Toll-like receptors and signalling in spermatogenesis and testicular responses to inflammation--a perspective

Abstract

It is self-evident that infection and inflammation in the reproductive tract can inhibit male fertility, but the observation that fertility may also be compromised by systemic inflammation and disease is more difficult to explain. Recent studies implicating microbial pattern-recognition receptors, such as the Toll-like receptors (TLRs), as well as inflammatory cytokines and their signalling pathways, in testicular function have cast new light on this mysterious link between infection/inflammation and testicular dysfunction. It is increasingly evident that signalling pathways normally involved in controlling inflammation play fundamental roles in regulating Sertoli cell activity and responses to reproductive hormones, in addition to promoting immune responses within the testis. Many of the negative effects of inflammation on spermatogenesis may be attributed to elevated production of inflammation-related gene products within the circulation and the testis, which subsequently exert disruptive effects on spermatogenic cell development and survival, as well as the ability of the Sertoli cells to provide support for spermatogenesis. These interactions have important implications for testicular dysfunction and disease, and may eventually provide new opportunities for therapeutic interventions.

Fig. 1

LPS dose–response data in adult male rats. Adult male rats were injected with varying doses of LPS (E. coli, serotype 0128:B8). Body temperature (A) was measured every hour for 3 h, at which time the rats were killed and blood and testes taken for analysis of testicular function. Testis weights were unaffected, but even at relatively low does of LPS, testicular interstitial fluid (IF) formation (B), intratesticular steroidogenesis (IF testosterone) (C) and pituitary secretion of luteinising hormone (LH) (D) was inhibited. All values are mean and SEM (n = 3–6 animals), except in panel A, where the error bars and some dose data have been removed for clarity (data previously unpublished). Note that subsequent experiments established that doses above 5 mg/kg frequently caused death through endotoxic shock within 12–24 h after administration.

Fig. 2

Toll-like receptors (TLRs) and signalling pathway interactions. The TLRs respond to a variety of pathogen-related molecules, usually forming homo- or heterodimers during signalling. TLR2 can self-associate or combine with either TLR1 or TLR6 to mediate responses to various bacterial lipoprotein (LP) classes. TLR4 forms a complex with the co-receptor proteins, MD2 and CD14, to facilitate binding of bacterial LPS. The TLRs are sub-divided into cell surface receptors (e.g. TLR2 and 4), which largely respond to bacterial proteins, lipoproteins and lipopolysaccharides, and intracytoplasmic endosomal receptors (e.g. TLR3), which recognise bacterial and viral nucleic acids, such as viral double-stranded RNA (dsRNA). Most TLRs signal via the adaptor protein, MyD88 (myeloid differentiation primary response protein 88), except TLR3, which acts through the adaptor protein TRIF (TIR domain-containing adaptor-inducing interferon β). Uniquely, TLR4 can interact with either MyD88 or TRIF, through engagement of the adaptor proteins, Mal (MyD88 adaptor-like) or TRAM (TRIF-related adaptor molecule). Downstream signalling involves the tumour necrosis factor receptor-associated factors 3 and 6 (TRAF3 and TRAF6), activation of the transcription factors, NFκB (nuclear factor kappaB) and IRF3 (interferon regulatory factor 3), or induction of the mitogen-activated protein kinases (MAP kinases) Jnk (Jun N-terminal kinase) and p38 and production of the Fos/Jun transcription factor AP1 (activated protein 1). These transcription factors interact to induce the expression of multiple pro-inflammatory genes, including interleukin (IL) 1α, tumour necrosis factor α (TNFα), inducible nitric oxide (iNOS), or the type 1 interferons (IFNα, IFNβ). Note that many details of the signalling pathways have been omitted or truncated for the sake of simplicity.

Fig. 3

Communication networks in the Sertoli cell. (A) Spermatogenesis and intraepithelial responses at the time of spermiation. Spermatogonia enter meiosis to become spermatocytes and pass through the tight junctions between adjacent Sertoli cells. At the end of meiosis, the resulting haploid spermatids undergo major structural differentiation and are released from the seminiferous epithelium as spermatozoa (spermiation), leaving behind most of their cytoplasm, which is phagocytosed by the Sertoli cells as a residual body (RB). Release and phagocytosis of the RBs coincides with a surge of DNA synthesis among the nearby spermatogonia and spermatocytes, reorganisation of the tight junctions to allow spermatocytes to pass into the luminal compartment, and increased inflammatory gene expression in the Sertoli cells and spermatogenic cells. (B) Hormonal and inflammatory signalling pathways in control of Sertoli cell function. Sertoli cell function is regulated by androgens, specifically testosterone (T) produced by the nearby Leydig cells and FSH from the anterior pituitary. Testosterone exerts direct genomic effects on target genes mediated via the androgen receptor (classical mechanism), and non-classical effects mediated through intracellular calcium mobilisation. FSH acts via a membrane receptor linked to adenylate cyclase, which produces cAMP and activates protein kinase A to stimulate production of Sertoli cell molecules, including lactate, transferrin, stem cell factor (SCF) and the FSH-regulating hormone, inhibin B. Sertoli cell Toll-like receptors (TLRs) respond to activation by appropriate ligands, which may be pathogen-derived or endogenous, and signal via a number of pathways, most critically the MyD88-dependent pathway to increase expression of inflammatory genes, including IL1α, IL6 and activin A. Although these separate signalling pathways control some functional outcomes in common, there is clear evidence for reciprocal inhibitory regulation between the two signalling pathways as well.