Mechanism of Pacemaker Activity in Zebrafish DC2/4 Dopaminergic Neurons.

Zebrafish models are used increasingly to study the molecular pathogenesis of Parkinson's disease (PD), owing to the extensive array of techniques available for their experimental manipulation and analysis. The ascending dopaminergic projection from the posterior tuberculum (TPp; diencephalic populations DC2 and DC4) to the subpallium is considered the zebrafish correlate of the mammalian nigrostriatal projection, but little is known about the neurophysiology of zebrafish DC2/4 neurons. This is an important knowledge gap, because autonomous activity in mammalian substantia nigra (SNc) dopaminergic neurons contributes to their vulnerability in PD models. Using a new transgenic zebrafish line to label living dopaminergic neurons, and a novel brain slice preparation, we conducted whole-cell patch clamp recordings of DC2/4 neurons from adult zebrafish of both sexes. Zebrafish DC2/4 neurons share many physiological properties with mammalian dopaminergic neurons, including the cell-autonomous generation of action potentials. However, in contrast to mammalian dopaminergic neurons, the pacemaker driving intrinsic rhythmic activity in zebrafish DC2/4 neurons does not involve calcium conductances, hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, or sodium leak currents. Instead, voltage clamp recordings and computational models show that interactions between three components - a small, predominantly potassium, leak conductance, voltage-gated sodium channels, and voltage-gated potassium channels - are sufficient for pacemaker activity in zebrafish DC2/4 neurons. These results contribute to understanding the comparative physiology of the dopaminergic system and provide a conceptual basis for interpreting data derived from zebrafish PD models. The findings further suggest new experimental opportunities to address the role of dopaminergic pacemaker activity in the pathogenesis of PD.SIGNIFICANCE STATEMENT Posterior tuberculum (TPp) DC2/4 dopaminergic neurons are considered the zebrafish correlate of mammalian substantia nigra (SNc) neurons, whose degeneration causes the motor signs of Parkinson's disease (PD). Our study shows that DC2/4 and SNc neurons share a number of electrophysiological properties, including depolarized membrane potential, high input resistance, and continual, cell-autonomous pacemaker activity, that strengthen the basis for the increasing use of zebrafish models to study the molecular pathogenesis of PD. The mechanisms driving pacemaker activity differ between DC2/4 and SNc neurons, providing: (1) experimental opportunities to dissociate the contributions of intrinsic activity and underlying pacemaker currents to pathogenesis; and (2) essential information for the design and interpretation of studies using zebrafish PD models.

NADPH oxidase 2 activity in Parkinson's disease.

Mitochondrial dysfunction and oxidative stress are strongly implicated in Parkinson's disease (PD) pathogenesis and there is evidence that mitochondrially-generated superoxide can activate NADPH oxidase 2 (NOX2). Although NOX2 has been examined in the context of PD, most attention has focused on glial NOX2, and the role of neuronal NOX2 in PD remains to be defined. Additionally, pharmacological NOX2 inhibitors have typically lacked specificity. Here we devised and validated a proximity ligation assay for NOX2 activity and demonstrated that in human PD and two animal models thereof, both neuronal and microglial NOX2 are highly active in substantia nigra under chronic conditions. However, in acute and sub-acute PD models, we observed neuronal, but not microglial NOX2 activation, suggesting that neuronal NOX2 may play a primary role in the early stages of the disease. Aberrant NOX2 activity is responsible for the formation of oxidative stress-related post-translational modifications of α-synuclein, and impaired mitochondrial protein import in vitro in primary ventral midbrain neuronal cultures and in vivo in nigrostriatal neurons in rats. In a rat model, administration of a brain-penetrant, highly specific NOX2 inhibitor prevented NOX2 activation in nigrostriatal neurons and its downstream effects in vivo, such as activation of leucine-rich repeat kinase 2 (LRRK2). We conclude that NOX2 is an important enzyme that contributes to progressive oxidative damage which in turn can lead to α-synuclein accumulation, mitochondrial protein import impairment, and LRRK2 activation. In this context, NOX2 inhibitors hold potential as a disease-modifying therapy in PD.

Quantification of functional recovery in a larval zebrafish model of spinal cord injury.

Human spinal cord injury (SCI) is characterized by permanent loss of damaged axons, resulting in chronic disability. In contrast, zebrafish can regenerate axonal projections following central nervous system injury and re-establish synaptic contacts with distant targets; elucidation of the underlying molecular events is an important goal with translational potential for improving outcomes in SCI patients. We generated transgenic zebrafish with GFP-labeled axons and transected their spinal cords at 10 days post-fertilization. Intravital confocal microscopy revealed robust axonal regeneration following the procedure, with abundant axons bridging the transection site by 48 h post-injury. In order to analyze neurological function in this model, we developed and validated new open-source software to measure zebrafish lateral trunk curvature during propulsive and turning movements at high temporal resolution. Immediately following spinal cord transection, axial movements were dramatically decreased caudal to the lesion site, but preserved rostral to the injury, suggesting the induction of motor paralysis below the transection level. Over the subsequent 96 h, the magnitude of movements caudal to the lesion recovered to baseline, but the rate of change of truncal curvature did not fully recover, suggesting incomplete restoration of caudal strength over this time course. Quantification of both morphological and functional recovery following SCI will be important for the analysis of axonal regeneration and downstream events necessary for restoration of motor function. An extensive array of genetic and pharmacological interventions can be deployed in the larval zebrafish model to investigate the underlying molecular mechanisms.

Regeneration of the zebrafish retinal pigment epithelium after widespread genetic ablation.

The retinal pigment epithelium (RPE) is a specialized monolayer of pigmented cells within the eye that is critical for maintaining visual system function. Diseases affecting the RPE have dire consequences for vision, and the most prevalent of these is atrophic (dry) age-related macular degeneration (AMD), which is thought to result from RPE dysfunction and degeneration. An intriguing possibility for treating RPE degenerative diseases like atrophic AMD is the stimulation of endogenous RPE regeneration; however, very little is known about the mechanisms driving successful RPE regeneration in vivo. Here, we developed a zebrafish transgenic model (rpe65a:nfsB-eGFP) that enabled ablation of large swathes of mature RPE. RPE ablation resulted in rapid RPE degeneration, as well as degeneration of Bruch's membrane and underlying photoreceptors. Using this model, we demonstrate for the first time that zebrafish are capable of regenerating a functional RPE monolayer after RPE ablation. Regenerated RPE cells first appear at the periphery of the RPE, and regeneration proceeds in a peripheral-to-central fashion. RPE ablation elicits a robust proliferative response in the remaining RPE. Subsequently, proliferative cells move into the injury site and differentiate into RPE. BrdU incorporation assays demonstrate that the regenerated RPE is likely derived from remaining peripheral RPE cells. Pharmacological disruption using IWR-1, a Wnt signaling antagonist, significantly reduces cell proliferation in the RPE and impairs overall RPE recovery. These data demonstrate that the zebrafish RPE possesses a robust capacity for regeneration and highlight a potential mechanism through which endogenous RPE regenerate in vivo.

Long-term RNAi knockdown of α-synuclein in the adult rat substantia nigra without neurodegeneration.

α-Synuclein plays a central role in the pathogenesis of Parkinson's disease (PD); interventions that decrease its expression appear neuroprotective in PD models. Successful translation of these observations into effective therapies will be dependent on the safety of suppressing α-synuclein expression in the adult brain. We investigated long-term α-synuclein knockdown in the adult rat CNS. 8-month old animals received either AAV-sh[Snca] (an RNA interference vector targeting the Snca mRNA transcript) or AAV-sh[Ctrl] (a control vector) unilaterally into the substantia nigra. No signs of systemic toxicity or motor dysfunction were observed in either experimental group over 12 months. Viral transgene expression persisted to 12 months post-inoculation, at which point Snca mRNA expression in substantia nigra dopaminergic neurons of animals that received AAV-sh[Snca] was decreased by ≈90%, and α-synuclein immunoreactivity by >70% relative to the control side. Stereological quantification of Nissl-labeled neurons showed no evidence of neurodegeneration in the substantia nigra 12 months after inoculation with either vector, and we observed abundant dopaminergic neurons with minimal α-synuclein immunoreactivity that appeared otherwise unremarkable in the AAV-sh[Snca] group. Despite the absence of neurodegeneration, some loss of TH expression was evident in nigral neurons after transduction with either vector, presumably a non-specific consequence of vector delivery, cellular transduction, or expression of shRNA or GFP. We conclude that long-term α-synuclein knockdown in the substantia nigra does not cause significant functional deficits in the ascending dopaminergic projection, or neurodegeneration. These findings are encouraging that it may be feasible to target α-synuclein expression therapeutically in PD.

Ablation of the pro-inflammatory master regulator miR-155 does not mitigate neuroinflammation or neurodegeneration in a vertebrate model of Gaucher's disease.

Bi-allelic mutations in the glucocerebrosidase gene (GBA1) cause Gaucher's disease, the most common human lysosomal storage disease. We previously reported a marked increase in miR-155 transcript levels and early microglial activation in a zebrafish model of Gaucher's disease (gba1-/-). miR-155 is a master regulator of inflammation and has been implicated in a wide range of different neurodegenerative disorders. The observed miR-155 upregulation preceded the subsequent development of widespread pathology with marked neuroinflammation, closely resembling human Gaucher's disease pathology. We now report similar increases of miR-155 expression in mammalian models of GD, confirming that miR-155 upregulation is a shared feature in glucocerebrosidase (GCase) deficiency across different species. Using CRISPR/Cas9 mutagenesis we then generated a miR-155 mutant zebrafish line (miR-155-/-) with completely abolished miR-155 expression. Unexpectedly, loss of miR-155 did not mitigate either the reduced lifespan or the robust inflammatory phenotypes of gba1-/- mutant zebrafish. Our data demonstrate that neither neuroinflammation nor disease progression in GCase deficiency are dependent on miR-155 and suggest that miR-155 inhibition would not be a promising therapeutic target in Gaucher's disease.

Astrocyte-specific DJ-1 overexpression protects against rotenone-induced neurotoxicity in a rat model of Parkinson's disease.

DJ-1 is a redox-sensitive protein with several putative functions important in mitochondrial physiology, protein transcription, proteasome regulation, and chaperone activity. High levels of DJ-1 immunoreactivity are reported in astrocytes surrounding pathology associated with idiopathic Parkinson's disease, possibly reflecting the glial response to oxidative damage. Previous studies showed that astrocytic over-expression of DJ-1 in vitro prevented oxidative stress and mitochondrial dysfunction in primary neurons. Based on these observations, we developed a pseudotyped lentiviral gene transfer vector with specific tropism for CNS astrocytes in vivo to overexpress human DJ-1 protein in astroglial cells. Following vector delivery to the substantia nigra and striatum of adult Lewis rats, the DJ-1 transgene was expressed robustly and specifically within astrocytes. There was no observable transgene expression in neurons or other glial cell types. Three weeks after vector infusion, animals were exposed to rotenone to induce Parkinson's disease-like pathology, including loss of dopaminergic neurons, accumulation of endogenous α-synuclein, and neuroinflammation. Animals over-expressing hDJ-1 in astrocytes were protected from rotenone-induced neurodegeneration, and displayed a marked reduction in neuronal oxidative stress and microglial activation. In addition, α-synuclein accumulation and phosphorylation were decreased within substantia nigra dopaminergic neurons in DJ-1-transduced animals, and expression of LAMP-2A, a marker of chaperone mediated autophagy, was increased. Together, these data indicate that astrocyte-specific overexpression of hDJ-1 protects neighboring neurons against multiple pathologic features of Parkinson's disease and provides the first direct evidence in vivo of a cell non-autonomous neuroprotective function of astroglial DJ-1.

Glucocerebrosidase 1 deficient Danio rerio mirror key pathological aspects of human Gaucher disease and provide evidence of early microglial activation preceding alpha-synuclein-independent neuronal cell death.

Autosomal recessively inherited glucocerebrosidase 1 (GBA1) mutations cause the lysosomal storage disorder Gaucher's disease (GD). Heterozygous GBA1 mutations (GBA1(+/-)) are the most common risk factor for Parkinson's disease (PD). Previous studies typically focused on the interaction between the reduction of glucocerebrosidase (enzymatic) activity in GBA1(+/-) carriers and alpha-synuclein-mediated neurotoxicity. However, it is unclear whether other mechanisms also contribute to the increased risk of PD in GBA1(+/-) carriers. The zebrafish genome does not contain alpha-synuclein (SNCA), thus providing a unique opportunity to study pathogenic mechanisms unrelated to alpha-synuclein toxicity. Here we describe a mutant zebrafish line created by TALEN genome editing carrying a 23 bp deletion in gba1 (gba1(c.1276_1298del)), the zebrafish orthologue of human GBA1. Marked sphingolipid accumulation was already detected at 5 days post-fertilization with accompanying microglial activation and early, sustained up-regulation of miR-155, a master regulator of inflammation. gba1(c.1276_1298del) mutant zebrafish developed a rapidly worsening phenotype from 8 weeks onwards with striking reduction in motor activity by 12 weeks. Histopathologically, we observed marked Gaucher cell invasion of the brain and other organs. Dopaminergic neuronal cell count was normal through development but reduced by >30% at 12 weeks in the presence of ubiquitin-positive, intra-neuronal inclusions. This gba1(c.1276_1298del) zebrafish line is the first viable vertebrate model sharing key pathological features of GD in both neuronal and non-neuronal tissue. Our study also provides evidence for early microglial activation prior to alpha-synuclein-independent neuronal cell death in GBA1 deficiency and suggests upregulation of miR-155 as a common denominator across different neurodegenerative disorders.

Live imaging of mitochondrial dynamics in CNS dopaminergic neurons in vivo demonstrates early reversal of mitochondrial transport following MPP(+) exposure.

Extensive convergent evidence collectively suggests that mitochondrial dysfunction is central to the pathogenesis of Parkinson's disease (PD). Recently, changes in the dynamic properties of mitochondria have been increasingly implicated as a key proximate mechanism underlying neurodegeneration. However, studies have been limited by the lack of a model in which mitochondria can be imaged directly and dynamically in dopaminergic neurons of the intact vertebrate CNS. We generated transgenic zebrafish in which mitochondria of dopaminergic neurons are labeled with a fluorescent reporter, and optimized methods allowing direct intravital imaging of CNS dopaminergic axons and measurement of mitochondrial transport in vivo. The proportion of mitochondria undergoing axonal transport in dopaminergic neurons decreased overall during development between 2days post-fertilization (dpf) and 5dpf, at which point the major period of growth and synaptogenesis of the relevant axonal projections is complete. Exposure to 0.5-1.0mM MPP(+) between 4 and 5dpf did not compromise zebrafish viability or cause detectable changes in the number or morphology of dopaminergic neurons, motor function or monoaminergic neurochemistry. However, 0.5mM MPP(+) caused a 300% increase in retrograde mitochondrial transport and a 30% decrease in anterograde transport. In contrast, exposure to higher concentrations of MPP(+) caused an overall reduction in mitochondrial transport. This is the first time mitochondrial transport has been observed directly in CNS dopaminergic neurons of a living vertebrate and quantified in a PD model in vivo. Our findings are compatible with a model in which damage at presynaptic dopaminergic terminals causes an early compensatory increase in retrograde transport of compromised mitochondria for degradation in the cell body. These data are important because manipulation of early pathogenic mechanisms might be a valid therapeutic approach to PD. The novel transgenic lines and methods we developed will be useful for future studies on mitochondrial dynamics in health and disease.

Different mechanisms regulate expression of zebrafish myelin protein zero (P0) in myelinating oligodendrocytes and its induction following axonal injury.

Zebrafish CNS axons regenerate robustly following injury; it is thought that CNS oligodendrocytes contribute to this response by expressing growth-promoting molecules. We characterized the mpz gene, which encodes myelin protein zero and is up-regulated in oligodendroglia following axonal injury. The 2.5-kb mpz mRNA is expressed from a single TATA box promoter. Four independent Tg(mpz:egfp) transgenic zebrafish lines, in which GFP was expressed under the mpz promoter and 10 kb of genomic 5'-flanking sequence, showed transgene expression in CNS oligodendrocytes from larval development through adulthood. Following optic nerve crush injury, the mpz:egfp transgene was strongly up-regulated in oligodendrocytes along the regenerating retinotectal projection, mirroring up-regulation of endogenous mpz mRNA. GFP-expressing oligodendroglia were significantly more abundant in the regenerating optic pathway, resulting from both transgene induction in oligodendroglial precursors and the birth of new cells. Up-regulation of the mpz:egfp transgene was not dependent on axonal regeneration, suggesting that the primary signal may be axonal loss, debris, or microglial infiltration. Deletion experiments indicated that an oligodendroglial enhancer located in the region from -6 to -10 kb with respect to the mpz transcriptional start site is dissociable from the cis-regulatory element mediating the mpz transcriptional response to axonal injury, which is located between -1 and -4 kb. These data show that different mechanisms regulate expression of zebrafish mpz in myelinating oligodendrocytes and its induction following axonal injury. The underlying molecular events could potentially be exploited to enhance axonal repair following mammalian CNS injury. The transgenic lines and cis-regulatory constructs reported here will facilitate identification of the relevant signaling pathways.

A human Tau expressing zebrafish model of progressive supranuclear palsy identifies Brd4 as a regulator of microglial synaptic elimination.

Progressive supranuclear palsy (PSP) is an incurable neurodegenerative disease characterized by 4-repeat (0N/4R)-Tau protein accumulation in CNS neurons. We generated transgenic zebrafish expressing human 0N/4R-Tau to investigate PSP pathophysiology. Tau zebrafish replicated multiple features of PSP, including: decreased survival; hypokinesia; impaired optokinetic responses; neurodegeneration; neuroinflammation; synapse loss; and Tau hyperphosphorylation, misfolding, mislocalization, insolubility, truncation, and oligomerization. Using automated assays, we screened 147 small molecules for activity in rescuing neurological deficits in Tau zebrafish. (+)JQ1, a bromodomain inhibitor, improved hypokinesia, survival, microgliosis, and brain synapse elimination. A heterozygous brd4+/- mutant reducing expression of the bromodomain protein Brd4 similarly rescued these phenotypes. Microglial phagocytosis of synaptic material was decreased by (+)JQ1 in both Tau zebrafish and rat primary cortical cultures. Microglia in human PSP brains expressed Brd4. Our findings implicate Brd4 as a regulator of microglial synaptic elimination in tauopathy and provide an unbiased approach for identifying mechanisms and therapeutic targets in PSP.

Full-field exposure of larval zebrafish to narrow waveband LED light sources at defined power and energy for optogenetic applications.

BACKGROUND: Optogenetic approaches in transparent zebrafish models have provided numerous insights into vertebrate neurobiology. The purpose of this study was to develop methods to activate light-sensitive transgene products simultaneously throughout an entire larval zebrafish. NEW METHOD: We developed a LED illumination stand and microcontroller unit to expose zebrafish larvae reproducibly to full field illumination at defined wavelength, power, and energy. RESULTS: The LED stand generated a sufficiently flat illumination field to expose multiple larval zebrafish to high power light stimuli uniformly, while avoiding sample bath warming. The controller unit allowed precise automated delivery of predetermined amounts of light energy at calibrated power. We demonstrated the utility of the approach by driving photoconversion of Kaede (398 nm), photodimerization of GAVPO (450 nm), and photoactivation of dL5**/MG2I (661 nm) in neurons throughout the CNS of larval zebrafish. Observed outcomes were influenced by both total light energy and its rate of delivery, highlighting the importance of controlling these variables to obtain reproducible results. COMPARISON WITH EXISTING METHODS: Our approach employs inexpensive LED chip arrays to deliver narrow-waveband light with a sufficiently flat illumination field to span multiple larval zebrafish simultaneously. Calibration of light power and energy are built into the workflow. CONCLUSIONS: The LED illuminator and controller can be constructed from widely available materials using the drawings, instructions, and software provided. This approach will be useful for multiple optogenetic applications in zebrafish and other models.

Hypokinesia and reduced dopamine levels in zebrafish lacking ß- and γ1-synucleins.

α-Synuclein is strongly implicated in the pathogenesis of Parkinson disease. However, the normal functions of synucleins and how these relate to disease pathogenesis are uncertain. We characterized endogenous zebrafish synucleins in order to develop tractable models to elucidate the physiological roles of synucleins in neurons in vivo. Three zebrafish genes, sncb, sncg1, and sncg2 (encoding ß-, γ1-, and γ2-synucleins respectively), show extensive phylogenetic conservation with respect to their human paralogues. A zebrafish α-synuclein orthologue was not found. Abundant 1.45-kb sncb and 2.7-kb sncg1 mRNAs were detected in the CNS from early development through adulthood and showed overlapping but distinct expression patterns. Both transcripts were detected in catecholaminergic neurons throughout the CNS. Zebrafish lacking ß-, γ1-, or both synucleins during early development showed normal CNS and body morphology but exhibited decreased spontaneous motor activity that resolved as gene expression recovered. Zebrafish lacking both ß- and γ1-synucleins were more severely hypokinetic than animals lacking one or the other synuclein and showed delayed differentiation of dopaminergic neurons and reduced dopamine levels. Phenotypic abnormalities resulting from loss of endogenous zebrafish synucleins were rescued by expression of human α-synuclein. These data demonstrate that synucleins have essential phylogenetically conserved neuronal functions that regulate dopamine homeostasis and spontaneous motor behavior. Zebrafish models will allow further elucidation of the molecular physiology and pathophysiology of synucleins in vivo.

Bioenergetics of neurons inhibit the translocation response of Parkin following rapid mitochondrial depolarization.

Recent studies delineate a pathway involving familial Parkinson's disease (PD)-related proteins PINK1 and Parkin, in which PINK1-dependent mitochondrial accumulation of Parkin targets depolarized mitochondria towards degradation through mitophagy. The pathway has been primarily characterized in cells less dependent on mitochondria for energy production than neurons. Here we report that in neurons, unlike other cells, mitochondrial depolarization by carbonyl cyanide m-chlorophenyl hydrazone did not induce Parkin translocation to mitochondria or mitophagy. PINK1 overexpression increased basal Parkin accumulation on neuronal mitochondria, but did not sensitize them to depolarization-induced Parkin translocation. Our data suggest that bioenergetic differences between neurons and cultured cell lines contribute to these different responses. In HeLa cells utilizing usual glycolytic metabolism, mitochondrial depolarization robustly triggered Parkin-mitochondrial translocation, but this did not occur in HeLa cells forced into dependence on mitochondrial respiration. Declining ATP levels after mitochondrial depolarization correlated with the absence of induced Parkin-mitochondrial translocation in both HeLa cells and neurons. However, intervention allowing neurons to maintain ATP levels after mitochondrial depolarization only modestly increased Parkin recruitment to mitochondria, without evidence of increased mitophagy. These data suggest that changes in ATP levels are not the sole determinant of the different responses between neurons and other cell types, and imply that additional mechanisms regulate mitophagy in neurons. Since the Parkin-mitophagy pathway is heavily dependent on bioenergetic status, the unique metabolic properties of neurons likely influence the function of this pathway in the pathogenesis of PD.

Frontotemporal degeneration, the next therapeutic frontier: molecules and animal models for frontotemporal degeneration drug development.

Frontotemporal degeneration (FTD) is a common cause of dementia for which there are currently no approved therapies. Over the past decade, there has been an explosion of knowledge about the biology and clinical features of FTD that has identified a number of promising therapeutic targets as well as animal models in which to develop drugs. The close association of some forms of FTD with neuropathological accumulation of tau protein or increased neuroinflammation due to progranulin protein deficiency suggests that a drug's success in treating FTD may predict efficacy in more common diseases such as Alzheimer's disease. A variety of regulatory incentives, clinical features of FTD such as rapid disease progression, and relatively pure molecular pathology suggest that there are advantages to developing drugs for FTD as compared with other more common neurodegenerative diseases such as Alzheimer's disease. In March 2011, the Frontotemporal Degeneration Treatment Study Group sponsored a conference entitled "FTD, the Next Therapeutic Frontier," which focused on preclinical aspects of FTD drug development. The goal of the meeting was to promote collaborations between academic researchers and biotechnology and pharmaceutical researchers to accelerate the development of new treatments for FTD. Here we report the key findings from the conference, including the rationale for FTD drug development; epidemiological, genetic, and neuropathological features of FTD; FTD animal models and how best to use them; and examples of successful drug development collaborations in other neurodegenerative diseases.

The advantages of frontotemporal degeneration drug development (part 2 of frontotemporal degeneration: the next therapeutic frontier).

Frontotemporal degeneration (FTD) encompasses a spectrum of related neurodegenerative disorders with behavioral, language, and motor phenotypes for which there are currently no effective therapies. This is the second of two articles that summarize the presentations and discussions that occurred at two symposia in 2011 sponsored by the Frontotemporal Degeneration Treatment Study Group, a collaborative group of academic and industry researchers that is devoted to developing treatments for FTD. This article discusses the current status of FTD clinical research that is relevant to the conduct of clinical trials, and why FTD research may be an attractive pathway for developing therapies for neurodegenerative disorders. The clinical and molecular features of FTD, including rapid disease progression and relatively pure molecular pathology, suggest that there are advantages to developing drugs for FTD as compared with other dementias. FTD qualifies as orphan indication, providing additional advantages for drug development. Two recent sets of consensus diagnostic criteria will facilitate the identification of patients with FTD, and a variety of neuropsychological, functional, and behavioral scales have been shown to be sensitive to disease progression. Moreover, quantitative neuroimaging measurements demonstrate progressive brain atrophy in FTD at rates that may surpass Alzheimer's disease. Finally, the similarities between FTD and other neurodegenerative diseases with drug development efforts already underway suggest that FTD researchers will be able to draw on this experience to create a road map for FTD drug development. We conclude that FTD research has reached sufficient maturity to pursue clinical development of specific FTD therapies.

A Critical Review of Zebrafish Neurological Disease Models-2. Application: Functional and Neuroanatomical Phenotyping Strategies and Chemical Screens.

Extensive phylogenetic conservation of molecular pathways and neuroanatomical structures, associated with efficient methods for genetic modification, have been exploited increasingly to generate zebrafish models of human disease. A range of powerful approaches can be deployed to analyze these models with the ultimate goal of elucidating pathogenic mechanisms and accelerating efforts to find effective treatments. Unbiased neurobehavioral assays can provide readouts that parallel clinical abnormalities found in patients, although some of the most useful assays quantify responses that are not routinely evaluated clinically, and differences between zebrafish and human brains preclude expression of the full range of neurobehavioral abnormalities seen in disease. Imaging approaches that use fluorescent reporters and standardized brain atlases coupled with quantitative measurements of brain structure offer an unbiased means to link experimental manipulations to changes in neural architecture. Together, quantitative structural and functional analyses allow dissection of the cellular and physiological basis underlying neurological phenotypes. These approaches can be used as outputs in chemical modifier screens, which provide a major opportunity to exploit zebrafish models to identify small molecule modulators of pathophysiology that may be informative for understanding disease mechanisms and possible therapeutic approaches.

A Critical Review of Zebrafish Neurological Disease Models-1. The Premise: Neuroanatomical, Cellular and Genetic Homology and Experimental Tractability.

The last decade has seen a dramatic rise in the number of genes linked to neurological disorders, necessitating new models to explore underlying mechanisms and to test potential therapies. Over a similar period, many laboratories adopted zebrafish as a tractable model for studying brain development, defining neural circuits and performing chemical screens. Here we discuss strengths and limitations of using the zebrafish system to model neurological disorders. The underlying premise for many disease models is the high degree of homology between human and zebrafish genes, coupled with the conserved vertebrate Bauplan and repertoire of neurochemical signaling molecules. Yet, we caution that important evolutionary divergences often limit the extent to which human symptoms can be modeled meaningfully in zebrafish. We outline advances in genetic technologies that allow human mutations to be reproduced faithfully in zebrafish. Together with methods that visualize the development and function of neuronal pathways at the single cell level, there is now an unprecedented opportunity to understand how disease-associated genetic changes disrupt neural circuits, a level of analysis that is ideally suited to uncovering pathogenic changes in human brain disorders.

Transient exposure to rotenone causes degeneration and progressive parkinsonian motor deficits, neuroinflammation, and synucleinopathy.

Individuals with Parkinson's disease (PD) typically receive a diagnosis once they have developed motor symptoms, at which point there is already significant loss of substantia nigra dopamine neurons, α-synuclein accumulation in surviving neurons, and neuroinflammation. Consequently, the point of clinical presentation may be too late to initiate disease-modifying therapy. In contrast to this clinical reality, animal models often involve acute neurodegeneration and potential therapies are tested concurrently or shortly after the pathogenic insult has begun rather than later when diagnostic clinical symptoms emerge. Therefore, we sought to develop a model that reflects the clinical situation more accurately. Middle-aged rats (7-9 months-old) received a single daily intraperitoneal injection of rotenone for 5 consecutive days and were observed over the next 8-9 months. Rotenone-treated rats showed transient motor slowing and postural instability during exposure but recovered within 9 days of rotenone cessation. Rats remained without behavioral deficits for 3-4 months, then developed progressive motor abnormalities over the ensuing months. As motor abnormalities began to emerge 3 months after rotenone exposure, there was significant loss of nigral dopaminergic neurons and significant microglial activation. There was delayed accumulation of α-synuclein in neurons of the substantia nigra and frontal cortex, which was maximal at 9 months post-rotenone. In summary, a brief temporally-remote exposure to rotenone causes delayed and progressive behavioral and neuropathological changes similar to Parkinson's disease. This model mimics the human clinical situation, in which pathogenesis is well-established by the time diagnostic motor deficits appear. As such, this model may provide a more relevant experimental system in which to test disease-modifying therapeutics.

Zebrafish models of Tauopathy.

Tauopathies are a group of incurable neurodegenerative diseases, in which loss of neurons is accompanied by intracellular deposition of fibrillar material composed of hyperphosphorylated forms of the microtubule-associated protein Tau. A zebrafish model of Tauopathy could complement existing murine models by providing a platform for genetic and chemical screens, in order to identify novel therapeutic targets and compounds with disease-modifying potential. In addition, Tauopathy zebrafish would be useful for hypothesis-driven experiments, especially those exploiting the potential to deploy in vivo imaging modalities. Several considerations, including conservation of specialized neuronal and other cellular populations, and biochemical pathways implicated in disease pathogenesis, suggest that the zebrafish brain is an appropriate setting in which to model these complex disorders. Novel transgenic zebrafish lines expressing wild-type and mutant forms of human Tau in CNS neurons have recently been reported. These studies show evidence that human Tau undergoes disease-relevant changes in zebrafish neurons, including somato-dendritic relocalization, hyperphosphorylation and aggregation. In addition, preliminary evidence suggests that Tau transgene expression can precipitate neuronal dysfunction and death. These initial studies are encouraging that the zebrafish holds considerable promise as a model in which to study Tauopathies. Further studies are necessary to clarify the phenotypes of transgenic lines and to develop assays and models suitable for unbiased high-throughput screening approaches. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.