TDRD3 Deficient Mice Exhibit Deficits in Neurogenesis and Synaptic Plasticity at Both Post-transcriptional and Behavioral Levels

Research Background

Topoisomerase 3b (top3b) is a dual-activity topoisomerase capable of resolving DNA and RNA topological problems. An increasing body of evidence indicates that top3b functions as a conserved complex with the tudor domain-containing 3 (tdrd3) protein in animals. Human genetic studies suggest that the loss or mutation of top3b is associated with mental and cognitive disorders, such as schizophrenia, autism, epilepsy, and intellectual disability. This hypothesis is supported by analyses of cultured neurons and various animal models, including mice, zebrafish, and fruit flies. Specifically, mice deficient in top3b exhibit behavioral phenotypes related to mental and cognitive disorders and defects in hippocampal neurogenesis and synaptic plasticity. However, the significance of tdrd3 in normal brain function in animal models has not been thoroughly investigated.

Research Origin

This study was conducted by Xingliang Zhu, Yuyoung Joo, Simone Bossi, Ross A. McDevitt, and others from institutions such as the National Institute on Aging at the National Institutes of Health, Florida Atlantic University, and Kumamoto University. The paper was published in the journal “Progress in Neurobiology” in 2024.

Research Procedure

Experimental Procedure

Step 1: Creation of Mouse Model

A gene trapping strategy was used to create tdrd3 deficient mice. The results showed that mRNA and protein levels of tdrd3 were undetectable in these mice. It was observed that the proportion of newborn tdrd3 deficient mice was significantly lower than the expected Mendelian ratio, suggesting that tdrd3 is essential for normal embryonic viability.

Step 2: Behavioral Tests

The study included a series of behavioral tests on the mice, such as the Morris water maze, continuous spontaneous alternation task, and fear conditioning test. The results showed that tdrd3 deficient mice exhibited cognitive deficits in these tasks, similar to top3b deficient mice. Additionally, tdrd3 deficient mice exhibited less anxiety behavior in the open field test and light-dark box test.

Step 3: Electrophysiological Tests

Long-term potentiation (LTP) and long-term depression (LTD) tests were performed on hippocampal slices. The results showed that CA1 neurons of tdrd3 deficient mice exhibited significantly weakened activity-dependent synaptic plasticity. Paired-pulse facilitation (PPF) tests did not show significant differences.

Step 4: Neurogenesis and Neuronal Morphology

The study found a significant reduction in the proliferation of newborn neurons in the hippocampus of tdrd3 deficient mice, particularly in Type II subtypes of neural stem cells. Using retroviral GFP labeling of newborn neurons, it was found that tdrd3 deficient mice exhibited significant morphological abnormalities in newborn neurons, such as increased neuronal intersections, reduced dendritic volume and diameter, and lower spine density.

Step 5: RNA Sequencing and Post-transcriptional Regulation

Through Pro-seq and RNA-seq sequencing analysis, the study observed that many genes related to neuronal function had significantly reduced mature mRNA levels in tdrd3 deficient mice, while their nascent RNA levels remained unchanged, suggesting accelerated mRNA turnover of these genes. Additionally, the study found post-transcriptional regulation defects in several GABA signaling pathway genes such as Gabra2, Gabra6, Neurod1, and Neurod2 in TDRD3 deficient mice.

Research Results

Main Findings

1. Cognitive and Emotional Behavior Deficits: Tdrd3 deficient mice exhibited memory deficits in cognitive tasks such as the Morris water maze and fear conditioning tests, potentially related to hippocampal dysfunction.
2. Anxiety Behavior Changes: Tdrd3 deficient mice showed significantly reduced anxiety in the open field and light-dark box tests, in contrast to the increased anxiety observed in top3b deficient mice.
3. Reduced Synaptic Plasticity: Electrophysiological tests indicated that CA1 neurons in tdrd3 deficient mice showed significantly weakened synaptic plasticity in LTP and LTD measurements.
4. Abnormal Morphology of Newborn Neurons: Retroviral GFP labeling revealed significant morphological abnormalities in newborn neurons of tdrd3 deficient mice, such as increased dendritic intersections, reduced volume and diameter, and lower spine density.
5. Dysregulation of Gene Expression: Sequencing analysis and RT-qPCR verification showed that the mature mRNA levels of several GABA signaling pathway-related genes were significantly reduced in tdrd3 deficient mice, while their nascent RNA levels remained nearly unchanged.

Research Conclusion

The study concluded that Tdrd3 plays a crucial role in normal brain function in mice, and its deficiency may contribute to cognitive and mental disorders through post-transcriptional regulatory pathways. The entire top3b-tdrd3 complex is indispensable for regulating the stability of gene expression, and deficiencies may lead to the dysregulation of GABA signaling and neurodevelopment-related gene expression.

Research Highlights

• Dual-Activity Topoisomerase Mechanism: Provides mechanistic insights into how the top3b-tdrd3 complex coordinates actions at both DNA and RNA levels.
• Synaptic Plasticity and Neuronal Structure: Reveals significant changes in synaptic plasticity and morphology of newborn neurons in tdrd3 deficient mice.
• Post-transcriptional Regulation: Demonstrates how the absence of tdrd3 affects the post-transcriptional regulation of numerous genes closely related to neuronal function.

Conclusion and Significance

This study not only reveals the crucial role of tdrd3 in brain function but also provides new perspectives for exploring the molecular mechanisms behind mental and cognitive disorders. By elucidating the dual role of the Top3b-Tdrd3 complex in gene transcription and post-transcriptional processing, this research lays the groundwork for understanding gene regulation in a broader neurobiological and pathobiological context. These findings may contribute to the future design of more effective therapeutic strategies targeting similar molecular mechanisms underlying mental and cognitive disorders.