Heterochromatin Protein 1 (HP1) inhibits stem cell proliferation induced by ectopic activation of the Jak/STAT pathway in the Drosophila testis.

Stem cells can divide asymmetrically with respect to cell fate, producing a copy of themselves (self-renewal), while giving rise to progeny that will differentiate along a specific lineage. Mechanisms that bias the balance towards self-renewal or extend the proliferative capacity of the differentiating progeny can result in tissue overgrowth and, eventually, the formation of tumors. Recent work has explored the role of heterochromatin and heterochromatin-associated proteins in the regulation of stem cell behavior under homeostatic conditions, but less is known about their possible roles in potentiating or suppressing stem cell overproliferation. Here we used ectopic activation of the Jak/STAT pathway in germline and somatic stem cells of the D. melanogaster testis as an in vivo model to probe the function of Heterochromatin Protein 1 (HP1) in stem cell overproliferation. Forced expression of HP1 in either early germ or somatic cells suppressed the overgrowth of testes in response to ectopic Jak/STAT activation. Interestingly, HP1 expression led to distinct phenotypes, depending on whether it was overexpressed in somatic or germ cells, possibly reflecting different cell-autonomous and non-autonomous effects in each cell type. Our results provide a new framework for further in vivo studies aimed at understanding the interactions between heterochromatin and uncontrolled stem cell proliferation, as well as the complex cross-regulatory interactions between the somatic and germline lineages in the Drosophila testis.

Brain Proteome Changes Induced by Olfactory Learning in Drosophila.

For more than 30 years, the study of learning and memory in Drosophila melanogaster (fruit fly) has used an olfactory learning paradigm and has resulted in the discovery of many genes involved in memory formation. By varying learning programs, the creation of different memory types can be achieved, from short-term memory formation to long-term. Previous studies in the fruit fly used gene mutation methods to identify genes involved in memory formation. Presumably, memory creation involves a combination of genes, pathways, and neural circuits. To examine memory formation at the protein level, a quantitative proteomic analysis was performed using olfactory learning and 15N-labeled fruit flies. Differences were observed in protein expression and relevant pathways between different learning programs. Our data showed major protein expression changes occurred between short-term memory (STM) and long-lasting memory, and only minor changes were found between long-term memory (LTM) and anesthesia-resistant memory (ARM).

Headcase promotes cell survival and niche maintenance in the Drosophila testis.

At the apical tip of the Drosophila testis, germline and somatic stem cells surround a cluster of somatic cells called the hub. Hub cells produce a self-renewal factor, Unpaired (Upd), that activates the JAK-STAT pathway in adjacent stem cells to regulate stem cell behavior. Therefore, apical hub cells are a critical component of the stem cell niche in the testis. In the course of a screen to identify factors involved in regulating hub maintenance, we identified headcase (hdc). Hub cells depleted for hdc undergo programmed cell death, suggesting that anti-apoptotic pathways play an important role in maintenance of the niche. Using hdc as paradigm, we describe here the first comprehensive analysis on the effects of a progressive niche reduction on the testis stem cell pool. Surprisingly, single hub cells remain capable of supporting numerous stem cells, indicating that although the size and number of niche support cells influence stem cell maintenance, the testis stem cell niche appears to be remarkably robust in the its ability to support stem cells after severe damage.

Decline in self-renewal factors contributes to aging of the stem cell niche in the Drosophila testis.

Aging is characterized by compromised organ and tissue function. A decrease in stem cell number and/or activity could lead to the aging-related decline in tissue homeostasis. We have analyzed how the process of aging affects germ line stem cell (GSC) behavior in the Drosophila testis and report that significant changes within the stem cell microenvironment, or niche, occur that contribute to a decline in stem cell number over time. Specifically, somatic niche cells in testes from older males display reduced expression of the cell adhesion molecule DE-cadherin and a key self-renewal signal unpaired (upd). Loss of upd correlates with an overall decrease in stem cells residing within the niche. Conversely, forced expression of upd within niche cells maintains GSCs in older males. Therefore, our data indicate that age-related changes within stem cell niches may be a significant contributing factor to reduced tissue homeostasis and regeneration in older individuals.

Drosophila Eph receptor guides specific axon branches of mushroom body neurons.

The conserved Eph receptors and their Ephrin ligands regulate a number of developmental processes, including axon guidance. In contrast to the large vertebrate Eph/Ephrin family, Drosophila has a single Eph receptor and a single Ephrin ligand, both of which are expressed within the developing nervous system. Here, we show that Eph and Ephrin can act as a functional receptor-ligand pair in vivo. Surprisingly, and in contrast to previous results using RNA-interference techniques, embryos completely lacking Eph function show no obvious axon guidance defects. However, Eph/Ephrin signaling is required for proper development of the mushroom body. In wild type, mushroom body neurons bifurcate and extend distinct branches to different target areas. In Eph mutants, these neurons bifurcate normally, but in many cases the dorsal branch fails to project to its appropriate target area. Thus, Eph/Ephrin signaling acts to guide a subset of mushroom body branches to their correct synaptic targets.

A somatic role for eyes absent (eya) and sine oculis (so) in Drosophila spermatocyte development.

Interactions between the soma and the germline are a conserved feature of spermatogenesis throughout the animal kingdom. In this report, we find that the transcription factors eyes absent (eya) and sine oculis (so), previously shown to play major roles during eye development [Cell 91 (1997), 881] are each required in the somatic cyst cells of the testis for proper Drosophila spermatocyte development. eya mutant testes exhibit degenerating young spermatocytes. Mosaic analysis reveals a somatic requirement for both eya and so, in that neither gene is required in the germline for spermatocyte development. Immunolocalization analysis supports this somatic role, since both proteins are localized within cyst cell nuclei as spermatocytes differentiate from amplifying spermatogonia. Using antibodies against known cyst cell markers, we demonstrate that cysts of degenerating spermatocytes in eya mutant testes are encysted, ruling out a role for eya in cyst cell viability. Finally, we have uncovered a genetic interaction between eya and so in the testis, suggesting that, as in the eye, eya and so may form a transcription complex responsible for the activation of target genes involved in cyst cell differentiation and spermatocyte development.