Mutation of COOH-terminal lysines in overexpressed alpha B-crystallin abrogates ischemic protection in cardiomyocytes

Neuropathy- and myopathy-associated mutations in human small heat shock proteins: Characteristics and evolutionary history of the mutation sites

Mutations in four of the ten human small heat shock proteins (sHSP) are associated with various forms of motor neuropathies and myopathies. In HspB1, HspB3, and HspB8 all known mutations cause motor neuropathies, whereas in HspB5 they cause myopathies. Several features are common to the majority of these mutations: (i) they are missense mutations, (ii) most associated disease phenotypes exhibit a dominant inheritance pattern and late disease onset, (iii) in the primary protein sequences, the sites of most mutations are located in the conserved α-crystallin domain and the variable C-terminal extensions, and (iv) most human mutation sites are highly conserved among the vertebrate orthologs and have been historically exposed to significant purifying selection. In contrast, a minor fraction of these mutations deviate from these rules: they are (i) frame shifting, nonsense, or elongation mutations, (ii) associated with recessive or early onset disease phenotypes, (iii) positioned in the N-terminal domain of the proteins, and (iv) less conserved among the vertebrates and were historically not subject to a strong selective pressure. In several vertebrate sHSPs (including primate sHSPs), homologous sites differ from the human sequence and occasionally even encode the same amino acid residues that cause the disease in humans. Apparently, a number of these mutations sites are not crucial for the protein function in single species or entire taxa, and single species even seem to have adopted mechanisms that compensate for potentially adverse effects of 'mutant-like' sHSPs. The disease-associated dominant sHSP missense mutations have a number of cellular consequences that are consistent with gain-of-function mechanisms of genetic dominance: dominant-negative effects, the formation of cytotoxic amyloid protein oligomers and precipitates, disruption of cytoskeletal networks, and increased downstream enzymatic activities. Future therapeutic concepts should aim for reducing these adverse effects of mutant sHSPs in patients. Indeed, initial experimental results are encouraging.

O-GlcNAcylation of αB-crystallin regulates its stress-induced translocation and cytoprotection

Under normal conditions, the ubiquitously expressed αB-crystallin functions as a chaperone. αB-crystallin has been implicated in a variety of pathologies, consistent with a build-up of protein aggregates, such as neuromuscular disorders, myofibrillar myopathies, and cardiomyopathies. αB-crystallins' cardioprotection is partially attributed to its translocation and binding to cytoskeletal elements in response to stress. The triggers for this translocation are not clearly understood. In the heart, αB-crystallin undergoes at least three significant post-translational modifications: phosphorylation at ser-45 and 59 and O-GlcNAcylation (O-linked attachment of the monosaccharide β-N-acetyl-glucosamine) at thr-170. Whether phosphorylation status drives translocation remains controversial. Therefore, we evaluated the role of αB-crystallins' O-GlcNAcylation in its stress-induced translocation and cytoprotection in cardiomyocytes under stress. Immunoblotting and precipitation experiments with anti-O-GlcNAc antibody (CTD110.6) and glycoprotein staining (Pro-Q Emerald) both demonstrate robust stress-induced O-GlcNAcylation of αB-crystallin. A non-O-GlcNAcylatable αB-crystallin mutant (αB-T170A) showed diminished translocation in response to heat shock and robust phosphorylation at both ser-45 and ser-59. Cell survival assays show a loss of overexpression-associated cytoprotection with the non-glycosylatable mutant to multiple stresses. While ectopic expression of wild-type αB-crystallin strongly stabilized ZsProSensor, a fusion protein rapidly degraded by the proteasome, the non-O-GlcNAcylatable version did not. Therefore, we believe the O-GlcNAcylation of αB-crystallin is a dynamic and important regulator of both its localization and function.

Mediators of ischemic preconditioning identified by microarray analysis of rat spinal cord

Spinal ischemia is a frequent cause of paralysis. Here we explore the biological basis of ischemic preconditioning (IPC), the phenomenon in which a brief period of ischemia can confer protection against subsequent longer and normally injurious ischemia, to identify mediators of endogenous neuroprotection. Using microarrays, we examined gene expression changes induced by brief spinal ischemia using a rat balloon occlusion model. Among the nearly 5000 genes assayed, relatively few showed two-fold changes, and three groups stood out prominently. The first group codes for heat shock protein 70, which is induced selectively and robustly at 30 min after brief ischemia, with increases up to 100-fold. A second group encodes metallothioneins 1 and 2. These mRNAs are increased at 6 and 12 h after ischemia, up to 12-fold. The third group codes for a group of immediate-early genes not previously associated with spinal ischemia: B-cell translocation gene 2 (BTG2), the transcription factors early growth response 1 (egr-1) and nerve growth factor inducible B (NGFI-B), and a mitogen-activated protein kinase phosphatase, ptpn16, an important cell signaling regulator. These mRNAs peak at 30 min and return to baseline or are decreased 6 h after ischemia. Several other potentially protective genes cluster with these induced mRNAs, including small heat shock proteins, and many have not been previously associated with IPC. These results provide both putative mediators of IPC and molecular targets for testing preconditioning therapies.

αB-Crystallin Modulates Protein Aggregation of Abnormal Desmin

αB-crystallin (CryAB) is the most abundant small heat shock protein in the heart. Upregulation of CryAB in desmin-related myopathy and its downregulation in end-stage congestive heart failure have both been reported. We previously demonstrated via cardiac-specific transgenesis that modest increases in normal CryAB are not detrimental to the heart, whereas expression of the R120G mutation of CryAB caused a desminopathy. It is generally believed that CryAB plays an important role in protecting the intermediate filaments, but the underlying mechanism is unclear. We hypothesized that CryAB protects the desmin filaments via preventing abnormal desmin protein from aggregating adversely. To test this hypothesis in vivo, mice expressing a desmin mutation that causes a desmin-related cardiomyopathy (D7) were bred into the R120G-CryAB transgenic (TG) background to examine the accumulation and aberrant aggregation of desmin protein. Despite lower mRNA expression of D7-des than in the D7-des TG hearts, the double-TG myocardium exhibited significantly higher desmin protein levels and dramatically more aberrant desmin aggregates than the D7-des TG hearts. The double-TG mice displayed a significantly stronger cardiac hypertrophic response, with the mice dying of congestive heart failure before 7 weeks. To explore the ability of wild-type (WT) CryAB to protect against mutant desmin, a desmin mutant was expressed in both the conventional and WT-CryAB stably transfected HEK cells. Significantly less aberrant desmin aggregation was observed in the WT-CryAB–overexpressing cells than in the HEK cells. The results suggest that CryAB modulates abnormal desmin aggregation and can serve a cardioprotective role.