ABNORMAL AXONAL GUIDANCE AND BRAIN ANATOMY IN MOUSE MUTANTS FOR THE CELL RECOGNITION MOLECULES CLOSE HOMOLOG OF L1 AND NgCAM-RELATED CELL ADHESION MOLECULE A. HEYDEN, a * F. ANGENSTEIN, a M. SALLAZ, b C. SEIDENBECHER a AND D. MONTAG a a Leibniz Institute for Neurobiology, Brenneckestr. 6, D-39118 Magde- burg, Germany b Fondation IDÉE-Institute for Children and Adolescents with Epilepsy, 89, rue Bellecombe, 69003 Lyon, France Abstract—Cell recognition molecules of the L1 family serve important functions in the developing and the mature ner- vous system. Mutations in genes encoding the L1 family members close homolog of L1 (CHL1) and NgCAM-related cell adhesion molecule (NrCAM) have been found to alter connectivity and morphology of several brain regions. In order to emphasize similarities and differences of these two structurally related molecules, null mutants for CHL1 and NrCAM were directly compared with respect to axonal guid- ance in the hippocampus and the olfactory bulb and the sizes of the ventricular system and the cerebellar vermis using a combined structural magnetic resonance imaging (MRI) and histological approach. The results demonstrate that the ab- sence of CHL1 leads to aberrant hippocampal mossy fiber projections whereas in both mutants, CHL1 and NrCAM, the guidance of the olfactory nerve projections is disturbed. Both mutations also alter the size of the ventricular system and the vermis with a specific profile of changes and partially oppo- site effects in each of the mutants. CHL1/NrCAM double- mutant mice do not show any enhancement of the single mutant’s phenotype but balance the opposing effects on the ventricular system. In summary, the results show that CHL1 and NrCAM both affect axonal guidance and the anatomy of the ventricular system and the cerebellar vermis but act dif- ferently on these processes. © 2008 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: L1 cell adhesion molecules, MRI, ventricle, cere- bellum, olfactory bulb, hippocampus. Neural cell recognition molecules of the L1 family, a sub- group of the immunoglobulin superfamily, mediate several aspects of nervous system development including cell ad- hesion and migration, axonal growth, fasciculation, guid- ance, and myelination (Brummendorf et al., 1998; Hortsch, 2000; Maness and Schachner, 2007). Mutations in the human L1 gene result in a complex syndrome termed CRASH (corpus callosum hypoplasia, mental retardation, adducted thumbs, spastic paraplegia and hydrocephalus) (Fransen et al., 1997; Weller and Gartner, 2001). Loss or truncation of the close homolog of L1 (CHL1) gene may also be responsible for mental retardation associated with the 3p-syndrome (Angeloni et al., 1999; Frints et al., 2003). Furthermore, mutations in the genes encoding L1, CHL1, and NgCAM-related cell adhesion molecule (NrCAM) have been implicated in a broad spectrum of psychiatric disor- ders including schizophrenia (Kurumaji et al., 2001; Saku- rai et al., 2002; Chen et al., 2005), autism (Petek et al., 2001; Bonora et al., 2005; Sakurai et al., 2006), and ad- diction (Ishiguro et al., 2006). L1 family-associated disorders are often accompanied by altered connectivity and morphology of several brain regions (Courchesne et al., 1993; Fransen et al., 1998a; Cannistraro et al., 2007; Chua et al., 2007). Accordingly, abnormal fiber projections were found in the hippocampus and the olfactory bulb of CHL1-deficient mice (Montag- Sallaz et al., 2002), and in the anterior commissure and the visual system of NrCAM-deficient mice (More et al., 2001; Falk et al., 2005; Williams et al., 2006). Additionally, de- pending on the genetic background, the brain ventricles of CHL1 mutant mice appeared enlarged (Montag-Sallaz et al., 2002). In NrCAM mutant mice, the size of particular cerebellar lobules was found to be reduced (Sakurai et al., 2001). Aberrant axonal guidance and altered morphology of the ventricular system and the cerebellum were also observed in L1 mutant mice (Dahme et al., 1997; Fransen et al., 1998b; Demyanenko et al., 1999). L1 family molecules are widely expressed throughout the brain in a distinct but overlapping pattern (Moscoso and Sanes, 1995; Hillenbrand et al., 1999; Lustig et al., 2001; Backer et al., 2002). The four transmembrane mem- bers of this family, namely, L1, CHL1, NrCAM, and neuro- fascin show considerable similarities of their protein struc- tures (Holm et al., 1996). Similar phenotypes of mutant mice of the L1 family concerning aberrant connectivity and anatomy of the brain (Dahme et al., 1997; Fransen et al., 1998b; Sakurai et al., 2001; Montag-Sallaz et al., 2002) might indicate possible overlapping functions of these mol- ecules. In support of this view, mild cerebellar defects in NrCAM-deficient and L1-deficient mice result in a more *Corresponding author. Tel: +49-391-6263217; fax: +49-391-6263- 229. E-mail address: Alexandra.Heyden@gmx.net (A. Heyden). Abbreviations: ANOVA, analysis of variance; CA3, cornu ammonis 3; CHL1, close homolog of L1; CRASH, corpus callosum hypoplasia, mental retardation, adducted thumbs, spastic paraplegia and hydro- cephalus; DAPI, 4=,6=-diamidino-2-phenylindole; DBA, Dolichos biflo- rus agglutinin; MRI, magnetic resonance imaging; NrCAM, NgCAM- related cell adhesion molecule; RARE, rapid acquisition relaxation enhanced; 3D, three-dimensional. Neuroscience 155 (2008) 221–233 0306-4522/08$32.00+0.00 © 2008 IBRO. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.neuroscience.2008.04.080 221