Developmental expression of the fragile X-related 1 proteins in mouse testis: association with microtubule elements.

Fragile X mental retardation 1 protein (FMRP) is the archetype of a class of cytoplasmic mRNA-binding proteins that includes the fragile X-related 1 and 2 proteins (FXR1P and FXR2P). Whereas absence of FMRP is the cause of fragile X syndrome, it is not known if FXR1P and FXR2P are associated with any pathology. It is also still elusive whether these homologous proteins can partially compensate for the absence of FMRP in the case of the fragile X syndrome. FXR1 is widely expressed in mammals and its expression pattern is complex since several mRNA variants and protein isoforms are detected. In mouse, we observed that the highest level of FXR1 is found in the adult testis. This tissue is an exception, since all known FXR1P isoforms, some of which have been considered as tissue specific, are detected in it. In young animals, changes in mRNA-spliced variants and their corresponding protein isoforms occur during spermatogenesis. Using biochemical, immunohistochemical and electron microscopic techniques, we show that FXR1P is associated with microtubule elements. Since the cytoskeletal framework is implicated in cellular plasticity as well as in mRNA transport, we propose new possibilities for the function(s) of the FXR proteins.