Functional binding of inner-arm dyneins with demembranated flagella ofChlamydomonas mutants

Chlamydomonas IC97, an intermediate chain of the flagellar dynein f/I1, is required for normal flagellar and cellular motility

Motile flagella (also called "motile cilia") play a variety of important roles in lower and higher eukaryotes, including cellular motility and fertility. Flagellar motility is driven by several species of the gigantic motor-protein complexes, flagellar dyneins, that reside within these organelles. Among the flagellar-dynein species, a hetero-dimeric dynein called "IDA f/I1" has been shown to be particularly important in controlling the flagellar waveform, and defects in this dynein species in humans cause ciliopathies such as multiple morphological abnormalities of the flagella and asthenoteratozoospermia. IDA f/I1 is composed of many subunits, including two HCs (HCα and HCβ) and three ICs (IC140, IC138, and IC97), and among the three ICs of IDA f/I1, the exact molecular function(s) of IC97 in flagellar motility is not well understood. In this study, we isolated a Chlamydomonas mutant lacking IC97 and analyzed the phenotypes. The ic97 mutant phenocopied several aspects of the previously isolated IDA-f/I1-related mutants in Chlamydomonas and showed slow swimming compared to the wild type but retained the ability to phototaxis. Further analysis revealed that the mutant had low flagellar beat frequency and miscoordination between the two (cis- and trans-) flagella. In addition, the mutant cells swam in a comparatively straight path compared to the wild-type cells. Taken together, our results highlight the importance of proper assembly of IC97 in the IDA-f/I1 complex for the regulation of flagellar and cellular motility in Chlamydomonas and provide valuable insights into both the molecular functions of IC97 orthologs in higher eukaryotes and the pathogenetic mechanisms of human ciliopathies caused by IDA-f/I1 defects.

IMPORTANCE

IDA f/I1 is a hetero-dimeric flagellar dynein that is particularly important for the regulation of flagellar waveform and whose defects are associated with human ciliopathies. IC97 is an evolutionarily conserved intermediate chain of IDA f/I1, but the detailed molecular functions of IC97 in flagellar motility have not been elucidated. In this study, mutational and biochemical analyses of the previously uncharacterized Chlamydomonas ic97 mutant revealed that IC97 is required for both the normal flagellar and cellular motility. In particular, IC97 appears to play an important role in both the control of flagellar beat frequency and the coordination between the two (cis- and trans-) flagella in Chlamydomonas. Our results provide important insights into the regulation of IDA-f/I1 activity by IC97 and the pathogenetic mechanisms of human ciliopathies caused by IDA-f/I1 defects.

Novel axonemal protein ZMYND12 interacts with TTC29 and DNAH1, and is required for male fertility and flagellum function

Male infertility is common and complex, presenting a wide range of heterogeneous phenotypes. Although about 50% of cases are estimated to have a genetic component, the underlying cause often remains undetermined. Here, from whole-exome sequencing on samples from 168 infertile men with asthenoteratozoospermia due to severe sperm flagellum, we identified homozygous ZMYND12 variants in four unrelated patients. In sperm cells from these individuals, immunofluorescence revealed altered localization of DNAH1, DNALI1, WDR66, and TTC29. Axonemal localization of ZMYND12 ortholog TbTAX-1 was confirmed using the Trypanosoma brucei model. RNAi knock-down of TbTAX-1 dramatically affected flagellar motility, with a phenotype similar to the sperm from men bearing homozygous ZMYND12 variants. Co-immunoprecipitation and ultrastructure expansion microscopy in T. brucei revealed TbTAX-1 to form a complex with TTC29. Comparative proteomics with samples from Trypanosoma and Ttc29 KO mice identified a third member of this complex: DNAH1. The data presented revealed that ZMYND12 is part of the same axonemal complex as TTC29 and DNAH1, which is critical for flagellum function and assembly in humans, and Trypanosoma. ZMYND12 is thus a new asthenoteratozoospermia-associated gene, bi-allelic variants of which cause severe flagellum malformations and primary male infertility.

Novel bi-allelic variants in DNAH2 cause severe asthenoteratozoospermia with multiple morphological abnormalities of the flagella

RESEARCH QUESTION

Multiple morphological abnormalities of the flagella (MMAF) is characterized by excessive immotile spermatozoa with severe flagellar abnormalities in the ejaculate. Previous studies have reported a heterogeneous genetic profile associated with MMAF. What other genetic variants might explain the cause of MMAF?

DESIGN

Whole-exome sequencing was conducted in a cohort of 90 Chinese patients with MMAF. The pathogenicity of identified mutations was assessed through electron microscopy and immunofluorescent examinations.

RESULTS

Three unrelated men with bi-allelic DNAH2 variants were identified. Sanger sequencing verified that the six novel variants originated from every parent. All these variants were located at the conserved domains of DNAH2 and predicted to be deleterious by bioinformatic tools. Haematoxylin and eosin staining and scanning electron microscopy revealed that spermatozoa harbouring DNAH2 variants displayed severely aberrant morphology mainly with absent and short flagella (≥78%). Moreover, transmission electron microscopy revealed the obvious absence of a central pair of microtubules and inner dynein arms in the spermatozoa with mutated DNAH2. Immunofluorescence data further validated these findings, showing reduced DNAH2 protein expression in the spermatozoa with DNAH2 variants, compared with normal spermatozoa. Intracytoplasmic sperm injection using spermatozoa from the three men with mutated DNAH2 resulted in blastocyst formation in all cases. Embryo transfer was carried out in two couples, both resulting in clinical pregnancy.

CONCLUSIONS

These experimental and clinical data suggest that bi-allelic DNAH2 variants might induce MMAF-associated asthenoteratozoospermia, which can be overcome through intracytoplasmic sperm injection. These findings contribute to the knowledge of the genetic landscape of asthenoteratozoospermia and clinical counselling of male infertility.

Mutations in PIH proteins MOT48, TWI1 and PF13 define common and unique steps for preassembly of each, different ciliary dynein

Ciliary dyneins are preassembled in the cytoplasm before being transported into cilia, and a family of proteins containing the PIH1 domain, PIH proteins, are involved in the assembly process. However, the functional differences and relationships between members of this family of proteins remain largely unknown. Using Chlamydomonas reinhardtii as a model, we isolated and characterized two novel Chlamydomonas PIH preassembly mutants, mot48-2 and twi1-1. A new allele of mot48 (ida10), mot48-2, shows large defects in ciliary dynein assembly in the axoneme and altered motility. A second mutant, twi1-1, shows comparatively smaller defects in motility and dynein assembly. A double mutant mot48-2; twi1-1 displays greater reduction in motility and in dynein assembly compared to each single mutant. Similarly, a double mutant twi1-1; pf13 also shows a significantly greater defect in motility and dynein assembly than either parent mutant. Thus, MOT48 (IDA10), TWI1 and PF13 may define different steps, and have partially overlapping functions, in a pathway required for ciliary dynein preassembly. Together, our data suggest the three PIH proteins function in preassembly steps that are both common and unique for different ciliary dyneins.

Motile cilia are hair-like organelles that protrude from many eukaryotic cells, and play vital roles in organisms including cell motility, environmental sensing and removal of infectious materials. Motile cilia are driven by gigantic motor protein complexes, called ciliary dyneins, defects in which cause abnormal ciliary motility, ultimately resulting in human diseases collectively called primary ciliary dyskinesia (PCD). Ciliary dyneins are preassembled in the cytoplasm before being transported into cilia, and preassembly requires a family of potential co-chaperones, the PIH proteins. Mutations in the PIH proteins cause defective assembly of ciliary dyneins and can result in PCD. However, despite their importance, the precise functions, and functional relationships, between the PIH proteins are unclear. In this study, using Chlamydomonas reinhardtii, we assessed the functional relationship between three PIH proteins with respect to dynein preassembly and motility. We found that these PIH proteins have complicated and related roles in dynein assembly, possibly with each playing common and unique roles in dynein assembly. Our results provide new information on each conserved PIH protein for dynein assembly and provide a new understanding of PCD caused by PIH mutations.

The ciliary inner dynein arm, I1 dynein, is assembled in the cytoplasm and transported by IFT before axonemal docking

To determine mechanisms of assembly of ciliary dyneins, we focused on the Chlamydomonas inner dynein arm, I1 dynein, also known as dynein f. I1 dynein assembles in the cytoplasm as a 20S complex similar to the 20S I1 dynein complex isolated from the axoneme. The intermediate chain subunit, IC140 (IDA7), and heavy chains (IDA1, IDA2) are required for 20S I1 dynein preassembly in the cytoplasm. Unlike I1 dynein derived from the axoneme, the cytoplasmic 20S I1 complex will not rebind I1-deficient axonemes in vitro. To test the hypothesis that I1 dynein is transported to the distal tip of the cilia for assembly in the axoneme, we performed cytoplasmic complementation in dikaryons formed between wild-type and I1 dynein mutant cells. Rescue of I1 dynein assembly in mutant cilia occurred first at the distal tip and then proceeded toward the proximal axoneme. Notably, in contrast to other combinations, I1 dynein assembly was significantly delayed in dikaryons formed between ida7 and ida3. Furthermore, rescue of I1 dynein assembly required new protein synthesis in the ida7 × ida3 dikaryons. On the basis of the additional observations, we postulate that IDA3 is required for 20S I1 dynein transport. Cytoplasmic complementation in dikaryons using the conditional kinesin-2 mutant, fla10-1 revealed that transport of I1 dynein is dependent on kinesin-2 activity. Thus, I1 dynein complex assembly depends upon IFT for transport to the ciliary distal tip prior to docking in the axoneme.

The regulation of dynein-driven microtubule sliding in Chlamydomonas flagella by axonemal kinases and phosphatases

The purpose of this chapter is to review the methodology and advances that have revealed conserved signaling proteins that are localized in the 9+2 ciliary axoneme for regulating motility. Diverse experimental systems have revealed that ciliary and eukaryotic flagellar motility is regulated by second messengers including calcium, pH, and cyclic nucleotides. In addition, recent advances in in vitro functional studies, taking advantage of isolated axonemes, pharmacological approaches, and biochemical analysis of axonemes have demonstrated that otherwise ubiquitous, conserved protein kinases and phosphatases are transported to and anchored in the axoneme. Here, we focus on the functional/pharmacological, genetic, and biochemical approaches in the model genetic system Chlamydomonas that have revealed highly conserved kinases, anchoring proteins (e.g., A-kinase anchoring proteins), and phosphatases that are physically located in the axoneme where they play a direct role in control of motility.

Novel 44-kilodalton subunit of axonemal Dynein conserved from chlamydomonas to mammals

Cilia and flagella have multiple dyneins in their inner and outer arms. Chlamydomonas inner-arm dynein contains at least seven major subspecies (dynein a to dynein g), of which all but dynein f (also called dynein I1) are the single-headed type that are composed of a single heavy chain, actin, and either centrin or a 28-kDa protein (p28). Dynein d was found to associate with two additional proteins of 38 kDa (p38) and 44 kDa (p44). Following the characterization of the p38 protein (R. Yamamoto, H. A. Yanagisawa, T. Yagi, and R. Kamiya, FEBS Lett. 580:6357-6360, 2006), we have identified p44 as a novel component of dynein d by using an immunoprecipitation approach. p44 is present along the length of the axonemes and is diminished, but not absent, in the ida4 and ida5 mutants, both lacking this dynein. In the ida5 axoneme, p44 and p38 appear to form a complex, suggesting that they constitute the docking site of dynein d on the outer doublet. p44 has potential homologues in other ciliated organisms. For example, the mouse homologue of p44, NYD-SP14, was found to be strongly expressed in tissues with motile cilia and flagella. These results suggest that inner-arm dynein d and its subunit organization are widely conserved.

Keeping an eye on I1: I1 dynein as a model for flagellar dynein assembly and regulation

Among the major challenges in understanding ciliary and flagellar motility is to determine how the dynein motors are assembled and localized and how dynein-driven outer doublet microtubule sliding is controlled. Diverse studies, particularly in Chlamydomonas, have determined that the inner arm dynein I1 is targeted to a unique structural position and is critical for regulating the microtubule sliding required for normal ciliary/flagellar bending. As described in this review, I1 dynein offers additional opportunities to determine the principles of assembly and targeting of dyneins to cellular locations and for studying the mechanisms that regulate dynein activity and control of motility by phosphorylation.

A novel subunit of axonemal dynein conserved among lower and higher eukaryotes

To elucidate the subunit composition of axonemal inner-arm dynein, we examined a 38 kDa protein (p38) co-purified with a Chlamydomonas inner arm subspecies, dynein d. We found it is a novel protein conserved among a variety of organisms with motile cilia and flagella. Immunoprecipitation using specific antibody verified its association with a heavy chain, actin and a previously identified light chain (p28). Unexpectedly, mutant axonemes lacking dynein d and other dyneins retained reduced amounts of p38. This finding suggests that p38 is involved in the docking of dynein d to specific loci.