To achieve and maintain fertility, the adult mammalian testis produces many generations of sperm. While testicular integrity is established in the fetus and develops further in juvenile life, sperm production does not ensue until much later in life, following the onset of puberty. Signals from the transforming growth factor-beta superfamily of proteins are vital for governance of testis development and spermatogenesis, and this review discusses our current understanding of the mechanisms and processes in which they have been implicated with a focus on the fetal and juvenile testis.

Growth factors modify the structure of the glycosaminoglycan (GAG) chains on biglycan leading to enhanced LDL binding. G-protein receptor-coupled agonists such as thrombin, signal changes the structure of proteoglycans produced by vascular smooth muscle cells (VSMCs). One component of classical G-protein-coupled receptor (GPCR) signaling invokes transactivation of protein tyrosine kinase receptors such as the epidermal growth factor receptor. Serine/threonine receptor growth factors such as transforming growth factor-(TGF)-␤ are potent activators of proteoglycan synthesis. We have used the model of proteoglycan synthesis to demonstrate that the signaling paradigm of GPCR signaling can be extended to include the transactivation of serine/ threonine receptor, specifically the TGF-␤ type I receptor (T␤RI) also known as activin-like kinase (ALK) V. Thrombin stimulated elongation of GAG chains and increased proteoglycan core protein expression and these responses were blocked by the T␤RI antagonist, SB431542 and T␤RI siRNA knockdown, as well as several protease-activated receptor (PAR)-1 antagonists. The canonical downstream response to TGF-␤ is increased C-terminal phosphorylation of the transcription factor Smad2 generating phospho-Smad2C (phosphorylation of Smad2 C-terminal region). Thrombin stimulated increased phospho-Smad2C levels, and the response was blocked by SB431542 and JNJ5177094. The proteolytically inactive thrombin mimetic thrombin-receptor activating peptide also stimulated an increase in cytosolic phospho-Smad2C. Signaling pathways for growth factor regulated proteoglycan synthesis represent therapeutic targets for the prevention of atherosclerosis, but the novel finding of a GPCR-mediated transactivation of a serine/ threonine growth factor receptor almost certainly has implications well beyond the synthesis of proteoglycans.Cardiovascular disease is the largest single cause of mortality, and its major underlying pathology is atherosclerosis (1). The process of atherosclerosis commences with the trapping of lipoproteins in the vessel wall by modified proteoglycans, specifically the glycosaminoglycan (GAG) 3 chain elongated and sulfated chondroitin and dermatan sulfate biglycan and decorin (2, 3), and it continues as an inflammatory disease (4). Proteoglycan synthesis and structure is regulated by vasoactive growth factors, and consequently, their receptors and signaling pathways are potential therapeutic targets (5, 6).G-protein-coupled receptors (GPCRs) are seven transmembrane receptors and are present on vascular smooth muscle cells (VSMCs) where they signal important actions such as vascular contraction, cellular migration and proliferation, and secretion (7,8). GPCR agonists include key vasoactive molecules associated with physiology and pathophysiology, including thrombin, endothelin-1, and angiotensin II (9). The current paradigm of GPCR signaling covers three major pathways; first, the classic pathway in which ligand engagement causes G-protein binding to the receptor and G ␣ complexes then ...

Bone morphogenetic proteins (BMPs), members of the transforming growth factor-␤ superfamily, extensively influence events that establish male fertility, affecting germ cells and somatic cells throughout fetal and postnatal life. BMP signals are relayed by SMAD proteins, transcription factors that translocate to the nucleus upon ligand stimulation. We show that BMP signaling in the testis may be regulated by selective expression of BMP-responsive and inhibitory SMADs, with expression differing between the first wave and adult spermatogenesis. Smad1, Smad5, Smad8, Smad4, Smad6, and Smad7 expression is ubiquitous during testis development but becomes cell-specific in the adult. Furthermore, regulated SMAD6 protein expression at the onset of spermatogenesis suggests differential responsiveness of spermatogonial subpopulations to ligands. In vitro, immature Sertoli cells and spermatogonia transduce BMP2 and BMP4 signals by means of SMAD1, SMAD5, and SMAD8. Based on these findings, we extrapolate these data to interpret BMP mutant testis phenotypes in terms of SMAD availability for signal transduction. Developmental Dynamics 237:97-111, 2008.