Protein kinase B (PKB/Akt) is a regulator of cell survival and apoptosis. To become fully activated, PKB/Akt requires phosphorylation at two sites, threonine 308 and serine 473, in a phoshpatidylinositol (PI) 3-kinasedependent manner. The kinase responsible for phosphorylation of threonine 308 is the PI 3-kinase-dependent kinase-1 (PDK-1), whereas phosphorylation of serine 473 has been suggested to be regulated by PKB/Akt autophosphorylation in a PDK-1-dependent manner. However, the integrin-linked kinase (ILK) has also been shown to regulate phosphorylation of serine 473 in a PI 3-kinase-dependent manner. Whether ILK phosphorylates this site directly or functions as an adapter molecule has been debated. We now show by in-gel kinase assay and matrix-assisted laser desorption-ionization time-of-flight mass spectrometry that biochemically purified ILK can phosphorylate PKB/Akt directly. Co-immunoprecipitation analysis of cell extracts demonstrates that ILK can complex with PKB/Akt as well as PDK-1 and that ILK can disrupt PDK-1/PKB association. The amino acid residue serine 343 of ILK within the activation loop is required for kinase activity as well as for its interaction with PKB/Akt. Mutational analysis of ILK further shows a crucial role for arginine 211 of ILK within the phosphoinositide phospholipid binding domain in the regulation of PKB-serine 473 phosphorylation. A highly selective small molecule inhibitor of ILK activity also inhibits the ability of ILK to phosphorylate PKB/Akt in vitro and in intact cells. These data demonstrate that ILK is an important upstream kinase for the regulation of PKB/Akt.Interaction of cells with the extracellular matrix results in the suppression of apoptosis and promotes cell cycle progression (1-4). The molecular basis for this anchorage-dependent cell growth and survival is an intensive area of study, since oncogenically transformed cells very often grow in an anchorage-independent manner. Alterations in anchorage-dependent signaling pathways are also likely to be of importance in tumor progression leading to metastasis. The integrin-linked kinase (ILK) 1 is an intracellular protein kinase that couples integrins and growth factors to downstream signaling pathways involved in the suppression of apoptosis and in promoting cell cycle progression. Cell-extracellular matrix interactions stimulate two major signaling pathways, leading to the regulation of cell cycle progression and cell survival. These are the Ras/Raf-MAP kinase pathway, and the protein kinase B/Akt (PKB/Akt) cell survival pathway (5). ILK regulates both the cell cycle, by stimulating the expression of cyclin D1 (6, 7) and cyclin A (7), and the activity of PKB/Akt, by stimulating the phosphorylation of PKB/Akt on serine 473 in a PI-3 kinase-dependent manner (8, 9), a requirement for full activation of this enzyme. Overexpression of ILK suppresses anoikis by activating PKB/ Akt (10), and ILK activity is constitutively up-regulated in tumor cells lacking expression of the PI(3,4,5)P 3 phosphatase tumor suppress...

Auditory signals are transmitted from the inner ear through the brainstem to the higher auditory regions of the brain. Neurons throughout the auditory system are tuned to stimulus frequency, and in many auditory regions are arranged in topographical maps with respect to their preferred frequency. These properties are assumed to arise from the interactions of convergent and divergent projections ascending from lower to higher auditory areas; such a view, however, ignores the possible role of descending projections from cortical to subcortical regions. In the bat auditory system, such corticofugal connections modulate neuronal activity to improve the processing of echo-delay information, a specialized feature. Here we show that corticofugal projections are also involved in the most common type of auditory processing, frequency tuning. When cortical neurons tuned to a specific frequency are inactivated, the auditory responses of subcortical neurons tuned to the same frequency are reduced. Moreover, the responses of other subcortical neurons tuned to different frequencies are increased, and their preferred frequencies are shifted towards that of the inactivated cortical neurons. Thus the corticofugal system mediates a positive feedback which, in combination with widespread lateral inhibition, sharpens and adjusts the tuning of neurons at earlier stages in the auditory processing pathway.

An understanding of the neural mechanisms responsible for auditory information processing is incomplete without a careful examination of substantial descending pathways. This study focuses on the functional role of corticofugal projections. Our work with the house mouse reveals that the focal electrical stimulation of the primary auditory cortex leads to profound changes in auditory response properties in the central nucleus of the inferior colliculus of the midbrain. Cortical stimulation does not impact on the collicular best frequencies when the best frequencies of stimulated cortical neurons and recorded collicular neurons are similar. Rather, collicular best frequencies are shifted toward the stimulated cortical best frequencies when cortical and collicular frequencies are different. Such a shift is unrelated to the differences in minimum thresholds between cortical and collicular neurons. In addition to frequency-specific shifts in collicular best frequencies, cortical stimulation elevates collicular minimum thresholds and reduces both dynamic ranges and response magnitudes if cortical and collicular best frequencies are different. If cortical and collicular best frequencies are similar but minimum thresholds are different, collicular minimum thresholds are shifted toward the stimulated cortical thresholds; dynamic ranges and response magnitudes may either increase or decrease in this scenario. Our results suggest that the corticofugal adjustment has a centre-surround organization with regard to both cortical best frequencies and cortical minimum thresholds. The midbrain processing of sound components in the centre of cortical feedback is largely enhanced while processing in the surround is suppressed.

The Jamaican mustached bat has delay-tuned neurons in the inferior colliculus, medial geniculate body, and auditory cortex. The responses of these neurons to an echo are facilitated by a biosonar pulse emitted by the bat when the echo returns with a particular delay from a target located at a particular distance. Electrical stimulation of cortical delay-tuned neurons increases the delay-tuned responses of collicular neurons tuned to the same echo delay as the cortical neurons and decreases those of collicular neurons tuned to different echo delays. Cortical neurons improve information processing in the inferior colliculus by way of the corticocollicular projection.

The brain selectively extracts the most relevant information in top-down processing manner. Does the corticofugal system, a "back projection system," constitute the neural basis of such top-down selection? Here, we show how focal activation of the auditory cortex with 500 nA electrical pulses influences the auditory information processing in the cochlear nucleus (CN) that receives almost unprocessed information directly from the ear. We found that cortical activation increased the response magnitudes and shortened response latencies of physiologically matched CN neurons, whereas decreased response magnitudes and lengthened response latencies of unmatched CN neurons. In addition, cortical activation shifted the frequency tunings of unmatched CN neurons toward those of the activated cortical neurons. Our data suggest that cortical activation selectively enhances the neural processing of particular auditory information and attenuates others at the first processing level in the brain based on sound frequencies encoded in the auditory cortex. The auditory cortex apparently implements a long-range feedback mechanism to select or filter incoming signals from the ear.