Nipah virus induces two inclusion body populations: Identification of novel inclusions at the plasma membrane.

Formation of cytoplasmic inclusion bodies (IBs) is a hallmark of infections with non-segmented negative-strand RNA viruses (order Mononegavirales). We show here that Nipah virus (NiV), a bat-derived highly pathogenic member of the Paramyxoviridae family, differs from mononegaviruses of the Rhabdo-, Filo- and Pneumoviridae families by forming two types of IBs with distinct localizations, formation kinetics, and protein compositions. IBs in the perinuclear region form rapidly upon expression of the nucleocapsid proteins. These IBperi are highly mobile and associate with the aggresome marker y-tubulin. IBperi can recruit unrelated overexpressed cytosolic proteins but do not contain the viral matrix (M) protein. Additionally, NiV forms an as yet undescribed IB population at the plasma membrane (IBPM) that is y-tubulin-negative but contains the M protein. Infection studies with recombinant NiV revealed that IBPM require the M protein for their formation, and most likely represent sites of NiV assembly and budding. The identification of this novel type of plasma membrane-associated IBs not only provides new insights into NiV biology and may open new avenues to develop novel antiviral approaches to treat these highly pathogenic viruses, it also provides a basis for a more detailed characterization of IBs and their role in virus assembly and replication in infections with other Mononegavirales.

Replication of a Nipah Virus Encoding a Nuclear-Retained Matrix Protein.

Nipah virus (NiV) matrix protein (NiV M) plays a major role in virus assembly. It undergoes nuclear transit before accumulating at the plasma membrane and recruiting nucleocapsids to the budding sites. Because nuclear NiV M cannot be detected in all cell types, we wondered whether it can reach the cell surface by bypassing the nucleus. Using an M mutant with a defective nuclear export signal (MNESmut), however, we revealed that the nuclear import of M is ubiquitous, because MNESmut was retained in the nuclei of all cell types tested. Because a functional nuclear transit is a general prerequisite for M surface transport, we wanted to characterize the effect of nuclear-retained M protein in a full viral context and generated a recombinant NiV-MNESmut. Mutant NiV-MNESmut caused increased cell-cell fusion and produced lower virus titers. As expected for an assembly defective NiV, perinuclear inclusions (IBperi) were formed, but inclusions at the plasma membrane (IBPM), which probably represent the viral assembly platforms, were not found. It is interesting to note that the transport-defective MNESmut was recruited to IBperi. This probably prevents overaccumulation of nonfunctional M proteins in the cytoplasm and nuclei of NiV-infected cells and thus provides first evidence that IBperi are functionally relevant aggresome-like compartments.

Nipah Virus Matrix Protein Influences Fusogenicity and Is Essential for Particle Infectivity and Stability.

UNLABELLED: Nipah virus (NiV) causes fatal encephalitic infections in humans. To characterize the role of the matrix (M) protein in the viral life cycle, we generated a reverse genetics system based on NiV strain Malaysia. Using an enhanced green fluorescent protein (eGFP)-expressing M protein-deleted NiV, we observed a slightly increased cell-cell fusion, slow replication kinetics, and significantly reduced peak titers compared to the parental virus. While increased amounts of viral proteins were found in the supernatant of cells infected with M-deleted NiV, the infectivity-to-particle ratio was more than 100-fold reduced, and the particles were less thermostable and of more irregular morphology. Taken together, our data demonstrate that the M protein is not absolutely required for the production of cell-free NiV but is necessary for proper assembly and release of stable infectious NiV particles. IMPORTANCE: Henipaviruses cause a severe disease with high mortality in human patients. Therefore, these viruses can be studied only in biosafety level 4 (BSL-4) laboratories, making it more challenging to characterize their life cycle. Here we investigated the role of the Nipah virus matrix protein in virus-mediated cell-cell fusion and in the formation and release of newly produced particles. We found that even though low levels of infectious viruses are produced in the absence of the matrix protein, it is required for the release of highly infectious and stable particles. Fusogenicity of matrixless viruses was slightly enhanced, further demonstrating the critical role of this protein in different steps of Nipah virus spread.

Isolation of Primary Porcine Bronchial Epithelial Cells for Nipah Virus Infections.

The Malaysian strain of Nipah virus (NiV) first emerged in 1998/99 and caused a major disease outbreak in pigs and humans. While humans developed fatal encephalitis due to a prominent infection of brain microvessels, NiV-infected pigs mostly suffered from an acute respiratory disease and efficiently spread the infection via airway secretions. To elucidate the molecular basis of the highly productive NiV replication in porcine airways in vitro, physiologically relevant cell models that have maintained functional characteristics of airway epithelia in vivo are needed. Here, we describe in detail the method of isolating bronchial epithelial cells (PBEpC) from pig lungs that can be used for NiV infection studies. After the dissection of primary bronchia and removal of the mucus and protease digestion, bronchi segments are cut open and epithelial cells are scraped off and seeded on collagen-coated cell culture flasks. With this method, it is possible to isolate about 2 × 106 primary cells from the primary bronchi of one pig lung which can be cryopreserved or further subcultured. PBEpC form polarized monolayers on Transwell membrane inserts as controlled by immunostainings of epithelial marker proteins. NiV infection causes rapid formation of syncytia, allowing productive NiV infections in living PBEpC cultures to be monitored by phase-contrast microscopy.

Measles and Nipah virus assembly: Specific lipid binding drives matrix polymerization.

Measles virus, Nipah virus, and multiple other paramyxoviruses cause disease outbreaks in humans and animals worldwide. The paramyxovirus matrix (M) protein mediates virion assembly and budding from host cell membranes. M is thus a key target for antivirals, but few high-resolution structures of paramyxovirus M are available, and we lack the clear understanding of how viral M proteins interact with membrane lipids to mediate viral assembly and egress that is needed to guide antiviral design. Here, we reveal that M proteins associate with phosphatidylserine and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] at the plasma membrane. Using x-ray crystallography, electron microscopy, and molecular dynamics, we demonstrate that PI(4,5)P2 binding induces conformational and electrostatic changes in the M protein surface that trigger membrane deformation, matrix layer polymerization, and virion assembly.