[Imaging of neuroendocrine tumors of the gastrointestinal tract : Value of (hybrid) imaging diagnostics in radiology]. / Bildgebung von neuroendokrinen Tumoren des Gastrointestinaltrakts : Bedeutung der (hybriden) bildgebenden Diagnostik in der Radiologie.

CLINICAL/METHODICAL ISSUE: Neuroendocrine tumors (NET) represent a heterogeneous group of rare tumors that predominantly arise in the gastrointestinal tract. At the time of initial diagnosis, the NET has already spread locoregionally in about half of the patients, and 27% of patients have already developed distant metastases. Since this plays a crucial role in therapy planning, accurate diagnostic imaging is important. STANDARD RADIOLOGICAL METHODS: Due to its high temporal and spatial resolution (multiphasic including arterial phase), computed tomography (CT) plays a decisive role in primary staging and follow-up care, while magnetic resonance imaging (MRI) with its excellent soft tissue contrast offers advantages in the assessment of parenchymal organs in the upper abdomen. METHODICAL INNOVATIONS: Somatostatin receptor (SSR) positron emission tomography (PET) provides additional functional information that not only helps to detect the primary tumor and distant metastases, but also has a significant influence on therapeutic management in a theranostic approach. PERFORMANCE: Hybrid imaging using SSR-PET/CT has proven to be particularly effective in the detection of NET. Compared to conventional imaging, it provides additional information in 68% of patients, which has a significant impact on clinical management. ACHIEVEMENTS: Imaging of NET requires the combined use of various methods such as ultrasound, CT, MRI, and PET/CT to enable accurate diagnosis and effective treatment planning. PRACTICAL RECOMMENDATIONS: SSR-PET/CT is a valuable tool for the accurate staging of neuroendocrine tumors of the gastrointestinal tract, especially with small metastases, while MRI with hepatocyte-specific contrast agent and diffusion-weighted imaging is useful for the specific assessment of liver metastases.

Validation of the standardization framework SSTR-RADS 1.0 for neuroendocrine tumors using the novel SSTR­targeting peptide [18F]SiTATE.

OBJECTIVES: Somatostatin receptor positron emission tomography/computed tomography (SSTR-PET/CT) using [68Ga]-labeled tracers is a widely used imaging modality for neuroendocrine tumors (NET). Recently, [18F]SiTATE, a SiFAlin tagged [Tyr3]-octreotate (TATE) PET tracer, has shown great potential due to favorable clinical characteristics. We aimed to evaluate the reproducibility of Somatostatin Receptor-Reporting and Data System 1.0 (SSTR-RADS 1.0) for structured interpretation and treatment planning of NET using [18F]SiTATE. METHODS: Four readers assessed [18F]SiTATE-PET/CT of 95 patients according to the SSTR-RADS 1.0 criteria at two different time points. Each reader evaluated up to five target lesions per scan. The overall scan score and the decision on peptide receptor radionuclide therapy (PRRT) were considered. Inter- and intra-reader agreement was determined using the intraclass correlation coefficient (ICC). RESULTS: The ICC analysis on the inter-reader agreement using SSTR-RADS 1.0 for identical target lesions (ICC ≥ 85%), overall scan score (ICC ≥ 90%), and the decision to recommend PRRT (ICC ≥ 85%) showed excellent agreement. However, significant differences were observed in recommending PRRT among experienced readers (ER) (p = 0.020) and inexperienced readers (IR) (p = 0.004). Compartment-based analysis demonstrated good to excellent inter-reader agreement for most organs (ICC ≥ 74%), except for lymph nodes (ICC ≥ 53%). CONCLUSION: SSTR-RADS 1.0 represents a highly reproducible and consistent framework system for stratifying SSTR-targeted PET/CT scans, even using the novel SSTR-ligand [18F]SiTATE. Some inter-reader variability was observed regarding the evaluation of uptake intensity prior to PRRT as well as compartment scoring of lymph nodes, indicating that those categories require special attention during further clinical validation and might be refined in a future SSTR-RADS version 1.1. CLINICAL RELEVANCE STATEMENT: SSTR-RADS 1.0 is a consistent framework for categorizing somatostatin receptor-targeted PET/CT scans when using [18F]SiTATE. The framework serves as a valuable tool for facilitating and improving the management of patients with NET. KEY POINTS: SSTR-RADS 1.0 is a valuable tool for managing patients with NET. SSTR-RADS 1.0 categorizes patients with showing strong agreement across diverse reader expertise. As an alternative to [68Ga]-labeled PET/CT in neuroendocrine tumor imaging, SSTR-RADS 1.0 reliably classifies [18F]SiTATE-PET/CT.

Diagnostic accuracy of SSR-PET/CT compared to histopathology in the identification of liver metastases from well-differentiated neuroendocrine tumors.

BACKGROUND: Histopathology is the reference standard for diagnosing liver metastases of neuroendocrine tumors (NETs). Somatostatin receptor-positron emission tomography / computed tomography (SSR-PET/CT) has emerged as a promising non-invasive imaging modality for staging NETs. We aimed to assess the diagnostic accuracy of SSR-PET/CT in the identification of liver metastases in patients with proven NETs compared to histopathology. METHODS: Histopathologic reports of 139 resected or biopsied liver lesions of patients with known NET were correlated with matching SSR-PET/CTs and the positive/negative predictive value (PPV/NPV), sensitivity, specificity, and diagnostic accuracy of SSR-PET/CT were evaluated. PET/CT reading was performed by one expert reader blinded to histopathology and clinical data. RESULTS: 133 of 139 (95.7%) liver lesions showed malignant SSR-uptake in PET/CT while initial histopathology reported on 'liver metastases of NET´ in 127 (91.4%) cases, giving a PPV of 91.0%. Re-biopsy of the initially histopathologically negative lesions (reference standard) nevertheless diagnosed 'liver metastases of NET' in 6 cases, improving the PPV of PET/CT to 95.5%. Reasons for initial false-negative histopathology were inadequate sampling in the sense of non-target biopsies. The 6 (4.3%) SSR-negative lesions were all G2 NETs with a Ki-67 between 2-15%. CONCLUSION: SSR-PET/CT is a highly accurate imaging modality for the diagnosis of liver metastases in patients with proven NETs. However, we found that due to the well-known tumor heterogeneity of NETs, specifically in G2 NETs approximately 4-5% are SSR-negative and may require additional imaging with [18F]FDG PET/CT.

Identification of SOX2 as a novel glioma-associated antigen and potential target for T cell-based immunotherapy.

Prognosis for patients suffering from malignant glioma has not substantially improved. Specific immunotherapy as a novel treatment concept critically depends on target antigens, which are highly overexpressed in the majority of gliomas, but the number of such antigens is still very limited. SOX2 was identified by screening an expression database for transcripts that are overexpressed in malignant glioma, but display minimal expression in normal tissues. Expression of SOX2 mRNA was further investigated in tumour and normal tissues by real-time PCR. Compared to cDNA from pooled normal brain, SOX2 was overexpressed in almost all (9 out of 10) malignant glioma samples, whereas expression in other, non-malignant tissues was almost negligible. SOX2 protein expression in glioma cell lines and tumour tissues was verified by Western blot and immunofluorescence. Immunohistochemistry demonstrated SOX2 protein expression in all malignant glioma tissues investigated ranging from 6 to 66% stained tumour cells. Human leucocyte antigen-A(*)0201-restricted SOX2-derived peptides were tested for the activation of glioma-reactive CD8+ cytotoxic T lymphocytes (CTLs). Specific CTLs were raised against the peptide TLMKKDKYTL and were capable of lysing glioma cells. The abundant and glioma-restricted overexpression of SOX2 and the generation of SOX2-specific and tumour-reactive CTLs may recommend this antigen as target for T-cell-based immunotherapy of glioma.

A successful approach for the detection of Fabry patients in Argentina.

Fabry disease is an X-linked lysosomal disorder caused by the deficiency of the lysosomal enzyme alpha-galactosidase A (alpha-Gal A). In males, the laboratory diagnosis is based on the demonstration of decreased levels of alpha-Gal A activity, while in females, the disease is diagnosed by the identification of a mutation in alpha-Gal A gene. Fabry disease in Argentina is underdiagnosed. To date, no comprehensive screening study of Fabry disease in our country has been reported. The present study aimed at developing a targeted screening for the detection of Fabry patients from Argentina based on the set of typical signs and symptoms. We received 121 blood samples from probable Fabry patients for enzymatic and genetic assay. We diagnosed six Fabry patients from six unrelated families, representing a yield of detection of 4.96%. The mutations detected in five of the families analysed were missense mutations: p.Leu243Trp, p.Asp155His, p.Leu415Pro, p.Cys94Tyr and p.Leu191Pro. After the detection of a Fabry patient, his/her relatives were also screened. In the course of these family studies, other 64 Fabry patients, 29 males and 35 females, were detected. To our knowledge, this is the first comprehensive screening of Fabry disease in Argentina. We detected 70 patients in a period of 2.5 years. The development of targeted protocols and the constitution of interdisciplinary groups for the identification of patients with Fabry disease are recommended to obtain a higher yield in the process.

True, true and related?

Several chorioretinal lesions have been observed that are associated with bone marrow transplantation (BMT), such as cotton-wool spots, macular stars, ischemic changes due to microangiopathy, "BMT retinopathy" and choroidal infiltration. Central serous retinopathy (CSR) has rarely been described in the BMT setting. We present a patient who underwent allogeneic BMT and subsequently developed severe chronic graft versus host disease (CGvHD) complicated with CSR.

Spontaneous regression of optic gliomas: thirteen cases documented by serial neuroimaging.

OBJECTIVE: To demonstrate spontaneous regression of large, clinically symptomatic optic pathway gliomas in patients with and without neurofibromatosis type 1 (NF-1). METHODS: Patient cases were collected through surveys at 2 consecutive annual meetings of the North American Neuro-Ophthalmology Society (NANOS) and through requests on the NANOSNET Internet listserv. Serial documentation of tumor signal and size, using magnetic resonance imaging in 11 patients and computed tomography in 2 patients, was used to evaluate clinically symptomatic optic pathway gliomas. All tumors met radiologic criteria for the diagnosis of glioma and 4 patients had biopsy confirmation of their tumors. In 3 patients, some attempt at therapy had been made many years before regression occurred. In one of these, radiation treatment had been given 19 years before tumor regression, while in another, chemotherapy had been administered 5 years before signal changes in the tumor. In the third patient, minimal surgical debulking was performed 1 year before the tumor began to shrink. RESULTS: Spontaneous tumor shrinkage was noted in 12 patients. Eight patients did not have NF-1. In an additional patient without NF-1, a signal change within the tumor without associated shrinkage was detected. Tumor regression was associated with improvement in visual function in 10 of 13 patients, stability of function in 1, and deterioration in 2. CONCLUSIONS: Large, clinically symptomatic optic gliomas may undergo spontaneous regression. Regression was seen in patients with and without NF-1. Regression may manifest either as an overall shrinkage in tumor size, or as a signal change on magnetic resonance imaging. A variable degree of improvement in visual function may accompany regression. The possibility of spontaneous regression of an optic glioma should be considered in the planning of treatment of patients with these tumors.

Optic neuropathy associated with laser in situ keratomileusis.

PURPOSE: To report 4 cases of optic neuropathy following laser in situ keratomileusis (LASIK). SETTING: Tertiary Care ophthalmic practices. METHODS: In this retrospective observational case series, 4 patients who developed acute visual loss following LASIK are reported. All had clinical evidence of optic neuropathy. Two had optic disc edema and 2 had normal appearing optic discs initially. None of the patients experienced significant visual recovery, and all developed optic atrophy in the affected eye. RESULTS: All patients had evaluations for alternative etiologies of their optic neuropathy, with negative results. All patients were therefore presumed to have experienced an ischemic optic neuropathy following LASIK. CONCLUSIONS: Patients who have LASIK may experience an acute anterior or retrobulbar optic neuropathy. The etiology is unknown but may be related to the marked increase in intraocular pressure that occurs during a portion of the procedure.

Bilateral and simultaneous ischemic optic neuropathy following hip replacement surgery.

Ischemic optic neuropathy, is an exceptional complication of surgery. Moreover, bilateral and simultaneous visual deficit in ischemic optic neuropathy is very rare. We describe two patients who suffered bilateral and simultaneous ischemic optic neuropathy after elective total hip replacement. Anemia and hypotension are the most likely risk factors.

Characterization of MAX.3 antigen, a glycoprotein expressed on mature macrophages, dendritic cells and blood platelets: identity with CD84.

MAX.3 is a monoclonal antibody that preferentially reacts with mature macrophages (MAC), monocyte-derived dendritic cells, megakaryocytes and platelets. In this study, we describe the characterization, purification and identification of the MAX.3 antigen. Immunoprecipitation and SDS/PAGE revealed different molecular masses of MAX.3 antigen in MAC (60-90 kDa) and platelets (58-64 kDa), whereas a similar size (45 kDa) was observed in both cell types after digestion with N-glycosidase F. Lectin affinity and sequential treatment with different glycosidases suggests complex type glycosylation of MAX.3 antigen in MAC and hybrid type glycosylation in platelets. Amino acid sequencing led to the identification of a corresponding cDNA clone and showed its identity to the sequence of the CD84 antigen, a member of the CD2 family of cell surface molecules. MAX.3/CD84 was further studied by immunohistochemistry and a variable expression was found on tissue MAC, confirming this antigen to be mainly a marker for MAC in situ.

A "stealth effect": adenocarcinoma cells engineered to express TRAIL elude tumor-specific and allogeneic T cell reactions.

BALB/c mammary adenocarcinoma cells engineered to express TNF-related apoptosis-inducing ligand (TRAIL)/APO-2 ligand (APO-2L) on their membrane (TSA-TRAIL) grow with kinetics similar to that of parental cells (TSA-pc) in vitro and in nu/nu mice. In contrast, TSA-TRAIL cells grow faster than TSA-pc in normal BALB/c mice. In DBA/2 mice, which differ from BALB/c mice at minor histocompatibility Ags, they also grow faster and display a higher percentage of tumor takes than TSA-pc. In fully histoincompatible C57BL/6 (B6) mice, TSA-TRAIL cells form evident tumors that are slowly rejected by most mice, but outgrow in a few. In contrast, TSA-pc cells are rejected at once by B6 mice. Since TRAIL/APO-2L induces apoptosis by interacting with a variety of specific receptors, this rapid growth in both syngeneic and allogeneic mice may be the result of an immunosuppressive mechanism. The following evidence supports this hypothesis: 1) TSA-TRAIL cells overcome the strong immunity against TSA-pc cells elicited in BALB/c mice by preimmunization with TSA cells engineered to release IL-4; 2) their rejection by B6 mice does not prime a CTL-mediated memory; 3) thymidine uptake by T lymphocytes unstimulated or stimulated by allogeneic cells is inhibited when TSA-TRAIL cells are added as third party cells; 4) CTL kill TSA-pc but not TSA-TRAIL cells in 48-h assays; and 5) activated lymphocytes interacting with TSA-TRAIL cells in vivo and in vitro undergo apoptosis.

A newly identified member of tumor necrosis factor receptor superfamily (TR6) suppresses LIGHT-mediated apoptosis.

TR6 (decoy receptor 3 (DcR3)) is a new member of the tumor necrosis factor receptor (TNFR) family. TR6 mRNA is expressed in lung tissues and colon adenocarcinoma, SW480. In addition, the expression of TR6 mRNA was shown in the endothelial cell line and induced by phorbol 12-myristate 13-acetate/ionomycin in Jurkat T leukemia cells. The open reading frame of TR6 encodes 300 amino acids with a 29-residue signal sequence but no transmembrane region. Using histidine-tagged recombinant TR6, we screened soluble forms of TNF-ligand proteins with immunoprecipitation. Here, we demonstrate that TR6 specifically binds two cellular ligands, LIGHT (herpes virus entry mediator (HVEM)-L) and Fas ligand (FasL/CD95L). These bindings were confirmed with HEK 293 EBNA cells transfected with LIGHT cDNA by flow cytometry. TR6 inhibited LIGHT-induced cytotoxicity in HT29 cells. It has been shown that LIGHT triggers apoptosis of various tumor cells including HT29 cells that express both lymphotoxin beta receptor (LTbetaR) and HVEM/TR2 receptors. Our data suggest that TR6 inhibits the interactions of LIGHT with HVEM/TR2 and LTbetaR, thereby suppressing LIGHT- mediated HT29 cell death. Thus, TR6 may play a regulatory role for suppressing in FasL- and LIGHT-mediated cell death.

Bone loss (osteopenia) in old male mice results from diminished activity and availability of TGF-beta.

One of the universal characteristics of the long bones and spines of middle-age and older mammals is a loss in bone mass (osteopenia). In humans, if this bone loss is severe enough, it results in osteoporosis, a skeletal disorder characterized by a markedly increased incidence of fractures with sequelae that may include pain, loss of mobility, and in the event of hip fracture, even death within a relatively few months of injury. An important contributing factor to the development of osteoporosis appears to be a diminution in the number and activity of osteoblasts responsible for synthesizing new bone matrix. The findings in the present and other similar studies suggest that this reduction in osteoblast number and activity is due to an age-related diminution in the size and osteogenic potential of the bone marrow osteoblast progenitor cell (OPC or CFU-f) compartment. We previously postulated that these regressive changes in the OPC/CFU-f compartment occurred in old animals because of a reduction in the amount and/or activity of TGF-beta1, an autocrine growth factor important in the promotion of OPC/CFU-f proliferation and differentiation. In support of this hypothesis, we now report that (1) the osteogenic capacity of the bone marrow of 24-month-old BALB/c mice, as assessed in vivo, is markedly reduced relative to that of 3-4-month-old animals, (2) that the matrix of the long bones of old mice contains significantly less TGF-beta than that of young mice, (3) that OPC's/CFU-f's isolated from old mice produce less TGF-beta in vitro than those recovered from young mice, and (4) that OPC's/CFU-f's from old mice express significantly more TGF-beta receptor (Types I, II, and III) than those of young animals and that such cells are more responsive in vitro to exogenous recombinant TGF-beta1. We also find that colony number and proliferative activity of OPC's/CFU-f's of young mice and old mice, respectively, are significantly reduced when incubated in the presence of neutralizing TGF-beta1 antibody. Collectively, these data are consistent with the hypothesis that in old male mice the reduction in the synthesis and, perhaps, availability from the bone matrix of TGF-beta1 contributes to a diminution in the size and development potential of the bone marrow osteoprogenitor pool.

LIGHT, a new member of the TNF superfamily, and lymphotoxin alpha are ligands for herpesvirus entry mediator.

Herpes simplex virus (HSV) 1 and 2 infect activated T lymphocytes by attachment of the HSV envelope glycoprotein D (gD) to the cellular herpesvirus entry mediator (HVEM), an orphan member of the tumor necrosis factor receptor superfamily. Here, we demonstrate that HVEM binds two cellular ligands, secreted lymphotoxin alpha (LTalpha) and LIGHT, a new member of the TNF superfamily. LIGHT is a 29 kDa type II transmembrane protein produced by activated T cells that also engages the receptor for the LTalphabeta heterotrimer but does not form complexes with either LTalpha or LTbeta. HSV1 gD inhibits the interaction of HVEM with LIGHT, and LIGHT and gD interfere with HVEM-dependent cell entry by HSV1. This characterizes herpesvirus gD as a membrane-bound viokine and establishes LIGHT-HVEM as integral components of the lymphotoxin cytokine-receptor system.

The receptor for the cytotoxic ligand TRAIL.

TRAIL (also known as Apo-2L) is a member of the tumor necrosis factor (TNF) ligand family that rapidly induces apoptosis in a variety of transformed cell lines. The human receptor for TRAIL was found to be an undescribed member of the TNF-receptor family (designated death receptor-4, DR4) that contains a cytoplasmic "death domain" capable of engaging the cell suicide apparatus but not the nuclear factor kappa B pathway in the system studied. Unlike Fas, TNFR-1, and DR3, DR4 could not use FADD to transmit the death signal, suggesting the use of distinct proximal signaling machinery. Thus, the DR4-TRAIL axis defines another receptor-ligand pair involved in regulating cell suicide and tissue homeostasis.

Visual loss following treatment of sphenoid sinus carcinoma.

A 71-year-old woman developed complete third nerve palsy and total blindness of the right eye one month after completing a course of radiotherapy for sphenoid sinus carcinoma over a 13-month period. Differential diagnosis included recurrence of the tumor, radiation-induced second neoplasm, empty sella with chiasmal prolapse and secondary chiasmal arachnoid adhesions, and radionecrosis. Magnetic resonance imaging demonstrated gadolinium contrast enhancement of the right intracranial optic nerve and chiasm, suggesting a radionecrosis process.

TGF-beta induced transdifferentiation of mammary epithelial cells to mesenchymal cells: involvement of type I receptors.

The secreted polypeptide transforming growth factor-beta (TGF-beta) exerts its multiple activities through type I and II cell surface receptors. In epithelial cells, activation of the TGF-beta signal transduction pathways leads to inhibition of cell proliferation and an increase in extracellular matrix production. TGF-beta is widely expressed during development and its biological activity has been implicated in epithelial-mesenchymal interactions, e.g., in branching morphogenesis of the lung, kidney, and mammary gland, and in inductive events between mammary epithelium and stroma. In the present study, we investigated the effects of TGF-beta on mouse mammary epithelial cells in vitro. TGF-beta reversibly induced an alteration in the differentiation of normal mammary epithelial NMuMG cells from epithelial to fibroblastic phenotype. The change in cell morphology correlated with (a) decreased expression of the epithelial markers E-cadherin, ZO-1, and desmoplakin I and II; (b) increased expression of mesenchymal markers, such as fibronectin; and (c) a fibroblast-like reorganization of actin fibers. This phenotypic differentiation displays the hallmarks of an epithelial to mesenchymal transdifferentiation event. Since NMuMG cells make high levels of the type I TGF-beta receptor Tsk7L, yet lack expression of the ALK-5/R4 type I receptor which has been reported to mediate TGF-beta responsiveness, we evaluated the role of the Tsk7L receptor in TGF-beta-mediated transdifferentiation. We generated NMuMG cells that stably overexpress a truncated Tsk7L type I receptor that lacks most of the cytoplasmic kinase domain, thus function as a dominant negative mutant. These transfected cells no longer underwent epithelial to mesenchymal morphological change upon exposure to TGF-beta, yet still displayed some TGF-beta-mediated responses. We conclude that TGF-beta has the ability to modulate E-cadherin expression and induce a reversible epithelial to mesenchymal transdifferentiation in epithelial cells. Unlike other transdifferentiating growth factors, such as bFGF and HGF, these changes are accompanied by growth inhibition. Our results also implicate the Tsk7L type I receptor as mediating the TGF-beta-induced epithelial to mesenchymal transition.

Peripheral ocular motor disorders.

In the reviewed period, articles on peripheral eye movement disorders covered interesting aspects. Localizing value of associated signs, repetitive presentations of palsies, and classical quotations are stressed for the oculomotor nerve. The superior oblique is correlated to central nervous system disorders when overacting in pediatric patients or when ocular torsion is matched to the perceived vertical tilt. The family "pseudo" brought two of its members: "pseudo" myasthenia and "pseudo" myotonia. Mitochondrial citopathies with ocular manifestations can overlap among the different clinical types, eg, Kearns-Sayre, MELAS (mitochondrial encephalopathy-lactic acidosis and strokelike episodes), MERFF (myoclonic epilepsy and ragged red fibers). The diagnostic value of DNA mutations is emphasized in those syndromes. Imaging of the carotid arteries provides useful hints in cases where the lumen is narrowed due to internal processes or external compression; its interpretation is not only of diagnostic but of prognostic value. Certain otorhinolaryngology surgical procedures can damage the orbital muscles and produce serious inconvenience to the ocular motility. Analyzing the involved structures the therapeutic gesture can be determined. Diplopia after cataract surgery or retinal detachment repair is due to different factors, anesthetics, or implant location and is implied in every case.

Inhibition of myogenic differentiation in myoblasts expressing a truncated type II TGF-beta receptor.

Transforming growth factor-beta (TGF-beta) is thought to play a role in mesenchymal cell development and, specifically, in muscle differentiation, yet its precise role in the latter process remains unclear. TGF-beta has been shown to both inhibit and induce myoblast maturation in vitro, depending on the culture conditions. Whether the type I or type II TGF-beta receptor mediates the various TGF-beta effects on myogenesis is not known. In the present study, C2C12 myoblasts were transfected with an expression vector for a truncated type II TGF-beta receptor, which has been shown to act as a dominant negative inhibitor of type II receptor signaling. In contrast to the parental cells, the transfected clones did not efficiently form myotubes or induce expression of MyoD, myogenin and several other differentiation markers following incubation in low serum media. However, some muscle differentiation markers continued to be expressed in the transfected cells suggesting that at least two pathways are involved in muscle cell differentiation. These cells could still growth arrest in low serum media, showing that decreased proliferation can be dissociated from differentiation. Unlike several oncogenes known to block myogenic differentiation, expression of the truncated TGF-beta receptor did not result in myoblast transformation. Injection of the parental or the transfected C2C12 cells into the limb muscle of nude mice revealed quantitative and qualitative differences in their behavior, and suggested that myoblasts expressing the truncated TGF-beta receptor cannot fuse in vivo. Finally, retrovirus-mediated expression of MyoD in the transfected cells restored their ability to form myotubes in vitro, indicating that inhibition of myoblast differentiation by the truncated TGF-beta receptor may depend on decreased MyoD expression. We propose that TGF-beta signaling through the type II receptor is required for several distinct aspects of myogenic differentiation and that TGF-beta acts as a competence factor in this multistep process.