Jennifer L. Wacker

Modulation of neurodegeneration by molecular chaperones

Many neurodegenerative disorders are characterized by conformational changes in proteins that result in misfolding, aggregation and intra- or extra-neuronal accumulation of amyloid fibrils. Molecular chaperones provide a first line of defence against misfolded, aggregation-prone proteins and are among the most potent suppressors of neurodegeneration known for animal models of human disease. Recent studies have investigated the role of molecular chaperones in amyotrophic lateral sclerosis, Alzheimers disease, Parkinsons disease and polyglutamine diseases. We propose that molecular chaperones are neuroprotective because of their ability to modulate the earliest aberrant protein interactions that trigger pathogenic cascades. A detailed understanding of the molecular basis of chaperone-mediated protection against neurodegeneration might lead to the development of therapies for neurodegenerative disorders that are associated with protein misfolding and aggregation.

Hsp70 and Hsp40 attenuate formation of spherical and annular polyglutamine oligomers by partitioning monomer

Protein conformational changes that result in misfolding, aggregation and amyloid fibril formation are a common feature of many neurodegenerative disorders. Studies with β-amyloid (Aβ), α-synuclein and other amyloid-forming proteins indicate that the assembly of misfolded protein conformers into fibrils is a complex process that may involve the population of metastable spherical and/or annular oligomeric assemblies. Here, we show by atomic force microscopy that a mutant huntingtin fragment with an expanded polyglutamine repeat forms spherical and annular oligomeric structures reminiscent of those formed by Aβ and α-synuclein. Notably, the molecular chaperones Hsp70 and Hsp40, which are protective in animal models of neurodegeneration, modulate polyglutamine aggregation reactions by partitioning monomeric conformations and disfavoring the accretion of spherical and annular oligomers.

Histone Deacetylase Inhibition Modulates Kynurenine Pathway Activation in Yeast, Microglia, and Mice Expressing a Mutant Huntingtin Fragment

The kynurenine pathway of tryptophan degradation is hypothesized to play an important role in Huntington disease, a neurodegenerative disorder caused by a polyglutamine expansion in the protein huntingtin. Neurotoxic metabolites of the kynurenine pathway, generated in microglia and macrophages, are present at increased levels in the brains of patients and mouse models during early stages of disease, but the mechanism by which kynurenine pathway up-regulation occurs in Huntington disease is unknown. Here we report that expression of a mutant huntingtin fragment was sufficient to induce transcription of the kynurenine pathway in yeast and that this induction was abrogated by impairing the activity of the histone deacetylase Rpd3. Moreover, numerous genetic suppressors of mutant huntingtin toxicity that are functionally unrelated converged unexpectedly on the kynurenine pathway, supporting a critical role for the kynurenine pathway in mediating mutant huntingtin toxicity in yeast. Histone deacetylase-dependent regulation of the kynurenine pathway was also observed in a mouse model of Huntington disease, in which treatment with a neuroprotective histone deacetylase inhibitor blocked activation of the kynurenine pathway in microglia expressing a mutant huntingtin fragment in vitro and in vivo. These findings suggest that a mutant huntingtin fragment can perturb transcriptional programs in microglia, and thus implicate these cells as potential modulators of neurodegeneration in Huntington disease that are worthy of further investigation.

Loss of Hsp70 Exacerbates Pathogenesis But Not Levels of Fibrillar Aggregates in a Mouse Model of Huntington's Disease

Endogenous protein quality control machinery has long been suspected of influencing the onset and progression of neurodegenerative diseases characterized by accumulation of misfolded proteins. Huntingtons disease (HD) is a fatal neurodegenerative disorder caused by an expansion of a polyglutamine (polyQ) tract in the protein huntingtin (htt), which leads to its aggregation and accumulation in inclusion bodies. Here, we demonstrate in a mouse model of HD that deletion of the molecular chaperones Hsp70.1 and Hsp70.3 significantly exacerbated numerous physical, behavioral and neuropathological outcome measures, including survival, body weight, tremor, limb clasping and open field activities. Deletion of Hsp70.1 and Hsp70.3 significantly increased the size of inclusion bodies formed by mutant htt exon 1, but surprisingly did not affect the levels of fibrillar aggregates. Moreover, the lack of Hsp70s significantly decreased levels of the calcium regulated protein c-Fos, a marker for neuronal activity. In contrast, deletion of Hsp70s did not accelerate disease in a mouse model of infectious prion-mediated neurodegeneration, ruling out the possibility that the Hsp70.1/70.3 mice are nonspecifically sensitized to all protein misfolding disorders. Thus, endogenous Hsp70s are a critical component of the cellular defense against the toxic effects of misfolded htt protein in neurons, but buffer toxicity by mechanisms independent of the deposition of fibrillar aggregates.

Disease-causing mutation in GPR54 reveals the importance of the second intracellular loop for class A G-protein-coupled receptor function.

The G-protein-coupled receptor (GPCR) GPR54 is essential for the development and maintenance of reproductive function in mammals. A point mutation (L148S) in the second intracellular loop (IL2) of GPR54 causes idiopathic hypogonadotropic hypogonadism, a disorder characterized by delayed puberty and infertility. Here, we characterize the molecular mechanism by which the L148S mutation causes disease and address the role of IL2 in Class A GPCR function. Biochemical, immunocytochemical, and pharmacological analysis demonstrates that the mutation does not affect the expression, ligand binding properties, or protein interaction network of GPR54. In contrast, diverse GPR54 functional responses are markedly inhibited by the L148S mutation. Importantly, the leucine residue at this position is highly conserved among class A GPCRs. Indeed, mutating the corresponding leucine of the α1A-AR recapitulates the effects observed with L148S GPR54, suggesting the critical importance of this hydrophobic IL2 residue for Class A GPCR functional coupling. Interestingly, co-immunoprecipitation studies indicate that L148S does not hinder the association of Gα subunits with GPR54. However, fluorescence resonance energy transfer analysis strongly suggests that L148S impairs the ligand-induced catalytic activation of Gα. Combining our data with a predictive Class A GPCR/Gα model suggests that IL2 domains contain a conserved hydrophobic motif that, upon agonist stimulation, might stabilize the switch II region of Gα. Such an interaction could promote opening of switch II of Gα to facilitate GDP-GTP exchange and coupling to downstream signaling responses. Importantly, mutations that disrupt this key hydrophobic interface can manifest as human disease.

Blood pressure is regulated by an α1D-adrenergic receptor/dystrophin signalosome

Hypertension is a cardiovascular disease associated with increased plasma catecholamines, overactivation of the sympathetic nervous system, and increased vascular tone and total peripheral resistance. A key regulator of sympathetic nervous system function is the α1D-adrenergic receptor (AR), which belongs to the adrenergic family of G-protein-coupled receptors (GPCRs). Endogenous catecholamines norepinephrine and epinephrine activate α1D-ARs on vascular smooth muscle to stimulate vasoconstriction, which increases total peripheral resistance and mean arterial pressure. Indeed, α1D-AR KO mice display a hypotensive phenotype and are resistant to salt-induced hypertension. Unfortunately, little information exists about how this important GPCR functions because of an inability to obtain functional expression in vitro. Here, we identified the dystrophin proteins, syntrophin, dystrobrevin, and utrophin as essential GPCR-interacting proteins for α1D-ARs. We found that dystrophins complex with α1D-AR both in vitro and in vivo to ensure proper functional expression. More importantly, we demonstrate that knock-out of multiple syntrophin isoforms results in the complete loss of α1D-AR function in mouse aortic smooth muscle cells and abrogation of α1D-AR-mediated increases in blood pressure. Our findings demonstrate that syntrophin and utrophin associate with α1D-ARs to create a functional signalosome, which is essential for α1D-AR regulation of vascular tone and blood pressure.

Cysteine String Protein (CSP) Inhibition of N-type Calcium Channels Is Blocked by Mutant Huntingtin

Cysteine string protein (CSP), a 34-kDa molecular chaperone, is expressed on synaptic vesicles in neurons and on secretory vesicles in endocrine, neuroendocrine, and exocrine cells. CSP can be found in a complex with two other chaperones, the heat shock cognate protein Hsc70, and small glutamine-rich tetratricopeptide repeat domain protein (SGT). CSP function is vital in synaptic transmission; however, the precise nature of its role remains controversial. We have previously reported interactions of CSP with both heterotrimeric GTP-binding proteins (G proteins) and N-type calcium channels. These associations give rise to a tonic G protein inhibition of the channels. Here we have examined the effects of huntingtin fragments (exon 1) with (huntingtinexon1/exp) and without (huntingtinexon1/nonexp) expanded polyglutamine (polyQ) tracts on the CSP chaperone system. In vitro huntingtinexon1/exp sequestered CSP and blocked the association of CSP with G proteins. In contrast, huntingtinexon1/nonexp did not interact with CSP and did not alter the CSP/G protein association. Similarly, co-expression of huntingtinexon1/exp with CSP and N-type calcium channels eliminated CSPs tonic G protein inhibition of the channels, while coexpression of huntingtinexon1/nonexp did not alter the robust inhibition promoted by CSP. These results indicate that CSPs modulation of G protein inhibition of calcium channel activity is blocked in the presence of a huntingtin fragment with expanded polyglutamine tracts.

α-Dystrobrevin-1 recruits α-catulin to the α1D-adrenergic receptor/dystrophin-associated protein complex signalosome

α1D-Adrenergic receptors (ARs) are key regulators of cardiovascular system function that increase blood pressure and promote vascular remodeling. Unfortunately, little information exists about the signaling pathways used by this important G protein-coupled receptor (GPCR). We recently discovered that α1D-ARs form a “signalosome” with multiple members of the dystrophin-associated protein complex (DAPC) to become functionally expressed at the plasma membrane and bind ligands. However, the molecular mechanism by which the DAPC imparts functionality to the α1D-AR signalosome remains a mystery. To test the hypothesis that previously unidentified molecules are recruited to the α1D-AR signalosome, we performed an extensive proteomic analysis on each member of the DAPC. Bioinformatic analysis of our proteomic data sets detected a common interacting protein of relatively unknown function, α-catulin. Coimmunoprecipitation and blot overlay assays indicate that α-catulin is directly recruited to the α1D-AR signalosome by the C-terminal domain of α-dystrobrevin-1 and not the closely related splice variant α-dystrobrevin-2. Proteomic and biochemical analysis revealed that α-catulin supersensitizes α1D-AR functional responses by recruiting effector molecules to the signalosome. Taken together, our study implicates α-catulin as a unique regulator of GPCR signaling and represents a unique expansion of the intricate and continually evolving array of GPCR signaling networks.

Differential regulation of GPR54 transcription by specificity protein-1 and partial estrogen response element in mouse pituitary cells

Precise spatial and temporal expression of the recently identified G-protein coupled receptor GPR54 is critical for proper reproductive function and metastasis suppression. However, regulatory factors that control GPR54 expression remain unknown. Thus, the identification of these cis-acting DNA elements can provide insight into the role of GPR54 in reproduction and cancer. Using luciferase reporter, electrophoretic mobility shift, and chromatin immunoprecipitation assays, we demonstrate that three SP1 sites and a partial estrogen response element modulate mouse GPR54 (mGPR54) promoter activity. Supporting experiments show transcription factor SP1 binds directly to the mGPR54 promoter region and activates gene expression. In conclusion, these novel findings now identify factors that regulate activity of the mGPR54 promoter, and these factors are highly conserved across multiple mammalian species.

Chaperone Suppression of Aggregated Protein Toxicity

Overwhelming experimental evidence supports the hypothesis that molecular chaperones are critical modulators of protein aggregation and toxicity in a number of protein misfolding diseases. However, the mechanism by which chaperone activity facilitates neuroprotection remains poorly understood. Early intermediates in the assembly process of Aβ aggregates have been found to be potent neurotoxins in vivo, and it is likely that prefibrillar intermediates of other disease proteins may have similar pathogenic effects. Accordingly, a key step in the pathogenesis of the various proteinopathies may stem from the aberrant interactions of altered protein conformations or prefibrillar intermediates with key cellular proteins, effectively sequestering their activity and triggering a cascade of events that culminates in neuronal dysfunction prior to the appearance of inclusions. The vast majority of animal studies have shown that chaperones facilitate neuroprotection in the absence of a visible effect on inclusion formation, suggesting that protective interactions may occur at the level of prefibrillar aggregation intermediates, or by preventing conformational changes that precede the formation of aggregation intermedites. It will be important to develop techniques that enable in vivo detection of early aggregation intermediates for the various protein misfolding diseases and determine how interaction of these intermediates with other cellular proteins, such as the molecular chaperones, alters pathogenesis. Ultimately, it is necessary to understand how the various components of the protein quality proteome work together to regulate the toxicity of misfolded proteins. Effective therapies will likely require the simultaneous modulation of numerous components of the cellular quality control apparatus, and the molecular chaperones will play a key role in these types of approaches. Because the molecular chaperones provide a first line of defense against misfolded proteins, and are likely to function at the earliest stages of disease pathogenesis, they are a particularly exciting prospect for therapeutic intervention.