Germ cells contribute to the function of the Sertoli cell barrier

Inositol hexakisphosphate kinase 1 is essential for cell junction integrity in the mouse seminiferous epithelium

Inositol hexakisphosphate kinases (IP6Ks) are enzymes that catalyse the synthesis of the inositol pyrophosphate 5-IP7 which is involved in the regulation of many physiological processes in mammals. The IP6K paralog IP6K1 is expressed at high levels in the mammalian testis, and its deletion leads to sterility in male mice. Here, we show that the loss of IP6K1 in mice causes a delay in the first wave of spermatogenesis. Testes from juvenile Ip6k1 knockout mice show downregulation of transcripts that are involved in cell adhesion and formation of the testis-specific inter-Sertoli cell impermeable junction complex known as the blood-testis barrier (BTB). We demonstrate that loss of IP6K1 in the mouse testis causes BTB disruption associated with transcriptional misregulation of the tight junction protein claudin 3, and subcellular mislocalization of the gap junction protein connexin 43. In addition to BTB disruption, we also observe a loss of germ cell adhesion in the seminiferous epithelium of Ip6k1 knockout mice, ultimately resulting in premature sloughing of round spermatids into the epididymis. Mechanistically, we show that loss of IP6K1 in the testis enhances cofilin dephosphorylation in conjunction with increased AKT/ERK and integrin signalling, resulting in destabilization of the actin-based cytoskeleton in Sertoli cells and germ cell loss.

Impact of Targeting β3 Receptor on Male Sex Hormonal Balance

BACKGROUND: Sympathetic stimulation has a significant impact on the physiology and pathology of the male reproductive system. β3 receptor is suspected to play a role in the regulation of fertility status in men. AIM: The study aims to investigate the role of the β3 receptor in regulating the fertility parameters (testosterone, estrogen, progesterone, and histology of testis) in male rats. MATERIALS AND METHODS: Male albino rats have been given either placebo (controls) or β3 agonist (Mirabegron). Testosterone, estrogen, and progesterone are measured before and after treatment for all cases and controls. Histology of testis is investigated for all the rats as well. RESULTS: β3 receptor activation caused a significant increase in testosterone plasma concentration and a significant reduction in estrogen plasma concentration. β3 agonist did not affect the progesterone plasma concentration. Histological sections showed that β3 activation resulted in degeneration of the spermatocytes and accumulation of edema between the seminiferous tubules in the testis. CONCLUSION: β3 receptor has a potentially important role in the fertility status of male rats via regulating sex hormonal profile and altering the histology of the testis.

Connexin43 in Germ Cells Seems to Be Dispensable for Murine Spermatogenesis

Testicular Connexin43 (Cx43) connects adjacent Sertoli cells (SC) and SC to germ cells (GC) in the seminiferous epithelium and plays a crucial role in spermatogenesis. However, the distinction whether this results from impaired inter-SC communication or between GC and SC is not possible, so far. Thus, the question arises, whether a GC-specific Cx43 KO has similar effects on spermatogenesis as it is general or SC-specific KO. Using the Cre/loxP recombinase system, two conditional KO mouse lines lacking Cx43 in premeiotic (pGCCx43KO) or meiotic GC (mGCCx43KO) were generated. It was demonstrated by qRT-PCR that Cx43 mRNA was significantly decreased in adult pGCCx43KO mice, while it was also reduced in mGCCx43KO mice, yet not statistically significant. Body and testis weights, testicular histology, tubular diameter, numbers of intratubular cells and Cx43 protein synthesis and localization did not show any significant differences in semi-quantitative Western blot analysis and immunohistochemistry comparing adult male KO and WT mice of both mouse lines. Male KO mice were fertile. These results indicate that Cx43 in spermatogonia/spermatids does not seem to be essential for successful termination of spermatogenesis and fertility as it is known for Cx43 in somatic SC, but SC-GC communication might rather occur via heterotypic GJ channels.

Disorganization of claudin-11 and dysfunction of the blood-testis barrier during puberty in a cryptorchid rat model

BACKGROUND

Cryptorchidism is known to impair spermatogenesis. The blood-testis barrier (BTB) becomes defined in seminiferous tubules around puberty and provides a suitable environment for germ cells. Little is known about the BTB in undescended testes (UDT).

OBJECTIVES

To determine the role of BTB during puberty in UDT using a non-surgical cryptorchid rat model.

MATERIAL AND METHODS

Unilateral cryptorchid male rats were intraperitoneally injected with non-steroidal antiandrogen during intrauterine development; the testes were harvested at 4, 5, and 6 weeks after birth. Testicular histology, expression levels of the BTB proteins (claudin-11, occludin, zonula occludens-1), and apoptotic cells were evaluated by immunohistochemistry, Western blotting, and TUNEL assay. The functionality of the BTB was investigated by electron microscopy using the lanthanum tracer method.

RESULTS

The testicular histology of undescended testes 6 weeks after birth showed maturation arrest at the spermatocyte level. The BTB protein distributions were altered in the UDT, with a noticeable difference in claudin-11(CLDN11) localization from 4 to 5 weeks after birth between control and UDT samples. BTB protein levels were similar. More apoptotic germ cells were detected in the adluminal compartment of tubules in the UDT than in the control testes. Electron microscopy showed that the lanthanum tracer was limited to the BTB of control testes, whereas it penetrated the BTB of UDT.

DISCUSSION

Here, loss of normal BTB function and impaired spermatogenesis were observed in UDT during puberty. CLDN11 is a pivotal tight junction protein belonging to the BTB. Tight junctions are considered as essential for normal spermatogenesis, and abnormal CLDN11 organization may cause UDT-associated male infertility.

CONCLUSION

CLDN11 disorganization within the BTB may cause spermatogenic impairment, possibly by limiting the BTB function.

Loss of connexin43 in murine Sertoli cells and its effect on blood-testis barrier formation and dynamics

Connexin43 (Cx43) is the predominant testicular gap junction protein and in cases of impaired spermatogenesis, Cx43 expression has been shown to be altered in several mammals. Amongst other functions, Cx43 is supposed to regulate junction formation of the blood-testis barrier (BTB). The aim of the present study was to investigate the expression pattern of different tight junction (TJ) proteins of the murine BTB using SC-specific Cx43 knockout mice (SCCx43KO). Adult homozygous male SCCx43KO mice (SCCx43KO-/-) predominantly show an arrest of spermatogenesis and SC-only tubules that might have been caused by an altered BTB assembly, composition or regulation. TJ molecules claudin-3, -5 and -11 were examined in adult wild type (WT) and SCCx43KO-/- mice using immunohistochemistry (IHC) and quantitative real-time PCR (qRT-PCR). In this context, investigation of single tubules with residual spermatogenesis in SCCx43KO-/- mice was particularly interesting to identify a potential Cx43-independent influence of germ cells (GC) on BTB composition and dynamics. In tubules without residual spermatogenesis, a diffuse cytoplasmic distribution pattern for claudin-11 protein could be demonstrated in mutant mice. Nevertheless, claudin-11 seems to form functional TJ. Claudin-3 and -5 could not be detected immunohistochemically in the seminiferous epithelium of those tubules. Correspondingly, claudin-3 and -5 mRNA expression was decreased, providing evidence of generally impaired BTB dynamics in adult KO mice. Observations of tubules with residual spermatogenesis suggested a Cx43-independent regulation of TJ proteins by GC populations. To determine initial BTB formation in peripubertal SCCx43KO-/- mice, immunohistochemical staining and qRT-PCR of claudin-11 were carried out in adolescent SCCx43KO-/- and WT mice. Additionally, BTB integrity was functionally analysed using a hypertonic glucose fixative. These analyses revealed that SCCx43KO-/- mice formed an intact BTB during puberty in the same time period as WT mice, which however seemed to be accelerated.

Regulation of blood-testis barrier assembly in vivo by germ cells

The assembly of the blood-testis barrier (BTB) during postnatal development is crucial to support meiosis. However, the role of germ cells in BTB assembly remains unclear. Herein, Kit^W/Kit^WV mice were used as a study model. These mice were infertile, failing to establish a functional BTB to support meiosis due to c-Kit mutation. Transplantation of undifferentiated spermatogonia derived from normal mice into the testis of Kit^W/Kit^WV mice triggered functional BTB assembly, displaying cyclic remodeling during the epithelial cycle. Also, transplanted germ cells were capable of inducing Leydig cell testosterone production, which could enhance the expression of integral membrane protein claudin 3 in Sertoli cells. Early spermatocytes were shown to play a vital role in directing BTB assembly by expressing claudin 3, which likely created a transient adhesion structure to mediate BTB and cytoskeleton assembly in adjacent Sertoli cells. In summary, the positive modulation of germ cells on somatic cell function provides useful information regarding somatic-germ cell interactions.-Li, X.-Y., Zhang, Y., Wang, X.-X., Jin, C., Wang, Y.-Q., Sun, T.-C., Li, J., Tang, J.-X., Batool, A., Deng, S.-L., Chen, S.-R., Cheng, C. Y., Liu, Y.-X. Regulation of blood-testis barrier assembly in vivo by germ cells.

Preparation of Testicular Samples for Histology and Immunohistochemistry

One approach to visualize internal structures of the testis is histological sectioning of the material. The use of testicular samples allows a detailed analysis of the structure of both seminiferous tubules and the interstitial space. It is worth noting that key role in the control of germ cell development is assigned to Sertoli cells. Thus, in this chapter the special reference is made on visualization of Sertoli cells in the seminiferous epithelium in which they create a specialized microenvironment to support the germ cell development through the formation of the blood-testis barrier (BTB). The use of transmission electron microscopy (TEM) allows a deeper insight into the BTB morphology, especially the organization of the basal ectoplasmic specialization (ES) and coexisting intercellular junctions.Equally important, immunohistochemistry (IHC) is an appropriate technique to detect the localization of various proteins in paraffin-embedded and fixed tissues, i.e. testicular samples. A proper fixation allows to stabilize structure of the seminiferous tubules and preserve cells against irreversible damage. As such localization of various junction proteins connecting adjoined Sertoli cells and present in germ cell-Sertoli cell interfaces is possible. Also immunofluorescence (IF) is helpful to detect the distribution and relative abundance of the junctional proteins, while immunocytochemistry (ICC) is a valuable technique to show a protein distribution within a single cell (e.g. in Sertoli cell culture).

Signaling pathways regulating blood-tissue barriers - Lesson from the testis

Signaling pathways that regulate blood-tissue barriers are important for studying the biology of various blood-tissue barriers. This information, if deciphered and better understood, will provide better therapeutic management of diseases particularly in organs that are sealed by the corresponding blood-tissue barriers from systemic circulation, such as the brain and the testis. These barriers block the access of antibiotics and/or chemotherapeutical agents across the corresponding barriers. Studies in the last decade using the blood-testis barrier (BTB) in rats have demonstrated the presence of several signaling pathways that are crucial to modulate BTB function. Herein, we critically evaluate these findings and provide hypothetical models regarding the underlying mechanisms by which these signaling molecules/pathways modulate BTB dynamics. This information should be carefully evaluated to examine their applicability in other tissue barriers which shall benefit future functional studies in the field. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Annexin A2 is critical for blood-testis barrier integrity and spermatid disengagement in the mammalian testis

Throughout spermatogenesis, two important processes occur at late stage VIII of the seminiferous epithelial cycle in the rat testis: preleptotene spermatocytes commence entry into the adluminal compartment and step 19 spermatids release from the seminiferous epithelium. Presently, it is not clear how these processes, which involve extensive restructuring of unique Sertoli-Sertoli and Sertoli-germ cell junctions, are mediated. We aimed to determine whether annexin A2 (ANXA2), a Ca²⁺-dependent and phospholipid-binding protein, participates in cell junction dynamics. To address this, in vitro and in vivo RNA interference studies were performed on prepubertal Sertoli cells and adult rat testes. The endpoints of Anxa2 knockdown were determined by immunoblotting, morphological analyses, fluorescent immunostaining, and barrier integrity assays. In the testis, ANXA2 localized to the Sertoli cell stalk, with specific staining at the blood-testis barrier and the concave (ventral) surface of elongated spermatids. ANXA2 also bound actin when testis lysates were used for immunoprecipitation. Anxa2 knockdown was found to disrupt the Sertoli cell/blood-testis barrier in vitro and in vivo. The disruption in barrier function was substantiated by changes in the localization of claudin-11, zona occludens-1, N-cadherin, and β-catenin. Furthermore, Anxa2 knockdown resulted in spermiation defects caused by a dysfunction of tubulobulbar complexes, testis-specific actin-rich ultrastructures that internalize remnant cell junction components prior to spermiation. Additionally, there were changes in the localization of several tubulobulbar complex component proteins, including actin-related protein 3, cortactin, and dynamin I/II. Our results indicate that ANXA2 is critical for the integrity of the blood-testis barrier and the timely release of spermatids.

The Mammalian Blood-Testis Barrier: Its Biology and Regulation

Spermatogenesis is the cellular process by which spermatogonia develop into mature spermatids within seminiferous tubules, the functional unit of the mammalian testis, under the structural and nutritional support of Sertoli cells and the precise regulation of endocrine factors. As germ cells develop, they traverse the seminiferous epithelium, a process that involves restructuring of Sertoli-germ cell junctions, as well as Sertoli-Sertoli cell junctions at the blood-testis barrier. The blood-testis barrier, one of the tightest tissue barriers in the mammalian body, divides the seminiferous epithelium into 2 compartments, basal and adluminal. The blood-testis barrier is different from most other tissue barriers in that it is not only comprised of tight junctions. Instead, tight junctions coexist and cofunction with ectoplasmic specializations, desmosomes, and gap junctions to create a unique microenvironment for the completion of meiosis and the subsequent development of spermatids into spermatozoa via spermiogenesis. Studies from the past decade or so have identified the key structural, scaffolding, and signaling proteins of the blood-testis barrier. More recent studies have defined the regulatory mechanisms that underlie blood-testis barrier function. We review here the biology and regulation of the mammalian blood-testis barrier and highlight research areas that should be expanded in future studies.