Motor neuron diseases (MNDs) are neurodegenerative diseases that involve loss of motor neurons in the brain and spinal cord. MNDs are debilitating and often fatal. Distal hereditary motor neuropathies (dHMNs) are a category of MND characterized by progressive, distal weakness without loss of sensation. The primary focus of our laboratory is to understand the functional consequences of mutations in small heat shock proteins (sHSPs) that result in dHMN. sHSPs comprise a family of 10 homologous proteins that are characterized by a central alpha-crystallin domain, are expressed ubiquitously, serve neuroprotective functions, and are upregulated by cell stress. To date, mutations in three sHSPs: HSPB1, HSPB3 and HSPB8, have been associated with dHMN. These mutations include HSPB1(R136W) and HSPB3(R7S). We propose that mutations reported in these proteins affect the same cellular pathway because they all lead to the same clinical phenotype and loss of motor neurons.
HSPB1 is the best characterized sHSP and is required for AU-rich element (ARE)-dependent mRNA decay. AREs are adenosine and uridine rich regions that are present in the 3’ untranslated region of a subset of mRNAs that signal for their rapid decay. We hypothesize that dHMN-associated mutations result in dysregulation of this critical mRNA decay pathway, and that mutations in HSPB1 and in HSPB3 result in an increased half-life of ARE-containing mRNAs. To determine the effect of sHSP mutations on ARE-mediated mRNA decay, we measured the rate of ARE-mediated mRNA decay in macrophages transfected with the wild-type HSPB1 gene or the mutant HSPB1(R136W) gene. However, these experiments must be replicated before conclusions can be drawn about the role of HSPB1(R136W) in ARE-mediated mRNA decay in vitro. To determine the effect of the HSPB1(R136W) on ARE-mediated mRNA decay in vivo, we developed transgenic mice expressing either the wild-type HSPB1 or mutant HSPB1(R136W) trangenes under the prion-protein promoter (PrP) to ensure high expression of the wild-type HSPB1 and mutant HSPB1(R136W) transgenes in neurons. We determined that expression of the PrP-driven HSPB1(R136W) transgene resulted in a subclinical motor neuropathy. We will use this mouse model in the future to determine the effect of mutant HSPB1(R136W) on ARE-mediated mRNA decay in vivo.
Little is known about the function of wild-type HSPB3 or about the effect of HSPB3(R7S) on that function. Therefore, we have begun to characterize wild-type and mutant HSPB3. Further studies must be performed to determine the effect of HSPB3 and HSPB3(R7S) on ARE-mediated mRNA decay in vitro and in vivo. Determining the effect of these small heat shock protein mutations on ARE-mediated mRNA decay in vitro and in vivo will further our mechanistic understanding of how small heat shock protein mutations lead to dHMN, and MNDs in general.A one-year embargo was granted for this item
