An Orchestrated Intron Retention Program in Meiosis Controls Timely Usage of Transcripts during Germ Cell Differentiation

Abstract

Global transcriptome reprogramming during spermatogenesis ensures timely expression of factors in each phase of male germ cell differentiation. Spermatocytes and spermatids require particularly extensive reprogramming of gene expression to switch from mitosis to meiosis and to support gamete morphogenesis. Here, we uncovered an extensive alternative splicing program during this transmeiotic differentiation. Notably, intron retention was largely the most enriched pattern, with spermatocytes showing generally higher levels of retention compared with spermatids. Retained introns are characterized by weak splice sites and are enriched in genes with strong relevance for gamete function. Meiotic intron-retaining transcripts (IRTs) were exclusively localized in the nucleus. However, differently from other developmentally regulated IRTs, they are stable RNAs, showing longer half-life than properly spliced transcripts. Strikingly, fate-mapping experiments revealed that IRTs are recruited onto polyribosomes days after synthesis. These studies reveal an unexpected function for regulated intron retention in modulation of the timely expression of select transcripts during spermatogenesis.

Keywords: alternative splicing; germ cell differentiation; intron retention; mRNA stability; spermatogenesis; transcriptome profiling.

Figure 1

An Extensive Alternative Splicing Program Characterizes the Meiotic Transition of Male Germ Cells (A) Representative images of Hoechst nuclear staining of purified cellular populations of meiotic pachytene spermatocytes (sp.cytes, upper panel) and post-meiotic round spermatids (sp.tids, lower panel). Scale bar, 25 μm. (B and C) Pie charts showing percentages of expression-regulated genes (B) and splicing-regulated exons (C) identified in sp.cytes compared with sp.tids (fold change ≥2; p ≤ 0.05). (D) Pie chart representing distribution of regulated splicing events in sp.cytes versus sp.tids among different splicing patterns. (E) Representative images of RT-PCR analyses for indicated AS events differentially regulated between sp.cytes and sp.tids. Schematic representation for each event analyzed is depicted above the representative agarose gel. Red and green boxes indicate respectively up- and downregulated events in sp.cytes compared with sp.tids. Black arrows in the scheme indicate primers used for the PCR analysis. See also Figure S1.

Figure 2

Intron Retention Is a Key Feature of the Male Meiotic Transcriptome (A) Bar graph showing percentages of events annotated in FAST-DB (dashed, light gray columns) and of those differentially regulated between spermatocytes (sp.cytes) and spermatids (sp.tids) (black columns) within each AS pattern. p Values above the graph indicate a significant enrichment for regulated events within specific AS pattern in our dataset compared with the reference database (modified Fisher's test). Red box highlights the IR pattern. (B) Bar graph representing percentages of up- and downregulated events among exonic and intronic splicing events. Enrichment in upregulated events in IR with respect to all other AS events was statistically significant (p = 4.9 × 10⁻⁶¹; χ² test). (C) (Top) Visualization of the RNA-seq reads profile of the intron-retaining genes Adam3, Adam18, and Stag3 in sp.cytes (upper graph) and sp.tids (lower graph). Sequence reads (vertical gray lines), exons (blue boxes), and introns (horizontal lines) are shown. (Bottom) Bar graphs showing results of qPCR analyses for the expression of the retained and non-retained introns (red and black dashed boxes, respectively) relative to their flanking exons in sp.cytes and sp.tids (mean ± SD, n = 3; ^∗p ≤ 0.05, ns = not significant; one-way ANOVA). (D) Bar graphs showing results of qPCR analyses for the expression of indicated introns relative to spliced product of their flanking exons in sp.cytes, and round and elongated sp.tids (mean ± SD, n = 4; ^∗∗p ≤ 0.01, ^∗∗∗p ≤ 0.001; ns, not significant; one-way ANOVA). See also Figures S2 and S3.

Figure 3

IRTs Are Highly Stable mRNAs Retained in the Nucleus of Meiotic Cells (A) Bar graph showing percentages of expression-regulated genes within the groups of intron-retention and other splicing-regulated genes. A significant association was found between the gene regulation and the type of event (p = 0.0156; χ² test). (B) Bar graph showing qPCR analysis using primers spanning exon-intron regions for EU-labeled RNA transcripts of indicated genes isolated from total RNA of seminiferous tubules of 20-dpp (days post-partum) mice treated or not for 1 hr with 1 μM FPD. Results are expressed as percentage of the EU-labeled RNA pulled down relative to control condition, arbitrarily set to 100% (dashed line in the graph, mean ± SD, n = 3). (C) Box plot showing the distribution of the mean percentages of EU-labeled RNA transcripts pulled down as described in (B) in the IR-regulated genes and other expressed genes. Whiskers indicate 1.5 interquartile range. p Value indicates a significant difference between means of the two groups (Welch's t test). (D) PCR analysis of the subcellular localization of the IRTs of Adam3, Hook2, and Izumo4 genes. The properly spliced genes Hspa4 and U6 were evaluated as control. See also Figure S4.

Figure 4

Specific cis-Acting Elements and Elevated Transcription Rate Feature Meiotic IRTs (A) Graphic representation of retained introns location within gene body. (B–D) Box plots representing comparison between retained introns and other introns for their length (B), GC content (C), and splice-site strength (D). Whiskers indicate 1.5 interquartile range. p Values indicate a significant difference between means of the two populations (Welch's t test). (E) Box plots showing distribution of FPKM values for IR and other expressed genes in spermatocytes (sp.cytes) and spermatids (sp.tids). p Values indicate a significant difference between means of the two groups in the two cell types (Welch's t test). (F) Bar graph showing percentages of intron-retaining and other expressed genes enriched for indicated histone marks in sp.cytes. p Values indicate a significant difference in the enrichment for different histone marks between the two groups (χ² test).

Figure 5

The Elevated Transcriptional Activity of Meiotic Spermatocytes Modulates IR Events (A) Histogram showing the total RNA per cell synthesized by spermatocytes (sp.cytes), round spermatids (sp.tids), elongated sp.tids, and spermatozoa (sp.zoa) (mean ± SD, n = 4, ^∗∗p ≤ 0.01, ^∗∗∗p ≤ 0.001; ns, not significant; one-way ANOVA test). (B) Representative images of immunofluorescence analysis of seminiferous tubule cross-sections from adult mice using anti-pSer2-RNAPII (H5) antibody. Hoechst staining was used to identify nuclear morphology. Insets show magnified images of meiotic spermatocytes (arrowhead), post-meiotic round spermatids (arrow), and elongated spermatids (square-headed arrow). Scale bar, 25 μm. (C) Representative western blot analysis for RNAPII and its Ser2-CTD phosphorylated form (pSer2-RNAPII) in nuclear-enriched extracts of sp.cytes and round sp.tids. HSP90 protein level was evaluated as loading control. (D) Upper panels show representative western blot analysis for total and pSer2-RNAPII in nuclear-enriched extracts obtained from seminiferous tubules treated or not for 24 hr with 10 μg/mL DRB. HSP90 was evaluated as loading control. Lower panels show representative images of agarose gel analysis for RT-PCR products of indicated IR events under the same experimental conditions. (E) Bar graphs showing results of qPCR analyses for indicated retained introns relative to their flanking exons in seminiferous tubules treated for 24 hr with 10 μg/mL DRB compared with the control condition, arbitrarily set to 1 (mean ± SD, n = 4; ^∗p ≤ 0.05, ^∗∗p ≤ 0.01, ^∗∗∗p ≤ 0.001; one-way ANOVA). See also Figure S5.

Figure 6

IRTs Are Stable during Spermatogenesis (A) Schematic representation of the timeline of the key phases of murine spermatogenesis and their transcriptional activity. Time points of the in vivo RNA labeling experiments with EU are depicted below. (B–D) EU staining with Alexa 594-azide of testicular paraffin-embedded cross-sections from EU-injected and PBS-injected (as control) mice harvested at 20 dpp (B), 21 dpp (C), and 29 dpp (D). Hoechst nuclear staining was performed to discriminate the different cellular types within the seminiferous tubules. Scale bar, 25 μm. (E) Bar graph showing qPCR analysis for EU-labeled RNAs pulled down at indicated time points. Results are expressed as percentage of the EU-labeled RNA captured 5 hr after injection at 20 dpp, arbitrarily set to 100% (mean ± SD, n = 3). (F and G) Box plot showing the distribution of the mean percentages of EU-labeled RNAs pulled down as described in (E) in the IR-regulated genes and other expressed genes. Whiskers indicate 1.5 interquartile range. p Values indicate a significant difference between means of the two groups (Welch's t test). See also Figure S6.

Figure 7

Meiotic IRTs Are Enriched in GO Terms Relative to Spermatogenesis and Are Actively Translated Several Days after Their Synthesis (A) Bar graphs showing the enrichment score of the GO functional clusters enriched in the groups of IR-regulated and other AS-regulated genes (p ≤ 0.05, high stringency). (B) Absorbance profile (OD = 260 nm) of sucrose gradient sedimentation of extracts from mitotic HEK293 cells (black line) and testes of 20-dpp mice (red line). (C) Bar graphs showing results of qPCR analysis for EU-labeled RNAs of indicated genes within the polysomal fraction derived from sucrose gradient fractionation of whole testes harvested at indicated time points after EU injection at 20 dpp. Results are expressed as percentage of the total EU-labeled RNA in all the fractions of the gradient (mean ± SD, n = 3). (D) Representative images of western blot analysis for ADAM3 in total extracts of spermatocytes (sp.cytes), and round and elongated spermatids (sp.tids). HSP90 was evaluated as loading control. See also Figure S7.