# Spatiotemporal regulation of thyroid hormone signaling in cone subtype specification in human retinal organoids

> **NIH NIH F31** · JOHNS HOPKINS UNIVERSITY · 2022 · $46,752

## Abstract

The human retina contains three subtypes of cone photoreceptors (blue, red, and green), which enable
trichromatic vision. Disruptions in cone development and cone maintenance lead to color vision defects and
retinopathies, yet the mechanisms that specify cone subtypes in humans are poorly understood. Cone subtypes
are specified in two steps: first, between blue or red/green cone fates, and then between green or red fates. In
human fetal retinas, blue cone specification precedes red/green cone specification. The Johnston lab advanced
human retinal organoid technology to study how this developmental decision occurs. Using human retinal
organoids, we previously showed that thyroid hormone signaling promotes red/green cone fate and inhibits blue
cone fate in the human retina.
 My project addresses how the retina intrinsically regulates thyroid hormone signaling to control the
temporal decision between blue and red/green cone fates through dynamic expression of deiodinase enzymes
and the transporter MCT8. DIO3, the inactivating enzyme, degrades both T3 and T4, and DIO2, the activating
enzyme, converts inactive thyroxine (T4) into active triiodothyronine (T3). Using RNA sequencing in organoids,
we found that DIO3 (inactivating) is highly expressed early, preceding and during blue cone specification,
whereas DIO2 (activating) is expressed late, during the onset of red/green cone specification. My preliminary
data indicate that DIO3 is expressed in retinal precursor cells (RPCs) early, DIO2 is expressed in terminally
differentiating cones later in development, and MCT8 (transporter) is broadly expressed in retinal organoids
throughout development.
 My data suggest the following hypothesis: early DIO3 expression in RPCs suppresses thyroid hormone
signaling. As retinal development progresses, RPC differentiation gradually decreases DIO3 levels. DIO2
expression by blue cones increases local thyroid hormone signaling until a threshold level is reached and
red/green cone specification commences. Red/green cones also express DIO2 to reinforce high thyroid hormone
signaling and the red/green cone choice. I predict that MCT8 (transporter) is essential for thyroid hormone
signaling but does not play a role in dynamic regulation. I will test this hypothesis by using IHC and RNA FISH
to determine the temporal and cell-type-specific expression of each thyroid hormone regulator (Aim 1), and then
determine the function of each in cone subtype specification through loss- and gain-of-function experiments that
will assess the global, spatial and temporal roles of each regulator (Aim 2). I will utilize CRISPR and
pharmacological inhibitors for loss-of-function experiments and overexpress thyroid hormone regulators with
temporal or cell-type specificity using viral constructs. This project will elucidate how the temporal mechanisms
behind cone cell subtype fate choice are regulated during human retinal development, broadly contributing to
our general understanding of gene ...

## Key facts

- **NIH application ID:** 10474346
- **Project number:** 5F31EY033187-02
- **Recipient organization:** JOHNS HOPKINS UNIVERSITY
- **Principal Investigator:** Christina Lynne McNerney
- **Activity code:** F31 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2022
- **Award amount:** $46,752
- **Award type:** 5
- **Project period:** 2021-09-01 → 2024-08-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10474346

## Citation

> US National Institutes of Health, RePORTER application 10474346, Spatiotemporal regulation of thyroid hormone signaling in cone subtype specification in human retinal organoids (5F31EY033187-02). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/10474346. Licensed CC0.

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