A tubulin alpha 8 mouse knockout model indicates a likely role in spermatogenesis but not in brain development

Abstract

Tubulin alpha 8 (Tuba8) is the most divergent member of the highly conserved alpha tubulin family, and uniquely lacks two key post-translational modification sites. It is abundantly expressed in testis and muscle, with lower levels in the brain. We previously identified homozygous hypomorphic TUBA8 mutations in human subjects with a polymicrogyria (PMG) syndrome, suggesting its involvement in development of the cerebral cortex. We have now generated and characterized a Tuba8 knockout mouse model. Homozygous mice were confirmed to lack Tuba8 protein in the testis, but did not display PMG and appeared to be neurologically normal. In response to this finding, we re-analyzed the human PMG subjects using whole exome sequencing. This resulted in identification of an additional homozygous loss-of-function mutation in SNAP29, suggesting that SNAP29 deficiency, rather than TUBA8 deficiency, may underlie most or all of the neurodevelopmental anomalies in these subjects. Nonetheless, in the mouse brain, Tuba8 specifically localised to the cerebellar Purkinje cells, suggesting that the human mutations may affect or modify motor control. In the testis, Tuba8 localisation was cell-type specific. It was restricted to spermiogenesis with a strong acrosomal localization that was gradually replaced by cytoplasmic distribution and was absent from spermatozoa. Although the knockout mice were fertile, the localisation pattern indicated that Tuba8 may have a role in spermatid development during spermatogenesis, rather than as a component of the mature microtubule-rich flagellum itself.

Differential requirements of tubulin genes in mammalian forebrain development

“…3). During the course of our analysis, a conditional deletion Tuba8 allele was reported by another group to have no brain phenotype (43). Consistent with this finding, the Tuba8 delex2/delex2 homozygous mice we analyzed are viable and fertile.…”

Mutation of the α-tubulin Tuba1a leads to straighter microtubules and perturbs neuronal migration

“…Hence, phenotypes would be expected to be different in coexpressing cells; however, this protein is not expressed in RMS migrating neurons ( Braun et al, 2010 ). Recent data also show that the Tuba8 -knockout mouse model presents no obvious defect in brain development but instead affected spermatogenesis ( Diggle et al, 2017 ). Specific molecular features may be expected for these two proteins, which we confirmed here by structural modeling.…”

Intrinsic and Extrinsic Factors Affecting Microtubule Dynamics in Normal and Cancer Cells

“…Interestingly, two classes show tissue-specific distribution, with αIII restricted mainly to testis and αVIII to heart and skeletal muscle, while the other two are widely distributed [34][35][36][37][38][39]. Although the tissue or cell type-specific functions of α-tubulin isotypes were only recently addressed for several isotypes (αIa in neuron migration [40], αIVa in platelets [41], and αVIII in brain and testis [42]), the role of α-tubulin isotypes in MT dynamics remains unknown.…”