Autoimmune responses and inflammation in type 2 diabetes

Obesity-induced insulin resistance is one of the largest noncommunicable disease epidemics that we are facing at the moment. Changes in lifestyle and greater availability of low nutritional value, high caloric food has led to the highest rates of obesity in history. Obesity impacts the immune system and obesity-associated inflammation contributes to metabolic diseases, such as type 2 diabetes. Both the adaptive and the innate immune system play a role in the regulation of glycemic control, and there is a need to understand how metabolic imbalances drive disease pathogenesis. This review discusses the cell types, mediators, and pathways that contribute to immunologic-metabolic crosstalk and explores how the immune system might be targeted as a strategy to treat metabolic disease.

A Dual Inhibitor of Cdc7/Cdk9 Potently Suppresses T Cell Activation

T cell activation is mediated by signaling pathways originating from the T cell receptor (TCR). Propagation of signals downstream of the TCR involves a cascade of numerous kinases, some of which have yet to be identified. Through a screening strategy that we have previously introduced, PHA-767491, an inhibitor of the kinases Cdc7 and Cdk9, was identified to impede TCR signaling. PHA-767491 suppressed several T cell activation phenomena, including the expression of activation markers, proliferation, and effector functions. We also observed a defect in TCR signaling pathways upon PHA-767491 treatment. Inhibition of Cdc7/Cdk9 impairs T cell responses, which could potentially be detrimental for the immune response to tumors, and also compromises the ability to resist infections. The Cdc7/Cdk9 inhibitor is a strong candidate as a cancer therapeutic, but its effect on the immune system poses a problem for clinical applications.

Themis integrates T cell receptor and cytokine signals in CD8+ T cells

T cell receptor (TCR) signaling controls T cell development in the thymus and T cell functions in the periphery. Themis is a T-cell lineage-specific protein with a critical role in T cell development, where it regulates TCR signal strength though modulation of Shp1 phosphatase activity. The role of Themis in mature peripheral T cells is unknown. To address this, we generated Themis conditional knockout (cKO) mice, with post-selection Themis deletion mediated by distal Lck-Cre. The cKO mice have reduced CD8+ T cell numbers. Themis-deficient CD8+ T cells on the OT-I TCR transgenic background show normal functional responses to agonistic pMHC ligands of different affinities. However, Themis-deficient OT-I CD8+ T cells have reduced cell surface expression of CD5 and increased cell surface expression of CD8, suggesting decreased signal from self pMHC. Moreover, cKO OTI CD8+ T cells display severe reduction in lymphopenia-induced (homeostatic) proliferation in vivo, as well as reduction in proliferative responses to low affinity pMHC and pro-inflammatory cytokines in vitro. Signals from low affinity pMHC and cytokines synergistically induce phosphorylation of Akt, metabolic changes, and induction of c-Myc in CD8 T cells. This does not occur in the absence of Themis. Themis performs these functions through the phosphatase Shp1, as double deletion of Themis and Shp1 in peripheral T cells rescues the CD8+ T cell maintenance. This work shows a novel role for cytokines and very low affinity pMHC in altering cellular metabolism to drive CD8+ T cell proliferation. It also demonstrates that Themis and Shp1 act in concert to integrate T cell receptor and cytokine signals.

Identification of Mediators of T-cell Receptor Signaling via the Screening of Chemical Inhibitor Libraries

The T-cell receptor (TCR) signaling pathway comprises a multitude of mediators that transmit signals upon the activation of the TCR. Different strategies have been proposed and implemented for the identification of new mediators of TCR signaling, which would improve the understanding of T-cell processes, including activation and thymic selection. We describe a screening assay that enables the identification of molecules that influence TCR signaling based on the activation of developing thymocytes. Strong TCR signals cause developing thymocytes to activate apoptotic machinery in a process known as negative selection. Through the application of kinase inhibitors, those with targets that affect TCR signaling are able to override the process of negative selection. The method detailed in this paper can be used to identify inhibitors of canonical kinases with established roles in the TCR signaling pathways and also inhibitors of new kinases yet to be established in the TCR signaling pathways. The screening strategy here can be applied to screens of higher throughput for the identification of novel druggable targets in TCR signaling.

Development of a screening strategy for new modulators of T cell receptor signaling and T cell activation

Activation of the T cell receptor (TCR) leads to the generation of a network of signaling events critical to the developmental decision making and activation of T cells. Various experimental approaches continue to identify new signaling molecules, adaptor proteins, and other regulators of TCR signaling. We propose a screening strategy for the identification of small molecules affecting TCR signaling based on the uncoupling of TCR stimulation from cellular responses in developing thymocytes. We demonstrate that this strategy successfully identifies inhibitors of kinases already shown to act downstream of TCR engagement, as well as new inhibitors. The proposed strategy is easily scalable for high throughput screening and will contribute to the identification of new druggable targets in T cell activation.

Target regulation of a motor neuron-specific epitope

In the adult rat nervous system, motor neurons are recognized specifically by a monoclonal antibody, MO-1. Because binding by MO-1 is lost following axotomy, contact with the target may regulate this motor neuron-specific epitope. To test this hypothesis, we examined the recovery of MO-1 immunoreactivity in hypoglossal neurons following unilateral damage to the hypoglossal nerve. During the first week following nerve crush, neurons in the ipsilateral hypoglossal nucleus lost all immunoreactivity for MO-1. Antibody binding returned with time, and by 4 weeks, 80% of the injured neurons had recovered the MO-1 epitope. Since motor neurons reinnervate their original targets readily following nerve crush, it appears that MO-1 binding is recovered when motor neurons return to their original target muscles in the tongue. When the hypoglossal nerve was cut and inserted into a foreign muscle nearby (the sternomastoid muscle), the MO-1 epitope was not detected in the injured neurons, even when examined 6 weeks after surgery. However, if the sternomastoid muscle was denervated prior to insertion of the hypoglossal nerve, thus allowing the hypoglossal nerve to synapse with this foreign target, increasing numbers of hypoglossal neurons reacquired MO-1 immunoreactivity with time. Our results suggest that the MO-1 epitope is only expressed in motor neurons that are in synaptic contact with skeletal muscle. Thus, a property that distinguishes mature motor neurons from other neuronal phenotypes appears to be regulated by direct synaptic interaction with the postsynaptic target.

Distinct neurotrophic responses of axotomized motor neurons to BDNF and CNTF in adult rats

In adult mammals, transection of the hypoglossal nerve results in a dramatic loss of choline acetyltransferase (ChAT) in the hypoglossal motor neurons without affecting their cell number. This injury-induced reduction in ChAT is prevented when brain-derived neurotrophic factor (BDNF) is applied to the proximal end of the transected nerve. In contrast, application of ciliary neurotrophic factor (CNTF) has no such effect, even though both factors are known to rescue developing motor neurons from cell death. These results suggest that BDNF may regulate the phenotypic expression of ChAT in mature motor neurons, and indicate that the sensitivity and response of motor neurons to such neurotrophic agents change with development.

A motor neuron-specific epitope and the low-affinity nerve growth factor receptor display reciprocal patterns of expression during development, axotomy, and regeneration

Somatic motor neurons begin to express the transmitter synthesizing enzyme, choline acetyltransferase (ChAT) and the low-affinity nerve growth factor receptor (NGFR) during embryonic development. However, as motor neurons mature in postnatal life, they lose immunoreactivity for NGFR and acquire a motor neuron-specific epitope that is recognized by the monoclonal antibody, MO-1. The present study was undertaken to examine the effect of nerve injury in adult rats on these three developmentally regulated markers in two populations of somatic motor neurons. Unilateral transection, ligation, or crushing of the sciatic nerve resulted in a loss of MO-1 binding and a concomitant rise in immunoreactivity for NGFR within axotomized motor neurons in lumbar levels of the spinal cord. These changes, detectable within 5 days following nerve injury, are reversed with reinnervation, but persist if reinnervation is prevented by chronic axotomy. Thus, regulation of the expression of NGFR and the MO-1 epitope appears to be critically dependent upon interactions between motor neurons and target muscles. These observations are also consistent with the idea that during regeneration, neurons may revert to a developmentally immature state; in motor neurons, this state is characterized by the presence of NGFRs and the absence of the MO-1 epitope. Transection of the hypoglossal nerve, a purely motor nerve, resulted in a similar loss of MO-1 binding and a selective rise in NGFR immunoreactivity in neurons within the ipsilateral hypoglossal motor nucleus. In addition, immunoreactivity for ChAT was also lost in axotomized hypoglossal motor neurons. In contrast, injury to the sciatic nerve, which bears both sensory and motor axons, did not result in any detectable change in ChAT immunoreactivity in spinal motor neurons.

Early stages in the development of spinal motor neurons

In order to identify early events in the differentiation of motor neurons, the expression of several developmentally regulated, neuronal molecules was investigated by immunohistochemistry on consecutive sections of cervical spinal cord. Motor neurons are among the first neurons to be born and to differentiate within the embryonic rat spinal cord. They undergo their terminal mitosis on embryonic days 10 and 11 (E10-11) and acquire detectable levels of the transmitter synthesizing enzyme, choline acetyltransferase, by E11.5. Staining with antibodies to the 68 kD neurofilament protein revealed motor neurons extending processes out the ventral root as early as E10.5. Monoclonal antibodies to two different epitopes on the cell adhesive molecule, NCAM, bound to myotomes on E10.5, and began to recognize ventral horn neurons by E11. Two other markers of developing neurons, the growth-associated protein, GAP-43, and the surface glycoprotein, TAG-1, were clearly detected on young motor neurons by E11.5. Thus, during the 36 hours following the final mitosis of their precursors, motor neurons acquire cytoskeletal, enzymatic, and cell surface components that distinguish them from other developing cells within the spinal cord. Not all of the newly acquired molecules continue to be expressed by motor neurons. Immunoreactivity for TAG-1 was lost by E12.5, followed by a gradual reduction of immunoreactivity for GAP-43 and the highly polysialylated form of NCAM. By E15, only antibodies to choline acetyltransferase (Phelps et al., J. Comp. Neurol. 307:1-10, 1990), and to neurofilaments, selectively stained motor neurons within the embryonic spinal cord. The transient presence of GAP-43, TAG-1, and the embryonic form of NCAM coincides with a period of vigorous axonal growth and declines when motor neurons reach their targets. This report describes the temporal sequence of early stages in the differentiation of the rodent motor neuronal phenotype. Some of these changes may be related to interactions with their synaptic partners.