AAV2 mediated retrograde transduction of corticospinal motor neurons reveals initial and selective apical dendrite degeneration in ALS

Abstract

Corticospinal motor neurons (CSMN) are the cortical component of motor neuron circuitry, which controls voluntary movement and degenerates in diseases such as amyotrophic lateral sclerosis, primary lateral sclerosis and hereditary spastic paraplegia. By using dual labeling combined with molecular marker analysis, we identified AAV2-2 mediated retrograde transduction as an effective approach to selectively target CSMN without affecting other neuron populations both in wild-type and hSOD1G93A transgenic ALS mice. This approach reveals very precise details of cytoarchitectural defects within vulnerable neurons in vivo. We report that CSMN vulnerability is marked by selective degeneration of apical dendrites especially in layer II/III of the hSOD1G93A mouse motor cortex, where cortical input to CSMN function is vastly modulated. While our findings confirm the presence of astrogliosis and microglia activation, they do not lend support to their direct role for the initiation of CSMN vulnerability. This study enables development of targeted gene replacement strategies to CSMN in the cerebral cortex, and reveals CSMN cortical modulation defects as a potential cause of neuronal vulnerability in ALS.

Motor Cortex Layer V Pyramidal Neurons Exhibit Dendritic Regression, Spine Loss, and Increased Synaptic Excitation in the Presymptomatic hSOD1G93AMouse Model of Amyotrophic Lateral Sclerosis

“…By contrast, LVPN basal arbor length was unchanged at both P21-P27 (WT, 2290 Ϯ 327 m, n ϭ 6; SOD1, 1497 Ϯ 366 m, n ϭ 9; p ϭ 0.61) and P28 -P40 (WT, 2930 Ϯ 444 m, n ϭ 14; SOD1, 2691 Ϯ 407 m, n ϭ 15; p Ͼ 0.99). There was no difference in LVPN soma size between SOD1 and WT at P21-P27 (WT, 1570 Ϯ 379 m 3 , n ϭ 6; SOD1, 1588 Ϯ 161 m 3 , n ϭ 9; p Ͼ 0.99) and P28 -P40 (WT, 2273 Ϯ 255 m 3 , n ϭ 14; SOD1, 1979 Ϯ 204 m 3 , n ϭ 15; p ϭ 0.68; but see Ozdinler et al, 2011;Jara et al, 2012).…”

“…Statistically significant differences were determined using two-way ANOVAs with Bonferroni's post-test, where *p Ͻ 0.05, **p Ͻ 0.01, ***p Ͻ 0.001, and ****p Ͻ 0.0001 were statistically significant. A power analysis (GPower3; Faul et al, 2007) showed that sample size could detect an effect size of 0.25 with p Ͼ 0.8.…”

“…Cu/Zn-superoxide dismutase (SOD1) mutations in familial ALS (Rosen et al, 1993) have provided key pathogenic insights into ALS, through the use of transgenic mice overexpressing the mutant human hSOD1 G93A gene, which exhibit progressive MN degeneration resembling human ALS (Gurney et al, 1994).…”

“…Golgi-impregnated human LVPNs show dendritic degeneration, "stumped" apical dendrites that do not reach the pia, dendritic spine loss and somatic swellings (Hammer et al, 1979). In addition, cortical hyperexcitation, proposed as a key determinant in clinical progression (Bae et al, 2013), may occur before clinical symptoms (Vucic et al, 2008) and separates ALS from other neuromotor disorders (Vucic et al, 2011).…”

“…In the mutant SOD1 mouse model, corticospinal neurons (a type of LVPN) display somatic abnormalities at postnatal day (P) 30 (Ozdinler et al, 2011) with cell loss, autophagic apical dendrite degeneration, and spine loss by P60 (Jara et al, 2012). Additionally, cortical cultures from SOD1 mice show intrinsic hyperexcitability (Pieri et al, 2009).…”