The Incoherent Fluctuation of Folate Pools and Differential Regulation of Folate Enzymes Prioritize Nucleotide Supply in the Zebrafish Model Displaying Folate Deficiency-Induced Microphthalmia and Visual Defects

Tsun-Hsien Hsiao; Gang-Hui Lee; Yi-Sheng Chang; Bing-Hung Chen; Tzu-Fun Fu

doi:10.3389/fcell.2021.702969

The Incoherent Fluctuation of Folate Pools and Differential Regulation of Folate Enzymes Prioritize Nucleotide Supply in the Zebrafish Model Displaying Folate Deficiency-Induced Microphthalmia and Visual Defects

Front Cell Dev Biol. 2021 Jun 29:9:702969. doi: 10.3389/fcell.2021.702969. eCollection 2021.

Authors

Tsun-Hsien Hsiao¹, Gang-Hui Lee¹, Yi-Sheng Chang^{2

3}, Bing-Hung Chen^{4

5

6

7}, Tzu-Fun Fu^{1

8}

Affiliations

¹ The Institute of Basic Medical Science, College of Medicine, National Cheng Kung University, Tainan, Taiwan.
² Department of Ophthalmology, College of Medicine, National Cheng Kung University Hospital, Tainan, Taiwan.
³ Department of Ophthalmology, College of Medicine, National Cheng Kung University, Tainan, Taiwan.
⁴ Department of Biotechnology, Kaohsiung Medical University, Kaohsiung, Taiwan.
⁵ Department of Medical Research, Kaohsiung Medical University Hospital, Kaohsiung, Taiwan.
⁶ Center for Biomarkers and Biotech Drugs, Kaohsiung Medical University, Kaohsiung, Taiwan.
⁷ Department of Biological Sciences, National Sun Yat-sen University, Kaohsiung, Taiwan.
⁸ Department of Medical Laboratory Science and Biotechnology, College of Medicine, National Cheng Kung University, Tainan, Taiwan.

Abstract

Objective: Congenital eye diseases are multi-factorial and usually cannot be cured. Therefore, proper preventive strategy and understanding the pathomechanism underlying these diseases become important. Deficiency in folate, a water-soluble vitamin B, has been associated with microphthalmia, a congenital eye disease characterized by abnormally small and malformed eyes. However, the causal-link and the underlying mechanism between folate and microphthalmia remain incompletely understood.

Methods: We examined the eye size, optomotor response, intracellular folate distribution, and the expression of folate-requiring enzymes in zebrafish larvae displaying folate deficiency (FD) and ocular defects.

Results: FD caused microphthalmia and impeded visual ability in zebrafish larvae, which were rescued by folate and dNTP supplementation. Cell cycle analysis revealed cell accumulation at S-phase and sub-G1 phase. Decreased cell proliferation and increased apoptosis were found in FD larvae during embryogenesis in a developmental timing-specific manner. Lowered methylenetetrahydrofolate reductase (mthfr) expression and up-regulated methylenetetrahydrofolate dehydrogenase (NADP⁺-dependent)-1-like (mthfd1L) expression were found in FD larvae. Knocking-down mthfd1L expression worsened FD-induced ocular anomalies; whereas increasing mthfd1L expression provided a protective effect. 5-CH₃-THF is the most sensitive folate pool, whose levels were the most significantly reduced in response to FD; whereas 10-CHO-THF levels were less affected. 5-CHO-THF is the most effective folate adduct for rescuing FD-induced microphthalmia and defective visual ability.

Conclusion: FD impeded nucleotides formation, impaired cell proliferation and differentiation, caused apoptosis and interfered active vitamin A production, contributing to ocular defects. The developmental timing-specific and incoherent fluctuation among folate adducts and increased expression of mthfd1L in response to FD reflect the context-dependent regulation of folate-mediated one-carbon metabolism, endowing the larvae to prioritize the essential biochemical pathways for supporting the continuous growth in response to folate depletion.

Keywords: eye diseases; folate; microphthalmia; mthfd1L; one-carbon metabolism; zebrafish.