Intercellular bridges are essential for transposon repression and meiosis in the male germline
Germ cell connectivity via intercellular bridges is a widely conserved feature but its functional significance is poorly understood. Intercellular bridges are essential for fertility in male mice as genetic ablation of a critical bridge component, TEX14, causes loss of germ cell connectivity and meiotic failure, however the underlying reasons are unknown. A screen for meiosis-defective mouse mutants yielded a sterile mutant harboring a point mutation in Tex14 that leads to a partial loss of germ cell intercellular bridges. We utilized the Tex14 hypomorphs with reduced intercellular bridges, along with Tex14-null mice that completely lack bridges to examine the roles of germ cell connectivity during adult spermatogenesis. Complete or partial loss of intercellular bridges does not severely deplete spermatogonia, but spermatocytes are unable to undergo meiotic DNA replication and enter meiotic prophase. Phenotypic severity correlates with the extent of intercellular bridge depletion, with Tex14 hypomorphs containing a relatively larger number of spermatocytes undergoing meiosis, thus allowing us to assess impacts on meiotic progression. Tex14-deficient germ cells that enter prophase form meiotic breaks and initiate synapsis, but later exhibit synaptic failures and retain unrepaired breaks. Strikingly, Tex14-deficient germ cells derepress LINE1 retrotransposons and accumulate LINE1-encoded proteins during meiosis. Single-cell RNA-sequencing of testes from Tex14 hypomorphs show that intercellular bridge depletion modestly impacts the RNA landscape in early germ cell stages, while mutant cells that progress to late spermatocyte and post-meiotic spermatid stages broadly upregulate retrotransposons and dysregulate gene expression. RNA-sequencing confirms extensive sharing of transcripts between normal spermatids and demonstrates its partial attenuation in Tex14 hypomorphs, indicating that intercellular bridges enable exchange of cytoplasmic material between connected germ cells in testes. Tex14 hypomorphs display a progressive decline in germ cell numbers starting from the spermatocyte stage, with a complete lack of elongating spermatids, and are therefore sterile. Our findings reveal that the regulation of meiosis is non-cell-intrinsic and inform a model in which intercellular bridges orchestrate critical meiotic events and protect germline genome integrity during spermatogenesis, possibly by facilitating the exchange of regulatory factors. Overall design: Single Cell RNA-seq of Tex14m/m and WT testis. Cells from each animal were counted and tagged with unique cell multiplexing oligo barcodes. Labelled cells from one wild type and one mutant were pooled equally and processed for cell encapsulation targeting 12,000 cells (~6,000 cells per mouse), followed by library preparation. This was done in duplicate to capture ~12,000 cells per mouse. Labelled cells from the second wild type and second mutant were pooled and processed similarly.