General mechanisms of axonal damage and its prevention

Axonal degeneration is a prominent pathological feature in multiple sclerosis observed over a century ago. The gradual loss of axons is thought to underlie irreversible clinical deficits in this disease. The precise mechanisms of axonopathy are poorly understood, but likely involve excess accumulation of Ca ions. In healthy fibers, ATP-dependent pumps support homeostasis of ionic gradients. When energy supply is limited, either due to inadequate delivery (e.g., ischemia, mitochondrial dysfunction) and/or excessive utilization (e.g., conduction along demyelinated axons), ion gradients break down, unleashing a variety of aberrant cascades, ultimately leading to Ca overload. During Na pump dysfunction, Na can enter axons through non-inactivating Na channels, promoting axonal Na overload and depolarization by allowing K egress. This will gate voltage-sensitive Ca channels and stimulate reverse Na-Ca exchange, leading to further Ca entry. Energy failure will also promote Ca release from intracellular stores. Neurotransmitters such as glutamate can be released by reverse operation of Na-dependent transporters, in turn activating a variety of ionotropic and metabotropic receptors, further exacerbating overload of cellular Ca. Together, this Ca overload will inappropriately stimulate a variety of Ca-dependent enzyme systems (e.g., calpains, phospholipases), leading to structural and functional axonal injury. Pharmacological interruption at key points in these interrelated injury cascades (e.g., at voltage-gated Na channels or AMPA receptors) may confer significant neuroprotection to compromised central axons and supporting glia. Such agents may represent attractive adjuncts to currently available immunomodulatory therapies.