Stem Cell Therapy for Amyotrophic Lateral Sclerosis

Introduction

Amyotrophic lateral sclerosis (ALS) is a terrifying diagnosis that strikes at the core of an individual’s function and productivity. The disease is characterized by progressive and selective degeneration of upper and lower motor neurons. This manifests as an insidious, inexorable decline in motor function, with progressively compromised strength, coordination, gait, swallowing, speech, and respiratory function. For nearly all those afflicted, this means first relying on canes or walkers, then wheelchair-dependence, progressing to ventilator-dependence and a bedbound status, all the while with sensation and cognition preserved. Complications arising from this decline lead to death in an average of 3–5 years from time of diagnosis. The estimated cost of this illness to society ranges from $256–433 million to over $1 billion, yet few treatment options exist. The only US Food and Drug Administration (FDA)-approved pharmacotherapy, riluzole, extends life span by a matter of mere months. In the more than two decades since approval of riluzole, no disease-modifying therapies have come to fruition.

The reasons behind this lack of progress lie in the fact that mechanisms of motor neuron loss in ALS remain a mystery. Many models exist to explain how motor neurons are selectively killed including excitotoxicity, loss of neurotrophic factors, impaired RNA metabolism, protein aggregation, inflammatory signaling, mitochondrial pathology, endoplasmic reticulum dysfunction leading to misfolded proteins, and many others. Cases with genetic underpinnings offer some clues, with approximately 15% of cases associated with mutations in genes such as Cu ²⁺ -Zn ²⁺ superoxide dismutase-1 (SOD1), transactive response DNA-binding protein 43 (TDP43), fused in sarcoma, and the recently identified hexanucleotide repeat in chromosome 9 open reading frame 72 (c9orf72). Indeed, various animal models expressing these mutations, particularly the mutant SOD1 protein, have been used in most of the preclinical studies described. However, the large majority of ALS cases appear sporadic in nature and many treatments targeted at the above-mentioned pathways have failed in large-scale clinical trials.

In this stark landscape of ALS therapeutics, stem cells have emerged as a promising new strategy to combat the multifaceted pathology of this disease. In the short time since stem cells were first described, there has been exponential growth in scientific understanding of stem cell biology as well as potential applications in disease. In this chapter, we will begin by describing the prevailing rationale underlying stem cell therapy, then touch on current experience using different types of stem cells in ALS.

Stem Cells in Amyotrophic Lateral Sclerosis: Microenvironment Modulation

Stem cells are defined by the fundamental property of being able to undergo asymmetric cell division, with one daughter cell able to differentiate into one or more different specific cell types while the other daughter cell retains the capacity for self-renewal. There are a number of different cell types that fall under the category of “stem cell,” each with unique properties and varying degrees of differentiation potential ( Fig. 9.1 ).

Early interest in using stem cell technology in ALS was focused on the possibility of regenerating the lost motor neuron pool. Experiments using murine stem cells showed that, after in vitro exposure to retinoic acid and the hedgehog agonist Hh-Ag1.3, these cells had the ability to differentiate into motor neurons and form neuromuscular junctions after transplantation into chick embryos. However, this enthusiasm was quickly tempered, as this initial success was not reproducible in rodent models of ALS. While stem cells can indeed be induced to form motor neurons in the spinal cord, these cells must integrate into local circuitry and receive input from interneurons and descending axons, grow new axons through an impermissible central nervous system (CNS), travel significant lengths in the periphery to connect with skeletal muscle, achieve remyelination, and finally form mature neuromuscular contacts. And while these are the minimum criteria for the creation of new motor units, successful functional improvements depend on formation of sufficient numbers of motor units on proper agonist/antagonist muscles within a time window prior to irreversible atrophy of muscle tissue. With our current understanding and abilities, this goal of motor system reconstruction is currently out of reach.

Furthermore, motor neuron replacement strategies face another challenge regarding inherent ALS pathophysiology. It is now understood that while motor neuron loss is the most obvious marker of ALS, factors within the motor neuron microenvironment contribute to the pathology. In chimeric mice expressing a mutant SOD1 protein in either motor neurons or astrocytes, motor neuron survival was preserved, despite mutant SOD1 expression, if surrounding astrocytes expressed the normal gene. In contrast, motor neurons expressing the wild-type SOD1 protein still exhibited degeneration if neighboring astrocytes expressed mutant SOD1. This has led to the widely held notion that motor neuron death in ALS is not a cell autonomous process. Accumulating evidence suggests interactions with interneurons, microglia, inflammatory signaling, vasculature, Schwann cells, and skeletal muscle play a role in ALS pathology. Transplanted motor neurons would still need to survive in this hostile environment in order for cell replacement to succeed.

Given this more nuanced and global understanding of ALS pathology, stem cells still rise to the forefront as a promising therapeutic option for ALS. Stem cells can succeed where pharmacotherapy has failed by having simultaneous impact on the multifactorial pathways leading to motor neuron death. Pharmacologic therapies often achieve their effects by targeting one specific signaling pathway, and pharmacologic action in nonaffected tissues may give rise to side effects. Stem cells, on the other hand, can act locally via intercellular and paracrine mechanisms, while retaining the ability to simultaneously influence many signaling cascades that may be deranged. Thus, the majority of current stem cell therapy investigations are aimed at modulating the local microenvironment and preventing the further loss of motor neurons.

Nearly every type of stem cell has been applied in ALS animal models ( Fig. 9.1 ), including embryonic stem cells (ESCs), olfactory ensheathing cells (OECs), bone marrow-derived mesenchymal stem cells (MSCs), peripheral blood stem cells (PBSCs), umbilical cord stem cells (UBSCs), and neural progenitor cells (NPCs). While results and progress in translation to humans have been variable, the fervor with which stem cell therapy in ALS is being pursued conveys a general sense of excitement and optimism.

Embryonic Stem Cells

The cell with the most diverse differentiation capability is the ESC. This cell, derived from the embryonic blastocyst, is considered totipotent, capable of forming all tissue types if given the correct environment. While the original interest in ESCs was for motor neuron replacement, it was found that human ESCs inefficiently formed mature motor neurons. Rather, these implanted cells were locally neuroprotective, perhaps through local secretion of transforming growth factor α and brain-derived neurotrophic factors.

The robust replicative potential of ESCs is in some ways a double-edged sword. Early use of ESCs in the mutant SOD1 rat model demonstrated tumor formation at sites of cell implantation ( Fig. 9.2 ). Later explorations of ESCs in this and other models of motor neuron degeneration would utilize in vitro differentiation of murine ESCs toward motor neuron fates prior to implantation.

Part of the difficulty with using ESCs also involves the social, ethical, and political ramifications of establishing and utilizing new cell lines derived from human embryos. Therefore, while ESCs possess extraordinary potential, the attendant caveats have made ESCs a less attractive option for clinical translation. To date, no clinical trials in humans have been conducted using human ESCs.