Cell Specificity in Translation

Transcription and translation are often thought to be coupled processes, with a rise in mRNA transcripts typically associated with an increase in translation. And while this is generally the case, specific cellular requirements often result in the decoupling of these processes, partially in order to bring about phenotypic change more rapidly than can be provided by alterations in transcription. This context usually occurs in tandem with the wider consideration of cellular growth, with protein production being particularly energetically expensive. This is needed to respond to different environmental stimuli, within such growth and temporal contexts, resulting in specificity in translation at a cellular level, and as such can provide focussed targets for novel therapeutics, with the potential to have limited off-target effects.

Historical Context

One of the earliest instances of cell specific translation was uncovered in the early 90s, linked to the developmental process. mRNA transcripts belonging to Pit-1, a transcription factor specific to pituitary cells, were found to be present in all 5 cell types of the pituitary gland tested (these being the somatotrophs, lactotrophs, thyrotrophs, gonadotrophs, and corticotrophs), yet the presence of Pit-1 protein, as determined via immunohistochemical co-localization techniques, was only observed within the somatotrophs, lactotrophs, and thyrotrophs (1). Interestingly, a case study of two individuals with pituitary dwarfism, whereby both individuals lacked these three cell types, hypothesised that their condition was in fact due to deficient/abnormal Pit-1 protein (2).

Later papers delved more deeply into the mechanisms behind cell specific translation. S-adenosylmethionine decarboxylase (or AdoMetDC), a key enzyme in the synthesis of polyamines, experiences a primarily monosomic distribution in T lymphocytes, although in other, nonlymphoid cell lines, it associates predominately with polysomes, averaging 7-9 ribosomes per transcript. In an elegantly simple experiment, researchers created a chimeric construct, replacing the 5’-transcript leader (5’-TL) of the human growth hormone gene with that of AdoMetDC’s. When expressed in the Jurkat T lymphocyte cell line, this transcript depicted identical monosomal distribution to that seen with AdoMetDC mRNA. However, when expressed in the nonlymphoid cell line Y1 (adrenocortical), such a distribution was again changed, being broadly polysomic. Mutational analysis impacting translation initiation of a uORF in the 5’-TL of this construct linked this factor to the phenomenology seen above (3). Further analysis by the same group on this construct revealed that not only was start codon context involved in this uORF regulation, but also stop codon context, and acted in a cis-dependant manner, possibly through nascent chain-ribosome interaction (4). Despite this advancement in mechanistic knowledge, the factors behind the differential activation in alternate cellular environments are not known.

Translational specificity has been reported genome-wide, as opposed to transcript specific effects. Rapamycin is a commonly accepted widescale translation repressor acting via inhibition of the mTORC1 complex. It was found that the variability in translation, at least in this instance, is related to the ability of mTORC1’s downstream substrates (4E-BPs and S6Ks) to recover their functionality following mTORC1’s inhibition. While S6Ks did not recover their functionality within the timeframe of these studies, 4E-BPs recovered their phosphorylated status and subsequent functionality, but only in HeLa, DU145, and MEF cells, while its status in U2OS, MCF7, and PC3 cells was unchanged (5).

The Brain Connection

Later research in the area of cell-specific translation has had a heavy focus on the brain. This is likely due to the broad heterogeneity observed within the central nervous system, with a significant diversity in relation to cell types observed in this organ. This is related not only to the variation seen in neurons themselves (which perform specialised functions, and often are receptive to, and secrete, varying molecular chemicals), but also to the supporting cells, such as microglia, astrocytes, and oligodendrocytes (which themselves may be in varying stages of development). Cells within the environment of the brain are often required to respond rapidly to novel stimuli, and it is this rapid response to which the process of translation is particularly apt for, not needing novel transcription events which are comparatively time-consuming. Additionally, synaptic protein production is inherently dependent on local translation, given the lack of local transcription in these regions of the neuron, being particularly distant from the cell body and nucleus, the hub of transcriptional activity.

One avenue of research from Hornstein et al., showed that, compared to the glial cells present in the brain, neuronal-specific transcripts have a greater tendency to be under translational control (6). This research also found that mature oligodendrocytes, with a function in myelin production requiring significant protein output, have a surprisingly low translational efficiency for their myelin-associated genes. This is perhaps likely due to the more housekeeping role of such cells, with their requirements and outputs being comparatively stable, as opposed to the more dynamic role neurons play. A further study, which also looked at differences in translation between neurons and their supporting cells, uncovered not just differential translation efficiencies between these cell types, but novel readthrough events within transcripts, events that were localised to distinct neuronal and astrocytic cell types (7). One interesting study has also been completed in vivo, uncovering strong variance in translation efficiency of specific transcripts across differing organs, as well as the insight that translation elongation rates slow with age (up to a 20% decrease between young and old mice)(8).

Neurons-1-scaled

Viral Tropism

Such cell specific phenomenology is especially apparent when analysing the tissue tropisms of viruses. When one thinks of the specificity of viruses and their target tissues, it is easy to pin their particularity onto the envelopes and capsids surrounding them, matching to their “tissue of choice” through cell-specific receptor binding. However, early research into viral tropisms dispelled this notion as an absolute for their propagation, highlighting that such interacting receptors, such as the human poliovirus receptor (PVR), is expressed in a wide variety of tissues, yet poliovirus replication is limited to a selection of tissues, including the neurons of the central nervous system (9). Delving further, differing cellular extracts (HeLa vs neuronal), devoid of surface receptors, were found to differentially promote translation initiation (10), suggesting there are indeed intracellular-specific factors that determine the effectiveness of translation of particular transcripts. A specific example of this was demonstrated by Pilipenko et al., with a targeted mutation to the internal ribosome entry site (IRES) of the GDVII strain of Theiler’s murine encephalomyelitis virus resulting in pronounced loss of binding to the neuronal homologue of polypyrimidine tract-binding protein (nPTB), as opposed to normal PTB. This was later associated with decreased translation initiation and replication in neurons, although such measures were unaffected in other cell types (11).

Conclusions and Future Perspectives

As a more detailed picture of cellular translation emerges, targeted therapeutics at this level of gene expression can become more of a realised possibility, instead of existing in the hypothetical space. Translation is coming to be realised not as a static, one-shoe-fits-all process, but constantly dynamic, with concepts like the heterogeneity of the ribosome gaining ever more traction. Yet, it is not only the structural organisation of the ribosomes that contribute to such cellular differences, but the interaction between the ribosome and the varying cellular cytosolic environments to which it inhabits, that determines ultimate gene expression. The elucidation of such mechanics will no doubt have implications for the areas of translation for which translation has long been associated with, such as developmental disorders, cancer, and viral infection. However, more importantly, treatments directed towards these foci have the potential to be highly specific, and as such, represent a step forward over the broad, non-specific treatments that exist for some of these associated conditions.

References

1.            Simmons DM, Voss JW, Ingraham HA, Holloway JM, Broide RS, Rosenfeld MG, et al. Pituitary cell phenotypes involve cell-specific Pit-1 mRNA translation and synergistic interactions with other classes of transcription factors. Genes Dev. 1990;4(5):695-711.

2.            Asa SL, Kovacs K, Halasz A, Toszegi AM, Szücs P. Absence of somatotrophs, lactotrophs, and thyrotrophs in the pituitary of two dwarfs with hypothyroidism: Deficiency of pituitary transcription factor-1? Endocr Pathol. 1992;3(2):93-8.

3.            Hill JR, Morris DR. Cell-specific translation of S-adenosylmethionine decarboxylase mRNA. Regulation by the 5′ transcript leader. J Biol Chem. 1992;267(30):21886-93.

4.            Hill JR, Morris DR. Cell-specific translational regulation of S-adenosylmethionine decarboxylase mRNA. Dependence on translation and coding capacity of the cis-acting upstream open reading frame. Journal of Biological Chemistry. 1993;268(1):726-31.

5.            Choo Andrew Y, Yoon S-O, Kim Sang G, Roux Philippe P, Blenis J. Rapamycin differentially inhibits S6Ks and 4E-BP1 to mediate cell-type-specific repression of mRNA translation. Proceedings of the National Academy of Sciences. 2008;105(45):17414-9.

6.            Hornstein N, Torres D, Das Sharma S, Tang G, Canoll P, Sims PA. Ligation-free ribosome profiling of cell type-specific translation in the brain. Genome Biology. 2016;17(1):149.

7.            Sapkota D, Lake AM, Yang W, Yang C, Wesseling H, Guise A, et al. Cell-Type-Specific Profiling of Alternative Translation Identifies Regulated Protein Isoform Variation in the Mouse Brain. Cell reports. 2019;26(3):594-607.e7.

8.            Gerashchenko MV, Peterfi Z, Yim SH, Gladyshev VN. Translation elongation rate varies among organs and decreases with age. Nucleic Acids Res. 2021;49(2):e9.

9.            Ren R, Racaniello VR. Human poliovirus receptor gene expression and poliovirus tissue tropism in transgenic mice. Journal of virology. 1992;66(1):296-304.

10.         Haller AA, Stewart SR, Semler BL. Attenuation stem-loop lesions in the 5′ noncoding region of poliovirus RNA: neuronal cell-specific translation defects. J Virol. 1996;70(3):1467-74.

11.         Pilipenko EV, Viktorova EG, Guest ST, Agol VI, Roos RP. Cell-specific proteins regulate viral RNA translation and virus-induced disease. The EMBO journal. 2001;20(23):6899-908.

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