No abstract

Stem cell self-renewal can be specified by local signals from the surrounding microenvironment, or niche. However, the relation between the niche and the mechanisms that ensure the correct balance between stem cell self-renewal and differentiation is poorly understood. Here, we show that dividing Drosophila male germline stem cells use intracellular mechanisms involving centrosome function and cortically localized Adenomatous Polyposis Coli tumor suppressor protein to orient mitotic spindles perpendicular to the niche, ensuring a reliably asymmetric outcome in which one daughter cell remains in the niche and self-renews stem cell identity, whereas the other, displaced away, initiates differentiation.

Stem cells generate many differentiated, short-lived cell types, such as blood, skin, and sperm, throughout adult life. Stem cells maintain a long-term capacity to divide, producing daughter cells that either self-renew or initiate differentiation. Although the surrounding microenvironment or "niche" influences stem cell fate decisions, few signals that emanate from the niche to specify stem cell self-renewal have been identified. Here we demonstrate that the apical hub cells in the Drosophila testis act as a cellular niche that supports stem cell self-renewal. Hub cells express the ligand Unpaired (Upd), which activates the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway in adjacent germ cells to specify self-renewal and continual maintenance of the germ line stem cell population.

Stem cells are rare cells that are uniquely capable of both reproducing themselves (self-renewing) and generating the differentiated cell types that are needed to carry out specialized functions in the body. Stem cell behaviour, in particular the balance between self-renewal and differentiation, is ultimately controlled by the integration of intrinsic factors with extrinsic cues supplied by the surrounding microenvironment, known as the stem cell niche. The identification and characterization of niches within tissues has revealed an intriguing conservation of many components, although the mechanisms that regulate how niches are established, maintained and modified to support specific tissue stem cell functions are just beginning to be uncovered.

In mammals, the PGC-1 transcriptional co-activators are key regulators of energy metabolism, including mitochondrial biogenesis and respiration, which have been implicated in numerous pathogenic conditions including neurodegeneration and cardiomyopathy. Here, we show that overexpression of the Drosophila PGC-1 homolog (dPGC-1/spargel) is sufficient to increase mitochondrial activity. Moreover, tissue-specific overexpression of dPGC-1 in stem and progenitor cells within the digestive tract extends lifespan. Long-lived flies overexpressing dPGC-1 display a delay in the onset of aging-related changes in the intestine, leading to improved tissue homeostasis in old flies. Together, these results demonstrate that dPGC-1 can slow aging both at the level of cellular changes in an individual tissue and also at the organismal level by extending lifespan. Our findings point to the possibility that alterations in PGC-1 activity in high-turnover tissues, such as the intestine, may be an important determinant of longevity in mammals.

The high risk human papillomaviruses (HPVs) are associated etiologically with the majority of human cervical carcinomas. These HPVs encode two viral oncoproteins, E6 and E7, which are expressed consistently in cervical cancers. The function of these viral oncoproteins during a productive infection is to ensure viral replication in cells that have normally withdrawn from the cell division cycle and are committed to terminal differentiation. Expression of the E7 oncoprotein has been shown to lead to the abrogation of various negative growth regulatory signals, including a p53-mediated G 1 growth arrest, TGF␤-mediated growth inhibition, and quiescence of suprabasal keratinocytes. Here we describe a novel mechanism by which E7 can uncouple cellular proliferation and differentiation. In contrast to normal, differentiating keratinocytes, HPV-16 E7-expressing keratinocytes show delayed cellular differentiation and elevated cdk2 kinase activity despite high levels of p21Cip1 and association of p21 Cip1 with cdk2. We show that the HPV E7 protein can interact with p21Cip1 and abrogate p21 Cip1-mediated inhibition of cyclin A and E-associated kinase activities. Based on these findings, we propose that this capacity of the HPV E7 oncoprotein to overcome p21 Cip1-mediated inhibition of cdk2 activity during keratinocyte differentiation contributes to the ability of E7 to allow for cellular DNA synthesis in differentiated keratinocytes.[Key Words: Human papillomavirus; cyclin dependent kinase inhibitor; cellular differentiation; cell division cycle]Received May 29, 1997; revised version accepted July 3, 1997.The papillomaviruses are small DNA viruses that have a pronounced tropism for epithelial cells where they cause benign hyperplasias or warts. Human papillomaviruses (HPVs) can be subdivided into two groups based on the propensity of the lesions to progress to malignancy. Low risk HPVs are associated with lesions with a very low frequency of malignant progression, whereas the lesions caused by the high risk HPVs can undergo carcinogenic progression (for review, see zur Hausen 1996), with more than 95% of all cervical carcinomas harboring high risk HPVs (Bosch et al. 1995). Although HPVs infect the dividing, basal cells of host epithelia, their replication is intimately tied to the cellular differentiation machinery of these cells. Expression of the viral capsid proteins and productive viral replication can only occur in the differentiated layers of the skin that are composed of cells that have withdrawn from the cell division cycle. Limited by their small genome size, with the exception of the viral E1 and E2 proteins, papillomaviruses do not encode the enzymes that are rate limiting for DNA replication. Rather, these viruses have developed multiple strategies to subvert key regulatory circuits that control host cell replication (for review, see Howley 1996). The precise mechanisms by which HPVs are able to reactivate the dormant replication machinery in these cells to replicate their own genomes are unknown. The E6 and E7 ...

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.

Cycles of protein phosphorylation are fundamental in regulating the progression of the eukaryotic cell through its division cycle. Here we test the complement of Drosophila protein kinases (kinome) for cell cycle functions after gene silencing by RNA-mediated interference. We observed cell cycle dysfunction upon downregulation of 80 out of 228 protein kinases, including most kinases that are known to regulate the division cycle. We find new enzymes with cell cycle functions; some of these have family members already known to phosphorylate microtubules, actin or their associated proteins. Additionally, depletion of several signalling kinases leads to specific mitotic aberrations, suggesting novel roles for familiar enzymes. The survey reveals the inter-digitation of systems that monitor cellular physiology, cell size, cellular stress and signalling processes with the basic cell cycle regulatory machinery.