Regulation of the cell adhesion molecule CD44 after nerve transection and direct trauma to the mouse brain

CD44 is a cell surface glycoprotein involved in cell adhesion during neurite outgrowth, leukocyte homing, and tumor metastasis. In the current study, we examined the regulation of this molecule 4 days after neural trauma in different forms of central and peripheral injury. Transection of the hypoglossal, vagus, or sciatic nerve led to the appearance of CD44-immunoreactivity (CD44-IR) on the surface of the affected motoneurons, their dendrites, and their axons. Fimbria fornix transection led to CD44-IR on a subpopulation of cholinergic neurons in the ipsi- and contralateral medial septum and diagonal band of Broca and colocalized with galanin-IR. Central projections of axotomized sensory neurons to the spinal cord (substantia gelatinosa, Clarke's column) also showed an increase in CD44-IR, which was abolished by spinal root transection. Nonneuronal CD44-IR was mainly restricted to sites of direct injury. In the crushed sciatic nerve, CD44-IR was found on the demyelinating Schwann cells and on infiltrating monocytes and granulocytes. Direct parasagittal transection of the cerebral cortex led to CD44-IR on resident astrocytes and on leukocytes entering the injured forebrain tissue. CD44-IR also increased on reactive retinal astrocytes and microglia after the optic nerve crush. Additional time points in the retina and hypoglossal nucleus (days 1, 2, and 14) and cerebral cortex (day 2) injury models also showed the same cell type pattern for the CD44-IR. Finally, polymerase chain reaction analysis confirmed the posttraumatic expression of CD44 mRNA and detected only the standard haematopoietic CD44 splice isoform both in direct and indirect brain injury models. Overall, the current study shows the widespread, graded appearance of CD44-IR on neurons and on nonneuronal cells, depending on the form of neural injury. Here, the ability of CD44 to bind to a variety of extracellular matrix and cell adhesion proteins and its common presence in different forms of brain pathology could suggest an important role for this cell surface glycoprotein in the neuronal, glial, and leukocyte response to trauma and in the repair of the damaged nervous system.