In this issue of Neuron, Gurnani and Silver (2021) report that activity across Golgi cells, a major type of inhibitory interneuron in the cerebellar cortex, is multidimensional and modulated by behavior. These results suggest multiple functions for inhibition in cerebellar computations.

In mammals, cerebellar neurons are categorized as glutamatergic or GABAergic, and are derived from progenitors that express the proneural genes atoh1 or ptf1a, respectively. In zebrafish, three atoh1 genes, atoh1a, atoh1b, and atoh1c, are expressed in overlapping but distinct expression domains in the upper rhombic lip (URL): ptf1a is expressed exclusively in the ventricular zone (VZ). Using transgenic lines expressing fluorescent proteins under the control of the regulatory elements of atoh1a and ptf1a, we traced the lineages of the cerebellar neurons. The atoh1+ progenitors gave rise not only to granule cells but also to neurons of the anteroventral rhombencephalon. The ptf1a+ progenitors generated Purkinje cells. The olig2+ eurydendroid cells, which are glutamatergic, were derived mostly from ptf1a+ progenitors in the VZ but some originated from the atoh1+ progenitors in the URL. In the adult cerebellum, atoh1a, atoh1b, and atoh1c are expressed in the molecular layer of the valvula cerebelli and of the medial corpus cerebelli, and ptf1a was detected in the VZ. The proneural gene expression patterns coincided with the sites of proliferating neuronal progenitors in the adult cerebellum. Our data indicate that proneural gene-linked neurogenesis is evolutionarily conserved in the cerebellum among vertebrates, and that the continuously generated neurons help remodel neural circuits in the adult zebrafish cerebellum.

Understanding the cell-type composition and spatial organization of brain regions is crucial for interpreting brain computation and function. In the thalamus, the anterior thalamic nuclei (ATN) are involved in a wide variety of functions, yet the cell-type composition of the ATN remains unmapped at a single-cell and spatial resolution. Combining single-cell RNA sequencing, spatial transcriptomics, and multiplexed fluorescent in situ hybridization, we identify three discrete excitatory cell-type clusters that correspond to the known nuclei of the ATN and uncover marker genes, molecular pathways, and putative functions of these cell types. We further illustrate graded spatial variation along the dorsomedial-ventrolateral axis for all individual nuclei of the ATN and additionally demonstrate that the anteroventral nucleus exhibits spatially covarying protein products and long-range inputs. Collectively, our study reveals discrete and continuous cell-type organizational principles of the ATN, which will help to guide and interpret experiments on ATN computation and function.

Background: Because of the structural and molecular similarities between the two systems, the lateral line, a fish and amphibian specific sensory organ, has been widely used in zebrafish as a model to study the development/biology of neuroepithelia of the inner ear. Both organs have hair cells, which are the mechanoreceptor cells, and supporting cells providing other functions to the epithelium. In most vertebrates (excluding mammals), supporting cells comprise a pool of progenitors that replace damaged or dead hair cells. However, the lack of regenerative capacity in mammals is the single leading cause for acquired hearing disorders in humans.

Results: In an effort to understand the regenerative process of hair cells in fish, we characterized and cloned an egfp transgenic stable fish line that trapped tnks1bp1, a highly conserved gene that has been implicated in the maintenance of telomeres' length. We then used this Tg(tnks1bp1:EGFP) line in a FACsorting strategy combined with microarrays to identify new molecular markers for supporting cells.

Conclusions: We present a Tg(tnks1bp1:EGFP) stable transgenic line, which we used to establish a transcriptional profile of supporting cells in the zebrafish lateral line. Therefore we are providing a new set of markers specific for supporting cells as well as candidates for functional analysis of this important cell type. This will prove to be a valuable tool for the study of regeneration in the lateral line of zebrafish in particular and for regeneration of neuroepithelia in general.

The developing nervous system consists of a variety of cell types. Transgenic animals expressing reporter genes in specific classes of neuronal cells are powerful tools for the study of neuronal network formation. We generated a wide variety of transgenic zebrafish that expressed reporter genes in specific classes of neurons or neuronal progenitors. These include lines in which neurons of specific neurotransmitter phenotypes expressed fluorescent proteins or Gal4, and lines in which specific subsets of the dorsal progenitor domain in the spinal cord expressed fluorescent proteins. Using these, we examined domain organization in the developing dorsal spinal cord, and found that there are six progenitor domains in zebrafish, which is similar to the domain organization in mice. We also systematically characterized neurotransmitter properties of the neurons that are produced from each domain. Given that reporter gene expressions occurs in a wide area of the nervous system in the lines generated, these transgenic fish should serve as powerful tools for the investigation of not only the neurons in the dorsal spinal cord but also neuronal structures and functions in many other regions of the nervous system.

The p2 progenitor domain in the ventral spinal cord gives rise to two interneuron subtypes: V2a and V2b. Delta-Notch-mediated cell-cell interactions between postmitotic immature neurons have been implicated in the segregation of neuron subtypes. However, lineage relationships between V2a and V2b neurons have not been reported. We address this issue using Tg[vsx1:GFP]zebrafish, a model system in which high GFP expression is initiated near the final stage of p2 progenitors. Cell fates were followed in progeny using time-lapse microscopy. Results indicate that the vast majority, if not all, of GFP-labeled p2 progenitors divide once to produce V2a/V2b neuron pairs,indicating that V2a and V2b neurons are generated by the asymmetric division of pair-producing progenitor cells. Together with evidence that Notch signaling is involved in the cell fate specification process, our results strongly suggest that Delta-Notch interactions between sister cells play a crucial role in the final outcome of these asymmetric divisions. This mechanism for determining cell fate is similar to asymmetric divisions that occur during Drosophila neurogenesis, where ganglion mother cells divide once to produce distinct neurons. However, unlike in Drosophila, the divisional axes of p2 progenitors in zebrafish were not fixed. We report that the terminal division of pair-producing progenitor cells in vertebrate neurogenesis can reproducibly produce two distinct neurons through a mechanism that may not depend on the orientation of the division axis.