Spatiotemporal patterns of neuronal subtype genesis suggest hierarchical development of retinal diversity

ERG responses to high-frequency flickers require FAT3 signaling in mouse retinal bipolar cells

Vision is initiated by the reception of light by photoreceptors and subsequent processing via downstream retinal neurons. Proper circuit organization depends on the multifunctional tissue polarity protein FAT3, which is required for amacrine cell connectivity and retinal lamination. Here, we investigated the retinal function of Fat3 mutant mice and found decreases in both electroretinography and perceptual responses to high-frequency flashes. These defects did not correlate with abnormal amacrine cell wiring, pointing instead to a role in bipolar cell subtypes that also express FAT3. The role of FAT3 in the response to high temporal frequency flashes depends upon its ability to transduce an intracellular signal. Mechanistically, FAT3 binds to the synaptic protein PTPσ intracellularly and is required to localize GRIK1 to OFF-cone bipolar cell synapses with cone photoreceptors. These findings expand the repertoire of FAT3's functions and reveal its importance in bipolar cells for high-frequency light response.

POU4F2 overexpression promotes the genesis of retinal ganglion cell-like projection neurons from late progenitors

Retinal ganglion cells (RGCs) are the projection neurons of the retina, and their death promotes an irreversible blindness. Several factors were described to control their genesis during retinal development. These include Atoh7, a major orchestrator of the RGC program, and downstream targets of this transcription factor, including Pou4f factors, that in turn regulate key aspects of terminal differentiation. The absence of POU4F family genes results in defects in RGC differentiation, aberrant axonal elaboration and, ultimately, RGC death. This confirms the requirement of POU4F factors for RGC development and survival, with a crucial role in regulating RGC axon outgrowth and pathfinding. Here, we have investigated in vivo whether ectopic Pou4f2 expression in late retinal progenitor cells (late RPCs) is sufficient to induce the generation of cells with RGC properties, including long-range axon projections. We show that Pou4f2 overexpression generates RGC-like cells that share morphological and transcriptional features with RGCs that are normally generated during early development and extend axonal projections up to the brain. In conclusion, these results show that POU4F2 alone is sufficient to promote the crucial properties of projection neurons that arise from retinal progenitors outside their developmental window.

Active DNA demethylation is upstream of rod-photoreceptor fate determination and required for retinal development

Retinal cell fate specification from multipotent retinal progenitors is governed by dynamic changes in chromatin structure and gene expression. Methylation at cytosines in DNA (5mC) is actively regulated for proper control of gene expression and chromatin architecture. Numerous genes display active DNA demethylation across retinal development; a process that requires oxidation of 5mC to 5-hydroxymethylcytosine (5hmC) and is controlled by the ten-eleven translocation methylcytosine dioxygenase (TET) enzymes. Using an allelic series of conditional TET enzyme mutants, we determine that DNA demethylation is required upstream of NRL and NR2E3 expression for the establishment of rod-photoreceptor fate. Using histological, behavioral, transcriptomic, and base-pair resolution DNA methylation analyses, we establish that inhibition of active DNA demethylation results in global changes in gene expression and methylation patterns that prevent photoreceptor precursors from adopting a rod-photoreceptor fate, instead producing a retina in which all photoreceptors specify as cones. Our results establish the TET enzymes and DNA demethylation as critical regulators of retinal development and cell fate specification, elucidating a novel mechanism required for the specification of rod-photoreceptors.

Evolution of rod bipolar cells and rod vision

Bipolar cells are vertebrate retinal interneurons conveying signals from rod and cone photoreceptors to amacrine and ganglion cells. Bipolar cells are found in all vertebrates and have many structural and molecular affinities with photoreceptors; they probably appeared very early during vertebrate evolution in conjunction with rod and cone progenitors. There are two types of bipolar cells, responding to central illumination with depolarization (ON) or hyperpolarization (OFF). In most vertebrate species, rod signals are conveyed to specialized rod bipolar cells, which sum signals from many rods and facilitate detection at the visual threshold. Lamprey, which diverged from all other vertebrates in the late Cambrian, have both rod ON and rod OFF bipolar cells, but mammals have only rod ON cells. Rod signals in mammals are conveyed to output neurons indirectly via AII (or A2) amacrine cells, which synapse onto cone ON and cone OFF bipolar-cells and then to ganglion cells. These findings raise the question of when during retinal evolution rod OFF bipolar cells were lost. Because physiological recordings have been made from rod OFF bipolar cells in both cartilaginous fishes (dogfish) and urodeles (salamanders), rod OFF bipolar cells and their circuits must have been retained in vertebrate progenitors at least until the Devonian. Recent evidence showing that zebrafish retina processes rod signals similar to those in mammals indicates that rod OFF bipolar cells were lost at least twice. The sole utilization of rod ON bipolar cells may have provided a selective advantage from increased signal-to-noise discrimination near the visual threshold. KEY POINTS: Rods and cones have many structural and molecular similarities to bipolar cells, which are retinal interneurons conveying signals from photoreceptors to the retinal output. Bipolar cells can be either ON (centre depolarizing) or OFF (centre hyperpolarizing) and either rod or cone dominant. Lamprey, which diverged from all other vertebrates 500 million years ago, have both ON and OFF bipolar cells, which can each be either rod or cone dominant. We argue that this configuration of separate rod/cone bipolar-cell pathways is representative of early vertebrates. Rod ON and rod OFF bipolars persisted at least until the progenitors of amphibians in the Devonian, but mammals and teleost fishes have only rod ON bipolar cells and convey rod OFF signals via a specialized amacrine cell. We argue that rod OFF bipolar cells were lost in at least two different lineages during vertebrate evolution, probably to increase the signal-to-noise of rod vision.

SMORE: spatial motifs reveal patterns in cellular architecture of complex tissues

Deciphering the link between tissue architecture and function requires methods to identify and interpret patterns in spatial arrangement of cells. We present SMORE, an approach to detect patterns in sequential arrangements of cells and examine their associated gene expression specializations. Applied to retina, brain, and embryonic tissue maps, SMORE identifies novel spatial motifs, including one that offers a new mechanism of action for type 1b bipolar cells. Structural signatures detected by SMORE also form a basis for classifying tissues. Together, our method provides a new framework for uncovering spatial complexity in tissue organization and offers novel insights into tissue function.

Genome-wide quantification of RNA flow across subcellular compartments reveals determinants of the mammalian transcript life cycle

Dissecting the regulatory mechanisms controlling mammalian transcripts from production to degradation requires quantitative measurements of mRNA flow across the cell. We developed subcellular TimeLapse-seq to measure the rates at which RNAs are released from chromatin, exported from the nucleus, loaded onto polysomes, and degraded within the nucleus and cytoplasm in human and mouse cells. These rates varied substantially, yet transcripts from genes with related functions or targeted by the same transcription factors and RNA-binding proteins flowed across subcellular compartments with similar kinetics. Verifying these associations uncovered a link between DDX3X and nuclear export. For hundreds of RNA metabolism genes, most transcripts with retained introns were degraded by the nuclear exosome, while the remaining molecules were exported with stable cytoplasmic lifespans. Transcripts residing on chromatin for longer had extended poly(A) tails, whereas the reverse was observed for cytoplasmic mRNAs. Finally, machine learning identified molecular features that predicted the diverse life cycles of mRNAs.

High temporal frequency light response in mouse retina requires FAT3 signaling in bipolar cells

Vision is initiated by the reception of light by photoreceptors and subsequent processing via downstream retinal neurons. Proper cellular organization depends on the multi-functional tissue polarity protein FAT3, which is required for amacrine cell connectivity and retinal lamination. Here we investigated the retinal function of Fat3 mutant mice and found decreases in physiological and perceptual responses to high frequency flashes. These defects did not correlate with abnormal amacrine cell wiring, pointing instead to a role in bipolar cell subtypes that also express FAT3. The role of FAT3 in the response to high temporal frequency flashes depends upon its ability to transduce an intracellular signal. Mechanistically, FAT3 binds to the synaptic protein PTPσ, intracellularly, and is required to localize GRIK1 to OFF-cone bipolar cell synapses with cone photoreceptors. These findings expand the repertoire of FAT3's functions and reveal its importance in bipolar cells for high frequency light response.

Spatial motifs reveal patterns in cellular architecture of complex tissues

Spatial organization of cells is crucial to both proper physiological function of tissues and pathological conditions like cancer. Recent advances in spatial transcriptomics have enabled joint profiling of gene expression and spatial context of the cells. The outcome is an information rich map of the tissue where individual cells, or small regions, can be labeled based on their gene expression state. While spatial transcriptomics excels in its capacity to profile numerous genes within the same sample, most existing methods for analysis of spatial data only examine distribution of one or two labels at a time. These approaches overlook the potential for identifying higher-order associations between cell types - associations that can play a pivotal role in understanding development and function of complex tissues. In this context, we introduce a novel method for detecting motifs in spatial neighborhood graphs. Each motif represents a spatial arrangement of cell types that occurs in the tissue more frequently than expected by chance. To identify spatial motifs, we developed an algorithm for uniform sampling of paths from neighborhood graphs and combined it with a motif finding algorithm on graphs inspired by previous methods for finding motifs in DNA sequences. Using synthetic data with known ground truth, we show that our method can identify spatial motifs with high accuracy and sensitivity. Applied to spatial maps of mouse retinal bipolar cells and hypothalamic preoptic region, our method reveals previously unrecognized patterns in cell type arrangements. In some cases, cells within these spatial patterns differ in their gene expression from other cells of the same type, providing insights into the functional significance of the spatial motifs. These results suggest that our method can illuminate the substantial complexity of neural tissues, provide novel insight even in well studied models, and generate experimentally testable hypotheses.

Thermal-plex: fluidic-free, rapid sequential multiplexed imaging with DNA-encoded thermal channels

Multiplexed fluorescence imaging is typically limited to three- to five-plex on standard setups. Sequential imaging methods based on iterative labeling and imaging enable practical higher multiplexing, but generally require a complex fluidic setup with several rounds of slow buffer exchange (tens of minutes to an hour for each exchange step). We report the thermal-plex method, which removes complex and slow buffer exchange steps and provides fluidic-free, rapid sequential imaging. Thermal-plex uses simple DNA probes that are engineered to fluoresce sequentially when, and only when, activated with transient exposure to heating spikes at designated temperatures (thermal channels). Channel switching is fast (<30 s) and is achieved with a commercially available and affordable on-scope heating device. We demonstrate 15-plex RNA imaging (five thermal × three fluorescence channels) in fixed cells and retina tissues in less than 4 min, without using buffer exchange or fluidics. Thermal-plex introduces a new labeling method for efficient sequential multiplexed imaging.

Multipotent progenitors instruct ontogeny of the superior colliculus

The superior colliculus (SC) in the mammalian midbrain is essential for multisensory integration and is composed of a rich diversity of excitatory and inhibitory neurons and glia. However, the developmental principles directing the generation of SC cell-type diversity are not understood. Here, we pursued systematic cell lineage tracing in silico and in vivo, preserving full spatial information, using genetic mosaic analysis with double markers (MADM)-based clonal analysis with single-cell sequencing (MADM-CloneSeq). The analysis of clonally related cell lineages revealed that radial glial progenitors (RGPs) in SC are exceptionally multipotent. Individual resident RGPs have the capacity to produce all excitatory and inhibitory SC neuron types, even at the stage of terminal division. While individual clonal units show no pre-defined cellular composition, the establishment of appropriate relative proportions of distinct neuronal types occurs in a PTEN-dependent manner. Collectively, our findings provide an inaugural framework at the single-RGP/-cell level of the mammalian SC ontogeny.

The endoplasmic reticulum: Homeostasis and crosstalk in retinal health and disease

The endoplasmic reticulum (ER) is the largest intracellular organelle carrying out a broad range of important cellular functions including protein biosynthesis, folding, and trafficking, lipid and sterol biosynthesis, carbohydrate metabolism, and calcium storage and gated release. In addition, the ER makes close contact with multiple intracellular organelles such as mitochondria and the plasma membrane to actively regulate the biogenesis, remodeling, and function of these organelles. Therefore, maintaining a homeostatic and functional ER is critical for the survival and function of cells. This vital process is implemented through well-orchestrated signaling pathways of the unfolded protein response (UPR). The UPR is activated when misfolded or unfolded proteins accumulate in the ER, a condition known as ER stress, and functions to restore ER homeostasis thus promoting cell survival. However, prolonged activation or dysregulation of the UPR can lead to cell death and other detrimental events such as inflammation and oxidative stress; these processes are implicated in the pathogenesis of many human diseases including retinal disorders. In this review manuscript, we discuss the unique features of the ER and ER stress signaling in the retina and retinal neurons and describe recent advances in the research to uncover the role of ER stress signaling in neurodegenerative retinal diseases including age-related macular degeneration, inherited retinal degeneration, achromatopsia and cone diseases, and diabetic retinopathy. In some chapters, we highlight the complex interactions between the ER and other intracellular organelles focusing on mitochondria and illustrate how ER stress signaling regulates common cellular stress pathways such as autophagy. We also touch upon the integrated stress response in retinal degeneration and diabetic retinopathy. Finally, we provide an update on the current development of pharmacological agents targeting the UPR response and discuss some unresolved questions and knowledge gaps to be addressed by future research.

Stage-specific dynamic reorganization of genome topology shapes transcriptional neighborhoods in developing human retinal organoids

We have generated a high-resolution Hi-C map of developing human retinal organoids to elucidate spatiotemporal dynamics of genomic architecture and its relationship with gene expression patterns. We demonstrate progressive stage-specific alterations in DNA topology and correlate these changes with transcription of cell-type-restricted gene markers during retinal differentiation. Temporal Hi-C reveals a shift toward A compartment for protein-coding genes and B compartment for non-coding RNAs, displaying high and low expression, respectively. Notably, retina-enriched genes are clustered near lost boundaries of topologically associated domains (TADs), and higher-order assemblages (i.e., TAD cliques) localize in active chromatin regions with binding sites for eye-field transcription factors. These genes gain chromatin contacts at their transcription start site as organoid differentiation proceeds. Our study provides a global view of chromatin architecture dynamics associated with diversification of cell types during retinal development and serves as a foundational resource for in-depth functional investigations of retinal developmental traits.

Spatial organization of the mouse retina at single cell resolution by MERFISH

The visual signal processing in the retina requires the precise organization of diverse neuronal types working in concert. While single-cell omics studies have identified more than 120 different neuronal subtypes in the mouse retina, little is known about their spatial organization. Here, we generated the single-cell spatial atlas of the mouse retina using multiplexed error-robust fluorescence in situ hybridization (MERFISH). We profiled over 390,000 cells and identified all major cell types and nearly all subtypes through the integration with reference single-cell RNA sequencing (scRNA-seq) data. Our spatial atlas allowed simultaneous examination of nearly all cell subtypes in the retina, revealing 8 previously unknown displaced amacrine cell subtypes and establishing the connection between the molecular classification of many cell subtypes and their spatial arrangement. Furthermore, we identified spatially dependent differential gene expression between subtypes, suggesting the possibility of functional tuning of neuronal types based on location.

Temporal regulation of neural diversity in Drosophila and vertebrates

The generation of neuronal diversity involves temporal patterning mechanisms by which a given progenitor sequentially produces multiple cell types. Several parallels are evident between the brain development programs of Drosophila and vertebrates, such as the successive emergence of specific cell types and the use of combinations of transcription factors to specify cell fates. Furthermore, cell-extrinsic cues such as hormones and signaling pathways have also been shown to be regulatory modules of temporal patterning. Recently, transcriptomic and epigenomic studies using large single-cell sequencing datasets have provided insights into the transcriptional dynamics of neurogenesis in the Drosophila and mammalian central nervous systems. We review these commonalities in the specification of neuronal identity and highlight the conserved or convergent strategies of brain development by discussing temporal patterning mechanisms found in flies and vertebrates.

Deletion of histone demethylase Lsd1 (Kdm1a) during retinal development leads to defects in retinal function and structure

Purpose

The purpose of this study was to investigate the role of Lysine specific demethylase 1 (Lsd1) in murine retinal development. LSD1 is a histone demethylase that can demethylate mono- and di-methyl groups on H3K4 and H3K9. Using Chx10-Cre and Rho-iCre75 driver lines, we generated novel transgenic mouse lines to delete Lsd1 in most retinal progenitor cells or specifically in rod photoreceptors. We hypothesize that Lsd1 deletion will cause global morphological and functional defects due to its importance in neuronal development.

Methods

We tested the retinal function of young adult mice by electroretinogram (ERG) and assessed retinal morphology by in vivo imaging by fundus photography and SD-OCT. Afterward, eyes were enucleated, fixed, and sectioned for subsequent hematoxylin and eosin (H&E) or immunofluorescence staining. Other eyes were plastic fixed and sectioned for electron microscopy.

Results

In adult Chx10-Cre Lsd1fl/fl mice, we observed a marked reduction in a-, b-, and c-wave amplitudes in scotopic conditions compared to age-matched control mice. Photopic and flicker ERG waveforms were even more sharply reduced. Modest reductions in total retinal thickness and outer nuclear layer (ONL) thickness were observed in SD-OCT and H&E images. Lastly, electron microscopy revealed significantly shorter inner and outer segments and immunofluorescence showed modest reductions in specific cell type populations. We did not observe any obvious functional or morphological defects in the adult Rho-iCre75 Lsd1fl/fl animals.

Conclusion

Lsd1 is necessary for neuronal development in the retina. Adult Chx10-Cre Lsd1fl/fl mice show impaired retinal function and morphology. These effects were fully manifested in young adults (P30), suggesting that Lsd1 affects early retinal development in mice.

Light-Seq: light-directed in situ barcoding of biomolecules in fixed cells and tissues for spatially indexed sequencing

We present Light-Seq, an approach for multiplexed spatial indexing of intact biological samples using light-directed DNA barcoding in fixed cells and tissues followed by ex situ sequencing. Light-Seq combines spatially targeted, rapid photocrosslinking of DNA barcodes onto complementary DNAs in situ with a one-step DNA stitching reaction to create pooled, spatially indexed sequencing libraries. This light-directed barcoding enables in situ selection of multiple cell populations in intact fixed tissue samples for full-transcriptome sequencing based on location, morphology or protein stains, without cellular dissociation. Applying Light-Seq to mouse retinal sections, we recovered thousands of differentially enriched transcripts from three cellular layers and discovered biomarkers for a very rare neuronal subtype, dopaminergic amacrine cells, from only four to eight individual cells per section. Light-Seq provides an accessible workflow to combine in situ imaging and protein staining with next generation sequencing of the same cells, leaving the sample intact for further analysis post-sequencing.

Contraction of axonal and dendritic fields in Sox5-deficient cone bipolar cells is accompanied by axonal sprouting and dendritic hyper-innervation of pedicles

Multiple factors regulate the differentiation of neuronal morphology during development, including interactions with afferents, targets, and homotypic neighbors, as well as cell-intrinsic transcriptional regulation. Retinal bipolar cells provide an exemplary model system for studying the control of these processes, as there are 15 transcriptionally and morphologically distinct types, each extending their dendritic and axonal arbors in respective strata within the synaptic layers of the retina. Here we have examined the role of the transcription factor Sox5 in the control of the morphological differentiation of one type of cone bipolar cell (CBC), the Type 7 cell. We confirm selective expression of SOX5 in this single bipolar cell type, emerging at the close of the first post-natal week, prior to morphological differentiation. Conditional knockout mice were generated by crossing a bipolar cell-specific cre-expressing line with mice carrying floxed Sox5 alleles, as well as the Gustducin-gfp reporter which labels Type 7 CBCs. Loss of SOX5 was confirmed in the bipolar cell stratum, in GFP+ Type 7 cells. Such SOX5-deficient Type 7 cells differentiate axonal and dendritic arbors that are each reduced in areal extent. The axonal arbors exhibit sprouting in the inner plexiform layer (IPL), thereby extending their overall radial extent, while the dendritic arbors connect with fewer cone pedicles in the outer plexiform layer, showing an increase in the average number of dendritic contacts at each pedicle. SOX5-deficient Type 7 CBCs should therefore exhibit smaller receptive fields derived from fewer if now hyper-innervated pedicles, transmitting their signals across a broader depth through the IPL.