Brain development in mice lacking L1-L1 homophilic adhesion

Area of Science:

• Neuroscience
• Molecular Biology
• Developmental Biology

Background:

• The L1 cell adhesion molecule (L1CAM) plays a critical role in neuronal development, including axon guidance and fasciculation.
• L1CAM mediates homophilic interactions (L1-L1) and heterophilic interactions with other cell surface molecules and extracellular matrix components.

Purpose of the Study:

• To investigate the specific functions of L1-L1 homophilic binding and other L1CAM interactions in vivo.
• To determine the necessity of L1-L1 binding for normal axon pathway development and its potential role in hydrocephalus.

Main Methods:

• Generation of a novel mouse line with a targeted deletion of the sixth Ig domain of L1CAM.
• In vitro binding assays to assess L1-L1 homophilic and heterophilic interactions (L1-integrin, L1-neurocan, L1-neuropilin).
• Assessment of axon guidance defects and hydrocephalus development in the modified mouse line and its backcrosses.

Main Results:

• The L1CAM deletion preserved overall L1 expression but abolished L1-L1 homophilic and L1-alpha5beta1 integrin binding.
• L1-neurocan and L1-neuropilin binding, as well as sema3a responses, remained intact.
• Unexpectedly, major axon guidance defects (corticospinal tract, corpus callosum) were absent in the modified mice.
• Backcrossing onto the C57BL/6 strain led to severe hydrocephalus, ultimately causing embryonic lethality.

Conclusions:

• L1-L1 homophilic binding and L1-alpha5beta1 integrin binding are not essential for the development of several major axon pathways.
• L1-L1 homophilic binding appears to be critical for preventing the development of X-linked hydrocephalus.