Cell organization, growth, and neural and cardiac development require αII-spectrin

Abstract

SummarySpectrin a2 (aII-spectrin) is a scaffolding protein encoded by the Spna2 gene and constitutively expressed in most tissues. Exon trapping of Spna2 in C57BL/6 mice allowed targeted disruption of aII-spectrin. Heterozygous animals displayed no phenotype by 2 years of age. Homozygous deletion of Spna2 was embryonic lethal at embryonic day 12.5 to 16.5 with retarded intrauterine growth, and craniofacial, neural tube and cardiac anomalies. The loss of aII-spectrin did not alter the levels of aI-or bI-spectrin, or the transcriptional levels of any b-spectrin or any ankyrin, but secondarily reduced by about 80% the steady state protein levels of bII-and bIII-spectrin. Residual bII-and bIII-spectrin and ankyrins B and G were concentrated at the apical membrane of bronchial and renal epithelial cells, without impacting cell morphology. Neuroepithelial cells in the developing brain were more concentrated and more proliferative in the ventricular zone than normal; axon formation was also impaired. Embryonic fibroblasts cultured on fibronectin from E14.5 (Spna2 2/2 ) animals displayed impaired growth and spreading, a spiky morphology, and sparse lamellipodia without cortical actin. These data indicate that the spectrin-ankyrin scaffold is crucial in vertebrates for cell spreading, tissue patterning and organ development, particularly in the developing brain and heart, but is not required for cell viability.

Critical roles of αII spectrin in brain development and epileptic encephalopathy

“…Immunoreactivity for αII spectrin was observed at the AIS, colabeling with ankyrin-G and βIV spectrin (Figure 1, B and C, and Supplemental Figure 1, C and E; supplemental material available online with this article; https://doi.org/10.1172/ JCI95743DS1). Consistent with previous studies (11), abundant αII spectrin expression was detected in the embryonic cerebral cortex ( Figure 1D and Supplemental Figure 1). The expression was enriched in the cortical plate but low in the intermediate zone and ventricular zone/subventricular zone.…”

“…The difference may reflect a partial knockdown with RNAi compared with a more complete loss of αII spectrin with CRISPR gene editing. In addition, the lack of a phenotype in heterozygous Sptan1-knockout mice argues that complete loss of αII spectrin is probably essential to study its function (11). Interestingly, αII spectrin RNAi experiments apparently showed no axonal defects (7).…”

“…-/-mice die before E16 with cardiac and neural tube malformations, and Sptan1 +/-mice are reportedly normal (11). Moreover, spectrins are membrane proteins that provide a membrane-cytoskeleton interface and mediate cell-cell interaction, adhesion, and mechanical stress adaptation, functions not easily recapitulated by in vitro cell culture (6).…”

“…Recent stochastic optical reconstruction microscopy (STORM) studies reveal that axons are supported by submembranous actin organized in a periodic ring structure linked by βII spectrins, the obligatory binding partner of αII spectrin, further suggesting that αII/βII spectrin complexes have indispensable structural roles in axon development (10). Because Sptan1 -/-mice die before embryonic day 16 (E16) with cardiac and neural tube malformations, while Sptan1 +/-mice have no detectable phenotypic abnormalities, studying αII spectrin function in vivo in rodents has been challenging (11).…”

Spectrin: Structure, function and disease

“…Mutated spectrin also causes some blood diseases such as hemolytic anemia. Spectrin also helps to maintain the Golgi apparatus and signal transduction pathways in many cell types [5].…”