# Plagl1 regulates the retinal progenitor cell to Müller glial cell transition

## TL;DR

This study shows that the Plagl1 gene is crucial for the transition of retinal progenitor cells into Müller glia, ensuring proper retinal development and function.

## Contribution

The study identifies Plagl1 as a novel regulator of the RPC-to-Müller glia transition through its role in chromatin accessibility and gene expression.

## Key findings

- Plagl1 loss disrupts chromatin and transcriptional programs in Sox9+ cells, leading to structural defects in Müller glia.
- Conditional deletion of Plagl1 in postnatal Müller glia disrupts cell positioning but not cell cycle exit.
- Plagl1 regulates the transition from retinal progenitor cells to Müller glia by controlling gene expression and chromatin accessibility.

## Abstract

Müller glia arise from late-stage retinal progenitor cells (RPCs) as a distinct lineage that diverges from neurogenic trajectories. Here, we identify the maternally imprinted gene Plagl1 as a key transcriptional regulator of gliogenesis in the murine retina. Plagl1 is expressed during the RPC-to-glia transition and is dynamically regulated in Müller glia following injury. To define its developmental role, we analyzed Plagl1⁺/⁻pat null mutant retinas at postnatal day 7 (P7), when central retinal gliogenesis is complete. In the absence of Plagl1, Sox9 ⁺ glial/precursor cells were displaced and proliferated ectopically, with structural dysmorphologies, reactive gliosis, and impaired visual processing persisting into later postnatal stages. Bulk RNA-seq and ATAC-seq revealed widespread reductions in chromatin accessibility and transcriptional dysregulation affecting epigenetic modifiers, translational machinery, fate-specifying transcription factors, cell cycle regulators, and signaling pathways. Single-cell pseudobulk analysis showed that Plagl1 loss disrupts chromatin, transcriptional, and translational programs specifically within Sox9 ⁺ cells, encompassing Müller glia and precursor populations, pinpointing these cells as the source of defects in Plagl1⁺/⁻pat retinas. Notch signaling was elevated in Plagl1-deficient glia, and genetic activation at P14 displaced Sox9 ⁺ glial cells, without inducing proliferation. Similarly, conditional deletion of Plagl1 in postnatal Müller glia at P14 disrupted positioning and not cell cycle exit, confirming a cell-autonomous requirement for Müller glia positioning that is independent of proliferation control. Since these conditional manipulations could only be performed at P14 at the earliest, they reveal Plagl1’s later functions in postmitotic glia and complement, rather than mirror, the earlier P7 mixed RPC/glial null phenotype. Together these findings establish Plagl1 as a critical regulator of the late-stage RPC to Müller glia transition, acting through coordinated control of chromatin accessibility and gene expression programs to ensure timely cell cycle exit. This function aligns with Plagl1’s broader tumor suppressor role in stabilizing postmitotic, differentiated cell states across tissues.

As the only glial cells in the retina, Müller glia play an essential role in maintaining tissue homeostasis and supporting retinal integrity and function. During development, Müller glia are derived from multipotent RPCs, which also give rise to the six types of retinal neurons, all of which differentiate in a defined temporal order. Müller glia are among the last retinal cells to differentiate, transitioning from cycling RPCs into post-mitotic glial cells. Here, we show that the RPC-to-Müller glial cell transition is orchestrated by the maternally imprinted gene Plagl1. Plagl1 encodes a transcription factor that is initially expressed in RPCs and later persists in retinal Müller glia. In the absence of Plagl1, Müller glia initiate glial-specific marker expression but fail to form a mature monolayer and continue to proliferate ectopically. Mechanistically, loss of Plagl1 compacts open chromatin, induces widespread changes in gene expression, and elevates several signaling pathways including Notch, which, when activated, disrupts the Müller glial monolayer. Thus, Plagl1 guides the RPC-to-Müller glial cell transition, providing new insights into this unique developmental process.