Wnt control of stem cells and differentiation in the intestinal epithelium

The intestinal epithelium represents a very attractive experimental model for the study of integrated key cellular processes such as proliferation and differentiation. The tissue is subjected to a rapid and perpetual self-renewal along the crypt-villus axis. Renewal requires division of multipotent stem cells, still to be morphologically identified and isolated, followed by transit amplification, and differentiation of daughter cells into specialized absorptive and secretory cells. Our understanding of the crucial role played by the Wnt/beta-catenin signaling pathway in controlling the fine balance between cell proliferation and differentiation in the gut has been significantly enhanced in recent years. Mutations in some of its components irreversibly lead to carcinogenesis in humans and in mice. Here, we discuss recent advances related to the Wnt/beta-catenin signaling pathway in regulating intestinal stem cells, homeostasis, and cancer. We emphasize how Wnt signaling is able to maintain a stem cell/progenitor phenotype in normal intestinal crypts, and to impose a very similar phenotype onto colorectal adenomas.     

Age-specific threats induce CRF expression in the paraventricular nucleus of the hypothalamus and hippocampus of young rats

Young animals respond to threatening stimuli in an age-specific way. Their endocrine and behavioral responses reflect the potential threat of the situation at a given age. The aim of the present study was to determine whether corticotropin-releasing factor (CRF) is involved in the endocrine and behavioral responses to threat and their developmental changes in young rats. Preweaning 14-day-old and postweaning 26-day-old rats were exposed to two age-specific threats, cat odor and an adult male rat. The acute behavioral response was determined during exposure. After exposure, the time courses of the corticosterone response and of CRF expression in the paraventricular nucleus of the hypothalamus (PVN) and in extrahypothalamic areas were assessed. Preweaning rats became immobile when exposed to cat odor or the male rat, whereas postweaning rats became immobile to cat odor only. Male exposure increased serum corticosterone levels in 14-day-old rats, but cat odor failed to increase levels at either age. Exposure induced elevation of CRF mRNA levels in the PVN that paralleled changes in corticosterone levels. CRF may thus play a role in endocrine regulation and its developmental changes during early life. Neither cat odor nor the adult male altered CRF mRNA levels in the bed nucleus of the stria terminalis (BNST) or the amygdala, but both stimuli increased levels in the hippocampus. Hippocampal CRF mRNA expression levels did not parallel cat odor or male-induced immobility, indicating that CRF is not involved in this response in young rats but may be involved in aspects of learning and memory.     

Protein kinase CK2 is required for dorsal axis formation in Xenopus embryos

Dorsal axis formation in Xenopus embryos is dependent upon asymmetrical localization of beta-catenin, a transducer of the canonical Wnt signaling pathway. Recent biochemical experiments have implicated protein kinase CK2 as a regulator of members of the Wnt pathway including beta-catenin. Here, we have examined the role of CK2 in dorsal axis formation. CK2 was present in the developing embryo at an appropriate time and place to participate in dorsal axis formation. Overexpression of mRNA encoding CK2 in ventral blastomeres was sufficient to induce a complete ectopic axis, mimicking Wnt signaling. A kinase-inactive mutant of CK2alpha was able to block ectopic axis formation induced by XWnt8 and beta-catenin and was capable of suppressing endogenous axis formation when overexpressed dorsally. Taken together, these studies demonstrate that CK2 is a bona fide member of the Wnt pathway and has a critical role in the establishment of the dorsal embryonic axis.     

Prenatal alcohol exposure reduces the proportion of newly produced neurons and glia in the dentate gyrus of the hippocampus in female rats

Prenatal alcohol exposure (PAE) alters adult neurogenesis and the neurogenic response to stress in male rats. As the effects of stress on neurogenesis are sexually dimorphic, the present study investigated the effects of PAE on adult hippocampal neurogenesis under both nonstressed and stressed conditions in female rats. Pregnant females were assigned to one of three prenatal treatments: (1) alcohol (PAE)—liquid alcohol (ethanol) diet ad libitum (36% ethanol-derived calories); (2) pair-fed—isocaloric liquid diet, with maltose–dextrin substituted for ethanol, in the amount consumed by a PAE partner (g/kg body wt/day of gestation); and (3) control—lab chow ad libitum. Female offspring were assigned to either nonstressed (undisturbed) or stressed (repeated restraint stress for 9 days) conditions. On day 10, all rats were injected with bromodeoxyuridine (BrdU) and perfused either 24 hours (cell proliferation) or 3 weeks (cell survival) later. We found that PAE did not significantly alter cell proliferation or survival, whereas females from the pair-fed condition exhibited elevated levels of cell survival compared to control females. Importantly, however, the proportion of both new neurons and new glial cells in the hippocampal dentate gyrus was reduced in PAE compared to control females. Exposure to stress did not alter neurogenesis in any of the prenatal treatment groups. In summary, compared to females from the control condition, prenatal dietary restriction enhanced the survival of new neurons, whereas PAE altered the differentiation of newly produced cells in the adult dentate gyrus. Alterations in hippocampal neurogenesis following PAE may contribute to learning and memory deficits seen in individuals with fetal alcohol spectrum disorders.     

Sex differences in salivary cortisol in response to acute stressors among healthy participants, in recreational or pathological gamblers, and in those with posttraumatic stress disorder

Sex differences in incidence and severity of some stress-related, neuropsychiatric disorders are often reported to favor men, suggesting that women may be more vulnerable to aberrant hypothalamic-pituitary-adrenal (HPA) axis responses to stress. In this review, we discuss several investigations that we, and others, have conducted assessing salivary cortisol as a measure of HPA function. We have examined basal cortisol among healthy men and women and also following acute exposure to stressors. Among healthy participants, men had higher basal cortisol levels than did women. In response to acute stressors, such as carbon dioxide or noise, respectively, cortisol levels were comparable between men and women or higher among women. We have also examined cortisol levels among those with problem eating, gambling, or posttraumatic stress disorder (PTSD). Women with restrained eating habits have higher basal cortisol levels than do women without restrained eating habits. Pathological gamblers have more aberrant stress response to gambling stimuli than do recreational gamblers, and these effects are more prominent among men than women. Men who have motor vehicle accident related PTSD, demonstrate more aberrant cortisol function, than do their female counterparts. Although these sex differences in cortisol seem to vary with type of stress exposure and/or pathophysiological status of the individual, other hormones may influence cortisol response. To address this, cortisol levels among boys and girls with different stress-related experiences, will be the subject of future investigation.     

Pre-experience of social exclusion suppresses cortisol response to psychosocial stress in women but not in men

Lack of social support and social exclusion is associated with adverse effects for mental and physical health. Additionally, women appear to be more vulnerable to social triggers of health disturbances. The hypothalamus–pituitary–adrenocortical-axis (HPA axis) might play a key role in this context as it has been shown both to relate to psychosocial conditions and health outcomes and to respond differentially depending on gender. In a previous experiment we found no effects of exclusion alone (operationalized via Cyberball) on cortisol secretion. Here we examine the effects of a social exclusion pre-experience on psychological and cortisol responses to a public speaking stressor. Subjects (33 m, 34 f) were randomly assigned to social exclusion (SE) or one of two control conditions (exclusion attributed to technical default (TD) and social inclusion (SI)). Afterwards salivary cortisol and psychological responses to a public speaking paradigm were assessed. Exclusion pre-treatment does not affect psychological responses to public speaking stress though with respect to cortisol significant. Cyberball by gender and Cyberball by gender by time interactions are found. SE-women show a blunted cortisol stress response to public speaking while cortisol responses of SE-men fall between SI-men and TD-men. Pre-experience of social exclusion leads to a blunted cortisol response to stress in women but not in men. This factor might contribute to the higher vulnerability to social triggers of health disturbances observed in women.     

Cannabinoid-hormone interactions in the regulation of motivational processes

There is a bi-directionality in hormone-cannabinoid interactions: cannabinoids affect prominent endocrine axes (such as the hypothalamic-pituitary-gonadal), and gonadal hormones modulate cannabinoid effects. This review will summarize recent research on these interactions, with a specific focus upon their implications for motivated behavior. Sexual behavior will serve as a “case study.” I will explore the hypothesis that ovarian hormones, in particular estradiol, may serve to release estrous behavior from endocannabinoid inhibition. Hormonal regulation of the endogenous cannabinoid system also affects processes that underlie drug abuse. This review will briefly discuss sex differences in behavioral responses to cannabinoids and explore potential mechanisms by which gonadal hormones alter cannabinoid reward. An examination of this research informs our perspective on how hormones and endocannabinoids may affect drug-seeking behavior as a whole and the development of addiction.     

Making heads from tails: Development of a reversed anterior-posterior axis during budding in an acoel

The anterior-posterior axis is a key feature of the bilaterian body plan. Although axis specification during embryogenesis has been studied extensively, virtually nothing is known about how this axis can be established post-embryonically, as occurs in budding animals. We investigated bud formation in the acoel Convolutriloba retrogemma, which reproduces by a remarkable process involving the formation of animals with linked but completely opposite body axes. Reverse axes are established anew during each round of budding and manifestations of the bud's new axis develop gradually, with regionalization of axial patterning genes (Hox and otx) and the establishment of organized musculature occurring secondarily, after bud initiation. A swath of tissue at the parent-bud boundary has no regenerative potential and appears devoid of inherent axial polarity. GSK-3 inhibitor trials suggest that Wnt/β-catenin or Hedgehog signalling may mediate the establishment of this unpolarized zone. Formation of unpolarized tissue may provide a buffer between opposing polarity cues and be a general mechanism by which budding animals establish and maintain linked body axes. In addition to elucidating the developmental basis of budding in a bilaterian, this study provides insight into convergence in animal budding mechanisms, redeployment of embryonic gene expression during budding, and Hox gene evolution.     

Additional sex combs-like 1 belongs to the enhancer of trithorax and polycomb group and genetically interacts with Cbx2 in mice

The Additional sex combs (Asx) gene of Drosophila behaves genetically as an enhancer of trithorax and polycomb (ETP) in displaying bidirectional homeotic phenotypes, suggesting that is required for maintenance of both activation and silencing of Hox genes. There are three murine homologs of Asx called Additional sex combs-like1, 2, and 3. Asxl1 is required for normal adult hematopoiesis; however, its embryonic function is unknown. We used a targeted mouse mutant line Asxl1^tm1Bc to determine if Asxl1 is required to silence and activate Hox genes in mice during axial patterning. The mutant embryos exhibit simultaneous anterior and posterior transformations of the axial skeleton, consistent with a role for Asxl1 in activation and silencing of Hox genes. Transformations of the axial skeleton are enhanced in compound mutant embryos for the polycomb group gene M33/Cbx2. Hoxa4, Hoxa7, and Hoxc8 are derepressed in Asxl1^tm1Bc mutants in the antero-posterior axis, but Hoxc8 expression is reduced in the brain of mutants, consistent with Asxl1 being required both for activation and repression of Hox genes. We discuss the genetic and molecular definition of ETPs, and suggest that the function of Asxl1 depends on its cellular context.     

Functional redundancy of EGF-CFC genes in epiblast and extraembryonic patterning during early mouse embryogenesis

During early mouse embryogenesis, multiple patterning and differentiation events require the activity of Nodal, a ligand of the transforming growth factor-beta (TGFβ) family. Although Nodal signaling is known to require activity of EGF-CFC co-receptors in many contexts, it has been unclear whether all Nodal signaling in the early mouse embryo is EGF-CFC dependent. We have investigated the double null mutant phenotypes for the EGF-CFC genes Cripto and Cryptic, which encode co-receptors for Nodal, and have found that they have partially redundant functions in early mouse development. Expression of Cripto and Cryptic is non-overlapping prior to gastrulation, since Cripto is expressed solely in the epiblast whereas Cryptic is expressed in the primitive endoderm of the late blastocyst and the visceral endoderm after implantation. Despite these non-overlapping expression patterns, Cripto; Cryptic double mutants display severe defects in epiblast, extraembryonic ectoderm, and anterior visceral endoderm (AVE), resulting in phenotypes that are highly similar to those of Nodal null mutants. Our results indicate that both Cripto and Cryptic function non-cell-autonomously during normal development, and that most if not all Nodal activity in early mouse embryogenesis is EGF-CFC-dependent.