Biomedical subjects

Michal Reichman-Fried

Publications and source records attributed to Michal Reichman-Fried.

6 recordsLinked to original sources

PubMed2006

Attraction rules: germ cell migration in zebrafish.

The migration of zebrafish primordial germ cell towards the region where the gonad develops is guided by the chemokine SDF-1a. Recent studies show that soon after their specification, the cells undergo a series of morphological alterations before they become motile and are able to respond to attractive cues. As migratory cells, primordial germ cells move towards their target while correcting their path upon exiting a cyclic phase in which morphological cell polarity is lost. In the following stages, the cells gather at specific locations and move as cell clusters towards their final target. In all of these stages, zebrafish germ cells respond as individual cells to alterations in the shape of the sdf-1a expression domain, by directed migration towards their target - the position where the gonad develops.

Animals↗

PubMed2006

Migration of zebrafish primordial germ cells: a role for myosin contraction and cytoplasmic flow.

The molecular and cellular mechanisms governing cell motility and directed migration in response to the chemokine SDF-1 are largely unknown. Here, we demonstrate that zebrafish primordial germ cells whose migration is guided by SDF-1 generate bleb-like protrusions that are powered by cytoplasmic flow. Protrusions are formed at sites of higher levels of free calcium where activation of myosin contraction occurs. Separation of the acto-myosin cortex from the plasma membrane at these sites is followed by a flow of cytoplasm into the forming bleb. We propose that polarized activation of the receptor CXCR4 leads to a rise in free calcium that in turn activates myosin contraction in the part of the cell responding to higher levels of the ligand SDF-1. The biased formation of new protrusions in a particular region of the cell in response to SDF-1 defines the leading edge and the direction of cell migration.

Actins↗

PubMed2005

Transition from non-motile behaviour to directed migration during early PGC development in zebrafish.

The migration of zebrafish primordial germ cells (PGCs) is directed by SDF-1a and serves as a model for long-range chemokine-guided cell migration. Whereas the development and migration of zebrafish PGCs have been studied in great detail starting at mid-gastrulation stages when the cells exhibit guided active migration [7-8 hours post fertilization (hpf)], earlier stages have not yet been examined. Here we show that the PGCs acquire competence to respond to the chemokine following discrete maturation steps. Using the promoter of the novel gene askopos and RNA elements of nanos1 to drive GFP expression in PGCs, we found that immediately after their specification (about 3 hpf) PGCs exhibit simple cell shape. This stage is followed by a phase at which the cells assume complex morphology yet they neither change their position nor do they respond to SDF-1a. During the third phase, a transition into a ;migratory stage' occurs as PGCs become responsive to directional cues provided by somatic cells secreting the chemokine SDF-1a. This transition depends on zygotic transcription and on the function of the RNA-binding protein Dead end and is correlated with down regulation of the cell adhesion molecule E-cadherin. These distinctive morphological and molecular alterations could represent a general occurrence in similar processes critical for development and disease.

Animals↗

PubMed2004

Involvement of Pax6 and Otx2 in the forebrain-specific regulation of the vertebrate homeobox gene ANF/Hesx1.

During early vertebrate development, ANF homeobox genes are expressed in the prospective forebrain. Their regulation is essential for correct morphogenesis and function of the prosencephalon. We identified a 1-kb fragment upstream of the chicken GANF gene sufficient to drive lacZ expression in the endogenous expression domain. Concordant with the high conservation of this sequence in five investigated species, this element is also active in the corresponding expression domain of the zebrafish orthologue. In vivo analysis of two in vitro-identified Otx2 binding sites in this conserved sequence revealed their necessity for activation of the chicken ANF promoter. In addition, we identified a Pax6-binding site close to the transcriptional start site that is occupied in vivo by Pax6 protein. Pax6 and GANF exhibit mutually exclusive expression domains in the anterior embryonic region. Overexpression of Pax6 in chick embryos inhibited the endogenous GANF expression, and in Pax6(-/-) mice the expression domain of the murine ANF orthologue Hesx1 was expanded and sustained, indicating inhibitory effects of Pax6 on GANF. However, a mutation of the Pax6 site did not abolish reporter activity from an electroporated vector. We conclude that Otx2 and Pax6 are key molecules involved in conserved mechanisms of ANF gene regulation.

Amino Acid Sequence↗

PubMed2004

Autonomous modes of behavior in primordial germ cell migration.

Zebrafish primordial germ cells (PGCs) are guided toward their targets by the chemokine SDF-1a. PGCs were followed during three phases of their migration: when migrating as individual cells, while remaining in a clustered configuration, and when moving as a cell cluster within the embryo. We found that individually migrating PGCs alternate between migratory and pausing modes. Pausing intervals are characterized by loss of cell polarity and correlate with subsequent changes in the direction of migration. These properties constitute an intrinsic behavior of PGCs, enabling erasure of prior polarity and re-sampling of the environment. Following migration arrest at a site of high SDF-1a levels, PGCs resume migration as a cluster. The seemingly coordinated cluster migration is a result of single-cell movement in response to local variations in SDF-1a distribution. Together, these behavioral modes allow the cells to arrive at specific destinations with high fidelity and remain at their target site.

Animals↗

PubMed2002

Guidance of primordial germ cell migration by the chemokine SDF-1.

The signals directing primordial germ cell (PGC) migration in vertebrates are largely unknown. We demonstrate that sdf-1 mRNA is expressed in locations where PGCs are found and toward which they migrate in wild-type as well as in mutant embryos in which PGC migration is abnormal. Knocking down SDF-1 or its receptor CXCR4 results in severe defects in PGC migration. Specifically, PGCs that do not receive the SDF-1 signal exhibit lack of directional movement toward their target and arrive at ectopic positions within the embryo. Finally, we show that the PGCs can be attracted toward an ectopic source of the chemokine, strongly suggesting that this molecule provides a key directional cue for the PGCs.

Animals↗