Thyroid hormone (TH) deficiency during development causes severe and permanent neuronal damage, but the primary insult at the tissue level has remained unsolved. We have defined locomotor deficiencies in mice caused by a mutant thyroid hormone receptor ␣1 (TR␣1) with potent aporeceptor activity attributable to reduced affinity to TH. This allowed identification of distinct functions that required either maternal supply of TH during early embryonic development or sufficient innate levels of hormone during late fetal development. In both instances, continued exposure to high levels of TH after birth and throughout life was needed. The hormonal dependencies correlated with severely delayed appearance of parvalbumin-immunoreactive GABAergic interneurons and increased numbers of calretinin-immunoreactive cells in the neocortex. This resulted in reduced numbers of fast spiking interneurons and defects in cortical network activity. The identification of locomotor deficiencies caused by insufficient supply of TH during fetal/perinatal development and their correlation with subtype-specific interneurons suggest a previously unknown basis for the neuronal consequences of endemic cretinism and untreated congenital hypothyroidism, and specifies TR␣1 as the receptor isoform mediating these effects.

Thyroid hormone is essential for brain development where it acts mainly through the thyroid hormone receptor α1 (TRα1) isoform. However, the potential for the hormone to act in adult neurons has remained undefined due to difficulties in reliably determining the expression pattern of TR proteins in vivo. We therefore created a mouse strain that expresses TRα1 and green fluorescent protein as a chimeric protein from the Thra locus, allowing examination of TRα1 expression during fetal and postnatal development and in the adult. Furthermore, the use of antibodies against other markers enabled identification of TRα1 expression in subtypes of neurons and during specific stages of their maturation. TRα1 expression was first detected in postmitotic cells of the cortical plate in the embryonic telencephalon and preceded the expression of the mature neuronal protein NeuN. In the cerebellum, TRα1 expression was absent in proliferating cells of the external granular layer, but switched on as the cells migrated towards the internal granular layer. In addition, TRα1 was expressed transiently in developing Purkinje cells, but not in mature cells. Glial expression was found in tanycytes in the hypothalamus and in the cerebellum. In the adult brain, TRα1 expression was detected in essentially all neurons. Our data demonstrate that thyroid hormone, unexpectedly, has the capacity to play an important role in virtually all developing and adult neurons. Because the role of TRα1 in most neuronal cell types in vivo is largely unknown, our findings suggest that novel functions for thyroid hormone remain to be identified in the brain.

T he Rel͞NF-B transcription factors have a central role in several cellular processes, including proliferation, cell adhesion, apoptosis, and regulation of the immune response (1). Under nonstimulating conditions, NF-B is retained in the cytoplasm in an inactive form because of its interaction with the inhibitory proteins I Bs. In response to an activating stimulus, I B is phosphorylated by an I B kinase (IKK) complex, which targets it for degradation by the proteasome, releasing NF-B, which translocates to the nucleus, where it regulates the transcription of target genes.Evidence gathered from studies during recent years have shown that NF-B has a growth inhibitory function in the skin. Initial studies showed that overexpression of the NF-B subunits p50 or p65 in the basal layer of the murine epidermis by using a keratin 14 (K14) promoter leads to hypoplasia of the epidermis, whereas expression of a superrepressor form of the inhibitor of NF-B type ␣ (I B␣) results in hyperplasia (2). Although K14-I B␣ mice die shortly after birth (day 7), we have shown that mice with keratin 5 (K5)-directed expression of I B␣ (K5-I B␣) to the basal layer of the epidermis survive to adulthood (3). Not only do these mice develop hyperplasia of the epidermis, but the skin phenotype of K5-I B␣ mice is also characterized by an intense neutrophil-dominated inflammation of the skin and an early development of squamous cell carcinomas (SCC).Recent data suggest that human sporadic SCC may show a block in NF-B signaling based on nuclear exclusion of the RelA NF-B subunit (ref. 4 and M.v.H., unpublished results). Furthermore, it was recently shown that coexpression of a superrepressor form of I B␣ and Ha-Ras results in neoplasia resembling invasive SCC in human keratinocytes transplanted to severe combined immunodeficient (scid͞scid) mice, supporting the relevance of a NF-B block in the induction of human SCC (4).Inflammation of the skin with neutrophil infiltration and hyperproliferation is also seen in female heterozygous Ikk␥-deficient mice, which develop a condition similar to the human X-linked disorder Incontinentia Pigmenti (IP) (5, 6). IKK␥ is the regulatory subunit of the IKK complex, which, in addition, contains the two catalytic subunits IKK␣ (IKK1) and IKK␤ (IKK2) (1). It is now well established that human IP results from loss-of-function mutations in the IKK␥ gene (7). Interestingly, subungual keratoacanthoma-like tumors and cases of SCC have been reported as late manifestations of IP (8-11).The catalytic IKK␤ subunit is necessary for activation of NF-B in response to proinflammatory stimuli (1). Skin-specific deletion of the IKK␤ gene, K14-Cre͞Ikk2 FL/FL , was recently shown to result in an inflammatory phenotype with concomitant hyperproliferation of the epidermis (12), very similar to what is seen in the K5-I B␣ mice. Because the K14-Cre͞Ikk2 FL/FL mice die between day 7 and day 9 after birth, it is currently not known whether they, similar to K5-I B␣ mice, are prone to develop SCC.Besides inf lammation and hyperprolifera...

A growth inhibitory role in skin development for the NF-kappaB proteins has been established in recent years. We have previously shown that inhibition of NF-kappaB by overexpression of degradation-resistant IkappaB-alpha in the skin results in the development of squamous cell carcinomas (SCC). In this paper, we characterize the progressive skin disease leading to cancer development in mice with inhibited NF-kappaB signaling in the skin. Increased proliferation and a strong inflammatory response were evident in transgenic skin. A mixed inflammatory cell infiltrate dominated by polymorphonuclear leukocytes was observed in concurrence with an upregulation of the proinflammatory cytokine tumor necrosis factor-alpha. This genetically engineered mouse mutation may be a useful tool to test the efficacy of cytokine therapies for SCC in the future.

Exposure of mammalian skin to UV light results in induced gene transcription, playing a role in inf lammation, immunosuppression, and tumor promotion. One important group of transcription factors induced by UV radiation is composed of members of the Rel͞NF-B family, which are known to play a major role in the transcriptional activation of many genes encoding inf lammatory cytokines, adhesion molecules, and viral proteins. However, the upstream events in the transduction of the UVB signal to Rel protein activity are, as yet, unknown. Here, we provide biochemical evidence that exposure of keratinocytes to UVB causes rapid association of tumor necrosis factor (TNF) receptor 1 with its downstream partner TRAF-2. The functional relevance of this association is demonstrated by experiments showing that expression of a dominant negative TNF receptor 1 or TRAF-2 protein inhibits UVB-induced Rel-dependent transcription. Inclusion of a neutralizing antibody toward TNF␣ has no effect on UVB activation of a Rel-responsive reporter gene. Therefore, UVB-induced activation of Rel proteins via TNF receptor 1, independent of ligand activation, is a key component in the UV response in keratinocytes.