Splice isoforms of phosducin-like protein control the expression of heterotrimeric G proteins

Phosducin-like protein PoPlp1 impacts cellulase and amylase expression and development in Penicillium oxalicum via the G protein-cAMP signaling pathway

In this study, a phosducin-like protein, PoPlp1, was identified and functionally studied in the cellulase-producing strain Penicillium oxalicum 114-2. PoPlp1 was proven to participate in several biological processes, including mycelium development, conidiation, and expression of cellulases and amylases. With deletion of Poplp1, morphology and development varied significantly in ΔPoplp1. Colony growth, glucose utilization, and the hydrolysis capability of starch and cellulose were limited, whereas conidiation was enhanced. Based on detection of the levels of expression of transcription factors involved in asexual development, we conjectured that PoPlp1 is involved in conidiation via the major factor BrlA. We explored the effect of PoPlp1 on cellulase and amylase expression and observed that cellulase and amylase activity and major gene transcription levels were all dramatically reduced in ΔPoplp1. Deletion of PoPlp1 caused a decrease in intracellular cAMP levels, and the cellulase gene expression level of ΔPoplp1 was restored to a certain extent through external addition of cAMP. These findings demonstrate that PoPlp1 may affect cellulase and amylase expression by regulating cAMP concentration. To comprehensively explore the mechanism of PoPlp1 in regulating multiple biological processes, we performed a comparative transcriptomic analysis between strains P. oxalicum 114-2 and ΔPoplp1. The major cellulase and amylase genes were all downregulated, congrent with the results of real-time quantitative polymerase chain reaction analysis. The genes involved in the G protein-cAMP signaling pathway, including several G-protein-coupled receptors, one regulator of G protein signaling, and two cAMP phosphodiesterases, were disrupted by deletion of PoPlp1. These results confirm the positive function of PoPlp1 in the G protein-cAMP signaling pathway. This functional analysis of PoPlp1 will be very beneficial for further study of the regulatory mechanisms of cellulase expression and other biological processes in P. oxalicum 114-2 via the G protein-cAMP signaling pathway.

Characterization of SPINK2, SPACA7 and PDCL2: Effect of immunization on fecundity, sperm function and testicular transcriptome

Testicular factors play a vital role in spermatogenesis. We characterized the functional role of rat Spink2, Spaca7 and Pdcl2 genes. Their primary, secondary and tertiary structure were deduced in silico. The genes of rat Spink2, Spaca7 and Pdcl2 mRNA were predominantly expressed in the testis. SPINK2, SPACA7 and PDCL2 protein expression was evident in all the cell types of testis and on spermatozoa. Ablation of each of these proteins by active immunization resulted in reduced fecundity and sperm count. Damage to the anatomical architecture of testis and epididymis was evident. In SPINK2 immunized rats, 283 genes were differentially regulated while it was 434 and 872 genes for SPACA7 and PDCL2 respectively. Genes that were differentially regulated in the testis of SPINK2 immunized rats primarily belonged to extracellular exosome formation, extracellular space and response to drugs. SPACA7 ablation affected genes related to extracellular space, oxidation-reduction processes, endoplasmic reticulum membrane and response to drugs. Differential gene expression was observed for nuclear function, protein binding and positive regulation of transcription from RNA polymerase II promoter in testis of PDCL2 immunized rats. Results of our study demonstrate the role of SPINK2, SPACA7 and PDCL2 in spermatogenesis and in important molecular processes that may dictate testicular function and other physiological responses as well.

PDCL2 is essential for spermiogenesis and male fertility in mice

Patients with teratozoospermia exhibit low phosducin-like protein (Pdcl2) expression. As a member of the phosducin family, chaperonin-related Pdcl2, a germline-specific gene, may be involved in germ cell protein folding. Given that PDCL2 is highly conserved in evolution, it may be indispensable for mammalian spermiogenesis; however, the function of PDCL2 in higher mammalian species remains unknown. To determine the role of PDCL2 in male fertility, we generated Pdcl2 knockout mice using CRISPR/Cas9. Our results revealed that Pdcl2 heterozygous (Pdcl2^+/−) male mice were normal, but male Pdcl2-null (Pdcl2^−/−) mice were infertile. Accordingly, Pdcl2^−/− male mice exhibited lower testis weight, epididymis weight, and sperm number than Pdcl2^+/+ mice. Moreover, Pdcl2^−/− mice displayed malformed and immotile sperm. Apoptotic cells were significantly enhanced in Pdcl2^−/− testes and epididymis when compared with those in wild-type mice. Mechanistically, PDCL2 can interact with the CCT complex, and dysfunction in this complex might lead to infertility in Pdcl2^−/− male mice. Collectively, these findings confirm that Pdcl2 knockout leads to male infertility in mice and that PDCL2 may function as a chaperone to promote protein folding during spermiogenesis.

A highly conserved zebrafish IMPDH retinal isoform produces the majority of guanine and forms dynamic protein filaments in photoreceptor cells

Inosine monophosphate dehydrogenase (IMPDH) is a key regulatory enzyme in the de novo synthesis of the purine base guanine. Dominant mutations in human IMPDH1 cause photoreceptor degeneration for reasons that are unknown. Here, we sought to provide some foundational information on Impdh1a in the zebrafish retina. We found that in zebrafish, gene subfunctionalization due to ancestral duplication resulted in a predominant retinal variant expressed exclusively in rod and cone photoreceptors. This variant is structurally and functionally similar to the human IMPDH1 retinal variant and shares a reduced sensitivity to GTP-mediated inhibition. We also demonstrated that Impdh1a forms prominent protein filaments in vitro and in vivo in both rod and cone photoreceptor cell bodies, synapses, and to a lesser degree, in outer segments. These filaments changed length and cellular distribution throughout the day consistent with diurnal changes in both mRNA and protein levels. The loss of Impdh1a resulted in a substantial reduction of guanine levels, although cellular morphology and cGMP levels remained normal. Our findings demonstrate a significant role for IMPDH1 in photoreceptor guanine production and provide fundamental new information on the details of this protein in the zebrafish retina.

The TRiC/CCT Chaperonin and Its Role in Uncontrolled Proliferation

The cell cycle is a sophisticated space-time regulated mechanism where a wide variety of protein modules and complexes associate functioning in a concerted manner to regulate and transfer the genetic material to daughter cells. CCT (chaperonin containing TCP-1, also known as TRiC) is a molecular machine that forms a high molecular weight complex (1000 KDa). CCT is emerging as a key molecule during mitosis due to its essential role in the folding of many important proteins involved in cell division (Cdh1, Plk1, p27, Cdc20, PP2a regulatory subunits, tubulin or actin) suggesting its involvement in uncontrolled proliferation. The assembly is formed by eight different subunits called CCTα, β, γ, δ, ε, ζ, η and θ in mammals corresponding to CCT1-8 in yeast. CCT/TRiC is organized in a unique intra- and inter-ring arrangement. The chaperonin monomers share a common domain structure including an equatorial domain, which contains all the inter-ring contacts, most of the intra-ring contacts and the ATP binding site, whose binding and hydrolysis triggers the conformational changes that take place during the functional cycle. All chaperonins display an open substrate-receptive conformation, where the unfolded protein is recognized and trapped, and a closed conformation where the substrate is isolated from the bulk of the intracellular environment. In this chapter we discuss the complex set of intra- and inter-ring allosteric signals during chaperonin function.

Chaperones and retinal disorders

Defects in protein folding and trafficking are a common cause of photoreceptor degeneration, causing blindness. Photoreceptor cells present an unusual challenge to the protein folding and transport machinery due to the high rate of protein synthesis, trafficking and the renewal of the outer segment, a primary cilium that has been modified into a specialized light-sensing compartment. Phototransduction components, such as rhodopsin and cGMP-phosphodiesterase, and multimeric ciliary transport complexes, such as the BBSome, are hotspots for mutations that disrupt proteostasis and lead to the death of photoreceptors. In this chapter, we review recent studies that advance our understanding of the chaperone and transport machinery of phototransduction proteins.

Archaeal Unfoldase Counteracts Protein Misfolding Retinopathy in Mice

Deregulation of cellular proteostasis due to the failure of the ubiquitin proteasome system to dispose of misfolded aggregation-prone proteins is a hallmark of various neurodegenerative diseases in humans. Microorganisms have evolved to survive massive protein misfolding and aggregation triggered by heat shock using their protein-unfolding ATPases (unfoldases) from the Hsp100 family. Because the Hsp100 chaperones are absent in homoeothermic mammals, we hypothesized that the vulnerability of mammalian neurons to misfolded proteins could be mitigated by expressing a xenogeneic unfoldase. To test this idea, we expressed proteasome-activating nucleotidase (PAN), a protein-unfolding ATPase from thermophilic Archaea, which is homologous to the 19S eukaryotic proteasome and similar to the Hsp100 family chaperones in rod photoreceptors of mice. We found that PAN had no obvious effect in healthy rods; however, it effectively counteracted protein-misfolding retinopathy in Gγ1 knock-out mice. We conclude that archaeal PAN can rescue a protein-misfolding neurodegenerative disease, likely by recognizing misfolded mammalian proteins.SIGNIFICANCE STATEMENT This study demonstrates successful therapeutic application of an archaeal molecular chaperone in an animal model of neurodegenerative disease. Introducing the archaeal protein-unfolding ATPase proteasome-activating nucleotidase (PAN) into the retinal photoreceptors of mice protected these neurons from the cytotoxic effect of misfolded proteins. We propose that xenogeneic protein-unfolding chaperones could be equally effective against other types of neurodegenerative diseases of protein-misfolding etiology.

CCT2 Mutations Evoke Leber Congenital Amaurosis due to Chaperone Complex Instability

Leber congenital amaurosis (LCA) is a hereditary early-onset retinal dystrophy that is accompanied by severe macular degeneration. In this study, novel compound heterozygous mutations were identified as LCA-causative in chaperonin-containing TCP-1, subunit 2 (CCT2), a gene that encodes the molecular chaperone protein, CCTβ. The zebrafish mutants of CCTβ are known to exhibit the eye phenotype while its mutation and association with human disease have been unknown. The CCT proteins (CCT α-θ) forms ring complex for its chaperon function. The LCA mutants of CCTβ, T400P and R516H, are biochemically instable and the affinity for the adjacent subunit, CCTγ, was affected distinctly in both mutants. The patient-derived induced pluripotent stem cells (iPSCs), carrying these CCTβ mutants, were less proliferative than the control iPSCs. Decreased proliferation under Cct2 knockdown in 661W cells was significantly rescued by wild-type CCTβ expression. However, the expression of T400P and R516H didn’t exhibit the significant effect. In mouse retina, both CCTβ and CCTγ are expressed in the retinal ganglion cells and connecting cilium of photoreceptor cells. The Cct2 knockdown decreased its major client protein, transducing β1 (Gβ1). Here we report the novel LCA mutations in CCTβ and the impact of chaperon disability by these mutations in cellular biology.

Heterotrimeric G protein-mediated signaling and its non-canonical regulation in the heart

Heterotrimeric guanine nucleotide-binding proteins (G proteins) regulate a multitude of signaling pathways in mammalian cells by transducing signals from G protein-coupled receptors (GPCRs) to effectors, which in turn regulate cellular function. In the myocardium, G protein signaling occurs in all cardiac cell types and is centrally involved in the regulation of heart rate, pump function, and vascular tone and in the response to hemodynamic stress and injury. Perturbations in G protein-mediated signaling are well known to contribute to cardiac hypertrophy, failure, and arrhythmias. Most of the currently used drugs for cardiac and other diseases target GPCR signaling. In the canonical G protein signaling paradigm, G proteins that are located at the cytoplasmic surface of the plasma membrane become activated after an agonist-induced conformational change of GPCRs, which then allows GTP-bound Gα and free Gβγ subunits to activate or inhibit effector proteins. Research over the past two decades has markedly broadened the original paradigm with a GPCR-G protein-effector at the cell surface at its core by revealing novel binding partners and additional subcellular localizations for heterotrimeric G proteins that facilitate many previously unrecognized functional effects. In this review, we focus on non-canonical and epigenetic-related mechanisms that regulate heterotrimeric G protein expression, activation, and localization and discuss functional consequences using cardiac examples where possible. Mechanisms reviewed involve microRNAs, histone deacetylases, chaperones, alternative modes of G protein activation, and posttranslational modifications. Some of these newly characterized mechanisms may be further developed into novel strategies for the treatment of cardiac disease and beyond.

Essential role of the chaperonin CCT in rod outer segment biogenesis

PURPOSE

While some evidence suggests an essential role for the chaperonin containing t-complex protein 1 (CCT) in ciliogenesis, this function remains poorly understood mechanistically. We used transgenic mice, previously generated in our lab, and characterized by a genetically-induced suppression of CCT in rod photoreceptors as well as a malformation of the rod sensory cilia, the outer segments, to gain new insights into this underlying molecular mechanism.

METHODS

The CCT activity in rod photoreceptors of mice was suppressed by overexpressing the chaperonin inhibitor, phosducin-like protein short, and the ensuing changes of cellular morphology were analyzed by light and electron microscopy. Protein expression levels were studied by fluorescent microscopy and Western blotting.

RESULTS

Suppressing the chaperonin made the photoreceptors incompetent to build their outer segments. Specifically, the CCT-deficient rods appeared unable to expand the outer segment plasma membrane, and accommodate growth of this compartment. Seeking the molecular mechanisms underlying such a shortcoming, we found that the affected rods could not express normal levels of Bardet-Biedl Syndrome (BBS) proteins 2, 5, and 7 and, owing to that deficiency, were unable to assemble the BBSome, a multisubunit complex responsible for ciliary trafficking. A similar effect in response to the chaperonin suppression was also observed in cultured ciliated cells.

CONCLUSIONS

Our data provide new evidence indicating the essential role of the chaperonin CCT in the biogenesis of vertebrate photoreceptor sensory cilia, and suggest that it may be due to the direct participation of the chaperonin in the posttranslational processing of selected BBS proteins and assembly of the BBSome.