A multitude of genes expressed solely in meiotic or postmeiotic spermatogenic cells offers a myriad of contraceptive targets

Abstract

Understanding mammalian spermatozoan development and the events surrounding fertilization has grown slowly, in part because of uncertainty about the number and identity of the cellular components involved. Determination of those transcripts expressed specifically by germ cells should provide an inclusive list of probable critical proteins. Here, total mouse testis transcript profiles were trimmed of transcripts found in cultures enriched in Sertoli or interstitial cells to yield a germ cell-enriched transcript profile. Monitoring of changes of this profile in the developing testis identified 1,652 genes whose transcript abundance increased markedly coincident with the onset of meiosis. Remarkably, 351 of these genes (approximately equal to 20%) appear to be expressed only in the male germline. Germ cell-specific transcripts are much less common earlier in testis development. Further analysis of the UniGene EST database coupled with quantitative PCR indicates that approximately 4% of the mouse genome is dedicated to expression in postmeiotic male germ cells. Most or many of the protein products of these transcripts are probably retained in mature spermatozoa. Targeted disruption of 19 of these genes has indicated that a majority have roles critical for normal fertility. Thus, we find an astonishing number of genes expressed specifically by male germ cells late in development. This extensive group provides a plethora of potential targets for germ cell-directed contraception and a staggering number of candidate proteins that could be critical for fertilization.

Fig. 1.

Heat maps showing the eight clusters of transcripts that change coincident with the formation of meiotic and postmeiotic male germ cells and the unclassified genes. Each row of the heat map represents a gene, and each column represents a time point in development (as labeled at the bottom). The color saturation represents differences in gene expression compared with day 1. Red indicates an increase in gene expression, whereas green indicates a decrease. The genes listed next to the heat maps are examples of known genes found in the respective clusters.

Fig. 2.

Testis developmental profiles for clusters 1–5. (Upper) The relative gene transcript abundance in testis from mice at ages days 4, 8, 11, 14, 18, 21, 26, and 29 and adult compared with testis at day 1 of age. The values are the mean relative abundance (compared with day 1) for all transcripts found in each of the clusters. Note the marked increase in relative transcript abundance for cluster 4 coincident with formation of spermatids. (Lower) Hematoxylin/eosin staining of sections of testis from the same mice at ages days 1, 11, 14, and 26 to show germ cell differentiation at each age.