PNPASE Regulates RNA Import into Mitochondria

“As a critical regulator of mitochondrial homeostasis, hPNPase is indispensable for development, with studies showing that constitutive knockout of the PNPT1 gene in mice results in embryonic lethality [4,38].”

“…for protein synthesis within the mitochondria; PNPT1 is implicated in mitochondrial RNA metabolism and, in some experimental systems, RNA import into mitochondria 24 ; and MTPAP is involved in the polyadenylation of mitochondrial RNA 25 , which is crucial for mitochondrial gene expression.”

“Polynucleotide phosphorylase (PNPase) facilitates the importation of miRNA into mitochondria, and the voltage-dependent anion channel (VDAC) participates in RNA and protein transportation through the external mitochondrion membrane [21,22].”

“On the contrary, there have been many recent publications demonstrating successful import of diverse RNA species into mitochondria (Rubio et al., 2008; Wang et al., 2010; Fan et al., 2019; Jeandard et al., 2019).”

“Finally, PNPT1 is involved in multiple metabolic RNA processes in mitochondria, and reduction of PNPT1 levels results in impaired mitochondrial processing and accumulation of large polycistronic transcripts, possibly due to its connection to the import of RNase P RNA into the mitochondria (Wang et al. 2010a; von Ameln…”

“…degrading c-myc mRNA (Sarkar et al., 2003; Sarkar et. al., 2005), miR-221, miR222, and miR-106b (Das et. al., 2010), and regulating translocation of small RNAs into the mitochondria, such as MRP (mitochondrial RNA processing), 5S rRNA, and RNase P (Wang et al., 2010; Wang et al., 2012).”

“Previous studies have provided evidence for a role of this enzyme in the import of RNA into the mitochrondria as well as mRNA processing and stability (Walter et al., 2002; Wang et al., 2010).”

“The RNA Degradosome maintains cellular homeostasis through mitochondrial RNA degradation (Wang et al. 2010; Borowski et al. 2013; Dhir et al. 2018; Liu et al. 2018).”

“Increased PNPase expression enhances miRNA import, whereas its depletion disrupts mitochondrial morphology and respiration (Wang et al. 2010; Shepherd et al. 2017; Li, Li, et al. 2025; Li, Wang, et al. 2025; Li, Xu, et al. 2025; Wang, Shimada, et al. 2012; Wang, Zhang, et al. 2012; Taylor et al.…”

“Human mitochondrial PNPase (hPNPase) was unexpectedly discovered in the intermembrane space of mitochondria as a peripheral membrane protein, where it functions as an RNA-importing enzyme [ 14 ].”

“Their transport may involve proteins such as Ago2 [134,135] and PNPase [135,136].”

“The M1GS strategy has been used to downregulate genes in both prokaryotes and eukaryotes, demonstrating its utility as an effective tool for disrupting genes’ functions [25-27].”

“Even though mitochondrial co-import of RNA and specific proteins has been described in multiple occasions in yeast and mammalian cell lines, these mechanisms are often highly disputed [10, 27, 28, 38, 97-99].”

“Separation-of-function mutations between its two key activities, mitochondrial RNA import, and exoribonuclease activity have been described (39).”

“The protein is essential in mice and in human cells (15, 17–19) and its overexpression triggers reactive oxygen species (ROS) accumulation in Hela cells (20).”

“Mitochondrial import of the MRP RNA and the H1 RNA ( originally described as the RNA component of RNase P) in human cells has been a controversial issue for a long time ( Wang et al., 2010 ; Cannon et al., 2015 ; Noh et al., 2016 ; reviewed in Weber-Lotfi and Dietrich, 2018 ) .”

“PNPase is shown to be able to recognize certain hairpin structures that seem to be import of RNA transport [33].”

“[20] Wang G, Chen HW, Oktay Y, Zhang J, Allen EL, Smith GM, et al.”

“Polynucleotide Phosphorylase (PNPase) is located in the inner mitochondrial membrane that expands into the intermembrane space, which can essentially induce the transport of ribosomal RNA and transfer RNA based on their hairpin-loop structure [4, 5].”

“suggested that a stem-loop sequence on the RNA was necessary for mitochondrial import [24].”

“Attaching these hairpins into non-imported RNAs can target RNAs for import, as shown with GAPDH [51].”

“Some have shown that PNPase is vital for mRNA and precursor stability and control of polyadeylation, while others have observed no effect of PNPase depletion (Nagaike et al, 2005; Slomovic and Schuster, 2008; Wang et al., 2010).”

“Objective: We hypothesized that PNPase could play a role in miRNA import (Wang et al. 2010).”

“In contrast, hPNPase localized to the mitochondria resides primarily in the intermembrane space and regulates aspects of mitochondrial RNA processing, homesostasis, and processing-independent RNA import into the mitochondria (61, 125).”

“In further in vitro experiments, stem-loop structures that function as targeting signals for PNPASEmediated organelle uptake were identified in the mammalian RNase P RNA and MRP RNA (Wang et al. 2010).”