Neurodegenerative diseases including amyotrophic lateral sclerosis (ALS) are characterized by the presence of protein inclusions in the affected neurons. Emerging data indicate that protein misfolding may be of mechanistic importance in these diseases.1 Mutations in the ubiquitously expressed superoxide dismutase (SOD1) gene account for 20% of cases of the familial form of ALS. More than 150 mutations in the SOD1 gene have been discovered, including the point mutations G93R and G85R.2 Recent studies also implicate SOD1 in the sporadic form of ALS and suggest a prionlike propagation of misfolded SOD1.3–5 Interestingly, some of the newly identified genes implicated in ALS, such as TARDBP and FUS, are also proteins that show a high propensity to misfold and prionlike activity.6 However, we still do not know the precise mechanism by which mutant proteins cause toxicity.5,7 The emerging consensus view is that multiple interacting pathophysiological factors, including protein misfolding, contribute to the neuronal toxicity in ALS.8,9 Despite progress in revealing multiple molecular processes involved in disease pathology, relatively little is known about when and how the disekease, which starts focally, spreads throughout the motor network.10–12 Interestingly, even in the subtypes of ALS caused by SOD1 mutations, there is considerable phenotypic heterogeneity. Ravits and La Spada12 hypothesized that despite disease heterogeneity, the disease poses common themes that may involve common mechanisms. They propose that ALS may in fact be an orderly, actively propagating process and that fundamental molecular mechanisms may be uniform. The zebrafish is emerging as a useful tool for studying neurological diseases relevant to humans. Previously, we had shown that mutant sod1 transgenic fish show the hallmarks of adult onset neurodegenerative ALS, including defective motor performance, motor neuron loss, a loss of neuromuscular connectivity, and muscle atrophy.13 The aforementioned observations demonstrate the usefulness of the zebrafish as a model for this disease. However, among the current limitations when working with in vivo models of ALS is the lack of a good readout for the presymptomatic course of the disease. The zebrafish offer great advantages in studying early disease processes, as they develop rapidly, reaching postembryonic life at around 3 days postfertilization (dpf), which is developmentally similar to the neonatal mouse (for a comparison of developmental stages in human, mouse, and zebrafish, see Table 1). Moreover, the embryonic and larval zebrafish spinal cord is functionally and anatomically similar to that of humans, yet it is also optically transparent and experimentally accessible, making it ideal for the study of spinal circuits in normal and pathophysiological conditions.14 TABLE 1 Comparison of Neural Developmental Stages in Humans, Mice, and Zebrafish In the current study, we monitored in vivo early neurological changes caused by mutant sod1 gene. The sod1 zebrafish ALS model harbors a fluorescent heat shock stress response (HSR) reporter gene (hsp70-DsRed). The HSR is an endogenous cellular pathway that attempts to refold the damaged proteins in stressed cells, although this response is not always sufficient or beneficial.15 Thus, the HSR-mediated DsRed fluorescence in the sod1 zebrafish model of ALS represents a useful tool for monitoring perturbations in cellular homeostasis caused by sod1 mutation. This facilitates the mapping of disease focality and spread through the central nervous system (CNS) by the spatiotemporal readout of the neuronal stress response in the spinal cord of mutant zebrafish and provides an understanding of the cells and networks involved in disease propagation in ALS. We present evidence that the HSR is an indicator of early pathogenic processes occurring in neurons. The HSR is first observed at embryonic stages, in discrete populations of inhibitory interneurons in the spinal cord, and is followed by dysregulation of glycine release from these inhibitory interneurons. Furthermore, we observe that following interneuron dysfunction, motor neurons start exhibiting neuronal stress. More interestingly, we show that motor neurons showing the HSR also show dysfunctional neuromuscular junctions (NMJs). Taken together, our observations suggest that the mutant sod1-induced HSR is a robust predictor of neuronal dysfunction and thus is a reliable marker of disease pathogenesis. Finally, we also show that the neuronal stress readout can be used to identify neuroprotective compounds such as riluzole and identify biological targets that may ameliorate early pathophysiological disease processes that are currently not well explored. Although the sod1 zebrafish model may by itself not be sufficient in developing new therapies for ALS, this model system would provide a rapid way to triage compounds for screening in higher vertebrate models, with the potential for more rapid identification of promising compounds for translation into human clinical trials.