The extracellular matrix glycoprotein tenascin-C (TN-C) has been suggested to play important functional roles during neural development, axonal regeneration, and synaptic plasticity. We generated a constitutively TN-C-deficient mouse mutant from embryonic stem cells with a floxed tn-C allele, representing a standard for future analysis of conditionally targeted mice. The gross morphology of the CNS was not detectably affected, including no evidence for perturbed nerve cell migration, abnormal oligodendrocyte distribution, or defective myelination. Despite the apparent normal histology of the hippocampus and normal performance in the water maze, theta-burst stimulation (TBS) of Schaffer collaterals elicited reduced long-term potentiation (LTP) in the CA1 region of TN-C-deficient mutants, as compared with wild-type littermates. However, high-frequency stimulation evoked normal LTP not only in CA1, but also at mossy fiber-CA3 and medial and lateral perforant path-granule cell synapses in the dentate gyrus. Low-frequency stimulation failed to induce long-term depression in the CA1 region of TN-C-deficient animals. Recordings of TBS-induced LTP in the presence of nifedipine, an antagonist of L-type voltage-dependent Ca2+ channels (VDCCs), did not affect LTP in TN-C-deficient mice, but reduced LTP in wild-type mice to the levels seen in mutants. Furthermore, chemical induction of a L-type VDCC-dependent LTP in the CA1 region by application of the K+ channel blocker tetraethylammonium resulted in impaired LTP in TN-C mutants. Thus, reduction in L-type VDCC-mediated signaling appears to mediate the deficits in certain forms of synaptic plasticity in constitutively TN-C-deficient mice.

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Declining pharmaceutical industry productivity is well recognized by drug developers, regulatory authorities and patient groups. A key part of the problem is that clinical studies are increasingly expensive, driven by the rising costs of conducting Phase II and III trials. It is therefore crucial to ensure that these phases of drug development are conducted more efficiently and cost-effectively, and that attrition rates are reduced. In this article, we argue that moving from the traditional clinical development approach based on sequential, distinct phases towards a more integrated view that uses adaptive design tools to increase flexibility and maximize the use of accumulated knowledge could have an important role in achieving these goals. Applications and examples of the use of these tools--such as Bayesian methodologies--in early- and late-stage drug development are discussed, as well as the advantages, challenges and barriers to their more widespread implementation.

The pituitary hormones LH, 1 thyrotropin, and pro-opiomelanocortin bear unique N-linked oligosaccharides that terminate with the sequence SO 4 -4-GalNAc␤1,4GlcNAc␤1,2Man␣ (1-6).The terminal GalNAc-4-SO 4 is recognized by a receptor, the Man/GalNAc-4-SO 4 receptor (7-9), which is located in hepatic endothelial cells and controls the circulatory half-life of LH and other glycoproteins bearing this structure (10, 11). A short circulatory half-life in conjunction with the regulated release from storage granules in the gonadotroph produces the episodic rise and fall seen in circulating LH levels (12, 13). This is essential for efficient activation of the LH receptor in the ovary and testis due to the down-regulation and internalization of the activated receptor by ␤-arrestin (14). Sequential actions of a hormone-specific ␤1,4 GalNAc-transferase and a GalNAc-4-sulfotransferase are required for the synthesis of GalNAc-4-SO 4 on LH and other glycoproteins (13,(15)(16)(17)(18)(19). Three key functions have been conserved throughout vertebrate evolution: 1) the peptide recognition determinant utilized by the GalNAc-transferase, 2) the GalNAc-transferase itself, and 3) the GalNAc-4-sulfotransferase. As a result, Nlinked structures terminating with ␤1,4-linked GalNAc-4-SO 4 occur on glycoprotein hormones from all vertebrate species (20). GalNAc-transferase and GalNAc-4-sulfotransferase activities with the same properties as those expressed in the pituitary have been detected in other tissues and cells (21). Furthermore, a number of other glycoproteins bearing N-and O-linked structures terminating with ␤1,4-linked GalNAc-4-SO 4 have been described (2,5,(22)(23)(24)(25)(26). Thus, oligosaccharides terminating with GalNAc-4-SO 4 may fulfill a number of different roles in vivo.Recently the HNK-1 sulfotransferase (HNK-1 ST), a glucuronyl-3-sulfotransferase that mediates synthesis of the HNK-1 epitope SO 4 -3-GlcA␤1,3Gal␤1,4GlcNAc (27) was cloned (28, 29). The HNK-1 structure is found in the central and peripheral nervous systems on glycolipids and glycoproteins involved in neural recognition and synaptogenesis (Ref. 30; for reviews, see Refs. 31 and 32). Expression of the HNK-1 structure is spatially and temporally regulated, suggesting an important role in axon outgrowth (33). It is a predominant autoantigen in demyelinating diseases of the peripheral nervous system (34). Recently this glycan has been recognized to be a crucial player in synaptic plasticity involving inhibitory interneurons in the hippocampus (35). The sulfate group of the HNK-1 glycan is essential for its function (36, 37).

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