Platelet-derived growth factor (PDGF)-B-deficient mouse embryos were found to lack microvascular pericytes, which normally form part of the capillary wall, and they developed numerous capillary microaneurysms that ruptured at late gestation. Endothelial cells of the sprouting capillaries in the mutant mice appeared to be unable to attract PDGF-Rbeta-positive pericyte progenitor cells. Pericytes may contribute to the mechanical stability of the capillary wall. Comparisons made between PDGF null mouse phenotypes suggest a general role for PDGFs in the development of myofibroblasts.

Platelet-derived growth factor (PDGF) affects the growth, migration, and function in vitro of mesenchymal cells, but little is known about its normal physiological functions in vivo. We show here that mice deficient for PDGF B die perinatally and display several anatomical and histological abnormalities. Kidney glomerular tufts do not form, apparently because of absence of mesangial cells. Instead, a single or a few distended capillary loops fill the glomerular space. The heart and some large arteries dilate in late-stage embryos. Most PDGF B mutant embryos develop fatal hemorrhages just prior to birth. Their hematological status includes erythroblastosis, macrocytic anemia, and thrombocytopenia. On the basis of these findings, we conclude that PDGF B has crucial roles in vivo in establishing certain renal and circulatory functions.

A mouse platelet-derived growth factor A chain (PDGF-A) null allele is shown to be homozygous lethal, with two distinct restriction points, one prenatally before E10 and one postnatally. Postnatally surviving PDGF-A-deficient mice develop lung emphysema secondary to the failure of alveolar septation. This is apparently caused by the loss of alveolar myofibroblasts and associated elastin fiber deposits. PDGF alpha receptor-positive cells in the lung having the location of putative alveolar myofibroblast progenitors were specifically absent in PDGF-A null mutants. We conclude that PDGF-A is crucial for alveolar myofibroblast ontogeny. We have previously shown that PDGF-B is required in the ontogeny of kidney mesangial cells. The PDGFs therefore appear to regulate the generation of specific populations of myofibroblasts during mammalian development. The two PDGF null phenotypes also reveal analogous morphogenetic functions for myofibroblast-type cells in lung and kidney organogenesis.

IntroductionTransforming growth factor beta (TGF-␤) is recognized as a highly pleiotropic family of growth factors involved in the regulation of numerous physiologic processes including development, hematopoiesis, wound healing, and immune response. The 3 isoforms of this growth factor that have been identified in mammals (TGF-␤1, -␤2, and -␤3) are encoded by distinct genetic loci and share a high level of homology. They act on virtually all cell types and mediate similar cellular responses in vitro, like regulation of proliferation, differentiation, apoptosis, and extracellular matrix synthesis. [1][2][3] In vivo, however, they demonstrate partly unique sets of physiologic functions due to different tissue distribution and temporal expression during development. [4][5][6] The TGF-␤ isoforms exert all their cellular functions through formation of a tetrameric complex with the 2 cell surface receptors T␤RI and T␤RII. Complex formation leads to phosphorylation of T␤RI on serine/threonine residues and propagation of the intracellular signal to the nucleus through a chain of phosphorylations of Smads, which regulate gene expression in cooperation with other transcription factors. 7 A growing body of evidence suggests TGF-␤ to be one of the major regulators of immune function, acting both by suppressive and stimulatory mechanisms on leukocytes to achieve a balanced immune response. [8][9][10] The suppressive mode of action has been highlighted by studies demonstrating inhibition of interleukin 1 (IL-1)-, IL-2-, and IL-7-dependent thymocyte proliferation by TGF-␤ 11-16 through autocrine and paracrine mechanisms, 13,17,18 whereas immunostimulatory functions were suggested by the capacity of TGF-␤ to induce cytokine expression in T cells and to promote effector expansion by inhibition of apoptosis. [19][20][21] Moreover, the influence of TGF-␤ on the development and function of other cells of the immune system, such as B cells, macrophages, and dendritic cells, has been reported. 10 Striking evidence for the importance of TGF-␤ in immune regulation was reported from studies on TGF-␤-null animals that demonstrated postnatal lethality and massive multifocal inflammation affecting multiple organs. 9,22,23 The uncontrolled inflammatory reaction has been ascribed to autoimmune mechanisms including autoantibodies and autoreactive T cells. [24][25][26][27] However, attempts to develop the phenotype by transplanting TGF-␤1-null bone marrow to healthy recipient mice unexpectedly resulted in minute inflammatory signs that did not cause clinical symptoms. 25 This raised the possibility that the presence of immune cells deficient for TGF-␤1 is not sufficient to cause the inflammatory phenotype. Alternatively, TGF-␤1-deficient donor cells may be responsive to endocrine or paracrine sources of TGF-␤1 produced by recipient tissues.Further evidence strongly suggests a role of TGF-␤ in the regulation of inflammation using dominant-negative transgenic For personal use only. on May 12, 2018. by guest www.bloodjournal.org From mouse models...

Specific inactivation of TGF␤ 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 TGF␤ 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, TGF␤ signaling is both sufficient and required for phosphorylation of CrkL, a signal adaptor protein implicated in the development of DiGeorge syndrome. Thus, TGF␤ signal modulation in neural crest differentiation might play a crucial role in the etiology of DiGeorge syndrome.Supplemental material is available at http://www.genesdev.org.Received July 16, 2004; revised version accepted December 23, 2004. During development, neural crest cells emerge from the dorsal part of the neural tube and emigrate to various locations within the embryo to generate most of the peripheral nervous system and a variety of other structures (Le Douarin and Dupin 2003). In particular, neural crest cells localized in the pharyngeal apparatus contribute to non-neural tissues, such as craniofacial bone and cartilage, thymus, parathyroid glands, and cardiac outflow tract and septum (Kirby and Waldo 1995;Jiang et al. 2000;Graham 2003). The function of neural crest cells in the generation of these tissues, however, has been debated (Graham 2003).Formation of the pharyngeal apparatus involves complex interactions of neural crest, ectoderm, endoderm, and mesoderm whose development must be coordinated (Graham 2003). Significantly, alterations to the development of this region are often associated with congenital human birth defects such as DiGeorge or Velocardiofacial syndrome. DiGeorge syndrome is the most common microdeletion syndrome in humans, characterized by cardiovascular defects plus thymic, parathyroid, and craniofacial anomalies (Lindsay 2001;Vitelli and Baldini 2003). Approximately 80% of the patients carry a variably sized deletion on chromosome 22 (del22q11). Ablation of genes affected by the microdeletion indicated two pathophysiological mechanisms causing DiGeorge syndrome: Mutations of the transcription factor Tbx1 lead to disturbed pharyngeal arch patterning. Consequently, neural crest cells are unable to populate the pharyngeal apparatus ( Previously, cell culture experiments allowed the identification of several growth factors able to instruct migratory and post-migratory neural crest cells to adopt specific lineages (Le Douarin and Dupin 2003;Lee et al. 2004). One of these factors is transforming growth factor (TGF) ␤, which elicits multiple responses in cultured neural crest stem cells (NCSCs). Depending on the cellular context, it can promote the generation of non-neural smooth-muscle-like cells or autonomic neurons, or induce apoptosis (Shah et al. 1996;Hagedorn et al. 1999Hagedorn et al. , 2000. Here we investigated the role of TGF␤ si...

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Regulating the choice between neural stem cell maintenance versus differentiation determines growth and size of the developing brain. Here we identify TGF-beta signaling as a crucial factor controlling these processes. At early developmental stages, TGF-beta signal activity is localized close to the ventricular surface of the neuroepithelium. In the midbrain, but not in the forebrain, Tgfbr2 ablation results in ectopic expression of Wnt1/beta-catenin and FGF8, activation of Wnt target genes, and increased proliferation and horizontal expansion of neuroepithelial cells due to shortened cell-cycle length and decreased cell-cycle exit. Consistent with this phenotype, self-renewal of mutant neuroepithelial stem cells is enhanced in the presence of FGF and requires Wnt signaling. Moreover, TGF-beta signal activation counteracts Wnt-induced proliferation of midbrain neuroepithelial cells. Thus, TGF-beta signaling controls the size of a specific brain area, the dorsal midbrain, by antagonizing canonical Wnt signaling and negatively regulating self-renewal of neuroepithelial stem cells.

The ribosomal protein S19 (RPS19) is located in the small (40S) subunit and is one of 79 ribosomal proteins. The gene encoding RPS19 is mutated in approximately 25% of patients with Diamond-Blackfan anemia, which is a rare congenital erythroblastopenia. Affected individuals present with decreased numbers or the absence of erythroid precursors in the bone marrow, and associated malformations of various organs are common. We produced C57BL/6J mice with a targeted disruption of murine Rps19 to study its role in erythropoiesis and development. Mice homozygous for the disrupted Rps19 were not identified as early as the blastocyst stage, indicating a lethal effect. In contrast, mice heterozygous for the disrupted Rps19 allele have normal growth and organ development, including that of the hematopoietic system. Our findings indicate that zygotes which are Rps19 ؊/؊ do not form blastocysts, whereas one normal Rps19 allele in C57BL/6J mice is sufficient to maintain normal ribosomal and possibly extraribosomal functions.The ribosomal proteins constitute a major component of cellular proteins and are known to be mandatory for cellular growth. A reduced level of one ribosomal protein is rate limiting for the assembly of ribosomes and may constitute a bottleneck for protein synthesis in tissues with a high proliferative activity. Recently, Draptchinskaia et al. found that the gene encoding ribosomal protein S19 (RPS19) was mutated in 25% of patients with Diamond-Blackfan anemia (DBA) (5). DBA is a rare, congenital, and chronic anemia that is characterized by the absence or decreased numbers of erythroid precursors in the bone marrow but an otherwise normal cellularity (1, 4). Approximately 30% of affected patients with DBA show one or several dysmorphic features including growth retardation, hand and/or limb malformations, urogenital anomalies, and congenital heart defects (1, 2, 10). Clinical expression in DBA is highly variable (14,21), and the role of RPS19 in the pathogenesis of the disease is presently unknown.RPS19 is part of the 40S ribosomal unit and shows a high degree of sequence conservation across mammalian species at the protein level (5). In addition to its implication in erythropoiesis, there are indications that RPS19 has extraribosomal functions.A previous study has shown that a dimer of RPS19 may mediate chemotaxis (13). Free RPS19 has also been shown to interact with fibroblast growth factor 2 (FGF-2), and a possible role for RPS19 in embryonic development has been suggested (17). We hypothesized that an Rps19 null mutant would be deleterious for development and that mice heterozygous for Rps19 may present with hematological abnormalities. To clarify the role of RPS19 in erythropoiesis, we created a null mutation for murine Rps19 by homologous recombination in embryonic stem (ES) cells. The targeted Rps19 was introduced on a C57BL/6J background. We present here the results from our studies of this mouse model. MATERIALS AND METHODSConstruction of the Rps19 targeting vector. An Rps19 intron probe was used t...