**6. Future perspectives and challenges**

Due to their master regulatory role, MAPKs have sparked a lot of interest and have been the main focus of multiple researchers worldwide over the last 30 years. As MAPK knowledge advances, it has become obvious that the external control of MAPK activity has the potential to modulate cellular behavior and survival. Moreover, MAPK signaling has been found to be altered or defective in many human diseases such as cancer; therefore, achievement of the control of

MAPK activity could provide an attractive intervention point for new therapeutic approaches. However, despite the tremendous amount of knowledge generated, there are fundamental questions that remain to be elucidated in order to transform the biomedical potential of MAPKs into a reality.

While the central core of MAPK signaling cascades has been extensively described, branches of the networks have not yet been completely identified. This is especially true in terms of the upstream sensors, where the picture is not well defined, especially in higher eukaryotes. In the immediate future, we therefore foresee that more sophisticated approaches using CRISPR/Cas9 and RNAi-based libraries will provide a means to perform systematic genome-wide genetic screens to reveal missing components of these pathways.

Additionally, there are other features of MAPK signaling that might have been overlooked. An example of such a feature is the peptide-mediated blockage of p38/ JNK interaction with importins, which reduces their nuclear export. The presence of this peptide impaired MAPK nuclear localization and decreased cell proliferation and tumor growth to a larger extent than the presence of commercial p38 inhibitors. These data open up a new perspective on MAPK regulation and need to be further examined as they could provide a new therapeutic intervention strategy to regulate MAPK activity [32]. It is clear that nuclear localization stimulates the encounter of MAPKs with defined chromatin loci, where their targets are located to provide specificity for gene induction; however, how the kinases are directed to these regions is not clear. Similarly, the molecular mechanisms of transcriptional termination have only recently been uncovered, and there are few reports regarding the targets and the control of MAPKs in termination, as many noncoding RNAs have been shown to be regulated by MAPKs such as Hog1/p38 and ERK2 [87, 88].

Remarkably, upstream MAPK pathway components as transcriptional regulators are also unclear, although the recruitment of several MAP2Ks (MEK1/2, MKK6) to chromatin has been detected. Furthermore, an extreme case has been reported in which, upon insulin stimulation, the entire signaling pathway from the insulin receptor to the ERK signaling cascades is recruited to insulin inducible loci [89]. The functions and consequences of such recruitment require further investigation. Due to their master regulatory role, MAPKs have generated a lot of interest. It is clear that controlling MAPK activity could provide a means of controlling cell behavior. Additionally, understanding the consequences of heterogeneity for MAPK-regulated events will be crucial for understanding differential responses to extracellular stimuli and therapeutic treatments. In conclusion, it is of utmost importance that MAPK-mediated mechanisms of controlling gene expression are fully characterized in order to further identify druggable targets/processes that are relevant to human diseases.
