5q- myelodysplastic syndromes: chromosome 5q genes direct a tumor-suppression network sensing actin dynamics.

Complete loss or interstitial deletions of chromosome 5 are the most common karyotypic abnormality in myelodysplastic syndromes (MDSs). Isolated del(5q)/5q- MDS patients have a more favorable prognosis than those with additional karyotypic defects, who tend to develop myeloproliferative neoplasms (MPNs) and acute myeloid leukemia. The frequency of unbalanced chromosome 5 deletions has led to the idea that 5q harbors one or more tumor-suppressor genes that have fundamental roles in the growth control of hematopoietic stem/progenitor cells (HSCs/HPCs). Cytogenetic mapping of commonly deleted regions (CDRs) centered on 5q31 and 5q32 identified candidate tumor-suppressor genes, including the ribosomal subunit RPS14, the transcription factor Egr1/Krox20 and the cytoskeletal remodeling protein, alpha-catenin. Although each acts as a tumor suppressor, alone or in combination, no molecular mechanism accounts for how defects in individual 5q candidates may act as a lesion driving MDS or contributing to malignant progression in MPN. One candidate gene that resides between the conventional del(5q)/5q- MDS-associated CDRs is DIAPH1 (5q31.3). DIAPH1 encodes the mammalian Diaphanous-related formin, mDia1. mDia1 has critical roles in actin remodeling in cell division and in response to adhesive and migratory stimuli. This review examines evidence, with a focus on mouse gene-targeting experiments, that mDia1 acts as a node in a tumor-suppressor network that involves multiple 5q gene products. The network has the potential to sense dynamic changes in actin assembly. At the root of the network is a transcriptional response mechanism mediated by the MADS-box transcription factor, serum response factor (SRF), its actin-binding myocardin family coactivator, MAL, and the SRF-target 5q gene, EGR1, which regulate the expression of PTEN and p53-family tumor-suppressor proteins. We hypothesize that the network provides a homeostatic mechanism balancing HPC/HSC growth control and differentiation decisions in response to microenvironment and other external stimuli.

p190 RhoGAP is the principal Src substrate in brain and regulates axon outgrowth, guidance and fasciculation.

The Src tyrosine kinases have been implicated in several aspects of neural development and nervous system function; however, their relevant substrates in brain and their mechanism of action in neurons remain to be established clearly. Here we identify the potent Rho regulatory protein, p190 RhoGAP (GTPase-activating protein), as the principal Src substrate detected in the developing and mature nervous system. We also find that mice lacking functional p190 RhoGAP exhibit defects in axon guidance and fasciculation. p190 RhoGAP is co-enriched with F-actin in the distal tips of axons, and overexpressing p190 RhoGAP in neuroblastoma cells promotes extensive neurite outgrowth, indicating that p190 RhoGAP may be an important regulator of Rho-mediated actin reorganization in neuronal growth cones. p190 RhoGAP transduces signals downstream of cell-surface adhesion molecules, and we find that p190-RhoGAP-mediated neurite outgrowth is promoted by the extracellular matrix protein laminin. Together with the fact that mice lacking neural adhesion molecules or Src kinases also exhibit defects in axon outgrowth, guidance and fasciculation, our results suggest that p190 RhoGAP mediates a Src-dependent adhesion signal for neuritogenesis to the actin cytoskeleton through the Rho GTPase.

The adhesion signaling molecule p190 RhoGAP is required for morphogenetic processes in neural development.

Rho GTPases direct actin rearrangements in response to a variety of extracellular signals. P190 RhoGAP (GTPase activating protein) is a potent Rho regulator that mediates integrin-dependent adhesion signaling in cultured cells. We have determined that p190 RhoGAP is specifically expressed at high levels throughout the developing nervous system. Mice lacking functional p190 RhoGAP exhibit several defects in neural development that are reminiscent of those described in mice lacking certain mediators of neural cell adhesion. The defects reflect aberrant tissue morphogenesis and include abnormalities in forebrain hemisphere fusion, ventricle shape, optic cup formation, neural tube closure, and layering of the cerebral cortex. In cells of the neural tube floor plate of p190 RhoGAP mutant mice, polymerized actin accumulates excessively, suggesting a role for p190 RhoGAP in the regulation of +Rho-mediated actin assembly within the neuroepithelium. Significantly, several of the observed tissue fusion defects seen in the mutant mice are also found in mice lacking MARCKS, the major substrate of protein kinase C (PKC), and we have found that p190 RhoGAP is also a PKC substrate in vivo. Upon either direct activation of PKC or in response to integrin engagement, p190 RhoGAP is rapidly translocated to regions of membrane ruffling, where it colocalizes with polymerized actin. Together, these results suggest that upon activation of neural adhesion molecules, the action of PKC and p190 RhoGAP leads to a modulation of Rho GTPase activity to direct several actin-dependent morphogenetic processes required for normal neural development.

Differential antigen expression during metamorphosis in the tripartite olfactory system of the African clawed frog, Xenopus laevis.

In the adult African clawed frog, Xenopus laevis, olfactory epithelium is housed in three separate nasal cavities: the principal cavity, the middle cavity, and the vomeronasal organ. The sensory epithelium in each of these cavities has distinct cellular features, and presumed physiological and behavioral functions, which arise during metamorphosis. Most notably, the middle cavity is formed de novo, and the principal cavity is transformed from a larval sensory epithelium with water exposure to an adult olfactory epithelium with air exposure. To understand the cellular nature of this plasticity more clearly, we characterized the staining patterns generated in the olfactory system of X. laevis with a new monoclonal antibody, anti-E7. The olfactory epithelium is first stained with anti-E7 during late embryonic development. Transection of the olfactory nerves during metamorphosis eliminates all staining and indicates that the staining is associated with mature or nearly mature olfactory receptor neurons. The antibody diffusely stains the vomeronasal organ throughout development and in adults. In the larval principal cavity, the olfactory receptor neurons are brightly stained, but this cellular staining is lost after metamorphosis. The mucus from Bowman's glands in the principal cavity, however, is intensely stained in adults. The middle cavity, throughout development and in adulthood, has the same staining characteristics as the larval principal cavity. Thus, the E7 antibody can distinguish the three areas of the olfactory epithelium, allowing measurement of sensory epithelium volume, and serves as an excellent marker for the changes in the sensory epithelium that occur during metamorphosis.

Steroid hormone enhancement of neurite outgrowth in identified insect motor neurons involves specific effects on growth cone form and function.

Dramatic reorganization of dendrites and axonal terminals is a hallmark of neuronal remodeling during metamorphosis in the hawkmoth, Manduca sexta. The dendritic and axonal arbors of leg motor neurons regress in late larval stages, then regrow during adult development. Ecdysteroids, the insect steroids that trigger metamorphosis, control both regression and outgrowth in vivo and stimulate neuritic growth in cultured pupal leg motor neurons. To identify subcellular targets of ecdysteroid action in these neurons, we examined the dynamic and structural features of branching and their modulation by ecdysteroids in vitro. Delayed treatment of pupal leg motor neurons with ecdysteroid led to a robust enhancement of neuritic branch accumulation accompanied by a subtle effect on total neuritic length. Repeated imaging revealed that branch formation occurred almost exclusively at the growth cone; interstitial branching was extremely rare. Ecdysteroid treatment significantly enhanced both the formation and retention of branches at the growth cone. Branches formed via two distinct processes: engorgement (of fine protrusions) and condensation (of lamellae) with the relative contributions of these mechanisms being unaltered by ecdysteroid. Confocal imaging of the cytoskeleton demonstrated that growth cones consisted of microtubule-based domains fringed by actin-based filopodia. Treated growth cones were larger and displayed increased numbers of microtubule-based branches, whereas filopodial density was unaffected. These findings indicate that ecdysteroid enhances neuritic branching by altering growth cone structure and function, and suggest that hormonal modulation of cytoskeletal interactions contributes significantly to neuritic remodeling during metamorphosis.

Regulation of axonal caliber, neurofilament content, and nuclear localization in mature sensory neurons by nerve growth factor.

The neuronal perikaryal response to axonal injury (axon reaction) includes reduction in axonal caliber beginning in the proximal portion of the nerve (somatofugal axonal atrophy), development of nuclear eccentricity, and chromatolysis. The means by which these events are triggered is unknown, but it has been argued that loss of a neurotrophic signal from the target of injured neurons plays a role. To date, the identity of this substance(s) remains unknown. In the present study, we have asked whether NGF normally functions to control axonal caliber of sensory neurons in the L4 and L5 dorsal root ganglia (DRG) of the adult rat. Two approaches were used: (1) NGF was continuously delivered to the proximal stump of a transected sciatic nerve to determine whether NGF administration would prevent the production of somatofugal axonal atrophy; and (2) NGF antisera were administered to normal animals to determine whether NGF deprivation would produce somatofugal axonal atrophy. In the first experiment, 9-week-old rats underwent a unilateral sciatic nerve transection at midthigh, and the proximal stump was connected to an osmotic pump containing either NGF or cytochrome C (as control). At 11 weeks of age, dorsal root fibers in lumbar DRG from the control group appeared smaller in caliber and less circular in shape than fibers from age-matched normal animals. Although smaller than those in normal animals, fibers from the NGF-treated nerves were larger than in axotomized controls. Mean axonal area and shape factor (an index in circularity) were measured and found to be decreased significantly (22% and 15%, respectively) from the control group. Fibers from the NGF-treated nerves were significantly (p less than 0.05) larger in axonal caliber and more circular in shape; mean values were only reduced by 11% and 10%, respectively. Quantitation of neurofilament (NF) numbers revealed that the larger calibers in the NGF-treated nerves result from a greater NF content. NGF treatment did not prevent the atrophy of motor fibers in the proximal ventral root. In the second experiment, 2 antisera to mouse NGF were given daily into the footpad for 11 or 12 d; control animals were given normal goat serum. Quantitation of axonal calibers in the L5 DRG demonstrated that mean axonal area and shape factor were significantly (p less than 0.05) reduced by 14% and 17% respectively. The axoplasm of atrophic fibers demonstrated a paucity of NFs.(ABSTRACT TRUNCATED AT 400 WORDS)