Biomedical subjects

Lukas Sommer

Publications and source records attributed to Lukas Sommer.

At least 19 recordsLinked to original sources

PubMed2006

Neural crest-derived cells with stem cell features can be traced back to multiple lineages in the adult skin.

Given their accessibility, multipotent skin-derived cells might be useful for future cell replacement therapies. We describe the isolation of multipotent stem cell-like cells from the adult trunk skin of mice and humans that express the neural crest stem cell markers p75 and Sox10 and display extensive self-renewal capacity in sphere cultures. To determine the origin of these cells, we genetically mapped the fate of neural crest cells in face and trunk skin of mouse. In whisker follicles of the face, many mesenchymal structures are neural crest derived and appear to contain cells with sphere-forming potential. In the trunk skin, however, sphere-forming neural crest-derived cells are restricted to the glial and melanocyte lineages. Thus, self-renewing cells in the adult skin can be obtained from several neural crest derivatives, and these are of distinct nature in face and trunk skin. These findings are relevant for the design of therapeutic strategies because the potential of stem and progenitor cells in vivo likely depends on their nature and origin.

Adipocytes↗

PubMed2006

Guidance of neural crest cell migration: the inhibitory function of the chondroitin sulfate proteoglycan, versican.

Neural crest cells are specialized multipotent embryonic stem cells found exclusively in vertebrates[1,2,3]. During embryonic development, these cells arise from the dorsal neural tube, undergo epithelial to mesenchymal transition, and subsequently migrate along stereotyped pathways to reach specific tissue targets, where they differentiate into a wide variety of cell types, such as glia and neurons of the peripheral nervous system, melanocytes, smooth muscle cells, craniofacial cartilage and bone tissues, or chromaffin cells of the adrenal medulla. In the trunk region, the ventrally migrating neural crest cells move through the somitic mesenchyme in a segmented pattern, presumably setting the basis for the metameric organization of sensory and sympathetic ganglia along the anterior-posterior axis later in development[4].

Animals↗

PubMed2006

Versican V0 and V1 guide migratory neural crest cells.

We previously showed the selective expression of the chondroitin sulfate proteoglycans versican V0 and V1 in barrier tissues that impede the migration of neural crest cells during embryonic trunk development (Landolt, R. M., Vaughan, L., Winterhalter, K. H., and Zimmermann, D. R. (1995) Development 212, 2303-2312). To test for an active involvement of these isoforms in the guidance process, we have now established protocols to isolate intact versican V0 and V1 in quantities sufficient for functional experiments. Using stripe choice assays, we demonstrate that pure preparations of either a mixture of versican V0/V1 or V1 alone strongly inhibit the migration of multipotent Sox10/p75NTR double-positive early neural crest stem cells on fibronectin by interfering with cell-substrate adhesion. We show that this inhibition is largely core glycoprotein-dependent, as the complete removal of the glycosaminoglycan chains has only a minor effect on the inhibitory capacity. Our findings support the notion that versican variants V0 and V1 act, possibly in concert with other inhibitory molecules such as aggrecan and ephrins, in directing the migratory streams of neural crest cells to their appropriate target tissues.

Animals↗

PubMed2006

DiGeorge syndrome and pharyngeal apparatus development.

DiGeorge syndrome is the most frequent microdeletion syndrome in humans, and is characterized by cardiovascular, thymic and parathyroid, and craniofacial anomalies. The underlying cause is disturbed formation of the pharyngeal apparatus, a transient structure present during vertebrate development that gives rise to endocrine glands, craniofacial tissue, and the cardiac outflow tract. The pharyngeal apparatus is composed of derivatives of ectoderm, endoderm, mesoderm and the neural crest. Thus, complex interactions between cell types from different origins have to be orchestrated in the correct spatiotemporal manner to establish proper formation of the pharyngeal system. The analysis of engineered mouse mutants developing a phenotype resembling DiGeorge syndrome has revealed genes and signalling pathways crucial for this process. Intriguingly, these mouse models reveal that interference with either of two distinct phases of pharyngeal apparatus development can contribute to the aetiology of DiGeorge syndrome.

Animals↗

PubMed2006

Growth factors regulating neural crest cell fate decisions.

Because of its unique ability to generate a wide variety of both neural and nonneural derivatives, the neural crest is an ideal model system to study the factors regulating cell lineage decisions in stem and progenitor cells. The use of various cell culture techniques and in vivo functional assays, including cell type-specific gene manipulation in mouse, helped to identify signaling factors involved in this process. Moreover, it became apparent that the biological functions of growth factors acting on neural crest cells depend on the context provided by the extracellular microenvironment. Thus, signaling molecules have to be viewed as parts of complex networks that change with time and location. Neural crest cells have to integrate these signals to ensure the generation of appropriate numbers of differentiating progeny. It will be important to determine how such signaling networks are established and how they elicit multiple signaling responses in neural crest cells to activate appropriate genetic programs.

Animals↗

PubMed2005

Compound developmental eye disorders following inactivation of TGFbeta signaling in neural-crest stem cells.

BACKGROUND: Development of the eye depends partly on the periocular mesenchyme derived from the neural crest (NC), but the fate of NC cells in mammalian eye development and the signals coordinating the formation of ocular structures are poorly understood. RESULTS: Here we reveal distinct NC contributions to both anterior and posterior mesenchymal eye structures and show that TGFbeta signaling in these cells is crucial for normal eye development. In the anterior eye, TGFbeta2 released from the lens is required for the expression of transcription factors Pitx2 and Foxc1 in the NC-derived cornea and in the chamber-angle structures of the eye that control intraocular pressure. TGFbeta enhances Foxc1 and induces Pitx2 expression in cell cultures. As in patients carrying mutations in PITX2 and FOXC1, TGFbeta signal inactivation in NC cells leads to ocular defects characteristic of the human disorder Axenfeld-Rieger's anomaly. In the posterior eye, NC cell-specific inactivation of TGFbeta signaling results in a condition reminiscent of the human disorder persistent hyperplastic primary vitreous. As a secondary effect, retinal patterning is also disturbed in mutant mice. CONCLUSION: In the developing eye the lens acts as a TGFbeta signaling center that controls the development of eye structures derived from the NC. Defective TGFbeta signal transduction interferes with NC-cell differentiation and survival anterior to the lens and with normal tissue morphogenesis and patterning posterior to the lens. The similarity to developmental eye disorders in humans suggests that defective TGFbeta signal modulation in ocular NC derivatives contributes to the pathophysiology of these diseases.

Animals↗

PubMed2005

Checkpoints of melanocyte stem cell development.

The bulge region of the adult hair follicle contains the niches for both epithelial and melanocyte stem cells. Recent evidence suggests that the development of melanocyte stem cells is controlled by a complex network of transcription factors, including Pax3, Sox10, and Mitf, and of regulatory extracellular cues such as Wnt. However, additional players are likely to be involved. It will be intriguing to identify these signals and to elucidate whether and how neighboring epithelial stem cells influence the balance between melanocyte stem cell maintenance and differentiation.

Animals↗

PubMed2005

Neural crest stem cell maintenance by combinatorial Wnt and BMP signaling.

Canonical Wnt signaling instructively promotes sensory neurogenesis in early neural crest stem cells (eNCSCs) (Lee, H.Y., M. Kleber, L. Hari, V. Brault, U. Suter, M.M. Taketo, R. Kemler, and L. Sommer. 2004. Science. 303:1020-1023). However, during normal development Wnt signaling induces a sensory fate only in a subpopulation of eNCSCs while other cells maintain their stem cell features, despite the presence of Wnt activity. Hence, factors counteracting Wnt signaling must exist. Here, we show that bone morphogenic protein (BMP) signaling antagonizes the sensory fate-inducing activity of Wnt/beta-catenin. Intriguingly, Wnt and BMP act synergistically to suppress differentiation and to maintain NCSC marker expression and multipotency. Similar to NCSCs in vivo, NCSCs maintained in culture alter their responsiveness to instructive growth factors with time. Thus, stem cell development is regulated by combinatorial growth factor activities that interact with changing cell-intrinsic cues.

Animals↗

PubMed2005

Inactivation of TGFbeta signaling in neural crest stem cells leads to multiple defects reminiscent of DiGeorge syndrome.

Specific inactivation of TGFbeta signaling in neural crest stem cells (NCSCs) results in cardiovascular defects and thymic, parathyroid, and craniofacial anomalies. All these malformations characterize DiGeorge syndrome, the most common microdeletion syndrome in humans. Consistent with a role of TGFbeta in promoting non-neural lineages in NCSCs, mutant neural crest cells migrate into the pharyngeal apparatus but are unable to acquire non-neural cell fates. Moreover, in neural crest cells, TGFbeta signaling is both sufficient and required for phosphorylation of CrkL, a signal adaptor protein implicated in the development of DiGeorge syndrome. Thus, TGFbeta signal modulation in neural crest differentiation might play a crucial role in the etiology of DiGeorge syndrome.

Adaptor Proteins, Signal Transducing↗

PubMed2005

Distinct disease mechanisms in peripheral neuropathies due to altered peripheral myelin protein 22 gene dosage or a Pmp22 point mutation.

Point mutations affecting PMP22 can cause hereditary demyelinating and dysmyelinating peripheral neuropathies. In addition, duplication and deletion of PMP22 are associated with Charcot-Marie-Tooth disease Type 1A (CMT1A) and Hereditary Neuropathy with Liability to Pressure Palsy (HNPP), respectively. This study was designed to elucidate disease processes caused by misexpression of Pmp22 and, at the same time, to gain further information on the controversial molecular function of PMP22. To this end, we took advantage of the unique resource of a set of various Pmp22 mutant mice to carry out comparative expression profiling of mutant and wild-type sciatic nerves. Tissues derived from Pmp22-/- ("knockout"), Pmp22tg (increased Pmp22 copy number), and Trembler (Tr; point mutation in Pmp22) mutant mice were analyzed at two developmental stages: (i) at postnatal day (P)4, when normal myelination has just started and primary causative defects of the mutations are expected to be apparent, and (ii) at P60, with the goal of obtaining information on secondary disease effects. Interestingly, the three Pmp22 mutants exhibited distinct profiles of gene expression, suggesting different disease mechanisms. Increased expression of genes involved in cell cycle regulation and DNA replication is characteristic and specific for the early stage in Pmp22-/- mice, supporting a primary function of PMP22 in the regulation of Schwann cell proliferation. In the Tr mutant, a distinguishing feature is the high expression of stress response genes. Both Tr and Pmp22tg mice show strongly reduced expression of genes important for cholesterol synthesis at P4, a characteristic that is common to all three mutants at P60. Finally, we have identified a number of candidate genes that may play important roles in the disease process or in myelination per se.

Animals↗

PubMed2005

Cortical development: the art of generating cell diversity.

The fascinating question of how the enormous diversity of neuronal and glial cells in the cerebral cortex is generated during development was recently discussed at a meeting on cortical development and stem cells in Greece. What emerged from this meeting is an equally fascinating answer, namely that precursor diversity at rather early stages of development anticipates later cell type diversity.

Animals↗

PubMed2004

The protooncogene Ski controls Schwann cell proliferation and myelination.

Schwann cell proliferation and subsequent differentiation to nonmyelinating and myelinating cells are closely linked processes. Elucidating the molecular mechanisms that control these events is key to the understanding of nerve development, regeneration, nerve-sheath tumors, and neuropathies. We define the protooncogene Ski, an inhibitor of TGF-beta signaling, as an essential component of the machinery that controls Schwann cell proliferation and myelination. Functional Ski overexpression inhibits TGF-beta-mediated proliferation and prevents growth-arrested Schwann cells from reentering the cell cycle. Consistent with these findings, myelinating Schwann cells upregulate Ski during development and remyelination after injury. Myelination is blocked in myelin-competent cultures derived from Ski-deficient animals, and genes encoding myelin components are downregulated in Ski-deficient nerves. Conversely, overexpression of Ski in Schwann cells causes an upregulation of myelin-related genes. The myelination-regulating transcription factor Oct6 is involved in a complex modulatory relationship with Ski. We conclude that Ski is a crucial signal in Schwann cell development and myelination.

Animals↗

PubMed2004

Multiple roles of canonical Wnt signaling in cell cycle progression and cell lineage specification in neural development.

Signaling by the canonical Wnt pathway has multiple functions in stem cells. It can either control stem cell expansion or--as we have recently demonstrated with neural crest stem cells-influence cell lineage decisions by promoting specific fates at the expense of others. Thus, the role of canonical Wnt in stem cells is dependent on cell--intrinsic properties that determine how a cell responds to Wnt. The molecular basis for the functional diversity of Wnt in different stem cell types remains to be elucidated.

Cell Cycle↗

PubMed2004

Efficient isolation and gene expression profiling of small numbers of neural crest stem cells and developing Schwann cells.

Schwann cells develop from multipotent neural crest stem cells and are important for neuronal survival, maintenance of axonal integrity, and myelination. We used transgenic mice expressing green fluorescent protein in a tissue-specific manner to isolate viable, pure populations of neural crest stem cells and developing Schwann cells, which are not readily accessible by microdissection. Starting with the minute amounts of RNA obtained, a two-round amplification procedure was used to achieve reproducible DNA array hybridizations. We validated our screening procedure by comparisons with the literature and by in situ hybridization. Stage-to-stage comparisons and hierarchical clustering for neural crest and five stages of Schwann cell development suggest a wealth of candidates for genes involved in stem cell regulation and in early Schwann cell development. The combination of methods applied in this study should be generally useful for isolating and profiling other stem cell and difficult to isolate cell populations.

Animals↗

PubMed2004

Instructive role of Wnt/beta-catenin in sensory fate specification in neural crest stem cells.

Wnt signaling has recently emerged as a key factor in controlling stem cell expansion. In contrast, we show here that Wnt/beta-catenin signal activation in emigrating neural crest stem cells (NCSCs) has little effect on the population size and instead regulates fate decisions. Sustained beta-catenin activity in neural crest cells promotes the formation of sensory neural cells in vivo at the expense of virtually all other neural crest derivatives. Moreover, Wnt1 is able to instruct early NCSCs (eNCSCs) to adopt a sensory neuronal fate in a beta-catenin-dependent manner. Thus, the role of Wnt/beta-catenin in stem cells is cell-type dependent.

Animals↗

PubMed2004

Wnt signaling and the regulation of stem cell function.

Canonical Wnt signaling plays a crucial role in controlling cell expansion in many types of stem cells. Recent studies, however, demonstrated that Wnt is not only a general stem cell growth factor but can also influence cell lineage decisions in certain stem cell types by promoting specific fates at the expense of others. Thus, Wnt signaling elicits multiple functions in stem cells. Wnt activity appears to depend on cell-intrinsic properties that might change with time during development, thereby altering the cellular response to Wnt. Moreover, the spatial context of a stem cell also determines how the cell interprets Wnt signal activity, in that synergistic or antagonistic signaling pathways can modulate Wnt signaling. How a stem cell integrates Wnt and other signals and how such signaling networks regulate stem cell function on the molecular level remains to be elucidated.

Animals↗

PubMed2002

Lineage-specific requirements of beta-catenin in neural crest development.

Beta-catenin plays a pivotal role in cadherin-mediated cell adhesion. Moreover, it is a downstream signaling component of Wnt that controls multiple developmental processes such as cell proliferation, apoptosis, and fate decisions. To study the role of beta-catenin in neural crest development, we used the Cre/loxP system to ablate beta-catenin specifically in neural crest stem cells. Although several neural crest-derived structures develop normally, mutant animals lack melanocytes and dorsal root ganglia (DRG). In vivo and in vitro analyses revealed that mutant neural crest cells emigrate but fail to generate an early wave of sensory neurogenesis that is normally marked by the transcription factor neurogenin (ngn) 2. This indicates a role of beta-catenin in premigratory or early migratory neural crest and points to heterogeneity of neural crest cells at the earliest stages of crest development. In addition, migratory neural crest cells lateral to the neural tube do not aggregate to form DRG and are unable to produce a later wave of sensory neurogenesis usually marked by the transcription factor ngn1. We propose that the requirement of beta-catenin for the specification of melanocytes and sensory neuronal lineages reflects roles of beta-catenin both in Wnt signaling and in mediating cell-cell interactions.

Animals↗

PubMed2002

Sox10 haploinsufficiency affects maintenance of progenitor cells in a mouse model of Hirschsprung disease.

Hirschsprung disease, or congenital megacolon, is characterized by aganglionosis of the terminal bowel, which leads to intestinal obstruction and chronic constipation. Several genes involved in the disease have been identified. In particular, haploinsufficiency of SOX10, which encodes a transcription factor, results in megacolon, often in combination with other disorders. Although Hirschsprung disease has been recognized as a neurocristopathy, the cellular mechanisms that lead to aganglionosis in affected individuals are unclear. Failure of mutant enteric progenitor cells to migrate into the gut, to survive, or to differentiate into appropriate cell types at the appropriate time and in correct numbers might contribute to the disease phenotype. In the present study, we use mice with a targeted deletion of Sox10 to study the etiology of Hirschsprung disease. We demonstrate that neural crest-derived enteric progenitors that are heterozygous for the Sox10 mutation colonize the proximal intestine and are unaffected in their survival capacity. However, unlike their wild-type counterparts, mutant enteric neural crest-derived cells are unable to maintain their progenitor state and acquire preneuronal traits, which results in a reduction of the progenitor pool size. Thus, the cells that normally colonize the hindgut are depleted in the Sox10 mutant, causing the distal bowel to become aganglionic.

Animals↗