The adhesion molecule L1 is a member of the immunoglobulin superfamily. L1 is involved in various recognition processes in the CNS and PNS, and binding to L1 can activate signal transduction pathways. Mutations in the human L1 gene are associated with a variable phenotype, including mental retardation and anomalous development of the nervous system, referred to as 'CRASH' (corpus callosum hypoplasia, retardation, adducted thumbs, spastic paraplegia, and hydrocephalus). We generated an animal model of these conditions by gene targetting. Mutant mice were smaller than wild-type and were less sensitive to touch and pain, and their hind-legs appeared weak and uncoordinated. The size of the corticospinal tract was reduced and, depending on genetic background, the lateral ventricles were often enlarged. Non-myelinating Schwann cells formed processes not associated with axons and showed reduced association with axons. In vitro, neurite outgrowth on an L1 substrate and fasciculation were impaired. The mutant mouse described here will help to elucidate the functions of L1 in the nervous system and how these depend on genetic influences.

Jagged1-mediated Notch signaling has been suggested to be critically involved in hematopoietic stem cell (HSC) selfrenewal. Unexpectedly, we report here that inducible Cre-loxP-mediated inactivation of the Jagged1 gene in bone marrow progenitors and/or bone marrow (BM) stromal cells does not impair HSC selfrenewal or differentiation in all blood lineages. Mice with simultaneous inactivation of Jagged1 and Notch1 in the BM compartment survived normally following a 5FU-based in vivo challenge. In addition, Notch1-deficient HSCs were able to reconstitute mice with inactivated Jagged1 in the BM stroma even under competitive conditions. In contrast to earlier reports, these data exclude an essential role for Jagged1-mediated Notch signaling during hematopoiesis. IntroductionHematopoietic stem cells (HSCs) exhibit self-renewing capacity as well as the ability to give rise to more committed progenitors that differentiate into all hematopoietic lineages. 1 The molecular mechanisms regulating stem cell self-renewal and/or differentiation are only poorly understood. Among the proteins that have been postulated to be involved in hematopoietic stem cell maintenance are the Notch receptors and their ligands. 2 Mammals have 4 Notch receptors (Notch1-4) that bind 5 different ligands (Jagged1-2, Delta-like 1-3-4). Expression of a constitutively active form of Notch1 (N1) in murine bone marrow progenitors can lead to increased HSC-self-renewal 3 or to the immortalization of stem cell like progenitors capable of undergoing lymphoid and myeloid differentiation both in vitro and in vivo. 4 In addition, coculture of murine or human HSCs with immobilized Notch ligands, or feeder cells expressing such ligands, can maintain or even enhance HSC self-renewal. [5][6][7][8][9] Recently, osteoblasts expressing the Notch ligand Jagged1 (J1) were identified as being part of the hematopoietic stem cell niche. Osteoblast-specific expression of the activated parathyroid hormonerelated protein receptor results in increased numbers of osteoblasts expressing high levels of J1. The increase in osteoblasts correlates with an increase in the number of HSCs, with evidence of N1 activation in vivo. 10 These results were interpreted to mean that J1-expressing osteoblasts regulate HSC homeostasis through N1 activation.To definitively assess the role of J1 in the hematopoietic system, we have generated inducible gene-targeted mice for J1. Surprisingly, inactivation of J1 in either bone marrow (BM) progenitors or BM stromal cells had no effect on HSC maintenance. In addition, N1-deficient HSCs transplanted into mice with inactivated J1 in the BM stroma reconstituted BM chimeras normally. Our data exclude an essential contribution of J1-mediated N1 signaling for HSC self-renewal or differentiation. Study design Generation and conditional inactivation of mice with a loxP-flanked J1A J1 genomic clone was isolated from a mouse genomic library using an oligonucleotide complementary to part of the first coding exon. LoxP sites were introduced into an XhoI site a...

The demyelinating toxin cuprizone is used increasingly in mouse studies of central nervous system remyelination. The value of this model for such studies depends on an accurate description of its quantifiable features. We therefore investigated histology and ultrastructure during the early oligodendrocyte differentiation phase of remyelination in mice given cuprizone and allowed to recover for 2 weeks. Limiting the dose of cuprizone to 0.2% overcame significant mouse morbidity and weight loss seen with a 0.4% dose, but the distribution of cuprizone‐induced demyelination was anatomically variable. The caudal corpus callosum and dorsal hippocampal commissure mostly demyelinated at this dose, but the rostral corpus callosum and rostral cerebellar peduncles did not. This variable response, together with small axon diameters and hence thin myelin sheaths, hindered analysis of the progress of early remyelination. The proportion of myelinated and unmyelinated axons in defined regions followed expected trends, but there was pronounced variation between animals. Furthermore, group mean G ratios did not change as expected during the early stages of remyelination, and regression analysis revealed a complex relationship between axon diameter and myelin sheath thickness during this period. We also noted axonal pathology that persisted for at least 2 weeks after cuprizone withdrawal.