Histone and DNA modifications are critical for both establishment of differentiation programs and maintenance of acquired differentiated phenotypes. Covalent modification of histone tails proved to be a convergent point of multiple signaling pathways and to embody an additional code superimposed on the genetic code.

G9a, also known as EHMT2 (euchromatic histone lysine N-methyltransferase 2) and KMT1C (lysine methyltransferase 1C), is the enzyme responsible for H3K9 mono- and dimethylation (H3K9me1 and H3K9me2) in mammalian euchromatin and is essential for early mouse development. Mammals possess only one paralog of G9a, called GLP or EHMT1/KMT1D. Both enzymes are characterized by a C-terminal catalytic SET domain preceded by ankyrin repeats that function as mono- and dimethyllysine binding modules. G9a knockout mice are embryonic lethal, and the derived embryonic stem cells show dramatically reduced levels of H3K9me1 and H3K9me2, histone marks that are generally associated with transcriptional silencing.

G9a has been implicated in the differentiation of a variety of cell and tissue types. G9a is essential for the differentiation and growth of thenocytes, with the expression of several tendon transcription factors being suppressed in G9a null cells. Furthermore, G9a cooperates with Sharp-1, a potent repressor of skeletal muscle differentiation, methylating MyoD and histones at the myogenin gene promoter; G9a deficiency impairs differentiation of monocytes and T helper cells.

In the nervous system, G9a has been shown to control cognition and adaptive responses by repression of nonneuronal genes. Its ortholog in Drosophila has been described as a regulator of peripheral dendrite development, classic learning, and memory genes. Moreover, G9a has been shown to influence neuronal subtype specification and regulate ethanol-induced neurodegeneration.

Alternative splicing not only is the most important source of mRNA and protein diversity but also greatly contributes to tissue- and species-specific protein patterns in multicellular eukaryotes.

Accumulating evidence points to the existence of multiple functional links between chromatin structure and pre-mRNA splicing. Recently G9a has been identified as a major regulator of VEGFA alternative splicing. The authors demonstrated separate roles for G9a in transcription and alternative splicing.

Here, researchers report that G9a is required for neuron differentiation in the cultured cells as well as in the developing mouse brain. This process correlates with increased inclusion of G9a alternative exon 10 (E10).

Although E10 inclusion does not affect G9a intrinsic catalytic activity, it results in increased H3K9me2 levels due higher nuclear localization of the enzyme and appears to be necessary for efficient differentiation. The alternatively spliced exon does not encode a nuclear localization signal (NLS), but it is predicted to enhance the exposure of the neighboring constitutive NLS to the external hydrophilic milieu.

Moreover, G9a regulates its own alternative splicing, suggesting a positive feedback loop that upregulates H3K9me2 during cell differentiation. . These findings indicate that by regulating its own alternative splicing, G9a promotes neuron differentiation and creates a positive feedback loop that reinforces cellular commitment to differentiation.

http://www.cell.com/cell-reports/abstract/S2211-1247(16)30184-X