Summary While normally dormant, Hair Follicle Stem Cells (HFSCs) quickly become activated to divide during a new hair cycle. The quiescence of HFSCs is known to be regulated by a number of intrinsic and extrinsic mechanisms. Here we provide several lines of evidence to demonstrate that HFSCs utilize glycolytic metabolism and produce significantly more lactate than other cells in the epidermis. Furthermore, lactate generation appears to be critical for the activation of HFSCs as deletion of lactate dehydrogenase (Ldha) prevented their activation. Conversely, genetically promoting lactate production in HFSCs through mitochondrial pyruvate carrier (Mpc1) deletion accelerated their activation and the hair cycle. Finally, we identify small molecules that increase lactate production by stimulating Myc levels or inhibiting Mpc1 carrier activity and can topically induce the hair cycle. These data suggest that HFSCs maintain a metabolic state that allow them to remain dormant and yet quickly respond to appropriate proliferative stimuli.

The D. melanogaster hematopoietic organ, called lymph gland, proliferates and differentiates throughout the larval period. The lymph gland of the late larva is comprised of a large primary lobe and several smaller secondary lobes. Differentiation into two types of hemocytes, plasmatocytes and crystal cells, is confined to the outer layer (cortical zone) of the primary lobe; the center of the primary lobe (medullary zone), as well as the secondary lobes, contain only proliferating prohemocytes. A small cluster of prohemocytes located at the posterior tip of the primary lobe serves as a signaling center (PSC) that inhibits precocious differentiation of the medullary zone. The larval lymph gland is stabilized by layers of extracellular matrix (basement membranes) that surround individual hemocytes, groups of hemocytes, as well as the lymph gland as a whole. In this paper we investigated the events shaping the lymph gland in the early pupa. The lymph gland dissociates and hemocytes disperse during the first 12h after puparium formation (APF), leaving behind empty husks of basement membrane. Prior to lymph gland dissociation, cells of the medullary zone differentiate, expressing the early differentiation marker Peroxidasin, as well as, in part, the late differentiation marker P1. Cells of the PSC spread out throughout the pupal lymph gland prior to their dispersal. Cells of the secondary lobes undergo a rapid phase of proliferation that lasts until 8h APF, followed by expression of Peroxidasin, and dispersal. These hemocytes do not express P1, indicating that they disperse prior to full maturation.

In some organs, adult stem cells are uniquely poised to serve as cancer cells of origin. It is unclear, however, whether tumorigenesis is influenced by the activation state of the adult stem cell. Hair follicle stem cells (HFSCs) act as cancer cells of origin for cutaneous squamous cell carcinoma (SCC) and undergo defined cycles of quiescence and activation. The data presented here show that HFSCs are unable to initiate tumors during the quiescent phase of the hair cycle, indicating that the mechanisms that keep HFSCs dormant are dominant to the gain of oncogenes (Ras) or the loss of tumor suppressors (p53). Furthermore, Pten activity is necessary for quiescence based tumor suppression, as its deletion alleviates tumor suppression without affecting proliferation. These data demonstrate that stem cell quiescence is a form of tumor suppression in HFSCs, and that Pten plays a role in maintaining quiescence in the presence of tumorigenic stimuli.

The heparin sulfate proteoglycan Trol (Terribly Reduced Optic Lobes) is the D. melanogaster homolog of the vertebrate protein Perlecan. Trol is expressed as part of the extracellular matrix (ECM) found in the hematopoietic organ, called the lymph gland. In the normal lymph gland, the ECM forms thin basement membranes around individual or small groups of blood progenitors. The pattern of basement membranes, reported by Trol expression, is spatio-temporally correlated to hematopoiesis. The central, medullary zone which contain undifferentiated hematopoietic progenitors has many, closely spaced membranes. Fewer basement membranes are present in the outer, cortical zone, where differentiation of blood cells takes place. Loss of trol causes a dramatic change of the ECM into a three-dimensional, spongy mass that fills wide spaces scattered throughout the lymph gland. At the same time proliferation is reduced, leading to a significantly smaller lymph gland. Interestingly, differentiation of blood progenitors in trol mutants is precocious, resulting in the break-down of the usual zonation of the lymph gland which normally consists of an immature center (medullary zone) where cells remain undifferentiated, and an outer cortical zone, where differentiation sets in. We present evidence that the effect of Trol on blood cell differentiation is mediated by Hedgehog (Hh) signaling, which is known to be required to maintain an immature medullary zone. Overexpression of hh in the background of a trol mutation is able to rescue the premature differentiation phenotype. Our data provide novel insight into the role of the ECM component Perlecan during Drosophila hematopoiesis.

Hemocytes (blood cells) are motile cells moving throughout the extracellular space and exist in all clades of the animal kingdom. Hemocytes play an important role in shaping the extracellular environment and in the immune response. Developmentally, hemocytes are closely related to the epithelial cells lining the vascular system (endothelia) and body cavity (mesothelia). In vertebrates and insects, common progenitors, called hemangioblasts, give rise to the endothelia and blood cells. In the adult animal, many differentiated hemocytes seem to retain the ability to proliferate; however, in most cases investigated closely, the bulk of hemocyte proliferation takes place in specialized hematopoietic organs. Hematopoietic organs provide an environment where undifferentiated blood stem cells are able to self renew, and at the same time generate offspring that differentiate into different blood cell types. Hematopoiesis in vertebrates, taking place in the bone marrow, has been subject to intensive research by immunologists and stem cell biologists. Much less is known about blood cell formation in invertebrate animals. In this review we will survey structural and functional properties of invertebrate hematopoietic organs, with a main focus on insects and other arthropod taxa. We will then discuss similarities, at the molecular and structural level, that are apparent when comparing the development of blood cells in hematopoietic organs of vertebrates and arthropods. Our comparative review is intended to elucidate aspects of the biology of blood stem cells that are more easily missed when focusing on one or a few model species.