Serious doubts over “Eggs forever?”

A recent commentary in this journal by Byskov et al. (2005) claims that, despite published results from numerous independent lines of investigation from our laboratory and others, there does not "exist any evidence for neo-folliculogenesis in the adult mammalian ovary." While we agree with Byskov et al. that our work represents a radical departure from the age-old dogma that mammalian females permanently lose the capacity for oocyte and follicle production during the perinatal period, careful examination of all of the available data leaves no doubt that adult female mammals retain the capacity for oogenesis and folliculogenesis. These findings do not change the fact that exhaustion of the oocyte pool occurs with advancing chronological age--a process responsible for driving the menopause in women--but rather question the basic mechanism underlying age-related ovarian failure. In this regard, studies of aging male mice have demonstrated that testicular atrophy is associated with a dramatic decline in the number, activity and quality of germline stem cells that maintain spermatogenesis during adulthood (Zhang et al., 2006). Therefore, to the contrary of the opinion of Byskov et al. that such a process would be "considered exceptional among stem cells," it is certainly reasonable to hypothesize that a similar deterioration of female germline stem cell function underlies the decline in oocyte quality and the onset of ovarian failure in aging females. Further, while we accept that a departure from conventional thinking can take years to gain widespread acceptance, we feel this resistance to change should not be construed as the sole means to voice opinions about the validity of our data or the maturity of our principal conclusion.     

Genetic variance modifies apoptosis susceptibility in mature oocytes via alterations in DNA repair capacity and mitochondrial ultrastructure

Although the identification of specific genes that regulate apoptosis has been a topic of intense study, little is known of the role that background genetic variance plays in modulating cell death. Using germ cells from inbred mouse strains, we found that apoptosis in mature (metaphase II) oocytes is affected by genetic background through at least two different mechanisms. The first, manifested in AKR/J mice, results in genomic instability. This is reflected by numerous DNA double-strand breaks in freshly isolated oocytes, causing a high apoptosis susceptibility and impaired embryonic development following fertilization. Microinjection of Rad51 reduces DNA damage, suppresses apoptosis and improves embryonic development. The second, manifested in FVB mice, results in dramatic dimorphisms in mitochondrial ultrastructure. This is correlated with cytochrome c release and a high apoptosis susceptibility, the latter of which is suppressed by pyruvate treatment, Smac/DIABLO deficiency, or microinjection of 'normal' mitochondria. Therefore, background genetic variance can profoundly affect apoptosis in female germ cells by disrupting both genomic DNA and mitochondrial integrity.     

The matrilins: modulators of extracellular matrix assembly

The matrilins form a family of oligomeric extracellular adaptor proteins that are most strongly expressed in cartilage but also present in many other extracellular matrices. Matrilins bind to different types of collagen fibrils, to other noncollagenous proteins and to aggrecan. They thereby support matrix assembly by connecting fibrillar components and mediating interactions between these and the aggrecan gel. The binding avidity of a matrilin can be varied by alternative splicing, proteolytic processing and formation of homo- and heterooligomers. Such changes in matrilin structure may lead to a modulation of extracellular matrix assembly. Some matrilins bind weakly to α1β1 integrin and cell surface proteoglycans, but even though matrilins play a role in mechanotransduction and matrilin-3 activates the expression of osteoarthritis-associated genes the physiological relevance of matrilin-cell interactions is unclear. Matrilin knockout mice do not display pronounced phenotypes, which points to a redundancy within the protein family or with functionally related proteins. In man, dominant mutations in the von Willebrand factor A like domain of matrilin-3 lead to a protein retention in the endoplasmic reticulum that causes multiple epiphyseal dysplasia by initiating a cell stress response. In contrast, a mutation in an EGF domain of matrilin-3 that is associated with hand osteoarthritis and disc degeneration does not interfere with secretion but instead with extracellular assembly of matrix structures. In this review we summarize such information on matrilin structure and function that we believe is important for the understanding of extracellular matrix assembly and for deciphering pathophysiological mechanisms in diseases causing skeletal malformations or cartilage degeneration.     

Telomere resolution in the Lyme disease spirochete.

The genus Borrelia includes the causative agents of Lyme disease and relapsing fever. An unusual feature of these bacteria is a genome that includes linear DNA molecules with covalently closed hairpin ends referred to as telomeres. We have investigated the mechanism by which the hairpin telomeres are processed during replication. A synthetic 140 bp sequence having the predicted structure of a replicated telomere was shown to function as a viable substrate for telomere resolution in vivo, and was sufficient to convert a circular replicon to a linear form. Our results suggest that the final step in the replication of linear Borrelia replicons is a site-specific DNA breakage and reunion event to regenerate covalently closed hairpin ends. The telomere substrate described here will be valuable both for in vivo manipulation of linear DNA in Borrelia and for in vitro studies to identify and characterize the telomere resolvase.     

Sexual transmission of single human immunodeficiency virus type 1 virions encoding highly polymorphic multisite cytotoxic T-lymphocyte escape variants

Antigenic variation inherent in human immunodeficiency virus type 1 (HIV-1) virions that successfully instigate new infections transferred by sex has not been well defined. Yet this is the viral "challenge" which any vaccine-induced immunity must deal with. Closely timed comparisons of the virus circulating in the "donor" and that which initiates new infection are difficult to carry out rigorously, as suitable samples are very hard to get in the face of ethical hurdles. Here we investigate HIV-1 variation in four homosexual couples where we sampled blood from both parties within several weeks of the estimated transmission event. We analyzed variation within highly immunogenic HIV-1 internal proteins encoding epitopes recognized by cytotoxic T lymphocytes (CTLs). These responses are believed to be crucial as a means of containing viral replication. In the donors we detected virions capable of evading host CTL recognition at several linked epitopes of distinct HLA class I restriction. When a donor transmitted escape variants to a recipient with whom he had HLA class I molecules in common, the recipient's CTL response to those epitopes was prevented, thus impeding adequate viral control. In addition, we show that even when HLA class I alleles are disparate in the transmitting couple, a single polymorphism can abolish CTL recognition of an overlapping epitope of distinct restriction and so confer immune escape properties to the recipient's seroconversion virus. In donors who are themselves controlling an early, acute infection, the precise timing of onward transmission is a crucial determinant of the viral variants available to compose the inoculum.     

The plasmids of Borrelia burgdorferi: essential genetic elements of a pathogen

The spirochete Borrelia burgdorferi, the causative agent of Lyme disease, has an unusual genome comprised of a linear chromosome and the largest plasmid complement of any characterized bacterium. Certain plasmid-encoded elements are required for virulence and viability, both in vitro and in vivo. The genetic tools to manipulate B. burgdorferi are sufficiently developed for precise molecular genetic investigations. B. burgdorferi now represents a prime system with which to address basic questions of plasmid biology and plasmid contributions to bacterial virulence and disease pathogenesis.     

The burgeoning molecular genetics of the Lyme disease spirochaete

Lyme disease is the most commonly reported vector-borne disease in North America and Europe, yet we know little about which components of the causative agent, Borrelia burgdorferi, are critical for infection or virulence. Molecular genetics has provided a powerful means by which to address these topics in other bacterial pathogens. Certain features of B. burgdorferi have hampered the development of an effective system of genetic analysis, but basic tools are now available and their application has begun to provide information about the identities and roles of key bacterial components in both the tick vector and the mammalian host. Increased genetic analysis of B. burgdorferi should advance our understanding of the infectious cycle and the pathogenesis of Lyme disease.     

Fetal and infant head circumference sexual dimorphism in primates

Studies have shown that after controlling for the effects of body size on brain size, the brains of adult humans, rhesus monkeys, and chimpanzees differ in relative size, where males have a greater volume of cerebral tissue than females. We assess whether head circumference sexual dimorphism is present during early development by evaluating sex differences in relative head circumference in living fetuses and infants within the first year of life. Head circumference is used as a proxy for brain size in the fetus and infant. Femur length is used as a proxy for body length in the fetus. Ultrasonography was used to obtain fetal measures, and anthropometry was used to obtain postnatal measures in humans, rhesus monkeys, baboons, and common marmosets. We show that statistically significant but low levels of head circumference sexual dimorphism are present in humans, rhesus monkeys, and baboons in early life. On average, males have head circumferences about 2% larger than females of comparable femur/body length in humans, rhesus monkeys, and baboons. No evidence for head circumference sexual dimorphism in the common marmoset was found. Dimorphism was present across all body size ranges. We suggest that head circumference sexual dimorphism emerges largely postnatally and increases throughout maturation, particularly in humans who reach adult dimorphism values greater than the monkeys. We suggest that brain dimorphism is not likely to impose an additional energetic burden to the gestating or lactating mother. Finally, some of the problems with ascribing functional significance to brain size sexual dimorphism are discussed, and the energetic implications for brain size sexual dimorphism in infancy are assessed.