Regulation of Translocator Protein 18 kDa (TSPO) Expression in Rat and Human Male Germ Cells

Gene regulation and signaling transduction in mediating the self-renewal, differentiation, and apoptosis of spermatogonial stem cells

ABSTRACT

Infertility has become one of the most serious diseases worldwide, and 50% of this disease can be attributed to male-related factors. Spermatogenesis, by definition, is a complex process by which spermatogonial stem cells (SSCs) self-renew to maintain stem cell population within the testes and differentiate into mature spermatids. It is of great significance to uncover gene regulation and signaling pathways that are involved in the fate determinations of SSCs with aims to better understand molecular mechanisms underlying human spermatogenesis and identify novel targets for gene therapy of male infertility. Significant achievement has recently been made in demonstrating the signaling molecules and pathways mediating the fate decisions of mammalian SSCs. In this review, we address key gene regulation and crucial signaling transduction pathways in controlling the self-renewal, differentiation, and apoptosis of SSCs, and we illustrate the networks of genes and signaling pathways in SSC fate determinations. We also highlight perspectives and future directions in SSC regulation by genes and their signaling pathways. This review could provide novel insights into the genetic regulation of normal and abnormal spermatogenesis and offer molecular targets to develop new approaches for gene therapy of male infertility.

The Role of Retinoic Acid in Spermatogenesis and Its Application in Male Reproduction

Spermatogenesis in mammalian testes is essential for male fertility, ensuring a continuous supply of mature sperm. The testicular microenvironment finely tunes this process, with retinoic acid, an active metabolite of vitamin A, serving a pivotal role. Retinoic acid is critical for various stages, including the differentiation of spermatogonia, meiosis in spermatogenic cells, and the production of mature spermatozoa. Vitamin A deficiency halts spermatogenesis, leading to the degeneration of numerous germ cells, a condition reversible with retinoic acid supplementation. Although retinoic acid can restore fertility in some males with reproductive disorders, it does not work universally. Furthermore, high doses may adversely affect reproduction. The inconsistent outcomes of retinoid treatments in addressing infertility are linked to the incomplete understanding of the molecular mechanisms through which retinoid signaling governs spermatogenesis. In addition to the treatment of male reproductive disorders, the role of retinoic acid in spermatogenesis also provides new ideas for the development of male non-hormone contraceptives. This paper will explore three facets: the synthesis and breakdown of retinoic acid in the testes, its role in spermatogenesis, and its application in male reproduction. Our discussion aims to provide a comprehensive reference for studying the regulatory effects of retinoic acid signaling on spermatogenesis and offer insights into its use in treating male reproductive issues.

Impact of Fetal Exposure to Endocrine Disrupting Chemical Mixtures on FOXA3 Gene and Protein Expression in Adult Rat Testes

Perinatal exposure to endocrine disrupting chemicals (EDCs) has been shown to affect male reproductive functions. However, the effects on male reproduction of exposure to EDC mixtures at doses relevant to humans have not been fully characterized. In previous studies, we found that in utero exposure to mixtures of the plasticizer di(2-ethylhexyl) phthalate (DEHP) and the soy-based phytoestrogen genistein (Gen) induced abnormal testis development in rats. In the present study, we investigated the molecular basis of these effects in adult testes from the offspring of pregnant SD rats gavaged with corn oil or Gen + DEHP mixtures at 0.1 or 10 mg/kg/day. Testicular transcriptomes were determined by microarray and RNA-seq analyses. A protein analysis was performed on paraffin and frozen testis sections, mainly by immunofluorescence. The transcription factor forkhead box protein 3 (FOXA3), a key regulator of Leydig cell function, was identified as the most significantly downregulated gene in testes from rats exposed in utero to Gen + DEHP mixtures. FOXA3 protein levels were decreased in testicular interstitium at a dose previously found to reduce testosterone levels, suggesting a primary effect of fetal exposure to Gen + DEHP on adult Leydig cells, rather than on spermatids and Sertoli cells, also expressing FOXA3. Thus, FOXA3 downregulation in adult testes following fetal exposure to Gen + DEHP may contribute to adverse male reproductive outcomes.

Oxidative Stress Disrupted Prepubertal Rat Testicular Development after Xenotransplantation

In the past two decades, testicular tissue grafting and xenografting have been well established, with the production of fertilization-competent sperm in some studies. However, few studies have been carried out to observe the development of grafted prepubertal testicular tissue of rats and compare the biological differences between in situ testis and grafted testis. In this study, we established the prepubertal testicular tissue xenografting model using a 22-day-old rat and evaluated certain parameters, including testicular histology, testosterone production, and ultrastructure of the grafted testes. We also assessed gene expression of cell proliferation markers, testicular cell markers, and antioxidative defense system. Our results showed that 47 days after transplantation, intratesticular testosterone concentration was not significantly altered; however, cell proliferation, spermatogenesis, and Sertoli cell markers in the transplanted testes were significantly disrupted compared with the control group, accompanied by aggravated apoptosis and oxidative damage. Moreover, the transplanted testes showed smaller tubular diameter and disrupted spermatogenic epithelium with apparent vacuoles, distorted and degenerated germ cells with obscure nuclear margin, and no spermatids in the center of the tubules. Although testis xenografting has been extensively tested and attained great achievement in other species, the prepubertal rat testicular tissue xenografting to immunodeficient mice exhibited obvious spermatogenesis arrest and oxidative damage. The protocol still needs further optimization, and there are still some unknown factors in prepubertal rat testes transplantation.

An FDA oncology analysis of toxicities associated with PBD-containing antibody-drug conjugates

With a new generation of antibody-drug conjugates (ADCs) that contain a drug-to-antibody ratio (DAR) of 2, the question remains whether advances in technology have resulted in more stable and tumor-specific ADCs. These ADCs are anticipated to cause minimal systemic exposures of payloads, with toxicities being evident mainly at tumor sites. We examined 15 ADCs with PBD-dimer payloads and a DAR of 2 and concluded that dose limiting toxicities in animals and in humans are generally related to the payload. Both the payloads and the ADCs had pro-inflammatory responses causing severe toxicities that were at times of low incidence, making it difficult to assess a cause-effect relationship. Due to their low incidence, single-patient cohorts may not detect these events and such design may not be suitable in first-in-human (FIH) trials. The commonly proposed approach by the sponsors for FIH dose selection was 1/6^th highest non-severely toxic dose (HNSTD) in monkeys. This approach resulted in an acceptable balance of safety and efficient dose escalation in phase 1 trials, when using data from repeat-dose toxicology studies and body surface area for scaling. No sponsor used the data generated in rodents or proposed novel approaches for FIH dose selection.

Effects of chronic antipsychotic drug exposure on the expression of Translocator Protein and inflammatory markers in rat adipose tissue

The precise effect of antipsychotic drugs on either central or peripheral inflammation remains unclear. An important issue in this debate is to what extent the known peripheral metabolic effects of antipsychotics, including increased adiposity, may contribute to increased inflammation. Adipose tissue is known to contribute to the development of systemic inflammation, which can eventually lead to insulin resistance and metabolic dysregulation. As a first step to address this question, we evaluated whether chronic exposure to clinically comparable doses of haloperidol or olanzapine resulted in the immune activation of rat adipose tissue. Samples of visceral adipose tissue were sampled from male Sprague-Dawley rats exposed to, haloperidol, olanzapine or vehicle (all n = 8), for 8 weeks. From these we measured a cytokine profile, protein expression of F4/80 (a phenotypic macrophage marker) and translocator protein (TSPO), a target for radiotracers putatively indicating microgliosis in clinical neuroimaging studies. Chronic olanzapine exposure resulted in significantly higher adipose IL-6 levels compared with vehicle-controls (ANOVA p = 0.008, Bonferroni post-hoc test p = 0.006); in parallel, animals exposed to olanzapine had significantly higher F4/80 expression when compared with vehicle-controls (Mann Whitney Test, p = 0.014), whereas there was no difference between haloperidol and vehicle groups (Mann Whitney test, p = 0.1). There were no significant effects of either drug on adipose TSPO protein levels. Nevertheless, we found a positive correlation between F4/80 and TSPO adipose protein levels in the olanzapine-exposed rats (Spearman's rho = 0.76, p = 0.037). Our data suggest that chronic exposure to olanzapine, but not haloperidol, increases production of the pro-inflammatory cytokine IL-6 in adipose tissue and increased macrophages expression (F4/80), in the absence of measurable changes in TSPO with respect to vehicle. This may have potentially important consequences in terms of metabolic dysregulation associated with long-term antipsychotic treatment.

An Updated View of Translocator Protein (TSPO)

Decades of study on the role of mitochondria in living cells have evidenced the importance of the 18 kDa mitochondrial translocator protein (TSPO), first discovered in the 1977 as an alternative binding site for the benzodiazepine diazepam in the kidneys. This protein participates in a variety of cellular functions, including cholesterol transport, steroid hormone synthesis, mitochondrial respiration, permeability transition pore opening, apoptosis, and cell proliferation. Thus, TSPO has become an extremely attractive subcellular target for the early detection of disease states that involve the overexpression of this protein and the selective mitochondrial drug delivery. This special issue was programmed with the aim of summarizing the latest findings about the role of TSPO in eukaryotic cells and as a potential subcellular target of diagnostics or therapeutics. A total of 9 papers have been accepted for publication in this issue, in particular, 2 reviews and 7 primary data manuscripts, overall describing the main advances in this field.