The Function of Fndc3 Genes in Mouse Spermatogenesis
Fibronectin type III domain-containing 3 (Fndc3) genes encode a novel family of cytosolic tail-anchored proteins found in metazoa, including humans and mice. Male mice lacking Fndc3a are sterile due to opening of spermatid intercellular bridges and loss of adhesion between spermatids and Sertoli cells. In the mouse testis, Fndc3a and Fndc3b are transcribed primarily in germ cells, with highest levels of Fndc3a transcripts in spermatocytes and Fndc3b transcripts in elongating spermatids. Analysis of mice with germ cell specific knockout of Fndc3a or Fndc3b revealed that Fndc3a, but not Fndc3b, is required in a germ cell-intrinsic manner for male fertility. Loss of Fndc3a in adult mice demonstrated that Fndc3a is also required for maintenance of adult spermatogenesis. Although lipids accumulate in Leydig cells in mice lacking Fndc3a in all tissues, Fndc3a is not required in Leydig cells for fertility. Possible mechanisms for loss of spermatid-Sertoli cell adhesion and intercellular bridge formation and maintenance in Fndc3a mutant mice were investigated. Defects in androgen signaling were not detected in sterile male Fndc3a mutant mice, indicating loss of spermatid-Sertoli cell adhesion was not due to loss of androgen signaling. Moreover, the testis of Fndc3a mutants displayed no defect in either the synthesis of ultra long chain polyunsaturated fatty acids (ULC-PUFA's), or the expression and formation of TEX14 rings required for maintenance of intercellular bridges, suggesting that failure in intercellular bridges is due to a novel defect. To begin to investigate how FNDC3A functions in spermatogenesis, interacting proteins were identified. FNBP4, SMURF1, and IQGAP1 were identified as WW-domain containing proteins involved in actin dynamics that can interact with FNDC3A and are co-expressed with Fndc3a in developing germ cells. Surprisingly, these interactions occur independently of conserved WW-domain binding motifs in FNDC3A. Instead, a novel conserved "KKLK sequence" within FNDC3 proteins was identified that is required for interaction. These results raise the possibility that loss of Fndc3a in spermatids leads to abnormal actin regulation resulting in opening of intercellular bridges and loss of spermatid-Sertoli adhesion. Models are presented for how FNDC3A may function in regulating the spermatid actin cytoskeleton for male fertility.