Novel functions of core cell cycle regulators in neuronal migration

p27kip1 Modulates the Morphology and Phagocytic Activity of Microglia

p27^kip1 is a multifunctional protein that promotes cell cycle exit by blocking the activity of cyclin/cyclin-dependent kinase complexes as well as migration and motility via signaling pathways that converge on the actin and microtubule cytoskeleton. Despite the broad characterization of p27^kip1 function in neural cells, little is known about its relevance in microglia. Here, we studied the role of p27^kip1 in microglia using a combination of in vitro and in situ approaches. While the loss of p27^kip1 did not affect microglial density in the cerebral cortex, it altered their morphological complexity in situ. However, despite the presence of p27^kip1 in microglial processes, as shown by immunofluorescence in cultured cells, loss of p27^kip1 did not change microglial process motility and extension after applying laser-induced brain damage in cortical brain slices. Primary microglia lacking p27^kip1 showed increased phagocytic uptake of synaptosomes, while a cell cycle dead variant negatively affected phagocytosis. These findings indicate that p27^kip1 plays specific roles in microglia.

A cancer-associated CDKN1B mutation induces p27 phosphorylation on a novel residue: a new mechanism for tumor suppressor loss-of-function

CDKN1B haploinsufficiency promotes the development of several human cancers. The gene encodes p27^Kip1, a protein playing pivotal roles in the control of growth, differentiation, cytoskeleton dynamics, and cytokinesis. CDKN1B haploinsufficiency has been associated with chromosomal or gene aberrations. However, very few data exist on the mechanisms by which CDKN1B missense mutations facilitate carcinogenesis. Here, we report a functional study on a cancer‐associated germinal p27^Kip1 variant, namely glycine9‐>arginine‐p27^Kip1 (G9R‐p27^Kip1) identified in a parathyroid adenoma. We unexpectedly found that G9R‐p27^Kip1 lacks the major tumor suppressor activities of p27^Kip1 including its antiproliferative and pro‐apoptotic functions. In addition, G9R‐p27^Kip1 transfection in cell lines induces the formation of more numerous and larger spheres when compared to wild‐type p27^Kip1‐transfected cells. We demonstrated that the mutation creates a consensus sequence for basophilic kinases causing a massive phosphorylation of G9R‐p27^Kip1 on S12, a residue normally never found modified in p27^Kip1. The novel S12 phosphorylation appears responsible for the loss of function of G9R‐p27^Kip1 since S12AG9R‐p27^Kip1 recovers most of the p27^Kip1 tumor suppressor activities. In addition, the expression of the phosphomimetic S12D‐p27^Kip1 recapitulates G9R‐p27^Kip1 properties. Mechanistically, S12 phosphorylation enhances the nuclear localization of the mutant protein and also reduces its cyclin‐dependent kinase (CDK)2/CDK1 inhibition activity. To our knowledge, this is the first reported case of quantitative phosphorylation of a p27^Kip1 variant on a physiologically unmodified residue associated with the loss of several tumor suppressor activities. In addition, our findings demonstrate that haploinsufficiency might be due to unpredictable post‐translational modifications due to generation of novel consensus sequences by cancer‐associated missense mutations.

Core cell cycle machinery is crucially involved in both life and death of post-mitotic neurons

A persistent dogma in neuroscience supported the idea that terminally differentiated neurons permanently withdraw from the cell cycle. However, since the late 1990s, several studies have shown that cell cycle proteins are expressed in post-mitotic neurons under physiological conditions, indicating that the cell cycle machinery is not restricted to proliferating cells. Moreover, many studies have highlighted a clear link between cell cycle-related proteins and neurological disorders, particularly relating to apoptosis-induced neuronal death. Indeed, cell cycle-related proteins can be upregulated or overactivated in post-mitotic neurons in case of acute or degenerative central nervous system disease. Given the considerable lack of effective treatments for age-related neurological disorders, new therapeutic approaches targeting the cell cycle machinery might thus be considered. This review aims at summarizing current knowledge about the role of the cell cycle machinery in post-mitotic neurons in healthy and pathological conditions.

Prenatal stress and genetic risk: How prenatal stress interacts with genetics to alter risk for psychiatric illness

Risk for neuropsychiatric disorders is complex and includes an individual's internal genetic endowment and their environmental experiences and exposures. Embryonic development captures a particularly complex period, in which genetic and environmental factors can interact to contribute to risk. These environmental factors are incorporated differently into the embryonic brain than postnatal one. Here, we comprehensively review the human and animal model literature for studies that assess the interaction between genetic risks and one particular environmental exposure with strong and complex associations with neuropsychiatric outcomes-prenatal maternal stress. Gene-environment interaction has been demonstrated for stress occurring during childhood, adolescence, and adulthood. Additional work demonstrates that prenatal stress risk may be similarly complex. Animal model studies have begun to address some underlying mechanisms, including particular maternal or fetal genetic susceptibilities that interact with stress exposure and those that do not. More specifically, the genetic underpinnings of serotonin and dopamine signaling and stress physiology mechanisms have been shown to be particularly relevant to social, attentional, and internalizing behavioral changes, while other genetic factors have not, including some growth factor and hormone-related genes. Interactions have reflected both the diathesis-stress and differential susceptibility models. Maternal genetic factors have received less attention than those in offspring, but strongly modulate impacts of prenatal stress. Priorities for future research are investigating maternal response to distinct forms of stress and developing whole-genome methods to examine the contributions of genetic variants of both mothers and offspring, particularly including genes involved in neurodevelopment. This is a burgeoning field of research that will ultimately contribute not only to a broad understanding of psychiatric pathophysiology but also to efforts for personalized medicine.

SOX2 is sequentially required for progenitor proliferation and lineage specification in the developing pituitary

Sox2 mutations are associated with pituitary hormone deficiencies and the protein is required for pituitary progenitor proliferation, but its function has not been well characterized in this context. SOX2 is known to activate expression of Six6, encoding a homeodomain transcription factor, in the ventral diencephalon. Here, we find that the same relationship likely exists in the pituitary. Moreover, because Six6 deletion is associated with a similar phenotype as described here for loss of Sox2, Six6 appears to be an essential downstream target of SOX2 in the gland. We also uncover a second role for SOX2. Whereas cell differentiation is reduced in Sox2 mutants, some endocrine cells are generated, such as POMC-positive cells in the intermediate lobe. However, loss of SOX2 here results in complete downregulation of the melanotroph pioneer factor PAX7, and subsequently a switch of identity from melanotrophs to ectopic corticotrophs. Rescuing proliferation by ablating the cell cycle negative regulator p27 (also known as Cdkn1b) in Sox2 mutants does not restore melanotroph emergence. Therefore, SOX2 has two independent roles during pituitary morphogenesis; firstly, promotion of progenitor proliferation, and subsequently, acquisition of melanotroph identity.

Summary: SOX2 has two independent roles during pituitary morphogenesis: promoting progenitor proliferation via SIX6 and determining melanotroph identity via PAX7.

Sphingomyelin Synthase 1 Regulates Neuro-2a Cell Proliferation and Cell Cycle Progression Through Modulation of p27 Expression and Akt Signaling

Sphingomyelin synthase (SMS) is a key enzyme involved in the generation of sphingomyelin (SM) and regulation of cell growth and survival. However, the effects of SMS on neuronal cell proliferation and cell cycle progression are not completely elucidated. In this study, we examined the direct effects of SMS1 in regulating cell cycle progression and proliferation of Neuro-2a cells that exhibit neuronal characteristics. Neuro-2a cells transfected with SMS-specific small hairpin RNA (shRNA) expressed significantly lower levels of SMS1. RNA interference-mediated depletion of SMS1 in Neuro-2a cells caused a significant decrease in SM levels. Decreased SMS1 levels resulted in reduced proliferation rate and morphological changes including neurite-like outgrowth. Also, silencing of SMS1 induced cell cycle arrest as shown by the increased percentage of cells in G0/G1 and decreased proportion of cells in S phase. These changes were accompanied by upregulation of cyclin-dependent kinase inhibitor p27 and decreased levels of cyclin D1 and phospho-Akt. Nuclear accumulation of p27 was also evident in SMS1-deficient cells. Furthermore, loss of SMS1 inhibited the migratory potential of Neuro-2a cells in association with decreased levels of matrix metalloproteinases. These results indicate that SMS1 plays an important role in mediating the key signaling pathways that are involved in the tight coordination of multiple cellular activities, including neuronal cell proliferation, cell cycle progression, and migration, and therefore may have significant implications in neurodegenerative diseases.