PERK is responsible for the increased phosphorylation of eIF2alpha and the severe inhibition of protein synthesis after transient global brain ischemia

The eukaryotic initiation factor-2 kinase pathway facilitates differential GADD45a expression in response to environmental stress

Phosphorylation of eukaryotic initiation factor-2 (eIF2) regulates general and gene-specific translation in response to diverse environmental stresses. Central to gene expression induced by eIF2 phosphorylation is the preferential translation of ATF4, a basic zipper transcription activator. Phosphorylation of eIF2 and its attendant induction of ATF4 can lead to different patterns of gene expression depending on the environmental stress. This is of fundamental importance because eIF2 kinases can induce the expression of genes involved in survival as well as in apoptosis. In this report, we explore the molecular basis for why there can be differential expression of GADD45a, a stress-responsive protein that regulates genome stability, apoptosis, and immune responses. We find that whereas ATF4 is required for GADD45a transcription during many different environmental stresses, GADD45a protein accumulates only during a limited number of stress arrangements. The basis for this difference between measurable GADD45a mRNA and protein lies in the observation that GADD45a protein is labile. Those stress agents that enhance ATF4-directed GADD45a transcription and impede the turnover of GADD45a protein by blocking ubiquitin/proteasome-mediated degradation elevate GADD45a protein levels. By comparison, those stress arrangements that trigger ATF4 levels and GADD45a transcription, but do not perturb the proteasome pathway, only elevate GADD45a mRNA levels. This study highlights the molecular mechanisms by which environmental stresses can differentially control central regulatory proteins targeted by the eIF2 kinase pathway.

Microcephaly and simplified gyral pattern of the brain associated with early onset insulin-dependent diabetes mellitus

Two families are presented with a child suffering from microcephaly with a simplified gyral pattern of the brain (SGP) and early onset insulin dependent diabetes mellitus (IDDM). The first patient was diagnosed postmortally with Wolcott-Rallison syndrome, after her younger brother developed IDDM, and a homozygous mutation in the eukaryotic translation initiation factor 2-alpha kinase 3 was found. The younger brother did not undergo magnetic resonance imaging (MRI). The patient from the second family has no EIF2AK3 mutation. SGP is considered to arise from decreased neuronal proliferation or increased apoptosis at an early stage of embryonal development, but insight into the pathways involved is minimal. EIF2AK3 is involved in translation initiation. It has been proposed that loss of function mutations reduce the ability of the cell to respond to endoplasmic reticulum stress, resulting in apoptosis of pancreatic Langerhans cells. Our findings suggest that in some cases, early onset IDDM and SGP can arise from common mechanisms leading to increased apoptosis.

Irreversible aggregation of protein synthesis machinery after focal brain ischemia

Focal brain ischemia leads to a slow type of neuronal death in the penumbra that starts several hours after ischemia and continues to mature for days. During this maturation period, blood flow, cellular ATP and ionic homeostasis are gradually recovered in the penumbral region. In striking contrast, protein synthesis is irreversibly inhibited. This study used a rat focal brain ischemia model to investigate whether or not irreversible translational inhibition is due to abnormal aggregation of translational complex components, i.e. the ribosomes and their associated nascent polypeptides, protein synthesis initiation factors and co-translational chaperones. Under electron microscopy, most rosette-shaped polyribosomes were relatively evenly distributed in the cytoplasm of sham-operated control neurons, but clumped into large abnormal aggregates in penumbral neurons subjected to 2 h of focal ischemia followed by 4 h of reperfusion. The abnormal ribosomal protein aggregation lasted until the onset of delayed neuronal death at 24-48 h of reperfusion after ischemia. Biochemical study further suggested that translational complex components, including small ribosomal subunit protein 6 (S6), large subunit protein 28 (L28), eukaryotic initiation factors 2alpha, 4E and 3eta, and co-translational chaperone heat-shock cognate protein 70 (HSC70) and co-chaperone Hdj1, were all irreversibly clumped into large abnormal protein aggregates after ischemia. Translational complex components were also highly ubiquitinated. This study clearly demonstrates that focal ischemia leads to irreversible aggregation of protein synthesis machinery that contributes to neuronal death after focal brain ischemia.

NFkappaB in neurons? The uncertainty principle in neurobiology

Nuclear factor kappaB (NFkappaB) is a dynamically modulated transcription factor with an extensive literature pertaining to widespread actions across species, cell types and developmental stages. Analysis of NFkappaB in a complex environment such as neural tissue suffers from a difficulty in simultaneously establishing both activity and location. Much of the available data indicate a profound recalcitrance of NFkappaB activation in neurons, as compared with most other cell types. Few studies to date have sought to distinguish between the various combinatorial dimers of NFkappaB family members. Recent research has illuminated the importance of these problems, as well as opportunities to move past them to the nuances manifest through variable activation pathways, subunit complexity and target sequence preferences.