Motor neuron trophic factors: therapeutic use in ALS?

Abstract

The modest effects of neurotrophic factor (NTF) treatment on lifespan in both animal models and clinical studies of Amyotropic Lateral Sclerosis (ALS) may result from any one or combination of the four following explanations: 1.) NTFs block cell death in some physiological contexts but not in ALS; 2.) NTFs do not rescue motoneurons (MNs) from death in any physiological context; 3.) NTFs block cell death in ALS but to no avail; and 4.) NTFs are physiologically effective but limited by pharmacokinetic constraints. The object of this review is to critically evaluate the role of both NTFs and the intracellular cell death pathway itself in regulating the survival of spinal and cranial (lower) MNs during development, after injury and in response to disease. Because the role of molecules mediating MN survival has been most clearly resolved by the in vivo analysis of genetically engineered mice, this review will focus on studies of such mice expressing reporter, null or other mutant alleles of NTFs, NTF receptors, cell death or ALS-associated genes.

Figure 1

Basic intracellular molecular pathways regulating MN survival during development. Cells die through apoptosis (Type I), autophagic cell death (Type II) or necrosis (not pictured). Two different pathways mediate apoptosis, called the intrinsic and extrinsic pathways. The intrinsic pathway involves activation of the pro-apoptotic multi-BH-domain proteins (MDP) Bak and Bax and inhibition of the anti-apoptotic MDPs Bcl-2 and Bcl-X_L by the pro-apoptotic BH3-domain-only proteins (BOPs). Bak/Bax activation promote mitochondrial permeability transition (mPT), loss of mitochondrial membrane potential (ψ_m), release of mitochondrial cytochrome c (cyt c) and activation of caspases (casp3, casp9), which dismantle the cell (lightening strike). Bak/Bax activation also depletes (ψ_m) by causing the transfer of Ca⁺⁺ from the endoplasmic reticulum (ER) to the mitochondrion (M). The extrinsic apoptotic pathway directly activates caspases (red arrow) or activates the intrinsic death pathway through the BOP Bid (purple arrow). Autophagic death (lightening strike) occurs independently of apoptosis and is characterized by the presence of abundant autophagic vacuoles (AV) and the activation of autophagic genes (ATGs) such as Beclin 1 (atg6). Cell death is regulated by extracellular anti-apoptotic and pro-apoptotic stimuli that operate through neurotrophic factor receptors (NTFRs) or death receptors (DRs). Activated DRs activate the extrinsic death pathway by assembling a death-inducing signaling complex (DISC). Activated NTFRs antagonize apoptotic, autophagic and necrotic death by activating cytosolic kinases such as phosphoinositide-3 kinase, Akt and B-Raf, inhibiting pro-apoptotic BOP activity, activating anti-apoptotic and inhibiting pro-apoptotic gene expression in the nucleus (N). Blue lines indicate anti-apoptotic function and black lines depict pro-apoptotic function; arrows indicate activation and flat lines indicate inhibition. For details see text.

Figure 2

Cellular mechanisms of neurodegeneration in fALS mutant SOD1 mice. MNs may succumb by intrinsic (cell autonomous) or extrinsic (cell non-autonomous) signaling changes in ALS. Intrinsic mechanisms include defects in transport and/or mitochondrial function, and extrinsic signaling changes include altered trophic, excitotoxic (reactive oxygen species (ROS) or glutamate) or death receptor (DR) signaling from other cell types. A MN is depicted in yellow, and cell types known to positively and negatively regulate MN survival are shown in purple and orange, respectively, including muscle cells (1), Schwann cells (2) endothelial cells in spinal, nerve or muscle tissue (3), descending supraspinal neurons (4), local interneurons (5), spinal sensory ganglia (6), endocrine glands (7), nerve- or muscle-derived fibroblasts (8), microglia (9) or astrocytes. See text for further details.

Figure 3

Neurotrophic signaling between MNs and other neuromuscular cell types in the context of ALS. (A) Expression of NTFs and NTFRs in cell types which may regulate MN survival during disease. MNs (in yellow) express nearly all NTFRs as well as several NTFs such as VEGF and NT-3. Expression of NTFs (blue type) and NTFRs (black type) by cell types known to positively and negatively regulate MN survival (purple and orange, respectively). Other NTF or NTFR sources not shown include oligodendrocytes in the ventrolateral funiculus and and smooth muscle cells of spinal, neural and muscular arterioles. Exogenously administered NTFs may bind NTFRs expressed by any of these cell types (B) Results of studies with transgenic or null mutant mice expressing elevated or reduced amounts of NTFs in specific cell types. Arrows indicate likely direction of signaling based on NTFR expression analysis. VEGF “KO” refers to HRE-VEGF mutant (see text). SC refers to Schwann cells. Two small arrows indicate a positive effect of NTF overexpression on lifespan of FALS mice; two dashes signify no such effect.

Figure 4

Subcellular degeneration of MNs in fALS and NTF KO mice. Cell types and colors are the same as those in figures 2 and 3. fALS mice exhibit degenerative changes (indicated by broken lines) at the neuromuscular junction (NMJ) and axon (1,2) before the soma (3). CNTF KO mice, similar to fALS mice, display reduced CNTF in the nerve and increased CNTFRα in the spinal cord, and a loss of countable MN soma (1) before neuromuscular changes at the axon and NMJ (2,3). Consistent with this scenario, systemic (subq=subcutaneous) CNTF treatment fails to ameliorate human ALS. NT-3 KO mice display a neuropathy that likely results from Schwann cell death and subsequent loss of axons and NMJs (1,2) before loss of MNs. Finally, HRE-VEGF mice (VEGF “KO” mice) exhibit a reduction in VEGF upon ischemic stress, which could arise from impaired vascular support of muscle, nerve or spinal tissue. Although the sequence of MN degeneration has not been elucidated, treatment with VEGF reduces both astrogliosis centrally and loss of NMJs peripherally.