Identification and characterization of a cell surface marker for embryonicrat spinal accessory motor neurons

The developing mammalian spinal cord contains distinct populations of motor neurons that can be distinguished by their cell body positions, by the exp ression of specific combinations of regulatory genes, and by the paths that their axons take to exit the central nervous system (CNS). Subclasses of s pinal motor neurons are also thought to express specific cell surface prote ins that function as receptors which control the guidance of their axons. W e identified monoclonal antibody (mAb) SAC1 in a screen aimed at generating markers for specific subsets of neurons/axons in the developing rat spinal cord. During early embryogenesis, mAb SAC1 selectively labels a small subs et of Isl1-positive motor neurons located exclusively within cervical segme nts of the spinal cord. Strikingly, these neurons extend mAb SAC1-positive axons along a dorsally directed trajectory toward the lateral exit points. Consistent with the finding that mAb SAC1 also labels spinal accessory nerv es, these observations identify mAb SAC1 as a specific marker of spinal acc essory motor neurons/axons. During later stages of embryogenesis, mAb SAC1 is transiently expressed on both dorsally and ventrally projecting spinal m otor neurons/axons. Interestingly, mAb SAC1 also labels the notochord and f loor plate during most stages of spinal cord development. The mAb SAC1 anti gen is a 100-kD glycoprotein that is likely to be the rat homolog of SC1/BE N/DM-GRASP, a homophilic adhesion molecule that mediates axon outgrowth and fasciculation. (C) 2001 Wiley-Liss, Inc.