formation, social interaction and neurodegeneration. NPAS4 has therefore been implicated in a number of neuropsychiatric or neurodegenerative diseases which are underpinned by defects in excitatory/inhibitory balance. Here we have explored a broad set of non-synonymous human variants in NPAS4 and ARNT2 for disruption of NPAS4 function. We found two variants in NPAS4 (F147S and E257K) and two variants in ARNT2 (R46W and R107H) which significantly reduced transcriptional activity of the heterodimer on a luciferase reporter gene. Furthermore, we found that NPAS4.F147S was unable to activate expression of the NPAS4 target gene BDNF due to reduced dimerisation with ARNT2. Homology modelling predicts F147 in NPAS4 to lie at the dimer interface, where it appears to directly contribute to protein/protein interaction. We also found that reduced transcriptional activation by ARNT2 R46W was due to disruption of nuclear localisation. These results provide insight into the mechanisms of NPAS4/ARNT dimerisation and transcriptional activation and have potential implications for cognitive phenotypic variation and diseases such as autism, schizophrenia and dementia.

Age- and location-dependent differences in store depletion-induced h-channel plasticity in hippocampal pyramidal neurons

Disruptions of endoplasmic reticulum (ER) Ca(2+) homeostasis are heavily linked to neuronal pathology. Depletion of ER Ca(2+) stores can result in cellular dysfunction and potentially cell death, although adaptive processes exist to aid in survival. We examined the age and region dependence of one postulated, adaptive response to ER store-depletion (SD), hyperpolarization-activated cation-nonspecific (h)-channel plasticity in neurons of the dorsal and ventral hippocampus (DHC and VHC, respectively) from adolescent and adult rats. With the use of whole-cell patch-clamp recordings from the soma and dendrites of CA1 pyramidal neurons, we observed a change in h-sensitive measurements in response to SD, induced by treatment with cyclopiazonic acid, a sarcoplasmic reticulum/ER Ca(2+)-ATPase blocker. We found that whereas DHC and VHC neurons in adolescent animals respond to SD with a perisomatic expression of SD h plasticity, adult animals express SD h plasticity with a dendritic and somatodendritic locus of plasticity in DHC and VHC neurons, respectively. Furthermore, SD h plasticity in adults was dependent on membrane potential and on the activation of L-type voltage-gated Ca(2+) channels. These results suggest that cellular responses to the impairment of ER function, or ER stress, are dependent on brain region and age and that the differential expression of SD h plasticity could provide a neural basis for region- and age-dependent disease vulnerabilities.

TorsinA rescues ER-associated stress and locomotive defects in C. elegans models of ALS

Molecular mechanisms underlying neurodegenerative diseases converge at the interface of pathways impacting cellular stress, protein homeostasis and aging. Targeting the intrinsic capacities of neuroprotective proteins to restore neuronal function and/or attenuate degeneration represents a potential means toward therapeutic intervention. The product of the human DYT1 gene, torsinA, is a member of the functionally diverse AAA+ family of proteins and exhibits robust molecular-chaperone-like activity, both in vitro and in vivo. Although mutations in DYT1 are associated with a rare form of heritable generalized dystonia, the native function of torsinA seems to be cytoprotective in maintaining the cellular threshold to endoplasmic reticulum (ER) stress. Here we explore the potential for torsinA to serve as a buffer to attenuate the cellular consequences of misfolded-protein stress as it pertains to the neurodegenerative disease amyotrophic lateral sclerosis (ALS). The selective vulnerability of motor neurons to degeneration in ALS mouse models harboring mutations in superoxide dismutase (SOD1) has been found to correlate with regional-specific ER stress in brains. Using Caenorhabditis elegans as a system to model ER stress, we generated transgenic nematodes overexpressing either wild-type or mutant human SOD1 to evaluate their relative impact on ER stress induction in vivo. These studies revealed a mutant-SOD1-specific increase in ER stress that was further exacerbated by changes in temperature, all of which was robustly attenuated by co-expression of torsinA. Moreover, through complementary behavioral analysis, torsinA was able to restore normal neuronal function in mutant G85R SOD1 animals. Furthermore, torsinA targeted mutant SOD1 for degradation via the proteasome, representing mechanistic insight on the activity that torsinA has on aggregate-prone proteins. These results expand our understanding of proteostatic mechanisms influencing neuronal dysfunction in ALS, while simultaneously highlighting the potential for torsinA as a novel target for therapeutic development.

Neuronal gap junction coupling as the primary determinant of the extent of glutamate-mediated excitotoxicity

In the mammalian central nervous system (CNS), coupling of neurons by gap junctions (electrical synapses) increases during early postnatal development, then decreases, but increases in the mature CNS following neuronal injury, such as ischemia, traumatic brain injury and epilepsy. Glutamate-dependent neuronal death also occurs in the CNS during development and neuronal injury, i.e., at the time when neuronal gap junction coupling is increased. Here, we review our recent studies on regulation of neuronal gap junction coupling by glutamate in developing and injured neurons and on the role of gap junctions in neuronal cell death. A modified model of the mechanisms of glutamate-dependent neuronal death is discussed, which includes neuronal gap junction coupling as a critical part of these mechanisms.

FUS-mediated alternative splicing in the nervous system: consequences for ALS and FTLD

Mutations in fused in sarcoma (FUS) in a subset of patients with amyotrophic lateral sclerosis (ALS) linked this DNA/RNA-binding protein to neurodegeneration. Most of the mutations disrupt the nuclear localization signal which strongly suggests a loss-of-function pathomechanism, supported by cytoplasmic inclusions. FUS-positive neuronal cytoplasmic inclusions are also found in a subset of patients with frontotemporal lobar degeneration (FTLD). Here, we discuss recent data on the role of alternative splicing in FUS-mediated pathology in the central nervous system. Several groups have shown that FUS binds broadly to many transcripts in the brain and have also identified a plethora of putative splice targets; however, only ABLIM1, BRAF, Ewing sarcoma protein R1 (EWSR1), microtubule-associated protein tau (MAPT), NgCAM cell adhesion molecule (NRCAM), and netrin G1 (NTNG1) have been identified in at least three of four studies. Gene ontology analysis of all putative targets unanimously suggests a role in axon growth and cytoskeletal organization, consistent with the altered morphology of dendritic spines and axonal growth cones reported upon loss of FUS. Among the axonal targets, MAPT/tau and NTNG1 have been further validated in biochemical studies. The next challenge will be to confirm changes of FUS-mediated alternative splicing in patients and define their precise role in the pathophysiology of ALS and FTLD.

Genetic dissection of a cell-autonomous neurodegenerative disorder: lessons learned from mouse models of Niemann-Pick disease type C

Understanding neurodegenerative disease progression and its treatment requires the systematic characterization and manipulation of relevant cell types and molecular pathways. The neurodegenerative lysosomal storage disorder Niemann-Pick disease type C (NPC) is highly amenable to genetic approaches that allow exploration of the disease biology at the organismal, cellular and molecular level. Although NPC is a rare disease, genetic analysis of the associated neuropathology promises to provide insight into the logic of disease neural circuitry, selective neuron vulnerability and neural-glial interactions. The ability to control the disorder cell-autonomously and in naturally occurring spontaneous animal models that recapitulate many aspects of the human disease allows for an unparalleled dissection of the disease neurobiology in vivo. Here, we review progress in mouse-model-based studies of NPC disease, specifically focusing on the subtype that is caused by a deficiency in NPC1, a sterol-binding late endosomal membrane protein involved in lipid trafficking. We also discuss recent findings and future directions in NPC disease research that are pertinent to understanding the cellular and molecular mechanisms underlying neurodegeneration in general.

Functional contribution of the transcription factor ATF4 to the pathogenesis of amyotrophic lateral sclerosis

Endoplasmic reticulum (ER) stress represents an early pathological event in amyotrophic lateral sclerosis (ALS). ATF4 is a key ER stress transcription factor that plays a role in both adaptation to stress and the activation of apoptosis. Here we investigated the contribution of ATF4 to ALS. ATF4 deficiency reduced the rate of birth of SOD1^G86R transgenic mice. The fraction of ATF4^−/−-SOD1^G85R transgenic mice that were born are more resistant to develop ALS, leading to delayed disease onset and prolonged life span. ATF4 deficiency completely attenuated the induction of pro-apoptotic genes, including BIM and CHOP, and also led to quantitative changes in the ER protein homeostasis network. Unexpectedly, ATF4 deficiency enhanced mutant SOD1 aggregation at the end stage of the disease. Studies in the motoneuron cell line NSC34 demonstrated that knocking down ATF4 enhances mutant SOD1 aggregation possibly due to alteration in the redox status of the cell. Our results support a functional role of ATF4 in ALS, offering a novel target for disease intervention.

Characterization of early pathogenesis in the SOD1(G93A) mouse model of ALS: part I, background and methods

Charcot first described amyotrophic lateral sclerosis (ALS) in 1869; however, its causes remain largely unknown and effective, long-term treatment strategies are not available. The first mouse model of ALS was developed after the identification of mutations in the superoxide dismutase 1 (SOD1) gene in 1993, and accordingly most of our knowledge of the etiology and pathogenesis of the disease comes from studies carried out using this animal model. Although numerous preclinical trials have been conducted in the mutant SOD1 mouse models, the results have been disappointing because they did not positively translate to clinical trials. One explanation may be that current understanding of when and where pathogenesis begins is insufficient to accurately guide preclinical trials. Further characterization of these early events may provide insight into disease onset, help in the discovery of presymptomatic diagnostic disease markers, and identify novel therapeutic targets. Here, we describe the rationale, approach, and methods for our extensive analysis of early changes that included an ultrastructural examination of central and peripheral components of the neuromuscular system in the SOD1(G93A) mouse and correlated these alterations with early muscle denervation, motor dysfunction, and motoneuron death. We also provide a discussion of published work to review what is known regarding early pathology in the SOD1 mouse model of ALS. The significance of this work is that we have examined early pathology simultaneously in both the spinal cord and peripheral neuromuscular system, and the results are presented in the companion paper (Part II, Results and Discussion). Our results provide evidence as to why a thorough characterization of animal models throughout the life span is critical for a strong foundation to design preclinical trials that may produce meaningful results.

Crosstalk between Endoplasmic Reticulum Stress and Protein Misfolding in Neurodegenerative Diseases

Under physiological conditions, the endoplasmic reticulum (ER) is a central subcellular compartment for protein quality control in the secretory pathway that prevents protein misfolding and aggregation. Instrumental in protein quality control in the ER is the unfolded protein response (UPR), which is activated upon ER stress to reestablish homeostasis through a sophisticated transcriptionally and translationally regulated signaling network. However, this response can lead to apoptosis if the stress cannot be alleviated. The presence of abnormal protein aggregates containing specific misfolded proteins is recognized as the basis of numerous human conformational disorders, including neurodegenerative diseases. Here, I will highlight the overwhelming evidence that the presence of specific aberrant proteins in Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), prion diseases, and Amyotrophic Lateral Sclerosis (ALS) is intimately associated with perturbations in the ER protein quality control machinery that become incompetent to restore protein homeostasis and shift adaptive programs toward the induction of apoptotic signaling to eliminate irreversibly damaged neurons. Increasing our understanding about the deadly crosstalk between ER dysfunction and protein misfolding in these neurodegenerative diseases may stimulate the development of novel therapeutic strategies able to support neuronal survival and ameliorate disease progression.

Ataxia, dementia, and hypogonadotropism caused by disordered ubiquitination

BACKGROUND

The combination of ataxia and hypogonadism was first described more than a century ago, but its genetic basis has remained elusive.

METHODS

We performed whole-exome sequencing in a patient with ataxia and hypogonadotropic hypogonadism, followed by targeted sequencing of candidate genes in similarly affected patients. Neurologic and reproductive endocrine phenotypes were characterized in detail. The effects of sequence variants and the presence of an epistatic interaction were tested in a zebrafish model.

RESULTS

Digenic homozygous mutations in RNF216 and OTUD4, which encode a ubiquitin E3 ligase and a deubiquitinase, respectively, were found in three affected siblings in a consanguineous family. Additional screening identified compound heterozygous truncating mutations in RNF216 in an unrelated patient and single heterozygous deleterious mutations in four other patients. Knockdown of rnf216 or otud4 in zebrafish embryos induced defects in the eye, optic tectum, and cerebellum; combinatorial suppression of both genes exacerbated these phenotypes, which were rescued by nonmutant, but not mutant, human RNF216 or OTUD4 messenger RNA. All patients had progressive ataxia and dementia. Neuronal loss was observed in cerebellar pathways and the hippocampus; surviving hippocampal neurons contained ubiquitin-immunoreactive intranuclear inclusions. Defects were detected at the hypothalamic and pituitary levels of the reproductive endocrine axis.

CONCLUSIONS

The syndrome of hypogonadotropic hypogonadism, ataxia, and dementia can be caused by inactivating mutations in RNF216 or by the combination of mutations in RNF216 and OTUD4. These findings link disordered ubiquitination to neurodegeneration and reproductive dysfunction and highlight the power of whole-exome sequencing in combination with functional studies to unveil genetic interactions that cause disease. (Funded by the National Institutes of Health and others.).

The cell biology of prion-like spread of protein aggregates: mechanisms and implication in neurodegeneration

The misfolding and aggregation of specific proteins is a common hallmark of many neurodegenerative disorders, including highly prevalent illnesses such as Alzheimer's and Parkinson's diseases, as well as rarer disorders such as Huntington's and prion diseases. Among these, only prion diseases are ‘infectious’. By seeding misfolding of the PrPC (normal conformer prion protein) into PrPSc (abnormal disease-specific conformation of prion protein), prions spread from the periphery of the body to the central nervous system and can also be transmitted between individuals of the same or different species. However, recent exciting data suggest that the transmissibility of misfolded proteins within the brain is a property that goes way beyond the rare prion diseases. Evidence indicates that non-prion aggregates [tau, α-syn (α-synuclein), Aβ (amyloid-β) and Htt (huntingtin) aggregates] can also move between cells and seed the misfolding of their normal conformers. These findings have enormous implications. On the one hand they question the therapeutical use of transplants, and on the other they indicate that it may be possible to bring these diseases to an early arrest by preventing cell-to-cell transmission. To better understand the prion-like spread of these protein aggregates it is essential to identify the underlying cellular and molecular factors. In the present review we analyse and discuss the evidence supporting prion-like spreading of amyloidogenic proteins, especially focusing on the cellular and molecular mechanisms and their significance.

The endocannabinoid system in normal and pathological brain ageing

The role of endocannabinoids as inhibitory retrograde transmitters is now widely known and intensively studied. However, endocannabinoids also influence neuronal activity by exerting neuroprotective effects and regulating glial responses. This review centres around this less-studied area, focusing on the cellular and molecular mechanisms underlying the protective effect of the cannabinoid system in brain ageing. The progression of ageing is largely determined by the balance between detrimental, pro-ageing, largely stochastic processes, and the activity of the homeostatic defence system. Experimental evidence suggests that the cannabinoid system is part of the latter system. Cannabinoids as regulators of mitochondrial activity, as anti-oxidants and as modulators of clearance processes protect neurons on the molecular level. On the cellular level, the cannabinoid system regulates the expression of brain-derived neurotrophic factor and neurogenesis. Neuroinflammatory processes contributing to the progression of normal brain ageing and to the pathogenesis of neurodegenerative diseases are suppressed by cannabinoids, suggesting that they may also influence the ageing process on the system level. In good agreement with the hypothesized beneficial role of cannabinoid system activity against brain ageing, it was shown that animals lacking CB1 receptors show early onset of learning deficits associated with age-related histological and molecular changes. In preclinical models of neurodegenerative disorders, cannabinoids show beneficial effects, but the clinical evidence regarding their efficacy as therapeutic tools is either inconclusive or still missing.

Alterations in the motor neuron-renshaw cell circuit in the Sod1(G93A) mouse model

Motor neurons become hyperexcitable during progression of amyotrophic lateral sclerosis (ALS). This abnormal firing behavior has been explained by changes in their membrane properties, but more recently it has been suggested that changes in premotor circuits may also contribute to this abnormal activity. The specific circuits that may be altered during development of ALS have not been investigated. Here we examined the Renshaw cell recurrent circuit that exerts inhibitory feedback control on motor neuron firing. Using two markers for Renshaw cells (calbindin and cholinergic nicotinic receptor subunit alpha2 [Chrna2]), two general markers for motor neurons (NeuN and vesicular acethylcholine transporter [VAChT]), and two markers for fast motor neurons (Chondrolectin and calcitonin-related polypeptide alpha [Calca]), we analyzed the survival and connectivity of these cells during disease progression in the Sod1(G93A) mouse model. Most calbindin-immunoreactive (IR) Renshaw cells survive to end stage but downregulate postsynaptic Chrna2 in presymptomatic animals. In motor neurons, some markers are downregulated early (NeuN, VAChT, Chondrolectin) and others at end stage (Calca). Early downregulation of presynaptic VAChT and Chrna2 was correlated with disconnection from Renshaw cells as well as major structural abnormalities of motor axon synapses inside the spinal cord. Renshaw cell synapses on motor neurons underwent more complex changes, including transitional sprouting preferentially over remaining NeuN-IR motor neurons. We conclude that the loss of presynaptic motor axon input on Renshaw cells occurs at early stages of ALS and disconnects the recurrent inhibitory circuit, presumably resulting in diminished control of motor neuron firing. J. Comp. Neurol. 521:1449-1469, 2013. © 2012 Wiley Periodicals, Inc.

The changing scene of amyotrophic lateral sclerosis

Several recent breakthroughs have provided notable insights into the pathogenesis of amyotrophic lateral sclerosis (ALS), with some even shifting our thinking about this neurodegenerative disease and raising the question as to whether this disorder is a proteinopathy, a ribonucleopathy or both. In addition, these breakthroughs have revealed mechanistic links between ALS and frontotemporal dementia, as well as between ALS and other neurodegenerative diseases, such as the cerebellar atrophies, myotonic dystrophy and inclusion body myositis. Here, we summarize the new findings in ALS research, discuss what they have taught us about this disease and examine issues that are still outstanding.

TDP-43 loss-of-function causes neuronal loss due to defective steroid receptor-mediated gene program switching in Drosophila

TDP-43 proteinopathy is strongly implicated in the pathogenesis of amyotrophic lateral sclerosis and related neurodegenerative disorders. Whether TDP-43 neurotoxicity is caused by a novel toxic gain-of-function mechanism of the aggregates or by a loss of its normal function is unknown. We increased and decreased expression of TDP-43 (dTDP-43) in Drosophila. Although upregulation of dTDP-43 induced neuronal ubiquitin and dTDP-43-positive inclusions, both up- and downregulated dTDP-43 resulted in selective apoptosis of bursicon neurons and highly similar transcriptome alterations at the pupal-adult transition. Gene network analysis and genetic validation showed that both up- and downregulated dTDP-43 directly and dramatically increased the expression of the neuronal microtubule-associated protein Map205, resulting in cytoplasmic accumulations of the ecdysteroid receptor (EcR) and a failure to switch EcR-dependent gene programs from a pupal to adult pattern. We propose that dTDP-43 neurotoxicity is caused by a loss of its normal function.

Neuroinflammation and proteostasis are modulated by endogenously biosynthesized neuroprotectin D1

Neurodegenerative diseases encompass complex cell signaling disturbances that initially damage neuronal circuits and synapses. Due to multiple protective mechanisms enacted to counteract the onset of neurodegenerative diseases, there is often a prolonged period without noticeable impairments during their initiation. Since severe cognitive deficit or vision loss takes place after that period there is an opportunity to harness endogenous protective mechanisms as potential therapeutic approaches. The activation of the biosynthesis of the docosanoid mediator neuroprotectin D1 (NPD1) is an early response to the upsurge of protein misfolding and other neuroinflammatory events. This overview discusses the potent neuroprotective and inflammation-modulating bioactivity of NPD1. This lipid mediator represents an early response to neurodegenerations, aiming to restore homeostasis.

Restricted transgene expression in the brain with cell-type specific neuronal promoters

Tissue-targeted expression is of major interest for studying the contribution of cellular subpopulations to neurodegenerative diseases. However, in vivo methods to investigate this issue are limited. Here, we report an analysis of the cell specificity of expression of fluorescent reporter genes driven by six neuronal promoters, with the ubiquitous phosphoglycerate kinase 1 (PGK) promoter used as a reference. Quantitative analysis of AcGFPnuc expression in the striatum and hippocampus of rodents showed that all lentiviral vectors (LV) exhibited a neuronal tropism; however, there was substantial diversity of transcriptional activity and cell-type specificity of expression. The promoters with the highest activity were those of the 67 kDa glutamic acid decarboxylase (GAD67), homeobox Dlx5/6, glutamate receptor 1 (GluR1), and preprotachykinin 1 (Tac1) genes. Neuron-specific enolase (NSE) and dopaminergic receptor 1 (Drd1a) promoters showed weak activity, but the integration of an amplification system into the LV overcame this limitation. In the striatum, the expression profiles of Tac1 and Drd1a were not limited to the striatonigral pathway, whereas in the hippocampus, Drd1a and Dlx5/6 showed the expected restricted pattern of expression. Regulation of the Dlx5/6 promoter was observed in a disease condition, whereas Tac1 activity was unaffected. These vectors provide safe tools that are more selective than others available, for the administration of therapeutic molecules in the central nervous system (CNS). Nevertheless, additional characterization of regulatory elements in neuronal promoters is still required.

Protein disulfide isomerases in neurodegeneration: from disease mechanisms to biomedical applications

Protein disulfide isomerases (PDIs) are a family of foldases and chaperones primarily located at the endoplasmic reticulum that catalyze the formation and isomerization of disulfide bonds thereby facilitating protein folding. PDIs also perform important physiological functions in protein quality control, cell death, and cell signaling. Protein misfolding is involved in the etiology of the most common neurodegenerative diseases, including Alzheimer, Parkinson, amyotrophic lateral sclerosis, Prion-related disorders, among others. Accumulating evidence indicate altered expression of PDIs as a prominent and common feature of these neurodegenerative conditions. Here we overview most recent advances in our understanding of the possible functional contribution of PDIs to neurodegeneration, depicting a complex and poorly understood scenario. Possible therapeutic benefits of targeting PDIs in a disease context and their use as biomarkers are discussed.

Epigenetic mechanisms governing the process of neurodegeneration

Studies elucidating how and why neurodegeneration unfolds suggest that a complex interplay between genetic and environmental factors is responsible for disease pathogenesis. Recent breakthroughs in the field of epigenetics promise to advance our understanding of these mechanisms and to promote the development of useful and effective pre-clinical risk stratification strategies, molecular diagnostic and prognostic methods, and disease-modifying treatments.

Hypoxia is a modifier of SMN2 splicing and disease severity in a severe SMA mouse model

Spinal muscular atrophy (SMA) is a progressive neurodegenerative disease associated with low levels of the essential survival motor neuron (SMN) protein. Reduced levels of SMN is due to the loss of the SMN1 gene and inefficient splicing of the SMN2 gene caused by a C>T mutation in exon 7. Global analysis of the severe SMNΔ7 SMA mouse model revealed altered splicing and increased levels of the hypoxia-inducible transcript, Hif3alpha, at late stages of disease progression. Severe SMA patients also develop respiratory deficiency during disease progression. We sought to evaluate whether hypoxia was capable of altering SMN2 exon 7 splicing and whether increased oxygenation could modulate disease in a severe SMA mouse model. Hypoxia treatment in cell culture increased SMN2 exon 7 skipping and reduced SMN protein levels. Concordantly, the treatment of SMNΔ7 mice with hyperoxia treatment increased the inclusion of SMN2 exon 7 in skeletal muscles and resulted in improved motor function. Transfection splicing assays of SMN minigenes under hypoxia revealed that hypoxia-induced skipping is dependent on poor exon definition due to the SMN2 C>T mutation and suboptimal 5' splice site. Hypoxia treatment in cell culture led to increased hnRNP A1 and Sam68 levels. Mutation of hnRNP A1-binding sites prevented hypoxia-induced skipping of SMN exon 7 and was found to bind both hnRNP A1 and Sam68. These results implicate hypoxic stress as a modulator of SMN2 exon 7 splicing in disease progression and a coordinated regulation by hnRNP A1 and Sam68 as modifiers of hypoxia-induced skipping of SMN exon 7.

Targeting neurodegenerative diseases: Drug discovery in a challenging arena

Neurodegenerative diseases represent one of the health care community’s truly unmet medical needs. They can be loosely classified into two categories, acute and chronic. One of the best known chronic neurodegenerative diseases, Alzheimer’s disease, represents a serious health care problem that may well exceed the limits of current fiscal and care giver resources. No disease-modifying therapeutic agents have been identified, and the few available symptomatic treatments possess limitations in their duration of action and side effects. Despite decades of drug discovery research and numerous clinical trials, no truly effective treatment for stroke, the most prevalent acute neurodegenerative disease, has been identified. This article summarizes two recent drug discovery projects, one targeting Alzheimer’s disease and the other targeting ischemic stroke. Both projects involved design, synthesis, and biological evaluation of a novel series of heterocyclic derivatives.

HSP-4 endoplasmic reticulum (ER) stress pathway is not activated in a C. elegans model of ethanol intoxication and withdrawal

Acute and chronic exposure of Caenorhabditis elegans to concentrations of ethanol in the range 250-350 mM elicits distinct behaviours. Previous genetic analysis highlights specific neurobiological substrates for these effects. However, ethanol may also elicit cellular stress responses which may contribute to the repertoire of ethanol-induced behaviours. Here, we have studied the effect of ethanol on an important arm of the cellular stress pathways, which emanates from the endoplasmic reticulum (ER) in response to several conditions including heat shock and chemical or genetic perturbations that lead to protein misfolding. HSP-4 is a heat shock protein and homologue of mammalian BiP. It is a pivotal upstream component of the ER stress response. Therefore, we used a C. elegans heat shock protein mutant, hsp-4, and a strain carrying a transcriptional reporter, Phsp-4::gfp, to test the role of the ER following chronic ethanol conditioning. We found no evidence for an overt ER response during acute or prolonged exposure to concentrations of ethanol that lead to defined ethanol-induced behaviours. Furthermore, whilst hsp-4 was strongly induced by tunicamycin, pre-exposure of C. elegans to low doses of tunicamycin followed by ethanol was not sufficient to induce an additive ER stress response. Behavioural analysis of an hsp-4 mutant indicated no difference compared to wild type in susceptibility to ethanol intoxication and withdrawal. There is a clear precedent for a significance of ER stress pathways particularly in clinical conditions associated with toxic or pathological effects of high doses of alcohol consumption. The concentrations of ethanol used in this C. elegans study equate to the highest blood alcohol levels measured in patients with chronic alcohol dependency. Taken together, these observations imply that the classic ER stress pathway in C. elegans is relatively refractory to induction by ethanol.

Hormesis: protecting neurons against cellular stress in Parkinson disease

Protein folding stress is a salient feature of the most frequent neurodegenerative diseases. Although the accumulation of abnormally folded proteins is a well-characterized event underlying the pathology, the way cells respond to this phenomenon is not well understood. Signs of endoplasmic reticulum (ER) stress are a common marker of neurodegeneration in many diseases, which may represent two contrasting processes: cell protection events due to activation of adaptive programs, or a chronic stress state that culminates in apoptosis to eliminate irreversibly injured cells. Autophagy has been proposed as a protective mechanism to overcome neurodegeneration that is also modulated by ER stress. In this issue of autophagy Bertrand Mollereau's group provides novel evidence indicating that engagement of nonharmful levels of ER stress protects against experimental Parkinson disease. At the mechanistic level, a homeostatic crosstalk between ER stress signaling and the autophagy pathway was proposed to mediate the therapeutic effects. This study, together with recent findings, supports the involvement of a "hormesis mechanism" to handle degeneration through preconditioning mediated by a dynamic balance between ER stress and autophagy. The implications for aging and future therapeutic development are discussed.

Spatial distribution and secular trends in the epidemiology of Alzheimer's disease

There are well-established differences in dementia incidence between communities and within communities over time. In part, these differences may be attributable to local improvements in dementia diagnosis and classification. Nevertheless, there are grounds for cautious optimism that there have been slight, but significant, recent reductions in dementia incidence. Possible causes include public health measures to reduce mortality attributable to stroke and heart disease, improved nutrition, and greater personal wealth. A life-course approach to dementia pathophysiology may help to elucidate the nature and timing of interventions that might delay dementia onset.

Reduced calreticulin levels link endoplasmic reticulum stress and Fas-triggered cell death in motoneurons vulnerable to ALS

Cellular responses to protein misfolding are thought to play key roles in triggering neurodegeneration. In the mutant superoxide dismutase (mSOD1) model of amyotrophic lateral sclerosis (ALS), subsets of motoneurons are selectively vulnerable to degeneration. Fast fatigable motoneurons selectively activate an endoplasmic reticulum (ER) stress response that drives their early degeneration while a subset of mSOD1 motoneurons show exacerbated sensitivity to activation of the motoneuron-specific Fas/NO pathway. However, the links between the two mechanisms and the molecular basis of their cellular specificity remained unclear. We show that Fas activation leads, specifically in mSOD1 motoneurons, to reductions in levels of calreticulin (CRT), a calcium-binding ER chaperone. Decreased expression of CRT is both necessary and sufficient to trigger SOD1(G93A) motoneuron death through the Fas/NO pathway. In SOD1(G93A) mice in vivo, reductions in CRT precede muscle denervation and are restricted to vulnerable motor pools. In vitro, both reduced CRT and Fas activation trigger an ER stress response that is restricted to, and required for death of, vulnerable SOD1(G93A) motoneurons. Our data reveal CRT as a critical link between a motoneuron-specific death pathway and the ER stress response and point to a role of CRT levels in modulating motoneuron vulnerability to ALS.

Altered gene expression, mitochondrial damage and oxidative stress: converging routes in motor neuron degeneration

Motor neuron diseases (MNDs) are a rather heterogeneous group of diseases, with either sporadic or genetic origin or both, all characterized by the progressive degeneration of motor neurons. At the cellular level, MNDs share features such as protein misfolding and aggregation, mitochondrial damage and energy deficit, and excitotoxicity and calcium mishandling. This is particularly well demonstrated in ALS, where both sporadic and familial forms share the same symptoms and pathological phenotype, with a prominent role for mitochondrial damage and resulting oxidative stress. Based on recent data, however, altered control of gene expression seems to be a most relevant, and previously overlooked, player in MNDs. Here we discuss which may be the links that make pathways apparently as different as altered gene expression, mitochondrial damage, and oxidative stress converge to generate a similar motoneuron-toxic phenotype.

Requirements for stress granule recruitment of fused in sarcoma (FUS) and TAR DNA-binding protein of 43 kDa (TDP-43)

Cytoplasmic inclusions containing TAR DNA-binding protein of 43 kDa (TDP-43) or Fused in sarcoma (FUS) are a hallmark of amyotrophic lateral sclerosis (ALS) and several subtypes of frontotemporal lobar degeneration (FTLD). FUS-positive inclusions in FTLD and ALS patients are consistently co-labeled with stress granule (SG) marker proteins. Whether TDP-43 inclusions contain SG markers is currently still debated. We determined the requirements for SG recruitment of FUS and TDP-43 and found that cytoplasmic mislocalization is a common prerequisite for SG recruitment of FUS and TDP-43. For FUS, the arginine-glycine-glycine zinc finger domain, which is the protein's main RNA binding domain, is most important for SG recruitment, whereas the glycine-rich domain and RNA recognition motif (RRM) domain have a minor contribution and the glutamine-rich domain is dispensable. For TDP-43, both the RRM1 and the C-terminal glycine-rich domain are required for SG localization. ALS-associated point mutations located in the glycine-rich domain of TDP-43 do not affect SG recruitment. Interestingly, a 25-kDa C-terminal fragment of TDP-43, which is enriched in FTLD/ALS cortical inclusions but not spinal cord inclusions, fails to be recruited into SG. Consistently, inclusions in the cortex of FTLD patients, which are enriched for C-terminal fragments, are not co-labeled with the SG marker poly(A)-binding protein 1 (PABP-1), whereas inclusions in spinal cord, which contain full-length TDP-43, are frequently positive for this marker protein.

Intercellular (mis)communication in neurodegenerative disease

Neurodegenerative diseases have been intensively studied, but a comprehensive understanding of their pathogenesis remains elusive. An increasing body of evidence suggests that non-cell-autonomous processes play critical roles during the initiation and spatiotemporal progression or propagation of the dominant pathology. Here, we review findings highlighting the importance of pathological cell-cell communication in neurodegenerative disease. We focus primarily on the accumulating evidence suggesting dysfunctional crosstalk between neurons and astroglia, neurons and innate immune system cells, as well as cellular processes leading to transmission of pathogenic proteins between cells. Insights into the complex intercellular perturbations underlying neurodegeneration will enhance our efforts to develop effective therapeutic approaches for preventing or reversing symptomatic progression in this devastating class of human diseases.

Iron and ER stress in neurodegenerative disease

Neurodegenerative disease is a condition in which subpopulations of neuronal cells of the brain and spinal cord are selectively lost. A common event in many neurodegenerative diseases is the increased level of endoplasmic reticulum (ER) stress caused by accumulation and deposits of inclusion bodies that contain abnormal aggregated proteins. However, the basis of how ER stress contributes to the selective neuronal vulnerability and degeneration remain elusive. Iron accumulation in the central nerve system is consistently present in many neurodegenerative diseases. In the past 5 years we have begun to show a relationship between polymorphisms in the HFE (high iron) gene and the risk of neurodegenerative disorders. Recent findings have suggested a connection between ER stress and iron metabolism and neurodegeneration. Here we review how the different levels of chronic ER stress contribute to the different fates of neurons, namely the adaptive response and neuronal death. And, we discuss the roles of iron and HFE genotype in selective neuronal vulnerability and degeneration through modifying the ER stress level.