Our laboratory is interested in how genetics and environment combine to regulate development of the vertebrate nervous system. The current focus of our research is on the roles of small heat shock proteins (sHsps) during development of the neuromuscular system both normally and in the context of environmental stress. Dominant mutations in the genes encoding either one of two sHsps – HSP27 or HSPB8 – can cause either either Charcot-Marie-Tooth disease (CMT) or Distal Hereditary Motor Neuropathy (dHMN) in humans. CMT is the most common inherited neuromuscular disease, affecting approximately 1 in 2500 individuals. It is characterized by degeneration of long motor and sensory axons, resulting in muscle atrophy and long-term disability. dHMN differs only in that motor, but not sensory, axons are affected. The median age of onset of CMT is approximately 16, but the cell biological events leading to degeneration presumably occur earlier. Furthermore, these diseases exhibit a great deal of heterogeneity, even within families, suggesting that non-genetic factors may play an important role.
We are using zebrafish as a model system to try to better understand the mechanisms by which HSP27 and HSPB8 cause axon degeneration. We have analyzed the expression patterns of all of the sHsps genes during embryonic and larval development, finding that both hsp27 and hspb8 are expressed at lows levels during development of the nervous system, but that they are also expressed at high levels in muscle. Both genes are also dramatically upregulated by heat shock (Marvin et al., 2008 and below).
We are beginning to test whether mutations in zebrafish hsp27 or hspb8 that mimic the mutations in the human diseases affect development of the motor axon projection and whether non-genetic factors exacerbate the phenotype. To test these hypotheses, we are generating transgenic lines that express the mutant variants of hsp27 and hspb8 in the zebrafish and using a combination of behavioral and histochemical analysis to determine the effects in embryos, juvenile, and adults. In addition, because hsp27 and hspb8 are expressed in both nervous system and muscle, we are interested in whether hsp27 and hspb8 mutations directly affect axons, or whether their effects on axons are secondary to effects on muscle. To determine this, we are misexpressing the disease variants specifically in neurons or specifically in muscle and analyzing the effects using the methods described above.
