Biology subjects

Hansen, K. G.

Publications and source records attributed to Hansen, K. G..

4 recordsLinked to original sources

bioRxiv2024

Transcription arrest induces formation of protective RNA granules in mitochondria

Mitochondrial gene expression regulation is required for the biogenesis of oxidative phosphorylation (OXPHOS) complexes, yet the spatial organization of mitochondrial RNAs (mt-RNAs) remains unknown. Here, we investigated the spatial distribution of mt-RNAs during various cellular stresses using single-molecule RNA-FISH. We discovered that transcription inhibition leads to the formation of distinct RNA granules within mitochondria, which we term inhibition granules. These structures differ from canonical mitochondrial RNA granules (MRGs) and form in response to multiple transcription arrest conditions, including ethidium bromide treatment, specific inhibition of the mitochondrial RNA polymerase, and depletion of the SUV3 helicase. Inhibition granules appear to serve a protective function, stabilizing certain mt-mRNAs during prolonged transcription inhibition. This phenomenon coincides with an imbalance in OXPHOS complex expression, where mitochondrial-encoded transcripts decrease while nuclear-encoded subunits remain stable. We found that cells recover from transcription inhibition via resolving the granules, restarting transcription and repopulating the mitochondrial network with mt-mRNAs within hours. We suggest that inhibition granules may act as a reservoir to help overcome OXPHOS imbalance during recovery from transcription arrest.

cell biology↗

bioRxiv2022

Single-nucleoid architecture reveals heterogeneous packaging of mitochondrial DNA

Cellular metabolism relies on the regulation and maintenance of mitochondrial DNA (mtDNA). Hundreds to thousands of copies of mtDNA exist in each cell, yet because mitochondria lack histones or other machinery important for nuclear genome compaction, it remains unresolved how mtDNA is packaged into individual nucleoids. In this study, we used long-read single-molecule accessibility mapping to measure the compaction of individual full-length mtDNA molecules at nucleotide resolution. We found that, unlike the nuclear genome, human mtDNA largely undergoes all-or-none global compaction, with the majority of nucleoids existing in an inaccessible, inactive state. Highly accessible mitochondrial nucleoids are co-occupied by transcription and replication machinery and selectively form a triple-stranded D-loop structure. In addition, we showed that the primary nucleoid-associated protein TFAM directly modulates the fraction of inaccessible nucleoids both in vivo and in vitro and acts via a nucleation-and-spreading mechanism to coat and compact mitochondrial nucleoids. Together, these findings reveal the primary architecture of mtDNA packaging and regulation in human cells.

molecular biology↗

bioRxiv2021

Balanced mitochondrial and cytosolic translatomes underlie the biogenesis of human respiratory complexes

Oxidative phosphorylation (OXPHOS) complexes consist of nuclear and mitochondrial DNA-encoded subunits. Their biogenesis requires cross-compartment gene regulation to mitigate the accumulation of disproportionate subunits. To determine how human cells coordinate mitochondrial and nuclear gene expression processes, we established an optimized ribosome profiling approach tailored for the unique features of the human mitoribosome. Analysis of ribosome footprints in five cell types revealed that average mitochondrial synthesis rates corresponded precisely to cytosolic rates across OXPHOS complexes. Balanced mitochondrial and cytosolic synthesis did not rely on rapid feedback between the two translation systems. Rather, LRPPRC, a gene associated with Leighs syndrome, is required for the reciprocal translatomes and maintains cellular proteostasis. Based on our findings, we propose that human mitonuclear balance is enabled by matching OXPHOS subunit synthesis rates across cellular compartments, which may represent a vulnerability for cellular proteostasis.

cell biology↗

bioRxiv2021

The unfolded protein response of the endoplasmic reticulum supports mitochondrial biogenesis by buffering non-imported proteins

Almost all mitochondrial proteins are synthesized in the cytosol and subsequently targeted to mitochondria. The accumulation of non-imported precursor proteins occurring upon mitochondrial dysfunction can challenge cellular protein homeostasis. Here we show that blocking protein translocation into mitochondria results in the accumulation of mitochondrial membrane proteins at the endoplasmic reticulum, thereby triggering the unfolded protein response (UPRER). Moreover, we find that mitochondrial membrane proteins are also routed to the ER under physiological conditions. The levels of ER-resident mitochondrial precursors is enhanced by import defects as well as metabolic stimuli that increase the expression of mitochondrial proteins. Under such conditions, the UPRER is crucial to maintain protein homeostasis and cellular fitness. We propose the ER serves as a physiological buffer zone for those mitochondrial precursors that cant be immediately imported into mitochondria while engaging the UPRER to adjust the ER proteostasis capacity to the extent of precursor accumulation.

cell biology↗