GFAP and amyotrophic lateral sclerosis: To this end, we immunostained spinal cord sections from the four ALS lines with aquaporin-4 (AQP4), a water channel protein, highly expressed in astrocytic end-feet (Nielsen et al, 1997), and glial fibrillary acidic protein (GFAP), a class-III intermediate filament and cell-specific marker that distinguishes astrocytes from other glial cells, combined with COL-IV.