Regulation of actin function by protein kinase A-mediated phosphorylation of Limk1.

Proper regulation of the cAMP-dependent protein kinase (protein kinase A, PKA) is necessary for cellular homeostasis, and dysregulation of this kinase is crucial in human disease. Mouse embryonic fibroblasts (MEFs) lacking the PKA regulatory subunit Prkar1a show altered cell morphology and enhanced migration. At the molecular level, these cells showed increased phosphorylation of cofilin, a crucial modulator of actin dynamics, and these changes could be mimicked by stimulating the activity of PKA. Previous studies of cofilin have shown that it is phosphorylated primarily by the LIM domain kinases Limk1 and Limk2, which are under the control of the Rho GTPases and their downstream effectors. In Prkar1a(-/-) MEFs, neither Rho nor Rac was activated; rather, we showed that PKA could directly phosphorylate Limk1 and thus enhance the phosphorylation of cofilin. These data indicate that PKA is crucial in cell morphology and migration through its ability to modulate directly the activity of LIM kinase.

Mutation of Prkar1a causes osteoblast neoplasia driven by dysregulation of protein kinase A.

Carney complex (CNC) is an autosomal dominant neoplasia syndrome caused by inactivating mutations in PRKAR1A, the gene encoding the type 1A regulatory subunit of protein kinase A (PKA). This genetic defect induces skin pigmentation, endocrine tumors, myxomas, and schwannomas. Some patients with the complex also develop myxoid bone tumors termed osteochondromyxomas. To study the link between the PRKAR1A mutations and tumor formation, we generated a mouse model of this condition. Prkar1a(+/-) mice develop bone tumors with high frequency, although these lesions have not yet been characterized, either from human patients or from mice. Bone tumors from Prkar1a(+/-) mice were heterogeneous, including elements of myxomatous, cartilaginous, and bony differentiation that effaced the normal bone architecture. Immunohistochemical analysis identified an osteoblastic origin for the abnormal cells associated with islands of bone. To better understand these cells at the biochemical level, we isolated primary cultures of tumoral bone and compared them with cultures of bone from wild-type animals. The tumor cells exhibited the expected decrease in Prkar1a protein and exhibited increased PKA activity. At the phenotypic level, we observed that tumor cells behaved as incompletely differentiated osteoblasts and were able to form tumors in immunocompromised mice. Examination of gene expression revealed down-regulation of markers of bone differentiation and increased expression of locally acting growth factors, including members of the Wnt signaling pathway. Tumor cells exhibited enhanced growth in response to PKA-stimulating agents, suggesting that tumorigenesis in osteoblast precursor cells is driven by effects directly mediated by the dysregulation of PKA.

Analysis of a new allele of limb deformity (ld) reveals tissue- and age-specific transcriptional effects of the Ld Global Control Region.

The mouse limb deformity (ld) phenotype is characterized by developmental failure of distal limb structures often associated with renal anomalies. It is caused by loss of the BMP-antagonist Gremlin in the limb buds, either through mutation of Grem1, or by loss of a transcriptional global control region (GCR) located in the neighboring Fmn1 gene. In this report, we describe a new allele of ld due to complete deletion of Fmn1, including its GCR. Unlike many other ld strains, these mice are viable and fertile as homozygotes. As expected, this genomic deletion causes loss of Gremlin in the developing limb buds, but effects in other tissues are variable. Specifically, Grem1 expression is retained in the developing lung and kidney, whereas expression is lost from the corresponding adult tissues. In contrast, expression in the brain appears to be unaffected by loss of the GCR. To provide information about long-range transcriptional effects of this region, effects of the deletion on the transcription of neighboring genes were also investigated. This analysis revealed that alterations in neighboring genes do occur, but only in a limited fashion. These data indicate that the predominant effect of the Ld GCR is to activate the expression of Grem1 in the developing limb buds, although it may serve a minor role in long-range transcriptional effects that extend beyond Fmn1 and Grem1.