Knocking Down FRMD4A, a Factor Associated with the Brain Development Disorder and a Risk Factor for Alzheimer’s Disease, Using RNA-Targeting CRISPR/Cas13 Reveals Its Role in Cell Morphogenesis

Genetic truncation or mutation of the gene encoding band 4.1, ezrin, radixin, and moesin (FERM) domain protein containing 4A (FRMD4A) is associated with brain developmental diseases, including microcephaly with global developmental delay. It has also been identified as a risk factor for Alzheimer's disease. By analogy with other FERM domain-containing proteins, FRMD4A is believed to regulate cell morphogenesis and/or cell polarization in central nervous system (CNS) cells; however, it remains unclear whether and how dysfunction of FRMD4A and/or its closely homologous protein FRMD4B causes abnormal morphogenesis in neuronal cells. Here, we describe for the first time the roles of FRMD4A and FRMD4B in process elongation in neuronal cells. Knockdown of Frmd4a or Frmd4b using specific RNA-targeting clustered regularly interspaced short palindromic repeat (CRISPR) and Cas13-fitted gRNAs led to decreased process elongation in primary cortical neurons. Similar decreases in neuronal marker expression were observed in the N1E-115 cell line, a model of neuronal differentiation. Furthermore, hesperetin, an aglycone of the citrus flavonoid hesperidin known to promote neuroprotective signaling, recovered the decreased process elongation induced by the knockdown of Frmd4a or Frm4b. Hesperetin also stimulated phosphorylation of mitogen-activated protein kinases/extracellular signal-regulated kinases (MAPKs/ERKs), which could help promote neuronal processes. These results suggest that FRMD4A and FRMD4B regulate process elongation through a possible signaling pathway linked to the sustained phosphorylation of MAPKs/ERKs. Crucially, this study reveals that, at the molecular and cellular levels, hesperetin can restore normal phenotypes when FRMD4A protein or FRMD4B protein is impaired.