Ancient origin of the rod bipolar cell pathway in the vertebrate retina.

Vertebrates rely on rod photoreceptors for vision in low-light conditions. The specialized downstream circuit for rod signalling, called the primary rod pathway, is well characterized in mammals, but circuitry for rod signalling in non-mammals is largely unknown. Here we demonstrate that the mammalian primary rod pathway is conserved in zebrafish, which diverged from extant mammals ~400 million years ago. Using single-cell RNA sequencing, we identified two bipolar cell types in zebrafish that are related to mammalian rod bipolar cell (RBCs), the only bipolar type that directly carries rod signals from the outer to the inner retina in the primary rod pathway. By combining electrophysiology, histology and ultrastructural reconstruction of the zebrafish RBCs, we found that, similar to mammalian RBCs, both zebrafish RBC types connect with all rods in their dendritic territory and provide output largely onto amacrine cells. The wiring pattern of the amacrine cells postsynaptic to one RBC type is strikingly similar to that of mammalian RBCs and their amacrine partners, suggesting that the cell types and circuit design of the primary rod pathway emerged before the divergence of teleost fish and mammals. The second RBC type, which forms separate pathways, was either lost in mammals or emerged in fish.

Ancient origin of the rod bipolar cell pathway in the vertebrate retina.

Vertebrates rely on rod photoreceptors for vision in low-light conditions. Mammals have a specialized downstream circuit for rod signaling called the primary rod pathway, which comprises specific cell types and wiring patterns that are thought to be unique to this lineage. Thus, it has been long assumed that the primary rod pathway evolved in mammals. Here, we challenge this view by demonstrating that the mammalian primary rod pathway is conserved in zebrafish, which diverged from extant mammals ~400 million years ago. Using single-cell RNA-sequencing, we identified two bipolar cell (BC) types in zebrafish that are related to mammalian rod BCs (RBCs) of the primary rod pathway. By combining electrophysiology, histology, and ultrastructural reconstruction of the zebrafish RBCs, we found that, like mammalian RBCs, both zebrafish RBC types connect with all rods in their dendritic territory, and provide output largely onto amacrine cells. The wiring pattern of the amacrine cells post-synaptic to one RBC type is strikingly similar to that of mammalian RBCs, suggesting that the cell types and circuit design of the primary rod pathway have emerged before the divergence of teleost fish and amniotes. The second RBC type, which forms separate pathways, is either lost in mammals or emerged in fish.

Ancient origin of the rod bipolar cell pathway in the vertebrate retina.

Vertebrates rely on rod photoreceptors for vision in low-light conditions1. Mammals have a specialized downstream circuit for rod signaling called the primary rod pathway, which comprises specific cell types and wiring patterns that are thought to be unique to this lineage2-6. Thus, it has been long assumed that the primary rod pathway evolved in mammals3,5-7. Here, we challenge this view by demonstrating that the mammalian primary rod pathway is conserved in zebrafish, which diverged from extant mammals ~400 million years ago. Using single-cell RNA-sequencing, we identified two bipolar cell (BC) types in zebrafish that are related to mammalian rod BCs (RBCs) of the primary rod pathway. By combining electrophysiology, histology, and ultrastructural reconstruction of the zebrafish RBCs, we found that, like mammalian RBCs8, both zebrafish RBC types connect with all rods and red-cones in their dendritic territory, and provide output largely onto amacrine cells. The wiring pattern of the amacrine cells post-synaptic to one RBC type is strikingly similar to that of mammalian RBCs. This suggests that the cell types and circuit design of the primary rod pathway may have emerged before the divergence of teleost fish and amniotes (mammals, bird, reptiles). The second RBC type in zebrafish, which forms separate pathways from the first RBC type, is either lost in mammals or emerged in fish to serve yet unknown roles.

Cellular and Molecular Determinants of Retinal Cell Fate.

The vertebrate retina is regarded as a simple part of the central nervous system (CNS) and thus amenable to investigations of the determinants of cell fate. Its five neuronal cell classes and one glial cell class all derive from a common pool of progenitors. Here we review how each cell class is generated. Retinal progenitors progress through different competence states, in each of which they generate only a small repertoire of cell classes. The intrinsic state of the progenitor is determined by the complement of transcription factors it expresses. Thus, although progenitors are multipotent, there is a bias in the types of fates they generate during any particular time window. Overlying these competence states are stochastic mechanisms that influence fate decisions. These mechanisms are determined by a weighted set of probabilities based on the abundance of a cell class in the retina. Deterministic mechanisms also operate, especially late in development, when preprogrammed progenitors solely generate specific fates.

Notch-mediated re-specification of neuronal identity during central nervous system development.

Neuronal identity has long been thought of as immutable, so that once a cell acquires a specific fate, it is maintained for life.1 Studies using the overexpression of potent transcription factors to experimentally reprogram neuronal fate in the mouse neocortex2,3 and retina4,5 have challenged this notion by revealing that post-mitotic neurons can switch their identity. Whether fate reprogramming is part of normal development in the central nervous system (CNS) is unclear. While there are some reports of physiological cell fate reprogramming in invertebrates,6,7 and in the vertebrate peripheral nervous system,8 endogenous fate reprogramming in the vertebrate CNS has not been documented. Here, we demonstrate spontaneous fate re-specification in an interneuron lineage in the zebrafish retina. We show that the visual system homeobox 1 (vsx1)-expressing lineage, which has been associated exclusively with excitatory bipolar cell (BC) interneurons,9-12 also generates inhibitory amacrine cells (ACs). We identify a role for Notch signaling in conferring plasticity to nascent vsx1 BCs, allowing suitable transcription factor programs to re-specify them to an AC fate. Overstimulating Notch signaling enhances this physiological phenotype so that both daughters of a vsx1 progenitor differentiate into ACs and partially differentiated vsx1 BCs can be converted into ACs. Furthermore, this physiological re-specification can be mimicked to allow experimental induction of an entirely distinct fate, that of retinal projection neurons, from the vsx1 lineage. Our observations reveal unanticipated plasticity of cell fate during retinal development.

Uncoupling of neurogenesis and differentiation during retinal development.

Conventionally, neuronal development is regarded to follow a stereotypic sequence of neurogenesis, migration, and differentiation. We demonstrate that this notion is not a general principle of neuronal development by documenting the timing of mitosis in relation to multiple differentiation events for bipolar cells (BCs) in the zebrafish retina using in vivo imaging. We found that BC progenitors undergo terminal neurogenic divisions while in markedly disparate stages of neuronal differentiation. Remarkably, the differentiation state of individual BC progenitors at mitosis is not arbitrary but matches the differentiation state of post-mitotic BCs in their surround. By experimentally shifting the relative timing of progenitor division and differentiation, we provide evidence that neurogenesis and differentiation can occur independently of each other. We propose that the uncoupling of neurogenesis and differentiation could provide neurogenic programs with flexibility, while allowing for synchronous neuronal development within a continuously expanding cell pool.

Imaging Subcellular Structures in the Living Zebrafish Embryo.

In vivo imaging provides unprecedented access to the dynamic behavior of cellular and subcellular structures in their natural context. Performing such imaging experiments in higher vertebrates such as mammals generally requires surgical access to the system under study. The optical accessibility of embryonic and larval zebrafish allows such invasive procedures to be circumvented and permits imaging in the intact organism. Indeed the zebrafish is now a well-established model to visualize dynamic cellular behaviors using in vivo microscopy in a wide range of developmental contexts from proliferation to migration and differentiation. A more recent development is the increasing use of zebrafish to study subcellular events including mitochondrial trafficking and centrosome dynamics. The relative ease with which these subcellular structures can be genetically labeled by fluorescent proteins and the use of light microscopy techniques to image them is transforming the zebrafish into an in vivo model of cell biology. Here we describe methods to generate genetic constructs that fluorescently label organelles, highlighting mitochondria and centrosomes as specific examples. We use the bipartite Gal4-UAS system in multiple configurations to restrict expression to specific cell-types and provide protocols to generate transiently expressing and stable transgenic fish. Finally, we provide guidelines for choosing light microscopy methods that are most suitable for imaging subcellular dynamics.

Generation of Thy1 Constructs for Pronuclear Injection.

With easy access to core facilities or commercial providers of pronuclear injections, generating simple Thy1-XFP transgenic mice (where XFP stands for any fluorescent protein) is now a possibility even for small laboratories. The generation of new Thy1 transgenic lines generally consists of five steps: (1) engineering and characterization of the desired fluorescent reporter protein, (2) cloning of the reporter protein into the Thy1 vector, (3) linearization and purification of the new Thy1 construct, (4) pronuclear injection to generate founders, and (5) screening of founder progeny to establish transgenic lines. Here, we provide a protocol for Steps 2 and 3. The sequence for a desired fluorescent reporter protein is cloned into the XhoI restriction site of the Thy1 vector. This usually involves blunt-end cloning because the traditional Thy1 vector does not carry an intact multiple cloning site. Following successful cloning, the DNA is prepared for pronuclear injection by linearizing it using EcoRI and PvuI restriction enzymes. The purified linearized DNA must then be sent to a facility specializing in pronuclear injection to generate transgenic founder mice.

Generation and Screening of Transgenic Mice with Neuronal Labeling Controlled by Thy1 Regulatory Elements.

Major progress has been made using in vivo imaging in mice to study mammalian nervous system development, plasticity, and disease. This progress has depended in part on the wide availability of two-photon microscopy, which is capable of penetrating deep into scattering tissue. Equally important, however, is the generation of suitable transgenic mouse models, which provide a "Golgi staining"-like labeling of neurons that is sparse and bright enough for in vivo imaging. Particularly prominent among such transgenic mice are the so-called Thy1-XFP mice (in which XFP stands for any fluorescent protein) that are used in numerous studies, especially to visualize spine plasticity in the cortex and remodeling in peripheral synapses. New generations of Thy1-XFP mice are now being generated at a high rate, and these have allowed previously difficult experiments to become feasible. Moreover, with easy access to core facilities or commercial providers of pronuclear injections, generating simple Thy1 transgenic mice is now a possibility even for small laboratories. In this introduction, we discuss the Thy1 regulatory elements used to generate transgenic lines with neuronal labeling. We provide a brief overview of currently available Thy1 transgenic mice, including lines labeling neuronal organelles or reporting neuronal function.

Generation of Tissue Sections for Screening Thy1 Mouse Lines.

New generations of Thy1-XFP transgenic mice (where XFP stands for any fluorescent protein) can now be readily generated, given the availability of core facilities or commercial providers of Thy1 pronuclear injections. Here, we provide a protocol for screening founder progeny. Transcardial perfusion is performed on 3-wk-old F1 mice that have been produced by crossing Thy1 transgenic founders and commercially obtained inbred mice. Cryosections are generated, and Thy1-driven expression is detected by histological characterization.

Imaging Acute Neuromuscular Explants from Thy1 Mouse Lines.

Because core facilities that generate transgenic founder mice for a reasonable fee are now available at most major research institutions, generating new Thy1-XFP transgenic animals (in which XFP stands for any fluorescent protein) is an option even for relatively small laboratories. Here, we provide a protocol for screening offspring of Thy1 transgenic founders. Acute neuromuscular explants are obtained from 3-wk-old F1 mice that have been produced by crossing Thy1 transgenic founders and commercially obtained inbred mice. Thy1-driven expression is detected by fluorescence microscopy.

Pervasive axonal transport deficits in multiple sclerosis models.

Impaired axonal transport can contribute to axon degeneration and has been described in many neurodegenerative diseases. Multiple sclerosis (MS) is a common neuroinflammatory disease, which is characterized by progressive axon degeneration-whether, when, and how axonal transport is affected in this condition is unknown. Here we used in vivo two-photon imaging to directly assay transport of organelles and the stability of microtubule tracks in individual spinal axons in mouse models of MS. We found widespread transport deficits, which preceded structural alterations of axons, cargos, or microtubules and could be reversed by acute anti-inflammatory interventions or redox scavenging. Our study shows that acute neuroinflammation induces a pervasive state of reversible axonal dysfunction, which coincides with acute disease symptoms. Moreover, perpetuated transport dysfunction, as we found in a model of progressive MS, led to reduced distal organelle supply and could thus contribute to axonal dystrophy in advanced stages of the disease.

An assay to image neuronal microtubule dynamics in mice.

Microtubule dynamics in neurons play critical roles in physiology, injury and disease and determine microtubule orientation, the cell biological correlate of neurite polarization. Several microtubule binding proteins, including end-binding protein 3 (EB3), specifically bind to the growing plus tip of microtubules. In the past, fluorescently tagged end-binding proteins have revealed microtubule dynamics in vitro and in non-mammalian model organisms. Here, we devise an imaging assay based on transgenic mice expressing yellow fluorescent protein-tagged EB3 to study microtubules in intact mammalian neurites. Our approach allows measurement of microtubule dynamics in vivo and ex vivo in peripheral nervous system and central nervous system neurites under physiological conditions and after exposure to microtubule-modifying drugs. We find an increase in dynamic microtubules after injury and in neurodegenerative disease states, before axons show morphological indications of degeneration or regrowth. Thus increased microtubule dynamics might serve as a general indicator of neurite remodelling in health and disease.

Multiparametric optical analysis of mitochondrial redox signals during neuronal physiology and pathology in vivo.

Mitochondrial redox signals have a central role in neuronal physiology and disease. Here we describe a new optical approach to measure fast redox signals with single-organelle resolution in living mice that express genetically encoded redox biosensors in their neuronal mitochondria. Moreover, we demonstrate how parallel measurements with several biosensors can integrate these redox signals into a comprehensive characterization of mitochondrial function. This approach revealed that axonal mitochondria undergo spontaneous 'contractions' that are accompanied by reversible redox changes. These contractions are amplified by neuronal activity and acute or chronic neuronal insults. Multiparametric imaging reveals that contractions constitute respiratory chain-dependent episodes of depolarization coinciding with matrix alkalinization, followed by uncoupling. In contrast, permanent mitochondrial damage after spinal cord injury depends on calcium influx and mitochondrial permeability transition. Thus, our approach allows us to identify heterogeneity among physiological and pathological redox signals, correlate such signals to functional and structural organelle dynamics and dissect the underlying mechanisms.

Amplification of progenitors in the mammalian telencephalon includes a new radial glial cell type.

The mechanisms governing the expansion of neuron number in specific brain regions are still poorly understood. Enlarged neuron numbers in different species are often anticipated by increased numbers of progenitors dividing in the subventricular zone. Here we present live imaging analysis of radial glial cells and their progeny in the ventral telencephalon, the region with the largest subventricular zone in the murine brain during neurogenesis. We observe lineage amplification by a new type of progenitor, including bipolar radial glial cells dividing at subapical positions and generating further proliferating progeny. The frequency of this new type of progenitor is increased not only in larger clones of the mouse lateral ganglionic eminence but also in cerebral cortices of gyrated species, and upon inducing gyrification in the murine cerebral cortex. This implies key roles of this new type of radial glia in ontogeny and phylogeny.

Increased radial glia quiescence, decreased reactivation upon injury and unaltered neuroblast behavior underlie decreased neurogenesis in the aging zebrafish telencephalon.

The zebrafish has recently become a source of new data on the mechanisms of neural stem cell (NSC) maintenance and ongoing neurogenesis in adult brains. In this vertebrate, neurogenesis occurs at high levels in all ventricular regions of the brain, and brain injuries recover successfully, owing to the recruitment of radial glia, which function as NSCs. This new vertebrate model of adult neurogenesis is thus advancing our knowledge of the molecular cues in use for the activation of NSCs and fate of their progeny. Because the regenerative potential of somatic stem cells generally weakens with increasing age, it is important to assess the extent to which zebrafish NSC potential decreases or remains unaltered with age. We found that neurogenesis in the ventricular zone, in the olfactory bulb, and in a newly identified parenchymal zone of the telencephalon indeed declines as the fish ages and that oligodendrogenesis also declines. In the ventricular zone, the radial glial cell population remains largely unaltered morphologically but enters less frequently into the cell cycle and hence produces fewer neuroblasts. The neuroblasts themselves do not change their behavior with age and produce the same number of postmitotic neurons. Thus, decreased neurogenesis in the physiologically aging zebrafish brain is correlated with an increasing quiescence of radial glia. After injuries, radial glia in aged brains are reactivated, and the percentage of cell cycle entry is increased in the radial glia population. However, this reaction is far less pronounced than in younger animals, pointing to irreversible changes in aging zebrafish radial glia.

In vivo imaging of disease-related mitochondrial dynamics in a vertebrate model system.

Mitochondria provide ATP, maintain calcium homeostasis, and regulate apoptosis. Neurons, due to their size and complex geometry, are particularly dependent on the proper functioning and distribution of mitochondria. Thus disruptions of these organelles and their transport play a central role in a broad range of neurodegenerative diseases. While in vitro studies have greatly expanded our knowledge of mitochondrial dynamics, our understanding in vivo remains limited. To address this shortcoming, we developed tools to study mitochondrial dynamics in vivo in optically accessible zebrafish. We demonstrate here that our newly generated tools, including transgenic "MitoFish," can be used to study the in vivo "life cycle" of mitochondria and allows identifying pharmacological and genetic modulators of mitochondrial dynamics. Furthermore we observed profound mitochondrial transport deficits in real time in a zebrafish tauopathy model. By rescuing this phenotype using MARK2 (microtubule-affinity regulating kinase 2), we provide direct in vivo evidence that this kinase regulates axonal transport in a Tau-dependent manner. Thus, our approach allows detailed studies of the dynamics of mitochondria in their natural environment under normal and disease conditions.

Axonal transport deficits and degeneration can evolve independently in mouse models of amyotrophic lateral sclerosis.

Axonal transport deficits have been reported in many neurodegenerative conditions and are widely assumed to be an immediate causative step of axon and synapse loss. By imaging changes in axonal morphology and organelle transport over time in several animal models of amyotrophic lateral sclerosis (ALS), we now find that deficits in axonal transport of organelles (mitochondria, endosomes) and axon degeneration can evolve independently. This conclusion rests on the following results: (i) Axons can survive despite long-lasting transport deficits: In the SOD(G93A) model of ALS, transport deficits are detected soon after birth, months before the onset of axon degeneration. (ii) Transport deficits are not necessary for axon degeneration: In the SOD(G85R) model of ALS, motor axons degenerate, but transport is unaffected. (iii) Axon transport deficits are not sufficient to cause immediate degeneration: In mice that overexpress wild-type superoxide dismutase-1 (SOD(WT)), axons show chronic transport deficits, but survive. These data suggest that disturbances of organelle transport are not a necessary step in the emergence of motor neuron degeneration.

The role of Pax6 in regulating the orientation and mode of cell division of progenitors in the mouse cerebral cortex.

Successful brain development requires tight regulation of sequential symmetric and asymmetric cell division. Although Pax6 is known to exert multiple roles in the developing nervous system, its role in the regulation of cell division is unknown. Here, we demonstrate profound alterations in the orientation and mode of cell division in the cerebral cortex of mice deficient in Pax6 function (Pax6(Sey/Sey)) or after acute induced deletion of Pax6. Live imaging revealed an increase in non-vertical cellular cleavage planes, resulting in an increased number of progenitors with unequal inheritance of the apical membrane domain and adherens junctions in the absence of Pax6 function. This phenotype appears to be mediated by the direct Pax6 target Spag5, a microtubule-associated protein, reduced levels of which result in the replication of the Pax6 phenotype of altered cell division orientation. In addition, lack of Pax6 also results in premature delamination of progenitor cells from the apical surface due to an overall decrease in proteins mediating anchoring at the ventricular surface. Moreover, continuous long-term imaging in vitro revealed that Pax6-deficient progenitors generate daughter cells with asymmetric fates at higher frequencies. These data demonstrate a cell-autonomous role for Pax6 in regulating the mode of cell division independently of apicobasal polarity and cell-cell interactions. Taken together, our work reveals several direct effects that the transcription factor Pax6 has on the machinery that mediates the orientation and mode of cell division.