Spermatogonial stem cells: characteristics and experimental possibilities

The continuation of the spermatogenic process throughout life relies on a proper regulation of self-renewal and differentiation of the spermatogonial stem cells. These are single cells situated on the basal membrane of the seminiferous epithelium. Only 0.03% of all germ cells are spermatogonial stem cells. They are the only cell type that can repopulate and restore fertility to congenitally infertile recipient mice following transplantation. Although numerous expression markers have been helpful in isolating and enriching spermatogonial stem cells, such as expression of THY-1 and GFRalpha-1 and absence of c-kit, no specific marker for this cell type has yet been identified. Much effort has been put into developing a protocol for the maintenance of spermatogonial cells in vitro. Recently, coculture systems of testicular cells on various feeder cells have made it possible to culture spermatogonial stem cells for a long period of time, as was demonstrated by the transplantation assay. Even expansion of testicular cells, including the spermatogonial stem cells, has been achieved. In these culture systems, hormones and growth factors are investigated for their role in the process of proliferation of spermatogonial stem cells. At the moment the best culture system known still consists of a mixture of testicular cells with about 1.33% spermatogonial stem cells. Recently pure SV40 large T immortalized spermatogonial stem cell lines have been established. These c-kit-negative cell lines did not show any differentiation in vitro or in vivo. A telomerase immortalized c-kit-positive spermatogonial cell line has been established that was able to differentiate in vitro. Spermatocytes and even spermatids were formed. However, spermatogonial stem cell activity by means of the transplantation assay was not tested for this cell line. Both the primary long-term cultures and immortalized cell lines have represented a major step forward in investigating the regulation of spermatogonial self-renewal and differentiation, and will be useful for identifying specific molecular markers.

LY6A/E (SCA-1) expression in the mouse testis

Recently, it was found by two research groups that LY6A, known widely in the stem cell community as stem cell antigen-1 or SCA-1, is expressed on testicular side population (SP) cells. Whether these SP cells are spermatogonial stem cells is a point of disagreement and, therefore, the identity of the LY6A-positive cells as well. We studied the expression pattern of LY6A in testis by immunohistochemistry and found it to be expressed in the interstitial tissue on peritubular myoid, endothelial, and spherical-shaped peritubular mesenchymal cells. To address the question whether LY6A has a function in spermatogenesis or testis development, we studied the testis of Ly6a(-/-) mice (allele Ly6a(tm1Pmf)). We found no morphological abnormalities or differences in numbers of spermatogonia, spermatocytes, Leydig cells, or macrophages in relation to the number of Sertoli cells. Therefore, we conclude that LY6A expression does not influence testis development or spermatogenesis and that spermatogonial stem cells are LY6A negative.

Gain-of-function amino acid substitutions drive positive selection of FGFR2 mutations in human spermatogonia

Despite the importance of mutation in genetics, there are virtually no experimental data on the occurrence of specific nucleotide substitutions in human gametes. C>G transversions at position 755 of FGF receptor 2 (FGFR2) cause Apert syndrome; this mutation, encoding the gain-of-function substitution Ser252Trp, occurs with a birth rate elevated 200- to 800-fold above background and originates exclusively from the unaffected father. We previously demonstrated high levels of both 755C>G and 755C>T FGFR2 mutations in human sperm and proposed that these particular mutations are enriched because the encoded proteins confer a selective advantage to spermatogonial cells. Here, we examine three corollaries of this hypothesis. First, we show that mutation levels at the adjacent FGFR2 nucleotides 752-754 are low, excluding any general increase in local mutation rate. Second, we present three instances of double-nucleotide changes involving 755C, expected to be extremely rare as chance events. Two of these double-nucleotide substitutions are shown, either by assessment of the pedigree or by direct analysis of sperm, to have arisen in sequential steps; the third (encoding Ser252Tyr) was predicted from structural considerations. Finally, we demonstrate that both major alternative spliceforms of FGFR2 (Fgfr2b and Fgfr2c) are expressed in rat spermatogonial stem cell lines. Taken together, these observations show that specific FGFR2 mutations attain high levels in sperm because they encode proteins with gain-of-function properties, favoring clonal expansion of mutant spermatogonial cells. Among FGFR2 mutations, those causing Apert syndrome may be especially prevalent because they enhance signaling by FGF ligands specific for each of the major expressed isoforms.

Establishment of cell lines with rat spermatogonial stem cell characteristics

Spermatogonial cell lines were established by transfecting a mixed population of purified rat A(s) (stem cells), A(pr) and A(al) spermatogonia with SV40 large T antigen. Two cell lines were characterized and found to express Hsp90alpha and oct-4, specific markers for germ cells and A spermatogonia, respectively. Expression of c-kit, normally expressed in A spermatogonia from late A(al) spermatogonia onwards, could not be detected in either cell line. Furthermore, no expression of vimentin (Sertoli cell marker) and alpha-smooth muscle actin (peritubular cell marker) could be found. Upon transplantation of these cell lines into recipient mice, the cells were found to be able to migrate to the basement membrane and to colonize seminiferous tubules. Taken together, we conclude that our cell lines have spermatogonial stem cell characteristics. These first spermatogonial cell lines with stem cell characteristics can now be used to study spermatogonial gene expression in comparison with more advanced germ cells.