Normal dendritic arborization in spinal motoneurons requires neurofilament subunit L

Neurofilaments, composed of three polypeptide subunits, NF-L, NF-M, and NF-H, are major cytoskeletal elements in large neurons with long axons. Neurofilaments play a critical role in the development of axonal diameter; however, their role in the development of dendrites is largely unknown. By overexpressing different neurofilament subunits, we previously demonstrated that alteration of neurofilament subunit composition resulted in dramatic changes in dendritic arborization. To further determine the role of neurofilaments in dendritic growth, we examined and compared the dendritic architecture of spinal cord neurons in young NF-L knockout (-/-), heterozygous (+/-), and wild-type (+/+) mice. We show that an absence or reduction in the expression of NF-L inhibited dendritic growth most dramatically in large motoneurons, mildly in medium neurons, but had no effect on small neurons. We also reveal that a decrease in NF-L leads to an increase in NF-M and NF-H subunits in cell bodies and their reduction in dendrites. These results demonstrate that NF-L is a critical intrinsic factor for dendritic growth in large motoneurons.