If a neuroscientist stated that a brain structure had efferent neurons, what can be concluded?

15.4.3 Motor neuron subclass diversification

MNs innervating each muscle are thought to be a distinct subtype predetermined to project and innervate that muscle. All MNs innervating the same muscle are called a motor pool. Pools of MNs that innervate muscles of similar embryonic origin occupy stereotypic location within the ventral spinal cord and commonly known as motor columns. For example, the medial motor column (MMC) consists of MNs that innervate long muscles of the back (MMCm) and MNs that innervate body wall musculature (MMCl). The MMC MNs are generated throughout the cervical, thoracic, and lumbar spinal cord. In contrast, the LMC consists of MNs that innervate limb muscles, and the LMC is found only in the lower cervical and lumbar level spinal cord. Similarly restricted in distribution is the column of PG visceral type MNs (PGMNs) that innervate neurons of the sympathetic ganglia. PGMNs are found only in the thoracic spinal cord.

The motor column subtype identity is determined by a combination of combinatorial LIM-HD transcription factor expression. All spinal MNs express Lhx3 or Lhx4 at the time of birth (Sharma et al., 1998). Those MNs that retain the expression of Lhx3/4 become MMCm, whereas those that downregulate Lhx3/4 expression do not (Sharma et al., 2000). The LMCl MNs express Lhx1 and their fate depends on retinoic acid signaling (Sockanathan and Jessell, 1998). Unlike MMC and LMC MNs, PG MNs do not express Islet2 (Tsuchida et al., 1994). What decides the MMCl versus LMCm fate is not clear. Which of these MN subtypes are generated at any axial level is determined by the anterior–posterior patterning of the early neural tube. For example, Hox9 paralogous group corresponds to the generation of PGC, whereas Hox6 and Hox10 paralogous groups correspond to the generation of LMCs consisting of MNs that would innervate muscles of the forelimb (Hox6) or the hindlimb (Hox10) (Dasen et al., 2003, 2005, 2008).

Introduction

Motor neurons (MN) are a diverse group of cells without which complex life would not be possible. MNs are responsible for integrating signals from the brain and the sensory systems to control voluntary and involuntary movements. Though MNs can be split into cranial and spinal subsets, this chapter will focus on spinal MNs, as they are a key target of disease and injury. As such, MNs are the focus of regenerative efforts to alleviate these public health burdens. During late gastrulation and neurulation, the developing spinal cord, termed the neural tube, is patterned into distinct progenitor domains. MNs are specified from progenitors in the ventral neural tube. Once specified, newly born MNs are further specified into columns, pools, and subtypes, forming a unique topography. From these columns and pools, axons reach out to their targets under varying guidance cues. All MNs are cholinergic cells which integrate with the motor control circuit, the sensory system, and their outlying targets to control movement. MNs are unique in that their targets lie outside the central nervous system (CNS), meaning that they require novel methods for seeking out and synapsing on them. Here, we present an overview of MN differentiation and development. We will focus mainly on signaling events, transcription factor markers, and the extracellular matrix (ECM) as they pertain to MN development. These cells are targets of permanent and often deadly diseases including amyotrophic lateral sclerosis, spinal muscular atrophy, multiple sclerosis, and injuries such as spinal cord injury. Only by understanding how these cells progress through development can we understand how to treat these maladies which currently have little hope of a cure. Further, by decoding the major events and players in development, we can better recapitulate them in vitro for cell replacement therapy, or harness the underlying principles for regeneration in the adult. Given the growing importance of the MN–glia interaction in a number of neurodegenerative diseases, we will also discuss the initial specification of oligodendrocyte precursor cells (OPCs) in detail, as they share a common progenitor with MNs.

Neuritin

SMN has been shown to influence the expression and subcellular localization of the Neuritin (also known as Cpg15) mRNA,61 which encodes a protein involved in neurite outgrowth and synaptogenesis.171 The RBP HuD binds an ARE present in the 3′-UTR of Neuritin mRNA and increases the stability of the transcript.172 Further, the Neuritin mRNA colocalizes with SMN and HuD in axonal granules, and SMN deficiency leads to a reduction in Neuritin mRNA levels in neurites of cortical neurons61 (Fig. 7.3). Thus, SMN may be driving the association of Neuritin mRNA with HuD, and in a setting of SMN deficiency, this association could be lost leading to altered stability and localization of the Neuritin mRNA. Importantly, overexpression of Neuritin in SMN-deficient zebrafish partially restored axonal outgrowth defects caused by SMN deficiency in this model61 (Fig. 7.3). However, the fact that Neuritin mRNA levels are reduced in the soma as well as the axon following SMN deficiency does not exclude the possibility that the axonal localization defects may be secondary to other defects in mRNA metabolism, including pre-mRNA splicing.

The Biochemical Function of Survival Motor Neuron

SMN forms a complex with many other proteins called gemins.59 The SMN complex, consisting of gemins 2–8 and unrip, is found in the cytoplasm.59–66 The SMN complex functions in placing the Sm protein ring onto small nuclear RNAs (snRNA) to form a small nuclear ribonuclear protein (snRNP) that together with the SMN complex is transported into the nucleus. snRNPs are critical for the correct splicing of all genes.63,65 Thus complete loss of SMN results in lethality as snRNPs are essential to the survival of any cell. Apart from SMN’s function in assembly of snRNA,59 SMN also functions in the assembly of the Lsm10, Lsm11, and Sm complex onto U7 snRNA.67–69 The U7snRNP processes the 3′-end of the histone message. Both of these functions have been clearly demonstrated and have biochemical assays that can be used to determine activity in various samples. In addition to this assembly function, SMN has been proposed to potentially function in assembly of other RNP complexes.59 SMN has been reported to bind many different proteins.16,58–60,64–70 Yet the question remains, how many of these reported interactions give rise to a SMN complex with a specific function?16 In a large number of cases the SMN colocalization studies only show a small proportion of the particular protein overlaps. In addition, coimmunoprecipitation (Co-IP) could be due to the fact that SMN is a sticky protein and is limited by the amount of the input in Co-IPs. It is also hard to eliminate the possibility that the interaction only occurs upon lysis of cells. It becomes key to know whether a function is associated with these putative other SMN complexes. The SMN protein in the axon has been associated with HuD, yet we do not know if SMN can assemble the HuD complex onto mRNA.58,69,70 Furthermore, the function of an SMN axonal complex must be determined with a rigorous biochemical assay. The lack of any abnormality of axons in vivo in mammalian models of axon patterning, along with no clear biochemical assay, does give concern. For example, overexpression of HuD corrected axonal defects in vitro in motor-neuron-like cells with reduced SMN but it is not known whether HuD overexpression can correct SMA motor neuron physiology in the mouse in vivo.58 However, the demonstration of SMN’s importance in axon repair later in development indicates other functions of SMN could be critical.71 Thus while the essential snRNP assembly function of SMN is well defined, the contribution of that function versus other proposed functions of SMN in SMA disease development is not. Therefore, there is an urgent need to define the parameters that cause SMA.

Development of Motor Neurons

Motor Nuclei

Trigeminal

The motor neurons innervating the muscles of mastication are myotopically organized within the motor trigeminal nucleus (Figure 1(a)). Jaw-closer motor neurons that constitute the majority of the motor neurons are dorsomedial to those innervating the jaw-openers. During mastication, jaw-closer and jaw-opener motor neurons receive alternating waves of excitatory glutamatergic input. In addition, during the jaw-opening phase of mastication, jaw-closer motor neurons are under phasic glycinergic inhibition. Such inhibition may dampen jaw-closer reflexes initiated by passive stretching of muscle spindles in jaw-closer muscle fibers, thus making for a smoother and more rapid widening of the mandible. The membrane properties of trigeminal motor neurons have been studied in detail by SH Chandler’s group. In the presence of serotonin, trigeminal motor neurons display plateau potentials and rhythmic bursting behavior, produced by conductance changes in L-type calcium channels. The membrane properties of these motor neurons are further dependent on Ca2+-dependent K+ channels and Na+/K+-ATPase pump currents, which regulate burst termination during the afterhyperpolarization. Serotonergic input to the motor trigeminal nucleus comes from brain stem raphe nuclei and could thus facilitate rhythmical bursting via these intrinsic motor neuron membrane properties to augment excitatory phasic input coming from a central pattern generator.

Figure 1. Major brain stem structures mediating consummatory responses of licking/mastication and swallowing. Sensory areas in blue, reticular formation structures in gray, and motor nuclei in red. C, jaw closure motor neurons in dorsal subdivisions of motor trigeminal nucleus; Es, esophageal generator neurons in the central subdivision of the nucleus of the solitary tract; Gi, nucleus gigantocellularis; IRt, intermediate zone of the medullary reticular formation; Mes V, mesencephalic trigeminal nucleus; NST, nucleus of the solitary tract; O, jaw opener motor neurons in the ventromedial subdivision of the motor trigeminal nucleus; OP, oropharyngeal generator neurons in the lateral and ventrolateral nucleus of the solitary tract; P, lingual protrudor motor neurons in dorsal subdivision of hypoglossal nucleus; PBN, waist area of parabrachial nucleus; PCRt, parvocellular zone of the medullary reticular formation; PnC, nucleus pontis caudalis; R, lingual retractor motor neurons in ventral subdivision of hypoglossal nucleus; t, solitary tract; V2, maxillary subdivision of principal trigeminal nucleus (a) or oral subdivision of spinal trigeminal complex (b); V3, mandibular subdivision of principal trigeminal nucleus (a) or oral subdivision of spinal trigeminal complex (b); 7, facial nucleus. Coronal sections adapted from Paxinos G and Watson C (2005) The Rat Brain in Stereotaxic Coordinates, 5th edn. Amsterdam: Elsevier.

Motor neurons

Motor neurons form the efferent division of the PNS. There are about 500,000 of them, carrying information from the CNS to peripheral effectors in the peripheral tissues and organ systems. Efferent fibers are the axons of motor neurons, and carry data away from the CNS. The two primary efferent systems are the somatic nervous system (SNS) and the autonomic (visceral) nervous system (ANS). The somatic nervous system includes the somatic motor neurons, which innervate skeletal muscles. The SNS is under conscious control. Cell bodies of somatic motor neurons are in the CNS. Axons travel through peripheral nerves, innervating skeletal muscle fibers at neuromuscular junctions.

The ANS is not under conscious control, with visceral motor neurons stimulating all peripheral effectors except for skeletal muscles. They innervate cardiac and smooth muscle, adipose tissue, and glands. Visceral motor axons in the CNS innervate other visceral motor neurons in the peripheral autonomic ganglia. The cell bodies of neurons innervate and control peripheral effectors. Preganglionic fibers are axons that extend from the CNS to autonomic ganglia. Postganglionic fibers are axons that connect ganglion cells with peripheral effectors.

Conclusion

SMN2 is the main SMA disease modifier in individuals with SMN1 deletions or mutations. Identification of modifying genes and pathways as well as molecules or drugs that upregulate the expression of FL-SMN2 RNA or stabilize the SMN protein is, without any doubt, the most promising strategy in SMA therapy. Although SMA is primarily caused by motor neuron dysfunction, there is increasing evidence that reduction of SMN, below a certain threshold, affects various molecular and cellular pathways, causing multiorgan dysfunction mainly correlated with severe SMA phenotype in humans and mice. In addition to SMN-dependent strategies, identification of SMA modifiers not only helps us to understand the most crucially disturbed pathways in SMA but also unravels ways to counteract SMA pathology. Combinatorial therapies may be crucial for lifelong functional maintenance of cellular integrity in SMN1-deleted individuals. Moreover, many of the pathways may reveal common molecular mechanism(s) underlying the pathogenesis of other neurodegenerative diseases and may be beneficial for drug development in a broader neurodegenerative context.

Development of the spinal cord

Motor neuron development results from ventral polarization of the neural tube. Motor neurons and dorsal root ganglion cells appear around E27. Cells expressing Nkx6.1, Olig2, and Pax6 form the so-called motor neuron subdomain. Neural crest cells give rise to peripheral nervous system sensory neurons, postganglionic neurons, Schwann cells, satellite cells of both the dorsal root ganglia and the autonomic ganglia, and the endocrine cells of the paraganglia, including adrenergic cells of the adrenals. The molecules involved in neural crest cell induction include the bone morphogenetic proteins and Wnt6 (Bayer and Altman, 2007a; Catala and Kubis, 2013).

The Motor Neuron Has Dendrites, a Cell Body, and an Axon

Motor neurons are large cells in the ventral horn of the spinal cord as shown in Figure 3.2.1. They have a number of processes called dendrites that bring signals to the motor neuron. The motor neuron also has one large process, the axon, that connects the motor neuron on one end with a muscle fiber on the other. Action potentials move along the axon so that activity in the motor neuron alters activity in the muscle.

Axons from neurons can be myelinated or unmyelinated. Myelin refers to a sheath that covers the axon, but not entirely. In the peripheral nervous system, Schwann cells make the myelin by wrapping themselves around the axon, forming a multilayered structure of multiple cell membranes of the Schwann cell. In the central nervous system, oligodendroglial cells make the myelin. The sheath is not continuous in either the peripheral or central nervous system. At the end of each Schwann cell, there is a gap in the myelin. This gap is called the Node of Ranvier (see Figure 3.2.2).

Like all cells, the motor neuron has a nucleus located in its cell body or soma. The soma is also sometimes referred to as the perikaryon from the Greek root “peri” meaning “around” or “surrounding” and “karyon” meaning “nut” or “kernel,” and referring to the nucleus. There is only one nucleus in the motor neuron and it is the site of mRNA transcription.