The nervous system's remarkable diversity in cell types, morphologies, and physiologies endows it with the capacity for complex neuronal function. Among various methods used to define cell types, transcriptomic profiling has emerged as a powerful approach. The retina, with its well-defined cell classes and accessibility, presents a unique opportunity to study cell-type specification. Comprising distinct neuronal classes and a glial class organized in three somatic layers, the retina facilitates synaptic connections and signal processing. The highest diversity is observed in retinal ganglion cells (RGCs) and amacrine cells (ACs), making them ideal candidates for studying general principles governing cell-type diversification in the nervous system. In this review, we delve into recent advancements in cell-type specification within RGC and AC classes.

Let's look at the findings from a recent study.

Postmitotic Specification of RGC Types

Single-cell transcriptomics has provided valuable insights into the postmitotic specification of RGC types. The retina's first-born cell class, RGCs, exhibit 46 distinct types in adult mouse retinas. Transcriptomic data taken at different developmental ages revealed that RGC fate specification occurs gradually during postmitotic maturation. The fate possibilities of RGC types decrease with age, suggesting a progressive restriction of cell fate. Interestingly, RGC precursors display multipotentiality, being correlated with multiple types at later developmental stages, indicating a period of fate exploration before final specification. As development progresses, fate coupling between RGC types diminishes, leading to a process of fate-decoupling that specifies terminal RGC types. Visual experience is implicated in the maturation of terminal RGC types, although the exact mechanisms of fate-decoupling remain to be fully elucidated.

Transcriptional Factors in RGC Fate Specification

Transcriptional factors play a crucial role in determining specific morphological and physiological features of RGC types. For example, Satb1 mediates the acquisition of a specific dendritic feature in ON-OFF direction-selective ganglion cells (ooDSGCs), distinguishing them from other RGC types. Additionally, Tbr1 controls the dendritic targeting of specific OFF RGC types in the inner plexiform layer (IPL). Transcriptional factors also contribute to fate specification in closely related RGC types. Tbx5, selectively expressed in upward-preferring ON DSGCs, guides their axonal projections to the medial terminal nucleus (MTN).

Complex Differentiation of Amacrine Cells

Amacrine cells (ACs), the major inhibitory cell class in the retina, are categorized into GABAergic (GA), Glycinergic (GL), and non-GABAergic/non-Glycinergic (nGnG) subclasses based on their neurotransmitter release. Postmitotically expressed transcriptional factors, such as Meis2 and Tcf4, are involved in the specification of individual AC subclasses. Moreover, fate-switching transcriptional factors, like Fezf1, determine the terminal cell fate between closely related AC types, such as ON and OFF starburst amacrine cells (SACs).

Conclusion and Future Perspectives

Single-cell transcriptomic studies have revolutionized our understanding of retinal cell-type composition and generation. The comprehensive molecular atlases generated using these approaches have provided valuable insights into the diversity of retinal cell types and the involvement of transcriptional factors in fate specification. To further our understanding, it is essential to validate findings using complementary techniques such as immunohistochemistry, genetic manipulations, and functional assays.

Looking ahead, new transcriptomic applications show promise in studying cell-type specification. Combining scRNA-seq with barcoding enables quantitative analysis of lineage trajectories, while Perturb-seq or CRISPR-seq allows high-throughput phenotypic investigation of candidate transcriptional factors. Integrating transcriptomics with epigenomics can shed light on transcriptional regulatory networks, and spatial transcriptomic methods offer valuable information on cell-cell interactions that influence cell-fate determination.

In conclusion, the integration of high-throughput transcriptomic approaches with complementary techniques opens up exciting avenues for exploring cell-type specification, with potential applications in diverse areas like regenerative medicine, disease modeling, tissue engineering, and evolutionary research. The continued advancement of transcriptomics will undoubtedly deepen our understanding of the intricate processes governing the organization and function of the retina.