**1. Introduction**

156 Amyotrophic Lateral Sclerosis

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Since its first description in 1874 by Charcot, the hallmark feature of ALS is the progressive degeneration of upper and lower motoneurons (Charcot, 1874). In the spinal cord, motoneuron degeneration starts long before symptom onset and advances in a size-related fashion, in which large-size alpha-motoneurons degenerate first followed by small-size alpha-motoneurons (Pun et al., 2006; Hegedus et al., 2007; Hegedus et al., 2008). There are conflicting reports regarding the survival of the smallest-sized spinal motoneurons, the gamma-motoneurons (Swash and Fox, 1974; Sobue et al., 1981). Despite its original description, the neuronal degeneration in ALS is not limited to motoneurons. Recent reports have shown evidence for degeneration of neurons in the brain (Karim et al., 1998; Lloyd et al., 2000; Maekawa et al., 2004) and interneurons in the spinal cord (Konno et al., 1986; Williams et al., 1990; Takahashi et al., 1993; Stephens et al., 2006).

Before their degeneration, spinal motoneurons experience progressive changes in their properties. These changes result from the pathological actions of the disease and the compensatory mechanisms of the nervous system to mitigate the neuronal malfunction. In this chapter, we describe the changes in the anatomical and electrical properties of spinal motoneurons in various genetic mouse models of ALS and critically analyze literature for the common and different pathological features across these models. We also present data from computer simulations showing the consequences of the alterations in properties of mutant motoneurons on cell excitability and dendritic processing of synaptic inputs. The presented computational analysis allowed for the identification of motoneuron alterations undetectable using standard electrophysiological methods. This information is essential for understanding motoneuron pathophysiology in ALS.

### **2. The changes in motoneuron anatomical properties**

One of the earliest abnormalities in transgenic mouse models of ALS is the change in anatomy of spinal motoneurons. In the G85R model, it has been shown that mutant

Electrophysiological Abnormalities in SOD1 Transgenic

to control (Sasaki and Maruyama, 1992).

motoneuron degeneration in ALS (see section 6).

**3.1 The change in input resistance** 

2003; Quinlan et al., 2011).

**3. The changes in motoneuron electrical properties** 

relative to WT.

Models in Amyotrophic Lateral Sclerosis: The Commonalities and Differences 159

slice preparation show that neonatal mutant motoneurons (P7) of the G93A (high expressor line), which expresses 25 copies of the human SOD1 gene, experience analogous alterations in their morphology and dendritic branching (Fig. 1), in which the soma surface area, numbers of dendritic branches and terminal dendrites increase significantly

Post-mortem morphological analysis of large neurons in the ventral horn of spinal cord indicated an increase in the initial segment diameter, by 36%, in ALS patients (Sasaki and Maruyama, 1992). Conversely, the soma area of these neurons was smaller by 16% relative

The similar findings on the anatomical alterations of neonatal motoneurons in various transgenic models of ALS indicate that the increase in cell size and dendritic overbranching are characteristic abnormalities in motoneuron pathophysiology in ALS. However, it is unclear whether the alteration in motoneuron anatomy is a disease mechanism or an adaptive response (see section 7 for discussion). Furthermore, it is important to understand the functional ramifications of the motoneuron anatomical alterations on the cell excitability and electrical properties and to what extent the anatomical alterations contribute to

Changes in a number of motoneuron electrical properties were reported in the transgenic mouse models of ALS. However, one challenge in the field of ALS is the large variability in data of mutant motoneuron electrical properties, which makes recording from a large number of cells is important to allow statistical tests to detect significant differences in electrophysiological properties between WT and mutant motoneuron samples. Table 1 summarizes the changes in electrical properties of mutant motoneurons observed in the various ALS transgenic mouse models. The summary shows that the changes in electrical properties involve the motoneuron passive properties (e.g., cell input resistance), the size and shape of the action potential spike and afterhyperpolarization, the amplitude of voltagesensitive ionic currents, and the motoneuron excitability and firing activity. However, some of these changes were inconsistent (i.e., not observed consistently) across the ALS transgenic models, whereas others were contradictory (i.e., opposite changes were observed). For instance, the decrease in input resistance of mutant motoneurons relative to WT is an inconsistent finding in ALS transgenic models (see table 1, column 7). This observation was seen in some studies (Bories et al., 2007; ElBasiouny et al., 2010; Quinlan et al., 2011), whereas other studies did not find a statistically significant difference between the input resistance of WT and mutant motoneurons (Pieri et al., 2003; Kuo et al., 2004; Kuo et al., 2005; van Zundert et al., 2008; Pambo-Pambo et al., 2009; Pieri et al., 2009; Filipchuk et al., 2010; Meehan et al., 2010). The changes in action potential size and width in mutant motoneurons, on the other hand, is a conflicting finding in which some studies reported an increase in spike height (van Zundert et al., 2008) and duration (Fuchs et al., 2008, 2009; Pambo-Pambo et al., 2009) relative to WT, whereas others reported reduction in spike height (Kuo et al., 2004; Pambo-Pambo et al., 2009; Filipchuk et al., 2010) and duration (Pieri et al.,

motoneurons exhibit an increase in the overall cell size and branching pattern of dendrites (Amendola et al., 2007; Amendola and Durand, 2008). These changes appear in transgenic mice at 10 days after birth (P10), long before disease onset and life end-stage (225 and 240 days, respectively) (Bruijn et al., 1997). More specifically, in mutant motoneurons the total dendritic surface area and total dendritic length are increased by 58% and 65% relative to wild-type (WT) cells, respectively. For dendritic branching, the number of dendritic branches, terminal branches, branching nodes, and maximum branching order are increased by 93%, 89%, 97%, 37%, respectively (Fig. 1). On the other hand, the soma size (diameter and surface area) and primary dendrites properties (number, diameter, and cross-sectional area) are similar in WT and mutant motoneurons. Interestingly, the increase in number of branches occurs primarily in short branches (dendritic branches with path length < 100µm) and most of the anatomical changes take place over the middle dendrites (200µm - 500µm from the soma). Surprisingly, the longest dendritic length from the soma was not increased in mutant motoneurons, indicating that dendritic overbranching occurs mainly within the cell circumjacent. In other words, the cell does not swell into larger space in the cord but develops more branching within its dendritic spatial field.

Fig. 1. Anatomical alterations in mutant motoneurons of the G85R (reconstructed from the whole-cord prep1) and G93A (high expressor line) (reconstructed from the slice prep.) transgenic models relative to WT cells. 1: data from Amendola and Durand (2008).

Similar early abnormalities in the anatomical properties of motoneurons have been found in other transgenic mouse models of ALS. In the G93A (low expressor line), which expresses 8 copies of the human SOD1 gene, mutant motoneurons at P9 exhibit increased cell size, dendritic overbranching, and dendritic complexity (Filipchuk et al., 2010). The total dendritic length and number of branching nodes and terminal dendrites in mutant motoneurons were significantly increased relative to WT. Similar to the G85R transgenic model, these anatomical changes in the G93A (low expressor line) model occur early in the disease (at P9) way before symptom development (between 195 and 240 days) and end stage (between 240 and 270 days). Preliminary data from our laboratory obtained in the slice preparation show that neonatal mutant motoneurons (P7) of the G93A (high expressor line), which expresses 25 copies of the human SOD1 gene, experience analogous alterations in their morphology and dendritic branching (Fig. 1), in which the soma surface area, numbers of dendritic branches and terminal dendrites increase significantly relative to WT.

Post-mortem morphological analysis of large neurons in the ventral horn of spinal cord indicated an increase in the initial segment diameter, by 36%, in ALS patients (Sasaki and Maruyama, 1992). Conversely, the soma area of these neurons was smaller by 16% relative to control (Sasaki and Maruyama, 1992).

The similar findings on the anatomical alterations of neonatal motoneurons in various transgenic models of ALS indicate that the increase in cell size and dendritic overbranching are characteristic abnormalities in motoneuron pathophysiology in ALS. However, it is unclear whether the alteration in motoneuron anatomy is a disease mechanism or an adaptive response (see section 7 for discussion). Furthermore, it is important to understand the functional ramifications of the motoneuron anatomical alterations on the cell excitability and electrical properties and to what extent the anatomical alterations contribute to motoneuron degeneration in ALS (see section 6).
