**7. MicroRNA and its biogenesis pathway**

**6. Link between miRNA and ALS**

140 Update on Amyotrophic Lateral Sclerosis

small RNA molecules are the controller of our genome.

the miRNAs which are culprits behind ALS [21].

In last few decades, neuroscience has made a remarkable progress. This has led to accumula‐ tion of humongous information including neuronal signalling and neuronal circuits. Neuro‐ science itself is complex and RNA interference (RNAi) molecules have made it much more intricate. The finer details are being searched to cope up with the neurodegenerative disorders. In the postgenomic era, we are still searching mysteries related to non-coding RNA. These

The microRNA molecules are acting as nodes and links of gene expression. These experts work in various biological functions. Using complementarity against cognate mRNA, this 21–22 nt stretch plays havoc. A miRNA can target several mRNAs, and in combination with other regulatory molecules, it can alter a cell's expression and activity. Neurology includes net‐ working between the cells of nervous system and other cells of the organism. Direct and indirect targets of one miRNA make a cell's system much more complex. ALS is a motor neuron disorder and it has been proved in various reports that ALS can be caused due to dysregulation of miRNAs and thus misexpression of proteins in the cells (**Figure 2**). This chapter deals with

**Figure 2.** In ALS, there are mutations in multiple genes like TDP-43, FUS–TLS and SOD1 and these results in activation of stress response involving phosphorylation of eIF2α and stress granule formation. In ALS, reduced pre-miRNA proc‐ essing and decreased levels of different miRNAs are caused by remodelling of Dicer complex in cytoplasm. Thus, the translation of target mRNAs is relieved, resulting in degeneration of motor neurons (adapted from Bicker S, Schratt G.

MicroRNAs in ALS: small pieces to the puzzle. EMBO J. 2015;34[3]:2601–3).

miRNAs, after being discovered in 1993 [13], has come a long way and established itself as one of key regulators at the post-transcriptional level. It gets expressed in wide varieties of organism from plants to humans. Many miRNAs are conserved across several species. These regulatory molecules are of 21–25 nt in length. Many components of miRNA-processing machinery can be found in Archaea and Eukarya, which supports its ancient presence [3]. Databases of different miRNAs are increasing day by day because of its association with several biological processes and diseases. miRNAs account for >3% of all human genes [22, 23]. At first it gained its initial importance because of its involvement in developmental biology. Now it has been noticed that normal miRNA population can be dysregulated in different diseases, and thus, they are being used as biomarker to detect diseases. Sequence-specific regulation by miRNA makes it more specific but it can diversify its function by binding to 3′ UTR sites of several mRNAs.

The miRNA genes can be operated from their own promoter or promoter of another gene [15]. Primary miRNA transcripts are transcribed by RNA pol II in nucleus [16, 17]. Primary transcripts are of multiple hairpin loops. In nucleus Drosha along with DGCR8 cleaves at 3′ end to form pre-miRNAs. Pre-miRNAs have a hanging 3′-OH end [24]. 70 nt structure premiRNAs get transported to cytoplasm from nucleus by Ran-GTP and Exportin-5 [24]. In the cytoplasm, RNase-III Dicer enzyme cut at the loop structure of the pre-miRNA to produce passenger strand and guide strand [24]. The thermodynamically unstable strand, i.e. guide strand, works as a mature miRNA [25]. These mature miRNAs get loaded into RISC complex [RNA-induced silencing complex]. RISC complex contains several proteins which are needed for ribonucleotide binding and cleaving the target mRNA. Argonaute protein, GW182 are some of the factors of RISC [8]. Once RISC factor is assembled, the miRNA guides the complex to its target on the basis of complementary base pairing. The complementarity is generally restricted to the second to eighth base from 5′ end of miRNA, which is called "seed" region [25]. This seed region binds to 3′ UTR of target mRNA [19, 20]. The fate of target mRNA depends upon the extent of base pairing to the cognate miRNA [19, 20]. The miRNA will direct the destruction of the target mRNA, who has the perfect or near-perfect pairing [19, 20]. If there is multiple partially complementary site, the accumulation of protein from target mRNAs will be inhibited.

#### **8. miRNA in mammals**

miRNAs can inhibit translation in many ways like co-translational protein degradation, inhibition of translation elongation, premature termination of translation, inhibition of translation initiation, deposition of mRNAs in cytoplasmic P-bodies and premature ribosome drop-off [8].

## **9. Post-initiation mechanisms**

Studies in the worm *C. elegans* and in mammalian cell cultures present evidences that miRNAs repress protein synthesis after translation is initiated. The contradictory observation that the targets of miRNAs appear to be actively translated while the corresponding protein product remains undetectable prompted the proposal that the nascent polypeptide chain might get degraded co-translationally. The identity of protease is unknown. Proteasome was excluded as a possibility because proteasome inhibitor does not restore the protein. Several other evidences lead to the suggestion that premature ribosome dissociation is caused by miRNAs [8].
