Degeneration and regeneration of the nervous system

Extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) are members of the mitogen-activated protein kinase super family that can mediate cell proliferation and apoptosis. The Ras-Raf-MEK-ERK signaling cascade controlling cell proliferation has been well studied but the mechanisms involved in ERK1/2-mediated cell death are largely unknown. This review focuses on recent papers that define ERK1/2 translocation to the nucleus and the proteins involved in the cytosolic retention of activated ERK1/2. Cytosolic retention of ERK1/2 denies access to the transcription factor substrates that are responsible for the mitogenic response. In addition, cytosolic ERK1/2, besides inhibiting survival and proliferative signals in the nucleus, potentiates the catalytic activity of some proapoptotic proteins such as DAP kinase in the cytoplasm. Studies that further define the function of cytosolic ERK1/2 and its cytosolic substrates that enhance cell death will be essential to harness this pathway for developing effective treatments for cancer and chronic inflammatory diseases.

How ERK1/2 activation controls cell proliferation and cell death: Is subcellular localization the answer?

Background The epididymis performs an important role in the maturation of spermatozoa including their acquisition of progressive motility and fertilizing ability. However, the molecular mechanisms that govern these maturational events are still poorly defined. This review focuses on recent progress in our understanding of epididymal function including its development, role of the luminal microenvironment in sperm maturation, regulation and novel mechanisms the epididymis utilizes to carry out some of its functions. Methods A systematic search of Pubmed was carried out using the search term 'epididymis'. Articles that were published in the English language until the end of August 2008 and that focused on the specific topics described above were included. Additional papers cited in the primary reference were also included. Results While the majority of these findings were the result of studies in animal models, recent studies in the human epididymis are also presented including gene profiling studies to examine regionalized expression in normal epididymides as well as in those from vasectomized patients. Conclusions Significant progress has been made in our understanding of epididymal function providing new insights that ultimately could improve human health. The data also indicate that the human epididymis plays an important role in sperm maturation but has unique properties compared with animal models.

New insights into epididymal biology and function

Apoptosis is a critically important biological process that plays an essential role in cell fate and homeostasis. An important component of the apoptotic pathway is the family of proteins commonly known as the B cell lymphoma-2 (Bcl-2). The primary role of Bcl-2 family members is the regulation of apoptosis. Although the structure of Bcl-2 family of proteins was reported nearly 10 years ago, however, it still surprises us with its structural and functional complexity and diversity. A number of studies have demonstrated that Bcl-2 family influences many other cellular processes beyond apoptosis which are generally independent of the regulation of apoptosis, suggesting additional roles for Bcl-2. The disruption of the regulation of apoptosis is a causative event in many diseases. Since the Bcl-2 family of proteins is the key regulator of apoptosis, the abnormalities in its function have been implicated in many diseases including cancer, neurodegenerative disorders, ischemia and autoimmune diseases. In the past few years, our understanding of the mechanism of action of Bcl-2 family of proteins and its implications in various pathological conditions has enhanced significantly. The focus of this review is to summarize the current knowledge on the structure and function of Bcl-2 family of proteins in apoptotic cellular processes. A number of drugs have been developed in the past few years that target different Bcl-2 members. The role of Bcl-2 proteins in the pathogenesis of various diseases and their pharmacological significance as effective molecular therapeutic targets is also discussed.

The mystery of BCL2 family: Bcl-2 proteins and apoptosis: an update.

Androgens and the androgen receptor (AR) play important roles in male fertility, although the detailed mechanisms, particularly how androgen/AR influences spermatogenesis in particular cell types, remain unclear. Using a Cre-Lox conditional knockout strategy, we generated a tissue-specific knockout mouse with the AR gene deleted only in Sertoli cells (S-AR-/y). Phenotype analyses show the S-AR-/y mice were indistinguishable from WT AR mice (B6 AR+/y) with the exception of testes, which were significantly atrophied. S-AR-/y mice were infertile, with spermatogenic arrest predominately at the diplotene premeiotic stage and almost no sperm detected in the epididymides. S-AR-/y mice also have lower serum testosterone concentrations and higher serum leuteinizing hormone concentrations than B6 AR+/y mice. Further mechanistic studies demonstrated that S-AR-/y mice have defects in the expression of anti-Mullerian hormone, androgen-binding protein, cyclin A1, and sperm-1, which play important roles in the control of spermatogenesis and/or steroidogenesis. Together, our Sertoli cell-specific AR knockout mice provide in vivo evidence of the need for functional AR in Sertoli cells to maintain normal spermatogenesis and testosterone production, and ensure normal male fertility.

https://www.pnas.org/content/pnas/101/18/6876.full.pdf

Infertility with defective spermatogenesis and hypotestosteronemia in male mice lacking the androgen receptor in Sertoli cells

Extracellular signal-regulated kinases 1/2 are serum-stimulated "Bim(EL) kinases" that bind to the BH3-only protein Bim(EL) causing its phosphorylation and turnover.

A reduction in plasma membrane cholesterol is one of the early events that either triggers or is closely associated with capacitation of mammalian spermatozoa. In this investigation, we have examined the effects of cholesterol efflux on tyrosine phosphorylation, lipid diffusion, and raft organization in boar spermatozoa. Results show that a low level of cholesterol efflux, mediated by 5 mM methyl-b-cyclodextrin (MBCD), enhances capacitation and induces phosphorylation of two proteins at 26 and 15 kDa without affecting sperm viability. Lipid diffusion rates under these conditions are largely unaffected except when cholesterol efflux is excessive. Low-density Triton X100-insoluble complexes (lipid rafts) were isolated from spermatozoa and found to have a restricted profile of proteins. Capacitation-associated cholesterol efflux has no effect on raft composition, but cholesterol depletion destabilizes them completely and phosphorylation is suppressed. During MBCD-mediated capacitation, the distribution of GM1 gangliosides on spermatozoa changes in a sequential manner from overlying the sperm tail to clustering on the sperm head. It is concluded that there is a safe window for removal of plasma membrane cholesterol from spermatozoa within which protein phosphorylation and polarized migration of lipid rafts take place. A preferential loss of cholesterol from the nonraft pool may be the stimulus that promotes raft clustering over the anterior sperm head. gamete biology, in vitro fertilization, sperm capacitation; sperm maturation

https://bioone.org/journals/Biology-of-Reproduction/volume-71/issue-1/biolreprod.103.026435/Cholesterol-Efflux-Alters-Lipid-Raft-Stability-and-Distribution-During-Capacitation/10.1095/biolreprod.103.026435.pdf

Cholesterol efflux alters lipid raft stability and distribution during capacitation of boar spermatozoa.

The mouse zona pellucida glycoprotein, mZP2, is thought to be the secondary receptor on eggs for retention of acrosome-reacted sperm during fertilization. Here, we present evidence that one of its complementary binding proteins on sperm is proacrosin/acrosin. mZP2 binds to proacrosin null sperm considerably less effectively than to wild-type sperm. Binding is mediated by a strong ionic interaction between polysulphate groups on mZP2 and basic residues on an internal proacrosin peptide. The stereochemistry of both sulphate groups and basic amino acids determines the specificity of binding. Structurally relevant sulphated polymers and suramin, a polysulphonated anticancer drug, compete with mZP2 for complementary binding sites on proacrosin/acrosin in solid-phase binding assays. The same competitors also displace attached sperm from the zona pellucida of eggs in an in vitro fertilization system. This combination of genetic, biochemical and functional data supports the hypothesis that mZP2-proacrosin interactions are important for retention of acrosome-reacted sperm on the egg surface during fertilization. Safe mimetics of suramin have potential as non-steroidal antifertility agents.

Interactions between mouse ZP2 glycoprotein and proacrosin; a mechanism for secondary binding of sperm to the zona pellucida during fertilization.

The actin cytoskeleton influences a wide range of functions in nonmuscle somatic cells, including shape, movement, and interactions with extracellular matrices. The role of actin in mammalian male germ cells, however, particularly during post-testicular development, is not well understood. In this paper, we examine 1) the distribution of 3 actin-regulatory proteins (thymosin beta10, destrin, and a testis-specific actin capping protein) involved in controlling the balance between actin monomers (G-actin) and actin filaments (F-actin), and 2) the distribution and polymerization status of actin in bull spermatozoa during epididymal maturation and following acrosomal exocytosis. Results show that in fixed, permeabilized testicular spermatozoa all 3 regulatory proteins (as determined by binding of specific antibodies) are localized primarily to the acrosomal domain but during epididymal maturation they become confined to the equatorial segment. Following ejaculation, however, they extend back into the acrosomal region. In spermatozoa induced to undergo an acrosome reaction with the calcium ionophore, A23187, further rearrangement occurs with destrin, thymosin beta10, and TS-ACP appearing in the postacrosomal domain. Actin is also found over the acrosome of testicular spermatozoa with both G- and F-actin present, although the 2 forms show slightly different patterns of distribution. Subsequently, actin in the sperm head is largely confined to the equatorial segment until F-actin appears in the postacrosomal domain of acrosome-reacted spermatozoa. This redistribution of actin and actin-regulatory proteins, coupled with changing levels of actin polymerization, suggest a continuing role for actin in both post-testicular sperm maturation and acrosomal exocytosis.

Actin and actin-binding proteins in bovine spermatozoa: potential role in membrane remodeling and intracellular signaling during epididymal maturation and the acrosome reaction.

Actin-capping proteins are ubiquitous components of mammalian cells. They are known to regulate the polymerization state of actin and hence indirectly control the activity of the cytoskeleton and cell shape. As part of our investigation into the molecular mechanisms that direct differentiation of a round spermatid into an elongating spermatozoa, we report on a testis-specific 1.7-kb transcript from rat testis with sequence similarities to the α subunit of actin-capping proteins (ACPs) from somatic cells. The transcript contains a putative cAMP-responsive motif (CREM) upstream of the initiation codon in the DNA sequence and is expressed postmeiotically, first appearing between 20 and 30 days of postnatal development. The primary amino acid sequence is 90% identical to that of a previously identified testis-specific mouse protein, gsg3, both showing approximately 40% homology to the α subunit of somatic ACPs. An affinity-purified polyclonal antibody to a synthetic peptide derived from the rat transcript identified a 32-kDa protein on Western blots of testicular extracts. Indirect immunofluorescent localization of the protein on frozen sections of adult rat testis showed that it is intracellular and accumulates asymmetrically in the cytoplasm of round spermatids coincident with the position of the developing acrosome. This spatial expression parallels the distribution of F-actin during sperm differentiation, supporting the hypothesis that testis-specific ACPs have an important role in determining the final shape of mature sperm heads. A disturbance in the expression of these ACPs may underlie many of the abnormalities in sperm morphology observed in infertile semen. Mol. Reprod. Dev. 49:81–91, 1998. © 1998 Wiley-Liss, Inc.

E. A. Howes

Papers

Extracellular signal-regulated kinases 1/2 are serum-stimulated "Bim(EL) kinases" that bind to the BH3-only protein Bim(EL) causing its phosphorylation and turnover.

Cholesterol efflux alters lipid raft stability and distribution during capacitation of boar spermatozoa.

Interactions between mouse ZP2 glycoprotein and proacrosin; a mechanism for secondary binding of sperm to the zona pellucida during fertilization.

Actin and actin-binding proteins in bovine spermatozoa: potential role in membrane remodeling and intracellular signaling during epididymal maturation and the acrosome reaction.

Expression of a testis-specific putative actin-capping protein associated with the developing acrosome during rat spermiogenesis.