mRNA Localization and Protein Synthesis in Neurons

Post by Sarah Hill

What's the science?

The proteins expressed in a cell drive its biological function. Neurons express ~12,000 different proteins that facilitate a number of functions, including rapid neurotransmission. Proteins are translated from a template mRNA, (protein synthesis), assuming that this occurs as a constant, steady-state process within the neuronal soma (cell body). However, unlike other round cells, neurons have a complex morphology with dendrites and axons branching out from the cell body forming synapses with other neurons. Proteins are required to enable functions such as neurotransmission at synaptic sites located a great distance from the cell body, on short notice. Previous research has demonstrated that a large fraction of postsynaptic (dendritic) proteins are translated locally within dendrites, offering an alternative mechanism to proteins being shuttled from the cell body to the dendrite where they are expressed. This week in Neuron, Fonkeu and colleagues offer new mechanistic insights on mRNA and protein turnover and propose a mathematical model that reliably incorporates the temporal and spatial dynamics of neuronal protein synthesis.

How did they do it?

To model the complex distribution of mRNA within a neuron, the authors first derived an equation that describes the production of an mRNA transcript in the cell nucleus, its subsequent transport to the soma and dendrites, and eventually, its degradation. They then derived a similar formula for neuronal protein distribution, based on a protein's translation from mRNA in the soma or dendrites, its potential transport throughout the cell, as well as its deterioration, noting that the protein distribution model eventually converges such that mRNA and protein synthesis, transport, and degradation balance out. To validate the model, the mRNA and protein formulas were applied using data for CaMKIIα, a key postsynaptic protein, important for synaptic plasticity. The CaMKIIα distribution predicted by the theoretical model was then compared to the distribution obtained experimentally through in situ hybridization and immunohistochemistry.     

What did they find?

Derivation of protein and mRNA distribution formulas enabled the authors to mathematically differentiate between distributions of proteins generated from somatic mRNA and those generated from dendritic mRNA. The distribution obtained experimentally closely matched that predicted by the model; the model indicated that 60% of CaMKIIα proteins were synthesized in dendrites. The model also successfully described three alternative protein distribution patterns, demonstrating its utility for a wide variety of protein and mRNA targets. Finally, they demonstrated the model's value in exposing how various formula parameters, such as mRNA transport velocity, diffusion coefficient, and degradation rate, affect final protein distribution. A slight decrease in mRNA transport velocity, for example, significantly alters the distribution outcome, while a mild to moderate change in the diffusion coefficient does not.                                     

What's the impact?

This is the first study to propose a model outlining the role of mRNA localization and dendritic translation of mRNA into protein far from the cell body in synaptic function. Given the high rates of mRNA and protein turnover, as well as the potential for mRNA localization in either somatic or dendritic sites, an overhaul of existing methods for predicting the distribution of proteins is needed to better capture the highly dynamic processes of neuronal transcription and translation. The model proposed in this study offers an improvement by better encompassing the range of biological nuances that regulate neuronal mRNA and protein stores.

Fonkeu et al. How mRNA Localization and Protein Synthesis Sites Influence Dendritic Protein Distribution and Dynamics. Neuron (2019).Access the original scientific publication here.

Integration of New Dentate Granule Cells Requires Lateral Dispersion

Post by Amanda McFarlan

What's the science?

The subgranular zone in the hippocampus regularly generates new dentate granule cells (neural stem cells) in the adult brain. Studies using chemical and transgenic approaches to target these cells have demonstrated that they migrate laterally along the subgranular zone. However, the mechanisms by which these cells migrate and integrate into the existing hippocampal neural circuit remain largely unknown. This week in Nature Communications, Wang and colleagues investigated the developmental stages of newly generated dentate granule cells in the hippocampus using in vivo imaging techniques in awake, behaving mice.

How did they do it?

To characterize the kinetics and critical period of dentate granule cell dispersion, the authors infected newly generated dentate granule cells in adult mice with a virus encoding the green fluorescent protein (GFP) and used immunohistochemistry at days 5, 6, and 7 post-infection to track the movement of these cells relative to the subgranular zone. Then, to assess the dynamics of dentate granule cell dispersion, they performed in vivo imaging of GFP-positive dentate granule cells in freely behaving mice for 2 days. To further investigate this dynamic process, they used a spinning disk confocal microscope to image virally-labelled dentate granule cells in cultured tissue from adult mice every 30 minutes for 3 days. 

Next, the authors set out to determine whether newly generated dentate granule cells were electrically coupled, by performing whole-cell paired recordings of adjacent GFP-positive dentate granule cells in acute hippocampal slices. They also explored whether these cells were connected via gap junctions by labelling dentate granule cells with GFP in mice deficient for connexin 43 (a gap junction channel protein) and using in vivo imaging techniques to follow the dispersion of these cells. Finally, the authors investigated the importance of lateral dispersion for the integration of dentate granule cells into existing neural networks by expressing either a dominant-negative variant of the connexin 43 gene (disrupts the formation of gap junctions) or GFP (control) in dentate granule cells. They compared changes in dendritic morphology, membrane properties, and evoked and spontaneous synaptic transmission between the connexin 43-deficient and control dentate granule cells.

What did they find?

The authors found that the proportion of GFP-positive dentate granule cells that remained within ±10° relative to the subgranular zone decreased rapidly between days 5 and 7 post-infection, suggesting that dentate granule cells display a lateral dispersion pattern within this time period. In vivo imaging data collected across 2 days revealed that the average distance travelled by a dentate granule cell was 300 µm and the average speed was 5 µm/h, suggesting that the migration of these cells is highly dynamic. The authors also determined that most dentate granule cells moved in one direction, although a small subset of cells moved both forwards and backwards. When imaged in cultured tissue, they found that 35% of GFP-positive dentate granule cells dispersed in a leap-frog manner, jumping over adjacent cells. 

Next, the authors determined that the frequency of electrically coupled cells decreased from day 5 to day 7 post-infection, suggesting that dentate granule cells become uncoupled following the cessation of lateral dispersion. They also found that bath application of carbenoxolone (a connexin blocker) disrupted the electrical coupling of dentate granule cells, suggesting that this coupling occurs through gap junctions. Additionally, in vivo imaging data revealed that lateral dispersion of dentate granule cells between days 5 and 7 post-infection is disrupted in connexin 43-deficient mice compared to controls, suggesting that this dispersion is dependent on electrical coupling via gap junctions. Finally, the authors showed that introducing connexin 43-deficiency in dentate granule cells between days 5 and 7 post-infection resulted in stunted dendritic growth compared to controls, while introducing connexin 43-deficiency in dentate granule cells after day 7 had no effect on dendritic growth compared to controls. Furthermore, they determined that connexin 43-deficient dentate granule cells had reduced evoked excitatory postsynaptic current amplitudes and a decrease in the frequency of spontaneous excitatory postsynaptic currents, suggesting that lateral dispersion of dentate granule cells is important for the integration of these cells into the neural circuit.

What's the impact?

This is the first study to show the developmental stages of newly generated dentate granule cells in the hippocampus of the adult mouse using in vivo imaging techniques. The authors demonstrated that lateral dispersion of the dentate granule cells early on in requires electrical coupling via gap junctions and is necessary for normal dendritic development and circuit integration of these cells. Altogether, these findings provide insight into the migration and integration of new hippocampal cells in the adult brain.

Wang et al. Lateral dispersion is required for circuit integration of newly generated dentate granule cells (2019).Access the original scientific publication here.

The Impact of Language Experience On Perceiving Speech in Noisy Situations

Post by Anastasia Sares 

What's the science?

Researchers have been trying to crack the code of human speech processing for a long time. Speech perception is often tested in a quiet lab setting, but in everyday life, we experience noisy environments and have to figure things out based on context. In addition, there may be differences in the way we process our native language versus a second language acquired later in life. This week in Brain and Language, Kousaie and colleagues looked at how these factors interacted during speech processing.

How did they do it?

To answer their question, the authors recruited three groups of people who spoke both English and French fluently. There was no difference between the groups in terms of language proficiency; only the age that they had learned their second language varied. The first group were simultaneous bilinguals, who had learned both languages from birth (their “second language” was defined as their less dominant one or the one they used less). The next group had learned at an early age, between 3-5 years old. The last group had learned “late,” between 6-9 years old.

The three groups performed a speech discrimination task, where they listened to sentences and had to repeat the final word. Some sentences were presented in the participant’s first language and some in their second language. Some sentences were “high context,” meaning it was easy to predict the last word based on the rest of the sentence (“Stir your coffee with a spoon”), while others had low context, meaning the last word was less predictable (“Bob could have known about the spoon”). Finally, some sentences were presented in quiet, whereas others were played with a background noise of babble-talk, much like you’d find at a café or a bar, for example.

Participants did the test in an MRI scanner, with the scanner turned off during the presentation of stimuli so that the scanner noise didn’t interfere with the speech perception (a technique called sparse-sampling).

What did they find?

Predictably, their performance was almost perfect when the sentences from either language were presented in quiet. The differences appeared when noise was introduced. While working in their first language, everyone benefited from high-context sentences to help them discriminate speech in noise. However, when working in their second language, the later learners did not benefit as much from high-context sentences. Keep in mind that all participants were highly proficient in both languages, and only differed on the age they learned them.

Looking at brain activity, the authors focused on this noisy second language condition. Simultaneous bilinguals showed increased activity in the left inferior frontal gyrus for low-context sentences in noise. This is likely due to the effort of the discrimination, made harder by the lack of context. The later learners, on the other hand, showed the most activity during high-context sentences! The authors suggest that this means their brains "gave up" in the low-context, noisy, second language condition since it was too demanding for them.

What's the impact?

This work is consistent with theories that our neural resources are limited, and that despite appearing perfectly fluent, people who have learned a second language later in life might be using more resources just to keep up in difficult listening situations. Finding a quiet place to talk might help them use their mental energies more effectively!

Kousaie et al. Language learning experience and mastering the challenges of perceiving speech in noise. Brain and Language (2019). Access the original scientific publication here.